J. Phys. Chem. B 2004, 108, 18535-18546
18535
Boron Sites in Borosilicate Zeolites at Various Stages of Hydration Studied by Solid State
NMR Spectroscopy
Son-Jong Hwang,*,† Cong-Yan Chen,‡ and Stacey I. Zones‡
DiVision of Chemistry and Chemical Engineering, California Institute of Technology,
Pasadena, California 91125, and CheVronTexaco Energy Technology Company, Richmond, California 94802
ReceiVed: May 27, 2004; In Final Form: August 9, 2004
The local structures of framework boron atoms in borosilicate zeolites B-β, B-SSZ-33 and B-SSZ-42 have
been studied in the course of hydration/dehydration by employing solid-state NMR methods. In particular,
characterization of trigonal boron sites has been studied in great detail. 11B MAS NMR spectra showed that
boron trigonally coordinated to the framework (B(OSi)3, denoted as B[3]) can be readily transformed to a
defective trigonal boron site (B(OSi)2(OH), denoted as B[3]-I) as a result of hydration. The presence of
B[3]-I sites was proven by utilizing a number of different NMR methods including 11B MAS NMR at two
different fields (11.7 and 19.6 T), 11B MQMAS, 11B CPMAS, and 11B 2D HETCOR experiments. The B[3]-I
species can be converted into B[3] upon dehydroxylation, but its presence can also be sustained even after
very high-temperature treatment (at least up to 500 °C). The formation of deboronated species, B(OH)3, in
distorted form was detected even under a mild hydration treatment. HETCOR NMR revealed that hydroxyl
protons with chemical shifts at 2.4 and 3.3 ppm in 1H NMR are correlated with B[3] and B[3]-I sites,
respectively. The presence of a new hydroxyl proton at 3.8 ppm in 1H NMR that showed selective correlation
with B[3]-I in HETCOR NMR was also identified.
I. Introduction
SCHEME 1
In recent years, much attention has been given to the syntheses
and characterization of borosilicate zeolites because their weaker
acidity is suitable for certain catalytic reactions that require mild
solid acids as catalysts.1-3 The borosilicate zeolites can also
provide a unique post-synthetic route for preparing aluminosilicate or other isomorphous forms of zeolites at certain Si/M
ratios (M represents trivalent metal ion such as Al and Ga) where
the trivalent metal ions cannot be incorporated directly into the
framework in synthesis.4 In the past two decades the change
of coordination geometry at the boron centers (tetrahedral
(B[4]) T trigonal (B[3])) upon hydration/dehydration treatments
of borosilicate zeolites, especially for the proton form where
the H+ becomes the charge compensating cation, has been
studied in detail both experimentally5-9 and computationally.10-11
Such a transformation of boron coordination and subsequent
variation of the local geometry were good rationales in explaining the weaker acidic borosilicate zeolites compared with the
corresponding Al substituted sites. Experimentally, use of high
resolution 11B solid-state NMR techniques has been most
efficient in investigating changes of structures around boron sites
by taking advantage of the element’s electric quadrupole
moment (I ) 3/2).12 The B(OSi-)3 types of trigonal geometry
render the quadrupolar coupling constant (Cqcc) as high as 2.6
MHz, which is large enough to yield second order quadrupole
broadening even under a fast magic angle spinning (MAS)
condition. The resulting anisotropic line shape is readily
distinguished from that of tetrahedrally coordinated boron sites,
where the Cqcc is measured to be an order of magnitude lower.
* Corresponding author. Fax: 1-626-567-8743. E-mail: sonjong@
cheme.caltech.edu.
† California Institute of Technology.
‡ ChevronTexaco Energy Technology Co.
The trigonal boron sites B[3] formed via dehydration from
the conversion of tetrahedral boron sites B[4] of the proton form
of borosilicate zeolites remain tightly anchored with the
framework through three B-O-Si bonds in dehydrated environments. However, the B[3] sites are very susceptible to
nucleophilic attack by water molecules when the materials
become hydrated (see Scheme 1).9 This attack leads to the
creation of B-OH bonds as found in B[4]-I species. Note that
the negative charge on the boron centers in the B[4]-I species
is balanced by coordination of OH2+. Successive hydrolysis
steps will eventually replace all the B-OSi bonds with B-OH
bonds, resulting in release of boron atoms from the framework
as B(OH)4- species and leaving a silanol nest (four SiOH
groups) in the framework. The deboronation reaction takes place
quickly even at ambient temperature when a dehydrated material
becomes rehydrated. For example, we found that the formation
of B(OH)4- species can be readily observed in a 11B MAS NMR
spectrum that is obtained after a drop of water is added to a
dehydrated borosilicate zeolite powder containing trigonal boron
sites. The vulnerability of B-OSi bonds to moisture is wellknown. The boron content was reported to be reduced up to
40% just by washing with the distilled water.9 To avoid the
deboronation, ion exchange of the proton of borosilicate zeolites
with alkali metal cations or NH4+ was employed because these
bulkier cations were proved to protect B[4] sites from the
coordination transformation even under dehydration conditions.8,9 A calcination process that uses ammonia gas was also
10.1021/jp0476904 CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/09/2004
18536 J. Phys. Chem. B, Vol. 108, No. 48, 2004
SCHEME 2
introduced to generate template free zeolites having all boron
sites in tetrahedral coordination.9 In any cases, once the catalytically active H-form of borosilicate zeolites is prepared, the
framework boron sites will be present in the B[3] form upon
dehydration. These B[3] sites are susceptible to the progressive
deboronation during a normal laboratory storage if great care
is not taken. In the process of hydrolysis, a number of different
types of boron species are expected to be formed as intermediates before the framework boron atoms are freed from B-OSi
linkages. They include both trigonal and tetrahedral boron
species, depending on the number of coordinating OH groups
as sketched in the structural moieties shown in Scheme 2.7,9
Currently, much of the spectroscopic details of the species in
Scheme 2 remain unresolved. As will be discussed next in this
report, a number of high-resolution NMR methodologies were
employed to elucidate the structural changes around boron atom
centers during hydration/dehydration.
First, the formation and transition of intermediates were
investigated at different stages of the hydration/dehydration
processes by acquiring 11B MAS NMR spectra after single pulse
(single quantum) excitation of the central transition (-1/2T 1/2).
Often, the second-order quadrupole interactions of trigonal
species (B[3], B[3]-I, B[3]-II) give rise to a crowded 1D
spectrum, especially when trigonal species coexist and render
overlap of characteristic quadrupole powder patterns. 11B MAS
NMR spectra obtained at a higher magnetic field (19.6 T) were
used to unravel the problem.
Second, a 11B 2D multiple quantum MAS (MQMAS)13-15
method was employed to obtain highly resolved isotropic line
components free of quadrupole interaction. Elimination of the
second order broadening effect in this technique is based on
proper correlation of the radio frequency (rf) driven symmetric
multiple quantum excitation with the single quantum coherence
under the MAS condition. As originally demonstrated by
Frydman et al.,13 the MQMAS NMR technique has been proven
to be extremely useful in examination of the half-integer
quadrupole nuclei and applied extensively to study amorphous
and crystalline inorganic solid materials. A recent and thorough
review on both theoretical and experimental aspects was reported
by Amoureux and Pruski.15
Third, a systematic approach was attempted to correlate 11B
and 1H NMR signals in this work. For this purpose, 1H-11B
cross-polarization (CP) based heteronuclear correlation spectroscopy16 (HETCOR) was utilized. Use of high MAS rate (>10
kHz) as well as low rf power was essential for this technique
in order to obtain efficient 1H-11B cross-polarization and 1H
spectral resolution that is high enough to show individual
correlation with 11B MAS powder patterns in the twodimensional contour display. Fild and Koller8,17 have reported
that internuclear distances between 1H and 11B in a calcined
and dehydrated B-β might be estimated using the rotational echo
double resonance (REDOR) NMR method. Information about
which hydroxyl groups are closely related to trigonal boron
centers can then be obtained via REDOR method. Both
HETCOR and REDOR techniques are used to probe heteronuclear dipole interactions under MAS conditions. Although
HETCOR NMR spectroscopy lacks in precise examination of
Hwang et al.
interatomic separations, it was proven to be more effective in
making selective associations between hydroxyl groups and
boron sites, especially when a number of different boron species
were populated. Such an individual correlation allowed us to
assign the hydroxyl group that is directly associated with the
B[3] center, as a possible framework Brønsted acid center.
