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Synergetic catalysis of bimetallic CuCo nanocomposites for selective hydrogenation of bio-derived esters Jun Wu, Guang Gao, Peng Sun, Xiangdong Long, and Fuwei Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02837 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017
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ACS Catalysis
Synergetic catalysis of bimetallic CuCo nanocomposites for selective hydrogenation of bio-derived esters Jun Wu†,‡, Guang Gao†, Peng Sun†, Xiangdong Long†,‡ and Fuwei Li*,† †
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China. ‡ University of Chinese Academy of Sciences, Beijing 100039, P. R. China. ABSTRACT: Bimetallic catalysts based on non-precious transition metals have attracted increasing attention because of their unique synergistic effects in the catalytic reactions, but the understanding of the nature of synergistic effects and their roles in a specific hydrogenation reaction remains lacking. Herein, a series of bimetallic CuxCoy/Al2O3 (x/y = 5:1, 2:1, 1:1, 1:2, 1:5) nanocomposite catalysts were fabricated via the successive calcination and reductive activation process of layered double hydroxides (LDHs) precursors. Their catalytic performance in the selective hydrogenation of bio-derived ethyl levulinate (EL) to 1,4pentanediol (1,4-PeD) depended sensitively on the chemical composition of bimetallic CuCo catalysts. The optimal bimetallic Cu2Co1/Al2O3 catalyst exhibited markedly improved catalytic activity and selectivity compared to monometallic Cu/Al2O3, as confirmed by its lower apparent activation energy barrier of 65.1 KJ mol-1 of the rate-determing step and its high selectivity of 93% to 1,4-PeD. Detailed characterization analyses and intrinsic catalytic studies revealed that the presence of CoOx species in the bimetallic CuxCoy/Al2O3 catalysts enhanced the metallic Cu dispersion and H2 activation ability. More importantly, the strong electronic interaction at the interface of Cu and adjacent CoOx species modified the chemical states of Cu species to create proper surface Cu0/Cu+ distributions and, particularly, provided synergic catalysis sites of Cu and electron-deficient CoOx species, which was primarily responsible for the excellent catalytic performance of bimetallic CuCo catalysts. The bimetallic CuCo catalysts exhibited good stability in both batch and fixed-bed continuous flow reactions. Furthermore, present CuCo nanocomposite catalyst could be applied to the highly selective hydrogenation of other carboxylic esters and lactones to synthesize valuable C4, C5 and C6 diols. KEYWORDS: synergic catalysis, CuCo nanocomposite, layered double hydroxide, selective hydrogenation, diol monomer
1. INTRODUCTION Lignocellulosic biomass is a sustainable and environmentally friendly organic carbon resource, the efficient and atomeconomic exploitation of which can provide various renewable biofuels and chemicals through specific catalytic processes. 1 Levulinic acid (LA)/ethyl levulinate (EL) as important biomass-derived platform chemicals could be produced from carbohydrates on a large-scale via the acid-catalyzed hydrolysis/alcoholysis valorization routes.2,3 Owing to the oxygencontaining bifunctional groups in LA/EL molecules, their catalytic conversions could afford numerous valuable chemicals including γ-valerolactone (GVL),4-7 1,4-pentanediol (1,4PeD),8,9 2-methyltetrahydrofuran (2-MTHF)10,11 and valerate esters.12-16 Especially, 1,4-PeD is a highly value-added C5 diol achieved from the selective ring-opening of GVL, which can be used as an attractive monomer for the synthesis of biodegradable polyesters as well as an intermediate for the production of fragrances and lubricants as other diols. 17-21 Moreover, from an economic and environmental perspective, EL seems to be a more appealing platform molecule for the selective synthesis of 1,4-PeD due to its energy-efficient separation and acid-free properties compared to LA.22 In contrast to the efficient production of GVL from LA/EL,4-7,23 the selective hydrogenation of EL to 1,4-PeD is more challenging and scarcely studied due to the inherent chemical stability of GVL and the further possible dehydration of 1,4-PeD to cycle ether of 2-MTHF.17,24 Previous researches in this field mainly focused on noble metals and their derived bimetallic catalysts.25-29 Typically, the monometallic Rh/SiO2
