Abstract
We report comparative structural changes of potassium-contained zeolite-W (K-MER, structural analogue of natural zeolite merlinoite) and monovalent extra-framework cation (EFC)-exchanged M-MERs (M = Li+, Na+, Ag+, and Rb+). High-resolution synchrotron X-ray powder diffraction study precisely determines that crystal symmetry of MERs is tetragonal (I4/mmm). Rietveld refinement results reveal that frameworks of all MERs are geometrically composed of disordered Al/Si tetrahedra, bridged by linkage oxygen atoms. We observe a structural relationship between a group of Li-, Na-, and Ag-MER and the group of K- and Rb-MER by EFC radius and position of M(1) site inside double 8-membered ring unit (d8r). In the former group, a-axes decrease reciprocally, c-axes gradually extend by EFC size, and M(1) cations are located at the middle of the d8r. In the latter group, a- and c-axes lengths become longer and shorter, respectively, than axes of the former group, and these axial changes come from middle-to-edge migration of M(1) cations inside the d8r channel. Unit cell volumes of the Na-, Ag-, and K-MER are ca. 2005 Å3, and the volume expansion in the MER series is limited by EFC size, the number of water molecules, and the distribution of extra-framework species inside the MER channel. EFC sites of M(1) and M(2) show disordered and ordered distribution in the former group, and all EFC sites change to disordered distribution after migration of the M(1) site in the latter group. The amount of water molecules and porosities are inversely proportional to EFC size due to the limitation of volume expansion of MERs. The channel opening area of a pau composite building unit and the amount of water molecules are universally related as a function of cation size because water molecules are mainly distributed inside a pau channel.
Highlights
Porous materials have been acknowledged as important specimens due to their pore characteristics, which are dependent on pore morphology, pore size, and pore size distribution [1,2,3]
The starting material, zeolite-W (K-MER), was synthesized under hydrothermal conditions according to Itabashi et al [23]
Chemical compositions derived from Rietveld refinement are Li6.9 Al6.9 Si25.0 O64.0 ·26H2 O, Na7.5 Al7.0 Si25.0 O64.0 ·20.0H2 O, Ag7.0 Al7.0 Si25.0 O64.0 ·22.2H2 O, and K6.42 Al6.5 Si25.8 O64.0 ·15.3H2 O for the Li, Na, Ag, and K-MER, respectively (Table S1)
Summary
Porous materials have been acknowledged as important specimens due to their pore characteristics, which are dependent on pore morphology, pore size, and pore size distribution [1,2,3]. Numerous experiments have been conducted to the understand fundamental factors of porous materials, and some have suggested characterization methods, e.g., the work of Liu et al (2014) [4]. Zeolite-W is a synthetic phase whose framework topology is the same as the natural small-pore zeolite, merlinoite (assigned zeolite code: MER) [15,16]. Many studies have provided information as to synthesis, crystal growth, and physiochemical characterization using methods such as Fourier transform infrared spectroscopy, thermogravimetric analysis, and solid-state NMR; there are few reports about the crystal structure of prepared products [19,20,21,22]. X-ray diffraction is one of the basic methods used to characterize products in order to understand heterogenetic cation sites, framework topology, and distributions of zeolitic water inside channels
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