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Kinetics, Catalysis, and Reaction Engineering

Nitrile Butadiene Rubber Hydrogenation over A Monolithic Pd/CNTs@Nickel Foam Catalysts: Tunable CNTs Morphology Effect on Catalytic Performance Zhao-Hui Luo, Miao Feng, Hui Lu, Xiao-Xin Kong, and Gui-Ping Cao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04688 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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Industrial & Engineering Chemistry Research

Nitrile Butadiene Rubber Hydrogenation over A Monolithic Pd/CNTs@Nickel Foam Catalysts: Tunable CNTs Morphology Effect on Catalytic Performance Zhao-Hui Luo, † Miao Feng, † Hui Lu, † Xiao-Xin Kong, † Gui-Ping Cao*† †UNILAB, State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China *Corresponding author: E-mail: [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: In heterogeneous catalyst systems, catalytic performance was often negatively influenced by mass transfer limitations in which macromolecular reactants cannot diffuse into the inner pores to attach active sites. Carbon nanotubes (CNTs) with no-porous structure were the promising supports to solve the problem. In this work, CNTs were directly prepared by CVD (Chemical Vapor Deposition) method on nickel foam (NF). The morphology of the as-synthesized CNTs were effectively tailored by varying temperature for growing CNTs. The prepared CNTs composites (CNTs@NF) with good mechanical strength were employed as palladium catalyst (Pd/CNTs@NF) for nitrile butadiene rubber (NBR) hydrogenation, where a high degree of hydrogenation (HD) was accomplished via Pd/CNTs@NF catalyst. The HD for 5th reuse was only slightly reduced, demonstrating its great recyclable activity. Remarkably, the structured Pd/CNTs@NF catalyst could be easily separated from the hydrogenated products, avoiding the necessity of filtration for industrialization. Keywords: Nickel foam; Carbon nanotubes; Monolithic support; Catalytic hydrogenation; Nitrile butadiene rubber; Structured catalyst.

1.Introduction Catalytic hydrogenation has been identified as a prospective industrial application to improve the mechanical and chemical properties of unsaturated polymer.1-8 The hydrogenation of acrylonitrile butadiene rubber (NBR) is a representative example to produce hydrogenated acrylonitrile butadiene rubber (HNBR), which possesses greatly enhanced thermal and oxidative resistance compared with NBR.9-11 High performance NBR hydrogenation is mostly carried out by using homogeneous noble metal catalysts. Nevertheless, those catalysts would be easily left in hydrogenated production, leading to high cost of separation and even the degradation of the HNBR latex.12-14 Therefore, the heterogeneous catalysts are considered as alternative desired catalysts for easy separation process. The most widely used heterogeneous catalysts in NBR hydrogenation are Rh/SiO2,15 Pd/SiO2,16 and Pd/Al2O3.17 With these catalysts, the hydrogenation degree (HD) of above 90 % can be achieved. However, for the absence of strong adsorption force or chemical bonding, the catalytic sites have poor attachment to the bare carrier, leading to the leaching of active site from the support surface. Thus, the catalyst above such as Pd/SiO2 was found to give decreasing HD from about 98.8 % to 33.3 % after one times recycling.15 Moreover, polymer


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hydrogenation process is completely different from that of small molecules, like benzene and ethylene.18 Due to the high-molecular weight of polymer coils and the high viscosity of polymer solution, mass transfer of the polymer coils from both bulk liquid phase and catalyst pores has been proved extremely challenging. Although those microporous or mesoporous materials could supply the high surface area to deposit active sites, polymer chains can hardly enter into those pores and subsequently be hydrogenated on the catalytic sites. Since that, hydrogenation of NBR with heterogeneous catalyst often needs harsher reaction conditions to enhance the external and pore diffusion, leading to the high operating cost. Herein, it remains a significant challenge to develop effective supports and catalysts that can eliminate the mass transfer limitation and can be easily separated from the modified products. Our previous works have demonstrated that carbon nanotube (CNT) supported palladium catalyst (Pd/CNT) performed an outstanding catalytic activity in hydrogenation of polystyrene (PS) compared with traditional powdered Pd/BaSO4 and Pd/AC catalysts.18 Thanks to the non-porosity structure and high external surface area of CNTs, the PS hydrogenation can be carried out under milder conditions. Unfortunately, the CNTs in pulverized form has some disadvantages in slurry phase operations. Notably, the agglomeration of those pulverized CNTs and difficulty of filtration significantly elevates the operating cost.19 Additionally, CNTs powders cannot be used directly in fixed-bed reactors due to a high pressure drop. To overcome these disadvantages, it is essential to incorporate CNTs on larger porous materials to serve as the structured monoliths for catalytic reaction. For a instance, CNT have been deposited on carbon-based carrier like activated carbon20 and graphite felt.21 More recently, CNT have also been grown on ceramic monoliths22 and SiC foam23 by chemical vapor decomposition (CVD) method. However, all of those structured supports retain considerable amount of micropores which is not accessible to polymer coils. Besides, for the absence of active species (like Fe, Co, Ni) in those structured composites, a tedious impregnation-calcination-reduction procedure was needed for CNTs growth, which would adversely impact the adherence between CNTs coatings and the substrate.24, 25 In order to facilitate polymer coils diffusion and CNTs synthesis, metallic supports, such as metal fibers26 and metal foam,27 have been proposed as the novel structured material in multiphase reactions due to its regular open structure and great mechanical properties. Especially, nickel foam (NF),28 a commercially available metallic support, has been a promising material in the structured catalytic system.29-31 For one thing, the



