Selectivity Modulation of Encapsulated Palladium Nanoparticles by Zeolite Microenvironment for Biomass Catalytic Upgrading

Strengthening the Metal-Acid Interactions by Using CeO2 as Regulators of Precisely Placing Pt Species in ZSM-5 for Furfural Hydrogenation

Understanding the synergism between the metal site and acid site is of great significance in boosting the efficiency of bi-functional catalysts in many heterogeneous reactions, particularly in biomass upgrading. Herein, a "confined auto-redox" strategy is reported to fix CeO2-anchored Pt atoms on the inner wall of a ZSM-5 cage, achieving the target of finely controlling the placements of the two active sites. Compared with the conventional surface-supported counterpart, the encapsulated Pt/CeO2@ZSM-5 catalyst possesses remarkably-improved activity and selectivity, which can convert >99% furfural into cyclopentanone with 97.2% selectivity in 6 h at 160 °C. Besides the excellent catalytic performance, the ordered metal-acid distribution also makes such kind of catalyst an ideal research subject for metal-acid interactions. The following mechanization investigation reveals that the enhancement is strongly related to the unique encapsulation structure, which promotes the migration of the reactants over different active sites, thereby contributing to the tandem reaction.

Confined Synthesis of Noble Metal Clusters Assisted by Liquid Film for Photocatalytic CO2 Reduction

The important concept of confined synthesis is considered a promising strategy for the design and synthesis of definable nanostructured materials with controllable compositions and specific morphology, such as highly loaded single-atom catalysts capable of providing abundant active sites for photocatalytic reactions. In recent years, researchers have been working on developing new confined reaction systems and searching for new confined spaces. Here, we present for the first time the concept of a bubble liquid film as a novel confined space. The liquid film has a typical sandwich structure consisting of a water layer, sandwiched between the upper and lower surfactant layers, with the thickness of the intermediate water layer at the micro- and nanometer scales, which can serve as a good confinement. Based on the above understanding and combined with the photodeposition method, we successfully confined synthesized Ag/TiO2, Au/TiO2, and Pd/TiO2 photocatalysts in liquid film. By HAADF-STEM, it can be seen that the noble metal morphologies are all nanoclusters of about 1 nm and are highly uniformly dispersed on the TiO2 surface. Compared with photodeposition in solution, we believe that the surfactant molecular layer restricts a limited amount of precursor to the liquid film, avoiding the accumulation of noble metals and the formation of large particle size nanoparticles. The liquid film, meanwhile, restricts the migration path of noble metal precursors, allowing for thorough in situ photodeposition and enables the complete and uniform dispersion of noble metal precursors, greatly reducing the photodeposition time. The uniform loading of the three noble metals proved the universality of the method, and the catalysts showed high activity for photocatalytic CO2 reduction. The rates of reduction of CO2 to CO over the Ag/TiO2 photocatalytic reached 230 μmol g-1 h-1.This study provides a new idea for the expansion of the confined reaction system and a reference for the study of liquid film as the confined space.

Ion-Sieve-Confined Synthesis of Size-Tunable Ru for Electrochemical Hydrogen Evolution

The controllable and low-cost synthesis of nanometal particles is highly desired in scientific and industrial research. Herein, size-tunable Ru nanoparticles were synthesized by using a novel ion-sieve-confined reduction method. The H2TiO3 ion-sieve was used to adsorb Ru3+ into the hydroxyl-enriched porous [TiO3]2- layers. The confined environment of the interlayer space facilitates Ru-Ru collision and bonding during annealing, achieving a precise reduction from Ru3+ to Ru0 without additional reductants. Owing to the confinement effect, Ru0 nanoparticles are uniformly embedded in the pores on the surface of the postannealed TiO2 matrix (Ru@TiO2). Ru@TiO2 exhibited a lower overpotential than Pt/C (57 vs 87 mV at 10 mA cm-2) for the HER in 0.1 M KOH solution. The confinement-induced reduction of metal ions was also preliminarily proved in ion-exchanged zeolites, which provides facile and abundant approaches for the size-controllable synthesis of nanometal catalysts with high catalytic activity.

