Nanofiber-like mesoporous alumina supported palladium nanoparticles as a highly active catalyst for base-free oxidation of benzyl alcohol

Microporous and Mesoporous Materials 266 (2018) 126–131

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Nanofiber-like mesoporous alumina supported palladium nanoparticles as a highly active catalyst for base-free oxidation of benzyl alcohol

T

Lijuan Chena,b,∗, Jingqing Yana, Zhanxin Tonga, Shiyi Yua, Jianting Tangb, Baoli Oua,b, Lijuan Yuea, Li Tiana,b a

School of Material Science and Engineering, Hunan University of Science and Technology, Xiangtan, Hunan Province, PR China Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of Education, Hunan University of Science and Technology, Xiangtan, Hunan Province, PR China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Palladium nanoparticles Nano-fibrous mesoporous alumina Benzyl alcohol oxidation Benzaldehyde

A nanofiber-like mesoporous alumina supported palladium catalyst, Pd/γ-Al2O3-fibr, was successfully prepared by a hydrothermal method and used in the aerobic oxidation of benzyl alcohol to benzaldehyde with molecular oxygen under base-free conditions. The material was characterized by N2 physisorption, FT-IR, X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and X-ray photo-electron spectroscopy (XPS). XRD and TEM results revealed that the palladium nanoparticles were uniformly dispersed in the framework of nanofiber-like mesoporous γ-alumina. The higher activity of Pd/γ-Al2O3-fibr compared to Pd/γ-Al2O3 and Pd/SBA15 in the aerobic oxidation of benzyl alcohol reaction suggested that the unique fibrous architecture of our material and synergic effect between Pd species and γ-Al2O3 are beneficial to enhance the reactivity of palladium NPs. Furthermore, the unique architecture of Pd/γ-Al2O3-fibr catalyst also offers it good mass-transfer property and accessibility, which make it an effective catalyst for aerobic oxidation of steric bulk alcohol substrate. The Pd NPs supported on nano-fibrous mesoporous alumina in this paper is demonstrated to be a recoverable noble metal-based nanocatalyst with easy accessibility and may be great potential as a promising candidate for numerous catalytic applications.

1. Introduction Benzaldehyde is of important value due to its application as intermediate and raw material in perfumery, pharmaceuticals, dyestuff and agrochemical industry. The tradition production of benzaldehyde via the toluene chlorination and subsequent hydrolysis process requires complicated preparation procedures and generates large amount of toxic acidic wastes, leading to serious environmental contaminations and costly separation process [1–3]. As an alternative green route to produce chlorine-free benzaldehyde, the aerobic oxidation of benzyl alcohol with molecular oxygen or air has attracted significant attention in recent years, being water the only by-product of the reaction. In this regards, the active catalysts concerning transition metal and noble metal such as Cu [4,5], Cr [6], Mn [7,8], Ru [9], Pt [10], Au [11,12], Pd [13–15], bimetallic Au-Pd [16,17] have been reported, among them, the Pd-based catalysts displays interesting and promising catalytic performance in both activity and selectivity, especially under the mild conditions. Due to the environmental friendly and safety property, aerobic

∗

oxidation of benzyl alcohol in water is more appreciable because it helps to reduce the amount of waste after reaction and avoid explosion and hazards associated with the application of other toxic and oxidisable organic solvent. Palladium nanoparticles supported on various supporting materials are attractive catalyst for aerobic oxidation of alcohols [18–21]. However, owing to the high surface energy of catalytically active Pd NPs, Pd NPs supported on numerous conventional supports usually suffered from the problems such as gradual growth of Pd NPs into inactive large particles and poor accessibility of active sites that entrapped in the pores and channels of supports. Thus, for the purpose of improving the accessibility of active sites to reactant and reduce the propensity of Pd NPs to coalesce, many heterogenized Pd NPs catalysts with well-designed nanocomposite structures were developed for catalytic applications, including hollow core-shell structured Pd NPs-based nanocatalysts [22–25], Pd NPs@ metal–organic frameworks [26–28], and Pd NPs/CNT nanohybrids [29,30]. These nanocatalysts show promising properties to suppress aggregation, leading to enhanced catalytic activity. However, the fabrication of these nanocomposites also suffers from problems such as difficult synthesis

Corresponding author. School of Material Science and Engineering, Hunan University of Science and Technology, Xiangtan, Hunan Province, PR China. E-mail address: [email protected] (L. Chen).

https://doi.org/10.1016/j.micromeso.2018.02.037 Received 14 October 2017; Received in revised form 20 January 2018; Accepted 26 February 2018 Available online 02 March 2018 1387-1811/ © 2018 Published by Elsevier Inc.

