CuO-ZrO2 catalysts for oxidation of glycerol to dihydroxyacetone

Journal Pre-proofs The effects of calcination temperature of support on Au/CuO-ZrO2 catalysts for oxidation of glycerol to dihydroxyacetone Yanxia Wang, Danping Yuan, Jing Luo, Yanfeng Pu, Feng Li, Fukui Xiao, Ning Zhao PII: DOI: Reference:

S0021-9797(19)31196-8 https://doi.org/10.1016/j.jcis.2019.10.017 YJCIS 25511

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Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

9 August 2019 4 October 2019 6 October 2019

Please cite this article as: Y. Wang, D. Yuan, J. Luo, Y. Pu, F. Li, F. Xiao, N. Zhao, The effects of calcination temperature of support on Au/CuO-ZrO2 catalysts for oxidation of glycerol to dihydroxyacetone, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.10.017

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The effects of calcination temperature of support on Au/CuOZrO2 catalysts for oxidation of glycerol to dihydroxyacetone Yanxia Wang †,‡, Danping Yuan†,‡, Jing Luo†,‡, Yanfeng Pu†, Feng Li†, Fukui Xiao*† and Ning Zhao*† † State

Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China

‡ University

of Chinese Academy of Sciences, Beijing 100049, PR China

Abstract Dihydroxyacetone (DHA) is a fine chemical and has been widely used in the cosmetics industry. In this work, DHA was synthesized with high selectivity over Au catalysts, also supported by Cu-Zr mixed oxide calcined at different temperatures. The effects of the calcination temperature of supports on the properties and catalytic performance for glycerol oxidation to dihydroxyacetone were also studied. BET and CO2-TPD measurements demonstrated that the increase in the support calcination temperature reduced the specific surface area of the catalyst and further reduced the surface basic sites of the catalysts. With increased support calcination temperature, the surface content of Au0 and the dispersion of Au first increase until the calcination temperature of the support was 600 oC and then decrease. It was also observed that the glycerol conversion is positively correlated with the surface content of Au0 and the dispersion of Au, while upon the increase of the amount of the basic sites, the catalytic

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activity increases first and then decrease. The suitable support calcination temperature is beneficial for the conversion of glycerol, and the best catalytic performance is obtained when the calcination temperature is 600 oC. Keywords: glycerol oxidation, dihydroxyacetone, Au based catalyst, calcination temperature, Cu-Zr mixed oxide. 1. Introduction Glycerol is a highly functional biomass platform molecule from which many valueadded chemicals can be obtained [1]. Dihydroxyacetone (DHA) is one of the glycerol derivatives and is widely used in the cosmetics, pharmaceuticals, chemical and other industries [2]. Many catalysts have been prepared for the oxidation of glycerol to DHA, among these supported noble metal-based catalysts are Pt [2-5], Au [6-8] and Pd [9] and are active for the reaction. Recently, Au-based catalysts have received extensive attention due to their excellent antioxidant capability [10, 11]. However, homogeneous base is added which presents environmental and economic impacts. Meanwhile, glycerol can be easily oxidized to the by-product (glyceric acid) under the basic medium. While in acidic medium, glycerol can be oxidized to dihydroxyacetone [12-20]. Xu et al. reported that the Au/CuO catalyst showed good glycerol conversion and DHA selectivity under non-basic conditions [21, 22]. Moreover, the catalytic activity of Au catalysts is significantly influenced by the nature of the supports [6, 15, 23, 24] e.g. CuO, as reducible oxide support, can enhance the catalytic activity via metal-support interaction and/or improve the dispersion of the Au. It was also reported that mixed 2

