ZrO2 catalyst for dimethyl oxalate hydrogenation

Accepted Manuscript Plasma-enhanced copper dispersion and activity performance of Cu-Ni/ZrO2 catalyst for dimethyl oxalate hydrogenation

ChuanCai Zhang, Denghao Wang, Mingyuan Zhu, Feng Yu, Bin Dai PII: DOI: Reference:

S1566-7367(17)30279-0 doi: 10.1016/j.catcom.2017.06.042 CATCOM 5108

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

30 January 2017 22 June 2017 23 June 2017

Please cite this article as: ChuanCai Zhang, Denghao Wang, Mingyuan Zhu, Feng Yu, Bin Dai , Plasma-enhanced copper dispersion and activity performance of Cu-Ni/ZrO2 catalyst for dimethyl oxalate hydrogenation. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/j.catcom.2017.06.042

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ACCEPTED MANUSCRIPT Plasma-enhanced copper dispersion and activity performance of Cu-Ni/ZrO2 catalyst for dimethyl oxalate hydrogenation ChuanCai Zhang a, Denghao Wang b, Mingyuan Zhu b, Feng Yu b, Bin Dai b, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China

b

Shool of Chemistry and Chemical Engineering, Shihezi University, Shihezi, Xinjiang 832000, PR China

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Corresponding author. Tel./fax: +86 -993-2057-210 E-mail address: [email protected](B.Dai)

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Abstract An efficient plasma-treated method of Cu-Ni/ZrO2 catalyst was prepared via the deposition–precipitation method for the selective hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG). Compared with the untreated sample, the plasma- treated Cu-Ni/ZrO2 catalyst exhibited enhanced Cu dispersion and significantly improved selectivity to EG and stability for long time on stream. An ethylene glycol selectivity of higher than 95% was obtained after 150 h of reaction. XRD and TEM measurements revealed that the mean

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copper particle size of the catalyst was remarkably reduced by plasma pretreatment. TPR and XPS measurements also indicated that copper dispersion was improved.

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Keywords: Cu-Ni/ZrO2, plasma treatment, Dimethyl oxalate, Hydrogenation, Ethylene glycol.

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1. Introduction The dispersion of the active components in heterogeneous catalysts has a crucial influence on their activity and stability and numerous studies have been devoted towards this goal [1-2]. The use of plasma in catalyst pretreatment for

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the enhanced dispersion of active components is one of these studies performed.

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Plasma can be utilized in a variety of applications [3-7], such as surface

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modification [8], catalyst precursor decomposition and organic template removal. It was found that the catalytic performance of the plasma pretreated catalyst was

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promoted by controlling the particle size and structure of the catalyst.

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Ethylene glycol (EG) is an important organic material with wide applications and considerable market potential. The coal–DMO–EG process for

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the production of EG in the coal chemical industry has been progressed rapidly over the past decade [9-10]. However, finding a suitable catalyst for the

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hydrogenation of dimethyl oxalate (DMO) to EG remains one of the key challenges. Promoting the conversion of DMO and the selectivity to EG by

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designing a novel efficient catalyst plays an important role in the production of EG. Copper catalysts have attracted increasing attention because of their good activity and selectivity. Different elements (e.g. nickel, boron, silver and cobalt) have been employed to modify copper [11-13], and different supports (e.g., SiO2, HMS, SBA-15, ZrO2, and TiO2) [14-19] have been incorporated in the catalysts to obtain high yileds of EG from DMO. Most of the studies reported in the literature have investigated the dispersion and specific surface area of the active copper,

ACCEPTED MANUSCRIPT which have been considered key factors for improving catalytic performance. Highly dispersed copper-based catalysts and the influence of copper dispersion in catalysts for EG synthesis were reported by Zhang et al. [20] and Zhu et al. [21]. These works have focused on the control of the particle size of catalytic active

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components during chemical preparation. However, a physical method is rarely

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used to improve the dispersion of the copper in catalysts for the hydrogenation of

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DMO to EG.

