Highly efficient catalytic conversion of chloromethane to light olefins over HSAPO-34 as studied by catalytic testing and in situ FTIR

Abstract

The catalytic conversion of chloromethane over HSAPO-34 was followed by in situ Fourier transform infrared (FTIR) spectroscopy and catalytic testing. Our detailed IR investigation allowed the identification of the active sites and the adsorbed species. HSAPO-34 has high activity and selectivity for the production of light olefins. The FTIR results demonstrated that the conversion starts at 350 °C with the formation of alkoxy groups as intermediate products. On adsorption and conversion of chloromethane, three distinct shifts of OH bands associated with the Si(OH)Al acidic sites were observed, attributed to the interaction between Si(OH)Al with CH 3Cl, HCl and C C groups. In the 350–500 °C temperature range, chloromethane is converted to ethylene, propylene, and butylenes with 70–80% selectivity. Increasing reaction temperature increases conversion and enhances the yield of lighter olefins. The detailed FTIR study of chloromethane conversion at 450 °C at different system chloromethane loadings reveals that increasing system CH 3Cl loading increased the conversion rate. This is attributed to the participation of gas-phase chloromethane molecules with adsorbed species enhancing the reaction rate, while at lower chloromethane loadings, the reactions may just occur between the adsorbed species. The IR studies also reveal the reversible breaking of Al O P bonds on adsorption of HCl, a main product of the reaction, to generate large amounts of P OH groups. The Al O P bonding was restored after removal of HCl by evacuation at 150 °C. These IR observations combined with the catalytic test results allow us to propose a realistic reaction mechanism.