Tuning the Microenvironment of Gold Nanoparticles Encapsulated within MIL‐101(Cr) for the Selective Oxidation of Alcohols with O₂: Influence of the Amino Terephthalate Linker

Abstract

This manuscript reports a comparative study of the catalytic performance of gold nanoparticles (NPs) encapsulated within MIL‐101(Cr) with or without amino groups in the terephthalate linker. The purpose is to show how the amino groups can influence the microenvironment and catalytic stability of incorporated gold nanoparticles. The first influence of the presence of this substituent is the smaller particle size of Au NPs hosted in MIL‐101(Cr)‐NH2 (2.45±0.19 nm) compared with the parent MIL‐101(Cr)‐H (3.02±0.39 nm). Both materials are highly active to promote the aerobic alcohol oxidation and exhibit a wide substrate scope. Although both catalysts can achieve turnover numbers as high as 106 for the solvent‐free aerobic oxidation of benzyl alcohol, Au@MIL‐101(Cr)‐NH2 exhibits higher turnover frequency values (12 000 h−1) than Au@MIL‐101(Cr)‐H (6800 h−1). Au@MIL‐101(Cr)‐NH2 also exhibits higher catalytic stability, being recyclable for 20 times with coincident temporal conversion profiles, in comparison with some decay observed in the parent Au@MIL‐101(Cr)‐H. Characterization by transmission electron microscopy of the 20‐times used samples shows a very minor particle size increase in the case of Au@MIL‐101(Cr)‐NH2 (2.97±0.27 nm) in comparison with the Au@MIL‐101(Cr)‐H analog (5.32±0.72 nm). The data presented show the potential of better control of the microenvironment to improve the performance of encapsulated Au nanoparticles.

Templateless Synthesis of Ultra‐Microporous 3D Graphitic Carbon from Cyclodextrins and Their Use as Selective Catalyst for Oxygen Activation

“…Since reactions involving reactive oxygen species are typically radical chain mechanisms, the presence of quencher in this proportion should be reflected in a significant change in the temporal conversion profile after its addition. As selective quenchers, p‐ benzoquinone to inhibit superoxide (O 2 ·− ) and dimethylsulfoxide (DMSO) to intercept hydroxyl radicals (·OH) were used. The results are presented in Figure b.…”

Encapsulation of Metal Nanoparticles within Metal–Organic Frameworks for the Reduction of Nitro Compounds

“…Of special interest for the present review is the case of noble MNPs hosted inside the pores of MOFs [20]. In this case, MOFs are used as supports and can stabilize the incorporated MNPs by providing a confined space that limits the growth of the MNPs by geometrical constraints [20,38,39,40]. These materials composed by MNPs incorporated inside MOFs (MNPs@MOFs) exhibit the intrinsic catalytic properties of the guest with some control in the accessibility to the site (shape selectivity) and some possible synergy by the presence of metal ions or functional groups at the linker that can cooperate to the catalysis [20,24].…”

Porous Metal Organic Frameworks as Multifunctional Catalysts for Cyclohexane Oxidation

“…It must be reminded that encapsulation of metal NPs inside MOFs have resulted frequently in the formation of very small (< 3 nm) noble metal NPs that remain stable without growing under reaction conditions. [84]…”

“…Confinement and polarity effects together with synergistic effects of having near the noble metal NPs [83] with Lewis acid sites should be exploited to show the superiority of MOFs compared to these simpler systems. It must be reminded that encapsulation of metal NPs inside MOFs have resulted frequently in the formation of very small (<3 nm) noble metal NPs that remain stable without growing under reaction conditions [84] …”