Palladium supported on bacterial biomass as a novel heterogeneous catalyst: A comparison of Pd/Al2O3 and bio-Pd in the hydrogenation of 2-pentyne

Nanoparticles of palladium supported on bacterial biomass, bio-Pd, were tested for their catalytic activity in the hydrogenation of 2-pentyne in a stirred tank reactor. The rates of reaction and the selectivity obtained using the novel materials were compared to those obtained with a conventional heterogeneous catalyst, palladium on alumina, under various reaction conditions. Under the same conditions, the initial rate of reaction over a 5 wt% bio-Pd catalyst in isopropanol was only 30% of that for a 5 wt% Pd/Al2O3 catalyst. However, the biomass-supported showed a high selectivity, displaying quantitative conversions of alkyne after 5 h with maximum product ratios of pentene/pentane in the range of 8-14 and cis/trans product ratios in the range of 6-14. Although the conventional 5% Pd/Al2O3 catalyst gave very high initial pentene/pentane and cis/trans ratios (26 and 10), these values fell rapidly over the course of the reaction and became slightly lower than those observed with bio-Pd at alkyne conversions greater than 70%. For example, at 92% alkyne conversion, the bio-Pd catalyst gave a cis/trans ratio of 2.5 and pentene/pentane ratio of 3.3, as opposed to respective values of 2.0 and 2.0 with 5% Pd/Al2O3. This is the first time that selectivity has been demonstrated for the bio-Pd in a multi-product reaction, thus showing promise of this material for use in catalytic syntheses where it is desirable to maximise yield of a specific component. The type of bacteria used as support in the 2-pentyne hydrogenation experiments was Desulfovibrio desulfuricans. The metal particles grow within the cell envelope, are regularly dispersed and are of uniform particle size, of similar to 1.7 nm as determined by chemisorption. The bio-Pd is also easily separated from the product mixture and remained active and selective when reused in a subsequent hydrogenation. (C) 2009 Elsevier Ltd. All rights reserved.