Al2O3–SiC foam catalysts and powder catalysts for the liquid-phase hydrogenation of benzaldehyde

Accepted Manuscript Title: A comparative study of Ni/Al2 O3 –SiC foam catalysts and powder catalysts for the liquid-phase hydrogenation of benzaldehyde Authors: Kai Li, Yilai Jiao, Zhenming Yang, Jinsong Zhang PII: DOI: Reference:

S1005-0302(18)30199-3 https://doi.org/10.1016/j.jmst.2018.09.018 JMST 1309

To appear in: Received date: Revised date: Accepted date:

15-3-2018 12-4-2018 2-5-2018

Please cite this article as: Li K, Jiao Y, Yang Z, Zhang J, A comparative study of Ni/Al2 O3 –SiC foam catalysts and powder catalysts for the liquid-phase hydrogenation of benzaldehyde, Journal of Materials Science and amp; Technology (2018), https://doi.org/10.1016/j.jmst.2018.09.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A comparative study of Ni/Al2O3–SiC foam catalysts and powder catalysts for the liquid-phase hydrogenation of benzaldehyde

a

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Kai Li a,b, Yilai Jiao a, Zhenming Yang a, Jinsong Zhang a*

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese

University of Chinese Academy of Sciences, Beijing 100049, China

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b

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Academy of Sciences, Shenyang 110016, China

*

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E-mail address: [email protected] (J. Zhang).

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Corresponding author. Tel.: +86 24 23971896, Fax: +86 24 23971896.

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Graphical abstract

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[Received 15 March 2018; Received in revised form 12 April 2015; Accepted 2 May 2018]

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Abstract In this study, Al2O3-washcoated SiC (Al2O3–SiC) foams and Al2O3 powder were employed as the supports of a Ni catalyst for the liquid-phase hydrogenation of benzaldehyde. A series of Ni/Al2O3–SiC foam catalysts and Ni/Al2O3 powder catalysts with a Ni loading

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from 10 wt% to 37 wt% of the weight of Al2O3 were first prepared by a deposition– precipitation (DP) method. The catalytic activity and recyclability of both kinds of catalysts

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were then compared. Although it had a smaller accessible surface area with the reactant, the foam catalyst with a Ni loading of 16 wt% exhibited a slightly higher conversion of

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benzaldehyde after 6 h (of 99.3%) in comparison with the Ni/Al2O3 catalyst with identical Ni

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loading (conversion of 97.5%). When the Ni loading increased from 16 wt% to 37 wt%, the

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reaction rate obtained with the foam catalyst increased significantly from 0.108 to 0.204 mol

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L−1 h−1, whereas the reaction rate obtained with the powder catalyst increased from 0.106 to

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0.123 mol L−1 h−1. Furthermore, the specific activity (moles of benzaldehyde consumed by 1 g min−1 of Ni) of the foam catalyst with a Ni loading above 30 wt% was superior to that of the

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powder catalyst because of its smaller Ni-particle size and higher mass-transfer rate. The

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foam catalyst displayed a high recyclability as a function of run times owing to the strong interaction between the Ni component and the Al2O3 coating. The conversion of benzaldehyde over the foam catalyst remained almost unchanged after being used 8 times. In comparison, a

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drop of 43% in the conversion of benzaldehyde with the powder catalyst was observed after being used 7 times due to the leaching of the Ni component.

Keywords: Foam catalyst; Powder catalyst; Benzaldehyde hydrogenation; Ni loading; 2

Recyclability

1. Introduction Benzyl alcohol is an important intermediate for the synthesis of pharmaceuticals and fine

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chemicals [1-3]. The liquid-phase hydrogenation of benzaldehyde is a green process for the production of benzyl alcohol because it occurs under milder conditions compared with the

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gas-phase hydrogenation process. The supported Ni catalyst has been widely used for the hydrogenation of benzaldehyde due to its high catalytic activity and relatively low price [4-6].

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However, there are several drawbacks for supported powder catalysts, such as the abrasion of

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catalyst particles under a vigorous agitating condition and agglomeration of catalyst particles

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in the low shear section in a stirred reactor [7]. In addition, the separation of the fine-particle

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catalysts from the reaction mixture is troublesome, time-consuming and costly.

