Palladium nanoparticles supported on mesoporous carbon nitride for efficiently selective oxidation of benzyl alcohol with molecular oxygen

Accepted Manuscript Title: Palladium Nanoparticles Supported on Mesoporous Carbon Nitride for Efficiently Selective Oxidation of Benzyl Alcohol with Molecular Oxygen Authors: Jie Xu, Jie-Kun Shang, Ye Chen, Yue Wang, Yong-Xin Li PII: DOI: Reference:

S0926-860X(17)30245-4 http://dx.doi.org/doi:10.1016/j.apcata.2017.05.036 APCATA 16258

To appear in:

Applied Catalysis A: General

Received date: Revised date: Accepted date:

26-1-2017 20-5-2017 28-5-2017

Please cite this article as: Jie Xu, Jie-Kun Shang, Ye Chen, Yue Wang, Yong-Xin Li, Palladium Nanoparticles Supported on Mesoporous Carbon Nitride for Efficiently Selective Oxidation of Benzyl Alcohol with Molecular Oxygen, Applied Catalysis A, Generalhttp://dx.doi.org/10.1016/j.apcata.2017.05.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Manuscript title:

Palladium Nanoparticles Supported on Mesoporous Carbon Nitride for Efficiently Selective Oxidation of Benzyl Alcohol with Molecular Oxygen Authors’ names: Jie Xu *, Jie-Kun Shang, Ye Chen, Yue Wang, Yong-Xin Li * Authors’ affiliation: Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Gehu Road 1, Changzhou, Jiangsu 213164, PR China Corresponding Dr. Jie Xu Tel.: +86-519-86330135; E-mail: [email protected] Prof. Yong-Xin Li Tel.: +86-519-86330135; E-mail: [email protected]

Graphical Abstract

1



Highlights 

Ordered mesoporous carbon nitride was utilized as a new support to load Pd nanoparticles.



Benzaldehyde yield of was 94% under 80 °C in selective oxidation of benzyl alcohol under molecular oxygen.



Basic sites of carbon nitride facilitated activation of benzyl alcohol.



Superior catalytic activity to Pd nanoparticles supported on other carbon-based materials.

2

Abstract Ordered mesoporous carbon nitride (CN) has been synthesized through a nanocasting approach and utilized as a catalyst support to load Pd nanoparticles. The physical and chemical properties of Pd/CN materials were characterized using various techniques, including N2 adsorption–desorption, XRD, small-angle X-ray scattering, TEM, XPS, FT-IR, O2-TPD, and CO2-TPD. The ordered mesoporous structures of CN have well remained after the introduction of Pd nanoparticles. In selective oxidation of benzyl alcohol under mild aerobic condition, Pd/CN materials showed excellent and recyclable catalytic conversions of benzyl alcohol. Under the same reaction condition, the catalytic activity acquired over Pd/CN was superior to those obtained over Pd/CNTs, Pd/AC, and Pd/mpg-C3N4 catalysts. The characterization revealed that mesoporous CN support with basic nitrogencontaining sites could not only disperse well the Pd nanoparticles but also facilitate in activating alcohol molecules. Furthermore, in a tandem reaction combining selective oxidation and Knoevenagel condensation, Pd/CN material also showed potential catalysis with its bifunctionality. Keywords: Carbon nitride; Mesoporous material; Pd nanoparticles; Selective oxidation; Benzyl alcohol

