Reaction mechanism of oxidative desulfurization of heterocyclic organic sulfides: a computational study

Catalytic oxidative desulfurisation over Co/Fe-γAl2O3 catalyst: performance, characterisation and computational study

The world faces the challenge to produce ultra-low sulfur diesel with low-cost technology. Therefore, this research emphasised on production of low sulfur fuel utilising nanoparticle catalyst under mild condition. A small amount of cobalt oxide (10-30 wt%) was introduced into the Fe/Al₂O₃ catalyst through the wet impregnation method. Cobalt modification induces a positive effect on the performance of the iron catalyst. Hence, the insertion of cobalt species into Fe/Al₂O₃ led to the formation of lattice fringes in all directions which resulted in the formation of Co₃O₄ and Fe₃O₄ species. The optimised catalyst, Co/Fe-Al₂O₃, calcined at 400 °C with a dopant ratio of 10:90 indicating the highest desulfurisation activity by removing 96% of thiophene, 100% of dibenzothiophene (DBT) and 92% of 4,6-dimethyl dibenzothiophene (4,6-DMDBT). Based on the density functional theory (DFT) on Co/Fe-Al₂O₃, two pathways with the overall energy of -40.78 eV were suggested for the complete oxidation of DBT.

A DFT study on catalytic oxidative desulfurization with H2O2 over Ti-MWW zeolite

The catalytic mechanism of Ti-MWW in oxidative desulfurization with H₂O₂ was investigated by quantum chemical calculations. A defect model (Ti-d) and a perfect model (Ti-p) were proposed for Ti-MWW, and two possible reaction pathways starting from Ti-d and Ti-p were considered. On Ti-d, the hydroperoxy bidentate intermediate TiOOH (η²) was formed by activating H₂O₂ at the Ti center. Afterwards, aromatic sulfides were oxidized to sulfoxides and to ultimate sulfones by TiOOH (η²). The order of oxidation reactivity was benzothiophene > dibenzothiophene > thiophene, conforming to experimental observations. The Ti-p pathway proposed for oxidation of sulfides with H₂O₂ resulted in higher energy barriers compared to the Ti-d pathway. Natural bond orbital charge analysis was carried out to understand the charge distribution. This work showed that the defective Ti-MWW model for oxidative desulfurization was more active than the perfect model. Graphical abstract Catalytic oxidative desulfurizationwith H₂O₂ over Ti-MWW zeolite.

Desulfination by 2'-hydroxybiphenyl-2-sulfinate desulfinase proceeds via electrophilic aromatic substitution by the cysteine-27 proton

Density functional theory shows that the rate-limiting desulfination step in biodesulfurization involves concerted electrophilic substitution with the Cys-27 proton.

Biodesulfurization is an attractive option for enzymatically removing sulfur from the recalcitrant thiophenic derivatives that comprise the majority of organosulfur compounds remaining in hydrotreated petroleum products. Desulfurization in the bacteria Rhodococcus erythropolis follows a four-step pathway culminating in C–S bond cleavage in the 2′-hydroxybiphenyl-2-sulfinate (HBPS) intermediate to yield 2-hydroxybiphenyl and bisulfite. The reaction, catalyzed by 2′-hydroxybiphenyl-2-sulfinate desulfinase (DszB), is the rate-limiting step and also the least understood, as experimental evidence points to a mechanism unlike that of other desulfinases. On the basis of structural and biochemical evidence, two possible mechanisms have been proposed: nucleophilic addition and electrophilic aromatic substitution. Density functional theory calculations showed that electrophilic substitution by a proton is the lower energy pathway and is consistent with previous kinetic and site-directed mutagenesis studies. C27 transfers its proton to HBPS, leading directly to the release of SO₂ without the formation of a carbocation intermediate. The H60–S25 dyad stabilizes the transition state by withdrawing the developing negative charge on cysteine. Establishing the desulfination mechanism and specific role of active site residues, accomplished in this study, is essential to protein engineering efforts to increase DszB catalytic activity, which is currently too low for industrial-scale application.

Mechanism of extractive/oxidative desulfurization using the ionic liquid inimidazole acetate: a computational study

The dual role of the ionic liquid 1-butyl-3-methyl-imidazolium trifluoroacetic acid ([C₄mim]TFA) as an extractant for thiophene (TH) and a catalyst for the oxidation of TH was explored at the molecular level by performing density functional theory (DFT) calculations. The calculated interaction energies demonstrated why [C₄mim]TFA is a better extractant for thiophene sulfone (THO₂) than for TH. Two pathways were proposed for the oxidation of TH to THO₂ with [C₄mim]TFA acting as a catalyst. In the dominant pathway, a peracid is formed which then oxidizes TH to the sulfoxide and sulfones. The presence of [C₄mim]TFA was found to greatly reduce the barrier to the oxidative desulfurization (ODS) of TH using H₂O₂ as an oxidant. Graphical Abstract Possible reaction mechanisms of TH with the aid of [C4mim]TFAᅟ.