(651g) Catalytic Consequences of Isolated and Paired Acid Sites in CHA Zeolites on Monomolecular Propane Cracking
Consequences of Isolated and Paired Acid Sites in CHA Zeolites on Monomolecular
Philip M. Kester, John R. Di Iorio, and Rajamani Gounder*
Charles D. Davidson
School of Chemical Engineering, Purdue University, West Lafayette, IN
Turnover rates for monomolecular
cracking and dehydrogenation of alkanes in zeolites are sensitive to local void
environments  and to the local density of Brønsted acid sites as inferred
from changes in bulk Si/Al ratio . Here, we utilize single tetrahedral-site chabazite
(CHA) zeolites of fixed bulk composition (Si/Al = 15) with controlled arrangements
of framework Al atoms (Al-O(-Si-O)x-Al) in paired (x = 1, 2) and
isolated (x ≥ 3) configurations  to decouple the effects of void
environment and active site proximity. Turnover rates of monomolecular propane cracking
(per H+, 748 K) increase systematically in CHA zeolites as the
fraction of Al pairs increases from 0 to 44%. Apparent rate constants (per H+)
on CHA with all isolated Al are constant, suggesting that all isolated Brønsted
acid sites are equivalently reactive. Apparent rate constants (per H+)
are an order of magnitude larger for paired Al sites (4.0 × 10-3 mol
(mol H+)-1 s-1 bar-1) than for
isolated Al sites (3.3 × 10-4 mol (mol H+)-1 s-1
bar-1). Furthermore, rate constants decrease upon partial Na+
poisoning of H+ sites in CHA zeolites containing paired Al sites, consistent
with the different reactivity of isolated and paired Brønsted acid sites. Infrared
spectra are used to detect and quantify paired proton sites in CHA zeolites
using molar extinction coefficients estimated from a suite of model samples of
varying paired acid site density, allowing for the quantification of the distribution
of residual protons in partially ion-exchanged CHA zeolites. Apparent
activation energies are independent of acid site proximity, while apparent
activation entropies are less negative at paired acid sites, suggesting that
transition states are later and more loosely bound at such sites. This work
illustrates that the catalytic behavior of zeolites for alkane activation can
be tuned by synthetically manipulating the local arrangement framework aluminum
atoms and, in turn, of active Brønsted acid sites.
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