| The chlorinated hydrocarbon vapor, as one kind of VOCs (Volatile Organic Compounds), is the pollutant in the air and harmful to the human health, how to deal with it has become a focus of the society. A number of add-on-control techniques, including the filtration, absorption, adsorption, membrane separation, ionization, ozonation and photo-catalysis, have been developed to remove it from contaminated air. Among these techniques, adsorption using porous materials as adsorbents is an effective technique for its removal. Porous materials, including zeolites, resins, activated carbons, are the most frequently utilized adsorbents. Due to their large surface area and pore volume, Metal organic frameworks (MOFs) become a new kind of adsorbent. Metal organic frameworks (MOFs), which are constructed of metal ions and organic linkers, are attracting considerable interest due to their high porosity and high adsorption capacity. Flexible MOFs (or called soft porous crystals (SPCs)) are one subclass of MOFs with mulitistable states, which have fascinating dynamic behavors upon the adsorption and desorption. As a prototypical material of SPCs, MIL-53 exhibits a reversible structural transformation (named breathing effect) between two distinct conformations (large pore (lp) phase and narrow pore (np) phase). The transformation, known as the breathing effect, is a common property of MIL-53 with different metal and makes MIL-53 have great potential in adsorption. Up until now, MIL-53 has been used to adsorb some volatile organic compounds (VOCs). However, the adsorption of the chlorinated hydrocarbon vapor in MIL-53 has not been reported and the mechanism of breathing effect is not very clear.In this work, the synthesizing, activation and water-stability analysis of MIL-53 were addressed using a combination of various experimental methods and computational chemistry. Besides, the adsorption experiments of the chlorinated hydrocarbon vapor in MIL-53 were carried out and the adsorption properties were studied. The main contents and findings are summarized as follows.1. Based on the different synthesis methods, activation methods of MIL-53 (Al) were studied comprehensively. The performance of activation method was evaluated by the specific surface area of the crystal. The mechanism of activation methods was confirmed by the loading induced structural transition which is the peculiar of MIL-53(Al)’s determined by X-ray diffraction. A combined activation method which has the best performance was proposed and specific experiment was performed to clear up the misunderstanding of the mechanism of activation assumed in the literature.2. The water stability and adsorption of MIL-53 were analyzed using density functional theory. According to the eigenvalue of the elasticity tensor and the lowest interaction energy, we find the np phase is the stable equilibrium state and the lp phase is the mestastable in humidity condition. The mechanical properties of a MIL-53(Al) and H2O composite in the both phase were addressed. The tensorial analysis of the elastic constants in the np phase provides a further support of adsorption induced stress mechanism of the breathing effect in MIL-53 and the practical mechanical properties of MIL-53(Al) in humidity condition. The adsorption of water molecules in MIL-53(Al) forms a reinforced wine-rack topological structure. The stable structure in the np phase eliminates the high anisotropy and anomalously low Young and shear moduli of the large pore phase. In addition, new soft modes which indicate the evolving path back to the large pore phase were found in the np phase.3. Based on the combined activation method, MIL-53(Al) synthesized by different methods was used for the chlorinated hydrocarbon vapor adsorption. Using the best synthesis method which has high adsorption capacity for the chlorinated hydrocarbon vapor, MIL-53(Al, Fe, Cr) were synthesized. The samples were characterized by XRD and N2 adsorption-desorption. The dynamic adsorption behavior of dichloromethane (CH2Cl2) and chloroform (CHCl3) on MIL-53 (Al, Fe, Cr) was studied at 298K,101 kPa using a fixed bed continuous flow reactor and a gas chromatography. The experimental results indicated that MIL-53(Al, Fe, Cr) have better adsorption capacity for CH2Cl2 than CHCl3, the adsorption capacity for the chlorinated hydrocarbon vapor was in the order MIL-53(Al)> MIL-53(Cr)> MIL-53(Fe), and the maximum adsorption amount for CH2Cl2 and CHCl3 on MIL-53(Al) were 16.88 mmol/g and 7.11 mmol/g, respectively. The results of adsorption experiments were supported by the Grand canonical Monte Carlo (GCMC) simulation. By the static adsorption of CH2Cl2 in MIL-53(Al), the properties of the equilibrium state and the dynamic were analyzed.4. Desorption energies of CH2Cl2 and H2O in MIL-53(Al) were investigated by the combination of experimental (differential scanning calorimeter, DSC) and computational (ab-initio calculations) methods. The differences of desorption energy and natural log of the frequency factor of CH2Cl2 and H2O in MIL-53(Al) were analyzed by a thermo active process using DSC measurements. The interaction energy of guest molecules with MIL-53(Al), which correspond to the desorption in the theromal active process, was explored using ab-initio calculation. As a results of the difference in the interaction energies of H2O and CH2Cl2 in MIL-53(Al), the site near the μ2-OH groups have two potential wells. Both experimentally and computationally, MIL-53 presents the preferential adsorption of CH2Cl2 than H2O. |
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