Abstract
An isomorphously substituted Al-free Fe-BEA zeolite with extremely high Fe content (~7 wt.%) was synthesized by a facile and industrially friendly method using an excess amount of NaOH. The obtained Fe-BEA zeolite was highly crystalline and showed a well-facetted bipyramidal morphology, similarly to beta zeolites synthesized in a fluoride medium. The chemical states of Fe in the above zeolite were investigated by diffuse reflectance ultraviolet-visible (UV-Vis) absorption spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, and X-ray absorption fine structure (XAFS) spectroscopy. The observed chemical states were similar to those of the Fe-BEA zeolite synthesized in the presence of fluoride (Fe-BEA-F). Considering the fact that more than 88% of the micropore volume of the calcined Fe-BEA zeolite was retained after hydrothermal treatment at 1000 °C for 5 h, and 53% of the tetrahedrally coordinated Fe3+ was retained after hydrothermal treatment at 700 °C for 20 h, the obtained Fe-BEA zeolite was concluded to be highly hydrothermally stable. The synthesized zeolite was evaluated in the selective catalytic reduction of NOx by ammonia (NH3-SCR), exhibiting greater catalytic activity than Fe-BEA-F throughout the reaction temperature range. Moreover, the potential of this catalyst as a hydrocarbon adsorbent for cold-start emission control was characterized by dynamic adsorption-desorption of toluene. Interestingly, only 66% of adsorbed toluene was desorbed from the Fe-BEA zeolite (cf. 96% for commercial beta zeolite), even though the gas stream did not contain oxygen, suggesting that hydrocarbon oxidation involved oxygen stored inside the Fe-BEA zeolite.
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