Sulfonic Acid-Functionalized Zeolite Beta: Bronsted Acid Catalysts for Reactions Involving Liquid Water

The utility of zeolites in biomass upgrading reactions can be limited in cases where stronger Bronsted acidity and/or hydrophobicity are required for effective catalysis. While sulfonated solids/sulfonic acid catalysts can be used in such cases, they forego the advantageous effects of confinement that are afforded by zeolites. Here, acidity, hydrophobicity, and confinement are integrated into a solid material by functionalizing the intrapore voids of zeolite Beta with phenethyl-sulfonic acid sites (PE-Betas). These PE-Betas, along with conventional H-Betas, mesoporous phenethyl-sulfonic acid-functionalized SiO2 gel (PE-SiO2), and commercial sulfonic acid catalysts (para-toluenesulfonic acid, Nafion NR50) are evaluated in the hydroxyalkylation/alkylation reaction of 2-methylfuran with acetone. The PE-Betas show superior turnover numbers (TON) due to the combination of high acid site strength and hydrophobicity that arises from the confined PE groups. Further, the hydrophobicity and TON over PE-Betas are invariant with acid site density, showing that these materials effectively decouple the active site density–hydrophilicity relationship that is a fundamental limitation of conventional zeolites. These sulfonic acid-functionalized zeolites represent a distinct class of water-tolerant, strong Bronsted acid catalysts that may be well suited for a wide range of biomass upgrading reactions that generate stoichiometric amounts of liquid water.