Palladium Supported on Carbon Nanotubes Decorated Nickel Foam

Subscriber access provided by Bibliothèque de l'Université Paris-Sud

Kinetics, Catalysis, and Reaction Engineering

Palladium Supported on Carbon Nanotubes Decorated Nickel Foam as the Catalytic Stirrer in Heterogeneous Hydrogenation of Polystyrene Miao Feng, Zhao-Hui Luo, Shan Yi, Hui Lu, Chong Lu, Chen-Yang Li, Jia-Li Zhao, and Gui-Ping Cao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03810 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 10, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 58 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

Industrial & Engineering Chemistry Research

Palladium Supported on Carbon Nanotubes Decorated Nickel Foam as the Catalytic Stirrer in Heterogeneous Hydrogenation of Polystyrene Miao Feng,† Zhao-Hui Luo,† Shan Yi,† Hui Lu,† Chong Lu,‡ Chen-Yang Li,† Jia-Li Zhao,† Gui-Ping Cao,*,† †

UNILAB, State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

‡

School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China *Corresponding author: E-mail: [email protected]

1

ACS Paragon Plus Environment

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 Carbon nanotubes supported palladium (Pd/CNTs) catalyst displayed excellent activity towards hydrogenation of polystyrene (PS) to produce polycyclohexylethylene (PCHE) with added commercial values. However, its application is limited by the costly catalyst filtration process and energy consuming elevated agitation speed. Herein, a dense, homogenous and strongly attached layer of CNTs was decorated on nickel foams and employed as the catalyst support for palladium nanoparticles. Structured catalyst (Pd/CNTs@NF) was directly used as the catalyst stirrer in a rotating foam stirrer reactor (RFSR) for PS hydrogenation. Owing to the enhanced mass-transfer process, hydrogenation degree reached 68.8 %, which was 45.1 % higher than that obtained in a slurry reactor using powdered Pd/CNTs catalyst under identical reaction condition. Outstanding attachment strength of CNTs on the foam and catalytic stability in 8 runs of recycling were also demonstrated. This work elucidates the promising feasibility of structured Pd/CNTs@NF catalyst as the catalytic stirrer in RFSR.

Keywords: Structured carbon nanotubes; Rotating foam stirrer reactor; Polystyrene hydrogenation

2


Page 2 of 58

Page 3 of 58 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


1. Introduction Hydrogenation of polymeric hydrocarbons is a commercially important process that imparts dramatically improved thermal and oxidative stability to those materials

1-4.

Heterogeneous catalytic hydrogenation of polystyrene (PS) is a representative example to produce polycyclohexylethylene (PCHE), which shows greatly enhanced heat and UV resistance compared with PS

5-7.

Unfortunately, high-molecular-weight of PS

molecular inevitably leads to large molecular size (10 - 60 nm) and high solution viscosity (10 - 105 mPa∙s). Both would cause severe liquid-solid mass transfer limitation and hence dramatically deteriorate the hydrogenating rate 8. Recently, Several attempts have been made to promote the hydrogenating rate, including the introduction of supercritical CO2 (sc-CO2)

9-11

and optimal design of the catalyst supports

8, 12, 13.

However, catalyst deactivation occurred in the CO2-expanded liquids as a consequence of CO formation via the reverse water-gas shift reaction

14, 15.

Additionally,

macroporous supports are believed to benefit the pore diffusion of PS, such as widepore silica

1, 13, 16

or monolithic TiO2 ceramic foam with open accessible pores

12.

Nevertheless, it remains challenging to obtain the highly dispersed Pd nanoparticles (NPs) on those supports since widening pores usually results in a decreased surface area. In the previous work, we proposed the utilization of carbon nanotubes (CNTs) as the catalyst support and the CNTs supported Pd catalyst (Pd/CNTs) displayed excellent catalytic hydrogenation activity. The synergistic effect of non-porosity and high effective surface area contributed to an enormous amount of accessible active sites and hence an accelerated reaction rate 7. Whereas, powdered slurry catalysts are not 3



Page 4 of 58

convenient for the industrialization of PS hydrogenation. In very viscous media slurry catalyst are moving with the liquid causing an inefficient liquid-solid mass transfer, therefore it is mandatory to elevate agitation speed to increase the refreshment rate of liquid film on catalyst surface

17.

On the other hand, powdered catalysts need to be

filtered from the slurry to avoid its loss and accelerated degradation of polymers

18.

Obviously, it is costly and energy-consuming to elevate the agitation speed and operate the filtration process. To address these issues, a novel rotating foam stirring reactor (RFSR) or monolithic stirrer reactor (MSR) has been proposed as an alternative to the conventional slurry reactor towards multiphase reactions 19-21. In this reactor, solid foams are generally used as the structured catalyst supports and meanwhile the stirrer by simply mounting them on a stirrer shaft. One significant advantage of the RFSR is that the solid phase is fixed and the catalyst does not need to be separated downstream from the reactor. Moreover, it shows great potential for further optimization of the liquid flow and therefore the mass transfer. Briefly, these foams strongly increase the mixing, especially in directions perpendicular to the flow, due to the continuous disruption of the flow pattern. Fluid streams split and recombine when passing the pores, and small turbulences behind the struts contribute to the mixing

22.

RFSRs also exhibit high rates of gas-liquid mass

transfer due to the formation of finely dispersed bubbles 17 and high rates of liquid-solid mass transfer because of the fast refreshment of the catalyst surface 23. The rational design of the catalytic stirrer, including its structure and macroscopic 4


Page 5 of 58 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


configuration, is believed to play an important role in enhancing the efficiency of mass/heat transfer process and hydrodynamics 17, 23, 24. Various porous materials, such as monoliths 25, cloths 26, and fibers 27 have been used as both the supports and stirrer in RFSRs to gain better catalyst activity. Among them, foam materials have attracted increasing attentions because of the commercial availability, low pressure drop (typically 101-2 Pa/m) and excellent mass transfer due to the interconnected open cells 28.

Herein, to combine the textual properties of CNTs and merits of RFSR, we therefore propose a strategy to design a catalytic stirrer including in-suit immobilization of CNTs on a solid foam and subsequent decoration of Pd NPs. Nickel foam, as a commercially available metal foam with three-dimensional interconnected struts and excellent mechanical strength, is comprehensively selected as the solid foam substrate in this work. CNTs can be directly immobilized on the foam surface via the facile chemical vapor deposition (CVD) method without the use of any external catalysts

29-34.

Moreover, excellent attachment strength between CNTs and the foam surface makes it practical to serve as the catalytic stirrer in a RFSR 35. In this paper, a RFSR with nickel foam based stirrer blades was designed for the heterogeneous hydrogenation of PS aiming at achieving a higher hydrogenation degree (HD). The nickel foams were at first decorated by CNTs (CNTs@NF) and then by Pd NPs to eventually form a structured catalyst (Pd/CNTs@NF). Factors like CVD duration, agitation speed, Pd amount and PS concentration that may influence the 5



hydrogenating rate have been systematically investigated. Besides, RFSR using structured Pd/CNTs@NF as the catalytic stirrer was compared with the slurry reactor using powdered Pd/CNTs catalyst under the same reaction condition. The objective of our work is to demonstrate the industrial feasibility of a RFSR with the structured Pd/CNTs@NF as its catalytic stirrer in PS hydrogenation or other heterogeneously catalytic polymers engaged reactions.

