A highly efficient Cu/La₂O₃ catalyst for transfer dehydrogenation of primary aliphatic alcohols

Abstract

La₂O₃-supported copper nanoparticles are sufficiently active for transfer dehydrogenation of primary aliphatic alcohols to aldehydes. When used for 1-octanol, the yield of 1-octanal can be as high as 63%. The catalyst is also highly effective for a wide variety of substrates, such as aromatic, aliphatic, and cyclic alcohols. The yield of the desired products approaches 100% and the turnover frequency is 86.5 h⁻¹. The synergistic effect between the basicity of La₂O₃ and the hydrogen spillover of Cu particle contributes significantly to the extremely high activity of the Cu/La₂O₃ catalyst for transfer dehydrogenation of primary aliphatic alcohols.

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Introduction

Catalytic transformation of alcohols to aldehydes or ketones is perhaps the most demanding reaction in organic and green chemistry.¹ Selective oxidation of alcohols using molecular oxygen and/or hydrogen peroxide as green oxidants has been used to replace the conventional oxidation process using stoichiometric quantities of inorganic oxidants (typically, KMnO₄ and K₂Cr₂O₇) which produce enormous amounts of toxic waste.^2,3 In this field, supported Au, Pd, Pt, and Ru nanoparticles exhibit very high activity for the oxidation of allylic and benzylic alcohols to aldehydes or ketones using molecular oxygen, but they are poorly active for the oxidation of primary aliphatic alcohols.^3–6 Only Ru-based catalysts afford desirable performance in the oxidation of primary aliphatic alcohols, with the yield of aldehydes in the range 70–90%.^6–10 However, the oxidation of alcohols using molecular oxygen often causes safety problems linked with the use of flammable organic solvents, particularly in large-scale production processes.^4–6

Transfer dehydrogenation of alcohols has been recently proposed to be a green and efficient reaction route, in which the alcohol is directly dehydrogenated to aldehyde or ketone and the readily available unsaturated organic molecule (as a hydrogen acceptor) removes the hydrogen.⁶ Unfortunately, only a few heterogeneous catalysts are known to be effective for this novel process, and they are active only for secondary and allylic/aromatic alcohols.^11–18 Hayashi and co-workers^11,12 reported that Pd/C catalyzed the dehydrogenation of allylic and aromatic alcohols with moderate yields (75–86%, 323 K, 2-4 days) using ethylene or nitrobenzene as hydrogen acceptor. Baiker and co-workers¹³ reported that transfer dehydrogenation of aromatic alcohols occurred over a Pd/Al₂O₃ catalyst using cyclohexene as hydrogen acceptor, but this catalyst was nearly inactive for aliphatic and alicyclic alcohols, and the yield of 1-octanal was only 1% in the case of 1-octanol. Ravasio and co-workers^14,15 revealed that a Cu/Al₂O₃ catalyst was highly active for the transfer dehydrogenation of allylic and alicyclic alcohols, but it was less active for 1-octanol, the yield of 1-octanal being only 4%. Over a Ru/AlO(OH) catalyst at 383 K for 24 h under argon atmosphere, 1-octanol was converted into a complex mixture containing 23% of 1-octanal.¹⁶ More recently, Kaneda and co-workers^17,18 found that Ag and Cu nanoparticles supported on a basic hydrotalcite (HT) showed excellent catalytic performance for acceptor-free dehydrogenation of benzylic and secondary aliphatic alcohols. However, they were not effective for primary aliphatic alcohols, and the conversion of 1-octanol was only 17%, although the selectivity of 1-octanal was 99% on the Ag/HT catalyst.¹⁷

Clearly, the majority of known catalysts are still poorly active for the dehydrogenation of primary aliphatic alcohols. Here we report that a Cu/La₂O₃ catalyst affords fairly high activity for the transfer dehydrogenation of primary aliphatic alcohols through a synergistic effect between the basicity of the support and the fast hydrogen spillover of Cu nanoparticles.