Microporous and Mesoporous Materials 266 (2018) 126–131

L. Chen et al.

procedures, the poor mechanical and chemical stability, poor control over the shell thickness and difficulty of obtaining a core particle of sufficiently small size, which greatly hinders their further application in catalysis field. Therefore, it is highly desirable to develop a simple and effective strategy to synthesize Pd NPs-based nanocomposite with high catalytic stability, uniform dispersion of small Pd NPs and improved accessibility. Porous alumina is one of promising support materials due to its essential properties of high surface area, tunable porosity and good chemical, mechanical, and especially thermal stability. It has been reported that porous alumina with different three-dimensional stacked configurations can not only function as effective barrier to prevent NPs from aggregation but also improve the chemical and thermal stability of supported NPs. For example, Au NPs immobilized on a novel alumina with architecture of thin sheets exhibit excellent sintering-resistant property, despite calcination at 700 °C, the size of supported Au NPs at predominate 2 ± 0.8 nm is unchanged [31]. Pd NPs supported on a spherical porous alumina, Al2O3(PB) exhibits high Pd distribution and thin Pd dispersion depth [32]. However, in these cases, the immobilization of metal NPs on porous alumina is achieved through an impregnation-reduction method, the supported precursors in oxidation state need to be reduced to metal NPs by a time-consuming or complex reduction procedure. Herein, we reported a controlled hydrothermal synthesis of in situ immobilization of Pd NPs in mesoporous alumina with stacked nanofiber like configuration. The hydrothermal treatment of mixture of colloidal Pd NPs, microporous graphitic microspheres, Al(NO3)3.9H2O and urea can generate boehmite (γ-AlO(OH)) nanofibers attached to graphitic microspheres, Pd NPs were thus homogeneously dispersed in the framework of boehmite nanofibers. This prepared mesoporous γAl2O3 of 3D nanofiber-stacked morphology can effectively stabilize the loaded Pd NPs, retaining the high dispersion of small Pd NPs despite calcination at 550 °C. The 3D hierarchical architecture containing large mesopores can also promote the mass diffusion of reactant/product. The resultant Pd/γ-Al2O3 nanocomposites can offer a highly recyclable, effective, environment-friendly and robust catalytic system for aerobic oxidation of benzyl alcohol to benzaldehyde in water.

0.12 mmol PdCl2 and 0.02 mL concentrated hydrochloric acid were added to 30 mL of water and 70 mL of ethanol under vigorous stirring to form a clear solution. 0.5 g of PVP was then added to the solution, and the solution was heated to boiling and refluxed for 3 h to generate a homogeneous dispersion of PVP-Pd NPs. Subsequently, 0.3 g of graphitic microspheres was ultrasonically dispersed in the aqueous solution containing 0.935 g of Al(NO3)3.9H2O and 1.2 g of urea (80 mL), and then another 40 mL of ethanol was added. Next, the resulting suspension was added with 10 mL of the above PVP-Pd NPs dispersion, sonicated for 1 h at room temperature and transferred into a 150 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 120 °C for 20 h, the black products were then collected, washed five times with water and ethanol. After being dried at 80 °C for 6 h, the dried products were calcined at 550 °C under static air for 3 h to produce Pd/γ-Al2O3-fibr catalyst. For comparison, Pd/γ-Al2O3 with similar Pd content was prepared by incipient wetness impregnation using commercial γ-Al2O3 support.

2. Experimental section

2.5. Catalytic test

2.1. Materials

Typical aerobic oxidation was carried out in a two-necked round flask provided with a reflux condenser and an electrically controlled magnetic stirrer, was loaded with 30 mg of solid catalyst and 1 mL (9.6 mmol) of benzyl alcohol in 40 mL of water. The flask was heated to 353 K in an oil bath with stirring rate of 800 rpm. Oxygen gas was bubbled into the liquid reaction mixture at a speed of 20 mL/min starting the reaction. After 2 h of reaction, the catalyst was separated by centrifugation, and the reaction solution was extracted with diethyl ether twice, 5 mL for each time. The combined organic layer was dried with MgSO4, analyzed using a Shimadzu 2014 gas chromatograph (GC) equipped with a HP-5 capillary column (30 m × 0.32 mm × 0.25 μm) to determine the yield of target product.

