Phosphotungstic acid H3[PMo12O40](PTA) with the Keggin structure has become well known as a solid superacid with pKa≈-13. Such a strong acidity is caused by delocalization of the negative charge of the anion on many oxygen atoms over the surface of the Keggin structure. High acidity of PTA and its good solubility in water and other polar solvents enables its use as a highly active homogeneous catalyst. However, in spite of relatively higher reaction rate, homogeneous catalysis has various drawbacks that limit its practical application. The main drawback is the difficult and expensive removal of the used catalyst from the reaction mixture and its recycling. PTA also demonstrated good catalytic activity as a heterogeneous catalyst of various organic reactions, e.g. hydrolysis, hydration and polymerization. Wide application of a pure superacid in catalysis is limited by its low surface area and solubility in polar solvents. The objective of this work is the synthesis and study of insoluble superacidic catalysts covalently embedded into the silica matrix. The catalyst PTA/SiO2 was synthesized by the sol-gel method. Tetraethoxysilane was co-condensed with PTA in acidic media in the presence of Pluronic P123 surfactant as a pore-forming agent. The obtained gel was air-dried and calcined at 500 °C producing a mesoporous material with a significant fraction of micropores in its structure. Isotherms of adsorption/desorption of nitrogen indicated cylindrical shape of the pores with necks that is typical for materials obtained with Pluronic P123 as a template. Cs-exchanged material was prepared by mixing PTA/SiO2 with a solution of CsCl. The cation exchange on cesium decreased the total pore volume due to a much higher volume of cesium ions as compared to protons. In addition, partial pore blocking by these ions restricted access to small pores thus reducing accessible surface area. Heteropolyacids are unstable in alkaline media that makes direct solid-state titration impossible. Surface acidity of the samples was determined by reversed titration. Dry samples were dispersed in a solution of pyridine in tetrahydrofuran. After equilibration, the solid phase was filtered, and the filtrate was titrated by HCl. PTA/SiO2 has a very high adsorption capacity on pyridine, which corresponds to 15 molecules of pyridine per [PMo12O40]3- anion. This number exceeded the number of available protons. The catalysts were successfully tested in the alkylation of mesitylene by alkenes. The use of superacidic materials in catalytic reactions can significantly improve the effectiveness of the processes.
| Identifer | oai:union.ndltd.org:ETSU/oai:dc.etsu.edu:asrf-1058 |
| Date | 05 April 2018 |
| Creators | Musrock, Henry, Nshizirungu, Patrick, Alorkpa, Esther, Vasiliev, Aleksey |
| Publisher | Digital Commons @ East Tennessee State University |
| Source Sets | East Tennessee State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Appalachian Student Research Forum |
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