CNTs catalysts

    Effect of loading method on selective hydrogenation of chloronitrobenzenes over amorphous Ni-B/CNTs catalysts Feng Li, Rui Ma, Bo Cao, Jinrong Liang, Hualin Song, Hua Song PII: DOI: Reference:

S1566-7367(16)30094-2 doi: 10.1016/j.catcom.2016.03.009 CATCOM 4611

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

13 November 2015 17 February 2016 15 March 2016

Please cite this article as: Feng Li, Rui Ma, Bo Cao, Jinrong Liang, Hualin Song, Hua Song, Effect of loading method on selective hydrogenation of chloronitrobenzenes over amorphous Ni-B/CNTs catalysts, Catalysis Communications (2016), doi: 10.1016/j.catcom.2016.03.009

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ACCEPTED MANUSCRIPT Effect of loading method on selective hydrogenation of chloronitrobenzenes over amorphous Ni-B/CNTs catalysts

Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry &

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Feng Lia*, Rui Maa, Bo Caoa, Jinrong Lianga, Hualin Songb, Hua Songa

Chemical Engineering, Northeast Petroleum University, Daqing 163318, Heilongjiang, China Key Laboratory of Cancer Prevention and Treatment of Heilongjiang Province, Mudanjiang

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Medical University, Mudanjiang 157011, Heilongjiang, China

Corresponding author. Address: College of Chemistry & Chemical Engineering, Northeast

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Petroleum University, Daqing 163318, China (F. Li). Tel: +86 0459 6504318 (F. Li). Fax: +86 0459 6504318 (F. Li).

E-mail address: [email protected] (F. Li)

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ACCEPTED MANUSCRIPT Abstract Carbon nanotubes (CNTs)-supported amorphous Ni-B catalysts were prepared by selectively

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depositing Ni-B particles inside or outside the CNTs (Ni-B-in/CNTs or Ni-B-out/CNTs).

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Hydrogenation of chloronitrobenzenes was carried out to test how the Ni-B particle-loading method over the CNTs affected the catalytic performance. Compared with the Ni-B-out/CNTs, the Ni-B-in/CNTs, because of the nanospace limitation inside the CNTs, restricted the size and

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aggregative behavior of the Ni-B particles better, and formed an amorphous Ni-B alloy with high

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thermal stability. The Ni-B-in/CNTs exhibited a much higher catalytic activity for hydrogenation of chloronitrobenzenes than the Ni-B-out/CNTs.

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Key word: Ni-B/CNTs; Amorphous alloy; Loading method; Chloronitrobenzenes; Hydrogenation

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ACCEPTED MANUSCRIPT 1. Introduction Metal-metalloid amorphous alloys have been widely used in catalytic reactions due to their

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short-range ordering structure [1–4]. However, poor thermal stability and/or low surface area have

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limited their application in industrial catalysis. Supported amorphous alloy catalysts have proven

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to be promising candidates for industrial applications owing to the great improvement in thermal stability and active surface area as a result of the high dispersion of the amorphous alloy particles

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on the support.

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After their discovery by Iijima, carbon nanotubes (CNTs) have attracted a great deal of interest due to their extraordinary physical and chemical properties [5]. Amongst their many

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potential applications, their use as catalysts or catalyst supports is of particular interest as CNTs

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display superior catalytic performance compared to traditional catalysts in reactions involving

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hydrogenation [6, 7], selective dehydrogenation [8], catalytic oxidation [9], or Fischer–Tropsch synthesis [10]. Improved catalytic performance has been reported when CNTs were used as supports to disperse metal catalysts on the outside surface of the channels [11]. Alternatively, it

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was also found that nanoparticles encapsulated in CNTs exhibit better catalytic performance compared with those located outside the CNTs for some reactions due to the confinement effect of the CNTs [12]. Therefore, the loading method for the active component on the CNTs is one of the most important factors. On the basis of research by Tessonnier et al. [13], two CNTs supported amorphous Ni-B catalysts were prepared with Ni-B particles confined inside the channels of the CNTs (Ni-B-in/CNTs) or loaded on the CNTs outer surface (Ni-B-out/CNTs). Hydrogenation of chloronitrobenzenes (CNBs) was chosen as a probe reaction to investigate how the loading method of Ni-B particles over CNTs affects the catalytic performance. 3

