TY - JOUR
T1 - DFT study on the accommodation and role of la species in ZSM-5 zeolite
AU - Li, Yanfeng
AU - Liu, Hui
AU - Zhu, Jiqin
AU - He, Peng
AU - Wang, Peng
AU - Tian, Huiping
N1 - Funding Information:
The financial supports by National Basic Research Program of China (973 program; No. 2010CB732301 ) and National Scientific Funding of China ( No. 20706005 ) are gratefully acknowledged. We are grateful to Prof. S.P. Huang for his valuable comments and suggestions.
PY - 2011/7
Y1 - 2011/7
N2 - Theoretical investigations on the location and structure of lanthanum species in lanthanum-modified ZSM-5 zeolites were carried out. The mechanism of change in the acid properties of the ZSM-5 zeolite after addition of lanthanum cations, and the influence of lanthanum species on the hydrothermal stability of the ZSM-5 zeolite were also studied. Based on optimized cluster models and thermodynamic analyses it is demonstrated that the introduced lanthanum cations lie in the symmetrical six-membered oxygen rings with Al situated at T11 sites in the straight channels and exist as La(OH)2+ up to a typical hydrocarbon cracking temperature of 950 K. This structure is further confirmed by the existence of weak intramolecular hydrogen bonds between the hydroxyls in La(OH)2+ species and the lattice oxygens. Similarly, we found that the introduced lanthanum cations transformed the strong Brønsted acid sites (Si-OH-Al) into the weak Brønsted acid sites. Accordingly, the weak Brønsted acid sites have O-H stretching frequencies of 3742 and 3762 cm-1 and are in the form Si-OLa(OH)2-Al. Our results also suggest that the hydrothermal stability of the ZSM-5 zeolite is improved by introducing lanthanum cations due to the strengthened Al-O bond, increased steric hindrances, weakened acidity and weak intramolecular hydrogen bonds. Finally, we extended our investigation by considering the influence of the lanthanum species on the deactivation rate of the ZSM-5 zeolite.
AB - Theoretical investigations on the location and structure of lanthanum species in lanthanum-modified ZSM-5 zeolites were carried out. The mechanism of change in the acid properties of the ZSM-5 zeolite after addition of lanthanum cations, and the influence of lanthanum species on the hydrothermal stability of the ZSM-5 zeolite were also studied. Based on optimized cluster models and thermodynamic analyses it is demonstrated that the introduced lanthanum cations lie in the symmetrical six-membered oxygen rings with Al situated at T11 sites in the straight channels and exist as La(OH)2+ up to a typical hydrocarbon cracking temperature of 950 K. This structure is further confirmed by the existence of weak intramolecular hydrogen bonds between the hydroxyls in La(OH)2+ species and the lattice oxygens. Similarly, we found that the introduced lanthanum cations transformed the strong Brønsted acid sites (Si-OH-Al) into the weak Brønsted acid sites. Accordingly, the weak Brønsted acid sites have O-H stretching frequencies of 3742 and 3762 cm-1 and are in the form Si-OLa(OH)2-Al. Our results also suggest that the hydrothermal stability of the ZSM-5 zeolite is improved by introducing lanthanum cations due to the strengthened Al-O bond, increased steric hindrances, weakened acidity and weak intramolecular hydrogen bonds. Finally, we extended our investigation by considering the influence of the lanthanum species on the deactivation rate of the ZSM-5 zeolite.
KW - Acidity
KW - Density functional theory
KW - Hydrothermal stability
KW - Lanthanum
KW - ZSM-5
UR - http://www.scopus.com/inward/record.url?scp=79954423527&partnerID=8YFLogxK
U2 - 10.1016/j.micromeso.2011.01.007
DO - 10.1016/j.micromeso.2011.01.007
M3 - Article
AN - SCOPUS:79954423527
SN - 1387-1811
VL - 142
SP - 621
EP - 628
JO - Microporous and Mesoporous Materials
JF - Microporous and Mesoporous Materials
IS - 2-3
ER -