Principle on Selecting the Coordination Ligands of Palladium Precursors Encapsulated by Zeolite for an Efficient Purification of Formaldehyde at Ambient Temperature

High-performance zeolite-supported noble metal catalysts with low loading and high dispersion of active components are the most promising materials for achieving the complete oxidation of formaldehyde (HCHO) at room temperature. In this work, palladium nanoparticles (Pd NPs) with different sizes were successfully encapsulated inside the silicalite-1 (S-1) zeolite framework by using diverse stabling ligands via the one-pot method. Thereafter, the rule on selecting the coordinative ligands for palladium was clarified: more N atoms, a short carbon chain, a smaller branch chain, and bidentate coordination are characteristics of an ideal ligand. Accordingly, the best-performing 0.2Pd@S-1(Ethylenediamine) catalyst exhibited outstanding performance for HCHO oxidation, achieving 100% conversion even at room temperature. High-resolution high-angle annular dark-field scanning transmission electron microscopy (HR HAADF-STEM) and density functional theory (DFT) calculations indicate that the chelate is formed by complexation of Pd2+ ions with ethylenediamine, displaying the smallest spatial site resistance simultaneously with the zeolite synthesis, resulting in Pd located mostly within the 5-membered ring (5-MR) channels of S-1 after calcination, thus limiting the growth of Pd clusters and promoting their dispersion.

Design of Bifunctional Nanocatalysts Based on Zeolites for Biomass Processing

Bifunctional catalysts consisting of metal-containing nanoparticles (NPs) and zeolite supports have received considerable attention due to their excellent catalytic properties in numerous reactions, including direct (biomass is a substrate) and indirect (platform chemical is a substrate) biomass processing. In this short review, we discuss major approaches to the preparation of NPs in zeolites, concentrating on methods that allow for the best interplay (synergy) between metal and acid sites, which is normally achieved for small NPs well-distributed through zeolite. We focus on the modification of zeolites to provide structural integrity and controlled acidity, which can be accomplished by the incorporation of certain metal ions or elements. The other modification avenue is the adjustment of zeolite morphology, including the creation of numerous defects for the NP entrapment and designed hierarchical porosity for improved mass transfer. In this review, we also provide examples of synergy between metal and acid sites and emphasize that without density functional theory calculations, many assumptions about the interactions between active sites remain unvalidated. Finally, we describe the most interesting examples of direct and indirect biomass (waste) processing for the last five years.

A General and Scalable Approach to Sulfur-Doped Mono-/Bi-/Trimetallic Nanoparticles Confined in Mesoporous Carbon

Metal nanoparticles confined in porous carbon materials have been widely used in various heterogeneous catalytic processes due to their enhanced activity and stability. However, fabrication of such catalysts in a facile and scalable way remains challenging. Herein, we report a general and scalable thiol-assisted strategy to synthesize sulfur-doped mono-/bi-/trimetallic nanoparticles confined in mesoporous carbon (S-M@MC, M = Pt, Pd, Rh, Co, Zn, etc.), involving only two synthetic steps, i.e., a hydrothermal process and pyrolysis. The strategy is based on coordination chemistry and hydro-phobic interaction that the metal precursors coordinated with the hydrophobic thiol ligands are located at the hydrophobic core of micelles, in situ confined in the hydrothermally prepared mesostructured polymer, and then converted into sulfur-doped metal nanoparticles confined in MC after pyrolysis. It is demonstrated that the S-PtCo@MC exhibits enhanced catalytic activity and improved durability toward acidic hydrogen evolution reaction due to the confinement effect and S-doping.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis

Zeolites with ordered microporous systems, distinct framework topologies, good spatial nanoconfinement effects, and superior (hydro)thermal stability are an ideal scaffold for planting diverse active metal species, including single sites, clusters, and nanoparticles in the framework and framework-associated sites and extra-framework positions, thus affording the metal-in-zeolite catalysts outstanding activity, unique shape selectivity, and enhanced stability and recyclability in the processes of Brønsted acid-, Lewis acid-, and extra-framework metal-catalyzed reactions. Especially, thanks to the advances in zeolite synthesis and characterization techniques in recent years, zeolite-confined extra-framework metal catalysts (denoted as metal@zeolite composites) have experienced rapid development in heterogeneous catalysis, owing to the combination of the merits of both active metal sites and zeolite intrinsic properties. In this review, we will present the recent developments of synthesis strategies for incorporating and tailoring of active metal sites in zeolites and advanced characterization techniques for identification of the location, distribution, and coordination environment of metal species in zeolites. Furthermore, the catalytic applications of metal-in-zeolite catalysts are demonstrated, with an emphasis on the metal@zeolite composites in hydrogenation, dehydrogenation, and oxidation reactions. Finally, we point out the current challenges and future perspectives on precise synthesis, atomic level identification, and practical application of the metal-in-zeolite catalyst system.

