锌基-MOFs对氨吸附制冷性能的分子模拟研究

Molecular Simulation for Refrigeration Based on ZIF-8(Zn)-Ammonia Sorption

本文采用巨正则蒙特卡洛(grand canonical Monte Carlo,GCMC)方法,基于UFF(universal force field)和TraPPE(transferable potentials for phase equilibria)力场,对ZIF-8(Zn)吸附NH_(3)进行了分子模拟研究,并结合分子模拟结果和吸附式制冷热力学循环模型,研究了ZIF-8(Zn)/NH_(3)工质对的吸附性能和制冷性能。研究表明:在等温条件下,ZIF-8(Zn)对NH_(3)的吸附量随压力的增大而提高,298 K和398 K下饱和吸附量分别达到0.305 g/g和0.231 g/g;同一温度下的总吸附热也随压力的增大而上升,这主要归因于NH_(3)分子间相互作用产生吸附热的增加,而ZIF-8(Zn)与NH_(3)相互作用的吸附热维持在较稳定的状态;NH_(3)在ZIF-8(Zn)中的吸附密度分布结果表明NH_(3)在金属位点处被大量吸附,难以通过ZIF-8(Zn)部分孔径较小的孔道,成功通过孔道的NH_(3)在ZIF-8(Zn)的SOD笼内部聚集;该吸附式制冷工质对制冷性能稳定,对温度有较好的适应性。制冷温度为283 K(通风工况)的制冷系数为0.43,制冷温度为243 K(冷冻工况)的制冷系数仍能达到0.38。

Metal-organic frameworks demonstrate significant application potential in sorption refrigeration driven by low-grade thermal energy owing to their ultrahigh specific surface area,large pore volume,and adjustable structure.Among the most structurally stable MOFs,ZIF-8(Zn)exhibits excellent structural stability and stable sorption properties.Herein,the grand canonical Monte Carlo molecular simulation method is adopted to simulate the sorption reaction between ZIF-8(Zn)and NH_(3),which is described by the universal force field and transferable potentials for phase equilibria force field parameters,respectively.Using both molecular simulation results and the adsorption refrigeration thermodynamic cycle model,we investigate the adsorption and refrigeration performances of the ZIF-8(Zn)/NH_(3) working fluid pair.The results show that the isothermal ammonia sorption capacity of ZIF-8(Zn)increases with pressure and that the sorption capacity reaches 0.305 g/g and 0.231 g/g at 298 K and 398 K,respectively.Moreover,the total heat of sorption at the same temperature increases with pressure owing to the increase in the reaction heat of the NH_(3) intermolecular interaction.The isothermal sorption heat from the interaction between ZIF-8(Zn)and NH_(3) is relatively stable at different pressures.Furthermore,the sorption density distribution diagram shows that numerous NH_(3) molecules are adsorbed at the metal sites.Some NH_(3) molecules fail to pass through the small channels of ZIF-8(Zn),whereas others accumulated inside the six-membered ring cage of ZIF-8(Zn)after passing through the holes.Finally,the sorption refrigeration system demonstrates stable refrigeration performance and good adaptability to refrigeration temperatures.Ideally,the coefficient of performance(COP)should be 0.43 when the refrigeration temperature is 283 K(ventilation conditions),and it should be able to reach 0.38 when the refrigeration temperature is 243 K(freezing conditions).