Control of micro/mesoporosity in non-hydrolytic hybrid silicophosphate xerogels

Abstract

Non-hydrolytic sol–gel reactions of acetoxysilanes with trimethylsilyl esters of phosphoric and phosphonic acids produce cross-linked matrices containing homogeneous dispersions of silicon and phosphoryl groups connected together by networks of Si–O–P(O) linkages. These polycondensation reactions proceed cleanly and under mild conditions for a wide variety of precursor silanes R_nSi(OAc)_4−n (R = alkyl, aryl; n = 1, 2) and phosphoryl compounds RP(O)(OSiMe₃)₂ (R = alkyl, aryl) to provide hybrid xerogels, the final properties of which are a sensitive function of the organic substituents and the Si : P ratio of the precursors. The reactions of bridged acetoxysilanes (AcO)₃Si–X–Si(OAc)₃ and phosphoryl reagents (Me₃SiO)₂P(O)–X–P(O)(OSiMe₃)₂ have also been investigated and found to produce gels that exhibit large surface areas (up to 700 m² g⁻¹). The presence of SiO₆ structural units in bridged-phosphoryl xerogels is related to their microporosity while the absence of such moieties in bridged-acetoxysilane networks is congruent with significant mesoporosity. Several important parameters are identified which can be used to tailor the properties of these hybrid matrices such that gels with specific polarity, porosity and surface area can be targeted at the time of synthesis.