Nanocatalysts (NCs), which are catalysts with nano size (at least less than 100 nm), exhibit superior activity in heterogeneous catalysis than those of their bulk materials [1]. Over the past few decades, rapid development in the synthesis of NCs has led to the generation of NCs with adjustable sizes, compositions, and new morphology. These materials with nanostructure and tunable properties have been extensively investigated in the area of catalysis including energy storage and environmental conservation [[2], [3], [4], [5]]. However, nanoparticles are prone to form aggregations during synthesis or catalytic reactions, leading to decreasing surface area, catalytic activity, and reduced exposure of active sites. Encapsulation of NCs in porous host materials such as zeolites, mesoporous silica, polymers, and metal-organic frameworks is an efficient method to prevent particle aggregation and to reduce particle diameter [[6], [7], [8], [9]]. The porous structure of these materials has enabled the access of reactants to the active sites of encapsulated NCs in the pores without aggregation, thereby enhancing the catalytic efficiency. The interconnected pores of the host materials can also improve the product selectivity in catalytic reactions over encapsulated NCs. Furthermore, the synergetic effect between NCs and the host materials can be expected to promote catalytic activity. As shown in Fig. 1, the raising enthusiasm in encapsulated catalysts can be recognized by the rapidly growing number of research publications and corresponding citations in the last decade.

Metal-organic frameworks (MOFs) containing metal nodes and organic linkers are one of the most promising porous crystalline host components, exhibiting considerable surface area, hierarchical porous framework, and structural adjustability [[10], [11], [12]]. MOFs-based materials have been widely utilized for explosives chemosensor, antimicrobial therapy and wound healing [[13], [14], [15], [16]]. Stable heterometallic Na-Eu-cluster-based MOFs was synthesized with high sensitivity for the detection of oridazole antibiotics and nitrophenol [13]. CuS/Co-ferrocene-MOF was found to be active for chemodynamic antibacterial therapy driven by near-infrared [14]. In addition, the properties of MOFs have enabled the improvement of catalytic activity in heterogeneous catalysis after the capsulation of active NCs guests, such as metal nanoparticles, metal oxides, single-atom catalysts, and quantum dots, in their framework pores. Several reviews have been published concerning the utilization of MOFs as hosts for NCs, and applications of the developed composited materials in heterogeneous catalysis [17,18]. However, there have been no review of recent work on incorporating NCs with different particle sizes in MOFs and enhancing catalytic activity compared to their individual counterparts in various applications. Methods and characterization techniques for accomplishing and observing successful incorporation are still in development, and a review of what has been achieved to date is significant for researchers interested in the fields of material design and heterogeneous catalysis.

The dimensions of catalysts have significant effects on the corresponding catalytic activity in various applications. Thanks to the tunability of the pore structure in MOFs, this review present nanocatalysts with different sizes confined in MOFs structure with enhanced catalytic activity for the first time. In this review, we start by briefly presenting the properties of MOFs including pore size and structure, and their tunability. Subsequently, we discuss the preparation, characterization, and applications of NCs incorporated MOFs. Various reported NCs-in-MOF composites including catalyst nanoparticles, clusters, quantum dots, and single atoms catalysts with varied sizes, compositions, and morphologies, are then discussed. The positive effects upon the encapsulation of NCs in MOFs including the influence of tunable chemical environment around nanocatalysts, catalytic stability and recyclability, and the confinement effect are also concluded and discussed (Fig. 2). Finally, we conclude the main content of this review and propose the future perspectives and challenges of NCs-in-MOFs composites for further investigation and practical applications. The purpose and significance of this paper is to give a comprehensive review regarding encapsulation strategies of nanocatalysts in MOFs, including “bottle-around-ship”, “ship-in-bottle,” and one-pot synthesis process, and the positive influences of the confinement effect, surface interactions, and metal-support cooperation in improving catalytic activity, stability, and selectivity to provide a fundamental and essential guidance for the rational construction and chemical environment modification of confined nanocatalysts.