Silica-Supported Phosphonic Acids as Thermally and Oxidatively Stable Organic Acid Sites

• Published in: Chemistry of materials 2016-09, Vol.28 (17), p.6166-6177
• Main Authors: Charmot, Alexandre, Solovyov, Andrew, DiPasquale, Antonio G, Katz, Alexander
• Format: Article
• Language: English
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.6b02027

Organic–inorganic materials consisting of organophosphonic-acid-supported-on-silica materials C3/SiO2 and C4/SiO2 are described, where C3 is propane-1,2,3-triphosphonic acid and C4 is butane-1,2,3,4-tetraphosphonic acid. Solid-state structures of both of these phosphonic acids are analyzed using single-crystal X-ray diffraction, and these data reveal extensive intermolecular hydrogen bonding and no intramolecular hydrogen bonds. Thermogravimetric analysis/mass spectroscopy (TGA/MS) data show a lack of combustion for these materials in air at temperatures below 400 °C, and only release of water corresponding to reversible organophosphonic acid condensation below 150 °C. A comparative series of silica-supported materials were synthesized, consisting of organophosphonic acid CX8, which represents a calixarene macrocycle that is decorated with a high density of organophosphonic-acid substituents on both the lower and upper rim, as well as polyvinylphosphoric acid (PVPA). Material CX8/SiO2 possesses a significantly lower thermal stability and lower combustion temperature of 300 °C in air, whereas PVPA demonstrates comparable thermal stability as observed with C3 and C4. TGA coupled with base-probe titration was used to determine the Brønsted acid site density of all silica-supported phosphonic acids at various coverages and temperatures. Material C4/SiO 2 -37% (corresponding to 37% (by mass) loading and half-monolayer coverage on silica) exhibited the highest Brønsted acid-site density of all materials, corresponding to 0.84 mmol/g at 150 °C, and 0.62 mmol/g at 300 °C. All supported phosphonic acids treated with pyridine at room temperature were strong enough acids to protonate pyridine at room temperature as exhibited by a distinct pyridinium cation band in the infrared spectrum; however, in contrast to much stronger acid sites in silica-supported phosphoric acid materials, almost all adsorbed pyridine was lost by 150 °C. Use of a stronger base for acid-site titration consisting of diisopropylamine (DIPA) demonstrates acid sites in all materials up to 300 °C, at which temperature the acid site was too weak to adsorb DIPA. Thus, these oxidatively stable materials are deemed to be useful in applications requiring weak Brønsted acid sites, while exhibiting high-temperature oxidative stability up to 400 °C.