CuNi/Co composites prepared by electroless deposition: structure and catalytic activity for the oxidation of cyclohexene with oxygen

Abstract

CuNi/Co composites with various Cu contents have been prepared by electroless deposition (ELD) of Cu on Co powers and characterized with ICP-OES, N₂ adsorption/desorption, XRD, SEM, TEM and XPS. Their catalytic performance was examined for the allylic oxidation of cyclohexene with oxygen under solvent-free conditions. The phase structure of the Cu deposit in the composites is transformed from an amorphous phase to a crystalline phase with the increase in Cu content. The composition and microstructure of the Cu deposit in the composite significantly affect its catalytic activity for the oxidation of cyclohexene. CuNi-ELD/Co-5 composite with 6.0 wt% Cu content and amorphous Cu deposit showed the highest catalytic performance with a 46.3% conversion of cyclohexene and 88.1% total selectivity to 2-cyclohexene-1-ol (Cy-ol), 2-cyclohexene-1-one (Cy-one), 2-cyclohexene-1-hydroperoxide (Cy-HP) and cyclohexene oxide (Cy-oxide).

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1 Introduction

Copper can be metalized in many ways, such as precipitation, physical vapor deposition (PVD), chemical vapor deposition (CVD), electro deposition (ED), electroless deposition (ELD) and so on.^1–5 Due to its low cost, fast deposition, uniform products, and simplicity, electroless copper deposition has been widely used in electromagnetic interference (EMI) shielding, through-hole plating in printed circuit boards and copper matrix composites.^6–9 In addition, uniform and homogeneous metal catalysts can be deposited on the surface of supports with electroless deposition. Suitable catalysts for various reactions can be prepared by changing the metallic components such as Co, Ag and Ni, in the plating solution.^10–12 For example, the plate-type copper-based catalyst prepared with electroless deposition exhibits much higher catalytic performance in methanol steam reforming than the commercial copper-based catalysts.^13–15

The oxidation of cyclohexene is a very important chemical process in the chemical industry, which can produce the chemical intermediates, such as alcohols, ketones, epoxides, and acids for the synthesis of polymers, drugs, agrochemicals and surfactants.¹⁶ Both C=C double bond and allylic C–H bond in cyclohexene are active sites for its oxygenation.¹⁷ The product distribution depends on the catalysts, oxidants, and solvents used for the reaction.^18–20 Environmentally benign oxidations use heterogeneous catalyst, solvent-free and clean oxidant, which has attracted considerable attentions.²¹ Compared with other oxidant, molecular oxygen is low cost, easily accessible and environmental benign. Various heterogeneous catalysts, including supported metal complexes, metal–organic frameworks, metal-containing molecular sieves and nanocatalysts have been used in the solvent-free oxidation of cyclohexene with molecular oxygen as the oxidant.^20–23 For example, the conversion of CrMCM-41 catalyzed cyclohexene oxidation with oxygen can reach 52.2%, and the total selectivity to Cy-ol, Cy-one and Cy-HP can be as high as 96.6% under 1 atm O₂ at 343 K.²⁴ [Cu(bpy)(H₂O)₂(BF₄)₂(bpy)] MOF exhibits promising catalytic activity and ∼90% selectivity to Cy-HP in the atmospheric O₂ at 318 K.²⁵

The Cu-coated metals and metal oxide powders prepared by electroless plating, such as W/Cu, Mo/Cu, Al₂O₃/Cu, and ZrW₂O₈/Cu composites, show both the high thermal and electrical conductivity of Cu and the characteristics of the support metal materials.^26–29 However, few studies have been conducted on the Co/Cu composite. In the present work, CuNi/Co composites with various Cu contents were prepared by Cu electroless deposition on Co powders. Sodium hypophosphite instead of formaldehyde was used as the reducing agent in the preparation due to its low pH, low cost, and relative safety.^30,31 The composition and structure of CuNi/Co composites were characterized with ICP-OES, N₂ adsorption/desorption, XRD, SEM, TEM and XPS. Their catalytic activity in the oxidation of cyclohexene with molecular oxygen under solvent-free conditions was discussed.

