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Chapter 22
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ACID AND BASE CATALYSIS ON ZEOLITES
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Jens Weitkamp and Michael Hunger
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Institute of Chemical Technology, University of Stuttgart, Stuttgart, Germany
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1. INTRODUCTION
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Surface acidity is perhaps the most important property of zeolites, if one judges from the
viewpoint of their application in catalysis. Indeed, it was the replacement of amorphous
silica–alumina catalysts by acid zeolites of the faujasite type in fluid catalytic cracking
(FCC) of heavy petroleum fractions almost 50 years ago that stood at the beginning
of the impressive success story of zeolite catalysts in the industrial practice. Today,
even two acid zeolite catalysts are employed in most FCC units [1,2], namely rareearth-stabilized ultrastable zeolite Y as the principal cracking component and H-ZSM-5
as an additive for improving the octane number of the gasoline produced and, at the
same time, enhancing the yield of propene as a by-product. Besides FCC, acid zeolite
catalysts conquered various other processes in petroleum refining and basic petrochemistry [3], such as isomerization of light gasoline [4], hydrocracking of heavy petroleum
distillates [5], catalytic dewaxing [5], alkylation of benzene with ethene or propene [6],
disproportionation of toluene [7], isomerization of xylenes [7] and numerous others.
In some of these processes, the acid zeolite is combined with a hydrogenation/
dehydrogenation component, typically a noble metal, to make the catalyst bifunctional.
In other instances, the principle of acid catalysis is combined with shape-selective catalysis by selecting a zeolite framework with the appropriate pore width and architecture,
which often allows one to suppress the formation of undesired products.
By proper modification techniques, zeolites with basic properties can also be prepared.
However, compared to their acid counterparts, such basic zeolites have so far gained
very little importance in industrial catalysis.
To arrive at the optimal zeolite catalyst for a given application its acidic or basic
properties have to be manipulated in the right manner and tailored for the envisaged
reaction. In this context, it is particularly important to clearly distinguish between (i) the
chemical nature of acid (or basic) sites in a zeolite, i.e., Brønsted versus Lewis sites;
(ii) their respective concentration (or, synonymously, their density) in the zeolite; and
(iii) their strength or strength distribution. Furthermore, in some instances, the relative
abundance of acid (or basic) sites at various locations (e.g., on the external surface or the
interior of the crystallites or in the large and small cavities of the faujasite framework)
may be relevant to catalysis, because it governs the accessibility of those sites for the
reactants.
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The current chapter aims at providing an introduction into the principles of acid and
base catalysis in zeolites and the experimental methods for characterizing the active sites.
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2. ACID CATALYSIS IN ZEOLITES
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2.1. Nature of acid sites
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Brønsted acid sites occurring on the surface of zeolites make these microporous solids
interesting materials for heterogeneous catalysis [3,8,9]. In aluminosilicate-type zeolites,
the 4+ charge on framework silicon atoms and the 2− charges on the four coordinating
oxygen atoms lead to neutral framework tetrahedra SiO4/2 . If, however, the silicon
cation in the framework is substituted by a cation with a 3+ charge, typically with
an aluminum cation, the formal charge on that tetrahedron changes from neutral to
1 − AlO4/2 − . This negative charge is balanced by a metal cation or a hydroxyl proton
forming a weak Lewis acid site or a strong Brønsted acid site, respectively [9–11].
Hydroxyl protons acting as Brønsted acid sites, i.e., as proton donors, are located on
oxygen bridges connecting a tetrahedrally coordinated silicon and aluminum cation
on framework positions (Figure 1(a)). These OH groups are commonly referred to as
structural or bridging OH groups (SiOHAl) [9,10].
Brønsted acidity is observed in a wide variety of other microporous catalysts that
have exchangeable cations, such as crystalline silicoaluminophosphates (SAPOs) [12],
ferrosilicates [13,14] and gallosilicates [15,16]. The Brønsted acid sites in these crystalline materials and in mesoporous materials with amorphous walls, such as MCM-41,
MCM-48 and SBA-15, i.e., materials resembling amorphous silica–alumina catalysts,
most likely arise from sites with local structures similar to those in zeolites. The best
description of Brønsted acid sites in zeolites is a weakly bound proton of a bridging
hydroxyl group between two tetrahedrally coordinated atoms, typically Si and Al. On
the other hand, Brønsted acid sites in amorphous materials are silanol groups involved
in a weak interaction with neighboring atoms acting as Lewis acid sites, i.e., as electron
pair acceptors, such as Al atoms [17].
Assuming equal local structures of bridging OH groups in zeolites substituted with
different metal atoms (SiOHT, T = Al, Ga, Fe, etc.), the acid strength of the hydroxyl
protons depends on the chemical behavior of the substituting atoms. The nature of these
metal atoms influences the acid strength and, hence, the catalytic activity of substituted
zeolites in a characteristic manner [18–20]. In addition, the impact of the Si−O−T bond
angle on the partial charge and the acid strength of the hydroxyl proton have to be
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(c)
(d)
H
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Si OH
Al OH
Si+
Al
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Figure 1. Schematic representation of the different types of hydroxyl groups and acid sites in
zeolites.
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considered. In zeolites, the Si−O−T bond angles vary from 137 to 177 for zeolite
ZSM-5, from 143 to 180 for mordenite and from 138 to 147 for zeolite Y [21,22].
Theoretical studies indicate that at a given Si−O−Al bond angle in the local structure
of SiOHAl groups, a corresponding partial charge and, therefore, a corresponding acid
strength occurs [23].
In another theory describing the chemical behavior of aluminosilicate-type zeolites,
the aluminum site distribution is considered to be the primary factor affecting the acid
strength of SiOHAl groups [24]. The key property is the lower electronegativity of
aluminum atoms in comparison with silicon atoms. For example, in FAU-type zeolites each framework aluminum atom is linked via oxygen bridges with four silicon
atoms, which in turn are connected with nine further T atoms in the next coordination
sphere [25]. These nine T atoms in the latter coordination sphere are called next nearest neighbors (NNNs). According to the NNN concept, the acid strength of SiOHAl
groups in aluminosilicate-type zeolites depends on the number of framework aluminum
atoms on NNN positions. The lower the number of these aluminum atoms (i.e., the
higher the number of silicon atoms), the higher is the acid strength. Figure 2 shows the
distribution of different numbers of aluminum atoms on NNN positions in FAU-type
zeolites for different nSi /nAl ratios [26]. A completely isolated AlO4 tetrahedron (highest
acid strength) has a 0-NNN configuration nSi /nAl 11. A FAU-type zeolite with the
maximum number of framework aluminum atoms nSi /nAl ≈ 1 is characterized by a
9-NNN configuration (lowest acid strength). In FAU-type zeolites with nSi /nAl ratios
between 1 and 11, there is a superposition of different NNN configurations and, hence,
a broad strength distribution of the Brønsted acid bridging OH groups.
The external surface of zeolite particles or framework defects are terminated by
silanol groups (Figure 1(b)), the acid strength of which is low. Removal of aluminum
atoms from the zeolite framework, e.g., by calcination, hydrothermal treatment or treatment with strong acids is the most important origin for the occurrence of framework
defects. Depending on the dealumination conditions, healing of the framework defects
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Figure 2. Distribution of aluminum atoms on next nearest neighbor (NNN) positions in FAU-type
zeolites plotted as a function of the number of framework aluminum atoms per unit cell (bottom
abscissa) or the nSi /nAl ratio (top abscissa) [26].
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by silicon migration, formation of silanol groups or formation of hydroxyl groups at
extra-framework aluminum species (Figure 1(c)) may occur [11]. In addition, there is
evidence for the formation of Lewis acid sites at extra-framework aluminum species and
framework defects (Figure 1(d)).
Finally, the interaction of Lewis and Brønsted acid sites in the framework of weakly
steamed zeolites can lead to very strong, perhaps even superacidic Brønsted sites [27–29].
Upon mild steaming of zeolite H-ZSM-5, Lago et al. [30] found a strongly enhanced
catalytic activity in n-hexane cracking, which was attributed to superacidic Brønsted
sites. A possible structural explanation is the partial hydrolyzation of framework aluminum atoms in the vicinity of bridging OH groups (SiOHAl) due to mild steaming.
These partially hydrolyzed framework aluminum atoms were viewed as strong electronwithdrawing centers for neighboring bridging OH groups creating Brønsted acid sites
with very high strength [30].
Since all the above-mentioned types of acid sites influence the catalytic and sorption
properties of zeolites, their profound investigation is a prerequisite for the successful
industrial application of these materials as solid catalysts [10,31–33].
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2.2. Principles of heterogeneous catalysis by solid acids
2.2.1. Bridging hydroxyl protons as catalytically active sites
It is widely agreed upon that in acid zeolites, Brønsted acidic bridging hydroxyl protons,
as described in the preceding section, act as catalytic sites. Ample experimental evidence
is available which justifies to attribute the catalytic activity to Brønsted rather than Lewis
acidity – though both types of sites are usually present in zeolites.
As early as 1971, Karge [34] reported on the alkylation of benzene with ethene on
H-mordenites which were pretreated at different temperatures. The concentration of both
Brønsted and Lewis acid sites in the mordenite samples was determined by quantitative
IR spectroscopy without and with pyridine as a probe molecule. A very clear correlation
was observed between the catalytic activity and the concentration of Brønsted acid sites,
whereas no such correlation existed for the Lewis acid sites. Later, Karge et al. [35,36]
studied the catalytic activities of H-mordenite and a series of mordenites exchanged
with all alkaline earth cations in the disproportionation of ethylbenzene into benzene and
diethylbenzenes. Interestingly an induction period occurs in this hydrocarbon reaction on
12-membered-ring zeolites: while on-stream, the catalyst is getting more and more active,
until it finally reaches a constant level of activity. These measured plateau activities
for the various zeolites increased in the series Ba-, Sr-, Ca-, Mg-, Be-, H-mordenite,
and the rates of ethylbenzene disproportionation could be clearly correlated with the
IR absorbances of the Brønsted acidic hydroxyl groups without and with pyridine as
probe. In a series of papers by Haag et al. [37–41] it was demonstrated that for carefully
prepared H-ZSM-5 zeolites a linear relationship exists between the concentration of
tetrahedrally coordinated framework aluminum atoms determined by 27 Al MAS NMR
(and hence bridging hydroxyl protons) and the rate of catalytic n-hexane cracking. Using
the dehydration of cyclohexanol as a test reaction for dealuminated, alkaline earthexchanged mordenites, Karge et al. [42] again found a linear relationship between the
rate of reaction and the concentration of Brønsted acid sites.
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Independent proof for the indispensable role of Brønsted acid sites was provided
by Karge et al. [43,44] who introduced La3+ ions into zeolite Y via solid-state ion
exchange: when a mixture of LaCl3 /NH4 -Y was heated at 675 K in the complete absence
of water vapor, La3+ cations migrated into the zeolite, but they did not give rise to acidic
hydroxyl protons (as detected by IR spectroscopy). The material thus obtained did not
show catalytic activity in the disproportionation of ethylbenzene. It was only after a
deliberate contact with water vapor that the Brønsted acid hydroxyl groups could form
according to the Hirschler–Plank mechanism (cf. Section 2.3), and the resulting zeolite
turned out to be a very active catalyst in the chosen hydrocarbon reaction.
The role of Lewis acid sites in catalysis is much less well documented. Evidence has
been reported that Lewis acid sites created in zeolite H-ZSM-5 by mild steaming may
have an electron-withdrawing effect on an adjacent Brønsted acid site, thereby lowering
its deprotonation energy and increasing its strength [30,41], i.e., exerting an indirect
effect on the catalytic activity of Brønsted acid sites. Lewis acid sites have also been
envisaged by some authors to potentially play a role in the formation of carbonaceous
deposits during the catalytic conversion of organic substrates [45,46].