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In recent years, structured catalysts with a large geometrical surface area and thin catalytic layer, which consists of active component and coating materials, such as activated carbon [8],

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silicon oxide [9] and alumina [10], have gained growing attention because they can enhance

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mass and heat transfer in catalytic reaction [11, 12]. Moreover, the attrition of structured catalysts is absent and they are easily separated from the reaction solution in the liquid-phase reaction [11]. Hence, structured catalysts have been considered as a promising alternative to

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the conventional catalyst for the three-phase catalytic system [13-16]. To date, monolithic honeycomb, membrane, cloths and foams have been used successfully as structured supports of the catalyst for multiphase reactions. Among them, monolithic honeycomb and solid foam materials have received the most extensive attention [17-19]. 3

Compared to monolithic honeycomb catalysts with the straight parallel channels, foam structured catalysts with three dimension interconnected structure show a higher radial mixing originated from the flow passing through its porous matrix, which can enhance mass and heat transfer for catalytic reactions more effectively [17, 20, 21]. Some attempts have been made

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to develop the foam catalyst both as the rotating stirrer and as the catalyst in the liquid-phase catalytic process [22-25]. Recently, foam structured catalysts based on SiC solid foams, which

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exhibit a high mechanical strength, a high thermal conductivity and an excellent resistance to

oxidation and corrosion [26-29], have been demonstrated to be a promising application in

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liquid-phase reactions. To the best of our knowledge, the study of the liquid-phase

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hydrogenation of benzaldehyde over foam catalysts has not been reported, yet.

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In this work, firstly, the SiC foam as the structured support was coated with a γ-alumina

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layer by a washcoating method. Then, the active component Ni was deposited on the Al2O3–

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SiC foam support by the deposition–precipitation (DP) method. The liquid-phase hydrogenation of benzaldehyde over the Ni/Al2O3–SiC catalyst as the stirrer blade was carried

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out in a stirred reactor. For comparison, γ-alumina powder supported Ni catalysts (Ni/Al2O3)

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were also prepared by the DP method. The as-prepared catalysts were characterized by N2 absorption–desorption, scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and H2 temperature-programmed reduction

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(H2-TPR). The influence of the Ni loading on catalytic performance of the two kinds of catalysts was systematically investigated. Furthermore, the recyclability of the Ni/Al2O3–SiC foam catalyst was compared with that of the Ni/Al2O3 powder catalyst in this work. 2. Experimental 4

2.1 Synthesis of SiC foams Rectangle-shaped SiC foams with dimensions of 8 mm × 10 mm × 60 mm were synthesized by a macromolecule pyrogenation method combined with controlled melt infiltrating reaction sintering. The detail of synthetic procedure of SiC foams was described in

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our previous work [30]. The three-dimensional (3D) reticulated SiC foams (15 pores per linear inch, PPI) possessed evenly distributed and well-connected millimeter-scale open pores

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(about 75% open porosity). In order to remove the residual Si from the surface of foam struts, pristine SiC foams were treated with a boiling NaOH solution (10 M) for 15 min. Afterwards,

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these foams were cleaned in the ultrasound bath for three times and then dried at 120 °C for 6

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h.

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2.2 Preparation of Al2O3 coating

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In order to improve its specific surface area, the bare SiC foam was coated with an Al2O3

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layer by a washcoating method [31]. The aqueous slurry, which was used for the washcoating, consisted of 15 wt% γ-alumina powder (AR, Zibo Nuoda Chemical Co., Ltd.) with the

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median diameter of about 15 μm, 10 wt% colloidal pseudo-boehmite (20 wt%, Aldrich),

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polyvinyl alcohol (AR, Sinopharm Chemical Reagent Co., Ltd), nitric acid (AR, Sinopharm Chemical Reagent Co., Ltd) and deionized water. Firstly, the slurry was mixed for 4 h by a ball-milling method. Then the pH value of the slurry was adjusted to about four with 8 M

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nitric acid. The SiC foam was immersed into the aqueous slurry for 30 s, and then was treated by a high-speed centrifugal machine to remove the excess of slurry. The coated SiC foam was dried at 300 °C for 30 min before the next washcoating. The desired amount of Al2O3 was obtained by repeating the above washcoating step. Finally, the washcoated SiC foams were 5

calcined at 600 °C for 2 h. The amount of Al2O3 coating was measured by the change in weight of the foam support before and after the washcoating. The coating thickness was almost linearly with increased amount of the loaded Al2O3 (Fig. S1 in Supplemental file). In this work, the Al2O3–SiC foam with Al2O3 coating of 0.4 g (thickness of about 25 μm) was