2. Experimental 2.1 Regents The nickel foams (~ 99 %, 75 PPI, a three-dimensional network of connected strands) applied in this study were provided by Kunshan Jia Yi Sheng Electronics Co., Ltd. The palladium precursor K2PdCl4 was purchased from Adamas Reagent Co., Ltd. High purity hydrogen (H2), nitrogen (N2) and the carbon source ethylene (C2H4) were supplied by Shanghai Siling Gas Co., Ltd. Commercial PS (GPPS-123) was presented by Shanghai SECCO Petrochemical Co., Ltd., with a number average, weight average, and viscosity average molecular weight of 90, 263, and 279 kg/mol, respectively, measured by Waters1515 Gel Permeation Chromatography (Waters Co., Ltd., USA) with tetrahydrofuran as the solvent at 35 ºC. The solvent, decahydronaphthalene (DHN), was purchased from Sinopharm Chemical Reagent Co., Ltd. All the regents were used without further treatment unless specified. The typical process of the preparation of Pd/CNTs@NF catalyst was displayed in 6


Page 6 of 58

Page 7 of 58 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


Figure 1. As can be seen from the schematic diagram, the preparation process can be divided into two steps, i.e., CNTs growth and Pd NPs loading, which were described in detail as follows.

Figure 1. Schematic illustration of Pd/CNTs@NF catalytic stirrer preparation

2.2 Preparation of CNTs decorated nickel foam (CNTs@NF) Prior to CNTs growth, nickel foams were cut into desired shape and dimensions (3.5 cm × 2.5 cm × 0.5 cm, with the weight of around 0.7 g). After being cleaned ultrasonically in anhydrous ethanol for 30 min and rinsed thoroughly with deionized water. CNTs were grown on the nickel foam via CVD method. Typically, nickel foams were firstly heated to 550 °C under a stream of N2 (VN2 = 200 mL/min). CNTs were synthesized by introducing a C2H4/N2 gas mixture (VC2H4 : VN2 = 100 mL/min : 180 mL/min). CNTs growth duration was ranged from 16 to 22 min. The obtained composite was denoted as CNTs@NF-t (t equaled the corresponding CVD duration). The deposition of CNTs layer on the foam surface was evidenced by a color change from silver to black. The amount of CNTs was determined by measuring the weight. In order to evaluate the attachment strength of CNTs to the foam surface, CNTs@NF composite was immersed into an ethanol solution and then vibrated by ultrasound with a frequency of 40 KHz. The weight loss of the composites after vibration for a certain period of time was measured as criterion. 7



2.3 Palladium deposition Palladium deposition was performed via wetness impregnation using K2PdCl4 dissolved in deionized water. Excess water was completely evaporated by placing the sample in a vacuum oven at 60 °C for 12 h, leaving behind the palladium precursor on CNTs@NF. The CNTs@NF adsorbing K2PdCl4 were further calcined under a stream of N2 at 400 ºC for 4 h and reduced at 300 ºC in a H2/N2 gas mixture (VH2 : VN2 = 150 mL/min : 100 mL/min) for 6 h. 2.4 Slurry catalyst preparation In order to compare the difference of catalytic activity between structured Pd/CNTs@NF catalyst in RFSR and powdered Pd/CNTs catalyst in the slurry reactor, CNTs on CNTs@NF were collected after completely dissolving the foam substrate by ultrasonic treatment in concentrated hydrochloric acid solution. Then, the obtained CNTs were thoroughly washed by deionized water. After that, the slurry Pd/CNTs catalyst was prepared under the same procedure as described in Section 2.3. 2.5 Catalyst characterization The samples of CNTs/NF-20 (CVD duration was 20 min) and Pd/CNTs@NF-20 were chosen as the representatives of the characterizations. Microstructures and morphologies of CNTs@NF were observed using a JOEL JSM-6360LV scanning electron microscopy (SEM) with the accelerating voltage of 150 kV. Transmission electron microscope (TEM) images were performed on JEOL JEM-2100 with a field emission source, and the accelerating voltage was 200 kV. TEM species were obtained 8


Page 8 of 58

Page 9 of 58 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


by ultrasonically suspending the samples in ethanol and depositing the droplets of suspension on a standard copper grid. Nitrogen physisorption was adopted to investigate the specific surface area (SSA) of the sample using an ASAP2010 static volumetric instrument (Micromeritics, USA). Samples were degassed at 300 °C for 3 h and measured at -196 °C. SSA was calculated using BET method. Degree of graphitization of as-synthesized CNTs was characterized by a Renishaw inVia Raman spectroscopy with a 532 nm laser. Raman spectrums of the CNTs were recorded from 500 to 2500 cm−1. The crystallite phase of Pd, Ni and CNTs in the catalysts was analyzed by the powder X-ray diffraction (XRD) technique by using a Rigaku D/Max 2550 X-ray diffractometer equipped with a high-speed array detection system. Cu Kα radiation (40 kV and 30 mA) was used as the X-ray source. The scanning 2θ range was 20 - 70 º. The average size of Pd NPs was calculated by Scherer equation based on the diffraction peak of Pd (1 1 1) crystal plane. Thermal stability and content of the assynthesized CNTs were measured by a PerkinElmer thermo gravimetric analysis under air atmosphere at a heating rate of 10 ºC/min from 30 to 800 ºC. 2.6 Catalytic hydrogenation of polystyrene (PS) The PS hydrogenation reaction, as shown in Eq. 1, was carried out in a mechanical stirred 500 mL autoclave in batch mode. CH CH2

n

H2 Pd NPs

CH CH2

n

(1)

9



Figure 2. Schematic illustration of the RFSR for PS hydrogenation reaction

As illustrated in Figure 2, structured Pd/CNTs@NF catalyst was mounted on the shaft as a catalytic stirrer. The reactor was equipped with four vertical baffles with a width of 10 mm and a thickness of 1.0 mm to reduce the vortex formation. Before the reaction, 120 g PS/DHN (3 wt%) solution was charged into the RFSR. After being flushed with N2 to remove air, the reactor was heated to 180 ºC. Hydrogenation of PS was initiated by flushing H2 to 5.8 MPa and adjusting the agitation rate. The samples of initial PS solution and hydrogenated reaction mixture were taken and analyzed by UV-vis spectrophotometer (UV-1800, SHIMADZU) at 261.5 nm to determine the concentration of aromatic rings. The conversion of the aromatic rings, namely hydrogenation degree (HD), was calculated as Eq. 2. 10


Page 10 of 58

Page 11 of 58 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


HD = (1 - cA/cA,0) × 100 % where

cA,0

is the concentration of aromatic rings at initial time;

(2)

cA

is the

concentration of aromatic rings at a certain time. In the slurry phase reactor, powdered catalyst (Pd/ CNTs) was directly dispersed into the reactant solution. To ensure an effective agitation, the catalytic stirrer foam Pd/CNTs@NF was replace by a stirrer blade, as shown in Figure 3. After that, PS hydrogenation over Pd/CNTs catalyst was performed under the same procedure as described above. All the experiments and variables in this work are summarized in Table 1.