Experimental

Catalyst preparation

The Cu/La₂O₃ catalyst was prepared by a deposition–precipitation method. 0.55 g copper acetate monohydrate (Cu(CH₃COO)₂·H₂O) was dissolved in 200 ml water, and 1.8 g commercially available La₂O₃ powder was then dispersed into the solution with stirring at room temperature. Subsequently, 40 ml sodium carbonate aqueous solution (0.1 M) was added dropwise to the mixture to achieve a final pH value of 8. The precipitate was further aged in the mother liquid for 2 h. After being filtered and washed thoroughly with water, the resultant solid was dried at 383 K overnight and calcined at 723 K for 4 h in air, giving the CuO/La₂O₃ precursor. 200 mg CuO/La₂O₃ sample was then loaded in a fixed-bed reactor and reduced with pure hydrogen (40 ml min⁻¹) at 573 K for 1 h, yielding the Cu/La₂O₃ catalyst.

Catalyst characterization

The actual loading of copper was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a Plasma-Spec-II instrument. 20 mg CuO/La₂O₃ was dissolved in concentrated hydrochloric acid and the mixture was diluted with water to meet the detection range of the instrument.

The N₂ adsorption–desorption isotherms were performed using a Micrometrics ASAP 2010 instrument at 77 K. Before the measurement, the sample was degassed at 573 K for 3 h. The surface area was calculated from a multipoint BET analysis of the adsorption isotherm.

The X-ray powder diffraction (XRD) patterns were recorded on a D/MAX 2500/PC powder diffractometer (Rigaku) operated at 40 kV and 200 mA, using Cu-Kα (1.5418 Å) radiation. In situ XRD measurement for the reduction of the CuO/La₂O₃ sample was performed in a high-temperature chamber installed in the diffractometer. The CuO/La₂O₃ sample was pressed into a flake and mounted in the chamber. After heating to 573 K in the flow of He, pure hydrogen was introduced into the chamber and kept at 573 K for 1 h and the XRD patterns were then recorded.

Transmission electron microscopy (TEM) images were recorded on a FEI Tecnai G² Spirit microscope operated at 120 kV. The specimen was prepared by ultrasonically dispersing the sample powder in ethanol and drops of the suspension were deposited on a clean carbon-coated copper grid and dried in air.

Temperature-programmed reduction (H₂-TPR) was conducted in a U-shape quartz reactor connected to a mass spectrometry (OmniStar 200). 100 mg CuO/La₂O₃ sample was heated to 673 K at a rate of 10 K min⁻¹ under He flow (50 ml min⁻¹) and maintained at this temperature for 1 h. After being cooled to room temperature, a 20 vol.% H₂/He mixture (30 ml min⁻¹) was introduced and the sample was heated to 723 K at a rate of 10 K min⁻¹ and kept at this temperature for 1 h.

The dispersion of copper in the Cu/La₂O₃ catalyst was determined by N₂O chemisorption.¹⁹ 50 mg of the CuO/La₂O₃ sample was loaded in a U-shape quartz reactor and treated with a 20 vol.% O₂/N₂ mixture (30 ml min⁻¹) at 673 K (10 K min⁻¹) for 0.5 h. After being cooled to room temperature, a 5 vol.% H₂/N₂ mixture (30 ml min⁻¹) was introduced and the sample was heated to 673 K at a rate of 5 K min⁻¹. Then, the sample was cooled to 363 K under He flow and further purged for 30 min. Subsequently, the sample was exposed to a 10 vol.% N₂O/He mixture for 30 min at 363 K and then cooled to room temperature under a He flow. A 5 vol.% H₂/N₂ mixture (30 ml min⁻¹) was introduced and the sample was heated to 573 K at a rate of 5 K min⁻¹. The dispersion of copper and the average size of copper particles were calculated by assuming 1.46 × 10¹⁹ copper atoms per m² and a N₂O/Cu ratio of 1:2.

The IR spectra of CO adsorption were recorded on a Bruker vector-22 FTIR spectrometer. The sample powder was pressed into a self-supporting wafer (20 mg) and mounted into the IR cell. The sample was first reduced with pure H₂ (40 ml min⁻¹) at 573 K for 1 h and then evacuated at 673 K (2 × 10⁻² Pa) for 5 h. After acquisition of the background spectrum, the sample was exposed to CO with increasing pressure (5–500 Pa) at room temperature. Then the cell was evacuated and heated from room temperature to 373 K. The spectra of CO adsorption on the Cu/La₂O₃ catalyst were obtained by subtracting the background spectrum.

