A periodic density functional theory study of the alkylation of benzene with propene in proton-exchanged mordenite has been achieved. The two different reaction routes that are usually proposed for this reaction, namely the direct and the step-by-step reaction pathways, have been investigated. The explicit consideration of the zeolite catalyst framework allows a better level of description of the interactions between the catalyst framework and the reaction than what is obtained with the cluster approach method. The direct reaction route is found to be the preferred one. It is observed that the cluster approach method, which does not describe the zeolite framework, is unable to qualitatively described the trend in activation energies. This is owing to the greater stabilization of larger transition state by the mordenite zeolite framework compared with smaller ones.