Scalable synthesis of mesoporous titania microspheres via spray-drying method

Aerosol-assisted synthesis of titania-based spherical and fibrous materials with a rational design of mesopores using PS-b-PEO

Surfactant-assisted synthesis is a promising technique for the tailor-made design of highly porous metal oxide based nanomaterials. There has been a demand for the comprehensive design of their morphology, porous structure and crystallinity to extend potential applications using metal oxide based materials such as titania (TiO₂). However, the porous structure is often deformed and/or destroyed during the process of crystallizing metal oxide frameworks. Herein, the aerosol-assisted synthesis of mesoporous TiO₂ powders was conducted in the presence of high-molecular-weight poly(styrene)-block-poly(ethylene oxide) (PS-b-PEO), which improved the stability of the derivative mesoporous structure with an increase in the thickness of the TiO₂ frameworks. To propose a rational synthetic route for stable and porous metal oxides, the resultant mesoporous structure and the textural morphology of the mesoporous TiO₂ powders were surveyed using PS-b-PEO with different lengths of PS and PEO chains. By a judicious choice of the molecular structure of PS-b-PEO, the morphological design of the fully crystallized anatase phase of TiO₂ from spherical to fibrous ones was achieved with control over the mesopore diameter.

Study on Continuous Hydrolysis in a Microchannel Reactor for the Production of Titanium Dioxide by a Sulfuric Acid Process

The development of a continuous hydrolysis process of titanium sulfate is an innovation to the traditional production process of titanium dioxide by the sulfuric acid process. In the experiment, a microchannel reactor was designed, and the hydrolysis rate of titanium sulfate, the particle size, and particle size distribution of metatitanic acid agglomerates were used as indicators to investigate the effect of operating conditions on the continuous hydrolysis of titanium sulfate. The results have shown that as the amount of dilution water increased, the hydrolysis rate of titanium sulfate decreased, and the particle size of primary aggregates of metatitanic acid increased from 39 to 54 nm. As the alkali mass concentration of dilution water increased, the hydrolysis rate of titanyl sulfate increased, and the particle size of primary aggregates of metastatic acid first decreased and then increased, and the particle size range was 40-48 nm. As the flow rate increased, the hydrolysis rate of titanyl sulfate increased, and the particle size of primary aggregates of metatitanic acid dropped from 59 to 43 nm. Compared with the batch hydrolysis operation, the continuous process has stronger anti-disturbance ability, significantly shorter operation time of the reaction section, and narrower particle size distribution of the product metatitanic acid.

Interfacial Assembly and Applications of Functional Mesoporous Materials

Functional mesoporous materials have gained tremendous attention due to their distinctive properties and potential applications. In recent decades, the self-assembly of micelles and framework precursors into mesostructures on the liquid-solid, liquid-liquid, and gas-liquid interface has been explored in the construction of functional mesoporous materials with diverse compositions, morphologies, mesostructures, and pore sizes. Compared with the one-phase solution synthetic approach, the introduction of a two-phase interface in the synthetic system changes self-assembly behaviors between micelles and framework species, leading to the possibility for the on-demand fabrication of unique mesoporous architectures. In addition, controlling the interfacial tension is critical to manipulate the self-assembly process for precise synthesis. In particular, recent breakthroughs based on the concept of the "monomicelles" assembly mechanism are very promising and interesting for the synthesis of functional mesoporous materials with the precise control. In this review, we highlight the synthetic strategies, principles, and interface engineering at the macroscale, microscale, and nanoscale for oriented interfacial assembly of functional mesoporous materials over the past 10 years. The potential applications in various fields, including adsorption, separation, sensors, catalysis, energy storage, solar cells, and biomedicine, are discussed. Finally, we also propose the remaining challenges, possible directions, and opportunities in this field for the future outlook.

Orderly Curled Silica Nanosheets with a Small Size and Macromolecular Loading Pores: Synthesis and Delivery of Macromolecules To Eradicate Drug-Resistant Cancer

Hierarchically organized silica nanomaterials have shown great promise for nanomedicine. However, the synthesis of silica nanomaterials with a small size and macromolecular loading pore is still a big challenge. Herein, orderly curled silica nanosheets (OCSNs) with a ∼42 nm diameter and orderly connected large channels (∼13.4 nm) were successfully prepared for the first time. The key to the formation of the unique structure (OCSNs) is using an oil/water reaction system with high concentrations of the surfactant and alkali. The prepared OCSNs exhibit a long blood circulation halftime (0.97 h) and low internalization in the reticuloendothelial system. Notably, the large superficial channels can concurrently house large guest molecules (siRNA) and chemotherapeutic drugs. Furthermore, drug-loaded OCSNs modified with polyglutamic acids can greatly increase the accumulation of incorporated siRNA and doxorubicin in solid tumors and restrain the growth of drug-resistant orthotopic breast cancer by inducing cell apoptosis. Overall, we report the preparation of hierarchically OCSNs; their small size and macromolecular loading pores are very promising for the delivery of large guest molecules and chemotherapeutic drugs for cancer therapy.

