Solvent Free Synthesis Of SAPO-18 Zeolite And Its Catalytic Performance For Methanol To Olefins

Methanol-to-olefin（MTO）is an important non-petrochemical route for the production of light olefins,which is expected to solve the shortage of petroleum resources in China.SAPO-18 zeolite with AEI structure exhibits excellent catalytic performance in the MTO reaction due to its large pore size and pore volume,and it is synthesized by hydrothermal method usually.There are many problems with this method,such as complex operation process,low productivity and cause environmental pollution easily.In this paper,from the perspective of green chemistry,SAPO-18zeolites were synthesized by solvent-free method to reduce the waste of water resources.In order to improve the catalytic lifetime and diene selectivity（ethylene and propylene）of zeolite,different templates were used for directed synthesis of SAPO-18 zeolites together.The crystal morphology and physicochemical properties of SAPO-18 zeolites were characterized by XRD,SEM and NH₃-TPD,and its catalytic performance was evaluated by MTO reaction.Finally,the zeolite was modified with metal cations to regulate the product distribution in the MTO reaction.The main research contents are as follows:1.A series of SAPO-18 zeolites were synthesized by solvent-free method.Firstly,the effects of template dosage,crystallization temperature and crystallization time on the synthesis of SAPO-18 zeolite were investigated.The results showed that the crystallization time has a significant effect on the synthesis of zeolites.With the extension of crystallization time,the crystallinity of zeolites gradually increased,until the crystallinity reached the maximum at 12 h.Then,continue to extend the crystallization time,the crystallinity gradually decreased and tended to be stable.SAPO-5 miscellaneous crystal phase would appear when the crystallization time was too long.Then,the effects of silicon content on the structure and acidity of zeolites were investigated.The results showed that pure SAPO-18 zeolites were synthesized while the Si O₂/Al₂O₃ ratio was in the range of 0.1～0.8.The sample with Si O₂/Al₂O₃ratio of 0.1 owned irregular crystal morphology.The sample with Si O₂/Al₂O₃ ratio of0.4 had the larger micropore volume and acidity density.It also had the longest catalytic life of 60 min in the MTO reaction.The sample with Si O₂/Al₂O₃ ratio of 0.6 had the smallest crystal size.In the MTO reaction,the diene selectivity up to 75%,it was the best.2.SAPO-18 zeolites were synthesized with the dual-template agent DIEA/TEAOH,DIEA/DEA systems respectively,and the effects of dual-template ratio and dual-template type on the synthesis,structure and acidity of molecular sieves were investigated.The results showed that the SAPO-18 zeolite with better phase purity was synthesized only under the ratio of DIEA/TEAOH=0.5/0.5 and DIEA/DEA=0.9/0.1.The SEM image showed that the sample synthesized under the condition of DIEA/TEAOH=0.5/0.5 were lamellar and uniformly distributed,and the NH₃-TPD result showed that it had stronger acidity.In the MTO reaction,the catalytic lifetime and diene selectivity of two samples were improved.The sample synthesized with DIEA/TEAOH=0.5/0.5 owned the longest catalytic time of 430 min and the highest diene selectivity of 86%.3.SAPO-18 zeolite was modified with metal cations by in-situ synthesis,and the effect of the introduction of metal heteroatoms on the structure and acidity of the zeolite was investigated.It was found that SAPO-18 zeolite was synthesized only in the introduction of Ni and Gd elements,but the crystal morphology of the zeolites became worse and the acidity weakened in varying degrees.The catalytic lifetime of zeolites was shortened after metal doping,while the selectivity of ethylene was improved effectively,indicating that metal cations modification was an effective means to regulate the distribution of products.