metal oxides supported Au catalysts showed better catalytic activity than monometallic oxides (MMO) supported Au catalysts [25] e.g. ZrO2 can improve the activity and stability of the catalysts [26, 27] by affecting the surface area and crystallite size of the CuO and thus inhibiting the sintering of CuO [28, 29]. In our previous study, Au/CuxZr1-xOy catalysts were prepared which showed good performance for glycerol oxidation in base-free conditions due to the interaction between Au and the Cu-Zr mixed oxides and the surface basicity. In general, by changing the calcination temperature of the supports, the surface properties of the supports can be tuned [30-35], thereby affecting the interaction between the supported metal and the supports and thus the particle size and chemical valence of the supported metal are modulated. In this present work, the effect of calcination temperature of Cu-Zr mixed oxide on the structure and catalytic performance of Au/Cu0.9Zr0.1 catalysts for glycerol oxidation to DHA was investigated. It was found that proper calcination temperature of the support might optimize the surface properties of the support and enhance the interaction between the Au and the Cu-Zr mixed oxides which then affected the catalytic activity. 2. Experimental 2.1 Preparation of catalysts The support was prepared by co-precipitation method. The precursors, Cu(NO3)2•6H2O (99.9%) was purchased from Beichen Fangzheng Chemical Reagent Co. Ltd.. Zr(NO3)4•3H2O (99.5%) and Na2CO3 (AR) were obtained from Sinopharm Chemical Reagent Co. Ltd.. The urea was obtained from Kermel Chemical Reagent Co. 3

Ltd., while the NaAuCl4•2H2O was obtained from Shanxi Kaida Chemical Engineering Co. Ltd.. The Cu0.9Zr0.1 precursors were prepared by using Na2CO3 as the precipitating agent and aging at 70 oC for 1 h. Then the as-prepared materials were dried overnight followed by calcination at 400 oC, 500 oC, 600 oC and 700 oC, respectively, for 4 h under air atmosphere. The supports were named as Cu0.9Zr0.1-T, with the calcination temperature represented as T. The Au was then loaded by the method of deposition-precipitation with urea [36]. The Au loading was 2 wt%. Typically, 2 ml of NaAuCl4 solution (10 mgAu/ml), 2.44 g of urea and 1 g of Cu-Zr mixed oxide powders were added to 50 mL of deionized water. Then, the suspensions were placed in a water bath at 80 oC and stirred for 6 h followed by aging at room temperature overnight. The solid sample was filtered and washed with deionized water until no Cl- was detected. The filter cake was then dried at 110 oC for 4 h and calcined at 200 oC for 5 h. The catalysts were named as Au/(Cu0.9Zr0.1-T). The actual loading of the catalyst was 2 wt% according to ICP results (Table 1). 2.2 Characterization of catalysts The specific surface areas and the pore structure properties of the catalysts were determined by N2 adsorption at 77 K using Micromeritics Tristar 3020. The specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method and the pore structure properties were determined by the method of Barrett-Joyner-Halenda (BJH).

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X-ray diffraction (XRD) patterns of the samples were obtained on a Bruker X-ray diffractometer Model D8 Advance with Cu Kα radiation (λ=0.15406 nm) operated at 30 mA and 40 KV and the 2θ range from 10 o to 80 o and the rate is 4 o/min. Transmission electron microscopy (TEM) experiments were determined on JEM2010 high-resolution transmission electron microscopy operated at 200.0 kV. The catalysts were dispersed in anhydrous ethanol and then dripped onto the carbon film, TEM characterization test after drying at room temperature. The particle size distribution of Au nanoparticles was obtained by counting 200 particles in the TEM images. The redox performance of the catalysts was measured by H2 temperatureprogrammed reduction experiments (H2-TPR), and the H2 consumption was recorded using a thermal conductivity detector (TCD). 0.05 g of the catalyst was placed in a quartz tube and pretreated at 200 °C for 30 minutes under an Ar atmosphere to remove gaseous impurities on the surface of the catalyst. After the sample was cooled to room temperature, a temperature-programmed reduction process was carried out using the 5 vol.% H2/Ar flow, and the temperature was raised to 500 oC at the heating rate of 10 oC/min,

and the consumption of H2 was recorded.

