Article
pubs.acs.org/jchemeduc
Heterogeneous Catalytic Chemistry by Example of Industrial
Applications
Josef Heveling*
Department of Chemistry, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa
ABSTRACT: Worldwide, more than 85% of all chemical products are manufactured with the
help of catalysts. Virtually all transition metals of the periodic table are active as catalysts or
catalyst promoters. Catalysts are divided into homogeneous catalysts, which are soluble in the
reaction medium, and heterogeneous catalysts, which remain in the solid state. A heterogeneous
metal catalyst typically consists of the active metal component, promoters, and a support
material. In some cases, the metallic state itself forms the active ingredient. However, this
situation is largely restricted to precious metal catalysts and to some base metals used under
reducing conditions. In most cases and especially in homogeneous catalysis, it is a metal
compound or a complex that forms the active catalyst. Catalysis can be rather puzzling as a given
metal can catalyze a variety of different chemical transformations, while the same substrate,
passed over different catalysts, can give different products. It is therefore helpful to be familiar
with the fundamentals of catalytic science before being exposed to the uncountable applications,
which form the backbone of industrial chemistry. Examples of practical importance are used in
this paper to highlight important principles of catalysis.
KEYWORDS: General Public, Public Understanding, Upper-Division Undergraduate, Curriculum, Inorganic Chemistry, Catalysis,
Green Chemistry, Industrial Chemistry
A
• In 2005 the value of the goods produced with the help of
catalysts amounted to ∼900 billion US$.3
Looking at these figures, one has to keep in mind that catalysts
employed for environmental abatement processes, such as
automotive exhaust catalysts, do not produce any goods of
economic value. A breakdown of catalyst usage by industry
sectors indicates that there is an almost even distribution across
four different sectors, namely, (i) the polymer industry (21%);
(ii) coal, oil, and gas refining (22%); (iii) manufacturing of
chemicals (27%); and (iv) environmental applications (30%).2
catalyst is a substance that accelerates the rate of a
chemical reaction. The catalyst is not consumed and
therefore does not appear in the overall reaction equation. A
catalyst promoter is an additive that improves the performance
of a catalyst, but has no catalytic activity for a given chemical
conversion. Often the catalyst is written above the reaction
arrow in square brackets. This indicates that a catalyst is needed
for the reaction to attain equilibrium within a reasonable time.
For example, the esterification of carboxylic acids with alcohols
takes place in the presence of acids or bases, as does the reverse
reaction, the hydrolysis of esters. In the equation below
(Scheme 1), the acid shows above the reaction arrow to
indicate that this reaction is acid catalyzed.
■
INDUSTRIAL CATALYTIC CONVERTERS
Various reactor types have been designed to facilitate catalytic
conversions on an industrial scale. Four examples are given in
Figure 1.4 Reactor A is a stirred tank reactor operated
batchwise. The catalyst is dissolved in the liquid phase, and it
could be an acid or a base, a metal salt, or a metal−organic
complex. Shown is a ruthenium alkylidene complex, which is
active for the olefin metathesis reaction.5 Catalysis in solution is
referred to as homogeneous catalysis. Homogeneous catalysts are
usually more active and selective than heterogeneous catalysts.
They are also easier to tailor for specific purposes, as the
reaction mechanism is often well understood. The major
disadvantage of homogeneous catalysis lies in the fact that it is
difficult to separate the products from the catalyst, as the
catalyst is present in the same phase. Also, homogeneous
catalysts do not lend themselves easily for continuous
operations. However, these problems can be overcome by
Scheme 1. Acid-Catalyzed Esterification
■
ECONOMIC BACKGROUND
The economic importance of catalysis reflects in the following
numbers:
• More than 85% of all chemical products are manufactured with the help of catalysts.1
• 15−20% of the economic activities in industrialized
countries depend directly on catalysis.2
• The commercial value of the catalysts produced annually
amounts to ∼14 billion US$.2
© 2012 American Chemical Society and
Division of Chemical Education, Inc.
