*4.2.1.1.3 Epitaxial growth synthesis (EGS)*

EGS is a synthetic strategy to encapsulate MOF particles as a core with the second layer of MOF. In addition, in most cases, the shell MOF topology is similar to the core MOF and active nanoparticles would be placed at the interface of the two layers prior to the epitaxial growth. Overall, this approach leads to effective integration of MOFs because synergies of various MOFs properties are simultaneously presented by an exceptional composite of MOFs @MOFs. Despite this method presenting a wide range of similarities to the CASS strategy, it can enhance the diffusion of the reaction medium toward the active sites due to the ability to fabricate a super-thin shell with a thickness size less than 10 nm [53, 54].

#### *4.2.2 Encapsulation of the MNPs in the micropores of MOFs*

Not only would MOFs provide a fully available pore space to optimize the diffusion of the reaction medium, but they also support the MNPs by enclosing them in their mesoporous matrix that increases the accessibility of active sites and enhances the catalytic activity [53]. Due to MOFs can play the role of host materials and be able to provide confined spaces for nucleation of MNPs, the encapsulation of MNPs into the cavities or channels of MOF matrix can be conducted by impregnating metal precursors in the pre-synthesized MOFs and subsequent reduction of the metal precursors in the micropores of MOFs [2]. Although the precise control of this encapsulation type of MNPs in MOFs seems to be difficult, the most effective pathway can be achieved by the double-solvent procedure prior to the reduction step. As a result of this double-solvent approach, the capillary force intensifies the mass transfer of metal precursors into the micropores of MOFs, which prevents from aggregation of MNPs and minimizes the dispersion of them on the outer surface of MOF [52].

The effective route to utilize noble metals (Pd, Ru, or Pt) in the MOFs-based catalysts is in the form of alloys with low-cost transition metals (Cu, Co, or Ni) owing to decreasing the essential amount of expensive noble metals during the catalyst's fabrication. Furthermore, the atomic and electronic specifications of their structures can be adjusted which leads to improved catalyst activity. In this approach, firstly low-cost transition MNPs are impregnated in the pre-synthesized MOFs and then are reduced by NaBH4. In the next step, the salts of noble metals were exposed into a solution of MOF in which the galvanic replacement reaction with the transition MNPs with the help of an excitement process like sonication can be started. Finally, it leads to the fabrication of alloy nanoparticles like Co-Ru which is encapsulated in MOF [2, 55].

#### **4.3 Encapsulation of MNPs in organic materials**

One of the most effective approaches of ultrasmall MNPs encapsulation with meticulously composed sizes in catalysts is the encapsulation of them in organic materials, which is presented in two parts as follows [2]:

#### *4.3.1 MNPs @organic capsules*

Organic capsules, such as dendrimers, which are a family of hyperbranched polymers, have a spherical structure, which is compressed on the exterior and creates hollow space in the interior. By applying organic groups, such as tertiary amines, the interior cavity can be put into particular operation, such as interception of metal ions from the solution through encapsulating them. In addition, the dendrimers can provide a monodic encapsulation of MNPs due to form mono dispersing of them with appropriate adjustment of ultrasmall sizes. Not only can dendrimers operate as effective encapsulating materials owing to the consuming amount of them being approximately the same as the metal ions amount, but also they are able to present a stable form that is unchanged for some months. Thus, these specifications convert them as useful stabilizers in the catalytic process to prepare a monotonous dispersion of nano-catalysts particles with suitable stability [2].

Polyamidoamine (PAMAM) is the most used dendrimer for encapsulating the ultrasmall MNPs. Not only can dendrimers, particularly PAMAM, provide homogeneous catalysis, but an effectual mass transfer will also happen at their exterior surface that leads to a significant improvement in the catalytic activity of the ultrasmall MNPs. To present applicable heterogeneous catalysts of encapsulated ultrasmall MNPs in dendrimers, it seems essential to utilize mesoporous supports, such as silica. Not only do they supply high surface area, but also they prepare constructive enclosures to intensify the stabilization of ultrasmall MNPs in harsh catalytic processes. When applying the silica mesoporous matrix as support, the electrostatic interaction and hydrogen bonding between silica molecules and dendrimers enforce MNPs@ dendrimers into the mesopores of the silica matrix. Not only do these supported catalysts without removing the dendrimer present highlighted stability, selectivity, and activity in a range of catalysis processes, they illustrate adjustable catalytic specifications that are possible to alter in order to enhance the catalyst activity by modifying the active groups on dendrimers, for instance, the tertiary amines can improve the activity of the catalyst due to their electron enrichment attributes. In addition, the unique spatial morphology of dendrimers in the shape of tree may have a significant impact on the reaction substrate stability, including reactant molecules or intermediate species that can lead to decrement of the level of activation energy and improve the turnover frequency [2, 19, 56]. Furthermore, click dendrimers consist of a great ratio of triazole rings, are the other type of dendrimers that can be fabricated from the azide-alkyne cyclo addition. Their rings provide an appropriate situation to accomplish the encapsulation by adsorbing metal ions and grafting the MNPs. In addition, by adding triethylene glycol (TEG) termini to the click dendrimer convert them to an effective soluble one in aqueous solutions which are suitable to fabricate the encapsulation of ultrasmall monometallic or bimetallic like Pt-Co in water solution. Thus, the yield of catalyst performance in an aqueous solution will dramatically enhance [2].

#### *4.3.2 Porous organic cages (POCs)*

POCs with intrinsic porosity have prominent attributes, such as high surface areas, shape durability, structural adjustability, and in particular including active practical groups in their cavities that lead them to encapsulate ultrasmall MNPs in an effective way. Moreover, POCs can exhibit either in a crystalline or an amorphous structure. Furthermore, the heterogeneous catalysts that are formed by encapsulating ultrasmall nanoparticles in these POCs would be homogenized in the solutions and have a significant influence on the enhancement of the performance of catalysts. Their specifications can be modified or developed through post-synthetic modification strategies, such as polymorph selection, modular co-crystallization, and the fabrication of composite materials. Not only does the cage wrapping significantly impact porosity, during this route by employing various crystalline polymorphs distinctive physical properties can be exhibited. In addition, to fabricate a cage in the correct form, at least it is essential to provide precursors with proper and precise geometry, for instance, a mild alter in bond angles of the precursors can lead to a distinct cage that might be in size and stoichiometry [2, 57, 58].

To fabricate POGs with polyhedral organic molecules, triamine is utilized to form the top and bottom prism that include a dialdehyde with a long alkyl chain and a thioether group at the three peripheral sides. Although the long alkyl chain provides a cage profoundly soluble in an oil phase due to thriving their hydrophobic properties, the thioether group leads the cage to adsorb the metal ions and prepare suitable sites to graft the MNPs. Therefore, these specifications develop POGs capability to utilize as promising catalysts, particularly in catalyzing organic reactions [2].
