Palladium Nanoparticles Anchored on Amine-Functionalized Silica

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Palladium Nanoparticles Anchored on AmineFunctionalized Silica Nanotubes as a High Effective Catalyst Jin Liu, Jufang Hao, Chencheng Hu, Baojiang He, Jiangbo Xi, Junwu Xiao, Shuai Wang, and Zheng-Wu Bai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10237 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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The Journal of Physical Chemistry

Palladium

Nanoparticles

Anchored

on

Amine-Functionalized Silica Nanotubes as a High Effective Catalyst ∥

Jin Liu†, Jufang Hao‡, Chencheng Hu§, Baojiang He , Jiangbo Xi†,*, Junwu Xiao§, Shuai Wang§, Zhengwu Bai †,* †

School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan,

430073, China. ‡

Staff Development Institute of China National Tobacco Corporation (CNTC), Zhengzhou

450008, China. §

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of

Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China. ∥

Zhengzhou Tobacco Research Institute of CNTC, Zhengzhou 450001, China

* Corresponding Authors. E-mail addresses: [email protected] (J. B. Xi), [email protected], [email protected] (Z.W. Bai).

ACS Paragon Plus Environment

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Abstract The catalytic performance of supported heterogeneous catalysts is mainly dependent on their constitutive components including active species and supports. Therefore, the design and development of effective catalysts with synergistic enhanced effect between active sites and supports is of great significance. A facile in situ

reduction

approach

to

prepare

amine-functionalized

silica

nanotubes

(ASNTs)-supported Pd (ASNTs@Pd) composite catalyst is demonstrated in this article. Benefiting from the intrinsic physical and chemical properties of the ASNTs support and deposited Pd nanoparticles (NPs), the as-prepared ASNTs@Pd catalyst exhibits superior catalytic activity, stability, and reusability toward nitroarenes reduction reactions. For catalytic reduction of 4-nitrophenol, the turnover frequency (TOF) is as high as 313.5 min-1, which is much higher than that of commercial Pd/C (5.0 wt. %) and many noble-metal based catalysts reported in the last 5 years. In addition, a high TOF of 57.4 min-1 was also realized by ASNTs@Pd catalyst for Suzuki coupling reaction.

1. INTRODUCTION Supported metal-based catalysts are widely used in chemical industry due to their high activity and/or selectivity in many important catalytic processes.1 Generally, the catalytic performance of supported heterogeneous catalysts is mainly dependent on their constitutive components including active species and supports.2 In line with this, extensive efforts have been focused on strategies to enhance the activity by increasing the density of supported metals, reducing the size of metal particles, and adjusting the morphologies and surface properties of support.3-5 As a result, effective heterogeneous catalysts were usually fabricated by immobilizing metal nanoparticles (NPs) on a variety of supports such as polymers, metal oxides, graphene and silica, etc.6-10 However, the loaded metal NPs in catalysts are apt to aggregate thus leading to decrease of their catalytic capability. This is mainly ascribing to of their large surface-to-volume ratio and higher surface free energy in comparison with their bulk counterparts.11 In order to overcome the problem mentioned above and improve the 2


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catalytic activity as well as stability, metal NPs-based catalysts were prepared by embedding the NPs on the inner surface or interlayer of the support.12-14 Bao group reported an effective strategy to enhance the catalytic activity of Rh NPs by confining the particles inside carbon-nanotube. The obtained catalyst exhibited a striking enhancement for the conversion of CO and H2 to ethanol due to the confinement effect.15 Wang and coworkers encapsulated Pd NPs in the interlayer of double-shelled graphene@carbon hollow spheres. The resulting catalyst exhibited an excellent chemical catalytic property.16 However, these catalysts were fabricated in a complicated way. Besides, the aggregation of NPs alternatively can also be prevented by controlling the morphology and modifying the surface of support materials, because the interaction between metal NPs and support can be strengthened.17 Sastry et al immobilized Pt and Pd NPs on 3-aminopropyltrimethoxysilane-functionalized Na-Y zeolite via a very strong complexation between amine and Pt/Pd. It was found that Pt/Pd NPs bound on the surface of the zeolite with a high coverage of amine were excellent heterogeneous catalysts and exhibited high activity, selectivity, and reusability for organic reactions.18 Ideally, an excellent support should have a functionalized surface that is beneficial to well nucleation and dispersity of metal NPs, a high specific surface area (SSA) that is favorable to good exposure and accessibility of active sites, and a hollow structure with porous wall that is advantageous to facile mass transportation. Thus, developing simple and reliable protocols to immobilize catalytically active metal NPs is a very important task in nanomaterials field. In this article, we report a facile method to fabricate amine-functionalized silica nanotubes (ASNTs) with highly dense Pd NPs anchored on the surfaces. Benefiting from the functionalized surface, high SSA, porous wall of the ASNTs support, as well as highly dense and ultrafine Pd NPs, the as-prepared ASNTs@Pd catalyst exhibited superior catalytic activity, stability, and reusability toward 4-nitrophenol (4-NP) reduction reaction. The turnover frequency (TOF) was as high as 313.5 min-1, which is much higher than that of commercial Pd/C (5.0 wt. %) and many noble-metal based catalysts reported in the last 5 years. In addition, a high TOF of 57.4 min-1 was also realized for Suzuki coupling reaction. 3



