Hydrothermal Catalytic Deoxygenation of Fatty Acids and Upgrading Algae Biocrude.

Abstract

This work addresses the production of renewable liquid transportation fuels from algae. Pt/C and PtSnx/C were used to deoxygenate free fatty acids, model compounds for components in biocrude oil, from liquified algae. All reactions were conducted in the aqueous phase.

Pt/C is an effective decarboxylation catalyst in the hydrothermal environment, but studies examining catalyst longevity are limited. Catalyst deactivation studies with Pt/C and butyric acid in a continuous flow reactor revealed a first order deactivation rate constant, kd, of 0.063+/-0.006 1/h. A combination of coking and molecular poisoning was responsible for the deactivation of the catalyst. Diffuse reflectance infrared Fourier transform spectroscopy and gas chromatography suggested the poison was an unsaturated C3 molecule.

Pt/C is effective for decarboxylating saturated fatty acids to the alkane products, but it suffers from low selectivity for unsaturated fatty acids, which are more abundant in plant-derived oils, in the absence of H2. Rather than decarboxylating the unsaturated feed to a hydrocarbon, the unsaturated acid hydrogenates to the fully saturated fatty acid. PtSnx/C catalysts showed selectivities up to three times higher than Pt for oleic acid (C18:1) decarboxylation and two times higher for linoleic acid (C18:2) decarboxylation. Even with the addition of Sn into the alloy system, however, catalyst activity and selectivity still declined with increasing fatty acid unsaturation. Interestingly, with both Pt and PtSnx in the absence of external H2, the resulting fuel molecule was fully saturated. Experiments in D2O showed that water served as a source of hydrogen.

More generally, many catalysts are unstable in the hydrothermal environment. To have societal impact, the catalysts need to be both active and stable for biocrude oil rather than just model compounds. Some catalyst metal and support combinations were chosen to upgrade algae biocrude oil based on previous activity towards deoxygenation and denitrogenation of model compounds found in biocrude oils and stability in near critical, supercritical water, and high temperature steam. The presence of H2 shifted the products from catalytically upgraded biocrude towards heavier, diesel appropriate molecules whereas the absence of H2 resulted in a wider range of hydrocarbons suitable for liquid fuels from gasoline to diesel.