(560em) Mechanistic Origins of the High-Pressure Inhibition of Methanol Dehydration Rates in Small-Pore Acidic Zeolites

Mechanistic Origins of the High-Pressure Inhibition of
Methanol Dehydration Rates in Small-Pore Acidic Zeolites

John R. Di Iorio,¹ Alexander J. Hoffman,²
Claire T. Nimlos,¹ Steven Nystrom,² David Hibbitts,*²
Rajamani Gounder*¹

¹Charles
D. Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall
Drive, West Lafayette, IN 47907, USA

²Department
of Chemical Engineering, University of Florida, 1030 Center Drive, Gainesville,
FL 32611, USA

            Turnover rates of
methanol dehydration to dimethyl ether (415 K, per proton), a well-studied
probe of acid strength and confinement in zeolite catalysts [1], increase as
the fraction of protons in paired configuration (Al−O(−Si−O)x−Al;
isolated: x ≥ 3,  paired: x = 1, 2) increases in small-pore, 8-membered
ring (8-MR) zeolites (e.g., CHA, AEI) [2]. Precise kinetic interpretation of
the origin of turnover rate differences between isolated and paired protons in
small-pore zeolites, however, is convoluted by the onset of kinetic inhibition
at high methanol partial pressures (>10 kPa, 415 K), a phenomenon not
observed in medium (e.g., MFI) or large-pore (e.g., BEA) zeolites. High
pressure inhibition of methanol dehydration rates (per proton) diminishes at
elevated reaction temperatures (>450 K) and coincides with the disappearance
of IR vibrational bands characteristic of methanol trimers and larger clusters
(~3370 cm^-1) measured from in situ IR spectra (343-473 K, 10
kPa CH₃OH). These results suggest that the formation of large
methanol clusters (>2 CH₃OH per cluster) inhibits methanol
dehydration rates and that inhibition is not caused by internal mass transport
limitations, which would become exacerbated at elevated reaction temperatures. Methanol
adsorption isotherms on CHA zeolites (293 K) indicate that micropore filling
(~2 CH₃OH per proton) occurs at similar gas-phase CH₃OH Gibbs
free energy, which accounts for differences in the stability of equilibrated
surface species at different temperatures, to those calculated at the onset of
dehydration rate inhibition (415 K, per H⁺). These observations are
supported by IR spectra measured during methanol dosing onto CHA zeolites
(0.01-3 CH₃OH/H⁺; 293 K) that shows the appearance of methanol
clusters at coverages >0.7 CH₃OH/H⁺, which increase in
intensity with increasing methanol coverage. Collectively, these observations
implicate methanol clusters as inhibiting species in methanol dehydration
catalysis in small-pore CHA zeolites, but kinetic interpretation of measured
dehydration rates is still convoluted by multiple candidate rate laws that can be
derived from different rate limiting steps, which functionally conform to the measured
rate data. Density functional theory is used to calculate the Gibbs free
energies of various methanol complexes (1-16 CH₃OH per cluster)
confined at protons within CHA as a function of temperature and methanol pressure,
which indicate that methanol trimers are preferentially stabilized in CHA at
methanol partial pressures that are consistent with experimentally observed
inhibition (>10 kPa, 415 K). DFT-predicted methanol dehydration rates (415
K) are computed and compared with prior experimental and computational
observations to further distinguish between plausible reaction mechanisms. These
results highlight the critical role that the zeolite framework plays in
stabilizing reactive intermediates and transition states, and how different
frameworks preferentially stabilize inhibitory species.

References:

[1] Carr, R. T., Neurock, M.,
Iglesia, E., J. Catal., 2011, 278, 78-93

[2] Di Iorio, J. R., Nimlos, C.
T., Gounder, R., ACS Catal., 2017, 7, 6663-6674