2. EXPERIMENTAL SECTION 2.1．．Preparation of amine-functionalized silica nanotubes (ASNTs) The templating agent N-miristoyl-D-alanine sodium salt (C14-D-AlaS) was synthesized according to the method in reported literatures.19,20 The ASNTs were prepared by employing a published method with improvements.21 Typically, the as-prepared C14-D-AlaS (0.32 g) was dissolved in deionized water (24 mL) under mild stirring. Then the acidity of the resulting solution was adjusted to pH 8-9 by adding diluted hydrochloric acid solution (8 mL, 0.01M). After the solution was stirred for an additional 60 min at 0-5 oC, amphiphilic molecules C14-D-AlaS spontaneously self-assembled to produce flat ribbons through the hydrophobic interaction of the hydrocarbon chains. Subsequently, a mixture of tetraethoxylsilane (TEOS) (1.46 g, 7.0 mmol) and 3-aminopropyltriethoxysilane (APTES) (0.23 g, 1.0 mmol) was added with stirring for 15 min, and the obtained mixture was then aged at 5 oC for 24 h. In this process, APTES and TEOS penetrated into the ribbons from their surfaces and convert the lipid walls into mesoporous ones through cocondensation and reassembly. At the same time, the chiral C14-D-AlaS packed parallel to their nearest neighbors at a zero twist angle in the presence of APTES and TEOS, and thus causes the formation of tubular silica.22 The product was then collected by filtration and washed with water. Finally, the template C14-D-AlaS was removed by extraction with a mixture consisting of ethanol, H2O and ethylenediamine (85:15:2, volume ratio) for 12 h affording ASNTs, which could be converted into SNTs via calcination at 600 oC in air for 6 h.23 ASNTs with different amount of free -NH2 can be synthesized by adjust the ratio of TEOS and APTES. 2.2．． Preparation of ASNTs@Pd composites In a typical process, 20 mg of as-prepared ASNTs were dispersed in deionized H2O (15 mL) with mild sonication. Then 1.2 mg of K2PdCl4 was added to the above suspension under magnetic stirring at 0-5 oC. After the K2PdCl4 was dissolved, 5mL of freshly prepared NaBH4 aqueous solution (32 mg/mL) was added. Then the mixture was stirred for an additional 15 min till the K2PdCl4 was completely reduced. The suspension was centrifuged and washed three times with deionized H2O. The 4