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employed as the structured catalyst support. 2.3 Ni deposition on Al2O3-SiC support

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The nickel was deposited on the Al2O3-washcoated SiC foam (Al2O3–SiC) by the DP method [10]. The Al2O3–SiC foam support was immersed into a mixed solution of 0.4 M

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Ni(NO3)2·6H2O (AR, Sinopharm Chemical Reagent Co., Ltd) and 0.83 M urea (AR,

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Sinopharm Chemical Reagent Co., Ltd) at 90 °C for 2–7 h. The amount of Ni on the catalytic

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coating was controlled by varying the DP time. After the DP procedure, the wet Ni/Al2O3–SiC

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catalyst was rinsed with deionized water for three times and then dried at 120 °C for 10 h. The

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dried catalyst was calcined in air at 400 °C for 2 h. After calcination, the Ni/Al2O3–SiC catalyst was further reduced in flowing H2 (100 mL/min) in a quartz tube furnace at 500 °C

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for 4 h. The amount of the loaded Ni was measured by inductively coupled plasma optical

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emission spectrometry (ICP-OES). In this paper, the foam catalyst is coded as w Ni/Al2O3– SiC, where w denotes Ni loading referred to the weight percentage of the Al2O3 coating. A series of foam catalysts with Ni loading from 10 wt% to 37 wt% of the Al2O3 coating was

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prepared and applied to the liquid-phase hydrogenation of benzaldehyde. 2.4 Preparation of powder catalysts To better understand characteristics of the foam catalyst (Ni/Al2O3–SiC), the Al2O3 powder supported nickel catalysts (Ni/Al2O3) with Ni loading from 10 wt% to 37 wt% were 6

also prepared by the DP method. Prior to deposition of the active species, a raw alumina powder (AR, Zibo Nuoda Chemical Co., Ltd.) above employed for washcoating was calcined at 600 °C for 2 h to obtain a stable γ-alumina phase. The median diameter of γ-alumina powder as catalyst support was around 15 μm. A mixed aqueous solution consisted of 0.4 M

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Ni(NO3)2·6H2O as the Ni precursor and 0.83 M urea as the precipitating agent was prepared. The solution was added to a stirred aqueous suspension including the Al 2O3 powder, which

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was kept at 90 °C by using a thermostatic water bath. After the DP procedure, the powder catalyst was filtrated and washed with deionized water for three times. Then the powder

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catalyst was dried at 120 °C overnight and calcined at 400 °C for 2 h with a heating rate of

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2 °C/min to decompose Ni(NO3)2·6H2O into nickel oxide (Fig. S2). After the calcination,

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2.5 Catalyst characterization

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powder catalysts were reduced under the same condition as mentioned in Section 2.3.

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The stability of the Al2O3 coating was evaluated using the ultrasonic vibration test. After the testing, the weight loss of the sample was measured. The ultrasonic vibration equipment

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with a power of 40 kHz was used. The ICP-OES experiment was performed (IRIS Intrepid) to

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determine the amount of active component Ni in the catalyst. The morphology of the foam catalyst support was characterized by using an Inspect F50 field emission scanning electron microscope. The crystal phase of the catalysts was analyzed by X-ray diffraction on an X-ray

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diffractometer (D/Max-2500PC) with Cukα radiation. The specific surface area, pore volume and average pore size of the catalysts were measured using N2 adsorption–desorption measurements at 77 K by using a Micromeritics 3Flex Surface Characterization Analyzer. The specific surface area (SBET) of the catalysts was calculated from the nitrogen isotherm by the 7

BET method. The average pore size (DBJH) and the pore volume (VBJH) were calculated by the BJH method. The mercury intrusion experiment was carried out with mercury porosimetry (Micromeritics AutoPore IV 9500). TEM images were obtained with an FEI Tecnai G2 F30. The H2-TPR analysis was carried out in a chemisorption analyzer (Micromeritics Autochem II

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2920) under 5% H2 in a H2/Ar mixture with a flow rate of 50 mL/min. The temperature was increased from 50 °C to 700 °C with a linear heating rate of 10 °C/min. The consumption of

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hydrogen was recorded as a function of temperature by using a thermal conductivity detector (TCD).

