Charge distribution in NaY zeolite from charge-transfer molecular dynamics
Introduction
Faujasites are the most widely used catalysts in the oil industry. Because of their importance as catalysts and molecular sieves, many attempts have been made to model their chemical and structural characteristics. Their catalytic activity strongly depends on the existence and location of acid sites, and their interaction with the adsorbed molecules. From the physical point of view one of their most important qualities is their electronic structure. Quantum-chemical simulations of zeolites have been recently performed [1]. However, this methodology is still quite expensive in computational terms and size and simulation times are rather limited. Experimentally, it is necessary to employ a series of techniques to characterise their structure. Recently, the usage of MAS NMR spectroscopy has shine some light on the existence of (nAl) of different kinds in zeolitic networks 2, 3. Even though in the last few years experimental studies have improved our understanding of these complex systems, there still are open questions to investigate, such as the combined effect of the existence and location of Brönsted and Lewis acid sites, and the role of water in their formation, which utlimately is related to proton transfer; the mechanisms of formation of extraframework alumina, and the catalytic role it plays. All these basic phenomena related to the catalytic properties of zeolites are amenable to numerical simulation.
The presence of aluminium in zeolites modifies the electronic environment and consequently produces electrostatic field gradients which are directly related to the catalytic activity [4]. Charge-transfer molecular dynamics (CTMD) [5]was developed to study systems in which bond making and breaking occurs, and in which electric charge is crucial and determines the way in which a number of phenomena such as diffusion and modifications of structure occur. In order to increase the realism of our simulations of silicates in general and of zeolites in particular we have fitted parameters for CTMD interaction potentials in order to represent the partially covalent nature of the Al–O bond, based on periodic ab initio calculations [6]. This potential along with the already existing one for the Si–O interaction allows us to simulate the zeolitic framework, improving the quality of the simulations relative to our own previous works 7, 8. In this Letter, we present and discuss one of the most interesting features of NaY zeolite, namely the charge distribution of Al, O, and Si atoms, as a step forward in the wider study of zeolite physico-chemical properties through CTMD simulations.
Section snippets
Interaction potential
Molecular dynamics simulations involve the solution of the classical equations of motion of a set of N particles in a box of volume V with periodic boundary conditions. Particles interact through pairwise additive potentials. In our simulations, the partially covalent Si–O and Al–O interactions in zeolite are described through an empirical CT scheme. The total potential energy is a sum of a Coulombic term, VCoul, plus a modified Morse term known as Hulbert and Hirschfelder potential, Vcov,
Results and discussion
As it was mentioned above, 54 silicon atoms were randomly subtituted by aluminium atoms, therefore all Si and Al atoms were in tetrahedral positions at the beginning of the simulation. As it should be expected given the thermodynamic conditions of the simulation there are not broken bonds at the end of the 20 ps of simulated time. On the other hand, all oxygen atoms are bridging either between two silicon atoms or one silicon and one aluminium atom. However, as we shall see from the charge
Conclusions
We have presented results of CTMD simulations of NaY as far as charge distributions is concerned. Charge distribution of Si atoms is intimately related to the Si chemical shift, provided there is a CT process between Al and O atoms, given that CT processes occur along a few interatomic distances. The comparison of Si charge distribution with experimental MAS NMR data, after the proper conversion of units gives outstanding good results. These results served us to further the validation of
Acknowledgements
E.M.M. thanks Professor José J. Fripiat for enlightning discussions and the Mexican Council of Science and Technology (CONACyT) for a postgraduate scholarship. This work was partially funded by DGAPA-UNAM Project IN128898.
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