Cellulose, a major constituent of biomass, is a promising source of sustainable energy. A key step in the conversion of cellulose to a platform chemical is glucose isomerization to fructose. Sn-Beta zeolite catalyzes this reaction with high yield. The effect of methanol as a reaction medium on glucose transformations catalyzed by Sn-Beta has not been quantified. Here, density functional calculations are employed to elaborate on the effect of methanol medium, specifically to determine how reaction pathways and energy barriers are affected by methoxylation of Sn or Si groups at the active sites in Sn-Beta. Calculations suggest that the presence of the neighboring silanol group is necessary for glucose isomerization. If the silanol group is altered by methoxylation glucose epimerization is promoted and will likely occur. These results provide additional understanding of the active site of Sn-Beta for glucose transformations and are insightful for novel catalyst design and development.

Lewis acidic zeolites are silica-based, crystalline microporous materials containing tetravalent heteroatoms (M4+=Ti, Sn, Zr, Hf) substituted in framework locations, and have been reported to catalyze a wide range of reactions involving oxygenates and hydrocarbons. The synthetic protocols used to prepare Lewis acid zeolites determine the structures of the active sites and the reaction pockets that confine them, which in turn influences reactivity, product selectivity, and catalyst stability. Specifically, aqueous-phase reactions of biomass-derived molecules, such as glucose isomerization, are sensitive to the hydrophobicity of confining environments, leading to changes in turnover rates. As a result, precise evaluation of the structure and behavior of reaction environments and confined active sites among catalysts of varying provenance or treatment history requires quantitative descriptions of active Lewis acid site densities, of densities of surface functional groups that determine the polarity of microporous confining environments, and of the kinetic behavior of these catalytic materials.<div><br></div><div>Methods for quantifying Lewis acid sites and silanol defects are developed here by analyzing infrared (IR) spectra collected after Lewis base (CD3CN, pyridine) titrations of Lewis acidic zeolite surfaces and are compared to vapor-phase methanol and water adsorption isotherms. Additionally, IR spectra collected under ex situ (flowing vapor-phase water) and in situ (aqueous-phase, 373 K, 0-50 wt% glucose) conditions are used to compare co-adsorbed water densities and structures within hydrophobic (low silanol density) and hydrophilic (high silanol density) confining environments within M-Beta zeolites. Under reaction conditions relevant for sugar conversion in aqueous media (353-398 K, 1-50 wt% glucose), hydrophilic reaction pockets stabilize liquid-like extended water structures within microporous environments, while hydrophobic channels stabilize vapor-phase water at lower intraporous water densities. Higher aqueous-phase glucose isomerization rates (368-383 K, 1-50 wt% glucose, per kinetically relevant active site) are observed on hydrophobic Ti-Beta (~6-12x, per Lewis acidic Ti) and Sn-Beta (~50x, per Lewis acidic Sn in open configuration) zeolites over their hydrophilic analogs. Higher turnover rates on hydrophobic M-Beta zeolites reflect the absence of an extended, hydrogen-bonded network of waters, which entropically destabilizes kinetically relevant hydride shift transition states by reducing the flexibility of their primary solvation spheres. These findings suggest catalyst design strategies to minimize the generation of silanol groups within confining reaction environments would lead to increases in turnover rates.<br></div><div><br></div><div>The methods derived herein can be applied to understanding the role of the confining environment and the associated co-adsorbed water on zeolitic materials of different topology and Lewis acid site identity. For example, the transient formation of silanol defects under aqueous-phase operating conditions is primarily responsible for the deactivation of Sn-Beta catalysts observed during aqueous-phase glucose isomerization. Further, quantifying the role of the confining environment geometry and hydrophobicity on aqueous-phase glucose isomerization rates can be used as guidance for catalyst design to increase reaction rates and selectivities toward specific isomerization products. These findings show that both the active site identity and its confining environment, which vary with zeolite topology and micropore polarity, combine to influence reactivity, selectivity and stability for aqueous-phase glucose isomerization catalysis.<br></div>

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<div><div><div><p>Lewis acid zeolites contain framework metal heteroatoms that catalyze sugar iso- merization reactions at different turnover rates depending on the local coordination around metal centers and the polarity of their confining secondary environments. Post-synthetic modification routes that react metal precursors with framework va- cancy defects in dealuminated Beta zeolites (Sn-Beta-PS-OH) are developed as an alternative synthetic strategy to the hydrothermal crystallization of Sn-Beta zeolites (Sn-Beta-HT-F). Post-synthetic routes provide the ability to systematically tailor the structural features of active and defect sites in Sn-zeolites, especially in composition ranges inaccessible to materials crystallized by hydrothermal routes (Si/Sn < 100; > 2 wt.