Tailoring Distinct Reactive Environments in Lewis Acid Zeolites for Liquid Phase Catalysis

Lewis acidic zeolites are microporous crystalline materials that offer promise as catalysts for the activation and conversion of biomass-derived precursors in the liquid phase due to their unique water tolerance and synthetic versatility. The active site environment in zeolite catalysts is multifaceted in nature and is composed of a primary catalytic binding site, the secondary pore structure that confines such binding sites, and occluded solvent and reactant molecules that interact with adsorbed species. Moreover, Lewis acidic heteroatoms can adopt structurally diverse coordination that selectively catalyze different classes of chemical transformations and can be difficult to control synthetically or characterize spectroscopically. Thus, precise mechanistic interpretation of liquid-phase zeolite catalysis necessitates the development of synthetic, spectroscopic, and kinetic methods that can decouple such complex active site structures and probe the interactions that occur between confined active sites, solvent and reactant molecules, and adsorbed intermediates and transition states. In this Account, we describe the development and application of synthetic, spectroscopic, and kinetic methods to investigate chemically distinct Lewis acid zeolite environments in siliceous zeolites for liquid-phase catalysis. Identification of unique Lewis acidic active site structures relied on the development of direct and indirect solid-state nuclear magnetic resonance (NMR) methods that probe the number and connectivity of framework Lewis acid sites for a diverse range of metal heteroatoms. Such methods enabled the quantitative comparison of catalytic turnover rates, on a per active site basis, measured on different catalysts in order to establish structure-function relationships between active site structure and reactivity. Rigorous normalization of turnover rate further permits comparison of catalytic turnover rates across materials of varying topology, metal heteroatom identity, solvent, and framework polarity to extract salient thermodynamic descriptors of catalysis through kinetic probes. Ex situ interrogation of alcohols adsorbed within hydrophobic and hydrophilic Sn-containing zeolites revealed that hydrophobic voids induce structural order on confined alcohol hydrogen-bonding networks, which give rise to enhanced turnover rates of liquid-phase transfer hydrogenation catalysis. This acceleration of turnover rates arises because ordered alcohol networks occluded within the pores of hydrophobic zeolites stabilize adsorbed transfer hydrogenation intermediates and transition states to a greater extent than liquidlike solvent networks observed in hydrophilic zeolites. The effects of confined solvent molecules can also influence catalysis, independent of framework polarity, due to differences in solvent polarity and substituent effects, which alter turnover rates via changes in how different solvent molecules interact with adsorbed intermediates and transition states. These observations underscore new opportunities to leverage specific interactions between active sites and solvent molecules to influence solvent organization and transition state stability at confined solid-liquid interfaces. This work illustrates the importance of quantitative methods that count distinct active site structures in order to compare catalytic materials on a per active site basis. This information can be used to develop new synthetic procedures that predictably manipulate the functionalization of both the primary binding site and the secondary reaction environment to tailor catalytic function for a desired chemistry. Collectively, these advances highlight strategies to engineer and characterize microporous catalysts with unique reaction environments in order to capture salient mechanistic features and navigate the complex free energy landscape of catalysis in condensed solvent systems.

Automated summary

Lewis acidic zeolites offer promise as catalysts for biomass-derived precursors due to their unique water tolerance and synthetic versatility. The multifaceted active site environment in zeolite catalysts is composed of primary binding sites, secondary pore structures, and occluded solvent and reactant molecules. Synthetic, spectroscopic, and kinetic methods are necessary for precise mechanistic interpretation of liquid-phase zeolite catalysis.