Gallophosphates

In 1982, Union Carbide reported the synthesis of the first phosphate-based molecular sieves (95, 96). This finding would include aluminophosphates (A1P04) and gallophosphates (GaP04) as well as silicoaluminum phosphates (SAPO) and metalloaluminum phosphates [MAPO, M = first-row transition metal (TM)]. The... 

Many gallophosphate molecular sieves are unstable to calcination in air or in the presence of moisture, which limits the utility of these materials because at room temperature (RT) the templates stuff the channels thus limiting access to the porosity. The ULM-16 structure is interesting because it is stable to 800°C under argon and up to 350-400°C in the presence of oxygen. This stability may reflect the fully connected framework of ULM-16. 

The structure of Cloverite (IZA structure code, CLO) has limited thermal stability in air, which is not surprising for gallophosphates. It has been shown that Cloverite is stable in ambient air or a vacuum up to 500°C, but if the molecular sieve is cooled below 100°C, the structure collapses (153). If the calcined Cloverite is immediately placed in hydrocarbon solvents it can be stored at RT for several months. 

Other Framework Compositions GrystaUine microporous frameworks have been reported with composihons of beryUophosphate [43], aluminoborate [44], alumi-noarsenate [45], galloarsenate [46], gallophosphate [47], antimonosilicate [48] and germanosilicate [49],... 

In 19851 a gallophosphate with a porous framework structure was reported by Parise (ref. 4). This was followed by the systematic synthesis of the GaPO family (refs. 5,6) in our laboratory and the structural characterization of several single crystals of the compounds (refs. 7 11). 

Recently, we also focused attention on the synthesis of aluminoarsenates and galloarsenates with open framework structures (refs. 12—14). While the aluminophosphates are labeled as AlPO s, the gallophosphates, aluminoarsenates and galloarsenates can be designated as AlPO s, AlAsO s and GaAsO s respectively. 

FIGURE 7.25 The framework of the synthetic gallophosphate, cloverite, represented as shaded spheres of van der Waals radii. (From C.R.A. Catlow (ed.) (1992) Modelling of Structure and Reactivity in Zeolites, Academic... 

The physical-chemical properties of a synthetic gallophosphate molecular sieve, the 30-A supercage cloverite , have been assessed [18]. Instead of attempting to list the burgeoning number of fullerene publications, attention is drawn to the formation and characterization of fullerene-like nanocrystals of tungsten disulfide [19,20]. Preparation, characterization and utilization of carbon nanotubes have been the subject of a number of reports from several laboratories [21-27]. 

The first breakthrough was provided by Flanigen et al. [7] who, playing on the similarity 2 Si4+ -o-A13+ -f P5+, synthesized microporous aluminophosphates (hereafter noted AlPOs) with structures related to those of zeolites. The structural studies [8] showed however a striking difference between the two families. As already mentioned, the framework of zeolites is built up exclusively from connected tetrahedra which can accept small amounts (<10% of substitution) of other metals, whereas in aluminophosphates and homologous gallophosphates, Al and Ga polyhedra can adopt five and sixfold coordinations, which change [9] the connectivity of the framework, and therefore the shape of the windows. 

The alumino and gallophosphates were formed [7] in the systems amine-Al(Ga)203—P205—H20 using the same way of the synthesis and the same range of temperatures and pressure. However, the introduction of phosphoric acid in the synthesis lowered the pH of the solutions to a neutral or weakly acidic medium which favours higher coordinations for aluminium and gallium ions. The combined effect of pH and of the shape of the amines lead to ca two hundred structure types. 

---