•Low-temperature wet-chemical route to niobates for piezoceramics.•Hexagonal-platelike KN-hydrate particles were prepared at 60 °C.•Hexagonal-platelike KNN-hydrate particles were prepared at 40 °C.•Hexagonal-platelike KN perovskite particles were obtained.Platelike alkaline niobates, which are an important class of templates for growing lead-free textured piezoceramic materials, are usually synthesized at temperatures greater than 900 °C from a melting process. We previously developed a hydrothermal route to niobate hydrate at temperatures above 100 °C and combined it with heat treatment to yield platelike niobate perovskite. In this contribution, we present the first report on near-room-temperature wet-chemical preparation of platelike potassium niobate (KN) and potassium sodium niobate (KNN) hydrate particles. Hexagonal-platelike KN-hydrate particles that were 1.5–4.0 μm wide and 0.1–0.35 μm thick were prepared via low temperature synthesis at 60 °C over a period of 24 h in a 9 mol/L KOH solution. Similarly, KNN hydrate particles with a hexagonal-platelike shape were prepared at 40 °C over a period of 48 h in 6 mol/L [OH−]. Sodium dodecyl benzene sulfonate (SDBS) surfactant was added as a shape modulator. Calcining the KN hydrate particles at 500 °C for 2 h transformed the crystals to a stable perovskite phase while maintaining the platelike morphology. The low-temperature wet-chemical route to alkaline niobates is expected to lead to a low-cost scalable method for the mass production of platelike template particles in the field of high-performance lead-free piezoceramics.

•Eight different proteinogenic amino acids have been employed as mesopore generating agents to prepare mesoporous zeolite A.•Mesopore formation mechanism mediated by amino acids is different from that by surfactants.•Isoelectric point of amino acids influences zeolite morphology, crystallization kinetics, and impurity content.•Hydropathy index of amino acids inversely affects mesopore size, indicating pore shrinkage effect by hydrophobic moieties.Hydrothermal crystallization of LTA zeolite has been conducted in the presence of amino acid in order to introduce mesoporosity. Eight different types of proteinogenic amino acids have been employed. The influence of their structure and physicochemical properties on the formation of hierarchically mesoporous LTA (MLTA) zeolite have been investigated. All eight amino acids bearing either basic, or acidic, or polar non-ionizable, or non-polar non-ionizable side chains were able to generate mesopores within LTA crystals. The isoelectric points of amino acids influenced the resulting MLTA's morphology, crystallization kinetics, and impurity content. Pure phase MLTA with controllable shape was obtained. Amino acid with a higher isoelectric point generated smaller and more spherical MLTA in less crystallization time, but induced slightly more impurity FAU phase. The mesopore size generally decreased with hydropathy index so that amino acids with non-polar, non-ionizable side groups generated 14–15 nm mesopores, 5–10 nm smaller than those generated by amino acids in other categories. The hydrophobic moieties in amino acids tended to shrink the mesopores, in contrast with their pore expansion effect in surfactant-mediated mesopore generation route, suggesting a different mesopore formation mechanism. Stability of amino acid during the hydrothermal zeolite synthesis and easy amino acid removal by washing with water was further confirmed. The findings discovered in this study paves a new way for tailoring hierarchical zeolite materials with controlled texture properties.Download high-res image (274KB)Download full-size image

Core–shell zeolite Y containing an ant-nest like hollow interior has been synthesized in the presence of natural amino acids. Time-resolved characterization of zeolite samples revealed the gradual development of three-dimensionally interconnected mesoporous channels via an intermediate stage of spatially aligned cavities in the core, which did not follow either a classical or a reversed crystal growth route. Comparative experiments suggested coordinative construction actions of amino acids, the interpenetration of which into the internal weak points of the aluminosilicate framework takes place simultaneously with zeolite crystallization. At the point of complete zeolite growth, amino acids in aggregated domains begin disassembling the zeolite interior forming a network of channels while maintaining the external zeolite shell thickness of 165 nm and morphologically similar to conventional faujasite. The core–shell zeolite Y samples exhibit superior crystallinity and enhanced catalytic activity for Friedel–Crafts benzylation of toluene.

Shape control in metal–organic frameworks still remains a challenge. We propose a strategy based on the capping agent modulator method to control the shape of ZIF-8 nanocrystals. This approach requires the use of a surfactant, cetyltrimethylammonium bromide (CTAB), and a second capping agent, tris(hydroxymethyl)aminomethane (TRIS), to obtain ZIF-8 nanocrystals with morphology control in aqueous media. Semiempirical computational simulations suggest that both shape-inducing agents adsorb onto different surface facets of ZIF-8, thereby slowing down their crystal growth rates. While CTAB molecules preferentially adsorb onto the {100} facets, leading to ZIF-8 particles with cubic morphology, TRIS preferentially stabilizes the {111} facets, inducing the formation of octahedral crystals. Interestingly, the presence of both capping agents leads to nanocrystals with irregular shapes and higher index facets, such as hexapods and burr puzzles. Additionally, the combination of ZIF-8 nanocrystals with other materials is expected to impart additional properties due to the hybrid nature of the resulting nanocomposites. In the present case, the presence of CTAB and TRIS molecules as capping agents facilitates the synthesis of metal nanoparticle@ZIF-8 nanocomposites, due to synergistic effects which could be of use in a number of applications such as catalysis, gas sensing and storage.

Mesoporous LTA zeolites have been hydrothermally synthesized by using amino acids as a mesoporogen. Amino acids of carnitine, lysine, or their salt derivatives were able to generate disordered mesopores of 10–20 nm within single crystalline LTA zeolites. Unlike other mesopore generating templates such as surfactants or polymers, the amino acid templates could be easily removed by washing with water, eliminating the energy-intensive calcination step. A possible crystallization process involving three-dimensional assembly of amino acid templates via hydrogen bonding and electrostatic interaction was proposed, as it was found that hydrogen bonding is crucial in hierarchical structure generation, and that hydroxyl group shielded acetyl carnitine yielded no mesopores. The obtained hierarchically mesoporous LTA zeolites exhibited a remarkably higher adsorption capacity (264 mg g−1) for catalase enzyme, and retained greater enzyme activity (>90%) than did conventional LTA.

Mesoporous LTA zeolites have been hydrothermally synthesized by using amino acids as a mesoporogen. Amino acids of carnitine, lysine, or their salt derivatives were able to generate disordered mesopores of 10–20 nm within single crystalline LTA zeolites. Unlike other mesopore generating templates such as surfactants or polymers, the amino acid templates could be easily removed by washing with water, eliminating the energy-intensive calcination step. A possible crystallization process involving three-dimensional assembly of amino acid templates via hydrogen bonding and electrostatic interaction was proposed, as it was found that hydrogen bonding is crucial in hierarchical structure generation, and that hydroxyl group shielded acetyl carnitine yielded no mesopores. The obtained hierarchically mesoporous LTA zeolites exhibited a remarkably higher adsorption capacity (264 mg g−1) for catalase enzyme, and retained greater enzyme activity (>90%) than did conventional LTA.