ZrO2 supported NiCo nanoparticles in subcritical water medium

Accepted Manuscript Selective hydrogenation of phenol by the porous Carbon/ZrO2 supported Ni-Co nanoparticles in subcritical water medium

Liang He, Zhaodong Niu, Rongrong Miao, Qiuling Chen, Qingqing Guan, Ping Ning PII:

S0959-6526(19)30088-5

DOI:

10.1016/j.jclepro.2019.01.077

Reference:

JCLP 15464

To appear in:

Journal of Cleaner Production

Received Date:

10 November 2018

Accepted Date:

08 January 2019

Please cite this article as: Liang He, Zhaodong Niu, Rongrong Miao, Qiuling Chen, Qingqing Guan, Ping Ning, Selective hydrogenation of phenol by the porous Carbon/ZrO2 supported Ni-Co nanoparticles in subcritical water medium, Journal of Cleaner Production (2019), doi: 10.1016/j. jclepro.2019.01.077

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4723 words

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Selective hydrogenation of phenol by the porous Carbon/ZrO2 supported Ni-Co

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nanoparticles in subcritical water medium

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Liang He a, b, c, Zhaodong Niu a, d, Rongrong Miao a, Qiuling Chen a, Qingqing Guan a* and

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Ping Ning a*

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a Faculty of Environmental Science and Engineering, Kunming University of Science and

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Technology, 650500, Kunming, Yunnan, China;

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b BiomassChem Group, Faculty of Chemical Engineering, Kunming University of Science

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and Technology, 650500, Kunming, Yunnan, China;

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c State Key Laboratory of Pulp and Paper Engineering, South China University of

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Technology, Guangzhou, Guangdong, 510640, China

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d Soil Environment Monitoring Room, Yunnan Environmental Monitoring Center, 650100,

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Yunnan, China.

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Abstract: Phenol is a typical aromatic organic byproduct in the coking industry, which can be

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also as a raw material for producing some useful chemicals by catalytic hydrogenation. In this

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paper, by constructing carbon/ZrO2 porous support with abundant vacancies for loading

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nickel and cobalt nanoparticles (i.e., NixCoy@C/ZrO2) with using Zr-based metal organic

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frameworks as the precursor, the significant phenol converting enhancement in both

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hydrothermalcatalytic activity and stability has been fulfilled. Specially, the physiochemical

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characteristics of the NixCoy@C/ZrO2 particles have been analyzed, which verified the

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presence of Ni-Co alloy and its high homogeneity and dispersity. The catalytic results showed

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that the Ni3Co1@C/ZrO2 catalyst has the best catalytic performance at the desired temperature

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of 200 oC for 240 min, and the conversion of phenol is 96.33% while the selectivity of

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cyclohexanol is 91.14%. Kinetic study showed that the activation energy was only 44.31

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kJ/mol, which reflected that the hydrogenation could be easily carried out with

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Ni3Co1@C/ZrO2 catalyst. These findings can provide a good guidance for designing and

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developing the non-noble-metal catalysts for achieving the industrial utilization of phenol in

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coking wastewater in the future. *

Corresponding author. Tel.: +86 18287160060. E-mail address: [email protected] (Qingqing Guan); [email protected] (Ping Ning).

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Keywords: Catalytic hydrogenation; Coking industrial effluent; Metal-organic frameworks;

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Ni-Co bimetallic catalyst; Subcritical water medium;

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Abbreviations NixCoy@C/ZrO2

Nickel and cobalt nanoparticles loaded on ZrO2 and Carbon co-supports, which’s atoms ratio is x to y.

SCWO

Supercritical water oxidation

MOFs

Metal organic frameworks

UiO-66

A Zr-based MOFs

GC-FID

A gas chromatography equipped with a flame ionization detector

FESEM

Field-emission scanning electron microscopy

HR-TEM

High-resolution transmission electron microscopy

HAADFS-TEM

High-angle annular dark-field scanning TEM

XRD

X-ray diffraction instrument

EDX

Energy dispersive X-ray spectroscopy

XPS

X-ray photoelectron spectroscopy

32 33

1. Introduction

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As a typical aromatic organic pollutant with carcinogenic, teratogenic, mutagenic

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properties, phenol is usually found in many waste effluents, especially in the coking

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wastewater (Feng et al., 2017; Ontañon et al., 2018). In order to solve the pollution problem

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caused by phenol, many new-type adsorbents, such as porous active carbon, polymeric resins,

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zeolite have been studied (Liu et al., 2013; Shabtai and Mishael, 2018; Yang et al., 2013).

