# Zirconium-Containing Metal-Organic Frameworks (MOFs) as Catalysts for Biomass Conversion
## 5 Conversion of Carbohydrates
### Levalinic Acid into Chemicals
#### 5.1.4 Synthesis of \(\gamma\)-Valerolactone (GVL)
Obviously, the use of two different alcohols is not convenient from a practical point of view, but it is still useful from an academic point of view to evaluate the role of different types of acid sites in each individual step. Indeed, it is interesting to note here that, while the esterification and ketalization reactions commented above are both catalyzed by Bronsted acid sites only, the conversion of LA to GVL in two steps requires the simultaneous presence of two types of acid sites in the catalyst: Bronsted acid sites for the first esterification step and Lewis acid sites for the ensuing MPV step. As commented above, UiO-66 is a highly active catalyst for the esterification of LA, due to the presence of strong Bronsted-induced acid sites. As it can be observed in Figure 5a, LA is fully esterified to _n_BuL in 5 h. Conversely, the Bronsted acidity of MOF-808 is very weak. Therefore, the activity of MOF-808 for the esterification of LA with _n_BuOH was only moderate (Figure 5b). However, when MOF-808 was submitted to a sulfation process to increase the Bronsted acidity of the solid [58,59], the catalytic performance of the resulting MOF-808-SO\({}_{4}\) catalyst significantly improved. Thus, the time needed to achieve full LA esterification passed from 14 h for MOF-808 to 11 h and 5 h for two MOF-808-SO\({}_{4}\) samples with increasing sulfation degree (Figure 5c,d). On the other hand, UiO-66 was not a good catalyst for the MPV reaction, which requires Lewis acid sites. The reasons for the poor performance of UiO-66 are two: (i) the small amount of Zr\({}^{4+}\) Lewis sites in UiO-66, associated with missing linker defects; and (ii) the small pores of this MOF, which hinders the formation of the bulky transition state of the MPV reaction. In sharp contrast, MOF-808 has both a large amount of available Lewis sites (all Zr\({}^{4+}\) centers can participate in the reaction) and much wider cavities to accommodate the sterically demanding transition state. Therefore, the catalytic activity of MOF-808 was much higher than UiO-66, as expected. Concerning sulfated MOF-808, the performance of the lowest sulfate sample was very good and comparable to pristine MOF-808, but it decreases somewhat for the deepest sulfated sample. This is probably due to the increasing  
Figure 5: One-pot two-step conversion of LA over: (**a**) UiO-66; (**b**) MOF-808; (**c**) MOF-808-1.47 SO\({}_{4}\); and (**d**) MOF-808-2.63 SO\({}_{4}\). Vertical line indicates the end of the first step (esterification) and the addition of _sec_-BuOH to start the second step (MPV). Reproduced from reference [57]. steric hindrance introduced by the sulfate groups in the MOF pores. Therefore, increasing the sulfation degree of MOF-808 has a positive effect on the first esterification step, but it does not compensate for the slowdown on the second MPV step. These results evidence the need to find the best equilibrium between Bronsted and Lewis acid sites in the solid catalyst to optimize the outcome of this multistep reaction.