Published: October 9, 2012
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In the oil refining industry, almost all chemical conversions are
catalyzed, and the E-factor is less than 0.1. However, when
ascending the value creation chain in the chemical industry, the
impact of catalysis decreases. The synthesis of pharmaceuticals,
for example, is clearly dominated by traditional, multistep,
stoichiometric organic chemistry, and burdened with a high Efactor; the quantity of waste produced can exceed the quantity
of the targeted active pharmaceutical ingredient by a factor
higher than 25 (see Figure 2).
Figure 2. The impact of catalysis on the environmental E-factor.
Figure 1. Some catalytic reactors (a, gaseous feed; b, gaseous or liquid
product; c, liquid feed; d, liquid product; e, off-gas; f, catalyst; g,
coolant). Examples for the catalysts used in each reactor type are
pictured at the bottom of the figure. See the text for a detailed
explanation.
Another convenient measure for the environmental impact of
chemical conversions is the atom utilization.7,8 Calculation of
the atom utilization (AU) is based on molecular weights (MW)
and defined as follows:
immobilizing homogeneous catalysts onto heterogeneous
supports. For a successful heterogenization of a transitionmetal complex, the reaction mechanism should predict that the
metal ligand chosen for bonding to the support remains firmly
attached to the metal. The cyclopentadienyl ligand is an
example for such a ligand.6
Reactors B, C, and D (Figure 1) are used for heterogeneous
catalytic processes. Example B represents a sparged stirred tank
reactor for gas−liquid reactions. The catalyst is suspended in
powder form. Shown below the reactor is a precious-metal
catalyst supported on activated carbon. The catalyst can readily
be filtered off and reused until it is no longer of sufficient
activity. Spend metal catalysts are normally returned to the
catalyst manufacturer for metal recovery. Examples C and D
show fixed-bed reactors. These operate in a continuous mode.
The solid catalyst is stationary and the gaseous or liquid feed is
passed over the catalyst bed. Multitube reactors (D) allow for
efficient heat removal in the case of exothermic reactions.
Heterogeneous catalysts for fixed-bed operations come in many
different forms, as determined by the macro- and the
microkinetics of the process. The artistic composition of
examples shown below the two reactors was taken from a
Clariant, formerly, Süd-Chemie catalogue, with permission.
Clariant is a catalyst manufacturer. Other companies producing
catalysts for the chemical industry and other applications
include BASF, Evonic, Johnson Matthey, Heraeus and HaldorTopsoe. Reactor types used for catalytic conversions are not
limited to those shown in Figure 1. See Henkel4 for a more
comprehensive selection.
AU =
MW of desired product
Sum of MWs of all products formed in the stoichiometric equation
The usefulness of the atom utilization for the comparison of
different chemical processes is demonstrated by example of two
routes to niacin or niacinamide. Niacin or nicotinic acid is a
vitamin of the B group. In vivo, the acid is converted into the
amide and both can be used as nutritional supplements. Lonza
AG (Switzerland) is a major supplier and has developed two
routes. The classical route is used in Switzerland, while a new
catalytic route is operated by Lonza Guangzhou in China. The
classical route starts with acetaldehyde and ammonia (Scheme
2). These are converted into 5-ethyl-2-methylpyridine by a
Scheme 2. Classical Route to Nicotinic Acid (Lonza AG,
Switzerland)
■
ENVIRONMENTAL IMPACT OF CATALYSIS
Catalysis is inherently a green technology. This is clearly
demonstrated by the influence catalysis has on the environmental factor (E-factor).7 The E-factor is defined as mass of
waste produced per mass of desired product.
kg of waste
E=
kg of desired product
Tschitschibabin pyridine synthesis,9 followed by oxidation with
nitric acid.10 As Scheme 2 shows, the environmental impact of
this route is contained by recycling of the nitrous oxide, but two
carbons are lost as carbon dioxide. The overall atom utilization
is 37%. It is important to keep in mind that the atom utilization
(in contrast to the E-factor) does not account for losses caused
by low selectivities, as these are not reflected in the overall
reaction equation. If a reactant A undergoes a variety of parallel
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or consecutive reactions and B is the desired product, the
selectivity S to B (in %) is defined as
SB =
Number of moles of A converted to B
100%
Total number of moles of A converted
The catalytic route (Scheme 3) starts with 2-methylpentanediamine, which is cyclized to 3-methylpiperidine over an
Scheme 3. Modern Catalytic Route to Niacinamide (Lonza
Guangzhou)
Figure 3. Schematic comparison of the thermodynamics and kinetics
of a catalyzed versus an uncatalyzed reaction.
