Base-free aqueous-phase oxidation of non-activated alcohols with molecular oxygen on soluble Pt nanoparticles

Abstract

Seven soluble metal nanoparticle catalysts including Pt, Ru, Rh, Pd, Ir, Ag and Au were synthesized and studied for the aqueous-phase selective oxidation of non-activated alcohols under atmospheric pressure of O₂. The effects of particle size were examined on the Pt catalysts with mean diameters of 1.5–4.9 nm. Pt nanoparticles efficiently catalyze the aerobic oxidation of alicyclic and aliphatic alcohols, in particular, primary aliphatic alcohols in the absence of any base. The particle sizes of the Pt catalysts strongly influence their activities, and the one of 1.5 nm exhibits much higher turnover frequencies. In comparison with the other metals examined in this work, it is concluded that Pt is the best metal of choice for the aerobic alcohol oxidation. Aliphatic primary alcohols reacts on the Pt catalysts more preferentially over their isomeric secondary alcohols with increasing their chain length or as they coexist. These steric effects, and the observed kinetic isotope effects with 1-C₄H₉OD and 1-C₄D₉OD are consistent with the general alcohol oxidation mechanism, which includes a sequence of elementary steps involving the formation of the alcoholate intermediates in quasi-equilibrated 1-C₄H₉OH dissociation on the Pt surfaces and the rate-determining hydrogen abstraction from the alcoholates. The inhibiting effects of hydroquinone, a typical radical scavenger, are indicative of the formation of radical intermediates in the H-abstraction step.

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1. Introduction

Oxidation of alcohols to carbonyl or carboxyl compounds has been extensively studied, as a result of its ubiquitous importance in production of fine and specialty chemicals.^1,2 Due to the ever-growing concerns over green chemistry and chemical processes, numerous efforts have been made to develop new catalytic protocols for the oxidation of alcohols particularly with O₂ as oxidant, in contrast to the traditional methods based on the use of stoichiometric amounts of noxious (inorganic or organic) oxidants.^2–4 Among these studies, the progress in the aerobic oxidation of alcohols in water is considerably notable.^5–9 However, very few catalysts are active for a wide range of alcohols in water, especially for non-activated alcohols, like alicyclic and aliphatic alcohols.^5a,6,7,9b Moreover, the oxidation of primary aliphatic alcohols in water frequently relies on the use of a strong base additive, such as KOH or K₂CO₃,^5a,6,7,9b which can cause the problems of corrosion and waste base disposal, etc.

Given these observed limitations, new catalysts more efficient for the green oxidation of alcohols with molecular oxygen are in need of development, for which soluble metal nanoparticle catalysts are worthy candidates to be examined. Soluble nanoparticles exhibit superior catalytic properties relative to their counterparts of traditional supported metal catalysts in hydrogenation and hydrogenolysis reactions,^10–13 such as low-temperature aqueous-phase Fisher-Tropsch synthesis on Ru nanoparticles,¹⁰ selective hydrogenation of o-chloronitrobenzene on Pt nanoparticles,¹¹ and selective hydrogenolysis of cellobiose (the simplest probe molecule for cellulose conversion) to C₆polyols on Ru nanoparticles in water.¹² Soluble nanoparticles have also been found to be efficient catalysts for other reactions, such as green synthesis of methyl formateviacarbonylation of methanol on Cu nanoparticles in the absence of base.¹⁴ These superior performances of the soluble nanoparticles most likely arise from their controllable sizes and morphologies as well as their unique accessibility to reactants.¹⁵

Recently, we have reported in a communication that a soluble Pt nanoparticle catalyst is efficient for the aerobic oxidation of a wide variety of activated and non-activated alcohols, in particular, non-activated aliphatic primary alcohols, in the absence of base.¹⁶ In this paper, we extend this preliminary study and present results on the effect of Pt nanoparticle size, the catalytic activities of other metal nanoparticles including noble metals of Ru, Rh, Ir, and Pd as well as Au and Ag, and the mechanistic examinations. These detailed studies are aimed at shedding light on the unique properties of Pt nanoparticles and the mechanism for the aerobic oxidation of nonactivated alcohols, specifically aliphatic alcohols, in water under base-free conditions.

