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

Abstract

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.

Phosphonic acid: preparation and applications

“…6 , the selected phosphonic acids prepared by the hydrolysis of phosphonate with concentrated HCl water solution present only few other functional groups. This methods was applied to prepare the alkylphosphonic acid 16 [ 120 ], arylphosphonic acid 17 [ 121 ] or 18 or block co-polymers possessing pendant arms functionalized with phosphonic acids (compound 19 [ 122 ]). The hydrolysis with 20% HCl solution (≈6 M) was also occasionally applied to coordination complexes bearing a phosphonate function as exemplified with compound 20 [ 123 ].…”

“…This reaction can be achieved in a non-protic solvent including CH 2 Cl 2 [ 173 ], acetonitrile [ 174 ], chloroform [ 175 ], DMF [ 176 ], pyridine [ 177 ] or collidine [ 178 ]. The use of BrSiMe 3 simultaneously as reagent and solvent was also reported [ 120 ]. The direct production of sodium salts of phosphonic acid is readily achieved by adding 2 N NaOH/water solution to the silylated phosphonate [ 178 ].…”

“…19 ), amphiphilic compounds that were used for self-assembled monolayers (e.g., compound 61 [ 187 ]), polymers (PEG derivative 62 [ 188 ] or functionalized polyethylene 63 [ 189 ]) were reported. The McKenna’s procedure was also applied to prepare polyphosphonic acids as exemplified by the compounds 64 [ 120 ], 65 [ 190 ], 66 [ 191 ] and the dendrimers 67 [ 192 ].…”

An Environmentally Benign Solid Acid Nanocatalyst for the Green Synthesis of Carboxylic Acid Ester

“…[3] In addition, recyclable solid acid catalysts are not corrosive compared to homogeneous catalysts and facilitate sustainable, organic synthesis under heterogeneous reaction conditions. [4] So far, an adequate number of recyclable solid acid catalysts have been developed and applied in the various organic transformations in the laboratory and industry, such as zeolite, [5] silica phosphoric acid, [6] tungstophosphoric, [7] Amberlyst-15, [8] Nafion-H, [9] heteropoly acids (HPAs), [10] silica sulfuric acid, [11] chalcogenides, [12] polymers, [13] activated carbon, [14][15][16] and silica-supported zirconium sulfate. [17] In addition, recyclable catalyst grafted on silica, [18] carbon, [19] and acid clay [20] correspondingly detailed for distinctive catalytic applications.…”