Microwave-assisted hydrothermal synthesis of submicron ZSM-22 zeolites and their applications in light olefin production

There is an increased demand for facile and rapid synthesis of one-dimensional pore zeolites with controlled crystal size in the nanometer range. Here, we report for the first time, the rapid synthesis of ZSM-22 (TON) by using microwave-assisted hydrothermal synthesis (MAHyS) to control nucleation and growth. ZSM-22 crystals were synthesized rapidly in 24 h (instead of 72 h in conventional synthesis). Effect of synthesis time under microwave irradiation on the crystallinity of TON zeolites from 8 to 24 h was investigated. The conventional hydrothermal synthesis was used for comparison. Under microwave-assisted synthesis, uniform crystal size with lower crystal aspect-ratio (length/width) was obtained and high agglomeration was prevented. Surface modification was applied by deposition of silica on the external surface of ZSM-22 using catalytic cracking of diphenyl silane. The protonated H-ZSM-22 prepared by the MAHyS and the surface-modified TON zeolites were applied in catalytic cracking of n-hexane. The modified MAHyS zeolites showed superior selectivity to propylene and lower selectivity to aromatics. Stability against deactivation was clearly observed over surface-modified ZSM-22 zeolites. The physico-chemical properties of modified zeolite-catalysts were characterized by using XRD, FE-SEM, N₂ adsorption-desorption, ²⁷Al MAS NMR and NH₃-TPD.