instinct three dimensional macroporous structure can facilitate the diffusion of polymer chains. For another, nickel can act as an active component for hydrocarbon decomposition. Thus, carbon nanotubes can be directly immobilized on nickel foam (CNTs@NF), which resolves the problem of adhesion between macroporous substrate and CNTs layers. Clearly, the excepted layer thickness, diameters and the adequate specific surface area of the prepared CNTs on the NF, which greatly influenced the catalytic performance, were strongly manipulated by CNTs growing process, like the feed composition and the growth time.32, 33 It was well known that, the addition of H2 in the initial stage during carbon deposition process could decrease the CNTs diameter,34 and the thickness of CNTs increased with the increasing duration.32 However, the growth temperature effect on the morphology of CNTs had never been reported over the nickel foam substrate. In this work, different growth temperatures were tested, i.e.450, 500, 550 and 600 ℃, to investigate difference in morphology of CNTs on their catalytic performance. The structured CNTs@NF carrier were successfully fabricated by CVD method and as-prepared Pd/CNTs@NF catalysts, for the first time, were applied in the hydrogenation of NBR. The result showed that the growth of CNTs layer on NF could be tunable by varying the growth temperatures. The prepared CNTs@NF could be a novel and effective support with desirable external surface and high mechanical stability. The Pd/CNTs@NF catalysts displayed excellent hydrogenation activity as well as good recyclable property.

2. Materials and methods 2.1.Materials The nickel foam (NF, 99 % purity, 75 PPI) was purchased from Changsha LYRUN New Material Co., Ltd. PdCl2 was obtained from Shanghai Jiulin Chemical Co., Ltd. Acetone and ethanol of analytical grade were purchased from Taitan Technology Chemical Co., Ltd. NBR (N31, ACN:33.5 wt %) was purchased from Shanghai Nessen International Trading Co., Ltd. Nitrogen (research grade; N2 ＞99.99%), methane (research grade; CH4 ＞ 99.95%) and hydrogen (research grade; H2 ＞ 99.999%) were purchased from Shanghai Siyuan Gas Industries Co., Ltd. All of the regents and solvents were used as received without further purification. 2.2.Synthesis of CNTs@NF monolith supports Prior to CNTs growth, the NF samples with the size of 5mm×5mm×5mm were


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sonicated for 30 min with anhydrous ethanol, washed with distilled water and then dried overnight in air at 100 ℃. For CNTs growth, the pretreated NF samples were placed in a quartz reactor (Figure 1). The prepared NF was firstly oxidized at 700 ℃ for 1 h in the flowing air and then reduced at 600 ℃ in a gas mixture of 50 % H2 /N2 (100/100 mL/min) for 1 h. Then, CNTs were synthesized for 8 min in a mixed gas of 25% CH4/N2 with a total flow rate of 250 mL/min under the desired temperature of 450, 500, 550 and 600 ℃, respectively. The resulting supports were denoted as CNTs@NF-x, where x was the CNTs growth temperature. By weighting the samples before and after CNTs growth, the carbon content (η1, wt %) was calculated as eq 1. The prepared CNTs@NF-x composites were put in 40 mL of deionized water, and the suspension was sonicated for different durations in a 135-W and 40 kHz ultrasonic bath. The sonicated composites were dried overnight in air at 100 ℃. By weighting the samples before and after ultrasound treatment, the carbon loss (η2, wt %) was calculated by eq 2 to evaluate the mechanical strength of CNTs@NF-x.

