Transition metal oxides supported on activated carbon for adsorptive desulfurization of diesel fuels

Abstract

In this study, period four transition metal oxides (Zn, Cr, Mn, Co, Fe, Cu, Ni), acting as intermediate Lewis acids, were supported on activated carbon (AC) and used in the adsorptive desulfurization (ADS) of model and commercial diesel in both batch-mode and fixed-bed applications. The Ni-oxide/AC had the highest desulfurization performance and removed 84.49 % of sulfur in model diesel and 95.17 % in commercial diesel at 30 °C. A trend in the ADS activity of period four transition metal oxides supported on AC was identified in both commercial and model diesel in the order of: Ni-oxide/AC > Cu-oxide/AC > Co-oxide/AC > Fe-oxide/AC > Mn oxide/AC > Cr-oxide/AC > Zn-oxide/AC. The metal oxides/AC adsorbents had a higher selectivity for the sulfur compound, in the order of 4-E,6 -MDBT > 2,4,6 -TMDBT > 4,6-DMDBT, then the unmodified AC adsorbent. There was no correlation between the Pearson hardness η metal oxide cation and the observed percentage of sulfur removal. However, a strong inverse linear relationship was observed sulfur removal decreased as the ionic covalent parameter (ICP) of the mixed metal oxide cations increased. Our findings suggested a novel concept in ADS of diesel fuel, i.e. that the ionic-covalent parameter (ICP) is a simple but more effective method of measuring the Lewis acidic strength of transition metal oxides. The adsorption kinetic of commercial diesel occurred via chemisorption in all the adsorbents used. Multi-component equilibrium isotherm modeling was used to identify how the competition interactions between sulfur compounds influenced the desulfurization performance. The results indicated that the incorporation of NiO onto AC significantly improved the synergistic interactions between the sulfur compounds and mitigated the competitive interaction between the more steric 4,6-DMDBT and 4-MDBT compounds. Kinetic and thermodynamic process modeling suggested that adsorption occurred though chemisorption and became more favorable with an increase in temperature. Spent adsorbent analysis suggested that the high desulfurization performance of the Ni(10%)O/AC adsorbent was attributed to the increase in the adsorbents Lewis acidity upon loading with NiO, which led to an increase in Ni-S acid-base interactions, π-complexation interactions, and breakage in the aromatic rings of the sulfur compounds.