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Although these adsorbents have good efficiency on the phenol removal, the residual phenol

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adsorbed on solid phase would still be potential hazard to the environment since it could not

41

be fully recycled yet (Wang et al., 2012). Recently, a simple method based on a supercritical

42

water oxidation (SCWO) technique has been studied for the direct gasification of phenol into

43

CO2, H2 and H2O (Xu et al., 2011). However, its industrial application is greatly limited by

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the problems associated with the higher energy consumption and pressure requirement of the

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equipment.

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Due to the simplicity of reaction pathways, the process for converting phenol into some

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useful chemicals, e.g., cyclohexanone, cyclohexanol, and cyclohexane, by the liquid-phase

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catalytic hydrogenation in the sub/supercritical water system at a relatively low temperature

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and pressure has attracted more attention (Guan et al., 2017). In order to facilitate the

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hydrogenation of phenol, the Pd-based catalyst has been firstly used in our investigation

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(Guan et al., 2017; Guan et al., 2016). The results indicated that the hydrogenation can be

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more easily carried out with the Pa-based catalyst (the active energy = 31.87±3.48 kJ/mol).

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Since then many other noble metal catalysts, e.g., group VIII: Rh, Pt, and Ru, have been also

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screened for the hydrogenation of phenol in sub/supercritical water system (Guan et al., 2016;

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Zhang et al., 2018; Zhou et al., 2017). Clearly, the high cost and the availability of these noble

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metal materials limits their use in the industrial-scale application. Thus, the recent effort has

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been shifted to explore some alternative catalysts based on the non-noble metals, e.g., Ni, Co,

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Cu, and Zn (Artetxe et al., 2016; Liu et al., 2017; Nabgan et al., 2016). In the previous work

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(Guan et al., 2016; Wang et al., 2018), the Ni-based bimetallic and tri-metallic alloy catalysts

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were prepared and used to the hydrogenation of phenol. It was observed that their catalytic

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activities were greatly reduced, probably due to the trend of metal particle’s agglomeration

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during the application.

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Attributed to its high surface area and porosity, the metal-organic frameworks (MOFs)

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have attracted considerable attention to be used as a porous carbon-based template

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(supporting matrix) for achieving an excellent distribution of nanoscale metal particles (Ding

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et al, 2017; Niu et al., 2018; Ramezanalizadeh and Manteghi, 2018). Recently, Cao and

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coworkers (Cao et al., 2017) developed a good structure stabilized Ru-based catalyst (i.e.,

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Ru/ZrO2@C) for the hydrogenation of levulinic acid, in which the C-based support was

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obtained by thermal decomposition of the Zr-based MOF (i.e., UiO-66). Wang et al. (Wang et

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al., 2018) reported a MOF based subnanometer Ni-Co bimetallic alloy catalyst, i.e.,

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NiCo/SiO2-MOF, for selective converting furfuryl alcohol into tetrahydrofurfuryl alcohol. It

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was found that there had a synergistic effect to enhance the catalytic ability in the presence of

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Ni and Co. Therefore, it is expected that the Ni-Co based metal catalyst supported by MOF

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might be effective to be used for efficiently converting phenol in sub/supercritical water

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system.

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In this study, the Ni- and Co-based metal catalysts (i.e., NixCoy@C/ZrO2) supported by a

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Zr-based MOF were synthesized. The main focuses were to characterize the structure and

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morphology of the catalysts and examine their catalytic activity and stability on the

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hydrogenation of phenol in subcritical water system. A robust mathematical model for

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describing the catalytic hydrogenation process of phenol was also established.

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2. Materials and methods

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2.1. Materials

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Zirconium (IV) chloride (ZrCl4, 98%, Macklin), 1,4-benzenedicarboxylic acid (H2BDC,

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99%, Macklin), nickel nitrate hexahydrate (Ni(NO3)2·6H2O, 99%, Sino-platinum Metals Co.,

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Ltd), cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 99%, Sino-platinum Metals Co., Ltd),

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acetone (99.5%, Sigma-Aldrich), HCl (36~38%, Shanghai Shabo Chemical Technology Co.,

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Ltd), N,N’-dimethylformamide (DMF, 99.5%, Macklin), glacial acetic acid (98%, Macklin),

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cetyltrimethylammonium bromide (CTAB, 99%, Sigma-Aldrich) and phenol (99.5%,

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Sigma-Aldrich). All agents were used without further purification.