ACCEPTED MANUSCRIPT 2. Experimental 2.1 Catalyst preparation

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Raw CNTs were refluxed in concentrated HNO3 for 6 h, then filtered and washed with

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distilled water, and dried at 373 K for 12 h. The Ni-B-in/CNTs precursor was synthesized as follows: 1.0 g of CNTs was impregnated with 3.5 ml of NiCl2 ethanol solution (0.54 M NiCl2·6H2O). Then, 2.6 ml H2O was added. The impregnated sample was oven dried at 303 K for

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12 h, and was denoted NiCl2-in/CNTs. The theoretical loading of Ni on the CNTs was 10 wt%. For

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the Ni-B-out/CNTs precursor, 1.0 g of CNTs was impregnated with 5.2 ml of ethanol. Then, 3.5 ml of NiCl2 aqueous solution (0.54 M NiCl2·6H2O) was added. The sample was dried and denoted

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NiCl2-out/CNTs. The above-prepared precursors (NiCl2-in/CNTs and NiCl2-out/CNTs) were

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reduced by adding 2 M KBH4 solution containing 0.20 M NaOH (Ni2+:BH4- in a molar ratio of 1:3)

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dropwise with vigorous stirring in an ice-water bath until no gas was released. The resulting samples (Ni-B-in/CNTs and Ni-B-out/CNTs) were then thoroughly washed with distilled water

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and ethanol and were kept in ethanol until use. In addition, Ni-in/CNTs was prepared from NiCl2-in/CNTs by H2 reduction at 673 K for 4 h. 2.2 Catalyst characterization The morphologies of the catalysts were characterized by a transmission electron microscope (TEM, JEOL JEM-4000EX). The Ni loading and Ni-B composition were analyzed using an inductively coupled plasma spectrometer (ICPS-7510, Shimadzu). The active surface area (Sact) was determined by H2 chemisorption using a dynamic pulse method [14]. The X-ray diffraction (XRD) patterns of the catalysts were recorded with a Rigaku D/max-2200 X-ray diffractometer with Cu Kα radiation. Differential scanning calorimetry (DSC) measurements were conducted on a

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ACCEPTED MANUSCRIPT Nezsch 200 F3 analyzer under N2 atmosphere at a rate of 10 K/min. Temperature programmed desorption of hydrogen (H2-TPD) was carried out in a CHEMBET 3000 (Quantachrome)

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instrument at a rate of 10 K/min. X-ray photoelectron spectroscopy (XPS) analyses were

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performed using Thermofisher Scientific K-Alpha X-ray photoelectron spectrometer with a nomochromatized Al Kα X-ray source. 2.3 Catalytic hydrogenation of CNBs

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Selective hydrogenation of CNBs was carried out in a 100 ml stainless autoclave at 417 K

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and 2.0 MPa. Typically, the autoclave was charged with 0.05 g of catalyst and 20 ml of 0.1 mol/l CNBs ethanol solution. Then, the autoclave was sealed and purged more than six times with

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hydrogen to exclude air. The reaction lasted for 1 h. Products were analyzed using a gas

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chromatograph (GC, Shimadzu GC-14C, FID, SE-30 capillary column), and were identified by

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gas chromatography/mass spectrometry (GC/MS, Agilent 5890). 3. Results and discussion