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2.6 Catalyst testing

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The scheme of the selective hydrogenation of benzaldehyde is presented in Scheme 1 [32].

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In this continuous hydrogenation reaction, benzyl alcohol as intermediate was the target

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product.

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The hydrogenation of benzaldehyde was carried out in a high-pressure vessel equipped with four baffles. The sketch of the structured catalyst (structured reactor) for the liquid-phase

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hydrogenation of benzaldehyde is illustrated in Fig. 1. In this rotating stirred reactor, the foam

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catalyst both as a stirrer blade and as the catalyst for the hydrogenation reaction was mounted at the endpoint of the mechanical agitating shaft. The powder catalyst was tested by using a stainless steel blade as the stirrer with the same dimension as the SiC foam blade. The dosage

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of the powder catalyst for each of hydrogenation experiments was 0.4 g, which was almost equal to the mass of the Al2O3 coating on the foam catalyst. All catalytic hydrogenation experiments were performed with both of the foam catalyst and the powder catalyst under the same condition. The benzaldehyde of 10 mL (≥ 98.5%, Aladdin) as a reactant and the 8

isopropanol of 140 mL (≥ 99.7%, Aladdin) as a solvent were used. The reaction pressure of 2 MPa, a rotating rate of 300 rpm and the reaction temperature at 90 °C were kept during the hydrogenation process. In order to monitor the course of the reaction, liquid products were withdrawn at regular intervals and analyzed by a gas chromatography (GC, Agilent 7890B)

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using n-nonane (≥ 99%, Aladdin) as the internal standard. The GC instrument was equipped with a capillary column (HP-5, 30 m × 250 μm × 0.25 μm) and a flame ionization detector

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(FID). Conversion of benzaldehyde (X) and selectivity (S) of benzyl alcohol are calculated by the following equations:

S=

nBOL, generated

nBAL, initial

nBAL, consumed

×100 ×100

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nBAL, consumed

(1) (2)

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X=

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where nBAL, initial and nBAL, consumed are moles of the initial benzaldehyde and the consumed

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3. Results and discussion

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benzaldehyde, respectively; nBOL, generated is moles of the generated benzyl alcohol.

3.1 Characterization of foam support

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An optical photograph of the foam support is displayed in Fig. 2(A). The color of the SiC

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foam changed from silver (Fig. 2(A-a)) to green (Fig. 2(A-b)) before and after removing the residual Si. Fig. 2(B) shows the morphology of the surface of the SiC foam strut before and after being treated with a boiling NaOH solution. After this processing, the residual Si was

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removed from the surface of foam struts, and a porous surface composed of SiC grains was exposed. The porous surface can enhance the adhesive strength between the Al2O3 coating and the strut. After the washcoating, a homogeneous layer of the Al2O3 coating on the SiC solid foam was obtained, as shown in Fig. 2(C). When the Al2O3 coating thickness was 9

increased to about 25 μm, the specific surface area of the foam support was increased from 0.4 to 24.3 m2/gfoam. The cross-section image of the Al2O3–SiC foam strut is presented in Fig. 2(D). A good adhesion between the foam strut and the Al2O3 coating was observed. There was no stratification or gap in the radial direction of the coating. The adhesion strength between

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Al2O3 coating and the foam strut was measured by the ultrasonic vibration testing. The weight loss of the as-prepared Al2O3–SiC support vs the ultrasonic time is plotted in Fig. S3. The

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weight loss of the foam support was only 3 wt% of the weight of the loaded Al2O3 after the testing for 80 min, indicating a good adhesion strength for the Al2O3 coating.

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3.2 Comparison between foam catalysts and powder catalysts

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3.2.1 Characterization of catalysts

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Fig. 3(A) shows XRD patterns of the Al2O3 support and Ni/Al2O3 catalysts with different

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Ni loading. When the Ni loading was 10 wt%, no obvious Ni characteristic peaks were

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observed, indicating that Ni particles were highly dispersed on the Al2O3 support [33]. The increase in the Ni loading led to the sharpening of Ni diffraction peaks. The three

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characteristic peaks at 44.3°, 51.8° and 76.2° were assigned to the diffractions of (111), (200)

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and (220) lattice planes (JCPDS No. 04-0850) [34], respectively. It can be speculated that the size of Ni particles on the Al2O3 support increased with increasing Ni loading [35]. XRD patterns for the SiC foam and the foam catalyst with different Ni loading are shown in Fig.