Here we report our studies on the following three borosilicate
zeolites at various stages of hydration using the NMR techniques discussed above: B-β (BEA*), B-SSZ-33 (CON), and
B-SSZ-42 (IFR). The B-β sample was most heavily studied at
each different dehydration level.
II. Experimental Section
1. Material Synthesis. The borosilicate zeolites B-SSZ-33,
B-SSZ-42, and B-β were synthesized by using the appropriate
structure directing agents (SDAs) in hydroxide form, as outlined
in our previous publications.18-21 Cab-o-sil M-5, sodium borate
decahydrate, and NaOH were used as silicon, boron, and alkali
metal sources, respectively. The compositions of these zeolites
were determined by Galbraith Laboratories (Knoxville, TN) via
elemental analyses using ICP methods. The molar Si/B ratios
of these zeolites are 17.0 (B-SSZ-33), 25.9 (B-SSZ-42), and
15.4 (B-β).
2. Calcination. The as-made borosilicate zeolites were
calcined in thin beds in a muffle furnace to remove the SDAs
occluded in zeolite channels. The temperature program was as
follows: 1 °C/min to 120 °C, hold for 2 h, 1 °C/min to 540 °C,
hold for 4 h, 1 °C/min to 600 °C, and hold for 4 h. The
calcinations were carried out in a steady flow of nitrogen
containing just a slight bleed of air over the bed of zeolite.
3. Dehydration. Calcined materials were hydrated in several
different manners after the calcinations, which will be noted
individually. The calcined and hydrated borosilicate zeolites
were first packed in a 4 mm ZrO2 NMR rotor, and the whole
ZrO2 rotor was placed inside a 15 cm long 5 mm glass NMR
tube. The glass NMR tube was then attached to a vacuum line
and evacuated to 10-3 Torr while heated at 1 °C/min up to a
target temperature (e.g., 120 °C) and then held at the temperature
for 4 h. When the dehydration temperature was set to be higher
than 120 °C, the sample was first held at 120 °C for 2 h in
order to remove most of water from the framework and then
further heated to the target temperature at 1 °C/min. Cooling to
room temperature was allowed to occur naturally inside the
heating furnace while the evacuation was maintained. Dry N2
gas was introduced to the sample before the rotor was exposed
to air, and the rotor was closed immediately with the tight
sealing kel-F cap. Such care was found to be efficient in
avoiding introduction of moisture.
4. NMR Analysis. Most of the solid-state NMR spectra were
collected using a Bruker DSX-500 spectrometer operating at
11.7 T with 500.2, 160.5, and 99.4 MHz for 1H, 11B, and 29Si
nuclei, respectively, and using a Bruker 4 mm CPMAS probe.
A typical spinning rate was 12 kHz and MAS spectra were
recorded after applying 4 µs-π/2 pulse for 1H and 29Si and
1 µs-π/8 pulse for 11B nucleus. The chemical shifts were
referenced to TMS and BF3‚O(CH2CH3)2. 11B MAS NMR
spectra at 19.6 T with 11B resonance frequency of 266.1 MHz
were obtained at the National High Magnetic Field Laboratory
in Tallahassee, FL, without 1H decoupling and at a 10 kHz
sampling spinning rate.
The 11B 2D MQMAS experiments were performed using the
standard z-filtered sequence22 at 13 kHz sample spinning. For
the 1H-11B CPMAS and 2D HETCOR experiments, the spinlocking during cross-polarization transfer was maintained in the
Boron Sites in Borosilicate Zeolites
J. Phys. Chem. B, Vol. 108, No. 48, 2004 18537
Figure 1. 1H and 11B MAS NMR spectra at 11.7 T of calcined B-β at various levels of hydration: (a) sample A, freshly calcined and then
vacuum-dried at 500 °C; (b) sample B, freshly calcined sample and then stored for one year in closed vial, no dehydration treatment; (c) sample
C, sample after vacuum-drying of sample B at room temperature; (d) sample D, sample after vacuum-drying of sample C at 120 °C; (e) sample E,
sample after vacuum-drying of sample D at 450 °C.
sudden-passage regime by applying very low rf pulse (1.3 kHz)
for boron.23-25 The adiabaticity parameter R ()υ21s/υQυr) was
estimated to be less than 0.0001 for trigonally coordinated boron
atoms. The sample was spun at 10 kHz.
III. Results and Discussion
1. Dehydration of Boron-β. Figure 1 shows 1H and 11B MAS
NMR spectra of calcined B-β zeolite at various levels of
hydration. Note that all spectra were obtained with the same
number of transients and normalized to the sample weights used
in the NMR measurements. This allows signals to be directly
compared and the relative changes among the steps of dehydration to be measured. Both bottom spectra (see Figure 1a) from
sample A, which was a freshly calcined and dehydrated sample,
can serve as reference spectra in future comparisons. These
spectra represent the local structure of boron atoms in well
dehydrated B-β framework, as represented by B[3] shown in
Scheme 1. The 1H signal at 2 ppm is assigned to the silanol
group (Si-OH) while the 11B signal represents the trigonal
boron sites coordinated to three OSi groups of the zeolite
framework. Such assignments are in good agreement with
previous reports.5-8 The 11B spectrum shows a MAS NMR line
shape (10 ppm ∼ -2 ppm) of the typical powder pattern that
originates from the second order quadrupole broadening of
trigonally coordinated 11B nuclei in a planar configuration.
Detailed NMR parameters for the quadrupole interaction will
be discussed below. A small and narrow peak around at -4
ppm represents tetrahedral boron groups B[4] (see Scheme 1).
The presence of B[4] groups can be attributed to Na+ ions that
exist in small quantity in the sample as confirmed by 23Na MAS
NMR and elemental analysis (not shown). The results also
indicate that the B-β sample studied here is dominated by the
H-form. Na+ ion is known to stabilize B[4] from coordination
conversion upon dehydration.8,9 Further investigation is in
progress to confirm the selective association of Na+ ion with
the B[4] unit.
Figure 1b depicts the spectrum collected from B-β sample
B, which was a freshly calcined material and then hydrated while
stored in a closed vial for 1 year. A stepwise dehydration was
performed on the same sample, resulting in samples C-E
(Figure 1, parts c-e, respectively). Rather drastically hydrated
samples were also studied and will be discussed later. Moisture
uptake by sample B is revealed by the presence of water peak
(∼5 ppm) in the 1H NMR shown in Figure 1b. From the signal
intensity of 1H NMR, the water content was measured to be
about 10 wt %. Concomitantly, structural modification of the
boron sites in sample B is well displayed by the 11B NMR
spectrum (see Figure 1b) upon hydration. Appearance of another
broad line shape ranging from 10 to 17 ppm in 11B MAS NMR
spectrum is eminent while the powder pattern of B[3] sites
shows reduced intensity compared to that of Figure 1a of sample
A. The broad line shape downfield (10-17 ppm) was also
reported in the literature8,17 and was interpreted as a formation
of nonframework trigonal boron species which bear no B-OSi links, such as B(OH)3. This implies that a mild hydration of
calcined borosilicate zeolite would induce the deboronation, as
also discussed in the Introduction section. Further investigation
was made to confirm the assignment of B(OH)3 to the broad
line shape by employing high-resolution NMR techniques and
will be discussed below. It is also worthwhile to note that the
increase of the -4 ppm peak indicates some conversion of B[3]
to tetrahedral boron sites B[4] or B[4]-I upon hydration (see
Scheme 1). The observation has been better analyzed by spectral
simulation and 2D MQMAS method (see below).