1 = ( mCNTs @ NF − mNF ) mNF 100%

(1)

Where mNF is the weight of NF and mCNTs@NF is the weight of the CNTs@NF supports.

2 = ( m2 − m1 ) m1 100%

(2)

Where m1, m2 are the weight of CNTs@NF-x composites before and after ultrasonic treatment, respectively. 2.3.Preparation of structured Pd/CNTs@NF catalyst Palladium NPs were deposited on CNTs@NF supports via wetness impregnation. Figure 1 showed the scheme of structured Pd/CNTs@NF catalyst preparing process. The precursor PdCl2 was firstly dissolved in the distilled water. After that, the CNTs@NF supports were added to the suspension and left standing for 12 h. Then, the excess water was evaporated under vacuum at 60 ℃ for 6 h, and the Pd/CNTs@NF catalysts were dried at 110 ℃ for 12 h, calcined at 350 ℃ for 4 h and subsequently reduced in 50 % of H2/N2 (100/100 mL/min) at 300 ℃ for 6 h. The prepared catalysts with the different CNTs growth temperatures were referred as the Pd/CNTs@NF-x composites.



Figure 1. The preparation of the CNTs@NF composites and Pd/CNTs@NF catalysts

2.4.Catalyst performance of NBR hydrogenation The hydrogenation reaction was carried out in 500 mL high pressure reactor (Figure 2). 2.0 g Pd/CNTs@NF-x catalyst and 100 g acetone solution containing 2.0 g NBR were added into the batch reactor. Before the hydrogenation reaction, the reactor was flushed with N2 for three times to remove air. Then, the mixture was stirred at 480 rpm at 70 ℃ with H2 pressure of 6 MPa for 6 h. Finally, the hydrogenated product was precipitated into ethanol for three times and dried at 60 ℃ for 12 h. To investigate the adsorption kinetics of NBR onto Pd/CNTs@NF-x catalysts. Those catalyst and NBR solution were also added into the same reactor above and stirred at 480 rpm at 70 ℃ for 6 h without H2. To evaluated the catalyst performance, hydrogenation degree (HD) was analyzed by Bromo-Iodometry method.35-37 Moreover, the powdered Pd/CNTs, the structured Pd/NF and the NF and CNTs supports were prepared and tested in NBR hydrogenation under the same procedure as described above. Additionally, to evaluate reusability of the designed catalyst, Pd/CNTs@NF-x catalysts were reused in further runs of NBR hydrogenation under the same reaction condition.


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Figure 2. Schematic diagram of the batch reactor over NBR hydrogenation.

2.5 characterization Surface area of CNTs@NF composites was calculated by the N2-adsorption isotherm obtained by the ASAP2010 static volumetric instrument (Micromeritics, USA) based on BET method. The average crystal size of palladium was determined in X-ray diffraction (XRD) using Scherrer equation based on Pd (111) crystal plane. The samples determined by a Rigaku D/MAX2550VB/PC diffractometer using Cu Kα radiation and operating at 40 KV and 30 mA were scanned in the 10°~80° (2θ) interval at a scanning rate of 0.5°/min. The morphology of the CNTs@NF composites was examined with JSM-6360LV scanning electron microscope SEM (JEOL, Japan) at an accelerating voltage of 15 KV. The average diameter and diameter distribution of CNTs@NF-x composites from SEM were based on the analysis of ca.4 images, estimating the diameter of about 100 carbon nanotubes. the Pd content in the Pd/CNTs@NF catalyst was determined by IRIS 1000 ICP-AES (Thermo Elemental, USA). The content and thermal stability of the growing CNTs were analyzed by thermogravimetric (TGA 4000, USA) method under air atmosphere at heating rate of 10 ℃/min from 100 to 800 ℃. The graphitization of those produced CNTs was characterized by InVia Raman Spectroscopy, the spectrum was recorded from 500 to 2100 cm-1. The structure of the HNBR and NBR samples were characterized by infrared (IR) spectroscopy with a Bruker Tensor 27 spectrophotometer in the wave-number range of 500 ~ 4000 cm-1. The average particle size of Pd NPs estimated from TEM was based on the analysis of ca.9 images, estimating the diameter of about 100 particles. The average particle size of Pd to calculate dispersion values assuming the spherical shapes described by Scholten et al38. The formula is as follow (eq 3).