Scalable Synthesis of Uniform Mesoporous Aluminosilicate Microspheres with Controllable Size and Morphology and High Hydrothermal Stability for Efficient Acid Catalysis

Mesoporous aluminosilicates are promising solid acid catalysts. They are also excellent supports for transition metal catalysts for various catalytic applications. Synthesis of mesoporous aluminosilicates with controllable particle size, morphology, and structure, as well as adjustable acidity and high hydrothermal stability, is very desirable. In this work, we demonstrate the scalable synthesis of Al-SBA-15 microspheres with controllable physicochemical properties by using the microfluidic jet-spray-drying technology. The productivity is up to ∼30 g of dried particles per nozzle per hour. The Al-SBA-15 microspheres possess uniform controllable micron sizes (27.5-70.2 μm), variable surface morphologies, excellent hydrothermal stability (in pure steam at 800 °C), high surface areas (385-464 m²/g), ordered mesopore sizes (5.4-5.8 nm), and desirable acid properties. The dependence of various properties, including particle size, morphology, porosity, pore size, acidity, and hydrothermal stability, of the obtained Al-SBA-15 microspheres on experimental parameters including precursor composition (Si/Al ratio and solid content) and processing conditions (drying and calcination temperatures) is established. A unique morphology transition from smooth to wrinkled microsphere triggered by control of the Si/Al ratio and solid content is observed. The particle formation and morphology-evolution mechanism are discussed. The Al-SBA-15 microspheres exhibit high acid catalytic performance for aldol-condensation reaction between benzaldehyde and ethyl alcohol with a high benzaldehyde conversion (∼56.3%), a fast pseudo-first-order reaction rate (∼0.1344 h^-1), and a high cyclic stability, superior to the commercial zeolite acid (H-ZSM-5). Several influencing factors on the catalytic performance of the obtained Al-SBA-15 microspheres are also studied.

Recent advances in the synthesis of hierarchically mesoporous TiO2 materials for energy and environmental applications

Because of their low cost, natural abundance, environmental benignity, plentiful polymorphs, good chemical stability and excellent optical properties, TiO₂ materials are of great importance in the areas of physics, chemistry and material science. Much effort has been devoted to the synthesis of TiO₂ nanomaterials for various applications. Among them, mesoporous TiO₂ materials, especially with hierarchically porous structures, show great potential owing to their extraordinarily high surface areas, large pore volumes, tunable pore structures and morphologies, and nanoscale effects. This review aims to provide an overview of the synthesis and applications of hierarchically mesoporous TiO₂ materials. In the first section, the general synthetic strategies for hierarchically mesoporous TiO₂ materials are reviewed. After that, we summarize the architectures of hierarchically mesoporous TiO₂ materials, including nanofibers, nanosheets, microparticles, films, spheres, core-shell and multi-level structures. At the same time, the corresponding mechanisms and the key factors for the controllable synthesis are highlighted. Following this, the applications of hierarchically mesoporous TiO₂ materials in terms of energy storage and environmental protection, including photocatalytic degradation of pollutants, photocatalytic fuel generation, photoelectrochemical water splitting, catalyst support, lithium-ion batteries and sodium-ion batteries, are discussed. Finally, we outline the challenges and future directions of research and development in this area.