% Sn), yet often result in incomplete or unselective Sn grafting within framework vacancy defects and form extraframework metal oxide domains and residual defect sites. The development of robust post-synthetic routes to prepare Sn-zeolites with intended active and defect structures has been limited by the dearth of characteri- zation techniques to unambiguously detect and quantify such structures present in stannosilicate materials, and of mechanistic links between such structures and the turnover rates of catalytic reactions.</p><p><br></p><p>The presence of framework Sn centers that can expand its coordination shell from four- to six-coordinate structures, and small extraframework tin oxide domains that cannot, were unambiguously detected from diffuse reflectance UV-Visible spectra of stannosilicate materials measured after dehydration treatments (523 K, 0.5 h) to discern ligand-to-metal charge transfer bands for tetrahedrally-coordinated Sn heteroatoms (< 220 nm, > 4.1 eV) and those for tin oxide domains (> 230 nm, < 4.1 eV). Liquid-phase grafting of stannic chloride in dichloromethane reflux (333 K) enables preparing Sn-Beta zeolites with higher framework Sn content (Si/Sn = 30– 144; 1.4–6.1 wt.% Sn) than grafting performed in isopropanol reflux (423 K, Si/Sn > 120; 1.6 wt.% Sn). This reflects competitive adsorption of isopropanol solvents with stannic chloride at framework vacancy defects during grafting procedures, consistent with infrared spectroscopy (IR) and temperature-programmed desorption (TPD) of dealuminated Beta samples after saturation with isopropanol at reflux temperatures (423 K), and not any limitations inherent to the structure of vacancy defects within dealuminated zeolite supports that would prevent reaction with metal precursors as often proposed.</p><p><br></p></div></div></div><div><div><div><p>This insight enabled preparing Sn-Beta zeolites with varying densities of residual defects, via dichloromethane-assisted grafting of stannic chloride to different extents, into dealuminated Beta supports of different initial Al content (Si/Al = 19–180) and mineralizing agent used for hydrothermal crystallization of the parent Al-Beta sam- ple (e.g., fluoride or hydroxide). Preparation of low-defect Sn-Beta zeolites using post-synthetic routes (Sn-Beta-PS-F) first required the synthesis of parent Al-Beta zeolites in fluoride media to minimize residual siloxy defects (OSi−) formed during crystallization, and dilute Al content (Si/Al > 100, < 0.6 Al (unit cell)−1), to min- imize the density of intrapore silanol groups formed after dealumination and high temperature oxidative treatment. The methanol packing density within microporous voids of Sn-Beta zeolites was assessed from relative volumetric uptakes at the point of micropore filling from single-component methanol (293 K) and nitrogen (77 K) adsorption isotherms, and decreased systematically among samples with increasing density of silanol groups. The total density of silanol groups within micropores and at external crystallite surface in Sn-Beta zeolites was quantified by H/D isotopic ex- change during temperature-programmed surface reactions (500–873 K), and within microporous voids from IR spectra measured after saturation of microporous binding sites with CD3CN (2275 cm−1, 303 K). In situ IR spectra collected at low methanol pressures (P/P0 < 0.2, 303 K) provide further evidence that methanol molecules ar- range in localized clusters within Sn-Beta-PS-F, but form extended hydrogen-bonded networks within Sn-Beta-PS-OH.</p><p><br></p></div></div></div><div><div><div><p>Glucose-fructose isomerization rate constants (373 K) were used to probe the lo- cal coordination of Sn heteroatoms and the polarity of the secondary environment as influenced by silanol defects within microporous cavities. Ex situ pyridine titration of Sn-Beta-HT-F samples suppressed isomerization rates (per total Sn, 373 K) after only a subset of Sn sites were poisoned, which correspond to the number of open Sn sites quantified ex situ via CD3CN IR (303 K), providing further evidence that open Sn sites are dominant active sites for glucose isomerization. First-order isomerization rate constants (373 K) decrease with increasing Sn content when normalized by total Sn density, and are invariant when normalized by the number of open Sn sites, be- cause open Sn sites are grafted preferentially within Sn-Beta-PS-OH (Si/Sn = 30–144; 1.4–6.1 wt.% Sn) at low Sn densities. Isomerization rate constants (per open Sn, 373 K), however, are lower by ∼4x and ∼15x on Sn-Beta-PS-F (Si/Sn = 284; 0.7 wt.% Sn) and Sn-Beta-PS-OH, respectively, than on Sn-Beta-HT-F. Open Sn sites catalyze aqueous-phase glucose isomerization at higher turnover rates (373 K) when their mi- croporous surroundings contain silanol defects present in low (hydrophobic) densities than high (hydrophilic) densities, which are characteristic of Sn-Beta-HT-F and Sn- Beta-PS-OH samples, respectively. This reflects reorganization of extended water networks, which are stabilized in high-defect, hydrophilic micropore environments, at kinetically relevant 1,2-hydride shift transition states that incurs entropic penal- ties that lower turnover rates. This thesis highlights the development of synthesis- structure-function relationships to guide the preparation of catalytic materials with intended active and defect site structures within confining reaction environments, the development of characterization techniques for the identification and quantification of such structures, and the influence of such structures on turnover rates of liquid-phase sugar isomerization.</p></div></div></div>