Specificity of sites within eight-membered ring zeolite channels for carbonylation of methyls to acetyls

Article abstract of DOI:10.1021/ja070094d

The acid-catalyzed formation of carbon-carbon bonds from C₁ precursors via CO insertion into chemisorbed methyl groups occurs selectively within eight-membered ring (8-MR) zeolite channels. This elementary step controls catalytic carbonylation rates of dimethyl ether (DME) to methyl acetate. The number of O-H groups within 8-MR channels was measured by rigorous deconvolution of the infrared bands for O-H groups in cation-exchanged and acid forms of mordenite (M,H-MOR) and ferrierite (H-FER) after adsorption of basic probe molecules of varying size. DME carbonylation rates are proportional to the number of O-H groups within 8-MR channels. Na⁺ cations selectively replaced protons within 8-MR channels and led to a disproportionate decrease in carbonylation turnover rates (per total H⁺). These conclusions are consistent with the low or undetectable rates of carbonylation on zeolites without 8-MR channels (H-BEA, H-FAU, H-MFI). Such specificity of methyl reactivity upon confinement within small channels appears to be unprecedented in catalysis by microporous solids, which typically select reactions by size exclusion of bulkier transition states.

Full text of DOI:10.1021/ja070094d

Published on Web 03/31/2007
Specificity of Sites within Eight-Membered Ring Zeolite
Channels for Carbonylation of Methyls to Acetyls
†
†
‡
‡
Aditya Bhan, Ayman D. Allian, Glenn J. Sunley, David J. Law, and
,
†
Enrique Iglesia*
Contribution from the Department of Chemical Engineering, UniVersity of California at
‡
Berkeley, Berkeley, California 94720, and BP Chemicals Limited, Hull Research and
Technology Center, Saltend, Hull HU 12 8DS, U.K.
Received January 10, 2007; E-mail: iglesia@berkeley.edu
1
Abstract: The acid-catalyzed formation of carbon-carbon bonds from C precursors via CO insertion into
chemisorbed methyl groups occurs selectively within eight-membered ring (8-MR) zeolite channels. This
elementary step controls catalytic carbonylation rates of dimethyl ether (DME) to methyl acetate. The number
of O-H groups within 8-MR channels was measured by rigorous deconvolution of the infrared bands for
O-H groups in cation-exchanged and acid forms of mordenite (M,H-MOR) and ferrierite (H-FER) after
adsorption of basic probe molecules of varying size. DME carbonylation rates are proportional to the number
+
of O-H groups within 8-MR channels. Na cations selectively replaced protons within 8-MR channels and
+
led to a disproportionate decrease in carbonylation turnover rates (per total H ). These conclusions are
consistent with the low or undetectable rates of carbonylation on zeolites without 8-MR channels (H-BEA,
H-FAU, H-MFI). Such specificity of methyl reactivity upon confinement within small channels appears to
be unprecedented in catalysis by microporous solids, which typically select reactions by size exclusion of
bulkier transition states.
Introduction
groups (or oxonium-ion derivatives) react slowly with CO via
insertion into the C-O bonds in bound methyls. The acetyls
formed then react with DME in methoxylation reactions that
Carbonylation provides a convenient route to functionalize
C1 intermediates via formation of carbon-carbon bonds at low
temperatures.1 Solid acid catalysts catalyze carbonylation of
alkanols and ethers via Koch-type reactions, in which surface
alkyl groups have been proposed to react with CO to form
regenerate methyl intermediates. Carbonylation rates (per total
-3
+
H ) were strongly influenced by the identity of the zeolite and
the density of OH groups and were strongly inhibited by the
presence of water. The basis for such marked effects of structure,
site density, and water remained unclear at the time of our earlier
reports.