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2.2. Methods

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2.2.1. Synthesis of NixCoy@C/ZrO2 catalysts

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The UiO-66 was prepared according to the method reported previously (Guan et al.,

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2017), in which 0.34 g of ZrCl4, 0.726 g of H2BDC, 30 mL of DMF, 24 mL of glacial acetic

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acid and 2.5 mL of HCl (35.5% wt.) were added into a 100 mL Teflon-lined stainless steel

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autoclave. The reaction was conducted at 120 oC for 3 h under autogenous pressure. Then, the

101

precipitate was isolated by filtration, followed by DMF washing (three times). Finally, the

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precipitate was dried at 80 oC for 24 h in a vacuum oven.

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The NixCoy@C/ZrO2 catalysts were prepared by a simple incipient-wetness

104

impregnation method (Zhang et al., 2015), in which 0.5 g UiO-66 and 0.15 g CTAB were

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dispersed in 10 mL H2O by the sonication at room temperature for 30 min. Then, the metal

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precursors (Ni(NO3)2·6H2O and Co(NO3)2·6H2O, 15 % wt.) were added and dispersed in

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another 10 mL H2O. The solution of metal precursor was added into the UiO-66 and CTAB

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mixtures for ultrasonically agitating about 30 min and then transferred into a water bath

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(60~70 oC) with a vigorous stirring until the slurry was coagulated. It was dried in a vacuum

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oven for overnight and then the solid was heated at 600 oC for 3 h with a nitrogen atmosphere.

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The prepared black powders were denoted as NixCoy@C/ZrO2, where x and y represent the

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molar ratio of Ni-Co.

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2.2.2. Hydrogenation of phenol

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A 6.6 mg of phenol, 6.6 mg of the NixCoy@C/ZrO2 catalyst and 0.653 mL of ultrapure

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water were added to a 4-mL stainless steel cylinder reactor (Swagelok Company, USA). The

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reactor was purged with hydrogen gas (0.2 MPa) for three times. Then, the reactor was heated

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to a desired temperature in a Techne fluidized sand bath (Techne, model SBL-2, USA). After

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reaction, the reactor was removed out and cooled to room temperature. The products were

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analyzed by a gas chromatography (Agilent, 7820A, USA) equipped with a flame ionization

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detector (GC-FID) and an HP-5 column (30 m × 0.32 mm).

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2.3. Methods and instruments for characterizing analysis

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The solid samples were examined by an X-ray diffraction (XRD) analysis using a Bruker

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D8 Advance diffractometer system (40 kV, 40 mA) with Cu Ka monochromatic radiation

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source (1.5406 Å) in the 2θ (range from 5 to 90o) at a scanning speed of 6o/min; the N2

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sorption isotherms measured at -196 oC using a the BET-N2 instrument (Micromeritics

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Trustar II 3020); the field-emission scanning electron microscopy (FESEM) images and

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element mapping collected by a Nova Nano SEM 450 instrument with an energy dispersive

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X-ray (EDX) detector; The high-resolution transmission electron microscopy (HR-TEM),

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high-angle annular dark-field scanning TEM (HAADFS-TEM) and energy-dispersive X-ray

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spectroscopy (EDX) conducted on a FEI Tecnai G2 TF30 S-Twin instrument; and the X-ray

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photoelectron spectroscopy (XPS) analysis carried out on an ULVAC PHI 5000 Versa

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Probe-II instrument.

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3. Result and discussion

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3.1. The primary information of NixCoy@C/ZrO2 catalysts

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3.1.1. Effect of Ni-Co ratio on the structure of catalysts

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Figure 1. XRD patterns of (a) UiO-66 and (b) NixCoy@C/ZrO2 catalysts (Note: the inset is

145

the peaks at around 44.50o).

