Magnetic Aerogels for Room-Temperature Catalytic Production of Bis(indolyl)methane Derivatives

Abstract

The potential of aerogels as catalysts for the synthesis of a relevant class of bis-heterocyclic compounds such as bis(indolyl)methanes was investigated. In particular, the studied catalyst was a nanocomposite aerogel based on nanocrystalline nickel ferrite (NiFe₂O₄) dispersed on amorphous porous silica aerogel obtained by two-step sol-gel synthesis followed by gel drying under supercritical conditions and calcination treatments. It was found that the NiFe₂O₄/SiO₂ aerogel is an active catalyst for the selected reaction, enabling high conversions at room temperature, and it proved to be active for three repeated runs. The catalytic activity can be ascribed to both the textural and acidic features of the silica matrix and of the nanocrystalline ferrite. In addition, ferrite nanocrystals provide functionality for magnetic recovery of the catalyst from the crude mixture, enabling time-effective separation from the reaction environment. Evidence of the retention of species involved in the reaction into the catalyst is also pointed out, likely due to the porosity of the aerogel together with the affinity of some species towards the silica matrix. Our work contributes to the study of aerogels as catalysts for organic reactions by demonstrating their potential as well as limitations for the room-temperature synthesis of bis(indolyl)methanes.

Keywords: aerogel; bis(indolyl)methanes; catalyst; ferrite; nanocomposite; organic synthesis.

Figure 1

Mechanism for the formation of bis(indolyl)methane 3 from indole 1 and 4-nitrobenzaldehyde 2. Red arrows suggest possible electron rearrangements.

Figure 2

Schematic of the production procedure of the aerogel catalysts: (a) sol–gel synthesis of the multicomponent gel by co-hydrolysis and co-gelation of the metal and silica precursors; (b) aerogel production by high-temperature supercritical drying of the multicomponent gel; (c) thermal treatments to promote the formation of magnetic NiFe₂O₄/SiO₂ nanocomposite aerogel catalysts.

Figure 3

Representative images of the aerogel catalyst at different stages of nanocomposite preparation: (a) optical image of a highly porous nickel-containing composite aerogel as obtained after supercritical drying; (b) corresponding SEM image and (c–f) energy-filtered images showing oxygen distribution (c), silicon distribution (d), Fe distribution (e), and nickel distribution (f). TEM images (scale bar is 100 nm) of the NiFe₂O₄/SiO₂ aerogel catalyst as obtained by calcination at 900 °C (g,h).

Figure 4

¹H NMR spectra of the reaction mixture as obtained without catalyst (red bottom curve); with the use of plain SiO₂ aerogel catalyst (green intermediate curve); and with the use of NiFeO₂-SiO₂ aerogel catalyst (top blue curve). Significant spectral ranges with corresponding attribution are included as a guide (catalyst amount: 5 mol %; run time: 1 week; solvent: CH₂Cl₂; reaction temperature: RT).

Figure 5

BIMs synthesis catalyzed by Ni-CAT aerogel catalysts (catalyst amount: 5 mol %; run time: 1 week; solvent: CH₂Cl₂; reaction temperature: RT): the composition of the resulting reaction mixture is represented as relative amounts of reactant 2 (grey bars) and product 3 (brown bars).

Figure 6

¹H-NMR spectra of pure BIM (green line), and the extracts after 1 (blue line) and 3 (red line) catalytic runs.