1.3 Applications of water stable MOFs 16
1.3.3 Heterogeneous catalysis 30
1.3.3.2 Catalysis of organic reactions 31
The use of MOFs with coordinatively unsaturated sites (CUS) has been widely explored for catalytic applications.195-199 In this case, one of the coordination sites of the metal ion is occupied by a weakly coordinated ligand, which can be removed without causing collapse of the crystalline structure. The terephthalate based Cr-MIL-101, UiO-66 and related materials are relevant examples of this type of water-stable MOFs.200-214 García and co-workers studied the influence of functional groups present in terephthalic acid on the catalytic activity of Cr-MIL‐101 in Lewis acid catalyzed reactions.215 The Cr-MIL‐101 materials were utilized as heterogeneous catalysts for epoxide ring opening by methanol, benzaldehyde acetalization by methanol and Prins coupling reaction. The observed results clearly showed the catalytic activity increased as the electron‐withdrawing ability of the substituents increased. Up to three folds of enhancement of reaction rate in the presence of the -NO2 substituent was found for some of these reactions. A similar study was carried out with functionalized UiO-66-X (X = -H, -NH2, -CH3, -OCH3, -F, -Cl, -Br, -NO2) MOFs for the cyclization of citronellal.208 While all materials were catalytically active, the rate was dramatically enhanced by the electron‐withdrawing groups on the linker (-F, -Cl, -Br,
-NO2) and UiO‐66‐NO2 was found to be the most active material. The functionalized Cr-MIL-101 MOFs were studied by Jainak et al. as heterogeneous catalysts for the diacetal formation from benzaldehyde and methanol (B-M reaction) as well as other aldehydes and alcohols. In this reaction, water is formed in the equilibrium reaction (Figure 1.19).210 The activity decreased in the order: Cr-MIL-101-NO2 > Cr-MIL-101 > Cr-MIL-101-NH2. Within different samples of un- functionalized Cr-MIL-101, the activity increased with increasing the surface area. Cr-MIL-101 has two terminal water molecules connected with its octahedral Cr(III) ions, which can be removed under high vacuum, thus creating Lewis acid sites. Cr-MIL-101 and its derivatives show extraordinary stability in water, which make them suitable for catalytic reactions involving water.
Fig. 1.19 Schematic representation of Brönsted acidity in functionalized Cr-MIL-101 MOFs for heterogeneous catalysis in the condensation reaction of aldehydes with alcohols. Reproduced with permission from ref. 210. Copyright 2014 American Chemical Society.
Water-stable Cr-MIL-101 and Al-MIL-53 MOFs anchored with Brönsted hydrogensulfate acid groups were investigated in the esterification of n-butanol with acetic acid.211 The hydrogensulphated catalyst namely, S-MIL-53(Al), showed the highest performance among the materials tested as acid catalysts in the esterification reaction and can be recycled with slight loss of activity. Control experiments confirmed that the activity of S-MIL-53(Al) was much higher than the same material treated with only one of the reactants (triflic anhydride or H2SO4). The results established that hydrogensulfate acid groups anchored on ordered structures became highly stable and consequently displayed high acidity and reactivity. Polyoxometalate (POM)-ionic- liquid-functionalized MIL-100 was developed by Liu et al. and used in biodiesel production through the esterification of oleic acid with ethanol.212 It was found that under the optimal
conditions, the conversion of oleic acid could reach 94.6%, demonstrating a great catalytic activity.
Another Keggin-type polyoxometalate (POM), [CuPW11O39]5–, a catalyst for air-based organic oxidation was encapsulated in the pores of MOF-199 (HKUST-1).213 This POM-MOF material, resulted a substantial synergistic stabilization of both the MOF and the POM and catalyzed the rapid chemo- and shape-selective aerobic oxidation of thiols to disulfides. García et al. also reported aerobic oxidation of thiols to disulfides using iron metal–organic frameworks as solid redox catalysts.214
Figure 1.20 The structures of Ce-BDC MOF (a), linear-polyethylenimine (PEI) (b), and the reaction of phosphonate-based nerve agents after hydrolysis (O-pinacolyl methylphosphonofluoridate, GD and a simulant (dimethyl 4-nitrophenyl phosphonate, DMNP) (c).
color codes: Carbon (gray), oxygen (red) and cerium (purple). Reproduced with permission from ref. 219. Copyright 2017 American Chemical Society.
In addition, Cr-MIL-101 encapsulated 12-tungstophosphoric heteropolyacid (HPW), HPW@MIL-101(Cr) was studied by Zhong et al. as a water-tolerant solid catalyst for hydrolysis.215 The catalytic activity and reusability of HPW@MIL-101 were evaluated in the liquid-phase hydrolysis of ethyl acetate. The specific activity of HPW@MIL-101 was found to be 377.2 mmol molacid−1 min−1, which is the highest observed in solid acid catalysts. The detoxification of organophosphate nerve agents using MOFs was examined by Farha et al..216-218 The fast rate of dimethyl 4-nitrophenyl phosphonate (DMNP) hydrolysis was observed for Ce-
UiO-66 compared to Zr-UiO-66 in N-ethylmorpholine buffering solution.219 Polyethylenimine (PEI), a linear polymer, was used to replace the buffer. The half-life for the degradation of O- pinacolyl methylphosphonofluoridate (known as GD) using Ce-UiO-66 was observed to be 3 min whereas Zr-UiO-66 showed ∼4 min half-life under the same conditions. DMNP was hydrolyzed at the P-O bond whereas GD was hydrolyzed at the P-F bond, which led to differences in hydrolysis rate between the simulant and agent.
The catalytic activity of Pd/MIL-101 was evaluated for the Suzuki-Miyaura coupling between 4-chloroanisole and phenylboronic acid in water.220 Here, MIL-101 acted as a support for the nanoparticles and showed high stability in water and other organic solvents. The Pd/MIL-101 catalyst gave 82% yield of 4-phenylanisole when the reaction was performed with tetrabutylammonium bromide using sodium methoxide as a base in 6 h. The catalytic activity of Pd/MIL‐101 for the Ullmann homocoupling reaction of 4‐chloroanisol in the absence of phenylboronic acid was also studied. Various aryl chlorides were examined as substrates for the Ullmann coupling reaction in air. The conversion was essentially quantitative with 100 % selectivity to the corresponding biphenyl compound at 80 °C. Cohen et al. synthesized highly crystalline Zr(IV)-based MOF containing open 2,2′-bipyridine (bpy) chelating sites.221 The resulting UiO-67-bpydc readily formed complexes with PdCl2 to produce a MOF that exhibited efficient and recyclable catalytic activity for the Suzuki-Miyaura cross-coupling reaction.
MOF-based bifunctional acid-base catalysts were designed for aldol condensation reaction.
The Hf-based MOF, Hf-MOF-808 was employed as a heterogeneous catalyst for the highly selective and efficient cross-aldol condensation of biomass-derived furanic carbonyls with acetone.222 Hf-MOF-808 could be also used in the one-pot synthesis of allylic alcohols by the sequential aldol condensation reaction to yield the α,β-unsaturated methyl ketone. In addition, proline-functionalized Zr-based UiO-67 and UiO-68 type MOF materials were also employed for the diastereoselective aldol addition.223 High yields (up to 97%) were achieved using ethanol as a solvent. Both MOFs showed reversed diastereoselectivity in aldol addition, preferring syn-aldol adduct formation for reaction of cyclohexanone with 4-nitrobenzaldehyde.