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Figure 1 shows the XRD patterns of UiO-66 and their derived catalysts. As seen in

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Figure 1a, the XRD patterns of UiO-66 matched well with the simulated standard patterns

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reported in the literature (Cavka et al., 2008). This indicates that the UiO-66 was successfully

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prepared in this work. Figure 1b shows that the similar diffraction peaks, i.e., at 30.21o (111),

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34.98o (200), 50.29o (220) and 59.98o (311), for all catalysts, which corresponds to the

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characteristic peaks from the tetragonal ZrO2 (Cao et al., 2017; Hengne and Rode, 2012; Yan

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et al., 2015). The XRD patterns of the Ni@C/ZrO2 catalysts shown in Figure 1b exhibits three

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diffraction peaks at around 44.50o, 51.86o and 76.38o, respectively, which agree to the (111),

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(200) and (220) of the face-centered cubic (fcc) crystal structure of Ni (JCPDS no. 04-0850)

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(Jia et al., 2017). Similarly, the XRD patterns of the Co@C/ZrO2 catalyst also matched well

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with the fcc metallic Co (JCPDS no. 15-0806) (Tong et al., 2017). For the patterns of the

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NixCoy@C/ZrO2 catalysts, the observed peak positions were slightly shifted and lied between

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the pure fcc Ni and Co, indicating the formation of NiCo alloy during the pyrolysis treatment

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(Wang et al., 2018). Meanwhile, the carbon structures of the MOFs also acted as a reduction

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reagent to reduce the metal ions to metal phases (Tong et al., 2017).

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Figure 2. (a) N2 sorption isotherms (filled symbols: adsorption; open symbols: desorption) of

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NixCoy@C/ZrO2 catalysts at -196 oC; and (b) the corresponding pore-size distribution curves.

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Figure 2 shows the N2 sorption isoterms of NixCoy@C/ZrO2 catalysts, in which some

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hysteresis loop were observed for the isotherms at relative pressure from 0.4 to 1.0 with

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typical type-IV curves. It indicates that the presence of the remarkable micro/mesoporous

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structures in the catalysts (Li et al., 2017). These mesoporous structures would be helpful to

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enhance the activity and stability of catalysts during hydrothermal hydrogenation reaction.

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Additionally, it could be also observed that the surface area (BET method) and pore volume

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of the NixCoy@C/ZrO2 catalysts were decreased sharply after pyrolysis treatment as shown in

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Table 1, indicating that the pore structure was collapsed and large bimetallic nanoparticles

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was formed (Long et al., 2016). It should be pointed out that all of the calculated surface area

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based on Langmuir method has high deviation, which reflects that the N2 monolayer

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adsorption and desorption process is not suitable for description of the pore structure of the

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NixCoy@C/ZrO2 catalysts.

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Table 1. Surface areas and pore distribution of the catalysts. Entry

Materials

SBET m2/g

SLangmuir m2/g

PV cm3/g

1

Co@C/ZrO2

24.4

—

0.134

2

Ni1Co3@C/ZrO2

19.0

—

0.112

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Ni2Co3@C/ZrO2

63.7

251.3±23.9

0.084

4

Ni1Co1@C/ZrO2

31.0

622.0±296.5

0.100

5

Ni3Co2@C/ZrO2

72.9

731.3±186.7

0.187

6

Ni3Co1@C/ZrO2

105.4

706.6±133.2

0.235

7

Ni@C/ZrO2

47.1

1145.4±764.0

0.184

179 180

3.1.2. Effect of Ni to Co ratios in the NixCoy@C/ZrO2 catalysts on the phenol hydrogenation

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Figure 3 shows the effect of Ni-Co ratio on the phenol hydrogenation (for the conversion

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and selectivity, respectively) at a given condition (200 oC, 240 min). It can be seen that the

183

poor conversion and selectivity were achieved in the single metal (Ni or Co) system, although

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the performance of single Ni containing catalyst was better than that of single Co containing

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catalyst. However, both the conversion and selectivity can be greatly improved in the

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catalysts co-existing Ni and Co metal ions (whatever in different ratios). When the ratio of Ni

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to Co in the catalyst is 3:1, the highest conversion of phenol (99.90%) was achieved, and the

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corresponding selectivity (to form cyclohexanol) was 95.3% (the best among all different

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Ni:Co ratios). Therefore, the Ni3Co1@C/ZrO2 catalyst was chosen in the rest study.

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Figure 3. Effect of Ni-Co ratio on the phenol hydrogenation (200 oC, 240 min).

