Study on catalytic performance of SAPO-44 molecular sieve with special morphology for methanol conversion to light olefins
SAPO-44 molecular sieve has the same CHA topological structure as SAPO-34 and has excellent catalytic performance in methanol conversion to light olefins(MTO)reaction.However,the single microporous pore structure of traditional SAPO-44 limits the molecular diffusion of reactants and products,and the mass transfer and diffusion performance of the molecular sieve needs to be further improved.SAPO-44 molecular sieve with special morphology was synthesized by adding ammonium chloride to the crystallization system by the hydrothermal method.XRD,SEM,N2 adsorption/desorption and NH3-TPD were used to study the effects of the amount of ammonium chloride added to the synthesis system on the crystal phase structure,morphology,pore structure and acid properties and other physical and chemical properties of SAPO-44 molecular sieve.The catalytic performance of SAPO-44 molecular sieve in MTO reaction was also investigated.The results show that pH value of molecular sieve crystallization system is reduced by adding ammonium chloride.The particle size of SAPO-44 molecular sieve synthesized without ammonium chloride is 14.79 μm,and the shape is traditional cube.The SAPO-44 molecular sieve synthesized adding ammonium chloride shows a new special shape morphology structure,and the particle size is reduced.When n(NH4Cl):n(Al2O3)= 0.25,SAPO-44 molecular sieve has a particle size of 7.58 μm and a flying saucer-shaped structure.When n(NH4Cl):n(Al2O3)= 0.50,the size of SAPO-44 molecular sieve is 9.41 μm,which shows similar spherical structure.The novel morphology is deposited in the molecular sieve to form mesoporous,constructs the micro-mesoporous structure,the specific surface area and pore volume are both increase,and the acid strength and acid content are also increase.In the MTO reaction,the reduction of micro-mesoporous structure and particle size generated by SAPO-44 molecular sieve with special morphology synthesized by adding ammonium chloride is conducive to enhancing the molecular diffusion efficiency and increasing the reaction stability by more than 1.4 times.Meanwhile,the enhancement of acidity forms more catalytic active centers.Moreover,the joint action of micro-mesoporous structure and small size particle can increase the selectivity of light olefins(ethylene + propylene)by more than 3.00%.
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