Figure 3. Schematic illustration of the slurry phase reactor for PS hydrogenation

reaction

11


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

Page 12 of 58

Table 1. Summary of Experiments Conducted in This Work. cPS,

Growth

mCNTs,

mPd,

Agitation

HD,

wt%

time, min

g

wt%

speed, rpm

%

nickel foam

3

0

0

0.5

180

500

0

RSFR

CNTs@NF

3

0

0

0.5

180

500

0

3

RSFR

Pd/NF

3

0

0

0.5

180

500

7.1

4

RSFR

Pd/CNTs@NF

3

16

0.15

0.5

180

500

55.2

5

RSFR

Pd/CNTs@NF

3

18

0.22

0.5

180

500

65.4

6

RSFR

Pd/CNTs@NF

3

20

0.30

0.5

180

500

68.8

7

RSFR

Pd/CNTs@NF

3

22

0.32

0.5

180

500

69.0

8

RSFR

Pd/CNTs@NF

3

20

0.30

0.5

180

300

39.6

9

RSFR

Pd/CNTs@NF

3

20

0.30

0.5

180

400

48.1

10

RSFR

Pd/CNTs@NF

3

20

0.30

0.5

180

600

53.6

11

RSFR

Pd/CNTs@NF

3

20

0.30

0.75

180

500

79.6

12

RSFR

Pd/CNTs@NF

3

20

0.30

1.0

180

500

87.0

13

RSFR

Pd/CNTs@NF

5

20

0.30

0.5

180

500

66.1

14

RSFR

Pd/CNTs@NF

7

20

0.30

0.5

180

500

28.3

15

RSFR

Pd/CNTs@NF

3

20

0.30

0.5

175

500

64

16

RSFR

Pd/CNTs@NF

3

20

0.30

0.5

170

500

60.3

Reaction

17

RSFR

Pd/CNTs@NF

3

20

0.30

0.5

165

500

52

temperature

18

RSFR

Pd/CNTs@NF

3

20

0.30

0.5

160

500

44.5

Trial

Reactor

Catalyst

1

RSFR

2

T, °C

12


Variables Existence of Pd

Amount of CNTs

Agitation speed

Amount of Pd PS concentration

Page 13 of 58 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


19

Slurry

Pd/CNTs

3

-

0.30

0.5

180

500

23.7

20

Slurry

Pd/CNTs

3

-

0.30

0.5

180

800

68.0

13


Reactor


3. Results and discussion 3.1 Characterization of catalytic Pd/CNTs@NF stirrer CNTs can be homogenously decorated on the foam surface, as evidenced by the color changing from silver to black after CVD process. Figure 4 shows the SEM images of original nickel foam and CNTs@NF-20 synthesized by CVD method using C2H4 as the carbon source with a duration of 20 minutes.

Figure 4. SEM imagines of nickel foam based samples: (a) skeleton of original nickel 14


Page 14 of 58

Page 15 of 58 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


foam; (b) surface of original nickel foam; (c) skeleton of CNTs@NF; (d) CNTs forests formed on nickel foam surface, inserted image shows the; (e) statistical distribution of CNTs outer diameter. The original nickel foam has a typical three-dimensional interconnected structure (Figure 4a). Grain boundaries of crystalline nickel were clearly observed on the smooth surface in the absence of any micropores (Figure 4b). It has been proposed that these grain boundaries could facilitate CNTs formation by serving as precipitation sites for diffused carbon in nickel grains 36. As shown in Fig 4c, after CVD process, the skeleton of nickel foam remains intact, indicating that the excellent mechanical strength of metal foam was well retained as sever corrosive metal dusting would break the bulk metal up to metal powder if the CVD conditions were not appropriately tuned. Foam surface appeared rougher after being uniformly covered by randomly oriented and compactly packed CNTs forests (Figure 4d). Average outer diameter of the as-prepared CNTs is 48.8 nm with a standard deviation of 9.8 nm by statistics of about 120 CNTs in total. The formation of CNTs forest from C2H4 decomposition on polycrystalline Ni have been systematically studied. Briefly, it starts with the formation of meta-stable Ni3C, which later decomposes into Ni NPs with proper dimensions (20 - 70 nm). After that, CNTs were formed on these sufficient Ni NPs 37, 38. To further confirm the quality of CNTs formed on nickel foam, TG and Raman characterizations were conducted. Figure 5a displays the thermal curve. CNTs with high crystallinity are much more resistant to oxidation than amorphous carbon, hence oxidation only occurs at a higher temperature. Weight loss was negligible below 480

15



ºC and the main weight loss occurred at a relatively higher temperature ranging from 480 to 615 ºC, which was ascribed to the oxidation of CNTs. This implies that most of the deposited carbon was in the form of CNTs rather than amorphous carbon. Figure 5b shows the Raman spectra. Two characteristic peaks were observed: the peak centered at 1592 cm−1 (G band) is assigned to the sp2-bonded C - C stretching (E2g) mode in a two-dimensional hexagonal lattice for graphene sheet, and another peak centered at 1348 cm−1 is usually termed as the D band, which is associated with the vibrations of carbon atoms with dangling bonds for the in-plane terminations of disordered graphite 39. The intensity ratio of the G and D band (IG/ID) reveals the degree of disorder in the graphite sheets and thus it can be used as a qualitative description of the crystallinity of CNTs 40. The value of IG/ID calculated with Lorentzian profile as the simplest fits was 1.02, suggesting the modest graphitization degree of the as-prepared CNTs. It is

believed to benefit both the adsorption of aromatic rings via strong π

stacking interaction and the anchoring of Pd NPs 41, 42.

100 a

Weight percentage, wt%

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

Page 16 of 58

95 CNTs oxidation ~ 20 wt% 480 - 615 C

90 85 80 0

200 400 600 Sample temperature, C

16


800

Page 17 of 58

b

G

D

IG/ID=1.02

Intensity, a.u.

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


500

1000

1500

2000

2500

-1

Raman shift, cm

Figure 5. (a) Thermal curve of structured CNTs@NF support in static air at a heating rate of 10 ºC/min from 30 to 800 ºC; (b) Raman spectra of CNTs grown on nickel foam. The catalytically active Pd NPs were deposited on the external surface of CNTs via a facile wetness impregnation method. X-ray diffraction was employed to determine the crystal structure. Figure 6 shows the XRD patterns of nickel foam based samples. The crystalline structures were identified according to the peaks of the reference crystalline compounds in the powder diffraction file database (Joint Committee on Powder Diffraction Standards, International Center for Diffraction Data). All the nickel foam based samples shows two distinct diffraction peaks at 44.5 º and 51.8 º, which are indexed to (1 1 1) and (2 0 0) crystal planes of Ni metal with face-centered cubic structure, respectively. A diffraction peak at 26.5 º attributed to (0 2 2) plane of hexagonal-structured graphite appears in sample CNTs@NF, verifying the welldefined crystalline structure of as-prepared CNTs. After Pd deposition, peaks at 40.1 º and 46.7 º could be observed and fit well with the characteristic (111) and (110) planes of Pd, respectively. The presence of such XRD peaks confirmed the existence and the zero valence of Pd. The average crystalline size was calculated to be approximately 6 17



nm by Scherrer equation.

JCPDS card NO.04-0850

C(002) Pd (111) Pd (200)

Pd/CNTs@NF

Intensity, a.u.

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

Page 18 of 58

CNTs@NF

Nickel foam Ni (111) Ni (200)

20

30

40 50 2 theta, 

60

70

Figure 6. X-Ray diffraction patterns of original NF, CNTs@NF and Pd/CNTs@NF, using the raw data without any other treatment (such as background subtraction and smoothing). The uniformity and size distribution of Pd NPs on CNTs@NF substrate was investigated by TEM (Figure 7). Pd NPs were highly dispersed on the external surface of CNTs with an average size of 5.9 nm, which matches well with the calculation results based on XRD patterns. Furthermore, the dispersion of Pd NPs was estimated to be 15.3 % using hemispherical model. 43High resolution TEM image (Figure 8) revealed the well crystallized (111) and (200) face of Pd with a lattice space of 0.22 nm and 0.20 nm, respectively.