The spectra of B-β sample C resulting from the first step of
dehydration, i.e., evacuation at room temperature, are shown
in Figure 1c. 1H NMR shows that most of the mobile water
molecules were readily removed and the silanol groups (2.0
ppm) become partly resolved. The broad line shape at 4.5 ppm
represents hydrogen bonded H2O groups. The shape and
intensity of the -4 ppm peak in the 11B MAS NMR spectrum
of sample C (Figure 1c) appeared to be nearly the same as that
of sample A (freshly calcined sample dehydrated in a vacuum
at 500 °C, see Figure 1a). Partial recovery of the B[3] peak is
noticeable in sample C, indicating that some tetrahedral boron
sites which were formed in sample B upon hydration (Figure
1b) underwent coordination conversion back to B[3] sites even
via such a mild dehydration treatment (also see Scheme 1). A
rather striking observation at this point is the reduction of a
broad peak (around 16 ppm) that was originally considered to
be a part of powder pattern of nonframework boron species (see
below). Such change became more eminent as further dehydration took place at higher temperatures. Spectra of sample D
(dehydrated at 120 °C) in Figure 1d show the drastic decrease
of hydrogen bonded water and complete disappearance of the
16 ppm peak in 11B NMR. Note that the broad resonance around
3 ppm shown in the 1H NMR spectrum of sample D in Figure
1d was noticeably reduced when the dehydration temperature
18538 J. Phys. Chem. B, Vol. 108, No. 48, 2004
Hwang et al.
Figure 2. Experimental 11B MAS NMR spectra of calcined B-β at 11.7 and 19.6 T along with simulated line shapes. (a) sample B, hydrated while
stored for 1 year in closed vial, no dehydration treatment (see Figure 1b); (b) sample D, vacuum-dried at 120 °C (see Figure 1d). Arrow 1: B[3]*
sites resolved in 19.6 T spectra; Arrow 2: B(OH)4- type species formed due to additional hydration; Arrow 3: 16 ppm peak (see text for details).
Note that both samples were exposed to air during the sample packing for the measurement at 830 MHz spectrometer.
was increased to 450 °C for sample E (Figure 1e). The change
was accompanied by an eminent reduction of a broad resonance
at 11 ppm in the 11B NMR spectra. Dehydration at 450 °C
(sample E) resulted in NMR spectra (Figure 1e) that are similar
to those of sample A which was a freshly calcined sample after
calcination at 500 °C (Figure 1a), where no noticeable difference
between the 1H NMR spectra but some minor distortion in 11B
NMR line shape were observed. These results have clearly
demonstrated the reversibility of the coordination transformation
of boron sites upon hydration/dehydration process. It reveals
that 11B MAS NMR is well suited to show the formation of
intermediate species during the hydration/dehydration process.
To make structural identification of these intermediates, a closer
NMR investigation was performed as follows.
2. Trigonal Boron Sites Other Than B[3]: Finding of
B[3]*. To analyze in greater detail the 11B NMR line shapes of
samples B and D (see Figure 1, parts b and d), a number of
different approaches were taken as discussed below.
First, numerical simulation of the line shape acquired at 11.7
T for sample B in Figure 1b was performed by using
QUASAR26 and by taking into account the possible presence
of B(OH)3 type nonframework trigonal boron species (denoted
as [B]-nf) as proposed by Fild et al.8 Inclusion of the line shape
for B[3]-nf (Cqcc ) 2.15 MHz, η ) 0.20, δiso ) 18.0 ppm) in
addition to the main trigonal site B[3] (Cqcc ) 2.56 MHz, η )
0.14, δiso ) 10.1 ppm) resulted in a reasonable fit of the
experimental spectrum as shown in the bottom part of Figure
2a. The resulting parameters obtained with an iterative fitting
process are consistent with the previous report.8 Note that
besides the trigonal boron sites, two different lines for tetrahedral
boron sites, B[4] and B[4]*, were introduced to fit the peak at
around -4 ppm region. The position of B[4] sites is assigned
to -4 ppm (also see Figure 1a). The downfield peak (-3.5 ppm)
is marked with B[4]* temporarily, and its identity will be
discussed later.
Second, 11B MAS NMR spectra of samples B and D that
were used for Figure 1, parts b and d were acquired using a
830 MHz spectrometer (19.6 T) at the National High Magnetic
Field Laboratory (NHMFL) in Florida. However, when spectra
were obtained at the higher field (19.6 T), the validity of the
decomposition of the low field (11.7 T) spectrum which is
depicted in the bottom part of Figure 2a appeared to be
uncertain. As evidently shown in Figure 2a, the 11B MAS NMR
spectrum of the same sample at higher magnetic field (19.6 T)
unveiled the existence of more boron species which could not
be characterized by the spectrum at 11.7 T alone. The remarkable enhancement in resolution at the higher magnetic field is
mainly due to the fact that the second order quadrupole coupling
is inversely proportional to the strength of operating magnetic
field.27 Direct observation of such hidden resonances at 19.6 T
became a crucial factor in determining the number and type of
spectral line components.
Third, the differences between the two spectra obtained at
two different fields were investigated next. Because the quadrupole induced shift is dependent on the magnetic field strength,
the position of peaks is expected to be varied at the higher field.
The comparison here was presented by employing a numerical
simulation of spectra at 19.6 T. Note that the simulated spectrum
underneath of the spectrum at 19.6 T in Figure 2a is simply a
projected spectrum produced by using the fitting parameters
reported above. The experimental spectrum at 19.6 T appeared
to be blurred because 1H decoupling was not possible for the
measurement. The direct comparison of the experimental
spectrum with the simulated line shape allowed us to extract
the newly observed and unidentified peaks that are marked with
arrows 1 and 2 in Figure 2a. The projected line position of B[3]nf species at 19.6 T is marked by a vertical broken line. Figure
2b shows the exact same array of experimental and simulated
spectra that were collected from B-β sample D (after a vacuum
dehydration at 120 °C). As discussed in the previous section,
the part of line shape of B[3]-nf species (namely, the 16 ppm
peak) was found to disappear in the spectrum of sample D at
11.7 T (see Figures 1d and 2b). In the spectra at 19.6 T,
however, it is interesting to observe the presence of a peak
centered at 16 ppm (marked with the broken line and arrow 3
at the same position in Figure 2). The intensity of this peak of
sample D (Figure 2b) appeared to be markedly reduced from
that of sample B in Figure 2a. For the dehydrated sample D, it
was necessary to introduce an additional powder pattern that
could explain the broad shoulder at 11 ppm shown in the
spectrum at 11.7 T and the presence of a peak (arrow 1) in the
spectrum at 19.6 T. The bottom half of Figure 2b shows the
newly constructed line components after an iteration using
QUASAR that resulted in a good fit for the spectrum at 11.7
T. The projected spectrum onto the 19.6 T field appeared to
reproduce the experimental line shape very well. In particular,
the position of arrow 1 is now well anticipated due to the newly
introduced powder pattern. The NMR parameters for this new
Boron Sites in Borosilicate Zeolites
powder pattern estimated from the line fitting are Cqcc ) 2.46
MHz, η ) 0.29, δiso ) 14.6 ppm, strongly indicating boron in
a trigonal coordination should be responsible for the line
component. Here B[3]* is used to distinguish this newly found
site from the other trigonal boron sites B[3] or B[3]-nf. Note
that NMR parameters found for B[3] in Figure 2b were identical
with those of B[3] in Figure 2a within experimental error range.
