Ethylbenzene is produced by alkylation over a split load of monoclinic silicalite alkylation catalysts having different silica/alumina ratios. A feedstock containing benzene and ethylene is applied to a multi-stage alkylation reaction zone having a plurality of series-connected catalyst beds. At least one catalyst bed contains a first monoclinic silicalite catalyst having a silica/alumina ratio of at least 275. At least one other catalyst bed contains a second monoclinic silicalite catalyst having a silica/alumina ratio of less than about 275. The alkylation reaction zone is operated at temperature and pressure conditions in which the benzene is in a gaseous phase to cause gas-phase alkylation of the aromatic substrate in the presence of the monoclinic silicalite catalysts to produce an alkylation product. The alkylation product is then withdrawn from the reaction zone for separation and recovery. The use of the split load of catalyst allows a higher purity ethylbenzene product to be produced at improved efficiencies than if only one of the catalysts were used by itself.