Raman spectra of the catalysts were recorded with a LabRAM HR Evolution Raman spectrometer with a 532 nm lighter laser. The scanning range of the Raman spectrogram is 100-1000 cm-1. X-ray photoelectron spectroscopy (XPS) analyses were performed using an ESCALAB 250 spectrometer with a monochromatic Al Kα radiation (hν =1486.6 eV), 5

operating at 12 kV and 15 mA. The C 1s peak (284.6 eV) was used as a reference to calibrate the elemental binding energies in the samples. The surface alkalinity of the catalysts was determined by the temperatureprogrammed desorption of CO2 (CO2-TPD) on GC-950 gas chromatograph equipped with a TCD detector. Typically, 0.10 g catalyst was loaded in the quartz tube purged with Ar gas at room temperature for 1 h. The temperature was then increased up to 200 oC

and purged with Ar for an extra hour to remove the impurities adsorbed on the

surface of the catalysts. After cooling to room temperature, the catalysts were exposed to CO2 atmosphere for adsorption for 1 h. The catalysts were thus flushed with Ar for 2 h to remove the physically adsorbed CO2. Finally, the catalyst was heated to 200 oC with a heating rate of 10 oC/min under Ar flow. The signals were recorded by a gas chromatograph. 2.3 Catalytic evaluation The liquid-phase oxidation of glycerol was carried out in a 70 ml Teflon-line stainless steel autoclave, in which 0.39 g catalyst and 20 mL glycerol aqueous solution (0.1 mol/L) were added (GLY/Au=50 mol/mol). The reactor was then pressurized to 0.2 MPa of O2 and heated to 50 oC for the reaction. The initial pH was about 6.5-7.0. After 4 h of the reaction, the autoclave was cooled to room temperature. The reaction mixture was centrifuged and analyzed by high-performance liquid chromatography (HPLC) equipped with an Aminex HPX-87H column and ultraviolet (210 nm) and refractive index (RI) detector. The mobile phase was diluted with H2SO4 solution (0.005 M). The

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gas-phase products were analyzed by DX 4000 portable infrared smoke infrared analyzer (detection limits>0.01 ppm). The glycerol conversion and the product selectivity were calculated as follows: Conv.% = Σ{(mol of product)(number of carbon atoms in the product molecule)} Σ{(mol of product or glycerol)(number of carbon atoms in the product or glycerol molecule)} × 100%. Sel.% =

{(mol of product)(number of carbon atoms in a product molecule)} Σ{(mol of product)(number of carbon atoms in a product molecule)} × 100%.

3. Results and discussion 3.1 Catalyst characterization 3.1.1 Textural properties The specific surface areas of the catalysts were shown in Table 1. The catalysts with supports treated at a lower temperature (400, 500 oC) showed larger specific surface areas (31 and 25 m2 g-1, respectively). However, when the supports were calcined at a higher temperature (600, 700 oC), the surface areas of the catalysts decreased to 20 and 6 m2 g-1, respectively. For example, the BET surface areas of the catalysts decreased with the increase of the calcination temperature of the support. Table 1. Texture, Au loading and base properties of Au/(Cu0.9Zr0.1-T) catalysts.

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Material

BET surface area（m2/g）

Total pore volume （cm3/g）

Au (wt%)

Basic sites (mmol/g)

Au/(Cu0.9Zr0.1-400)

31

0.15

1.95

0.086

Au/(Cu0.9Zr0.1-500)

25

0.10

2.00

0.047

Au/(Cu0.9Zr0.1-600)

20

0.08

2.01

0.043

Au/(Cu0.9Zr0.1-700)

6

0.03

1.98

0.039

3.1.2 Structural characterizations Fig.1 presented the XRD patterns of the Au/(Cu0.9Zr0.1-T) catalysts. Apparently, when the calcination temperature of the supports increased from 400 oC to 500 oC, only the diffraction peaks of CuO were observed. A further increase in the calcination temperature to 600 oC, new diffraction peaks of tetrahedral ZrO2 were detected. For the support calcined at 700 oC, the monoclinic ZrO2 appeared, while the diffraction peaks of tetrahedral ZrO2 became weaker. The different crystal forms of ZrO2 also exhibit a different amount of basic sites. Moreover, the diffraction peaks of CuO increased with the increase of the calcination temperature of the support. Higher calcination temperature was favorable in achieving well-crystallized particles while growth of the particles might take place, leading to pore expansion and reduced specific surface area, which was consistent with the results of BET surface areas.

