Morphology control of ionic-liquid-templated ZSM-22 and ZSM-5 zeolites using a two-step process and its effect on toluene methylation

ZSM-22 (TON) and ZSM-5 (MFI) zeolites were synthesized using 1-ethyl-3-methylimidazolium bromide ([EMIM]Br) and 1-butyl-3-methylimidazolium bromide ([BMIM]Br), respectively, as structure-directing agents. A two-step crystallization method, i.e., crystallizing at different temperatures under static conditions and/or agitation by rotation, was used to tune the zeolite particle size. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images show good crystallinity and a variety of particle sizes of the zeolites. Temperature programmed desorption of ammonia (NH₃-TPD) along with ²⁷Al and ²⁹Si solid-state NMR indicate that the acid site concentration and the acid strength were similar for all the zeolite samples and that Al was incorporated into the framework. The shape-selective capability of the zeolites was evaluated by toluene methylation. All of the ZSM-22 zeolites showed higher para-xylene (p-xylene) selectivity than the ZSM-5 zeolites. The ZSM-22 sample with a smaller particle size exhibited the highest para-xylene (p-xylene) selectivity of up to 92%. In contrast, the highest p-xylene selectivity among the ZSM-5 zeolites only reached 53%. The enhancement of the p-xylene selectivity is ascribed to the one-dimensional structure of the ZSM-22 zeolite. This work provides an alternative method of promoting high p-xylene selectivity in toluene methylation. Furthermore, as an alternative to modifying MFI zeolites by post-treatments, rational design of ZSM-22 zeolites, e.g., incorporating mesopores and uniform nanosized particles, is a promising way to achieve high selectivity during toluene methylation.