11B-MAS NMR approach to the boron adsorption mechanism on a glucose-functionalised mesoporous silica matrix

Abstract

[EN] Boron chemistry has raised much interest because, despite the difference between necessities and toxicity being very narrow, it is still widely used in industrial processes. In a previous work we reported an adsorbent for boron extraction from water by the functionalisation of a UVM-7 mesoporous silica matrix with gluconamide moieties. The ability of this material to adsorb boron is based on its well-known affinity for coordinating the cis-diols present in attached saccharide. Although much research on the formation of boron esters with sugars and sugar derivatives in solution has been done, very few reports have dealt with the adsorption mechanism of boron onto functionalised materials in the solid state. Hence an in-depth study of the adsorption process was conducted in this paper. For this purpose, several solids with increasing boron contents were prepared from the gluconamide-functionalised UVM-7 material and an isothermal boron adsorption curve was obtained. The B-11 and C-13 MAS NMR techniques were used to characterise the obtained solids, and the simulation of the boron NMR and isothermal adsorption combined data was accomplished to enlighten the boron adsorption process. A model with three different coordination environments, two possible adsorption sites and the presence of oligomeric boron species allowed us to reproduce not only the isothermal boron adsorption curve, but also the evolution of the integrated areas for the signals in the B-11 MAS NMR spectra obtained for the different boron-containing gluconamide-functionalised UVM-7 materials.