Vapor phase coupling of n-butanol over the mixed catalyst system PdZn/SiO2 + TiO2

Abstract

Coupling fermentation derived oxygenates via Guerbet-type reactions offers a potential route for producing fuels and chemicals from agricultural feedstocks. In this work the vapor phase reactions of n-butanol over a bimetallic PdZn/SiO₂ catalyst and physical mixtures of PdZn/SiO₂ and TiO₂ were studied. The bimetallic catalyst was highly selective for n-butanol dehydrogenation without the subsequent decarbonylation of butanal which is characteristic of monometallic Pd nanoparticles. When combined with TiO₂, a known aldol condensation catalyst, the bifunctional system performs Guerbet-type coupling reactions and produces mixtures of C₈ oxygenates and higher-order products including C₇, C₈, and C₁₂ hydrocarbons. Results show that within the reaction network PdZn/SiO₂ performs dehydrogenation/hydrogenation reactions and decarbonylates C₈ aldehydes to form C₇ hydrocarbons. TiO₂ catalyzes aldol condensation and alcohol dehydration reactions responsible for producing C₈ and C₁₂ hydrocarbons. Based on the developed understanding of the function of each catalyst, it was shown that increasing the Brønsted acidity of the TiO₂ catalyst resulted in an increase in the production of C₈ hydrocarbons relative to C₁₂ hydrocarbons. This work demonstrates the ability of bimetallic Pd-based catalysts that are selective for alcohol dehydrogenation to participate in Guerbet-type coupling reactions and that their combination with an appropriate aldol condensation/dehydration catalyst is an effective strategy to produce higher molecular weight oxygenates and hydrocarbons from renewable resources.