(703b) Effects of Zeolite Framework on Direct Reduction of Ester to Ether Using H2 over Pt-Zeolite Catalysts

The direct reduction of esters using H₂ from biomass-derived esters offers a sustainable method to produce ethers. This chemistry occurs on homogeneous catalysts and costly metal hydrides and has not been reported in the heterogenous catalysis sector. We recently demonstrated that the direct hydrogenation of propyl acetate by H₂ can produce ethyl propyl ether with selectivities between 6-18% over heterogeneous catalysts, Pd nanoparticles supported on Nb₂O₅ and WO₃ having Brønsted acid sites. More recently, metal identity studies over metal-incorporated FAU having Brønsted acidic sites (i.e., Pt-, Pd- and Ru-FAU) demonstrated that Pt-FAU results in much greater rates and ether selectivities (34%) among tested metals.

In this contribution, we investigated zeolite framework effects on ether formation chemistry using Pt-based zeolites. We synthesized Pt-zeolite catalysts with varying pore diameters and channel dimensionality (Pt-CHA, Pt-BEA, Pt-MOR, Pt-FER, Pt-MFI, Pt-FAU). The ether formation rate over large pore Pt-zeolite (Pt-FAU) is an order of magnitude higher than another smaller pore Pt-zeolite (e.g., Pt-MFI, and -BEA). The heat of adsorption and activation energy explain the ether formation rate trend: dispersive interactions between the transition state with surrounding molecules (ester or ester-derived intermediate) within zeolite pore stabilize the transition state, thus ether formation rates are improved. Lower channel dimensionality increases the possibility that the reactant meets the metal site first to facilitate the ether formation pathway compared to acid-catalyzed side reactions because ether formation is a metal-acid reaction whereas side reactions (e.g., hydrolysis and transesterification) are acid-catalyzed reactions. Consequently, the lower channel dimensionality zeolites (e.g., Pt-FER) have high ether selectivity of up to 50%.

Pt-based zeolite would be an attractive material for ether formation because it improves selectivities of up to 50% of the highest ether selectivity ever achieved in the flow reactor. The DOW Chemical Company financially supported this research, filling the patent.