2 Experimental

2.1 Preparation and characterization of CuNi-ELD/Co catalyst

All reagents were obtained from commercial sources and used as received. In a typical synthesis, Co powder (300 meshes, 99.5%), 0.192 g CuSO₄·5H₂O (≥99%), 0.08 g NiSO₄·6H₂O (≥98.5%), 1.8 g NaH₂PO₂·H₂O (≥99.0%), 1.4 g Na₃C₆H₅O₇·2H₂O (≥99.0%) and 80 mL distilled water were added into a 250 mL round bottom flask and the pH value was adjusted to 9 with NaOH (≥96%). The amounts of Co powder were adjusted to obtain desired ratios between Co and Cu. The reaction was allowed to continue for 2 h at 333 K under stirring. The precipitate was filtered, washed with distilled water three times and air-dried at 373 K for 3 h. The color of precipitate was changed from Co gray to Cu red of the product, CuNi-ELD/Co. The chemical composition of CuNi-ELD/Co composite including the metal content of Cu, Co and Ni was determined with ICP-OES (Thermo Fisher SCIENTIFIC ICAP 6000 SERIES). The phase structure of the catalyst was determined with X-ray diffraction (XRD) on a Bruker-AXS D8 ADVANCE with 2θ Cu Kα in the range of 30–100°. A scanning electron microscope (SEM, Hitachi S4800) was used to examine the surface morphology of the catalysts. Transmission electron microscopy (TEM) images were obtained on a JEM-2010. An X-ray photoelectron spectroscope (XPS, VG Microtech 3000 Multilab) was used to determine the electronic properties of the catalyst. The binding energy values were referenced to the C 1s peak of contaminant carbon at 284.6 ± 0.2 eV. The N₂ adsorption–desorption isotherms of the catalyst at 77 K were determined with a Micromeritics' ASAP 2020 Accelerated Surface Area and Porosimetry analyzer after it was automatically in situ degassed at 473 K.

2.2 Oxidation of cyclohexene

Cyclohexene (Aladdin 99%) and high-purity oxygen (99.999%) were used as received. In a typical reaction, certain amounts of substrate and catalyst were charged into a 50 mL stainless steel reactor with a Teflon inner liner and heated to the desired temperature in an oil bath. A certain amount of O₂ gas was then introduced into the reactor. The reaction was conducted under stirring with a magnetic stirrer. The reactor was allowed to cool to room temperature and depressurized. The catalyst was removed by filtering the mixture through a filter. The filtrate was collected and diluted with ethanol. The composition of the reaction mixture and major oxidation products including Cy-ol, Cy-one, and Cy-oxide were analyzed with gas chromatograph (GC, SHIMADZU GC-14C, column RTX-50). Product Cy-HP was analyzed by triphenylphosphine reduction.²⁵ The by-products were quantified with GC-MS. The conversion was defined as the total moles of all products divided by the moles of initial cyclohexene reactant. The selectivity was defined as the moles of a specific product divided by the total moles of products formed.^32,33 Note: the use of compressed O₂ in the presence of organic substrates requires appropriate safety precautions and must be performed in the suitable equipment.

3 Results and discussion

3.1 Characterization of CuNi-ELD/Co

The chemical compositions of the catalyst samples are listed in Table 1. Composites CuNi-ELD/Co-1–7 were prepared with a Ni/Cu mole ratio of 1 : 2.5 and various Cu/Co mole ratios ranging from 1 : 62 to 1 : 5. Composites CuNi-ELD/Co-8–11 were prepared with a Cu/Co mole ratio of 1 : 16 and various Ni/Cu ratios ranging from 1 : 1 to 1 : 5. It is clear that the chemical compositions of the catalyst prepared with Cu electroless deposition under different conditions are consistent with the theoretical values, indicating the complete reduction of Cu²⁺ ions. Ni²⁺ is used as the catalyst for the oxidation of hypophosphite to obtain continuous Cu deposition. Small portion of Ni²⁺ can be co-deposited Ni in the Cu deposits during the deposition (Scheme S1†).^31,34 The Cu deposition rate decreases with the increase of reaction time and eventually stopped when the molar ration of Ni²⁺/Cu²⁺ decrease to a limit. Thus, a certain amount of Ni²⁺ in the reaction substrate is necessary to maintain the Cu deposition rate.³⁵ A initial Ni²⁺/Cu²⁺ mole ratio of 1/2.5 can ensure the completion of the whole Cu deposition within the reaction time (Table 1 entries 2–8). The mole ratios of Ni/Cu in the produced deposit are in the range of 1/10 to 1/5 (Table 1 entries 2–8) and decreases with the decrease of Ni²⁺/Cu²⁺ mole ratio in the reaction substrate (Table 1 entries 9–12), indicating that the deposit on Co surface is a Cu–Ni alloy.^30,36 No significant change was found in the Cu/Co mole ratios. NaH₂PO₂·H₂O was used as a reducing agent and small amount of phosphorus could also be co-deposited in the deposit to form Cu–Ni–P alloy during the copper electroless deposition.^31,36 Phosphorus was not detected in all the CuNi/Co-ELD composite samples by the energy dispersive X-ray analysis (EDX), due to its low content. The presence of phosphorus in the deposits was confirmed by the XPS analysis.