Spherical Mesoporous Materials from Single to Multilevel Architectures

Mesoporous materials with various structures have attracted considerable attention due to their distinctive properties such as large pore sizes, high surface areas, tunable pore structures, and controllable framework compositions. Among them, spherical mesoporous materials (SMMs) are of great interest owing to the unique spherical shape, which show the closed packing nature and lowest surface energy. The open mesopores and short channels of SMMs not only increase the density of high accessible active sites but also facilitate the mass diffusion with short length. These characteristics are particularly useful for applications in catalysis, adsorption, energy storage and conversion, biomedicine, and so on. In addition, the creation of a spherical shape is conformable to the law of natural selection because objects in nature tend to minimize energy, while the sphere is one of the most perfect matter structures. Therefore, the design and synthesis of SMMs are very important from both fundamental and technological viewpoints. Compared to the simple single-level, SMMs with more complex multilevel structures inevitably bring unusual mechanical, electrical, and optical properties, which are highly desired for practical applications. For example, the construction of core-shell structured SMMs has inspired great attention as they can combine multiple components into one functional unit, exhibiting ameliorated or new physicochemical properties, which cannot be obtained from the isolated one. The presence of a hollow cavity in the yolk-shell structure allows sufficient exposure of the core while maintaining the protective ability of the shell, which is conducive to retaining the distance-dependent properties of the core. Multishelled hollow structures consisting of two or more mesoporous shells are expected to show superior activities in various applications compared to their bulk counterparts because more active interfaces and unique compartmentation environments can be provided. Therefore, SMMs from single to multilevel structure represent a class of advanced nanostructured materials with unique structures and fascinating properties. In this Account, we highlight the progresses on the synthesis and applications of SMMs from single to multilevel architectures. The synthetic strategies have been summarized and categorized into (i) the modified Stöber method, (ii) the hydrothermal strategy, (iii) the biphase stratification approach, (iv) the nanoemulsion assembly method, (v) the evaporation induced aggregating assembly (EIAA) method, and (vi) the confined self-assembly strategy. Special emphasis is placed on the synthetic principles and underlying mechanisms for precise control of SMMs over the particle sizes, pore sizes, pore structures and functionalities as well as different levels of architectures. Moreover, the implementation performances in catalysis, drug delivery, and energy related fields have been highlighted. Finally, the opportunities and challenges for the future development of SMMs in terms of synthesis and applications are proposed.

Controllable synthesis of mesoporous carbon microspheres with renewable water glass as a template for lithium-sulfur batteries

Mesoporous carbon microspheres (MCMs) were prepared via a spray drying-assisted template method using resorcinol-formaldehyde as the carbon precursor and water glass as the template. The pore structure could be controlled by adjusting the hydrolysis time, hydrolysis temperature, concentration of the water glass and reactant ratio. Water glass could be recycled after use, making this strategy environmentally friendly and cost-effective. MCMs with three-dimensional interconnected networks, high surface area (852-1549 m² g^-1), large pore volume (1.7-2.1 cm³ g^-1) and controllable pore diameter (3.8-15.1 nm) were constructed and have good electrical conductivity and a large volume for sulfur loading. The S/MCM composites with abundant residual nanochannels could not only benefit for the diffusion of electrolyte but also improve the utilization of sulfur and buffer the volume expansion of sulfur. The MCMs with relatively small mesopores manifest a high reversible capacity and rate performance owing to the strong confinement effect of polysulfides. MCM-1 delivered an initial capacity of 888.7 mA h g^-1 under 0.5C with a capacity retention of 700.5 mA h g^-1 after 100 cycles. The good electrochemical performance confirms that mesoporous carbon microspheres can be an excellent host material for sulfur cathodes.

Chemical Crosslinking Assembly of ZSM-5 Nanozeolites into Uniform and Hierarchically Porous Microparticles for High-Performance Acid Catalysis

Hierarchically porous zeolites combining the advantages of desirable mass transport of nanozeolites and easy separation and handling of micro-zeolites are ideal candidates in catalytic applications. Facile routes for the assembly of zeolite microparticles with hierarchical porosity and high mechanical strength are much expected. Herein, based on a microfluidic jet spray drying technology, we report a facile and scalable chemical crosslinking assembly strategy for the synthesis of hierarchical zeolite microparticles by directly using the conventional as-synthesized nanozeolite suspension as a precursor. This route not only avoids the energy-intensive centrifugal separation process of nanozeolites but also significantly increases the uniformity and mechanical strength of the microparticles. The soluble aluminosilicate species act as a stabilizer to improve the droplet stability during the drying process and then as a "cross-linker" to chemically bind and interconnect zeolite nanoparticles to form robust bodies after drying and calcination. Zeolite microparticles with variable morphologies (spherical, bowl-like, and dimpled) and uniform and controllable sizes (from 70 to 108 μm) can be obtained by adjusting the experimental parameters. The particle formation mechanism is discussed based on the zeolite microparticles obtained from the purified nanozeolite suspension as a control. The zeolite microparticles possess emerged uniform mesopores (∼6 nm) and a well-maintained high surface area, large pore volume, high microporosity, and strong acidity of the original nanozeolites. As a result, they exhibit excellent acid catalytic performances in acetolysis of epichlorohydrin and catalytic cracking of low-density polyethylene, far better than those of the commercial ZSM-5.