catalyst only provided GVL as the primary product in the aqueous phase hydrogenation of LA whereas bimetallic RhMoOx/SiO2 catalyst with a proper Mo/Rh atomic ratio showed a relatively high selectivity of 70% to 1,4-PeD. This remarkable change of product selectivity was attributed to the synergistic effect between the closely contacting Rh and Mo species.25 Besides, bimetallic Pt-Mo/HAP catalyst was shown effective for the selective hydrogenation of LA to 1,4-PeD.30 Despite these catalytic systems showed moderate or relatively high performance for the conversion of LA to 1,4-PeD, the seeking of earth-abundant and cheaper alternatives based on the first-row transition metals to replace the scarce and expensive precious metals would confirm to the idea of sustainability of green chemistry. More recently, Cu/ZrO2 and skeletal CuAlZn catalysts have been attempted for the hydrogenation of EL/GVL to 1,4-PeD.31,32 However, they are still facing low catalytic activity, rigorous reaction conditions and poor reusability, which significantly restrict their practical application. Therefore, the development of efficient and stable heterogeneous catalysts consisted of non-precious metals with specific catalytically active sites is still a challenge for the enhanced selective conversion of EL to 1,4-PeD. Cu-based bimetallic catalysts are believed to provide a potential solution to address this issue. They exhibited attractive catalytic performance in the hydrogenative transformation of C-O bonds, such as the hydrogenation/hydrogenolysis reactions of dimethyl oxalate/sugar polyols to glycols,19,33,34 and the catalytic conversions of CO2/syngas to methanol/higher alcohols.35-38 In general, the catalytic performance of bimetal-
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lic catalysts is closely associated with their chemical compositions which is crucial in creating unique geometric and electronic effect.39-41 Furthermore, the introduction of second metal promoters could particularly modify the surface structure of active metals to promote the formation of new catalytically active sites for the hydrogenation reactions due to their strong interaction.40,42-44 However, basic understanding on the nature of interaction between two metal species in a bimetallic catalyst and its contribution to a specific selective hydrogenation reaction remains limited. In this sense, for the selective hydrogenation of EL to 1,4-PeD, the precise design of Cu-based bimetallic catalysts that could provide synergistically catalytic active sites for the respective activation of H2, oxygenated reactant and intermediates would be favorable to deeply understand the mechanism of synergic catalysis. Inspired by our previous works and recent reports,45-48 layered double hydroxides (LDHs) are more appealing precursors to create efficient and stable Cu-based bimetallic catalysts with controlled microstructure via regulating the chemical compositions of brucite-like layers. More importantly, the characteristic of atomiclevel uniform dispersion of metal ions in the layers could maximize the interaction between different metal species and create more synergistically catalytic active sites in the resultant bimetallic catalysts. In this work, a series of structure versatile bimetallic CuCo nanocomposite catalysts derived from LDHs precursors were prepared and evaluated for the selective hydrogenation of EL to 1,4-PeD. The relationships between the catalyst structure and intrinsic catalytic activity were established and discussed based on the detailed characterizations and reaction investigations. Moreover, the reaction pathway, recyclability and substrate tolerance of catalyst were investigated. 2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The CuxCoyAl-LDH hydrotalcite precursors with different chemical compositions [(Cu2++Co2+)/Al3+ = 2:1, x/y = 1:0, 5:1, 2:1, 1:1, 1:2, 1:5, 0:1] were prepared by a one-pot co-precipitation method. For instance, a mixed aqueous solution (0.225 M) includes Cu(NO3)2, Co(NO3)2 and Al(NO3)3 and the NaOH (1.5 M) aqueous solution were dropped concurrently into Na2CO3 (0.15 M) solution at 30 oC while the pH of the suspension was kept at about 10. After aging at 65 oC for 24 h, the slurry was filtered and washed with deionized water until the solution was near neutral condition and dried at 80 oC for overnight. Then, the CuxCoyAl-LDO mixed metal oxides (x/y = 1:0, 5:1, 2:1, 1:1, 1:2, 1:5, 0:1) were obtained by calcination of the CuxCoyAl-LDH precursors at 500 oC for 4 h in air. Subsequently, the corresponding CuxCoy/Al2O3 (x/y = 1:0, 5:1, 2:1, 1:1, 1:2, 1:5, 0:1) nanocomposite catalysts were achieved after the CuxCoyAl-LDOs were reduced at 300 oC for 3 h under H2 flow. Similarly, a series of reference hydrotalcite derived nanocomposite catalysts: CuZn/Al2O3, CuMn/Al2O3, CuNi/Al2O3 [(Cu2++M2+)/Al3+ = 2:1, Cu2+: M2+ = 1:1, M = Ni, Mn, Zn] and NiCo/Al2O3 [(Ni2++Co2+)/Al3+ = 2:1, Ni2+: Co2+ = 1:1] were fabricated with the same procedures as mentioned above. In addition, the γ-Al2O3 supported bimetallic CuCo (Cu: 28 wt%, Co: 26 wt%) and different noble metal (metal loading: 5 wt%) catalysts were prepared using the incipient wetness impregnation (IM) protocol at 60 oC. After drying at 80 oC for overnight, calcined at 500 oC for 4 h in air and further reduced at 250 oC under H2 flow, the referenced catalysts were ob-