2.4. Characterization X-ray diffraction patterns of the synthesized samples were measured by Brucker AXS D8 Advance diffractometer using Cu Kα radiation with a scanning speed of 10° min−1. Transmission electron microscopy (TEM) images of the samples were collected on a JEM 3010 instrument operated at 200 kV. More than 100 particles for each sample were randomly counted to determine the particle size distributions. N2 adsorption and desorption isotherms were obtained on a micromeritics ASAP 2010 instrument at 77 K, samples were degassed at 473 K for 4 h under high vacuum prior to measurements. Specific surface areas and pore distribution were calculated by Rrunauer-Emmett-Teller (BET) and Barret-Joyner-Hallender (BJH) methods, respectively. Elemental analysis of samples was performed with an inductively coupled plasmaoptical emission spectrophotometer Shimadzu ICPs-7500. IR spectra were recorded on KBr pellets by a Nicolet Niclet 6700 spectrophotometer. The binding energy of Pd was determined by X-ray photoelectron spectroscopy (XPS) using monochromatic Al Kα radiation (ESCALAB 250).

Anhydrous D-glucose was purchased from the national pharmaceutical industry of china, polyvinylpyrrolidone K15 (PVP, Mw∼10,000) was purchased from Aldrich, Al(NO3)3.9H2O, concentrated hydrochloric acid, PdCl2, ethanol (> 99.5%) and benzyl alcohol were analytical grade and used as received without further purification. commercial γ-Al2O3 powder was obtained from Nanjing Chemical Reagent Company (SBET = 185 m2/g). 2.2. Preparation of graphitic microspheres (GM) The carbon microspheres (GM) were prepared through the hydrothermal treatment of aqueous glucose solution. In a typical synthesis, 4 g of anhydrous D-glucose was dissolved in 30 mL deionized water to form transparent solution, then the solution was sealed into a 40 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 12 h. The dark brown products were collected by centrifugation, washed repeatedly with deionized water and ethanol, and air-dried at 80 °C overnight. The graphitic microspheres were obtained by pyrolyzing the carbon microspheres at 800 °C for 2 h under N2 flow.

3. Results and discussion 3.1. Catalyst preparation and characterization After hydrothermal treatment of the suspension containing aluminum precursor, PVP-Pd NPs and graphitic microspheres, centrifugation of the reaction mixture can produce a colorless supernatant and a black solid reflecting that the Pd NPs were completely immobilized in the resulting boehmite gel. The TEM images of graphitic microspheres, boehmite gel with Pd NPs and Pd/γ-Al2O3-fibr were shown in Fig. 1. As shown in Fig. 1(b) and (c), in the presence of graphite microspheres, boehmite particles grown on the surface of graphite microspheres to produce boehmite nanofiber, the boehmite nanofibers enwrapped

2.3. Preparation of Pd/Al2O3-fibr catalyst Pd/γ-Al2O3-fibr catalyst with Pd loading of 1.0 wt% was prepared by a controlled hydrothermal synthesis method. In a typical synthesis, 127

Microporous and Mesoporous Materials 266 (2018) 126–131

L. Chen et al.

Fig. 1. TEM images of graphitic microspheres (GM) (a), Pd NPs dispersed in fibrous boehimite grown on graphitic microspheres (Pd-boehmite@GM) (b), Pd NPs distribution of Pd-boehmite@GM (c), Pd NPs supported nanofiber-like mesoporous alumina (Pd/γAl2O3-fibr) (d), Pd NPs distribution of Pd/γ-Al2O3fibr (e), Pd/γ-Al2O3-fibr at higher magnification and HRTEM image of Pd NPs (inset) (f).

550 °C, the resultant Pd/γ-Al2O3-fibr catalyst exhibits γ-alumina phase (JCPDS file No. 10–425), indicating the conversion from boehmite phase to γ-alumina (Fig. 2c). FT-IR spectra of graphitic microspheres, as-prepared Pdboehmite@GM before calcination and Pd/γ-Al2O3-fibr are presented in Fig. 3. In all the samples, the band at 3450 cm−1 is attributed to OH stretching vibration, and the band at 1640 cm−1 corresponds to physically absorbed water. The absorption peaks at 3000-2700 cm−1 and 1723 cm−1 caused by CeH and C=O vibrations, originated to PVP molecule on the surface of Pd-boehmite@GM (Fig. 3 (b)). After calcination, in IR spectrum of Pd/γ-Al2O3-fibr, the absorption peak at 1723 cm−1 corresponds to C=O vibration disappears, The adsorption at around 1550 cm−1 corresponds to the longitudinal phonon vibration of γ-Al2O3, as reported in literature [35], the peak at 778 cm−1 corresponds to AleO stretching vibrations, and the wide valley between 1000 and 450 cm−1 can be assigned to the feature of γ-Al2O3 [36] (Fig. 3 (c)). The disappearance of absorption peaks correspond to CeH and C=O vibrations in Fig. 3c suggests that the organic molecules on the surface of Pd-boehmite@GM have been completely removed by calcinations. Fig. 4 displays N2 adsorption-desorption isotherms and pore size distribution of graphitic microspheres and Pd/γ-Al2O3-fibr The N2 adsorption and desorption of graphitic microspheres exhibits classic type I

