Layered Silicates Exhibiting MOF-Like Gate-Opening Behaviors in Liquid-Phase Adsorptions: Experimental and Theoretical Investigations.

Layered silicates, including clay minerals, can be used as liquid-phase adsorbents in many important applications. However, because their two-dimensional interlayer space is narrow and not entirely opened due to the presence of interlayer species, guest species are forced to penetrate while expanding the interlayer space, which limits their adsorption performances compared with microporous materials such as MOFs and zeolites. Herein, as reported for the adsorption of gaseous species on flexible MOFs, we report a layered silicate that exhibits gate-opening adsorption in liquid phases. This layered silicate, synthesized via dilute acid treatment of the parent sodium-type, exhibits an abrupt increase in the basal spacing (layer thickness + interlayer space) to reach a plateau even at an earlier stage of benzoic acid adsorption from acetonitrile, whereas a typical layered silicate, magadiite, exhibits a gradual increase in the basal spacing as adsorption progress under identical conditions. The layered silicate shows an excellent adsorption capacity and rate for benzoic acid uptake from acetonitrile, which is considerably higher than that of magadiite. With comprehensive adsorption tests using different adsorbates and solvents, we propose that the layered silicate has zeolite-like but distorted, flexible open microchannels within each layer, and the intralayer microchannels can effectively and rapidly accommodate the solvent (acetonitrile) molecules, which are capable of expanding the framework to initiate the adsorption of aromatic compounds. The density function theory calculation revealed the adsorption mechanism, where the layered silicate accommodates acetonitrile in the intralayer microchannel followed by the interlayer space, and the former selectively plays a role as the adsorption site of aromatic compounds via exchange with acetonitrile.

Hydrophilicity of Organically Modified Montmorillonite and Effect on Benzene Adsorption by the Molecular Dynamics Method.

Interlayer modification of layered materials with organocations has been known to endow the nanocomposite with hydrophobicity, and adsorption of aromatic compounds in the aqueous phase has been investigated for decades by using montmorillonite, a representative layered clay mineral, as the host material. Usage of the organocation has been believed to be effective due to the π-π interaction with the aromatic adsorbate, the presence of which is not verified spectroscopically in the water-immersed state. Considering that the organocation is generally regarded as a pillar to keep the interlayer space, the interaction between the organocation and adsorbate has not yet been clarified sufficiently. In the present study, we revealed the role of the organocation by the molecular dynamics method, where tetramethylammonium (TMA) and trimethylphenylammonium (TMPA) ions were selected as the representative and simple organocations, and benzene was the adsorbate to exclude the effect of the substitution group. Both H2O and benzene molecules were introduced in the interlayer of TMA- or TMPA-modified montmorillonite to model the water-immersed adsorption structure. It was found that H2O is preferentially distributed on the clay surface, followed by the center of the interlayer when the amount of H2O is large. In the adsorption model, benzene was vertically adsorbed on the clay surface. Radial distribution function analysis revealed that benzene is distributed around both the methyl and the phenyl groups in the TMA and TMPA cations, but the orientation of the phenyl ring is not consistent with that of benzene. Thus, benzene was found not to form the π-π interaction in montmorillonite modified with the TMPA cations in the water-immersed state. Furthermore, the surface was partly covered with the phenyl group in the TMPA cation, decreasing the adsorption area. Therefore, the experimental suggestion that benzene is adsorbed on the clay surface was reproduced by our simulation, and the interaction between the organocation and benzene and surface occupancy should be paid attention to maximize the adsorption property.

High Efficient CO2 Separation at High Pressure by Grain-Boundary-Controlled CHA Zeolite Membrane Investigated by Non-Equilibrium Molecular Dynamics.