Table 1 Chemical compositions and BET surface areas of CuNi-ELD/Co catalyst samples

Entry	Catalyst	nCu^2+/nCo theoretical	nCu/nCo actual	nNi^2+/nCu^2+ theoretical	nNi/nCu actual	Wt%			BET surface area (m^2 g^−1)
						Cu	Co	Ni
1	Co powder	—	—	—	—	—	—	—	0.9
2	CuNi-ELD/Co-1	1 : 62	1 : 61	1 : 2.5	1 : 5	1.7	98.0	0.3	1.8
3	CuNi-ELD/Co-2	1 : 47	1 : 49	1 : 2.5	1 : 7	2.2	97.5	0.3	1.5
4	CuNi-ELD/Co-3	1 : 31	1 : 31	1 : 2.5	1 : 6	3.3	96.2	0.5	1.6
5	CuNi-ELD/Co-4	1 : 23	1 : 24	1 : 2.5	1 : 10	4.3	95.3	0.4	1.8
6	CuNi-ELD/Co-5	1 : 16	1 : 17	1 : 2.5	1 : 7	6.0	93.2	0.8	1.5
7	CuNi-ELD/Co-6	1 : 10	1 : 9	1 : 2.5	1 : 10	10.7	88.3	1.0	1.0
8	CuNi-ELD/Co-7	1 : 5	1 : 5	1 : 2.5	1 : 7	17.5	80.2	2.3	0.9
9	CuNi-ELD/Co-8	1 : 16	1 : 14	1 : 1	1 : 11	7.0	92.4	0.6	—
10	CuNi-ELD/Co-9	1 : 16	1 : 15	1 : 2	1 : 9	6.9	92.5	0.6	—
11	CuNi-ELD/Co-10	1 : 16	1 : 14	1 : 4	1 : 17	7.3	92.3	0.4	—
12	CuNi-ELD/Co-11	1 : 16	1 : 15	1 : 5	1 : 19	6.6	93.1	0.3	—

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Fig. 1 shows the XRD patterns of CuNi-ELD/Co composite samples. The Co powder consists of a cubic phase (JCPDS, no. 15-0806) and a hexagonal phase (JCPDS, no. 05-0727). All diffraction peaks of the composites with Cu contents ranging from 1.7 wt% to 4.3 wt% (CuNi-ELD/Co-1–4) are from the Co powder, and no trace of a crystalline Cu phase is detected (Fig. 1A).

Fig. 1 XRD patterns of catalyst samples: (A) (a) Co powder; (b) CuNi-ELD/Co-1; (c) CuNi-ELD/Co-2; (d) CuNi-ELD/Co-3; (e) CuNi-ELD/Co-4; (f) CuNi-ELD/Co-5; (g) CuNi-ELD/Co-6; (h) CuNi-ELD/Co-7; (B) (a) CuNi-ELD/Co-8; (b) CuNi-ELD/Co-9; (c) CuNi-ELD/Co-5; (d) CuNi-ELD/Co-10; (e) CuNi-ELD/Co-11.