graphitic microspheres forming a three-dimensional gel network, the Pd NPs with average size of 3.3 ± 0.4 nm are highly dispersed in boehmite gel. After calcination at 550 °C, boehmite is converted to γ-Al2O3, and the graphitic microspheres were removed from the network of gel leaving some voids in the randomly stacked γ-Al2O3 nanofibers. As shown in Fig. 1d and e, Pd NPs with average size of 4.8 ± 0.5 nm are well dispersed in γ-Al2O3 support, revealing a somewhat increase in Pd particle size after calcination at 550 °C. HRTEM image shows that Pd/γAl2O3-fibr exhibit the lattice spacing of 0.227 nm corresponding to Pd (111) crystal plane. Fig. 2 shows the XRD patterns of different samples. The two broad peaks of graphite microspheres, centered at 23° and 44° of 2θ value, could be indexed to (002) and (100) respectively, indicating the existence of lower ordered graphitic carbon [33,34]. In the as-prepared Pd-boehmite@GM(after hydrothermal process), boehmite phase (γAlOOH) (JCPDS file No. 21–1307) appeared, simultaneously, the diffraction peaks of Bragg angles of 40.1°, 46.5° and 68.1° correspond to (111), (200) and (220) facet of Pd crystal. The XRD pattern of Pdboehmite@GM revealed the presence of all the expected characteristic peaks of Pd NPs along with the peaks corresponding to amorphous graphitic carbon and boehmite, indicating the successful immobilization of Pd NPs on boehmite. After calcination of Pd-boehmite@GM at

γ: γ-Al2O3 phase

b: boehmite phase

( a)

γ γ

Pd(111) b Pd(200) b

b

Transmittance(%)

intensity(a.u.)

(b) γγ

(c)

b Pd(220)

(b)

1723

1640

(c)

778

(a) 10

20

30

40

2θ(degree)

50

60

70

544 3000

80

2000

Wavenumber cm

Fig. 2. XRD patterns of (a) graphitic microspheres (GM); (b) as-prepared Pdboehmite@GM; (c) Pd/γ-Al2O3-fibr.

1000

-1

Fig. 3. FTIR spectra of (a) graphitic microspheres; (b) Pd-boehmite@GM (before calcination) and (c) Pd/γ-Al2O3-fibr.

128

Microporous and Mesoporous Materials 266 (2018) 126–131

L. Chen et al. 160

0.020

400

120

9

0.016

350

60 40

5 4 3 2 1 0 0.6

20

0.7

0.8

0.9

1.0

1.1

pore width(nm)

0.2

0.4

0.6

0.8

250 200

0

0.0

40

50

0.2

0.4

0.6

0.8

1.0

Relative pressure(P/P0) 0.0035

0.0025

400

0.0020

0.0030

0.0015 0.0010 0.0005 0.0000 -0.0005 10

20

30

40

50

60

70

pore width(nm)

0 0.2

30

(b)

0.0030

0

0.0

20 pore width (nm)

500

0.0035

100

10

100

0.4

0.6

0.8

3

3

200

0.004

50

1.0

3

pore volume(cm /g.nm)

3

300

0.008

0.000

150

(b) Pd/γ-Al2O3-fibr

400

0.012

pore volume(cm /g.nm

0.0

N2 adsorption(cm /g)

N2 adsorption(cm /g)

500

300

3

6

pore volume(cm /g.nm)

3

80

7

3

pore volume(cm /g.nm)

100

N2 adsorption(cm /g)

8

3

N2 adsorption(cm /g)

(a)

450

(a) graphitic microspheres

140

300

200

0.0025 0.0020 0.0015 0.0010 0.0005 0.0000 -0.0005

0

10

100

1.0

20

30 40 pore width (nm)

50

60

70

0

relative pressure(P/P0)

0.0

0.2

0.4

Fig. 4. N2 adsorption-desorption isotherms and pore size distributions (inset) of (a) graphitic microspheres and (b) Pd/γ-Al2O3-fibr.

