Tungsten-Promoted Mesoporous Group 4 (Ti, Zr, and Hf) Transition-Metal Oxides for Room-Temperature Solvent-Free Acetalization and Ketalization Reactions

Abstract

Tungsten (0–30%)-promoted University of Connecticut mesoporous materials (UCT-X, X = 55 (Ti), 50 (Zr), and 56 (Hf)) were synthesized in one step using a recently discovered approach. The approach relies on inverse micelle formation and unique NOx chemistry to control the sol–gel chemistry of inorganic components. The tungsten-promoted mesoporous transition-metal (TM) oxides consist of monodispersed nanocrystalline particle aggregates with a particle size of ∼12 nm. The mesopores are formed by connected intraparticle voids of these aggregates, and the pore sizes vary between 3.8 and 6.1 nm, depending on the tungsten loading and TM oxide support. The materials were closely examined using PXRD, HRTEM, XPS, FTIR, Raman, and N2 sorption. The tungsten exist as very small (<1 nm) nano-WOx clusters that are well-dispersed on the TM support. Nano-WOx clusters were found to promote the formation of Brønsted acid sites. Catalytic activity was tested for solvent-free room-temperature acetalization and ketalization reactions. A 20% tungsten loading was found to be the optimum loading for the reactions. The catalyst can convert benzaldehyde with a conversion of 99% and yield of 93% to the corresponding acetal and can convert cyclohexanone with a conversion of 96% and yield of 94% to the corresponding ketal. The strong adsorption of products and intermediates on the mesopores was found to be the reason for the moderate performance in the conversion of other substrates.