Preparation Of High Performance SAPO Molecular Sieves And Catalytic Performance Of Dimethyl Ether To Olefins

With the intensification of global energy crisis and the increasingly prominent of environment issue,conventional non-environmentally benign thermal-cracking of distilled oil route for the production of light olefins is being ignored than ever before.Conversion of methanol(directly or indirectly via dimethylether)to olefin(MTO or DTO)from natural gas,coal and biomass has sparked continuing interest in the past decades.SAPO-18,an alternative MTO or DTO catalyst,is considered as the most likely catalyst to substitute SAPO-34 with relatively shorter lifetime and faster coking formation rate because of its high conversion of methanol and dimethylether,high selectivity to light olefins and long lifetime.However,relatively expensive templates are needed during the preparation of SAPO-18,meanwhile,the obtained samples always exhibit relatively small crystal size,which make its cost and separation are high and difficult in industry.Thus,it's a great challenge to explore some cheap and abundant templates,and increase the particle size appropriately for its practical applications.Firstly,SAPO-18 zeolites with various morphologies were synthesized by employing sodium dodecyl benzene sulfonate(SDBS)and N,N?-diisopropylethylamine(DIEA)as morphology controlling agent and template,respectively.The influence of the addition of SDBS on the physicochemical properties and catalytic performance of the obtained samples were studied systematically.The results showed that the addition of lower concentration of SDBS resulted in the zeolite with sheet-like morphology,while higher concentration of SDBS led to aggregated spherical zeolites.The crystal defects and the number of weak acid sites also increased with the morphology variation.When the zeolites were used as catalysts for the conversion of DTO,enhanced catalytic activity and stability were observed,which were attributed to the higher weak acid density possessed on the zeolites with crystal defects.Furthermore,SAPO-18 molecular sieve was first synthesized by using trace amount of DIEA under the assistance of TEA,making it possible that the preparation cost can be sharply decreased.The results demonstrate that the particle size increases with the increasing of TEA ratio in the TEA-DIEA dual templates;TEA tend to promote more Si atoms incorporation into the SAPO framework by means of mechanism ?(1Si?1P);the novel synthesized SAPO-18 exhibits higher light olefins selectivity and lower rate of coke formation in the DTO reaction compared with the conventional synthesized SAPO-18.Finally,a series of SAPO-18/34 intergrowth and SAPO-18 zeolites were hydrothermally synthesized by employing diethylamine(DEA)and DIEA as dual templates.The effects of DIEA/DEA ratio and crystallization time were investigated.The crystalline products changed from SAPO-34/5,to SAPO-18/34 intergrowth,and finally to pure SAPO-18;meanwhile,the crystal size and acidity decreased with the increasing of DIEA/DEA ratio.Furthermore,with the prolonging of crystallization time,the metastable AEI phase converted into more stable CHA phase,also,the Si content and acidity increased.For DTO reaction,the samples synthesized with shorter crystallization time exhibited higher selectivity to light olefins,lower selectivity to alkanes and lower carbon deposition rate,because the decrease in CHA/AEI ratio,particle size and acidity may suppress the unfavorable hydrogenation and polyreaction of light olefins.