Solvothermal Synthesis Approach to Aluminophosphates

So far, a large number of nondense aluminophosphates have been successfully synthesized via the solvothermal synthesis system. Most of them have anionic frameworks except for those crystallized from hydrothermal synthesis systems, such as AlP04-5, A1P04-11, and A1P04-21. The stoichiometries of these aluminophosphates include 

Besides the composition of the initial gel, the type and properties of the alcohol solvent play an important role in the formation of the final structure of aluminophosphates as well. For example, Xu and coworkers studied in detail the correlation between the crystallized products and the properties of more than 20 alcohols for the system 5 Et3N-Al203-1.8 P2Os-X ROH at 180 °C and the results are summarized in Table 3.7[29] They found that the structure of the product mainly depends on the polarity of solvent. Among 

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. 

Because Al and P atoms in the final stmcture come from the isolated aluminum and phosphorus sources in the parent mixture, respectively, the formation reaction of Al-O-P linkage must have happened during the crystallization process. Considering the importance of the introduction of organic amines to the synthesis system, we can 

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