The CO2 permeability and selectivity of CHA-type zeolite membranes in the separation of a CO2/CH4 mixture gas at high pressure were evaluated using non-equilibrium molecular dynamics (NEMD). It was found that in a perfectly crystalline, defect-free CHA membrane, the adsorption of CH4, which diffuses slowly in the pores, hinders CO2 permeation. Therefore, an increase in the amount of CH4 adsorbed at high pressure decreases the CO2 permeability and significantly reduces the CO2 selectivity of the CHA membrane. CHA membranes with grain boundaries parallel to the permeation direction were found to show higher CO2 selectivity than perfectly crystalline CHA membranes at high pressure, as the blocking effect of CH4 on CO2 permeation occurring within the grain boundary is not significant. This paper is the first to show that the CO2 permeability of CHA membranes with controlled grain boundaries can exceed the intrinsic performance of fully crystalline zeolite membranes at high pressure.

Estimation of Adsorbed Amounts in Organoclay by Machine Learning.

Adsorption properties of organoclay have been investigated for decades focusing on the morphology and physicochemical properties of two-dimensional interlayers. Experimental studies have previously revealed that the adsorption mechanisms depend on the molecular species of the organocation and adsorbate, making it difficult to estimate the adsorbed amount without experiments. Considering that the adsorption of aromatic compounds has been reported by using various clays, organocations, and adsorbates, machine learning is a promising method to overcome the difficulty. In the present study, we collected adsorption data from the literature and constructed models to estimate the adsorbed amount of the organoclay by random forest regression. The composition of the clay, molecular descriptors of the organocation and adsorbate obtained by the RDKit, and experimental conditions were used as the explanatory variables. Simple model construction by using all the experimental data resulted in low R 2 and a mean absolute error. This problem was solved by the correction of the adsorbed amount data by the Langmuir or Freundlich equation and the following model construction at various equilibrium concentrations. The plots of the adsorbed amount estimated by the latter model were located close to the corresponding adsorption isotherm, while that by the former was not. Thus, it was revealed that the adsorbed amount was estimated quantitatively without understanding the adsorption mechanisms individually. Feature importance analysis also revealed that the combination of the organocation and adsorbate is important at high equilibrium concentrations, while the clay should be selected carefully as the concentration gets lower. Our results give an insight into the rational design of the organoclay including the synthesis and adsorption properties.

Solvent-Solute Interaction Effect on Permeation Flux through Forward Osmosis Membranes Investigated by Non-Equilibrium Molecular Dynamics.

The relationship between the solvent-solute interaction and permeation properties is fundamental in the development of the forward osmosis (FO) membrane. In this study, we report on the quantitative reproduction of the permeation flux, which has different solvent-solute interactions, through the modeled FO membrane by non-equilibrium molecular dynamics (NEMD). The interaction effect was investigated by changing the interatomic interaction between the solute and the solvent. The calculated permeation through the semi-permeable modeled FO membrane, in which the interaction between solvent and solution is equal to that between solutions, was consistent with the theoretical curve derived from the combination of the permeation flux and Van't Hoff equations. These results validate the NEMD for the evaluation of permeation in FO. On the other hand, the permeation is much derived from the theoretical values when the interaction between the solvent and solute atoms is relatively large. However, the simulated permeation was consistent with the theoretical curve, correcting the solution concentration by the coordination number of the solvent atoms to the solute atoms. Our results imply that permeation flux through the FO membrane is significantly changed by the interaction between the solute and the solvent and can be theoretically predicted by calculating the coordination number of the solvent to the solute, which can be readily estimated by equilibrium molecular dynamics simulation.

Intercalation-Induced Ordered Nanostructure in the Interlayer Modified with Methylviologen by Molecular Dynamics Simulation.

