Microporous inorganic/organic hybrids via oxysilylation of a cubic symmetry nanobuilding block [(HMe₂SiOSiO_1.5)₈] with R_xSi(OEt)_4−x

Abstract

The B(C₆F₅)₃ catalyzed Piers-Rubinsztajn (oxysilylation) reaction of the cubic symmetry Q-cage [(HMe₂SiOSiO_1.5)₈] with ethoxysilanes in hexane forms microporous 3-D networks coincident with ethane evolution. Slow drying provides monoliths whereas fast drying provides powders. The reaction is most efficient if initiated at 60°C for 5 min and then allowed to progress at ambient. The products offer high specific surface areas [SSAs > 700 m²/g, e.g. with Si(OEt)₄], with micropores of 0.6–2.0 nm, mesopores of 2–40 nm, total pore volumes ≈ 0.5 cc/g, and thermal stabilities to 320°C. Changes in reaction conditions (times, solvent volumes, catalyst concentrations) do not significantly change product properties pointing to rapid and complete reaction as evidenced by the near absence of residual Si–H under all conditions for 1:1: Si–H:Si–OEt ratios. RSi(OEt)₃ gives similar 3-D microporous gels. Smaller R groups give higher SSAs than those with large R groups; however, R = n-octyl is not porous. Rigid, bridged compounds [(EtO)₃Si–R–Si(OEt)₃] (R = phenyl, biphenyl) offer high SSAs whereas flexible bridges [R = (CH₂)_{2 or 8}] give reduced SSAs. All materials were characterized by FTIR, TGA, XRD and BET. XRD studies show periodicities suggesting some long range ordering as might be expected for reactions with cubic symmetry Q₈ cages. However, the fast reaction rates likely generate kinetic rather than thermodynamic products that cannot be expected to exhibit high degrees of ordering. Gel affinities for specific organics were studied showing some preferential ability to absorb specific solvents for example the 1:1 OHS:vinylSi(OEt)₃ gels were more selective for toluene than the 1:1 OHS:TEOS gels.