In this work, an efficient ZrO2-supported Cu-Ni bimetallic catalyst was

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synthesized via dielectric barrier discharge (DBD) plasma preparation

synthesized

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methodology for the hydrogenation of DMO to EG. The Cu-Ni/ZrO2 catalyst by DBD showed excellent activity

towards the

selective

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hydrogenation of DMO to EG because of having a smaller particle size of its active components than the non plasma-treated catalyst. We believe that plasma

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pretreatment is expected to become an additional strategy for decreasing the particle size of active catalytic components and preparing more active and

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selective heterogeneous catalysts for other important reactions as well.

2. Experimental

2.1. Catalyst preparation The Cu-Ni/ZrO2 catalyst was first prepared using the deposition– precipitation method [22]. Typically, a certain amount of 10 wt.% zirconium nitrate solution was added drop-wise to a well-stirred NH4OH solution (5 M, ammonia excess of 50%) in a three-necked flask, and the mixture was treated under ultrasonic conditions. The base concentration was 0.55 M after the

ACCEPTED MANUSCRIPT addition of the Zr(NO3)4 solution. The mixture was refluxed at 100 °C for 24 h. Then, 80 mL of mixed aqueous solution containing 6.48 g of Cu(NO3)2·3H2O and 0.7 g of Ni(NO3)2·6H2O was slowly added to the aforementioned mixture and treated under ultrasonic conditions. Subsequently, NH4OH solution was further added to increase the pH to 11. The system was maintained at 100 °C for 6 h

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under vigorous stirring. The precipitate was filtered, washed and dried at 120 °C

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overnight. The solid catalyst precursor thus obtained was calcined at 400 °C for 5

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h and is denoted as Cu-Ni/ZrO2.

Subsequently, 1.5 g of Cu-Ni/ZrO2 catalyst was placed in a quartz plate in Ar

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atmosphere and a glow discharge plasma (CTP-2000K; Nanjing Suman Electronic Co., Ltd.) was applied, five times every 15 min. The discharge parameters were as

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follows: frequency, 14.3 kHz; discharge voltage, 60 V; and anodic current, 150 mA. The obtained sample is denoted as Cu-Ni/ZrO2-DBD.

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2.2. Catalyst characterization

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The mass fraction of metal elements in catalysts was measured by atomic absorption spectroscopy (AAS). The specific surface area of the catalysts was

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analyzed based on N2 adsorption-desorption isotherms at 77K (Micromeritics ASAP 2020 apparatus) and estimated using the Brunauer-Emmett-Teller (BET) method. The dispersion of the metal was estimated based on the amount of hydrogen consumed by temperature programmed reduction (H2-TPR) before and after the applied N20 titration experiment as described

elsewhere [23,24]. The

wide angle X-ray powder diffraction (XRD) analyses of the catalysts were performed in the scanning range of 10~90° using a Bruker D8 ADVANCE X-ray diffractometer (Cu K (λ=0.15418nm) radiation). The particle morphology of catalysts was observed using a JEM 2010 transmission electron microscope (TEM)

ACCEPTED MANUSCRIPT with an accelerating voltage of 200 Kv. X-ray photoelectron spectra (XPS) were recorded on an Axis Ultra spectrometer with Mg K X-ray emission source (1253.6 eV) , where the binding energies were calibrated based on the standard C1s value (284.5 eV). The redox behavior of the solid catalysts was investigated using the H2 temperature programed reduction (H2-TPR) method (Micromeritics

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ASAP 2720 instrument).

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2.3. Catalytic activity measurements

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Catalytic activity and stability evaluation was conducted in a high-pressure fixed-bed micro-reactor. The catalyst sample (1.0 g) was placed into the reactor,

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and 5% H2/Ar gas mixture was introduced into the reactor at the rate of 40

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mL/min to reduce the catalyst. The reduction temperature was programmed to 230 °C at a ramp rate of 3 °C/min and maintained at 230 oC for 3 h. The sample

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temperature was then decreased to the reaction temperature of 200 °C. The liquid feedstock (15 wt.% DMO in methanol, methanol as solvent will not react in

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this catalyst and reaction conditions.) was pumped into an evaporator, and the produced gaseous stream after mixed with the hydrogen stream was then fed

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into the reactor. The hydrogenation of DMO was performed under the following conditions: hydrogen pressure (PH2) of 2.5 MPa; H2/DMO molar ratio (H2/DMO) of 120 and weight liquid hourly space velocity (WLHSV) of 0.2–1.0 h−1. A Shimadzu GC-9A gas chromatograph equipped with a WondaCap WAX capillary column (30 m × 0.53 mm × 1.0 µm) and flame ionization detector was used to analyze the effluent gas products from the reactor.

