Lipid formation by aqueous Fischer-Tropsch-type synthesis over a temperature range of 100 to 400 degrees C.

The formation of lipid compounds during an aqueous Fischer-Tropsch-type reaction was studied with solutions of oxalic acid as the carbon and hydrogen source. The reactions were conducted in stainless steel vessels by heating the oxalic acid solution at discrete temperatures from 100 to 400 degrees C, at intervals of 50 degrees C for two days each. The maximum lipid yield, especially for oxygenated compounds, is in the window of 150-250 degrees C. At a temperature of 100 degrees C only a trace amount of lipids was detected. At temperatures above 150 degrees C the lipid components ranged from C12 to > C33 and included n-alkanols, n-alkanoic acids, n-alkyl formates, n-alkanals, n-alkanones, n-alkanes, and n-alkenes, all with essentially no carbon number preference. The n-alkanes increased in concentration over the oxygenated compounds at temperatures of 200 degrees C and above, with a slight reduction in their carbon number ranges due to cracking. It was also noted that the n-alkanoic acids increased while n-alkanols decreased with increasing temperature above 200 degrees C. At temperatures above 300 degrees C synthesis competes with cracking and reforming reactions. At 400 degrees C significant cracking was observed and polynuclear aromatic hydrocarbons and their alkylated homologs were detected. The results of this work suggest that the formation of lipid compounds by aqueous FTT reactions proceeds by insertion of a CO group at the terminal end of a carboxylic acid functionality to form n-oxoalkanoic acids, followed by reduction to n-alkanoic acids, to n-alkanals, then to n-alkanols. The n-alkenes are intermediate homologs for n-alkan-2-ones and n-alkanes. This proposed mechanism for aqueous FTT synthesis differs from the surface-catalyzed stepwise FT process (i.e., gaseous) of polymerization of methylene reported in the literature.