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3(B). No obvious Ni diffraction peaks were observed in foam catalysts with the Ni loading below 22 wt% because Ni particles were highly dispersed on the coating and their diffraction peaks were weakened by peaks of the SiC crystal phase with the very strong intensity. Although the Ni loading increased to 30 wt%, only a weak Ni diffraction peak was observed 10

on the XRD pattern. N2 adsorption–desorption properties of foam catalysts and powder catalysts are presented in Table 1. For both kinds of catalysts, their specific surface area and pore volume decreased with the increase in Ni loading due to the deposition of some Ni particles into the pore of the

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catalytic support. Because the BET surface area of the SiC foam (0.4 m2/g) was much lower than that of the Al2O3 powder (212.6 m2/g), the BET surface area of Ni/Al2O3–SiC foam

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catalysts was lower than that of Ni/Al2O3 powder catalysts. Additionally, the average pore size of foam catalysts was nearly half than that of the powder catalyst. The reason for this is that

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the average pore size of the Al2O3 originated from the pseudo-boehmite was smaller than that

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of the Al2O3 powder (Table S1), and some pore of the raw alumina powder in the slurry for

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washcoating was filled with the colloidal pseudo-boehmite during the washcoating. In

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addition, it was found that based on the mass of Al2O3 coating, the foam catalyst had a higher

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specific surface area compared to the powder catalyst (Table S1) because of the higher specific surface area of the Al2O3, which was transformed by the pseudo-boehmite. N2

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adsorption–desorption isotherms and the pore size distribution for the Al2O3 support, the

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Al2O3–SiC support and the 30 wt% Ni/Al2O3–SiC catalyst are presented in Fig. S4. All the isotherms belong to well-defined type IV with the hysteresis loop characteristic of the mesoporous structure. The mercury intrusion experiment for the 30 wt% Ni/Al2O3–SiC

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catalyst was carried out to further characterize the micro-structure of the foam catalyst. As shown in Fig. S5, the cumulative Hg intrusion plot showed that the foam catalyst had a pore size range of the mesoporous-scale. 3.2.2 Catalytic hydrogenation performance 11

Fig. 4(A) shows the conversion of benzaldehyde over foam catalysts with different Ni loading as a function of reaction time. It was clear that the catalytic activity of the foam catalyst increased greatly with increased Ni loading. The 37 wt% Ni/Al2O3–SiC catalyst exhibited the highest catalytic activity due to the largest amount of Ni loading, which

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provided more catalytic active sites for the hydrogenation reaction. Conversion of benzaldehyde over the 16 wt% and 37 wt% Ni/Al2O3 powder catalysts as a function of the

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reaction time is shown in Fig. 4(B). Evidently, the activity of the powder catalyst was higher

than that of the foam catalyst with identical Ni loading in the first 1 h of the reaction, which

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was attributed to the larger accessible surface area of the powder catalyst with the reactant

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[36]. When the Ni loading was 16 wt%, the conversion of benzaldehyde after 6 h (of 99.3%)

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over the foam catalyst was slightly higher than that over the powder catalyst (of 97.5%),

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suggesting that the foam catalyst had a similar rate of reaction to the powder catalyst.

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Moreover, for the two catalysts, a similar selectivity of benzyl alcohol as the target product was observed (Fig. S6), and its tendency toward over-hydrogenation into toluene was

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observed when nearly all the benzaldehyde was converted. When the Ni loading increased

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from 16 wt% to 37 wt%, the reaction rate obtained with the foam catalyst increased significantly from 0.108 to 0.204 mol L−1 h−1, whereas the reaction rate obtained with the powder catalyst increased from 0.106 to 0.123 mol L−1 h−1. The benzaldehyde conversion of

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76.7% after 2 h for the 37 wt% Ni/Al2O3–SiC was much higher than that for the 37 wt% Ni/Al2O3 (see inset in Fig. 4(B)). The results indicate that for the foam catalyst, it took a shorter time to achieve the complete conversion of benzaldehyde compared with that for the powder catalyst. 12