18


Page 19 of 58 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


Figure 7. TEM images of Pd NPs deposited on the CNTs surface of CNTs@NF: (a) 0.5 wt% Pd loading; (c) 0.75 wt% Pd loading; (e) 1.0 wt% Pd loading; (g) 0.5 wt% Pd loading after 9 runs of reactions; statistical size distribution of Pd NPs: (b) 0.5 wt% Pd loading; (d) 0.75 wt% Pd loading; (f) 1.0 wt% Pd loading; (h) 0.5 wt% Pd loading after 9 runs of reactions. 19



Figure 8. High-resolution image of Pd NPs, showing clear inter planar spacing of 0.22 nm and 0.20 nm corresponding to Pd (111) and Pd (200), respectively. Strong attachment of CNTs to nickel foam substrate is essential for its application as the catalytic stirrer in RFSR. Figure 9 shows the carbon weight after vibration by ultrasound for a certain period of time. Carbon content decreases slightly from the initial 20.2 wt% to 18.9 wt% after ultrasonic treatment for 80 min. Then it remains constant, indicating that initial structure of the composites is retained. Vast majority of CNTs (~ 94 wt%) is remained and it reveals a remarkably firm attachment strength between CNTs and nickel foam substrate. In reality, the working liquid remains transparent and colorless without any trace of detached CNTs in the product liquid after 10 h reaction as shown in the inset images.

20


Page 20 of 58

Page 21 of 58

20.4 20.1

Carbon content, %

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


19.8 19.5 19.2 18.9 18.6

0

30

60

90

120

150

180

Vibration time, min Figure 9. Stability of CNTs attachment against ultrasonic treatment with a frequency of 40 KHz, inset imagine shows that working solution remains transparent and colorless without any trace of detached CNT after PS hydrogenation reaction.

3.2 Catalytic performance 3.2.1 Influence of CVD duration

21



250

a

200

0.21

150

0.14

100

0.07

50

Carbon weight, g

2

0.28

Specific surface area, m /g C

0.35

0.00

16

18

20

22

0

Growth time, min Trial 5

Trial 4

Trial 6

Trial 7

b

150

60

120

45

90

30

60

15

30

0

P d /C

P d /C

NTs@ NF-1 6

P d /C

NTs@ NF-1 8

P d /C

NTs@ NF-2 0

Specific rate, mmol/(g Pdh)

75

Hydrogenation degree, %

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

Page 22 of 58

0 NTs@ NF-2 2

Figure 10. (a) Carbon weight obtained after different CVD duration and corresponding specific surface area expressed in terms of m2/g C (CVD condition: 550 ºC temperature, VC2H4 : VN2 = 100 mL/min : 180 mL/min carbon source, 16 ~ 22 min duration); (b) Hydrogenation degree and specific rate expressed in terms of mmolaromatic rings/gPd/h 22


Page 23 of 58 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


over Pd/CNTs@NF-t catalysts (t = 16 ~ 22min, trial 4 ~ 7 in Table 1, 10 h reaction duration). To exclude the involvement of nickel foam or CNTs@NF supports in the catalysis, experiments using them as catalysts (Trial 1 & 2 in Table 1) were carried out and no HD of PS could be detected, suggesting that Pd was the only active site for PS hydrogenation. The carbon weight increased dramatically from 0.15 to 0.30 g with the increasing CVD duration from 16 to 20 min (Figure 10a). Prolonged CVD duration facilitated the formation of CNTs with a higher growth density and a longer average nanotube length, therefore a denser and thicker CNTs forests could be formed. However, it only obtained a marginal weight increase of 0.02 g after a CVD duration of 22 min, suggesting the occurrence of carbon capsulation onto the Ni NPs 44. In other words, the Ni NPs were capsulated by excess amount of carbon generated by composition of C2H4 and hence no longer catalyzed the CNTs growth. It should be noted that despite the increasing CNT amount, the specific surface area normalized for the CNTs amount was identical with the value of approximately 190 m2/g CNTs, which is in good agreement with the reported values 45, 46. The corresponding Pd supported catalysts were tested in PS hydrogenation and the activity was displayed in Figure 10b. As is well-know, heterogeneously catalytic hydrogenation of PS was a complex process, during which only a part of the aromatic rings on the PS chains could be adsorbed and catalyzed at one time, and the partially hydrogenated PS coils need to conformationally rearrange the chains for many times in order to get all the aromatic rings saturated 8, 47. The RFSR equipped with the structured Pd/CNTs@NF as the catalytic stirrer displayed an 23



excellent activity towards PS hydrogenation. Highest HD of 69 % was achieved over Pd/CNTs@NF-22 catalytic stirrer with a specific rate of 126.5 mmolaromatic rings/gPd/h. Lowest HD of only 7.1 % was achieved directly employing nickel foam as the catalytic stirrer (Trial 3 in Table 1), revealing the positive effect of CNTs forests on improving the catalytic activity. Moreover, an extension in CVD duration also leads to an increased HD. This trend could be interpreted by the increased surface area contributed by CNTs forests facilitating higher dispersion of Pd NPs at a fixed loading, which was evidenced by the fact that HD over Pd/CNTs@NF-20 (68.8 %) was close to that over Pd/CNTs@NF-22 (69 %) for there existed only a negligible difference between the two samples in surface area.

3.2.2 Influence of agitation speed

Trial 8

Trial 9 Trial 6/17 Trial 10 RFSR RFSR

60

Trial 18

Slurry Reactor Slurry Reactor

150 120

45

90

30

60

15

30

0

300

400

500 600 Agitation speed, rpm 24


800

0


75


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

Page 24 of 58

Page 25 of 58 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


Figure 11. Hydrogenation degree and specific rate expressed in terms of mmolaromatic rings/gPd/h

as the function of the agitation speed in RSFR and slurry reactor. (Trial 6, 8

~ 10, 17 ~18 in Table 1, 10 h reaction duration). Figure 11 shows the catalytic performance of RFSR as a function of the agitation speed within the range of 300 to 600 rpm. Increasing the rotating speed from 300 rpm to 500 rpm elevated the HD of PS from 39.6 % to 69.0 %, indicating the improved mass transfer at higher agitation speeds. However, at 600 rpm, HD declined to 53.5 %. This trend can be explained by the following fact: within the range of the low agitation speeds, an increase in the stirrer speed caused an increasing liquid flow through the foam matrix, which improved the liquid-solid mass transfer and hence increased the opportunity of contact between the reactants and the active Pd sites localized on the CNTs surface 48. Another effect that played an important role was the gas-liquid mass transfer of hydrogen. The number of bubbles breaking when passing the foam window was increased and could led to the formation of finer dispersed bubbles which enhanced the gas-liquid mass transfer 49. Nevertheless, if the stirrer speed is excessively fast, more violent swirling flow might occur and less reaction liquid flowed through the foam matrix, which further deteriorated the contact between the liquid-solid interfaces. In other words, the residence time of the reactants was too short to react sufficiently, which led to the decline of catalytic activity 28. Thus, a moderate agitation speed, 500 rpm, was desired to increase the HD of PS and was used unless specified for the subsequence tests. Similar phenomenon has been reported in previous works, suggesting the similarity in hydrodynamics and mass transfer between different reactions carried in

25



the RFSR 28, 48, 49. Owing to the diversity of properties of reaction mediums, different optimum agitation speeds were desired to achieve the highest catalytic activity. In the present work, the optimum agitation speed (500 rpm) was higher than the reported values in the previous works (300 ~ 450 rpm), which could be attributed to the intrinsic properties of polymer engaged reactions with severe mass transfer limitation.