The temperature-programmed desorption of CO₂ (CO₂-TPD) was conducted in a U-shape quartz reactor connected to a mass spectrometry (OmniStar 200). 100 mg CuO/La₂O₃ sample was heated at a rate of 10 K min⁻¹ to 723 K under He flow (30 ml min⁻¹) and maintained at this temperature for 1 h in order to remove the surface impurities. Then the sample was reduced with pure H₂ at 573 K for 1 h. After being cooled to room temperature under He flow, the sample was exposed to a mixture of 30 vol.% CO₂/He (20 ml min⁻¹) for 0.5 h. Subsequently, the sample was purged with He (50 ml min⁻¹) for 2 h and then heated to 1073 K at a rate of 10 K min⁻¹. Desorption of CO₂ was monitored by the mass spectrometry.

The X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were performed on an ESCALAB MK-II X-ray photoelectron spectrometer (VG Scientific Ltd, UK). The CuO/La₂O₃ sample was pressed into a thin disc and mounted on a sample rod placed in a pretreatment chamber, in which the sample was reduced by pure H₂ at 573 K for 1 h. Then the sample was transferred into the analysis chamber where the spectra of Cu 2p, Cu L3VV levels were recorded. The charging effect was corrected by adjusting the binding energy of C 1 s to 284.6 eV.

Catalytic test

Transfer dehydrogenation of alcohols was conducted with a stainless steel autoclave (50 ml). Typically, the reaction mixture contained 2 mmol alcohol, 4 mmol styrene (hydrogen acceptor), 200 mg Cu/La₂O₃ catalyst and 8 ml mesitylene. The reaction was performed at 363–423 K for a certain period (0.5–24 h) under N₂ atmosphere. The yield of products was analyzed by Agilent GC-7890T (HP-5 capillary column) using decane as an internal standard. The turnover frequency (TOF) was defined as the moles of product formed per mole of Cu per hour, and the dispersion of copper was determined by N₂O chemisorption measurement.

Results and discussion

Fig. 1 shows the XRD patterns of the samples. Bare La₂O₃ showed typical diffraction peaks of hexagonal structure (JCPDS 05-0602) and the average crystallite size was 94 nm. The Cu/La₂O₃ catalyst also exhibited the diffraction peaks of hexagonal La₂O₃, but lanthanum carbonate (La₂O₂CO₃, JCPDS 37-0804) and oxyhydroxide (LaOOH, CPDS 19-0656) phases were also detected. This is caused by the interaction of lanthanum oxide with water and carbon oxide during copper loading and calcination process.²⁰ The absence of diffraction lines of Cu species indicates that they were highly dispersed on the surface of La₂O₃, or they were too small to be detected. The Cu/La₂O₃ catalyst had a surface area of 22 m² g⁻¹, and the copper loading was 8.4 wt%.

Fig. 1 XRD patterns of La₂O₃ (a) and Cu/La₂O₃ (b).

Fig. 2 shows the H₂-TPR profiles of bulk CuO and CuO/La₂O₃. The reduction of bulk CuO occurred at 539 K, whereas the reduction of CuO in the CuO/La₂O₃ sample shifted to a much lower temperature region – the main reduction took place at 432 K with a small shoulder at 415 K. The minor reaction peak was ascribed to the reduction of finely dispersed CuO species, while the major peak corresponded to the reduction of CuO species strongly interacting with La₂O₃. The H₂ consumption for the CuO/La₂O₃ sample was estimated as 1.25 mmol g⁻¹, which is very close to the stoichiometric value (1.32 mmol g⁻¹) for the total reduction of CuO species, indicating the sole presence of metallic copper in the Cu/La₂O₃ catalyst.

Fig. 2 H₂-TPR profiles of CuO (a) and CuO/La₂O₃ (b).

Fig. 3 shows the TEM images of the Cu/La₂O₃ catalyst. The La₂O₃ support was composed of spherical nanoparticles with an average size of about 30 nm. The Cu particles could not be clearly observed, probably due to the small size and the poor contrast between Cu and La₂O₃. However, the N₂O chemisorption measurement revealed that the mean size of copper particle was about 6 nm, giving a copper dispersion of about 17%.