3

1. Introduction The selective oxidation of alcohols in liquid phase is one of the most challenging reactions in green chemistry. Traditionally, industry process for this purpose uses a large excess of stoichiometric metal oxidants such as chromate and permanganate, which inevitably releases significant amount of pollutants [1, 2] and also demands laborious work-up procedure to remove them. Alternatively, catalytic oxidation under molecular oxygen (O2) promoted by heterogeneous catalysts offers a facile, low-cost, and ecologically benign strategy [3, 4]. Unfortunately, there are only a few satisfactory cases, where usually selectivity was sacrificed in order to achieve a high activity. In this sense, developing a robust heterogeneous catalyst affording high selectivity and activity is the core topic for the selective oxidation of alcohols under aerobic condition. A large number of catalyst systems have been developed for the selective oxidation of alcohols; however, up to now, supported noble metal nanoparticles including Au [5-7], Pd [8-10], Pt [11, 12], and their alloys [1, 13, 14] have been widely recognized as the most efficient catalysts. Wherein, the catalytic activity of such noble metal nanoparticles intrinsically depends on their size, shape, valence, crystallinity and multiple physicochemical properties. More specifically, small-sized nanoparticles with high dispersion generally lead to high activity [15]. On the other hand, it is commonly believed that catalyst support plays a crucible role in determining the above characters of metal nanoparticles [13]. Moreover, an ideal support could also stabilize the nanoparticles by reducing their mobility or agglomeration [16, 17], and even provide synergy to activate reactants through interactions between the metal and support itself. Hence, it is of high importance to develop or select appropriate support that can optimize the various natures of metal nanoparticles, and eventually upgrade their activity in oxidative reactions The past decade has witnessed the robust development of graphitic carbon nitride (g-CN) materials [18-20]. g-CN is a classical carbon-based material, yet

4

featuring unique combination of diverse physicochemical properties, which are responsible for its versatile applications in photocatalysis [21-23], fuel cells [24, 25], gas adsorption [26-28], etc. Of particular note, g-CN, especially mesoporous g-CN with high surface area and rich accessible porosity, possesses plentiful inherent nitrogen-containing groups, which enable it to be a typical solid base [29, 30]. Many research groups including ours have revealed that mesoporous g-CN samples could catalyze a series of base-mediated reactions, including Knoevenagel condensation [31-33], transesterification [29, 34], and even CO2-activating processes [35-37]. Meanwhile, mesoporous g-CN materials have been recently utilized as catalyst supports for Pt, Au, and Pd nanoparticles in several organic reactions such as aldehyde–amine–alkyne coupling [38], hydrogenation of phenol [17, 39, 40], and low-temperature oxidation of carbon monoxide [41]. The corresponding work revealed that, due to its abundant basic sites (i.e. nitrogen-containing groups), g-CN could favorably anchor the metal nanoparticles and offer high catalytic activities [42]. Despite the previous success, the exploitation of mesoporous g-CN as a support for wide catalytic application is still limited. More importantly, the effect of nature of g-CN on the catalytic activity is rarely discussed in detail. In this work, we have prepared ordered mesoporous g-CN material (CN) with high surface area, which was then successfully applied as a support for Pd nanoparticles. The Pd nanoparticles could be well dispersed on the surface of CN material. In the following aerobic selective oxidation of benzyl alcohol to benzaldehyde, the Pd/CN materials exhibited remarkably high conversion (~94%) under 80 °C. Additionally, the catalytic performance of Pd/CN has been compared with Pd catalyst supported on other porous carbon-based materials. 2. Experimental 2.1. Catalyst preparation The mesoporous carbon nitride material was synthesized using carbon tetrachloride (CTC) and ethylenediamine (EDA) as precursors, and ordered 5

mesoporous silica FDU-12 as a hard template (see the supporting information for the detailed synthetic route of FDU-12), through a nanocasting method as we reported previously [31, 34]. In brief, 1.0 g of FDU-12 powder was added into a well-mixed solution containing 6.0 g of CTC and 3.6 g of EDA. The mixture was heated under refluxing at 90 °C for 6 h to induce polymerization of the precursors, and then dried overnight. Next, the obtained dark brown solid was heated from room temperature to 600 °C at a ramp of 3 °C min−1 and the temperature was kept at 600 °C for another 5 h under nitrogen atmosphere. Afterwards, the sample was immersed into NH4HF2 aqueous solution (4 mol L−1, 100 mL) to remove the template. The obtained black solid was then centrifuged and washed with water and ethanol, and dried at 50 °C under vacuum overnight. The resultant mesostructured CN sample was labeled as CN. 0.2 g of CN was dispersed into 20 mL of distilled water with the aid of ultrasonication for ca. 30 min. After that, 4 mL of PdCl2 aqueous solution (0.02 mol L−1) was added into the dispersion and subjected to further ultrasonication for 30 min. Next, 40 mL of fresh NaBH4 aqueous solution (2 mg mL−1) was dropwisely added into the above mixture under vigorous stirring. After 1 h, the mixture was filtrated and the separated solid was then washed by water for three times, followed by drying at 60 °C under vacuum for 2 h. The obtained solid was named as mPd/CN, where m indicated the nominal loading amount (wt%). 2.2. Sample characterization Nitrogen adsorption–desorption isotherms were measured at −196 °C using a Micromeritics ASAP 2020 analyzer. The specific surface area was calculated according to the Brunauer–Emmet–Teller (BET) method. X-ray diffraction (XRD) pattern was recorded on a Rigaku D/max 2500 PC X-ray diffractometer with a Cu-Kα radiation (λ = 1.5418 Å). Small angle X-ray scattering (SAXS) pattern was analyzed on a Bruker NanoSTAR U SAXS system using Cu-Kα radiation (40 kV, 35 mA).

