Aluminophosphates water

To evaluate properties of basic catalysts, the Knoevenagel condensation over aluminophosphate oxynitrides was investigated [13]. In this reaction usually catalysed by amines, the solid catalysts function by abstraction of a proton from an acid methylene group, which is followed by nucleophilic attack on the carbonyl by the resultant carbanion, re-protonation of oxygen and elimination of water. The condensation between benzaldehyde and malononitrile is presented below. 

Hydration of the NH4-form of SAPO-34 and SAPO-37, that is, of materials that were ammoniated at the bridging OH groups, caused a coordination of water molecules exclusively to Al atoms in =P-O-A1= bridges. This process led to a hydrolysis of the framework (220). No hydrolysis of the silicoaluminophosphate framework occurred, provided that not only the bridging OH groups (SiOHAl), but also the aluminophosphate framework (=P-O-A1=) was covered by ammonia. The latter finding may explain the stabilizing effect of preloaded ammonia on silicoalumino-phosphates toward hydration and weak hydrothermal treatments as recently observed for H-SAPO-34 (227). 

A typical synthesis procedure involves the following steps (i) alumina is slurried in water, (ii) phosphoric acid is diluted in water, (iii) the phosphoric acid solution is added to the alumina slurry, (iv) the aluminophosphate precursor mixture is aged at ambient conditions, (v) an organic is added to the precursor mixture and aged with rapid agitation to form the final gel, which (vi) is charged into the autoclave and heated. The gel composition can be written as... 

Synthesis of large AFI-tvpe crystals. For these studies of the crystallization of large AFI-type aluminophosphate crystals, the reaction temperature T, the concentration of tripropylamine Pr3N, and the amount of water were chosen as operational variables. Synthesis runs were performed with each of these "factors" at a high (+) and a low (-) level, for molar compositions with b Pr,N - A1 0-, - P203 - c H20 as indicated in Table I ... 

The amount of water present during the synthesis of large AFI-type aluminophosphate crystals was found to be of significant importance. However, yield of AlP0 -5 and crystal size were countercurrently influenced. It follows tnat at least with tripropyl amine, either few large crystals or maximum conversions into small particle AlP0 -5 is possible. 

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.

Naturally, structures (d) and (f) do not exhaust all possible states of low-coordinated A1 atoms on the surface of the oxides considered. The calculations, however, seem quite sufficient to suggest that water molecule coordination by a LAS is energetically less favorable for aluminophosphate than for aluminosilicate surfaces. This conclusion is also in accordance with IR data, which indicate that LASs of the both oxides quite similarly interact with pyridine, whereas the LASs of aluminophosphates do not coordinate C02 molecules (136). Indeed, in the case of a sufficiently strong base (pyridine), adsorption interaction appears stronger than the structural coordination and therefore stabilizes the A1 atom in the adsorption state. On the contrary, for C02, which is certainly a very weak base, the interaction is strong enough in the case of aluminosilicates but is insufficient for the adsorption stabilization of aluminum in aluminophosphates. 

Fig. 18.2. The aluminophosphate AIPO-5. AIO4 and PO4 tetrahedra, which are strictly alternating, are represented in gray, and water molecules have been omitted for clarity.

Aluminophosphate A1PQ4 5. AlPO -5 was obtained from compositions containing pseudoboehmite (Pural SB, Condea) which was added to aqueous phosphoric acid (Merck, 85%) and deionized water. Finally, tripropyl amine (Aldrich) was continuously added to the stirred solution. The crystallization time was limited to 72 hours. 

Although an alcohol instead of water was used in the solvothermal system, a small amount of water was still unavoidably introduced due to the use of 85 wt% of phosphoric acid in water as phosphorus source and boehmite (6-coordinated Al) as A1 source. If a fluoride source was used as mineralization agent, a small amount of water could be introduced by the fluorine source as well. Studies indicate that the small amount of water played an important role in the formation of the structures of aluminophosphates from solvothermal systems.[74]... 

Compared with the amount of organic solvent, the amount of water in the solvothermal synthesis system is little. However, it plays an extremely important role in the successful formation of the final structures. Previous studies revealed that the synthetic reaction would not happen without the presence of this small amount of water. Another direct proof showing the importance of this small amount of water is the variation of structure type crystallized from the aluminum isopropoxide-phosphoric acid-triethylamine-tetra-ethylene glycol-water (extra added) system with the variation of the amount of extra added water, keeping the other parameters constant. By a gradual increase in the amount of the extra added water, one-dimensional aluminophosphate chain, two-dimensional aluminophosphate sheet, three-dimensional aluminophosphate open-framework JDF-20 and AIPO4-5, and dense-phase cristobalite sequentially crystallized from this system under conditions of identical crystallization temperature and time. 

---