(460g) Kinetics and Mechanism of Adsorption of Aromatic Sulfur Compounds On MOFs From Liquid Fuels

Metal-Organic Frameworks (MOFs) constitute the novel kind of well-ordered 3D “hybrid” organic-inorganic polymers that contain metal coordination centers surrounded by organic “linker” structural units. MOFs are of a strong current interest to the Chemistry and Chemical Engineering communities due to unique combination of 1) well-defined nano- and mesoporous structure; 2) extremely high specific surface area, up to 5,000 m²/g, and 3) presence of metal cations that can form adsorption or catalytic active sites. MOFs feature moderate to high stability toward oxygen, temperature and non-water solvents, and they have been extensively tested for selective adsorption and separation of many gases of industrial importance such as hydrogen, carbon dioxide, acetylene etc. However, separation of organic compounds from non-water solutions was studied in much lesser extent.

Large-ring aromatic sulfur compounds e.g. benzothiophene (BT), dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) are present in raw petroleum, refinery napthas and commercial liquid fossil fuels. Aromatic sulfur compounds cause air pollution, degradation of catalytic converters of combustion engines, and deactivation of fuel-reforming catalysts of fuel cells. Adsorption capacity of some large-ring aromatic sulfur compounds from model liquid fossil fuels with certain MOFs was previously reported. One popular group of MOFs contains anions of 1,3,5-benzenetricarboxylic acid as organic “linker”; two well-known MOFs of this group are Cu-containing Basolite C300 aka HKUST-1 and Fe-containing Basolite F300 aka MIL-100(Fe).

Here, we report thermodynamic and kinetic studies of molecular mechanisms of temperature-dependent reversible adsorption/desorption of BT, DBT, 4-MDBT and 4,6-DMDBT on C300 and F300 MOFs from model liquid fuel in tetradecane n-C₁₄H₃₀ (model diesel fuel or refinery naphtha). Adsorption of BT, DBT, 4-MDBT and 4,6-DMDBT on Basolite C300 and F300 proceeds without formation of molecular products in liquid phase, as found by the RP-HPLC with the UV absorbance detection. Adsorption capacity follows the trend at 25 and 75 °C: BT > DBT > 4-MDBT > 4,6-DMDBT, consistently with molecular size and diameters of major kinds of nanopores present in C300 MOF. Kinetics of adsorption at 25 and 75 °C proceeds on timescale of hours, is more complex than either zeroths, first or second order rate law, and is due to adsorption on different sites in C300 and F300 MOFs. Two types of adsorption kinetics are observed, depending on initial molar concentration of aromatic sulfur compound: i) preferential adsorption of aromatic sulfur compound and ii) preferential adsorption of solvent n-C₁₄H₃₀, followed by preferential adsorption of aromatic sulfur compound. Experimentally determined excess adsorption isotherms of binary solutions “aromatic sulfur compound in n-C₁₄H₃₀“ on MOF and calculated thermodynamic equilibrium constants support the assignments of adsorption sites.