Migration of zeolite-encapsulated Pt and Au under reducing environments

Simulations reveal accelerated migration of Pt@zeolite by reducing adsorbates and the importance of PtCO in early stages of particle growth.

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...

The mechanism of MOF as the heterogeneous catalyst for propene hydroformylation: The DFT study

Metal-organic framework which was composed of metal center and organic linkers possessing the similar structure with the homogeneous metal-complex catalyst for hydroformylation, thus it could be potentially used as the...

Advances in Catalytic Applications of Zeolite-Supported Metal Catalysts

Zeolites possessing large specific surface areas, ordered micropores, and adjustable acidity/basicity have emerged as ideal supports to immobilize metal species with small sizes and high dispersities. In recent years, the zeolite-supported metal catalysts have been widely used in diverse catalytic processes, showing excellent activity, superior thermal/hydrothermal stability, and unique shape-selectivity. In this review, a comprehensive summary of the state-of-the-art achievements in catalytic applications of zeolite-supported metal catalysts are presented for important heterogeneous catalytic processes in the last five years, mainly including 1) the hydrogenation reactions (e.g., CO/CO₂ hydrogenation, hydrogenation of unsaturated compounds, and hydrogenation of nitrogenous compounds); 2) dehydrogenation reactions (e.g., alkane dehydrogenation and dehydrogenation of chemical hydrogen storage materials); 3) oxidation reactions (e.g., CO oxidation, methane oxidation, and alkene epoxidation); and 4) other reactions (e.g., hydroisomerization reaction and selective catalytic reduction of NO_x with ammonia reaction). Finally, some current limitations and future perspectives on the challenge and opportunity for this subject are pointed out. It is believed that this review will inspire more innovative research on the synthesis and catalysis of zeolite-supported metal catalysts and promote their future developments to meet the emerging demands for practical applications.

Hierarchical Zeolites-confined Metal Catalysts and Their Enhanced Catalytic Performances

The confinement of metal species within hierarchical zeolites combines the acidic/basic sites of zeolites, the enhanced mass transfer of mesoporous system, and the inside active metal sites, leading to high activity, unique selectivity, and superior stability in chemicals synthesis, energy and environment catalysis. To date, review on this emerging topic is rarely reported. Herein, we classify five metals-hierarchical zeolites composite (metal@hierarchical zeolites) according to the location of metals on hierarchical structure, including metals located on micropores, intercrystalline mesopores, intracrystalline mesopores, hollow nanobox and mesoporous shells. The synthesis and catalysis applications of metal@hierarchical zeolites composite are provided, highlighting the rational design of catalyst preparation, the improved catalytic efficiency and stability of metal species. Finally, we discuss the current limitations and future opportunities for this emerging field. This Review is expected to inspire more developments and applications of metal@hierarchical zeolites.