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precipitate was dried in vacuum to give ASNTs@Pd catalyst. SNTs@Pd catalyst was prepared with the same manner using SNTs instead of ASNTs. Bare Pd NPs were prepared similarly in the absence of ASNTs support. 2.3．． Catalytic study of ASNTs@Pd catalyst Nitroarene reduction reaction. 200 mg of NaBH4 was added into a freshly prepared solution of nitroarene substrate (3 mL, 20 mM) in water or H2O/ethanol=1/9 (v/v). 2 mg of ASNTs@Pd catalyst was then added to the solution that was stirred. The reaction progress was monitored by measuring the reaction mixture by thin-layer chromatography (TLC) in short-time intervals. When traced the reaction, 20 µL of reaction mixture was withdrawn and diluted to 4 mL with deionized H2O. The diluted solution was filtered and the filtrate was subjected to UV/Vis detection or high-performance liquid chromatography (HPLC) measurement. The catalytic activity of Pd NPs was also tested similarly by adding Pd NPs aqueous suspension (2 mL, 65 μg/mL) into 4-NP and NaBH4 solution. Suzuki coupling reaction. 0.5 mmol of iodobenzene, 1 mmol of phenylboronic acid or substituted phenylboronic acid and 1 mmol of K2CO3 were added into 10 mL of ethanol and then the resulting solution was preheated to reflux (ca. 78 °C) under stirring. Upon addition of 2 mg of ASNTs@Pd catalyst, the reaction mixture was detected by TLC at regular intervals. After completion of the reaction, the catalyst was filtered off via filtration. The filtrate was analyzed with HPLC. 3. Results and Discussion 3.1 Structural properties of ASNTs@Pd catalyst The preparation process of the proposed ASNTs@Pd catalyst is schematically illustrated in Figure 1. ASNTs were firstly synthesized by using C14-D-AlaS as the template, APTES as the co-structure-directing agent, and TEOS as the silicon source. 19,21,24

The crude ASNTs were then extracted with a solution of ethylenediamine in a

mixture of water and ethanol to remove the templatling agent and meanwhile to create mesopores. Afterwards, ualtrafine Pd nanoparticles were deposited on the surface of the mesopours support (ASNTs) with K2PdCl4 as the Pd precursor and NaBH4 as the reducing agent to gain ASNTs@Pd catalyst. Figures 2a and 2b, 5



respectively, show representative scanning electron microscopy (SEM) images of the ASNTs and ASNTs@Pd catalyst, which exhibit a uniform tubular structure with a length of 2-3 µm. The hollow morphology and fine structure of the ASNTs@Pd were further characterized by transmission electron microscopy (TEM). According to Figure 2c and Figure S1, the wall thickness of the ASNTs is found to be in the range of 20-30 nm and the outer diameter is ca. 200 nm. As can be seen in Figure 2d that is a highly magnified TEM image, the ultrafine Pd NPs with an average size of ca. 3 nm were well dispersed on the surface of ASNTs. The interplanar spacing between lattice fringes is 0.195 nm that corresponds to (200) planes of Pd (the inset in Figure 2d).25 In addition, the specific surface area (SSA) and pore-size distribution of ASNTs@Pd catalyst were estimated by measuring N2 adsorption-desorption isotherm. Based on Brunauer-Emmett-Teller (BET) method, the SSA of the ASNTs@Pd catalyst was found to be 375.5 m2/g. Moreover, the ASNTs@Pd catalyst showed a mesoporous property the average pore-size of which approximately was 5.7 nm (Figure 3a). The mesopores should be originated from the tube wall of the ASNTs after the template was removed. The SNTs@Pd catalyst without amino group on the surface was synthesized in the same way to prepared ASNTs@Pd catalyst except that the support (ASNTs) were calcined, during which amino group was removed. It should be noted that only aggregated metal clusters rather than well-dispersed Pd NPs were observed from the TEM images (Figure S2). This observation reveals that the Pd NPs nucleation and subsequent growth behavior were altered in comparison with the fabrication of ASNTs@Pd catalyst.26 The well dispersion of Pd NPs on ASNTs support should be attributed to strengthened metal-support interaction between amino groups on ASNTs surface and Pd species.27 Taken together, these findings suggest the amino groups acted as adsorption and coordination centers for Pd species. Upon addition of NaBH4, the Pd precursor (K2PdCl4) was reduced and simultaneously in situ immobilized on the ASNTs support.9

6


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Figure 1 Schematic of the synthetic route of ASNTs@Pd composite.

Figure 2 SEM and TEM of ASNTs support and ASNTs@Pd catalyst. (a) SEM images of ASNTs; (b) ASNTs@Pd composite; (c) TEM image of ASNTs@Pd; (d) high-magnification TEM image ASNTs@Pd composite, in which the inset in Figure 2d is the high-magnification TEM image of a single Pd NP.