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The location and size of Ni-B particles were characterized by TEM analysis. The TEM image of Ni-B-in/CNTs shows that most of the Ni-B particles are evenly distributed within the CNT nanochannels (Fig. 1 (a)). The TEM image of the Ni-B-out/CNTs clearly shows that the Ni-B particles are dispersed on the outer surfaces of the CNTs (Fig. 1 (b)). Moreover, the Ni-B particles in the Ni-B-in/CNTs are uniformly sized (about 8 nm), while those in the Ni-B-out/CNTs are non-uniformly sized and range from 12 to 26 nm. The physicochemical properties of the Ni-B-in/CNTs, Ni-B-out/CNTs and Ni-in/CNTs are listed in Table 1. Significant Ni component loss occurred in the Ni-B-out/CNTs, but not in the Ni-B-in/CNTs and Ni-in/CNTs. The Ni component loss is attributed mainly to the reaction

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ACCEPTED MANUSCRIPT between Ni2+ and BH4-, and it was inhibited by intratubal loading compared with extratubal loading.

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Figure 2 presents the XRD patterns of the CNTs, Ni-B-in/CNTs, and Ni-B-out/CNTs. The

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three peaks at 26.1°, 43.1°, and 53.5° observed in the three profiles can be indexed as the (002), (100), and (004) diffractions of hexagonal graphite [15], respectively, indicating the graphitization structure of the CNTs is not destroyed during the acid treatment. The characteristic diffraction

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peak of amorphous Ni-B alloy is reportedly a dispersion peak near 2θ = 45°. For the

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Ni-B-in/CNTs and Ni-B-out/CNTs, except for the CNTs diffraction peaks, no other crystalline peak was observed, which indicates that the Ni-B exists as an amorphous phase[16].

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The thermal stabilities of Ni-B-in/CNTs and Ni-B-out/CNTs were compared by DSC. Both

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catalysts exhibit two exothermic peaks, which correspond to phase transitions at around 704 and

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741 K for Ni-B-in/CNTs and 692 and 735 K for Ni-B-out/CNTs (Fig. 3 (a)). Li et al. reported that when the heat treatment temperature rose to 573 K, the amorphous Ni-B alloy started to transform

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to metallic Ni, crystal Ni2B, Ni3B alloy, and when increased further to 683 K, the substances decomposed to metallic Ni and free B [17]. Therefore, the thermal stability of the supported amorphous Ni-B alloys was improved greatly. The crystallization temperatures of Ni-B-in/CNTs were about 10 K higher than those of Ni-B-out/CNTs. The higher thermal stability of Ni-B-in/CNTs may be attributed to the confinement effect of CNTs on the amorphous Ni-B alloy. Figure 3 (b) shows the H2-TPD profiles of the Ni-B-in/CNTs and Ni-B-out/CNTs. The Ni-B-in/CNTs has a significantly lower H2 desorption temperature versus the Ni-B-out/CNTs (669 K vs. 697 K) and a larger H2 desorption peak area. H2 is a reactant in hydrogenation, and when the desorption temperature is far higher than the hydrogenation temperature (413 K), the lower

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ACCEPTED MANUSCRIPT desorption temperature is more favorable for CNBs hydrogenation [18]. Moreover, the desorbed quantity of H2 increases under the same Ni loading amount, indicating the active centers in

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Ni-B-in/CNTs are more scattered and abundant.

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The interaction between Ni and B was characterized by XPS (Fig. 4). For the two catalysts, both Ni and B were present in two states. However, Ni and B, which were at different states in the two catalysts, did not show a significant difference in binding energy (BE). In Ni 2p3/2 (Fig. 4 (a)),

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the peaks around 852.5 and 855.7 eV are assigned to metallic Ni and oxidized Ni, respectively. In

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B 1s (Fig. 4 (b)), the peaks around 188.4 and 192.3 eV are assigned to elemental B and oxidized B, respectively. In comparison with the standard BE of pure Ni (853.1 eV) and pure B (187.1 eV),

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the BEs of elemental Ni and B shifted by –0.6 and +1.3 eV, respectively, indicating some

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electrons transferred from B to Ni in the amorphous Ni-B alloy to form electron-rich Ni and

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electron-deficient B [19]. The degree of BE shift for metallic Ni is less than that of metallic B, which could be attributed to the relatively larger atomic weight of the Ni atom versus the B atom

