The feed or product may also be an aromatic hydrocarbon e.g. benzene or naphthalene, optionally substituted by at least one (e.g. 1-3) alkyl or alkenyl group e.g. of 1-20 carbons, such as 1-6 carbons, especially methyl, ethyl or isopropyl; examples are benzene, toluene xylene, cumene and styrene.