acylium ions, which form carboxylic acids upon subsequent
hydrolysis.2
,4-6
Carbonylation of methanol and DME on zeolites
and polyoxometalate clusters occurs concurrently with extensive
deactivation and hydrocarbon formation. We have recently
reported the selective carbonylation of dimethyl ether (DME)
to methyl acetate, a precursor to acetic acid, at low temperatures
Here, we present evidence for the unique reactivity of CH3
groups located within eight-membered ring (MR) channels in
mordenite (MOR) and ferrierite (FER) in carbonylation reac-
+
tions. H groups (and the CH3 groups formed via reactions of
(
400-460 K) on acidic zeolites, without detectable homologa-
+
7,8
DME with H ) within 10-MR and 12-MR channels in H-MFI,
tion reactions or catalyst deactivation. Kinetic and spectro-
scopic probes showed that carbonylation involves the initial
formation of methyl groups via DME reactions with acidic OH
groups and subsequent propagation reactions, in which methyl
H-BEA, and H-FAU gave undetectable carbonylation rates.7
,8
We show here that these marked effects of zeolite structure and
+
+
H
density reflect the initial presence of H and the ultimate
presence of CH3 groups during DME-CO reactions, within
8-MR channels, and their higher reactivity compared with those
within less constrained 10-MR, 12-MR, or mesoscopic channels
for CO insertion reactions. These findings provide evidence of
unprecedented clarity for transition state specificity and for the
remarkable effects of spatial constraints in catalytic reactions
within microporous solids.
†
Department of Chemical Engineering, University of California at
Berkeley.
‡
BP Chemicals Limited.
(
(
1) Bagno, A.; Bukala, J.; Olah, G. A. J. Org. Chem. 1990, 55, (14), 4284.
2) Stepanov, A. G.; Luzgin, M. V.; Romannikov, V. N.; Zamaraev, K. I. J.
Am. Chem. Soc. 1995, 117 (12), 3615.
(
(
(
3) Sunley, G. J.; Watson, D. J. Catal. Today 2000, 58 (4), 293.
4) Wegman, R. W. J. Chem. Soc., Chem. Comm. 1994, (8), 947.
5) Xu, Q.; Inoue, S.; Tsumori, N.; Mori, H.; Kameda, M.; Tanaka, M.;
Fujiwara, M.; Souma, Y. J. Mol. Catal. A: Chem. 2001, 170 (1-2), 147.
6) Stepanov, A. G.; Luzgin, M. V.; Romannikov, V. N.; Sidelnikov, V. N.;
Zamaraev, K. I. J. Catal. 1996, 164 (2), 411.
(
(
(
Experimental Methods
7) Cheung, P.; Bhan, A.; Sunley, G. J.; Iglesia, E. Angew. Chem., Int. Ed.
4 4
Catalyst Synthesis. NH -MOR (Si/Al ) 10, Zeolyst) and NH -FER
2
006, 45 (10), 1617.
(
Si/Al ) 33.5, Zeolyst) were treated in flowing dry air (zero grade,
8) Cheung, P.; Bhan, A.; Sunley, G. J.; Law, D. J.; Iglesia, E. J. Catal. 2007,
45 (1), 110.
-
1
2
Praxair) at 773 K (0.0167 K s ) for 3 h to remove organic species
10.1021/ja070094d CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 4919-4924 4919
9

A R T I C L E S
Bhan et al.
adsorbed upon contact with ambient air and to convert exchanged
Results and Discussion
+
+
NH
4
cations to H . NH
4
-MOR (Si/Al ) 10, Zeolyst, ∼14 g) was
Distribution of H+ Sites in MOR and FER. H-MOR
consists of 12-MR main channels with intersecting 8-MR
channels (side pockets). Al substitution into the neutral silicate
framework creates a charge imbalance compensated by cations
(e.g., O-H group that act as Brønsted acids). The shift in
frequency for the antisymmetric O-H stretch in H-MOR during
adsorption of probe molecules of varying size has been used to
+
partially exchanged with Na using 0.5 L of aqueous NaNO
3
solu-
tions (99%, EMD Chemicals, 0.014-2.44 M) at 353 K for 12 h and
then washed in 2 L of deionized water and isolated by filtration. The
resulting solids were treated at 393 K overnight in ambient air and
-
1
then for 3 h at 773 K (0.0167 K s ) in flowing dry air (zero grade,
Praxair). NH
-MOR (Si/Al ) 10, Zeolyst, ∼ 2.5 g) was exchanged
with Co cations using 0.1 L of aqueous Co(NO ‚6H O (98+%,
4
2
+
3
)
2
2
Sigma-Aldrich, CAS 10226-22-9, 0.1 M) solutions at 353 K for 24 h
and then washed in 0.5 L of deionized water and isolated by filtration.