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tained and denoted as CuCo/γ-Al2O3-IM and 5% M/γ-Al2O3 (M = Pd, Pt, Rh, Ru, Ir). 2.2. Catalyst Characterization. Powder X-ray diffraction (XRD) patterns of the catalysts were obtained using an X’Pert Promultipurpose diffractometer (PANalytical, Inc.) with Nifiltered Cu Kα radiation (0.15046 nm) at room temperature from 5° to 80° (wide angle). Elemental analysis was performed over the inductively coupled plasma atomic emission spectroscopy (ICP-AES) with a Perkin-Elmer OPTIMA 3300 DV spectrometer (Norwalk, CT, U.S.A.). Transmission electron microscopy (TEM), high-angle annular dark-field scanning-transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDX) measurements were conducted on a JEM-2010 TEM with an accelerating voltage of 200 kV. The morphology of catalyst was observed with field emission-scanning electron microscopy (JEOL JSM-6701F FE-SEM). H2 temperature-programmed reduction (H2-TPR) profiles were recorded on a unit DAX-7000 instrument (Huasi Technology Co., Ltd, China). Prior to testing, the fresh oxide catalyst precursor (about 30 mg) was pretreated at 200 °C for 1 h under Ar flow, then the temperature was increased from room temperature to 900 °C (10 °C min− 1) under 5% H2/Ar flow (40 mL min− 1). In situ X-ray photoelectron spectra (XPS) and Cu LMM Auger electron spectroscopy (XAES) measurements were conducted on the ESCALAB 250xi spectrometer equipped with a Al Kα X-ray radiation source (hν = 1486.6 eV) with 20 eV pass energy. Before recording the XPS profiles, the fresh oxide precursor was reduced at 300 oC for 2 h with 5% H2/Ar (40 mL min − 1) in the pretreatment chamber. After the temperature was cooled down to 30 oC, the pretreatment chamber was evacuated to 10− 3 Pa. Finally, the fresh activated catalyst was transferred to the analytic chamber (10− 9 Pa) via the transfer chamber (10− 6 Pa) for XPS analysis without exposure to air. Binding energies of all elements were calibrated using the C1s peak at 284.6 eV as a reference. In situ Fourier-transform infrared (FT-IR) spectra of CO adsorption were recorded using a Bruker Tensor 27 spectrometer equipped with a diffuse reflectance attachment at a resolution of 4 cm-1. About 50 mg catalyst was placed into an in situ cell. Prior to each measurement, the catalyst precursor was pretreated online at 300 oC for 2 h in 5% H2/N2 flow (100 mL min− 1) and evacuated for 1 h at this temperature. After cooling down to room temperature, the background spectrum of the sample was recorded. Then, the sample was exposed to 5% CO/He flow for 30 min and evacuated for 30 min. The IR spectrum of sample was collected and referenced to the corresponding background spectrum. 2.3. Catalytic Performance. The batch selective hydrogenation reaction of EL was formed in a 100 mL stainless steel autoclave with a steering speed of 800 rpm. In a typical catalytic run, 5 mmol of tant (EL or other esters), 80 mg of catalyst and 20 mL of vent were introduced into the autoclave. Then the reactor was pressured with pure H2 to 5 MPa after fully purging out the air, and the reaction was triggered at 160 °C. After a period of reaction, the reaction solution was cooled down to room perature and centrifuged. Qualitative analysis of products was achieved by GC-MS (Agilent 5975C/7890A) with a HP-5MS column. The corresponding conversion of reactant and the selectivity of different products were analyzed based on an
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Table 1. Physicochemical Properties of CuxCoy/Al2O3 Catalysts with Different Chemical Compositions catalyst
Cua (wt %)
Coa (wt %)
SbBET (m2 g-1)
VbPore (cm3 g-1)
DbPore (nm)
dcCu (nm)
DdCu (%)
SAeCu (m2 g-1)
dfCu (nm)
Cu/Al2O3
58.8
0
50.9
0.25
35.2
6.4
12.1
46.3
8.2
Cu5Co1/Al2O3
48.6
8.7
63.4
0.41
30.5
5.3
19.8
62.5
5.1
Cu2Co1/Al2O3
36.9
16.7
82.4
0.63
28.3
4.0
29.0
69.4
3.4
CuCo/Al2O3
27.7
25.5
74.6
0.58
25.9
3.6
42.2
75.9
2.4
Cu1Co2/Al2O3
19.2
34.7
85.9
0.69
27.5
3.3
65.5
81.6
1.5
Cu1Co5/Al2O3
9.2
42
92.9
0.69
23.6
3.1
80.4
48.0
1.2
Co/Al2O3
0
54.8
120.9
1.03
23.1
-
-
-
a
Determined by ICP-AES. bDetermined by N2 physisorption. cParticle size calculated from Cu (111) diffraction peak in XRD by using Scherrer equation. dMetallic Cu dispersion estimated from N2O dissociative chemisorption. eMetallic Cu specific area obtained from N2O dissociative chemisorption. fParticle size determined by N2O dissociative chemisorption.