7



X-ray photoelectron spectroscopy (XPS) was measured to investigate the surface composition and chemical states of the catalysts. In the spectrum of ASNTs@Pd displayed in Figure 3b, the signals of O, Si, C, Pd and N were observed at binding energies of 530 eV (O 1s), 101 eV (Si 2p), 150 eV (2s), 282 eV (C 1s), 335 eV (Pd 3d) and 400 eV (N 1s) in the survey scan, respectively (Figure 3b), which match the element compositions in the ASNTs@Pd catalyst. In comparison, no N 1s signal could be detected in the SNTs@Pd catalyst, indicating that the amine was completely removed when ASNTs were calcined. The high-resolution XPS spectra of ASNTs@Pd catalyst were also measured to determine the chemical valence states of N and Pd elements and the results are depicted in Figure 3c and 3d. The N 1s XPS spectrum (Figure 3c) show that nitrogen in ASNTs@Pd catalyst is predominantly in three states: free amine (C-NH2, 399.7 eV), Pd coordinated amine (-NH2·Pd (II), 400.4 eV), and ammonium ion (C-NH3+, 401.6 eV), respectively.28 It should be noted that the positively shifted binding energy of free amine is as high as 0.7 eV, which should be attributable to the decrease of electron cloud density around N atoms via -NH2·Pd (II) coordination. This observation is in good agreement with that in a previously reported literature.29 The observed peaks of Pd 3d are located at binding energy around 335.4 eV and 340.6 eV arising from the Pd 3d5/2 and Pd 3d3/2 orbital, respectively, which are the characteristic peaks of Pd element (Figure 3d). The two weak peaks centered at 337.2 eV and 342.3 eV are assigned to Pd (II) which correspond to Pd (II)·NH2complex30 and the partially oxided Pd species in ASNTs@Pd composite.31 Furthermore, ultraviolet photoelectron spectroscopy (UPS) was also conducted to determine the work function, which can be calculated by subtracting the width of the He I UPS spectra.32 The work functions of ASNTs and ASNTs@Pd were calculated to be 4.6 eV and 3.5 eV (Figure S3), both of which were lower than that of Pd (5.12 eV).33 The lowest work function of ASNTs@Pd should be ascribed to the interactions of NH2- and Pd NPs. The prepared catalyst was further characterized by X-ray diffraction (XRD) analysis. As can be seen from Figure S4, the sample exhibited a broad diffraction peak (ca. 23°), which should attribute to the amorphous silica of ASNTs support.9 In addition, no characteristic diffraction peak of Pd was detected in 8


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the XRD patterns recorded in the 2θ range of 10~90°. It is maybe due to the low content (1.52 wt%) and ultrafine diameter (~3 nm) of Pd NPs in ASNTs@Pd catalyst.34 The Pd loading amount of ASNTs@Pd catalysts was 1.52 wt.%, which was detected by the inductively coupled plasma mass spectrometry (ICP-MS) analysis.

Figure 3 BET and XPS analysis of ASNTs@Pd catalyst. (a) Nitrogen adsorption-desorption isotherms and pore-size distributions of ASNTs@Pd composite (inset); (b) XPS survey spectra of ASNTs@Pd composite and SNTs@Pd composite; (c) N 1s spectrum of ASNTs@Pd composite; (d) Pd 3d spectrum of ASNTs@Pd composite.

3.2 .Catalytic performance of ASNTs@Pd catalyst. To evaluate the catalytic performance of the ASNTs@Pd composites, 4-NP reduction reaction and Suzuki coupling reaction were chosen as the model reactions (Figure 4a). In order to demonstrate the catalytic activity of ASNTs@Pd, the support ASNTs were introduced into the reaction solutions to determine the adsorption and catalytic capability towards the reactants in the absence of Pd NPs. With respect to 9



