Effect of cofeeding n-butane with methanol on aromatization performance and coke formation over a Zn loaded ZSM-5/ZSM-11 zeolite

Abstract

The aromatization performance of cofeeding n-butane with methanol was investigated in a fixed bed reactor over a Zn loaded ZSM-5/ZSM-11 zeolite. Coke deposited on the catalyst was characterized by TG, TPO, FTIR and Raman technique. During the 48h life-test under the conditions of 510°C, 0.4MPa, WHSV (CH2)=2h−1, pure n-butane reaction performance of the catalyst changes little, but high LPG and low aromatics yields are obtained. Only under the suitable n-butane/methanol ratio, the relatively good aromatics selectivity can be obtained and maintained during the reaction process. Compared with methanol aromatization, the integrated process exhibits a much slower rate of deactivation. Addition of methanol in n-butane inhibits the dry gas production while promotes the LPG production. Water generated from methanol dehydration may occupy the catalytic active sites for olefins aromatization over the catalyst and thus suppress the production of total aromatics. In fact, water mainly suppresses benzene formation and this inhibiting effect increases with n-butane content in the feed, in contrast to its promoting effect on formation of C8 and C9 aromatics. Increasing methanol content in the feed, a simultaneous increase in coke content of the spent catalyst and preference for coke deposition in micropore is observed. Furthermore, the fraction of aliphatics in coke decreases, while those of less-condensed aromatics and polyaromatics incease, which is associated with the increased deactivation of catalyst.