Molecular Simulation Study On Selective Adsorption Separation Of Low-Concentration Gas In MOFs

The enriching of low-concentration coal mine gas(CMM) is a challenging issue, adsorption separation technology requires less investment, low operating and superior energy efficiency, in which a highly selective adsorbent is the key. Metal-organic frameworks(MOFs) with abundant structure and adsorption-desorption characteristics may become the best option for the future of adsorption field. At present, there are few studies on the adsorption separation of CH4/N2 in MOFs, particularly the information about the separation of CMM are far from sufficient.Based on Molecular Simulation, Dynamic Simulation and Ideal Adsorption Solution Theory, this paper has studied CH4 concentration from CMM by MOFs. Combined with the feature of low methane concentration in CMM, the adsorption, diffusion and separation mechanism of CMM in MOFs were studied. Results are as follows:(1) Revealing the adsorption behavior of CH4 and N2 in MOFs. Namely, gas molecules are preferentially adsorbed at the center of small cages and near the strong binding sites at low pressure, the uptake correlates well with surface area at medium pressure, the amount adsorbed are controlled by surface area and pore volume in the high pressure regions. There is significant adsorption over a large volume region, the adsorbed molecules mainly accumulate in the center of large channel.(2) By studying adsorption and diffusion properties of an equimolar of CH4/N2 mixture in MOFs. Indicating IAST can accurately predict adsorption behaviors of CH4/N2 in MOFs. MOFs show better separation performance for CH4/N2 mixture than traditional materials. In particular, CH4/N2 selectivity in ZIF-7 is the most prominent. The main reason is that the richness of micro-pores in ZIF-7 with sizes comparable to those of the adsorbate molecules has a favorable effect for gas separation.(3) Research indicates the adsorption selectivity correlates well with the reciprocal of adsorbility(△AD) 1/△AD. That is the difference in AD of two components adsorbed in material should be larger, the porosity of material should be smaller, its adsorbility(△AD) will be much greater. Pointing out that △AD can serve as a criterion for preliminary screening MOFs for CH4/N2 separation applications.(4) Adsorption selectivity, diffusion selectivity and permselectivity at 298 K and 1000 kPa of CH4/N2 in ZIF-68 are about 3.96, 6.07 and 24.01, respectively. Result demonstrates that ZIF-68 has a good performance for separating CH4/N2 mixture; Findings indicate the quantitative relationship between the CH4/N2 selectivity and surface area, pore size, pore volume of the structure properties of MOF, the structural properties of the best separation selectivity: surface area 61~1000 m2/g, pore size 0.25~0.5 nm, pore volume 0.1~0.5 cm3/g.(5) Exploring the effect of temperature(273K, 298 K and 323K), moisture content(1% H2 O and 5% H2O) and CO2 on the CH4/N2 separation. It is shown that the selectivity of CH4/N2 in MOFs is decreased with temperature increasing; the CH4/N2 separation factor reduces as the moisture content increases. H2 O competes with CH4 on the adsorption sites, which leads to a decrease of CH4 adsorption capacity, adsorbed water molecules provide additional adsorption sites for N2, resulting in an increase of N2 adsorption. There are different behaviors of CO2 effect on the CH4/N2 selectivity: CO2 shows a negligible effect on the adsorption selectivity of CH4/N2 in the most MOFs; the selectivity of CH4 over N2 is significantly increased as the pressure increases in Cu-BTC, PCN-26, MIL-47; the presence of CO2 enhances the separation performance of Pd(2-pymo)2 and ZIF-68 for CH4/N2 mixture.(6) Investigating the adsorption separation properties of extraction and ventilation gas in ZIF-68. Results suggest the selectivities of CH4 over N2 and O2 increase with the pressure increasing. At 298 K and 8000 kPa, the adsorption selectivities of ZIF-68 for CH4/N2 and CH4/O2 in ventilation gas are up to 16.48 and 15.37 respectively. Results reveal that ZIF-68 with large adsorption selectivity separates efficiently methane from CMM mixtures and is the most promising materials for CH4/N2 separation. A new adsorption site IV in the center of the HPR cages was observed, pores formed by phenyl rings have a strong affinity and offer more preferential adsorption sites towards CH4. IAST can be a good alternative to estimate the adsorption behavior of CMM mixtures in ZIF-68.This paper has studied the adsorption separation properties of CMM mixtures in MOFs, It can provide a theoretical basis for the experimental study and even industrial application of MOFs for adsorption separation. In addition, a SCI paper about research results was published.