2. Experimental

2.1 Synthesis of various metal nanoparticle catalysts

Pt, Rh, Ru and Ir nanoparticles were synthesized in a similar way reported previously in the literature.¹⁷ Here, as an example, the synthesis of Pt nanoparticles is briefly described. 150 mg of NaOH (3.75 mmol) was added into a glycol solution of H₂PtCl₆·6H₂O (150 mg, 0.2895 mmol Pt in 15 mL glycol) with vigorous stirring to obtain a transparent yellow solution. Then, the solution was heated at 160 °C for 3 h with a N₂ (purity: 99.9995%) flow for taking away H₂O and some organic products formed during the preparation process. After the solution was cooled to room temperature, 1.288 g of poly(N-vinyl-2-pyrrolidone) (PVP; AR grade, MW = 30000; 11.6 mmol) was dissolved in it with vigorous stirring. Afterwards, the solution was dialyzed overnight (through a cellulose ester dialysis membrane with a cutoff molecular-weigh of 12000) using deionized water to completely remove glycol, followed by dilution to 150 mL using deionized H₂O to form Pt nanoparticle solution. The pH value of the solution was measured to be ∼7.

For the Pt nanoparticles, six samples with varying particle sizes were synthesized by a seeded-growth method. H₂ was used to reduce H₂PtCl₆ on seeds. For example, 2 ml aqueous solution of H₂PtCl₆ (3.86 × 10⁻² mmol Pt) was added to 20 ml aqueous solution containing Pt nanoparticles of 1.5 nm in diameter (obtained following the procedure described above) as seeds (3.86 × 10⁻² mmol Pt) with stirring. Then the solution was placed into a 60-ml stainless steel autoclave, and reduced by H₂ (3.0 MPa) at 25 °C for 1 h. Subsequently, the resulting black solution containing platinum nanoparticles of 2.0 nm in diameter was diluted to 40 ml, and used for preparing larger Pt particles. For all the solutions of Pt nanoparticles of 2.1, 3.0, 3.5, 4.3 and 4.9 nm prior to alcohol oxidation reactions, additional 0.171, 0.257, 0.300, 0.321 and 0.332 g PVP were added, respectively, to keep their PVP/Pt ratios at 40/1.

For the synthesis of Pd nanoparticles, PdCl₂ was first converted to H₂PdCl₄ in hydrochloric acid solution. The H₂PdCl₄ solution (0.03 mol/L, 6.42 ml, 0.1926 mmol) and PVP (0.855 g, 7.7 mmol) were then introduced to a mixture of water (8.58 ml) and methanol (15 ml), followed by changing the pH value of this solution to 9 using methanolic NaOH solution (0.1 mol/L). Then, the solution was stirred under reflux for 3 h, and turned black. Afterwards, the black solution was dialyzed overnight using deionized water, and then was diluted to 100 mL by deionized H₂O. The pH value of the solution was measured to be ∼7.

The synthesis of Au and Ag nanoparticles was carried out in the same way. As an example, Au nanoparicles were synthesized as follows: HAuCl₄·4H₂O (49.9 mg, 0.0963 mmol) and PVP (0.427 g, 3.85 mmol) were added to 5 ml deionized water and stirred for 30 minutes in ice-water bath, to which a newly prepared aqueous solution of NaBH₄ (0.96mmol, 5 ml) was then rapidly added under vigorous stirring. Immediately, the solution turned black. The resulting solution was then dialyzed overnight in deionized water. Afterwards, the black solution was diluted to 50 ml with deionized water to form Au nanoparticle solution (1.93 × 10⁻³ mol/L).

It was mentioned that the complete reduction of the metallic salts to metals, for the synthesis of the above metal nanoparticles, was examined by UV spectroscopy before the dialysis procedure.

