Post-Synthesis Of Hierarchical SAPO-34 By TEAOH-H₃PO₄ Etching

SAPO-34 zeolite showed excellent performance in methanol to olefin（MTO）reaction.However,the formation of confined compounds in SAPO-34 cages restricts the diffusion of raw materials and products,lead to rapid deactivation.Hierarchical SAPO-34 zeolite,which own microporous,mesopores and macropores structure,greatly enhances the mass transfer efficiency and prolongs the lifetime of the catalyst.Post-treatment is an important method for preparing hierarchical SAPO-34 zeolite.In this work,hierarchical SAPO-34 samples were synthesized with TEAOH and TEAOH-H₃PO₄ as etchant.Effects of temperature,etching time and etchant composition were investigated.Samples were characterized by XRD、SEM、ICP、NH₃-TPD、TG and N₂ adsorption-desorption.The MTO evaluation reaction was carried out in a fixed-bed reactor.The main results are as follows:（1）Hierarchical SAPO-34 zeolite was prepared by using tetraethylammonium hydroxide（TEAOH）solution as etchant.The etching rate of SAPO-34 zeolite increased with the increase of temperature,but the etching decreased with the increase of temperature,and the etching and recrystallization occurred simultaneously at 200℃.As the same temperature（25,50,100 and 150℃）,with the prolongation of etching time,the pores on the surface of the cube SAPO-34 grains increased and gradually fragmented,the crystallinity of the sample decreased,the specific surface area and micropore volume decreased,the mesoporous volume and mesoporous surface area increased,the Si/Al and P/Al were higher than those of the original sample and the Si/Al increased with time.The crystallinity of the sample did not decrease significantly with time at 200℃,but new crystalline phases appeared in the sample T-200-8,and Si/Al and P/Al decreased significantly with the formation of new crystalline phases.Samples through etching by TEAOH,the lifetime was prolonged,the olefin selectivity was comparable to that of the original sample in the MTO reaction.（2）Hierarchical SAPO-34 zeolite was prepared by using TEAOH-H₃PO₄ mixed as a etchant at 100 and 200℃,respectively.The results show that the yield was decreased with the time and increased with the amount of H₃PO₄ at 100℃;the yield was increased with the time and amount of H₃PO₄ at 200℃.When the TEAOH:H₃PO₄ greater than 1,at the same temperature,with the prolongation of etching time,the pores of the cube SAPO-34 grain surface increase and gradually fragment,the crystallinity of the sample decreases,the specific surface area and micropore volume decrease,the mesoporous volume and mesoporous surface area increase,the Si/Al and P/Al are higher than that of the original sample and the Si/Al increases with the extension of time.When the molar ratio of TEAOH to H₃PO₄ is 0.5:1,new crystalline phases appear in the samples at 100℃and 200℃,and the Si/Al and P/Al decrease significantly with the formation of new crystalline phases.The samples obtained from the mixed solution at 100℃and 200℃showed good performance in the MTO reaction,and the samples performed best in the MTO reaction at the mole ratio of TEAOH to H₃PO₄ is 2:1 at200℃.Catalyst lifetime reached 210 min,ethylene and propylene selectivity reached 81.93%.