Para-xylene maximization: Part IX—Activation of toluene methylation catalysts with palladium

The importance of increasing para-xylene production as a primary material for producing polyester fibers has activated this research. Although alkylation is an acid catalyzed reaction, we found that Pd can act as a promoter to activate this reaction by virtue of the presence of a vacant d-orbital in the Pd atom. In this work, a series of catalysts containing 0.1, 0.2 or 0.3%Pd in H-ZSM-5 zeolite is examined for alkylating toluene with methanol, aiming to maximize para-xylene at temperature range 300–500 °C in hydrogen using a continuous-flow fixed-bed reactor. The acid site strength distribution of the catalysts was characterized by temperature programmed desorption (TPD) of ammonia, and palladium dispersion in the catalysts by hydrogen chemisorption. The diffusion extent, in the current catalysts during the current reaction was compared via estimation of the Thiele modulus (ΦL). The para-xylene selectivity was found to increase systematically with increasing the Pd content in the catalysts, although, para-xylene production was the highest on the 0.2%Pd/H-ZSM-5 catalyst. The heavy undesired trimethylbenzenes were the lowest to be formed on the 0.3%Pd containing catalyst.