Preparation Of Phosphomolybdates And The Application In Catalytic Oxidative Deep Desulfurization Of Diesel Fuel

With people's environmental awareness increasing, the control target of sulfur content in fuel oil continues to increase. In fact, zero-emission of sulfur in fuel oil will be the ultimate goal for the future of the world. The conventional hydrodesulfurization process (HDS) is inefficient for removing of thiophene sulfurs and has become the bottleneck for deep desulfurization of fuel oil. However, the oxidative desulfurization method (ODS) has been widely concerned for its mild reaction conditions, efficiency and easy to take off thiophene sulfur which is difficult to be removed by HDS. In the paper, eight types of phosphomolybdates have been prepared by double decomposition, and the samples'constructions and thermal properties have been characterized by Fourier transform infrared spectroscopy (FT-IR), powder X-ray diffraction (XRD), ultraviolet-visible spectroscopy (UV-vis), thermogravimetry-differential scanning calorimetry (TG-DSC) and scanning electron microscopy (SEM). Moreover, the oxidative desulfurization of model oil (benzothiophene and dibenzothiophene) and diesel fuel catalyzed by the samples, the separation methods for the oxidized diesel fuel and the catalytic reaction mechanism of oxidative desulfurization catalyzed by phosphomolybdates have been discussed.Metallic phosphomolybdates have been prepared by mixing of phosphomolybdic acid and lanthanum chloride, cerium chloride, vanadium oxide chloride and chromium chloride via double decomposition reaction, respectively. FT-IR analysis shows that the phosphomolybdates keep the Keggin structures. XRD analysis indicates that the additions of lanthanum, cerium, vanadium and chromium are favorable for the structure formation and improve of the phosphomolybdates. UV-vis. analysis shows that the characteristic absorption peaks become a little red shift with mixing of metallic elements into the counter positions, which cut down the transition energies of the Mo-Ob-Mo and Mo-Oc-Mo bonds. TG-DSC analysis shows that the surface and heteropoly anion structures of the phosphomolybdates have been changed with mixing of lanthanum, cerium, vanadium and chromium, respectively.The catalytic oxidative desulfurizations of model oil and diesel fuel have been investigated by phosphomolybdic acid, lanthanum phosphomolybdate, cerium phosphomolybdate, vanadium oxide phosphomolybdate and chromium phosphomolybdate, respectively. The results show that the amount of catalyst, initial concentration of H2O2, reaction temperature and reaction time are impacted on the oxidative desulfurization, and the oxidative reactions of dibenzothiophene (DBT) and benzothiophene (BT) are in conformity with apparent-first order kinetics. The conversion rates of DBT and BT under microwave radiation heating are significantly higher than those of traditional heating with the same reaction conditions. When catalyzed by lanthanum phosphomolybdate and extracted once with V(DMF)/V(diesel) of 1/4, the better desulfurization rate of diesel fuel reaches 72.2 % with the recovery rate of 97.6 %, and the sulfur content in the diesel fuel reduces from 994 g/g to 276 g/g, which meets the European III standard.Quaternary ammonium phosphomolybdates have been prepared by mixing of phosphomolybdic acid and tetramethyl ammonium chloride (TMAC), dodecyl trimethyl ammonium chloride (DTAC), hexadecyl trimethyl ammonium chloride (HTAC) and octadecyl trimethyl ammonium chloride (OTAC) via double decomposition reaction, respectively. FT-IR analysis shows that the quaternary ammonium phosphomolybdates keep the Keggin structures. XRD anslysis indicates that HPMo clusters are finely dispersed with mixing of quaternary ammonium salts, and the dispersion levels increase with longer alkyl chains in the catalysts. UV-vis analysis shows that the bond strengths of Mo=O, Mo-Ob-Mo and Mo-Oc-Mo lower with mixing of quaternary ammonium salts, which indicates the favorable ability for oxygen supply. TG-DSC analysis