On Oxidative And Adsorptive Desulfurization Of Thiophenic Sulfurs From Model Oil

For effectively controlling the environmental pollution caused by theS-content in fuel oils, more stringent environmental regulations to lower theS-content in fuel oils are continuously introduced by governments. Thetraditional industrial process is the hydrodesulfurization (HDS), which iseffective for non-thiophenic S-compounds, but less effective for thedesulfurization of thiophene (Th), benzothiophene (BT), dibenzothiophene(DBT) and their derivatives. Therefore, some new technologies different fromHDS such as adsorptive desulfurization (ADS), extractive desulfurization(EDS), oxidative desulfurization (ODS) and other processes have beenproposed. In this paper, we studied the catalysis of three catalysts in theextractive and catalytic oxdative desulfurization (ECODS) process, proposedan integrated ODS process, and investigated the adsorption behavior of fourmetal-organic frameworks (MOFs) in model oil.Firstly, three Keggin-type phosphotungstates were synthesized andcharacterized by elemental analysis, X-ray diffraction and infrared spectra.Their catalysis in an ECODS process was studied with ionic liquid (IL)[Bmim]PF₆as extractant and H₂O₂as oxidant. The main factors affecting the desulfurization process were investigated, including temperature,hydrophobicity of IL, variety of sulfur compounds, as well as the amount ofcatalyst, IL and H₂O₂. All the three catalysts show high catalytic activity.Under the optimal conditions (40℃, m(DBT-oil):m([Bmim]PF₆=4:1,n(H₂O₂):n(S):n(catalyst)=300:100:1,1.5h), for the three catalysts, theirS-removal of DBT-oil with its initial S-content being1000ppm can achieve99.8%. It is found that the hydrophobic IL [Bmim]PF₆is more suitable thanhydrophilic IL [Bmim]BF₄as the extractant in the ECODS system. For thepresent ECODS process, a new interpretation is proposed, in which IL isassumed as a reaction phase, and the amount of the extracted S-compound andthe peroxidized catalyst wherein greatly affect the desulfurization rate.Besides, the IL with the dissolved catalyst can be reused many times andregenerated easily.Secondly, we proposed a novel integrated ODS process, in which theoxidant H₂O₂is in situ generated by oxidizing isopropanol with oxygen, andthe oxidative desulfurization process is helped by a reaction-controlled phasetransfer catalyst. Some influencing factors for the S-conversion were studied,viz., time, temperature, various S-compounds, and the amount of isopropanol,initiator, oxygen, and catalyst. It is found that the catalysis of[Ï€-C₅H₅NC₁₆H₃₃]₃[PW₁₂O₄₀] is much superior to H₃PW₁₂O₄₀and[(C₄H₉)₄N]₃[PW₁₂O₄₀], under the same conditions (70℃, m(DBT-oil)/m(isopropanol)=4:1, m(AIBN)/m(isopropanol)=4%, n(S) /n([Ï€-C₅H₅NC₁₆H₃₃]3[PW₁₂O₄₀])=50:1, P_O2=1.4MPa,6h), the S-conversioncan reach to98.9%,34.9%,18.7%, respectively, because of its gooddispersivity in oil and adsorptivity for S-compounds. For the integrated ODSprocess, the rate determining step is the induction period of the oxidation ofisopropanol, and all measures favoring the production of the isopropanolradicals can improve the reaction rate markedly. Both BT and DBT can beremoved efficiently at mild conditions in6h with S-conversion above96%,however, the S-conversion of3-methylthiophene (3-MT) is relatively low, theS-conversion for which is only43.3%at90°C in6h. The reactivity ofdifferent S-compounds follows the order DBT> BT>3-MT, which is inaccordance with their decreasing electron density on the sulfur atom. Theoxidation products of DBT in the integrated ODS process is characterized asDBTO and DBTO₂. Under the same conditions, the S-conversion withisopropanol can reach98.9%, which is much higher than the S-conversionwithout isopraponal, i.e.5.1%. So the present integrated ODS process isbetter than the traditional ODS process. And the oxidation products can beseparated easily via settling or filtration. Besides, the dissolved isopropanol inthe treated oil can be removed easily by washing with water.Finally, four MOFs (Cu-BDC, Cu-BTC, Cr-BDC and Cr-BTC) weresynthesized using two different metal ions (Cu²⁺and Cr³⁺)and two differentcarboxylate ligands (H₃BTC and H₂BDC). Their adsorption behaviors forthiophenic sulfurs in model oils are systemically investigated at mild temperatures. And Cu-BTC, Cr-BDC and Cr-BTC are found to be superior tothe conventional zeolites or activated carbons adsorbents, their adsorptioncapacity of DBT-S can reach56.1,41.2, and30.7mg S/(g MOF) at30°C in1h (mDBT-oil:mMOF=200:1). The results show that the adsorption of these MOFsis a result of combined effects of physisorption and chemisorption. Theiradsorption behaviors are controlled by manifold factors involving appropriateframework structures, suitable pore size and shape, and exposed Lewis acidsites matching the given S-compound. The adsorption capacity of differentsulfur compounds follows the order: DBT> BT>3-MT, which is mainlyascribed to their Ï€ electron number and the electron density on sulfur atom.The used MOF can be easily regenerated by solvent washing and recycled atleast five times.