Fig. 5(A) shows the catalytic performance of the foam catalyst and the powder catalyst vs the Ni loading for the hydrogenation of benzaldehyde after 2 h. The foam catalyst showed a selectivity of benzyl alcohol comparable with that of the powder catalyst. However, the increase in activity of the foam catalyst with increased Ni loading was much higher than that

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of the powder catalyst. When the Ni loading was ≤ 22 wt%, the powder catalyst was more active than the foam catalyst because of its higher accessibility to the reactant, as mentioned

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above. Nevertheless, as the Ni loading increased to 30 wt%, the foam catalyst showed a

higher catalytic activity than the powder catalyst. The probable reason for this is that for a

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catalyst with high Ni loading, the advantage of the large accessible surface area for the

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comparison with the foam catalyst [37].

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powder catalyst was overwhelmed by the shortcoming of its lower mass-transfer rate in

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To gain better insight into the catalytic activity of the two catalysts, the activity of the

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catalyst was also evaluated by the specific activity (which was normalized by the moles of benzaldehyde consumed by 1 g min−1 of Ni) [36]. The specific activity of the foam catalysts

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and powder catalysts with different Ni loading is shown in Fig. 5(B). It can be seen that the

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specific activity of both kinds of catalysts decreased with increased Ni loading. Fig. 6 shows the TEM micrographs of the catalysts and the size distribution of Ni-particle on the catalysts. The lattice fringes associated with Ni (111) and Ni (200) lattice planes in metallic Ni were

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clearly observed, as shown in Fig. 6(B). It was observed that the average size of Ni particles of both kinds of catalysts increased with increasing Ni loading, but it was smaller for the Ni/Al2O3–SiC catalyst than for the Ni/Al2O3 catalyst with identical Ni loading. Therefore, the increase in the average size of Ni particles was responsible for the reduction of the specific 13

activity of the catalysts with increasing Ni loading [4, 38]. However, the extent of decline in the specific activity with the increase in Ni loading was evidently different for the two kinds of catalysts. The specific activity of the powder catalyst decreased drastically, whereas the activity of the foam catalyst decreased slowly with

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increased Ni loading. These results indicate that compared with the powder catalyst, the foam catalyst exhibits a better mass-transfer performance, which may be attributed to the formation

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of finer bubbles around the foam stirrer and the faster refreshment rate of the foam catalyst

surface [39]. This was mainly because a high mass-transfer rate was required to match the

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high Ni loading in the catalyst to ensure its relatively stable specific activity. When the Ni

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loading increased from 10 wt% to 22 wt%, the difference in the value of specific activity

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between the two kinds of catalysts was reduced from 3.38 to 0.47 mmol gNi−1 min−1.

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Furthermore, when the Ni loading increased to 30 wt%, the specific activity of the foam

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catalyst became superior to that of the powder catalyst, which was attributed to its smaller Ni-particle size [4] and higher mass-transfer rate [40, 41]. It was reported that the gas–liquid

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and liquid–solid mass-transfer rate for the foam catalyst were higher than that for the powder

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catalyst in a stirred reactor [39]. 3.2.3 Recyclability of the foam catalyst and the powder catalyst The recyclability of 30 wt% Ni/Al2O3–SiC and 30 wt% Ni/Al2O3 for the liquid-phase

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hydrogenation of benzaldehyde was examined. As shown in Fig. 7(A), the conversion of benzaldehyde over 30 wt% Ni/Al2O3–SiC was almost unchanged after being used 8 times, indicating that the foam catalyst had a stable hydrogenation activity as a function of run times. However, a drop of 43% in the conversion of benzaldehyde for 30 wt% Ni/Al2O3 was 14

observed after being used 7 times, as shown in Fig. 7(B). Fig. 8 shows the XRD patterns for a fresh powder catalyst and the powder catalyst that was used 7 times, both after having been treated at 600 °C for 2 h. It was observed that with the diffraction peak of alumina as the reference, the intensity of the NiO diffraction peak of the reused catalyst was evidently

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weaker than that of the fresh catalyst. The ratios of the intensity and area of the top three peaks of NiO to those of maximum peak of the alumina are presented in Table 2. It was