Figure 1. XRD patterns of (A) as-prepared CuxCoyAl-LDH hydrotalcite precursors; (B) as-calcined CuxCoyAl-LDO mixed metal oxides and (C) as-reduced CuxCoy/Al2O3 catalysts with different Cu/Co mole ratios (x/y) of: (a) 1:0; (b) 5:1; (c) 2:1; (d) 1:1; (e) 1:2; (f) 1:5; (g) 0:1.
internal standard method by gas chromatograph (Agilent GC7890A) equipped with a capillary column AT-SE-54 (25 m × 1.5 mm × 0.1 μm) and FID detector. 3. RESULTS AND DISCUSSION 3.1. Characterization of Catalysts. 3.1.1. Physicochemical Properties of Catalysts. As shown in Figure S1, the N2 adsorption-desorption isotherms and corresponding pore size distribution profiles of hydrotalcite derived CuxCoy/Al2O3 catalysts with different chemical compositions were recorded. They exhibited typical IV isotherms with H3-type hysteresis loops, ascribing to the characteristics of mesorporous structure. Besides, as the content of cobalt increasing, the BET specific surface area of bimetallic CuxCoy/Al2O3 increased gradually from 63.4 m2 g-1 to 92.9 m2 g-1, which was obviously larger than that of Cu/Al2O3 with a value of 50.9 m2 g-1 (Table 1). Moreover, the pore volumes of bimetallic CuxCoy/Al2O3 catalysts exhibited a similar trend. The above phenomena suggested the addition of an appropriate amount of Co into the monometallic Cu catalyst could modify the textural properties and produce more mesorporous structure in the bimetallic Cu xCoy/Al2O3 catalysts, which can make the reactant molecules more easily accessible to the surface active species of bimetallic catalysts. The surface area and particle dispersion of metallic copper species in the CuxCoy/Al2O3 catalysts were measured by N2O dissociative chemisorption. As shown in Table 1, the metallic copper dispersion (DCu) increased dramatically from 19.8% of Cu5Co1/Al2O3 catalyst to 80.4% for the Cu1Co5/Al2O3 sample compared to the value of 12.1% for monometallic Cu/Al 2O3 with the rising of Co ratio in bimetallic CuxCoy/Al2O3 catalysts.
Additionally, a similar variation tendency was observed in the corresponding metallic copper surface area (SACu) which enhanced gradually from 62.5 m2 g-1 for Cu5Co1/Al2O3 to the maximum of 81.6 m2 g-1 of Cu1Co2/Al2O3 and then diminished to 48.0 m2 g-1 for Cu1Co5/Al2O3. Nevertheless, it was noteworthy that both SACu and DCu for the bimetallic CuxCoy/Al2O3 catalysts with various chemical compositions were markedly higher than that of Cu/Al2O3. Besides, with the introduction of Co component, an obvious decrease of the particle size of active Cu species was found in bimetallic CuxCoy/Al2O3 catalysts, implying its enhanced nanoparticle dispersion. Furthermore, in the hydrogenation reactions dominated by Cu-based catalysts, the active Cu sites with relatively large SACu and high DCu are usually favorable to improve the catalytic performance.19 3.1.2. Structural and Morphological Characteristics of Catalysts. XRD patterns shown in Figure 1A revealed that the CuxCoyAl-LDH hydrotalcite precursors with different chemical compositions were fabricated via one-pot co-precipitation method. Typical diffraction peaks were observed in all samples at 2θ = 11.7o, 23.7o, 34.7o, 39.5o, 47.0o, 60.5o, 61.8o, which were ascribed to the characteristic reflections of (003), (006), (009), (015), (018), (110) and (113) basal planes of hydrotalcite structure, respectively.49,50 In addition, no other crystalline impurity phase was found due to the precise control of fabrication parameters during the structural topology transformation of hydrotalcite precursors. A series of hydrotalcite derived CuxCoyAl-LDO mixed metal oxides were generated after their parent CuxCoyAl-LDH precursors calcined at 500 °C for 4 h in air. As shown in
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Figure 2. TEM images of (A) as-calcined Cu2Co1Al-LDO; (B) as-reduced Cu2Co1/Al2O3 catalyst, the scale in both A and B is 50 nm; (C) HRTEM image of as-reduced Cu2Co1/Al2O3 catalyst; (D) HAADF-STEM image of as-reduced Cu2Co1/Al2O3 catalyst and the corresponding EDX elemental mappings of Cu, Co, Al and O.
