Alkali promotion of alumina-supported ruthenium catalysts for hydrogenation of levulinic acid to γ-valerolactone

Gamma-valerolactone is a biomass-derived chemical with high potential as a platform intermediate that can be readily synthesized by hydrogenation of levulinic acid, which is a common biomass intermediate, using supported ruthenium catalysts. In previous work we demonstrated that the addition of potassium to supported ruthenium catalysts dramatically enhanced the ruthenium activity for levulinic acid hydrogenation. In this work, a systematic study was carried out with alkali metal promoters Na+, K+, and Cs+, to study the origin of promotion and to determine the most active alkali metal and the optimal ratio of promoter to ruthenium. Series of catalysts were prepared by impregnating various amounts of alkali onto 2 wt% Rut alumina. Catalysts were characterized by H-2 chemisorption, X-ray photoelectron spectroscopy, CO Fourier transfer infrared spectroscopy, and Pyridine-Fourier transfer infrared spectroscopy and then evaluated for levulinic acid hydrogenation. Strong promotional effects in the form of rate increases of up to one order of magnitude with no change in selectivity were observed for all alkali metals. With the same atomic ratio of dopant to ruthenium, the promotion follows the order: Na+ < K+ < Cs+. Alkali promoters enrich the electron density of the ruthenium surface in proportion to their polarizability, with the best promoter being Cs+, for which the rate per gram of catalyst increased by one order of magnitude with the addition of 3 wt% Cs. For each alkali metal, ruthenium electron density increased in proportion to the loading, but for the K+ and Cs+ series, the rate of reaction turned down at the highest loadings, suggesting that for these two more highly interacting promoters, ruthenium site blockage plays a stronger role in the rate than does electronic alteration of ruthenium. (C) 2017 Elsevier Inc. All rights reserved.