Palladium-Mediated Hydrogenation of Unsaturated Hydrocarbons with Hydrogen Gas Released during Anaerobic Cellulose Degradation

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Among five hydrogenation catalysts, palladium on charcoal was the most reactive one when suspended in anaerobic culture medium, and Lindlar catalyst (Pd on CaCO3) was the most reactive one when suspended in the gas phase of culture tubes. Palladium on charcoal in the culture medium (40 to 200 mg 10 ml−1) completely inhibited growth of Neocallimastix frontalis and partly inhibited Ruminococcus albus. Lindlar catalyst (40 to 200 mg per tube) suspended in a glass pouch above the culture medium did not affect the rate of cellulose degradation or the ratio of fermentation products by these organisms. Acetylene added to tubes containing Lindlar catalyst in pouches, and either of the two organisms in monoculture or coculture with Methanospirillum hungatei, was reduced to ethylene and then ethane, followed by hydrogen production. Similar results were obtained with 1-pentene. Neither acetylene nor 1-pentene affected cellulose degradation but both inhibited methanogenesis. In the presence of Lindlar catalyst and propylene or 1-butene, fermenter-methanogen cocultures continued to produce methane at the same rate as controls and no olefin reduction occurred. Upon addition of bromoethanesulfonic acid, methanogenesis stopped and olefin reduction took place followed by hydrogen evolution. In a gas mixture consisting of propylene, 1-butene, and 1-pentene, the olefins were reduced at rates which decreased with increasing molecular size. These results demonstrate the technical feasibility of combining in one reactor the volatile fatty acid production by anaerobic digestion with chemical catalyst-mediated reductions, using the valuable by-product hydrogen.

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