2.2 Characterization of metal nanoparticles

The metal nanoparticles were characterized by transmission electron microscope (TEM). The TEM micrographs were recorded on a Hitachi H-9000 high resolution transmission electron microscope (HRTEM) at 300 kV. Samples were prepared by dropping the metal nanoparticle solutions onto carbon-coated Cu grids. More than 200 particles for each sample were randomly counted to determine the particle size distributions.

2.3 Oxidation of alcohols

The typical procedure for the alcohol oxidation is described briefly. A mixture of 15 ml catalyst solution (1.93 × 10⁻³ mol/L, containing 2.895 × 10⁻² mmol metal), and alcohol (e.g. 1-butanol, 1.45 mmol) was stirred in a flask at 80 °C for 1 h under atmospheric pressure of O₂ (1 atm, O₂ balloon). After cooling to room temperature, diethyl ether was used to extract the reaction system.

For examining the effect of a radical scavenger, varying amounts of hydroquinone (Sinapharm Chemical Reagent Co., Ltd, AR grade, >99.0%) in the range of 0.145mmol and 1.45mmol were introduced to a mixture of 15 ml catalyst solution (1.93 × 10⁻³ mol/L, containing 2.895 × 10⁻² mmol Pt), and 1-butanol (1.45 mmol), and then reacted at 80 °C for 1 h under atmospheric pressure of O₂ (1 atm, O₂ balloon).

For measuring the kinetic isotope effect, deuterated 1-butanols, 1-C₄H₉OD (Aldrich Chemical Company, 98 atom% D) and 1-C₄D₉OD (Acros Organics, 99.5 atom% D), were used as reactants instead of 1-C₄H₉OH. The oxidation reactions were carried out with a mixture of 15 ml catalyst solution (1.93 × 10⁻⁴ mol/L, containing 2.895 × 10⁻³ mmol Pt), and 1-butanol (0.145 mmol) at 40 °C under atmospheric pressure of O₂ (1 atm, O₂ balloon).

The reactants and products were analyzed by a gas chromatograph (Agilent 6820) equipped with a FID detector and a HP innowax column (30 m × 0.32 mm × 0.5 μm), and by GC-MS (Agilent 6809/5973i). The metal contents in the organic phases were measured after the alcohol oxidation reactions by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, PROFILE SPEC, LEEMAN LABS). The measured results showed that the metal contents for all the reactions with the metal nanoparticle catalysts in this work were below the detection limit (<1 μg–mg/ml), showing no leaching of the metals into the reaction media.

3. Results and discussion

As stated earlier, our recent communication reported the synthesis of a soluble Pt nanoparticle catalyst with a mean diameter of 1.5 nm, viareduction by glycol and stabilization by poly(N-vinyl-2-pyrrolidone) (PVP).¹⁶ The Pt catalyst is stable and exhibits high efficiencies in the absence of base for the selective oxidation of not only activated alcohols, e.g.benzyl alcohol, 1-phenylethanol and cinnamyl alcohol, but also non-activated alicyclic alcohols and aliphatic alcohols.¹⁶

Table 1 shows the oxidation conversions and yields for representative non-activated alcohols on the Pt nanocluster (1.5 nm) catalyst in water under atmospheric pressure of O₂. Alicyclic alcohols, i.e.cyclopentanol, cyclohexanol and cyclooctanol, were oxidized to the corresponding cyclic ketones in good yields, being 97.1%, 78.4%, and 88.8%, respectively (Table 1, entries 1–3). This Pt catalyst also efficiently catalyzed the conversion of secondary aliphatic alcohols to ketones, for example, oxidation of 2-octanol gave a 94.2% yield to 2-octanone (Table 1, entry 4). For primary aliphatic alcohols, the most inactive alcohols, Pt was highly active under the identical conditions for their oxidation with O₂ in the absence of any bases (Table 1, entries 5–7). 1-butanol and 1-hexanol almost quantitatively converted to 1-butanoic acid and 1-hexanoic acid, respectively, achieving yields of as high as 99.7% and 99.3%, and 1-octanol to 1-octanoic acid with a yield of 94.5%, similar to that for the 2-octanol oxidation to 2-octanone. These results contrasted clearly with the previous findings that undesirable inorganic bases as co-catalysts are prerequisite for the aerobic oxidation of aliphatic primary alcohols.^5a,6,7,9b For example, Uozumi and coworkers reported that amphiphilic resin-supported Pt nanoparticles catalyzed oxidation of 1-octanol in the presence of equimolar K₂CO₃.⁷ These results demonstrate the efficiency of the Pt catalyst for the aqueous-phase oxidation of non-activated alcohols with O₂.

