Catalytic Performance Of Cr/ZSM-5 For Dehydration Of Methanol

In this work,CrO_x nanoparticle-and H⁺-modified Na ZSM-5 was prepared by impregnating Na ZSM-5 in chromium nitrite aqueous solution to form Cr/HNa ZSM-5composites.The prepared materials were characterized by FT-IR,SEM,XRD,TEM,UV-vis,XPS,²⁹Si-²⁷Al MAS NMR,NH₃-（CO₂-）TPD and N₂ adsorption-desorption techniques.The results showed that the Cr/HNa ZSM-5 material was formed by the high dispersion of Cr O_x nanocrystals on the surface of the molecular sieve and the partial substitution of H⁺for Na⁺.The catalytic performance of the prepared Cr/HNa ZSM-5 composites for the synthesis of dimethyl ether from gas-phase methanol dehydration was evaluated.The results showed that the composite material can efficiently catalyze the methanol dehydration reaction at a lower temperature.The relationship between the catalytic activity and the acidity and basicity of the catalyst surface was studied.It was confirmed that the excellent catalytic performance was derived from the acidic and basic sites of activated methanol,which generated CH₃⁺and CH₃O^-intermediates,respectively.The effects of catalyst preparation conditions and reaction conditions,such as catalyst calcination temperature,metal loading,reaction temperature and methanol flow rate on the reaction were investigated.The optimum calcination temperature of catalyst preparation was determined to be 250°C,and the Cr loading amount was 0.064%.The optimum reaction conditions were as follows:reaction temperature 170°C,methanol flow rate 0.02 m L/min.Under the optimum conditions,the conversion of methanol was 93%and the selectivity was100%.The results of the catalyst life test showed that the catalyst worked continuously for 500 hours without any decrease in catalytic activity.The composition and structure of the catalyst before and after the reaction did not change significantly,and there was no carbon deposition.Cr/HNa ZSM-5 composites exhibit excellent operational stability.In addition,the adsorption behavior of methanol on the catalyst surface was measured by in-situ infrared spectroscopy,and the nature of the interaction between the acid-base active sites on the catalyst surface and methanol was confirmed,that is,the formation of CH₃⁺and CH₃O^-intermediates and the combination of the two to form dimethyl ether.