Hydrophobic Modification Of HZSM-5 Zeolite And Its Catalytic Performance For Dehydration Of Crude Methanol To Dimethyl Ether

With the increasing global energy demand and the increasing environmental pollution caused by conventional fossil fuels,the world has shifted its focus to finding alternative fuels.Dimethyl ether has become a clean fuel that can replace natural gas and liquefied petroleum gas as civil fuels due to its advantages of easy compression,easy storage,high combustion efficiency and low pollution.At the same time,dimethyl ether has a high cetane number and can be used as an alternative to diesel.Therefore,the synthesis of dimethyl ether has become a hot topic in the field of green chemistry.In the existing dimethyl ether synthesis process,the methanol vapor phase dehydration process has been widely used because of its simple operation,mature process and low cost.The key to methanol dehydration to dimethyl ether is the choice of catalyst.Compared with other commonly used catalysts,molecular sieves can combine adsorption,diffusion and chemical reactions by virtue of their special pore structure,change the path of reaction,and play a certain shape selectivity for molecules in the reaction system.The synthesis method is simple,easy to modify,cheap,and more in practical applications.However,the surface of HZSM-5 molecular sieve contains Si(Al)-OH,so the hydrophobicity of molecular sieve is not high.Therefore,in the process of catalytic methanol dehydration to dimethyl ether,water molecules and methanol will form competitive adsorption on its surface,thus reducing the yield of dimethyl ether.Therefore,the hydrophobic modification of HZSM-5 zeolite and its catalytic performance for the dehydration of crude methanol to dimethyl ether were systematically studied in this paper.The main research contents are as follows:(1)Using trimethylchlorosilane(TMCS)as a modification reagent,HZSM-5 molecular sieve was modified by silane under different conditions by vapor deposition method.Through the analysis of orthogonal experimental data,it is known that the most important factor affecting the modification effect is the addition ratio of modifier,followed by the modification time,and finally the modification temperature.It is concluded that the comprehensive performance of the modified sample is the highest when the modification reagent addition ratio(mHZSM-5:mTMCS)is 1:2,the modification time is 12 h,and the modification temperature is 100℃.The water absorption of the unmodified HZSM-5 molecular sieve was 14.74%,while the water absorption of the modified sample was 3.15%.In the reaction of dehydration of crude methanol to dimethyl ether,the conversion rate of unmodified HZSM-5 zeolite was 67.22%,while the conversion rate of modified sample was 80.24%,the conversion rate of methanol was increased by 13.02%,and the catalyst stability was higher during the reaction.The samples were characterized by SEM,BET,XRD,FTIR,NH3-TPD,Py-IR,TG and so on.The results showed that the skeleton structure of the modified molecular sieve remained well and was not destroyed,and the hydrophobic properties and methanol conversion rate were improved.(2)HZSM-5 molecular sieve was modified by citric acid under different conditions by liquid phase reflux with citric acid solution as modification reagent.Through the analysis of orthogonal experimental data,it is found that the most important factor affecting the modification effect is the modification temperature,followed by the concentration of citric acid solution,and finally the modification time.It is concluded that the modified sample has the highest comprehensive performance when the concentration of citric acid solution is 1 mol/l L,the modification temperature is 80℃ and the modification time is 12 h.The water absorption of the unmodified HZSM-5 molecular sieve was 14.74%,while the water absorption of the modified sample was 5.15%.In the reaction of crude methanol dehydration to dimethyl ether,the conversion rate of unmodified HZSM-5 molecular sieve was 67.22%,while the conversion rate of modified sample was 79.86 0%,the conversion rate of methanol was increased by 13.02%,and the catalyst stability was higher during the reaction.Through SEM,BET,XRD,FTIR,NH3-TPD,Py-IR,TG and other characterization,it was found that the skeleton structure of the modified molecular sieve remained well and was not destroyed.The hydrophobicity and methanol conversion rate of the modified sample were improved.