TPSR study: effect of microwave radiation on the physicochemical properties of Fe/ZSM-5 in the methanol to hydrocarbons (MTH) process

The effect of microwave radiation on 0.5Fe/ZSM-5 catalysts (0.5 wt% Fe) active in the conversion of methanol to hydrocarbons was studied. Radiation was applied as a post-synthesis step using the solid catalyst only. No solvents or other liquid-phase was used. Changes in surface properties effected by the use of radiation were monitored using temperature programmed surface reaction (TPSR) and other methods of characterization used included p-XRD, SEM, BET, FT-IR, and TGA. This work is what we believe is the first time that such a study has been carried out for methanol conversion. A series of 0.5Fe/ZSM-5 based catalysts were prepared by the impregnation of HZSM-5 zeolite (Si/Al = 30 or 80) with iron with subsequent microwave radiation at various power levels in the range 0–700 W. The duration of radiation exposure was constant at 10 s. It was found that microwave radiation induced few changes in the bulk properties of the 0.5Fe/ZSM-5 catalysts, though there were some changes noted in their textural properties. In contrast, their surface and catalytic behavior were markedly changed. Microwave irradiation enhanced crystallinity and mesoporous growth, decreased coke and methane formation, and decreased surface area and micropore volume as the microwave power level was increased from 0 to 700 W. From the TPSR data, it was observed that exposure to microwave radiation affects the amounts and maximum desorption temperatures (Tmax) of the evolved aliphatic and aromatic hydrocarbons. Microwave radiation shifted the Tmax values of the MTH products over the HZSM-5(30) and HZSM-5(80)-based catalysts to lower and higher values, respectively. Effects of microwave radiation were more pronounced on HZSM-5(80)-based catalysts than HZSM-5(30)-based catalysts. More C2–C5 aliphatic hydrocarbons evolved than aromatics (BTX). The MeOH-TPSR profiles showed that methanol was converted to DME and subsequently converted to aliphatic and aromatic hydrocarbons. It is reasonable to suggest that microwave radiation could be a useful post-synthesis modification step to produce catalysts that show reduced coke and methane formation and increase catalyst activity and selectivity.