An interesting phenomenon also appeared in the study of proximity effect: the content of 3-ethyltoluene in the oil phase was significantly higher than that of 4-ethyltoluene (Fig. ). It has been reported that the nonselective isomerization reaction at the acidic sites on the zeolite surface is generally considered to be the main reason for the decrease in the selectivity of para-aromatic products in the toluene alkylation reaction . Therefore, 3-ethyltoluene might also be formed by isomerization of 4-ethyltoluene on the surface of Z5.