# Zirconium-Containing Metal-Organic Frameworks (MOFs) as Catalysts for Biomass Conversion
## 2 Metal-Organic Frameworks (MOFs) and Their Use in Catalysis  
MOFs are a subclass of coordination polymers that are characterized by being crystalline, porous solids formed by the linkage of metal ions (or metal oxoclusters) and polydentate organic molecules by metal coordination bonds, forming a crystalline network defining pores of strictly regular dimensions [4; 5]. These metal-ligand coordination bonds are stronger and more directional than the hydrogen bonds or \(\pi\)-\(\pi\) stacking interactions typically found in other amorphous, non-porous coordination polymers--hence, the convenience for practical reasons of differentiating the MOFs within the large family of coordination polymers [6]. What has drawn interest in these materials in recent years is that it is possible to fine-tune the framework topology, as well as to control the shape, dimensions, and connectivity of the structural pores, by properly selecting the structural units (metals and organic ligands) and the way in which they are connected. By a careful selection of the structural units forming the MOF, it has been possible to prepare ultrahigh porous materials that have largely surpassed the specific surface areas and pore volumes found in other porous compounds (such as zeolites, mesoporous silicas, or carbons). Thus, some MOFs feature astonishing specific surface areas over 7000 m\({}^{2}\)g\({}^{-1}\) and pore volumes of 4 cm\({}^{3}\)g\({}^{-1}\) or larger, such as the compounds known as NU-110 (7140 m\({}^{2}\)g\({}^{-1}\) and 4.40 cm\({}^{3}\)g\({}^{-1}\), respectively) [7] or DUT-60, holding the ultrahigh porosity world-wide record attained so far with 7800 m\({}^{2}\)g\({}^{-1}\) and 5.02 cm\({}^{3}\)g\({}^{-1}\)[8]. When designing a material for heterogeneous catalysis, large specific surface areas and wide pores are a must for ensuring a fast diffusion of reactants and products in and out of the pores to reach the active sites located at the internal surface of the catalyst. Therefore, it is not surprising that since the discovery of MOFs, these compounds have been considered for catalytic applications [9; 10; 11]. Moreover, relevant chemical properties, such as Lewis/Bronsted acidity, hydrophilicity/hydrophobicity, the presence of specific functional groups, or the nature and magnitude of noncovalent interactions inside the pores, can be largely determined by the chemical composition of the material. This large tunability of MOFs opens up the possibility to use these materials in size/shape selective reactions. In this way, if we have a mixture of two or more compounds that can be potentially converted in a reaction, only those molecules whose dimensions are small enough to enter the pores of the MOF will reach the catalytic active sites and will be converted, while larger molecules (or with an unappropriated shape) will remain unchanged in the reaction medium. Likewise, if different products or transition states can be competitively formed during a reaction, only those transition states or products that can fit inside the MOF pores will be viable. In other words, by finely tuning the pore size and shape of a MOF, as well as other properties such as the chemical environment inside the cavities, we can drive the reaction towards the formation of the desired product among different possibilities, thereby achieving very high selectivity, emulating the behavior of enzymatic catalysis. Nevertheless, it is always a good practice to compare the results obtained with MOF catalysts with those previously reported in the scientific literature with other catalysts (both homogeneous and heterogeneous), as this allows us to fairly appraise the potential of the MOF for any given reaction. In the present review on biomass conversion routes over MOFs, we will focus our interest on two families of compounds, namely UiO-66 [12] and MOF-808 [13].