Intercalation of phenol in montmorillonite, a representative layered material, has historically been investigated, and modification of the interlayer with methylviologen (Mont-MV) results in color change due to formation of a charge-transfer complex. Its detailed nanostructure, however, has yet been revealed owing to its small gallery height and poor crystallinity. In the present study, we performed molecular dynamics simulation to investigate structural changes in Mont-MV by the intercalation of phenol. The value of the basal spacing of Mont-MV was well consistent with that reported experimentally, and the MV cations were distributed horizontally. Successive intercalation of phenol revealed that the interlayer swelled nonlinearly and both the MV cations and phenol molecules were tilted, which were roughly parallel to each other. The obtained ordered nanostructure was similar to that reported in the charge-transfer complex crystal of the MV cation and the naphthol derivative. Thus, the parallel orientation in the interlayer was found to be the key for the color reaction. Combined with the fact that the phenol molecules interacted with the Mont layers, the role of the MV cation was found to be that of a pillar for providing sufficient gallery height, and the formation of the charge-transfer complex is the secondarily derived function.

Inhomogeneity of Organically Modified Montmorillonite Revealed by Molecular Dynamics Simulation.

The modification of an interlayer of layered materials by intercalation with an organoammonium ion has been a promising method to control the polarity of the two-dimensional nanospace. Montmorillonite is one of the best-known examples, and the modification with octadecyltrimethylammonium ion (Mont-C18) results in adsorption of anthracene and pyrene together with specific excimer emission, while the nanostructure is yet to be uncovered at the molecular level because the gallery height is only ca. 27 Å. We, herein, investigated the nanostructure of this nanocomposite by molecular dynamics (MD) simulation, combined with analysis of molecular orientations against the Mont layer. The gallery height of Mont-C18 was well consistent with the experimental value, which was linearly increased along with the intercalation of anthracene. Anthracene was segregated on the Mont layer with its short and long molecular axes vertical in the early and late stages, respectively. In contrast, C18 was initially rather horizontal, forming the so-called pseudotrimolecular layer. Pushed out by anthracene, distribution and orientation of C18 were gradually changed: the third molecular layer was distinctly observed in the center of the interlayer in the early stage, and the orientation was changed to vertical in the late stage. Thus, the continuous increase in the gallery height is ascribed to soft response of C18 to the intercalation. Summarizing the abovementioned results, it was concluded that Å-order inhomogeneity is introduced in the interlayer by the intercalation of anthracene, which is significant in ideal design of the two-dimensional nanospace.

Electrically Induced Alignment of Semiconductor Nanosheets in Niobate-Clay Binary Nanosheet Colloids toward Significantly Enhanced Photocatalysis.

Aqueous binary colloids of niobate and clay nanosheets, prepared by the exfoliation of their mother layered crystals, are unique colloidal systems characterized by the separation of niobate and clay nanosheet phases, where niobate nanosheets form liquid crystalline domains with the size of several tens of micrometers among isotropically dispersed clay nanosheets. The binary colloids show unusual photocatalytic reactions because of the spatial separation of photocatalytically active niobate and photochemically inert clay nanosheets. The present study shows structural conversion of the binary colloids with an external electric field, resulting in the onsite alignment of colloidal nanosheets to improve the photocatalytic performance of the system. The colloidal structure is reshaped by the growth of liquid crystalline domains of photocatalytic niobate nanosheets and by their electric alignment. Niobate nanosheets are assembled by the domain growth process and then aligned by AC voltage, although clay nanosheets do not respond to the electric field. Photocatalytic decomposition of the cationic rhodamine 6G dye, which is selectively adsorbed on clay nanosheets, is examined for the niobate-clay binary nanosheet colloids with or without domain growth and electric field. The fastest decomposition is observed for the electrically aligned colloid without the domain growth, whereas the sample with the domain growth and without the electric alignment shows the slowest decomposition. The results demonstrate the improvement of the photocatalytic performance by changing the colloidal structure, even though the sample composition is the same.

Aqueous synthesis of protectant-free copper nanocubes by a disproportionation reaction of Cu2O on synthetic saponite.

Here, we report a synthesis of Cu nanocubes by photoreduction of CuSO4. Because synthetic saponite (one of the layered clay minerals) was used as the adsorbent, the nanocubes contained no capping agents or protectants, and the disproportionation reaction of Cu2O with H2SO4 was found to be the key for morphological control.