A Computational Study of the Translational Motion of Protons in Zeolite H-ZSM-5
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A Computational Study of the Translational Motion of Protons in Zeolite H-ZSM-5 M.E. Franke1, M. Sierka2, J. Sauer2, U. Simon1 1 Aachen University of Technology, Institute of Inorganic Chemistry, Professor Pirlet Str. 1, Aachen, Germany 2 Humboldt University of Berlin, Institute of Chemistry/Quantum Chemistry, Jägerstr. 10/11, Berlin, Germany
ABSTRACT The potential energy profile for proton translational motions between two neighboring Alsites in zeolite H-ZSM-5 is calculated by a combined quantum mechanics-interatomic potential function approach. The potential energies of the six stable intermediate proton positions and the five transition structures along this path show an almost symmetrical trend reaching the maximum in the middle between these sites. Therefore, the maximum barrier will decrease with decreasing SiO2/Al2O3 ratio of the zeolite. For the SiO2/Al2O3 ratio examined (190) an activation energy of ~ 210 kJ/mol is calculated. This is much lower than the energy of deprotonation, about 1300 kJ/mol. The deprotonation energy of an Al-OH-Si bridge is obviously partially compensated by the proton affinity of the Si-O-Si bridges.
INTRODUCTION The interest in the proton mobility in zeolites is due to the enormous application potential of their protonic form, e.g. in heterogenous catalysis and chemical sensing. The proton motion and the corresponding activation barrier may have a significant influence on the properties and the activity, respectively, of the protons in the catalyst, which are again given by the structural environment. However, up to now no unifying concept has been developed to relate the overall activity of a zeolite catalyst to the framework structure and composition. In this paper an attempt has been made to enhance the understanding of the structure property relation in zeolites by means of theoretical calculations. In H-zeolites the protons are strongly bound to the anionic, negatively charged zeolite lattice forming a Si-OH-Al bridge, called Brønsted site (in figure 1 the Al atom of the corresponding Brønsted site is illustrated in black color). Proton mobility arises either from onsite jumps between the four oxygen atoms surrounding one aluminum site or from inter-site translational motions between two neighboring Brønsted sites. The on-site mobility has been analyzed in detail by theoretical [1-3] and experimental [4-8] methods. Translational motions have been studied by impedance spectroscopy on H-ZSM-5. For SiO2/Al2O3 ratios between 30 and 1000 activation energies between 89 and 126 kJ/mol have been found [9-11]. This is much less than the energy required to completely remove the proton from the zeolite, about 1300 kJ/mol [12]. Obviously, on its translational path the proton is stabilized by interactions with Si-OSi framework groups. To obtain specific theoretical estimates of the activation barriers, the structures and energies of intermediate stable states and of transition structures of the proton moving from one Al-site to another are determined by quantum chemical ab initio techniq
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