Trapping of BTX compounds by SiO2, Ag–SiO2, Cu–SiO2, and Fe–SiO2 porous substrates

Adsorption isotherms of BTX aromatic hydrocarbons (benzene, toluene, and p-xylene) on pristine (SiO2) and metal-doped (Ag–SiO2, Cu–SiO2 and Fe–SiO2) mesoporous and microporous substrates were measured and interpreted. These adsorbents were synthesized by the sol–gel procedure and their BTX sorption isotherms were obtained by the gas chromatographic technique (GC) at several temperatures in the range 423–593 K. The uptake amount of these hydrocarbon adsorptives on SiO2, Ag–SiO2, Cu–SiO2 and Fe–SiO2 mesoporous and microporous substrates was temperature-dependent. Additionally, the interaction of BTX molecules with the pore walls was evaluated by means of the corresponding isosteric heat of adsorption (qst), which was found to follow the next increasing sequence: qst (benzene) < qst (toluene) < qst (p-xylene). In general, the isosteric heat of adsorption of aromatic BTX compounds on microporous silica depicted an increasing tendency when the amount adsorbed was raised. This was a consequence of the existence of cohesive interactions (adsorbate–adsorbate) besides of the adhesive ones (adsorbate–adsorbent). The inclusion of silver or iron atoms within the SiO2 structure leads to an increased adsorbed amount of BTX molecules on the solid surface if compared with the Cu–SiO2 adsorbent. The adsorption of benzene, but not of toluene and p-xylene, molecules on pristine SiO2 is facilitated by the pore size of this substrate since this is the highest of all materials.