(337c) Catalytic Conversion of Polyols to Olefins

Sharkey, B. E. - Presenter, University of Massachusetts
Jentoft, F. C., University of Massachusetts

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 ReOx
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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