The Investigation Of Metal Organic Frameworks(MOFs) For Adsorption Desulfurization

The sulfur compounds in fossil fuels can corrode the equipment and pipe run, deactivate or poison the catalyst and drop the products quality. And they will pollute the environment and harm to people’s health if they were discharged in the air. At these points, it will be necessary to carry out the desulfurization process for fossil fuels to decrease the sulphur emissions and realize the high efficient and clean utilization of coal resource. Existing desulfurization techniques mainly include hydrogenation desulfurization, oxidative desulfurization, extraction desulfurization and adsorption desulfurization. Adsorption desulfurization has drawn much attention, due to its advantages of low energy consumption, mild operating condition, desulfurization of high precision, easy regeneration.Metal organic frameworks is a new porous material rising in recent ten years, which is composed of aromatic acid or alkali nitrogen-oxygen polydentate organic ligands, through coordination bonds with the inorganic metal centers to hybrid form three-dimensional network structure of crystals. MOFs are considered promising adsorbent, due to their variety, easy functional, large porosity and high specific surface area, low crystal density, and adjustable pore size. The study on the adsorption desulfurization performance and mechanism of MOFs will provide guidance for its application in adsorption desulfurization area.MIL-101 was synthesized by hydrothermal method in this work. The fixed bed was employed to carry out the dynamic adsorption experiments for dimethyl sulfide, ethyl mercaptan and hydrogen sulfide at ambient temperature to study the adsorption behavior of MIL-101 and discuss its adsorption mechanism. The density functional theory was used to evaluate influence of coordinatively unsaturated metal site（contain configuration and metal ion） and organic ligand（with and without substituent group） of MIL-101 on the desulfuration performance. The main results are listed as follows:MIL-101 prepared from hydrothermal method owns large specific surface area and pore volume, which BET surface area is 3294.20 m2/g. Its micro pore volume is as high as 1.34 cm³/g. Sample has uniform crystal size, good dispersion, and particle size about 100 nm; Crystal skeleton can withstand high temperature as high as 300 ℃. The dynamic adsorption experiment of fixed bed indicates that activation process has great influence on desulfurization performance of MOFs. Preferred activation method of MIL-101 is high temperature nitrogen purge, and the optimum activation temperature is 170 ℃. Moreover, temperature has different effects for MIL-101 adsorbing three sulfide. For adsorbing dimethyl sulfide, the breakthrough time is reduced with increasing adsorption temperature. For adsorbing ethyl mercaptan and hydrogen sulfide, the breakthrough time is plus with increasing adsorption temperature. Adsorption capacity is 24.2 mg S/g, 14.9 mg S/g and 5.8 mg S/g MOFs for dimethyl sulfide, ethyl mercaptan and hydrogen sulfide, respectively. The characterized of XRD found that the framework of MIL-101 wasn’t damaged after breakthrough experiments. Results of TG-MS showed that dimethyl sulfide was adsorbed on the MIL-101 in its molecular form, whereas hydrogen sulfide could react with MIL-101.The theoretical calculation of density functional theory（DFT） demonstrated that CUS was the main position to absorb sulfides, and the average adsorption energies over CUS of MIL-101 weaken with the increased number of sulfide. The adsorption sequence over CUS was CH₃SCH₃>CH₃CH₂SH>H₂S, which was coincidence with the order of sulfur capacity from experiment results. The contribution of each fragment of MOFs to the adsorption of three sulfur compounds were also investigated using DFT. The results revealed that, MOFs with CUS have the strongest binding strength with sulfur compounds, MOFs with NH2-BDC substituent group ligand comes second, followed by that with saturated metal center, and the organic ligands without substituent group has the weakest adsorption strength. Moreover, metal centers of MOF-74 and MOF-199（M and M-M） have stronger adsorption strength than M3 O of MIL-101. It was also found that, among different metal ions（Fe, Zn and Cu）, MOFs with unsaturated Fe has the strongest adsorption strength for sulfur compounds.