Table 1 Aerobic oxidation of various non-activated alcohols on a soluble Pt nanoparticle catalyst with a mean diameter of 1.5 nm^a

Entry	Alcohol	Product	Conversion (%)	Yield (%)
a Reaction conditions: 80 °C, 1.0 bar O2, 24 h, 0.579 mmol alcohol, 2.895 × 10^−2 mmol Pt, 15 ml water, PVP:Pt = 40:1.
1			100	97.1
2			88.4	78.4
3			99.1	88.8
4			95.2	94.2
5			100	99.7
6			100	99.3
7			96.0	94.5

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By referring to the previous studies on the alcohol oxidation by soluble metal nanoparticle catalysts in water, to our knowledge, only two kinds of catalysts have been reported to date, i.e., Pd and Au nanoparticles stabilized by microgels and polymer, respectively.^8,9 The Pd catalyst was only active for 1-phenylethanol in the absence of a base additive,^8a and essentially not for non-activated alcohols such as 2-octanol and cyclohexanol.^8b The Au catalyst was active for a wide range of alcohols but K₂CO₃ or KOH is indispensable.^9b It is thus apparent that our Pt catalyst represents the first example that soluble metal nanoparticles are capable of catalyzing the aerobic oxidation of, in particular, non-activated primary alcohols with high efficiencies in the absence of any bases.

To further understand the activity of the Pt catalyst, we examined the effects of the Pt particle size and the activity of other metal nanoparticles for comparison. Six Pt samples were synthesized, and their TEM images (Fig. 1) show that the particles are roughly spherical and possess mean diameters of 1.5, 2.1, 3.0, 3.5, 4.3, and 4.9 nm respectively, with narrow size distributions. The activity of these Pt nanoparticles depends on their size. As shown in Fig. 2, the activity normalized by total Pt atoms in the aerobic 1-butanol oxidation increased gradually from 4.1 to 8.3 h⁻¹ with decreasing the particle size from 4.9 to 2.1 nm, and then sharply increased to 24.5 h⁻¹ at the particle size of 1.5 nm. The activity normalized by exposed surface Pt atoms (estimated by using the mean diameters of the Pt particles and by assuming that they are spherical) (Fig. 2), i.e. turnover frequency (TOF), was also much higher at 1.5 nm than the TOFs at other sizes. Such size effect was found prevalently with nanoparticle catalysts for many reactions, such as low-temperature CO oxidation on Au,¹⁸ low-temperature aqueous-phase Fisher-Tropsch synthesis on Ru,¹⁰ and aerobic oxidation of alcohols on Pd.¹⁹ Such effect has been generally ascribed to the existence of more coordinately unsaturated sites on smaller particles and the changes in their electronic properties, however, the exact underlying reasons as well as the exact active sites (or structures and sizes of these particles) are still being discussed. For example, Kiely and Hutchings, et al.²⁰ very recently found that the presence of 0.5 nm bilayer Au particles is correlated to the high catalytic activity for the extensively studied CO oxidation reaction on Au/Fe₂O₃, leading to deeper insights into the Au catalysts.

Fig. 1 TEM micrographs and size distributions of Pt nanoparticles with different diameters (scale bar = 10 nm). (a) 1.5 ± 0.2 nm. (b) 2.1 ± 0.2 nm. (c) 3.0 ± 0.4 nm. (d) 3.5 ± 0.4 nm. (e) 4.3 ± 0.4 nm. (f) 4.9 ± 0.4 nm.