6

Transmission electron microscopy (TEM) experiments were conducted on a JEOL 2010 electron microscope operating at 200 kV. Fourier transform infrared (FT-IR) spectra were obtained on a Bruker Tensor 27 spectrometer using transmission mode. Each sample was mixed with KBr and crushed into a wafer and the spectrum was measured at room temperature. X-ray photoelectron spectra (XPS) were recorded on a Thermo ESCALAB 250XI spectrometer using an Mg/Al anode as an X-ray source. CO2 temperature-programmed desorption (CO2-TPD) was performed on a Quantachrome ChemBET-3000 analyzer. Prior to analysis, the sample was heated at 300 °C for 1 h under helium gas and then cooled down to the room temperature. Afterwards, pure CO2 was purged into the sample for ca. 0.5 h until saturated adsorption of CO2, which was monitored by thermal conductivity detector (TCD). After that, the gas path system was swept by pure helium (100 mL min−1) for ca. 0.5 h at room temperature. Finally, TPD data based on the signals was recorded from 50 to 400 °C with a ramping rate of 10 °C min−1 under the flow of helium (100 mL min−1). Oxygen temperature-programmed desorption (O2-TPD) experiments were performed on a Quantachrome ChemBET-3000 analyzer. The operation procedure of O2-TPD is the same to that of CO2-TPD. Wherein, pure O2 was applied as an adsorbate and was purged into the sample for ca. 0.5 h until saturated adsorption, which was also checked by TCD. Eventually, the TPD data was recorded from 75 to 450 °C under the flow of helium (100 mL min−1). 2.3. Catalytic test The selective oxidation reactions of benzyl alcohol (BZA) were carried out in twonecked flask (50 mL) in an oil bath, equipped with a condenser. 1.0 mL of benzyl alcohol (9.6 mmol), and 300 μL of decane as an internal standard, were dissolved into 10 mL of toluene, followed by addition of 25 mg of catalyst. Afterwards, the mixture was heated up to 80 °C, and flushed with pure oxygen with a flow rate of 20 mL min−1

7

under vigorous stirring. After reaction, the mixture was centrifuged, and the products were analyzed by GC equipped wih a FID detector. The carbon balance for the calculation method is 100±5%. The values of both the catalytic conversion of BZA and selectivity to the target product were based on three parallel experiments. The quasi TOF value (h−1) for each catalyst is calculated as follows:

TOF=

nBZA, converted nPd  t

=

nBZA, fed  Conv.%  Sel.% Wcatal  m t M Pd

where nBZA, fed, Wcatal, m, and MPd are the molar amount (mmol) of benzyl alcohol, weight (g) of total supported catalyst, loading amount (wt%) of Pd, and formula weight (g mol−1) of Pd, respectively. The quasi TOF value is calculated under relatively low catalytic conversions (<35%) of BZA, and based on the catalytic activity at 3 h of reaction. 3. Results and discussions 3.1. Structure characterization a

Determined by the adsorption branches.

