Hollow Alveolus-Like Nanovesicle Assembly with Metal-Encapsulated Hollow Zeolite Nanocrystals

Hierarchical zeolite-encapsulated metal nanoparticles for heterogeneous catalysis

Zeolites, characterized by their highly porous structure, have become integral to modern industry and environmental science due to their broad applications in adsorption, separation, and catalysis. Recent advancements in zeolite synthesis, particularly through hydrothermal methods and the incorporation of metal nanoparticles, have significantly expanded their utility. This review delves into the innovative strategies for encapsulating metal nanoparticles within zeolite matrices, enhancing catalytic reactions' efficiency, selectivity, and durability. Challenges such as nanoparticle agglomeration and catalyst deactivation are addressed through hierarchical zeolite encapsulation, which provides a novel route for the development of multifunctional materials. By examining methods ranging from in situ encapsulation to post-synthetic recrystallization, this review highlights the versatility and potential of metal@zeolite catalysts in various applications, including organic synthesis, pollutant treatment, and energy conversion. The review underscores the importance of optimizing the interaction between metal nanoparticles and the zeolite framework to achieve superior catalytic performance, offering new directions for research in catalytic science and industrial process optimization.

Preparation of Metal-Supported Nanostructured Zeolite Catalysts and their Applications in the Upgrading of Biomass-Derived Furans: Advances and Prospects

The development of platform chemicals derived from biomass, in particular, 5-hydroxymethylfurfural (5-HMF) and furfural (FUR), is of crucial importance in biorefinery. Over the past decades, metal-supported nanostructured zeolites, in particular, metal-supported hierarchically porous zeolites or metal-encapsulated zeolites, have been extensively elaborated because of their multiple functionalities and superior properties, for example, shape-selectivity, (hydro)thermal stability, tunable acidity and basicity, redox properties, improved diffusion, and intimacy of multiple active sites. In this review, the effects of such properties of metal-supported nanostructured zeolites on the enhanced catalytic performances in furanic compound upgrading are discussed. In addition, the recent rational design of metal-supported nanostructured zeolites is exemplified. Consequently, the ongoing challenges for further developing metal-supported nanostructured zeolites-based catalysts and their applications in HMF and FUR upgrading are identified.

Review and perspectives on TS-1 catalyzed propylene epoxidation

Titanium silicate zeolite (TS-1) is widely used in the research on selective oxidations of organic substrates by H2O2. Compared with the chlorohydrin process and the hydroperoxidation process, the TS-1 catalyzed hydroperoxide epoxidation of propylene oxide (HPPO) has advantages in terms of by-products and environmental friendliness. This article reviews the latest progress in propylene epoxidation catalyzed by TS-1, including the HPPO process and gas phase epoxidation. The preparation and modification of TS-1 for green and sustainable production are summarized, including the use of low-cost feedstocks, the development of synthetic routes, strategies to enhance mass transfer in TS-1 crystal and the enhancement of catalytic performance after modification. In particular, this article summarizes the catalytic mechanisms and advanced characterization techniques for propylene epoxidation in recent years. Finally, the present situation, development prospect and challenge of propylene epoxidation catalyzed by TS-1 were prospected.

Amino-Acid-Assisted Synthesis of Hollow Hierarchical FER Zeolite with Improved Catalytic Performance

Hierarchical zeolites are highly-desired catalysts in the petrochemical industry due to their shorter diffusion length, faster diffusion rate, and better accessibility to active acid sites compared with conventional zeolites. Herein, we report a simple amino-acid-assisted method to synthesize urchin-like hollow hierarchical FER zeolites with abundant mesopores and macroporous inner cavities. An amino acid (i. e. L-lysine) is used to facilitate the agglomeration of primary gel nanoparticles. The preferential nucleation and crystal growth at the external surfaces together with the lagged crystallization of the inner core of the agglomerates results in the formation of hollow inner cavities after the exhaustion of interior materials. Thanks to the unique hierarchical structure and more accessible acid sites, the hollow hierarchical FER zeolite exhibits improved catalytic performance over the conventional one in the skeletal isomerization of 1-butene to isobutene.