Figure 1B, the reflection peaks ascribed to hydrotalcite structure completely disappeared accompanied with the appearance of characteristic diffraction peaks for metal oxides. As for monometallic CuAl-LDO catalyst precursor (Figure 1B, a), the broad and weak diffraction peak at 2θ = 35.5o with a shoulder at 38.9o was observed, indicating the presence of CuO crystalline phase (JCPDS 80-1268). In the case of bimetallic Cu5Co1Al-LDO with high copper content (Figure 1B, b), it could be found a faint reflection peak of CuO at 2θ = 35.5o and plenty of strong diffraction peaks at 2θ = 31.3o, 36.8o, 45o, 65.4o assigned to (220), (311), (400) and (440) lattice planes of Co3O4 spinel (JCPDS 78-1970), respectively. However, no obvious diffraction peak of CuO was observed in other bimetallic CuxCoyAl-LDOs, suggesting CuO species was highly dispersed in amorphous Al2O3 matrix, which might be attributed to the strong interaction between mixed metal oxides. Additionally, the intensity of reflection peaks belonging to Co3O4 spinel enhanced gradually with the increase of cobalt ratio in the CuxCoyAl-LDO precursors (Figure 1B, b-g). On the other hand, the presence of Co 3O4 spinel was further demonstrated by FT-IR spectroscopy. As depicted in Figure S2, for the Co-rich bimetallic CuxCoyAl-LDO samples, two typical characteristic IR absorption bands of Co 3O4 spinel were observed in the wave number range of 400 cm-1-1000 cm-1: One band was located at 669 cm-1 (ν1) ascribed to Co2+ species in the tetrahedral coordination environment, another one was at 566 cm-1 (ν2), which was the characteristic of octahedrally coordinated Co3+ species.51 Moreover, as the content of Cu increased in CuxCoyAl-LDO samples (Figure S2, a-f), especially for the Cu5Co1Al-LDO, the absorption band of Co3+ species (ν2) broadened and shifted gradually to a lower wavenumber, which was possibly resulted from the increased crystallinity of CuO species. This result was also consistent with above XRD patterns (Figure 1B, b). The XRD patterns of the reduced Cu xCoy/Al2O3 catalysts were displayed in Figure 1C. As for the Cu/Al 2O3 catalyst, several strong diffraction peaks at 2θ = 43.4o, 50.4o, 74.1o were assigned to (111), (200) and (220) lattice planes of metallic Cu (JCPDS 89-2838), respectively (Figure 1C, a). While for the
bimetallic CuxCoy/Al2O3 catalysts (Figure 1C, b-f), the intensity of diffraction peaks for metallic Cu decreased gradually as the Co ratio increased. This tendency indicated the increase of Cu dispersion as also confirmed by N2O chemisorption (Table 1). Besides, Co3O4 spinel species were still existed in all these samples with relatively broader and weaker diffraction peaks compared with their corresponding as-calcined CuxCoyAlLDO precursors, suggesting the Co3O4 spinel was partially reduced into CoOx species during the activation process in H2 atmosphere, which would be discussed in the following sections. The morphology of CuxCoyAl-LDH hydrotalcite precursors were characterized by FE-SEM. As shown in Figure S3, large amounts of smooth and randomly oriented hexagonal platelike nanoparticles with the diameter of 70-350 nm were clearly observed in the CuxCoy/Al-LDH samples with different chemical compositions. Additionally, the gradual introduction of cobalt component obviously decreased the particle size of CuxCoyAl-LDH precursors. Furthermore, TEM was used to reveal the microstructure of typical bimetallic Cu 2Co1/Al2O3 catalyst. As we can see from Figure 2A, numerous closedpacked nanoflakes were observed in the calcined Cu2Co1AlLDO catalyst precursor, which inherited the structure characteristics of its parent hydrotalcite. After the reductive activation in H2 