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obvious that for the reused catalyst, the ratio of the area of NiO peaks to the area of the Al2O3 maximum peak is lower than that of the fresh catalyst, suggesting that the leaching of Ni

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occurred during the hydrogenation reaction. The amount of Ni on the fresh catalyst and the

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reused powder catalyst, determined by ICP-OES, is also shown in Table 2. When the 30 wt%

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Ni/Al2O3 catalyst was used 7 times, the Ni content in the powder catalyst decreased from 117

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to 83 mg. From the above result, it can be inferred that the leaching of the active metal Ni in

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the powder catalyst is an important reason for deactivation. The H2-TPR experiment was carried out for analyzing the reason of the leaching of Ni

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particles in the Ni/Al2O3 catalyst. Fig. 9 shows H2-TPR profiles for the 30 wt% Ni/Al2O3

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catalyst and the 30 wt% Ni/Al2O3–SiC foam catalyst. The reduction behavior of the foam catalyst was considerably different from that of the powder catalyst, indicating that the NiO-support interaction greatly depended on the type of the catalyst support. There were four

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reduction peaks in the 30 wt% Ni/Al2O3 catalyst, which might refer to four types of nickel oxide species. The peak at lower-temperature agreed with the reduction of relatively free nickel oxide species, showing that there was weak interaction between NiO and the Al2O3 support. The broad peak that started to appear at around 400 °C was reasonably assigned to 15

the small crystallite of NiO that had a strong interaction with the support [42]. The 30 wt% Ni/Al2O3–SiC foam catalyst showed only a broad peak with the peak maximum at around 430 °C, indicating that the foam catalyst had a stronger interaction between the active component and the catalyst support compared with the powder catalyst. Hence, the active

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leading to a high stable catalytic performance for the foam catalyst.

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component Ni on the foam catalyst was not easy to lose during the hydrogenation reaction,

4. Conclusion

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The SiC foam coated with a stable Al2O3 layer (Al2O3–SiC) as the catalyst support was

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first prepared by the washcoating method. Then Ni/Al2O3–SiC foam catalysts and Ni/Al2O3

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powder catalysts were prepared for the liquid-phase hydrogenation of benzaldehyde by the DP

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method. Conversion of benzaldehyde after 6 h (of 99.3%) over 16 wt% Ni/Al2O3–SiC foam

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catalyst was slightly higher than that over 16 wt% Ni/Al2O3 (of 97.5%), suggesting that 16 wt% Ni/Al2O3–SiC had a similar reaction rate to the powder catalyst. When the Ni loading

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increased from 16 wt% to 37 wt%, the reaction rate obtained with the foam catalyst increased

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significantly from 0.108 to 0.204 mol L−1 h−1, whereas the reaction rate obtained with the powder catalyst increased from 0.106 to 0.123 mol L−1 h−1. Therefore, when the Ni loading was ≥ 30 wt%, the specific activity of the foam catalyst was superior to that of the powder

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catalyst due to its smaller Ni-particle size and higher mass-transfer rate. The foam catalyst displayed a high recyclability as a function of run times owing to the strong interaction between the Ni component and the Al2O3 coating. The conversion of benzaldehyde over the foam catalyst remained almost unchanged after being used 8 times. In comparison, a drop of 16

43% in the conversion of benzaldehyde with the powder catalyst was observed after being used 7 times, which was attributed to the leaching of Ni component.

Acknowledgements

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The authors gratefully acknowledge the financial support of the project from the National Key Research & Development Plan (No. 2017YFB0310405). The authors would like to thank

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Prof. G.D. Wen for GC measurements and Dr. X.D. Yang for H2-TPR experiments.

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Figure List

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Fig. 1. Sketch of the rotating foam stirrer reactor for the liquid-phase hydrogenation of

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benzaldehyde.

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Fig. 2. (A) Optical photograph of the SiC solid foam (15 PPI) and the foam support: (a) the pristine SiC foam (containing residual Si), (b) the bare foam and (c) the Al2O3–SiC foam; (B) SEM images of the foam strut surface before and after removing residual Si; (C) the top-view

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image of Al2O3–SiC foam; (D) the cross-section image of a strut of the Al2O3–SiC foam.

