Synthesis And Modification Of SAPO-34 Molecular Sieve With Microwave And Its Application In MTO Reaction

Methanol is an important industrial raw material,China is rich in coal resource,Coal to methanol is a cheap way.The key to produce ethylene propylene from methanol is the synthesis and modification of catalyst.In this paper,SAPO-34molecular sieve was synthesized by microwave,synthesis and compared with traditional hydrothermal crystallization to synthesize SAPO-34.The effects of microwave on crystallinity,specific surface,surface acid content and acid strength of SAPO-34 molecular sieve were investigated.The catalytic properties of SAPO-34molecular sieves with different silicon to aluminum ratios obtained by two kinds of synthesis methods were tested by a fixed bed MTO reactor.The surface acidity and specific surface area of SAPO-34 molecular sieves synthesized by microwave temperature programmed synthesis were investigated after modified with zinc oxide.The changes in pore size and the effect of MTO reaction on the catalytic lifetime and selectivity of low olefins were studied.Different molar ratios of ammonium chloride and glacial Acetic Acid were added to the crystallization solution of SAPO-34molecular sieve.The changes of the surface acidity of SAPO-34 molecular sieve were influenced by the way of Si atoms entering into the molecular sieve skeleton by regulating the pH of crystallization solution.Compared with the traditional hydrothermal crystallization method,the SAPO-34 molecular sieve synthesized by microwave temperature programmed synthesis has higher crystallinity,larger specific surface area,more weak acid content,less moderate acid content and longer catalytic life in MTO reaction.The selectivity to ethylene is relatively high.This is mainly due to the rapid heating of microwave and the rapid crystallization and nucleation of molecular sieve,which can form SAPO-34 molecular sieve with smaller grain size,so that the specific surface area of molecular sieve will be larger.The selectivity of ethylene to ethylene is higher in MTO reaction.SAPO-34 molecular sieve was modified with zinc oxide,and the modified material ZnO followed the Loewenstain rule to replace AlO₂^-tetrahedron with the same charge in the form of ZnO₂^2-tetrahedron,which resulted in a net negative charge in the framework and enhanced the adsorption ability of the modified SAPO-34molecular sieve for proton hydrogen.As a result,a stronger B acid center was produced,which increased the selectivity of SAPO-34 molecular sieve modified by zinc oxide for low olefins,especially propylene in MTO reaction,but also enhanced the coke deposition rate of the zeolite to reduce the catalyst life.By adding ammonium chloride and glacial Acetic Acid to the crystallization solution of SAPO-34 molecular sieve,both of these modified substances can reduce the crystallization liquid pH.it is found that under the condition of low pH,The surface acidity of modified SAPO-34 molecular sieve enhanced the selectivity of low alkenes,especially propylene,but it was found that SAPO-5 was formed in the crystal system of SAPO-34 with the increase of glacial Acetic Acid content.This will reduce the selectivity of molecular sieve to low olefins in MTO reaction.