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2.2.2. The adsorbate: alkoxy species and carbocations
Under the influence of a Brønsted acid site, a hydrocarbon reactant may be transformed
into a carbocation (or a species resembling a carbocation). In the current context,
the most important carbocations are classical, tricoordinated carbenium ions and nonclassical, tetra- or pentacoordinated carbonium ions [47]. The chemistry of carbocations
in solution has been elucidated in remarkable detail as a result of studies in liquid
superacids [48,49]. In the channels of a zeolite a given carbocation may assume a
structure and electronic state that differs from those in a liquid superacid. In particular, as
theoretical studies indicate, a considerable degree of covalent bonding occurs in a zeolite
between a carbocation-like species and the anionic framework oxygen [50]. It is hence
getting customary to describe such adsorbates as alkoxy species, at least in their ground
states, rather than as genuine ion pairs. The excited states of such alkoxy species are
envisaged to be (i) carbocation-like in their chemical behavior and (ii) transition states in
the acid-catalyzed conversion of adsorbed alkoxy species which may be looked upon as
intermediates in zeolite-catalyzed reactions of hydrocarbons [51–54]. Consideration of
carbocation reactions known from studies in liquid superacids provides fruitful guidance
in the understanding of reactivity patterns in the pores of acid zeolites [55]. In what
follows, selected hydrocarbon conversions in acid zeolites are therefore discussed in
terms of carbocation chemistry.
In Figure 3, various possibilities are shown for the formation of an alkylcarbenium ion
from an alkane. The dotted lines in the alkylcarbonium ions are meant to represent twoelectron-three-center bonds. Route 1 comprises the attack of a proton at a C–H--bond
followed by abstraction of dihydrogen. Alternatively, the alkane can donate a hydride
ion to another carbenium ion (route 2) which results in a new alkane and carbenium ion.
This intermolecular hydride transfer plays an important role in numerous acid-catalyzed
hydrocarbon reactions, e.g., in cracking (vide infra) or the alkylation of isobutane with
butenes [56,57]. Alkenes readily add a proton (route 3) to give an alkylcarbenium ion.
If one starts with an alkane, route 3 is accessible only if there is an efficient way for
the conversion of alkanes into alkenes. This is not the case on merely (monofunctional)
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Route:
02
H
H
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+H
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CH2
H
+H2
–H
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–H2
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R2
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Figure 3. Formation of alkylcarbenium ions from alkanes and alkenes. The dotted line in the
formulae for alkylcarbonium ions stand for two-electron-three-center bonds.
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acidic zeolites, but on bifunctional catalysts in which the acid component is combined
with a noble metal.
For an understanding of the chemistry of carbocations, their relative stabilities are
important (Figure 4): the stability increases in the series primary < secondary < tertiary
alkylcarbenium ions. Due to their high energy content, primary alkylcarbenium ions
are usually avoided in reaction paths via carbocations. Particularly unfavorable is the
formation of the methyl and ethyl cations, and this explains why, for example, hardly
any methane and C2 hydrocarbons are formed in acid-catalyzed processes like fluid
catalytic cracking [2,58].
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H
C
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H
H
R
H
C
R
R
C
C
R
A secondary
carbenium
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A tertiary
carbenium
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Figure 4. Relative stability of alkyl carbenium ions (R is an alkyl group).
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2.2.3. Skeletal isomerization
Sketched in Figure 5 is an acid-catalyzed cycle for a simple skeletal isomerization of
2-methylpentane into 3-methylpentane. Note that in the chosen reaction, the number of
branchings of the carbon skeleton remains constant, just the position of the branching
changes. For such reactions, the term type A isomerization is customary. The catalytic
cycle starts from the 2-methylpentyl-(2) cation. An intramolecular shift of a hydride
ion (step I) gives the 2-methylpentyl-(3) cation. Next is the intramolecular shift of the
alkyl group (step II) which results in the 3-methylpentyl-(2) cation. The latter undergoes
(step III) an intermolecular hydride transfer (route 2 in Figure 3) with a new reactant
molecule, thereby closing the cycle and forming a 3-methylpentane product molecule.
For starting the catalytic cycle, the protonation of either 2-methylpentane (route 1 in
Figure 3) or an alkene impurity (route 3 in Figure 3) may be considered.
Isomerization reactions in which the number of branchings increases or decreases
are classified as type B isomerizations. Type B isomerizations are generally slower
than those of type A. An example for a type B isomerization is the conversion of
n-hexane into 2-methyl- and 3-methylpentane. If one attempted to interpret such a
skeletal rearrangement by a path analogous to the one depicted in Figure 5, a primary
carbenium ion would inevitably have to be involved. For energetic reasons, however, this
is considered to be unlikely. Various other mechanistic models were therefore designed
which avoid the occurrence of primary carbenium ions. Today, acid-catalyzed type B
rearrangements are generally believed to proceed via protonated cycloalkylcarbonium
ions with a three-membered ring, i.e., protonated cyclopropanes (PCPs). A PCP can be
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Figure 5. Simplest catalytic cycle for the acid-catalyzed skeletal type A isomerization of
2-methylpentane into 3-methylpentane. ZO− stands for an anionic framework oxygen of the
zeolite; the hydrogen atoms on carbon atoms 1, 2 and 5 are omitted for clarity.
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+
H
H HH
H
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H
H
R.C.
+
+
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H
H
H
R.O.
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carbenium ions
R.C.
R.O.
H
H H
H C
H
C C
H
H
C H
C C H
H
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H
C
C C
H
H
Secondary
Edge-protonated
carbenium ions
PCPs
+
H
H
Corner-protonated
PCPs
Figure 6. Mechanistic model for the acid-catalyzed type B rearrangement of a secondary n-hexyl
cation into the 2-methylpentyl-(2) and the 3-methylpentyl-(3) cations via protonated cyclopropanes
(PCPs). The hydrogen atoms on carbon atoms 1, 5 and 6 in the hexyl-(2) cation were omitted for
clarity. R.C., ring closure; R.O., ring opening. The dotted lines in the formulae for PCPs stand for
two-electron-three-center bonds.
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either edge- or corner-protonated. In Figure 6 the widely accepted pathway of type B
rearrangements via PCPs is depicted for the example of branching of an n-hexyl cation.
Experimental evidence in favor of a mechanism via PCBs in type B rearrangements
of alkylcarbenium ions has been obtained on two ways that are entirely independent
from each other:
1. Brouwer and Oelderik [59,60] studied the isomerization of n-pentane and labeled
n-butane-(1)-13 C in the liquid superacid HF-SbF5 at room temperature. Under
these conditions, n-pentane was readily isomerized into isopentane, whereas no
conversion of n-butane into isobutane occurred. However, isotopic scrambling of
n-butane-(1)-13 C into n-butane-(2)-13 C did take place at a rate similar to that of the
type B isomerization of n-pentane. Figure 7 shows that the mechanism via PCPs
indeed predicts isopentane and n-butane-(2)-13 C as products from n-pentane and
n-butane(-1)-13 C, respectively, whereas there is no way for branching of n-butane
into isobutane.
2. Weitkamp [61,62] investigated the skeletal isomerization of the series of n-alkanes
with 6–15 carbon atoms on a bifunctional zeolite Pt/Ca-Y under conditions where
the reactions at the acid sites were rate-controlling. For long-chain n-alkanes with
eight or more carbon atoms, the rate of formation of the isomer with the methyl
branching at the end of the main chain, i.e., in the 2-position, was found to be
ca. one half of the rates of formation of the isomers with the methyl branching at
an internal position (e.g., 3-methyl-, 4-methyl-, 5-methyl- and 6-methyldodecane
from n-tridecane). It was, moreover, demonstrated that exactly this result is predicted by the branching mechanism via PCPs (if quantitatively handled with a
number of straightforward assumptions), while a more conventional carbocation
mechanism analogous to the one shown in Figure 5 completely fails to interpret
the experimental finding.
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H
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Figure 7. Interpretation of results obtained in the isomerization of n-pentane (A, top) and n-butane(1)-13 C (B, bottom) in liquid HF-SbF5 , after Brouwer and Oelderik [59,60]. The face-protonated
cyclopropanes are meant to represent the entire set of edge- and corner-protonated cyclopropanes
shown in Figure 6. R.C., ring closure; R.O., ring opening. Letters a, b and c above the arrows
indicate which bond in the PCP is opened.
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Today, with an installed capacity of ca. 750 × 106 t a−1 , fluid catalytic cracking
(FCC [2,3]) of heavy vacuum gas oil (i.e., the overhead product of vacuum distillation
of the residue from atmospheric petroleum distillation) is the second most important
catalytic process in the world (after catalytic hydrotreating). As already mentioned, the
principal cracking component in the catalyst is rare-earth-stabilized ultrastable zeolite
Y. Motor gasoline, i.e., hydrocarbons in the range from C5 to about C10 or C11 , is the
main product, but considerable amounts of C4 and C3 hydrocarbons are formed as well.
Of these, propene is particularly desired, since it represents a base chemical with an ever
increasing consumption.
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2.2.4.1. Bimolecular cracking
Of course, a hydrocarbon like n-hexane is a product rather than a reactant in the FCC
process. It is nevertheless illustrative to discuss the mechanism of catalytic cracking for
a model hydrocarbon like n-hexane (Figure 8). A secondary hexyl cation forms from
n-hexane either by direct protonation or from a hexene impurity. Next is a type B,
i.e., branching rearrangement (step I) via PCPs, as shown in detail in Figure 6, to give
the tertiary 2-methylpentyl-(2) cation. Via intramolecular hydride shift (step II), the latter
is in equilibrium with the secondary 2-methyl-(4) cation. This can undergo a carbon–
carbon bond cleavage through -scission (step III). -Scission means that the second
next carbon–carbon bond relative to the positively charged carbon atom is broken. The
moieties resulting from that particular -scission are propene, which is desorbed, and
the secondary propyl cation. In the final step IV, the latter undergoes an intermolecular
hydride transfer with an n-hexane molecule which results in desorbed propane and a
new secondary hexyl cation, whereby the catalytic cycle is closed.
It is generally believed that, in hydrocarbon cracking on acid zeolites, step IV in
Figure 8 is rate-controlling. Note also that step IV is a bimolecular reaction, and this is
why the long-established classical mechanism sketched in Figure 8 is also referred to as
“bimolecular cracking”. Of course the crucial step in that mechanism which brings about
a diminution of the carbon number of the reactant molecules is -scission (step III).
Since -scission is, in all probability, not rate-controlling in catalytic cracking on acid
zeolites, the system is not very suitable for studying the chemical features of -scission
in detail.
Fortunately, there is an alternative catalytic system in which -scission is rateand selectivity-controlling, and this is hydrocracking over bifunctional zeolites with a
sufficiently strong hydrogenation/dehydrogenation component like palladium or platinum (“ideal hydrocracking”, see, e.g., Ref. [63]. An ideal hydrocracking catalyst is,
e.g., Pt/Ca-Y zeolite). Systematic studies of ideal hydrocracking of various model alkanes led to a detailed insight into the -scission of alkylcarbenium ions. Based on these
studies, a classification of -scissions has been advanced [64] which is by now broadly
adopted in the literature (see Figure 9).
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Figure 8. Simplest catalytic cycle for the acid-catalyzed cracking of n-hexane into propene and
propane. ZO− stands for an anionic framework oxygen of the zeolite.
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The salient feature of type A -scissions is that they start from a tertiary carbenium
ion and give again a tertiary one. Type B -scissions start from a secondary carbenium
ion and give a tertiary one or vice versa. Type C -scissions start from a secondary
carbenium ion and lead again to a secondary one, whereas type D -scissions start from
a secondary carbenium ion and give a primary one. In much the same way as type A
isomerizations are faster than those of type B (cf. Section 2.2.3), the rates of -scissions
strongly decrease from type A to type D. Note that three branchings in an -position
and at least eight carbon atoms, two branchings in a - or in an -position and a
minimum of seven carbon atoms and one branching in the -position and a minimum of
six carbon atoms are required for -scissions of type A, type B and type C, respectively.
The -scission shown in step III of Figure 8, for instance, would be classified as type
C. The terminology introduced in Ref. [64] was later slightly extended by Buchanan
et al. [65] who distinguished various subcases of type D -scissions and found it even
useful to introduce one further type of -scissions (type E, primary → tertiary) in an
attempt to interpret their results of acid-catalyzed cracking of C5 - to C8 -alkenes on
zeolite H-ZSM-5.
Type A -scissions are very fast and are likely to proceed on acid zeolites even at
temperatures below 100 C, i.e., as a side reaction in isobutane/butene alkylation [57,66].
On the contrary, type D -scissions, which would lead to a primary carbenium ion, are
so slow that they are unlikely to occur to any significant extent in the FCC process.
This explains why the cracking in FCC comes to a complete end at the level of C5 C4
and C3 hydrocarbons in FCC and virtually no methane or C2 hydrocarbons are formed.
