SBA-15 pore-width decrease via a one- or a two-step covalent bonding of a Fischer tungsten carbene as measured by N2 sorption

Abstract

The chemical attachment of Fischer tungsten carbene molecules around the cylindrical walls of SBA-15 causes a pore-width diminution with respect to the diameters of the voids existing in the pristine substrate. The attachment is made via a bridging unit of aminopropyltriethoxysilane (APTES) that, at its ethoxysilane end, anchors on the SBA-15 surface while, at its amino end, reacts with the tungsten carbene. The attachment can either be made in a single stage by adding the whole molecule, i.e. bridging + carbene units, or stepwise, i.e. the bridging unit is firstly attached to the silica surface and afterward the carbene group is linked to the amine end of the previous species. A remarkable result is that a uniform pore-width decrease of 0.30 nm is achieved when the ethoxysilane bridging units are anchored on the SiO2 surface; this decrease being independent of the pore size. An additional decrease of 0.37 nm is observed when the tungsten carbene is fixed to the amine end of the bridging molecule; the total pore-width decrease is thus 0.67 nm. In turn, the one-stage insertion of the whole carbene molecules causes a uniform pore-width decrease of 0.78 nm. The central cores of the functionalized pore entities remain free for the molecular transport of species that are taking place in catalytic, separation, and controlled release processes. The chemical anchoring of molecules on the surface of SBA-15 pores can be tailor sized and occlude the existing micropores, thus representing important and fundamental industrial applications. Copyright � 2008 John Wiley & Sons, Ltd.