indicates that the surface and heteropoly anion structures of the phosphomolybdates have been changed with mixing of quaternary ammonium salts and the thermal stabilization of the quaternary ammonium phosphomolybdates has been cut down. SEM anslysis shows that the catalysts form the loose structure like the molecular sieves, which leads to the higher dispersion of the catalytic active centers and higher catalytic properties.The catalytic oxidative desulfurizations of model oil and diesel fuel have been investigated by tetramethyl ammonium phosphomolybdate, dodecyl trimethyl ammonium phosphomolybdate, hexadecyl trimethyl ammonium phosphomolybdate and octadecyl trimethyl ammonium phosphomolybdate, respectively. The results show that the amount of catalyst, initial concentration of H2O2, reaction temperature and reaction time are impacted on the oxidative desulfurization, and the catalytic oxidative reactivity towards DBT is much higher than that of BT under the same reaction conditions. The oxidative reactions of dibenzothiophene (DBT) and benzothiophene (BT) are in conformity with apparent-first order kinetics. The apparent activation energies of DBT catalyzed by dodecyl trimethyl ammonium phosphomolybdate, hexadecyl trimethyl ammonium phosphomolybdate and octadecyl trimethyl ammonium phosphomolybdate are 31.4 kJ/mol, 26.8 kJ/mol and 22.5 kJ/mol, respectively, and the apparent activation energies of BT are 45.7 kJ/mol, 54.5 kJ/mol and 62.4 kJ/mol, respectively. As the alkyl chains in the tetramethyl ammonium phosphomolybdate are too short to form stable emulsion system, so the catalytic reactivy of the catalyst is low. Octadecyl chains are more favorable to wrap up DBT to the catalytic center and form stable emulsion system with the better conversion rates of DBT. The shorter dodecyl chains can wrap up BT more suitably and bring smaller steric hindrance, which displays the better conversion rates of BT. Under the condition of m(catalyst)/m(diesel) 1.8 %, v(H2O2)/v(diesel) 2.5 %, 70°C and 3 h, the desulfurization rate of diesel fuel reaches 88.7 % with the recovery rate of no less than 99 % catalyzed by octadecyl trimethyl ammonium phosphomolybdate.The effects of the separation methods, such as the types of extractant, extractant dosage and extraction times, for the desulfurization rates and the recovery rates of the diesel fuel have been investigated. The results show that the diesel fuel can get the high desulfurization rate and the high recovery rate with using DMF as the extractant. To cut down the solvent/diesel ratio and increase the extraction times, the total amount of the extraction agent can be reduced and the higher desulfurization rate and recovery rate can be got. With the extraction times increasing, the desulfurization rates increase, but the recovery rates of diesel decrease significantly. Therefore, the high desulfurization rate and the high recovery rate are contradictory for the oxidative desulfurization of diesel fuel.The oxidized products of DBT and BT have been traced by gas chromatography (GC) and gas chromatography/mass spectrum (GC/MS), and the reaction mechanism of oxidative desulfurization of diesel fuel has been speculated. Firstly, hydrogen peroxide attacks nucleophilicly the active center MoVI of the catalyst to form the active hydrogen peroxide MoVI. Secondly, the sulfur atom in the sulfur compounds attacks nucleophilicly the hydrogen peroxide MoVI and gives out one water molecule to form the MoVI peroxy radicals. Thirdly, the MoVI peroxy radicals attack the sulfur atoms to form sulfoxides and give out the active center MoVI. Then, the sulfoxides can be oxidized to sulfones with a similar oxidation process of the previous, and the leased active site MoVI enters a new cycle of catalytic oxidation. As the catalytic oxidation of diesel fuel is a multi-phase reaction and the efficient catalytic emulsion system can be formed with mixing of quaternary ammonium salts, the catalytic reactivity of quaternary ammonium phosphomolybdates are much higher than those of metallic phosphomolybdates.