The surface areas and porosity of CN and its supported Pd catalysts were analyzed by N2 adsorption–desorption characterization. The CN sample shows typical type IV curves with a steep H1 hysteresis loop in the range of p/p0=0.64–0.80 (Fig. 1A), inferring that the material possesses mesoporous structures with a relatively concentrated pore size distribution (PSD). After loading Pd species, the CN materials exhibit similar isothermal curves, despite decreases in total adsorption quantity. The corresponding PSD curves (Fig. 1B) calculated from the adsorption branches indicate that both CN and Pd/CN samples have narrow pore size distribution, located in the range of 6–7 nm. Table 1 lists the textual parameters of all the materials. The surface area and pore volume of the parent CN sample are 591 m2 g−1 and 1.34 cm3 g−1, respectively. In the case of Pd catalysts, with increasing the loading amount, the surface area as well as pore volume decreases monotonously. Notwithstanding, the pore sizes of Pd/CN samples are ca. 7 nm, slightly lower than that of the CN support. 8

Since FDU-12 material possesses a large unit cell parameter (a0), commonly than 20 nm, SAXS instead of SAXRD technique was employed to probe the orderness of porosity [43]. As displayed in Fig. 2, FDU-12 exhibits two well-resolved and intensive scattering peaks, which are located at q = 0.384 and 0.68 nm−1, indexed as (111) and (311) planes of a face-centered cubic (fcc) structure [44, 45]. The corresponding unit cell parameter (a0) of FDU-12 is ca. 28.3 nm and the SAXS pattern confirms that FDU12 owns well-ordered mesopores, which agrees well with the characterization results of the N2 adsorption–desorption isotherms and PSD curves (Fig. S1 and Table S1). Two obvious scattering peaks could also be identified in the case of CN, showing that the material has successfully replicated the ordered fcc symmetry of FDU-12. It is also found that the intensity of the two peaks obtained over CN is relatively lower than FDU-12, and meanwhile the scattering vector of primary peaks seems be shifted towards higher value (q = 0.398 nm−1 and a0 = 27.3 nm). The phenomenon is mainly due to the shrinkage of CN walls and partial collapse of the mesopores of CN [27, 34]. As for Pd/CN materials, it is clearly observed that upon loading Pd species, the parent orderness of mesoporous structures of CN has been well retained. The morphology of FDU-12, CN and Pd/CN materials was observed by TEM. The image of FDU-12 (Fig. S2) reveals highly ordered honeycomb-like patterns. Likewise, the CN sample shows ordered arrays in the TEM images (Fig. S2). Directly calculated from the images, the pore size of CN is ca. 8 nm, suggesting that CN has ordered mesopores, in good agreement with the above N2 adsorption–desorption and SAXS characterization results. After loading Pd species, the original ordered mesopores of the CN support seem to aggregate (Fig. 3). The images of 2Pd/CN, 4Pd/CN, and 6Pd/CN demonstrate that the Pd nanoparticles are well dispersed on the surface of CN support. The mean particle sizes measured by their histograms are ca. 3.6, 4.2, and 4.6 nm, respectively. Compared with 2Pd/CN and 4Pd/CN, a small amount of Pd nanoparticles are found to agglomerate in the case of 6Pd/CN. Nevertheless, the good dispersion of Pd species on the support implies that the mesoporous CN material is able to anchor the Pd nanoparticles, as reported in the previous work [15, 9

38, 46] involving the noble-metal nanoparticles supported on mesoporous CN materials. Fig. 4 shows the XRD patterns of CN and Pd/CN materials. All the sample exhibit major peaks at ca. 2θ = 25.2 °, corresponding to the (002) planes, characteristic interplanar stacking structures of a graphite-like material [47]. It is worth noting that, no apparent diffraction peaks can be detected at the lower loading amount of Pd species, suggesting that the Pd nanoparticles are highly dispersed on CN material. As the loading amount of Pd increases up to 6wt%, a weak and broad peak centered at 2θ = 40 ° is found, which is indexed as the (111) planes of metallic Pd (PDF# 46-1043). Obviously, the Pd nanoparticles underwent potential aggregation on the CN support under a higher loading amount, which is in accordance with the aforementioned TEM result. Besides Pd/CN materials, Pd catalysts supported on the other materials, i.e. activated carbon (AC) and carbon nanotubes (CNTs) were also synthesized by means of the similar method. The surface areas and pore sizes of two supported catalysts are summarized in Table 1. As provided in Fig. 4, the XRD patterns of 4Pd/AC and 4Pd/CNTs show notable diffraction lines pertaining to Pd substance. According to Scherrer’s equation the mean crystalline size of Pd particles in aspect of (111) reflection for 4Pd/AC and 4Pd/CNTs is ca. 8.0 nm and 8.9 nm, respectively, confirming that the Pd particles suffer evident agglomeration on the two supports at the same loading amount. The bulk and surface chemical composition were analyzed by FT-IR, and XPS characterization, respectively. The FT-IR spectra (Fig. 5) of CN and Pd/CN samples reveal similar bands. The major peaks located at 1308 and 1596 cm−1 are assigned to aromatic C–N stretching bonds, and aromatic ring modes [48], respectively. The broad bands centered at 3431 cm−1 are associated with the stretching modes of N–H bonds in the aromatic functionalities and adsorbed water molecules [49, 50]. Compared with that of the mesoporous CN support, the intensity of the bands at 3431 cm−1 becomes much lower in the case of Pd/CN materials. One direct reason is the decrease of the surface area of mesoporous CN after loading with Pd. 10