substrates in the gas phase. Wilhelm Normann proved him
wrong; in 1902, he patented the catalytic hydrogenation of fats
and oils in the liquid phase.15 By 1914 not less than 25 plants
for the saturation of fats and oils (fat hardening) were running
worldwide, mainly for the production of margarine and
shortening.16,17
■
MECHANISTIC ASPECTS OF CATALYSIS ON METAL
SURFACES
Today’s understanding of double-bond hydrogenation is based
on the mechanism originally proposed by Horiuti and Polanyi18
for the hydrogenation of ethylene on a Pt(111) surface (Figure
4). This mechanism is instructive, as the principle reaction steps
acidic catalyst. 3-Methylpiperidine is dehydrogenated to 3picoline over palladium.11 It follows an ammonoxidation
reaction to 3-cyanopyridine using a vanadium oxide containing
catalyst. Finally, selective enzymatic nitrile hydrolysis leads to
pharma-grade niacinamide.12 Notably, the ammonia set free in
the first step is re-incorporated in the third step. The only
byproducts are hydrogen and water, and the overall atom
utilization is 75%. Hydrogen can be recovered for further use,
and water is environmentally beneficial. The process shown is
therefore a prime example of modern green chemistry. The
starting material, 2-methylpentanediamine, is marketed by
Invista (previously DuPont) under the trade name Dytek A.
It is obtained by hydrogenation of 2-methylglutaronitrile, which
is a structural isomer of adipodinitrile and, therefore, a
byproduct of the Nylon 6.6 manufacturing route.
■
THERMODYNAMIC AND KINETIC ASPECTS OF
CATALYSIS
The thermodynamic and kinetic implications of catalysis are
schematically demonstrated in Figure 3. A catalyst provides a
new route with a lower Gibbs energy of activation (ΔG⧧) for
the product formation from given starting materials (reactants).13 In this way, a catalyst accelerates the reaction. The
Gibbs energy for the reaction (ΔG⊖) remains unchanged. This
means a reaction that is thermodynamically unfavorable cannot
be made favorable with the help of a catalyst. (Catalysis is a
kinetic, not a thermodynamic, phenomenon.) All the catalytic
intermediates of the reaction must be less stable than the
product. If an “intermediate” is more stable than the desired
product, the reaction will stop there (dotted line in Figure 3),
and this intermediate will be the final product.
The addition of hydrogen to the double bond of ethylene, for
example, is thermodynamically favorable, but does not proceed
at room temperature at any appreciable rate. At the turn of the
19th century, Paul Sabatier discovered that such reactions could
be greatly accelerated by the addition of finely divided nickel.14
However, he maintained that this would only be possible with
Figure 4. Schematic presentation of the mechanism for the
hydrogenation of ethylene on a platinum surface.
involved offer a good starting point for the understanding of
many other reactions taking place on metal surfaces.
The following steps describe the mechanism:
• Physisorption of the reactants on the metal surface.
(Physisorption takes place via van der Waals forces.)
• Chemisorption of the reactants. (Chemisorption involves
the formation of chemical bonds with the catalyst
surface.) Ethylene forms an equilibrium between the πbonded and the di-σ-bonded form, and the H−H bond
of hydrogen is cleaved. According to Somorjai,19 it is
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mainly π-bonded ethylene that, in the following step,
reacts further to the ethyl intermediate.
• Formation of the ethyl intermediate. This step must be
reversible, as the complete range of deuterated ethanes
(and not only DH2C−CH2D) is observed in case D2 is
used instead of H2.20
• Formation of the second C−H bond.
• Desorption of the product.
The key step is the activation of the reactants by chemisorption
to the platinum surface. This facilitates the catalytic route of
lower activation energy for the formation of the products.
