CO2置换CH4水合物技术中主、客体分子间作用的DFT研究

天然气水合物是新型清洁能源，围绕CO₂置换CH₄水合物技术的研究对天然气水合物的资源开采和减少全球碳排放具有重要意义。其中，置换机理的解析是CO₂置换CH₄水合物技术的关键问题，对提升置换效率具有重要作用。为深入阐述置换机理的本质，采用量子力学（QM）方法对水合物中主、客体双分子聚体间的相互作用进行模拟。利用不同的密度泛函理论（DFT）方法对双分子聚体的结构及单点能进行计算分析，在对CO₂置换CH₄水合物过程的研究中，获得QM方法下进行几何结构优化和单点能计算的较优的计算参数。采用对称性匹配微扰理论（SAPT）进行能量分析，解析主、客体相互作用中各分子的贡献，并通过计算波函数信息分析约化密度梯度函数（RDG）、独立梯度模型（IGM）和静电势，定向研究主、客体分子间最主要的相互作用。研究结果表明CO₂置换CH₄水合物过程中主、客体分子间的作用主要由静电作用贡献，色散和诱导作用占比较小；在置换过程中，客体分子由CH₄转变为CO₂时色散作用影响减弱，静电作用影响加强。因此，静电作用是置换过程的关键，提高与H₂O的静电作用是提升置换效率的有效方法。所得结果为CO₂置换CH₄水合物技术的发展提供了理论指导。

A DFT study of the interaction between host and guest molecules in the replacement of CH4 in natural gas hydrate by CO2

Natural gas hydrate is a new type of clean energy. Studies of the replacement of CH₄ in natural gas hydrate by CO₂ have profound significance for both the exploitation of natural gas hydrate resources and the reduction of global carbon emissions. The micro-mechanism is a key issue in the replacement technology, and plays an important role in maximizing replacement efficiency. In this work, quantum mechanics (QM) methods have been used to simulate the interaction between host and guest dimers in hydrates to elaborate the replacement mechanism. By comparing the geometry and single point energy results calculated using different density functional theories (DFT), the optimal structure and calculated energy parameters for the process of the replacement of CH₄ by CO₂ were obtained. Decomposition energies were calculated by symmetry adapted perturbation theory (SAPT) in order to analyze the contribution of each molecule to the interaction between host and guest species in the hydrate. The reduced density gradient function (RDG), independent gradient model (IGM), and electrostatic potential results were obtained by analysis of wavefunction information in order to probe the key interactions between host and guest. The results showed that the interaction between the host and guest molecules during the replacement of CH₄ by CO₂ is mainly provided by electrostatic interaction, with only minor contributions from dispersion and induction effects. In the replacement process, the influence of dispersion effects was reduced when the guest molecule was changed from CH₄ to CO₂, and the electrostatic interaction was enhanced. The results indicated that electrostatic interaction is the major factor controlling the replacement of CH₄ by CO₂, and the increased electrostatic interaction between CO₂ and H₂O enhances the replacement efficiency. This study can provide theoretical guidance for the development of the necessary technology for the replacement of CH₄ in natural gas hydrates by CO₂.