This indicates that these samples with low Cu contents may be amorphous Cu phase or the amount of Cu crystal may be blow the detection limit of XRD. To confirm this, Co powder was mixed with same amounts of commercial Cu nanoparticles (10–30 nm, 99.9%) as those in CuNi-ELD/Co-1–4 samples. The XRD patterns of the mixtures of Cu nanoparticles and Co powder showed two weak peaks at 2θ = 43.3° and 50.5° in Fig. S1,† which were ascribed to the (111) and (200) planes of the cubic Cu phase (JCPDS, no. 04-0836). This indicates that XRD can detect the Cu crystal with contents ranging from 1.7 wt% to 4.3 wt% in Co powder. It has also been reported that the XRD can identify Cu crystal in the composites with contents as low as 1.5 wt%.³⁷ Therefore, the Cu phase in the composites of CuNi-ELD/Co-1–4 is amorphous.³⁸ Further increasing the Cu content to 6.0 wt% in CuNi-ELD/Co-5 led to a weak and broad diffraction peak at 2θ = 43.3°, which was ascribed to a preferred orientation (111) plane of the cubic Cu phase (JCPDS, no. 04-0836) (Fig. 1A). This indicates that the Cu deposits in CuNi/Co-ELD-5 are composed of amorphous and crystalline phases. The peaks at 2θ = 43.3°, 50.5°, 74.1° and 89.9° of CuNi-ELD/Co-6 and CuNi-ELD/Co-7 can be assigned to the (111), (200), (220), and (311) planes of cubic Cu phase respectively, indicating the intensive formation of the high crystallized Cu phase with further increased Cu content. No copper oxide phase was detected in the deposits. These results suggest that the formation of the phase structure of Cu deposit is attributed to the content of copper, the deposition rate, and the presence of nickel. Various amount of Co powder were added to the reactor to produce different composite with various Cu contents. Large amount of Co powder results in a low relative content of Cu in the reaction substrate and large surface areas with more nucleation sites available for Cu deposition. The high Cu deposition rate leads to a rapid copper nucleation rate, which favoured the formation of the amorphous Cu phase. Whereas small amount of Co powder results in a high relative content of Cu and less nucleation sites available. The crystal growth becomes dominant, which promoted the formation of crystalline phase. However, a single metal can be rarely converted into amorphous form.³⁹ The Nickel atoms in the copper lattice can increase the defects in the copper deposit, which is necessary for the formation of amorphous Cu structure.³⁰ The XRD patterns of CuNi-ELD/Co-8–11 with different Ni²⁺/Cu²⁺ mole ratio are similar to that of CuNi-ELD/Co-5, suggesting that the initial Ni²⁺/Cu²⁺ mole ratio does not affect the structure of deposits (Fig. 1B).

SEM was used to examine the morphologies of the produced composite. The Co particles have irregular shapes with smooth surfaces (Fig. 2a). Some of the Co particles were coated with Cu nanoparticles and some were not during the copper electroless deposition (Fig. 2b), indicating the ununiform Cu distribution on the Co powders. This might be caused by the absence of the preliminary treatment for the sensitization-activation before the deposition process. The occurrence of Cu electroless deposition suggests that there are special species on the surface of Co particles acting as Cu nucleation sites. Thus, the inadequate nucleation sites may also cause the uneven Cu distribution. The composition and acting mechanism of nucleation sites are not clear.

Fig. 2 SEM images of catalyst samples: (a) Co powder; (b)–(f) CuNi-ELD/Co-5.

The amorphous Cu phase has honeycomb-like porous morphology (Fig. 2c) and the crystalline Cu phase exhibits sphere-like morphology (Fig. 2d), which is consistent with the results of XRD analysis. The size of spherical particles in the crystalline Cu phase is not uniform and they are agglomerated tightly (Fig. 2e), which is similar to those in the Cu-coated Al₂O₃ composites produced with electroless plating at the pH 11.5.²⁸ This indicates initiation of the Cu coating layer formed on the surface of Co particles. In addition, some spherical Cu crystalline particles are embedded in the amorphous Cu phase (Fig. 2f), indicating the change of Cu phase from amorphous phase to crystalline phase with the increase of Cu content. These observations indicate that the chemical composition of CuNi-ELD/Co composite is closely related to the surface morphology of Cu deposit (Table 1). N₂ adsorption–desorption was used to determine the BET surface areas of the CuNi-ELD/Co composites (data not shown here). The uncoated Co powder sample has low surface area, which can be attributed to the large particle size (300 meshes). The porous structure and high dispersion of amorphous Cu layer in CuNi-ELD/Co-1–4 composites significantly increased their surface areas. The crystalline Cu spherical particles grew and agglomerated to form coating layer and the Cu dispersion is decreased with the increase of Cu content. Therefore, the surface areas of CuNi-ELD/Co-5–7 samples are gradually decreased with the increase of Cu content.

Fig. 3 illustrates the TEM images, selected electron diffraction patterns and EDS analysis of CuNi-ELD/Co-5 catalyst sample where the amorphous phase and crystalline phase co-exist in the Cu coating layer. A ∼2 μm particle was observed and Co and Cu co-existed in this particle (Fig. 3a). The selected electron diffraction pattern of this sample is ascribed to the (111), (220) and (222) orientations of a cubic Co phase (Fig. 3a). A ∼250 nm Cu particle with a porous structure was observed in this particle (Fig. 3b). The EDX analysis of the Cu particle indicates that it is the segregation of free copper. The selected electron diffraction pattern of the Cu particle displays the blurry ring characteristics of the amorphous structure, which is consistent with the results of XRD and SEM analysis.