Confinement in a Zeolite and Zeolite Catalysis

ConspectusZeolites, accompanied by their initial discovery as natural mines and the subsequent large-scale commercial production, have played indispensable roles in various fields such as petroleum refining and the chemical industry. Understanding the characteristics of zeolites, in contrast to their counterparts with similar chemical compositions and the origin thereof, is always a hot and challenging topic. Zeolites are known as intrinsic confined systems with ordered channels on the molecular scale, and structural confinement has been proposed to explain the unique chemical behaviors of zeolites. Generally, the channels of zeolites can regulate the diffusion of molecules, leading to a visible difference in molecular transportation and the ultimate shape-selective catalysis. On the other hand, the local electric field within the zeolite channels or cages can act on the guest molecules and change their energy levels. Confinement can be simply interpreted from both spatial and electronic issues; however, the nature of zeolite confinement is ambiguous and needs to be clarified.In this Account, we make a concise summary and analysis of the topics of confinement in a zeolite and zeolite catalysis from two specific views of spatial constraint and a local electric field to answer two basic questions of why zeolites and what else can we do with zeolites. First, it is shown how to construct functional sites including Brønsted acid sites, Lewis acid sites, extraframework cation sites, and entrapped metal or oxide aggregates in zeolites via confinement and how to understand the specific role of confinement in their reactivity. Second, the multiple impacts of confinement in zeolite-catalyzed reactions are discussed, which rationally lead to several unique processes, namely, Brønsted acid catalysis confined in zeolites, Lewis acid catalysis confined in zeolites, catalysis by zeolite-confined coordinatively unsaturated cation sites, and a cascade reaction within the confined space of zeolites. Overall, confinement effects do exist in zeolite systems and have already played extremely important roles in adsorption and catalysis. Although confinement might exist in many systems, the confinement by zeolites is more straightforward thanks to their well-ordered and rigid structure, deriving unique chemical behaviors within the confined space of zeolites. A zeolite is a fantastic scaffold for constructing isolated sites spatially and electrostatically confined in its matrix. Furthermore, zeolites containing well-defined transition-metal sites can be treated as inorganometallic complexes (i.e., a zeolite framework as the ligand of transition-metal ions) and can catalyze reactions resembling organometallic complexes or even metalloenzymes. The local electric field within the confined space of zeolites is strong enough to induce or assist the activation of small molecules, following the working fashion of frustrated Lewis pairs. The tactful utilization of structural confinement, both spatially and electronically, becomes the key to robust zeolites for adsorption and catalysis.

Recent Advances in Catalytic Confinement Effect within Micro/Meso-Porous Crystalline Materials

Micro/meso-porous crystalline materials with a well-defined pore structure, such as zeolites, carbon nanotubes, and metal-organic frameworks, are of great significance in the development of catalytic systems for scientific and industrial demands. The confinement effect aroused by pore features of porous crystalline materials has triggered great interest in heterogeneous catalysis. Catalytic reactions in confined spaces exhibit unique behaviors compared to those observed on bulk materials. More interestingly, chemical reactivity can be modulated in different ways by the confinement effect, despite the fact that the mechanism on how the confinement effect changes the reaction remains unclear. In this review, a systematic discussion and fundamental understanding is provided concerning the concept of confinement effect, highlighting the impact of confinement effects on diffusion, adsorption/desorption, and catalytic reaction in typical micro/meso-porous crystalline materials, including zeolites, carbon nanotubes, and metal-organic frameworks. Relevant studies demonstrate that confinement effect affords not only shape selectivity against reactants/products, but also modulates surface electron distribution of active species confined within porous environments, thereby successively affecting the catalytic reactivity, selectivity, and stability. This review provides a useful guide for researchers attempting to design excellent porous crystalline catalysts based on the concept of confinement effect in heterogeneous catalysis.

