Chlorine-nickel interactions in gas phase catalytic hydrodechlorination: catalyst deactivation and the nature of reactive hydrogen

The gas phase hydrodechlorination of chlorobenzene and 3-chlorophenol (wher e 473 K less than or equal to T less than or equal to 573 K) has been studi ed using a 1.5% w/w Ni/SiO2 catalyst which was also employed to promote the hydrogenation of benzene, cyclohexene and phenol. In the former two instan ces the catalyst was 100% selective in removing the chlorine substituent, l eaving the aromatic ring intact. While the dechlorination of chlorobenzene readily attained steady state with no appreciable deactivation, the turnove r of 3-chlorophenol to phenol was characterised by both a short and a long term loss of activity. Chlorine coverage of the catalyst surface under reac tion conditions was probed indirectly by monitoring, via pH changes in an a queous NaOH trap, HCl desorption after completion of the catalytic step. Co ntacting the catalyst with the chlorinated reactants was found to severely limit and, depending on the degree of contact, completely inhibit aromatic ring reduction although a high level of hydrodechlorination activity was ma intained. Hydrogen temperature programmed desorption (TPD) reveals the exis tence of three forms of surface hydrogen which are tentatively assigned as: (i) hydrogen bound to the surface nickel; (ii) hydrogen at the nickel/sili ca interface; (iii) spillover hydrogen on the silica support. The effect of chlorine-nickel interactions on the resultant TPD profiles is presented an d discussed. The (assigned) spillover hydrogen appears to be hydrogenolytic in nature and is responsible for promoting hydrodechlorination while the h ydrogen that is taken to be chemisorbed on, and remains associated with, th e surface nickel metal participates in aromatic hydrogenation. Hydrodechlor ination proceeds via an electrophilic mechanism, possibly involving spillov er hydronium ions. The experimental catalytic data are adequately represent ed by a kinetic model involving non-competitive adsorption between hydrogen and the chloroaromatic, where incoming chloroaromatic must displace the HC l that remains on the surface after the dechlorination step. Kinetic parame ters extracted from the model reveal that chlorophenol has a higher affinit y than chlorobenzene for the catalyst surface but the stronger interaction leads to a greater displacement of electron density at the metal site and t his ultimately leads to catalyst deactivation.