Aluminophosphates model clusters

One more type of cluster can be obtained by intracluster saturation of the broken bonds (41). This model was used in the comparative description of the surface centers of aluminophosphates and aluminosilicates (see Section V). 

The catalytic activity of aluminophosphates has been discussed by Tada et al. (133-135). It seems also of interest to perform a theoretical investigation of possible active centers (both BAS and LAS) in these systems and to compare them with the respective centers in aluminosilicates. Such a comparison implies certain requirements both to the scheme of computations and to the choice of the cluster models. Most important is that the procedure of saturating the dangling bonds of a cluster should affect the results to a minimal extent. A simple way of attaining this aim is to construct closed clusters with terminal bonds mutually saturating each other (41). 

Fig. 21. Model dusters of aluminosilicates and aluminophosphates. The lengths of bonds marked by crossing were varied to minimize the energy of the system. Optimization of the bond lengths in structure (d) gave rise to activationless migration of the Al atom through the 0,0203 plane inside the cluster, with simultaneous loss of the water molecule.

Kramer and co-workers used ab initio calculations of H4TO4 (T = Si, Al, P) clusters to derive parameters for the rigid ion potential model. The potential energy surface of the clusters was scanned along two modes of distortion, and the resulting potential curves were fitted using Eq. [15]. The set of parameters was refined by the use of experimental data on a-quartz. This procedure resulted in a parameterization that well reproduced both structure and elastic moduli of silicates, aluminosilicates, and aluminophosphates. Subsequently, this approach was extended to protonated forms of zeolites. ... 

VPI-5 is an aluminophosphate framework with very large one-dimensional pores defined by 18-member ring[22]. The crystal structure report[7j of synthesized VPI-5 revealed the possible role of water molecules as templates. The use of VPI-5 as a molecular sieve and as a catalyst primarily depend on the removal of the occluded water molecules without the destruction of the framework structure. Prasad et al.[23] reported from their TGA experiment that the seven distinct types of water molecules could be desorbed from VPI-5 in a step-wise fashion, in the temperature range of 35 to 60°C. The cluster model calculations[24] have revealed the actual electronic interaction of water molecules with VPI-5 framework. CG technique also indicated that the void volume in VPI-5 could be controlled by the partial removal of water molecules. 

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