Zirconium-containing metal organic frameworks as solid acid catalysts for the esterification of free fatty acids: Synthesis of biodiesel and other compounds of interest

Abstract

Zr-containing metal-organic frameworks (MOFs) formed by terephthalate (UiO-66) and 2-aminoterephthalate ligands (UiO-66-NH 2 ) are active and stable catalysts for the acid catalyzed esterification of various saturated and unsaturated fatty acids with MeOH and EtOH, with activities comparable (in some cases superior) to other solid acid catalysts previously reported in literature. Besides the formation of the corresponding fatty acid alkyl esters as biodiesel compounds (FAMEs and FAEEs), esterification of biomassderived fatty acids with other alcohols catalyzed by the Zr-MOFs allows preparing other compounds of interest, such as oleyl oleate or isopropyl palmitate, with good yields under mild conditions. 2

Room temperature synthesis of biodiesel using sulfonated graphitic carbon nitride

“…After finding optimum catalyst loading of Sg-CN, it was imperative to compare our results with the reported acid catalyst. We were pleasantly surprised to see that Sg-CN completed the reaction within 4 h at room temperature which was not precedence in earlier reports (Table 2)1336373839…”

Conversion of levulinic acid into chemicals: Synthesis of biomass derived levulinate esters over Zr-containing MOFs

“…We have recently found (Cirujano et al, 2014) a similar trend in the esterification of fatty acids of increasing chain length and degree of unsaturation with EtOH and MeOH, and attributed to a higher adsorption of the fatty acid (or fatty ester product) on the surface of the solid, which caused the progressive (reversible) deactivation of the catalyst. A similar effect could also explain the observed reactivity trend in the esterification of fatty alcohols with LA.…”

“…This could be due to a cooperative action of the (basic) amino groups that are located adjacent to the (acid) Zr sites of UiO-66-NH 2 , thus leading to a bifunctional acid-base catalyst, as already proposed for cross-aldol condensation (Vermoortele et al, 2011), CO 2 cycloaddition to styrene oxide (Kim et al, 2013), or fatty acid esterification over UiO-66-NH 2 materials (Cirujano et al, 2014). This could lead to a dual activation mechanism, in which levulinic acid would adsorb onto the Zr acid sites, thus increasing the electrophilic character of the carboxylic carbon atom.…”

“…In many cases, the Lewis acid character of the MOF comes from the creation of a coordination vacancy upon thermal removal of a solvent molecule (usually H 2 O) initially bound to the metallic nodes. However, in spite of lacking coordination vacancies in their ideal crystalline structure, Zr-containing UiO-66 type MOFs (Cavka et al, 2008) also display the typical reactivity expected for a Lewis acid catalyst (Cirujano et al, 2014;Vermoortele et al, 2011;Vermoortele et al, 2012). According to recent characterization studies (Valenzano et al, 2011), the catalytic activity of these Zr-MOFs would arise from the occurrence of a (thermal) dehydroxylation of the [Zr 6 O 4 (OH) 4 ] 12+ clusters to [Zr 6 O 6 ] 12+ exposing µ 3 -vacancies, together with the systematic formation of crystalline defects associated to linker deficiencies.…”

Pd@UiO‐66‐Type MOFs Prepared by Chemical Vapor Infiltration as Shape‐Selective Hydrogenation Catalysts

“…Most probably, the well-known Lewis acidity of the [Zr 6 O 6 ] clusters of UiO-66 is responsible for the uncontrolled decomposition of the precursor. [24][25][26][27][28] Analysis of this material by ICP-OES revealed a final loading of 16.4 wt.-% Pd. Despite the relatively high metal loading, no additional diffraction lines corresponding to metallic Pd were detected in the corresponding XRD patterns (see Figure S5 in Supporting Information), which is in agreement with the small size of the individual Pd particles.…”