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 423-593K. 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 (q_st), which was found to follow the next increasing sequence: q_st (benzene)<q_st (toluene)<q_st (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.