When examining the spectra in Figure 2a, the peak marked
with arrow 1 in the spectrum at 19.6 T now indicates that the
B[3]* species is also present in the hydrated sample B. This
result strongly suggests that the interpretation of the spectrum
at 11.7 T (Figure 2a) should include contributions from the
powder patterns of both B[3]-nf and B[3]* groups as well as
that of B[3]. Koller recently also used this approach to explain
11B MAS spectral line shapes of a calcined B-β at different
dehydration temperatures.17 We found from the spectrum at 19.6
T in Figure 2a that the two humps at 16 and 12.5 ppm (not the
isotropic chemical shifts), temporarily assigned to be B[3]-nf
and B[3]*, respectively, show roughly the same contribution
in the signal intensity. However, a reasonable fitting of the
experimental spectrum at 11.7 T in Figure 2a could not be
obtained by taking into account equal or similar contributions
from both B[3]-nf and B[3]* components. As a result, we came
to the conclusion that the spectrum can be properly interpreted
by excluding the contribution of the particular powder pattern
of B[3]-nf and finding out a way to explain the line shape in
the region between 13 and 20 ppm: a Gaussian type peak with
its center at around 16 ppm. A new label on this peak is given
as P1 for the sake of distinction from the previous assignment.
At this moment, we can associate the identity of P1 with a boron
species that is formed in a relatively highly hydrated environment and is readily transformed to a trigonal boron site (most
likely B[3]) even under mild dehydration treatment. From the
Gaussian type line shape, it is proper to speculate that the
coordination geometry of this boron site is deformed from the
typical plane structure of trigonal borons and is closer to
tetrahedral coordination. In addition, the chemical shift of P1
(∼16 ppm) strongly indicates that the peak might be closely
related to those defective tetrahedral boron sites B[4]-II or B[4]III (see Scheme 2). Note that chemical shifts of these sites are
expected to be between the two extremes of -4 and 19.2 ppm
of the isotropic chemical shifts for B(OSi)4- and B(OH)4- type
structures, respectively.
To better characterize the signal around P1, resolution
enhancement was carried out by employing the 2D MQMAS
NMR method. Figure 3 shows 11B 2D MQMAS spectra at 11.7
T of samples B and D discussed in Figure 2. As marked with
arrows, the assignment of bands in the 2D contour plots can be
readily made by matching up components found in the analyses
of 1D MAS NMR spectra (see Figure 2). In the resolution
aspect, there is no additional information gained from the 2D
method. However, like other typical 2D MQMAS spectra, the
shape and position of a band reveal valuable information about
the quadrupole interaction of the responsible structure. For
example, trigonally coordinated boron sites B[3] and B[3]*
display bands dispersed widely along the anisotropic axis in
agreement with the quadrupole parameters reported above while
the tetrahedral boron sites B[4] and B[4]* show rather circular
bands with minor elongation along the chemical shift axis
because of nominally weak quadrupole interactions (Cqcc ∼0.2
MHz). Note that separation of two tetrahedral boron sites is
clearly revealed in the 2D MQMAS spectra, which is well
consistent with the spectral simulation shown in Figure 2. The
appearance of B[4]* signal is eminently associated with a mild
J. Phys. Chem. B, Vol. 108, No. 48, 2004 18539
Figure 3. 11B 2D MQMAS spectra of calcined B-β at 11.7 T: (a)
sample B, hydrated while stored for 1 year in closed vial (Figures 1b
and 2a); (b) sample D, vacuum-dried at 120 °C (Figures 1d and 2b).
Note that the projection onto the isotropic axis for Figure 3b is omitted
because of its similarity to that of Figure 3a. The spinning sideband is
marked with an asterisk.
hydration of calcined B-β, as pointed out in the previous
discussion on Figures 1b and 2a. The formation of B[4]* cannot
be unambiguously correlated with the appearance of the P1 or
the B[3]* signals. The slight downfield shift of its position from
the position of B[4] could provide a clue in speculating its
possible structure. De Ruiter et al.28 pointed out that tetrahedral
boron species such as B(OH)x(OSi)y (x + y ) 4) could be
responsible for downfield peak shifting. The isotropic chemical
shifts of those entities are unknown to our best knowledge. The
postulated species include the B[4] associated with H+(H2O)n
as the counterions or the B[4]-I structure which is formed by
replacing one of the B-O-Si bonds in B(OSi)3 with OH (see
Scheme 1). The B[4]* site is easily removed in sample C (see
Figure 1c) after room-temperature evacuation as learned from
the previous discussion on Figures 1 and 2. Whichever is the
identity of tetrahedral boron sites characterized by -3.5 ppm
in 11B NMR, its flexibility on the transformation between
trigonal and tetrahedral coordination is found to be remarkable.
For P1, its band shape is not unambiguously distinguished from
that of B[3]* in Figure 3a for the hydrated sample B. However,
the disappearance of P1 in the dehydrated sample D is evidently
demonstrated in Figure 3b. Comparison of these two MQMAS
spectra allows us to conclude that, unlike the other two trigonal
sites, the P1 site has a rather circular contour shape without
much dispersion along the anisotropic axis. It is, therefore, more
likely that P1 is not associated with part of the powder pattern
of a trigonal boron site but represents a tetrahedrally coordinated
boron species as we initially speculated (see above). As a matter
of fact, the identity of P1 was better characterized when the
zeolite was further hydrated (see below).
By now,our most of the boron species in the weakly hydrated
B-β (sample B) represented by 11B MAS spectrum in Figure
2a have been identified, except for the signal marked with arrow
2 in 11B MAS spectrum at 19.6 T (Figure 2). The peak at ∼19.0
ppm can be assigned as a B(OH)4- type species because the
isotropic chemical shift matches well with that of boric acid in
aqueous solution (19.2 ppm). It is quite rational to expect the
formation of B(OH)4- at the end of the hydration steps, provided
that the hydration level is sufficiently high. Its presence was
not observed in the spectrum at 11.7 T (Figure 2a), indicating
that the hydration level of sample B might have changed before
experiments at 19.6 T were performed. A similar mismatch has
already been observed by detection of a peak at 16 ppm (arrow
3) in the spectrum at 19.6 T of the dehydrated sample while
any indication of its presence does not exist in the spectrum at
18540 J. Phys. Chem. B, Vol. 108, No. 48, 2004
Hwang et al.
Figure 4. 11B MAS NMR spectra of calcined B-β zeolite at 11.7 T: (a) sample F, sample B (see Figure 1b) hydrated by exposing to air at room
temperature; (b) sample G, sample F further hydrated by soaking in water (55 mg of distilled water were added to 61 mg of sample F);
(c) sample H, sample G vacuum-dried at 120 °C; (d) sample I, sample H vacuum-dried at 450 °C. (e) 2D MQMAS spectrum of sample F.
