| Hydrogen sulfide (H2S) is a hazardous substance of industrial gas such as natural gas, refinery gas, synthesis gas and so on. H2S cannot only cause catalyst poisoning and corrosion to pipelines in the process of transport, but also it is considered as one of the main contaminants leading to air pollution, greenhouse effect and ozone depletion. Therefore, H2S is one of the environmental pollutants that should be removed and controlled. Refer to the removal of H2S, green wet desulfurization has now become a hot research with some advantages, such as recycle of desulfutization agent, recovery of sulfur from the exhanst gas and no secondary pollution. H2S removal by heteropolycompounds (HPCs) is a new method faced on natural gas purification and recovery of sulfur. HPCs have been used as a catalyst in some certain reactions for a long period, but not been applied to contaminants treatment for their oxidation-reduction. Based on the abundance of Mo and W element, V-substitued phosphomolybdic acid and transition metal doped HPC solutions were developed as the desulfurizers, and it is also extended to the field of ionic liquid (IL) which is used as a green solvent. HPC and functionalized ionic liquid green desulfurization were constructed. The content of this thesis contains the following five parts:Ⅰ) Synthesis, Desulfuration and Air Regeneration of Dawson-type Molybdovanadophosphoric HeteropolyacidDawson-type molybdovanadophosphoric heteropolyacids H6+n[P2Mo18-nVnO62](n=1～4) were prepared using the method of ethyl ether extraction, and the absorption efficiency of H2S by their solutions was measured under different conditions reflecting the effects of the number of vanadium, absorption temperature, concentrations of H2S gas and heteropolyacid. Then, desulfurizer was regenerated by air with steam under microwave irradiation, and the regenerability was investigated, compared with single air regeneration. As a result, H2S removal efficiency decreases with the number of V atom, temperature and gas flow increasing, and with the concentration of HPA decreasing. Microwave-assisted air regeneration improves with the temperature, irradiation time and power increasing, therefore, the conditions of microwave-assisted air regeneration were proposed that the temperature, irradiation time and power was intended to be55℃,2h and695W, respectively. Characterized by FT-IR, the characteristic peak of desulfurizer H7[P2Mo17VO62] was found to be changed after H2S absorption and microwave-assisted air regeneration, which indicates that the valence state of Mo decreases and increases, respectively. The changes of the valence state for Mo and V during absorption and regeneration did not be detected by XPS. The absorbents were tested by redox potentiometry and chemical oxygen demand (COD), which revealed that the redox in absorption and regeneration, and microwave-assisted air regeneration was further compared with single air regeneration. The result shows that the redox potential of HPA solution decreases in the process of absorption, and increases in the process of regeneration, and the redox potential for microwave-assisted air regeneration is higher than that for single air regeneration. The COD value after absorption, microwave-assisted air regeneration and single air regeneration is187.2,120.1and136.8mg O2/L, respectively, and this indicates that the regeneration of heteropolyacid is promoted by foregoing microwave assisted air regeneration method which can activate O2. In summary, microwave assisted air regeneration is proved to be a new method superior to single air regeneration and recyclable forever without any change in the structure of this absorbent.II) Effects of Fe-, Cu-, Zn-, Mn-and Cr-doped H4PMo11VO40for the removal of H2S in wet oxidationA series of new transition metal (Cu, Fe, Zn, Mn and Cr)-doped H4PMo11VO40(M11PV1) were prepared by the method of hydrothermal synthesis and the synthetic heteropolycompounds were abbreviated as M11PV1Cu, M11PV1Fe, M11PV1Zn, M11PV1Mn and M11PV1Cr. Their structures and thermostability were confirmed by FT-IR, XRD and TGA-DSC techniques. The CV curves of M11PV1Cu, M11PV1Fe, M11PV1Zn, M11PV1Mn and M11PV1Cr were compared with M11PV1, and the deoxidization peaks of Cu (0) to Cu (Ⅱ) and Fe (0) to Fe (Ⅲ) were only observed, and the result shows that a special connection is set up between Cu2+/Fe3+and PMo11VO404-. H2S removal efficiency of M11PVlCu, M11PVlFe, M11PV1Zn, M11PV1Mn, M11PV1Cr and M11PV1was studied, and the mechnism of oxidation desulfurization was analyzed. The experimental results demonstrates that M11PV1Cu has the best H2S absorption efficiency (＞87%, and the top efficiency is nearly100%) which presents in ’bridge type’, and it shows that the efficiency increases firstly and then keeps one value for some time and at last decreases. The H2S absorption efficiency of M11PVlFe, M11PV1Zn, M11PV1Mn and M11PV1Cr decreases between85%～50%, while that of M11PV1is stable between70%～65%. The XPS spectra of elements Mo, Cu and V shows that after300-min absorption, all of Cu2+ions have already been reduced to Cu+, whilst Mo6+and V5+have been partly reduced to Mo5+and V4+, and the H2S absorption efficiency of M11PV1Cu is still very high (＞85%), and higher than that of M11PV1, even Cu+did not have oxidizability with H2S, and it indicates that Cu2+/Cu+can adsorb or store H2S. The XPS spectra of S yielded indicates that H2S can be oxidized to sulfur, and no S2-suggests that there is no Cu2S, and all of H2S are oxidated to S by Mo and V elements. The’bridge type’ also suggests that Cu+can adsorb or store H2S, or its capacity is better than Cu2+.