Fig. 1 XRD patterns of Au/(Cu0.9Zr0.1-T) catalysts. (•) monoclinic CuO, (‫ )٭‬monoclinic ZrO2, (♦) tetrahedral ZrO2. 3.1.3 TEM 8

Fig. 2 showed the TEM images of the Au/(Cu0.9Zr0.1-T) catalysts and the corresponding particle size distributions. Based on the measurement of more than 200 Au particles, the average particle size of Au in the Au/(Cu0.9Zr0.1-T) catalysts were in the range of 2.15 to 2.92 nm. The calcination temperature of the support changed the surface properties, thereby affecting the Au particle size e.g the average particle sizes of the Au decreased and then increased upon increase of the calcination temperature of the supports. When the calcination temperature of the support was up to 600 oC, the smallest average Au particle size was observed. In general, the smaller average Au particle size was beneficial to improve the catalytic activity [24, 37, 38].

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Fig. 2 TEM images of Au/(Cu0.9Zr0.1-T) catalysts and the corresponding particle size distributions. 3.1.4 H2-TPR and Raman spectra The reduction properties of Au/(Cu0.9Zr0.1-T) catalysts were explored by H2-TPR and the results were shown in Fig. 3a. Because most of the Au exhibited as a zero valence state, and ZrO2 could not be reduced under this condition, the reduction peaks could be attributed to the Cu species which could reflect the interaction between Cu and Zr. All the catalysts showed two reduction peaks in the range of 100-400 oC. It was well established that the peak centered at lower temperature could be attributed to the reduction of highly dispersed CuO. While the peak centered at higher temperature could be attributed to the reduction of crystalline CuO [36-42]. The reduction temperatures of the catalysts with support treated at increased temperature also shifted to higher values e.g. when the calcination temperature of the support increased to 700 oC, the reduction temperature of Cu species increased to over 315 oC which meant the 10

decreased lattice oxygen mobility and enhanced interaction between CuO and ZrO2 [39, 41]. In addition, lower reduction peak temperature meant higher redox ability. Therefore, the redox ability of the four samples was in the order of: Au/(Cu0.9Zr0.1-400)> Au/(Cu0.9Zr0.1-600)>Au/(Cu0.9Zr0.1-500) > >Au/(Cu0.9Zr0.1-700). Fig. 3b showed the Raman spectra of Au/(Cu0.9Zr0.1-T) catalysts. The Raman bands at around 290 cm-1, 345 cm-1 and 626 cm-1 could be attributed to the vibrations of the oxygen atoms in the lattice (Cu-O and Cu-O-Cu bonds) [43]. No obvious Raman features of ZrO2 were detected. However, compared with literature [44], the bands of the catalysts shifted to lower frequencies due to the interaction between Cu and Zr. As the calcination temperature was increased from 400 to 700 oC, the broad band with a maximum at 275.8 cm-1 shifted to 271 cm-1, indicating the enhanced interaction between CuO and ZrO2.

