| The pollutants emitted by vehicular engines greatly affect the air quality, especially the major source of sulfur oxide (SOX), including sulfur dioxide (SO2) and other combustion related pollutants from sulfur compound containing fuels. These ultimately lead to a series of environmental issues, such as smog, global warming and water pollution, which may badly threaten human health. Diesel sufur content of our country is much higher than developed countries. In order to meet the stringent environmental requirements and the low sulfur standard of diesel, researchers should be committed to developing variety of deep desulfurization methods. At present, the main adopted ways to reduce sulfur content in diesel are hydrodesulfurization (HDS) and non-hydrodesulfurization (NHDS). Hydrodesulfurization requires high operation cost and severe operation conditions. Consequently, the oxidative desulfurization (ODS) of diesel would become the main technique to produce ultra low sulfur diesel. Hydrogen peroxide (H2O2) in aqueous solution is the commonly used oxidant in ODS, but the use of H2O2 suffers from some shortcomings, such as difficulty separation with fuels and the low utilization efficiency of H2O2. Taking environmental and economic concerns into account, the utilization of oxygen (O2) as the oxidant for oxidative desulfurization process (ODS) is most desirable, especially the advantages of air has received universal attention. The new system for avoiding the shortcoming of high cost and poor desulfurization efficiency has attracted much attention. Adding catalyst could improve oxidative desulfurization rate.This work is the first to use oxygen (O2) as oxidant, heteropoly compound as catalyst to realize deep desulfurization under mild conditions. Several heteropoly compounds type catalysts were synthesized, characterized, and evaluated for ODS. The influencing factors on ODS were investigated, and the mechanism of ODS was proposed. In this paper, heteropoly compound type catalyst was successfully synthesized, which keeps the catalytic performance of heteropolyacid. The catalyst shows excellent catalytic performance on ODS, and it can be recovered after reaction. Our present work developed a new and green catalyzed system to realize deep desulfurization of refractory sulfur compounds from fuel oil, which should provide a new pathway for efficient oxidative desulfurization under eco-sustainable conditions. This article includes the following four chapters.(I) A series of remarkable crystalline compounds [Cu2(BTC)4/3(H20)2]6[HnXM12O40]·(C4H,2N)2(X=Si5 P; M=W, Mo) were obtained from simple one-step hydrothermal reaction of copper nitrate, benzentricaboxylate (BTC), and different Keggin polyoxometalates (POMs). Characterization of the compounds by XRD, SEM, TG and IR was conducted, which confirmed the existence of POM anion in MOFs structure. And then they were used as catalysts in oxidative desulfurization of dibenzothiphene (DBT) with H2O2 or O2 acting as oxidant, and CH3CN or H2O, extractant. The phosphotungstic acid incorporated metal-organic frameworks (HPW@MOFs) were selected as the most effective catalyst. The effect of reaction conditions on the desulfurization was studied. Under the favourable operating conditions, the DBT conversion was 86%at 180 min when H2O2 was used as oxidant. In case of oxidization with O2, the DBT conversion reaches 90% at 240 min. The efficiency of desulfurization with air was studied and it is possible to use air as green oxidant in desulfurization. Catalyst HPW@MOFs recyclability was also investigated, this composite material could be recycled for twice with a slight decrease of sulfur removal. Moreover, very positive desulfurization efficiency was obtained for desulfurizing real diesel catalyzed by HPW@MOFs. The mechanism of the ODS was proposed.(II) A trilacunary Keggin phosphotungstic reaction-controlled phase transfer catalyst (RCPTC) [C7H7(CH3)3N]9PWO34(Q9PW9) was successfully synthesized. This catalyst was shown to be effective for removing the most refractory sulfur compounds (dibenzothiophene, benzothiophene, thiophene and 4,6-dimethyldibenzothiophene) from model oil through oxidative desulfurization (ODS) process. Nearly 100% dibenzothiophene (DBT) removal rate was achieved under optimal conditions (70℃; catalyst dosage:0.2015g; O/S molar ratio:20; pre-immersion time of catalyst in H2O2:10 min), and the catalyst could be easily separated from the reaction solution. The recycled catlayst was characterized by IR and XRD, indicating that the structure of the polyoxometalate anion must be retained intact after reusing. Catalyst Q9PW9 recyclability was also investigated, this composite material could be recycled for three consecutive ODS cycles with only a slight decrease of sulfur removal. Moreover, a reaction mechanism for DBT oxidation was elaborated and very positive desulfurization efficiency was achieved for actual diesel catalystic ODS with Q9PW9.(Ⅲ) The heterogeneous catalyst, chromium terephthalate metal organic framework (MIL-101) encapsulated phosphotungstic acid (HPW), HPW@MIL-101, was prepared by one step method. And it was used as catalyst in oxidative desulfurization system in which O2 acted as oxidant for the removal of dibenzothiophene (DBT). The effects of oxygen flow rate, sulfur content and reaction temperature on the desulfurization were studied. The results show that the DBT conversion is up to 74% for 60 min under the optimal reaction condition (sulfur content:350 ppm oxygen flow rate:90 mL·min-1, the amount of catalyst:1% the mass of normal octane; reaction temperature:75℃). The DBT conversion doesn’t decrease obviously, after reused for three times with this catalyst. And it is possible to use air as green oxidant in desulfurization.(IV) The Keggin structure copper phosphotungstate catalyst showed considerable catalytic activity in the oxidation of dibenzothiophene (DBT) using molecular oxygen and air as oxidant under mild reaction conditions. The influence of the catalyst dosage and oxidant flow rate on desulfurization was investigated. The oxidation activity of air was close to molecular oxygen at same conditions. A remarkable DBT conversion appeared when the air flow rate was 800 mL·min-1. Nearly 97% DBT conversion was achieved in 30 min. This provides a new pathway to realize green oxidative desulfurization of fuels with oxygen as oxidant. |
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