Controllable Synthesis Of Hierarchical SAPO-34 Zeolites By Acid Post-treatment And Investigation On Relationship Between Structure And Performance

Methanol to Olefin（MTO）process is a viable and practical way in China,because it not only decrease the domestic methanol production capacity but also increase the supply of light olefins.Catalyst SAPO-34 is key in MTO technology.However,zeolite SAPO-34 suffers a serious coke formation due to micropores,which lead to its short single-pass catalytic lifetime.Nowadays,introduction of mesoporous or macropores into mocroporous SAPO-34 is an effective way to reduce coke formation rates.Up to now,more attention have been paid on expensive in-situ synthesis in prensence of dual-templates that are hard to applied on industry due to the cost.In this work,we synthesized hierarchical SAPO-34 with high relative crystallinity by controlling the acidic post-treatment conditions.Effects of the different acid（nitric acid,oxalic acid and butanedioic acid）on development of hierarchical pores on SAPO-34 crystals and their catalytic performance in MTO were investigated in detail.After etching in nitric acid and oxalic acid,the surfaces of treated SAPO-34 present two kinds of pore patterns,i.e.scattered pores on the top and bottom surfaces and an Butterfly-shaped,touching triangles pore pattern on the four side surfaces.And the single-run lifetime of the resultant zeolites was significantly prolonged in the MTO process without sacrificing the excellent methanol conversion and C⁼_2-4 selectivity.However,no hierarchical SAPO‐34 was obtained after a treatment with butanedioic acid,and with this sample,fast deactivation was detected after 100 min.Futhermore,the SAPO-34 was treated with more environmental friendly solid oxalic acid,and the symmetric,touching triangle pore patterns on four side surfaces and scattered pores on the two left surfaces can be obtained without affecting the relative crystallinity.The prepared hierarchical pores SAPO-34 also contains 10 and 1000 nm etching channels besides the inherent micro-pores,and it can also control the etching channels can be adjusted to a certain extent by changning the post-treatment parameters.Both the liquid and solid phase acid can result in the partial dissolution of the SAPO-34 to form the hierarchical pores.The content of related elements,the chemical bond,acid sites and the coordination of the elements change accordingly.The effects of the post-treatment parameters on the catalytic performance were investigated in details,which suggested an optimized condition of 90^oC treatment in solid phrase oxalic acid/SAPO-34 at a mass ratio of 1:20 for 6 h.The resultant hierarchical SAPO-34 shows a complete methanol conversion and high light olefins selectivity.Profited from the improvement of mass diffusion in the hierarchical structure,the single-run lifetime of hierarchical SAPO-34 reaches570 min under condition of 400^oC,1 atm and methanol space velocity of 1 h^-1 in the MTO reaction,which was improved more than 75%compared with the parent one,indicating a promising prospect in industrial application.