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Fig. 3 a) H2-TPR and b) Raman spectra of Au/(Cu0.9Zr0.1-T) catalysts. 3.1.5 XPS The effects of the calcination temperature of the supports on the surface composition and the state of different species were explored by XPS and the results were shown in Fig. 4 and Table 2. Two types of Au species could be detected for all catalysts (Fig. 4). The binding energy for Au4f7/2 of about 84.1-84.3 eV which corresponded to Au0 could be found [24, 45, 46]. Moreover, a small amount of Au species appeared as Auᵟ+ at the binding energy of 85-85.2 eV. As for the XPS spectra of Cu 2p for Au/(Cu0.9Zr0.1-400) and Au/(Cu0.9Zr0.1-500), the binding energy was between Cu2+ and CuO due to the overlapping of CuO (933.6 eV) and Cu2+ (934.4 eV) [47]. While for Au/(Cu0.9Zr0.1600) and Au/(Cu0.9Zr0.1-700) catalysts, the binding energy of Cu 2p was at 934.2 eV that could be assigned to Cu2+ species [31]. The transformation of Cu species from CuO to Cu2+ would cause increased activity of the catalysts. The binding energy of Zr 3d5/2 located at 182 eV changed a little with the increase of the calcination temperature of the supports. The increased calcination temperature of supports leads to increased Zr and Au concentration on the surface which might be as a result of the gold which is

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preferentially adsorbed on the Cu2+ sites during the preparation process and thus resulted in shielding of the copper species. Three kinds of oxygen species could be found on the surface of catalysts from O1s XPS spectra: lattice oxygen Olatt (529-530.0 eV), chemisorbed oxygen Oads (531.3531.9 eV), and hydroxyl oxygen Ohyl (532.7-533.5 eV). It was clear that Au/(Cu0.9Zr0.1600) had the highest lattice oxygen concentration among the four catalysts (see Table 2) which might be active in the oxidation reaction.

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Fig. 4 XPS spectra of Au/(Cu0.9Zr0.1-T) catalysts. As could be seen in Table 2, upon the increase of the calcination temperature of the supports, the surface content of the Au0 increased firstly e.g. the Au/(Cu0.9Zr0.1-600) catalyst exhibited the highest surface content of Au0 (90%). When the calcination temperature of the support is further increased to 700 oC, the surface content of Au0 decreased to 81%. This might suggest that increasing the calcination temperature of the support facilitated the reduction of the Au oxide species. However, a higher calcination temperature of the support was not favorable for the reduction of Au oxides. Table 2. XPS dates of Au/(Cu0.9Zr0.1-T) catalysts. Catalyst

Au0 (%)

Auᵟ+ (%)

Cu/Zr atomic ratio

Au atomic ratio

Olatt/O (%)

Au/Cu0.9Zr0.1-400

79

21

0.356/0.033

0.010

54.0

Au/Cu0.9Zr0.1-500

85

15

0.366/0.038

0.018

55.8

Au/Cu0.9Zr0.1-600

90

10

0.320/0.053

0.032

56.5

Au/Cu0.9Zr0.1-700

81

19

0.250/0.079

0.052

53.1

3.1.6 CO2-TPD CO2-TPD profiles were displayed in Fig. 5. Because the calcination temperature of catalysts was 200 oC, only the desorption profiles below 200 oC assigned to the weakly 14

basic sites arising from the surface OH group [48, 49] were considered. It was clear that with the increase of the calcination temperature of the support, the amount of the basic sites decreased constantly (Table1, Fig. 5), which meant that the calcination temperature of support tuned the surface base property of the catalysts effectively. The change of the surface basicity could be explained by the changes of the specific surface area of the catalyst [48] e.g. with the increase of the calcination temperature of the supports, the specific surface areas decreased, which could decrease the exposure of the basic sites.

Fig. 5 CO2-TPD profiles of Au/(Cu0.9Zr0.1-T) catalysts. 3.2 Catalytic performance The catalytic performance for liquid-phase oxidation of glycerol over different catalysts was listed in Table 3. It was observed that the supports were inactive for the reaction. Apparently, the calcination temperature of supports greatly affected the activity e.g. with the increase of the calcination temperature of the support, the glycerol conversion increased firstly until the calcination temperature of the support was 600 oC and then decreased. Under the optimized conditions, the DHA yield was up to 80%. 15

Compared with the Au/MWCNT catalyst [50] which was active in the presence of NaOH (Table 3, Entry 9), the DHA yield obtained in this work was higher. Moreover, the Au/(Cu0.9Zr0.1-600) catalyst showed similar DHA yield and better DHA selectivity when compared with Au/CuO in base-free conditions as reported by Xu et al [8] (Table 3, Entry 10). Table 3. Catalytic activity of glycerol oxidation over different catalysts.

