Titration and quantification of open and closed Lewis acid sites in Sn-Beta zeolites that catalyze glucose isomerization

Abstract

Methods to quantify framework Lewis acidic Sn4+ sites in zeolite Beta (Sn-Beta) with four Lewis base titrants (pyridine, deuterated acetonitrile, n-propylamine, ammonia) were developed using infrared (IR) spectroscopy or temperature programmed desorption (TPD). Integrated molar extinction coefficients (E, cmμmol−1) were measured for IR bands of pyridine adsorbed to Lewis acidic Sn sites (E(1450cm−1)=1.42±0.30) and of CD3CN adsorbed to open (E(2316cm−1)=1.04±0.22) and closed (E(2308cm−1)=2.04±0.43) Sn sites, and differ from analogous E values for Lewis acidic Al sites by up to 3.6×. TPD of Sn-Beta samples after saturation with NH3 or n-propylamine (NPA) and purging to remove physisorbed species also enabled quantification of Lewis acid sites, which is seldom performed despite analogous methods that use such titrants to quantify Brønsted sites in solid acids. These four Lewis bases titrated Lewis acidic Sn sites with equimolar stoichiometry and counted similar numbers of Sn sites on low-defect Sn-Beta samples synthesized in fluoride media (Sn-Beta-F) with high Sn content (Si/Sn <150). NPA binding on residual silanol defect sites, however, caused overestimation of Sn sites by TPD on Sn-Beta-F samples with low Sn content (Si/Sn >175) or high defect Sn-Beta samples prepared via post-synthetic insertion of Sn atoms into framework vacancy defects of dealuminated Beta zeolites (Sn-Beta-OH). Molar ratios of open-to-closed Sn sites varied widely (0.29–1.64) among the eight Sn-Beta samples studied here. Open Sn sites have been proposed as the dominant active sites for aqueous-phase glucose–fructose isomerization via intramolecular 1,2-hydride shift, consistent with successive poisoning of Sn-Beta samples with pyridine prior to measurement of initial glucose isomerization rates that suppressed reactivity at pyridine uptakes similar to the number of open Sn sites counted ex-situ by CD3CN. Measured first-order glucose isomerization rate constants in water (per open Sn, 373K), which reflect free energy differences between isomerization transition states and two coordinated water molecules at Sn sites, were ∼50× higher on hydrophobic Sn-Beta-F than on hydrophilic Sn-Beta-OH zeolites. The characterization methods reported herein enable normalization of initial glucose isomerization turnover rates on Sn-Beta zeolites by their number of open Sn sites, as required prior to interpreting the catalytic consequences of structural heterogeneities introduced by differences in sample preparation or treatment history.