Fig. 3 TEM images, the selected electron diffraction patterns and EDS analysis of CuNi-ELD/Co-5 catalyst sample.

The XPS spectra of CuNi-ELD/Co catalyst samples are presented in Fig. 4. Two major Co 2p peaks at binding energies of 781.1 eV (Co 2p_3/2) and 797.1 eV (Co 2p_1/2) are found in the spectra of all samples. The Co 2p doublet separation energy is 16.0 eV. This indicates that the surface Co of the particle is mainly in oxidation state. A thin oxide film on the surface of Co powders can form in alkaline solutions, containing Co(OH)₂ and a certain amount of CoO from the dehydration of Co(OH)₂, while Co remained in the metal state in the bulk of particles.^40–42 The Cu 2p_3/2 and 2p_1/2 peaks are found at binding energies of 932.7 eV and 952.3 eV, respectively. The Cu 2p doublet separation energy is 19.6 eV. These demonstrate that the deposit is metallic Cu.^43,44 Although trace amount of Ni was detected in the deposits by EDX as discussed above, no significant change of the binding energy of metallic Cu was observed with the change of Ni amount. This might be explained that the amount of Ni in the deposits is not high enough to affect the properties of metallic Cu. The Ni 2p_3/2 and 2p_1/2 show at binding energies of 856.0 eV and 873.6 eV. In addition, a shake-up satellite peak was found at 861.5 eV. The Ni 2p doublet separation energy is 17.6 eV. These peaks can be assigned to the trivalent Ni cation in N₂O₃,^45,46 indicating that the Ni in the deposit is in oxidation state. However, more recent work argued that the Ni might present as Ni(OH)₂,^47,48 and NiOH⁺ was detected on the surface of the NiP alloy electroless plated on Al substrate.⁴⁹ A major peak at 133.3 eV is found in the P 2p spectra of all the CuNi-ELD/Co samples except CuNi-ELD/Co-7 composite, which could be assigned to the oxidized P species. This indicates the presence of phosphorus in the deposit.

Fig. 4 XPS spectra of CuNi-ELD/Co catalyst samples: (a) CuNi-ELD/Co-1; (b) CuNi-ELD/Co-2; (c) CuNi-ELD/Co-3; (d) CuNi-ELD/Co-4; (e) CuNi-ELD/Co-5; (f) CuNi-ELD/Co-6; (g) CuNi-ELD/Co-7.

The oxidized P species produced from the hydrolysis of H₂PO₂⁻ are hard to be removed, even though the sample was thoroughly washed (Scheme S1†).^{45,47,48,50–52} A minor peak at 130.0 eV is found in the P spectra of CuNi-ELD/Co-5 and CuNi-ELD/Co-6 samples, which could be assigned to the elemental P species (Scheme S1†).^{45,47,48,50,52} The XPS peak for metal Ni was also detected at 853.0 eV in CuNi-ELD/Co-5 and CuNi-ELD/Co-6.^45–47,50 These indicate the covalent interaction between Ni⁰ and P⁰ and trace amount of Cu–Ni–P alloy in CuNi-ELD/Co-5 and CuNi-ELD/Co-6 catalyst samples. The positive shift of the binding energy of Ni⁰ in the alloy can be attributed to the electron donation of Ni⁰.

3.2 Catalytic performances in the oxidation of cyclohexene

The major products of the oxidation of cyclohexene are Cy-ol, Cy-one, Cy-oxide and intermediate Cy-HP. Meanwhile, several by-products including 1,2-cyclohexanediol, hexanediol, adipic acid and cyclohexanone are also produced (Scheme 1). The possible reaction pathways of the oxidation of cyclohexene over CuNi-ELD/Co catalyst are shown in Scheme 2.^20–22 Cyclohexene subjects to a complex radical-chain reaction and cyclohexenyl peroxyl radical (Cy-OO˙) is the main chain propagator. This is evidenced by the fact that the addition of a free-radical scavenger (hydroquinone) essentially stops the reaction and profoundly affects the product distribution (Table 2 entry 10).⁵³ Cy-HP is initially formed as the key primary product, and decomposes to Cy-ol and Cy-one or to Cy-one and water (Scheme 2 step 1 and 2). Cy-HP can also be converted to Cy-ol and Cy-oxide through the epoxidation of cyclohexene, and the Cy-oxide can further react with water to produce 1,2-cyclohexanediol (Scheme 2 step 3 and 4). Small fractions of Cy-ol, Cy-one, Cy-oxide and 1,2-cyclohexanediol products can be oxidized to form deep oxygenated by-products.

