# Zirconium-Containing Metal-Organic Frameworks (MOFs) as Catalysts for Biomass Conversion
## 7 Conversion of Other Biomass-Derived Compounds  
### Isomerization of Terpenoids: Citronellal to Isoplegol Conversion  
Menthol is a very important naturally occurring monoterepenoid compound largely used in the production of pharmaceuticals, agrochemicals, cosmetics, toothpastes, chewing gum, and other products. Natural (1\(R\),2\(S\),5_R_)\(-\)menthol is obtained from peppermint oil, but it is also manufactured by Takasago from myrcene [72]. An important step in this reaction is the carbonyl-ene isomerization of citronellal to isopulegol shown in Scheme 8, which is usually carried out catalytically using aqueous ZnBr\({}_{2}\) or tris(2,6-diarylphenoxy)aluminum catalysts. Citronellal can be converted into a mixture of four isomers, of which (-)-isoplegol is the target compound for the menthol synthesis. Therefore, several heterogeneous Lewis acid catalysts have been explored to improve the final stereoselectivity of the process. \begin{table}
\begin{tabular}{c c c c c} \hline
**Carbonyl** & **Time** & **Conv. ** & **Select. ** & **Select. ** \\
**(h)** & **(\%)** & **Dioxane (\%)** & **Dioxolane (\%)** \\ \hline \hline \multirow{4}{*}{CHO} & \multirow{4}{*}{3} & \multirow{4}{*}{100} & \multirow{4}{*}{57.8} & \multirow{4}{*}{42.2} \\  
[PERSON] et al. [73] first showed that the Cu\({}^{2+}\) Lewis acid sites of copper trimesate MOF known as HKUST-1 can catalyze the isomerization reaction of various terpene derivatives, including \(\alpha\)-pinene oxide to camphonicle aldehyde and citronellal to isopulegol, as well as the rearrangement of the ethylene acetal of 2-bromopropophenone. In the case of citronellal isomerization, the selectivity to (-)-isopulegol ranged between 65 and 69%. Later, we and others have shown that other MOFs can also catalyze this reaction, including Cr\({}^{3+}\) in MIL-101 [74] or Zr\({}^{4+}\) sites in UiO-66 [22, 52] or MOF-808 [58]. The isomerization of citronellal is usually carried out in the presence of a Lewis acid catalyst, attaining high selectivities to the desired (-)-isopulegol isomer. However, clear differences are observed when the reaction is catalyzed by Bronsted acid sites. Isopulegol is still the major product obtained, but the final diastereoselectivity is considerably lower. This trend was clearly observed by comparing the catalytic activity of pristine and sulfated MOF-808 catalysts for this reaction. Pristine MOF-808 produced a very low citronellal conversion (only 8% conversion after 8 h of reaction), but a very high selectivity to isopulegol of 85%. Conversely, after the sulfation of MOF-808 to increase the Bronsted acidity of the catalyst, the activity increased considerably (98% citronellal conversion after 1.5 h of reaction time), but at the expense of isopulegol selectivity, which dropped to 55% [58]. As we have discussed above, UiO-66 type compounds provide a rare opportunity for tuning the Lewis/Bronsted acid properties by simply adjusting the hydration degree of the material, as depicted in Scheme 2. Accordingly, we explored the catalytic activity of hydrated and dehydrated UiO-66 containing 7% of missing linker defects for the citronellal isomerization, and the results obtained are shown in Figure 8. It is clearly observed that the catalytic activity of UiO-66 is higher for the dehydrated solid. Thus, the calculated rate constant of hydrated UiO-66 increases from 0.076 to 0.360 h\({}^{-1}\) upon dehydration, i.e., an almost 5-fold increase. This higher activity for the dehydrated material versus hydrated UiO-66 indicates that Lewis acid sites catalyze the isomerization of citronellal more efficiently than Bronsted acid centers. Meanwhile, the selectivity to isopulegol also increases for dehydrated (Lewis acid) UiO-66, from 75% to 86%. The participation of the Zr\({}^{4+}\) sites of UiO-66 in citronellal isomerization was also suggested by [PERSON] et al. by using first-principle kinetic calculations [75].