0.6

0.8

1.0

Relative pressure(P/P0)

Fig. 5. N2 adsorption-desorption isotherms and pore size distributions (inset) of (a) Pd/γAl2O3 and (b) Pd/γ-Al2O3-fibr.

isotherms characterized by a sharp uptake under low relative pressures in the range P/P0 = 10−3-10−2 followed by a plateau at higher pressures, indicating its structural feature of microporous materials. The pore size distribution calculated from Horvath–Kawazoe (HK) method indicates that graphitic microspheres has narrow pore size distribution at around 0.5 nm. The BET surface area of graphitic microspheres is 420 m2/g, and the t-plot method demonstrated that the micro-pore surface area is 398 m2/g. The total pore volume, micropore pore volume and mesopore pore volume of graphitic microspheres are 0.21 cm3/g, 0.19 cm3/g and 0.02 cm3/g, respectively. The N2 physisorption result reveals that graphitic microspheres template has high specific surface area with abundant micropores, which made graphitic microspheres good matrix for boehmite sol adsorption. The subsequent growth of boehmite particles in 3-D without preferred orientation produces boehmite nanofibers attached to graphitic microspheres, the Pd NPs was thus highly dispersed in the resulting boehmite gel. After calcinations, the graphitic microsphere templated boehmite is converted into γ-Al2O3 with the removal of template and other organic residues, rendering mesopores in structure. The adsorption-desorption isotherm of Pd/γ-Al2O3-fibr displays type IV isotherm with H2 hysteresis loop, which indicates mesoporous characteristic of materials and ink-bottle mesopores [37]. This type of pores is generally formed between agglomerated primary crystallites, which are developed with the randomly stacked alumina nanofibers. The BET surface area of Pd/γAl2O3-fibr is 205 m2/g, the total pore volume and average pore size of Pd/γ-Al2O3-fibr is 0.69 cm3/g and 21.3 nm, respectively. As shown in Fig. 5, compared with commercial mesoporous γ-Al2O3, the nanofiberlike mesoporous γ-Al2O3 exhibits broader pore size distribution and larger average pore size. The mesoporous structural features of nanofiber-like mesoporous γ-Al2O3 is more beneficial to the diffusion of reactants and products, especially for the substrates with high steric hindrance. Fig. 6 shows the XPS spectra in the regions of N 1s and Pd 3d of asprepared Pd-boehmite@GM (before calcination) and Pd/γ-Al2O3-fibr. The N 1s XPS spectrum at binding energy of 411.7 eV in Pd-boehmite@GM is associated with the remaining PVP component in Pd-boehmite@GM. This N 1s XPS peak assigned to PVP component can not be observed in Pd/γ-Al2O3-fibr, suggesting that the organic residues has been completely removed after calcinations, which is consistent with the FTIR results (Fig. 3). The Fit-Gaussian of Pd (3d5/2) peaks of Pdboehmite@GM and Pd/γ-Al2O3-fibr can be divided into two

(a)

intensity/(a.u.)

N 1s

Pd-boehmite@GM Pd/γ-Al2O3-fibr 392

394

396

398

400

402

Binding energy/eV

404

406

(b)

0

Pd

2+

Pd

intensity(a.u.)

Pd-boehmite@GM

0

Pd Pd/γ-Al2O3-fibr

348

346

344

2+

Pd

342

340

338

336

334

332

330

binding energy/eV Fig. 6. XPS spectra in the regions of N 1s (a) and Pd 3d (b) of Pd-boehmite@GM and Pd/ γ-Al2O3-fibr.

129

Microporous and Mesoporous Materials 266 (2018) 126–131

L. Chen et al.

Table 1 Aerobic oxidation of benzyl alcohol and (4-(tert-butyl) phenyl) methanol by various Pd-based catalysts.a Entry

Catalyst

Pd loading (wt%)

Substrate

t(h)b

Product

Yield (%)c

1 2 3 4 5 6 7e 8

– Pd/γ-Al2O3-fibr Pd-boehmite@GM Pd/γ-Al2O3 Pd/SBA-15d Pd/γ-Al2O3-fibr Pd-PVP Pd/γ-Al2O3

