Investigation of Hydroisomerization Catalysts and Reaction Conditions to produce Branched-Chain Fatty Acids

Abstract

Fatty acids (FAs) from vegetable oils can be used as feed to produce renewable hydrocarbons and bio-lubricants. Naturally occurring FAs are, however, susceptible to oxidative attack and have operating temperature limitations due to poor cold flow properties. Converting FAs into saturated branch-chain isomers would alleviate these restrictions. Combining the hydrogenation and skeletal isomerization of C=C double bonds in FAs required to form saturated branch-chain FAs into one single hydroisomerization reactor would significantly reduce the complexity and costs of production of these renewable hydrocarbons. This project thus investigated possibility of hydroisomerization of oleic acid to produce saturated branched-chain FA isomers in a single reactor by developing and testing hydroisomerization catalysts. In this process, acidic solid surfaces promote isomerization reactions, while hydrogenation is catalyzed by active metals. Bifunctional Pt on H+-Beta zeolite catalysts were synthesized through incipient wetness impregnation and tested in a batch reactor. Hydroisomerization was unsuccessful as hydrogenation of double bonds was favoured over the competing skeletal isomerization reaction, resulting in a saturated straight-chain FA. The melting point of products increased from 13 °C to 62 °C. Due to these findings, preliminary exploration of effects on skeletal isomerization of oleic acid to form bc-isomers was instead investigated by omitting platinum. Reactions were conducted in the presence of N2 or H2 gas at varying pressures. Triphenylphosphine was added to some experiments to block the external acid sites of the zeolite, and deionized water was added as a co-catalyst. Four zeolites of interest, 12.5 silica-alumina ratio H+-Beta, 19 silica-alumina ratio H+-beta, SAPO-11 and MCM-41, were also tested. These zeolites represented varying pore sizes and acidity (Silica to Alumina ratio). It was shown that the pore size has a greater effect on catalytic activity than acidity. A maximum bc-isomer yield of 63 wt% was achieved with 5 wt% H+-Beta zeolite in oleic acid under a H2 pressure of 15 bar and reactor set-point of 280 ºC, stirred for 6 h in a batch reactor. Product analysis indicated that trans-isomers and lactones were also produced as by-products of the skeletal isomerization.