A Combined Experimental/Computational Study on the Adsorption of Organosulfur Compounds over Metal-Organic Frameworks from Fuels

This work investigates the adsorption of organosulfur compounds in model fuels over metal organic frameworks (MOFs) using a combined experimental/computational approach. Adsorption isotherms of three MOFs, MIL-101(Cr), MIL-100(Fe), and Cu-BTC, follow the Langmuir isotherm models, and Cu-BTC shows the highest adsorption capacity for both dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT), ascribing to the highest density of adsorption sites and fairly strong adsorption sites on Cu-BTC. Experimental results show adsorption selectivity of various compounds in model fuels follows the order of quinoline (Qu) > indole (In) > DBT > 4,6-DMDBT > naphthalene (Nap), which is consistent with the order of calculated binding energies. Adsorption capacities of thiophenic compounds decrease significantly with the introduction of Qu, In, or water due to their strong competitive adsorptions over the coordinatively unsaturated Cu sites on Cu-BTC. The binding energies of Qu, In, H2O, and DBT are calculated as -56.04, -41.01, -50.27, and -27.52 kJ/mol, respectively. The experimental and computational results together suggest that the adsorption strength of thiophenic compounds over Cu-BTC is dominated by the interaction of both the conjugated pi system (pi-M) and the lone pair of electrons on sulfur atom (sigma-M) of thiophenes, with the coordinatively unsaturated sites (CUS) on Cu-BTC. Alkyl groups on 4- and/or 6-positions of thiophenic compounds function as both eletron donor to increase sigma-M interaction and steric inhibitor to decrease (sigma-M interaction. MOFs with strong and highly dense CUS can be promising materials for ADS of fuels.