Methanol conversion to light olefins over commercial zeolites

Abstract

The conversion of methanol to light olefins (MTO) over commercial narrow pore Silicoaluminophosphate SAPO-34, and petensil type ZSM-5 and Silicalite S-115, was studied in a fixed bed tubular reaction system. Nitrogen was used as carrier gas. Experiments were careied out at various temperatures, space velocities (WHSV) and methanol-to-nitrogen (wt) ratio to study their effects on catalyst activity, selectivity, product distribution and coke deposition. ZSM-5 and S-115 were modified by impregnation with various metal elements to investigate their effects on catalyst active life time and selectivity to lower olefins. At 400 C and 4 hr-1 WHSV SAPO-34 exhibited higher initial selectivity to lower olefins than both ZSM-5 and S-115 but also deactivated more rapidly ( 2hrs). This was attributed to the catalyst's narrow pore size which enhanced coke formation leading to poisoning of active sites and blockade of pore channel entrances. The optimum operating conditions for S-115 and ZSM-5 were found to be 375?C and 450?C respectively at 4 hr?¹ WHSV and 2.67 methanol-to-nitrogen (wt) ratio. S-115 showed higher overall selectivity to olefins than ZSM-5 at optimum conditions. Active life-on-stream was also longer for S-115 than ZSM-5 with 58.1 and 25.5 hr respectively. Since both zeolites have identical pore-sizes and structures, the better performance of S-115 was attributed to its higher silica/alumina ratio of 300 as compared to ZSM-5 which had a silica/alumina ratio of 40. Modification of the two zeolites with various elements showed the degree of selectivity to light olefins to vary according to the modifying element. With S-115, the selectivity was found to vary according to the order Ba > La > Ag > Cu > Ca > Sr > Ga > Cd. In, while for ZSN-5, the selectivity decreased in the order in Ag > Cu > Cd. Optimum Baloading on S-115 for high olefin yield was found to be 5.2% by impregnation and 0.03% by ion-exchange.