The survey spectrum of XPS of the CN support (Fig. S3) indicates that the material is composed of carbon and nitrogen, along with a small of amount of oxygen that might come from adsorbed water molecules. The fine N 1s spectrum (Fig. 6A) can be deconvoluted into three peaks. The lowest energy (398.3 eV) contribution is indexed as nitrogen atoms (Na) bonded with sp2-hybrized carbon atoms in aromatic ring, whereas the peak located at 400.5 eV corresponds to nitrogen atoms (Nb) trigonally bonded to sp2-hybrized carbon atoms [27, 32]. The last minor signal centered at ca. 404.1 eV is generally attributed to quaternary nitrogen (Nq) and/or charging effect of XPS [30, 51]. As for CN-supported Pd catalysts, they show similar N 1s spectra with three independent peaks. Notwithstanding, it is evident that the relative intensity for the primary peaks undergoes variation. Specifically, upon loading of Pd, the concentration of Na atoms decreases whereas the percentage of Nb increases continuously. This implies that the supported Pd should be interacted with the N atoms of CN material, and the actively anchoring sites thereof might be the nitrogen atoms bonded with sp2-hybrized carbon atoms in aromatic ring. Besides N element, the Pd 3d spectra (Fig. 7) of the Pd/CN materials have also been deconvoluted for further analysis. The spectra could be separated into four peaks. Wherein, two peaks appearing at 341.3, and 336.0 eV are ascribed to Pd0 3d3/2, and Pd0 3d5/2 signals, respectively; the other peaks with binding energies of 343.4 and 338.0 eV are assigned to Pd2+ 3d3/2, and Pd2+ 3d5/2 signals [16, 46]. According to the peak areas integrated in Pd 3d spectra, the molar percentage of Pd0 in total Pd element of 2Pd/CN is about 21%. By comparison, the value increases to 31% in 4Pd/CN. As the supported Pd catalysts are to be evaluated in selective oxidation reaction, O2-TPD experiments are performed and the resultant profiles are given in Fig. 8. For the parent CN sample, there presents a broad desorption peak in the range of 150– 400 C. This means that the mesoporous CN material has appreciable adsorption of oxygen molecules, which should results from chemical adsorption, and/or physical 11

adsorption due to its accessible mesopores. After loading Pd species, the location of peak is shifted towards low temperatures while the peak area shows no significant variation. On the other hand, after introduction of Pd, the overall surface area and pore volume of mesoporous CN decrease apparently (Table 1). According to the comparison, it can be excluded that the adsorption of oxygen molecules is mainly derived from the textural properties of the materials. That is, Pd/CN catalysts have superior oxygen-activating capability to the pristine CN support. 3.2. Catalytic activity a

Reaction conditions: VBZA = 1 mL, Vtoluene = 10 mL, T = 80 °C, t = 3 h, and Wcatal. = 25

