Department of Chemistry, Chemical Theory Center, and Minnesota Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431, USA.
Phys Chem Chem Phys. 2018 Jul 11;20(27):18503-18514. doi: 10.1039/c8cp01788c.
We report the synthesis and characterisation of a HY/MgO zeolite/oxide nanocomposite material with high crystallinity and highly dispersed, highly basic MgO sites. Preparation was optimized in order to preserve sample crystallinity, to avoid the formation of mesoporosity and to minimize the formation of separate Mg-containing phases. These features were checked by means of electron microscopy, X-ray powder diffraction, porosimetry and IR spectroscopy. A highly dispersed material was obtained, comprising nanoclusters of magnesium oxide and hydroxide hosted by the microporous zeolite framework. The location and structure of the Mg-containing clusters have been studied by means of a combination of Rietveld refinement of XRPD data and high quality quantum mechanical simulations. The refinement has shown the presence of magnesium and oxygen atoms in the double six-membered ring cages, consistent with the presence of mononuclear Mg moieties. However, the composition and IR spectroscopy demonstrate that other Mg species must exist, likely located in the zeolite pores. In order to propose candidate structures for these species, several hypothetic periodic models of the material were built by placing (MgO)n clusters in different locations of the zeolite structure, taking into account the material composition and other constraints imposed by the experimental observations. Periodic structures with P1 symmetry were optimized at the B3LYP-D*/DZVP level with the CRYSTAL code and classified according to their stability. Two families of possible sites were identified: highly solvated (MgO)n units in narrow cavities and less coordinated clusters in the supercages. The stability of these clusters appears to be regulated by the ability of Mg2+ and O2- ions to interact with the pore walls and by the formation of Mg-OH species as result of the reaction of Mg-O couples with remaining acidic protons. The reactivity of four representative models with CO2 has been simulated at the B3LYP-D*/TZVP level. CO2 forms very stable linear end-on adducts with low coordinated Mg ions in most cases. Isolated sites give rise to bridge bidentate complexes in agreement with previous spectroscopic observations. The formation of hydrogen-carbonates is observed only on specific sites, through a process having a low adsorption energy because of the high deformation of the adsorption site.
我们报告了一种 HY/MgO 沸石/氧化物纳米复合材料的合成与表征，该材料具有高结晶度和高度分散、高碱性 MgO 位。为了保持样品结晶度、避免介孔形成和尽量减少单独的含 Mg 相的形成，对制备进行了优化。通过电子显微镜、X 射线粉末衍射、孔隙率测定和红外光谱对这些特性进行了检查。得到了一种高度分散的材料，由镁氧化物和氢氧化物的纳米簇组成，由微孔沸石骨架承载。通过结合 XRD 数据的 Rietveld 精修和高质量量子力学模拟研究了含 Mg 簇的位置和结构。精修表明，镁和氧原子存在于双六元环笼中，与单核 Mg 部分的存在一致。然而，组成和红外光谱表明，必须存在其他 Mg 物种，可能位于沸石孔中。为了提出这些物种的候选结构，根据材料组成和实验观察到的其他限制，通过将 (MgO)n 簇放置在沸石结构的不同位置，构建了几种假设的周期性模型。使用 CRYSTAL 代码在 B3LYP-D*/DZVP 水平上对具有 P1 对称性的周期性结构进行了优化，并根据其稳定性进行了分类。确定了两种可能的位置家族：在狭窄空腔中的高度溶剂化 (MgO)n 单元和在超笼中的配位较少的簇。这些簇的稳定性似乎受到 Mg2+ 和 O2- 离子与孔壁相互作用的能力以及 Mg-O 对与剩余酸性质子反应形成 Mg-OH 物种的能力的调节。在 B3LYP-D*/TZVP 水平上模拟了四种代表性模型与 CO2 的反应性。在大多数情况下，CO2 与低配位的 Mg 离子形成非常稳定的线性端到端加合物。孤立的位点产生桥联双齿配合物，与先前的光谱观察结果一致。仅在特定位置观察到形成碳酸氢盐，这是由于吸附位置的高变形，吸附能较低。
