Synthesis Of CHA Zeolites In Ionic Liquids

Silicoaluminophosphate zeolite SAPO-34 with a CHA framework is important microporous crystal with uniform and intricate channels,high specific surface area and adsorption capacity,and high thermal and hydrothermal stabilities,which have been widely used in catalysis,adsorption,ion-exchange,separation,etc.However,the performance of SAPO-34 molecular sieve has a great relationship with its morphology.Therefore,it is important to develop synthetic strategies to control the morphology of molecular sieves.With the emphasis on the environment and energy saving in recent years,some new molecular sieve synthesis methods have emerged.Ionothermal synthesis need little water during the reaction,which reduces the generation of waste water and the hidden dangers of the high-pressure zone.However,to date there are few studies on the synthesis of SAPO-34 molecular sieve in ionic liquid.In this paper,hierarchical porous SAPO-34 molecular sieves were successfully synthesized in ionic liquid.The effects of various factors on the morphology and performance of SAPO-34molecular sieves were investigated,and the formation mechanism of the hierarchical porous SAPO-34 molecular sieve in ionic liquids was revealed.In this paper,a triclinic SAPO-34 zeolite with hierarchically porous structure was successfully prepared in 1-ethyl-3-methylimidazole chloride([EMIm]Cl).The effects of crystallization temperature,crystallization time,triethylamine,ionic liquid,hydrofluoric acid and silicon source on the synthesis and morphology of triclinic SAPO-34 zeolite were investigated.The optimal conditions for preparation of triclinic SAPO-34 zeolite with hierarchically porous structure were determined to be as follows:the molar ratio was:39IL:1TEA:1Al:3P:0.73HF:0.1Si,the crystallization time was48 h,and the crystallization temperature was 180°C.The formation mechanism of triclinic SAPO-34 zeolite with hierarchically porous structure was investigated.It was found that the formation mechanism accorded with Ostwald ripening theory.During the synthesis of the molecular sieve,the smaller grains will gradually dissolve into the surrounding medium due to the larger curvature and higher energy content,and then precipitate on the surface of the larger grains,forming spherical agglomerates composed of small grains.With the extension of crystallization time,the crystals continued to grow,and finally formed hierarchical porous triclinic SAPO-34 molecular sieve with more thermodynamically stable cubic crystals.The accumulation of grains constitutes the existence of mesopores and macropores.In order to further investigate the effect of ionic liquid types on the synthesis of triclinic SAPO-34 molecular sieve,the structural orientation of ionic liquids was studied in detail.The triclinic SAPO-34 molecular sieves were successfully synthesized in[EMIm]Br,[BMIm]Br and[BMIm]Cl,respectively.The larger size of the ionic liquid cation,the more conducive to the generation of large pore molecular sieves.On the short alkyl substituent chain of ionic liquid cations the intensity of the interaction is strong because of the high electron density.The strong interaction obviously promotes the growth of the{111}plane of the CHA framework and,therefore,SAPO-34 with large-sized cubic crystals morphology is obtained.The Cl~-system can provide an environment of weak coordination and strong polarity,speed up the reaction involving ionic liquid cations,and then affects the morphology of triclinic SAPO-34molecular sieve.The effects of phosphoric acid,TEA,template species and ionic liquid in[BMIm]Cl on the synthesis and morphology of triclinic SAPO-34 molecular sieves were investigated.By adjusting the type of the template agent,nano-sized SAPO-34catalysts were prepared under diethylamine or morpholine.When the template agent was N-methylimidazole,the sheet-shaped SAPO-34 molecular sieve was prepared.Finally,Me APO-34(Me=Mg,Mn,Zn,Co)molecular sieve with CHA topology was successfully prepared by ionothermal synthesis.The morphology,metal content and acidity of Me APO-34 molecular sieves are significantly different.In addition,the differences in morphology and structure of SAPO-34 molecular sieve prepared by different methods were investigated.Compared with the SAPO-34 molecular sieve synthesized by hydrothermal method,the external specific surface area,mesopore volume and average diameter of the SAPO-34 molecular sieve synthesized by ionothermal synthesis are significantly increased,and the crystal size is significantly reduced.Due to the difference in the weak/strongly acidic site ratios of the samples,different catalytic properties were exhibited in the catalytic conversion reaction of methanol.SAPO-34 molecular sieve synthesized by ionothermal synthesis has high content of weak acid and low content of strong acid,which makes the reaction mainly occur in the first stage to form dimethyl ether.