Furthermore, as Figure 9 suggests, type A -scissions require relatively bulky precursors,
and it is questionable whether these can be formed under the spatial constraints in
the pores of 10- (or 8-) membered ring zeolites. Indeed, while type A and type B
-scissions govern the cracking chemistry in large-pore zeolites [67,68], type B and
type C -scissions are more likely to occur in medium-pore zeolites [64].
Evidence has repeatedly been reported that -scission proceeds rather sluggishly, if
the carbon–carbon bond to be broken forms part of a naphthenic ring. For example, this
is one of the reasons for the fact that ring opening of bicyclic hydrocarbons into aliphatic
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Aliphatic
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Figure 10. Possible role of the orbital orientation for the easiness of -scission, after Ref. [71].
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products with the same carbon number through catalytic cracking or hydrocracking has
so far been unsuccessful [69,70]. An interesting interpretation for the reluctance of cyclic
carbenium ions to undergo -scissions has been advanced (see Figure 10) by Brouwer
and Hogeveen [71]: in an aliphatic carbenium ion, there is free rotation around the
-bond, and in the most stable conformation, the -bond to be broken and the vacant
p-orbital are ideally coplanar. This results in a maximal orbital overlap in the transition
state of -scission. In a naphthenic carbenium ion, by contrast, the -bond forms part
of the ring and is fixed in a position perpendicular or near-perpendicular to the vacant
p-orbital, so that the situation is very unfavorable for orbital overlap.
The chemistry of FCC is complex, and beside carbon–carbon bond cleavage, a number
of other acid-catalyzed reactions occur at the high process temperatures around 500 C.
Among these side reactions are hydrogen transfer, aromatization and the formation of
carbonaceous deposits (or, synonymously, coke) which deactivate the zeolite catalyst
and are continuously burnt off with air in the fluidized-bed regenerator of the FCC unit.
The formation of carbonaceous deposits is almost ubiquitous in hydrocarbon reactions
over acidic catalysts. An excellent account on coke formation on zeolite catalysts and
the techniques for its characterization may be found in Ref. [72].
2.2.4.2. Monomolecular cracking
From results of a careful investigation of n-hexane and 3-methylpentane cracking on
zeolite H-ZSM-5, Haag and Dessau [73] were led to conclude that a second principal
mechanism may be operative in acid-catalyzed cracking, especially if the pores are sufficiently narrow to inhibit the spatially demanding bimolecular hydride transfer involved
in the bimolecular mechanism (step IV in Figure 8). As opposed to the latter mechanism,
the new mechanism was referred to as monomolecular cracking in the original work. The
essential features of this mechanism, for which the terms “Haag–Dessau cracking” or
“protolytic cracking” are also customary today, are sketched in Figure 11. An excellent
discussion on monomolecular cracking may be found in Ref. [74].
In step I, the alkane reactant Cn C2n+2 is protonated to give an alkylcarbonium ion
Cn H2n+3 + . Note that this step is identical with the first step of route 1 in Figure 3.
Depending on which -bond in the reactant alkane has been protonated, the alkyl
carbonium ion collapses (step II) into dihydrogen, a smaller alkane (for instance methane
or ethane) and an alkene (step III). In the last step, the free Brønsted acid site is restored.
A convincing demonstration of the occurrence of monomolecular cracking was published by Krannila et al. [75]: when extrapolated to zero conversion, cracking of
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Ck H2k+1
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ion
Alkane or
dihydrogen
Figure 11. Monomolecular or Haag–Dessau cracking of an alkane, after Ref. [73]. ZO− designates
an anionic framework oxygen of the zeolite.
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normal-butane n = 4 on zeolite H-ZSM-5 at 798 K gave a product distribution of
15 mol% H2 m = 0, 20 mol% CH4 m = 1, 17 mol% C2 H6 m = 2, 0 mol% C3 H8
m = 3, 15 mol% C2 H4 k = 2, 16 mol% C3 H6 k = 3 and 17 mol% C4 H8 k = 4. The
monomolecular mechanism nicely supplements the classical bimolecular mechanism and
accounts for the formation of small amounts of dihydrogen, methane, ethane and ethene
in fluid catalytic cracking. As already pointed out by Haag and Dessau in the original work, the relative importance of monomolecular cracking increases with decreasing
width of the zeolite pores, with increasing reaction temperature, at low hydrocarbon
partial pressure and, in particular, at low partial pressure of alkenes. Protolytic Haag–
Dessau cracking has also been invoked to be operative in the ring opening of naphthenes
to a synthetic steamcracker feedstock consisting of ethane, propane and n-butane [76].
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2.2.5. Further hydrocarbon reactions on acid zeolites
There is a number of additional acid-catalyzed hydrocarbon reactions in which zeolites
are of high industrial relevance already today or likely to find commercial application in
the near future. Of the latter category is isobutane/butene alkylation into a high-quality
component for motor gasoline. La- or rare-earth-exchanged zeolites X or Y seem to
be among the most promising solid catalysts, if appropriate regeneration procedures
are incorporated into the process. The current mechanistic views on the mechanism of
isobutane/butene alkylation in acid zeolites may be found in Refs. [56,57,66,77].
Another class of highly relevant acid-catalyzed hydrocarbon reactions are those
in which an aromatic reactant is involved. Belonging to this category are Friedel–
Crafts alkylations of benzene (i) with ethene to give ethylbenzene (worldwide capacity
ca. 26 × 106 ta−1 ), (ii) with propene for the manufacture of cumene (worldwide capacity
ca. 10 × 106 ta−1 ) and (iii) with long-chain linear alkenes (ca. C10 H20 to C18 H36 ) for the
production of linear alkylbenzenes (LABs) which are intermediates in the manufacture of
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detergents. The replacement of conventional Friedel–Crafts catalysts such as aluminum
chloride or solid phosphoric acid by acid zeolites like H-ZSM-5 and, more recently,
H-MCM-22 is another impressive success story. For details including the current mechanistic views on benzene alkylation on zeolite catalysts and the potential role of pockets
on the external surface of zeolite MCM-22 in catalysis, Refs. [6,7] are particularly
recommended.
Another aromatic chemical that relies to a very large extent on acid zeolite catalysis
is para-xylene (worldwide production capacity ca. 28 × 106 t a−1 . A whole family of
zeolite-catalyzed processes is used to satisfy the para-xylene market, including xylene
isomerization followed by isomer separation via liquid-phase adsorption on zeolites,
toluene disproportionation and transalkylation of toluene with C9 or higher aromatics.
Here again, reference can be made to an excellent recent review that covers both the
industrial and mechanistic aspects of alkylaromatics conversion on zeolite catalysts [7].
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2.2.6. Bifunctional catalysis
To make a zeolite bifunctional, a small amount of a cation containing a noble metal,
such as PtNH3 4 2+ or PdNH3 4 2+ , is usually introduced into the acid form by ion
exchange in aqueous suspension. Upon thermal decomposition of the ammine complex
(mostly in air or oxygen) and reduction of the metal ion with hydrogen under suitable
conditions [78], very small clusters of the noble metal occur inside the zeolite channels
or cavities. In a good bifunctional catalyst, the average distance between the two kinds of
active centers, i.e., the acid and metal (or hydrogenation/dehydrogenation) sites, should
be below a critical value; otherwise, mass transfer effects between both types of active
sites become rate-controlling for the overall catalytic reaction, which is usually undesired.
Weisz [79] derived a so-called intimacy criterion which allows one to estimate this critical
distance between both types of sites. In his model of bifunctional catalysis, alkenes are
believed to be formed from an alkane reactant at the metal sites, and diffusion of these
alkene intermediates is envisaged as the mechanism of mass transfer between both types
of catalytic sites. As an alternative, it has sometimes been speculated that hydrogen
spillover rather than diffusion of olefinic intermediates could be the mechanism of mass
transfer between both types of sites. A clear experimental discrimination between both
models is difficult, but in one case, i.e., hydrocracking of n-dodecane on a Pd/Ca-Y
zeolite, olefinic intermediates could be unambiguously detected [80].
Typical refinery processes that rely on bifunctional zeolite catalysts are hydrocracking
of heavy vacuum gas oil [3,5] and isomerization of light gasoline [3,4]. Both reactions
are conducted under hydrogen pressure of several tens to more than 100 bar. Hydrocracking has sometimes been described as catalytic cracking with hydrogenation reactions
superimposed which might suggest that essentially the same product distributions occur
in both processes with the main difference that the alkenes produced in FCC are hydrogenated to alkanes in hydrocracking. This would be a severe oversimplification, and in
reality the differences are much more far-reaching. In particular, essentially no coke is
formed in hydrocracking, and it is ideally suited for making diesel fuel and jet fuel, while
a catalytic cracker inevitably cracks the heavy gas oil all the way down to gasoline.
From a mechanistic point of view, the essential difference is that a very efficient and
fast route is available in bifunctional catalysis for the interconversion of alkanes and
alkylcarbenium ions via alkenes (route 3 in Figure 3). As a consequence, the steps at the
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acid sites are rate- and selectivity-controlling and the measurable product distributions
furnish valuable insight into carbocation chemistry.
For example, on bifunctional zeolites with a strong hydrogenation/dehydrogenation
component, such as Pt/Ca-Y or Pd/La-Y, long-chain alkanes with eight or more carbon
atoms can be isomerized without disturbing cracking or hydrocracking reactions. This is
not only of interest for studying the mechanistic details of carbocation rearrangements
(cf. Section 2.2.3), but it has also led to a refinery process for improving the coldflow properties of waxes or base oils for lubricants [5] (“isomerization dewaxing”).
Pt/H-SAPO-11 or Pt/H-ZSM-23 zeolites have been quoted as potential catalysts in such
processes [5].
When a long-chain n-alkane, e.g., n-hexadecane, is converted on a typical bifunctional
zeolite catalyst in the presence of hydrogen, the following products gradually appear
upon increasing the conversion: isohexadecanes with a single branching, isohexadecanes
with two branchings and hydrocracked products. As long as the hexadecane conversion
is below 100%, the carbon number distribution of the hydrocracked products is strictly
symmetrical (see Figure 12). This indicates a pure primary cracking selectivity, i.e., all
cracked products are desorbed from the acid sites before they can undergo a secondary
cracking step. Figure 12 also shows that a completely different distribution of the cracked
products is observed on the monofunctional SiO2 –Al2 O3 –ZrO2 catalyst: already at a
medium cracking conversion of 54%, severe secondary cracking occurs and C4 to C6
hydrocarbons dominate the product pattern. An intermediate behavior is shown by a
bifunctional catalyst with a weak hydrogenation/dehydrogenation component (sulfided
CoO–MoO3 ).
A careful evaluation of all features of the product distributions from long-chain
n-alkanes on bifunctional zeolites with a strong hydrogenation/dehydrogenation led to
the reaction network shown in Figure 13: at mild conditions, the n-alkane is converted
into its monobranched isomers. In a consecutive reaction, these are isomerized a second
time to give dibranched isomers. At even more severe conditions, the latter isomerize
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Figure 12. Typical carbon number distributions in catalytic cracking and hydrocracking of
n-hexadecane at medium cracking conversion XCr ≈ 50–55%. Data for catalytic cracking after
Ref. [81], data for hydrocracking from Ref. [82].
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isoalkanes
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β-scission
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Figure 13. Reaction network for isomerization and hydrocracking of a long-chain n-alkane in a
large-pore bifunctional zeolite catalyst.
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again into tribranched species which, however, are not desorbed but undergo the very fast
type A -scission (cf. Figure 9). In a parallel path, some type B -scission occurs starting
from dibranched carbon skeletons. A more detailed discussion of the isomerization and
hydrocracking paths in large-pore zeolites may be found in Refs. [63,83].
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2.2.7. Shape-selective catalysis
Zeolites are unique catalytic materials in that their pore widths are strictly uniform and
of the same order of magnitude as the dimensions of the reactants, intermediates, transition states and products of the reactions they catalyze. There are countless examples
for shape-selectivity effects occurring in acid and bifunctional zeolites. These are commonly classified into mass transfer effects (also referred to as reactant or product shape
selectivity) and intrinsic chemical effects (or restricted transition state shape selectivity).