Fig. 3 TEM images of the Cu/La₂O₃ catalyst.

Fig. 4 shows the FTIR spectra of CO absorption on the Cu/La₂O₃ catalyst. Generally, the band of CO absorption on Cu²⁺ site is above 2140 cm⁻¹; weakly bound CO on Cu⁰ shows a clear stretching frequency below 2110 cm⁻¹; while CO adsorption on Cu⁺ is located between 2110 and 2135 cm⁻¹.²¹ The Cu⁰–CO band is easily decomposed at 298–373 K, but the Cu⁺–CO band is relatively stable and can be present at 373 K.²² CO adsorption at room temperature on the Cu/La₂O₃ catalyst showed a band at 2098 cm⁻¹, indicating the presence of a Cu⁰–CO band. This is similar to the previous observation of CO adsorption on Cu⁰ at 2100 cm⁻¹ on a Cu/TiO₂ catalyst.²³ As the pressure of CO increased, the band slightly shifted to 2102 cm⁻¹ and the intensity was enhanced significantly. Upon heating, the Cu⁰–CO band weakened, and it totally disappeared at 373 K. This further confirms that the copper species are present as Cu⁰ in the Cu/La₂O₃ catalyst.

Fig. 4 FTIR spectra of CO adsorbed on the Cu/La₂O₃ catalyst. CO pressure of 5 Pa (a), 20 Pa (b), and 100 Pa (c), followed by evacuation at 323 K (d) and 373 K (e).

Fig. 5 shows the X-ray photoelectron spectra of Cu 2p and Cu L3VV Auger in the Cu/La₂O₃ catalyst. Obviously, the disappearance of the satellite peak and the binding energy of Cu 2p_3/2 at 932.0 eV shows that the copper exists as Cu⁰ or Cu⁺ (Fig. 5A). The Cu⁰ and Cu⁺ species cannot be distinguished by the binding energy of Cu 2p_3/2 because they are almost identical, but they can be easily distinguished from the Cu L3VV Auger lines, where Cu⁰ and Cu⁺ are separated by 2.0 eV.²⁴ The kinetic energy of Cu L₃VV at 918.3 eV confirmed that Cu⁰ is the main species in the Cu/La₂O₃ catalyst.²⁵ These findings are in line with the H₂-TPR and FTIR results, which showed the sole presence of metallic Cu after hydrogen reduction at 573 K.

Fig. 5 XPS spectra of Cu 2p (A) and Cu L3VV Auger (B) in the Cu/La₂O₃ catalyst.

Fig. 6 shows the CO₂-TPD profile of the Cu/La₂O₃ catalyst. Desorption of CO₂ occurred at 354, 390 and 501 K, and the relative areas were 21, 41 and 38%, respectively. This indicates that there are three types of adsorption sites for CO₂ with different basicity strengths. Previous CO₂-TPD^26,27 and FTIR^26–28 studies have readily revealed that the hydroxy groups on the surface of La₂O₃ have a weakly basic character; CO₂ adsorption on the medium strength basic sites (bidentate carbonates) gave CO₂ desorption at intermediate temperature, and CO₂ adsorption on the strong strength basic sites (unidentate carbonates) desorbed at high temperature. Therefore, the desorption peak at 354 K is ascribed to the weak basic sites (OH groups), while the desorption peak at 390 K and 501 K represents the moderately basic (La–O²⁻ pairs) and strongly basic (O²⁻ ions) sites on the Cu/La₂O₃ catalyst, respectively.

Fig. 6 CO₂-TPD profile of the Cu/La₂O₃ catalyst.

