Study On Vapor-Phase Carbonylation Of Dimethoxymethane To Synthesize Methyl Methoxyacetate Over ZSM-5 Zeolites

The research and development of methanol and its derivatives utilization processes to get valuable chemicals are hot topics in coal chemical industry,which can reduce demand of petroleum and increase self-sufficient of essential chemicals for our nation.Methyl methoxyacetate is a very valuable organic chemical intermediate which can be used to synthesize ethylene glycol efficiently by hydrogenation and hydrolysis.The synthesis of methyl methoxyacetate by the vapor-phase carbonylation of methanol-based dimethoxymethane can build a new and green coal chemical route.Traditionally,homogenous catalysts such as Rh-or Ir-containing organometallic complexes and halogen-containing promoters were used as homogeneous catalyst in the carbonylation reaction.However,with the increase of environmental awareness and catalyst manufacturing cost,it is of great importance to develop a noble metals-and halide-free and efficient heterogeneous catalyst.The researchers have found that carbonylation process could be achieved through Koch pathway by using solid acid as catalyst.Zeolites are one promising catalyst family for carbonylation reactions,because of their advantages of green synthesis,low cost,unique porous structure,and wide adjustable acidity,has recently attracted extensive research attention.The synthesis of methyl methoxyacetate can be achieved under mild conditions through gas-phase carbonylation with zeolites as catalysts.However,these studies still focused on trying catalyst and discussing rudimentary mechanism by far.The structure-activity relationship is vague,and dimethoxymethane conversion and product selectivity are still poor.The catalytic performance and reaction mechanism of zeolite are closely related with unique porous structure and the distribution of acid sites.Based on this,this work selected ZSM-5 zeolite as catalyst for dimethoxymethane carbonylation,because of simple synthesis process and adjustable acidity.IVThe reaction conditions,acid characteristics and the porous structure over ZSM-5 zeolite were studied detailedly.The primary research contents and results are as follows:(1)A range of mixed solution of different concentrations of dimethoxymethane(DMM),methyl formate(MF),methanol(Me OH),and methyl methoxyacetate(MMAc)were prepared,and diethylene glycol dimethyl ether(DGDE)was selected as internal standard.All solutions were analyzed quantitatively by Agilent gas chromatography.The mole calibration factors were mesured and mathematical formulas were established by analyzing the linear relationship of the peak area of raw material products and internal standard.The effects of reaction temperature,pressure,GHSV,and time on the performance of DMM carbonylation were studied by a fixed-bed reactor system.It was found that the better catalytic performance was achieved under 448 h-1 of GHSV(DMM)and 5000 h-1 of GHSV(CO)at 110 ℃ and 0.6 MPa for 7 hours.(2)ZSM-5 zeolites with different morphologies and Si/Al ratios,such as NB,NQ,NE,NZ,and NC,were prepared by adjusting the synthesis method and changing the ratio of raw materials.We used XRD,SEM,N2 physical adsorption-desorption,NH3-TPD,Py-FTIR,27 Al MAS NMR and other methods to characterize the product’s structure,texture,acid properties,and the distribution of aluminum species.The results showed that zeolites with different morphologies showed large differences in acidic characteristics and aluminum species distribution.At the same time,regulating the Si/Al ratios could further change the ratio of Br?nsted and Lewis acid and adjust the spatial distribution of active aluminum species.The chess-like zeolite with a Si/Al ratio of 30 exhibited the highest catalytic activity with 11.3% DMM and 58.8% MMAc selectivity.That might be attributed to appropriate amount of medium-strong Br?nsted acid sites and the higher proportion of aluminum species in ZSM-5 channel intersections.(3)On the basis of the above studies,the texture and acid properties of ZSM-5 zeolites were further adjusted by changing the amount of Na OH adding in the synthetic gel system.It was found that increasing the amount of Na OH helped to change the mesopores pore volume and the specific surface area,meanwhile,more aluminum species were introduced into the framework,more Br?nsted acid sites were exposed.The increase of medium-strong Br?nsted acid sites can raise the DMM conversion by providing more active acid sites,whereas the mesopores can improve the selectivity to MMAc by shortening the product diffusion path,weakening the steric constraints and suppressing the carbon resistance.ZSM-5 zeolite with 0.6 g Na OH adding in the synthetic gel shows the best catalytic activity,11.9% DMM conversion and 69.3% MMAc selectivity were obtained.