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...

Efficient Microwave Oxidation of Alcohols Using Low‐Loaded Supported Metallic Iron Nanoparticles

“…Almost complete selectivity to the targeted product (aldehydes or ketones) was found regardless of the substrate employed (Table 3). Our results with supported metallic iron nanoparticles are a clear improvement on reported protocols for the oxidation of alcohols, [13,17] providing exceedingly higher TONs as well as much reduced reaction times (from 8-24 h to a maximum of 1 h) with nearly identical conversions and selectivities to those reported using conventional heating.…”

“…[15] Alternative systems have been developed based on greener oxidants, including hydrogen peroxide and molecular oxygen, as well as heterogeneous catalysts such as Fe 2 O 3 nanoparticles, [13] Ag-NPs supported on hydrotalcites, [14] Au-NPs supported on metal oxides [16] and Pd-NPs supported on SBA-15. [17] Following our microwave-assisted approach, we report the preparation of highly active and stable supported Fe-NPs on a range of supports and their catalytic activities in the oxidation of alcohols under microwave irradiation. To our knowledge, this is the first report of directly prepared and catalytically active low-loaded supported Fe-NPs.…”

Solvent-free aerobic oxidation of hydrocarbons and alcohols with Pd@N-doped carbon from glucose

“…Compared with homogeneous catalysts, the heterogeneous catalysts, which are convenient for recycling from the reaction environment and readily used in flow reactors, are more popular, in particular in large-scale production 16 . During the past decade, great progress has also been made in heterogeneous Pd catalysts for oxidation processes, for example, Pd/hydroxyapatite 17 , Pd/mesoporous silicas 18,19 polymer-supported Pd 20,21 , carbon nanotubesupported Pd 22,23 , Pd supported on metal oxide 24 , Pdcontaining metal-organic frameworks 25 etc. In the field of oxidation, the current Pd-based catalysis was focusing mainly on the oxidations associating with dehydrogenation processes (alcohol oxidation 26,27 , Wacker oxidation 28,29 , oxidative couplings 30 , etc.).…”

Sustainable Preparation of Supported Metal Nanoparticles and Their Applications in Catalysis

“…[5,14] It also has the added advantage of inhibiting particle growth to a particular size regime as well as reducing particle aggregation. [5-7, 10, 14] Furthermore, by selecting and manipulating the textural properties of the porous support (sometimes in unison with a reduction step), it should be possible to control the size and shape of the resulting nanoparticles.…”