Table 1 summarizes the reaction results of transfer dehydrogenation of alcohols on the Cu/La₂O₃ catalyst. 1-Octanol, which is a representative of aliphatic primary alcohols, was readily and selectively converted into 1-octanal with a yield of about 10% even at 363 K (entry 1). The yield of 1-octanal rapidly increased to 35% at 403 K for 10 h and 63% at 423 K for 2 h (entries 2 and 3). The TOF increased rapidly with temperature, and it reached 13.7 h⁻¹ at 423 K, clearly demonstrating the high activity of the Cu/La₂O₃ catalyst. Here, the TOF is as high as 0.2 h⁻¹ at 363 K, which is almost 20 times larger than that obtained over a Cu/Al₂O₃ catalyst (TOF = 0.01 h⁻¹) at the same reaction temperature.^14,15 The TOF of 1.5 h⁻¹ at 403 K is about 10 times larger than that obtained over the most active Ru/AlO(OH) (TOF = 0.16 h⁻¹)¹⁶ and Cu/HT (TOF = 0.1 h⁻¹ for heptanol)¹⁸ catalysts. Moreover, the Cu/La₂O₃ catalyst was also highly active for the dehydrogenation of other aliphatic primary alcohols. 1-Hexanol and 1-decanol were selectively converted to 1-hexanal (yield 48%) and 1-decanal (yield 45%) at 423 K (entries 4 and 5). This extremely high activity of the Cu/La₂O₃ catalyst may rely on the active Cu particles and the basic nature of La₂O₃, which favors the effective and selective dehydrogenation of alcohols.

Table 1 Transfer dehydrogenation of alcohols over the Cu/La₂O₃ catalyst^a

Entry	Substrate	Product	T/K	t/h	Conv./%	Sel./%	TOF/h^−1
a Reaction conditions: alcohol (2 mmol), styrene (4 mmol), Cu/La2O3 (200 mg), mesitylene (8 ml), N2 atmosphere.
1	1-Octanol	1-Octanal	363	24	12	97	0.2
2	1-Octanol	1-Octanal	403	10	35	100	1.5
3	1-Octanol	1-Octanal	423	2	63	100	13.7
4	1-Hexanol	1-Hexanal	423	8	48	100	2.6
5	1-Decanol	1-Decanal	423	10	45	100	2.0
6	Benzyl alcohol	Benzaldehyde	423	3	99	100	14.3
7	1-Phenylethanol	Acetophenone	423	0.5	99.5	100	86.5
8	2-Octanol	2-Octanone	423	1	99.5	100	43.3
9	Cyclohexanol	Cyclohexanone	423	1	77	100	33.5

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This methodology can also be applied to the conversion of benzylic and secondary aliphatic alcohols. As expected, the Cu/La₂O₃ catalyst was quite active for aromatic alcohols. Benzyl alcohol and 1-phenylethanol were converted to the corresponding aldehydes and ketones quantitatively and the TOFs were much higher than those for primary aliphatic alcohols (entries 6 and 7). When used for benzyl alcohol, 99% yield of benzaldehyde was obtained within 3 h with a TOF of 14.3 h⁻¹. Previously, the reaction of benzyl alcohol using of Pd/C–ethylene system only gave a 43% yield of benzyl aldehyde in 4 days at 323 K,¹¹ and the Pd/Al₂O₃–cyclohexene system gave an 18% yield of benzyl aldehyde within 15 h at 353 K.¹³ Clearly, the reaction rate of Cu/La₂O₃ catalyst is at least 5 times larger than the previous catalysts. In the case of 1-phenylethanol, the yield of acetophenone was 99.5% and the TOF reached 86.5 h⁻¹ (entry 7). This value is also much higher than those reported for Cu catalysts with or without a hydrogen acceptor: Cu/Al₂O₃ (TOF = 8.7 h⁻¹)^14,15 and Cu/HT (TOF = 4.4 h⁻¹).¹⁸ In addition, the Cu/La₂O₃ catalyst was also highly effective for the dehydrogenation of aliphatic secondary alcohols. 2-Octanol was quantitatively converted to 2-octanone within 1 h (TOF = 43.3 h⁻¹, entry 8). However, this reaction took 15 h at 383 K with a Ru/AlO(OH) catalyst (TOF = 1.5 h⁻¹),¹⁶ 4 h at 353 K with a Cu/Al₂O₃ catalyst (TOF = 1.5 h⁻¹),^14,15 and 6 h at 403 K with a Cu/HT catalyst (TOF = 2.2 h⁻¹).¹⁸ More importantly, cyclohexanol, which is poorly reactive over noble metals (Pd and Ru) even under aerobic conditions,^5,6 was converted to cyclohexanone with 77% yield within 1 h (TOF = 33.5 h⁻¹, entry 9). In contrast, the Pd/Al₂O₃–cyclohexene system only gave 1% yield of cyclohexanone within 5 h at 353 K,¹³ and it took 12 h at 353 K over a Ru/AlO(OH) catalyst to achieve total conversion to cyclohexanone.¹⁶

