Two-dimensional Hexagonal Mesoporous Silica Research Articles

Water Filling and Emptying Kinetics in Two-Dimensional Hexagonal Mesoporous Silica of the Same Pore Diameter but Different Pore Lengths.

The effect of pore length on the water filling and emptying rates was studied using mesoporous silica (MPS) with same pore diameter but different pore lengths. The pore diameter of the synthesized MPS was ∼8 nm, whereas the average pore lengths were 460, 1,770, and 4000 nm. The gravimetric method was employed to record the time course of the adsorbed mass of water in MPS at 298 K and 1 atm. In both the filling and emptying processes, the relaxation curves (time course of adsorbed mass of water per unit mass of sample) were not significantly related to the pore length. This independence of the initial adsorption and desorption rates on the pore length suggests that the surface of the MPS aggregates is the bottleneck in the overall adsorption and desorption processes and that the initial mass flux in each nanopore is inversely proportional to the pore length. Furthermore, because the relaxation times to reach the equilibrium state were independent of the pore length, the mass flux of water uptake, release, and transport probably increase with an increase in the pore length during the entire adsorption and desorption processes. A transport model to describe these phenomena was proposed.

Synthesis of Hf/SBA-15 Lewis acid catalyst for converting glycerol to value-added chemicals

Hf/SBA-15 catalyst containing highly ordered two-dimensional hexagonal mesoporous silica with a large surface area is prepared in a facile one-step sol–gel method with hafnia content ranging from 2.5 to 10%. The synthesized catalyst is characterized by X-ray diffraction analysis, N2 adsorption–desorption isotherm, scanning electron microscope and transmission electron microscope. The oxidation state and Lewis acidic properties of 10% Hf/SBA-15 are investigated by diffuse reflectance UV–Vis technique, X-ray photoelectron spectroscopy and pyridine-adsorbed FT-IR analysis. The catalytic activity of Hf/SBA-15 is employed in the acetalization of glycerol with acetophenone and benzaldehyde, order of activity of the catalyst is 10 > 5 > 2.5% Hf/SBA-15. The 10% Hf/SBA-15 is given 43 and 21% selectivity of six-membered (1,3-dioxane) and five-membered (1,3-dioxolane) acetals. Hence, this catalyst is further exploited in the acetalization of glycerol with substituted acetophenone and benzaldehyde. The catalyst could be recycled for five times without a significant decrease in activity.

Adsorption and Desorption of Water in Two-Dimensional Hexagonal Mesoporous Silica with Different Pore Dimensions

We have investigated the adsorption and desorption of water in two-dimensional hexagonal mesoporous silica (MPS) with three different pore dimensions. All the adsorption–desorption isotherms exhibited type V behavior with significant hysteresis. Hysteresis was observed in the transient region between the layer adsorption state and the pore filling state, where capillary condensation and evaporation occurred. When the relative humidity is changed in steps from the onset point of capillary condensation and capillary evaporation, the relaxation curves of water uptake and release were measured at 298 K using a gravimetric method. For all MPSs, the relaxation rates of water uptake and release increased as the magnitude of the stepwise change in relative humidity increased. The relaxation curves of some MPSs could be well fitted by the exact solution of the Fickian diffusion equation at a small stepwise change in relative humidity. On the other hand, the adsorbed mass is proportional to the square root of time at large stepwise changes in relative humidity. The relationship between the mass flux of water uptake or release and the difference in chemical potential between the initial and final states of moist gases depends on the structural properties of the mesopores.

Biomimetic Mn-Catalases Based on Dimeric Manganese Complexes in Mesoporous Silica for Potential Antioxidant Agent.

Two new structural and functional models of the Mn-catalase with formula [{Mn(III)(bpy)(H2O)}(μ-2-MeOC6H4CO2)2(μ-O){Mn(III)(bpy)(X)}]X, where X = NO3 (1) and ClO4 (2) and bpy = 2,2'-bipyridine, were synthesized and characterized by X-ray diffraction. In both cases, a water molecule and an X ion occupy the monodentate positions. The magnetic properties of these compounds reveal a weak antiferromagnetic behavior (2J = -2.2 cm(-1) for 1 and -0.7 cm(-1) for 2, using the spin Hamiltonian H = -2J S1·S2) and negative zero-field splitting parameter DMn (-4.6 cm(-1) and -3.0 cm(-1) for 1 and 2, respectively). This fact, together with the nearly orthogonal orientation of the Jahn-Teller axes of the Mn(III) ions explain the unusual shape of χMT versus T plot at low temperature. Compound 1 presents a better catalase activity than 2 in CH3CN-H2O media, probably due to a beneficial interaction of the NO3(-) ion with the Mn complex in solution. These compounds were successfully inserted inside two-dimensional hexagonal mesoporous silica (MCM-41 type) leading to the same hybrid material ([Mn2O]@SiO2), without the X group. The manganese complex occupies approximately half of the available pore volume, keeping the silica's hexagonal array intact. Magnetic measurements of [Mn2O]@SiO2 suggest that most of the dinuclear unit is preserved, as a non-negligible interaction between Mn ions is still observed. The X-ray photoelectron spectroscopy analysis of the Mn 3s peak confirms that Mn remains as Mn(III) inside the silica. The catalase activity study of material [Mn2O]@SiO2 reveals that the complex is more active inside the porous silica, probably due to the surface silanolate groups of the pore wall. Moreover, the new material shows catalase activity in water media, while the coordination compounds are not active.

