dc.description.abstract | Metal–organic frameworks (MOFs), a relatively new class of microporous crystalline materials, have a very large number of applications in industry, such as gas capture, gas storage, catalysis, and selective separation, etc. Both MIL-53 series and UiO-66 series are also such porous materials with similar properties, which have gained a lot of attention due to useful properties including structural stability in water, flexible pore openings, and outstanding selectivity for specific gases.
On the other hand, computer simulations are good tools to help us understand details of MOF structure. Density functional theory (DFT), one kind of first principles simulations, is a common way to implement quantum mechanical simulation of periodic systems. First principles calculations are based on quantum mechanics and are performed without any assumptions. Monte Carlo (MC) simulation, another simulation method using random sampling to find numerical results, calculates molecule state and distribution of the location in thermodynamics by statistics.
In this work, we investigated these two classes of MOFs, MIL-53(Al) and UiO-66(Zr), by using membranes synthesis to observe the structural performance of MIL-53(Al) and computer simulation to observe the structural performance of UiO-66(Zr).
Experimentally, we demonstrated PSF polymer-based Mixed-Matrix Membranes. The MIL-53(Al) series, ML-53(Al) and NH2-MIL-53(Al), were added to the MMMs as a co-filler, containing different loading ratio. The membranes were characterized by X-ray diffraction, SEM images, FTIR analyses and DSC analyses to investigate the interactions inside membranes. The results show that the two added fillers can maintain the original characteristics without any agglomeration phenomena, and can effectively combine with the polymer film.
The section of simulation, DFT was used to minimize the total energy of the system by allowing all the atoms and lattices in the cell to relax, thereby optimizing the geometry. The systems calculated include the empty UiO-66(Zr), m- xylene, and p-xylene molecule. After optimizing the geometry with DFT, the structure was used to find the most likely location of adsorbate in UiO-66(Zr) by MC calculation. Finally, the UiO-66(Zr) uptaking m-xylene molecule and UiO-66(Zr) uptaking p-xylene molecule were studied again using DFT after finding the location to calculate the heat of adsorption. The results showed the heat of adsorption is 53.68 kJ/mol for UiO-66(Zr) uptaking m- xylene, and 60.16 kJ/mol for UiO-66(Zr) uptaking p-xylene molecule, respectively. Compared to the molecule with a small diameter(CO2, CH4), m-xylene and p-xylene have a higher adsorption energy, which confirms the good interaction between the 1,4-benzenedicarboxylate (BDC) linkers and benzene ring of xylene.
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