3. Results and discussion 3.1. Catalyst chemical composition, textural and structural properties

ACCEPTED MANUSCRIPT The chemical composition, textural and structural properties of the catalyst, with and without DBD plasma pretreatment, are summarized in Table 1. The copper content of Cu-Ni/ZrO2 and Cu-Ni/ZrO2-DBD catalysts determined by AAS are 25.1 and 24.7 wt%, respectively. The nickel content of the two catalysts are 1.98 and 1.95 wt%, respectively. These results show that the content of copper

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and nickel in the catalyst were not reduced after plasma treatment, in agreement

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with the literature [9].

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BET measurements were performed to investigate the effect of DBD plasma pretreatment on the texture of the Cu-Ni/ZrO2 catalyst. The BET surface area of

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Cu-Ni/ZrO2 increased from 110.1 to 133.9 m2g−1 after DBD plasma pretreatment and the pore volume also slightly increased from 0.30 to 0.33 cm3g−1. The plasma

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pretreatment of the catalyst was responsible for the slight increase in the SBET and

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Vp values due to the cleaning of the pore system. The metal dispersion of

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Cu-Ni/ZrO2 and Cu-Ni/ZrO2-DBD catalysts are 10.7 and 13.4%, respectively. These results show that plasma treatment can effectively improve (25% increase)

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the dispersion of metal in the catalyst. Table 1.

Sample

Chemical composition, textural and structural properties of the catalysts.

Cu Ni SBET loading loading (m2 g-1 ) (wt%) (wt%)

Vp Dp (cm3 g-1) (nm)

metal dispersion (%) a

dCu b (nm),

Cu /Zrc

Ni/Zr

mol/mol

mol/mol

Cu-Ni/ZrO2

25.14

1.98

110.1

0.30

9.6

10.7

16.9

0.458

0.051

Cu-Ni/ZrO2 -DBD

24.73

1.95

133.9

0.33

9.2

13.4

13.7

0.531

0.059

a

metal dispersion(%) was determined by N 2O titration method.

b the c

Cu particle size of reduced catalysts determined by Scherrer' equation

Elemental ratio on the catalyst surface determined by XPS.

3.2. Crystalline phases and morphology

ACCEPTED MANUSCRIPT The X-ray diffraction (XRD) patterns of the Cu-Ni/ZrO2 and Cu-Ni/ZrO2-DBD solid catalysts are shown in Fig. S1. Several strong diffraction peaks are located in the 2 range of 10–90°. The first strong diffraction peaks of the catalysts shown at 30.5 and 52° are attributed to the presence of t-ZrO2 (tetragonality, JCPDS A previous study [11] showed that t-ZrO2 was more suitable than

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50-1089).

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amorphous ZrO2 as a carrier for the copper-based catalyst in the hydrogenation

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of ester to alcohol. The diffraction peaks centered at 35.5, 38.7, 48.7 and 61.5° are characteristic ones of CuO (tenorite, JCPDS 05-0661). The diffraction peaks of CuO

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in Cu-Ni/ZrO2 obviously decrease after DBD plasma pretreatment. These findings

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reveal that DBD plasma pretreatment lead to smaller copper particle sizes. The weakened and broadened CuO diffraction peaks indicated that DBD catalyst

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preparation method promotes copper dispersion in the catalyst. These results are consistent with those reported in previous studies [9]. Moreover, the CuO

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diffraction peaks of the Cu-Ni/ZrO 2 -DBD catalyst shifted to smaller 2theta values, of approximately 0.5°, compared with those of the Cu-Ni/ZrO2 catalyst. The

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profiles of the XRD peaks were broadened and shifted to low 2theta values probably because of synergistic effects between copper and nickel induced after the DBD plasma pretreatment [25]. The morphology of the two catalysts can be observed in the HRTEM images presented in Fig. S2. In particular, Fig. S2a shows that some irregular dark particles, which are ascribed to copper-containing phases, are unevenly distributed on the surface of the Cu-Ni/ZrO2 catalyst. The d spacing of

ACCEPTED MANUSCRIPT approximately 0.298 nm, which corresponds to the t-ZrO2 (011) face, is shown in Fig. S2b, result which is consistent with the XRD one. In the case of Cu-Ni/ZrO2-DBD (Fig. S2c), the small dark copper nanoparticles were uniformly distributed on the surface of the support. This finding clearly demonstrates that

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CuO particle size decreases after DBD pretreatment compared with that of the

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untreated catalyst, thus indicating the improved dispersion of CuO.