11.7 T (see Figure 2b). We speculate that these two samples
packed in NMR rotors most likely have adsorbed some moisture
before the NMR measurement at 19.6 T at NHMFL in Florida,
which may explain the presence of the unexpected peaks present
in both spectra at 19.6 T (see Figure 2, parts a and b). 1H NMR
spectra of these two samples at 19.6 T are not available to show
the degree of hydration. As presented in the following section,
the formation of B(OH)4- has been observed at 11.7 T when
the calcined B-β samples were further hydrated. Although the
phenomena can be explained on the basis of the known behavior
of zeolite samples, it should still be pointed out that, because
of the aforementioned differences in the exact hydration level
of the samples, the direct comparison of two spectra obtained
at different magnetic fields might not be completely accurate.
3. Dehydration after Deboronation. When the calcined B-β
zeolite was exposed to ambient air, rapid adsorption of moisture
took place. A further hydration treatment by soaking the sample
in water resulted in significant dissolution of boron atoms from
the framework. B-β sample B presented in Figure 1b was
employed in the following hydration/dehydration course with
an aim of investigating the structural changes in boron sites
after being subjected to such severe hydration conditions. 11B
MAS NMR spectra collected in this course are presented in
Figure 4. After being exposed to ambient atmosphere overnight,
the water content of the resulting sample F rose to about 2.5
times higher than that of the parent sample B, as quantified
from 1H MAS NMR spectra (not shown). The resulting changes
on boron sites are clearly revealed in Figure 4a. First of all, the
11B MAS NMR powder patterns that are characteristic of
trigonal boron sites, B[3] and B[3]*, have disappeared. The
coordination conversion to B[4] sites appeared to be markedly
enhanced while a broad peak at 16 ppm showed a strong growth
at the same time. The identity of this 16 ppm peak was discussed
in the previous section and it was assigned as P1 (see Figure
3a). Note that right beside the 16 ppm peak there is a broad
line shape widely stretched up to 5 ppm on the 11B chemical
shift axis, covering the region where NMR line shape of B[3]*
used to reside. Acquisition of a 2D MQMAS spectrum allowed
us to unveil the spectroscopic line shape of P1 more clearly
(see Figure 4e), despite additional complexities. Unlike our
initial speculation (see discussion on Figure 3 above), P1 is not
characterized by a Gaussian type line shape but a powder pattern
showing sizable dispersion along the anisotropic axis. Figure
4e also shows the emergence of an additional resonance, which
position in the isotropic dimension (δiso) is at ∼21 ppm. The
projected spectrum onto the axis also indicates the presence of
additional component. Its appearance explains the formation of
boron sites in different structural form as a result of the advanced
hydration. Estimation of their quadrupole coupling parameters
could be readily made by applying the following equations.14,15
17
δiso ) δF2 + (δF1 - δF2) ,
27
x
SOQE ) Cqcc
1+
η2
) 8.246 × 10-3νoxδF1 - δiso
3
where δF1 and δF2 stand for the centers of gravity that can be
read from the 2D contour plot, SOQE is the second-order
quadrupole effect, and νo is the Larmor frequency. The isotropic
chemical shifts (δiso) were determined to be 16.7 and 19.2 ppm,
respectively, for these two sites. Both show similar SOQE values
of approximately 1.8 MHz. Consequently, the interpretation on
P1 needs to be reconsidered to include not only the defective
B[4]-II or B[4]-III but also trigonal sites. In any case, the
responsible structure should be associated with a severely
distorted form, i.e., with high asymmetry parameter (η). A closer
investigation on the identity is currently underway. It should
be noted that the signal intensities over the two regions (B[4]
vs P1+B(δiso∼21 ppm)) were measured to be 45:55 at this particular
hydration level. When sample F was further hydrated with the
water vapor over 1 M aqueous KCl solution in a closed vessel,
the water content of the resulting sample rose only about 20%
more than that of sample F, implying that sample F was already
well hydrated under the ambient conditions. The P1 resonance
shows narrowing in line width and downfield shift of its position
by 0.8 ppm, indicating that its structural change moves toward
the formation of the deboronated species, B(OH)4-.
After adding distilled water to sample F, the 11B MAS NMR
spectrum of the resulting hydrated sample G (Figure 4b) clearly
shows the formation of B(OH)4- (19 ppm) species, if not in
the form of B(OH)3 present in aqueous solution,29 implying
dissolution of boron species from the zeolite framework. In
addition to the complete disappearance of the P1 peak, the
reduction of the B[4] peak was also observed in Figure 4b.
About 20% of total boron atoms was measured to remain in
the framework as indicated by B[4] signal.
Boron Sites in Borosilicate Zeolites
Figure 5. 29Si MAS NMR spectra of calcined B-β zeolite: (a) sample
A, sample freshly calcined and then vacuum-dried at 500 °C (see Figure
1a); (b) sample F, sample B hydrated by exposing to air at room
temperature (see Figure 4a); (c) sample G, sample F further hydrated
by soaking in water (55 mg of distilled water were added to 61 mg of
sample F; see Figure 4b); (d) sample I, sample H further vacuumdried at 450 °C (Figure 4d).
Sample G then underwent the similar dehydration process
as described in Figure 1. Two subsequent spectra shown in
Figure 4, parts c and d, represent the structural transformation
of boron species that are present in the zeolite after such
deboronation/dehydration treatments. Without further spectral
analysis, the 11B MAS NMR spectra strongly suggest that boron
atoms were incorporated back into the framework, at least in
the form of B(OSi)3, i.e., B[3]. The formation of B[3]* species
was observed in significant amounts as shown in Figure 4c.
The powder pattern of B[3] after evacuation at 450 °C appeared
to be broadened and distorted compared to that of sample E in
Figure 1e, which could be attributed to the formation of B[3]*
species as an intermediate in large quantity. The 29Si MAS NMR
spectrum of each sample in Figure 5 reveals, in turn, the changes
in B-OSi coordination. The very freshly calcined material after
dehydration at 500 °C, sample A, shows the majority of its
silicon signal at -111 ppm with a very small portion at -102
ppm, representing Si-(OSi)4 (Q4) and the silanol group HOSi-(OSi)3 (Q3) associated with the B[3] units and/or isolated
silanol groups, respectively.28 The formation of SiOH groups
observed by 29Si MAS NMR can serve as a direct measure of
the hydrolysis of B-OSi bonds caused by hydration. Figure
5b shows a substantial growth of the peak at -102 ppm,
indicating that significant amount B-OSi bonds underwent the
J. Phys. Chem. B, Vol. 108, No. 48, 2004 18541
hydrolysis in sample F by being exposed to moisture from the
ambient atmosphere. For sample G, which was prepared by
further hydration via soaking in water, there is no significant
change found in 29Si MAS spectrum except slight decreasing
of the Q4/Q3 ratio when deboronation was accomplished by
soaking (see Figure 5c). The change of the ratio was attributed
to the additional dissolution of B[4] units in further hydrated
sample G. This result strongly suggests that the boron species
giving rise to the P1 resonance in 11B MAS NMR (Figure 4a)
should bear no B-OSi linkage. The identity of the P1 peak
can now be postulated to be extraframework boron species such
as B(OH)3, but with close association with the silanol nest and/
or (H2O)n in such a way that a mild dehydration can regenerate
B-OSi bonds. Figure 5d shows that, with sample I, the dehydration at 450 °C nearly completes the recondensation of
B-OSi bonds, as compared to the spectrum of Figure 5a for
sample A (the freshly calcined sample which was then vacuumdried at 500 °C), although minor modification seems to take
place in the framework silicon atoms. This is in good agreement with distorted line shape of B[3] sites in 11B MAS NMR
(Figure 4d).