The resulting solids were dried at ambient temperatures overnight and
then treated in flowing dry air (zero grade, Praxair) for 3 h at 773 K
-1
detect two types of O-H species, with bands at ∼3610 cm
+
-1
+
for H in 12-MR channels and at ∼3590 cm for H sites in
9
-11
8
-MR side pockets.
We have used singular value decom-
position (SVD) and spectral analysis methods reported by
-
1
(
0.0167 K s ). Na and Co contents were measured by inductively
1
2-14
Garland et al.
to determine band positions and relative
coupled plasma optical emission spectroscopy (ICP-OES) (Galbraith
Laboratories); the fraction of ion exchange for these samples is reported
in Figure 2.
concentrations of O-H groups in 8-MR and 12-MR channels
of MOR (sections 1.1-1.3; Supporting Information). The
+
fraction of H sites within 8-MR and 12-MR channels in
Catalytic Carbonylation of Dimethyl Ether. Steady-state DME
carbonylation rates and selectivities were measured in a packed-bed
reactor (8.1 mm i.d.; 9.5 mm o.d.) equipped with a thermocouple held
within a concentric thermowell (1.6 mm) aligned along the tube center.
Catalyst samples (0.5 g, 125-250 µm aggregates) were treated in
H-MOR was determined to be 0.55 and 0.45 ((0.05), respec-
tively, for this sample.
Eder and Lercher1
5-17
have shown that alkane adsorption is
localized on Brønsted acid sites within zeolite channels via
hydrogen bonding between alkanes and Si-OH-Al groups at
temperatures below 373 K. n-Hexane adsorption on H-MOR
led to an asymmetric O-H band with maxima centered at 3590
-
1
flowing dry air for 3 h at 773 K (0.0167 K s ) and cooled to reaction
3
-1
temperature (438 K) in flowing He (1.67 cm s ) before reaction with
3
-1
premixed reactant mixtures (2% DME/93% CO/5% Ar; 1.67 cm s );
reactants were dried before use by flowing through a CaH bed (0.5 g,
9.99%, Aldrich) held at ambient temperature. Heat-traced lines (423-
73 K) were used to transfer the reactor effluent into a mass
2
-
1
and 3610 cm ; the intensity of this band decreased to only
9
4
0
.67 of its initial value as the n-C6H14 pressure increased to 10
mbar and the band shifted to lower frequencies, indicating that
n-hexane perturbs only O-H groups within 12-MR channels.
The inability of n-hexane to interact with H sites in 8-MR
channels of FER was also reported by van Well et al.1 Figure
1 shows the O-H region infrared spectra for H-MOR (Si/Al )
10, Zeolyst) after exposure to n-hexane at 303 K. A broad band
typical of hydrogen-bonded (perturbed) OH groups appeared
at lower wavenumbers upon hexane dosing. The pronounced
asymmetry of the perturbed hydroxyl groups confirmed that
n-hexane molecules did not interact with OH groups within
spectrometer (MKS Spectra Minilab; 1-90 amu) and a gas chromato-
graph (Agilent 6890) equipped with a methyl siloxane capillary column
+
(
HP-1, 50 m × 0.32 mm × 1.05 µm) connected to a flame ionization
8,19
1
detector and a Porapak Q packed column (80-100 mesh, 12 ft × /
8
in) connected to a thermal conductivity detector. Pyridine (Sigma-
Aldrich, 99.9%, CAS 110-88-1) was introduced into DME/CO/Ar
3
-1
reactant streams (2/93/5; 1.5 cm s ) using a saturator maintained at
2
3
-1
73 K using a He flow (0.17 cm s ) to titrate Brønsted acid sites and
to determine their concentration and their role in DME carbonylation
reactions.
Infrared Spectroscopic Studies of Proton-Exchanged and Cation-
Exchanged Zeolites. Infrared spectra were measured in the 4000-
8-MR channels. Band deconvolution analysis of infrared spectra
measured at increasing n-hexane pressures (shown in the inset
of Figure 1) showed that 55-60% of the H sites are located
in the 8-MR channels and that n-hexane perturbs only H sites
within 12-MR channels.