Fig. 2 The catalytic activity (normalized by total Pt atoms) and turnover frequency (TOF, normalized by exposed surface Pt atoms) as a function of Pt nanoparticle size. Reaction conditions: 80 °C, 1.0 bar O₂, 1 h, 1.45 mmol butanol, 2.895 × 10⁻² mmol Pt, 15 ml water, PVP:Pt = 40:1.

Table 2 shows the TOFs on Ru, Rh, Pd, Ir, Au and Ag nanoparticles for the aerobic 1-butanol oxidation. These catalysts were characterized by TEM, and possessed mean diameters of 1.1, 1.3, 2.4, 5.2, 3.2, and 4.0 nm respectively (Fig. 3). For comparison, the results of the Pt catalysts (presented in Fig. 2) were also listed in Table 2. Because of the known particle size effect, the comparison with the Pt catalysts was made for the different catalysts with similar sizes. Ru, Rh, and Ag were not active, and gave negligible or slight TOFs (0.07–0.26 h⁻¹; Table 2, entries 1–3). Ir and Pd exhibited about 2.0 and 2.4 times lower TOFs than Pt at a comparable particle size of ∼5 and 2.1 nm, respectively (Table 2, entries 4–5). As mentioned above, Au was active for the aliphatic alcohols only in the presence of strong bases, required presumably for H abstraction from the alcoholic OH groups.^9b Similarly, our Au nanoparticle catalyst (3.2 nm) is almost inactive (Table 2, entry 6), but its activity was dramatically increased upon addition of K₂CO₃ (Table 2, entry 7). In contrast, the activity of the Pt catalyst (1.5 nm) remained essentially unchanged in the presence of K₂CO₃ (Table 2, entry 9). Such a difference between the Au and Pt catalysts reflects their intrinsic difference in activating the alcoholic OH groups to form alcoholate intermediates. Taken together, these results clearly show the superiority of the Pt nanoparticles for catalyzing the oxidation of alcohols with O₂.

Table 2 Aerobic oxidation of 1-butanol on various soluble metal nanoparticle catalyst^a

Entry	Catalyst	Particle size (nm)	TOF (h^−1)
a Reaction conditions: 80 °C, 1.0 bar O2, 1 h, 1.45 mmol 1-butanol, 2.895 × 10^−2 mmol metallic catalysis, 15 ml water, PVP:Metal = 40:1. b 300 mol%K2CO3 (K2CO3:1-butanol = 3:1) was added before the reaction. c 100 mol% K2CO3 (K2CO3:1-butanol = 1:1) was added before the reaction.
1	Ru	1.1 ± 0.2	0.26
2	Rh	1.3 ± 0.2	0.07
3	Ag	4.0 ± 2.6	0.1
4	Pd	2.4 ± 0.5	6.7
5	Ir	5.2 ± 1.1	8.1
6	Au	3.2 ± 0.7	0.62
7^b			76.0
8	Pt	1.5 ± 0.2	37.1
9^c			40.0
10	Pt	2.1 ± 0.2	16.0
11	Pt	3.0 ± 0.4	16.4
12	Pt	3.5 ± 0.4	16.3
13	Pt	4.2 ± 0.4	18.2
14	Pt	4.9 ± 0.4	16.3

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Fig. 3 TEM micrographs and size distributions of metal nanoparticles. (a) Ru, 1.1 ± 0.2 nm (scale bar = 30 nm). (b) Rh, 1.3 ± 0.2 nm (scale bar = 30 nm). (c) Ag, 4.0 ± 2.6 nm (scale bar = 20 nm). (d) Pd, 2.4 ± 0.5 nm (scale bar = 30 nm). (e) Ir, 5.2 ± 1.1 nm (scale bar = 30 nm). (f) Au, 3.2 ± 0.7 nm (scale bar = 30 nm).

