Para-xylene Maximization—Part VI: Shape-Selective Platinum-Promoted Methylation of Toluene

摘要

The need for increased production of para-xylene, which is the primary material for producing the polyester fibers, activated this research. Although the alkylation reaction is acid catalyzed, we found that a Pt promoter activates this reaction by virtue of the presence of a vacant d-orbital in the Pt atom. In this work, a series of catalysts containing 0.1, 0.2, or 0.3% Pt in H-ZSM-5 zeolite was tested for alkylating toluene with methanol, aiming to produce the xylenes and maximizing para-xylene production in a temperature range of 300°C-500°C in the presence of hydrogenflow in a continuous-flow fixed-bed reactor. The catalysts were characterized by temperature programmed desorption (TPD) of ammonia for acid sites distribution analysis and platinum dispersion in the catalysts by hydrogen chemisorption. Moreover, the diffusion resistance extent in the current catalysts during the alkylation reaction has been evaluated via estimation of the Thiele modulus, Ф_L. The selectivity for para-xylene production was found to increase systematically with increasing the Pt content in the catalysts, whereas the unloaded zeolite did not follow this order. The Ф_L values calculated were accordingly found to increase also with increasing Pt content in the catalysts. Although para-xylene was the highest on the 0.3% Pt/H-ZSM-5 catalyst, the heavy undesired trimethylbenzenes were the lowest to be formed on this catalyst.