Separation Of Similar C5-C7 Molecules With Metal-Organic Frameworks Based On Tetrathiafulvalene-type Ligands

The selective separation of structurally similar C5-C7 cycloalkanes and chain alkanes is an important process in the chemical industry and the development of adsorption separation technologies based on novel porous materials has the potential to provide a cleaner solution to traditional energy intensive distillation techniques.Metalorganic framework(MOFs)materials based on tetrathiafulvalene-type ligands with non-uniform pore channels and minimum pore sizes have the potential to achieve high selectivity separation of C5-C7 similar molecules through mechanisms such as shape recognition and kinetic recognition.In this work,a series of tetrathiafulvalene-based MOFs were synthesised based on metal substitution and ligand derivatisation strategies.and their performance on three typical separation targets was systematically investigated.The relationship between adsorbent structure and separation performance was explored,and the separation mechanism was elucidated at the microscopic level.Firstly,two M2TTFTB(M=Zn,Cd)materials with tiny pore size differences were constructed using tetrathiafulvalene tetrabenzoic acid ligands coordinated with two divalent metal ions,Zn2+and Cd2+,respectively.Zn2TTFTB(6.3 A)>Cd2TTFTB(5.3?).In addition,the degassing temperature of M2TTFTB with the retention of bound water was determined by thermogravimetric curves,preventing the appearance of open metal sites making the structure of M2TTFTB more stable.In addition,fluorinefunctionalised Cd2TTFTB-F materials were synthesised for the first time via fluorinefunctionalised tetrathiofuorvalene tetrabenzoic acid ligands modified with fluorine functional groups and Cd2+.Single-crystal structural analysis showed that 1/2 of the F atoms extend into the pore interior,further reducing the pore size at the narrowest part of the pore(4.52 ?)compared to Cd2TTFTB,making Cd2TTFTB-F promising for partial system exclusion separation.The separation performance of Cd2TTFTB and Cd2TTFTB-F for aromatics/cycloalkanes was investigated by single component vapor adsorption and equimolar mixed vapor fixed bed penetration experiments,and the deeper separation mechanism was explored.The results of single component vapor adsorption isotherms showed that Cd2TTFTB adsorbed on toluene(TOL)and methylcyclohexane(MCH),and the single component adsorption isotherms of MCH at different temperatures revealed its possible kinetic diffusion limitation on MCH,while Cd2TTFTB-F had almost no adsorption on methylcyclohexane at 303 K and 333 K.The results of the single component vapour adsorption isotherms showed that Cd2TTFTB was able to adsorb toluene(TOL)and methylcyclohexane(MCH)at different temperatures.The diffusion kinetics of TOL and MCH in Cd2TTFTB at different temperatures showed significant differences in the diffusion rates of them in Cd2TTFTB,with the kinetic selectivity of TOL/MCH at 363 K reaching 73.21,which demonstrated that Cd2TTFTB was able to separate toluene/methylcyclohexane by the kinetic mechanism,and the results of experimental column breakthrough further verified that the kinetic separation performance of Cd2TTFTB for TOL and MCH.The results of experimental column breakthrough based on Cd2TTFTB-F show that it has the expected size exclusion performance for both toluene/methylcyclohexane and benzene/cyclohexane systems,and the presence of the exclusion effect gives Cd2TTFTB-F excellent separation selectivity and competitive separation performance.DFT calculations showed that both toluene and methylcyclohexane were adsorbed by Cd2TTFTB through C-H…O hydrogen bond interactions,while Cd2TTFTB-F adsorbed toluene through C-H…O and C-H…F hydrogen bond interactions.For the cyclopentane/2,2-dimethylbutane system,which have similar boiling points and molecular sizes,the adsorption separation of them is achieved by the special pore structure of M2TTFTB.The single-component vapour adsorption isotherms show differences in the static adsorption of cyclopentane and 2,2-dimethylbutane between Zn2TTFTB and Cd2TTFTB,with more significant differences in the heat of adsorption between cyclopentane and 2,2-dimethylbutane for Cd2TTFTB,indicating a relatively large difference in adsorption energy.The results of experimental column breakthrough showed that Cd2TTFTB was able to achieve dynamic separation of cyclopentane,2,2dimethylbutane with the highest dynamic selectivity(8.80)at 333 K.The DFT calculations showed that both cyclopentane and 2,2-dimethylbutane were adsorbed on Cd2TTFTB through C-H…O hydrogen bonding interactions with adsorption energy cyclopentane>2,2-dimethylbutane.Finally Cd2TTFTB-F was used for the separation of n-hexane/3methylpentane/2,2-dimethylbutane(nHEX/3MP/22DMB)and shows extremely low adsorption capacity for 22DMB compared with nHEX and 3MP at low temperature.the results of experimental column breakthrough showed that Cd2TTFTB-F got the best separation effect on the three components at 363 K.DFT calculations showed that nHEX and 3MP were adsorbed on Cd2TTFTB-F through multiple C-H…O and C-H…F hydrogen bond interactions.