D = 1021 

6  M   site d   metal  N

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(3)

where D is the dispersion (Pdsurface/Pdtotal), %, M is the atomic mass of Pd (106.42 g/mol), ρsite is the Pd surface site density (10.2 Pd atoms/nm2), d is the palladium particle size, nm, ρmetal is the metal density (12.02 g/cm3 for Pd) and N is the Avogadro’s constant.

3. Results and discussion 3.1. Characterization of the NF and CNTs@NF supports

Figure 3. The profile and SEM image of the bare Ni foam as received. (a) Profile of Ni foam; (b), (c) morphology of the Ni foam.

The geometry and the typical morphology of the NF were shown in Figure 3. The prepared NF showed the three-dimensional network with thickness of 10-15 μm and width of 170 μm. The NF owned the homogeneous open-pore structure, the size of pores formed by the NF framework was about 100~210 μm. The crystal size of Ni was on the order of 1~10 μm, and grain boundaries could be clearly seen on its surface (Figure 3c). According to the profile of NF and bulk density of Ni metal (7.78 g/cm3), the value of surface area per gram Ni was estimated to be 0.8 m2/g.


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Figure 4. SEM imagines of the CNTs@NF supports and statistical diameter distribution of as-prepared CNTs grown at different temperatures of 450, 500, 550 and 600 ℃ for 8 min using a gas mixture containing 25 % CH4/N2 with a total flow rate of 210 mL/min, respectively. (a) CNTs@NF-450; (b) CNTs@NF-500; (c) CNTs@NF-550; (d) CNTs@NF-600;

Figure 4 presented the typical morphologies of the CNTs@NF produced at 450, 500, 550 and 600 ℃, and the corresponding diameter distributions of the as-prepared CNTs. Clearly, the surface of those NF was almost covered with randomly oriented and compactly packed CNTs.3 With CNTs growth temperatures increased, the color of the CNTs@NF composites gradually deepened and the surface of NF gradually became rougher, but the original three-dimensional macroporous structure remained unchanged after CNTs growth (Figure S1, Supporting Information). Noticeably, the growing temperatures exerted a great influence on the morphology of CNTs. CNTs-layer of CNTs@NF-450 were not completely covered over the NF surface. And when the growth was carried out at 600 ℃, CNTs@NF possessed relatively uniform CNTs-layer. What’s more, we can see that, those CNTs were interconnected with each other and consequently created several pores, thereby also increasing the surface areas of CNTs@NF composites. The mean diameter of the as-prepared CNTs showed a decreasing trend with the increase of CNTs growth temperatures. The CNTs grown at 450 ℃ possessed the largest size (average 51.6 nm) among the four supports and the smallest in CNTs@NF-600 (average 38.7 nm). As indicated in Table1, the amount of carbon was obviously increased by approximately 12.9 times for CNTs@NF-600 (16.8 %) compared to that of CNTs@NF-450, which was probably due to the higher catalytic activity of Ni at relatively higher temperatures.39, 40 This result demonstrated that not only the content of the produced CNTs but also its average diameters and



density can be controlled by adjusting the growth temperatures.


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Table 1. Physicochemical properties of the resulting CNTs@NF composites Carbon content (wt %) a

Support

BET surface area (m2/g) b

Surface area per gram carbon (m2/g) b

Average CNTs size (nm)c

Graphite degree (%) d

Weight loss after sonication (%)

IG/ID Value in Raman spectra

NF 0.8 CNTs@NF-450 1.3 2.6 185 51.6 58.1 2.3 0.89 CNTs@NF-500 8.1 15.5 191 44.3 69.8 7.4 0.97 CNTs@NF-550 10.6 21.1 199 41.9 81.4 12.3 1.05 CNTs@NF-600 16.8 35.8 213 38.7 91.3 1.2 1.21 a Carbon content determined by eq 1. b Total specific surface area and average pore size determined by BET method. c Average CNTs size calculated by SEM images. d graphite degree of XRD determined by eq 4.

Table.2. Properties of Pd/CNTs@NF catalysts with different growing temperatures

Catalyst

Actual Pd loading (wt %)a

Pd/CNTs@NF-450 Pd/CNTs@NF-500 Pd/CNTs@NF-550 Pd/CNTs@NF-600

0.497 0.489 0.502 0.499

Average Pd size (nm)b

Pd dispersion

12.7 10.8 9.6 4.7

7.1 8.3 9.4 19.1

HD

TOF

(%)

(s-1)d

(%)c

a

Adsorption amount (mg/m2)

31.4 53.7 70.7 85.2

0.12 0.20 0.24 0.35

0.58 0.63 0.97 2.20

Determined with ICP-AES. b Calculated by TEM images. c On the basis of the spherical model of eq 3. d Calculated from eq 5.