3.2.3 Influence of Pd amount

Trial 6

Trial 11

Trial 12

150

80

120

60

90

40

60

20

30

0

0.50

0.75

1.00


100


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

Page 26 of 58

0

mPd/mPS, wt% Figure 12. Catalytic hydrogenation of PS over Pd/CNTs@NF-20 catalyst with different Pd loading (Trial 6, 11 ~ 12 in Table 1, 10 h reaction duration). As shown in Figure 12, HD increased gradually from 68.8 % to 87 % with the increasing relative Pd amount from 0.50 wt% to 1.00 wt%, indicating more catalytically active sites were formed and hence led to the improvement of hydrogenation 26


Page 27 of 58

performance. However, the specific hydrogenation rate expressed in terms of rate per gram Pd decreases with increasing Pd amount. As confirmed by TEM (Figure 7), increased deposited amount of Pd could result in the larger size of the Pd NPs and hence a lower proportion of surface Pd atoms or a lower dispersion, which consequently depressed the catalytic efficiency of the active component. In order to maintain the sufficient utilization of noble Pd metal, the mass ration of Pd to PS in the subsequent tests was fixed at 0.5 wt%.

3.2.4 Influence of PS concentration

Trial 6

Trial 13

Trial 14

210

60

175

48

140

36

105

24

70

12

35

0

3

5 PS concentration, wt%

7


72


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


0

Figure 13. Catalytic hydrogenation of PS over Pd/CNTs@NF-20 catalyst with different PS loading (Trial 6, 13 ~ 14 in Table 1, 10 h reaction duration). The viscosity of PS-DHN solution with different concentration varies greatly due to the instinctive property of polymer solution. In order to investigate the influence of 27



solution viscosity, PS-DHN solutions with various concentration were used as the reactant and catalytic activity over Pd/CNTs@NF-20 was represented in Figure 13. With the increased concentration, the movement of polymer chain in the solvent becomes much more challenging due to the severe steric hindrance. Under this circumstance, hydrogenating rate is mainly limited by the liquid-solid mass transfer, during which PS coils transfer from bulk liquid phase to external surface of the solid catalyst particles. Increasing the PS concentration from 3 wt% to 5 wt%, only a marginal decrease of HD from 68.8 % to 66.1 % occurred, however, HD decreased dramatically to 28.3 % in 7 wt% PS-DHN solution. This result indicates that hydrogenating rate has been greatly depressed at a PS concentration higher than 5 wt% (i.e. 7 wt%). This could be further confirmed by the specific hydrogenating rate under different PS concentration. The specific rate at first increased to 202 mmolaromatic rings/gPd/h,

which was the result of concentration effect since the hydrogenation

displayed reaction order of 1 with respect to PS concentration. Dramatic decrease of specific rate to 123 mmolaromatic rings/gPd/h at PS concentration of 7 wt% verified that the mas transfer started to play a major role in determining the reaction rate.

3.2.5 Comparison with slurry reactor In order to compare the catalytic performance of slurry reactor, powdered Pd/CNTs catalysts containing the same amount of Pd and CNTs as those of Pd/CNTs@NF were employed as the catalyst for PS hydrogenation, as shown in Figure 11. At an agitation 28


Page 28 of 58

Page 29 of 58

speed of 500 rpm, the RFSR with Pd/CNTs@NF catalytic stirrer obviously outperformed the slurry reactor, which obtained a much lower HD of 23.7 %. In a slurry reactor, the powdered catalysts are moving with the liquid, which could cause a low refreshment rate of liquid film on catalyst surface and hence a low liquid-solid mass transfer 23. RFSR was proved to be a promising alternative to the slurry phase reactor owing to the enhanced liquid-solid transfer with less energy consumption. Subsequently, the slurry phase experiment was conducted at higher agitation speed, i.e., 800 rpm, at which a close HD of 68.0 % could be obtained. This further confirmed the conclusion that during the slurry phase operation, more energy was needed to maintain a higher agitation speed for effective liquid-solid mass transfer. Moreover, the corresponding turn-over frequency (TOF) was calculated to investigate the intrinsic catalytic activity of Pd NPs. The TOF of Pd NPs on CNTs@NF was 0.025 s-1, similar to that of Pd/CNTs (0.024 s-1).

75 Hydrogenation degree, %

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


a

60 45

measured data at 180 C measured data at 175 C measured data at 170 C measured data at 165 C measured data at 160 C

30 fit curve for 180 C fit curve for 175 C fit curve for 170 C fit curve for 165 C fit curve for 160 C

15 0 0

2

4 6 Reaction duration, h 29


8

10


-2.0

-2.2

b

Ea = 55.8 kJ/mol 2

180 C

R = 0.9775

175 C

-2.4

170 C

lnk

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

Page 30 of 58

-2.6 165 C

-2.8 0.264

160 C

0.268

0.272

0.276

0.280



1/RT    mol/J Figure 14. (a) Reaction duration of PS hydrogenation over Pd/CNTs@NF-20 catalyst at different temperatures in the RFSR; (b) Arrhenius plots of PS hydrogenation reaction over Pd/CNTs@NF-20 catalyst in the RFSR. (Trial 6, 15 ~ 18 in Table 1, 10 h reaction duration). Furthermore, PS hydrogenation experiments were conducted at various reaction temperatures ranging from 160 to 180 ºC to investigate the kinetics and activation energy, as shown in Figure 14a. According to our previous work 7, PS hydrogenation displays a reaction order of 1 with respect to PS assuming the constant H2 solubility in the liquid phase, therefore the reaction rate can be expressed as -rA  

dcA  kcA dt

(3)

Where t is the reaction time, h; k is the apparent reaction constant; cA is the concentration of aromatic rings in the solution, mol/L. The kinetic parameters were regressed by non-liner regression analysis against the experimental data. The best fit curves were represented by the dotted curves and the estimated parameters were summarized in Table 2. 30


Page 31 of 58 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


Table 2. Kinetics Parameters of PS Hydrogenation over Pd/CNTs@NF Catalyst. Temperature, ºC

k, h-1

R2

160

0.0590

0.9981

165

0.0739

0.9985

170

0.0923

0.9984

175

0.1028

0.9985

180

0.1166

0.9983

Figure 14b shows the Arrhenius plot and the activation energy (Ea) of PS hydrogenation was calculated to be 55.8 kJ/mol, indicating a similar apparent activity for the hydrogenation of aromatic rings on PS chains as compared with the reported values for other Pd supported catalysts 7. The performance of RFSR towards PS hydrogenation was also compared with other researchers’ works or our previous works, as summarized in Table 3. Pd/CNTs@NF displays a dramatically improved specific rate even under a much milder agitation. This could be stemmed from the intensified mass transfer process in RFSR. Table 3. Comparison with other researchers’ work. No

Catalyst Form

T, ºC

PH2, MPa

Agitation

Specific rate,

speed, rpm

mmolaromatic rings/gPd/h

1 16

Slurry

150

5.0

1200

75

13

Slurry

150

7.0

1200

77

3 12

Structured

180

5.8

1000

154

47

Slurry

150

5.8

1000

188

5

Slurry

180

5.8

500

45

6

RFSR

180

5.8

500

202

2

Data of No.1 was originated from Ref 16. Data of No.2 was originated from Ref 13. Data of No.3 was originated from Ref 12. Data of No.4 was originated from Ref 7. 3.2.6 Stability of RFSR