mg. Initially, a blank test for the selective oxidation of benzyl alcohol was carried out at 80 °C. The conversion of alcohol is lower than 1% (Table 2), verifying that the reaction could rarely occur in the absence of catalyst. Also, after the addition of mesoporous CN, the conversion shows no essential improvement. Using Pd/CN catalysts instead, the catalytic activity demonstrates significant promotion. Specifically, with the loading amount of Pd, the catalytic conversion of benzyl alcohol increases progressively. The variation correlates well the abovementioned O2-TPD results. That is, the Pd nanoparticles have effectively enhanced the ability to activate oxygen molecules. For 4Pd/CN, the catalytic conversion is up to 94% at 3 h. However, no apparent increase of the catalytic activity is received using Pd/CN with higher loading amounts, which might be due to agglomeration of partial Pd nanoparticles under excessive loading amounts. The influence of reaction conditions on the catalytic performances has been further investigated. As the amount of benzyl alcohol increases from 0.4 to 0.8 mL (Table S2), its conversion improves, and the maximum conversion is obtained under 1 mL of benzyl alcohol. Further adding the alcohol, the conversion decreases monotonously. Since the catalytic reaction proceeds in the presence of toluene (10 mL), under the low amount of feedstock (< 0.8 mL), the solvent with high 12

concentration might impede the adsorption of benzyl alcohol on the surface of heterogeneous catalyst, thus undermining catalytic activity. The effects of reaction temperature and time on the catalytic performances are provided in Fig. S4. In the wide temperature range (70–90 °C) surveyed, the highest conversion is obtained under 80 °C. In the case of reaction time, the conversion is ca. 76% at 1 h. Upon prolonging the reaction, the conversion increases continuously but seems to level off after 3 h. Additionally, the catalyst amount also exhibits a noticeable relationship with the final catalytic performance (Table S3). As the catalyst amount is increased, the conversion of BZA is elevated progressively. Whereas, the catalytic conversion reaches its maximum when using 25 mg of 4Pd/CN. Besides reaction temperature and time, recyclability is also a crucial parameter to evaluate a heterogeneous catalyst. Regarding this issue, a series of recycling experiments were subjected on 4Pd/CN catalyst. As displayed in Fig. 9, after the first run, the conversion of benzyl alcohol shows a slight decrease of ca. 5%. However, the conversions acquired in the next consecutive catalytic tests are all above 87%, confirming that the present CN-supported Pd catalysts are relatively stable in such heterogeneously oxidative system. To further verify that whether the catalysts really acted as heterogeneous catalysts or not, a hot-filtration experiment was carried out. The reaction conditions (including reaction temperature, catalyst amount, substrate amount, etc) of hot-filtration experiment was identical with those applied in the ordinary catalytic experiment. However, after the reaction time reached 1h, the Pd/CN catalyst was immediately separated from the reaction mixture by instant filtration, while the liquid-phase filtrate (i.e. reaction mixture without any catalyst) continued to proceed for another 3 h. The resultant catalytic conversion of benzyl alcohol as a function of reaction time was plotted in Fig. S4. It is evident that the conversions show no appreciable growth after 1 h, revealing that the reaction was completely stopped by the removal of the catalyst. The hotfiltration test simply but effectively excludes the potential leaching problem for this heterogeneous catalyst. Combining the above recycling and hot-filtration 13

experiments, it can be confirmed that the Pd/CN catalysts really act as heterogeneous catalysts in the selective oxidation reactions of benzyl alcohol. Table 3 Selective oxidation of various alcohols under molecular oxygen a. Time (h)

Conv.(%) b

1

4

94

2

3

92

3

2.5

93

4

4

54

5

10

87

Entry

a

Reactant

Product

Reaction conditions: 1 mL of alcohol, 25 mg of 4Pd/CN, Vtoluene = 10 mL, and T = 80

°C. b The selectivity to the corresponding aldehyde for each entry was >99%. The data was the mean value based on the three parallel experiments. In order to test the catalytic versatility of Pd/CN, we submitted several BZAbased substrates with various substituents for the selective oxidation to aldehydes. As listed in Table 3, the representative analogue of BZA could be smoothly transformed into the corresponding aldehydes with the aid of 4Pd/CN under molecular oxygen. The catalytic activity for each case depends on the property of substituent. The alcohols with electron-donating substituents (entries 2–3) demonstrate high catalytic conversions (>92%) in shorter reaction time. In comparison, under the same reaction conditions like entry 1, only 54% of 414