Ternary alloy (FeCoNi) nanoparticles supported on hollow porous carbon with defects for enhanced oxygen evolution reaction

Low-cost non-noble metal nanoparticles are promising electrocatalysts that can catalyze oxygen evolution reaction (OER). Various factors such as poor activity and stability hinder the practical applications of these materials. The electroactivity and durability of the electrocatalysts can be improved by optimizing the morphology and composition of the materials. Herein, we report the successful synthesis of hollow porous carbon (HPC) catalysts loaded with ternary alloy (FeCoNi) nanoparticles (HPC-FeCoNi) for efficient OER. HPC is firstly synthesized by a facile carbon deposition method using the hierarchical porous zeolite ZSM-5 as the hard template. Numerous defects are generated on the carbon shell during the removal of zeolite template. Subsequently, FeCoNi alloy nanoparticles are supported on HPC by a sequence of impregnation and H₂ reduction processes. The synergistic effect between carbon defects and FeCoNi alloy nanoparticles endows the catalyst with an excellent OER performance (low overpotential of 219 mV; Tafel slope of 60.1 mV dec^-1) in a solution of KOH (1 M). A stable potential is maintained during the continuous operation over 72 h. The designed HPC-FeCoNi presents a platform for the development of electrocatalysts that can be potentially applied for industrial OER.

Defect-Enriched Hollow Porous Carbon Nanocages Enable Highly Efficient Chlorine Evolution Reaction

Metal-free carbon-based catalysts are crucial for the electrocatalytic chlorine evolution reaction (CER) to reduce the usage of noble metals and industrial cost. However, the corresponding catalytic activity of high overpotential and low durability hinders their wide application. Here, a hollow porous carbon (HPC) nanocage with a controlled oxygen electronic state around designed carbon defects for CER activity is reported. Alkali etching can bring defects in zeolite with a hollow structure. In a hard template strategy, the type of carbon defects is directly related to etching degree of the zeolite template. More importantly, the oxygen atoms can be "borrowed" from the zeolite framework by the defective carbon. The electron density around unsaturated O atoms can be decreased on the minor defects in carbon compared with that on large defects which is favorable for the adsorption of Cl^- . Consequently, the as-synthesized HPC nanocages with minor defects show excellent electrocatalytic performance for CER with a low overpotential of 94 mV at current density of 10 mA cm^-2 with good stability, which is superior to the commercial precious metal catalyst of dimensionally stable anode (DSA), and the best in the reported carbon materials. The designed carbon materials provide an option for metal-free industrial catalysts with significant CER activities.

Structure engineering of alveoli-like ZSM-5 with encapsulated Pt nanoparticles for the enhanced benzene oxidation

Inspired by the alveolar configuration, an alveoli-like ZSM-5 and the corresponding platinum encapsulated nanocomposite (Pt@PZ5) were fabricated via a dual-template method and a controlled selective desilication-recrystallization strategy. The dimensions of the central cavity, interconnected zeolitic vesicles, and mesoporous shell could be tuned by adjusting the synthesis parameters, as verified by scanning electron microscopy, transmission electron microscopy, nitrogen physisorption investigations, X-ray photoelectron spectroscopy, and X-ray diffraction techniques. Thanks to these properties and merits, the alveoli-like Pt@PZ5 showed the highest catalytic performance with excellent stability, obtaining 100% benzene conversion at 180 °C. Adsorption experiments combined with a finite-element simulation study uncovered that the alveolar architecture could expedite the accumulation of reactants and boost mass transfer; the conversion of intermediates in the voids could be further facilitated, giving optimal catalytic performance. Additionally, the alveolar architecture is resistant to metal sintering (5-20 nm) and leaching, even after calcination at 850 °C for 360 min. This work provides an alveolar concept into the rational design of efficient catalysts for fundamental catalytic action.

In situ synthesis of Pt nanoparticles encapsulated in Silicalite-1 zeolite via a steam-assisted dry-gel conversion method

In this work, Pt nanoparticles (NPs) were directly encapsulated into MFI-type zeolite (Pt@S-1) via a steam-assisted dry-gel conversion method. The synthesis process included the disaggregation of Pt immobilized SiO2-SH spheres...