It is well known that the acid–base properties of the catalyst play a central role in alcohol dehydrogenation.^27,29 In order to facilitate the dehydrogenation of alcohols, organic or inorganic bases are often added to the reaction media as co-catalysts. In particular, strong bases like KOH or NaOH or sodium alkoxides are frequently used as promoters in H-transfer reaction of alcohols for promoting the reaction rates. Meanwhile, the basic component also inhibits over-oxidation of primary alcohols to carboxylic acids under aerobic conditions, improving the selectivity. This can be alternatively achieved by using basic solid oxides, such as MgO, Mg–Al, and Cu–Mg–Al hydrotalcite.^27,30 Pillai et al.³¹ has reported that a palladium catalyst supported on hydrotalcite (a basic MgO-like material) showed comparable performance to that of Pd/MgO, whereas Pd catalysts supported on acidic materials (silica, alumina and zeolite-β) were not active for alcohol oxidation. Probably, the La₂O₃ support here acts as a strong base with a large number of basic sites,³² which favors proton abstraction from the substrate alcohol under anaerobic conditions.

However, a blank test with La₂O₃ alone only gave 1.5% conversion of 1-octanol, implying that La₂O₃ favors the dehydrogenation of alcohol but the finely dispersed Cu particles are also needed to transfer hydrogen species. A mechanistic study on a Cu/Al₂O₃ catalyst for alcohol dehydrogenation revealed that the rate-determining step is the formation of a carbocation-type intermediate by abstracting the OH group in the substrate alcohol.¹⁵ Because the primary carbocation is highly unstable and the removal of the second hydrogen atom from the α carbon normally does not occur, the primary aliphatic alcohols are almost inactive over the Cu/Al₂O₃ catalyst. Concerning the current Cu/La₂O₃ catalyst, however, the reaction might follow an alternative route. La₂O₃ provides available basic sites for alcohol adsorption and hydrogen abstraction from the substrate alcohol. It is likely that the basic sites of La₂O₃ can display nucleophilic reactivity and abstract a proton from the alcohol to form a reactive negatively charged intermediate (alkoxide). Simultaneously, the Cu particles catalyze the hydrogenation of styrene based on the fast hydrogen spillover and the facile release of hydrogen from its surface. Unlike noble metals, which tend to adsorb hydrogen strongly with the formation of stable metal hydride species,^13,17 Cu adsorbs hydrogen only weakly and thus favors a fast spillover.

Therefore, it seems reasonable that transfer dehydrogenation of alcohols occurs at the Cu–La₂O₃ boundaries, following a bifunctional process (Scheme 1). The substrate alcohol dehydrogenates to alkoxide on the basic sites of La₂O₃, and then the dissociated hydrogen atom migrates to the nearby Cu. Subsequently, the extraction of α-hydrogen from the alkoxide intermediate occurs on a Cu particle or Cu–La₂O₃ interface, forming the aldehyde or ketone. Finally, hydrogenation of styrene takes place on Cu, yielding ethylbenzene. In summary, the basic sites of La₂O₃ are mainly responsible for proton abstraction from alcohol, while the Cu particles promote the hydrogenation of styrene.

Scheme 1 A possible reaction pathway for transfer dehydrogenation of alcohols on the Cu/La₂O₃ catalyst.

Conclusions

Basic La₂O₃-supported Cu nanoparticles are highly effective for the transfer dehydrogenation of various alcohols, especially aliphatic primary alcohols. When used for 1-octanol, the yield of 1-octanal was as high as 63%. This unique performance is attributed to the synergistic effect between the basicity of La₂O₃ and the efficient spillover effect of Cu particles. The basic sites of La₂O₃ abstract protons from the substrate alcohol, while the Cu particles accelerate the extraction of α-hydrogen from the intermediate and simultaneously catalyze the hydrogenation of styrene.

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