The hydrogenation of 1,3-pentadiene over an alumina-supported palladium catalyst: an FTIR study

The hydrogenation of a mixture of cis- and trans-1,3-pentadiene over a 1% Pd/Al2O3 catalyst at 303 K has been studied using infrared spectroscopy to monitor the changes in the composition of the gas phase over the catalyst as a function of time. The reaction is seen to occur as a consecutive process, with the terminal double bond hydrogenated in advance of the internal double bond. Vibrational assignments have been confirmed through ancillary calculations for a number of C-5 molecules. The reaction profile is consistent with the catalyst presenting two distinct reaction sites: hydrogenation of the terminal double bond occurs at Site alpha, whilst Site beta is responsible for hydrogenation of the internal double bond. Trans-pent-2-ene is identified as the only reaction intermediate. From comparative studies of the hydrogenation of pentenes over the catalyst, the absence of any cis-pent-2-ene in the reaction mixture is tentatively attributed to cis-1,3-pentadiene isomerising at Site alpha to form trans-1,3-pentadiene. The effect of toluene-d(8) to act as a chemical modifier was also investigated and shown to selectively poison Site beta, Site alpha being unperturbed.