Catalytic Conversion of Polyols to Olefins

Catalytic Conversion of Polyols to Olefins

Bryan E. Sharkey and Friederike C. Jentoft^*

Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003, USA

*Corresponding author: fcjentoft@umass.edu

Hydroxyl groups are abundant in the cellulosic fraction of lignocellulosic biomass. To obtain fuels or a wider variety of chemicals from lignocellulose-derived feedstocks, the hydroxyl groups in these molecules must be removed or replaced by other, more useful functionalities. This conversion is best achieved after hydrolysis of biomass, which delivers a stream consisting of sugars or, after hydrogenation, sugar alcohols. Neighboring OH groups in sugars or sugar alcohols can be converted via deoxydehydration (DODH), that is, the simultaneous removal of one oxygen atom and a molecule of water (Eq. 1). The product is an olefin and thus a versatile chemical intermediate. The oxidation of the reductant, which is needed in stoichiometric quantities, and the elimination of a molecule of water make the reaction thermodynamically feasible.

R-CHOH-CH₂OH + Red à R-CH=CH₂ + RedO + H₂O          (Eq. 1)

Original catalysts for this conversion were molecular (homogeneous) and include high-oxidation-state rhenium,^1,2 molybdenum³, or vanadium complexes.^4,5 Heterogeneous catalysts have been reported only in recent years, and exclusively contain rhenium as the active transition metal species.^6,7 These catalysts suffer from leaching and contain expensive rhenium. In the present work, a second generation of rhenium-containing catalysts and a first generation of molybdenum-containing catalysts were developed and characterized.

Perrhenate or molybdate were deposited on various oxidic supports via incipient wetness impregnation or equilibrium adsorption of ammonium salts. The materials were optionally calcined. The conversion of diols was performed in benzene at temperatures of 135 to 200 °C, using various reductants including triphenylphosphine. The catalysts were characterized by various spectroscopic methods.

Catalysts consisting of oxide-supported ReO_x were highly selective in converting alkane diols to the respective terminal olefins. Decanediol could be converted to decenes with yields of more than 80% (at 100% diol conversion). Differences in conversion and yield could in part be explained by adsorption of both the diol and the olefin on the supports. The reaction progressed rapidly and selectively at 150 °C, whereas at higher temperature (175 °C), double bond shift occurred and internal olefins were produced in addition to 1-decene. The reductant efficiency was excellent with the ratio of decenes to triphenylphosphine oxide being close to one. Titania and silica were identified as the best oxide supports with respect to product yields, surpassing oxides of iron, zirconium, and aluminum.

Leach tests indicated some loss of rhenium, in particular for silica-supported catalysts. Consistently, the activity of recovered catalysts was generally lower than that of fresh catalysts, but there was always residual activity even after several uses. Spectroscopic characterization gave some indication that the structure of the supported oxo-rhenium species determines the proclivity for leaching.

A first generation of molybdenum-containing catalysts showed promise although they were less active than rhenium catalysts, and a higher reaction temperature was required to achieve DODH of decanediol to decene. More side products were observed, which included partially deoxygenated molecules and heavy species.

The activity of both oxo-rhenium and oxo-molybdenum species supports the hypothesis that an active catalyst must contain a metal that can form a dioxo unit on the surface and can cycle between two states by changing its oxidation state by ± 2. Deoxydehydration is confirmed as an attractive route to convert 1,2 diols into α-olefins.

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