N

Fig. 3. (A) XRD patterns of the Al2O3 powder and Ni/Al2O3 catalysts with different Ni

A

loading and (B) XRD patterns of (a) the bare SiC foam, (b) 10 wt% Ni/Al2O3–SiC, (c) 16 wt%

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Ni/Al2O3–SiC, (d) 22 wt% Ni/Al2O3–SiC and (e) 30 wt% Ni/Al2O3–SiC.

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Fig. 4. Conversion of benzaldehyde over (A) foam catalysts and (B) powder catalysts with different Ni loading as a function of reaction time. Reaction conditions: benzaldehyde 10 mL, isopropanol 140 mL, 90 °C, 2 MPa H2, stirring rate at 300 rpm.

23

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Fig. 5. (A) Conversion of benzaldehyde and selectivity of benzyl alcohol over foam catalysts

and powder catalysts with different Ni loading after 2 h and (B) the specific activity of both

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kinds of catalysts versus the Ni loading.

Fig. 6. (A) TEM image of 10 wt% Ni/Al2O3 and (B) the high-magnification image of a Ni particle on the 10 wt% Ni/Al2O3. TEM images of (C) 10 wt% Ni/Al2O3–SiC, (D) 16 wt% 24

Ni/Al2O3, (E) 16 wt% Ni/Al2O3–SiC, (F) 22 wt% Ni/Al2O3, (G) 22 wt% Ni/Al2O3–SiC, (H) 30 wt% Ni/Al2O3 and (I) 30 wt% Ni/Al2O3–SiC. TEM images of 37 wt% Ni/Al2O3 (dm=23 nm) and 37 wt% Ni/Al2O3–SiC (dm=21.7 nm) were provided in Fig. S7. (dm refers to the mean

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size of Ni particles)

Fig. 7. Performance of hydrogenation of benzaldehyde over (A) 30 wt% Ni/Al2O3–SiC and

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(B) 30 wt% Ni/Al2O3 as a function of run times. Reaction conditions: benzaldehyde 10 mL,

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isopropanol 140 mL, 90 °C, 2 MPa H2, after 4 h of the reaction, stirring rate at 300 rpm.

Fig. 8. XRD patterns for the fresh 30 wt% Ni/Al2O3 catalyst and the reused 30 wt% Ni/Al2O3 25

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catalyst, both after being treated at 600 °C for 2 h.

Fig. 9. H2-TPR profiles of the unreduced 30 wt% Ni/Al2O3 catalyst and the unreduced 30 wt%

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Ni/Al2O3–SiC catalyst.

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Scheme 1. Reaction pathway of the hydrogenation of benzaldehyde.

27

Table 1 Textural properties of supports and catalysts. Ni loading a

VBJH

DBJH

(m2/gsample)

(cm3/gsample)

(nm)

–

212.6

0.79

12

10 wt% Ni/Al2O3

11

141.6

0.56

14.4

16 wt% Ni/Al2O3

15.3

132.1

0.52

13.8

22 wt% Ni/Al2O3

22.6

125.3

0.47

13

30 wt% Ni/Al2O3

29

120.1

0.43

12.3

Al2O3–SiC

–

24.3

0.09

8.3

10 wt% Ni/Al2O3–SiC

10.5

22.1

0.08

7.7

16 wt% Ni/Al2O3–SiC

15.6

19.7

0.07

7.1

22 wt% Ni/Al2O3–SiC

21.8

18.2

0.05

6.8

30 wt% Ni/Al2O3–SiC

16.4

0.04

6.4

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loading was determined by ICP-OES

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a Ni

29.7

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Al2O3

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SBET

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Samples

Table 2 Results from XRD and ICP-OES characterization for the fresh 30 wt% Ni/Al2O3 and the reused 30 wt% Ni/Al2O3 after being treated at 600 °C for 2 h.

NiO/Al2O3 (fresh)

1.56

1.78

1.13

3.03

1.74

NiO/Al2O3 (used 7 times)

1.42

1.49

0.99

2.29

1.04

Intensity ratio of the top three peaks of NiO and the maximum peak of Al2O3.

b Area

117 83

ratio of NiO diffraction peak (at 37.3° and 62.9°, respectively) to the maximum peak of Al2O3.

Ni content was measured by ICP-OES.

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c

Ni content c (mg)

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a

Area ratio b

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Intensity ratio a

Catalyst

29