Assembly of Silicalite-1 Crystals Like Toy Lego Bricks into One-, Two-, and Three-Dimensional Architectures for Enhancing Its Adsorptive Separation and Catalytic Performances

Many researchers have contributed to the assembly of zeolitic nanosheets and nanocrystallites into three-dimensional (3D) networks as it can remarkably improve the catalytic and/or adsorptive performances of zeolites. However, the applications of these synthesized materials are seriously limited because of low hydrothermal stability. A highly interesting strategy, but a great challenge, is the alignment of well-crystallized zeolite crystals into desirable architectures. Here, well-crystallized silicalite-1 crystals are assembled like toy Lego bricks into one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) architectures, and the assembly mechanism is investigated by combining elaborate experiments, in situ spectroscopy, and theoretical calculations. A 1D architecture was formed by stacking crystals along the b axis with the assistance of ethanol that is selectively adsorbed on (100) and (001) crystal facets. Such adsorption increases the condensation energy barriers along a and c axes, but facilitates the condensation between (010) facets. The assembly of the crystals into well-arrayed 2D architectures is achieved using both ethanol and benzaldehyde because of their preferable adsorption on the (001) facet. When an amphiphilic copolymer (P123) was further added in the gel along with the substitution of ethanol by 1-propanol, a 3D network was fabricated by the agglomeration and self-pillaring of the 2D Lego bricks possibly with P123 aggregates as the substrate matrix. Excitingly, upon alignment of crystals into 2D architectures, the adsorptive selectivity of 1-butanol (2 wt %) to water of silicalite-1 increases by 45.3 times, while into 3D networks, the catalytic activity for the Beckmann rearrangement of cyclohexanone oxime elevates by 79% along with a great enhancement of catalytic stability.

Metal-Organic Framework-Based Hierarchically Porous Materials: Synthesis and Applications

Metal-organic frameworks (MOFs) have been widely recognized as one of the most fascinating classes of materials from science and engineering perspectives, benefiting from their high porosity and well-defined and tailored structures and components at the atomic level. Although their intrinsic micropores endow size-selective capability and high surface area, etc., the narrow pores limit their applications toward diffusion-control and large-size species involved processes. In recent years, the construction of hierarchically porous MOFs (HP-MOFs), MOF-based hierarchically porous composites, and MOF-based hierarchically porous derivatives has captured widespread interest to extend the applications of conventional MOF-based materials. In this Review, the recent advances in the design, synthesis, and functional applications of MOF-based hierarchically porous materials are summarized. Their structural characters toward various applications, including catalysis, gas storage and separation, air filtration, sewage treatment, sensing and energy storage, have been demonstrated with typical reports. The comparison of HP-MOFs with traditional porous materials (e.g., zeolite, porous silica, carbons, metal oxides, and polymers), subsisting challenges, as well as future directions in this research field, are also indicated.

Versatile Hollow ZSM-5 Nanoreactors Loaded with Tailorable Metal Catalysts for Selective Hydrogenation Reactions

Zeolites are one of the most commonly used materials in the chemical industry, acting as catalysts or catalyst supports in different applications. Recently, the synthesis and functionalization of hollow zeolites have attracted many research interests, owing to the unique advantages of their hollow morphology. In the development of more sustainable processes, the hollow zeolites are often endowed with additional stability, selectivity, and activity. Herein, we present a stepwise synthetic protocol to prepare a range of complex hollow ZSM-5 catalysts with catalytic nanoparticles. Solid ZSM-5 crystals were first synthesized from Stöber silica spheres. This solid ZSM-5 sample was then loaded with transition metals via the impregnation method. A subsequent hollowing process was carried out in hydrothermal conditions in which hollow ZSM-5 crystals with confined transition metals inside were synthesized. More specifically, after the encapsulation of transition metals inside hollow ZSM-5, two different approaches have been further devised to allow the deposition of noble metals into the interior cavities or onto the exterior surfaces of the hollow ZSM-5. The deposition of Pt on the exterior surface was carried out by mixing the hollow ZSM-5 sample with presynthesized Pt nanoparticles. Loading of Pd in the interior was achieved by the galvanic replacement reaction between the Pd ions and embedded transition metals inside the hollow ZSM-5 sample. The catalytic performance of these reactor-like nanocatalysts has been evaluated with hydrogenation reactions in both liquid and gas phases, and their compositional and structural merits have been illustrated. For the hollow ZSM-5 sample with Pd loaded inside, liquid-phase selective hydrogenation of styrene over 4-vinylbiphenyl has been achieved with the ZSM-5 shell acting as a molecular sieve. The deposition of Pt on the exterior has improved the C₂-C₄ product yield when tested for the gas-phase CO₂ hydrogenation reaction.