-
1
4
00 cm region on self-supporting wafers (∼20-40 mg) held within
+
a quartz vacuum cell with NaCl windows. Spectra were acquired using
a Nicolet NEXUS 670 infrared spectrometer equipped with a Hg-
Cd-Te (MCT) detector. Each spectrum was obtained by averaging 64
+
-
1
The selective titration of OH groups within 12-MR channels
scans collected at 2 cm resolution. Samples were treated in flowing
1
0
20
3
-1
of MOR by pyridine and alkylpyridines has also been
reported. We have also dosed pyridine and 2,6-dimethylpyridine
dry air (∼1.67 cm s , zero grade, Praxair) at 723 K for 2 h, evacuated
at 723 K for 2 h using a diffusion pump (<0.01 Pa dynamic vacuum;
Edwards E02), and cooled to ambient temperatures (∼303 K) in a
vacuum before measuring spectra.
(9) Datka, J.; Gil, B.; Kubacka, A. Zeolites 1997, 18 (4), 245.
(
10) Maache, M.; Janin, A.; Lavalley, J. C.; Benazzi, E. Zeolites 1995, 15 (6),
Infrared Spectroscopic Studies of Propane, n-Hexane, Pyridine,
and 2,6-Dimethyl Pyridine Probe Molecules. n-Hexane (Sigma-
Aldrich, g 99%), pyridine (Sigma-Aldrich, g99.9%), and 2,6-dimethyl
pyridine (Sigma-Aldrich, 99+%) were dosed onto H-MOR (Si/Al )
507.
(11) Makarova, M. A.; Wilson, A. E.; van Liemt, B. J.; Mesters, C.; de Winter,
A. W.; Williams, C. J. Catal. 1997, 172 (1), 170.
(
12) Gao, F.; Allian, A. D.; Zhang, H. J.; Cheng, S. Y.; Garland, M. J. Catal.
2006, 241 (1), 189.
(
13) Allian, A. D.; Tjahjono, M.; Garland, M. Organometallics 2006, 25 (9),
1
0, Zeolyst) at ambient temperatures (∼303 K) to measure the number
2182.
+
of H in 8-MR and 12-MR channels. n-Hexane, pyridine, and 2,6-
dimethyl pyridine were purified by multiple freeze-thaw cycles before
dosing from the vapor phase. Sample treatment protocols were identical
to those described above. The pressure of the probe molecules was
increased stepwise by dosing from a control volume; infrared spectra
were collected 180s after each dose without intervening evacuation.
Pressures were measured with a MKS BARATRON pressure transducer
(14) Chen, L.; Garland, M. Appl. Spectrosc. 2003, 57 (3), 331.
(
15) Eder, F.; Stockenhuber, M.; Lercher, J. A. Sorption of light alkanes on
H-ZSM5 and H-Mordenite. Zeolites: A Refined Tool For Designing
Catalytic Sites; Elsevier Science: 1995; Vol. 97, pp 495.
(16) Eder, F.; Lercher, J. A. J. Phys. Chem. B 1997, 101 (8), 1273.
17) Eder, F.; Stockenhuber, M.; Lercher, J. A. J. Phys. Chem. B 1997, 101,
(
5414.
(18) van Well, W. J. M.; Cottin, X.; de Haan, J. W.; Smit, B.; Nivarthy, G.;
Lercher, J. A.; van Hooff, J. H. C.; van Santen, R. A. J. Phys. Chem. B
1998, 102 (20), 3945.
(type 127). n-Hexane and propane (Praxair, 20%; balance He) were
(
19) van Well, W. J. M.; Cottin, X.; Smit, B.; van Hooff, J. H. C.; van Santen,
R. A. J. Phys. Chem. B 1998, 102 (20), 3952.
also dosed onto H-FER (Si/Al ) 33.5, Zeolyst) at ambient temperatures
(
20) Nesterenko, N. S.; Thibault-Starzyk, F.; Montouillout, V.; Yuschenko, V.
V.; Fernandez, C.; Gilson, J.-P.; Fajula, F.; Ivanova, I. I. Microporous
Mesoporous Mater. 2004, 71 (1-3), 157.
+
(
∼303 K) to determine the concentration of H in 8-MR and 10-MR
channels of FER.
