# Zirconium-Containing Metal-Organic Frameworks (MOFs) as Catalysts for Biomass Conversion
## 6 Conversion of Lipids  
### Esterification of Free Fatty Acids (FFAs)  
Biodiesel is a mixture of methyl and ethyl esters of long chain fatty acids (FAMEs or FAEEs) obtained from oils or fats (triglycerides), with properties similar to that of diesel obtained from oil (gasoil) [1,2]. However, compared with gasoil derived from petroleum, biodiesel has the main advantages of being biodegradable and it emits much fewer SO\({}_{\kappa}\), CO, and solid particles when burnt in diesel engines. Therefore, biodiesel is considered to be less toxic than petroleum diesel. At the industrial scale, biodiesel is produced by the transesterification of tryglycerides using stoichiometric amounts of bases, such as sodium methoxide/potassium hydroxide. However, the triglycerides used as raw materials must have low water content to avoid catalyst neutralization or dilution, together with a low amount of free fatty acids to avoid saponification side reactions and the formation of emulsions, which complicate the separation of the target esters. Thus, the direct esterification of FFA with low molecular weight alcohols (methanol, ethanol) is an alternative route for preparing biodiesel compounds when raw materials with high FFA concentrations are used (such as wastes and byproducts coming from biomass processing plants or from food residues) [60]. This direct esterification of FFA with alcohols is usually carried out in the presence of an acid catalyst, such as sulfuric, phosphoric, hydrochloric, or organic sulfonic acids [61]. However, it is much preferred to develop new solid acid catalysts to allow the recovery of the catalyst, to facilitate the isolation of the target esters, and to reduce equipment corrosion associated with the use of mineral acids. As we will show throughout this section, metal-organic frameworks containing various metal cations (mainly Zr4+ or Hf4+) have shown a high potential for this reaction.  
While [PERSON] et al. [62] and [PERSON] et al. [63] described the transesterification of triglycerides or fatty esters using MOF catalysts, our group focused on the esterification of free fatty acids. We reported on the use of Zr-containing UiO-66 compounds, namely UiO-66 and UiO-66-NH\({}_{2}\), for the methanol and ethanol esterification reaction with a series of saturated fatty acids. These included saturated fatty acids with 12, 16, or 18 carbon atoms (C12 lauric acid; C16 palmitic acid; and C18 stearic acid), as well as unsaturated fatty acids with 18 carbon atoms (C18:1 oleic acid; C18:2 linoleic acid; and C18:3 \(\alpha\)-linolenic acid) [64]. The results obtained in terms of the pseudo-first order rate constant are shown in Figure 6. In all the reactions studied, full acid conversion was achieved over UiO-66 and UiO-66-NH\({}_{2}\) and the ester was the only product detected. Moreover, both MOFs were found to be stable and reusable under the experimental conditions used. As can be observed in Figure 6, the reaction rate progressively decreases with the increase in the chain length and unsaturation degree of the fatty acid, which was attributed to a stronger adsorption of the reaction substrates or products on the catalyst surface and progressive deactivation of the acid centers [64]. Nevertheless, this deactivation is fully reversible and the catalysts recovered the initial activity by simply washing with ethanol.