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Figure 5. XRD patterns of the NF and CNTs@NF supports prepared at different growth temperature. (a) overview diffraction pattern; (b) graphite reflection of the NF and CNTs@NF carriers.

The X-ray diffraction patterns of the directly reduced NF and the prepared CNTs@NF-x supports were presented in Figure 5. As shown in Figure 5a, the diffraction patterns at 2θ of 44.7º, 52.1º and 76.7º observed in all samples corresponded to metallic Ni (111), (200) and (220),41-43 indicating the face-centered cubic structure was well preserved after CNTs growth. After CNTs growth, a characteristic peak appeared around 26.5º with a relatively low intensity, which was attributed to (022) plane of hexagonal-structured graphite.44, 45 The XRD result and subsequent characterization of Raman spectra (Figure 7a) and TEM image (Figure 9) indicated that those carbon deposited on

NF was mainly composed of carbon

nanotube. Clearly, the intensity of the graphite peaks increased with the growth temperatures in Figure 5b. The interplanar distances (d002) of the CNTs@NF carries grown from 450 to 600 ℃ were decreased from 0.339, 0.338, 0.337 to 0.3362 nm, respectively. All those values were very close to crystalline graphite (0.3348 nm). Based on

the

mathematical model,46, 47 the graphitization (G) of carbon deposits were calculated via eq 4 in Table 1. Clearly, the graphite degree dramatically increased when increasing CNTs synthesis temperatures. As we know, the difference in graphitization of CNTs among these carriers will have a great influence on the dispersion of Pd NPs and subsequently the activity for hydrogenation. G = ( 0.344 − d 002 ) / ( 0.344 − 0.3354 )

(4)

where 0.344 and 0.3354 nm are the crystal plane spacing distance of nongraphite and ideal graphite, respectively.


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Figure 6. The properties of the CNTs@NF-x supports. (a) CNTs@NF samples before and after ultrasonic treatment; (b) Nitrogen-adsorption-desorption isotherms; (c) Raman characterizations; (d) TG characterizations.

Well-known, a catalyst support must maintain the great structural stability in strong shear force surroundings like fixed bed reaction system. Therefore, the attachment strength between NF and growth CNTs was tested in an ultrasonic bath. The aqueous solution immersed CNTs@NF-600 carriers were both clear before and after ultrasonic treatment for 0.5 h and 2 h. (Figure S2, Supporting Information). As shown in Figure 6 a and Table 1, less than 3 % of carbon detached over CNTs@NF-600 composites, confirming the strong anchoring of the nanotubes immobilized on the NF. This would be due to the fact that CNTs could be evenly dispersed on the NF surface, nested in the macropores of the NF and interconnected with adjacent nanotubes. Nitrogen physisorption was used to study the porous structure of the CNTs@NF composites. Specific surface area of CNTs@NF-x were also shown in Table 1. It was found that those carriers had a large increase in specific surface area after being decorated with CNTs. Notably, the CNTs@NF-600 composite possessed a BET surface area as high as 35.8 m2/g, which was 44.8 times larger than that of the bare NF (0.8 m2/g). Meanwhile, based on the amount of CNTs on each sample, the



surface area per gram of CNTs was calculated to be approximate 200 m2/g which was in accordance with those reported literatures.28, 31 To further study the graphitization of the prepared CNTs, Raman spectra of CNTs@NF-x were collected, as depicted in Figure 6 c. All the samples showed two bands appearing at 1340 ~1350 cm-1 (D band, related to the vibrations of defects) and 1550 ~1600 cm-1 (G band, assigned to the crystalline graphitic carbon structure of CNT). The intensity ratio of IG/ID is considered as a parameter to determine the degree of graphitization of carbon materials.48-50 The value of IG/ID increased from 0.89 to 1.21, indicating an enhanced crystalline perfection of the obtained CNTs. The Raman results greatly agreed with the XRD analysis. To evaluate the thermal stability and verify the carbon content, TG characterizations of the as-produced CNTs@NF samples were tested and shown in Figure 6d. From TG curves, the weight of the CNTs@NF decreased initially, followed by an increasing trend. The phenomenon could be explained by the fact that the oxidation of NF (with mass increase due to the formation of NiO) was supposed to go on simultaneously with the gasification of CNT (with mass loss due to the formation of CO2).30 Note that the peak of burn-off of CNTs@NF composites shifted to higher temperatures, showing the higher degree of crystalline perfection of the synthesized CNTs. This result was in good accordance with the Raman analysis. 3.2. Characterization of Pd/CNTs@NF catalysts