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3.2. Characterization of Ni3Co1@C/ZrO2 catalyst

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The surface morphology of Ni3Co1@C/ZrO2 catalyst (after the pyrolysis treatment) was

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examined by TEM as shown in Figure 4. It can be observed that the Ni-Co alloy nanoparticles

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were homogeneously dispersed on the support (i.e., porous C & ZrO2 mixtures) with the

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particle sizes range from 16 to 52 nm (average size = 38.8 nm). The thickness of graphite

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shell is about 6 nm covered on the surface of Ni-Co alloy nanoparticles (see Figure 4b).

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Figure 4c shows that the interplanar spacing of nanoparticle lattice was 0.205 nm, which is

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between the (111) plane spacing of metallic Ni (0.201 nm) and Co (0.214 nm) (Li et al.,

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2017). Figures 4e and 4f show the HAADF-STEM image and the corresponding EDX line

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scanning profiles of the Ni3Co1@C/ZrO2 catalyst, respectively. The EDX line profiles

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indicate that the weight ratio of Ni to Co in catalysts is about 3:1, which is consistent with the

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initial addition of Ni and Co during the preparation.

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Figure 4. Morphology characterization of the Ni3Co1@C/ZrO2 catalyst: (a-c) HRTEM images

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and (d) corresponding particle size distribution, (e) HAADF-STEM image, and (f)

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corresponding EDX line profiles.

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The microstructure of the synthesized Ni3Co1@C/ZrO2 catalyst and its corresponding

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elemental mapping images were further examined by both SEM and EDX, as shown in Figure

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5. Some amorphous nanosheets can be clearly observed after the pyrolysis treatment (Figure

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5a), which obtain the exposed active metal sites on the surface. As shown in Figures 5b and c,

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the elemental distribution of Ni and Co is obviously overlapped, further indicating the

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formation of Ni-Co alloy nanoparticles. Additionally, it has been also revealed that the Zr and

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O elements have a well-proportioned distribution and thus confirmed that the formed ZrO2

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could be acted as a nanoscale co-support for loading Ni and Co metal species (see Figures 5e

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and 5f).

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Figure 5. Characterization of the Ni3Co1@C/ZrO2 catalyst: (a) Low magnification SEM

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image and (b-f) corresponding EDX mapping images.

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Figure 6 shows the electronic properties of Ni3Co1@C/ZrO2, Ni@C/ZrO2 and

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Co@C/ZrO2 catalysts analyzed by XPS, in which two main peaks at binding energies of

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854.3 eV (2p3/2) and 873.0 (2p1/2) eV are observed in the Ni 2p XPS spectra (see Figure 6a).

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They are corresponded to the Ni(0) and Ni(II), respectively (Long et al., 2016; Wang et al.,

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2018). The results also indicate that the Ni(II) ions have been reduced to Ni(0) during the

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pyrolysis treatment. The peaks area ratio between Ni(0) and Ni(II) is 25:75 and most of Ni is

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present in an oxidation state, indicating that the NiCo nanoparticles were partially oxidized.

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Compared to the monometallic Ni@C/ZrO2 catalyst, the Ni 2p3/2 peaks for Ni3Co1@C/ZrO2

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catalyst are shifted to the high binding energy by 0.42 eV (Ni(0)) and 0.59 eV (Ni(II)) (see

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Figure 6b). The similar shifts also exhibited by Ni3Co1@C/ZrO2 and Co@C/ZrO2 catalysts

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(see Figure 6c and d). All above results further confirmed the formation of NiCo alloy

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nanoparticles (Jia et al., 2017) and are consistent with the results from the XRD, TEM and

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SEM analysis on the Ni3Co1@C/ZrO2 catalyst examination.

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Figure 6. XPS spectra of Ni3Co1@C/ZrO2, Ni@C/ZrO2 and Co@C/ZrO2 catalyst: (a) Ni 2p,

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(b) Ni 2p3/2, (c) Co 2p, and (d) Co 2p3/2.

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3.3. Catalytic activity of Ni3Co1@C/ZrO2

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The Ni3Co1@C/ZrO2 catalyst was applied to the hydrogenation of phenol at the

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temperature of 120, 150, 180 and 200 oC, respectively, as shown in Figure 7, in which the

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loading of phenol was 2 wt% (20,000 mg/L). It can be seen that the catalytic conversion of

244

phenol is 51.29% at 120 oC for 5 h in the presence of Ni3Co1@C/ZrO2 catalyst, which is much

245

higher than about 40% by using Ni3Co1@ N-doped carbon catalyst (at 100oC and for 12 h)

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reported previously (Li et al., 2017). A near complete conversion of phenol can be achieved at

247

180oC for 240 min or 200 oC for 200 min. This verified that the present Ni3Co1@C/ZrO2

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catalyst has a high efficiency on the hydrogenation of phenol.