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[20]. On the basis of the peak areas for reduced Ni and oxidized Ni (the peak area of Ni0 to Ni2+ was denoted "R"), a comparison of the two catalysts shows that the Ni-B-in/CNTs (R = 0.51) contain more reduced Ni than the Ni-B-out/CNTs (R = 0.38). As shown in Table 2, both the Ni-B-in/CNTs and Ni-B-out/CNTs have higher selectivity than the Ni-in/CNTs for hydrogenation of CNBs. According to the principles of catalysis, the catalytic activity and reactive selectivity are mainly decided by the amount and nature of the active centers. From the perspective of microscopic structures, the equilibrium positions of atoms in an amorphous alloy do not show the inherent translational periodicity like crystalline metals, but instead are characterized by short-range ordering and long-range disordering, which are favorable

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ACCEPTED MANUSCRIPT for the improvement of the catalytic activity of surface Ni atoms [21]. Also, in amorphous Ni-B alloys, Ni is in an electron-rich state and B is in an electron-deficient state. In the-NO2 groups of

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CNBs, electrons on the N atoms are transferred to O atoms, and consequently, the electron-rich

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active Ni atoms absorb more -NO2 by adsorbing the electron-deficient N atoms, while the electron-deficient B atoms react with the O atoms in -NO2, further enhancing the adsorption. After the hydrogenation, the N atoms in the resulting -NH2 groups, because of electron enrichment,

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depart from the active Ni atoms, thus avoiding excessive hydrogenation.

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The CNBs hydrogenation selectivities are not significantly different between Ni-B-in/CNTs and Ni-B-out/CNTs, but the Ni-B-in/CNTs is significantly more reactive than the Ni-B-out/CNTs.

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From the perspective of catalyst structures, the amorphous Ni-B alloy in the Ni-B-in/CNTs is

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more thermally stable than that in the Ni-B-out/CNTs, and the Ni-B particles are smaller and have

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more active centers. From the perspective of reaction space, the Ni-B particles in the Ni-B-out/CNTs are distributed outside the CNTs, and the hydrogenation of CNBs also occurs

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outside the CNTs. While the Ni-B particles in the Ni-B-in/CNTs are distributed inside the CNTs, and the hydrogenation of CNBs also occurs inside the CNTs. The nanospace inside the CNTs as a reaction place has a space-limiting effect that will induce a gas-phase, high-pressure reaction. Guan et al. [22] reported that CNTs as nanoreactors could enrich the reactants, modifier, and additive, and thus, the Pd nanocatalyst inside the channels of the CNTs achieves higher activity and enantioselectivity than those of the Pd nanocatalyst outside the CNTs. Shi et al. [23] applied Co/CNTs to the selective epoxidation of styrene, and found that when Co was loaded inside the CNTs, the styrene oxide yield was 93 %, and when the Co was loaded outside the CNTs, the styrene oxide yield was only 50 %. The above results indicate that the intratubal space not only

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ACCEPTED MANUSCRIPT restricts the size and aggregation of Ni-B particles, but also serves as a special nanoreactor, thus improving the catalytic performance. The different behaviors between the isomers (m-, o- and

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p-CNB) can be attributed to the electron-withdrawing and electron-donating effects of the

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substituents [24-26].

Results from tests on the reusability of the three catalysts are listed in Table 3. Ni-B-in/CNTs

catalytic activity even after 4 recycled uses.

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4. Conclusions

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versus Ni-B-out/CNTs could prevent the loss of active components and thus maintain very high

Amorphous Ni-B/CNT catalysts were prepared by selective depositing of Ni-B particles

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inside or outside the CNTs. The Ni-B particles deposited inside the CNTs are more catalytically

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active than those deposited outside the CNTs. This high activity was mainly attributed to the

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unique properties of the CNTs channels, as they can efficiently inhibit the size and aggregation of Ni-B particles, thereby providing more active centers. Moreover, when the active components

CNBs.

