Molecular dynamics and quantum chemical investigations of partially cystalline/amorphous material derived from zeolite ZSM-5

Atashi Basu Mukhopadhyay1, Christina Oligschleger2, Michael Dolg1

1 Institute for Theoretical Chemistry, University of Cologne
2 Fachhochschule Bonn-Rhein-Sieg

contact: a.basu@uni-koeln.de

Studies of zeolite-based amorphous materials are important for technological applications such as catalysis, ion-exchange and ceramic chemistry. Partial amorphization can be used to tune specific properties. In our investigations of partially amorphous materials derived from zeolite ZSM-5 we applied molecular dynamics using classical interaction potentials and canonical ensembling.

In order to generate partially amorphous structures the silicious crystalline configuration was heated to high temperatures, then equilibrated and finally quenched to 300 K followed by equilibration. The expected (local) minimum configurations were stored and then quenched to zero temperature using a combined steepest-descent-conjugate-gradient algorithm.  The extent of amorphization was quantified as the percentage of energy crystallinity (PEC),
PEC = (Eamorphous-Econfiguration) / (Eamorphous-Ecrystalline) * 100

For the detected local minima the dynamic matrices were calculated and diagonalized in order to obtain eigenvalues (squares of eigenfrequencies) and eigenvectors (types of motion).

Structural properties: The structural properties of the partially amorphous materials (black lines) were analyzed by means of pair-distribution functions and bond angle distributions. A comparison to the crystalline ZSM-5 revealed the main building principles. Slightly distorted SiO4 tetrahedra were found to be the basic units. The analysis of the connectivity between the tetrahedra mainly points to corner sharing, but a small percentage of edge sharing seems also to be present.

An important quantity for zeolites is the internal surface area (ISA). For its (approximate) determination the system is modelled as an ensemble of intersecting hard spheres with radii Rcoord depending on the coordination number (CN). The ISA was then determined using the so-called probe-atom model,
ISA = 1/M  ( sumi=1N  4 pi [Rcoord(i)+rprobe]2  pi/p  ).

Here rprobe denotes the probe-atom radius, p the total number of sample points homogenously distributed on the surfaces of the spheres and pi the number of points on sphere i not being inside other spheres. The partial amorphization of zeolite ZSM-5 lead to the following effects: For large probe radii the ISA decreases due to the reduction of the number of large pores, whereas for small probe radii the ISA increases due to the increase in under-coordination and an increasing tendency to convert large rings into smaller rings.

Vibrational properties: The relative contributions of the motions of structural subunits to the total vibrational density of states (VDOS) was analyzed by projecting the eigenvectors onto the vibrational modes of the isolated structural subunits Si-O-Si  and SiO4.

For structures with PEC of above/below 60% the intensity of the so-called Boson peak decreases/increases. We attempt an explanation  in terms of the Maxwell counting of floppy modes. The former effect is associated with a decrease of the concentration of 10-fold rings and a general lowering of symmetry by puckering of large rings. The latter behavior is related to an increasing participation of under-coordinated centers in the relevant low-frequency motions.

Finally, the struture and relative stability of edge-sharing SiO4 tetrahedra vs. the common corner-sharing SiO4 tetrahedra was investigated by quantum chemical ab initio techniques for the model systems W-silica and alpha-quartz.

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