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.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis.

Direct conversion of methane (CH4) to C1-2 liquid oxygenates is a captivating approach to lock carbons in transportable value-added chemicals, while reducing global warming. Existing approaches utilizing the transformation of CH4 to liquid fuel via tandemized steam methane reforming and the Fischer-Tropsch synthesis are energy and capital intensive. Chemocatalytic partial oxidation of methane remains challenging due to the negligible electron affinity, poor C-H bond polarizability, and high activation energy barrier. Transition-metal and stoichiometric catalysts utilizing harsh oxidants and reaction conditions perform poorly with randomized product distribution. Paradoxically, the catalysts which are active enough to break C-H also promote overoxidation, resulting in CO2 generation and reduced carbon balance. Developing catalysts which can break C-H bonds of methane to selectively make useful chemicals at mild conditions is vital to commercialization. Single atom catalysts (SACs) with specifically coordinated metal centers on active support have displayed intrigued reactivity and selectivity for methane oxidation. SACs can significantly reduce the activation energy due to induced electrostatic polarization of the C-H bond to facilitate the accelerated reaction rate at the low reaction temperature. The distinct metal-support interaction can stabilize the intermediate and prevent the overoxidation of the reaction products. The present review accounts for recent progress in the field of SACs for the selective oxidation of CH4 to C1-2 oxygenates. The chemical nature of catalytic sites, effects of metal-support interaction, and stabilization of intermediate species on catalysts to minimize overoxidation are thoroughly discussed with a forward-looking perspective to improve the catalytic performance.

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Single Atom Catalysts for Selective Methane Oxidation to Oxygenates.

Zeolite-encapsulated metal clusters have been shown to be an effective bifunctional catalyst for tandem catalysis. Nevertheless, the efficient encapsulation of nanometric metal species into a high-aluminum ZSM-5 zeolite still poses a significant challenge. In this contribution, we have prepared well-dispersed and ultra-small Ru clusters encapsulated within a high-aluminum ZSM-5 zeolite (with a Si/Al ratio of ∼30–40) via an in situ two-stage hydrothermal synthesis method. Small Ru clusters with an average size of ∼1 nm have been identified by scanning transmission electron microscopy and hydrogen chemisorption. Shape-selective hydrogenation experiments with different probe molecules reveal a predominant encapsulation (∼90%) of metal clusters within the MFI zeolite cavities, which significantly enhances thermal stability of metal clusters against sintering. 27Al magic angle spinning nuclear magnetic resonance and Brønsted acid site (BAS) titration experiments show the successful incorporation of aluminum species (>99%) into the zeolite framework and build-up of intimacy between the Ru clusters and BASs at a sub-nanometric level. The resulting Ru@H-ZSM-5 shows an enhanced activity and stability for the crucial hydrodeoxygenation (HDO) of phenol to cyclohexane, in biomass valorization. This synthesis strategy could be of great help for the rational design and development of zeolitic bifunctional catalysts and could be extended to other crystalline porous materials. 

Enhanced Catalytic Performance through In Situ Encapsulation of Ultrafine Ru Clusters within a High-Aluminum Zeolite

Liquid organic hydrogen carriers (LOHCs) are promising hydrogen carriers that play an important role in the hydrogen economy. However, designing an efficient catalyst for realizing hydrogen storage with cost-effective and low-temperature is still a great challenge. Herein, we report a Ru single-atoms supported on *BEA zeolite catalyst (Ru(Na)/Beta), with the assistance of hydrogen spillover, which can remarkably enhance the hydrogenation of N-ethylcarbazole(NEC), N-propylcarbazole(NPC) and 2-methylindole (2-MID) at lower temperatures with lower Ru content (0.5 wt%). Notably, the obtained Ru(Na)/Beta catalyst exhibits excellent activity in the hydrogenation of NEC with the hydrogen uptake of 5.69 wt% and a conversion rate of > 99% within 1.5 h for the 6 MPa H2 at 100 °C, whereas the hydrogen uptake on traditional Ru/Al2O3 is only 2.97 wt% with the conversion rate of 67 % under the same conditions. It is found that highly dispersed Ru single-atoms boost hydrogen activation and the strong acid sites (Brønsted and Lewis) of zeolites promote the hydrogen spillover on the hydrogenation with N-heterocycles. Moreover, the synergistic effect of Ru single atoms and *BEA zeolite is crucial for accelerating the hydrogenation rate and lowering the activation energy (45.7 vs. 88.3 kJ/mol) compared with traditional Ru-based catalysts. 

Synergistic catalysis of Ru single-atoms and zeolite boosts high-efficiency hydrogen storage