31



80 68.8 %


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

Page 32 of 58

66.3 %

66.3 %

60

40

20

0

0

1

2

3

4

5

6

7

8

Reused times Figure 15. Stability of Pd/CNTs@NF-20 catalyst (Reaction condition: 3 wt% PS DHN reactant solution, 180 ºC reaction temperature, 500 rpm agitation speed, 10 h reaction duration). The stability of catalytic stirrer Pd/CNTs@NF was tested by the reuse experiments and leaching test. After each run, the catalytic stirrer was thoroughly washed by DHN. As shown in Figure 15, during 4 reused tests, there is a marginal fading of HD from 68.8 to 66.3 % and then HD remains unchanged for further tests, suggesting the outstanding stability of the RFSR. Furthermore, the leaching test was performed as follows: a mixed solution containing fresh PS-DHN solution and hydrogenated product solution was poured into the slurry reactor without the addition of any catalyst and then maintained under the reaction condition. No HD could be detected after 10 hours operation, indicating negligible Pd NPs in the solution and the strong interaction between the Pd NPs and the CNTs surface. After 9 runs of hydrogenation reactions, the

32


Page 33 of 58 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


Pd NPs were still confirmed to be uniformly dispersed on the CNTs surface (Figure 7d). The statistical particle size distribution of Pd NPs indicated that the average size was 6.0  1.0 nm, close to that of fresh Pd/CNTs@NF (5.8  0.8 nm). In conclusion, the excellent catalytic stability is the direct consequence of well-attached CNTs forests on nickel foam and the strong anchoring of Pd NPs on the CNTs surface.

4. Conclusion In this paper, structured catalyst support for Pd NPs was fabricated by in-situ immobilization of CNTs layer on the nickel foam surface via CVD method. The resultant structured catalyst (Pd/CNTs@NF) was directly employed as the catalytic stirrer in a RFSR for PS hydrogenation. Factors including CVD duration, agitation speed, Pd amount and PS concentration that may influence the activity of the RFSR have been systematically investigated. Experimental results show that HD as high as 68.8 % could be obtained over the RFSR equipped with Pd/CNTs@NF-22 as its catalytic stirrer at an agitation speed of 500 rpm. Moreover, the catalytic activity of RFSR could be improved by optimizing the agitation speed and the structure of its catalytic stirrer (CNTs amount and Pd loading). Compared with the slurry reactor using powdered Pd/CNTs catalyst, a HD of only 23.7 % could be achieved at identical reaction condition. This confirms the enhanced liquid-solid mass transfer in RFSR and hence allows the reaction to be carried out under milder agitation with less energy consumption. Furthermore, it has also demonstrated the outstanding attachment strength of CNTs on the foam and catalytic stability in 8 runs of recycling. Our work 33



suggests the promising feasibility of structured Pd/CNTs@NF catalyst as the catalytic stirrer in RFSR with the merits of eliminating the filtration operation and enhancing liquid-solid mass transfer. We believe it may also have a wide range of catalytic applications in other heterogeneously catalyzed polymers refinement reactions.

Author Information Corresponding Author E-mail: [email protected]

Acknowledgements We gratefully acknowledge the financial support provided by the National Science Foundation of China (21576091), 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).

34


Page 34 of 58

Page 35 of 58 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


References (1) Hucul, D. A.; Hahn, S. F. Catalytic Hydrogenation of Polystyrene. Advanced Materials. 2000, 12, 1855-1858. (2) Maheshwari, S.; Michael Tsapatsis, A.; Bates, F. S. Synthesis and Thermodynamic Properties

of

Poly(cyclohexylethylene-b-dimethylsiloxane-b-cyclohexylethylene).

Macromolecules. 2007, 40, 6638-6646. (3) Wang, H.; Yang, L.; Rempel, G. L. Homogeneous Hydrogenation Art of Nitrile Butadiene Rubber: A Review. Polymer Reviews. 2013, 53, 192-239. (4) Wang, H.; Rempel, G. L. Aqueous-phase catalytic hydrogenation of unsaturated polymers. Catalysis Today. 2015, 247, 117-123. (5) Bates, F. S.; Fredrickson, G. H.; Hucul, D.; Hahn, S. F. PCHE-based pentablock copolymers: Evolution of a new plastic. AIChE J. 2001, 47, 762-765. (6) Zhao, J.; Hahn, S. F.; And, D. A. H.; Meunier, D. M. Thermal and Viscoelastic Behavior of Hydrogenated Polystyrene. Macromolecules. 2001, 34, 1737-1741. (7) Han, K. Y.; Zuo, H. R.; Zhu, Z. W.; Cao, G. P.; Lu, C.; Wang, Y. H. High Performance of Palladium Nanoparticles Supported on Carbon Nanotubes for the Hydrogenation of Commercial Polystyrene. Ind. Eng. Chem. Res. 2013, 52, 1775017759. (8) Jason S. Ness; Jason C. Brodil; Frank S. Bates; Stephen F. Hahn; Dennis A. Hucul; Marc A. Hillmyer. Molecular Weight Effects in the Hydrogenation of Model Polystyrenes Using Platinum Supported on Wide-Pore Silica. Macromolecules. 2002, 35



35, 602-609. (9) Xu, D.; Carbonell, R. G.; Roberts, G. W.; Kiserow, D. J. Phase equilibrium for the hydrogenation of polystyrene in CO2-swollen solvents. Journal of Supercritical Fluids. 2005, 34, 1-9. (10)Dong, L. B.; Carbonell, R. G.; Roberts, G. W.; Kiserow, D. J. Determination of polystyrene-carbon dioxide-decahydronaphthalene solution properties by high pressure dynamic light scattering. Polymer. 2009, 50, 5728-5732. (11)Dong, L. B.; Turgman-Cohen, S.; Roberts, G. W.; Kiserow, D. J. Effect of polymer size on heterogeneous catalytic polystyrene hydrogenation. Ind. Eng. Chem. Res. 2010, 49, 11280-11286. (12)Han, K. Y.; Cao, G. P.; Zuo, H. R.; Guo, W. Z.; Zhu, Z. W.; Lu, C.; Wang, Y. H. Hydrogenation of commercial polystyrene on Pd/TiO2 monolithic ceramic foam catalysts: catalytic performance and enhanced internal mass transfer. Reaction Kinetics Mechanisms & Catalysis. 2015, 114, 501-517. (13)Pan, D.; Shi, G.; Zhang, T.; Yuan, P.; Fan, Y.; Bao, X. New understanding and controllable synthesis of silica hollow microspheres with size-tunable penetrating macroporous shells as a superior support for polystyrene hydrogenation catalysts. Journal of Materials Chemistry A. 2013, 1, 9597-9602. (14)Xu, D.; Carbonell, R. G.; Douglas J. Kiserow; Roberts, G. W. Hydrogenation of Polystyrene in CO2-Expanded Solvents: Catalyst Poisoning. Ind. Eng. Chem. Res. 2005, 44, 6164-6170. 36


