Title : Catalytic performance of modified delaminated zeolite ITQ 2 in selective catalytic reduction of nitrogen oxides with ammonia
Abstract:
The emission of nitrogen oxides (NOx) from stationary and mobile sources is among very significant environmental issues. According to the Best Available Techniques, currently the most efficient method to reduce NOx production is selective catalytic reduction with ammonia (NH3-SCR). This technology aims to transform NOx into nitrogen and water vapor on the surface of the catalyst. The commercial SCR systems typically use vanadium-based catalysts, for example V2O5-TiO2 promoted with MoO3 or WO3. However, weak activity below 300 °C and the narrow temperature window of 300-400 °C, combined with sulfur dioxide oxidation above 300 °C and easy deactivation by alkali compounds, limits further application of these catalysts on the industrial scale. Therefore, the goal of our work was to design an alternative catalyst free from the disadvantages mentioned above.
Zeolites modified with transition metals are generally considered as the leading alternative catalysts for V2O5-TiO2. It results from their high activity in broad temperature window, high stability, and resistance to poisons present in the flue gas, such as SO2. Despite the interests in zeolites as the precursors of new NH3-SCR
catalysts, only few studies considered application of ITQ-2 in this reaction.
ITQ-2 is a delaminated zeolite, belonging to the MWW family. It is obtained from 2D precursor, MCM-22 (P) by swelling and subsequent delamination. ITQ-2 exhibits so-called “house of cards” structure and very high specific surface area of ca. 700 m2 · g-1. Well-defined pore structure and the abundance of acidic sites makes ITQ-2 an excellent candidate for new catalyst of NH3-SCR.
The catalytic activity of metal-zeolites is determined by the speciation of the active phase [6]. To date, research on the incorporation of metals into zeolitic frameworks has mainly focused on the methods such as ion-exchange or impregnation [7]. However, in the case of ion-exchange usually multiple repetitions of the procedure are required to obtain the desired amount of the active phase. Impregnation results in pore-blocking upon aggregation of metal oxides in the channels. In contrast, one-pot synthesis of zeolites reduces the preparation procedure to only one step and yields well-dispersed metal species. To our knowledge, no one has examined catalytic performance of one-pot synthesized Fe-ITQ-2 in NH3-SCR.
According to the above-mentioned, our work describes the design of the new NH3-SCR catalyst, Fe-ITQ-2, prepared by one-pot synthesis. We analyzed the dependence of catalytic performance on Si/Fe molar ratio in the samples. We compared the obtained results to the activity of the reference, Fe-ion-exchanged ITQ-2. The physicochemical properties of the catalysts were analyzed using ICP-OES, X-ray diffraction, UV-Vis and FT-IR spectroscopy, low-temperature sorption of N2, SEM microscopy, and NH3-TPD.
Our work has demonstrated that one-pot synthesis results enables preparation of highly active catalyst in low- temperature range of the reaction. The effect can be ascribed to the presence of isolated Fe3+ cations, incorporated into the zeolitic framework and highly acidic character of Fe-ITQ-2. Our study provides the foundation for a new way of preparing catalysts of NH3-SCR supported on MWW zeolites.