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Figure 7. Conversion of phenol at different temperatures in subcritical water with

251

Ni3Co1@C/ZrO2.

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Figure 8 shows the percent conversion of phenol and yield of products at 120, 150, 180

253

and 200 oC, respectively. It can be seen that the main products from hydrogenation of phenol

254

contains cyclohexanol, cyclohexane, cyclohexanone and benzene, in which cyclohexanol was

255

a dominant product. At 120 oC, the yield of cyclohexanol reached to 52.69%, and the

256

selectivity of cyclohexanol was also significantly improved with the time increased (see

257

Figure 8a). The highest yield of cyclohexanol (ca. 91.14%) was achieved at 200 oC and 300

258

min, similar to the data collected with 200 and 240 min, as shown in Figure 8d. The results

259

indicate that the reaction rate of hydrogenation of phenol to cyclohexanol increased gradually

260

with the temperature increase. In order to obtain a higher yield of cyclohexanol, the reaction

261

temperature and time should be fixed within 200 oC for 200 min.

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Figure 8. The conversion of phenol and yield of products catalyzed by Ni3Co1@C/ZrO2 at (a)

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120 oC, (b) 150 oC, (c) 180 oC, and (d) 200 oC.

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3.4. Chemical reaction kinetics of phenol hydrogenation

268 269

3.4.1. Establishment of the model

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In order to deeply insight the hydrogenation kinetics of phenol in subcritical water

271

system, a quantitative model was developed. Based on the above analysis, there are two

272

postulated pathways for the hydrogenation of phenol, as shown in Figure 9a. In the first route,

273

the phenol is hydrogenated to cyclohexanone, and then transformed into cyclohexanol. A part

274

of cyclohexanol is further hydrogenated to cyclohexane. The second route is that the hydroxyl

275

group in phenol is directly removed to form benzene. As shown in Figure 8, due to the

276

selectivity of cyclohexanone, cyclohexane and benzene is much poor than that of

277

cyclohexanol (i.e., k1, k3, k4 ≪ k2), the above reaction routes could be simplified as an

278

efficient directed reaction from phenol to cyclohexanol (see Figure 9b).

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Figure 9. Reaction routes of phenol hydrogenation in HTW with Ni3Co1@C/ZrO2.

281 282

According to the simplified route, the rate of phenol, i.e., Cphenol can be given as below:

283

―

𝑑𝐶𝑝ℎ𝑒𝑛𝑜𝑙 𝑑𝑡

= 𝑘 ∙ 𝐶𝑛𝑐𝑦𝑐𝑙𝑜ℎ𝑒𝑥𝑎𝑛𝑜𝑙

(1)

284

where, the k and n represents the reaction rate constant and reaction order (n≠1), respectively.

285

Integrate Eq. (1) to have ―𝑛 ― (1 ― 𝑛) ∙ 𝑘 ∙ 𝑡] 𝐶𝑝ℎ𝑒𝑛𝑜𝑙 = [𝐶1𝑝ℎ𝑒𝑛𝑜𝑙,0

286

1 (1 ― 𝑛)

(2)

287

where, Cphenol,0 represent the original concentration of phenol.

288

Thus, the real-time concentration of produced cyclohexanol, Ccyclohexanol can be described as

289

follow, i.e., ―𝑛 ― (1 ― 𝑛) ∙ 𝑘 ∙ 𝑡] 𝐶𝑐𝑦𝑐𝑙𝑜ℎ𝑒𝑥𝑎𝑛𝑜𝑙 = 𝐶𝑝ℎ𝑒𝑛𝑜𝑙,0 ― [𝐶1𝑝ℎ𝑒𝑛𝑜𝑙,0

290

1 (1 ― 𝑛)

(3)

291

where, Ccyclohexanol represent the real-time concentration of produced cyclohexanol.

292

According to Arrhenius equation, the reaction rate constant can be described as: 𝐸𝑎

293

(4)

𝑙𝑛𝑘 = 𝑙𝑛𝐴 ― 𝑅𝑇

294

where, Ea and A represents the activation energy and frequency factor, respectively.

