On configuration of exchanged La3+ on ZSM-5: A theoretical approach to the improvement in hydrothermal stability of La-modified ZSM-5 zeolite

Abstract

Density functional calculations have been employed to investigate the locating and binding of lanthanum cation, i.e., La͑OH) 2 ϩ , on HZSM-5 zeolite. Through geometry optimization, it was determined that lanthanum ions are favorably accommodated in the two 6-T rings of the straight channels ͑Clusters 1 and 2, see Sec. III A for details͒. Cluster 1 was found to exist in prior to Cluster 2 due to the preference of Al substitution in the T11 site ͑Cluster 1͒ rather than in the T8 site ͑Cluster 2͒. Geometry-optimization of Cluster 1 containing another two lanthanide ions Nd 3ϩ and Yb 3ϩ was also carried out and it was found that a monotonic decrease in Ln-O bond length will take place as the atomic number increases, conforming well to the rule of lanthanide contraction. Some of the optimized parameters are comparable to the corresponding experimental values in Y zeolite, which confirms that the optimized configurations are acceptable. The average frequencies of hydroxyls attached to La 3ϩ or Yb 3ϩ in Cluster 1 fall at 3609.16 and 3579.76 cm Ϫ1 , respectively, with the gap of these two frequencies close to that in the sodalite cage of Y zeolite. Compared to H-form zeolite, the charges on both Al and O atoms in Ln-ZSM-5 zeolite show an obvious increase, which will undoubtedly lead to a stronger mutual interaction and hence enhance the stability of the ͓AlO 4 ͔ Ϫ anion. Moreover, the Ln͑OH) 2 ϩ seem to have thickened the zeolite framework, which can effectively retard the process of dealumination. Through the evaluation of the possibility for dimer formation, it turned out that when the exchange degree arrived to approximately 0.28, lanthanum monomers began to aggregate into dimers, and were completely converted into dimers when the exchange degree approached 0.60.

Promotional effects and mechanism of second cations on activity and stability of Co-MOR for nitrous oxide decomposition: UV–Vis spectroscopy and EXAFS analysis

“…For example, La ions were found to retard dealumination from the zeolite and thus stabilize Co-mordenite catalysts for NO reduction [10]. In contrast, the addition of a noble metal greatly increased the resistance to water deactivation and the author suggested synergetic effect existed in the bimetallic system [14].…”

“…It has been shown that addition of a second metal, such as rare earth metal La or noble metal Pd or Pt, to Co-zeolites could improve their stability in the presence of water vapor [10][11][12][13][14]. For example, La ions were found to retard dealumination from the zeolite and thus stabilize Co-mordenite catalysts for NO reduction [10].…”

Enhancement on the hydrothermal stability of ZSM-5 zeolites by the cooperation effect of exchanged lanthanum and phosphoric species

“…two substituted Al sites close to each other. The 6-T rings in straight channels provide the most suitable environment for exchanged La cations, with Al situated at Al 11 site [8]. Combined with previous experimental and theoretical results [16][17][18], the same T 11 site along with a nearby T 12 site was chosen to be occupied by two Al atoms (see Fig.…”

“…Such situation heavily restricts any improvement on the current catalyst systems. Theoretical calculations, which have been successfully applied in dealing with the interaction of zeolite-exchanged species previously [8,[11][12][13][14][15], will be attempted to resolve the above problems in this paper.…”

“…Nevertheless, RSCC process still has to be operated at relatively high temperatures (about 680 8C); moreover, the used catalysts are regenerated under steaming environments (about 800 8C), which will cause severe collapse to the zeolite framework and in turn a remarkable drop in catalytic activity [3][4][5][6]. Rare earth ions, confirmed to be effective in retarding the dealumination processes [1,2,7,8], are introduced into the ZSM-5 zeolites. Another modifier H 3 PO 4 was selected out by Derewiński and Lischke et al to improve the reaction selectivity catalysed by the ZSM-5 zeolites [9,10].…”