Fabrication of Unique Mixed-Valent Co^ICo^II and Cu^ICu^II Metal–Organic Frameworks (MOFs) for Desulfurization of Fuels: A Combined Experimental and Theoretical Approach toward Green Fuel

Abstract

Herein, metal-organic framework (MOF)-based adsorbents are designed with distinct hard and soft metal building units, namely, [Co₂^ICo^II(PD)₂(BP)] (<b>Co_PD-BP</b>) and [Cu₂^ICu^II(PD)₂(BP)] (<b>Cu_PD-BP</b>), where H₂PD = pyrazine-1,4-diide-2,3-dicarboxylic acid and BP = 4,4'-bipyridine. The designed MOFs were characterized via spectral and SCXRD techniques, which confirm the mixed-valent states (+1 and +2) of the metal ions. Topological analysis revealed the rare <b>ths</b> and <b>gwg</b> topologies for Co MOF, while Cu-MOF exhibits a unique <b>8T21</b> topology in the 8-c net (point symbol for net: {4²⁴·6⁴}). Moreover, severe environmental issues can be resolved by effectively removing heterocyclic organosulfur compounds from fuels via adsorptive desulfurization. Further, the developed MOFs were investigated for sulfur removal via adsorptive desulfurization from a model fuel consisting of dibenzothiophene (DBT), benzothiophene (BT), and thiophene (T) in the liquid phase using <i>n</i>-octane as a solvent. The findings revealed that <b>Cu_PD-BP</b> effectively removes the DBT with a removal efficiency of 86% at 300 ppm and an operating temperature of 25 °C, with a recyclability of up to four cycles. The adsorption kinetic analysis showed that the pseudo-first-order model could fit better with the experimental data indicating the physisorption process. Further, the studies revealed that adsorption capacity increased with the increasing initial DBT concentration with a remarkable capacity of 70.5 mg/g, and the adsorption process was well described by the Langmuir isotherm. The plausible reason behind the enhanced removal efficiency shown by <b>Cu_PD-BP</b> as compared to <b>Co_PD-BP</b> could be the soft-soft interactions between soft sulfur and soft Cu metal centers. Interestingly, density functional theory (DFT) studies were done in order to predict the mechanism of binding of thiophenic compounds with <b>Cu_PD-BP</b>, which further ascertained that along with other interactions, the S···π and S···Cu interactions predominate, resulting in a high uptake of DBT as compared to others. In essence, <b>Cu_PD-BP</b> turns out to be a promising adsorbent in the field of fuel desulfurization for the benefit of mankind.