# Zirconium-Containing Metal-Organic Frameworks (MOFs) as Catalysts for Biomass Conversion
## 7 Conversion of Other Biomass-Derived Compounds
### Reduction of Carbonyl Compounds: Synthesis of Hydroxysteroids  
One important route for the reduction of carbonyl compounds to the corresponding alcohol is the so-called Meerwein-Ponndorf-Verley (MPV) reaction using a secondary alcohol as a hydride source [77; 78; 79]. Interestingly, it can also be used in the opposite reaction, known as Oppenauer oxidation, to oxidize an alcohol with a sacrificial carbonyl compound, as shown in Scheme 9. One main advantage of the MPVO reaction is that it is a chemoselective process. This means that the reaction can be carried out even with molecules containing other reducible functional groups besides the target carbonyl, such as conjugated and unconjugated unsaturated C-C bonds, esters, or nitro groups. This is not always the case with other existing methods for the reduction of carbonyls, such as direct hydrogenation with H\({}_{2}\) or reduction with other reducing agents, such as NaBH\({}_{4}\). In the case of the oxidation reaction (Oppenauer), no overoxidation to the carboxylic acid takes place. Another advantage of MPVO reactions is that they occur under mild conditions, which is highly desirable for the synthesis of natural products. Various zirconium-containing MOFs have been successfully applied so far for MPVO reactions. Thus, [PERSON] et al. reported on the use of UiO-66-NO\({}_{2}\), the nitro analog of UiO-66, for the Oppenauer oxidation of geraniol with furfural [52]. Later, the same group demonstrated that the defects induced in UiO-66-NO\({}_{2}\) by treating the catalyst with HCl produced an enhancement of the catalytic activity of this material, UiO-66-NO\({}_{2}\).10\({}_{\text{HCI}}\), for the MPV reduction of _tert_-butylcyclohexanone with \({}^{\text{i}}\)PrOH compared to defective UiO-66-10\({}_{\text{HCI}}\)[76]. Meanwhile, MOF-808 was also used for the MPV reduction of several \(\alpha\),\(\beta\)-unsaturated carbonyls, including the terpenoid \(R\)-carvone, using (S)-1-indanol as a  
Figure 8: (**a**) Time-conversion plots of citronellal over hydrated (\(\Circle\)) and dehydrated (\(\blacksquare\)) UiO-66 with 7% of missing linker defects. Parts (**b,c**) show citronellal conversion (\(\blacksquare\)) and yields to isopulegol (\(\Circle\)) and other isopulegols (\(\lx@math@degree\)) for hydrated (part (**b**)) and dehydrated (part (**c**)) UiO-66. Reproduced from reference [22]. reducing agent [53]. We recently demonstrated the excellent performance of MOF-808 as a catalyst for the diastereoselective reduction of substituted cyclohexanones [23; 80]. Also, as commented above, the reduction of levulinic acid or a levulinate ester can be carried out through a MPV reaction as well using MOF-808 as a catalyst [57]. In the following, we will describe in more detail an interesting use of MOF-808 as a heterogeneous catalyst for the reduction of ketosteroids to the corresponding hydroxysteroids. As we have discussed above, UiO-66 has only a moderate activity for the MPV reaction. The reason for this limited performance is that only the Zr\({}^{4+}\) ions at defect sites participate in the reaction. Additionally, the MPV reaction requires the formation of a bulky transition state, which can barely fit inside the reduced space of UiO-66 pores. Accordingly, we have shown that UiO-66 was a poor catalyst to reduce cyclohexanone to cyclohexanol through a MPV reaction with \({}^{1}\)PrOH as a hydride source; only 12% cyclohexanone conversion was attained after 1 h of reaction time [23]. On the other hand, all Zr\({}^{4+}\) sites in MOF-808 can participate in the MPV reaction, and the pore cavities are much wider to accommodate the transition state. Therefore, a much higher cyclohexanone conversion was achieved under the same reaction conditions: 69% conversion after 1 h. And this activity improved further when defective sites were introduced by combining tritopic (trimesic acid) and ditopic ligands (isophthal or pyridine dicarboxylic acid) during the synthesis of MOF-808; the conversion of cyclohexanone increased to 91 and 93% after 1 h, respectively.