atmosphere, a plenty of black metal nanoparticles were generated and well-dispersed on Al2O3 support (Figure 2B). Moreover, the high-resolution TEM (HRTEM) image of Cu2Co1/Al2O3 catalyst revealed the presence of two interlayer spacing of 0.209 nm and 0.284 nm within two adjacent nanoparticles, which were attributed to the lattice planes of Cu (111) and Co3O4 spinel species (220), respectively (Figure 2C). The close proximity of copper to spinel species in the bimetallic Cu2Co1/Al2O3 catalyst would be expected to trigger strong interaction at the interface, thus facilitating to generate new catalytically active species for the target hydrogenation reaction. Furthermore, the elemental mappings of as-calcined bimetallic Cu2Co1Al-LDO and reduced Cu2Co1/Al2O3 catalyst determined by HAADF-STEM and EDX analysis presented the uniform and homogeneous distribution of copper, cobalt,
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aluminum and oxygen elements (Figures S4 and 2D), which was originated from their close contact and the strong interaction. Table 2. The Selective Hydrogenation of EL with Various Catalysts in 1,4-Dioxanea conversion
selectivity (%)
(%)
1,4-PeD GVL Othersc
entry
catalyst
1
Cu/Al2O3
>99
52
48
2
Cu5Co1/Al2O3
>99
84
14
1
3
Cu2Co1/Al2O3
>99
93
5
1.6
4
CuCo/Al2O3
>99
89
9
1.8
5
Cu1Co2/Al2O3
>99
80
17
2.6
6
Cu1Co5/Al2O3
>99
72
26
1.2
7
Co/Al2O3
11
0
26
0
8b
Cu/Al2O3+ Co/Al2O3
>99
29
71
0
9
CuCo/γ-Al2O3IM
>99
42
57
0
10
CuZn/Al2O3
>99
24
75
0
11
CuMn/Al2O3
>99
31
69
0
12
CuNi/Al2O3
>99
24
76
0
13
NiCo/Al2O3
>99
20
80
0
14
5%Pd/γ-Al2O3
88
0
64
1.0
15
5%Pt/γ-Al2O3
>99
17
82
0
16
5%Rh/γ-Al2O3
>99
0.4
96
0
17
5%Ru/γ-Al2O3
>99
16
83
0
18
5%Ir/γ-Al2O3
77
0
95
0
19
Al2O3
0
0
0
0
0
a
Reaction conditions: EL 5 mmol, 1,4-dioxane 20 mL, catalyst 0.08 g, 160 °C, H2 5 MPa, 12 h. bPhysical mixture of monometallic Cu/Al2O3 and Co/Al2O3 catalysts and their mole ratio is in accordance with bimetallic Cu2Co1/Al2O3 catalyst. cOthers contain ethyl 4-hydroxypentanoate (4-HPE) or 2-MTHF.
3.2. The Evaluation of Catalytic Performance. As shown in Table 2, a variety of transition metal based monometallic and bimetallic catalysts fabricated by different protocols were used to evaluate their catalytic performance in the selective hydrogenation of EL to 1,4-PeD. We can clearly see that the hydrotalcite derived CuxCoy/Al2O3 catalysts with different chemical compositions presented excellent catalytic activities for the selective hydrogenation of EL, whose main products were 1,4-PeD and GVL (Table 2, entries 1-6). For the monometallic Cu/Al2O3 catalyst, a medium selectivity of 52% to 1,4-PeD was obtained accompanied with the selectivity of 48% to GVL. In the case of bimetallic Cu xCoy/Al2O3 (x/y = 5:1, 2:1, 1:1, 1:2, 1:5) catalysts under the identical reaction conditions, the primary product was 1,4-PeD and its selectivity enhanced remarkably compared with Cu/Al2O3 as deduced from their product distributions. In concrete, with the introduction of cobalt component, the selectivity of 1,4-PeD firstly increased from 84% of Cu5Co1/Al2O3 to the maximum value of 93% for the best Cu2Co1/Al2O3 catalyst, and then declined gradually to 72% of Cu1Co5/Al2O3 catalyst. Meanwhile the selectivity of GVL presented an opposite trend. Interestingly,
Figure 3. The optimization of reaction parameters for the selective hydrogenation of EL over the bimetallic Cu2Co1/Al2O3 catalyst. Reaction conditions: (A) EL 5 mmol, catalyst 0.08 g, 1,4dioxane 20 mL, H2 5 MPa, 12 h; (B) EL 5 mmol, catalyst 0.08 g, 1,4-dioxane 20 mL, 160 °C, 12 h; (C) EL 5 mmol, 1,4-dioxane 20 mL, 160 °C, H2 5 MPa, 12 h; (D) EL 5 mmol, catalyst 0.08 g, 160 °C, H2 5 MPa, 12 h. Others contain 4-HPE and 2-MTHF.