In the former case, the size of a reactant or product molecule is too large to move freely
through the pores of the zeolite. In the limiting case, such molecules cannot enter the
pores or escape from them at all. The fundamentals of shape-selective catalysis have
been covered in previous review articles, e.g., in Ref. [84].
Use can be made of certain shape-selective reactions for probing the pore width of
zeolites. The most widely employed reactions for this purpose are (i) the competitive
cracking of an equimolar mixture of n-hexane and 3-methylpentane on acid zeolites
(from the conversions of the two reactants, the constraint index (CI) is calculated), (ii) the
isomerization of n-decane on bifunctional zeolites (the yield ratio of 2-methylnonane
and 5-methylnonane at ca. 5% n-decane conversion is the refined or modified constraint
index CI∗ ) and (iii) hydrocracking of butylcyclohexane on bifunctional zeolites (the
yield ratio of isobutane and n-butane gives the spaciousness index (SI)). While CI and
CI∗ are particularly useful for probing the pore width of 10-membered-ring zeolites, SI
is often the method of choice for 12-membered-ring zeolites. For a discussion of the
underlying reaction mechanisms and shape-selectivity effects, reference can be made to
a recent review [85].
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Finally, it should be mentioned that molecular simulation techniques recently contributed to a much more detailed understanding of shape-selectivity effects on a theoretical basis (for a review see Ref. [86]).
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Bridging OH groups acting as Brønsted acid sites in zeolites are mostly generated by
either of the procedures represented by Eqns. 1 and 2, where ZO− stands for the negatively charged zeolite framework in the vicinity of framework aluminum atoms [87,88]:
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NH4 + + Na+ ZO−
NH4 + ZO−
−Na+
≈573–673 K
−NH3
H+ ZO−
(Eqn. 1)
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i.e., aqueous ion exchange with an ammonium salt followed by thermal decomposition
of the ammonium ions inside the zeolite, or [87,88]:
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LaH2 On 3+ + 3 Na+ ZO−
−3
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Na+
LaH2 On 3+ ZO− 3
≈573 K
−n−2H2 O
LaOHH2 O
H+ ZO− 3 −→ LaOH2 + H+ 2 ZO− 3
2+
(Eqn. 2)
i.e., aqueous ion exchange with the salt of a multivalent metal cation (frequently used
are Mg2+ Ca2+ La3+ or mixed rare-earth cations) followed by thermal dehydration.
The series of reactions shown in Eqn. 2 is usually referred to as the Hirschler–Plank
mechanism [89,90]: the removal of most of the water molecules from the cations gives
rise to strong electrostatic fields inside the zeolite pores, because the multivalent cation
has to neutralize more than one, typically two or three negative charges fixed in the
zeolite framework at a significant distance from each other. Under the influence of
these strong local electrostatic fields, residual water molecules dissociate into a hydroxyl
proton bound to a bridging oxygen atom (SiOHAl) and an OH group bound to the
extra-framework cation. The latter OH group is non-acidic. It is seen from Eqn. 2 that a
maximum of two Brønsted acid sites can be formed per three-valent cation introduced.
The direct ion exchange with mineral acids [87,88]:
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H+ + Na+ ZO−
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−Na+
H+ ZO−
(Eqn. 3)
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is generally less favored, because an exposure of zeolites to such acids often leads to
undesired effects like framework dealumination or, in the case of aluminum-rich zeolites,
to a complete framework collapse.
Finally Brønsted sites are inevitably formed, when cations of metals nobler than
hydrogen are reduced by molecular hydrogen [1,2]:
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PdNH3 4 2+ + 2 Na+ ZO−
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PdNH3 4 2+ ZO− 2
Pd2+ ZO− 2
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−2 Na+
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−4 NH3
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(Eqn. 4)
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(b)
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Si
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O
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H
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Si
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–H2O
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Si
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Si
O
O
Si
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Si
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Si
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Figure 14. Mechanisms of the dehydroxylation of zeolites and formation of Lewis acid sites [11].
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Regardless of the method used for their generation (Eqns. 1–4), the chemical nature of
the Brønsted acid sites is the same, viz., bridging hydroxyl groups consisting of a proton
bound to a framework oxygen connecting SiO4 and AlO4 tetrahedra.
Upon severe heat treatment T ≥ 773 K, the Brønsted acid sites are degraded by
dehydroxylation, and water is split off with the concomitant formation of Lewis acid
sites. Up till now, the precise chemical nature of Lewis acid sites in zeolites is a matter
of research. Lewis acid sites can be attributed to extra-framework aluminum (EFAL)
species of octahedral or tetrahedral coordination as well as tri-coordinated aluminum
atoms partially dislodged in the framework [27,91]. Scherzer and co-workers [92] suggested AlO+ AlOH2 + and AlO(OH) as EFAL species on extra-framework positions
in dealuminated zeolites. Similarly Kühl [87,93] concluded from X-ray spectrometry
that AlO
+ units removed from the zeolite framework are transformed into cationic
extra-framework species, which act as so-called “true” Lewis acid sites. Framework
Lewis acid sites have been suggested to consist of positively charged silicon ions in
the neighborhood of tricoordinated aluminum atoms. Gonzales et al. [94] studied the
formation of framework Lewis sites via dehydroxylation routes (a) and (b) in Figure 14.
Route (b), which contains a defect SiOH group in the vicinity of the bridging OH group,
is significantly less endothermic than and, hence, preferred over route (a), which starts
with two neighboring bridging OH groups.
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2.4. Characterization of acid sites
2.4.1. Test reactions
Cracking of heavy petroleum fractions on acidic zeolite catalysts is among the most
important commercial processes, and consequently cracking of model hydrocarbons
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has frequently been employed for characterizing the catalytic activity. For the most
common zeolites, the cracking rates of hydrocarbons were often found to increase
linearly with the framework aluminum content, i.e., with the density of Brønsted acid
sites. In particular for zeolites H-ZSM-5, the linear increase of the rate of cracking of
n-hexane with the aluminum content has been found to extend over a particularly wide
range of framework compositions [37,41]. For zeolite H-Y, a similar linear increase
of the cracking rate was found for materials with an nSi /nAl ratio larger than 14 [95].
Since most research groups dealing with heterogeneous catalysis are equipped to perform
hydrocarbon cracking reactions, the use of such test reactions has been quite popular in
an attempt to characterize the density of acid sites in zeolites.
The frequently applied alpha test consists of the measurement of the cracking rate of
n-hexane under specified conditions [96]. The correlation between the catalytic activity
and the aluminum content of zeolites H-ZSM-5 and H-Y indicates that this test reaction
can indeed provide information on the density of Brønsted acid sites under favorable
conditions. Upon mild steaming of zeolite H-ZSM-5, however, a significant increase of
the cracking rates for n-hexane in comparison with non-steamed zeolites H-ZSM-5 with
equal acid site densities was found [30]. In addition, the cracking rates may also depend
on the zeolite structure, even for non-steamed catalysts [97]. These few examples show
that great care must be applied, when drawing conclusions on the density of Brønsted
acid sites from results of the alpha test.
Disproportionation of ethylbenzene on acidic zeolite catalysts is another test reaction
that has been studied in detail on a large-pore zeolite, viz., La,Na-Y, by the Catalysis
Commission of the International Zeolite Association [98]. It was among their main
objectives to provide reference data for newcomers in catalysis on zeolites, who want
to test their experimental skill in both the preparation of acid zeolite catalysts and the
performance of catalytic experiments. Figure 15 shows a schematic drawing of the
experimental setup recommended by the Commission [98]: the catalyst, which may be
diluted with an inert material, is placed in the fixed-bed reactor made from glass or quartz.
It is held in its position by glass frits or quartz wool plugs. The temperature is measured
with a thermocouple located inside the catalyst bed. The carrier gas (dry nitrogen or
helium) is loaded in a thermostated saturator with the vapors of the reactant and then sent
through the catalyst bed from top to bottom. Product analysis is preferentially carried
out by an on-line sampling valve and a capillary gas chromatograph.
The products occurring in the acid-catalyzed disproportionation are benzene, unvonverted ethylbenzene, ortho-, meta-, and para-diethylbenzene and – at elevated
conversions – triethylbenzenes. These can be readily separated using standard capillary
columns containing, e.g., CP-Sil5 or Carbowax as stationary phases [98]. Interestingly,
an induction period occurs in the disproportionation of ethylbenzene on large-pore
zeolites, i.e., the catalysts gain activity while on stream. It is only after a certain timeon-stream that the catalyst arrives at a constant (or virtually constant) activity. It is this
plateau in the catalytic activity that can be used for the characterization of the density of
acid sites.
Karge et al. [35,36,99,100] investigated the disproportionation of ethylbenzene on
a broad variety of acid zeolites. They found that, in favorable cases, the catalytic
activity can be correlated with the density of Brønsted acid sites. They moreover
observed that the induction period occurs only on large-pore zeolites, but it is absent
on medium-pore zeolites [99–103]. Also, somewhat higher temperatures are needed
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Gas for gas
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valve
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loop
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ethylbenzene
Off-gas
F1
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stream
Catalyst
Thermostated
bath
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Figure 15. Scheme of the equipment suitable for ethylbenzene disproportionation on solid catalysts. Transfer lines between the reactor and the gas chromatograph should be heated (adapted
from Ref. [98]).
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+
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⊕
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H⊕
Bz
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ZO
Ө
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⊕
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E-Bz
DE-Bz
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H
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⊕
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Figure 16. Catalytic cycle suggested for ethylbenzene disproportionation in large-pore zeolites
according to the Streitwieser–Reif mechanism [101].
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for ethylbenzene disproportionation in medium-pore zeolites than in large-pore zeolites. This has been interpreted in terms of different disproportionation mechanisms,
viz., a dealkylation/ re-alkylation path with free ethene as an intermediate in mediumpore zeolites, as opposed to a mechanism via diphenylmethane intermediates (and no
free ethene) in large-pore zeolites. The latter mechanism (Figure 16) is a modification
of the Streitwieser–Reif mechanism originally proposed for liquid Friedel/Crafts-type
catalysts [104].
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2.4.2. Titration with bases
The titration of acid sites of a solid with bases like n-butylamine and Hammett indicators
enables one, at least in principle, to collect information on both the density and strength
of the sites. The method was introduced by Benesi [105] and has been subsequently
modified [106–108]. Generally, the catalyst surface is titrated with an amine, such as
n-butylamine, in a non-aqueous solvent, and a series of Hammett indicators with different
pKa values is employed. Upon protonation of the indicators by the Brønsted acid sites of
the catalyst under study, a color change is observed, often from yellow to red. Typical
indicators are, e.g., phenylazonaphthylamine pKa = +33, benzeneazodiphenylamine
pKa = +15, dicinnamalacetone pKa = −30 and anthraquinone pKa = −82 [109].
The application of colored indicators in liquid acid–base titrations is a routine method.
When dealing with liquid acids, the underlying assumptions are fulfilled, i.e., chemical
equilibrium is achieved at each time, and the amount of indicator is much too small to
disturb the equilibrium appreciably. In the titration of Brønsted acid sites on the surface
of a solid, however, these assumptions have been severely questioned. An important
topic is the effect of different non-aqueous solvents with different dielectric constants
r , since the energy required to separate charged species is inversely proportional to
r [10]. Streitwieser and Kim demonstrated how large this effect can be [110]. They
compared the basicity of a series of amines in tetrahydrofuran (THF, r = 76) with their
basicities in dimethyl sulfoxide (DMSO, r = 467) and acetonitrile r = 359. While
the protonation of amines in DMSO and acetonitrile results in spatially separated ionic
species, the corresponding protonation products in THF are similar to ion pairs since the
energy required for separating charged species in a medium with low dielectric constant
r is too high. Finally, an additional limitation in the applicability of Hammett indicators
for the characterization of acidic zeolites is the size of the indicator molecules. Often
they cannot enter the channels and cages of, e.g., medium-pore zeolites and thus react
only with sites on the external surface and in the pore mouth region [111]. Because of
these fundamental limitations and weaknesses and the cumbersome and time-consuming
nature of the experiments, the surface titration with bases is no longer a popular technique
for characterizing the surfaces of acid zeolite catalysts.