Sabatier postulated that a good catalyst provides an optimum
strength of bonding between the reactants and the catalyst
surface (Sabatier’s principle).21 Both too weak and too strong
bonds will slow down or prevent the reaction. This principle
was convincingly demonstrated by Rootsaert and Sachtler,22 as
illustrated in Figure 5. The model reaction is the decomposition
strike the correct balance and provide the optimum strength of
bonding between the reactant and the catalyst surface for the
reaction to occur.
For ethylene hydrogenation the situation translates into the
picture shown in Figure 6.23,24 First-row transition metals and
Figure 6. Rate of ethylene hydrogenation over various metals relative
to rhodium (Rh = 1; adapted from ref 24).
second- and third-row transition metals appear on separate
volcano curves. The group VIII metals (periodic groups 8−10)
are most active and among these the platinum group metals.
It is well-known from practical applications that base metals
are less active hydrogenation catalysts than the more expensive
platinum group metals (PGMs). Base metals generally require
more stringent reaction conditions than PGMs to achieve
acceptable results. However, for bulk chemical applications, the
catalyst price is a determining factor, and for this reason, nickel
and cobalt are often preferred to precious metal catalysts (Ru,
Rh, Pd, and Pt).
It should be mentioned that the exact chemical nature of the
chemisorbed species that play an active role in a given catalytic
conversion is often difficult to establish. The precise molecular
structure of the intermediates depends on many factors. These
include
• Composition of the catalytically active metal or alloy.
• Crystallite size of the metal or alloy.
• Exact metal surface structure (e.g., type of exposed
crystal phase, presence of kinks and steps).
• Adsorbate induced restructuring.19
• Presence of other chemisorbed species.
• Nature of the catalyst support.
• Presence of catalyst promoters.
• Temperature and pressure.
A catalyst surface must be seen as a flexible, dynamic system
that is able to accommodate a sequence of catalytic
intermediates. The multiplicity of possible surface sites makes
it difficult to determine the exact chemical environment of each
intermediate that contributes to the catalytic cycle. For an
advanced treatment of this important aspect of heterogeneous
catalysis, see van Santen and Neurock.25
Figure 5. Demonstration of Sabatier’s principle by example of the
dehydrogenation of formic acid. The plot shows the reaction
temperature required to achieve a given rate in the metal-catalyzed
dehydrogenation reaction versus the enthalpy of formation of the
metal formates (adapted from ref 22).
of formic acid to carbon dioxide and hydrogen over various
metals. It is reasonable to assume that the metal formate shown
is the reaction intermediate. The strength of the bond between
the formate and the catalyst surface can be estimated by the
enthalpy of formation (ΔfH⊖) of the corresponding metal
formate salt. ΔfH⊖ is plotted on the x axis of the graph shown
in Figure 5. The reaction temperature required to reach a
prefixed reaction rate (log r = 0.8) for the decomposition of
formic acid over the corresponding metal catalysts appears on
the y axis. The result is a volcano curve. The most active metals
appear at the top of the volcano curve at the lowest
temperatures required to achieve the preset reaction rate. To
the left (Au and Ag), bond formation between the catalyst
surface and the formate is too weak, as indicated by the
corresponding ΔfH⊖ values; to the right (W, Fe, Co, Ni, and
Cu), the enthalpy of formation is highly negative and bond
formation is too strong. In the first case, the intermediate does
not form at a sufficiently high rate; in the second case, the
intermediate is too stable and does not decompose at a
sufficiently high rate. The platinum group metals in the middle
■
CONVERSIONS BY METALLIC-STATE CATALYSIS
It is necessary to clearly distinguish between catalysis by metals
and catalysis in the metallic state. The general term “metal
catalysis” is usually understood to include catalysis by metal
compounds, such as metal oxides, salts, and organometallic
complexes. Homogeneous transition-metal catalysis is always
facilitated by metal compounds. A border case between
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synthesis. As indicated in Figure 7, a line drawn across the
periodic table separates copper and nickel, separates the base
metals from the PGMs up to ruthenium, divides ruthenium
from rhodium and osmium, and finally crosses down between
osmium and rhenium. Under normal catalytic reaction
conditions, metals to the left of this line dissociatively
chemisorb carbon monoxide, whereas metals to the right will
chemisorb CO molecularly (without breaking the carbon−
oxygen bond).29 In other words, on iron, cobalt, and nickel CO
dissociates, whereas it remains undissociated on the coinage
metals (Cu, Ag, and Au) and the PGMs (except Ru).
