Silica gel-based adsorbents prepared via homogeneous and heterogeneous routes: adsorption properties and recycling as heterogeneous catalysts

Silica gel-based adsorbents were prepared via homogeneous and heterogeneous routes using two silane coupling reagents, 3-glycidoxypropyltrimethoxysilane and γ-chloropropyltrimethoxysilane. Characterization results showed that amino contents of the adsorbents prepared via the homogeneous route were higher than those of the adsorbents prepared via the heterogeneous route for both silane coupling reagents. The adsorption capabilities of the resulting four types of adsorbents for Hg(II), Cu(II), Au(III), Ni(II), Pb(II) and Ag(I) ions were compared. Good adsorption capability for Au(III) was observed for the new adsorbents and the maximum static saturated adsorption capacities for Au(III) could reach 0.67 mmol g−1. Due to the formation of Au(0) particles in the adsorption process, which hampered the reusability of the spent adsorbents, alternative recycling of the spent adsorbents after Au(III) adsorption was sought. The spent adsorbents were treated with NaBH4 and used as catalysts in the reduction of 4-nitrophenol to 4-aminophenol. After three catalytic cycles at 298 K, the k values indicated minimal decrease of catalytic activity. © 2017 Society of Chemical Industry