4. Boron SSZ-33 and SSZ-42. The formation of B[3]*
species in the process of dehydration was not limited only to
B-β. Two other borosilicate zeolites, B-SSZ-33 and B-SSZ-42,
were examined after calcination, hydration and dehydration at
120 °C. Figure 6 shows experimental and simulated 11B MAS
NMR spectra of these two different materials at 11.7 T. All of
the NMR parameters acquired from numerical simulations of
spectra for various boron species in these samples are compiled
in Table 1. The numbers and types of the components involved
here look very similar to those observed in B-β (see Figure 2b)
except the contribution of B[4] species. Additional investigation
with 23Na MAS NMR (not reported here) confirmed that the
amount of sodium in borosilicate zeolites shows a good
correlation with the signal intensity of B[4] species. The results
support the previous findings that alkali metal ions protect B[4]
sites from the coordination transformation to B[3] species.8,9,28
The relative amount of B[3]* vs B[3] seems to be proportional
to the amount of B[4] species, but more cases should be
examined before generalizing the trend. The asymmetric
parameter η of B[3]* appears to be slightly larger for B-β than
that of other zeolites, although it is not proper to take the
difference seriously because of low accuracies in the determination due to the blurred line shape. Note that the P1 resonance
was also observed with a B-SSZ-33 sample (see Figure 6a).
Figure 6. Experimental 11B MAS NMR spectra at 11.7 T along with simulated line shapes: (a) B-SSZ-33 and (b) B-SSZ-42. Both samples were
dehydrated at 120 °C in a vacuum.
18542 J. Phys. Chem. B, Vol. 108, No. 48, 2004
Hwang et al.
TABLE 1: Boron Species Found during the
Hydration/Dehydration Treatment of Borosilicate Zeolites
zeolite
B-β
(BEA*)
chemical
shift
species (ppm)
B[3]
B[3]*
B[4]-I
B[4]
B[3]-nf
B-SSZ-33 B[3]
(CON)
B[3]*
B[4]
B-SSZ-42 B[3]
(IFR)
B[3]*
B[4]
a
10.0
14.6
-3.5
-3.8
18.0
16.7a
19.2a
10.1
15.3
-3.2
-3.6
10.7
14.8
-3.4
quadrupole
parameters
Cqcc and η
2.53 MHz, 0.15
2.46 MHz, 0.29
2.15 MHz, 0.20
1.8 MHz (SOQE)
1.8 MHz (SOQE)
2.54 MHz, 0.13
2.46 MHz, 0.2
2.6 MHz, 0.18
2.4 MHz, 0.2
rel
intens
vs B[3]
ref
1
0.26
0.02
0.04
0.28
8, 17, 28
17
1
0.3
0.1
0.03
1
0.4
0.3
2
2
8, 17
Denoted as P1, the identity is yet unknown (see text).
The P1 peak disappeared when the material was evacuated at
higher temperatures. When both zeolites were dehydrated at
higher temperatures, no significant differences in either 1H or
11B MAS NMR spectra were observed up to the dehydration
temperature of 500 °C in contrast to those of B-β sample (see
Figure 1). It was generally observed for all three zeolites that
the 11B NMR line shape of B[3]* species got smeared out to
become a part of the dominating B[3] peak when the evacuation
process took place over 350 °C. The results clearly imply the
conversion of B[3]* to B[3]. However, the overall line shape
of 11B MAS NMR spectra for samples evacuated above 350
°C appeared to be distorted and shows a broad tail on the left
side of the powder pattern of B[3]. The change is well illustrated
between parts d and e of Figure 1 and more evidently in Figure
4d. The line shape can be better explained with inclusion of
another powder pattern which severely deviates from the planar
structure (η > 0.6) and occupies about 10% of the main B[3]
sites in the case of B-β sample E (Figure 1e). The results indicate
that roughly 30% of B[3]* species could remain unconverted
but significantly altered from its original geometry as a result
of the high-temperature treatment. No noticeable change in the
B[4] peaks at about -4 ppm was detected after the hightemperature dehydration.
5. 1H NMR and 11B CPMAS NMR: Characterization of
B[3]*. A 1H MAS NMR spectrum of B-β zeolite was obtained
at each step of the dehydration process and Figure 1 has already
illustrated the extent of dryness of the samples as a function of
the dehydration temperature. Besides the eminent spectral
change showing the disappearance of surface adsorbed water,
1H MAS NMR spectra showed various changes of surface
hydroxyl groups that could be correlated with the changes
observed in 11B NMR spectra (see Figure 1). A closer examination of 1H NMR spectra is performed in this section.
The spectrum of B-β sample D in Figure 1d is redisplayed
in Figure 7a after deconvolution into five different components: 1.7, 2.0, 2.2, 3.0, and 5.0 ppm. The broad peak at 5.0
ppm was shifted to upfield (4.6 ppm) and disappeared completely at evacuation temperatures greater than 350 °C while
the majority of upfield peaks showed no significant decrease
in intensity or variation of chemical shift as the dehydration
temperature was raised. Figure 7b shows a plot of intensity vs
dehydration temperature for the decomposed lines. Note that
the 3.0 ppm peak showed a minor upper field shift of about 0.2
ppm at higher temperatures and a gradual 66% loss of intensity
by 450 °C. A 1H MAS NMR spectrum of a relatively well
dehydrated β zeolite sample has been known to give rise to a
number of different lines due to hydroxyl groups in different
environments.30 Regardless of the type of heteroatoms (Al or
B), a 1H NMR spectrum would show the presence of mainly
two classes of hydroxyl groups: bridging hydroxyls that are
responsible for the Brønsted acid sites and isolated SiOH groups.
It is also known that the weak acid sites of borosilicate zeolites
is closely related to low values of isotropic chemical shifts (2-3
ppm) of the possible bridging hydroxyl groups in the NMR
spectra.8,31 Unlike aluminosilicate zeolites, the proton form of
borosilicate zeolites undergoes B[4] to B[3] transformation upon
dehydration, which most likely precludes the existence of the
bridging hydroxyl groups (B-O(H+)-Si) around B[3] units (see
Scheme 1). In addition, B[4] units that are still present even
after dehydration should be most likely charge compensated by
Na ions, not H+. The decomposed lines in Figure 7 can be
assigned thus as follows: strongly adsorbed surface water (5.0
ppm), different types of hydroxyl groups located around the
trigonal boron sites B[3] and B[3]* (3.0, 2.2, and 2.0 ppm) and
the isolated SiOH groups (1.7 ppm). From a similarly treated
B-β zeolite, Fild et al.8 also reported three 1H NMR lines at
2.0, 2.3, and 3.0 ppm that were proved by REDOR NMR
experiments to be closely located to the framework trigonal
boron sites, i.e., B[3]- - -O(H)Si groups. In terms of position
and behavior of individual lines upon dehydration, very similar
experimental results were also obtained from 1H NMR measurements of B-SSZ-33 and B-SSZ-42 in this study. A noticeable
difference in the relative intensities of individual peaks was
observed among B-β, B-SSZ-33, and B-SSZ-42. For example,
upon dehydration at 250 °C, at which most of the strongly bound
surface water is removed, B-SSZ-42 showed a remarkably small
amount (∼30%) of a hydroxyl peak at 2.2 ppm compared to
that found in B-SSZ-33 or B-β samples. However, such
variations in the distribution of individual hydroxyl groups could
not be correlated with differences observed from 11B MAS NMR
experiments (e.g., B-SSZ-42 contains the highest amount of B[4]
units.).
Unlike aluminosilicate zeolites, not much effort has been
made to characterize 1H NMR signals of borosilicate zeolites.
