Direct formation of cyclohexene via the gas phase catalytic dehydrohalogenation of cyclohexyl halides

The gas phase dehalogenation of cyclohexyI chloride and cyclohexyl bromide (where 423 K less than or equal to T less than or equal to 523 K) promoted using silica and zeolite supported nickel catalysts in the presence of hydrogen is presented as a viable one step route for the production of cyclohexene. Cyclohexene is generated via the internal elimination of the corresponding hydrogen halide where the process is 100% selective at T less than or equal to 473 K. At higher temperatures, cyclohexane and benzene were isolated in the product mixture as a result of the combination of catalytic hydrogenolysis, hydrogenation and dehydrogenation steps. Cyclohexene yield was subject to a short term reversible decline with time-on-stream due to a surface poisoning by the hydrogen halide that was produced but the presence of hydrogen served to displace the inorganic halide and extend the productive lifetime of the catalyst. Bromine removal from cyclohexyl bromide was found to be more facile while the use of a higher loaded (15.2% (w/w) as opposed to 1.5% (w/w)) nickel silica is shown to result in appreciably higher dehydrohalogenation rates. Both Na/Y and Ni-Na/Y zeolites promoted cyclohexene formation but exhibited an irreversible deactivation which is attributed to pore blockage by occluded coke. With a view to optimising catalyst efficiency, the effect on cyclohexene yield of varying such process parameters as reaction time and temperature and reactant(s) partial pressures were studied and the catalytic data are compared with thermodynamic predictions. (C) 1999 Elsevier Science B.V. All rights reserved.