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2.4.3. Temperature-programmed desorption of bases
Temperature-programmed desorption (TPD) of bases is again designed to furnish information on both the density and strength of acid sites on the surface of a solid (see
also Chapter 17). The principle is simple and readily conceivable: at first the solid
is completely degassed, e.g., by evacuation at 773 K. Thereafter, a gaseous base like
ammonia or pyridine vapors are adsorbed, typically at 373 K. The experiment now starts
by heating the base-loaded solid in a stream of inert gas like helium, argon or nitrogen in a temperature-programmed manner. The amount of base desorbed is detected
gravimetrically [112], volumetrically [112], by gas chromatography [113] or mass spectrometry [114]. In principle, the area under the desorption curve gives the number of
acid sites, while the temperatures at which desorption occurs is related to the acid
strength. Figure 17 shows curves obtained for the temperature-programmed desorption
of ammonia (TPDA) from H-SAPO-5 (a), H-SAPO-11 (b) and H-ZSM-5 (c) [115]. The
desorption peaks occurring at 438 and 513 K for H-SAPO-5 and at 438 and 548 K for
H-SAPO-11 were assigned to two kinds of Brønsted acid sites with medium strength.
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Desorption temperature (K)
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Figure 17. Temperature-programmed desorption of ammonia (TPDA) from the silicoaluminophosphates H-SAPO-5 (a) and H-SAPO-11 (b) and zeolite H-ZSM-5 (c) [115].
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For zeolite H-ZSM-5, two desorption peaks were detected at 478 and 673 K, which were
attributed to Brønsted acid sites of medium and high strength, respectively.
The example is quite representative, in that the strength of the acid sites from which the
probe molecules desorb is simply correlated in a qualitative manner with the temperature
of the maximum in the desorption peak. Since the desorption spectra are usually poorly
resolved, curve deconvolution techniques must be applied. Even then it is difficult to
compare results that were obtained with different adsorbate–zeolite systems. There is
always an uncertainty as to what extent the desorption spectra are affected by a hindered
diffusion of the desorbed base and a re-adsorption on its way out of the pores [116,117].
The integral of the desorption peaks in Figure 17 were utilized to determine the density of the Brønsted acid sites in the materials under study [115]. However, deriving
site densities from TPDA curves is not free from problems. The primary difficulty is
that ammonia interacts with both Brønsted and Lewis acid sites, and it is not possible
to distinguish between the nature of the sites from which the base has been desorbed.
Juskelis et al. [118], showed that ammonia desorbs from CaO, which is usually considered as a base, at a higher temperature than from strongly acidic USY zeolite. Since most
calcined zeolites contain extra-framework aluminum species and framework defects, the
amount of desorbed ammonia is by no means equal to the number of Brønsted acid sites.
Woolery et al. [119] suggested an interesting method for eliminating ammonia adsorbed
on Lewis acid sites: upon exposing the calcined zeolites to ammonia, the samples are
mildly steamed. It is claimed that water displaces ammonia from the Lewis acid sites,
but not from the Brønsted acid sites. It was demonstrated that the method works well for
high-silica zeolites yielding a good correlation between the Brønsted acid site densities
obtained by TPDA and other methods.
An interesting alternative approach for the determination of the density of Brønsted
acid sites on solid catalysts is the temperature-programmed desorption of reactive
amines [120–123]. Alkylammonium ions, which are formed by protonation of amines at
Brønsted acid sites, react in a very narrow temperature range via a reaction similar to
the Hofmann elimination reaction [10,123]:
R-CH2 -CH2 -NH2 + ZOH → R-CH2 -CH2 -NH3 + · · ·ZO−
(Eqn. 5)
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R-CH2 -CH2 -NH3 + · · ·ZO− → R-CH2 = CH2 + NH3 + ZOH
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Turbomolecular pump
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Figure 18. Scheme of the equipment used for TPD-TGA measurements [123].
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A typical experiment is performed in vacuum with the sample placed in a microbalance
as shown in Figure 18. This arrangement allows the simultaneous determination of the
gas phase partial pressure and the sample weight. After saturating the zeolite sample
with the amine at 298 K, e.g., with isopropylamine, and evacuation for 1 h, all molecules
except those that are bound to a framework aluminum atom are desorbed. Isopropylamine
molecules in excess of one per framework aluminum atom do not react below 500 K and
leave the sample until a coverage of 1:1 is reached. The remaining complexes decompose
via Eqns. 5 and 6 to propene and ammonia between 625 and 700 K. Desorption of the
amine, propene and ammonia can be observed, e.g., by mass spectrometry at m/e = 30,
41 and 17, respectively [10].
An important feature of the study of Brønsted acid sites of zeolites by TPD of
amines is that the results are independent of which alkylamine is used to probe the site
density, as long as the amine is small enough to enter the zeolite pores and cages. For
H-ZSM-5 zeolites with varying framework aluminum contents, e.g., a reaction of one
amine molecule per aluminum atom was found [121]. The temperature at which the
decomposition occurs depends on the nature of the alkyl group rather than on the type
of the solid acid under study. Hence, the technique cannot be used to determine the
strength of Brønsted acid sites [123].
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2.4.4. Microcalorimetry
When an acid is neutralized by a reaction with a base, the heat of adsorption is evolved,
which is larger when the acid site is the stronger. Therefore, the heat of adsorption of
basic molecules on acidic zeolites can be utilized to characterize the strength of Brønsted
acid sites. Heats of adsorption are usually determined in two ways: either by calculating
the isosteric heats from adsorption isotherms, measured at different temperatures, or by
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measuring the heat of adsorption directly with a calorimeter at a chosen temperature. In
the case of zeolites, the adsorption heats obtained via these two methods differ significantly. These differences are caused by the fact that in calculating isosteric adsorption
heats using the Clausius–Clapeyron equation, it is difficult to evaluate a derivative from
an experimental curve. Direct calorimetric measurements are free from such deviations
and give more reliable results. Enthalpies of adsorption, determined by calorimetric measurements, thus provide valuable insight into the mechanism of adsorption and, hence,
into the nature of adsorption sites [10,124–126].
The study of surface sites on solid catalysts by gas adsorption microcalorimetry
requires an equipment for the simultaneous determination of the adsorbed amount of gas
molecules and of the adsorption heat. Generally, the adsorption heat is measured with
an isothermal and differential microcalorimeter, such as the Tian-Calvet microcalorimeter [127]. Figure 19 shows the scheme of such equipment with a pump for evacuating
the sample volume, a system for dosing the adsorbate and the microcalorimeter allowing
the measurement of adsorption heat in the left sample chamber in comparison with the
reference chamber on the right side. In such a way, it is possible to perform the adsorption experiment at constant temperature. The amount of adsorbed probe molecules is
determined by the manometer linked to the calorimeter. With this manometer, the quantity na of the adsorbed gas molecules and thus the adsorption isotherm can be measured.
Simultaneously, the variation in the signals of the thermal sensors of the calorimeter
gives the amount of heat Qint developed by the adsorption process. If the adsorption heat
is measured in such a way that no external heat is transferred to the calorimeter excluding
the one by adsorption, the true differential heat of adsorption qdiff is obtained by [126]:
qdiff = dQint /dna
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In most cases, this differential heat qdiff , i.e., the molar heat of each dose of adsorbates,
is plotted as a function of na . Hence, the ratio of the amount of heat evolved for each
increment to the number of moles of adsorbed probe molecules in equal periods is
identical to the average value of the differential enthalpy of adsorption.
As an example, Figure 20 shows the differential heats of adsorption for pyridine on
four H-ZSM-5 zeolites with aluminum contents of 180, 370, 530 and 600 mol g−1 [128].
In each case, the differential heats are constant at ca. 200 kJ mol−1 until a coverage
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Figure 19. Scheme of the equipment used for the microcalorimetric measurement of the adsorption
heat [129].
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Figure 20. Differential heats of adsorption of pyridine on zeolites H-ZSM-5 with aluminum
contents of 180 (•), 370 , 530 and 600 mol g−1 (square) [128].
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of one probe molecule per framework aluminum atom is reached. The points of drop
down of the differential heat for increasing the coverage agree well with the number
of Brønsted acid sites in the H-ZSM-5 zeolites under study. Generally, for the zeolites
with the lower aluminum contents, a higher strength of the Brønsted acid sites (see
Section 2.1) and, therefore, a higher differential adsorption enthalpy at a low coverage
is expected. The differential heats in Figure 20, however, are average values for all
Brønsted acid sites and do not give information on the spectrum of sites with different
strengths. This is due to the irreversible adsorption of the base molecules at Brønsted
acid sites for temperatures of T < 600 K. Hence, the adsorbate molecule interacts with
the Brønsted acid site which is first available in the pore system of the zeolite under
study rather than with the strongest one.
The limitation of microcalorimetry for the investigation of the strength of Brønsted acid
sites in zeolites was demonstrated by Kresnawahjuesa et al. [18] comparing aluminumand iron-containing H-ZSM-5 zeolites. Both samples had the same density of Brønsted
acid sites. In the case of H-[Fe]ZSM-5, these acid sites are formed in the vicinity of
iron atoms at framework positions in the ratio of 1:1. Differential heats of adsorption
for ammonia and pyridine on H-[Fe]ZSM-5 were found to be identical to those obtained
for H-[Al]ZSM-5 with values of ca. 150 kJ mol−1 for ammonia and ca. 200 kJ mol−1 for
pyridine. Generally, it is accepted that the acid strength and hence the catalytic activity
of zeolite H-[Fe]ZSM-5 is significantly lower than those of zeolite H-[Al]ZMS-5. For
H-[Al]ZSM-5, e.g., a rapid oligomerization of propene was found already at room temperature, while this reaction on H-[Fe]ZSM-5 required heating to 370 K [18]. Hence,
the assumption that acid sites, which are catalytically more active, show higher heats
of adsorption for base molecules has to be examined for each adsorbate/acid site system [10]. For a discussion of further effects influencing the results of microcalorimetric
measurements, see Refs. [126,129]. A survey on applications of microcalorimetry for
the characterization of zeolite catalysts is given in Ref. [130].
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2.4.5. FTIR spectroscopy
Infrared spectroscopy is based on the interaction of electromagnetic radiation with
compounds that possess a permanent or induced dipole moment leading to an excitation
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of vibration states. Due to the dipole moment of OH groups, Brønsted acid sites in
zeolites can directly be studied by Fourier transform IR spectroscopy (FTIR), while the
study of Lewis acid sites always requires the use of probe molecules. To characterize
Brønsted acid sites in zeolite catalysts, often the fundamental stretching vibrations of
hydroxyl groups are investigated using the IR transmission technique [131,132]. This
technique requires the preparation of zeolite samples in the shape of thin wafers with a
thickness of ca. 10 mg cm−2 . These wafers are measured in IR transmissions cells, such
as shown in Figure 21. It allows the study of activated zeolite catalysts under vacuum
conditions without or upon adsorption of probe molecules. In some cases, e.g., if the
material under study cannot be pressed to thin wafers or if the transmission is too weak,
the diffuse reflection technique is applied. The pros and cons of this technique are
described in Refs. [132,133].
The fundamental stretching vibrations OH of hydroxyl groups in dehydrated zeolites
cover a range of 3200–3800 cm−1 . Table 1 gives a survey on the stretching vibrations of
the most important types of OH groups in zeolites. For reviews on bending vibrations
and overtone and combination bands see Refs. [130,132,134]. For certain zeolites, the
stretching vibrations of bridging OH groups (SiOHAl) are split into two characteristic
ranges, called high-frequency (HF) and low-frequency (LF) band. The HF band is caused
by non-interacting SiOHAl groups in large cages or pores consisting of 10-membered
oxygen rings or larger. Examples are bridging OH groups in the supercages of faujasitetype zeolites and in 10- or 12-ring pores. In contrast, the LF band is due to SiOHAl
groups in small structural units, such as in sodalite cages of faujasite-type zeolites or in
hexagonal prisms. The latter type of hydroxyl protons interacts with oxygen atoms in
their vicinity, e.g., via hydrogen bonding or electrostatic interactions. As an example,
Figure 22 shows the experimental FTIR spectrum of dehydrated zeolite H-Y. Generally,
a shift of the stretching vibrations to lower frequencies occurs with decreasing −OH O
distance of the hydrogen bonding [135]. This wavenumber shift can also be observed for
internal silanol groups or extra-framework metal OH groups interacting with framework
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Figure 21. IR cell for the study of thin wafers of zeolites in the transmission mode after Karge
et al. [132].
