(269a) Introducing Catalytic Diversity into Single-Site Zeolites of Fixed Composition Via Synthetic Control of Active Site Proximity

Introducing
Catalytic Diversity into Single-Site Zeolites of Fixed Composition via
Synthetic Control of Active Site Proximity

John
R. Di Iorio, Claire T. Nimlos, Rajamani Gounder*

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

Distinct
Brønsted acid site (H⁺) ensembles within a given zeolite arise from
differences in the arrangement (Al−O(−Si−O)_x−Al)
of framework Al atoms between isolated (x ≥ 3) or paired configurations
(x = 1, 2), the latter defined functionally by their ability to exchange
extraframework divalent cations. The effects of proton proximity on hydrocarbon
catalysis have been previously studied in MFI zeolites,^1-2
but the concomitant changes in Al
distribution among different crystallographically
distinct tetrahedral sites (T-sites) and void environments (i.e., straight and
sinusoidal channels, and their intersections) have precluded unambiguous
kinetic assessments of proton proximity in MFI. Here, we use chabazite
(CHA) zeolites, a high-symmetry framework with only 1 unique lattice T-site, to
develop a predictive relationship that describes how synthetic methods can be
developed to prepare zeolites, of fixed topology and elemental composition,
with systematically different framework Al arrangements. Such arrangements, in
turn, generate different proton active site ensembles that markedly influence
turnover rates for the Brønsted acid-catalyzed
dehydration of methanol to dimethyl ether, a known probe reaction of solid
acids.³ Specifically, we extend previously reported synthetic procedures⁴
to crystallize CHA zeolites at similar composition (Si/Al = 15), but with
fractions of paired Al sites that systematically varied from the limit of site
isolation (0% paired Al) to nearly half of the sites (44% paired Al) in paired
arrangements. This series of CHA zeolites is used to demonstrate that first and
zero-order rate constants (415 K, per total H⁺) of methanol
dehydration are one order of magnitude larger on paired than on isolated
protons. In situ IR spectra measured
during steady-state dehydration catalysis are used to observe the formation of
surface methoxy species at paired Al sites in CHA, which are not observed on
isolated protons in CHA or MFI, providing evidence that alternate dehydration
pathways are accessible on paired protons in CHA zeolites. These results represent a step toward the synthetic control
of active site atomic arrangement and establish a predictive
synthesis-structure-function relation for Brønsted acidic CHA zeolites, demonstrating that catalytic
diversity can arise solely from differences in the atomic arrangement of active
sites, even among single T-site zeolites of fixed composition.

References:

1.     
Janda, A.; Bell, A. T.
J. Am. Chem. Soc. 2013, 135, 19193-19207

2.     
Bernauer, M.; Tabor,
E.; Pashkova, V.; Kaucký,
D.; Sobalík, Z.; Wichterlová,
B.; Dedecek, J. J.
Catal. 2016,
344, 157-172.

3.     
Carr, R. T.; Neurock,
M.; Iglesia, E. J.
Catal. 2011,
278, 78-93.

4.     
Di Iorio, J. R.; Gounder, R. Chem. Mater. 2016, 28, 2236-2247