Figure 7. XRD patterns of Pd/CNTs@NF-x catalysts

The X-ray diffraction patterns of Pd/CNTs@NF-x catalysts were illustrated in Figure 7. The reflections displayed at 2θ of 44.7º, 52.1º and 76.7º were assigned to metallic Ni and 2θ of 26.5º was attributed to the formation of CNTs in all samples,


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indicating that loading of Pd did not change the structure of CNTs@NF. After deposition of Pd onto CNTs@NF-x composites, a small pattern appeared around 40.0º and 46.5º, indicating the (111) and (200) planes of the face-centered cubic palladium (JCPDS 46-1043). This result demonstrated that the Pd NPs were successfully immobilized onto CNTs@NF supports. By using Scherrer equation, average crystal size of palladium grains of Pd/CNTs@NF catalysts with the CNTs growth temperatures of 450, 500, 550 and 600 ℃ were 14.8 nm, 12.4 nm, 10.2 nm and 6.7 nm, respectively.

Figure 8. TEM micrographs and the corresponding Pd size distribution of the Pd/CNTs@NF-x catalysts. (a)(b)(c)(d) the Pd/CNTs@NF with CNTs growing temperatures of 450, 500, 550 and 600 ℃, respectively. (a1) (b1) (c1) (d1) the insets of the CNTs and Pd morphology taken in other spots at higher magnification. (a2) (b2) (c2) (d2) the statistical diameter distribution of deposited Pd particles.

In order to investigated the effects of crystallite size and surface metal site on catalytic activity, typical TEM images of Pd/CNTs@NF-x catalysts were obtained in Figure 8, and the sizes and dispersion of Pd particles were summarized in Table 2. Generally, all the prepared CNTs almost had a tubular structure with hollow inner core closed both their ends. Notably, however, when the growth was carried out at 600 ℃, the small amount of CNTs had changed into two-dimensional graphitic carbon with bits of nanosheets. The mean diameter of the grown CNTs determined by



TEM were 45 nm which was coincided with the SEM results. Most of the Pd grains with a size range of 2-18 nm existed on the external surface of CNTs, which NBR coils could easily reach. Specially, Pd NPs on Pd/CNTs@NF-600 possessed the smallest size (average 4.7 nm) of the four Pd/CNTs@NF-x catalysts and the largest in Pd/CNTs@NF-450 (average 12.7 nm). This result agreed with the conclusion of the XRD analysis in Figure 7. On the basis of eq 3, palladium dispersions on Pd/CNTs@NF-x were calculated to be 7.1 %, 8.3 %, 9.4 % and 19.1 %, respectively. The HR TEM images (Figure S3, Supporting Information) showed d-spacing for adjacent lattice fringes were 0.22 nm and 0.20 nm, which agreed well with the (111) and (200) lattice spacing of face-centered cubic (fcc) Pd, respectively. 3.3 Catalytic performance of Pd/CNTs@NF catalysts.

Figure 9. (a) NBR hydrogenation over different composites. (b) color change of reaction solution before and after hydrogenation over the Pd/CNTs@NF-600. Reaction conditions: 70 ℃ reaction temperature, 1.00 gcat/gNBR, 2 wt % NBR-acetone, 6.0 MPa initial H2 pressure, 480 rpm agitation rate).

The as-prepared Pd/CNTs@NF-x catalysts were used in catalytic hydrogenation of NBR to evaluate its catalytic performance. Firstly, NBR hydrogenation behavior over 0.5 wt % Pd/CNTs@NF-600, the CNTs@NF-600 support, 0.5 wt % Pd/NF, the NF composite and CNTs in powders were carried out under the same reaction condition mentioned above. As shown in Figure 9 a, the relationship between HD and reaction time were tested during the NBR hydrogenation. Generally, no HD of NBR could be detected with the powdered CNTs, the nickel foam and the CNTs@NF-600 composites in the absence of Pd, indicating that metal Pd was the only active site for NBR hydrogenation. Specifically, HD of NBR over Pd/CNTs@NF-600 was 85.2%,


