Abstract

Aim: To use the microwave radiation as the post-treatment method to tune the acidic properties of Fe/ZSM-5 catalyst.

Background: ZSM-5 zeolite is a widely used standard catalyst in the methanol conversion to olefins and high-octane gasoline range hydrocarbons. However, the coke deposition and the concentration of acid sites determine its overall catalytic activity. Thus, the concentration of acid sites more precisely, the number of Brønsted acid sites, is vital in determining the activity of ZSM-5 zeolite and product distribution in the methanol-to-hydrocarbons reaction.

Objectives: (1) To modify ZSM-5 using an iron solution to form Fe/ZSM-5 via the impregnation method.

(2) To tune the acidic properties of Fe/ZSM-5 using microwave radiation.

(3) To check and determine the concentration of acidic sites using n-propylamine temperature-programmed surface reaction.

(4) To check the effect of microwave radiation and acidic properties in the methanol conversion (product distribution) via methanol temperature-programmed surface reaction.

Methods: Two sets of zeolites were used, with iron being introduced by ion-exchange from ferric nitrate (Fe(NO₃)₃ 9H₂O, Sigma Aldrich, ≥ 98% metal) solutions. These two series were designated as Z (0.5FeZ_10/0-700) and X (0.5FeX_10/0-700) after microwave treatment. The Z and X series possess the Si/Al framework ratio of 30 and 80, respectively. The TPSR studies were then conducted for characterization and catalytic tests.

Results: From the C₃H₉N-TPSR experiments, it was found that the concentration of Brønsted acid sites decreased with increasing microwave power level. Both X and Z series exhibited high selectivity to propene than ethene. Microwave treated catalysts (0.5FeZ_10/280 and 0.5FeX_10/462) with decreased concentration of Brønsted acid sites showed the highest propene/ethene ratios of 1.67 and 5.27, respectively.

Conclusion: From the results obtained, it was found that the amount of methane evolved (as a measure of coke deposited) and the concentration of Brønsted acid sites decreased with increasing microwave power level (0-700 Watts). High selectivity to propene was found when using both X series (0.5FeX_10/0-700) and Z series (0.5FeZ_10/0-700) as catalysts. After decreasing the concentration of Brønsted acid using microwave treatment, the highest P/E ratios were observed for 0.5FeX_10/280, 0.5FeX_10/462, 0.5FeZ_10/280 and 0.5FeZ_10/462 catalysts.

It is reasonable to suggest that microwave radiation could be a feasible post-synthesis modification step to produce ZSM-5 based catalysts that exhibit reduced concentration of Brønsted acid sites, reduced methane formation, increased catalytic activity and selectivity.

Keywords: Fe/ZSM-5, microwave radiation, propene, Temperature Programmed Surface Reaction (TPSR), brønsted acid sites.

Graphical Abstract