Effect Of Acid Treatment On The Structure,acidity Of SAPO-34 Molecular Sieve And Its Catalytic Performance In MTO Reaction

Light olefins such as ethylene and propylene are important basic petrochemical raw materials and have been widely used in modern chemical industry.However,with the increasing shortage of petroleum resources,people have to develop an alternative non-petroleum route to produce light olefins.Therefore,in recent decades,methanol to olefins?MTO?reaction technology,which takes methanol as the reactant,light olefins such as ethylene and propylene as final production,greatly alleviates the shortage of oil resources,and thus has received extensive attention from scientists.SAPO-34 molecular sieve with unique cage structure?CHA?and moderately acidic strength exhibits excellent catalytic activity and higher light olefins selectivity in MTO reaction.However,its narrow 8 membered ring pore structure and acidity will result in the generation of carbon deposition,which will lead to the rapid deactivation of the catalyst.Studies have shown that the acidity of SAPO-34 molecular sieve can be modified by adjusting the silicon environment via acid treatment,then solve the inherent defect and improve the catalytic reaction performance.In addition,acid treatment can also dissolve and remove skeleton elements to generate hierarchical pore structure,which could improve the diffusion of reactants and products,enhance the carbon capacity of SAPO-34 molecular sieve,and improve the single lifetime in MTO reaction.Therefore,the main work of this paper focuses on the effects of treatment of oxalic acid and citric acid solutions with different concentrations on SAPO-34molecular sieve acidity,pore structure and MTO reaction performance,as well as the effects of treatment of oxalic acid and citric acid solutions on the removal selectivity of skeleton elements and the yield of SAPO-34 zeolite.The main results are listed as follows:?1?SAPO-34 zeolite was treated with oxalic acid solution.The results showed that oxalic acid treatment can produce macropores in the crystals via skeleton etching,form holly on the crystal surfaces,consequently which is favorable for diffusivity and carbon deposition capacity.Furthermore,silicon species of Si?OSi?₁?OAl?₃ in the SAPO-34 framework were extracted selectively,at the same time the acid strength and acid density of SAPO-34 zeolite obviously decreased.Consequently,The samples with 0.01 mol/L oxalic acid treatment exhibit higher selectivity of C₂H₄ and longer catalyst single-run lifetime in MTO reaction than that of the parent SAPO-34.?2?SAPO-34 zeolite was treated by citric acid solution.The results showed that citric acid can also produce macropores in the crystals via skeleton etching,form lamellar channel on the crystal surfaces.Furthermore,silicon species of Si?OSi?₁?OAl?₃ in the SAPO-34 framework were extracted selectively,at the same time the acidity of SAPO-34 zeolite decreased.It's worth noting that the effects of acid treatment on structure and component of zeolite caused by citric acid were more serious than those caused by oxalic acid treatment.The results of MTO evaluation showed that citric acid treatment could only improve the single-run lifetime of catalyst,but the selectivity of ethylene was not improved.