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
(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
(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 (0.5FeZ10/0-700) and X (0.5FeX10/0-700) 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 (0.5FeZ10/280 and 0.5FeX10/462) with decreased
concentration of Brønsted acid sites showed the highest propene/ethene ratios of 1.67 and
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
(0.5FeX10/0-700) and Z series (0.5FeZ10/0-700) as catalysts. After decreasing the concentration of
Brønsted acid using microwave treatment, the highest P/E ratios were observed for 0.5FeX10/280,
0.5FeX10/462, 0.5FeZ10/280 and 0.5FeZ10/462 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.