Concerning the reactivity of the aliphatic primary and secondary alcohols, although the primary alcohols are generally accepted as the most inactive ones, there are inconsistent results reported in the literature. Taking 1-octanol and 2-octanol as examples, Hutchings and coworkers²¹ found that 1-octanol oxidation readily proceeds on Au-Pd/TiO₂, whereas 2-octanol is completely inactive under identical conditions. However, Kaneda reported that 2-octanol is more reactive than 1-octanol with hydroxyapatite-supported Pd catalysts.⁴ In this regard, we systematically examined the oxidative reactivity of different aliphatic primary and secondary alcohols on the Pt catalyst (1.5 nm). As shown in Table 3, the TOFs were lower for 1-propanol and 1-butanol than 2-propanol and 2-butanol, respectively, whereas 1-hexanol and 1-octanol converted more rapidly than 2-hexanol and 2-octanol as these alcohols were individually used as reactants under identical conditions (Table 3, entries 1–8). Such a difference in the reactivity was noticed more clearly from the TOF ratios of the primary alcohols to secondary alcohols, which increased from 0.88 to 2.84 with increasing the carbon number of the alcohols from three (i.e. propanols) to eight (i.e. octanols). It is thus apparent that the reactivity of the primary and secondary alcohols depends on their chain length and steric hindrance; the primary alcohols with longer carbon chains tend to react more preferentially over the corresponding secondary alcohols. Such dependence was further confirmed by competitive oxidation of equimolar mixtures of the primary and secondary alcohols. As shown in Table 3 (entries 9–12), the coexistence of the primary and secondary alchohols led to more preferential reaction of the primary alcohols over the corresponding secondary alcohols, and accordingly an increase in their TOF ratios compared to the ratios obtained with the individual alcohols. This is exampled by the oxidation of equimolar mixture of 1-propanol and 2-propanol, affording a 2.36 times greater reactivity of 1-propanol than 2-propanol, in contrast to the value of 0.88 obtained with individual reactants under identical conditions. Such steric effects of the alcohols are consistent with the alcohol oxidation mechanism, which is proposed to generally involve alcoholate intermediates derived from dissociative chemisorption of the alcohol molecules on the catalyst surfaces, and their subsequent conversion to carbonyl products in the presence of O₂.²

Table 3 Turnover frequencies (TOF) for the aerobic oxidation of primary and secondary alcohols on Pt catalyst of 1.5 nm^a

Entry	Alcohol	TOF (h^−1) (1-alcohol)	TOF (h^−1) (2-alcohol)	TOF ratio (1-alcohol/2-alcohol)
a Reaction conditions: 80 °C, 1.0 bar O2, 1 h, 1.45 mmol alcohol, 2.895 × 10^−2 mmol Pt, PVP:Pt = 40:1, 15 ml water. b Equimolar mixture of 1-alcohol and 2-alcohol, i.e., 7.24 × 10^−4 mol for each alcohol.
1	1-propanol	40.7		0.88
2	2-propanol		46.2
3	1-butanol	46.4		0.90
4	2-butanol		51.3
5	1-hexanol	42.3		1.45
6	2-hexanol		29.1
7	1-octanol	32.4		2.84
8	2-octanol		11.4
9^b	1-propanol + 2-propanol	26.9	11.4	2.36
10^b	1-butanol + 2-butanol	29.3	10.8	2.71
11^b	1-hexanol + 2-hexanol	25.6	6.6	3.88
12^b	1-octanol + 2-octanol	20.4	5.5	3.71