295

According to equations (3) and (4), the real-time concentration of produced cyclohexanol can

296

be written as

297

―𝑛 𝐶𝑐𝑦𝑐𝑙𝑜ℎ𝑒𝑥𝑎𝑛𝑜𝑙 = 𝐶𝑝ℎ𝑒𝑛𝑜𝑙,0 ― [𝐶1𝑝ℎ𝑒𝑛𝑜𝑙,0 ― 𝐴 ∙ (1 ― 𝑛) ∙ 𝐸𝑋𝑃[ ―𝐸𝑎 (𝑅𝑇)] ∙ 𝑡]

1 (1 ― 𝑛)

(5)

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3.4.2. Parameters determination and model evaluation

300

In order to calculate the parameters (i.e., Ea, A and n) in equation (5), the real-time

301

concentration data of cyclohexanol at different temperature and time (shown in Figure 7)

302

were used. Through the fitting process, the value of the above parameters can be obtained, in

303

which Ea is 44.31 kJ/mol, A is 135.98 and the reaction order (n) is 1.15, respectively. The

304

value of activation energy in the present catalytic system is comparable to that of Pd-based

305

catalyst and is much lower than that of the monometallic Ni-based catalyst reported

306

previously (Zhao et al., 2012). The results suggest that there is a synergetic effect in the

307

presence of both Co and Ni in the catalyst on the selective hydrogenation of phenol to

ACCEPTED MANUSCRIPT 308

cyclohexanol. The reaction order is nearly 1, indicating that the hydrogenation of phenol in

309

subcritical water media is a pseudo first-order process. From Figure 10a, it can be seen that

310

there is a good correlation between the predicted value and the measured value (R-square

311

value = 0.906). Moreover, in order to further explore the robustness of the developed model,

312

the relative deviations (RD, %) between the predicted and measured values was also

313

calculated (shown in Figure 10b). It can be seen that most of RD data of predicted values is

314

within 10.0%, indicating that the present model can provide a good description for the phenol

315

hydrogenation. The kinetics results theoretically confirm that the hydrogenation of phenol can

316

be easily carried out with the present Ni- and Co-based catalyst.

317 318

Figure 10. The correlation (a) and relative deviation (b) between the predicted and measured

319

data.

320

3.5. The main challenge for industrial application

321

According to the catalytic performance of the Ni3Co1@C/ZrO2 nanopowders exhibited

322

above, it can be known that the catalytic activities can be comparable with some noble metal

323

catalysts for hydrogenation of phenol. At the desired temperature at 200 oC and time with 200

324

min, the highest conversion of phenol and the selectivity of the main product, i.e.,

325

cyclohexanol, can be achieved. Besides, due to that the reaction in the hydrogenation was

326

carried out in water system with 0.2 MPa of H2; the minimum pressure-resistant capacity of

327

the reaction vessel should be up to 1.75 MPa, which is the main challenge for industrial

328

application. However, compared to the SCWO technique which is usually performed at 374.3

329

oC

330

acceptable.

with the pressure of 22.06 MPa at least, the operation condition of the present work can be

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4. Conclusion

333 334

The characterization and performenace of porous C/ZrO2 supported Ni-Co bimetal

335

catalysts were studied and the results showed that Ni3Co1@C/ZrO2 bimetallic alloy catalyst

336

has an outstanding performance for hydrogenation of phenol. The calculated activation

337

energy (Ea) is only 44.31 kJ/mol, which reflected that the hydrogenation can be easily carried

338

out with Ni3Co1@C/ZrO2 catalyst. The results also suggest that there is a synergetic effect in

339

the presence of both Co and Ni in the catalyst on the selective hydrogenation of phenol to

340

cyclohexanol. The present findings can provide a technical feasibility for using the

341

NixCoy@C/ZrO2 catalyst to directly convert phenol in coking industrial effluent in the future.

342 343

Acknowledgements

344 345

This work was jointly supported by the National Natural Science Foundation of China

346

(21767015) and the Foundation of State Key Laboratory of Pulp& Paper Engineering, China

347

(No. 201745; No. 201810).

348 349

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ACCEPTED MANUSCRIPT Highlights * Phenol is a typical aromatic organic byproduct of the coking industries. * The Ni3Co1@C/ZrO2 catalyst has been prepared. * Phenol can be easily converted into cyclohexanol with Ni3Co1@C/ZrO2 catalyst. * The Ni-Co alloy and its dispersity play an important role on the selective catalysis.