Regardless of the detailed mechanisms involved,29 the full
hydrogenation of undissociated CO over a copper catalyst leads
to methanol, whereas the hydrogenation over the base metals
iron, cobalt, and nickel leads to hydrocarbons and water. The
fact that commercial nickel catalysts can produce methane with
a selectivity of up to 96% is explained by the high abundance of
chemisorbed hydrogen relative to chemisorbed carbon
(carbidic surface carbon) on a nickel surface. As a result,
almost all the surface carbon is rapidly hydrogenated to
methane.29 This is in contrast to the situation found on iron
(and cobalt). The coverage by carbidic carbon is extensive and
the reduced availability of hydrogen favors hydrocarbon chain
growth to give alkanes, and unsaturated compounds (olefins)
form as byproducts. In terms of Sabatier’s principle iron and
cobalt provide the optimum bond strength for the chemisorbed
species to form Fischer−Tropsch products, whereas nickel
provides the optimum bond strength for the formation of
methane.
The reverse reaction of methanation (double arrow in Figure
7) is known as steam reforming and is of even greater industrial
importance. Most of the synthesis gas used for processes such
as methanol synthesis, Fischer−Tropsch synthesis, and hydroformylation (the conversion of olefins into aldehydes) is
produced by nickel-catalyzed steam reforming.
homogeneous and heterogeneous catalysis would be the
catalysis induced by colloidal nanoparticles kept in suspension.26 Only groups VIII and IB of the periodic table (all the
metals shown in Figure 7) display catalytic activity of practical
Figure 7. Synthesis gas (CO/H2) products obtained over various
metals depending on the mode of CO chemisorption (associative or
dissociative; chemisorption of hydrogen not shown).
significance in the metallic state, especially for hydrogenation
and oxidation reactions. Other transition elements are too
difficult to reduce and to maintain in the metallic state.24 In the
metallic state, iron, cobalt, nickel, copper, and ruthenium
catalyze hydrogenation reactions, including the hydrogenation
or hydrogenolysis of carbon monoxide. (Hydrogenolysis, in
contrast to hydrogenation, involves the cleavage of an
interatomic connection in the substrate.) Iron and ruthenium
are unique in their capability of converting nitrogen to
ammonia, a reaction that can be regarded as the hydrogenolysis
of N2. Silver is used for some large-scale selective oxidation
reactions, such as the conversion of ethylene to ethylene oxide
and the oxidation of methanol and ethanol to the
corresponding aldehydes. In contrast, oxidations over Rh, Pd,
Ir, and Pt are prone to result in deep oxidation, which is the
total oxidation to CO2 and water. These four metals are also
extremely useful for many conversions involving hydrogen,
such as hydrogenations, dehydrogenations, hydrogenolyses, and
naphtha reforming. The catalytic activity of gold appears to be
restricted to the nanostate,27 as confirmed by numerous more
recent reports.28
Metallic-state catalysis over group VIII and IB elements
includes many important large-scale processes. Examples are
ammonia synthesis (Fe), ammonia oxidation (Pt−Rh),
Fischer−Tropsch synthesis (Fe, Co), methanol synthesis
(Cu), and refinery processes such as platforming.
■
AUTOMOTIVE EXHAUST CATALYSIS
As indicated above, oxidations over Rh, Pd, Ir, and Pt are prone
to result in deep oxidation. Environmental abatement catalysts
make use of this. The most important single application is
probably automotive exhaust catalysis. It is a remarkable
achievement that the deep oxidation of hydrocarbons and
carbon monoxide and the reduction of nitrous oxides can be
done in a single catalytic converter.30 This is possible by the use
of platinum or palladium or both combined with rhodium. A
schematic presentation is given in Figure 8. Rhodium is able to
dissociatively adsorb NO in preference to O2, whereas
palladium and platinum are responsible for the splitting of
oxygen.31 The chemisorbed nitrogen atoms combine to N2, and
the chemisorbed oxygen atoms oxidize carbon monoxide and
hydrocarbons (HC) to CO2 and water.