Crystallization and Melting Behavior of Erythritol In and Around Two-Dimensional Hexagonal Mesoporous Silica

The crystallization and melting behavior of erythritol in and around two-dimensional hexagonal mesoporous silica SBA-15 with a pore diameter range of 7.5–9.2 nm were investigated. Erythritol inside the mesopores did not crystallize but became amorphous when the temperature decreased from 423 K, a value above the fusion point in the bulk, at 1 K min–1. Subsequently, as the temperature increased at 1 K min–1, the amorphous cold-crystallized and then fused around 320–360 K. If the temperature decreased from around the fusion point in the mesopores at the same cooling rate, erythritol inside the mesopores become a crystal. The XRD pattern of the erythritol inside the mesopores exhibited the crystal structure at 353 K, suggesting that the crystal grew around the nuclei of crystal. The heterogeneity of mesopores induces partial crystallization or partial melting, resulting in smooth phase transition. However, because the relaxation time of dynamic properties of erythritol confined in mesopores becomes longer du...

Effect of withdrawal speed on film thickness and hexagonal pore-array dimensions of SBA-15 mesoporous silica thin film.

Two-dimensional hexagonal mesoporous silica thin films of SBA-15 were synthesized on Si substrates via dip-coating using an evaporation-induced self-assembly process. The effect of the withdrawal speed on the thicknesses, one-dimensional pore alignments, and two-dimensional hexagonal pore arrays of the films was elucidated. Detailed analyses of FE-SEM and TEM images and XRD and XRR patterns of the synthesized thin films clarified that the pore sizes, interplanar spacings, and film thicknesses depend on the withdrawal speed. Furthermore, the same films were synthesized on Si substrates with microtrenches. The local flow of coating solutions around microtrenches affects the pore direction as well as the film thickness. In order to form well-ordered mesoporous silica thin films with large surface areas, it is important to control the synthetic conditions such as the local flow of the coating solutions as well as the physicochemical properties of the silica precursor solutions or template molecules.

Adsorption–Desorption and Transport of Water in Two-Dimensional Hexagonal Mesoporous Silica

The adsorption–desorption isotherms and relaxation rates of water in Zr-doped two-dimensional hexagonal mesoporous silica with pore diameters of 3.8 nm were measured at 283–298 K using a gravimetric method. A type V adsorption–desorption isotherm with significant hysteresis was obtained. The adsorption layer was formed on the pore surface at low relative pressure of water vapor, capillary condensation was initiated at around 0.4–0.6 relative humidity, and then pore filling was completed near the saturated vapor pressure. The relaxation rate of water uptake in the capillary condensation state was much lower than that in the layer-adsorption state or pore-filling state. The isotherms and relaxation rates during desorption had a similar trend. Effects of the hydrophilicity of mesopores and temperature on the adsorption–desorption kinetics suggest that the water transport mechanism in mesopores includes activated processes. In the capillary condensation and evaporation states, the relaxation curve could be estimated by the Fickian diffusion equation at a small stepwise change in relative humidity. The relaxation curves at large stepwise change in relative humidity suggest that the transport mechanism could be liquid water flow due to capillary action.

Water Adsorption–Desorption Behavior of Two-Dimensional Hexagonal Mesoporous Silica around Freezing Point

Water Adsorption–Desorption Behavior of Two-Dimensional Hexagonal Mesoporous Silica around Freezing Point

G132 メソポーラスシリカSBA-15薄膜の細孔構造制御(OS-7:マイクロ・ナノ熱工学(3))

In this study, two-dimensional hexagonal mesoporous silica thin films of SBA-15 were synthesized via dip-coating using an evaporation-induced self-assembly process. The effect of the withdrawal speed on the thickness of the films, one-dimensional pore alignments and two-dimensional hexagonal pore arrays was elucidated. The film thickness increases with increasing withdrawal speed and also increases upon decreasing the withdrawal speed to ultraslow region where capillary rise of the solution occurs. Detailed analyses of FE-SEM and TEM images and XRD patterns of the synthesized thin films clarified that the pore sizes and interplanar spacing also depend on the withdrawal speed; this suggests that the film thickness and hexagonal pore-array dimensions can be controlled without changing the physicochemical properties of the silica precursor solutions or the sizes of the template molecules.

