Vol.11 no.3 2019
10/58

Research paper : Challenge towards synthesis of non-silica-based hybrid mesoporous materials (T. KIMURA)−117−Synthesiology - English edition Vol.11 No.3 (2018) not only CnTMA, but also alkyl polyoxyether (CnEOm) or polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer (EOnPOmEOn) could be used.[16][17] By using the changes in alkyl chain length and different polymerization number, it was confirmed that the pore size could be controlled in the range from around 2 nm to little less than 10 nm. At this stage, the surfactant was removed by low-temperature (for example 400 °C) firing, and therefore, I could only make products with bridged organic groups with relatively high heat resistance, such as methylene groups or ethylene groups. Using a similar synthesis method, for example, I attempted synthesis from benzene bridged phosphonates that could be functionally designed and had high heat resistance. However, I was unable to obtain mesoporous aluminum phosphonates with high structural order. The details will be explained later, but this presented the limit of synthesizing from organically bridged phosphonates. That is, I reached the conclusion that it was difficult to extend this substance unless there was a breakthrough in the precision reactivity control of the starting material.To solve the second issue, the development of an extraction method of the surfactant, efforts were made by trial and error. As a result, it was found that CnEOm and EOnPOmEOn could be dissolved (removed) simply by heating in an acetone solvent.[18][19] I achieved a situation in which a hybrid framework could be designed without considering heat resistance of bridged organic groups, and this was an extremely important result that pushed the research forward. It is thought that the phosphate (P-OH) group on the solid surface showing acidity acted as a catalyst to break down the EO and PO units. In mesoporous aluminophosphate, the stability of the mesoporous structure was so low that the structural order gradually degenerated even when there was steam present. Therefore, in a case of aluminum phosphonates that contained an aluminophosphate-like framework, it was important to do acetone treatment so there would be few water (H2O) molecules as possible. It was thought impossible to conduct performance evaluation for mesospace with hydrophilic environment unless this low stability was resolved. Fortunately, it was found that the stability of a mesoporous structure increased with the introduction of bridged organic groups. As a hydrophobic organic group was introduced proximal to the aluminophosphate-like framework, hydrolysis by H2O molecules was inhibited to some degree. As a result of introducing an organic group, the material surface became somewhat hydrophobic. Understanding this as being a local structure, the aluminophosphate-like structure that is the point of absorption of H2O molecules will remain exposed at the pore surface. That is, the road to performance evaluation of a hydrophilic surface environment or its proximity was not closed.3.2 Reactivity control of starting material: Possibility of phosphonate compoundsThe reactivity of bisphosphonate is changed dramatically when organic groups with high electron density such as benzene linkers are bridged. In this case, the reactivity with AlCl3 was decreased, and it was thought that mesoporous materials could not be synthesized.[20] Here, synthesis was done using a mixed solvent of ethanol-water, but it was necessary to consider the solubility of organically bridged phosphonates in the solvent. For example, xylene bridged phosphonate was not dissolved in an ethanol-water mixed solvent, and even preparation of a precursor solution could not be done. In synthesis from bisphosphonates bridged by simple alkyl groups, mesoporous materials could be synthesized in an optimal synthetic condition even if organic groups were slightly different. When the property changed greatly as in benzene linker, the optimal synthetic condition could no longer be applied. For example, in the synthesis of mesoporous metal phosphate, the importance of appropriately selecting the difference between acid and alkaline of the starting material has been reported.[21] In accordance with reactivity in the early reaction, in this synthesis system, when bisphosphonate ester (H5C2O)2OP-R-PO(OC2H5)2 was used as a starting material instead of its acid form (HO)2OP-R-PO(OH)2 (R = bridged organic group), its reactivity with AlCl3 was too high and a gel was formed. Considering that the reactivity of phosphonates was insufcient, it was noted that reactivity with AlCl3 could be designed appropriately if there was a similar compound with reactivity somewhere in between that of phosphonates and esters.3.3 Diversification of organic groups: From alkyl group to aromatic compoundsThough it is referred to as a phosphonate compound here, phosphonic acid is made by treating phosphonate esters in excessive amount of aqueous hydrochloric acid solution. If the treatment is done in conditions in which hydrochloric acid is lacking against the number of ester groups, a compound in an intermediate condition, that is, a phosphonate compound in which acid and esters coexist in the same molecular structure is obtained. I thought it would be possible to conduct continuous reaction control if the percentage was changed arbitrary. Here, the actual synthesis of mesoporous aluminum phosphonates from benzene bridged phosphonate esters when AlCl3 was used as the aluminum source will be explained. Synthesis was done using as starting material phosphonate compounds with different degrees of hydrochloride treatment on bisphosphonate esters. As a result I succeeded in obtaining a mesoporous lm with extremely high structural order.[22] The result of TEM observation is shown in Fig. 6. It can be seen that the mesopores are arranged evenly throughout the lm.I also obtained a mesoporous lm using a similar approach from phosphonate compounds bridged by aromatic

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