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which had approximately 4.63 times for the Pd/NF catalyst with HD of 18.4% under the same conditions. The result strongly confirmed that CNTs immobilized on the NF enhanced the internal diffusion and elevated the catalytic activity. This may due to the fact that CNTs grown on NF supplied a significant increase surface area compared to the bare NF. The large surface area of the CNTs@NF-x support would be sufficient to deposit Pd NPs with high dispersion. What’s more, the color of reaction solution after hydrogenation was still transparent and no CNTs powders were found in such viscous reaction media (Figure 9 b), demonstrating the great mechanical strength of the CNTs@NF-600 support and easy separation property of the Pd/CNTs@NF-600 catalyst.

Figure 10. Catalytic NBR hydrogenation over Pd/CNTs@NF-x catalysts under the same reaction conditions mentioned above. (a) the relationship between HD and reaction time. (b) HD after 6 h reaction time.

Further analysis on the catalytic behavior of the Pd/CNTs@NF-x catalysts with different CNTs growing temperatures were tested under the same reaction conditions mentioned above. As shown in Figure 10 a, HD increased with CNTs growth temperatures increasing from 450 to 600 ℃. In particular, the Pd/CNTs@NF-600 had 2.71 times higher HD than Pd/CNTs@NF-450 catalyst. The structures of NBR and HNBR products catalyzed by the Pd/CNTs@NF-600 composites were characterized by FTIR (Figure S4, Supporting Information), the decrease of olefin peaks about 970 cm-1 and still presence of -C≡N peak around 2235 cm-1 indicated the hydrogenation of C=C bonds instead of -C≡N groups. 3.4 Mechanism NBR hydrogenation over Pd/CNTs@NF catalysts. Firstly, to investigate the catalytic performance based on amounts of active sites exposed to reactants, turnover frequencies (TOF) were calculated by eq 5 in Table 2. It can be seen that the TOF value after HD of approximately 10 % on the stream of



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Pd/CNTs@NF-600 catalyst was the highest (0.35 s-1) among the four catalysts as it was about 1.46, 1.75 and 3.18 times higher than those of the Pd/CNTs@NF-550, Pd/CNTs@NF-500 and Pd/CNTs@NF-450, respectively. Differences in TOF values over four catalytic systems implied that Pd particle size was expected to have a significant influence on the hydrogenation kinetics among the catalysts. This could be explained that the content of CNTs with smaller diameter and greater crystallinity increased with its increasing growth temperatures, which consequently offered higher effective surface area to absorb Pd NPs with better deposition and contributed to the diffusion or contact of NBR coils.7

TOF =

c  nNBR t  nPd

(5)

Where c is the conversion of NBR, nNBR is the mole of the NBR coils, t is the reaction time and nPd is the mole of active Pd at surface.

Figure 11. Adsorption kinetics of NBR. (Experiment condition: 2wt% NBR/acetone solution, 2.0 g Pd/CNTs@NF-x catalyst, 70 ℃, 480 rpm agitation speed, 6h adsorption duration)

Then, to investigate the interaction between the Pd/CNTs@NF-x catalysts and NBR reactants, the adsorption kinetics of NBR onto Pd/CNTs@NF-x catalysts were tested. As shown in Figure 11, the plots represented the adsorbed NBR chains on catalyst surface versus time. Clearly, the adsorptions of NBR on Pd/CNTs@NF-x were observed to reach apparent equilibrium within 3 h and CNTs prepared at different temperatures indeed show distinct adsorption ability with NBR chains. Specially, to quantitatively investigate the effect of CNTs growth temperature on the NBR adsorption, the specific adsorbed amount of NBR coils was expressed in the term of mgNBR/m2 catalyst surface. The highest equilibrium specific adsorbed amount of NBR (2.2 mg/m2) was achieved on Pd/CNTs@NF-600 which was about 3.6 times higher


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than that of Pd/CNTs@NF-450. For one thing, the average size of the CNTs@NF-450 is large and the entanglement seriously occurred, leading to the steric hindrance of the adsorption of NBR. For another, it’s known that interaction between CNTs surface and carbon-carbon double bond is mostly derived from the π﹣π interaction of sp2 hybrid carbon atoms.51,