Bioinspired Assembly of Double Honeycomb-Like Hierarchical Capsule Confined Encapsulation with Functional Micro/Nanocrystals

Inspired by "micro/nanoreactor" effect of cellular organelle on specific biochemical reactions, a double honeycomb-like hierarchical capsule confined encapsulation with functional micro/nanocrystals is designed. The bioinspired hierarchical capsules derived from polymeric composite microspheres are successfully fabricated through a combination of selective chemical etching and pyrolysis. In situ introduction of functional guests (including organometallic molecules, tetraethoxysilane, or metal-organic frameworks (MOFs)) into internal cellular structure of microspheres is first put forward by phase inversion method. The development of selective etching creates honeycomb-like structure on the outside surface of capsule and allows sulfur to homogeneously distribute into matrix. With the novel approach, the hierarchical channels (micro-meso-macropore) of composite capsule enhance transportation of reactants and dispersion of active sites, and thus exhibit superior photocatalytic oxidation and electromagnetic absorbing. The promising strategy will be applied more generally to encapsulate different species into hierarchical capsule with tailored properties and functionalities.

Endowing Zeolite LTA Superballs with the Ability to Manipulate Light in Multiple Ways

Advances in zeolites research emerging from interdisciplinary efforts have opened new opportunities beyond conventional applications. Colloids drive much current research owing to their distinct collective behaviors, but so far, using zeolites as a colloidal building block to construct ordered superstructures remains unexplored. Herein we show that self-assembly of colloidal zeolite LTA superball (ZAS) by tilted-angle sedimentation forms macroscopic films with micro-mesoporosity and 3D long-range periodicity featuring a photonic band gap (PBG) that is tunable through the superball geometry and responds reversibly to chemical vapors. Remarkably, self-assembly of ZAS at elevated temperature forms 3D chiral photonic crystals that enable negative circular dichroism, selective reflection of right-handed circularly polarized (CP) light and left-handed CP luminescence based on PBG. We present a novel class of functional colloids and zeolite-based photonic crystals with the ability to manipulate light in several ways.

Recent advances in zeolite-encapsulated metal catalysts: A suitable catalyst design for catalytic biomass conversion

Metal clusters and nanoparticles, which have been used to tune the acidity of zeolite support, are beneficial for promoting the catalytic performance of various reaction processes, including biomass conversion. However, catalytic instabilities resulting from metal coalescence, sintering and leaching are major problems that need to be resolved. Therefore, metal encapsulation within the zeolite structure has been proposed as a feasible solution for this issue, particularly for biomass conversions that require high temperatures. In this current review, recent developments in metal confinement techniques are described along with experimental examples of biomass upgrading reactions. The present and future perspectives of zeolite-encapsulated metal catalysts in biomass conversions are also given.

Encapsulating soluble active species into hollow crystalline porous capsules beyond integration of homogeneous and heterogeneous catalysis

Homogeneous molecular catalysts and heterogeneous catalysts possess complementary strengths, and are of great importance in laboratory/commercial procedures. While various porous hosts, such as polymers, carbons, silica, metal oxides and zeolites, have been used in an attempt to heterogenize homogeneous catalysts, realizing the integration of both functions at the expense of discounting their respective advantages, it remains a significant challenge to truly combine their intrinsic strengths in a single catalyst without compromise. Here, we describe a general template-assisted approach to incorporating soluble molecular catalysts into the hollow porous capsule, which prevents their leaching due to the absence of large intergranular space. In the resultant yolk (soluble)-shell (crystalline) capsules, the soluble yolks can perform their intrinsic activity in a mimetic homogeneous environment, and the crystalline porous shells endow the former with selective permeability, substrate enrichment, size-selective and heterogeneous cascade catalysis, beyond the integration of the respective advantages of homogeneous and heterogeneous catalysts.

Core–shell structured catalysts for thermocatalytic, photocatalytic, and electrocatalytic conversion of CO2

An in-depth assessment of properties of core–shell catalysts and their application in the thermocatalytic, photocatalytic, and electrocatalytic conversion of CO₂into synthesis gas and valuable hydrocarbons.

