Quantification of Intraporous Hydrophilic Binding Sites in Lewis Acid Zeolites and Consequences for Sugar Isomerization Catalysis

Abstract

Aqueous-phase sugar isomerization is catalyzed at lower turnover rates when Lewis acid active sites are confined within high-defect zeolite micropores, because intraporous silanol groups serve as hydrophilic binding sites that stabilize extended water networks during catalysis that entropically destabilize isomerization transition states relative to their precursors. The number of intraporous silanol defects varies widely among Lewis acid zeolites prepared by different hydrothermal and post-synthetic routes, and after subsequent activation treatments prior to catalysis, thus leading to unintended or unpredictable consequences for reactivity. Here, we develop a suite of methods to characterize and quantify intraporous silanol groups in Lewis acid zeolites and provide a link between the number of such groups and glucose isomerization rate constants. Sn-Beta zeolites were prepared with systematically varying silanol density by increasing the extent of Sn grafting into various dealuminated Beta supports, which were derived from parent Al-Beta zeolites synthesized both with varying initial Al content (Si/Al = 19–180, 0.4–3.2 Al (unit cell)−1) and defect density via the mineralizing agent used (F–, OH–). The packing density of methanol within micropores was used as a reporter of the intraporous silanol density and was quantified from relative methanol (293 K) and dinitrogen (77 K) uptakes at the point of micropore filling from single-component adsorption isotherms; methanol packing densities decreased systematically from 0.98 to 0.07 among Sn-Beta samples with increasing SiOH density. In situ IR spectra (CH3OH reduced pressure < 0.2, 303 K) indicate that methanol arranges in isolated clusters within low-defect Sn-Beta micropores (<1 silanol (unit cell)−1), but forms extended hydrogen-bonded networks within high-defect micropores (>1 silanol (unit cell)−1). On each sample, the total number of extracrystalline and intraporous SiOH groups was quantified by H/D isotopic exchange with D2 via temperature-programmed surface reaction (500–873 K), while the number of intraporous SiOH groups was quantified from strongly H-bound CD3CN in IR spectra (2275 cm–1, 303 K). Aqueous-phase first-order glucose isomerization rate constants (per defect-open Sn, 373 K) were 4 times higher on Sn-Beta prepared post-synthetically from dealuminated Beta supports whose parent Al-Beta zeolites were initially low-defect (<0.6 Al (unit cell)−1) than high-defect (>0.6 Al (unit cell)−1). These findings constitute a synthesis-structure-function relationship that provides specific guidance to minimize the density of intraporous defects when using post-synthetic routes to prepare catalytic materials with reactivity comparable to their hydrothermally crystallized hydrophobic analogues.

Topics & Concepts