Highly Efficient Aerobic Oxidation of Alcohols Using a Recoverable Catalyst: The Role of Mesoporous Channels of SBA‐15 in Stabilizing Palladium Nanoparticles

Abstract

Dedicated to Professor Dieter Enders on the occasion of his 60th birthdayEver-increasing environmental concerns has resulted in much attention being recently directed toward the development of new protocols for the aerobic oxidation of alcohols using transition-metal catalysts.[1] Among them, palladium-based catalysts show very interesting and promising catalytic activity, and different types of palladium-based homogeneous [2] and heterogenous [3] catalysts in the form of metal complexes or nanoparticles [4] have been developed for this purpose. Accordingly, the application of palladium-based catalysts has also been well documented for the asymmetric oxidation of alcohols.[5] Although, significant progress has been achieved in improving catalytic activity, selectivity, and substrate scope, there is still the major problem that palladium agglomeration and the formation of palladium black can cause catalyst deactivation in many cases. Recently, Tsuji and co-workers have shown that novel pyridine derivatives with 2,3,4,5-tetraphenylphenyl substituents and higher dendritic units at the 3-position significantly suppress the formation of palladium black and give the highest reported turnover numbers (TON) of 1480 in the homogeneous palladium-catalyzed oxidation of alcohols in air.[2o]Very recently, we explored a new silica-based palladium(II) interphase catalyst for the aerobic oxidation of alcohols. [3g] However, this method requires high catalyst concentrations (up to 5 mol %) and it suffers from the disadvantage of a significant reduction in its reactivity after three reaction cycles. Furthermore, this catalyst did not show good catalytic activity in the aerobic oxidation of allylic alcohols. Quite recently, the use of palladium nanoparticles dispersed in an organic polymer has also been demonstrated in the aerobic oxidation of alcohols. [4a,b] However, these heterogeneous Pd systems also suffer from high catalyst loading (typically substrate/catalyst ratios are ca. 20:1) and also the organic polymers used in these systems are potentially susceptible to oxidative degradation under aerobic oxidation conditions, thus restricting catalyst recovery over a long period. Moreover, it is well known that the small particle size as well as the high surface area of nanoparticles means they are very mobile and thermodynamically susceptible to agglomeration and the formation of larger inactive particles.[6] Ordered mesoporous structures (such as MCM-41 [7] and SBA-15 [8] ) with regular channel structures and pore diameters in the range of 2 to 30 nm, their easy separation from the reaction mixtures, and their relatively high surface area, would seem to be ideal for forming a scaffold in which three-dimensional dispersions of metal nanoparticles could be supported. Furthermore, because the majority of the nanoparticles are usually formed inside the channels of ordered porous materials, the support prevents agglomeration while providing the inherent advantages of a heterogeneous catalyst such as easy recovery and product se...

Catalytic oxidation of benzyl alcohol on Au or Au–Pd nanoparticles confined in mesoporous silica

“…Hu et al [48] reported that gold nanoparticles confined in the walls of mesoporous silica catalysts were stable under the reaction condition for aerobic oxidation of alcohol. Karimi et al [8] demonstrated that the Pd/SBA-15 catalyst with organic ligands can be prepared by using Pd(OAc) 2 as a precursor and the palladium particles can be distributed inside the channels of SBA-15 to produce a durable catalyst. In our work, the grafting method provided DAuPdS sample a technique of uniformly distributing the Results were obtained for the oxidation of benzyl alcohol after 2 h, 4 h, 6 h and 8 h of reaction.…”

“…The present stringent ecological standards have forced researchers to develop new environmentally benign methods. Accordingly, a remarkable number of metal-catalyzed aerobic processes, especially based on Pd [7][8][9], Ru [10][11][12], and Cu [13,14] complexes, have been developed. In recent years, the unexpectedly high activity of Au as a lowtemperature CO oxidation catalyst [15] has initiated intensive research in the use of Au nanoparticles for the liquid-phase oxidation of alcohols [16][17][18][19][20].Gold and palladium Mesoporous SBA-15 Nanoparticles Oxidation Benzyl alcohol…”

Characteristics of Au/HMS catalysts for selective oxidation of benzyl alcohol to benzaldehyde

“…These materials are not only expensive but also have serious toxicities. Several homogenous Pd [4][5][6], Cu [7,8] or Ru [9][10][11] catalysts are able to serve for the selective oxidation of alcohols, but they require the use of organic solvents or high-pressure oxygen. The present stringent ecological standards have forced researchers to develop new environmental friendly methods.…”

A green oxidation protocol for the conversion of secondary alcohols into ketones using heterogeneous nanocrystalline titanium (IV) oxide in polyethylene glycol

“…Hydrogen peroxide was found to be an excellent oxidizing agent. The reason is that active oxygen availability in case of hydrogen peroxide is higher in comparison to other oxidants (15).…”

“…In addition to this, various heterogeneously catalyzed protocols were reported, including the use of silver nanoparticles supported on hydrotalcites (7), metallic iron nanoparticles on MCM-41 (8), and highly expensive gold and palladium nanoparticles supported on metal oxides (9Á12). However, most of the systems could be applicable for the oxidations of only activated substrates by using molecular O 2 (g) (13), or large quantities of additives such as bases and electron transfer mediators are required (14,15). Besides this, lethal V 2 O 5 and ruthenium catalyst have also been used as catalysts in the toxic solvents such as toluene (16,17).…”