Adsorptive Desulfurization Of Metal-Organic Frameworks Via Molecular Simulation

Metal-Organic Frameworks(MOFs)have attracted much attention due to their advantages such as adjustable structure,large surface area,and pore volume in adsorptive desulfurization.In recent years,the numbers of MOFs are increasing,and experimental research on screening ideal candidates is relatively not feasible due to low efficiency and long-time period.Therefore,high-throughput screening calculation methods which is based on Monte Carlo(MC)molecular simulation has broken through the traditional limitations and deeply explores the adsorptive desulfurization performance and adsorption mechanism of MOFs.First,under the condition of model oil with a concentration of 224.6 ppm(thiophene as sulfide,and n-octane as oil),high-throughput screening of 716 CoRE-MOFs(Compution-ready,Experimental-MOFs)has been performed based on the kinetic size of thiophene(4.6 A).The results show that Henry selectivity cannot predict adsorptive desulfurization performance because intermolecular interactions are not considered.Based on GCMC selectivity and thiophene adsorption capacity,pure components of thiophene and n-octane,and their binary mixed components in FAHFUR and PUQXUV were studied.Pure component results show that under low pressure,thiophene in FAHFUR was adsorbed on square-like and vase-like channels,while n-octane was mainly adsorbed on square-like channel.And n-octane passed through square-like and vase-like channels with the increasing pressure.For PUQXUV,thiophene containing heteroatom S could enter hydrophilic pores,and it could also enter hydrophobic pores due to alkyl interactions on the ring,but n-octane could only enter the hydrophobic pores.For binary components,in FAHFUR,due to the competition between thiophene and n-octane,the density of thiophenes in square-like channels is much lower than that of pure components.The thiophene density distribution in hydrophobic channels of PUQXUV is very low.It shows that the structure-property relationship of CoRE-MOFs has guiding significance for adsorptive desulfurization.Second,adsorption and separation of thiophene,benzene and n-octane(1:1:998)in 716 CoRE-MOFs were studied via high-throughput screening method.The results show that MOFs with LCD at 5-8 A,VF at 0.2-0.5 and GSA at 0-1000 m2/g can efficiently adsorb thiophene,while LCD at 5.8-12 A,VF at 0.3-0.85,and GSA at 500-3500 m2/g can effectively adsorb benzene.We selected EYOQAL and PUQXUV due to their excellent adsorptive desulfurization performance,the adsorption isotherms and density distribution of thiophene-benzene(50%-50%)and thiophene-benzene-n-octane(1%-1%-98%)were studied.The binary component results show that the adsorption capacity of thiophene is much larger than that of benzene.For EYOQAL,under low pressure,thiophene was mainly distributed in tetragonal channels,and benzene was mainly distributed in hexagonal channels.Under high pressure,due to the size entropy effect,thiophene and benzene compete on hexagonal channels.For PUQXUV,due to the large kinetic size of benzene,benzene has little effect on thiophene adsorption.The ternary component results show that thiophene absorbed more with increasing fugacity,which in tetragonal and hexagonal pores.The adsorption of n-octane decreased slightly at the whole range of pressure,which was only distributed in hexagonal channels.It can be observed that benzene molecule has a small adsorption capacity at the whole range of pressure.In PUQXUV,the adsorption capacity of benzene and n-octane is saturated at low pressure due to the pore-limiting effect of the structure.The adsorption capacity of thiophene increases with increasing pressure,and the presence of benzene and n-octane have little effect on thiophene adsorptionThird,Mil-47 is the candicate for adsorptive desulfurization due to excellent effect,which has been widely studied in experiments.In molecular simulation,it is a great challenge to accurately reproduce the adsorption of MIL-47 due to the accuracy of force field,while the polarization force field is significant to simulate the adsorptive desulfurization experiments.The polarization force field can be divided into Fluctuation-Charge model,Drude oscillator model and induced point dipole polarization model.We use the additional atomic polarizability and adjust LJ energy parameters of benzothiophene.The adsorption of benzothiophene and n-octane in MIL-47 are investigated via GCMC method at 298.15 K,308.15 K and 318.15 K.Without polarizable force field,the isotherm of benzothiophene is much less than that of experiment.The adsorption isotherm under the condition of polarizable force field at 298.15 K is slightly higher than that of experiment.At 308.15 K and 318.15 K,the polarizable force field has good predictions for the adsorption behavior.Owing to the competition between two absorbates,more benzothiophene and less n-octane are distributed in MIL-47 with increasing temperature.The uptake of pure benzothiophene decreases with increasing temperature,which is an exothermic process.