the Co/Al2O3 catalyst (entry 7) showed an extremely low catalytic activity with a conversion of 11% for EL hydrogenation and no 1,4-PeD produced, indicating the sole cobalt species could not provide the catalytically active sites for the selective synthesis of 1,4-PeD. The above reaction results seem to reveal that there is some interaction existed between copper and cobalt species in the bimetallic CuxCuy/Al2O3 catalysts with various chemical compositions, which is responsible for their high catalytic performance. To further demonstrate this hypothesis, a control experiment consisted of the physical mixture of monometallic Cu/Al2O3 and Co/Al2O3 catalysts was conducted where a low selectivity of 29% to 1,4-PeD was obtained (entry 8). Besides, other Cu-based catalysts CuM/Al2O3 (M = Zn, Mn, Ni) with Cu/M mole ratio of 1:1 as well as NiCo/Al2O3 catalyst derived from hydrotalcite precursors presented poor selectivity to 1,4-PeD (entries 10-13). Furthermore, as expected, the bimetallic CuCo/γ-Al2O3-IM catalyst prepared by incipient wetness impregnation method only gave a medium selectivity of 42% of 1,4-PeD (entry 9). On the other hand, the single Al2O3 support provided no any catalytic activity (entry 19), indicating the indispensable roles of Cu and CoOx species in the conversion of EL to 1,4-PeD. Moreover, Al2O3 supported noble metal (Pd, Pt, Rh, Ru and Ir) catalysts displayed inferior catalytic performance for the selective synthesis of 1,4-PeD, their primary product was GVL (entries 14-18). These contrast experiments further proved that the direct and specific interaction between proximate Cu and CoOx species of hydrotalcite derived bimetallic CuxCoy/Al2O3 catalysts was essential to enhance the catalytic activity and selectivity, thus producing a high yield of desired 1,4-PeD. To examine the reaction pathways of catalytic conversion of EL to 1,4-PeD, various reaction parameters such as temperature, H2 pressure, catalyst loading and solvent effect were systematically evaluated over the optimized bimetallic Cu2Co1/Al2O3 catalyst. As shown in Figure 3A, at a low temperature of 120 oC, the main product was GVL with a selectivity of 82% whereas the selectivity of target product was only 11%, although EL was converted completely. Besides, about 6.6% selectivity of ethyl 4-hydroxypentanoate (4-HPE)
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Scheme 1. Tandem Reaction Pathways for Catalytic Conversion of EL to 1,4-PeD.
Figure 4. (A) Time-course plots of the selective hydrogenation of EL in 1.4-dioxane over the bimetallic Cu2Co1/Al2O3 catalyst. Reaction conditions: EL 5 mmol, catalyst 0.08 g, 1,4-dioxane 20 mL, 160 °C, H2 5 MPa. Others contain 4-HPE and 2-MTHF; (B, C) -Ln(1-C) vs reaction time plots of the rate-determing step over Cu/Al2O3 and Cu2Co1/Al2O3 catalysts at 140 oC-160 oC, respectively. C refers to conversion; (D) Arrhenius plots of the ratedeterming step over Cu/Al2O3 and Cu2Co1/Al2O3 catalysts. Reaction conditions for (B, C) and (D): GVL 5 mmol, catalyst 0.08 g, 1,4-dioxane 20 mL, H2 5 MPa.
Table 3. TOF Values and Initial Reaction Rates of the Rate-determing Step over CuxCoy/Al2O3 Catalysts with Different Chemical Compositions entry
catalyst
TOF (h-1)a
initial reaction rate (μmol gcat-1 min-1)b
1
Cu/Al2O3
8.0
150.0
2
Cu5Co1/Al2O3
10.4
264.6
3
Cu2Co1/Al2O3
11.1
311.4
4
CuCo/Al2O3
8.8
269.4
5
Cu1Co2/Al2O3
7.7
252.6
6
Cu1Co5/Al2O3
8.7
168.0
Reaction conditions: GVL 5 mmol, catalyst 0.08 g, 1,4-dioxane 20 mL, 160 °C, H2 5 MPa, 1 h. aTOF is calculated from the mole of 1,4-PeD generated per mole surface metallic Cu site determined from N2O chemisorption. bInitial reaction rate is calculated from the mole of 1,4-PeD generated per gram catalyst.
