The Synthesis Of Dual-acidic Deep Eutectic Solvents And Their Application In Deep Desulfurization Of Fuels

Fuel oil is one of the most important energy sources in people's life and industrial production.However,sulfur oxides(SOx)caused by combustion of sulfur compounds contained in fuels is one of the main sources of atmospheric pollutants.With the global environmental problem intensifying,countries around the world have passed regulations to limit the maximum sulfur concentration in oil products,among which the EU and China have stipulated that the sulfur content of fuel oil must not exceed 10 ppm.As the traditional process in industry,hydrodesulfurization(HDS)technology requires high reaction pressure and temperature with expensive catalysts.Nevertheless,aromatic sulfur compounds,such as benzothiophene(BT),dibenzothiophene(DBT),and 4,6-dimethyldibenzothiophene(4,6-DMDBT),are difficult to remove using HDS.Deep eutectic solvents(DESs)are synthesized based on hydrogen bond interactions and its structure is designable.By screening the structure and acidity of hydrogen bond acceptor(HBA)and hydrogen bond donor(HBD),DESs severs as both extractant and catalyst were obtained.In this paper,amino acids and organic strong acids were selected as HBA and HBD,respectively.Dual-acidic DESs were synthesized and studied in "one-pot"desulfurization system with hydrogen peroxide as oxidant.Four dual-acidic DESs were synthesized using a one-step method,with L-Pyroglutamic acid(L-Pyro)and L-proline(L-Pro)as HBAs,and trifluoracetic acid(TFA)and formic acid(For)as HBDs.The desulfurization efficiency of L-Pyro/TFA,L-Pro/TFA,L-Pyro/For,and L-Pro/For,were investigated in different oxidative desulfurization(ODS)system,among which L-Pyro/TFA was further studyed as the extractant and catalyst owing to its excellent properties for removing aromatic sulfur compounds.Hydrogen bond formation of DES between L-Pyro and TFA was confirmed using 1H NMR and infrared spectra.The factors affecting the desulfurization efficiency,such as the DES/model oil volume ratio(VDES/Voil),reaction temperature,and H2O2/DBT molar ratio(n(H2O2)/n(DBT)),were explored.The sulfur removal rates of DBT,4,6-DMDBT,and BT were up to 99.7%,99.6%,and 99.2%,respectively,under the optimum conditions,which were VDES/VOil=1:20,n(H2O2)/n(DBT)=3,and T=60?.Furthermore,the reaction equilibrium could still reach at 45min even the the initial sulfur concentration increased to 1000 ppm.Adding cyclohexane,p-xylene,1-octene and cyclohexene to the model oil to explore the effect on desulfurization rates,it was found that adding cyclohexene caused the sulfur removal rate to decrease sharply.The desulfurization efficiency reached 98%after DES regeneration,achieved by simply washing with ultrapure water.The fluorine contained in DESs is not accordant with the concept of"green chemistry".Therefore,TFA was replaced by p-toluenesulfonic acid(p-TsOH)as the HBD and six ternary DESs were synthesized with polyethylene glycol(PEG)added as the diluent.The effects of relative molecular weight and the amount of PEG to extraction desulfurization and ODS process,were explored.Under the optimum experimental conditions,which were VDES/VOil=1:20,n(H2O2)/n(DBT)=5,and T=70?,for L-Pyro/2TsOH/2PEG200,the desulfurization efficiency of DBT,4,6-DMDBT,and BT could reach up to 99.4%,96.54%and 83.02%,respectively.The desulfurization capability of DES was regeneration and restored by washing with water.In this paper,the "one-pot" desulfurization process for these two types of DESs follows the first-order reaction kinetics model through dynamics calculation and analysis,and the reaction selectivity to organic sulfur follows the order of DBT>4,6-DMDBT>BT.The HBDs of dual-acidic DESs would exist in the form of peroxy acid state after the H2O2 was added,and organic thiophene sulfur were oxidized to the corresponding sulfones.