Synthesis Design And Catalytic Performance Evaluation Of Hybrid Zeolite Catalysts

The methanol conversion to light olefin process has been considered as one of the most important non-oil routes to obtain olefins,due to its high methanol conversion,high olefin selectivity and broad sources of methanol from coal,biomass and nature gas.The most commonly used catalysts in the reaction of methanol conversion to light olefin are microporous zeolites with frameworks of CHA and MFI.The SAPO-34 and SSZ-13?CHA framework?catalysts show a high selectivity to light olefins?C₂-C₄?and are appropriate for the methanol-to-olefin?MTO?process,while the ZSM-5?MFI framework?catalyst presents a high selectivity to propylene and is suitable for the methanol-to-propylene?MTP?process.Recent studies on the structure analysis and catalytic performance evaluation of the catalysts used in the methanol conversion to light olefin process indicated that the pore and textural structure,acidity,crystal size and morphology showed strong influences to the reaction,and many researches focused on the structure modification of catalysts on account of the enhancement of the target olefin selectivity.However,most of the zeolite catalysts possessed only one framework and showed limited possibility of structure improvements.The hybrid zeolites with combination of duel-/multiple frameworks or morphologies exhibit tunable balance of skeletal porosity and additional macro-/mesoporosity,diversed acidity or multiple active sites and controllable morphologies.However,both synthesis study and catalytic application of hybrid zeolites are very limited.There are many issues,such as nucleation and growth process control and reactive features,needed to be resolved,and the structure-property relationship investigation of hybrid zeolites is highly desired.In view of this,two types of hybrid zeolites with core-shell structures,which are layered ZSM-5/bulked ZSM-5 and layered silicalite-1/bulky SSZ-13,will be designed synthesized by combined using of epitaxial growth and secondary growth approaches,and the methanol-to-olefin?MTO?reaction and methanol-to-propylene?MTP?reaction are chosen to evaluate the catalytic performance of the obtained hybrid zeolite catalysts.?1?The layered ZSM-5/bulked ZSM-5 hybrid zeolites?LBZ5?were prepared by epitaxial growth of layered ZSM-5 nanosheets over bulky ZSM-5 crystals The systematic balance of zeolitic microporosity and interlayered mesoporosity was achieved by elaborate regulation of the ratio between layered phase and bulky phase,resulting in a series of layered-bulky ZSM-5hybrid composites with the controllable thickness of the layered shell ranging from 98 to 307nm.The crystallinity,textural structure,acidity,composition and morphology of the series LBZ5 samples were systematically characterized.The catalytic performance of the LBZ5catalysts were evaluated by the methanol-to-propylene?MTP?reaction,comparing with the parent bulky ZSM-5?BZ5?and the lamellar ZSM-5?LZ5?.And the results indicated that the LBZ5 materials to be candidate catalysts for the MTP process with prolonged catalytic lifetime,superior coke toleration,and enhanced propylene selectivity compared with the parent bulky ZSM-5?BZ5?and layered ZSM-5?LZ5?due to their well-retained zeolitic framework,hierarchical meso-/microporosity by the layered-bulky composite structure,and appropriate strong acidity.In particular,the sample of LBZ5₇0 revealed the highest propylene selectivity?43.0%?among all the ZSM-5 catalysts,and the sample of LBZ5₃0presented not only the considerably high propylene selectivity?41.4%?but also the longest catalytic lifetime?171 h?and the largest P/E ratio?9.4?.?2?The layered silicalite-1/bulked SSZ-13 hybrid zeolites?LS/BSZ13?were synthesized by secondary growth of layered silicalite-1 nanosheets over bulky SSZ-13 crystals.In order to overcome the growth compatibility,the parent SSZ-13 crystals were ion exchanged with C_22-6-6Br₂ surfactant.The thickness of silicalite-1 layers was controlled by changing the additional amount of the parent SSZ-13 crystals.The crystallinity,textural structure,acidity,composition and morphology of the series LS/BSZ13 samples were systematically characterized.The catalytic performance of the LS/BSZ13 catalysts was evaluated by the methanol-to-olefin?MTO?reaction,comparing with the parent bulky SSZ-13.The results showed that the lifetime and olefins selectivity of LS/BSZ13 were improved due to the outer surface passivation by the layered silicalite-1 and the inhibition of mass transfer diffusion resistance by the hierarchical layered mesoporosity.The sample obtained under the ion-exchange condition of 0.30 M C_22-6-6Br₂ showed the significant improvement in the catalytic performance.