– 1.0 1.0 1.0 1.0 1.0

Benzyl alcohol

2 2 2 2 2 6 6 6

benzaldehyde

NR 74 67 56 36 60 39 23

a b c d e

(4-(tert-butyl) phenyl) methanol

1.0

4-tert-butyl benzaldehyde

Reaction conditions: Substrate (9.6 mmol), solid catalyst (30 mg), water (40 mL), p (O2) = 1 atm, T = 80 °C. Reaction time. Determined by GC. SBA-15 was synthesized as reported (SBET = 870 m2/g), and Pd NPs was loaded on SBA-15 by impregnation-reduction method. Performed with 2.8 μmol of Pd for 9.6 mmol of (4-(tert-butyl) phenyl) methanol.

catalyst exhibits enhanced activity than commercial γ-Al2O3, suggesting the impact of morphology of support on activity (Table 1, entry 3). The enhanced activity of Pd/γ-Al2O3-fibr is probably attributed to its unique nanofiber-stacked morphology and relatively “free” state of Pd NPs on nanofiber-like mesoporous alumina. A similar phenomena is also observed for other heterogeneous Pd catalyst of nanofiber-like morphology, for example, Pd NPs on the fibrous meso-structured carbon exhibits higher activity compared to Pd@CMK-3 and Pd/C in the aerobic oxidation of alcohols on water [44]. The structure features of high porosity and relative larger inter-crystalline mesopores of nanofiber-like mesoporous γ-Al2O3 can lead to uniform distribution of the Pd NPs on alumina and efficient mass transfer. As expected, the Pd/γAl2O3-fibr catalyst is also be found to show higher catalytic performance for selective oxidation of (4-(tert-butyl) phenyl) methanol into 4tert-butyl benzaldehyde than Pd-PVP and Pd/γ-Al2O3, suggesting that the morphology of Pd/γ-Al2O3-fibr is obviously advantageous to the mass transfer of such a bulky substrates (Table 1, entry 6). In many cases, the base additives in reaction system are necessary for the palladium catalyzed selective oxidation of alcohols. However, the aerobic oxidation of benzyl alcohol can proceed smoothly without any base additives over our Pd/γ-Al2O3-fibr catalyst in water. Furthermore, the Pd/γ-Al2O3-fibr catalyst can be recovered by simply centrifugation and reused at least eight times without activity loss. These results have demonstrated that Pd/γ-Al2O3-fibr has high potential as a heterogeneous selective alcohol oxidation catalyst.

components, including a major peak at binding energy of 335.7 for metallic Pd0 species and a smaller peak at binding energy of 337.0 eV for Pd2+ species. The peak area integration results indicate that Pd0/ Pd2+ ratio in Pd-boehmite@GM and Pd/γ-Al2O3-fibr are 61/39 and 66/ 34, respectively. The amount of Pd0 in Pd/γ-Al2O3-fibr is somewhat higher than that in Pd-boehmite@GM, which indicates that during calcination under insufficient air, the removal of graphitic microspheres template by calcinations generates some amount of CO as a reducing agent, leading to partial reduction of Pd2+ to Pd0. The Pd 3d5/2 XPS spectra indicate that main oxidation state of palladium on Pd-boehmite@GM and Pd/γ-Al2O3-fibr are metallic Pd (Pd0). 3.2. Catalytic activity results The catalytic activity of Pd/γ-Al2O3-fibr towards the aerobic selox of benzyl alcohol was examined and its performance was compared with other Pd catalysts under identical reaction conditions. As summarized in Table 1, we found that Pd/γ-Al2O3-fibr exhibits a higher catalytic activity than Pd-PVP, Pd-boehmite@GM, or Pd/SBA-15 catalysts, and affords benzaldehyde in yield of 74% within 2 h (Table 1, entry 2). The less efficiency of Pd-boehmite@GM than Pd/γ-Al2O3-fibr is due to the presence of residual PVP, which is confirmed by the presence of N 1s signal in the XPS spectrum of Pd-boehmite@GM. The presence of residual PVP on Pd-boehmite@GM not only reduces the exposure of surface Pd sites, but also influences the interaction between Pd and boehmite, the similar influence of residual PVP was also observed in benzyl alcohol oxidation catalyzed by ceria-zirconia mixed oxides supported bimetallic AuePd catalysts [17]. It is generally believed that aerobic selective oxidation of alcohols over noble metal NPs based catalyst is initiated by oxidative dehydrogenation of alcohol taking place on metallic sites, the surface properties of supports have important effect on the performance of noble metal based catalysts [38]. Alumina is known to have both acidic and basic surface sites [39–41]. In oxidant-free dehydrogenation of alcohols catalyzed by heterogeneous silver catalysts on different supports, bifunctional support (Al2O3) is observed to give higher activity than basic inorganic oxides (MgO and CeO2) and acidic to neutral (SiO2) support, a mechanic explanation is that the basic sites at silveralumina interface facilitate binding of the alcohol substrate to give the alkoxide intermediate on alumina, and protonic OH groups at the interface facilitate the removal of hydride species from the silver sites to regenerate coordinatively unsaturated sites on the silver clusters [42]. In our research, Pd/γ-Al2O3-fibr exhibits higher activity than Pd/SBA15 is probably due to the beneficial effect of the bifunctional γ-Al2O3 support on the dehydrogenation of alcohols. The higher performance of Pd NPs deposited on alumina-grafted SBA-15 than on pure SBA-15 for cinnamyl alcohol selective oxidation also confirms the synergic effect between Pd sites and alumina surface [43]. It is noted that under the similar Pd loadings, the Pd/γ-Al2O3-fibr