Samples

Conv. of glycerol /%

DHA

OA

GA

Au/Cu0.9Zr0.1-400a

53.7

94.4

1.5

1.4

0.5

0.5

0.2

1.5

27.3

Cu0.9Zr0.1-400a

0.3

30.0

46.7

0

0

0

0

23.3

--

Au/Cu0.9Zr0.1-500a

69.0

95.1

1.1

1.7

0.4

0.2

0.1

1.4

32.1

Cu0.9Zr0.1-500a

0.4

23.2

44.8

6.4

0

0

0

25.6

--

Au/Cu0.9Zr0.1-600a

72.7

95.5

1.1

1.5

0.4

0.2

0.1

1.2

36.8

Cu0.9Zr0.1-600a

0.3

23.3

44.4

6.7

0

0

0

25.7

--

Au/Cu0.9Zr0.1-700a

60.6

95.8

1.3

1.3

0.3

0.2

0.1

1

39.0

Cu0.9Zr0.1-700a

0.3

24.0

50.1

0

0

0

0

25.9

--

Au/MWCNTb

93

60

0

13

0

26

1

0

3930

Au/CuOc

97.8

81.8

1.0

0.9

0

0

0

a

Sel. /% GLD GLA TA+HA CO2

TOF h-1

16.2 40.6

Reaction conditions: 20 mL 0.1 M GL, GLY/Au = 50 mol/mol, 50 oC, PO2 = 0.2 MPa,

stirring speed = 700 r/min, 4 h. b Reaction conditions: 60 °C, PO2= 3 bar, 150 mL of glycerol 0.3 M, catalyst =700 mg, NaOH/glycerol=2mol/mol. c Reaction conditions: 20 mL 0.1 M GL, GL/Au = 50 mol/mol, stirring speed = 900 rpm, 4 h. OA= Oxalic acid, GA= Glycolic acid, GLD = Glyceraldehyde, GLA = Glyceric acid, TA = Tartronic acid, HA = Hydroxypyruvic acid. TOF= GL conversion rate (mol/h)/surface Au atoms (mol). The dispersion (D) of Au was estimated from D=1/d, where d is the volume-surface 16

mean diameter of Au particle measured by TEM. TOF values are calculated under the reaction for 1 h. 3.3 Structure-Performance relationship According to the results of XRD (Fig. 1), H2-TPR (Fig. 3a) and Raman (Fig. 3b), it was revealed that with the increase in the calcination temperature of the support, the Au particle size decreased and then increased (Fig. 2). However, the surface content of Au0 firstly increased then decreased (Fig. 3, Table 2). Meanwhile, the basic sites decreased constantly (Fig. 5). Combined with the evaluation results (Table 3), the catalytic activity of the catalyst might be influenced by the Au particle size, surface content of Au0 and the basic sites.

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Fig. 6. Structure and performance relationship of Au/(Cu0.9Zr0.1-T) catalysts. a) Particle size of Au nanoparticle Vs catalytic activity, b) Surface content Au0 Vs catalytic activity, c) Basic sites of catalysts Vs catalytic activity. It was clear that the glycerol conversion showed a linear relationship with the Au particle size e.g. the smaller the Au particle size, the better the catalytic activity (Fig. 6a). As a result, the Au/(Cu0.9Zr0.1-600) with the smallest Au particle size (2.15 nm) showed the best catalytic performance. Moreover, the glycerol conversion showed a linear relationship with the surface content of the Au0 i.e despite the similar particle size of Au/(Cu0.9Zr0.1-500) and Au/(Cu0.9Zr0.1-600), the better catalytic activity of Au/(Cu0.9Zr0.1-600) over Au/(Cu0.9Zr0.1-500) might be attributed to the higher surface content of active sites Au0 than that of Au/(Cu0.9Zr0.1-500) (Fig. 6b)[24, 48]. As previously reported the higher the content of Au0, the more the content of electron-rich gold species, the better the ability to activate molecular oxygen by nucleophilic attack [51]. In addition, the basicity of the catalysts also affected the catalytic behaviours (Fig. 6c). i.e. with the increase of the amount of the basic sites, the catalytic activity firstly increased and then decreased. On one hand, too much basic sites might accelerate the 18

