新型掺杂多孔芳香骨架材料的储氢性能模拟

基于PAF-301分子模型通过Li掺杂或B取代等模式设计了几种新型多孔芳香骨架(PAFs)材料,采用量子力学和分子力学方法对新材料的储氢性能进行研究.由量子力学计算得到了不同分子片段与H2之间的结合能,并结合DDEC方法计算了各分子片段的原子电荷分布.利用巨正则蒙特卡洛(GCMC)模拟方法计算了77和298 K下H2在不同PAFs材料中的吸附平衡性质.结果表明,H2直接与苯环的结合能较低,但掺杂Li原子能够提高H2与六元环的结合能,同时Li原子体现出较高的正电性质,B原子取代苯环中的两个C原子后,使得原有C原子电负性增强;77 K下PAF-301Li具有最高的储氢性能,而PAF-C4B2H4-Li2-Si和PAF-C4B2H4-Li2-Ge体现出较好的常温储氢性能,各种材料的常温储氢性能远低于其低温储氢性能.通过77 K下H2在PAFs材料中的等位能面分布和吸附平衡质心密度分布对H2在PAFs材料中的优先吸附位置进行分析,发现在PAF-301和PAF-301Li骨架中,由于中心能量较低的等位能区域范围较宽,H2在其中存在四个明显的吸附高密度分布区域,而其它三种PAFs晶胞中心能量较低的等位能区域范围较窄,使得H2在其中只存在两个明显的吸附高密度分布区域.

Simulation on the Hydrogen Storage Properties of New Doping Porous Aromatic Frameworks

Several new porous aromatic frameworks (PAFs) were designed by Li doping or B substitution based on the PAF-301 molecular model. The hydrogen storage capacities of these materials were investigated using quantum mechanics and molecular mechanics methods. First, the binding energies between H₂ and the different molecular fragments were calculated using quantum mechanics, and the atomic charge distributions of the molecular fragments were calculated by the density-derived electrostatic and chemical charge (DDEC) method. Then, the adsorption equilibrium properties of H₂ on the different PAFs were calculated at 77 and 298 K using grand canonical Monte Carlo (GCMC) simulations. The results indicate that the binding energy between H₂ and benzene without Li doping is poor, while the binding energies between H₂ and Li-doped six-member rings are improved. Li atoms doped into the benzene ring result in higher positive charges, and the electronegativity of the original carbon atoms in the benzene ring increase after its two carbon atoms are replaced with two boron atoms. Among these new materials, PAF-301Li has the highest hydrogen storage capacity at 77 K, while PAF-C₄B₂H₄-Li₂-Si and PAF-C₄B₂H₄-Li₂-Ge have better hydrogen storage capacities at room temperature than at 77 K. However, the hydrogen storage capacities of these various materials at room temperature are far below the capacities at cryogenic temperature. The preferential adsorption sites for H₂ on the PAFs at 77 K were analyzed through the potential energy surfaces and mass center density distribution of the adsorption equilibrium. It was found that there are four obvious high-density adsorption regions in the frameworks of PAF-301 and PAF-301Li because of their wide low-energy regions in the crystal center, while there are only two distinct high-density adsorption regions in the other three PAFs because of their narrow low-energy regions in the unit cell center.