Scheme 1 The oxidation of cyclohexene over CuNi-ELD/Co catalyst.

Scheme 2 The possible reaction pathways of the oxidation of cyclohexene over CuNi-ELD/Co catalyst.

Table 2 Catalytic performances in the oxidation of cyclohexene over CuNi-ELD/Co catalysts^a

Entry	Catalyst	Conversion (%)	Selectivity (%)
			Cy-ol	Cy-one	Cy-HP	Cy-oxide	Others
a Reaction conditions: cyclohexene 5 mL, molar ratio of CuNi-ELD/Co and cyclohexene 1 : 250, oxygen 2 MPa, temperature 349 K, time 6 h. Side products-1,2-cyclohexanediol, hexanediol, adipic acid and cyclohexanone were identified by comparison with standard samples (retention time in GC) and other products were detected by GC-MS.b Reactions were carried out in presence of small amount (3 wt% of cyclohexene) of hydroquinone.c The preparation method was described in ESI.
1	—	7.5	8.6	22.9	64.8	1.7	2.0
2	Co powder	17.9	9.0	23.4	58.9	2.7	6.0
3	CuNi-ELD/Co-1	23.0	9.3	25.0	57.4	3.1	5.2
4	CuNi-ELD/Co-2	33.9	9.7	26.7	51.5	4.1	8.0
5	CuNi-ELD/Co-3	39.3	10.5	29.2	45.0	4.4	10.9
6	CuNi-ELD/Co-4	35.3	10.3	28.7	48.1	3.8	9.1
7	CuNi-ELD/Co-5	46.3	9.0	28.1	46.5	4.5	11.9
8	CuNi-ELD/Co-6	26.0	5.9	20.0	62.1	4.1	7.9
9	CuNi-ELD/Co-7	23.6	8.4	24.0	59.0	3.0	5.6
10^b	CuNi-ELD/Co-5	1.3	23.6	25.2	25.1	25.7	0.4
11^c	Ni-ELD/Co-5	19.7	8.2	26.9	54.1	2.6	8.2
12^c	CuCo-ELD/Co-5	40.7	9.0	29.5	43.4	4.5	13.6
13	CuNi-ELD/Co-8	43.6	9.1	27.9	45.9	4.6	12.5
14	CuNi-ELD/Co-9	45.1	8.7	27.8	46.1	4.7	12.7
15	CuNi-ELD/Co-10	49.4	16.2	33.9	24.2	5.1	20.6
16	CuNi-ELD/Co-11	47.6	17.4	34.1	30.1	4.7	13.7
17	Al powder	17.7	6.3	20.6	66.0	3.1	4.0
18^c	CuNi-ELD/Al-5	27.2	8.3	24.2	55.2	3.7	8.6
19^c	Cu-ELD/Co-5	34.1	9.7	28.8	45.1	3.1	13.3

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The catalytic performances of the CuNi-ELD/Co catalysts were examined for the oxidation of cyclohexene with oxygen under solvent-free conditions, and the results are summarized in Table 2. The conversion of the oxidation of cyclohexene in the absence of catalyst was 7.5% (Table 2 Entry 1). The conversion was increased to 17.9% with Co powders only as catalyst (Table 2 entry 2). A conversion of 23.0% was obtained over CuNi-ELD/Co-1 with 1.7 wt% Cu content (Table 2 entry 3). The conversions varied between 30% and 40% over CuNi-ELD/Co-2–4 catalysts with the Cu contents increasing from 2.2 wt% to 4.3 wt% (Table 2 entries 4–6). CuNi-ELD/Co-5 catalyst with 6.0 wt% Cu content exhibited the highest conversion of 46.3% among all composites (Table 2 entry 7). Further increasing Cu wt% in CuNi-ELD/Co-6 and CuNi-ELD/Co-7 decreased the conversions to less than 30% (Table 2 entries 8 and 9). The variation of Cy-HP selectivity is contrary to that of cyclohexene conversion, suggesting that the decomposition of Cy-HP depends on the catalytic activity of the catalyst and plays an aforementioned dominant role in the total oxidation rate. These results indicate that the Cu deposits on Co powder substrate can significantly improve the catalytic activity. Both the Cu content and the microstructure of Cu deposits can affect its catalytic activity for the oxidation of cyclohexene. The Cu deposits in the CuNi-ELD/Co-1–5 catalyst samples are mainly composed of amorphous phase, and the active sites increase with the increase of Cu content. Thus, reaction rate and cyclohexene conversion are increased with the increase of Cu content from 1.7 wt% to 6.0 wt%. The crystallized Cu phase in CuNi-ELD/Co-6 and CuNi-ELD/Co-7 catalysts leads to lower Cu dispersion and less active sites, which reduces the reaction rate and decreases cyclohexene conversion. Furthermore, amorphous alloy possesses unique short-range order and long-range disordered atomic arrangement, which results in an isotropic and homogeneous structure and affords a uniform dispersion of active sites in an identical chemical environment and high concentration of coordinatively unsaturated sites. These are essential for the bonding and activation of the reactants and are beneficial for high catalytic activity.^54–56