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To probe the kinetic relevance of the elementary steps, as stated above, involving the alcoholate formation and hydride abstraction from the alcoholates, the oxidation of 1-butanols, fully deuterated (1-C₄D₉OD) and deuterated only at the hydroxyl group (1-C₄H₉OD), were examined. Kinetic relevance of the 1-butanol dissociation to form alcoholates would lead to normal kinetic isotope effects (KIE) for both 1-C₄H₉OD and 1-C₄D₉OD reactants, whereas normal KIE would be seen only for 1-C₄D₉OD rather than 1-C₄H₉OD if the reaction rate is limited by the hydride abstraction from the alcoholates. Table 4 shows the TOFs and kinetic isotope effects (KIE, denoted as k_H/k_D) measured at 40 °C on the Pt catalyst (1.5 nm) with similar conversions of ∼5%. These conditions were chosen to fully keep the reactions in the kinetic region. Undeuterated 1-butanol (1-C₄H₉OH) and 1-C₄H₉OD showed similar TOFs, affording a KIE of 0.94. In contrast, much smaller TOF was obtained with 1-C₄D₉OD relative to 1-C₄H₉OH, and the corresponding KIE was as great as 2.50. It is thus apparent from these measured KIE values that the dissociative chemisorption to form the alcoholate intermediates is quasi-equilibrated, and the hydride abstraction from the alcoholates is the kinetically relevant step during the 1-butanol oxidation on Pt.

Table 4 Kinetic isotopic effects for aerobic 1-butanol oxidation on Pt catalyst of 1.5 nm^a

Entry	Alcohol	TOF (h^−1)
a 40 °C, 1.0 bar O2, 0.145 mmol alcohol, 15 ml water, 2.895 × 10^−3 mmol Pt, PVP:Pt = 40:1, ∼5% 1-butanol conversion.
1	1-C4H9OH	1.5
2	1-C4H9OD	1.6
3	1-C4D9OD	0.6
4	k1-C4H9OH/k1-C4H9OD	0.94
5	k1-C4H9OH/k1-C4D9OD	2.50

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Further, the involvement of radical intermediates was suggested by the inhibiting effect of a radical scavenger on the TOFs in the aerobic 1-butanol oxidation on Pt. As shown in Fig. 4, upon addition of a slight amount of radical scavenger, 0.145 mmol hydroquinone (i.e. 1-butanol:hydroquinone = 10:1), the TOF decreased sharply from 37.1 to 18.9 h⁻¹, which then became negligible (3.3 h⁻¹) in the presence of an equivalent amount of the scavenger. Such a marked scavenger effect indicates that the kinetically relevant hydride abstraction from the alcoholates forms radical-type intermediates involving in the 1-butanol oxidation mechanism.

Fig. 4 Effect of radical scavenger, hydroquinone, on aerobic oxidation of 1-butanol catalyzed by soluble Pt nanoparticles (1.5 nm in diameter). Reaction conditions: 80 °C, 1.0 bar O₂, 1 h, 1.45 mmol butanol, 2.895 × 10⁻² mmol Pt, 15 ml water, PVP:Pt = 40:1.

4. Conclusions

Soluble Pt nanoparticles efficiently catalyze the selective oxidation of non-activated alcohols, in particular, primary aliphatic alcohols with atmospheric O₂ in the absence of any base. Their activities depend on their particle size in the range of 1.5–4.9 nm, which reach the highest value on the catalyst with a mean diameter of 1.5 nm. At comparable particle sizes, the Pt catalysts are more active than other metal catalysts including Ru, Rh, Pd, Ir, Ag and Au, showing the superiority of Pt for the aerobic alcohol oxidation. The reactivity of isomeric aliphatic alcohols (primary and secondary alcohols) on Pt depends on their carbon-chain length. The primary alcohols are more reactive than the secondary alcohols with increasing their chain length; when they coexist, the primary alcohols always react preferentially over the secondary alcohols. Such steric effects are consistent with the documented reaction mechanism of the aerobic alcohol oxidation involving alcoholate intermediates derived from dissociative adsorption of the alcohol molecules on the catalyst surfaces. Normal and no kinetic isotope effects were observed for 1-C₄D₉OD and 1-C₄H₉OD, respectively, which suggest the quasi-equilibrated alcoholate formation and the rate-determining hydrogen abstraction from the alcoholate intermediates on the Pt catalysts.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Nos. 20533010, 20673005, 20773005, 20825310).

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