Fischer Michael
Fachgebiet Kristallographie, Fachbereich Geowissenschaften, Universität Bremen, Klagenfurter Straße, 28359 Bremen, Germany.
Phys Chem Chem Phys. 2016 Jun 21;18(23):15738-50. doi: 10.1039/c6cp02289h. Epub 2016 May 26.
Porous aluminophosphates (AlPOs) and silicoaluminophosphates (SAPOs) with zeolite-like structures have received considerable attention as potential adsorbents for heat transformation applications using water adsorption/desorption cycles. Since a detailed experimental characterisation of the water adsorption properties has only been performed for some of these materials, such as AlPO-18 (AEI topology) and SAPO-34 (CHA topology), more systematic insights regarding the influence of the pore topology and (for SAPOs) the arrangement of the framework protons on the affinity towards water are lacking. To study the relationships between structure and properties in more detail, the interaction of water with six structurally different AlPOs (with AEI, AFX, CHA, ERI, GIS, RHO topologies) and their SAPO analogues was investigated using dispersion-corrected density-functional theory (DFT-D) calculations. Different possible locations of silicon atoms and charge-balancing protons were considered for the SAPO systems. The calculations for SAPOs at low water loadings (one H2O molecule per framework proton) revealed that the interaction energies exhibit a considerable variation, ranging from -75 to -100 kJ mol(-1) (per water molecule). The differences in interaction energy were rationalised with the different structural environment of the framework protons at which the water molecules are adsorbed. At high water uptakes (near saturation), interaction energies in the range of -65 kJ mol(-1) were obtained for all AlPOs, and there was no evidence for a marked influence of pore size and/or topology on the interaction strength. The interaction of water with SAPOs was found to be approximately 5 kJ mol(-1) stronger than for AlPOs due to an increased contribution of electrostatic interactions. An analysis of the structural changes upon water adsorption revealed striking differences between the distinct topologies, with the materials with GIS and RHO topologies being distorted much more drastically than the systems based on double six-ring (d6r) units. Moreover, the direct coordination of water molecules to framework aluminium atoms occurs more frequently in these materials, an observation that points towards a reduced structural stability upon hydration.
具有类沸石结构的多孔磷酸铝（AlPOs）和硅磷酸铝（SAPOs）作为利用水吸附/解吸循环进行热转换应用的潜在吸附剂受到了广泛关注。由于仅对其中一些材料（如AlPO - 18（AEI拓扑结构）和SAPO - 34（CHA拓扑结构））进行了水吸附特性的详细实验表征，因此缺乏关于孔拓扑结构（对于SAPOs而言）以及骨架质子排列对水亲和力影响的更系统见解。为了更详细地研究结构与性能之间的关系，使用色散校正密度泛函理论（DFT - D）计算研究了水与六种结构不同的AlPOs（具有AEI、AFX、CHA、ERI、GIS、RHO拓扑结构）及其SAPO类似物的相互作用。对于SAPO体系，考虑了硅原子和电荷平衡质子的不同可能位置。低水负载量（每个骨架质子一个H₂O分子）下SAPOs的计算结果表明，相互作用能呈现出相当大的变化，范围为 - 75至 - 100 kJ mol⁻¹（每个水分子）。相互作用能的差异通过水分子吸附位置处骨架质子的不同结构环境得到合理解释。在高水摄取量（接近饱和）时，所有AlPOs的相互作用能范围为 - 65 kJ mol⁻¹，没有证据表明孔径和/或拓扑结构对相互作用强度有显著影响。由于静电相互作用的贡献增加，发现水与SAPOs的相互作用比与AlPOs的相互作用大约强5 kJ mol⁻¹。对水吸附时结构变化的分析揭示了不同拓扑结构之间的显著差异，具有GIS和RHO拓扑结构的材料比基于双六元环（d6r）单元的体系扭曲程度大得多。此外，在这些材料中，水分子与骨架铝原子的直接配位更频繁发生，这一观察结果表明水合作用下结构稳定性降低。
