Insight Into Effect Of Acidity And Pore Structure On Zeolite-Catalyzed Carbonylation Of Dimtheyl Ether

Ethanol is widely used as a clean fuel or fuel additive because it could contribute to reducing the emission of CO,hydrocarbons and solid particulates,which plays an essential role in solving environmental pollution and reducing oil consumption.Recently,a new synthetic route of ethanol from syngas has been proposed,consisting of carbonylation of dimethyl ether（DME）to methyl acetate（MA）and MA hydrogenation.This route has attracted much attention,owing to its wide raw material sources,mild reaction conditions.Zeolite with 8-member ring（8-MR）shows excellent MA selectivity,in which Br（?）nsted acid sites（BAS）serve as active centers.In this paper,the influence of acid distribution,acid strength,and pore structure of zeolite on DME carbonylation together were explored.Distribution of BAS at different channels of mordenite（MOR）was altered by size difference and competitive location effect between inorganic template（Na⁺）and organic template（cyclamine）.The sample synthesized by using 4-methylpiperidine as an organic template possesses the highest percentage of BAS at 8-MR and exhibits the greatest DME carbonylation catalytic performance.Regulatory mechanism of dual-templates on BAS distribution was elucidated by the combination of experiments and theoretical calculations:competitive effect exists between cyclamine and Na⁺when bonded with the framework[Al O₄]^-groups.Cyclic amines prefer to locate in the 12-MR exclusively in single molecular form due to the spatial constraints.As a result,enrichment of the smaller Na-[Al O₄]in 8-MR was achieved.So,the percentage of BAS at 8-MR increased.Isomorphous substitution of heteroatom was applied to modify the acidity of MOR by introducing boron atom through one-step hydrothermal synthesis method.Acidity characterization showed that boron atom preferred locate at 12-MR compared with Al atom.The amount of Al at 8-MR increased with the increase of boron content due to the competitive location effect at 12-MR between trivalent metals.Incoporating a proper content of boron,for instance,with the B/Al of 0.15 in zeolite,could improved the amount of BAS at 8-MR.As a result,the catalytic performance for DME carbonylation was enhanced.Strength of BAS at 8-MR was controlled by adjusting Si/Al ratios of MOR to investigate the influence of acid strength on DME carbonylation,which was realized by treating with oxalic acid and ammoniumhexafluoro silicate.NH₃-TPD-FTIR was used to calculate the heat of NH₃ desorption from BAS at 8-MR,and the results showed that the strength of BAS at 8-MR decreased with the increase of Si/Al ratios.By correlating the heat of NH₃ desorption from BAS at 8-MR with the turnover frequency of MA formation,a positive correlation between the strength of BAS and reacitivity was stablished.Theoretical calculation was performed to determine the activation barrier of the CO insertion step to form acetyl group on the models with different acid strength.The results suggested that the stronger acid strength of BAS at 8-MR can reduce the activation barrier of rate-limiting step,thus enhancing the reactivity of DME carbonylation.SSZ-13 zeolite was applied to explore the influence of pore structure on DME carbonylation,and it was found that SSZ-13 showed excellent stability.Acidity modification was realized by Na and Cs cations selectively exchange with BAS at different channels.The subsequent experimental and theoretical studies proved that the BAS at 8-MR serves as the active site for DME carbonylation over SSZ-13.By correlating product selectivity with the pore parameters,we found that the pore structure of SSZ-13 was a key factor that affected MA selectivity.The reduce of cage size could inhibit the formation of side-reaction intermediates and result in an improvement of MA selectivity.