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4-NP reduction reaction, the reaction progress can be monitored by observing color changes of the reaction solution as well as measuring Uv-vis spectrometry.35 Herein, the catalytic activity of catalyst was estimated by turnover frequency (TOF), which was defined as the amount (mmol) of the substrate (4-NP/iodobenzene) that was converted into the relevant product (4-AP/biphenyl) by 1 mmol active metal per min.36 Upon addition of ASNTs@Pd catalyst (Pd content 1.52 wt.%) into the 4-NP/NaBH4 aqueous solution, the bright yellow mixture became brown, and then turned colorless gradually within 40 s (Figure 4b and 4c). Accordingly, the absorption band at 400 nm disappeared completely, and a new absorption band at 300 nm appeared, indicative of the reduction of 4-NP and formation of 4-aminophenol (4-AP) (Figure 4c).37,38 It should be noted that the ASNTs@Pd catalyst was uniformly dispersed in the aqueous solution during the reaction process, showing an excellent hydrophilic property (Figure 4b). The TOF value is as high as 313.5 min-1, which is much higher than that of commercial Pd/C (5.0 wt.%) and many noble-metal based catalysts reported in the last 5 years (Table 1).39-51 Moreover, we have further extended the ASNTs@Pd catalyst to the reduction of various nitroarenes to examine the common applicability of the reaction. The ASNTs@Pd-catalyzed reduction of nitroarenes into anilines by NaBH4 in water are summarized in Table 2. It is noteworthy

that

nitrobenzene

and

substituted

nitrobenzenes

with

electron-withdrawing group, such as chloro-, cyano and carboxyl functionalities were all converted to the corresponding aniline in high yield (entries 1–4).

10


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Table 2 Catalytic reduction of nitrobenzene and substituted nitrobenzenes into amines by NaBH4 catalyzed by ASNTs@Pda.

T (min)

Yield (%)b

1

5

95.2

2

2

96.8

3

1

97.4

4

1.5

99.1

Entry

a

Reactant

Product

Standard reaction conditions: 0.06 mmol nitrobenzene or substituted nitrobenzenes,

2 mg ASNTs@Pd, 3 mL H2O/ethanol=1/9 (v/v), and 6 mmol (100 equiv.) NaBH4 at room temperature. b

HPLC content.

For comparison purposes, we tested the catalytic activity of ASNTs support and Pd NPs without the ASNTs support, respectively. As shown in Figure S5, ASNTs showed negligible adsorption to 4-NP and no obvious catalytic activity for the reduction of 4-NP even though a large excess of reducing agent (NaBH4) was fed and the reaction solution was stirred as long as 1 h. While bare Pd NPs without ASNTs support and SNTs@Pd catalyst (Pd content 0.81 wt.%) exhibited moderate catalytic activity, with a TOF of 43.0 min-1 and 112.6 min-1 respectively. In order to elaborate the influence of the amount of free -NH2 on the catalytic process, we prepared ASNTs with different amounts of free -NH2 by adjusting the fed ratio of TEOS to APTES (the optimal molar ratio for TEOS / APTES is 7: 1), in which APTES acted as -NH2 source. The as-prepared catalysts with less free -NH2 (TEOS/APTES = 7/0.5) and more free -NH2 (TEOS/APTES = 7/2) were denoted as ASNTs-0.5 and ASNTs-2, respectively. Accordingly, the corresponding ASNTs@Pd catalysts were also 11



Page 12 of 25

prepared in the same way, which were denoted as ASNTs-0.5@Pd and ASNTs-2@Pd. As can be seen from the SEM images (Figures S6a and S6c), most of ASNTs-0.5@Pd composites exhibit tubular structure except a few randomly aggregated bulk ones and twisted ribbons, while we note that ASNTs-2@Pd composites exhibit similar tubular morphology except there are a minority of randomly aggregated bulk silica. The as-obtained ASNTs-0.5@Pd and ASNTs-2@Pd were also employed as catalysts for 4-NP reduction as control experiments. The results revealed that the reduction of 4-NP can be completed in 90 s and 70 s when the reaction was catalyzed by ASNTs-0.5@Pd and ASNTs-2@Pd, respectively (Figure S6b and S6d), although the Pd contents (1.49 wt.% for ASNTs-0.5@Pd and 1.53 wt.% for ASNTs-2@Pd) were comparable to that of ASNTs@Pd. The catalytic activities of ASNTs-0.5@Pd and ASNTs-2@Pd were much lower than that of ASNTs@Pd. The inferior catalytic activities should attributable to less uniform tubular morphology rather than the amount of free -NH2 of ASNTs supports in ASNTs-0.5@Pd and ASNTs-2@Pd composites. Suzuki coupling reaction is regarded as an effective route to construct C-C bond to give biaryls.52-55 Therefore, the catalytic activity of ASNTs@Pd catalyst for Suzuki coupling reaction (the substrates were iodobenzene and phenylboronic acid) was also evaluated. The results in the present study show that the conversion yield of iodobenzene (0.5 mmol) amounted to 98.4% (Figure S7) within 30 min with a TOF up to 57.4 min-1. In addition, an excellent catalytic activity of ASNTs@Pd was observed for the Suzuki coupling reaction with differently substituted phenylboronic acids as the reactants, including 3-methylphenylboric acid, 4-ethylphenylboronic acid, 4-ethoxyphenylboronic