An obvious hurdle is related to the fact that the chemical shifts
of the three lines corresponding to the hydroxyl groups around
B[3] sites are not well distinguished from other types of
hydroxyl groups present in zeolites. It is worthwhile to note
that the 1H NMR spectrum of deboronated and dehydrated
B-SSZ-33 exhibits deconvoluted line components very similar
to those observed in parent B-SSZ-33 (see Figure 1S in the
Supporting Information for data from deboronated and dehydrated B-SSZ-33). It is quite striking to observe lines at chemical
shifts of 4.5, 3.0, 2.3, and 2.0 ppm that are exactly identical to
those observed before deboronation, although the contributions
of 4.5 and 3.0 ppm peaks are found to be particularly greater.
These results support the finding that the presence of B[3] right
next to SiOH groups makes almost invisible contributions in
deterring the chemical shifts of the hydroxyl groups. Consequently, the 1H NMR signal of the B[3]- - -HOSi group can be
only investigated through boron atoms, and 1H-11B double
resonance NMR offers an excellent tool. Much of the detailed
results of 1H NMR studies will be published elsewhere. In the
present work, we focus on better characterization of the B[3]*
species using 11B CPMAS experiments and 2D HETCOR NMR.
The cross-polarization method was used in both one- and twodimensional fashions to show which hydroxyl sites are closely
located around the two known trigonal boron species, B[3] and
B[3]*. Since all three zeolites gave similar results, only the
experimental spectra from B-SSZ-33 are presented in Figure
Boron Sites in Borosilicate Zeolites
J. Phys. Chem. B, Vol. 108, No. 48, 2004 18543
Figure 7. (a) 1H MAS NMR spectrum of B-β sample D (dehydrated at 120 °C, see Figure 1d) and its deconvoluted components. (b) Dependence
of peak intensities on the dehydration temperature.
Figure 8. (a) 11B CPMAS NMR spectra of B-SSZ-33 at different cross-polarization contact times after the sample was dehydrated at 120 °C.
(b) 2D HETCOR spectrum of B-SSZ-42 after dehydration (0.5 ms contact time).
8a. The cross polarizing efficiency was found to be higher for
B[3]* as the corresponding powder pattern appears even at 0.2
ms of cross-polarization contact time. The growth of powder
pattern for B[3] species becomes evident after increasing contact
times to greater than 0.5 ms, and the spectrum at 5.0 ms
resembles closely to the MAS spectrum shown in Figure 6a.
Note that B[4] sites are also cross polarized from protons but
with much lower efficiency, supporting the hypothesis that the
BO4- unit could have Na+ as the countercation. The individual
correlations can be mapped out efficiently by means of the 2D
HETCOR method. Figure 8b shows a 2D HETCOR spectrum
of dehydrated B-SSZ-42 zeolite. The spectrum was obtained
using 0.5 ms contact time and a sample spinning rate of 10
kHz. Under restriction of the contact time used in this experiment (0.5 ms), a few observations are summarized below:
(1) Surface bound water (∼4.2 ppm) and one type of hydroxyl
group (3.0 ppm) show evident correlation with the B[3]* band,
indicating that they are closely located to B[3]* units.
(2) The hydroxyl group at 3.0 ppm is also responsible for
cross-polarization of B[3] units.
(3) B[3] units reveal a rather weak correlation with the 2.2
ppm peak, meaning that the corresponding hydroxyl group is
weakly tied to the B[3] sites.
(4) No correlation from the 2.0 and 1.7 ppm peaks was
observed.
From all these results, it can be concluded that the hydroxyl
groups responsible for the 3.0 ppm resonance in 1H NMR are
most closely associated with the SiOH- - -B[3] sites while
geometrically the distance from the hydroxyl proton to B[3]*
seems to be shorter than the distance to B[3]. However, the 2D
correlation contour maps exhibited significant variations at
different contact times and different dehydration temperatures.
When a 2D spectrum was acquired with a longer contact time
(5 ms) for a borosilicate zeolite that was dehydrated at 450 °C,
the 1H lines projected on the 1H spectrum axis (figure not shown
here) appeared to be very close to those in the 1D MAS
spectrum (Figure 1e). This is due to the nature of the crosspolarization process which makes the loosely correlated spins
actively contact at longer contact periods. In this case of longer
contact time, B[3] species were dominantly correlated with all
the individual 1H lines. The contribution of the 1.7 ppm peak
to the contour map has not yet been detected unambiguously.
On the other hand, when examining the mildly hydrated B-β
sample C (see Figure 1c for the corresponding 1H NMR
spectrum), only the B[3]* powder pattern was correlated with
a broad 1H line at 4-6 ppm region. A series of 2D HETCOR
spectra were then obtained at each different stage of the
dehydration process in order to closely investigate the hydroxyl
groups around the triognal boron sites. B-β zeolite was used
for this study (see Figure 9 for 2D spectra). The collection of
projected lines onto the 1H axis is presented in Figure 10. The
results are discuused below.
i. Only B[3]* sites show correlation with the ∼4 ppm 1H
resonance that is thought to represent surface bound water (4-6
18544 J. Phys. Chem. B, Vol. 108, No. 48, 2004
Figure 9.
11
Hwang et al.
B 2D HETCOR spectra of calcined B-β evacuated at (a) room temperature and (b) 120, (c) 250, (d) 350, and (e) 450 °C.
Figure 10. 1H MAS NMR spectra of B-β observed via 2D 1H-11B HETCOR spectroscopy at different stages of dehydration (also see Figure 9).
Evacuation temperatures were (a) room temperature and (b) 120, (c) 250, (d) 350, and (e) 450 °C. (f) Deconvolution of part b.
ppm) (Figures 9a and 10a). It is a fairly striking observation
considering that hydrogen bonded water groups (H2O)n surround
any hydroxyl groups and trigonal boron sites. If the mobility
of water groups or a proton exchange process prohibits the
efficient cross-polarization to trigonal borons, there is no reason
for B[3]* sites to be selectively cross polarized. Therefore, it is
Boron Sites in Borosilicate Zeolites
probable that the broad 1H lines around 4-6 ppm might not
only represent the surface bound water groups but also B-OH
groups both in the trigonal and tetrahedral defective sites as
shown in Scheme 2. In fact, the 1H spectrum in Figure 10a can
be decomposed to two lines at 5.6 and 4.1 ppm with 2:3 ratio
in intensity. The 5.6 ppm peak was removed when dehydration
at 120 °C was performed while the presence of the 4.1 ppm
peak was observed even after evacuation at 250 °C (see below).
ii. Upon removal of most of the surface bound water (Figures
9b and 10b), two proton lines at 3.3 and 2.4 ppm show selective
correlations to mainly B[3]* and B[3] sites, respectively.
Correlation between 3.3 ppm and B[3] sites seems to be present
to some extent but the strength appears to be less when
compared to the case of B-SSZ-42 (Figure 8b). The 1H projection spectrum in Figure 10b can be decomposed to four lines
at 3.8, 3.3, 2.4, and 2.0 ppm as shown in Figure 10f. The
components are consistent with our previous fit of 1H spectra
(see Figure 7a) except for the 1.7 ppm peak, which was assigned
to the isolated SiOH groups. The result indicates that most of
hydroxyl groups are closely associated with trigonal boron sites.
