AN INFRARED SPECTROSCOPIC STUDY OF THE MECHANISM OF CHLOROMETHANE CONVERSION TO HIGHER HYDROCARBONS ON HZSM5 CATALYST

Investigation of the reaction mechanism of chloromethane on ZSM5 is a new topic. In this work an in situ FTIR technique was employed to stud y the conversion processes of chloromethane, the active sites on HZSM5 , and the desorption state of surface species. The catalytic conversio n of chloromethane to higher hydrocarbons was also studied. It is demo nstrated that chloromethane can be reversibly adsorbed on acidic sites of HZSM5 at room temperature. At 100 degrees C chloromethane is irrev ersibly and dissociatively adsorbed on the strong acidic sites of HZSM 5, on which surface methoxyl is formed as proved by infrared character istic C-H stretchings of -CH3 at 2960 and 2870 cm(-1). Alkoxyls are pr oduced and adsorbed on the catalyst surface as characterized by the in frared absorption bands of -CH2- groups at 1460 and 2930 cm(-1). At 10 0 degrees C the adsorbed methoxyl and alkoxyls are the main surface sp ecies, and a small amount of aromatics might exist as detected by a ch aracteristic absorption band at 1510 cm(-1). Between 100 and 200 degre es C the adsorbed surface methoxyl and alkoxyls are converted to aroma tics, and the occupied OH groups partially appear. At temperature high er than 300 degrees C the adsorbed aromatics are thermally desorbed in to the gas phase. Aromatics and alkanes are the main products in catal ytic conversion. These results reveal that the formation of aromatics from methoxyl and alkoxyls is easier than the desorption of aromatics from HZSM5 catalyst. An alkoxyl mechanism is proposed for the conversi on of chloromethane on HZSM5 based upon the experimental results and t he three assumptions: (a) The primary C-C bond is formed from surface methoxyl groups via the methoxyl group polarization and C-H bond weake ning. (b) The adsorbed alkoxyls are converted to aromatics via hydroge n transfer and bond rearrangement similar to the conventional carbeniu m ion mechanism for the aromatization of olefins and alkanes on HZSM5. The hydrogen atoms from the aromatization stimulate the desorption of alkoxyls to alkanes. (c) At temperature higher than 300 degrees C sur face reactions and desorption of adsorbed species take place simultane ously, determining the product distribution in the catalytic conversio n.