On the mechanism of aromatic acylation over zeolites

Two possible mechanisms have been proposed for liquid phase acylation reactions using Brønsted acid zeolite catalysts such as H-Beta. For the acylation of anisole by acetic anhydride, Corma et al. [A. Corma, M.J. Climent, H. Garcia, J. Primo, Applied Catalysis 49 (1989) 109] have suggested that an acylium cation is formed by the protonation of an acyl species interacting with the proton of the zeolite, and that the acylium species attaches to the nucleophilic aromatic ring to form the acylated product. Based on desorption studies [M.L.M. Bonati, R.W. Joyner, M. Stockenhuber, Catalysis Today 81 (2003) 653; M.L.M. Bonati, R.W. Joyner, G.S. Paine, M. Stockenhuber, Studies in Surface Science and Catalysis 154C (2004) 2724], we have suggested that the acylating agent is ketene, which we have shown is formed in situ by the decomposition of acetic anhydride into acetic acid and ketene. The electrophilic ketene then reacts with the anisole in a rather similar manner to the acylium species proposed in the Corma mechanism. We show that these two mechanisms can be differentiated using deuterated reagents, and report on a series of experiments that show that the acylium cation mechanism is to be preferred in the liquid phase acylation at 333 K. We argue, however, that oligomerisation of ketene within the zeolite pores occurs in parallel to the acylation, and plays an important role in the deactivation (poisoning) of the H-Beta catalyst.