Note that there is a difference of about 0.2-0.3 ppm in line
positions measured with these two different methods, i.e., the
direct 1H MAS NMR and the projected 1H spectrum of 2D
HETCOR NMR. Despites of slight upfield shift caused by
removal of surface bound water, the 3.8 ppm peak is believed
to originate from the same hydroxyl proton that was observed
at 4.1 ppm peak in Figure 10a. The line width of the 3.8 ppm
was measured to be ∼ 2 ppm wide. Its contribution appears to
be as high as 50% of the 1H spectrum. It is observed in the 2D
spectrum (Figure 9b) that the 3.8 ppm shows strong correlation
only with the B[3]* sites. The peak was no longer present in
spectra for dehydration temperatures greater than 350 °C (Figure
10, parts d and e). This observation is in good agreement with
Koller’s report on an additional proton site at 4.3 ppm observed
from a calcined and dehydrated B-β using 1H{11B} REDOR
NMR experiments.17 According to Koller, the 4.3 ppm resonance appears to show the strongest correlation with trigonal
boron sites. Using the REDOR method alone is, however, not
possible to resolve which of the two trigonal sites (B[3] and
B[3]*) is correlated. The identity of the 3.8 ppm peak is yet to
be explored. Again, the 2D HETCOR method proves to be
remarkably instrumental because it provides detailed insights
into the structures that are directly related to the 11B nuclei,
which are especially in the trigonal geometry.
iii. When the surface bound water is completely removed from
the zeolite (Figure 10, parts d and e), the positions of hydroxyl
groups in the projected spectra can be more clearly observed.
The 3.8 ppm peak is no longer present. At 450 °C, measurable
reduction of the 3.3 ppm peak is also detected, and at this point
the selective correlation pairs, OH(3.3 ppm)- - -B[3]* and OH(2.4
ppm)- - -B[3], are more clearly represented in the 2D contour plot
(see Figure 9e). The gradual depletion of the 3.3 ppm peak now
can be directly correlated to the reduction in intensity of the
B[3]* group after evacuation at 450 °C. Considering the fact
that the B[3]* species appeared to be an intermediate formed
in the process of hydration/dehydration of borosilicate zeolites,
the hydroxyl group OH(3.3 ppm) is found to be a critical
component of its formation and deformation. All of the
spectroscopic evidence presented in this study strongly suggests
that the identity of B[3]* species should be attributed to
trigonally coordinated boron atoms with one or two of B-OSi
bonds being replaced by B-OH(3.3 ppm). Having directly coordinated hydroxyl groups, it is rational to expect that B[3]*
species have cross-polarization efficiency superior to B[3] sites
J. Phys. Chem. B, Vol. 108, No. 48, 2004 18545
SCHEME 3
even under hydrated circumstances, which agrees well with our
experimental observations. Using 1H-27Al CPMAS NMR
method, Roberge et al. have drawn a similar conclusion and
proposed the presence of tetrahedral Al site bearing a direct
Al-OH bond in their study of Al-β zeolite.32 Conversion of
B[3]* to B[3] can now be understood as a consequence of
dehydroxylation when dehydration takes place at higher temperatures (over 400 °C). Unresolved issues regarding the
incomplete conversion of B[3]* to B[3] and severe alteration
of B[3]* geometry can be explained as follows. For example,
when two surrounding hydroxyl groups, Si-OH, are involved
in forming Si-O-Si bond by ejecting a H2O molecule (Scheme
3), the B[3]* species would be isolated and could be stable at
high-temperature such as 450 °C. In this case, a significant
structural distortion around B[3]* sites might occur, resulting
in a considerable change of the asymmetric parameter of B[3]*
species. Note that the structure of B[3]* is depicted with one
B-OH bond in Scheme 3 without determining how many OH
groups were coordinated to B[3]* sites. However, if B[3]* bears
two hydroxyl groups (B(OSi)(OH)2, see B[3]-II in Scheme 2),
it is unlikely to have such structural distortion as the dehydroxylation proceeds. The B[3]* site can now be assigned to
B[3]-I, a defective trigonal boron site (Scheme 2). At this
moment, we have not obtained a firm experimental evidence
that could conclusively determine whether B[3]* can be assigned
to either B[3]-I or B[3]-II. Note that the assignment of B[3]*
to B[3]-II was recently proposed by Koller.17 The formation of
B[3]-I was not detected by Koller. It is also probable that the
3.8 ppm peak is responsible for hydroxyl groups in the boron
geminal structure, B(OSi)(OH)2, and B[3]-I and B[3]-II are
positioned in 11B NMR too close with each other to be
distinguished in our measurements reported here. It is postulated
that B[3]-II species be completely converted to either B[3]-I or
B[3] species during dehydration below 350 °C. Further investigation is underway to make conclusive identification.
iv. The association of the 2.0 ppm peak with any of trigonal
boron sites was not conclusively observed in our current setup
(0.5 ms contact time) although the OH(2.0 ppm) shows a correlation
band with B[3] sites at a longer contact times (not shown here).
This observation is in disagreement with previous reports8,17
which showed that 2.0 ppm peak is associated with the trigonal
boron sites to a similar extent as the 2.4 ppm peak. As
demonstrated in their 1H{11B} REDOR experiments using
dehydrated B-β, however, the distances from the framework
trigonal boron sites to different hydroxyl sites (3.0, 2.3, and
2.0 ppm) appear to be reasonably distinct, with OH(3.0 ppm) being
the shortest while OH(2.0 ppm) is the longest. Discrimination
between OH(2.4 ppm) and OH(2.0 ppm) could be investigated quantitatively by means of REDOR NMR. The presence of two
different types of bridging hydroxyl groups in aluminosilicate
zeolites have been known and exploited in versed methods.30,32-36
It is interesting to learn whether the two hydroxyl groups that
are associated with B[3] sites in borosilicate zeolites would show
acidic character similar to that of aluminosilicate zeolites.
18546 J. Phys. Chem. B, Vol. 108, No. 48, 2004
IV. Conclusions
The formation and the evolution of a number of trigonal and
tetrahedral boron sites during the hydration/dehydration process
were investigated in detail in this work by employing multinuclear and multidimensional solid-state NMR methods. NMR
evidence supports the presence of a B(OSi-)2(OH) species,
B[3]-I, a defect site that showed stability up to 450 °C. Distorted
B(OH)3 type species were detected while the formation of
B(OSi-)(OH)2 species was not unambiguously concluded.
Boron shows a great degree of flexibility in coordination
conversion between trigonal boron, B[3], and tetrahedral boron,
B[4], in the framework of calcined borosilicate zeolites upon
change of the hydration level. Boron can be removed from the
framework of the proton form of dehydrated borosilicate zeolites
even under mild hydration treatment. The deboronation mechanism is related to successive substitutions of B-O-Si bonds
with B-OH. Even after a great extent of deboronation via
hydration, reoccupation of boron into the framework position
can be achieved by dehydration treatment. 1H-11B crosspolarization and two-dimensional heteronuclear correlation
spectroscopies provide powerful tools to selectively identify the
proton resonances corresponding to hydroxyl groups in B[3]-I
(3.8 and 3.3 ppm) and silanol groups near to B[3] sites
(2.4 ppm).
Acknowledgment. We thank ChevronTexaco Energy Technology Co. for supporting this work. Acquisition of NMR
spectra at 19.6 T was made possible by help from Drs. Hyungtae
Kwak and Z. Gan at the NHMFL in Florida. Their contribution
is greatly acknowledged. Helpful discussion from Prof. Mark
E. Davis at California Institute of Technology is also acknowledged. The NMR facility at Caltech was supported by the
National Science Foundation under Grant 9724240 and partially
supported by the MRSEC Program of the National Science
Foundation under Award DMR-0080065.
Supporting Information Available: Figure 1S, showing 1H
MAS NMR spectra of SSZ-33 after deboronation and dehydration at 250 °C. This material is available free of charge via the
Internet at http://pubs.acs.org.
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