Ru/C-Catalyzed Hydrogenation of Aqueous Glycolic Acid from Microalgae - Influence of pH and Biologically Relevant Additives

Ethylene glycol (EG) is obtained by a novel, two-step approach combining a biotechnological and a heterogeneously catalyzed step. First, microalgae are cultivated to photobiocatalytically yield glycolic acid (GA) by means of photosynthesis from CO2 and water. GA is continuously excreted into the surrounding medium. In the second step, the GA-containing algal medium is used as feedstock for catalytic reduction with H-2 to EG over a Ru/C catalyst. The present study focuses on the conversion of an authentic algae-derived GA solution. After identification of the key characteristics of the algal medium (compared to pure aqueous GA), the influence of pH, numerous salt additives, pH buffers and other relevant organic molecules on the catalytic GA reduction was investigated. Nitrogen- and sulfur-containing organic molecules can strongly inhibit the reaction. Moreover, pH adjustment by acidification is required, for which H2SO4 is found most suitable. In combination with a modification of the biotechnological process to mitigate the use of inhibitory compounds, and after acidifying the algal medium, over Ru/C a EG yield of up to 21 % even at non-optimized reaction conditions was achieved.