Deactivation Pathways of the Catalytic Activity of Metal–Organic Frameworks in Condensation Reactions

Abstract

In the present study we have selected three different condensation reactions as model reactions, namely the hydroxylalkylation of anisole by paraformaldehyde to bis(methoxyphenyl)methane, the Pechmann condensation of phenols with ethyl acetoacetate (EAA) to coumarins and the Knoevenagel condensation of two aldehydes with three active methylene compounds to form α,β‐unsaturated esters and nitriles, using two related Fe‐containing metal–organic frameworks (MOFs), namely commercial Fe(BTC) (BTC: 1,3,5‐benzenetricarboxylate) and synthetic MIL‐100(Fe) as the catalysts. The main aim of this study was to determine the nature of the poisons, the MOF structural stability in connection with the substrate, and the variations in the product selectivity. We have found that undesired intermediates (bisarylmethyl cation in the case of hydroxyalkylation) or byproducts (benzoic acid in the case of Knoevenagel condensation) can poison the MOF by being strongly adsorbed within the MOFs and blocking the pores. In the Pechmann condensation, besides pore blocking, a low structural stability of Fe(BTC) was reflected in the collapse of the crystal structure, while using polyhydroxy aromatic compounds because of their ability to act as ligands for Fe3+, replacing trimesate ligand. MIL‐100(Fe) was considerably more robust for this reaction.

Adsorption of Azo-Dye Orange II from Aqueous Solutions Using a Metal-Organic Framework Material: Iron- Benzenetricarboxylate

“…The X-ray diffraction pattern of the Fe(BTC) material shows several sharp and clearly-resolved reflections; these reflexes in the XRD profile provide evidence of the formation of a large scale structure, indicating the highly crystalline organization of the Fe(BTC) structure as shown in Figure 1a, which is in good agreement with previously reported studies [30,34,35]. The SEM image (Figure 1b) shows that the powders were irregular particles, with sizes on the micrometer scale.…”

Direct Synthesis, Structural Features, and Enhanced Catalytic Activity of the Basolite F300-like Semiamorphous Fe-BTC Framework

“…At the same time, no significant metal leaching was detected, The reaction mixture of the first cycle only contained 0.3 ppm of Fe, which should be considered negligible. Reaction mixture of the second and third cycles had even lower iron content (0.015 and 0.042 ppm, respectively), whereas both the reaction mixture of the blank experiment and that of the Basolite F300, which has no leaching after similar reaction conditions, 11,13,16,22,24 gave an Fe concentration of 0.035 ppm. That so low metal leaching 24 contrasts to that found in the series of isostructural M-MOF-74 materials, tested under exactly the same conditions 7 .…”

Polyoxometalates confined in the mesoporous cages of metal–organic framework MIL-100(Fe): Efficient heterogeneous catalysts for esterification and acetalization reactions

“…MIL-100(Fe) exhibits a very important porosity and a significant amount of accessible Lewis acid sites upon dehydration [16]. Besides its low toxicity and environmentally-friendly character, MIL-100(Fe) as host matrix to support homogeneous catalyst possesses several unique characteristics including [16][17][18][19][20][21]: i) high thermal (up to 280 °C in air) and hydrothermal stability, ii) 3D mesoporous system allowing easy diffusion of the reactants, and iii) low cost materials. Because of these excellent properties, MIL-100(Fe) has been applied in several different fields, including gas adsorption/separation, adsorption removal of heavy metal ions, dye, and N/S-heterocyclic contaminants, thin-film modifications, and heterogeneous catalysis .…”