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3.3. Reducibility of the catalysts and XRD studies of reduced catalysts The H2−TPR traces of the Cu-Ni/ZrO2 and Cu-Ni/ZrO2-DBD catalysts are

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shown in Fig. 1. Two partially overlapping (T<250 oC) and one very weak

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reduction peaks (~300-400 oC) of the pretreated and untreated Cu-Ni/ZrO2 catalysts are evident. The two obvious overlapping reduction peaks belong to the

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reduction of copper oxide, while the weak reduction peak at high temperature s is attributed to the reduction of nickel oxide. The TPR trace of nickel oxide is

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located at 300-400℃, and is weak due to the low nickel content in the catalyst (Table 1). On the other hand, the

TPR trace of copper oxide (133 – 215 oC) in

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the case of Cu-Ni/ZrO2 catalyst shows maximum reduction peaks at T = 187.4 °C and 167.2 °C.

In the case of Cu-Ni/ZrO2-DBD catalyst, the two partially

overlapping reduction peaks slightly shift to lower temperatures compared with those of Cu-Ni/ZrO2 catalyst (the main peak at 186.5 °C and the shoulder peak at 160.5 °C). Moreover, the area under the TPR trace of the Cu-Ni/ZrO2-DBD catalyst is larger than that of Cu-Ni/ZrO2 catalyst. This finding suggests that copper dispersion increased in the case of Cu-Ni/ZrO2-DBD catalyst.

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0.40 0.35

Intensity (a.u)

0.30

Cu-Ni/ZrO2 0.25

Cu-Ni/ZrO2-DBD

0.20 0.15

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0.10 0.05

100

200

300

400

500

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T(℃)

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Fig. 1. H2 -TPR profiles of the Cu-Ni/ZrO2 and Cu-Ni/ZrO2 -DBD catalysts.

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To investigate the crystalline structure of the reduced catalyst, XRD patterns

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of the reduced Cu-Ni/ZrO2 and Cu-Ni/ZrO2-DBD catalysts are shown in Fig. 2. The diffraction peaks located at 30.5, 50.5 and 60.0° are characteristic diffraction

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peaks of t-ZrO2. The peak at 2θ of 36.4° corresponds to the Cu 2O phase (JCPDS 03-0892), while peaks at 2θ of 43.5, 50.7and 74.7°could be attributed to the Cu

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phase (JCPDS 03-1018). It is seen that XRD diffraction peaks intensities of Cu and Cu2O phases in the Cu-Ni/ZrO2-DBD catalyst are weaker than those present in the

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Cu-Ni/ZrO2 catalyst. It was calculated from the Cu(111) face that the mean cuprous crystallite sizes in the reduced Cu-Ni/ZrO2 and Cu-Ni/ZrO2-DBD catalysts were 16.9 and 13.7 nm after using the Scherrer equation.

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

(111) 







Intensity (a.u.)



Cu2O

Cu-Ni/ZrO2-DBD 

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

 t-ZrO2

 Cu

10

20

30

40

50

60

70

80

90

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2 Theta (Degree)

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Cu-Ni/ZrO2

Fig. 2. XRD patterns of the reduced Cu-Ni/ZrO2 and Cu-Ni/ZrO2-DBD catalysts.

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3.4. Catalysts surface chemical composition

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XPS analysis provided further evidence for the enhanced Cu and Ni metal dispersion after plasma pretreatment of the Cu-Ni/ZrO2 catalyst. The Cu/Zr and Ni/Zr atom ratios on the surface of Cu-Ni/ZrO2 and Cu-Ni/ZrO2-DBD catalysts

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were calculated on the basis of XPS quantitative analysis and results are shown in

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Table 1. The Cu/Zr and Ni/Zr ratios in the Cu-Ni/ZrO2 were found to be 0.458 and 0.051, respectively. However, the ratios of Cu/Zr and Ni/Zr in the

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Cu-Ni/ZrO2-DBD catalyst increased to 0.531 and 0.059, respectively. This finding further proves that more metal elements are exposed on the surface of the Cu-Ni/ZrO2 after plasma-assisted preparation of the catalyst.

