Improved atom efficiency via an appreciation of the surface activity of alumina catalysts: Methyl chloride synthesis

The hydrochlorination of methanol over a commercial grade η-alumina catalyst has been investigated via temperature-programmed reaction (TPR). Experiments were conducted over a temperature range of 295–1000 K using: (i) a methanol-only feed stream and (ii) a methanol/HCl mixed feed stream in a 1:1 mole ratio. Methanol-only studies showed activity for the formation of dimethyl ether (DME) above 450 K, consistent with temperature-programmed desorption studies. A rapid decline in DME activity and consumption of methanol at higher temperatures were observed and are attributed to a deactivation pathway, involving the conversion of methoxy species to surface formate species. At elevated temperatures these processes lead to the deposition of carbon on the substrate. The introduction of HCl to the reaction stream resulted in the formation of methyl chloride (MC) over a temperature range of 400–750 K. Relatively small quantities of DME by-product were also observed over a similar temperature range. The coincidence of the reaction profiles of both MC and DME imply that the same reactive methoxy species are involved in both processes. The rate of reaction for the formation of both products declines rapidly above 700 K. This is attributed to the methoxy decomposition pathway to formate and, ultimately, to carbon retention by the catalyst.The effect of carbon deposition was also investigated via a subsequent cooling process carried out immediately after TPR experiments. Reaction profiles showed comparable activity for the formation of both products with respect to initial TPR experiments. Taken together, these results indicate an optimum reaction temperature for the formation of methyl chloride from the hydrochlorination of methanol. At such temperatures, carbon deposition is minimized and high conversion is maintained.