Spectroscopic Characterization of Dealuminated H-Mordenites: The Role of Different Aluminum Species on the SCR of NO with Methane

• Published in: Journal of catalysis 1997-06, Vol.168 (2), p.511-521
• Main Authors: Lezcano, M., Ribotta, A., Miró, E., Lombardo, E., Petunchi, J., Moreaux, C., Dereppe, J.M.
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts: preparations and properties; Chemistry; Exact sciences and technology; General and physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1006/jcat.1997.1697

In order to understand the role of different aluminum species in the selective catalytic reduction of nitrogen oxides with methane over H-mordenites, solids with varying Si/Al ratios (5.9–16.9) were prepared by acid leaching. They were thoroughly characterized before and after reaction. The distribution of Al was determined through27Al MAS NMR. All the samples presented three signals, one at 54 ppm corresponding to lattice AlIV, another at 0 ppm associated with octahedrally coordinated Al, and a broad band, BB (ca 100 ppm wide), assigned to aluminum containing species. As the spinning rate increased up to 11.3 kHz, a decrease of the BB intensity and an increase of the AlIVsignal took place, while the AlVIslightly increased. The best estimate of lattice aluminum was obtained from the AlIVpeak intensity. Despite the high spinning rate employed, it was possible to observe only between 70–80% of the total Al present in the samples. The catalysts were also analyzed by XRD, FTIR, and129Xe NMR of physisorbed Xenon. By correlating the variation of theacell constant with Al/u.c., only qualitative information was obtained. The IR band shift at ca 572 and 588 cm−1at higher wave lengths, and the decrease of the bands intensity at 650 and 730 cm−1with decreasing Al content were examined. These changes in the IR spectra are a clear indication of the dealumination process carried out in the samples, thus supplementing the27Al MAS NMR results and supplying information on the dealumination mechanism as well.129Xe NMR results shows that nonlattice aluminum may interrupt the free exchange of molecules between the main channels and side pockets. The turnover frequency of NO disappearance remains constant with varying lattice aluminum content. The catalysts were partially deactivated after being on stream at 650°C due to the additional dealumination occurring at high temperatures in the reacting stream. Both in fresh and used catalysts, only the sites related with lattice aluminum were active in the reaction under study. The nonlattice, polymeric species, generated during dealumination hinder the access of the reactant molecules to the active sites.