Transhydrogenation of pentane and 1-hexyne over CrOx/Al2O3 and potassium-doped CrOx/Al2O3 catalysts

The transhydrogenation of pentane (P) and 1-hexyne (1HY) was investigated over 4% CrO_x/Al₂O₃ and potassium-doped 4% CrO_x/Al₂O₃ catalysts over a range of temperatures (523–773?K) with a 5:1 P:1HY ratio. Over the CrO_x/Al₂O₃ catalyst, transhydrogenation clearly occurred at temperatures below 625?K where the yield of alkenes was higher for the co-fed system than for a combination of the individual yields. Due to the acidic nature of the alumina, many of the products were alkylated olefins and alkylated hydrocarbons formed by coincident alkylation and isomerisation. When pentane was added to a feed containing 1-hexyne, the extent of carbon deposition was reduced. By comparing transhydrogenation to limited hydrogen 1-hexyne hydrogenation at 623?K, it was shown that the processes of hydrogenation and transhydrogenation were different, with hydrogenation favouring alkanes, while transhydrogenation favoured alkenes. This may be because pentane dehydrogenation only releases two hydrogen atoms, which only allows 1-hexyne to hydrogenate to 1-hexene. Therefore, if the rate of alkene isomerisation and desorption is faster than that of pentane dehydrogenation, only alkenes will be observed. The latter proposal would suggest that the dehydrogenation/hydrogenation process is closely coupled and would be consistent with pentane influencing 1-hexyne surface chemistry. The effect of the potassium doping was to increase the yield of alkenes. The reason for this may be related to changes in the nature of the surface chromia species. The potassium also neutralised the acid sites on the alumina, reducing the extent of alkylation and hydrogenolysis, which suppressed the formation of other alkynes in the product mix.