■
METAL-DEPENDENT PRODUCT FORMATION
FROM SYNTHESIS GAS
The conversion of synthesis gas (mixtures of CO/H2 in
different ratios) provides an interesting case concerning the
change in product selectivity depending on the catalytic
element used. This is demonstrated in Figure 7. The product
selectivity changes along the first transition-metal series from
left to right. Iron and cobalt (and also ruthenium) convert
synthesis gas into a mixture of hydrocarbons by a process
known as Fischer−Tropsch synthesis. Nickel is a selective
methanation catalyst, and copper is used for methanol
Figure 8. Schematic presentation of the key aspects of an automotive
catalytic converter (HC = hydrocarbons; adsorption of CO and HC
not explicitly shown).
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The introduction of the automotive exhaust catalyst shows
that legislative pressure can trigger innovative solutions and
create business opportunities on a large scale (even for metal
brokers and speculators). To understand some of the price
fluctuation seen for rhodium over the last four decades,32 it has
to be kept in mind that the annual mining output of rhodium is
only about 25 tons. This constitutes ca. 1% of the annual gold
production. The annual production of platinum amounts to
approximately 130 tons. In 2008, more than 75% of the total
rhodium output was used for the manufacturing of
autocatalysts.33
Scheme 5. Selective Oxidation of Hydroxymethylimidazoles
to Formylimidazoles
■
PEDAGOGICAL ASPECTS
The content of this article can be used as the basis for an
introduction to heterogeneous catalysis. For example, it could
lay the foundation for a course on descriptive heterogeneous
catalysis or industrial process chemistry. It also could serve as
an amendment to a course on homogeneous catalysis. Students
are provided with a background on economic and ecological
aspects of catalysis, types of catalysts used in industrial practice,
modification of catalysts (to improve selectivity), the
importance of surface chemistry for the understanding of
catalytic reactions at a molecular level, and thermodynamic and
kinetic implications. Examples of industrial importance are used
for demonstration purposes. After they have been introduced to
catalysis using the concept of this paper, learners should be able
to answer the following question: Why do certain metals
selectively catalyze certain transformations?
Parts of this paper could be given as exercises. Depending on
their level, students could be asked to match catalyst types to
given catalytic converters (Figure 1). After Scheme 2 has been
explained, students should be able to calculate the atom
utilization for the reaction of Scheme 3 and compare the two
processes. After they have been introduced to one of the
Figures 5 and 6, they could be asked to find an interpretation
for the remaining figure. On the basis of the information
contained in Figure 7, students should be able to propose
possible catalysts for other reactions of carbon monoxide.
Examples for other reactions, including those of CO, are found
elsewhere.38
■
SELECTIVE OXIDATION CATALYSIS UNDER MILD
CONDITIONS
Deep oxidation, or over oxidation, is likely to occur with PGMs
under oxidative conditions, unless special precautions are taken.
However, for selective oxidation reactions, high-oxidation-state
base-metal oxides can be used as catalysts. These must be able
to switch between two oxidation states (redox catalysis).
Examples are MoO3, V2O5, and Sb2O5. The reactions take place
at elevated temperatures in the gas phase. These conditions can
be prohibitive for fine chemical applications, due to the
temperature sensitivity of complex organic molecules. However,
for applications in the fine chemicals industry, palladium and
platinum catalysts can be modified by incorporation of bismuth
or lead, resulting in improved selectivity for partial oxidations.
Although the exact role of bismuth and lead is still a matter of
speculation, Besson et al.34 provided an instructive suggestion
by example of the selective oxidation of an hydroxyl group (see
Scheme 4).