Rational Design of Multifunctional Fe@γ-Fe2 O3 @H-TiO2 Nanocomposites with Enhanced Magnetic and Photoconversion Effects for Wide Applications: From Photocatalysis to Imaging-Guided Photothermal Cancer Therapy

Titanium dioxide (TiO₂ ) has been widely investigated and used in many areas due to its high refractive index and ultraviolet light absorption, but the lack of absorption in the visible-near infrared (Vis-NIR) region limits its application. Herein, multifunctional Fe@γ-Fe₂ O₃ @H-TiO₂ nanocomposites (NCs) with multilayer-structure are synthesized by one-step hydrogen reduction, which show remarkably improved magnetic and photoconversion effects as a promising generalists for photocatalysis, bioimaging, and photothermal therapy (PTT). Hydrogenation is used to turn white TiO₂ in to hydrogenated TiO₂ (H-TiO₂ ), thus improving the absorption in the Vis-NIR region. Based on the excellent solar-driven photocatalytic activities of the H-TiO₂ shell, the Fe@γ-Fe₂ O₃ magnetic core is introduced to make it convenient for separating and recovering the catalytic agents. More importantly, Fe@γ-Fe₂ O₃ @H-TiO₂ NCs show enhanced photothermal conversion efficiency due to more circuit loops for electron transitions between H-TiO₂ and γ-Fe₂ O₃ , and the electronic structures of Fe@γ-Fe₂ O₃ @H-TiO₂ NCs are calculated using the Vienna ab initio simulation package based on the density functional theory to account for the results. The reported core-shell NCs can serve as an NIR-responsive photothermal agent for magnetic-targeted photothermal therapy and as a multimodal imaging probe for cancer including infrared photothermal imaging, magnetic resonance imaging, and photoacoustic imaging.

Aerosol processing: a wind of innovation in the field of advanced heterogeneous catalysts

Aerosol processing technologies represent a major route of innovation in the mushrooming field of heterogeneous catalysts preparation.

Synthesis of Uniquely Structured Yolk-Shell Metal Oxide Microspheres Filled with Nitrogen-Doped Graphitic Carbon with Excellent Li-Ion Storage Performance

Novel structured composite microspheres of metal oxide and nitrogen-doped graphitic carbon (NGC) have been developed as efficient anode materials for lithium-ion batteries. A new strategy is first applied to a one-pot preparation of composite (FeO_x -NGC/Y) microspheres via spray pyrolysis. The FeO_x -NGC/Y composite microspheres have a yolk-shell structure based on the iron oxide material. The void space of the yolk-shell microsphere is filled with NGC. Dicyandiamide additive plays a key role in the formation of the FeO_x -NGC/Y composite microspheres by inducing Ostwald ripening to form a yolk-shell structure based on the iron oxide material. The FeO_x -NGC/Y composite microspheres with the mixed crystal structure of rock salt FeO and spinel Fe₃ O₄ phases show highly superior lithium-ion storage performances compared to the dense-structured FeO_x microspheres with and without carbon material. The discharge capacities of the FeO_x -NGC/Y microspheres for the 1st and 1000th cycle at 1 A g^-1 are 1423 and 1071 mAh g^-1 , respectively. The microspheres have a reversible discharge capacity of 598 mAh g^-1 at an extremely high current density of 10 A g^-1 . Furthermore, the strategy described in this study is generally applied to multicomponent metal oxide-carbon composite microspheres with yolk-shell structures based on metal oxide materials.

Ti3+ Self-Doped Blue TiO2(B) Single-Crystalline Nanorods for Efficient Solar-Driven Photocatalytic Performance

Ti³⁺ self-doped blue TiO₂(B) single-crystalline nanorods (b-TR) are fabricated via a simple sol-gelation method, cooperated with hydro-thermal treatment and subsequent in situ treatment method, and afterward annealed at 350 °C in Ar. The structures are characterized by X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (UV-vis), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The prepared b-TR with narrow band gap possesses single-crystalline TiO₂(B) phase, Ti³⁺ self-doping, and one-dimensional (1D) rodlike nanostructure. In addition, the improved photocatalytic performance is studied by decomposition of Rhodamine B (RhB) and hydrogen evolution. The degradation rate of RhB by Ti³⁺ self-doped blue TiO₂(B) single-crystalline nanorods is ∼6.9- and 2.1-times higher compared with the rates of titanium dioxide nanoparticles and pristine TiO₂(B) nanorods under visible light illumination, respectively. The hydrogen evolution rate of b-TR is 26.6 times higher compared with that of titanium dioxide nanoparticles under AM 1.5 irradiation. The enhanced photocatalytic performances arise from the synergetic action of the special TiO₂(B) phase, Ti³⁺ self-doping, and the 1D rod-shaped single-crystalline nanostructure, favoring the visible light utilization and the separation and transportation of photogenerated charge carriers.