Effect of acid modification of ZSM-5 catalyst on performance and coke formation for methanol-to-hydrocarbon reaction

To study effects of different acid treatments with HZSM-5 zeolites on the chemistry of framework, acid nature, performance, and coke formation for methanol-to-hydrocarbon reaction, HZSM-5 with different Si/Al ratio were post treated with two different acid solutions (tartaric acid and ammonium hexafluorosilicate). After the post treatment with hexafluorosilicate (HZn-HF), it was found the extensive decrease in all kinds of acid sites owing to the non-selective dealumination, whereas the noticeable decrease in weak Lewis acid sites was found after the post treatment with tartaric acid (HZn-TA) resulted by the selective removal of the surface alumium. The result of the distingushed removal of Al from the bare HZSM-5 zeolites (HZn) also attributed the change in product selectivity for the MTH reaction as follows; high carbon distributions to the C2-C4 range olefins and low carbon distribution to C10+ species from the HZn-HF samples whereas high carbon distribution to BTEX from the HZn-TA samples. Analysis of the spent catalysts revealed the amount and chemistry of carbon deposits formed during the MTH reaction are directly related to the acidic nature of the HZSM-5 catalysts. For example, the amount of carbon deposit from the spent catalysts were found to be increased in the order: HZn-TA > HZn > HZn-HF. Therefore, among the bare and acid modified HZ catalysts, the HZn-HF type catalyst showed the least carbon deposition despite requiring the longest reaction time for the methanol conversion to reach 80%.