# 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)  
LA conversion to GVL is usually achieved by one of the following two routes [43,44]: (i) the reduction of LA to \(\gamma\)-hydroxyvaleric acid followed by intramolecular cyclization and dehydration; and (ii) the dehydration of LA to angelica lactone catalyzed by acids and the hydrogenation of the C\(-\)C bond (Scheme 6). However, both routes use expensive noble metal catalysts and relatively high pressures (40-60 bar), and the water content of the LA feedstock used must be very low (<2.5 wt%) to avoid metal leaching and loss of catalytic activity. A third promising route has then been developed to overcome these limitations, consisting of a chemoselective reduction of the keto group of LA through a Meerwein-Ponndorf-Verley (MPV) catalytic transfer hydrogenation using a secondary alcohol as sacrificial source of hydrogen [45,46]. This route avoids the use of noble metal catalysts and the use of H\({}_{2}\) as a reducing agent. Both ZrO\({}_{2}\)[47] and Ni-Raney [48] were initially evaluated for this reaction. However, severe limitations were found, including the use of very elevated amounts of catalyst and sacrificial alcohol excess, and the process only worked smoothly when a levilinate ester was used as a starting product, but not directly from LA. Later, Zr- and Hf-beta zeolites [47,49,50,51] were used as catalysts for the MPV synthesis of GVL, starting either from LA or its esters. We then wanted to evaluate the potential of Zr MOFs as catalysts for this reaction. Indeed, [PERSON] and co-workers [52] first demonstrated the use of UiO-66 as an MPV catalyst for the conversion of geraniol with furural, and later other Zr- and Hf-MOFs, such as MOF-808 [23,53,54,55,56] and DUT-67 [23], were successfully used as well. In particular, [PERSON] [56] and [PERSON] [55] showed the use of Zr- and Hf-MOFs for the synthesis of GVL from ethyl levulinate, while we explored the direct synthesis from LA through various routes using UiO-66, MOF-808, and sulfated MOF-808 as catalysts [57]. Figure 4: (a) Time–conversion plots of LA over various UiO-66 catalysts with different missing linker defects; (b) TOF\({}_{\text{tot}}\), calculated as mols of LA converted per total amount of Zr in the catalyst and per hour; and (c) TOF\({}_{\text{def}}\), calculated as mols of LA converted per amount of Zr associated with linker defects and per hour. Both TOF\({}_{\text{tot}}\) and TOF\({}_{\text{def}}\) are calculated at short reaction times (LA conversion lower than 30%). Reproduced from reference [42]. [PERSON] et al. [56] reported a maximum GVL yield of 92.7% after 2 h of reaction when ethyl levulinate (EL) was reacted with 2-propanol (IPA) in a 1:100 EL:IPA molar ratio at 200 \({}^{\circ}\)C in the presence of UiO-66. Meanwhile, MOF-808 was found to be the most active Zr-catalyst at 130 \({}^{\circ}\)C in terms of the GVL formation rate (94.4 mmol g\({}^{-1}\) min\({}^{-1}\)), which is very relevant for the discussion below. [PERSON] et al. [55] reported the use of Hf-MOF-808 for the conversion of EL to GVL with IPA (~1:52 EL:IPA molar ratio), attaining a 94% yield after 8 h of reaction at 120 \({}^{\circ}\)C.  
We explored different routes for converting LA into GVL [57]. The first route consisted of a two-step process in which LA was first esterified with \(n\)-butanol to \(n\)-butyl levulinate (\(n\)BuL), and the results are shown in Figure 5 (data points before the vertical straight line in parts a-d) over UiO-66 (a), MOF-808 (b), and two sulfated MOF-808s with different degrees of sulfation (c and d). The sulfation process used was adapted from [58], as detailed in [57]. Once the esterification was completed, the resulting \(n\)BuL was reduced to GVL through a MPV reaction with _sec_-butanol (data points after the vertical line). Note that we used a primary alcohol in the first step to accelerate the esterification reaction, while a secondary alcohol was used to facilitate the ensuing MPV reaction.