Page 36 of 58

Page 37 of 58 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


(15)Dong, L. B.; Mcvicker, G. B.; Kiserow, D. J.; Roberts, G. W. Hydrogenation of polystyrene in CO2-expanded liquids: The effect of catalyst composition on deactivation. Applied Catalysis A: General. 2010, 384, 45-50. (16)Chen, J.; Hu, Y.; Cai, A.; Cheng, T.; Wu, Z.; Liu, H.; Bao, X.; Yuan, P. The mesopore-elimination treatment and silanol-groups recovery for macroporous silica microspheres and its application as an efficient support for polystyrene hydrogenation. Catal Commun. 2018, 111, 75-79. (17)Tschentscher, R.; Nijhuis, T. A.; Schaaf, J. V. D.; Kuster, B. F. M.; Schouten, J. C. Gas-liquid mass transfer in rotating solid foam reactors. Chemical Engineering Science. 2010, 65, 472-479. (18)García-Bordejé, E.; Kvande, I.; Chen, D.; Rønning, M. Synthesis of composite materials of carbon nanofibres and ceramic monoliths with uniform and tuneable nanofibre layer thickness. Carbon. 2007, 45, 1828-1838. (19)Boldrini, D. E.; Jhon, F. S. M.; Tonetto, G. M.; Damiani, D. E. Monolithic Stirrer Reactor: Performance in the Partial Hydrogenation of Sunflower Oil. Ind. Eng. Chem. Res. 2012, 51, 12222–12232. (20)Wu, D.; Guo, Y.; Geng, S.; Xia, Y. Synthesis of Propylene Carbonate from Urea and 1,2-Propylene Glycol in a Monolithic Stirrer Reactor. Ind. Eng. Chem. Res. 2013, 52, 1216-1223. (21)Leon, M. A.; Tschentscher, R.; Nijhuis, T. A.; van der Schaaf, J.; Schouten, J. C. Rotating foam stirrer reactor: effect of catalyst coating characteristics on reactor 37



performance. Ind. Eng. Chem. Res. 2011, 50, 3184-3193. (22)Stemmet, C. P.; Meeuwse, M.; Schaaf, J. V. D.; Kuster, B. F. M.; Schouten, J. C. Gas-Liquid mass transfer and axial dispersion in solid foam packing. Chemical Engineering Science. 2007, 62, 5444-5450. (23)Tschentscher, R.; Spijkers, R. J. P.; Nijhuis, T. A.; Schaaf, J. V. D.; Schouten, J. C. Liquid-Solid Mass Transfer in Agitated Slurry Reactors and Rotating Solid Foam Reactors. Ind. Eng. Chem. Res. 2010, 49, 10758-10766. (24)Fan, X.; Ou, X.; Xing, F.; Turley, G. A.; Denissenko, P.; Williams, M. A.; Batail, N.; Pham, C.; Lapkin, A. A. Microtomography-based numerical simulations of heat transfer and fluid flow through β -SiC open-cell foams for catalysis. Catalysis Today. 2016, 278, 350-360. (25)Boldrini, D. E.; Damiani, D. E.; Tonetto, G. M. Mathematical modeling of the partial hydrogenation of vegetable oil in a monolithic stirrer reactor. AIChE J. 2015, 60, 3524-3533. (26)Yu, M. M.; Sheintuch, M. Catalytic fibers and cloths. Applied Catalysis: A General. 2002, 231, 1-16. (27)Mikkola, J. P.; Aumo, J.; Murzin, D. Y.; Salmi, T. Structured but not overstructured: Woven active carbon fibre matt catalyst. Catalysis Today. 2005, 105, 325330. (28)Mu, C.; Huang, K.; Cheng, T.; Wang, H.; Yu, H.; Peng, F. Ni foams decorated with carbon nanotubes as catalytic stirrers for aerobic oxidation of cumene. Chemical 38


Page 38 of 58

Page 39 of 58 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


Engineering Journal. 2016, 306, 806-815. (29)Chinthaginjala, J. K.; K. Seshan, A.; Lefferts, L. Preparation and Application of Carbon-Nanofiber Based Microstructured Materials as Catalyst Supports. Ind. Eng. Chem. Res. 2007, 46, 3968-3978. (30)Jeong, N.; Lee, J. Growth of filamentous carbon by decomposition of ethanol on nickel foam: Influence of synthesis conditions and catalytic nanoparticles on growth yield and mechanism. Journal of Catalysis. 2008, 260, 217-226. (31)Ping, D.; Wang, C.; Dong, X.; Dong, Y. Co-production of hydrogen and carbon nanotubes on nickel foam via methane catalytic decomposition. Applied Surface Science. 2016, 369, 299-307. (32)Zhu, Q. C.; Du, F. H.; Xu, S. M.; Wang, Z. K.; Wang, K. X.; Chen, J. S. Hydroquinone resin induced carbon nanotubes on Ni foam as binder-free cathode for Li-O2 batteries. ACS Applied Materials & Interfaces. 2015, 8, 3868. (33)Li, J.; Zhao, Y.; Zou, M.; Wu, C.; Huang, Z.; Guan, L. An effective integrated design for enhanced cathodes of Ni foam-supported Pt/carbon nanotubes for Li-O2 batteries. ACS Applied Materials & Interfaces. 2014, 6, 12479-12485. (34)Lin, X.; Lu, X.; Huang, T.; Liu, Z.; Yu, A. Binder-free nitrogen-doped carbon nanotubes electrodes forlithium-oxygen batteries. Journal of Power Sources. 2013, 242, 855-859. (35)Chinthaginjala, J. K.; Thakur, D. B.; Seshan, K.; Lefferts, L. How Carbon-NanoFibers attach to Ni foam. Carbon. 2008, 46, 1638-1647. 39



(36)Jarrah, N. A.; Ommen, J. G. V.; Lefferts, L. Mechanistic aspects of the formation of carbon-nanofibers on the surface of Ni foam: A new microstructured catalyst support. Journal of Catalysis. 2006, 239, 460-469. (37)Benito, S. P.; Lefferts, L. The production of a homogeneous and well-attached layer of carbon nanofibers on metal foils. Carbon. 2010, 48, 2862-2872. (38)Chinthaginjala, J. K.; Lefferts, L. Influence of hydrogen on the formation of a thin layer of carbon nanofibers on Ni foam. Carbon. 2009, 47, 3175-3183. (39)Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S.; Cançado, L. G.; Jorio, A.; Saito, R. Studying disorder in graphite-based systems by Raman spectroscopy. Physical Chemistry Chemical Physics Pccp. 2007, 9, 1276. (40)Gilkes, K.; Prawer, S.; Nugent, K.; Robertson, J.; Sands, H.; Lifshitz, Y.; Shi, X. Direct quantitative detection of the sp 3 bonding in diamond-like carbon films using ultraviolet and visible Raman spectroscopy. Journal of Applied Physics. 2000, 87, 7283-7289. (41)Lin, D.; Xingt, B. Adsorption of phenolic compounds by carbon nanotubes: role of aromaticity and substitution of hydroxyl groups. Environmental Science & Technology. 2008, 42, 7254. (42)Sheng, G. D.; Shao, D. D.; Ren, X. M.; Wang, X. Q.; Li, J. X.; Chen, Y. X.; Wang, X. K. Kinetics and thermodynamics of adsorption of ionizable aromatic compounds from aqueous solutions by as-prepared and oxidized multiwalled carbon nanotubes. Journal of Hazardous Materials. 2010, 178, 505-516. 40


Page 40 of 58

Page 41 of 58 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


(43)Bulushev, D. A.; Zacharska, M.; Lisitsyn, A. S.; Podyacheva, O. Y.; Hage, F. S.; Ramasse, Q. M.; Bangert, U.; Bulusheva, L. G. Single Atoms of Pt-Group Metals Stabilized by N-Doped Carbon Nanofibers for Efficient Hydrogen Production from Formic Acid. Acs Catalysis. 2016, 6. (44)Ji, J.; Duan, X.; Qian, G.; Zhou, X.; Chen, D.; Yuan, W. In situ production of Ni catalysts at the tips of carbon nanofibers and application in catalytic ammonia decomposition. Ind. Eng. Chem. Res. 2013, 52, 1854-1858. (45)Lehman, J. H.; Terrones, M.; Mansfield, E.; Hurst, K. E.; Meunier, V. Evaluating the characteristics of multiwall carbon nanotubes. Carbon. 2011, 49, 2581-2602. (46)Gupta, V. K.; Kumar, R.; Nayak, A.; Saleh, T. A.; Barakat, M. A. Adsorptive removal of dyes from aqueous solution onto carbon nanotubes: a review. Advances in Colloid & Interface Science. 2013, 193-194, 24-34. (47)Rosedale, J. H.; Bates, F. S. Heterogeneous catalytic hydrogenation of poly(vinylethylene). J. Am. Chem. Soc. 1988, 110, 3542-3545. (48)Min, Q.; Li, K.; Jiang, C.; Xu, W.; Yang, Z.; Zhang, J. Selective hydrogenation of cinnamaldehyde over a rotating stirrer reactor made of SiC foam supported Al2O3 and Pt catalysts. Catal Commun. 2016, 83, 62-65. (49)Truong-Phuoc, L.; Truong-Huu, T.; Nguyen-Dinh, L.; Baaziz, W.; Romero, T.; Edouard, D.; Begin, D.; Janowska, I.; Pham-Huu, C. Silicon carbide foam decorated with carbon nanofibers as catalytic stirrer in liquid-phase hydrogenation reactions. Applied Catalysis A: General. 2014, 469, 81-88. 41