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Intensity(a.u.)

Cu2p xps

934.2eV

954.1eV

933.4eV 953.3eV

970

960

950

940

Intensity(a.u.)

930

920

854.3eV

Ni2p xps 871.8eV 871.3eV

875

870

865

860

855

Binding Energy(eV)

850

845

840

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880

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853.8.eV

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Fig. 3. Cu2p and Ni2p XPS spectra of Cu-Ni/ZrO2 and Cu-Ni/ZrO2-DBD catalysts.

The Cu2p and Ni2p XPS spectra of the calcined Cu-Ni/ZrO2 and

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Cu-Ni/ZrO2-DBD catalysts are shown in Fig. 3. The binding energies (BE) of

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Cu2p3/2 and Ni2p3/2 in the Cu-Ni/ZrO2 catalyst are located at 933.4 eV and 853.8 eV, respectively. However, the Cu2p3/2 and Ni2p3/2 BEs in the Cu-Ni/ZrO2-DBD

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shifted to 0.8 eV and 0.5 eV of the high-energy region, respectively. This finding suggests that synergistic effects between copper and nickel in the Cu-Ni/ZrO2

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catalyst were enhanced by the DBD plasma pretreatment applied. 3.5. Catalytic activity and stability

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The catalytic performance of Cu-Ni/ZrO2 and Cu-Ni/ZrO2-DBD with varied WLHSVs is reported in Fig. S3. The DMO conversion and EG selectivity of both catalysts decrease with increasing WLHSV. However, the plasma-pretreated catalyst shows higher activity and selectivity than the untreated catalyst. This result becomes more apparent when WLHSV increases. This finding might be attributed to the better copper dispersion achieved on the plasma-treated catalyst. The long-term stability of the Cu-Ni/ZrO2 and Cu-Ni/ZrO2-DBD catalysts for

ACCEPTED MANUSCRIPT the selective conversion of DMO to EG is shown in Fig. 4. The EG selectivity of plasma-treated catalyst appears significantly better than that of the untreated catalyst under the specified reaction conditions of P(H2)=2.5 MPa, T=200 °C, H2/DMO=120, and WLHSV=0.3 h−1. More precisely, the EG selectivity is higher than 96% after 150 h in reaction feed stream compared with that of untreated

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catalyst (86.6% after 90 h of reaction) . On the other hand, only small differences

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are observed in the conversion of DMO between the two catalysts (Fig. 4).

100

100

95

95

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Selectivity(%)

90

85

85

80

80

75

Cu-Ni/ZrO2

75

70

Cu-Ni/ZrO2

70

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Conversion (%)

90

Cu-Ni/ZrO2-DBD

65

65

Cu-Ni/ZrO2-DBD

60 0

25

50

75

100

125

60 150

Comparison of the DMO conversion and EG selectivity observed over the

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Fig. 4.

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Time (h)

Cu-Ni/ZrO2 and Cu-Ni/ZrO2-DBD solids with time on reaction stream.

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4. Conclusions

The DBD plasma pretreatment of Cu-Ni/ZrO2 catalyst was shown to be an effective method to enhance catalytic activity and stability for the EG formation via

the

conversion of DMO. Compared with

the

untreated catalyst,

Cu-Ni/ZrO2-DBD showed enhanced EG selectivity from 90% to more than 96% and at the same time prolonged lifetime stability (an increase from 90 h to more than 150 h was obtained). The reduced mean copper particle size and the concomitant increase in the number of exposed active Cu and Ni metals per gram

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Graphical abstract

ACCEPTED MANUSCRIPT Highlights 

·Cu-Ni/ZrO2 catalyst was prepared via the deposition–precipitation method.

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·plasma-treated Cu-Ni/ZrO2 shows high activity and stability for DMO hydrogenation. ·Plasma enhanced copper dispersion and more copper was exposed on the catalyst

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surface .

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·Cu-Ni/ZrO2-DBD catalyst can maintain the excellent activity over a lifetime of 150h.

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