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Table 1. Wavenumber OH of the fundamental stretching vibrations and assignments of hydroxyl
groups in dehydrated zeolites (∗ , additional interaction of the hydroxyl proton, e.g., by hydrogen
bonding)
OH cm−1 Abbreviation
3780
MeOH
3745–3720
SiOH
3665–3690
AlOH
3570–3610
CaOH∗ MgOH∗ AlOH∗
3600–3660
SiOHAl
3580–3550
SiOH∗ Al
3550–3470
SiOH∗
3250
SiOH∗ Al
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Type of hydroxyl groups
Terminal metal OH groups in
large cages and on the
external surface, such as
AlOH groups
Terminal silanol groups on the
external surface or at lattice
defects
OH groups at extra-framework
aluminum species
Cation OH groups located in
sodalite cages of zeolite Y
and in channels of ZSM-5,
hydrogen bonded
HF band, bridging OH groups
in large cages and channels
of zeolites
LF band, bridging OH groups
in small cages of zeolites,
hydroxyl protons interacts
with framework oxygen
Hydrogen-bonded SiOH
groups, internal silanols
Disturbed bridging OH groups
in zeolite H-ZSM-5, H-Beta
and H-MCM-22, hydroxyl
proton interacts with
framework oxygen
References
[131,132,136]
[131,132,137,138,140]
[131,132,141,142]
[131,143,144]
[131,132,145,146]
[131,132,145,146]
[147–149]
[150]
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oxygen atoms in their neighborhood. In Table 1, the hydrogen bonded or disturbed
hydroxyl groups are marked by asterisks.
An additional parameter influencing the stretching vibrations of non-interacting bridging OH groups in zeolites is the mean electronegativity of the zeolite framework. In
dependence on the nSi /nAl ratio of the zeolites under study, the mean electronegativity
increases with increasing silicon content or decreases with increasing aluminum content
in the framework [132]. Correspondingly, the stretching vibrations of SiOHAl groups
in zeolites with a high mean Sanderson electronegativity (high nSi /nAl ratio) occur at
lower frequencies than those of zeolites with low mean electronegativity (low nSi /nAl
ratio). The mean Sanderson electronegativity S m of the zeolite framework is defined
as the geometric means of the electronegativities Si of the atoms i. For zeolites of the
composition HAlO2 SiO2 x S m is calculated by Eqn. 8 [151,152]:
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S m = SH SAl SO 2x+2 SSi x 1/3x+4
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SiOHAl
HF SiOHAl
LF
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SiOH
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3600
3500
3400
Wavenumber (cm–1)
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Figure 22. Experimental (top) and deconvoluted (bottom) FTIR spectrum of zeolite H-Y. The
sample wafer had a density of 5 mg cm−2 [139].
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with SH = 355 SAl = 222 SO = 521 SSi = 284 and the nSi /nAl ratio x. Utilizing this
principle, Jacobs and Mortier [153] could rationalize the stretching vibrations of most
of the non-interacting bridging OH groups in dehydrated zeolites.
In principle, the concentration of different types of hydroxyl groups can be determined
by the intensities of their IR bands, i.e., by the integrated absorbance. To calculate
the OH concentration, however, the extinction coefficient has to be determined by an
independent measurement. The extinction coefficient is a function of the wavenumber
and varies with a band shift, e.g., caused by a change of the framework aluminum content.
In the literature, therefore, the extinction coefficients given for bridging OH groups
vary significantly [23,154]. A more suitable way for obtaining OH concentrations by
IR spectroscopy is the quantitative adsorption of probe molecules and an observation of
the response in the FTIR spectrum. Adsorption of probe molecules is also an interesting
approach for the study of the strength of Brønsted acid sites in zeolites.
Reviews on the application of probe molecules for the study of Brønsted and Lewis
acid sites in zeolites and other solid catalysts are given in Refs. [33,155]. Linear correlations between the strength of Brønsted acid sites in solid catalysts and the wavenumber
shift OH of the stretching vibrations of OH groups were reported for benzene [156],
acetone [157], CO [158,159] and ethene [160]. Upon low-temperature (T ca. 100 K)
adsorption of CO on different Y-type zeolites, e.g., Lavalley et al. [158] observed
wavenumber shifts of the HF band of bridging OH groups of OH = 160–302 cm−1
corresponding to the following sequence of acid strength: H,Li,Na-Y 302 cm−1 >
H,K,Na-Y 220 cm−1 > H,Rb,Na-Y 168 cm−1 > H,Cs,Na-Y 160 cm−1 .
Based on an empirical relation first established by Paukshtis and Yurchenko [161]:
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PA kJ mol−1 = 1390 − 4425 log OH /SiOH (Eqn. 9)
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the proton affinity PA of the hydroxyl groups on solid acids can be estimated by
measuring the adsorbate-induced wavenumber shift OH of the OH groups under study
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H-Beta
H-ZSM-5
H-Y
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HF
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600
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200
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400
500
600
700
ΔνSiOH (cm–1)
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SiOH
Figure 23. Plots of the wavenumber shifts of bridging OH groups and SiOH of silanol
groups of zeolites H-Y, H-mordenite, H-Beta and H-ZSM-5 obtained upon 1:1 adsorption of
the following probe molecules: (1) O2 , (2) N2 , (3) N2 O, (4) CO2 , (5) CO, (6) C4 H4 S C2 H2 ,
(7) C2 H4 C6 H6 C4 H6 , (8) C4 H4 O C3 H6 , (9) HC2 CH3 , (10) H2 O, (11) CH3 CN CH3 CO, (12)
CH3 OH, (13) CH3 CH2 OH, (14) CH3 2 O, (15) CH3 CH2 2 O, (16) THF, (17) NH3 (, data for
H-Y; , data for H-mordenite; , data for H-Beta; , data for H-Beta/H2 O; , data for H-ZSM-5).
For comparison, experimental data of probe molecules in HF are given [162].
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and of silanol groups SiOH . The plot of adsorbate-induced wavenumber shifts shown
in Figure 23 [162] indicates the difference in the PA values between zeolites H-Beta,
H-ZSM-5 and H-mordenite in comparison with zeolite H-Y, HF and silanol groups.
From this plot, a difference of the proton affinities of the above-mentioned zeolites of
ca. 62 kJ mol−1 can be estimated, which is comparable to the difference between H2 SO4
and CF3 SO3 H [163].
Probe molecules, such as ammonia and pyridine, which are protonated in an interaction
with Brønsted acid sites, but are coordinatively bound at Lewis acid sites allow the
separation of both types of surface sites by FTIR spectroscopy [164,165]. The protonation
of pyridine by Brønsted acid sites on solid acids is accompanied by the appearance of
a characteristic band at ca. 1540 cm−1 . Coordination of this probe molecule at Lewis
acid sites leads to a band at ca. 1450 cm−1 [159,165]. Quantitative evaluation of the
integrated absorbance of these bands allows the calculation of the number of Brønsted
and Lewis acid sites. Also in this case, the extinction coefficient has to be determined
by an independent measurement.
Upon adsorption of CO and H2 at Lewis acid sites, characteristic shifts of the stretching
vibrations of the probe molecules were observed [33]. Adsorption of CO at Lewis acid
extra-framework aluminum species in zeolite H-ZSM-5 was found to be accompanied by
the occurrence of a doublet at CO = 2230 and 2220 cm−1 corresponding to band shifts
of CO = +87 and +77 cm−1 , respectively. Utilizing H2 as probe molecule interacting
with the same surface sites, a doublet occurs at HH = 4027 and 4002 cm−1 corresponding
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to band shifts of HH = −133 and −158 cm−1 , respectively. Comparison of these values
indicates that H2 is a superior probe molecule for evaluating Lewis acid sites of zeolites
since its spectroscopic response is nearly twice that of CO [33].
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2.4.6. NMR spectroscopy
With a probability of 99.85%, hydroxyl protons consists almost exclusively of 1 H
isotopes with a nuclear spin of I = 1/2 which renders them directly accessible for
NMR spectroscopy. Hydroxyl groups acting as Brønsted acid sites of zeolites catalysts,
therefore, can be investigated by 1 H MAS NMR spectroscopy in a direct manner. The
MAS technique (MAS, magic angle spinning) is the prerequisite for obtaining highly
resolved solid-state NMR spectra allowing the separation of signals due to different types
of hydroxyl groups in zeolite catalysts [32,166,167]. There are different experimental
techniques for the study of calcined and dehydrated zeolite samples by the MAS NMR
technique [168]. The simplest way is the transfer of the dehydrated powder material in a
gas-tight MAS rotor using a glove box purged with a dry and inert gas, such as nitrogen.
Using the vacuum device shown in Figure 24(a), the samples can be dehydrated and
loaded with probe molecules directly inside the MAS rotor. Subsequently, the rotor is
sealed with a gas-tight cap using the sealing rod. In another approach, the sample is
prepared in a glass insert like an ampoule (see Figure 24(b)). Upon treatment, the glass
insert is sealed by fusing and can be inserted into the MAS rotor. Nowadays, glass
inserts are offered for all commercial MAS rotor systems.
The 1 H MAS NMR signals of hydroxyl groups in calcined solid catalysts cover a range
of isotropic chemical shifts 1H of ca. 0–15 ppm (see Table 2). The lowest chemical shifts
have been observed for non-interacting metal OH groups such as AlOH groups at the
outer surface of -Al2 O3 [169] and MgOH groups in the supercages of zeolite Y [170].
SiOH groups at the outer surface of silicate or aluminosilicate particles or at framework
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(b)
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Vacuum line
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valve
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rotor filled
with catalyst
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Figure 24. Scheme of the equipment for the preparation of the zeolite sample inside an MAS
NMR rotor (a) and of a glass ampoule suitable as insert for MAS rotors (b) [168].
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Table 2. 1 H NMR shifts and assignments of hydroxyl groups in solid catalysts
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05
1H (ppm)
Abbreviation
−05 to 0.5
MeOH
1.2–2.2
SiOH
2.4–3.6
AlOH
2.8–6.2
3.6–4.3
CaOH∗ MgOH∗ AlOH∗ LaOH∗
SiOHAl
4.6–5.2
SiOH∗ Al
5.2–8.0
SiOH∗ Al
up to 16 ppm
SiOH∗
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Type of hydroxyl groups
Terminal metal OH groups in
large cages and on the
external surface, such as
AlOH groups
Terminal silanol groups on the
external surface or at lattice
defects
OH groups at extra-framework
aluminum species
Cation OH groups located in
sodalite cages of zeolite Y
and in channels of ZSM-5,
hydrogen bonded
Bridging OH groups in large
cages and channels of
zeolites
Bridging OH groups in small
cages of zeolites, hydroxyl
protons interacts with
framework oxygen
Disturbed bridging OH groups
in zeolite H-ZSM-5, H-Beta
and H-MCM-22, hydroxyl
proton interacts with
framework oxygen
Hydrogen-bonded SiOH
groups, internal silanols
References
[170,177]
[171,173–175,177]
[173–175,177–179]
[169,170,180,181]
[32,166,171–173,182]
[32,154,166,182]
[173,175,177,178,183]
[184,185]
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Hydroxyl protons involved in a hydrogen bonding or electrostatic interaction with neighboring oxygen atoms
are marked by an asterisk.
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defects in zeolites are responsible for 1 H MAS NMR signals at 1H = 12–22 ppm
[171–175]. Hydroxyl groups located in small structural units, such as in narrow pores
or in the small sodalite cages of zeolite Y, are often involved in a hydrogen bonding
or electrostatic interaction with neighboring oxygen atoms. According to Yesinowski
et al. [176], the signals of hydroxyl protons involved in a hydrogen bonding are shifted
to higher chemical shifts in a quantitative manner.
1
H MAS NMR signals of bridging OH groups in zeolites occur at 1H = 36–43 ppm
and 1H = 46–52 ppm. These signals are due to bridging OH groups located in large
structural units, such as supercages of zeolite Y or 10- and 12-ring pores of zeolite
H-ZSM-5 and H-mordenite (SiOHAl), and in the small structural units, such as sodalite
cages SiOH∗ Al, respectively [32]. The larger chemical shift of the SiOH∗ Al groups
is caused by a weak hydrogen bonding or electrostatic interaction with neighboring
framework oxygen atoms in the small structural units. Like the wavenumber of the HF
band in FTIR spectroscopy (see Section 2.4.5), the chemical shift 1H of non-interacting
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SiOHAl groups depends on the mean Sanderson electronegativity S m of the zeolite
framework and, therefore, on the framework nSi /nAl ratio [151,152].
