458119 Quantifying Lewis Acid Sites in Zeolites That Catalyze Glucose Isomerization

Wednesday, November 16, 2016: 3:55 PM
Imperial B (Hilton San Francisco Union Square)
James W. Harris, Michael J. Cordon, Juan C. Vega-Vila, Fabio H. Ribeiro and Rajamani Gounder, School of Chemical Engineering, Purdue University, West Lafayette, IN

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


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.

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