nitrobenzyl alcohol (entry 4) is converted in 4 h. To achieve a good catalytic conversion, the reaction has to be extended for ca. 10 h (entry 5). Apparently, substituents with electron-withdrawing groups would hinder the activation of hydroxyl in the catalytic reactions. Concerning the catalytic mechanism of selective oxidation of BZA catalyzed by Pd or other noble metal species, it has been widely recognized that the first step is the activation of alcohol on Pd and formation of alkoxide thereafter [52-54]. Subsequently, the alkoxide undergoes β-elimination (i.e. dehydrogenation) and then transforms into aldehyde, which is also regarded as the rate-determining step in the overall catalytic oxidation of alcohol. Wherein, the electron-donating substituents are able to facilitate the formation of carbocationtype transition state. The effect of substitution of BZA on the catalytic activity in selective oxidation has also previously observed over Pd nanoparticles supported on hydrotalcite [55] and mesoporous carbon [56] materials. We have also tentatively used mesoporous CN as a support to prepared Au and Pt catalysts (see the supporting information for their detailed preparation route). Interestingly, neither of the two supported noble metal catalysts exhibits remarkable activity in the selective oxidation of benzyl alcohol (Table 4). Furthermore, for the sake of detailed comparison, we have prepared Pd catalysts applying mesoporous gC3N4 (mpg-C3N4) as a support. The mpg-C3N4 material was synthesized using cyanamide as a precursor and commercial colloid silica nanoparticles as hard templates. It should be stressed that, although both mpg-C3N4 (also named as mpC3N4) and mesoporous CN are called as mesoporous carbon nitride in many papers, the two kinds of materials are actually different according to the precursors employed and physicochemical properties [20, 57]. The first group is mainly prepared using cyanamide, dicyandiamide, or melamine as a precursor. The synthesized materials possess high concentration of nitrogen element and present a yellow color [19, 30]. Whereas, the other group is commonly fabricated using CTC and EDA as precursors, and the materials own higher content of carbon than mpgC3N4 [29, 58]. 15

As for mpg-C3N4, it has been well documented that the material could be exploited as a novel catalytic support to load noble metals for many catalytic reactions [38, 41, 42]. In particular, Pd/mpg-C3N4 catalysts have been found to be able to promote a variety of hydrogenation reactions with high activity and selectivity, including phenol to cyclohexanone [40], quinoline to 1,2,3,4tertrahydroquinoline [46], nitrile to amines [59], etc. However, in the case of selective oxidation, light irritation is usually demanded for mpg-C3N4-based catalysts to gain high activity [22, 23, 60]. In this work, as listed in Table 4, under the same loading amount of Pd, the conversion of benzyl alcohol received over 4Pd/mpg-C3N4 is ca. 2%. The corresponding TOF value for 4Pd/mpg-C3N4 is only 7 h−1 whereas the value gained over 4Pd/CN is up to 672 h−1, showing that the mpg-C3N4-supported Pd catalyst can barely promote the oxidation reaction. In addition, despite a 100% selectivity to benzaldehyde, the catalytic conversion received over Pd catalysts supported on activated carbon (4Pd/AC) and carbon nanotubes (4Pd/CNTs) is quite limited, In sharp contrast, even in the solvent-free reaction condition with 4 mL of benzyl alcohol, the 4Pd/CN still shows a moderate activity, affording a conversion of 48% at 3 h. a

Unless specified, the reaction conditions are as follows: VBZA = 1 mL, Vtoluene = 10 mL,

T = 80 °C, t = 3 h, and Wcatal. = 25 mg. The catalytic conversions were the mean values based on the three parallel experiments. b VBZA = 6 mL. c VBZA = 2 mL. The above comparison using various carbon-based supports evidences the advantage of mesoporous CN material. From the TEM images, it can be found that, the Pd nanoparticles supported on mpg-C3N4 material have also a small mean size (ca. 4.4 nm, Fig. S5) like those on mesoporous CN support. Therefore, the effect of size of Pd nanoparticles on the catalytic activity of the final supported catalysts could be excluded. CO2-TPD experiments have been performed to elucidate the basicity of various supported Pd catalysts. As depicted in Fig. 10, both the bare CN and its supported Pd

