Aerobic oxidation of alcohols using various types of immobilized palladiumcatalyst: the synergistic role of functionalized ligands, morphology of support, and solvent in generating and stabilizing nanoparticles

Abstract

Preparation and characterization of a variety of immobilized palladium catalyst, based on either ligand functionalized amorphous or ordered mesoporous silica, is described. The resulting Pd-loaded materials act as efficient catalyst for the oxidation of a variety of alcohols using molecular oxygen and air. Our studies show that in the case of supported palladium catalyst on hybrid amorphous silica, the nature of ligand and the solvent could effectively control the generation of nanoparticles. Furthermore, we have found that nanoparticles with smaller size and higher activity were generated from the anchored palladium precursor when the aerobic oxidation of alcohols was carried out in a,a,a-trifluorotoluene (TFT) instead of toluene. On the other hand, in the case of aerobic oxidation reactions by using supported palladium catalyst on hybrid SBA-15, the combination of organic ligand and ordered mesoporous channels resulted in an interesting synergistic effect that led to enhanced activity, prevention of Pd nanoparticles agglomeration, and finally generation of a durable catalyst.

Reaction Conditions‐Dependent Formation of Catalytically Active Palladium Complexes or Palladium Nanoparticles on a Silica Support

“…Because the observed PdNPs are small and unanticipated, they could be easily overlooked if specific analysis for nanoparticles was not performed after synthesizing the catalytic material. The similar catalytic activity exhibited by the immobilized PdNPs and the immobilized Pd complex after an induction period also emphasizes the importance of further analysis to identify the potential catalytically active species, especially for fundamental studies . The observation of an induction period described in our work may be used as a way to identify the original transition metal species present on the surface of a support.…”

“…The similar catalytic activity exhibited by the immobilized PdNPs and the immobilized Pd complex after an induction period also emphasizes the importance of further analysis to identify the potential catalytically active species, especially for fundamental studies. [51] The observation of an induction period described in our work may be used as a way to identify the original transition metal species present on the surface of a support. As a final note, in addition to being important in terms of developing an understanding of catalytic mechanisms, the process leading to PdNPs provides a simple, reproducible, onepot, room temperature preparation method for the formation of monodisperse PdNPs supported on a silica surface.…”

Mechanistic Studies of Alcohol Selective Oxidation

“…Specifically, production of crotonaldehyde and cinnamaldehyde is enhanced 10-fold over that achievable using a conventional γ-alumina support (Figure 2.7), which is attributed to the greater dispersion achievable over the higher area alumina [96], and its ability to stabilize singlesite Pd 2+ catalytic centers. Amorphous silica and mesoporous SBA-15, SBA-16, and KIT-6 have also been employed to support Pd nanoparticles for alcohol selox [35,97,98]. These catalysts show high thermal stability and recyclability, and careful comparison of initial rates and evaluation of surface metal and oxide contents by XPS and CO chemisorption provided conclusive proof that surface PdO was the active site, independent of silica support architecture [35].…”

Palladium supported on hollow magnetic mesoporous spheres: a recoverable catalyst for hydrogenation and Suzuki reaction

“…The results above 40 prompted the authors to investigate the reaction of aryl bromides. HMMS-salpr-Pd also afforded a satisfactory yield for bromides ( Table 4, Entry 6-10 and [18][19][20][21][22]. Bromoarenes with either electron-with drawing substituents, such as -NO 2 and -CH 3 , or electron-donating substituents, such as -CH 3 and -NH 2 coupled read- 45 ily with arylboronic acids in excellent yields.…”