Deep Adsorption Desulfurization Performances Of Combined Metal-organic-framework Porous Materials

Metal organic frameworks（MOFs）materials have the advantages of high porosity,large specific surface area,regular pore channels,adjustable pore sizes,topological diversity,and tailorability etc.,exhibiting extremely application potential in the field of fuels adsorptive desulfurization.However,the simple metal species and monotonic pore structure of MOFs restrict the top boundary of adsorption desulfurization performance.Moreover,the sulfurs adsorption and separation abilities of many water instable MOFs from water-containing fuels could give a high discount.Compared with individual MOF,the MOF-on-MOF composites have the virtues of adjustable components,hierarchical pore structure,and synergistic adsorption,etc.Thus,a series of MOF-on-MOF heterojunction composites were constructed through the internal extended,epitaxy,and phase-competition-driven growth methods in this paper,then systematically investigated the performances of thiophenic sulfurs adsorption and separation from water-containing fuels.Specific contents include:（1）Microporous Cu-BTC as core,mesoporous MOF-818 as shell and polyvinylpyrrolidone（PVP）as structural guiding agent,the（MOF-818）-on-（Cu-BTC）was synthesized through the liquid-phase internal extension growth method.The mesoporous MOF-818 shell layer could enhance the rapid adsorption and diffusion of thiophenic sulfurs during the pore channel.Moreover,its shell layer structure similar with“bricks wall”delayed the attacking time of water molecules towards Cu-BTC core layer,enhancing the water stability of the（MOF-818）-on-（Cu-BTC）.The results of characterization and batch adsorption tests indicated that the synergistic effects of Lewis acid-base,π-π,and hierarchical pores are beneficial to improve the adsorption performance of the materials.The maximum saturation capacities of thiophene（Th）,benzothiophene（BT）,and dibenzothiophene（DBT）over（MOF-818）-on-（Cu-BTC）could reach 220,98,and 104 mg/g,respectively.Immersed in water for seven days,the（MOF-818）-on-（Cu-BTC）still maintained high desulfurization capacity and structural integrity,demonstrating good water stability.At the same time,the desulfurization capacity of the（MOF-818）-on-（Cu-BTC）were still more than 93%after fifth recycle.（2）MOF-5 as core,MIL-88B as shell,the Fe/Ni/Cu/Zn-（MIL-88B）-on-（MOF-5）multi-metallic core-shell materials were prepared through phase-competitive growth and one-pot hydrothermal synthesis method utilizing Fe³⁺,Ni²⁺,Cu²⁺,and Zn²⁺as the multi-metallic ions sources.The results of N₂ adsorption-desorption,scanning electron microscopy,powder X-ray diffraction,inductively coupled plasma spectroscopy,X-ray photoelectron spectroscopy,and other characterizations as well as the results of the batch adsorption tests indicated that the synergistic effects of dual MOFs with multi-metallic ions could enhance the adsorption desulfurization performance.The one-pot hydrothermal synthesis method could avoid excessive structural defects and pore blockage,attributing a morphology integrity of the MOFs crystals.When the molar ratios of Cu²⁺and Ni²⁺were 1.00,0.75,and 0.50,the maximum adsorption amounts of the Fe/Ni/Cu/Zn-（MIL-88B）-on-（MOF-5）towards Th,BT,and DBT could reach 105,115,and 124 mg/g,respectively.Langmuir and pseudo-second-order kinetic models could describe the adsorption process well.The adsorption thermodynamics indicated that the adsorption processes of DBT and BT were dominated by physical adsorption and accompanied by chemisorption,while the adsorption process of Th was mainly physical.The crystal morphology and structure of the Fe/Ni/Cu/Zn-（MIL-88B）-on-（MOF-5）were maintained in good integrity after soaking in water for five days,and the adsorption capacities of Th,BT,and DBT can still reach 81,77,and 77%of the original ones.Furthermore,there was still a 92%desulfurization capacity of Fe/Ni/Cu/Zn-（MIL-88B）-on-（MOF-5）after fifth cycle of adsorption desulfurization.（3）Cu-BTC-NH₂ as the core,ZIF-8-NH₂ as the shell and PVP as the surfactant,the（NH₂-ZIF-8）-on-（Cu-BTC-NH₂）core-shell MOF-on-MOF was constructed by the epitaxial growth method.The results of systematic characterizations such as scanning electron microscopy,powder X-ray diffraction and fourier infrared spectroscopy analysis,and adsorption desulfurization tests demonstrated that the（NH₂-ZIF-8）-on-（Cu-BTC-NH₂）has excellent performances of adsorption and water stability,and the maximum desulfurization capacity can reach 135 mg/g.The-NH₂functional groups on the surface of dual MOFs generated hydrogen bonding force with thiophenic sulfur molecules,enhancing the ability of adsorption desulfurization.Due to the water shielding protective effect of the ZIF-8-NH₂ shell layer,the（NH₂-ZIF-8）-on-（Cu-BTC-NH₂）containing 25 wt%water still possessed good water resistance and adsorption property.In addition,the（NH₂-ZIF-8）-on-（Cu-BTC-NH₂）still retained more than 90%of Th adsorption capacity after fifth desulfurization cycle,exhibiting a good regeneration performance.This thesis could provide a new research idea for the development of controlled synthesis of MOF-on-MOF materials and their applications in deep adsorption desulfurization of fuels.