Adsorptive desulfurization using period 4 transition metals oxide: A study of Lewis acid strength derived from the adsorbent ionic-covalent parameter

Abstract

Pearson’s Hard and Soft Acids and Bases (HSAB) principle has been widely utilized in describing the adsorption trend of sulfur compounds when using transition metal/metal oxides as adsorbents. This study is the first to report on the use of the ionic covalent parameter (ICP) for explaining the adsorption trend of sulfur compounds. Period 4 transition metal oxides (Zn-oxide, Cr-oxide, Mn-oxide, Co-oxide, Fe-oxide, Cu-oxide, Ni-oxide), which act as intermediate Lewis acids, were incorporated into activated carbon (AC), and used in the adsorptive desulfurization (ADS) of model and commercial diesel samples. The study findings show that desulfurization activity in model diesel increased in proportion to the concentration of adsorbent Lewis acid sites. With commercial diesel, the loading of metal oxides significantly enhanced the sulfur adsorption capacity of AC and increased its selectivity for steric sulfur compounds. The results showed a poor correlation between sulfur removal and the Pearson hardness ηth of the intermediate metal oxide cations. However, a strong inverse linear relationship was observed: sulfur removal decreased as the ICPth of the adsorbent metal oxide increased. The study findings suggest a novel concept, i.e. that the ICP is a more accurate measure of the Lewis acidic strength of transition metal oxides with varying crystal structures and metal geometries, and their subsequent desulfurization performance. Kinetic modelling showed that chemisorption was the controlling mechanism. The fixed-bed ADS of commercial diesel when using unmodified AC is best described by the Yoon-Nelson and Thomas models. Adsorption when using the Ni-oxide/AC adsorbent was correlated to the filtration advection–dispersion equation.