PHOTODEGRADATION-ADSORPTION METHOD: INTRODUCTION

INTRODUCTION

The presence of sulfur compounds in fuels is a major source of atmospheric pollution (Juan et al. 2010). Recently, many attentions have been focused on decreasing the sulfur content of gas oil used in diesel engines (Matsuzawa et al. 2002). Hydrogenation desulfurization is the predominant clean-fuel technique and contributes to the removal of thiols, thioethers, bisulfides and other linear sulfides. This method cannot successfully remove aromatic thiophenes and their derivatives. Moreover, it needs high operating pressures and temperatures and highly pure hydrogen (Ping et al. 2009; Campos-Martin et al. 2010). The oxidative desulfurization (ODS) is considered to be one of the promising new methods for deep desulfurization of the fuels. It can be carried out under very mild conditions, room temperature and atmospheric pressure. Thiophene and its derivatives are major sulfur compounds in the fuels with boiling point higher than 100°C. They usually sum up to 50-80% of the total sulfur content in diesel and jet fuels.

Benzothiophene is a model for sulfur heterocycles whose presence in fuels is undesirable. Given the legal regulations limiting the sulfur content in fuels, there is a large interest in developing efficient desulfurization procedures in organic media ( Liu et al. 2008; Dai et al. 2008; Alvaro et al. 2005). Heterogeneous photocatalysis involving semiconductors, mainly titanium dioxide has attracted considerable attentions in recent years for destruction of undesirable organic pollutants with the help of solar or artificial light (Qamar, 2010). TiO2 has been most widely used photocatalyst because it is easily available, inexpensive, non-toxic, and shows relative high chemical and photocatalytical stability ( Qourzal et al. 2009). TiO2 can be used in the form of fine powder or crystals dispersed in the solution. However, the need to filter TiO2 after the reaction makes such a process troublesome and more costly. In order to solve this problem, many researchers have examined methods for fixing TiO2 on supporting materials including glass beads, fiberglass and zeolite (Nikazar et al. 2006).

Zeolites are a family of crystalline aluminosilicate minerals whose rigid framework defines channels and cavities of strictly regular dimensions in the nanometer scale. Clinoptilolite is an abundant naturally occurring zeolite, formed by the diversification of volcanic ash in lake and marine waters millions of years ago (Faghihian and Vafadar, 2007; Alvaro et al. 2004). In this study, at the first stage, we report a photocatalytic oxidation system using TiO2-clinoptilolite as photocatalyst and its application for photodegradation of benzothiophene in hexane solution. In the second part, the removal of sulfur-containig photodegradation products was examined by two zeolite; clinoptilolite and P-zeolite.