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(641a) Ordered Hydrogen-Bonded Alcohol Networks Confined in Lewis Acid Zeolites Accelerate Transfer Hydrogenation Turnover Rates

Johnson, B. - Presenter, Massachusetts Institute of Technology
Roman, Y., MIT
The disruption of ordered water molecules confined within hydrophobic reaction pockets alters the energetics of adsorption and catalysis, but a mechanistic understanding of how nonaqueous solvents influence catalysis in microporous voids remains unclear. Here, we use kinetic analyses coupled with IR spectroscopy to study how hydrogen-bonded alkanol networks confined within hydrophobic and hydrophilic zeolite catalysts modify reaction free energy landscapes. Hydrophobic Beta zeolites containing framework Sn atoms catalyze the transfer hydrogenation reaction of cyclohexanone in a 2-butanol solvent 10x faster than their hydrophilic analogues (Figure 1a). This rate enhancement stems from the ability of hydrophobic Beta zeolites to inhibit the formation of extended liquid-like 2-butanol oligomers (~3300 cm-1) and stabilize dimeric H-bonded 2-butanol networks (~3510 cm-1; Figure 1b). These different intraporous 2-butanol solvent structures manifest kinetically as differences in the activation and adsorption enthalpies and entropies that comprise the free energy landscape of transfer hydrogenation catalysis. The ordered H-bonding solvent network present in hydrophobic Sn-Beta stabilizes the transfer hydrogenation transition state to a greater extent than the liquid-like 2-butanol solvent present in hydrophilic Sn-Beta, giving rise to higher turnover rates on hydrophobic Sn-Beta. Additionally, reactant adsorption within hydrophobic Sn-Beta is driven by the breakup of intraporous solvent-solvent interactions, resulting in positive enthalpies of adsorption that are partially compensated by an increase in the solvent reorganization entropy. These results emphasize how hydrophobic zeolite pores order intraporous H-bonded alcohol networks, similar to how enzymes use hydrophobic and hydrophilic residues to regulate solvent structure, and highlight the importance of framework polarity on catalysis in the liquid-phase. Moreover, the ability of the zeolite pore to control the structure of confined nonaqueous solvents offers a unique dimension for the design and engineering of microporous adsorbents and catalysts beyond the active site.