Adsorption of polycyclic aromatic hydrocarbons (PAHs) from isooctane solutions by mesoporous molecular sieves: Influence of the surface acidity

Abstract

Three different mesoporous molecular sieves of MCM-41 type (a pure silica and two aluminum doped silica with Si/Al molar ratios of 10 and 30) were evaluated for adsorption of polycyclic aromatic hydrocarbons (PAHs) from an organic solution. The aluminum containing samples were synthesized at room temperature using a sol–gel method with two reaction steps: acid hydrolysis of the metallic alkoxides (pH 2) and alkaline condensation of the hydrolysis products (pH 10). The removal of the organic template (dodecylamine hydrochloride) was performed by solvent extraction (ethanol/HNO 3 0.1 M) followed by calcination at 550 °C for 3 h. Surface chemistry of synthesized samples was analysed by X-ray photoelectronic spectroscopy (XPS). Acidity values (proton concentration) were obtained using thermogravimetric analysis of the adsorbed cyclohexylamine (CHA) at 250 °C on the calcined materials. Equilibrium adsorption properties of naphthalene, anthracene and pyrene (PAHs) in isooctane were measured at 25, 40 and 60 °C by the contact method (finite bath). The Langmuir model correlates well the equilibrium data, indicating decreasing adsorption maximum capacity ( q m) in the following order: Al-MCM-41(10) > Al-MCM-41(30) > MCM-41. For all samples, PHA adsorption capacity increased with the number of aromatic rings. The estimated heats of adsorption (−Δ H) show a regular increase with increasing number of π-electrons in the aromatic structure. These results indicate that electrostatic interactions between the π-electrons of aromatic rings and the acidic centers of the MCM-41 materials, caused by both isomorphic and extra-framework Al, may play a significant role in adsorption of PAHs in addition to van der Waals forces.