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were deposited inside the CNTs, the CNT channels serve as nanoreactors for the hydrogenation of

Acknowledgements

The authors acknowledge the financial supports from the National Natural Science Foundation of China (21276048), the Education Department of Heilongjiang Province (12541060), and the Youth Foundation of Northeast Petroleum University (2013NQ112).

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ACCEPTED MANUSCRIPT References [1] W. Wang, S. Yang, Z. Qiao, P. Liu, K. Wu, Y.Yang, Catal. Commun. 60 (2015) 50-54.

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ACCEPTED MANUSCRIPT List of Tables Table 1 Physicochemical properties of Ni-B-in/CNTs, Ni-B-out/CNTs and Ni-in/CNTs.

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Table 2 Hydrogenation of CNB over Ni-B-in/CNTs, Ni-B-out/CNTs and Ni-in/CNTs.

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Table 3 Reusability of Ni-B-in/CNTs, Ni-B-out/CNTs and Ni-in/CNTs in the hydrogenation of m-CNB.

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List of Figures

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Fig. 1. TEM images of Ni-B-in/CNTs and Ni-B-out/CNTs.

Fig. 2. XRD patterns of Ni-B-in/CNTs and Ni-B-out/CNTs.

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Fig. 3. (a) DSC and (b) H2-TPD profiles of Ni-B-in/CNTs and Ni-B-out/CNTs.

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Fig. 4. XPS spectra of Ni-B-in/CNTs and Ni-B-out/CNTs: (a)Ni 2p3/2; (b) B 1s.

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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ACCEPTED MANUSCRIPT Table 1 Physicochemical properties of Ni-B-in/CNTs, Ni-B-out/CNTs and Ni-in/CNTs SBET

(wt%)

(atomic ratio)

2

(m /g)

Ni-B-in/CNTs

9.2

Ni58.6B41.4

Ni-B-out/CNTs

8.6

Ni-in/CNTs

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Vpore

dpore

Sact

(cm /g)

(nm)

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(m /g Ni)

80.64

0.360

17.45

17.83

Ni61.7B38.3

77.15

0.329

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89.53

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Composition

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Ni loading

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16.64

15.46

15.21

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Selectivity (%)

m-CNB

37.64

94.1

91.9

o-CNB

35.76

89.4

90.5

p-CNB

30.48

76.2

m-CNB

29.88

74.7

89.9

o-CNB

28.36

p-CNB

24.52

m-CNB

34.68

o-CNB

32.16

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Conversion (%)

p-CNB

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Ni-B-out/CNT

Reaction rateb

93.6

70.9

89.3

61.3

94.2

86.7

85.2

80.4

84.9

73.1

88.7

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Ni-B-in/CNT

x-CNB

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Catalyst

29.24

Reaction conditions: 0.05 g catalyst; 2 mmol CNB; T=413 K; P=2.0 MPa; reaction time 1 h.

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The CNB mole consumption per g catalyst and per hour, mmol CNB g cat-1 h-1.

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Ni loading (wt%)

Reaction rate

Conversion (%)

Selectivity (%)

Ni-B-in/CNT

1

—

37.64

94.1

91.9

2

—

37.04

92.6

92.4

3

—

35.68

89.2

92.1

4

8.7

33.80

84.5

92.6

Ni-B-out/CNT

4

7.1

21.36

53.4

91.2

Ni-in/CNT

4

9.7

31.12

77.8

86.5

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Reaction conditions are similar to Table 2.

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Table 3 Reusability of Ni-B-in/CNTs, Ni-B-out/CNTs and Ni-in/CNTs in the hydrogenation of m-CNB a

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Graphical abstract

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ACCEPTED MANUSCRIPT Research Highlights > Ni-B particles were selectively deposited inside or outside CNTs.

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> CNTs channels efficiently restricted the size and aggregative of Ni-B particles.

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> Thermostability of amorphous Ni-B alloy was enhanced when deposited inside CNTs.

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> Ni-B inside CNTs showed higher hydrogenation activity than Ni-B outside CNTs.

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