acid,

4-tert-butylphenylboronic

acid,

and

3,4,5-trimethoxyphenylboronicacid (Table 3, entries 1-5). All these phenylboronic acid with an electron-donating group demonstrated a high reactivity towards the Suzuki coupling reaction, indicating high catalytic activity of ASNTs@Pd catalyst.

12


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Table 1 Comparison in catalytic performance for 4-NP reduction with ASNTs@Pd and other catalysts reported within recent 5 years. Mass of

Amount of

Metal

Conversion

TOF

catalysts (mg)

4-NP (mmol)

content (wt.%)

time (min)

value

ASNTs@Pd

2

6×10-2

1.52

0.67

313.5

This work

SNTs@Pd

2

6×10-2

0.81

3.5

112.6

This work

Pd/C (5.0wt. %)

2

6×10-2

5

1.1

58.04

This work

Pd@HCS

3

3×10-4

1.71

5

0.12

[39]

0.2

3×10-4

7.2

4.42

0.5

[40]

Pt-PDA/RGO

0.06

2.7×10-4

6.1

12

1.2

[41]

Fe3O4@SiO2-Pd

-

4.5×10-4

-

5

10.19

[42]

MSNCs-NH2-Ag

0.02

3×10-4

15.43

4.83

2.17

[43]

0.1

3×10-4

1.97

28

0.98

[44]

Pd-ZnO

5

3×10-4

0.05

1.5

8.64

[45]

Pd@NC

2.5×10-2

1×10-4

2

6.5

3.27

[46]

2

0.04

23.1

0.17

57.62

[47]

Pd/MPC

0.03

3×10-4

5.11

6.83

3.06

[48]

TiO2@GOS@Au

0.44

7.4×10-3

63.8

1.5

3.47

[49]

CMF@PDA/Pd

492

1x10-2

0.447

0.25

1.587

[50]

Pd-rGO-CNT

5

3×10-4

1.12

0.33

1.71

[51]

Catalysts

Pd/HAM@γAlOOH

Au/TiO2 hybrid nanofibers

Fe3O4@P(MBAA m-co-MAA)@Ag

13


Reference


Page 14 of 25

Table 3 Catalytic performance of ASNTs@Pd in the Suzuki coupling reactions a.

T (min)

Yield (%)b

1

25

96.1

2

20

97.2

3

20

95.9

4

15

96.3

20

98.7

Entry

Reactant

Product

5

a

Standard reaction conditions: 0.5 mmol iodobenzene, 1 mmol substituted

phenylboronic acid, 1 mmol K2CO3，2 mg ASNTs@Pd, 10 mL ethanol at 80oC. b

HPLC content.

The stability of the ASNTs@Pd catalyst was further studied by repeated uses of the recycled catalyst in the same condition. In each cycle, the catalyst was recovered from the reaction mixture by filtration, washed with deionized water and lyophilized. As shown in Figure 4d, almost the same conversions (100%) were observed in the first three successive cycles, and even a 90% conversion was still maintained in the eighth run. The excellent stability should be attributed to the unique immobilization of Pd NPs on amine-functionalized support,56 which was further evidenced by TEM image of the recycled catalyst. As shown in Figure S8, only a slight degree of particles aggregation was observed, and most of the Pd NPs are still uniformly dispersed on the ASNTs support. Notably, the SNTs support catalyst (SNTs@Pd) 14


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showed a lower Pd loading amount (0.81%) even if the fed ratio of support/K2PdCl4 was the same as that for the preparation of ASNTs@Pd. Furthermore, only 89% conversion of 4-AP was achieved in the fourth run for SNTs@Pd (Figure S9), exhibiting an unsatisfied recyclability due to the lack of -NH2 group. These results demonstrate

that

there

was

a

strong

metal-support

interaction

between

amine-functionalized support and Pd NPs, which was beneficial to the nucleation and anchoring of Pd NPs. In other words, the aminopropyl group on the surface of ASNTs not only benefited to the stability but also improved the distribution of Pd NPs.