Water adsorption–desorption isotherms of two-dimensional hexagonal mesoporous silica around freezing point

Zr-doped mesoporous silica with a diameter of approximately 3.8nm was synthesized via an evaporation-induced self-assembly process, and the adsorption–desorption isotherms of water vapor were measured in the temperature range of 263–298K. The measured adsorption–desorption isotherms below 273K indicated that water confined in the mesopores did not freeze at any relative pressure. All isotherms had a steep curve, resulting from capillary condensation/evaporation, and a pronounced hysteresis. The hysteresis loop, which is associated with a delayed adsorption process, increased with a decrease in temperature. Furthermore, the curvature radius where capillary evaporation/condensation occurs was evaluated by the combined Kelvin and Gibbs–Tolman–Koening–Buff (GTKB) equations for the modification of the interfacial tension due to the interfacial curvature. The thickness of the water adsorption layer for capillary condensation was slightly larger, whereas that for capillary evaporation was slightly smaller than 0.7nm.

α-Amylase immobilization capacities of mesoporous silicas with different morphologies and surface properties

α-Amylase was encapsulated in several mesoporous materials (folded sheet mesoporous silica (FSM), cubic mesoporous silica (KIT-6), and two-dimensional hexagonal mesoporous silica (SBA-15)) that differed morphologically in terms of particle shape, pore size, and pore structure. The encapsulation capacity and thermal stability of encapsulated α-amylase were examined. The amount of α-amylase encapsulated increased with increasing pore size in the following order: SBA-15 < KIT-6 < FSM. Nitrogen adsorption experiments were performed before and after α-amylase encapsulation in mesoporous silicas with pore sizes larger than the size of α-amylase, confirming that α-amylase was encapsulated in the pores. Among mesoporous silicas with similar pore sizes, FSM was found to have the highest capacity for α-amylase encapsulation both per weight and per surface area of silica. Furthermore, α-amylase encapsulated in FSM demonstrated high thermal stability at 90 °C relative to the thermal stability of free α-amylase or free α-amylase encapsulated in other mesoporous silicas. Zeta potential measurements showed that the FSM surface had an isoelectric point that was lower than that of other mesoporous silicas, and hydrophilicity measurements showed that its surface was more hydrophilic. The surface properties of FSM contributed to the high thermal stability of the α-amylase encapsulated within the pores.

Two-Dimensional Hexagonal Mesoporous Silica Thin Films Prepared from Block Copolymers: Detailed Characterization and Formation Mechanism

Silica thin films with a two-dimensional (2D) hexagonal structure were produced from TEOS and the F127 triblock copolymer PEO106PPO70PEO106. The structure of the films was deduced from transmission electron microscopy (TEM) performed on microtomed sections and from a 2D X-ray scattering technique adapted for film characterization. Both methods are complementary and allow for a detailed characterization of the film structure. The pore channels within the coatings are organized in domains, which spin and curve inside the film, but align with the surface plan at the air/film and film/substrate interfaces. The formation of the film was monitored by in situ time-resolved SAXS experiments to follow the structural evolution during the first minutes after deposition. Cylindrical micelles, which gives rise to a wormlike structure, form first at the air/film interface and extend toward the film/substrate interface via solvent evaporation. When most of the liquid phase is evaporated, the well-aligned domains located at both interfaces are obtained by a self-arrangement of the micelles parallel to the interfaces. This rearrangement occurs when the film is considered to be dry. The atom content profiles in the as-prepared films were obtained by Rutherford backscattering (RBS) and show homogeneous distribution of atoms throughout the film thickness. To stiffen the network and remove the surfactant, various postsynthesis treatments were applied to the coatings, that is, thermal treatment, base-catalyzed condensation, and solvent extraction. Each method induces shrinkage, leading to 2D-centered rectangular films through a contraction of the initial 2D hexagonal structure. As-prepared and treated films exhibit excellent optical transparency and high degree of organization. Ellipsometry measurements and N2 adsorption/desorption isotherms were thus used to estimate the refractive index and the porosity of the films. Refractive index down to 1.2 was obtained in the film that was pretreated under a NH3 atmosphere before solvent extraction of the surfactant. This shows the efficiency of this treatment to minimize the network shrinkage. N2 adsorption−desorption measurement performed on films calcined at 350 °C gives a surface area of 800 m2 g-1 and a porosity of 63%.