4920 J. AM. CHEM. SOC.
9
VOL. 129, NO. 16, 2007

Carbonylation of Methyls on 8-MR Zeolite Channels
A R T I C L E S
Figure 1. Infrared spectra of the OH stretching region of HMOR (Si/Al
)
10, Zeolyst) upon n-hexane adsorption at 303 K. Inset shows the
percentage of H sites determined from band deconvolution analysis in
+
8
-MR (b) and 12-MR (1) channels with increasing n-hexane pressure.
+
on H-MOR to determine the concentration of H in 8-MR and
1
2-MR channels. Contact with pyridine led to bands at 1545
-
1
cm , assigned to pyridine on Brønsted sites (B), and at 1491
-
1
cm , assigned to pyridine on Brønsted and Lewis sites (B +
L). Band deconvolution of infrared spectra recorded at increasing
pyridine and 2,6-dimethyl pyridine pressures showed that these
+
strong bases titrate all H sites within 12-MR channels in MOR,
but they perturb only a small fraction (<15%) of the OH groups
within 8-MR channels, possibly those located near the intersec-
tions of 8-MR and 12-MR channels. The fraction of the initial
OH groups perturbed by these molecules is consistent with the
number of OH groups in 12-MR channels measured by spectral
deconvolution of the O-H bands without adsorbed molecules
and from perturbation of O-H bands upon adsorption of
n-hexane (Figure S4; Supporting Information). These indepen-
dent measurements confirmed that 0.55 ((0.05) of the OH
groups in this H-MOR sample (Si/Al ) 10, Zeolyst) reside
within 8-MR channels.
Figure 2. Infrared spectra (-) and deconvoluted bands corresponding to
+
H
in 8-MR and 12-MR channels (- -) of (a) H100Na0MOR, (b) H83Na17-
MOR, (c) H73Na27MOR, (d) H59Na41MOR, (e) H45Na55MOR, and (f) H29-
Co32MOR.
n-Hexane adsorption on H-FER (Si/Al ) 33.5, Zeolyst) per-
turbed >85% of the OH groups, as also reported by van Well
Distribution of H+ Sites in Cation-Exchanged MOR
Samples. Figure 2 shows the infrared spectra in the OH
1
8,19
13
et al.,
who also concluded, based on C CP MAS NMR
+
2+
stretching region for H-MOR, as Na and Co replaced some
studies, that n-hexane adsorption on H-FER was similar to that
on NaZSM-22, (structural code TON; 10-MR one-dimensional
channel system) and that n-hexane adsorbed only on sites located
within 10-MR channels of H-FER. This conclusion was con-
firmed by configurational bias Monte Carlo simulations, in which
n-hexane was found to adsorb only within 10-ring channels in
FER even at high pressures (6.3 MPa). In contrast, propane
interacts preferentially with OH groups within 8-MR channels
of FER, because of stronger dispersive interactions within
smaller 8-MR channels compared to 10-MR channels. Singular
value decomposition (SVD) analysis of the OH region infrared
spectra at different propane pressures was used to deconvolute
+
of the H species, a process that significantly decreased DME
carbonylation turnover rates (per total H ). The OH bands in
MOR became more symmetrical with increasing Na content,
+
8
+
because of a preferential weakening of the band corresponding
-
1
+
to OH groups within 8-MR (at 3592 cm ) as Na replaced
+
H ; these data indicate that Na exchange occurred selectively
by replacement of H within 8-MR side pockets. The selective
replacement of H in side pockets of MOR by Na was
+
+
+
1
0,21
2+
+
+
previously reported.
Co , in contrast to Na , replaced H
with similar probability in 8-MR and 12-MR channels. Taken
together with the sharp decrease in carbonylation rates (per total
+
-
1
H ) upon Na exchange (Figure 3), these data implicate a specific
O-H bands within 8-MR (∼3590 cm ) and 10-MR (∼3600
-
1
reactivity of CH3 intermediates within 8-MR channels in
cm ) channels in H-FER. These deconvolution methods for
spectra measured in the presence of propane or n-hexane showed
that the fraction of the OH groups within 8-MR channels in
H-FER (Si/Al ) 33.5, Zeolyst) was 0.11 ((0.04).