4. Conclusion Pd NPs supported on nanofiber-like mesoporous alumina with 1% weight percent of Pd was prepared by sol-immobilization method. This unique nano-fibrous morphology of mesoporous alumina is found to be an effective support to stabilize Pd NPs, and the resulting composite Pd/ γ-Al2O3-fibr material was found to be an active catalyst for aerobic oxidation of benzyl alcohol with molecular oxygen in water without presence of any base additives, which is relatively low for such reaction. Remarkably, the current Pd/γ-Al2O3-fibr catalyst shows higher catalytic activity compared to Pd/γ-Al2O3 and Pd/SBA-15 under identical reaction conditions. The higher reactivity of Pd/γ-Al2O3-fibr catalyst arises from the combination of enhanced accessibility of active surface Pd species and synergic effect between Pd sites and alumina surface. Besides the easy recovery and reusability of this catalyst, the success application of this catalyst for more steric bulk alcohol substrates also demonstrates that Pd/γ-Al2O3-fibr catalyst has high potential as a heterogeneous catalyst for aerobic oxidation of alcohols. Acknowledgement This work is supported financially by the National Natural Science Foundation (NSF) of China (Grants no. 21406058 and no.51306047), 130

Microporous and Mesoporous Materials 266 (2018) 126–131

L. Chen et al.

Hunan Provincial Science and Technology Department of China (no. 2014NK3060), Hunan Natural Science Foundation (no. 13JJB009) and State Key Lab for Powder Metallurgy (no. 20110920).

Hydrogen Energy 40 (2015) 4951–4959. [22] L. Zhang, N. Zhou, B. Wang, C. Liu, G. Zhu, J. Colloid. Interface Sci. 421 (2014) 1–5. [23] Z. Zhang, F. Xiao, J. Xi, T. Sun, S. Xiao, H. Wang, S. Wang, Y. Liu, Sci. Rep. 4 (2014) 4053, http://dx.doi.org/10.1038/srep 04053. [24] J.X. Xu, G.J. Chen, R. Yan, D. Wang, M.C. Zhang, W.Q. Zhang, P.C. Sun, Macromolecules 44 (2011) 3730–3738. [25] N. Zhang, Y.J. Xu, Chem. Mater. 25 (2013) 1979–1988. [26] M. Zhao, K. Deng, L. He, Y. Liu, G. Li, H. Zhao, Z. Tang, J. Am. Chem. Soc. 136 (2014) 1738–1741. [27] Y.Z. Chen, Q. Xu, S.H. Yu, H.L. Jiang, Small 11 (2015) 71–76. [28] L.Y. Chen, H.R. Chen, R. Luque, Y.W. Li, Chem. Sci. 5 (2014) 3708–3714. [29] G. Zhan, H.C. Zeng, Chem. Mater. 27 (2015) 726–734. [30] X.M. Gu, W. Qi, X.Z. Xu, Z.H. Sun, L.Y. Zhang, W. Liu, X.L. Pan, D.S. Su, Nanoscale 6 (2014) 6609–6616. [31] J. Wang, A.H. Lu, M.R. Li, W.P. Zhang, Y.S. Chen, D.X. Tian, W.C. Li, ACS Nano 7 (2013) 4902–4910. [32] P.G. Tang, Y.Y. Chai, J.T. Feng, Y.J. Feng, Y. Li, D.Q. Li, Appl. Catal. Gen. 469 (2014) 312–319. [33] Y. Mi, W. Hu, Y. Dan, Y. Liu, Mater. Lett. 62 (2008) 1194–1196. [34] J. Cheng, Y. Wang, C. Teng, Y. Shang, L. Ren, B. Jiang, Chem. Eng. J. 242 (2014) 285–293. [35] A.N. Richard, O.K. Ronald, Infrared Spectra of Inorganic Compounds, Academic Press, Inc., New York and London, 1971 pp. 209. [36] K.M. Parida, Amaresh C. Pradhan, J. Das, N. Sahu, Mater. Chem. Phys. 113 (2009) 244–248. [37] H.M. Gobara, S.A. Hassan, Petrol. Sci. Technol. 27 (2009) 1555–1571. [38] T. Mallat, A. Baiker, Chem. Rev. 104 (2004) 3037–3058. [39] C. Morterra, G. Magnacca, Catal. Today 27 (1996) 497–532. [40] M. Digne, P. Sautet, P. Raybaud, P. Euzen, H. Toulhoat, J. Catal. 226 (2004) 54–68. [41] P.O. Scokart, P.G. Rouxhet, Comparison of the acid-base properties of various oxides and chemically treated oxides, J.Colloid. Int. Sci. 86 (1982) 96–104. [42] K. Shimizu, K. Sugino, K. Sawabe, A. Satsuma, Chem. Eur J. 15 (2009) 2341–2351. [43] C.M.A. Parlett, L.J. Durndell, A. Machado, G. Cibin, D.W. Bruce, N.S. Hondow, K. Wilson, A.F. Lee, Catal. Today 229 (2014) 46–55. [44] B. Karimi, H. Behzadnia, M. Bostina, H. Vali, Chem. Eur J. 18 (2012) 8634–8640.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

