Transhydrogenation of pentane with 1,5- and 2,4-hexadiene over CrOx/Al2O3

Abstract

Transhydrogenation of pentane (P) and 1,5-hexadiene (1,5HD) and pentane and 2,4-hexadiene (2,4HD) was studied over a CrO_x/alumina catalyst at 523–773?K. Thermodynamic stability differences between the conjugated (2,4-hexadiene) and non-conjugated (1,5-hexadiene) isomers indicated that transhydrogenation was favoured between pentane and 1,5-hexadiene but not pentane and 2,4-hexadiene (+?ve ?G). At 773?K a significantly enhanced alkene yield was observed for the P/1,5HD system, clearly showing the effect of transhydrogenation. The yield of alkenes was?~?50% and included alkylated and isomerized alkenes. Alkylation and isomerization were significant reactions under reaction conditions. Pentane was shown to affect the chemistry of 1,5HD and vice versa with the conversion of pentane significantly enhanced at all reaction temperatures, indicating a molecular interaction between the reactants even when transhydrogenation was not obvious. In contrast, no effect on the conversion of pentane was observed when the co-feed was 2,4HD. An unexpected effect of pentane on 2,4HD conversion was observed, with all reactions of cis-2,4-hexadiene (including alkylation and isomerization) being completely inhibited at low reaction temperatures (573?K and 523?K) by the presence of pentane, suggesting that pentane competes for the same sites as cis-2,4-hexadiene. Transhydrogenation activity between pentane and 1,5-hexadiene was less obvious at the lower reaction temperature, which appeared to be a kinetic effect. Direct hydrogenation of 1,5-hexadiene revealed that 1,5HD sampled the same hydrogen population for hydrogenation and transhydrogenation. Comparisons of transhydrogenation of 1-hexyne, 1,5-hexadiene, and 2,4-hexadiene with pentane have revealed significant differences in the adsorption and reaction chemistry of the three isomers.