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 42


Page 42 of 58

Page 43 of 58 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


Table of contents

43



Figure 1. Schematic illustration of Pd/CNTs@NF catalytic stirrer preparation

44


Page 44 of 58

Page 45 of 58 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


Figure 2. Schematic illustration of the RFSR for PS hydrogenation reaction

45



Figure 3. Schematic illustration of the slurry phase reactor for PS hydrogenation

reaction

46


Page 46 of 58

Page 47 of 58 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


Figure 4. SEM imagines of nickel foam based samples: (a) skeleton of original nickel foam; (b) surface of original nickel foam; (c) skeleton of CNTs@NF; (d) CNTs forests formed on nickel foam surface, inserted image shows the; (e) statistical distribution of CNTs outer diameter.

47



Weight percentage, wt%

100 a 95 CNTs oxidation ~ 20 wt% 480 - 615 C

90 85 80 0

200 400 600 Sample temperature, C

b

500

800

G

D

IG/ID=1.02

Intensity, a.u.

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

Page 48 of 58

1000

1500

2000

2500

-1

Raman shift, cm

Figure 5. (a) Thermal curve of structured CNTs@NF support in static air at a heating rate of 10 ºC/min from 30 to 800 ºC; (b) Raman spectra of CNTs grown on nickel foam.

48


Page 49 of 58

JCPDS card NO.04-0850

C(002) Pd (111) Pd (110)

Pd/CNTs@NF

Intensity, a.u.

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


CNTs@NF

Nickel foam Ni (111) Ni (200)

20

30

40 50 2 theta, 

60

70

Figure 6. X-Ray diffraction patterns of original NF, CNTs@NF and Pd/CNTs@NF, using the raw data without any other treatment (such as background subtraction and smoothing).

49



Figure 7. TEM images of Pd NPs deposited on the CNTs surface of CNTs@NF: (a) 0.5 wt% Pd loading; (c) 0.75 wt% Pd loading; (e) 1.0 wt% Pd loading; (g) 0.5 wt% Pd loading after 9 runs of reactions; statistical size distribution of Pd NPs: (b) 0.5 wt% Pd loading; (d) 0.75 wt% Pd loading; (f) 1.0 wt% Pd loading; (h) 0.5 wt% Pd loading after 9 runs of reactions. 50


Page 50 of 58

Page 51 of 58 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


Figure 8. High-resolution image of Pd NPs, showing clear inter planar spacing of 0.22 nm and 0.20 nm corresponding to Pd (111) and Pd (200), respectively.

51



20.4 20.1

Carbon content, %

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

Page 52 of 58

19.8 19.5 19.2 18.9 18.6

0

30

60

90

120

150

180

Vibration time, min Figure 9. Stability of CNTs attachment against ultrasonic treatment with a frequency of 40 KHz, inset imagine shows that working solution remains transparent and colorless without any trace of detached CNT after PS hydrogenation reaction.

52


Page 53 of 58

250

a

200

0.21

150

0.14

100

0.07

50

Carbon weight, g

2

0.28

Specific surface area, m /g C

0.35

0.00

16

18

20

22

0

Growth time, min Trial 5

Trial 4

Trial 6

Trial 7

b

150

60

120

45

90

30

60

15

30

0

P d /C

P d /C

NTs@ NF-1 6

P d /C

NTs@ NF-1 8

P d /C

NTs@ NF-2 0


75


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


0 NTs@ NF-2 2

Figure 10. (a) Carbon weight obtained after different CVD duration and corresponding specific surface area expressed in terms of m2/g C (CVD condition: 550 °C temperature, VC2H4 : VN2 = 100 mL/min : 180 mL/min carbon source, 16 ~ 22 min duration); (b) HD and specific rate expressed in terms of mmolaromatic

rings/gPd/h

over Pd/CNTs@NF-t

catalysts (t = 16 ~ 22min, trial 4 ~ 7 in Table 1, 10 h reaction duration). 53



Trial 8

Trial 9 Trial 6/17 Trial 10 RFSR RFSR

60

Trial 18

Slurry Reactor Slurry Reactor

150 120

45

90

30

60

15

30

0

300

400

500 600 Agitation speed, rpm

800


75


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

Page 54 of 58

0

Figure 11. HD and specific rate expressed in terms of mmolaromatic rings/gPd/h as the function of the agitation speed in RSFR and slurry reactor. (Trial 6, 8 ~ 10, 17 ~18 in Table 1, 10 h reaction duration).

54


100


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


Trial 6

Trial 11

Trial 12

150

80

120

60

90

40

60

20

30

0

0.50

0.75

1.00


Page 55 of 58

0

mPd/mPS, wt% Figure 12. Catalytic hydrogenation of PS over Pd/CNTs@NF-20 catalyst with different Pd loading (trial 6, 11 ~ 12 in Table 1, 10 h reaction duration).

55



Trial 6

Trial 13

Trial 14

210

60

175

48

140

36

105

24

70

12

35

0

3

5 PS concentration, wt%

7


72


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

Page 56 of 58

0

Figure 13. Catalytic hydrogenation of PS over Pd/CNTs@NF-20 catalyst with different PS loading (trial 6, 13 ~ 14 in Table 1, 10 h reaction duration).

56


Page 57 of 58


75

a

60 45

measured data at 180 C measured data at 175 C measured data at 170 C measured data at 165 C measured data at 160 C

30 fit curve for 180 C fit curve for 175 C fit curve for 170 C fit curve for 165 C fit curve for 160 C

15 0 0

-2.0

-2.2

2

4 6 Reaction duration, h

b

8

10

Ea = 55.8 kJ/mol 2

180 C

R = 0.9775

175 C

-2.4

170 C

lnk

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


-2.6 165 C

-2.8 0.264

160 C

0.268

0.272

0.276

0.280



1/RT    mol/J Figure 14. (a) Reaction duration of PS hydrogenation over Pd/CNTs@NF-20 catalyst at different temperatures in the RFSR; (b) Arrhenius plots of PS hydrogenation reaction over Pd/CNTs@NF-20 catalyst in the RFSR. (trial 6, 15 ~ 18 in Table 1, 10 h reaction duration).

57



80 68.8 %


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

Page 58 of 58

66.3 %

66.3 %

60

40

20

0

0

1

2

3

4

5

6

7

8

Reused times Figure 15. Stability of Pd/CNTs@NF-20 catalyst (Reaction condition: 3 wt% PS DHN reactant solution, 180 °C reaction temperature, 500 rpm agitation speed, 10 h reaction duration).

58


Palladium Supported on Carbon Nanotubes Decorated Nickel Foam

Recommend Documents