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 ∼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.