Probing the Lewis Acidity and Catalytic Activity of the Metal–Organic Framework [Cu₃(btc)₂] (BTC=Benzene‐1,3,5‐tricarboxylate)

Abstract

An optimized procedure was designed for the preparation of the microporous metal–organic framework (MOF) [Cu3(btc)2] (BTC=benzene‐1,3,5‐tricarboxylate). The crystalline material was characterized by X‐ray diffraction, optical microscopy, SEM, X‐ray photoelectron spectroscopy, N2 sorption, thermogravimetry, and IR spectroscopy of adsorbed CO. CO adsorbs on a small number of Cu2O impurities, and particularly on the free CuII coordination sites in the framework. [Cu3(btc)2] is a highly selective Lewis acid catalyst for the isomerization of terpene derivatives, such as the rearrangement of α‐pinene oxide to campholenic aldehyde and the cyclization of citronellal to isopulegol. By using the ethylene ketal of 2‐bromopropiophenone as a test substrate, it was demonstrated that the active sites in [Cu3(btc)2] are hard Lewis acids. Catalyst stability, re‐usability, and heterogeneity are critically assessed.

MOFs as multifunctional catalysts: One-pot synthesis of menthol from citronellal over a bifunctional MIL-101 catalyst

“…Less time was required to achieve full conversion over the MIL-101 catalysts, while yielding higher selectivities to the desired isopulegol product (2): e.g., 74% selectivity for Cr 3+ -MIL-101 after 18 hr, as compared to 65-69% for [Cu 3 (BTC) 2 ] after (at least) 48 hr, depending upon the preparation procedure. 29 The performance of Pd@MIL-101 and the support Cr 3+ -MIL-101 for the isomerization of citronellal is similar to that of Ir/H-Beta and the H-Beta support, both in terms of activity and selectivity to isopulegol (2), being the Beta catalysts only slightly more active than the MIL-101…”

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

“…This high variability, along with the possibility to introduce new functionalities in a pre-formed MOF by post-synthesis modification (Wang and Cohen, 2009;Zhang et al, 2009), allow to finely tune the chemical compositions, chemical environment and pore structures of the materials and thus, their reactivity. The Lewis acid catalytic properties of MOFs have already been demonstrated for many reactions, including cyanosilylation of carbonyl compounds (Fujita et al, 1994;Henschel et al, 2008;Horike et al, 2008;Schlichte et al, 2004), epoxide methanolysis (Wee et al, 2012), isomerizations of α-pinene oxide and citronellal (Alaerts et al, 2006;Cirujano et al, 2012), Friedländer condensation (Pérez-Mayoral and Cejka, 2011), alkene cyclopropanation (Corma et al, 2010b), etc. 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.…”

Chemical instability of Cu3(BTC)2 by reaction with thiols

“…[3] Since its origin many research works have been carried out with this MOF. For instance, Cu 3 (BTC) 2 have been used widely as heterogeneous catalysts in acetalization of aldehydes with methanol, [4] quinoline synthesis, [5] -pinene oxide rearrangement, [6] trimethylsilylazide addition to carbonyl compounds, [7] oxidation of benzylic compounds with t-butylhydroperoxide, [8] ring opening of epoxides with methanol, [9] alkylation of amines with dimethyl carbonate, [10] and cycloaddition of phenylacetylene with benzyl azide among other reactions. [11] Considering that the use of MOFs as solid catalysts is a field of growing importance and the ample use of Cu 3 (BTC) 2 as solid catalysts, it is of interest to delineate the scope and conditions in which this material can be used in catalysis.…”