Reduction Modification Of Cu-BTC For Improved Adsorptive Removal Of Disulfide

Metal-organic framework materials(MOFs)are widely utilized as adsorption materials to achieve the adsorption and separation of different components due to their high specific surface area,excellent tunability of the framework and pore size.Aiming at efficient removal of disulfides from light petroleum products,the study on the reduction of metal-organic framework Cu-BTC was conducted in order to enhance the adsorption of disulfides.Through experimental evaluation,Na2S2O3 was employed as a reducing system,and the conditions of reduction were optimized to achieve excellent adsorption performance of S(0.46)/Cu-BTC/24h.Furthermore,the structure evolution and underlying mechanism during the reduction modification on Cu-BTC were explored.This research provides an effective method for the efficient removal of disulfides in light hydrocarbons and an approach to the development of high-performance adsorbent materials,and highlights the structural modification and performance enhancement of functional materials through Cu valence regulation.First of all,the optimized conditions for the reduction modification of Cu-BTC using Na2S2O3 system were obtained through the evaluation of sulfide adsorption performance.Multi-technical analyses and characterizations were performed on the samples before and after modification.From the Raman,XPS and XAFS results,it is indicated that Cu(Ⅱ)species in the Cu-BTC structure can be partially reduced to Cu(Ⅰ)ones via Na2S2O3 modification.With an increase of Cu(Ⅰ)/Cu(Ⅱ)molar ratio,the DMDS adsorption capacities of modified Cu-BTC samples increase.The S(0.46)/Cu-BTC/24h sample with Cu(Ⅰ)/Cu(Ⅱ)=1.79 shows the highest DMDS adsorption capacity of 146.66 mg-S/g,which is much larger than that of the parent Cu-BTC sample(86.23 mg-S/g).As the Cu(Ⅰ)/Cu(Ⅱ)molar ratio continues to increase,their adsorption capacities decrease.The adsorption capacity of the S(2)/Cu-BTC/24h sample with Cu(Ⅰ)/Cu(Ⅱ)=2.75 is largely reduced to 17.81 mg-S/g.On this basis,the evolution of the composition and structures of Cu-BTC materials during the reduction modification process were explored.The BET specific surface area of the modified sample S(0.46)/Cu-BTC/24h having the highest adsorption capacity increases from 845.45 m2/g of the parent Cu-BTC to 1544.60 m2/g,and the mesopore volume changes from 0.04 m3/g to 0.08 m3/g.Meanwhile,the BET specific surfacearea of the over-reduced sample of S(2)/Cu-BTC/24h is largely reduced to only 58.26 m2/g,due to the serious destruction of the crystal structure caused by the excessive dosage of Na2S2O3.In addition,XAFS characterization was employed to examine the fine structure of Cu-BTC materials before and after modification,and the determined results are in good accordance with the model structure constructed by DFT simulation.Furthermore,the thermodynamics,kinetics and dynamic adsorption of DMDS on S(0.46)/Cu-BTC/24h were studied.In the range of 288-308 K,the values of Langmuir model parameter b and Freundlich model parameter K0 both decrease with the increase of temperature,indicating that the adsorption of DMDS on S(0.46)/Cu-BTC/24h is reduced as the temperature is raised,that is,lower temperature is more conducive to adsorption.With regards to the adsorption of DMDS on S(0.46)/Cu-BTC/24h,ΔG(-12.14～-11.85 kJ/mol)and ΔH(-7.72 kJ/mol)are found to be negative,suggesting the spontaneously exothermic adsorption process.Simultaneously,the adsorption of DMDS on modified Cu-BTC samples shows larger rate as compared with the parent Cu-BTC.The apparent diffusivity for the liquid phase adsorption of DMDS on Cu-BTC,S(0.46)/Cu-BTC/1h,S(0.46)/Cu-BTC/6h and S(0.46)/Cu-BTC/24h are(2.92-4.24)×10-13 cm2/s,(3.40-8.46)×10-13 cm2/s,(4.01-9.66)×10-13 cm2/s,and(5.14-10.63)×10-13 cm2/s,respectively.At 298 K,the breakthrough adsorption capacity of DMDS on S(0.46)/Cu-BTC/24h is 105.29 mg-S/g,which was 1.66 times higher than that of the parent Cu-BTC sample.Finally,DFT calculation method was utilized to simulate the adsorption behavior of DMDS on Cu-BTC structures before and after modification.The calculation results show that all of the modified Cu(Ⅰ)/Cu(Ⅱ)-BTC structures have stronger adsorption energy with DMDS compared with the parent Cu(Ⅱ)-BTC structure.Moreover,the Cu(Ⅰ)/Cu(Ⅱ)-BTC structures finds the highest guest-host interaction energy of 171.87 kJ/mol at Cu(Ⅰ)/Cu(Ⅱ)=2,which is consistent with the experimental result that the S(0.46)/Cu-BTC/24h sample(having Cu(Ⅰ)/Cu(Ⅱ)=1.79)exhibits the highest DMDS adsorption capacity.