Investigation Of Metal-Organic Framework On Adsorbing/Desorbing Organic Solvent Molecules

As a new type of porous material, Metal-Organic Frameworks (MOFs)are superior to traditional inorganic porous. Through changing the functional groups of organic ligands, we can control the aperture geometries and properties of the three-dimensional frameworks to use in the selective adsorption of organic solvent molecules-Because the interactions between aperture and organic solvent molecules are different if the structures and properties are different, which will cause the amounts of the organic solvent molecules adsorbed are different. For these reasons, this kind of material can be applied to control and recycle of organic waste gas.Three kinds of different structural MOFsZn(BDC)(4,4'-Bipy)_0.5(DMF)(H₂O)_0.5(MOF508;BDC=1,4-benzenedicarboxylicacid, 4,4'-Bipy=4,4'-bipyridine, DMF= dimethyl formamide), Zn₃(Htaa)₃(H₂O)₃(C₄H₈O₂) (J370;Htaa=2-(5H-tetrazol-5-yl)acetic acid, C₄H₈O₂=1,4-dioxane)Zn₄(OH)₂(1,2,4-BTC)₂(H₂O)₂(DEF)₃(J372;1,2,4-BTC=1,2,4-benzenetricarbo xylic acid, DEF=N,N-diethyl formaide)were synthesized by the method of solvothermal synthesis. Through XRD, SEM and TGA, we determined their structures and thermal stabilities. Their properties of adsorption/desorption on organic solvents molecules were invested here too.In order to study adsorbate properties on adsorption / desorption, we select the MOF508 as adsorbent, the organic solvents molecules which have different chain length (CH₃OH, C₂H₅OH, n-C₃H₇OH) or different structures (n-C₃H₇OH, i-C₃H₇OH) or different sizes (CH₂C1₂, CHC1₃, CC1₄) as adsorbate. The longer the chain length, the greater the values of adsorption at low relative pressure, whereas the result was opposite under high relative pressure; The branched-chain alcohols was not adsorbed at the beginning, the amount of i-PrOH adsorption was gradually increased as the pressure increased; The smaller the organic solvent molecules, the greater the values of adsorption. Due to the flexibility of MOF508, some organic solvents molecules can be adsorbed even though their sizes were larger than the pore. In addition, an unambiguous two-step isotherm was observed in the CS₂ adsorption and desorption isotherm with the intermediated state, which framework was highly shrunk. As a further increase in the adsorption pressure leaded to a reopening of the total porosity and complete pore filling.Apart from MOF508 which coordinated with BDC and 4,4'-Bipy by Zn²⁺, the adsorption/desorption of J370 and J372 were also studied here, which was carried out on investigating the ability in adsorbing different alcohols. Because of the different organic ligands, the apertures were different in the three kinds of MOFs. The results showed that the amount of organic solvent molecules adsorbed was positive to the specific surface area of MOFs. The capillary condensation appeared when the alcohols were adsorbed in the MOF508 which had big specific surface area and narrow aperture. Whereas this phenomenon didn't show in the J370 with small specific surface area and narrow aperture or J372 with big specific surface area and wide aperture. The interactions between alcohols and the wall of MOF508 or J372 were stronger than that of J370. From the above results, the selective adsorption of organic solvent molecules was more effective in the MOF508.In order to design a kind of MOFs which has the selective absorbency on organic solvents molecules, we can change the organic ligands to control the further system structure of organic framework. It is expected to synthesize a new class of specific framework can be used to treat organic waste gas.