J.A.B. Satrio, L.K. Doraiswamy, Chem. Eng. J. 82 (2001) 43–56. D.V. McGrath, R.H. Grubbs, J.W. Ziller, J. Am. Chem. Soc. 113 (1991) 3611–3613. D.A. Knight, T.L. Schull, Synth. Commun. 33 (2003) 827–831. P. Gamez, I.W.C.E. Arends, J. Reedijk, R. Sheldon, Chem. Commun. 19 (2003) 2414–2415. X. Feng, P.P. Lv, W. Sun, X.Y. Han, L.F. Gao, G.X. Zheng, Catal. Commun. 99 (2017) 105–109. G. Wu, X. Wang, J. Li, N. Zhao, W. Wei, Y. Sun, Catal. Today 131 (2008) 402–407. Y. Su, L.C. Wang, Y.M. Liu, Y. Cao, H.Y. He, K.N. Fan, Catal. Commun. 8 (2007) 2181–2185. Q.H. Tang, X.N. Gong, C.M. Wu, Y.T. Chen, A. Borgna, Y.H. Yang, Catal. Commun. 10 (2009) 1122–1126. E.J. García-Suárez, M. Tristany, A.B. Garcia, V. Collière, K. Philippot, Microporous Mesoporous Mater. 153 (2012) 155–162. C. Zhou, Y. Chen, Z. Guo, X. Wang, Y. Yang, Chem. Commun. 47 (2011) 7473–7475. V.R. Choudhary, A. Dhar, P. Jana, R. Jha, B.S. Uphade, Green Chem. 7 (2005) 768–770. V.R. Choudhary, R. Jha, P. Jana, B.S. Uphade, Green Chem. 9 (2007) 267–272. N. Jamwal, M. Gupta, S. Paul, Green Chem. 10 (2008) 999–1003. Y.T. Chen, Z. Guo, T. Chen, Y. Yang, J. Catal. 275 (2010) 11–24. G. Wu, X. Wang, N. Guan, L. Li, Appl. Catal. B Environ. 136 (2013) 177–185. Y. Chen, H. Wang, C.J. Liu, Z. Zeng, H. Zhang, C. Zhou, X.L. Jia, Y.H. Yang, J. Catal. 289 (2012) 105–117. C.M. Olmos, L.E. Chinchilla, A. Villab, J.J. Delgado, H.Y. Pan, A.B. Hungría, J.J. Calvino, L. Prati, X.W. Chen, Appl. Catal. Gen. 525 (2016) 145–157. K. Mori, T. Hara, T. Mizugaki, J. Am. Chem. Soc. 126 (2004) 10657–10666. P. Li, H. Liu, Y. Yu, Chem. Asian J. 8 (2013) 2459–2465. R. Ji, S.R. Zhai, W. Zheng, Z.Y. Xiao, RSC Adv. 6 (2016) 70424–70432. S. Ghosh, A.L. Teilout, D. Floresyona, P. Oliveira, A. Hagège, H. Remita, Int. J.

131