was detected, which was derived from the selective hydrogenation of ketone group in EL and as an intermediate for GVL formation.52 As the temperature increasing from 120 oC to 160 oC, the selectivity of 1,4-PeD enhanced dramatically to the highest 93% at 160 oC. Moreover, the higher temperature of 170 oC was facilitated to induce side reaction, where about 3.5% of 2-MTHF produced due to the further dehydration and cyclization reaction of 1,4-PeD.10 As deduced from Figure 3B, the H2 pressure has also a great effect on the product distribu-
tions. A notable increase in the selectivity of 1,4-PeD was observed with H2 pressure arising from 1 MPa to 5 MPa, and no obvious further increase if it mounted up to 6 MPa. It was found that the bimetallic Cu2Co1/Al2O3 catalyst presented very high activity even at a low catalyst amount of 20 mg over which a full conversion of EL was obtained (Figure 3C). Moreover, the selectivity of 1,4-PeD enhanced gradually with the catalyst loading increasing from 20 mg to 80 mg, possibly ascribed to the increased catalytically active sites. On the other hand, solvent effect revealed the remarkable change of product distribution in the selective hydrogenation of EL (Figure 3D), suggesting 1,4-dioxane was the most suitable solvent for the catalytic synthesis of 1,4-PeD from EL. Furthermore, the kinetic study on the selective hydrogenation of EL to 1,4-PeD was conducted over bimetallic Cu2Co1/Al2O3 catalyst under the optimized reaction conditions of 160 oC, 5 MPa H2 in 1,4-dioxane. As depicted in Figure 4A, the selectivity of GVL declined gradually, while the selectivity of 1,4-PeD continued to increase until to the maximum value of 93% during the reaction of 12 h. In the initial reaction stage of 0.5 h, a small amount of 4-HPE intermediate was detected. Notably, no EL could be found even in 10 minutes, indicating the step of EL to GVL is quite easy to proceed in our catalytic system. Additionally, if the reaction was conducted under more mild conditions (such as at a low catalyst loading, Table S1), a relatively low conversion of EL was obtained where the primary products were GVL and 4-HPE but no 1,4-PeD was detected. These reaction results reveal that the catalytic conversion of EL to 1,4-PeD is a multi-step tandem reaction process (Scheme 1): EL is firstly hydrogenated to 4-HPE which can be quickly transformed into GVL via the preferred intramolecular esterification reaction, then the slow hydrogenation of GVL to 1,4-PeD takes place which is the rate-determing step for the hydrogenation of EL to 1,4-PeD. In order to further investigate the intrinsic catalytic activities of CuxCoy/Al2O3 catalysts with different Cu/Co mole ratios, their corresponding turnover frequency (TOF) values and initial reaction rates of the rate-determing step were evaluated at a low conversion (99
1,4-pentanediol 92
8
γ-butyrolactoneb
96
1,4-butanediol 95
5d
γ-hexalactoneb
92
1,4-hexanediol 99
0
δ-valerolactone
>99
1,5-pentanediol 99
0
This work was supported by the National Natural Science Foundation of China (21503242, 21522309 and 21373246) and the Chinese Academy of Sciences. The authors acknowledge Prof. Haichao Liu and Mr. Xianrui Gu from Peking University for in situ FT-IR characterizations and discussion.
REFERENCES
a
Reaction conditions: reactant 5 mmol, catalyst 0.08 g, 1,4dioxane 20 mL, 160 °C, H2 5 MPa, 12 h. b170 °C. cGVL. dTHF.
4. CONCLUSIONS In conclusion, highly efficient and stable hydrotalcite derived CuxCoy/Al2O3 nanocomposite catalysts were fabricated and used for the multi-step tandem hydrogenation of EL to 1,4PeD. The catalytic activity and selectivity depended sensitively on the chemical composition of bimetallic CuCo catalysts. The optimized Cu2Co1/Al2O3 catalyst presented obviously improved catalytic performance compared to monometallic Cu/Al2O3. The combination of systematic characterizations and intrinsic catalytic activity studies was used to explain the promoting effects of CoOx species to active Cu sites in the bimetallic CuCo catalysts. XRD and H2-TPR profiles suggested the presence of strong interaction between Cu and CoO x species. The increased Cu dispersion and H2 activation ability in the bimetallic CuxCoy/Al2O3 catalyst was confirmed by N2O chemisorption and H2-TPD results. More importantly, a strong electronic interaction at the interface of Cu and adjacent CoO x species as deduced from the in situ XPS and FT-IR spectra, which can not only modify the chemical states of active Cu species and produce an appropriate Cu 0/Cu+ distribution, but also create synergistically catalytic active sites of Cu and electron-deficient CoOx species. It plays significant roles in the markedly improved catalytic performance of bimetallic CuCo catalysts due to the promoted adsorption and activation of H2, oxygenated reactant (EL) and particularly key intermediate (GVL). Additionally, the optimized bimetallic CuCo catalyst presented stable recyclability in different reactors and good substrate tolerance. The deep understanding of such synergic catalysis will provide significant clues for rational fabrication of bimetallic catalysts toward hydrogenation and even other catalytic reactions.
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