Zeolites are used in the chemical and separation industries for their exceptional selectivity, adsorption capacity, regenerability, and stability in gas and liquid phase processing. Here, we developed an explicit solvation method for predicting solvent/condensed phase effects on adsorption free energies in microporous media such as zeolites based on the hybrid quantum mechanical/molecular mechanical free energy perturbation (QM/MM-FEP) technique. Our explicit solvation method for zeolite systems, called eSZS, aims to capture site-specific interactions during the adsorption process at the Brønsted acid sites of H-MFI zeolite while still considering the diverse configuration space of the solvent molecules. This strategy is ideal for chemical reactions or adsorbates that interact with the microporous medium in few distinct adsorbate/transition state configurations, i.e., the harmonic or similar approximations are acceptable for the adsorbate/transition state while such approximations break down for the solvent molecules that require extensive configuration space sampling. In this way, our approach effectively overcomes the limitations of implicit solvation models and classical force field methods for describing solvation effects on chemical reactions within porous materials such as zeolites. Specifically, in this study, we investigated various aspects of our hybrid QM/MM approach, including QM cluster size dependencies in a periodic electrostatically embedded cluster model (PEECM), rules for link atoms at the QM/MM boundary, and functional and basis set considerations for converged and reasonably accurate gas and aqueous phase methanol and ethanol adsorption free energy predictions in H-MFI. For gas phase adsorption of methanol and ethanol in H-MFI at a Brønsted acid site in T12 position, we compute adsorption free energies at 298 K of -0.61 and -0.75 eV, respectively, using a PEECM containing 50 Si and 1 Al atom with ωB97x-D/def2-TZVP level of theory. For solvent effect calculations, we sample the aqueous phase using grand canonical Monte Carlo (GCMC) simulations to (1) obtain a mean field of electrostatic interactions in the reaction system and (2) perform a rigorous free energy perturbation calculation. Similar to the experimentally and computationally observed endergonic solvation effects observed for hydrocarbon adsorption on metal surfaces, we also observe that a condensed aqueous environment destabilizes methanol and ethanol at these acid sites in H-MFI at 298 K. Specifically, the computed solvation free energies of adsorption (ΔΔG_solv) for methanol and ethanol are +0.44 and +0.54 eV, respectively. From this study, it is evident that adsorbates (methanol and ethanol) are competing with water for adsorption space inside the H-MFI zeolite, leading to an endergonic solvation effect. We expect that the endergonic, aqueous solvent effect during adsorption in microporous zeolites is highly tunable by changing the pore size and hydrophobicity of the microporous material as this will affect the water density inside the pore structure.

分子筛因其卓越的选择性、吸附能力、再生性和在气液相处理中的稳定性，在化工和分离工业中得到广泛应用。在此，我们开发了一种显式溶剂化方法，用于基于杂化量子力学/分子力学自由能扰动（QM/MM-FEP）技术预测微孔介质如分子筛中溶剂/凝聚相效应对吸附自由能的影响。我们的针对分子筛系统的显式溶剂化方法称为eSZS，在H-MFI分子筛的Brønsted酸位点处捕捉特定位点相互作用的同时，仍然考虑了溶剂分子多样化的构象空间。这种策略非常适合于化学反应或与微孔介质以少数特定吸附态/过渡态配置相互作用的吸附质，即对于吸附物/过渡态而言谐波近似是可接受的，但对于需要广泛构象空间采样的溶剂分子则不适用。通过这种方式，我们的方法有效地克服了隐式溶剂化模型和经典力场方法在描述多孔材料如分子筛内部化学反应中溶剂效应方面的局限性。具体来说，在这项研究中，我们探讨了杂化QM/MM方法的各个方面，包括周期静电嵌入簇模型（PEECM）中的QM簇尺寸依赖性、QM/MM边界处连接原子的规则以及用于H-MFI中甲醇和乙醇气相吸附自由能预测收敛且合理准确的能量水平和基组考虑。对于H-MFI T12位置Brønsted酸位点上甲醇和乙醇的气相吸附，我们使用包含50个Si和1个Al原子的PEECM，在ωB97x-D/def2-TZVP理论层次下分别计算得到在298 K时的吸附自由能为-0.61 eV 和 -0.75 eV。对于溶剂效应计算，我们使用巨正则蒙特卡洛（GCMC）模拟采样水相以（1）获取反应系统中的平均静电相互作用场以及（2）进行严格的自由能扰动计算。类似于实验和计算观察到的烃类在金属表面吸附时的吸热溶剂化效应，我们还观察到，在H-MFI中这些酸位点上，298 K下的水性环境使甲醇和乙醇变得不稳定。具体来说，所计算出的甲醇和乙醇的溶剂化吸附自由能（ΔΔG_solv）分别为+0.44 eV 和 +0.54 eV。从这项研究中可以看出，吸附质（甲醇和乙醇）与水在H-MFI分子筛内部争夺吸附空间，导致吸热溶剂化效应。我们预期，在微孔分子筛中的吸附过程中，由于改变孔径和微孔材料的疏水性会直接影响孔结构内的水密度，因此这种吸热、水性溶剂效应是高度可调的。