# Zirconium-Containing Metal-Organic Frameworks (MOFs) as Catalysts for Biomass Conversion
## 5 Conversion of Carbohydrates
### Levalinic Acid into Chemicals
#### 5.1.3 Direct Ketalization of LA  
Another reaction that effectively exploits the Bronsted-induced acidity of defective UiO-66 is the direct ketalization of LA. In this process, LA is reacted with a polyalcohol in the presence of an acid catalyst to form the corresponding ketal. Although seemingly a simple process, direct ketalization is complicated by the occurrence of the competing esterification of either the resulting levulinic ketal, or the starting LA, or both, thus yielding a mixture of products, namely ketal, ester, and ketal-ester, which lower the final yield of the desired ketal (see Scheme 5). For this reason, most of the reported studies on ketalization address the use of ethyl levulinate instead of LA to minimize competing (trans)esterification side reactions and to improve the final ketal selectivity [38; 39; 40]. Meanwhile, the direct ketalization of LA with various polyols was reported by [PERSON] and [PERSON] over Amberlyst-15 and \(p\)-TSA catalysts [41]. Those authors concluded that selectivity to ketal depends on the catalyst structure and the number, nature, and accessibility of the active sites. Figure 3: Logarithmic plot of the reaction rate constant of LA esterification (log \(k\)) versus the pH of a methanol dispersion of various catalysts, as indicated as follows: _para_-toluene sulfonic acid (\(\blacktriangle\)); Amberlyst-15\({}^{\text{(b)}}\) (\(\blacktriangleright\)); UiO-66-NO\({}_{2}\) (\(\blacktriangledown\)); UiO-66-HAc (\(\blacktriangle\)); UiO-66-SO\({}_{3}\)H (\(\square\)); various Zr- and Hf-DUT-67 (\(\lozenge\)). Samples of UiO-66 (\(\blacksquare\)) and UiO-66-NH\({}_{2}\) (\(\Circangledown\)) are numbered as follows. (1) Hydrated UiO-66 (7% of defects); (2) Dehydrated UiO-66 (7% of defects); (3) Hydrated UiO-66 (2% of defects); (4) Dehydrated UiO-66 (2% of defects); (5) Hydrated UiO-66-NH\({}_{2}\) (8.8% of defects); (6) Dehydrated UiO-66-NH\({}_{2}\) (8.8% of defects). Reproduced with permission from reference [22]. Copyright (2020) American Chemical Society. Given that the direct ketalization (and esterification) of LA is a largely unexplored reaction, we decided to test the effectivity of the Bronsted-induced acidity of defective UiO-66 for this reaction [42]. Our results showed that UiO-66 indeed catalyzes the direct ketalization of LA with 1,2-propanediol, achieving ketal selectivities over 90% at 90% conversion under optimized conditions. The results obtained depended on the alcohol excess used, thermal pre-activation of the catalyst, as well as the number of linker defects of UiO-66. This is clearly observed in Figure 4, where time-conversion plots of LA are shown for UiO-66 samples containing between 6 and 18% linker defects (part a). The calculated TOFs also increase linearly with the total number of defects of the catalyst, ranging from 9.2 to 20.3 h\({}^{-1}\) from the least to the most defective materials (part b). Interestingly, the intrinsic activity per defective Zr\({}^{4+}\) site was found to be roughly constant for all materials and around 68 \(\pm\) 5 h\({}^{-1}\) (part c), which demonstrates that all active sites are isolated and independent from one another, so that UiO-66 can be considered a true single-site catalyst [11]. Finally, it is also worth mentioning here that the activity of the other Zr-MOF considered in the present study, MOF-808, afforded a very limited activity for this reaction; only 35% LA conversion was achieved after 23 h of reaction, with an almost equal distribution of ester and ketal products and a very low TOF of 0.055 h\({}^{-1}\). This result was not unexpected, given the very weak character of the Bronsted acid sites in MOF-808, as discussed above in Section 3. This provides further proof that Bronsted acid sites are the true active sites for both esterification and ketalization reactions.