# Zirconium-Containing Metal-Organic Frameworks (MOFs) as Catalysts for Biomass Conversion
## 5 Conversion of Carbohydrates
### Levalinic Acid into Chemicals
#### 5.1.2 Synthesis of Levulinate Esters  
Given the interest in LA and its derivatives, one of the first reactions studied in our group dealing with biomass conversion over MOF catalysts was the esterification of LA with various alcohols [20]. We reported on the use of UiO-66 and its derivative, UiO-66-NH\({}_{2}\) (containing 2-aminoterephthalate linkers), for the conversion of LA to ethyl levulinate, \(n\)-butyl levulinate, and various long-chain alkyl levulinates. The relevance of that work, which we will revise now, was that we established, for the first time, that a direct correlation existed between the concentration of missing linker defects and catalytic activity for this type of reaction. Later, we also investigated the reaction mechanism and the influence of amino substituents in the terephthalate ring with ab initio methods [36], and we demonstrated that the active sites for this reaction were Brensted-induced acid sites formed by strongly polarized water molecules adsorbed onto coordinatively unsaturated Zr\({}^{4+}\) sites associated with missing linker defects [22]. Thus, our approach consisted of preparing several batches of UiO-66 and UiO-66-NH\({}_{2}\), each of them having a variable amount of missing linker defects comprised between 2.5 and 13.2% for UiO-66, and between 2.1 and 8% for UiO-66-NH\({}_{2}\). The exact amount of missing linker defects in each sample was calculated from the corresponding thermogravimetric curve (TGA), following the method proposed by [PERSON] et al. [37]. These linker defects were formed randomly during the synthesis of the MOF, and are most likely due to small differences in nucleation and crystal growth rates during the synthesis from batch to batch. After checking that all catalysts were highly crystalline, we observed that all of them were active for the esterification of LA with ethanol. However, a large variation of the catalytic performance existed from batch to batch. Thus, calculated pseudo-first order kinetic rate constants varied between 0.61 h\({}^{-1}\) and 0.07 h\({}^{-1}\) for UiO-66; i.e., an almost 9-fold increase from the less active to the most active catalyst batch. Similar results, although with smaller variations, were found for UiO-66-NH\({}_{2}\), with kinetic rate constants ranging from 0.33 h\({}^{-1}\) to 0.43 h\({}^{-1}\). The time-yield plots obtained for all the catalyst batches are shown in Figure 2.  
The linear dependence of the kinetic rate constant on the concentration of the defects of the catalyst is evident from the plot shown in Figure 2c, for both the UiO-66 and UiO-66-NH\({}_{2}\) series of catalysts. Note also that, for a given defect concentration, UiO-66-NH\({}_{2}\) catalysts are, in general, slightly more active that UiO-66. This was first attributed [20] to a cooperative bifunctional acid-base effect of the (Lewis acid) Zr\({}^{4+}\) sites and the (basic) amino groups of UiO-66-NH\({}_{2}\). Later [36], we used a combination of experimental and ab initio calculations to show that the amino groups in the terephthalate linkers do not participate directly in the esterification reaction. However, their presence in the terephthalate linkers increases the hydrophilic character of the solid and favors water adsorption (more water is adsorbed and the interaction is stronger than in the UiO-66 lacking amino groups). And these adsorbed water molecules do participate directly in the esterification reaction. Meanwhile, catalyst particle size has a different effect on UiO-66 (catalytic activity independent of particle size) and UiO-66-NH\({}_{2}\) (lower activity as the particle size increases), as shown in Figure 2d. This indicates that there is no diffusion control of the esterification reaction in UiO-66. On the contrary, the amino groups reduce the free pore openings of UiO-66-NH\({}_{2}\), due to the large size of -NH\({}_{2}\) groups (of aminoterephthalate linkers) with respect to -H (in terephthalate linkers). This prevents or at least hinders the diffusion of LA into the MOF pores of UiO-66-NH\({}_{2}\).