Enhanced porosity of Ni@HSZ for dry reforming of methane

A novel Ni@hollow silicate zirconia (Ni@HSZ) is prepared via a hydrothermal approach to re-use the SiO₂ in Ni@SiO₂@ZrO₂ with TBAOH as an etchant and template.

Functionalization of Hollow Nanomaterials for Catalytic Applications: Nanoreactor Construction

Hollow nanomaterials have attracted a broad interest in multidisciplinary research due to their unique structure and preeminent properties. Owing to the high specific surface area, well-defined active site, delimited void space, and tunable mass transfer rate, hollow nanostructures can serve as excellent catalysts, supports, and reactors for a variety of catalytic applications, including photocatalysis, electrocatalysis, heterogeneous catalysis, homogeneous catalysis, etc. Based on state-of-the-art synthetic methods and characterization techniques, researchers focus on the purposeful functionalization of hollow nanomaterials for catalytic mechanism studies and intricate catalytic reactions. Herein, an overview of current reports with respect to the catalysis of functionalized hollow nanomaterials is given, and they are classified into five types of versatile strategies with a top-down perspective, including textual and composition modification, encapsulation, multishelled construction, anchored single atomic site, and surface molecular engineering. In the detailed case studies, the design and construction of hierarchical hollow catalysts are discussed. Moreover, since hollow structure offers more than two types of spatial-delimited sites, complicated catalytic reactions are elaborated. In summary, functionalized hollow nanomaterials provide an ideal model for the rational design and development of efficient catalysts.

Highly dispersed nickel catalysts via a facile pyrolysis generated protective carbon layer

Highly dispersed nickel catalysts were synthesized by simple Ni(acac)2 impregnation followed by pyrolysis of organic ligands, which produces a protective carbon coating on the Ni nanoparticles, reducing the metal mobility and sintering. The synthesized catalyst shows high thermal stability and enhanced CO methanation activity compared to the catalyst prepared by the traditional impregnation/calcination method.

Strategies to control zeolite particle morphology

Methods to synthesize zeolites with different crystal habits and assemble zeolite crystals into specific structures are reviewed for the rational design of zeolite particle morphologies.

Ultrasmall Metal Nanoparticles Confined within Crystalline Nanoporous Materials: A Fascinating Class of Nanocatalysts

Crystalline nanoporous materials with uniform porous structures, such as zeolites and metal-organic frameworks (MOFs), have proven to be ideal supports to encapsulate ultrasmall metal nanoparticles (MNPs) inside their void nanospaces to generate high-efficiency nanocatalysts. The nanopore-encaged metal catalysts exhibit superior catalytic performance as well as high stability and catalytic shape selectivity endowed by the nanoporous matrix. In addition, the synergistic effect of confined MNPs and nanoporous frameworks with active sites can further promote the catalytic activities of the composite catalysts. Herein, recent progress in nanopore-encaged metal nanocatalysts is reviewed, with a special focus on advances in synthetic strategies for ultrasmall MNPs (<5 nm), clusters, and even single atoms confined within zeolites and MOFs for various heterogeneous catalytic reactions. In addition, some advanced characterization methods to elucidate the atomic-scale structures of the nanocatalysts are presented, and the current limitations of and future opportunities for these fantastic nanocatalysts are also highlighted and discussed. The aim is to provide some guidance for the rational synthesis of nanopore-encaged metal catalysts and to inspire their further applications to meet the emerging demands in catalytic fields.

Encapsulation of Metal Nanoparticle Catalysts Within Mesoporous Zeolites and Their Enhanced Catalytic Performances: A Review

