Thermal evolution of silicophosphate gels: Porosity and structure
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| Year of publication | 2014 |
| Type | Conference abstract |
| MU Faculty or unit | |
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| Description | Non-hydrolytic sol-gel reactions are viable alternatives to classical aqueous techniques in the area of synthesis of multimetallic oxides and inorganic-organic hybrid materials in the form of xerogels, nanoparticles, and thin films. We developed a non-hydrolytic sol-gel route based on acetic acid ester elimination providing silicophosphate hybrid inorganic-organic materials. The polycondensation reactions between Si(OAc)4 and OP(OSiMe3)3 lead to microporous xerogels with surface areas up to 568 m2 g-1. The structure of xerogels was built up exclusively from Si-O-P bonds and contained octahedrally coordinated silicon atoms, which are characteristic for crystalline silicon phosphates.1 We changed starting precursors to acetoxysilanes and phosphonic acid esters with bridging alkyl or aryl groups (AcO)3Si-(CH2)x-Si(OAc)3 (x = 1-3, 6), (Me3SiO)2P(O)-3R-P(O)(OSiMe3)2 (3R = C2H4, C6H4). Silicon in acetoxysilanes with bridging organic groups was not able to acquire hexacoordination in contrast to Si in Si(OAc)4. The change of the structure of the xerogels, which were in this case built up from CSiO3 tetrahedra, was accompanied by the modification of textural properties - the hybrid silicophosphates displayed significant mesoporosity. The resulting samples were chemically modified with POCl3, water and methanol in order to introduce Brönsted acidic P-OH groups onto the surface of xerogels. The products of chemical modification resembled solid phosphoric acid catalysts and were utilized in methylstyrene dimerization. The mesoporous silicophosphate samples with a high number of P-OH groups on the surface provided excellent yields and selectivities in this catalytic test reaction. The prepared xerogels were characterized by solid-state 13C, 29Si, 31P NMR, IR, surface area analysis, TGA and XRD. |
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