As discussed above, the Cu–Ni deposits can significantly improve the catalytic activity, suggesting that the Cu–Ni alloy may be the catalytic active center. To prove it, Ni-ELD/Co-5 was prepared with a Ni/Co mole ratio of 1 : 16, which is same as that of CuNi-ELD/Co-5 composite. The conversion of cyclohexene over the Ni-ELD/Co-5 is 19.7% (Table 2 entry 11), which is close to that over Co powder, suggesting that the Ni deposits alone cannot show any catalytic activity and the Cu species in Cu–Ni alloy are the main active sites. CuCo-ELD/Co-5 with a Cu/Co mole ratio of 1 : 16 was also prepared. Co²⁺ instead of Ni²⁺ was used as the catalyst for the oxidation of hypophosphite. The conversion of cyclohexene over CuCo-ELD/Co-5 is 40.7% (Table 2 entry 12), which is slightly lower than that over CuNi-ELD/Co-5 composite. To further investigate the effects of Ni species on the catalytic activity, CuNi-ELD/Co-8–11 composites with different Ni contents were prepared and their catalytic activities in the oxidation of cyclohexene were determined. The conversion was increased slightly from 43.6% to 49.4% (Table 2 entries 13–16) with the decrease of the Ni content from 0.6 wt% to 0.3 wt% (Table 1 entries 9–12), indicating that the appropriate amount of Ni species in Cu–Ni deposits can improve catalytic activity. In all, Cu–Ni alloy in the CuNi-ELD/Co composite is the catalytic active center, where Cu species are the major catalytic active component and Ni species are a promoter.

To investigate the effect of the support metal on the catalytic activity, CuNi-ELD/Al-5 composite with a Cu/Al mole ratio of 1 : 16 was prepared and its catalytic performance in the oxidation of cyclohexene was determined. The conversion of cyclohexene over Al powder is 17.7% (Table 2 entry 17), which is close to that over Co powder. However, but CuNi-ELD/Al-5 composite exhibited lower conversion of 27.2% compared with CuNi-ELD/Co-5 composite (Table 2 entry 18). These results indicate that the Co powder with the cobalt oxide formed on the surface in alkaline solutions and Cu deposits have a synergistic effect on the oxidation of cyclohexene. It has been reported that the synergistic effect between Cu and Co oxides plays a key role in the significantly enhanced activity of the Cu–Co catalyst.^57–59 In addition, the conversion of cyclohexene over the Cu-ELD/Co-5 composite prepared with formaldehyde as the reducing agent and a Cu/Co mole ratio of 1 : 16 is lower than that over CuNi-ELD/Co-5 (Table 2 entry 19), indicating that environmental benign Cu electroless deposition with sodium hypophosphite as reducing agent is more effective.

The effect of reaction temperature and time on the catalytic activity was also examined and the results are listed Table 3. When the temperature was increased from 339 to 349 K, the conversion of cyclohexene increased from 9.8% to 46.3%, and the total selectivity to Cy-ol, Cy-one, Cy-HP and Cy-oxide decreased from 96.5% to 88.1% (Table 3 entries 1–4). This is caused by the formation of by-products such as 1,2-cyclohexanediol, hexanediol and adipic acid at high temperature. The selectivity to Cy-HP decreased from 62.5% to 46.5%, whereas the selectivity to Cy-one increased from 23.4% to 28.1% with the increase of reaction temperature, indicating that Cy-HP rapidly decomposed into Cy-ol and Cy-one at high reaction temperatures. The opposite changes of selectivity to Cy-HP and to Cy-ol and Cy-one and higher selectivity to Cy-one than to Cy-ol confirm the aforementioned reaction pathways. Suitable reaction temperature is required in the oxidations of cyclohexene over CuNi-ELD/Co catalyst. The carbonization occurs through deep oxidations at higher temperature, whereas no product is produced at lower temperature.