+
carbonylation steps. The number of H sites within 8-MR
(
21) Veefkind, V. A.; Smidt, M. L.; Lercher, J. A. Appl. Catal. A: General
2000, 194, 319.
J. AM. CHEM. SOC.
9
VOL. 129, NO. 16, 2007 4921

A R T I C L E S
Bhan et al.
Figure 3. DME carbonylation rates (438 K, 3.34 cm s-1 g-1, 0.93 MPa
3
+
CO, 20 kPa DME, 50 kPa Ar) plotted against the number of total H sites
in (i) H-MOR (Si/Al ) 10, Zeolyst) and cation-exchanged MOR samples
reported in Figure 2 (b); (ii) H-MOR samples with Si/Al ) 6.5 and Si/Al
∼
10 (9); (iii) H-MFI (Si/Al ) 12.2), H-BEA (Si/Al ) 12.5), H-USY (Si/
3
-1
Figure 4. DME carbonylation rates per unit mass (438 K, 3.34 cm s
Al ) 3), amorphous silica-alumina (Si/Al )3), and H-FER (Si/Al ) 10
and 33.5) (2); and (iv) H-MOR (Si/Al ) 10, Zeolyst) after chemical
dealumination by oxalic acid treatments with Si/Al ratios of 14-17 (1).
Details regarding synthesis and treatment protocols for (ii), (iii), and (iv)
were reported in refs 7 and 8.
-1
g
H
, 0.93 MPa CO, 20 kPa DME, 50 kPa Ar) plotted against the number of
+
sites per unit mass in 8-MR channels of MOR (2; Si/Al ) 10, Zeolyst)
and FER ([; Si/Al ) 33.5, Zeolyst) and 12-MR channels of MOR (b).
Inset shows DME carbonylation rates plotted against the total number of
+
H
sites in these samples (9).
channels decreased upon exchange by a factor of ∼10 when
+
+
only 0.55 of the (total) H species were replaced by Na in
+
H-MOR. In contrast, the fraction of H species within 12-MR
channels was essentially unchanged by Na exchange (0.45 (
.05) when 0.55 of the total H were replaced by Na .
+
+
+
0
Correlation between DME Carbonylation Rates and the
Number of OH Groups. Figure 4 shows DME carbonylation
rates (per g) as a function of the number of OH groups (per g)
located within 8-MR (MOR, FER) or 12-MR (MOR) channels
for samples with varying Na or Co content. Carbonylation rates
+
increased in parallel with the number of H sites within 8-MR
+
channels. In contrast, no correlation with the number of H sites
+
within 12-MR (MOR) channels or with the total number of H
sites is evident from these data (Figure 4, inset). Such specificity
in the reactivity for O-H sites of identical composition when
placed within a more restrictive environment appears to be
21
unprecedented for inorganic systems. Veefkind et al. reported
that rates of ethanol reactions with NH3 are ∼1.5 times higher
+
+
on H sites within 8-MR pockets in H-MOR than on H sites
within 12-MR channels; these authors proposed that monoethyl
amine intermediates were selectively stabilized within 8-MR
channels. We find here that CH3 intermediates confined within
+
Figure 5. DME carbonylation rates (b; per total H ) and pyridine uptake
2; Pyridine/Al) plotted against the time-on-stream (h) during pyridine
(
titration experiments on HMOR (Si/Al ) 10, Zeolyst; 423 K, 1 atm of
total pressure).
8
-MR channels react with CO much more rapidly than similar
species within 10-MR (FER) or 12-MR (MOR) channels,
apparently because of specific stabilization of the acetyl-like
transition states or of CO molecules as they approach CH3
groups when such events occur within more constrained spatial
and conclusions are also consistent with the marked differences
in turnover rates (per total H ) for H-MOR with nominally
+
similar Si/Al ratios but different origin (Figure 3) and with the
+
lower turnover rates (per total H ) in dealuminated MOR
+
geometries. The unique reactivity of H (and CH3) within 8-MR
samples with high Si/Al ratios; dealumination led to the
preferential loss of accessible Al sites within 8-MR channels,
either by removing them or by making such channels less
channels is consistent with the high DME carbonylation rates
on H-MOR and H-FER, which contain 8-MR channels and with
the much lower and often undetectable reactivity of OH (CH3)
sites with similar local composition in H-MFI and H-BEA
zeolites and in H-USY and amorphous silica-alumina samples
8
accessible.