Scheme 4. Speculative Mechanism for the Pt−Bi Catalyzed
Oxidation of Alcohols (adapted from ref 34)
■
CONCLUSION
Catalysis, and heterogeneous catalysis in particular, is a mature
field of science. Catalysis grows and diversifies further as the
world economy grows and diversifies. It contributes significantly to value creation in the real economy. From an
environmental point of view, it impacts positively on the
viability of many human activities, not only those of a pure
chemical nature. To continue educating the catalysis experts of
tomorrow at schools and universities and to raise public
awareness for the importance of catalysis is therefore of great
significance. To the lay person and to many undergraduate
students, catalysis appears as a rather complex field, not the
least because of the huge number of different catalytic materials
and the many different catalytic processes that go with them.
Often students are exposed to specific applications of catalysis
(e.g., descriptive heterogeneous catalysis) before they have
been exposed to the basic principles of the relevant surface
chemistry. This often contributes to the perception that
catalysis is a “magic” art, rather than a science. Teaching
important fundamentals of catalysis at an early stage can be
expected to contribute to a deeper understanding. This, in turn,
will allow the student to put forthcoming information into
perspective. In the paper at hand, an attempt was made to
explain some important concepts of catalysis in a brief, but
According to this proposal, dehydrogenation of the alcohol
to the aldehyde takes place on the noble metal surface. The
chemisorbed hydrogen reacts with a higher-oxidation-state
bismuth oxide species, giving water and a lower-oxidation-state
bismuth species. The lower-oxidation-state bismuth is reoxidized by the oxidizing agent used for the overall reaction.
The addition of bismuth keeps the platinum surface free of
excess oxygen, thus preventing over oxidation of the substrate.
The left part of the catalytic cycle exemplifies the action of a
common base-metal oxidation catalyst: bismuth switches
between two oxidation states and (in the case of Scheme 4)
oxidizes hydrogen to water. Depending on the substrate, Pt−Bi
catalysts allow for very high selectivities under mild reaction
conditions. Instead of air, hydrogen peroxide can also be used
as the oxidizing agent. Examples for the oxidation of
hydroxymethylimidazoles to formylimidazoles are shown in
Scheme 5.35,36 Formylimidazoles are important intermediates in
the synthesis of pharmaceuticals. Further examples for the
successful application of Pt−Bi oxidation catalysts were
reported by Anderson et al.37
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(28) Bond, G. C.; Louis, C.; Thompson, D. T. Catalysis by Gold;
Imperial College Press: London, 2006.
(29) Campbell, I. M. Catalysis at Surfaces; Chapman and Hall:
London, 1988; pp 132−142, 205−215.
(30) Gandhi, H. S.; Graham, G. W.; McCabe, R. W. J. Catal. 2003,
216, 433−442. Twigg, M. V. Platinum Met. Rev. 2011, 55, 43−53.
(31) Thomas, J. M.; Thomas, W. J. Principles and Practice of
Heterogeneous Catalysis; VCH: Weinheim, 1997; pp 576−590.
(32) Sharelynx Gold. http://www.sharelynx.com/gold/RareMetals.
php#rhodium, accessed May 22, 2012.
(33) http://www.platinum.matthey.com./publications/pgm-marketreviews/archive/platinum-2009/Pt2009.html (Supply and Demand),
accessed Sept. 28, 2012.
(34) Besson, M.; Lahmer, F.; Gallezot, P.; Fuertes, P.; Flèche, G. J.
Catal. 1995, 152, 116−121.
(35) Bessard, Y.; Heveling, J. US Pat. US6’469’178, 2002.
(36) Heveling, J.; Wellig, A. Eur. Pat. EP916’659, 2002; Eur. Pat.
EP913’394, 2005.
(37) Anderson, R.; Griffin, K.; Johnston, P.; Alsters, P. L. Adv. Synth.
Catal. 2003, 345, 517−523.
(38) Wittcoff, H. A.; Reuben, B. G.; Plotkin, J. S. Industrial Organic
Chemicals, 2nd ed.; Wiley-Interscience: Hoboken, NJ, 2004.
understandable manner. Applications of industrial and general
importance were used for demonstration purposes.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The author declares no competing financial interest.
■
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