Ionothermal Synthesis,characterization And Catalytic Performance Of Extra-large-pore Silicoaluminophosphate Molecular Sieve

Due to the larger pore size than the molecular sieves currently used in industry,extra-large-pore zeolites can better overcome the diffusion limitation problem in the reaction process,and thus they have broad application prospects in dealing with bulky molecules involved in the catalysis,adsorption and separation process.In recent years,the synthesis of extre-large-pore zeolites mostly requires germanium as a skeleton atom,which makes it costly to synthesize and poor in structural stability.Therefore,it is of great scientific and practical value to carry out relevant research on synthesis and catalytic applications in depth.In this paper,the ionothermal synthesis,physicochemical properties and catalytic performance of aluminophosphate(AIPO)and silicoaluminophosphate(SAPO)molecular sieves in methanol to hydrocarbon(MTH)were studied by comparing the similar LTA structure with-CLO structure.The ionothermal synthesis of extra-large-pore aluminophosphate molecular sieve DNL-1 was studied by using 1,6-hexanediamine and several quaternary ammonium compounds with different types of cations and anions as co-structure-directing agents(co-SDAs).It was found that the structure of co-SDA had no significant effect on the crystallization process,but its concentration and basicity had significant effects on the structure selectivity and crystallization kinetics of the product.Combined with various characterization methods,we deduced that alkaline co-SDAs acted as an acidity regulator in the ionothermal system to facilitate the formation of the d4r unit,and influence their subsequent assembly mode around IL cations,and thus,direct the crystallization of-CLO and LTA structures at different concentrations of co-SDA.The Si substitution of AlPO molecular sieves with LTA and-CLO structure in ionothermal system was studied.By combining ICP?NH3-TPD?NMR?BET and other characterizations,it was found that with the increase of silicon concentration in raw materials,the silicon content in zeolites increased gradually,but the content was low(SiO2 wt%<5%).The acidity of zeolites increased first and then decreased slightly with the decrease of specific surface area.The silicon in SAPO-LTA mainly via the SM2 and SM3 mechanism to form Si islands,while the Si in SAPO-CLO mainly via the SM2 mechanism.The catalytic performance of SAPO-LTA and SAPO-CLO molecular sieves with different Si contents synthesized by ionothermal method in MTH reaction was compared.The initial methanol conversion of SAPO-LTA zeolite increased from 3%to 97%with the increase of silicon content,and its catalytic stability is also improved,but the overall deactivation rate is fast.The products are mainly light olefins,and the total initial selectivity is higher than 75%,indicating that the 8-membered ring of the LTA structure has a certain shape selectivity of light olefins.The catalytic activity of SAPO-CLO molecular sieves increases with increasing silicon content,but the corresponding catalytic activity and catalytic stability are worse than SAPO-LTA molecular sieves.The total initial selectivity of light olefins in the products are over 62%,and the alkanes are obviously increased,indicating that SAPO-CLO molecular sieve is more favorable to the formation of large-sized hydrocarbon molecules than SAPO-LTA.The reason of catalyst deactivation was investigated by XRD and GC-MS characterization.It was shown that the structure of SAPO-LTA deactivated catalyst was intact,and the catalytic activity was maintained after three regenerations.The main reason for the deactivation was the formation of polycyclic aromatic hydrocarbons in the structure.The activity of the SAPO-CLO deactivated catalyst could not be recovered after calcination at high temperature,and the cause of the deactivation was mainly the irreversible collapse of the structure.