Table 3 Catalytic oxidation of cyclohexene with oxygen in different reaction conditions

Entry	Catalyst	T (K)	Time (h)	Conversion (%)	Selectivity (%)
					Cy-ol	Cy-one	Cy-HP	Cy-oxide	Others
a Reaction conditions: cyclohexene 5 mL, molar ratio of CuNi-ELD/Co-5 and cyclohexene 1 : 250, oxygen 2 MPa.b Reaction conditions: cyclohexene 5 mL, catalyst 50 mg, oxygen balloon, in ref. 21.c Reaction conditions: cyclohexene 19.7 mmol, catalyst 3.0 mg, oxygen 1 atm, in ref. 60.d Reaction conditions: cyclohexene 6 mmol, 10 mL toluene, catalyst 60 mg, oxygen 1 atm, in ref. 61.
1^a	CuNi-ELD/Co-5	339	6	9.8	9.1	23.4	62.5	1.5	3.5
2^a	CuNi-ELD/Co-5	344	6	12.9	9.2	24.6	59.2	2.0	5.0
3^a	CuNi-ELD/Co-5	346	6	27.6	9.1	26.7	52.6	3.8	7.8
4^a	CuNi-ELD/Co-5	349	6	46.3	9.0	28.1	46.5	4.5	11.9
5^a	CuNi-ELD/Co-5	349	5	39.2	8.6	27.3	49.4	3.9	10.8
6^a	CuNi-ELD/Co-5	349	4	14.2	8.8	25.1	57.7	2.2	6.2
7^a	CuNi-ELD/Co-5	349	2	3.2	9.6	24.7	63.3	1.4	1.0
8^b	[Co2(DOBDC) (H2O)2]·8H2O	353	20	32.8	39.3	51.2	—	—	—
9^c	MG@NSal-Co	343	12	46.8	8.7	77.2	—	5.3	8.8
10^d	MSS-SH-Au^0	373	8	61.0	4.0	63.0	—	14.0	19.0

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Reaction time also affects the conversion of cyclohexene and selectivity of the reaction. The conversion increased from 3.2% to 46.3% when the reaction time was extended from 2 h to 6 h (Table 3 entries 4–7). The selectivity to Cy-HP of a 2 h reaction was 63.3% and that of a 6 h reaction was decreased to 46.5%, suggesting that Cy-HP is the primary product at the initial stage of reaction. Meanwhile, the selectivity to Cy-one increased from 24.7% to 28.1% with the increase of reaction time from 2 h to 6 h. The free radicals accumulate as the reaction proceeds, which gradually increases the reaction rate, Cy-HP decomposition rate and the formation rates of Cy-ol and Cy-one. The total selectivity to Cy-ol, Cy-one, Cy-HP and Cy-oxide decreased linearly because of the occurrence of peroxidation as the reaction time was increased. Therefore, the oxidation was conducted at 6 h to obtain desired conversion and selectivity under optimized reaction conditions.

In full, the CuNi-ELD/Co-5 composite shows compatible catalytic activity as the catalysts previously reported for the oxidation of cyclohexene (Table 3 entries 8–10). High conversion and selectivity of the oxidation of cyclohexene can be obtained over CuNi-ELD/Co catalyst under solvent-free conditions.

4 Conclusions

In summary, CuNi-ELD/Co composites with different compositions and microstructures were prepared by electroless deposition and used for the first time as a catalyst for the oxidation of cyclohexene. The amorphous phase can be formed as a dominating phase in Cu deposits with low Cu content and possesses a honeycomb-like porous morphology. The microstructure of the Cu deposit changes from amorphous phase to cubic sphere-like crystalline phase with the increase of Cu content. Both Cu content and microstructure of the CuNi-ELD/Co composite can significantly affect its catalytic activity for the oxidation of cyclohexene. CuNi-ELD/Co-5 catalyst with 6.0 wt% Cu content and amorphous phase shows the best catalytic performance in the oxidation of cyclohexene under solvent-free conditions. Compared with the traditional catalysts, CuNi-ELD/Co composite prepared by electroless deposition is low coast and environmentally benign with compatible catalytic activity. It is a promising catalyst for the oxidation of hydrocarbon compounds.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (NSFC 21163011), the Natural Science Foundation of Inner Mongolia (2013MS0208), and the Science Research Projects of Inner Mongolia University (NJ10072). Thanks are also due to Zhanli Chai of School of Chemistry and Chemical Engineering, Inner Mongolia University, for helpful discussions.

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Footnote

† Electronic supplementary information (ESI) available: Additional catalyst preparation method and characterization results. See DOI: 10.1039/c4ra13064b

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