Ⅲ) An excellent recycling strategy for highly efficient removal of H2S by [Bmim]3PMo12O40dissolved in [Bmim]ClA kind of functionalized ionic liquid [Bmim]3PMo12O40([Bmim]3PM) was prepared by the method of precipitation. The structure and thermostability of [Bmim]3PM was confirmed by FT-IR, XRD and TGA-DSC. An environmentally benign approach has been proposed for the removal of H2S using a Keggin-type heteropolyanion-based ionic liquid [Bmim]3PM dissolved in a green solvent-ionic liquid (IL). The results show that80℃, H2S removal efficiency by [Bmim]3PM-IL of0.005mol/L is in the following sequence:[Bmim]3PM-[Bmim]Cl＞[Bmim]3PM-[Bmim]BF4＞H3PM012O40-H2O＞[Bmim]3PM-[Bmim]PF6＞[Bmim]3PM-[Bmim]NTf2＞[Bmim]3PM-H2O＞H2O. In the condition of80℃, different concentration of [Bmim]3PM (0,0.001,0.005and0.01mol/L), H2S removal efficiency by [Bmim]3PM-[Bmim]Cl and [Bmim]3PM-[Bmim]BF4was investigated, and the result shows that when the concentration of [Bmim]3PM is0mol/L, the H2S removal efficiency of the two desulfurizers is very low; when the concentration is0.001mol/L, H2S removal efficiency of [Bmim]3PM-[Bmim]Cl is nearly100%, but that of [Bmim]3PM-[Bmim]BF4is only40%; when the concentration is0.01mol/L, that of [Bmim]3PM-[Bmim]BF4reaches100%. The temperature (45～180℃) does not affect the H2S removal efficiency, but the viscosity of ionic liquids at low temperature affects the removal of H2S. Therefore,[Bmim]3PM-ILs are suitable to desulfurization at high temperature. The sulfur capacity of [Bmim]3PM-[Bmim]Cl (230.9g·L-1) is higher than that of [Bmim]3PM-[Bmim]BF4(62.2g·L-1) or H3PMo12O40-H2O (59.0g·L-1), which indicates that [Bmim]Cl as a solvent can improve the oxidation of H2S by [Bmim]3PM. Microscopic observation, turbidity measurement and quantum chemical calculations were used to analyse the factors (solubility of [Bmim]3PM in ILs and stabilization energy for H2S-Anion) that influence H2S removal efficiency, the results suggest that [Bmim]3PM is well dissolved in [Bmim]Cl and high stabilization energy for H2S-Cl-increases the residence time of H2S in [Bmim]Cl, which makes the reaction between H2S and [Bmim]3PM more sufficient. H2S is oxidated to elemental S and Mo6+is reduced to lower valence state in the absorption stage according to UV-vis and FTIR data, respectively. Using air,[Bmim]3PM-[Bmim]Cl can be recycled for more than six times without any obvious decrease in the removal efficiency of H2S, but the time for keeping100%reduces to the half.IV) H2S Absorption Capacity and Regeneration Performance of Amine Fe-based Ionic LiquidAmine Fe-based ionic liquid1.6Et3NHCl·FeCl3was synthetized with ideal H2S absorption capacity and good thermostability. H2S removal efficiency was tested under the condition with concentration of H2S being832mg/m3, temperature ranging from40to180℃, and gas flow of100,300,400or500mL/min. The results show that when the gas flow is less than400mL/min, H2S removal efficiency can reach100%; H2S removal efficiency increases with the increasing in temperature and tends to approach an asymptotic value. Under the optimal conditions, the sulfur capacity of1.6Et3NHCl·FeCl3is6.36g/L, higher than that of [Bmim]FeCl4. Comparing the FT-IR spectra before and after H2S absorption, redox reaction between1.6Et3NHCl·FeCl3and H2S is confirmed. The interaction between H2S and1.6Et3NHCl·FeCl3/[Bmim]FeCl4/H2O has been studied at the molecular level using density functional theory, and the influence of the substrate on H2S absorption was illustrated to be responsible for the enhancement of H2S absorption by aminal group. The product after H2S absorption is orthorhombic crystal sulfur (α), which is the same as the product from traditional aqueous phase oxidation desulfurization.1.6Et3NHCl·FeCl3ionic liquid can be reused efficiently after quick regeneration by air flow.V) H2S Absorption Capacity Studies of Ionic Liquid-MDEA-H2O Combined DesulfurizersThree kinds of functionalized ionic liquids [Bmim]HC03,[TMG]L and [MEA]L were synthetized which was referred to previous methods. The structure and thermostability of the synthetic ionic liquids (ILs) was confirmed by FT-IR spectrum and thermogravimetry characterization, respectively. The new combined deoxidizer was prepared by mixing ionic liquid ([Bmim]HC03,[TMG]L,[MEA]L,[Bmim]Cl or [Bmim]BF4) with methyldiethanolamine (MDEA) aqueous solution according to certain proportion. In the conditions of different ionic liquids, absorption temperature and combined proportion, H2S absorption and bubble eliminated capacity by deoxidizers and ILs were measured, and regeneration performance of optimized deoxidizer was studied. The concentration of SO42-in regenerated deoxidizer was analyzed by ion chromatography after deeply oxidation by O3, and the mechanism of absorption was analyzed by density functional theory. The results show that absorption capacity is in the sequence that [Bmim]Cl-MDEA-H2O＞[Bmim]HCO3-MDEA-H2O＞[Bmim]BF4-MDEA-H2O＞MDEA-H2O＞[TMG]L-MDEA-H2O＞[MEA]L-MDEA-H2O, and the major factor is attributed to the the stability of IL-MDEA;[Bmim]HCO3performs the best capacity of bubble eliminated;[Bmim]Cl-MDEA-H2O,[Bmim]HCO3-MDEA-H2O and [Bmim]BF4-MDEA-H2O can be regenerated by air basicly, and high stability of IL-H2S brings higher the absorption efficiency, but smaller regeneration. |
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