Figure 4 Illustrations for the catalytic performance of ASNTs@Pd catalyst. (a) Reduction reaction of 4-NP with NaBH4 and preparation of biphenyl with Suzuki coupling reaction; (b) photographs showing the color change of 4-NP (20 mM) solution, (1) before addition of ASNTs@Pd; (2) reaction was ongoing in the presence of ASNTs@Pd; (3) after completion of the reaction. (c) UV-Vis absorption spectra of the reaction solution of 4-NP reduced under the catalysis of ASNTs@Pd in different durations; (d) conversion efficiency of 4-NP reduced within 40 s with ASNTs@Pd as the catalyst for consecutive reactions of eight-time cycle.

15



Page 16 of 25

Based on the results obtained in studying the ASNTs@Pd catalyst, we believe that the excellent catalytic performance should be resulted from the synergistic effects of the large surface area of ultrafine Pd NPs and amine-functionalized mesoporous ASNTs support. Firstly, the large surface area and well dispersion of Pd NPs provided highly dense active sites and thus reinforced the catalytic activity. Besides, the unique physical and chemical properties of the support also synergistically improved the catalytic activity: the hydrophilic nature of the ASNTs support could make the ASNTs@Pd catalyst well dispersed in aqueous media, consequently leading to an easier access of the reactants to active sites during the catalytic process; and the ASNTs@Pd catalyst possessed a unique tubular and mesoporous structure, which could be advantageous to diffusion and mass transportation of the reactants, thus improving catalytic performance.57 In a word, the well dispersion of Pd NPs, and the high SSA, mesoporous tubular structure as well as excellent hydrophilic property of the support facilitated the exposure and accessibility of active sites and for the ASNTs@Pd catalyst and as a result, the catalytic activity was enhanced.43

4. CONCLUSIONS In summary, an effective ASNTs@Pd catalyst composed of amine-functionalized tubular silica and highly dense Pd NPs were fabricated via a facile method. The tubular

nanocatalyst

prepared

with

this

method

had

a

hydrophilic

amine-functionalized surface, large SSA and mesoporous tube wall. These structural characteristics were favorable for well dispersion of Pd NPs, facile mass transportation of reactants and easy access of reactants to active sites. Owing to the superior properties, a high catalytic activity of ASNTs@Pd could be achieved and as a result, the nitroarenes reduction and Suzuki coupling reactions were implemented with very high yields. The TOF for 4-NP reduction was much higher than those gained by conventional noble-metal-based catalysts which have been reported in literatures. It is envisioned that the fabrication strategy of ASNTs@Pd provides a facile and versatile approach for designing tubular silica-based composites, which are applicable in electrocatalysis, drug/gene delivery, and adsorption for water-treatment. 16


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■ ASSOCIATED CONTENT Supporting Information The following additional information is reported in the Supporting Information: Materials, Characterization, Uv-vis spectra, TEM images, XRD curves, UPS data and HPLC data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J. B. Xi), [email protected], [email protected] (Z.W. Bai). ACKNOWLEDGMENTS This research was financially supported by the Natural Science Foundation of Hubei Province (No. 2016CFB263) and National Natural Science Foundation of China (No. 51772110, 21401060, 21771069 and 51373127). The authors would like to acknowledge the Analytical and Testing Center of Huazhong University of Science and Technology and the Wuhan National Laboratory for Optoelectronics for providing SEM, TEM, and XPS measurements. The authors also would like to show great gratitude to Prof. Rong Chen and Dr. Jizhou Jiang for their invaluable suggestions. Author Contributions J. B. Xi and Z. W. Bai conceived the idea and co-wrote the manuscript. J. Liu, C. C. Hu and J. B. Xi carried out the materials synthesis and the chemical catalysis. J. F. Hao, B. J. He, J. W. Xiao, S. Wang and J. B. Xi performed the materials characterizations. All of the authors discussed the results and commented on the manuscript. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

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Palladium Nanoparticles Anchored on Amine-Functionalized Silica

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