(574c) Quantifying Lewis Acid Sites in Zeolites That Catalyze Glucose Isomerization

Quantifying Lewis Acid Sites in Zeolites that
Catalyze Glucose Isomerization

James W.
Harris, Michael J. Cordon, Juan C. Vega-Vila, Fabio H. Ribeiro, Rajamani
Gounder^*

School of Chemical Engineering, Purdue University, 480
Stadium Mall Drive, West Lafayette, IN 47907

*rgounder@purdue.edu

Pure-silica molecular sieves with tetravalent framework
heteroatoms (M = Hf⁴⁺,Sn⁴⁺, Ti⁴⁺,
Zr⁴⁺) catalyze intermolecular and intramolecular
Meerwein-Ponndorf-Verley alcohol oxidation and Oppenauer aldehyde reduction
(MPVO) cycles, which mediate stereoselective glucose isomerizations. Precise assessment of the reactivity of
Lewis acid zeolites of different provenance, which invariably contain different
densities of silanol defects and of open ((HO)-M-(OSi)₃) and closed
(M-(OSi)₄) sites, require methods to accurately quantify these
structures. Integrated molar extinction coefficients were determined for infrared
(IR) peaks of deuterated acetonitrile (CD₃CN, 303 K) bound to open (2316
cm^-1) and closed (2308 cm^-1) Sn sites, and of pyridine
(423 K) bound to Lewis acidic Sn sites (1450 cm^-1, 1610 cm^-1)
in Sn-Beta zeolites [1]. Identical numbers of Lewis acidic Sn sites were titrated
by CD₃CN and pyridine among Sn-Beta zeolites (>10 samples) of
varying synthetic origin and composition, and by ammonia and n-propylamine only
among samples with high Sn content (Si/Sn ≤ 150) and low-defect density. Pyridine
and CD₃CN did not distinguish isolated Ti sites of different
coordination in IR spectra. We will discuss our latest results using carbon
monoxide (CO, 100 K) to distinguish open and closed Sn and Ti sites in IR
spectra, by extending similar reports on Zr-Beta [2].  

The number of open Sn sites counted ex situ (CD₃CN IR) agreed quantitatively with the number
of active sites in Sn-Beta zeolites that catalyze aqueous-phase glucose-fructose
isomerization (373 K, 1% w/w glucose) at initial
reaction times [1], consistent with experimental [3] and theoretical [4] evidence
that open Sn sites are the dominant active sites for this reaction. Isomerization
rate constants were measured on Sn-Beta and Ti-Beta in kinetic regimes that are
first-order or zero-order in glucose concentration, and reflect free energy
differences between the same kinetically-relevant 1,2-hydride shift
isomerization transition state relative to two bound water molecules or to one bound
glucose molecule at open Sn sites, respectively. Rate constants in both regimes
(per active M, 373 K) were ~10-50x higher on hydrothermally-synthesized, low-defect
M-Beta-F zeolites (Si/M = 100-220) than on post-synthetically prepared,
high-defect M-Beta-OH zeolites (Si/M = 30-580), suggesting that isomerization
rate enhancements within hydrophobic voids are caused by preferential
stabilization of 1,2-hydride shift transition states. The number of silanol
defects quantified from CD₃CN IR spectra (2275 cm^-1)
correlated linearly with the molar water uptake determined from adsorption
isotherms (293 K, P/P₀ = 0.2) for low-defect M-Beta-F samples,
providing a quantifiable descriptor of hydrophobicity. These findings provide
guidance to prepare more reactive M-Beta zeolites by maximizing the number of
open sites while minimizing residual silanol defects.

[1] J. W. Harris et al. J. Catal., 335 (2016) 141-154.

[2] V. L. Sushkevich et al., J. Phys. Chem. C, 119 (2015)
17633-17639

[3] R. Bermejo-Deval et al. ACS Catal., 4 (2014) 2288-2297.

[4] N. Rai et al. ACS Catal., 3 (2013) 2294-2298; Y.P. Li et
al. ACS Catal., 4 (2014) 1537-1545.