+
Figure 5 shows DME carbonylation rates (per total H ) for
HMOR (423 K; 1 bar total pressure) upon pyridine addition to
DME/CO/Ar reactant mixtures. DME carbonylation rates de-
7
with higher Al content (Si/Al ) 3) (Figure 3). These findings
4922 J. AM. CHEM. SOC.
9
VOL. 129, NO. 16, 2007

Carbonylation of Methyls on 8-MR Zeolite Channels
A R T I C L E S
creased by less than 15% even after adsorption of 0.5 pyridine
molecules per OH group. These data indicate that either pyridine
did not interact strongly with CH3 species prevalent during
(chemical shifts of 59.9 ppm on H-MFI, 60.7 ppm on HMOR,
2
6-28
49 ppm on H-ZSM-11, 56 ppm on H-FAU),
in sharp
contrast with the marked reactivity differences that we report
here for their reactions with CO. Zeolites (H-MFI, H-MOR,
H-FER) with markedly different carbonylation turnover rates
chemisorbed DME with a saturation stoichiometry of 0.5
+
steady-state DME carbonylation or the reactivity of H (CH3)
in 8-MR channels, inaccessible to pyridine because of steric
constraints, contributed most of the active sites relevant to
+
7,8
steady-state DME carbonylation catalysis; the reactivity of H
((0.05) DME/Al at 423 K; thus, all OH groups, irrespective
(
CH3) in 12-MR channels, which are readily titrated by pyridine,
of their spatial environment, form saturation coverages of CH3
groups at conditions of carbonylation catalysis. Hence, we
propose here that DME carbonylation on acidic zeolites and its
rate-determining elementary step (insertion of CO into CH3
groups) represent an example of transition state selectivity in
which charged transition states are stabilized to a much greater
extent than adsorbed (neutral) CH3 reactants and CH3CO
products. The charged transition state benefits from concerted
interactions with lattice oxygens within the smaller 8-MR
channels. These interactions become weaker within larger 10-
MR and 12-MR channels. In contrast, neutral reactants and
products are stabilized by local covalent bonds with framework
oxygen atoms vicinal to Al sites. These bonds are only weakly
affected by more distant oxygen atoms, the precise location of
which varies with zeolite structure and specifically with their
presence within 8-MR, 10-MR, and 12-MR channels. We
consider these effects of confinement on the transition state for
carbonylation of methyls to acetyls to differ in concept and detail
from size exclusion effects denoted in previous studies as
must be therefore significantly lower than that of species of
similar composition located within 8-MR channels.
The fractional change in the intensity of the OH infrared
bands upon exposure to pyridine was very similar ((10%) to
the number of pyridine molecules adsorbed per Al site during
DME carbonylation. These data indicate that extinction coef-
ficients for OH groups located in 8-MR and 12-MR channels
are similar. Makarova et al.11 reported that OH groups in 8-MR
channels have a slightly higher extinction coefficient (3.8 µmol
-
1
g ) than that for OH groups in 12-MR channels (2.5 µmol
-
1
g ). Our deconvolution procedures differ from those in the
previous study; we use only one primary component to describe
the high-frequency and low-frequency OH bands (compared to
three primary components used by Makarova et al. ) and allow
the peak shape to be optimized instead of choosing arbitrary
shapes (65% Lorentzian and 35% Gaussian used by Makarova
et al. ). Thus, we have not used these previously reported
extinction coefficients. We note that a difference in extinction
coefficients of 1.5 between these two bands changes only the
slope and not the linearity of the correlation between carbonyl-
11
1
1
29
“restricted transition state shape selectivity” and also from the
“active site shape selectivity” described by Anderson and
+
30
ation rates and the number of H sites in 8-MR channels (Figure
Klinowski in which the zeolite structure determines the relative
4
).
abundance of adsorbed intermediates by steric restrictions on
isomerization pathways. The specificity of 8-MR channels for
CH3*-CO reactions reported here represents an unprecedented
example of molecular reactivity controlled by confinement
within microporous solids with conceptual analogues in enzyme
Implications for Carbonylation Catalysis. Zecchina et
al.22,23 used infrared spectroscopy to probe neutral and hydrogen-
bonded complexes formed by interactions of H2O with acidic
OH groups in H-MFI, H-MOR, and H-Nafion. Bands for
+
-1
-1
31
[H(H2O)n] ionic species (at 1860-1650 cm and 1450 cm )
catalysis.
