One-step Synthesis Of Hierarchical SAPO-34 Zeolite And Its MTO Catalytic Performance

SAPO-34 zeolites can catalyze methanol to olefins with high yields of ethylene and propylene.However,in the methanol to olefin（MTO）process,SAPO-34 has high methanol conversion efficiency,but because of its single micropore structure and low diffusion efficiency,it is easy to form carbon deposition,resulting in premature deactivation of zeolite.Therefore,in this work,esoporous structure were introduced into microporous SAPO-34 crystal to form a heirarchical SAPO-34 zeolites to improve the catalytic life of the catalyst.The heirarchical SAPO-34 zeolites were synthesized by different soft templates in one step,and the MTO catalytic performance of the samples was comprehensively analyzed.Heirarchical SAPO-34 zeolites were synthesized by hydrothermal and microwave methods using morpholine（MOR）and tetraethyl ammonium hydroxide（TEAOH）as microporous templates,cetyltrimethyl ammonium bromide（CTAB）,polyethylene glycol（PEG2000）and uncalcined mesoporous SiO₂ zeolites as mesoporous agents.Meanwhile,modern analytical techniques such as XRD,SEM,BET,NH₃-TPD and FT-IR were used to analyze the phase of the synthesized heirarchical SAPO-34 zeolites.Finally,the MTO catalytic performance of the synthesized samples was tested to investigate the catalytic life and low carbon selectivity of the synthesized zeolites.Firstly,the synthesis of hierarchical SAPO-34 zeolites using CTAB as mesoporous agent and different microporous templates was studied.Through a series of characterization,it was found that the catalytic lifetime of the samples synthesized with a single cheap MOR template was shorter than that of the samples synthesized by TEAOH templates,but the mixed template of MOR and TEAOH at 3:1 ratio could also reach the lifetime of the samples synthesized with single template TEAOH,thus greatly reducing the cost of the catalyst.Meanwhile,it was found that with the addition of CTAB,the crystal shape of the synthesized zeolites gradually changed from cube to stacked sphere.When the addition of CTAB was 0.02,the synthesized samples showed the maximum specific surface,external specific surface area and total pore volume,which were 878 m²/g,108 m²/g,0.458 cm³/g respectively,and the catalytic time of MTO was up to 280 min.Secondly,the synthesis of hierarchical SAPO-34 zeolites using uncalcined MCM-41 and MCM-48 zeolite as silicon sources and mesoporous agents simultaneously was studied in this work.The hierarchical SAPO-34 zeolite synthesized with MCM-48 as mesoporous agent presented high resistance to carbon deposition and long catalytic life up to 578 min when catalyzed by MTO.Similarly,hierarchical SAPO-34 zeolites were synthesized using PEG2000 as mesoporous agent.when the amount of polyethylene glycol was 0.02,the sample reached the maximum catalytic lifetime 240 min.Finally,compared with the hydrothermal synthesis of SAPO-34 zeolites,the crystallinity and catalytic life of SAPO-34 zeolites synthesized by microwave method decreased with the addition of CTAB,which was different from the previous experimental study.Therefore,although microwave method is time-saving,it is not suitable for the synthesis of heirarchical SAPO-34 zeolites.