Because of the high affinity of the carbonate ion (CO3 2-) to the hydrotalcite-like compounds or LDHs (layered double hydroxides), it was difficult to deintercalate carbonate ions and convert them into other LDHs with an easier anion-exchangeability. By treatment with dilute acids such as HCl, the conversion is limited, and, at higher concentrations, changes in shape and weight loss occurred. We found that the addition of NaCl dramatically enhanced the deintercalation of carbonate ions by dilute HCl solution (0.0025−0.005 N). It made the carbonate ions very rapidly deintercalate from the hydrotalcite-like compounds at 25 °C without any weight loss. Scanning electron microscopy (SEM) revealed that no morphological change occurred. Furthermore, by changing the HCl/NaCl ratio, it was possible to regulate the exchange ratio. Protonation of carbonate ions in the interlayer space and successive ion exchange with a large excess of Cl- ions was assumed to be the mechanism for this enhanced deintercalation.

A higher-order nanostructured polysiloxane was prepared by the sol−gel reaction of 3-aminopropyltrimethoxysilane catalyzed by a strong acid such as hydrochloric acid or nitric acid. In the X-ray diffraction (XRD) profiles, this compound had three peaks:  one prominent peak and two minor peaks. The d-value ratios of the three peaks were 1:1/√ 3:1/2. This strongly indicates the formation of a hexagonal phase. The peaks shifted due to a change in the humidity and the resulting product was completely dispersed in water. In addition, the d-values of each peak of the anion-exchanged product changed while depending on the bulk of the counteranion. Such behaviors cannot be observed for hexagonal mesoporous silica. Therefore, we estimated that this hexagonal phase came from a stacking of rodlike polymer with Si−O−Si framework at the core and ammonium groups extruding outside. The transmission electron microscopy (TEM) images showed a stripe pattern, indicating that the rodlike polysiloxane had parallel stacking. The scanning electron microscopy (SEM) image showed that the aggregate of the polysiloxane lined up in a regular direction, suggesting that the nano-ordered rodlike structure influences the micro-ordered regular structure. This rodlike polysiloxane with a hexagonal phase belongs to a new type of higher-ordered material, based on siloxane bonds.

For the homogeneous precipitation of hydrotalcite-like compounds (layered double hydroxides; LDHs), HMT (hexamethylenetetramine) was used and the conditions for obtaining the LDHs were investigated. The well-crystallized 1–5 μm LDHs were obtained in a pressure vessel after a 1-day treatment at 140 °C. The resulting LDHs contained a carbonate anion, which was successfully deintercalated using an NaCl–HCl mixed solution without any morphological change.

Improving the stability of porous materials for practical applications is highly challenging. Aluminosilicate zeolites are utilized for adsorptive and catalytic applications, wherein they are sometimes exposed to high-temperature steaming conditions (∼1000 °C). As the degradation of high-silica zeolites originates from the defect sites in their frameworks, feasible defect-healing methods are highly demanded. Herein, we propose a method for healing defects to create extremely stable high-silica zeolites. High-silica (SiO2/Al2O3 > 240) zeolites with *BEA-, MFI-, and MOR-type topologies could be stabilized by significantly reducing the number of defect sites via a liquid-mediated treatment without using additional silylating agents. Upon exposure to extremely high temperature (900–1150 °C) steam, the stabilized zeolites retain their crystallinity and micropore volume, whereas the parent commercial zeolites degrade completely. The proposed self-defect-healing method provides new insights into the migration of species through porous bodies and significantly advances the practical applicability of zeolites in severe environments.

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