**1. Introduction**

MOFs and the related researches have become more and more important not only in chemistry but also in general science and technology. MOFs are a class of porous materials composed of metal-containing nodes connected by organic linkers through strong chemical bonds. The union of these two building units produces different coordination modes, depending on the symmetry of the linker and the coordination number of the metal center. The flexibility or rigidity of the added linker can allow the articulation of the clusters into a highly crystalline three-dimensional framework, which can exhibit higher surface area and pore volume than most porous zeolites [1]. Depending on the architecture of the obtained MOFs, they can be synthesized with high purity and also, they can be engineered to have a high skeletal density but constructed from relatively light elements. Therefore, most of the important related work is aimed at designing compounds possessing very large pores and high surface areas in order to load these materials with atoms, molecules, or even biomolecules. Due to these loading possibilities, wide applications of MOFs have emerged in different fields, such as in catalysis [2–4], guest adsorption (molecular recognition) [5], drug delivery [6, 7], gas storage [8–13], optical applications [14–16], composites [17], water treatment [18, 19], and sensor technologies [20], among others [21–26].

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Some materials as metals in solution (transition metal complexes or metal salts) have been used in catalysis with excellent results. These materials are able to catalyze a variety of organic reactions, in many cases, reaching high yields and regenerating the material after the reaction. However, in many cases the metals are hardly recovered and/or decompose during the reaction due to the conditions. To achieve control these limitations, researchers have developed methods using porous materials as carriers, to achieve well-isolated, uniform single sites that don't interact between them, preventing the decomposition [2, 27]. Active sites on MOFs are located at the metal nodes on the crystalline structure; when the reaction occurs, the framework protects their active sites and increases the efficiency and resistance of catalyst [28].

Given the variety of metallic nodes and organic linkers, it is possible to control the synthesis of MOFs to design them with modular properties, functionalized with specific sites or specific assets to catalyze organic reactions. In this chapter, we present the main results of research with MOFs in the field of catalysis, with special focus on design, relationship between structure and activity, formation of active sites and limitations of these materials.