Metal nanoparticles (NPs) exhibit desired activities in various catalytic reactions. However, the aggregation and sintering of metal NPs usually cause the loss of catalytic performance in practical reaction processes. Encapsulation of catalytically active metal NPs on/within a high-surface-area inorganic support partially resolve such concerns. Microporous zeolites, owing to their rigid frameworks and porous structural features, have been considered as one of ideal inorganic supports. Metal NPs can be easily encapsulated and stabilized within zeolitic frameworks to prevent unwished aggregation during the catalysis. Unfortunately, sole microporous nanochannels (generally <1 nm) in conventional zeolites are not easy to be accessed. The introduction of another set of nanochannel (e.g., mesopore), known as mesoporous zeolites, can greatly improve the mass-transfer efficiency, which is structurally beneficial for most catalytic reactions. The coexistence of micropores and mesopores in inorganic supports provides the synergetic advantages of both fine confinement effect for metal NPs and easy diffusion for organic reactants/intermediates/products. This review focuses on the recent advances in the design and synthesis of mesoporous zeolites-encapsulated metal NP catalysts as well as their desired catalytic performances (activity and stability) in organic reactions. We first discuss the advantages of mesoporous zeolites as the supports and present general strategies for the construction of mesoporous zeolites. Then, the preparation methods on how to encapsulate NP catalysts within both microporous and mesoporous zeolites are clearly demonstrated. Third, some recent important cases on catalytic applications are presented to verify structural advantages of mesoporous zeolite supports. Within the conclusion, the perspectives on future developments in metal NP catalysts encapsulated within mesoporous zeolites are lastly discussed.

High-Density Ultra-small Clusters and Single-Atom Fe Sites Embedded in Graphitic Carbon Nitride (g-C3N4) for Highly Efficient Catalytic Advanced Oxidation Processes

Ultra-small metal clusters have attracted great attention owing to their superior catalytic performance and extensive application in heterogeneous catalysis. However, the synthesis of high-density metal clusters is very challenging due to their facile aggregation. Herein, one-step pyrolysis was used to synthesize ultra-small clusters and single-atom Fe sites embedded in graphitic carbon nitride with high density (iron loading up to 18.2 wt %), evidenced by high-angle annular dark field-scanning transmission electron microscopy, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and ⁵⁷Fe Mössbauer spectroscopy. The catalysts exhibit enhanced activity and stability in degrading various organic samples in advanced oxidation processes. The drastically increased metal site density and stability provide useful insights into the design and synthesis of cluster catalysts for practical application in catalytic oxidation reactions.

A Topotactic Synthetic Methodology for the Synthesis of Nanosized MFI Zeolites with Hierarchical Structures

Much effort has been invested in the designed synthesis of zeolites with nanosized and hierarchical structures in recent decades, on account of increasing demands in practical applications, especially catalysis. Herein, a new topotactic synthetic strategy is demonstrated to synthesize nanosized and hierarchical zeolites in a one-step procedure. By using silica spheres as the adjustable amorphous precursors and tetrapropylammonium hydroxide as a structure-directing agent, effortless control of both size and porosity can be achieved in this system with no extra templates. With a simple hydrothermal process, hierarchical zeolite spheres can be modified with acid cites (Al species incorporated in the framework). Benefitting from its mesoporosity, palladium nanoparticles are incorporated into the nanosized hierarchical zeolite, which makes the materials suitable for use in a cascade catalysis reaction of benzimidazole derivatives, including independent acid catalysis and hydrogenation sites. The nanocomposites show exceptional activity and stability in catalysis and recycling reaction. This strategy can be developed into other versatile and practicable scaffolds for advanced zeolite catalytic nanoreactor systems.

Synthesis of Engineered Zeolitic Materials: From Classical Zeolites to Hierarchical Core-Shell Materials

The term "engineered zeolitic materials" refers to a class of materials with a rationally designed pore system and active-sites distribution. They are primarily made of crystalline microporous zeolites as the main building blocks, which can be accompanied by other secondary components to form composite materials. These materials are of potential importance in many industrial fields like catalysis or selective adsorption. Herein, critical aspects related to the synthesis and modification of such materials are discussed. The first section provides a short introduction on classical zeolite structures and properties, and their conventional synthesis methods. Then, the motivating rationale behind the growing demand for structural alteration of these zeolitic materials is discussed, with an emphasis on the ongoing struggles regarding mass-transfer issues. The state-of-the-art techniques that are currently available for overcoming these hurdles are reviewed. Following this, the focus is set on core-shell composites as one of the promising pathways toward the creation of a new generation of highly versatile and efficient engineered zeolitic substances. The synthesis approaches developed thus far to make zeolitic core-shell materials and their analogues, yolk-shell, and hollow materials, are also examined and summarized. Finally, the last section concisely reviews the performance of novel core-shell, yolk-shell, and hollow zeolitic materials for some important industrial applications.