appeared in H-MOR even at the lowest H2O pressures, indicat-
ing that OH groups within narrow 8-MR channels in MOR
stabilize ionic clusters more effectively than within larger
channels in H-MFI. Hence, it appears plausible that the strong
water inhibition of DME (and methanol) carbonylation reflects
the selective formation of unreactive [(CH3)(H2O)n] cationic
clusters within 8-MR channels, where CO insertion into CH3
groups selectively occurs. Haw et al.24 have proposed that high
dielectric constant solvents favor proton transfer in zeolites by
stabilizing ion-pair structures. Similarly, 8-MR channels that
selectively stabilize ionic H2O clusters may also provide a more
polar environment, which is likely to favor ionic transition states
involved in CO insertion into bound methyl groups.
Clark et al. have described the dearth of firm evidence for
transition state selectivity, wherein the local shape of the zeolite
pore influences local reactions to form a given product by
selective stabilization of the required transition state without
detectable effects on bound reactants (CH3) or products (CH3-
CO). Previous 13C MAS NMR studies did not detect any spectral
differences among CH3 groups within various zeolite structures
The evidence presented indicates a direct correspondence
+
between DME carbonylation rates and the number of H
sites present within 8-MR channels in MOR and FER. In light
of this, improved catalysts will require zeolite frameworks in
which (i) 8-MR channels and pockets can be accessed from
external crystal surfaces via larger interconnected channels
7
,8
+
+
and (ii) H (and framework Al atoms) can be selectively placed
within such 8-MR channels and pockets. The data presented
provide a compelling experimental demonstration of transi-
tion state selectivity and a unique opportunity to design
microporous structures to match the stringent spatial constraints
required to stabilize specific transition states in catalytic
reactions.
2
5
Acknowledgment. The authors dedicate this article to the
memory of Dr. John P. Collins. His leadership within the BP
technical staff was instrumental in the birth and growth of the
Methane Conversion Cooperative program that led to the
findings reported herein. The authors acknowledge financial
support from BP as part of the Methane Conversion Coop-
(
26) Derouane, E. G.; Gilson, J. P.; Nagy, J. B. Zeolites 1982, 2 (1), 42.
(
(
(
(
22) Zecchina, A.; Geobaldo, F.; Spoto, G.; Bordiga, S.; Ricchiardi, G.; Buzzoni,
(27) Ivanova, I. I.; Corma, A. J. Phys. Chem. B 1997, 101 (4), 547.
(28) Jiang, Y. J.; Hunger, M.; Wang, W. J. Am. Chem. Soc. 2006, 128 (35),
11679.
R.; Petrini, G. J. Phys. Chem. 1996, 100 (41), 16584.
23) Zecchina, A.; Spoto, G.; Bordiga, S. Phys. Chem. Chem. Phys. 2005, 7
(8), 1627.
(29) Csicsery, S. M. Zeolites 1984, 4 (3), 202.
(30) Anderson, M. W.; Klinowski, J. J. Am. Chem. Soc. 1990, 112 (1), 10.
(31) Mesecar, A. D.; Stoddard, B. L.; Koshland, D. E. Science 1997, 277 (5323),
202.
24) Haw, J. F.; Xu, T.; Nicholas, J. B.; Goguen, P. W. Nature 1997, 389 (6653),
32.
25) Clark, L. A.; Sierka, M.; Sauer, J. J. Am. Chem. Soc. 2004, 126 (3), 936.
8
J. AM. CHEM. SOC.
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VOL. 129, NO. 16, 2007 4923

A R T I C L E S
Bhan et al.
erative Research Program and helpful technical discussions
with Dr. Patricia Cheung (University of California at Berkeley),
Professor Avelino Corma (Universidad Politecnica de Valencia),
and Professor Johannes Lercher (Technical University of
Munich).
Supporting Information Available: Band deconvolution
methods used. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA070094D
4924 J. AM. CHEM. SOC.
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VOL. 129, NO. 16, 2007