Zeolite Fixed Metal Nanoparticles: New Perspective in Catalysis

ConspectusLoading metal nanoparticles on the surface of solid supports has emerged as an efficient route for the preparation of heterogeneous catalysts. Notably, most of these supported metal nanoparticles still have shortcomings such as dissatisfactory activity and low product selectivity in catalysis. In addition, these metal nanoparticles also suffer from deactivation because of nanoparticle sintering, leaching, and coke formation under harsh conditions. The fixation of metal nanoparticles within zeolite crystals should have advantages of high activities for metal nanoparticles and excellent shape selectivity for zeolite micropores as well as extraordinary stability of metal nanoparticles immobilized with a stable zeolite framework, which is a good solution for the shortcomings of supported metal nanoparticles.Materials with metal nanostructures within the zeolite crystals are normally denoted as metal@zeolite, where the metal nanoparticles with diameters similar to those of industrial catalysts are usually larger than the micropore size. These metal nanoparticles are enveloped with the zeolite rigid framework to prevent migration under harsh reaction conditions, which is described as a fixed structure. The zeolite micropores allow the diffusion of reactants to the metal nanoparticles. As a result, metal@zeolite catalysts combine the features of both metal nanoparticles (high activity) and zeolites (shape selectivity and thermal stability), compared with the supported metal nanoparticles.In this Account, we describe how the zeolite micropore and metal nanoparticle synergistically work to improve the catalytic performance by the preparation of a variety of metal@zeolite catalysts. Multiple functions of zeolites with respect to the metal nanoparticles are highlighted, including control of the reactant/product diffusion in the micropores, the adjustment of reactant adsorption on the metal nanoparticles, and sieving the reactants and products with zeolite micropores. Furthermore, by optimizing the wettability of the zeolite external surface, the zeolite crystals could form a nanoreactor to efficiently enrich the crucial intermediates, thus boosting the performance in low-temperature methane oxidation. Also, the microporous confinement weakens the adsorption of C₁ intermediates on the metal sites, accelerating the C-C coupling to improve C₂ oxygenate productivity in syngas conversion. In particular, the zeolite framework efficiently stabilizes the metal nanoparticles against sintering and leaching to give durable catalysts. Clearly, this strategy not only guides the rational design of efficient heterogeneous catalysts for potential applications in a variety of industrial chemical reactions but also accelerates the fundamental understanding of the catalytic mechanisms by providing new model catalysts.

Incorporation of Active Metal Species in Crystalline Porous Materials for Highly Efficient Synergetic Catalysis

The design and development of efficient catalytic materials with synergistic catalytic sites always has long been known to be a thrilling and very dynamic research field. Crystalline porous materials (CPMs) mainly including metal-organic frameworks and zeolites with high scientific and industrial impact have recently been the subject of extensive research due to their essential role in modern chemical industrial processes. The rational incorporation of guest species in CPMs can synergize the respective strengths of these components and allow them to collaborate with each other for synergistic catalysis, leading to enhanced catalytic activity, selectivity, and stability in a broad range of catalytic processes. In this review, the recent advances in the development of CPMs-confined active metal species, including metal nanoparticles, metal/metal oxides heteroparticles, metal oxide, subnanometric metal clusters, and polyoxometalates, for heterogeneous catalysis, with a particular focus on synergistic effects between active components that result in an enhanced performance are highlighted. Insights into catalysts design strategies, host-guest interactions, and structure-property relationships have been illustrated in detail. Finally, the existing challenges and possible development directions in CPMs-based encapsulation-structured synergistic catalysts are discussed.

Synthesis of a Stable Solid Acid Catalyst from Chloromethyl Polystyrene through a Simple Sulfonation for Pretreatment of Lignocellulose in Aqueous Solution

A stable solid acid catalyst, SCPR140-1, was synthesized from chloromethyl polystyrene resin (CPR) and used for catalytic pretreatment of corncob in aqueous solution. Under the optimized pretreatment condition, 73.07 % of xylose was directly obtained, and the enzymatic digestibility of treated residue reached up to 94.65 %, indicating that the SCPR140-1 had high selectivity for xylose production and effectively deconstructed the structure of corncob. The -CH₂ Cl group of CPR was substituted by -SO₃ H through the sulfonation, and the -SO₃ H was stably bound on the catalyst during the pretreatment process. Compared with other similar reports, the SCPR140-1 was not only synthesized through a simpler process but also had a more stable catalytic activity during multiple recycling runs.

In Situ Encapsulation of Pt Nanoparticles within Pure Silica TON Zeolites for Space-Confined Selective Hydrogenation

Straightforward encapsulation of Pt clusters (∼2 nm) into the pure silica TON-type zeolite (ZSM-22) was reached in a dry gel conversion route, where the ionic liquid template was removed via the hydrocracking-calcination-reduction approach. The obtained Pt@ZSM-22 series possessed high crystallinity, large surface area, and ultrafine Pt clusters inside the zeolite crystals. They exhibited remarkable activity in the semi-hydrogenation of phenylacetylene into styrene; the lead sample with 0.2 wt % Pt loading afforded a large turnover number up to 117,787. The preferential high affinity of the pure silica ZSM-22-encapsulated Pt clusters toward the substrate phenylacetylene rather than the hydrogenated product was derived from the unique space-confinement effect of zeolite microchannels, which is responsible for such excellent performance.