The incorporation of secondary metal ions into Cu-exchanged SSZ-13 zeolites could improve their catalytic properties in selective catalytic reduction of NOx with ammonia (NH3-SCR), but their essential roles remain unclear at the molecular level. Herein, a series of Cu-Sm-SSZ-13 zeolites have been prepared by ion-exchanging Sm ions followed by Cu ions, which exhibit superior NH3-SCR performance. The NO conversion of Cu-Sm-SSZ-13 is nearly 10% higher than that of conventional Cu-SSZ-13 (175–250 °C) after hydrothermal ageing, showing an enhanced low-temperature activity. The Sm ions are found to occupy the six-membered rings (6MRs) of SSZ-13 by X-ray diffraction Rietveld refinement and aberration-corrected scanning transmission electron microscopy. The Sm ions at 6MRs can facilitate the formation of more active [ZCu2+(OH)]+ ions at 8MRs, as revealed by temperature-programmed reduction of hydrogen. X-ray photoelectron spectroscopy and density functional theory (DFT) calculations indicate that there exists electron transfer from Sm3+ to [ZCu2+(OH)]+ ions, which promotes the activity of [ZCu2+(OH)]+ ions by decreasing the activation energy of the formation of intermediates (NH4NO2 and H2NNO). Meanwhile, the electrostatic interaction between Sm3+ and [ZCu2+(OH)]+ results in a high-reaction energy barrier for transforming [ZCu2+(OH)]+ ions into inactive CuOx species, thus enhancing the stability of [ZCu2+(OH)]+ ions. The influence of the ion-exchanging sequence of Sm and Cu ions into SSZ-13 is further investigated by combining both experiments and theoretical calculations. This work provides a mechanistic insight of secondary ions in regulating the distribution, activity, and stability of Cu active sites, which is helpful for the design of high-performance Cu-SSZ-13 catalysts for the NH3-SCR reaction. 

Unveiling Secondary-Ion-Promoted Catalytic Properties of Cu-SSZ-13 Zeolites for Selective Catalytic Reduction of NO<i><sub>x</sub></i>

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 NOx 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.

Advances in Catalytic Applications of Zeolite-Supported Metal Catalysts.

Slice-shaped nanometer SAPO-34 molecular sieve with relatively low silicon content, preparation method and application thereof

The invention discloses a nanosheet-shaped SAPO-34 molecular sieve with a larger specific surface area as well as an ultrafast preparation method and an application of the nanosheet-shaped SAPO-34 molecular sieve and belongs to the technical field of molecular sieves. The nanosheet-shaped SAPO-34 molecular sieve is rapidly synthesized by adopting a reaction kettle wholly made of a stainless steel material and introducing seed crystals, and the crystallization time of the SAPO-34 molecular sieve is successfully shortened to 5 min due to excellent heat transmission rate of the stainless steel reaction kettle and seed crystal induced fast crystal nucleation. The synthetic method is simple, the crystallization time is extremely short, the synthesis cost is greatly reduced, the synthesis time is greatly shortened, and meanwhile, the liquid waste pollution caused by synthesizing is effectively reduced. The yield of synthesized nanosheet-shaped SAPO-34 samples is very high and can be higher than 90%; the nanosheet-shaped SAPO-34 molecular sieve has very high light olefin selectivity in MTO (methanol to olefins) reaction, especially, the total yield of ethylene and propylene can be higher than 85%, and the nanosheet-shaped SAPO-34 molecular sieve is very suitable for industrial scale-up application.

Nanpsheet-shaped SAPO-34 molecular sieve as well as ultrafast preparation method and application thereof

A SAPO-34 molecular sieve of a larger specific surface area and a hollow alumina-rich hierarchical pore structures and application thereof in a reaction that uses methanol to prepare low carbon olefin, belongs to the technical field of molecular sieves. The product by the invention is obtained by taking triethylamine as a template agent, adopting a traditional hydrothermal or solvothermal synthetic method, taking water or alcohol as a solvent, and introducing polyethylene glycol polymer in a high pressure reactor under an autogenous pressure through an in-situ alumina-rich method. A synthetic SAPO-34 molecular sieve sample is in a hollow and hierarchical pore structures cubic shape, the average crystal size is 5-10 [mu]m, and the mesoporou size is 2-15 nm. According to the invention, the yield of the synthetic SAPO-34 molecular sieve sample is extremely high, and can reach over 90%. The invention has extremely high selectivity of low carbon olefin in a reaction that uses methanol to prepare olefin (MTO), particularly the total yield of ethylene and propylene can reach over 85%, and the SAPO-34 molecular sieve is extremely suitable for industrial amplification application.

SAPO-34 molecular sieve of larger specific surface area and hollow alumina-rich hierarchical pore structures and application thereof

The invention discloses a catalyst as well as a preparation method and application thereof. The catalyst comprises a carrier and an active component, the carrier is a pure silicon molecular sieve, andthe active element in the active component comprises Pt,wherein Pt is loaded in the carrier in the form of a sub-nano Pt cluster (the number of Pt atoms in the cluster is 1-10, and the average numberof Pt atoms is 3), zn is linked with the carrier through a Zn-O-Si bond in the form of unit point + divalent Zn ions, and is connected with the platinum cluster through a Zn-O-Pt bond. The preparation method of the catalyst adopts the direct hydrogen reduction, the prepared catalyst encapsulates the sub-nano platinum clusters and the unit point Zn ions into a pure silicon molecular sieve, and the catalyst is applied to the propane dehydrogenation catalytic reaction and shows the performance far higher than that of other catalysts.

Wang Ning

Papers

Advances in Catalytic Applications of Zeolite-Supported Metal Catalysts.

Slice-shaped nanometer SAPO-34 molecular sieve with relatively low silicon content, preparation method and application thereof

Nanpsheet-shaped SAPO-34 molecular sieve as well as ultrafast preparation method and application thereof

SAPO-34 molecular sieve of larger specific surface area and hollow alumina-rich hierarchical pore structures and application thereof

Catalyst and preparation method and application thereof