In the literature [173,175,177,178,183], signals of disturbed bridging OH groups
occurring at 5.2–8.0 ppm are described for zeolites H-ZSM-5, H-Beta and H-MCM-22.
These signals are explained by perturbed bridging OH groups, which are involved in a
hydrogen bonding with neighboring oxygen atoms in the pores of the above-mentioned
zeolites. Similarly, the 1 H MAS NMR spectra of as-synthesized zeolites of the structure
type NON, DDR, AFI (SSZ-24) and MFI show a signal at 10.2 ppm, which does not
originate from the organic structure-directing agents [184]. This signal was assigned to
silanol groups involved in internal hydrogen bonding between defect sites and neighboring framework oxygen atoms. The same effect was observed for the layered material
RUB-18 containing strongly hydrogen-bonded silanol groups occurring at the resonance
position of 15.9 ppm [185].
Typical 1 H MAS NMR spectra of dehydrated zeolites Y are shown in Figure 25 [166].
The spectrum of zeolite 83Mg,Na-Y consists of signals of MgOH groups at −05 and
0.5 ppm, silanol groups at 1.8 ppm and bridging OH groups in the supercages and in
the sodalite cages at 3.9 and 4.8 ppm, respectively (Figure 25(a)). In the spectrum of
zeolite 83Ca,Na-Y, an additional signal due to CaOH groups in the sodalite cages occurs
at 2.8 ppm (Figure 25(b)). Lanthanum hydroxyl groups of lanthanum cations and oxide
complexes located in the sodalite cages cause a signal at 5.6 ppm in the spectrum of
zeolite 73La,Na-Y (Figure 25(c)). The dealumination of zeolite H-Y is accompanied by
the formation of hydroxyl groups at extra-framework aluminum complexes leading to a
signal at ca. 2.6 ppm in Figure 25(d). The decrease of the framework aluminum content
due to dealumination leads to an increase of the mean framework electronegativity and,
therefore, to a resonance shift of the signal of bridging OH groups in the supercages
from 3.9 ppm for the parent zeolite H-Y to 4.2 ppm for the dealuminated material
(Figure 25(d)).
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(b)
83Mg, Na-Y
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(c)
83Ca, Na-Y
–0.5
5.6
3.9
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(d)
73La, Na-Y
Steamed H-Y
3.9
3.9
2.8
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2.6
36
1.8
4.2
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1.8
0.5
0.0
40
1.8
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1H (ppm)
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Figure 25. 1 H MAS NMR spectra of magnesium-exchanged zeolite Y (83Mg,Na-Y) (a), calciumexchanged zeolite Y (83Ca,Na-Y) (b) and lanthanum-exchanged zeolite Y (73La,Na-Y) (c) dehydrated at 433 K, and of steamed zeolite H-Y (d) dehydrated at 673 K [166].
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A suitable way to quantify the concentration of hydroxyl groups in solid catalysts by
H MAS NMR spectroscopy is the comparison of the signal intensities of the sample
under study with the intensity of an external intensity standard. For quantitative studies,
the repetition time of the pulse experiments has to be large in comparison with the
spin-lattice relaxation times T1 of the different OH species, which are of the order of
1–10 s [177]. Often, a well-characterized and stable zeolite, such as a dehydrated zeolite
35H,Na-Y, is used as an intensity standard [106,186]. The total concentration cOH of the
hydroxyl groups in the zeolite catalyst under study can be calculated by Eqn. 10 [187]:
1
09
cOH = cst mst AOH /m Ast 10
(Eqn. 10)
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with the concentration cst , the weight mst and the total integral Ast of the standard “st”,
and the weight m and the total integral AOH of the zeolite catalyst under study. In the
case of different signals of OH groups in the 1 H MAS NMR spectrum, a separation of
the signals via suitable simulation software must be performed.
The ability to protonate basic probe molecules, such as pyridine [32,106,166,188,189],
ammonia [166,190] and trimethylphosphine [178,191–194], or to form a hydrogen bonding to these molecules is utilized to distinguish Brønsted acid sites with high strength
(e.g., SiOHAl) and very low strength (e.g., SiOH) by solid-state NMR spectroscopy.
A more quantitative comparison of the acid strength of Brønsted acid sites is possible by
the application of weak base molecules, which generally interact via hydrogen bonding
with Brønsted acid sites of zeolites. The adsorbate-induced resonance shift of the
MAS NMR signals caused by the interacting surface OH groups or due to the interacting
functional groups of the probe molecules depends on the strength of the Brønsted acid
site: a high value corresponds to a high acid strength. For the study of the strength
of Brønsted acid sites in acidic zeolites, Jaenchen et al. [195] and Huang et al. [196] utilized deuterated acetonitrile and observed resonance shifts varying from 5.1 ppm for
zeolite H-Y to 7.1 ppm for zeolite H-ZSM-5. Another probe molecule for characterizing
the strength of Brønsted acid sites in zeolites is 13 C-2-acetone [197–199]. Based on the
experimentally determined dependence of the resonance positions of the carbonyl atom
of 13 C-2-acetone molecules dissolved in aqueous sulfuric acids of varying concentration,
a scale of the Brønsted acid strength was introduced [200]. According to this scale,
bridging OH groups in acidic zeolites, such as in zeolite H-ZSM-5, are as strong as 80%
H2 SO4 [200].
The interaction of hydroxyl groups in zeolites with perchloroethene has been investigated by Sachsenroeder et al. [201] to quantify the deprotonation energy of hydroxyl
groups in zeolites. The adsorbate-induced 1 H MAS NMR shift 1H was used to
determine the deprotonation energy EDP of SiOHAl groups contributing to weakly
hydrogen-bonded complexes [201]:
1
EDP kJ mol−1 = − log1HSiOHAl /1HSiOH A
(Eqn. 11)
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where 1HSiOH and 1HSiOHAl are the 1 H MAS NMR shifts induced by the adsorbate
molecule for the resonance positions of the silanol and bridging OH groups, respectively. A is a constant given by 0.00226 [161]. Utilizing this technique, deprotonation
energies of SiOHAl groups in zeolites 30H,Na-Y and H-ZSM-5 of EDP = −146 and
−179 kJ mol−1 , respectively, were calculated.
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Typical probe molecules for solid-state NMR investigations of Lewis acid sites
in zeolites are 13 C-2-acetone 13C = 233 ppm [198], 13 C-enriched carbon monoxide
13C ≈ 770 ppm [202,203], 15 N-pyridine 15N = 265 ppm [204,205], trimethylphosphine (31P = −32 to −67 ppm) [191,206,207] and trimethylphosphine oxide 31P =
37 ppm [208,209]. 13 C 15 N or 31 P MAS NMR signals occurring at the chemical shifts
given in parentheses upon adsorption of the above-mentioned probe molecules indicate
the presence of Lewis acid sites.
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The nature of basic sites in zeolites is less well defined than that of acid sites. This is
related to the fact that most basic zeolites contain alkali cations acting as weak Lewis
acid sites as well as basic framework oxygen atoms. In most reactions catalyzed by basic
zeolites, both Lewis acid sites and base sites are involved. The strength of the basic
sites must be high enough to stabilize anionic or polarized species that take part in the
catalytic cycle [9,210].
Utilizing the Sanderson principle of equalization of electronegativities for framework
atoms of zeolites (see Section 2.4.5) [151], the charge on the framework atoms and
cations in zeolites can be estimated [152,211]. Since the mean electronegativity of the
zeolite framework is calculated by the geometric average of the electronegativities of
all atoms contributing to the framework (see Eqn. 8), no influence of the local structure
is considered. However, this principle is useful for the study of the general chemical
behavior of zeolites with different framework aluminum contents and extra-framework
cations. Barthomeuf [211,212] utilized the Sanderson principle to estimate the charge
q0 on the framework oxygen atoms in basic zeolites exchanged with different alkali
metal cations (Figure 26). For comparison, also the charges at hydroxyl protons −qH and oxygen atoms −q0 of zeolites in the H+ -form are given. For a given framework
aluminum content, Barthomeuf [211,212] found increasing mean charges −q0 on the
oxygen atoms, i.e., an increase of the base strength in the sequence Li- < Na- < K- < Rb< Cs-zeolites. This sequence is opposite to the electronegativities of the corresponding
cations. Furthermore, the basicity of the framework oxygen atoms increases for a given
cation with increasing framework aluminum content due to the lower electronegativity
of aluminum in comparison with silicon [151]. Hence, the zeolite with the cation of
the lowest electronegativity, i.e., cesium, and the highest framework aluminum content
is the catalyst with highest base strength. Such a material is, e.g., cesium-exchanged
zeolite X [212].
Considering the above-mentioned influence of alkali metals on the basicity, zeolites
are used as base catalysts in their alkali-exchanged or -impregnated forms. These alkaliexchanged zeolites possess basic framework oxygen atoms of relatively low strength,
which limits their applicability in organic syntheses. However, since their base sites are
resistant to poisoning by water or carbon dioxide, they can be handled in air [213]. A
technique for generating stronger base sites in the cavities of zeolites is impregnation
with various alkali salts, such as cesium hydroxide. Hathaway and Davis [214] created
intrazeolitic alkali oxide clusters by impregnation methods. The base sites of the guest
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Figure 26. Charge on the framework oxygen −q0 and on the hydroxyl proton −qH of H-form
(a, b), Li-form (c), Na-form (d), K-form (e), Rb-form (f) and Cs-form (g) zeolites calculated by
the Sanderson principle of equalization of electronegativities (adapted from Ref. [211]).
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oxides were shown to be stronger than those of the zeolitic framework. Another approach
is the impregnation of zeolite Y with NaN3 followed by controlled thermal decomposition
yielding tetrahedral Na4 3+ clusters in the sodalite cages, Nay 0 clusters in the supercages
and metallic clusters on the external surface [215]. In olefin isomerization, side-chain
alkylation of toluene and aldol condensations, a correlation between the concentration
of the Nay 0 clusters in the supercages and the catalytic activity was observed. However,
these materials are sensible toward contact with air or water, which limits their utilization
in catalysis.
Ono and Baba [216] developed the following procedure for obtaining basic zeolites:
alkali-exchanged zeolite Y was immersed in a solution of metallic Na, Yb or Eu in
liquid ammonia and the solvent was removed by evacuation. Heating in vacuum at
suitable temperatures (about 450 K) leads to zeolites, the base strength of which strongly
depends on the type of additional alkali cation present. It increases with the amount of
guest compound. For Eu/K-Y, a maximum initial rate in the olefin isomerization was
observed at a Eu-loading of 8 wt% [216]. EXAFS investigations on Yb/K-Y revealed
that the local structures of the Yb species change drastically upon evacuation at around
500 K, viz., from a highly dispersed state to aggregated particles [217].
One reason for the limited application of zeolite catalysts in the synthesis of fine
chemicals resides in their small pore openings, which prevent bulky molecules from
reaching the active sites. Therefore, the use of mesoporous MCM-41 materials as carriers
for basic guest species has been proposed [218,219]. By impregnation of MCM-41 with
cesium acetate in aqueous or methanolic solution and subsequent calcination, finely
dispersed cesium oxide clusters were obtained in the pores of the carrier, as long as
the cesium content did not exceed 10 wt.%. The impregnated MCM-41 material was
active in typically base-catalyzed reactions like Knoevenagel condensations or Michael
additions. However, the impregnated material did not show a good thermal and chemical
stability. After repeated calcination or after the use as catalyst, aggregation of the cesium
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oxide particles and a significant reduction of the specific surface area were observed.
More stable materials were obtained by impregnating MCM-41 simultaneously with
cesium acetate and lanthanum nitrate [220,221]. In this case, a CsLaOx guest compound
was formed in the channels of MCM-41, the base strength of which was, however, lower
than that of the CsOy guest oxide.
Another way of modifying mesoporous supports is to functionalize the silanol
groups by anchoring organic bases. By forming a covalent bond between the inorganic host and the organic guest species, a higher stability against leaching was
aimed at [222]. By reacting MCM-41 with 3-chloropropyl-triethoxysilane and piperidine, pyrrolidine, pyrimidine or triazabicyclo[4,4,0]dec-5-ene in subsequent steps, various organic bases could be bound to the surface of MCM-41 [222–227]. Other
groups varied the synthesis conditions and bound the silanol groups of MCM-41
with 3-trimethoxysilylpropyl(trimethyl)ammonium chloride. Subsequently, the chloride was exchanged by hydroxide ions, whereby materials with free OH− ions were
obtained [228,229].
