Transition metal-catalyzed deoxydehydration: missing pieces of the puzzle

Abstract

Deoxydehydration (DODH) is a transformation that converts a vicinal diol into an olefin with the help of a sacrificial reductant. The reaction has drawn interest in the past 25 years for its potential to upgrade polyols from biomass to chemicals or fuels. This minireview is organized in 7 sections, and, while providing a comprehensive survey of the literature in tabular form, focuses on aspects that are not extensively discussed in prior reviews. The first three brief sections consist of an introduction to DODH, followed by an overview of present research thrusts and a listing of prior reviews and the patent literature. The fourth section addresses reaction thermodynamics. The fifth section provides a survey of catalysts investigated for DODH, most of which are rhenium, molybdenum, and vanadium compounds. These catalysts have been used in heterogeneous and homogenous catalysis alike. The catalyst compositions are discussed including the effect of counterions that are not part of the active metal moiety. The sixth section reviews rate laws that have been formulated and the steps identified as rate controlling, by experiment or theory. Catalyst reduction and olefin extrusion emerge as key steps. In this context, the somewhat mysterious trends among alcohol reductants are inspected. It emerges that the DODH field would benefit from benchmark reactions that will quantitatively connect the collected catalytic data. The seventh section considers phase chemistry, separations, and energy input.