adsorption of organic and inorganic species (by-products) on the surface of the catalysts [52] and decrease the catalytic activity. On the other hand, it was found that the coadsorbed hydroxide on the gold surface acted as a Bronsted base to assist the deprotonation of ethanol to form the adsorbed alkoxide to activate the ethanol [20]. Similarly, the basic sites (surface OH) could also activate the glycerol. Thus, the Au/(Cu0.9Zr0.1-700) catalyst with the least amount of basic sites showed a lower glycerol conversion, while the Au/(Cu0.9Zr0.1-600) catalyst with an appropriate amount of basic sites exhibited the best catalytic activity. 4. Conclusions Au supported on Cu and Zr mixed oxides calcined at different temperature catalysts were prepared by previously reported methods [33, 36], which showed excellent activity in the oxidation of glycerol to DHA under non-alkaline condition [8]. This work had demonstrated that the different calcination temperatures of the supports changed the crystal structure of CuO-ZrO2 [32], the surface composition and the basicity of the catalysts [46, 48], further affected the dispersion and chemical valence state of supported Au. The glycerol conversion also showed a volcano trend with the increased calcination temperature of support, which meant that the catalytic performance was closely related to the Au particle size, surface content of Au0 and the surface basicity of the catalyst, the results similar to those reported in literature [20, 24, 37-38, 51]. The best performance (C=72.7%, S=95.5%) was obtained when the calcination temperature of the support was 600 oC, which catalyst had the smallest Au particle size, the largest surface content of Au0, and a suitable number of surface basic sites. These correlations 19

would provide a basis for a better understanding of the surface catalysis involved in glycerol oxidation in base-free conditions over Au catalysts. Acknowledgements This work was financially supported by the Science Foundation for Young Scientists of Shanxi Province, China (201701D221052), Independent Research Project of the State Key Laboratory of Coal Conversion (2018BWZ002), the National Natural Science Foundation of China (21776294, 21802158) and Natural Science Foundation of Shanxi Province (201801D121070). References [1] S. Carrettina, P. McMorna, P. Johnston, K. Griffinb, C.J. Kiely, G.A. Attard, G.J. Hutchings, Top. Catal. 27 (2004) 131-136. [2] H. Kimura, K. Tsuto, Appl. Catal., A. 96 (1993) 217-228. [3] L.S. Ribeiro, E.G. Rodrigues, J.J. Delgado, X.W. Chen, M.F.R. Pereira, J.J.M. Órfão, Ind. Eng. Chem. Res. 55 (2016) 8548-8556. [4] W.B. Hu, D. Knight, B. Lowry, A. Varma, Ind. Eng. Chem. Res. 49 (2010) 1087610882. [5] H. Kimura, Appl. Catal., A. 105 (1993) 147-158. [6] Y. Meng, S.H. Zou, Y.H. Zhou, W.Z. Yi, Y. Yan, B. Ye, L.P. Xiao, J.J. Liu, H. Kobayashi, J. Fan, Catal. Sci. Technol. 8 (2018) 2524-2528. [7] L. Prati, A. Villa, C. Campione, P. Spontoni, Top. Catal. 44 (2007) 319-324. [8] S.S. Liu, K.Q. Sun, B.Q. Xu, ACS Catal. 4 (2014) 2226-2230.

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Graphical abstract

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Declaration of interest statement 25

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. There is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled“The effects of calcination temperature of support on Au/CuO-ZrO2 catalysts for oxidation of glycerol to dihydroxyacetone”.

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