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3.2.1. Test reactions
Catalytic test reactions were frequently used for characterizing solid bases. The selectivities of several reactions give evidence for the presence of base sites with different strengths. Aramendia et al. [230] compared the transformation of 1-phenylethanol,
2-propanol and 2-methyl-3-butyn-2-ol on basic catalysts. These authors showed that
some of the alcohols (in particular 1-phenylethanol and 2-propanol) can undergo both
dehydrogenation and dehydration on basic sites, hence no unambiguous conclusions
concerning the nature of the active sites can be drawn from the measured selectivities.
On the other hand, in agreement with Handa et al. [231], it was found that selectivities in
the decomposition of 2-methyl-3-butyn-2-ol do allow the discrimination between acidic
and basic catalysts. Since base-catalyzed reactions have relatively low rates in comparison with acid-catalyzed reactions, in many cases minor traces of acidic protons due, for
example, to silanol groups, may change the selectivity of the reaction dramatically [232].
In order to overcome this problem, basic zeolite catalysts are often prepared with a slight
excess of alkali cations.
A further test reaction which finds an ever widespread application is the Knoevenagel condensation (Figure 27) [233–236]. The important advantage of this liquid-phase
reaction is that it can be performed with reactants having different acidities. However,
problems with diffusional hindrance limit its application for the characterization of basic
zeolites. Corma et al. [233] studied the Knoevenagel condensation of benzaldehyde
with cyanoacetate, ethyl acetoacetate and ethyl malonate on alkali-exchanged X and Y
zeolites. They found an order of the reactivity, which agrees with the increase of the
charges on the framework oxygen atoms of the zeolite catalysts under study as estimated
by the mean framework electronegativities, i.e., Li- < Na- < K- < Cs- and Y- < X-type
zeolites. Corma et al. [233] concluded that most of the base sites in alkali-exchanged
zeolites Y and X have pKb ≤ 103 and sites with pKb ≤ 13 are present in zeolite Cs-X
only. This catalyst was found to be more active than pyridine pKb ≤ 88 and less
active than piperidine pKb ≤ 111. By comparing the Knoevenagel condensation on
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R2 = H, Me
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Figure 27. Mechanism of the Knoevenagel condensation [210].
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zeolite Na-X and germanium-substituted faujasite nAl /nGe = 103 it was shown that
the latter catalyst is more active [237]. It was concluded that most of the base sites in this
germanium-modified zeolite have pKb ≈ 112 and additional sites with pKb ≤ 133 exist.
Double bond isomerization of 1-butene and 2,3-dimethylbut-1-ene leading to 2-butene
and 2,3-dimethylbut-2-ene, respectively, is useful for the characterization of strong
solid bases at low temperatures [231,238,239]. In the case of the isomerization of
1-butene, the cis/trans ratio of the 2-butenes gives a measure of the base strength.
While the isomerization is a useful test reaction for determining the relative activities
of strong solid bases, only very little or no conversion at all was found for alkaliexchanged zeolites [230]. Two decades ago, Dessau [240] introduced the dehydration
of acetonylacetone as catalytic test reaction. Under specified conditions, the ratio of the
selectivities to methylcyclopentenone and dimethylfuran is taken as a measure of the
base strength. Alcaraz et al. [241] showed that the dehydration of acetonylacetone allows
the catalytic characterization of materials exhibiting acidic as well as basic sites over a
broad range of acid and base strengths.
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3.2.2. Analytical and spectroscopic methods
The most widely applied technique for the investigation of base sites on solid catalysts is the use of molecular probes and their study by temperature-programmed desorption experiments, FTIR spectroscopy, X-ray photoelectron spectroscopy and NMR
spectroscopy [242–244]. A frequently employed molecular probe is carbon dioxide. Its
adsorption on alkali-exchanged zeolites, however, is not straightforward: different adsorbate structures may occur, and on strongly basic guest compounds surface carbonates
can be formed [244]. Both these effects complicate the TPD curves and FTIR spectra
of CO2 on basic zeolites [245]. Knözinger and Huber [243] published a survey of the
application of carbon monoxide, pyrrole, acetylenes and deuterated chloroform as FTIR
probes for the investigation of basic solids. As an example, Figure 28(a) shows the
FTIR spectra of carbon monoxide adsorbed on alkali-exchanged zeolites Y [243]. For
these zeolites, a good correlation was found between the wavenumber shift CO of
the stretching frequency and the cation radius of the exchanged cations (Figure 28(b)),
which has the inverse sequence of the electronegativity. The main drawback of C–H
and N–H acids used as probe molecules is the possible dissociation of the C–H or N–H
bonds on strongly basic surface sites. As this leads to a disappearance of the analytical
signal, those strong base sites may not be detected by the probe molecules [243].
Suitable methods for the direct investigation of basic oxygens in zeolitic materials are
X-ray photoelectron spectroscopy (XPS) and 17 O NMR spectroscopy. With XPS, the
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(a)
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Figure 28. FTIR spectra of alkali-exchanged zeolites Y loaded with carbon monoxide (a) and
wavenumber shift CO of the C–O stretching vibration recorded at ca. 90 K as a function of the
radii of alkali metal cations introduced by ion exchange (b) [243].
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binding energies of O1s electrons are evaluated, which could be shown to correlate with
the base strength of framework oxygens in zeolites [246,247]. During the last years, the
introduction of superconducting magnets with magnetic fields of up to 18.8 T and new
techniques of high-resolution solid-state NMR spectroscopy opened new possibilities
of investigating 17 O nuclei in solid materials [248–250]. Using high magnetic fields,
double-oriented rotation (DOR) of powder samples and 2-dimensional multiple-quantum
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MAS spectroscopy (MQMAS), the problems of strong signal broadening by the secondorder quadrupolar interaction of 17 O nuclei with a nuclear spin of I = 5/2 may be
overcome [248–250]. For low-silica faujasites with a framework nSi /nAl = 1 (LSX),
which contain Si–O–Al bridges only, the spectra show four lines due to oxygen atoms
at the four different crystallographic positions. For oxygens at O-1 sites in zeolite LSX,
a correlation was found between the isotropic value of the chemical shift, 17O , and the
cation radius r Å in hydrated zeolites Li-LSX, Na-LSX and Cs-LSX [250]:
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(Eqn. 12)
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Eqn. 12 indicates that the electronic shielding of the nuclei of framework oxygen atoms
in zeolites LSX is affected by the introduction of alkali cations with different radii.
Considering the influence of the cation radius on the wavenumber shift CO of carbon
monoxide acting as probe molecule for alkali-exchanged zeolites Y (Figure 28(b)), also
the isotropic value of the chemical shift 17O of the framework oxygen atoms may be
utilized as an adequate measure of the base strength of zeolites.
Like for FTIR spectroscopy, there is a number of probe molecules for solid-state NMR
spectroscopic studies of base sites in zeolites. A survey on these molecular probes is
given in Table 3. Sánchez-Sánchez et al. [251,252], e.g., applied pyrrole and chloroform
as NMR probes for basic zeolites. Figure 29 shows the 1 H MAS NMR spectra of pyrrole
adsorbed on various alkali-exchanged zeolites Y and X [251]. The hydrogen atoms
at the rings of the pyrrole molecules are not influenced by the different zeolites and
cause the two signals at ca. 6–7 ppm. The 1 H NMR shift of the hydrogen atoms at the
ring nitrogens, on the other hand, covers a range of 8.4–11.5 ppm and indicates the
different base strengths of the framework oxygen atoms contributing to the H-bondings
with the pyrrole molecules. An important advantage of the probe molecule pyrrole is
its remarkable sensitivity (large shift range) and the good resolution of the MAS NMR
spectra.
Regardless of the spectroscopic method applied, drawbacks of using such probe
molecules are that most of these are not totally unreactive in the presence of strong base
sites and that they can form different adsorption structures complicating an evaluation
of the spectra. An interesting technique, which seems to be free from these problems, is
the application of methoxy groups directly formed at the basic framework oxygens from
methyl iodide, as spectroscopic probes. Applying 13 C MAS NMR spectroscopy, Bosacek
et al. [254–256] found a correlation between the 13 C NMR shift 13C of these surface
methoxy groups bound to zeolite oxygens in bridging positions and the mean Sanderson
electronegativity Sint of the zeolite framework. According to this correlation, a low
chemical shift of methoxy groups corresponds to a high base strength of the framework
oxygen atoms. Methoxy groups bound at framework oxygen atoms of alkali-exchanged
zeolites Y and X cover a range of chemical shifts of 54.0–56.5 ppm. For zeolites Y and
X impregnated with alkali compounds, additional signals occur at 50.0 and 52.3 ppm
due to methoxy groups bound at strongly basic guest compounds [257]. According
to Krawietz et al. [259], guest compounds formed by impregnation of a support with
cesium hydroxide or acetate are a mixture of cesium oxide Cs2 O, peroxide Cs2 O2 and superoxide CsO2 . The 13 C NMR signals observed at ca. 50 and 52 ppm could,
therefore, be a hint at the presence of different basic guest compounds on impregnated
zeolites Y and X.
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Table 3. Probe molecules applied for the NMR characterization of base sites in zeolites
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Probe molecule
Resonance/effects
References
Trichloromethane
Hydrogen-bonded trichloromethane
at 1H = 755 (Li-Y) to 8.23 ppm
(Cs;Na-Y-90)
Hydrogen-bonded trifluoromethane
at 1H = 662 (Li-Y) to 7.6 ppm
(Cs,Na-Y-90)
Hydrogen-bonded pyrrole at
1H = 84 (Li-Y) to 11.5 ppm
(K-X)
Hydrogen-bonded chloroform at
1H = 745 (H-Y) to 8.70 ppm
(Na,Ge-X)
Hydrogen-bonded 13 C-chloroform
at 13C = 779 (H-Y) to
81.7 ppm (Na,Ge-Y)
Methoxy groups occurring at
13C = 585 (Na-ZSM-5) to
54.0 ppm (Cs,Na-X)
13C = 102–112 ppm
for nitromethane at mixed
magnesium–aluminum oxides
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[253]
[251]
[252]
[252]
[254–257]
[258]
C NMR shifts are referenced to tetramethylsilane 1H = 0 ppm 13C = 0 ppm.
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Figure 29. 1 H MAS NMR spectra of pyrrole adsorbed on various alkali-exchanged zeolites X
and Y [251].
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4. A LOOK TO THE FUTURE
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Intense worldwide research, that paralleled the successful use of acid and bifunctional
zeolites in industrial catalysis, has led to a detailed understanding of Brønsted acidity
in such materials. There is general consensus about the paramount role of bridging
hydroxyl protons as catalytically active centers in acid and bifunctional zeolite catalysts.
Nevertheless, there is much room for further progress even in acid and bifunctional
catalysis on zeolites. A few examples are the creation of Brønsted acid sites in zeolites
with a significantly higher strength that would enable one to carry out certain processes,
e.g., isomerization of light gasoline, at lower temperatures, where the position of thermodynamic equilibrium is more favorable. Other processes, such as isobutane/butene
alkylation or ring opening of aromatics are still waiting for zeolite catalysts that are
good enough for an industrial application.
Also needed are the development of methods which will lead to a deeper understanding
of Lewis acidity and basicity of zeolites and their role in heterogeneous catalysis. We
firmly expect progress along these lines in the years to come. The advent of zeolite-like
materials with sufficiently strong Brønsted basic sites would be a true landmark event
in heterogeneous catalysis. It appears, however, as if this will remain a desideratum for
an extended time in the future.
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ACKNOWLEDGMENTS
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Financial support by Deutsche Forschungsgemeinschaft, Volkswagenstiftung Hannover,
and Fonds der Chemischen Industrie is gratefully acknowledged.
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5. REFERENCES
[1] R. von Ballmoos, D.H. Harris and J.S. Magee, in Handbook of Heterogeneous Catalysis,
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Chapter No: 22
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Query No
Contents
AU1
Please update all “in press” references.
AU2
Please provide the initials of Kubo in Ref. [11].
AU3
Please update Ref. [86].
AU4
Please update Ref. [172].
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