# Zirconium-Containing Metal-Organic Frameworks (MOFs) as Catalysts for Biomass Conversion
## 3 Structure and Properties of UiO-66 and MOF-808: Origin of Lewis and Bronsted-Induced Acidity  
UiO-66 (UiO stands for University of Oslo) is a robust zirconium terephthalate (1,4 benzenedicarboxylate) formed by hexameric [Zr\({}_{6}\)O\({}_{4}\)(OH)\({}_{4}\)] octahedral oxoclusters connected by 12 terephthalate linkers into a face-centered cubic packing. This arrangement of structural units leads to tetrahedral and octahedral cavities of about 8 and 11 A, respectively; both of them accessible through triangular windows of about 6 A in diameter (see Figure 1). The high coordination number of the inorganic building units to 12 organic linkers bestows a remarkable thermal, chemical, and mechanical stability [14], which is seldom found in other MOFs. An interesting property of UiO-66 is that Zr\({}^{4+}\) ions can be replaced by other tetravalent cations (Hf [15], Ce [16], Ti [17], etc. ), while terephthalate linkers can be replaced by other linkers with the same geometrical connectivity, including substituted terephthalate lates, and extended linear carboxylates, such as biphenyl-4_4_\({}^{\prime}\)_-dicarboxylate in UiO-67, _p_-terphenyl-4_,4_\({}^{\prime\prime}\)_-dicarboxylate in UiO-68, or the large family of PIZOFs with even longer linkers [18; 19]. This affords a battery of isoreticular derivatives available, i.e., materials having the same network topology and pore architecture as UiO-66, but with a tailored pore size and chemical environment inside the cavities, and thus, with tunable properties. In the structure of UiO-66 and related solids, Zr\({}^{4+}\) ions (or other M\({}^{4+}\) cations) lack coordination vacancies available for substrate binding, and thus, in the _ideal_ UiO-66 structure, Zr\({}^{4+}\) ions should not behave as Lewis acid sites. However, _real_ UiO-66 compounds usually present missing linker defects, either randomly formed during the self-assembly formation of the MOF [20] or intentionally created during or after the synthesis [21]. In this way, the Zr\({}^{4+}\) ions associated with these missing linker defects feature coordinative unsaturation sites (_cus_), with the ability to accept electron density from species adsorbed inside the pores. In UiO-66, each terephthalate linker is connected to four Zr\({}^{4+}\) ions. A missing linker defect can be imagined as the removal of one of the twelve negatively charged terephthalate linkers connecting the Zr\({}_{6}\) oxoclusters. When this happens, two of the four Zr ions initially connected by the missing linker are saturated by \({}^{-}\)OH (or Cl\({}^{-}\)) anions to preserve the electrical neutrality of the material, thus leaving two Zr\({}^{4+}\)_cus_ per missing linker molecule, as shown in Scheme 2. As we have discussed in previous works [22], when the MOF is hydrated, these _cus_ will have strongly coordinated water molecules, which will introduce Bronsted-induced acidity in the material. These adsorbed water molecules will be highly polarized due to the high charge-to-radius ratio of Zr\({}^{4+}\) cations, so the acidity is noticeable. However, these water molecules can be removed by thermal dehydration, exposing the Zr\({}^{4+}\) sites and making them available as Lewis acid sites. Therefore, it is possible to tune the acid properties of UiO-66-type materials by simply controlling the hydration degree of the material. Throughout this review, we will see examples in which both types of sites can be used effectively to catalyze specific reactions for converting biomass into chemicals. A key aspect to take into account in UiO-66 is that the active sites are associated with defects, so: (i) not all the Zr\({}^{4+}\) sites will participate in the reaction, and therefore (ii) the catalytic activity will increase with the number of missing linker defects. In other words, we should not expect any catalytic activity for an \"ideal\", defect-free UiO-66. Figure 1: Structure and pore cavities in UiO-66 and MOF-808. Figure adapted from references [12; 13]. Copyright 2008 and 2014, American Chemical Society.