Gaz ayırma için cof ve mof membranlarda denge ve denge dışı yaklaşımların karşılaştırmalı moleküler dinamik çalışması

This thesis study presents a comprehensive investigation of covalent organic frameworks (COFs) and metal organic frameworks (MOFs) as gas separation membranes, using a non-equilibrium approach called concentration gradient-driven molecular dynamics (CGD-MD). First, hydrogen (H2) and methane (CH4) flux was simulated through two distinct COF membranes, COF-300 and COF-320, for which experimental data are available in the literature. The results were in strong agreement with the experimental permeability and selectivity values. Leveraging the same methodology, thirteen COFs were screened, revealing promising candidates such as NPN-1 and PCOF-2 for H2/CH4 separation. We then compared our findings with simulations utilizing the well-known approach that merges grand canonical Monte Carlo (GCMC) and equilibrium molecular dynamics (EMD) to predict gas adsorption and diffusion parameters in COF membranes. Our results showed that when the pore sizes of COF membranes are below 10 A˚ , the two methods can give different results. The GCMC+EMD approach predicts CH4 selectivity, whereas the CGD-MD method indicates H2 selectivity. It shows that the choice of method is important for evaluating gas transport performance of COF membranes. In the second part, the pressure-induced amorphization of zeolitic imidazolate framework-8 (ZIF-8) was studied. Simulations showed that the atomic charge assignment models and the presence of guest CH4 molecules inside the pores, strongly affect the onset and pathway of structural collapse. Different simulation runs with random initial velocities produced different results, highlighting the stochastic nature of the amorphization process. We performed pair distribution function (PDF) g(r) analysis to capture the loss of long-range order during the pressure-induced amorphization of ZIF-8. The crystallinity of ZIF-8 is preserved up to 0.8 GPa, but further pressure increases result in the loss of PDF peaks from approximately 7 to 30 A˚ , indicating the degradation of long-range order and the transition to an amorphous state. Incorporating a moderate number of adsorbed CH4 molecules (≤54) into ZIF-8 does not affect the initial amorphization pressure. However, when the number of adsorbed CH4 molecules increases to a saturated loading (400), the amorphization onset shifts to approximately 1.0 GPa. The CGD-MD method was finally applied to crystalline and created amorphized ZIF-8 membranes for CO2/CH4 separation. The crystalline form showed higher permeability for both gases, with 23,380 Barrer for CO2 and 1,746 Barrer for CH4. In contrast, the structure obtained at 0.4 GPa (aZIF-8@0.4 GPa) had lower permeabilities of 11,617 Barrer for CO2 and 591 Barrer for CH4. Despite this reduction, aZIF-8@0.4 GPa exhibited higher CO2/CH4 ideal selectivity (19.65) compared to the crystalline form (13.39). Mean residence time analysis confirmed that CO2 interacts more strongly and remains longer in the frameworks than CH4. In conclusion, this study compares GCMC+EMD and CGD-MD methods for gas transport in porous membranes and shows how amorphization can be used to improve their gas separation performance. These results help to understand the gas transport in porous materials and highlight the value of non-equilibrium simulations for designing future COF and MOF membranes.