Synthesis And Characterization Of Small Crystal SAPO-34 Zeolite At A Low Cost

SAPO-34,a CHA type silicoaluminaphosphate molecular sieve,contains 8-membered ring ellipsoidal cages,with a pore diameter of 0.43 nm.The remarkable pore structure of SAPO-34 makes it an ideal shape-selective catalyst.Due to its special shape-selective catalysis,relatively mild acidity and good thermal stability,SAPO-34 has been widely used in MTO,where it shows an excellent selectivity to light olefins and almost complete conversion of methanol.However,SAPO-34,when applied as a MTO catalyst,has the shortages of short lifetime and poor resistance to coke composition because of diffusion limitation by small pore entrance.It is worth mentioning that reducing the particle size of SAPO-34 is an effective way to improve the catalytic activity and lifetime of catalysts,since small crystallites possess the higher capacity to decrease diffusion limitations and more abundant acidic sites on their external surface.The synthesis of small crystallites on the basis of the lower production cost,however,is an ongoing concern.In this work,the influence of synthetic conditions,such as the concentration of reactants,temperature and synthesis time,has been investigated in details.SAPO-34 molecular sieves were synthesized hydrothermally by using TEA as template.Besides,those methods,including an accurate selection and effective combination of templates,addition of crystallites seeds and modifiers such as HF,were successfully applied to fabricate SAPO-34 molecular sieves with perfect morphology and ideal size distribution.TEAOH has been recognized as the best structure-directing agent and tends to synthesize nano-SAPO-34.But its high price limits its wider application.TEA,another template,can also direct formation of CHA structure and its much lower price reduce synthesis costs.Nevertheless,SAPO-34 synthesized by using TEA only has large crystal size and relative strong acidity.Here,a combination of TEA and TEAOH has been used in synthesis of SAPO-34 to achieve modification in morphology,size distribution and acidity,in terms of increasing the number of nucleus.In addition,TEAB can be used as asubstitution for TEAOH and mixed with TEA to adjust crystal size distribution and to facilitate perfect crystallization of SAPO-34 molecular sieves.During the synthesis of SAPO-34,HF is an excellent modifier.Adding HF can increase the solubility of both pseudoboehmite and Ludox by giving rise to P-Al-F complex and SiF6-,which plays an important role in increasing nucleus and obtaining small crystallites with the uniform size distribution.Also,some characterizing methods such as XRD,FT-IR were utilized to prove that Si atoms can directly take part in the formation of CHA structure both in the formation of nucleus and in the growth of crystal grains,instead of incorporating framework structures of AlPO4 according to Si substitution for P or P-Al.The role of HF in promoting solubility of pseudoboehmite further strengthens the interaction of P and Al,which,to some extent,inhabits participation of Si in formation of framework structures of SAPO-34.In that case,the acidic sites decrease with reduction of the number of Si incorporated in frameworks.The seed-assisted synthesis of SAPO-34 has been also studied in depth.Although this routes cannot still get rid of utilization of organic structure-directing agent(OSDA)in case that without the adjustment of OSDA,gel maybe too acidic to allow SAPO-34 to crystalize.Because of the relatively small surface area,seeds cannot absorb nutrients of the gel for the secondary growth.For one thing,they may be partially dissolved in the liquid while nutrients can be adsorbed on the interface between the liquid and the residual of seed exposed to the gel for further growth.For another,seeds can be dissolved continually to supply active species for nucleus.In other words,it is the role of seeds in supplying of active sites and active species that promotes the quantity of crystal nucleus to achieve the reduction of crystal size in a short time.In addition,seed-induced agent can not only make great contribution to size reduction of SAPO-34 crystal,but makes it possible to obtain pure phase of SAPO-34 when the ratio of SiO2/Al2O3 of the starting gel is lower.The results can be attributed to its cation concentration of TEA+ and abundant composite units devoted to the synthesis system where the ability of Si incorporation is promoted in spite of lower concentration of Si species.