52

Thus, the difference adsorption performance could be

reasonably related to the difference perfection degree of sp2 hybrid carbon atoms, which macroscopically reflected in the difference graphitization degree of CNT. In other words, the higher graphitization degree, the more perfect structure of the graphene sheet constituting the CNTs surface, and hence a stronger adsorption of NBR coils onto the CNTs surface in Pd/CNTs@NF-600 catalyst. On the base of the hydrogenation degree, the TOF values, the catalyst structure and the adsorption experiments, the schemes of NBR hydrogenation over the Pd/CNTs@NF-x catalysts were proposed in Figure 12. It’s well known that, during the heterogenous catalytic systems, reactant substrates need to diffuse from bulk phase to catalyst surface and then adsorb onto the supports, after interaction with the catalytic active sites, the products would desorb from the catalyst surface and finally diffuse back to the bulk phase.53 With Pd/NF catalyst, Pd NPs were mainly dispersed in the inner pores of NF supports. The steric hindrance was potentially found which NBR coils cannot diffuse into, leading to the low hydrogenation activity consequently.18 However, owing to the non-porous and wide-open structure of CNTs, almost Pd grains were dispersed on the external surface of CNTs. Thus, the pore diffusion limitation has been completely eliminated, resulting in relatively high hydrogenation activity over the Pd/CNTs@NF-x catalysts. And among the Pd/CNTs@NF-x catalyst, with higher surface area, lower diameter and better crystallinity of CNTs growing at 600 ℃, the Pd grains have a higher degree of dispersion (Figure 12 b). Therefore, more NBR coils could be uniformly adsorbed at the same CNT and less entanglement simultaneously occurred. Consequently, the NBR coils could be efficiently catalyzed over the Pd/CNTs@NF-600 catalyst.



Figure 12. Proposed scheme of selective hydrogenation of NBR coils. (a) NBR hydrogenation over Pd/CNTs@NF-450 catalysts; (b) NBR hydrogenation over Pd/CNTs@NF-600 catalysts.

3.5 Stability of Pd/CNTs@NF catalysts in NBR hydrogenation system To investigate the stability of Pd/CNTs@NF-x catalysts, the Pd/CNTs@NF-600 composite were tested in NBR hydrogenation for five-time use. As shown in Figure 13, after reused for 5 times, the HD decreased slightly, followed by a stable trend. In particular, it was found that HD of Pd/CNTs@NF-600 catalyst was 80.1% for the 5th reuse, indicating its great reusability. The great catalytic stability of Pd/CNTs@NF catalysts was ascribed to the well-attached CNTs layer on NF and the strongly anchored Pd NPs on CNTs.

Figure 13. Catalytic NBR hydrogenation over the fresh Pd/CNTs@RNF-600 and reused catalysts

under the same reaction conditions mentioned above.


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4.Conclusion The structured CNTs@NF composites were successfully synthesized by directly growing carbon nanotubes (CNTs) under temperatures of 450, 500, 550 and 600 ℃ on the macroporous nickel foam (NF). Tested by the ultrasonic treatment, it was found that CNTs@NF possess good mechanical strength, especially in CNTs@NF-450 and CNTs@NF-600. Thanks to the high external surface area supplied by CNTs, the as-prepared Pd/CNTs@NF-600 showed the extremely higher catalytic activity over the Pd/NF catalysts in NBR hydrogenation. Moreover, compared with the catalysts of Pd/CNTs@NF-550, Pd/CNTs@NF-500 and Pd/CNTs@NF-450, the higher HD was achieved over Pd/CNTs@NF-600 under the same conditions. Remarkably, the structured Pd/CNTs@NF-x catalyst could be easily separated from the hydrogenated products, avoiding the necessity of filtration and additional cost. As a macro-structured CNTs support, CNTs@NF composite has potential in industrializing the heterogeneous reactions. Author Information Corresponding Author E-mail: [email protected] Acknowledgments Support from the Non-governmental International Science and Technology Cooperation Program (10520706000) from the Science and Technology Commission of Shanghai Municipality, the State Key Laboratory of Chemical Engineering open fund (SKL-ChE-09C07), and National University Student Innovation Program (201710251007 and x17009) and funding from the National Science Foundation of China (21576091) are gratefully acknowledged. Supporting Information for Publication The SEM images and profile of the CNTs@NF composites, images of different CNTs@NF before and after ultrasonic treatment, the HR TEM images of the Pd/CNTs@NF catalysts, FTIR spectra of NBR and HNBR products are presented in S1, S2, S3 and S4 in supporting information for publication, respectively.



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TOC Graphic




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CNTs

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