16

catalysts demonstrate intensive desorption peaks centered at ca. 220 °C, which are attributed to weak chemical adsorption of acidic CO2 on the surface. It is also of interest to compare the profiles of 4Pd/mpg-C3N4 and 4Pd/CN. Although the mpgC3N4 supported catalyst presents a broad peak in the range of 175–325°C, the intensity is much lower. Moreover, the peak area of 4Pd/mpg-C3N4 is substantially smaller than that of 4Pd/CN. The noticeable difference in basic quantity of the supported catalyst indeed originates from the intrinsic nature of mpg-C3N4 and mesoporous CN. In fact, the motif unit for yellow g-C3N4 is π-conjugated tri-s-triazine heterocycles that theoretically have almost no active basicity [51, 61], and the basic sites of mpg-C3N4 are actually contributed from uncondensed amino group at the edges of graphitic sheets [62]. For mesoporous CN prepared from CTC and EDA, the tectonic units have been widely regarded as basic pyridine, pyrrole and other nitrogen-containing heterocycles [29, 31]. Based on the characterization results of CO2-TPD, it can be inferred that the catalytic activity of supported Pd catalysts in the oxidation reaction ought to originate from the basicity of the supports. For the similar carbon-based mesoporous materials, a support with higher quantity of active basic sites (such as –NH2, –NH–, and pyridine) could provide superior catalytic activity. Very recently, Wang et al have found that, in selective hydrogenation [46] and oxidation [63] reactions, nitrogencontaining carbon materials as basic supports could efficiently activate organic substrate molecules. In addition, Arai et al have reported that basic nitrogen-doped activated carbon could promote the selective oxidation of alcohols [64]. More specifically, the basic nitrogen-containing groups on the surface of carbon materials adsorb phenol or alcohol via O–H…N or O…H–N interactions, and thereafter facilitate the scission of O–H bond of the substrate [63]. Based on this viewpoint, it can be smoothly explained that Pd catalysts supported on CN exhibit better activity than those on mpg-C3N4, because the former support owns higher content of active basicity, which can essentially contribute to the activation of benzyl alcohol.

17

a

Reaction conditions: VBZA= 1 mL, Vmethyl cyanoacetate = 1 mL, Vtoluene= 10 mL, T = 90 °C, t

= 3 h, and 25 mg of 4Pd/CN. Since the above CO2-TPD characterization result and our previous work have illustrated that the mesoporous CN materials have active basic sites in the forms of aromatic N-containing groups, we thus submitted a one-pot tandem reaction to further examine the catalytic performance of the Pd/CN material. That is, the benzyl alcohol is initially oxidized into benzaldehyde, followed by a Knoevenagel condensation involved by methyl cyanoacetate. As shown in Table 5, the conversion of benzyl alcohol is up to 90% after 4 h; simultaneously, nearly one third of methyl cyanoacetate has been converted, yielding the target molecule, i.e. methyl -2-cyano3-phenylacrylate. We tentatively used malononitrile as a more active methylene substrate and anticipated higher catalytic activity in such a tandem reaction. Unexpected, the conversion of alcohol in the first stage was dramatically deteriorated. One possible reason for this phenomenon is that the supported Pd nanoparticles were poisoned by malononitrile. Nevertheless, the test has clearly verified that Pd/CN materials are potential bifunctional heterogeneous catalysts possessing both oxidative and basic capability. 4. Conclusion In summary, Pd nanoparticles have been supported on ordered mesoporous CN and other carbon-based materials via a impregnation method. The Pd nanoparticles disperse well on the mesoporous CN materials and the mean sizes are 4–5 nm. In heterogeneously selective oxidation of benzyl alcohol with molecular oxygen, the Pd/CN materials manifest high and reproductive catalytic activity, affording a maximum yield of benzaldehyde of 94% under 80 °C at 3 h. The introduction of Pd species effectively improves the oxygen-activating ability of CN. Among several carbon-based materials, the Pd catalysts supported on CN exhibit the highest catalytic activity, and the excellent performance is probably due to the rich basic sites on the mesoporous CN that allow efficient activation of benzyl alcohol.

18

Furthermore, Pd/CN catalysts demonstrate potential catalysis in a tandem reaction of benzyl alcohol oxidation and Knoevenagel condensation. Owing to the oxidative– basic bifunctional merits, the Pd/CN materials are expected to be exploited in wider oxidative organocatalysis. Acknowledgments This work was supported by National Natural Science Foundation of China (21203014), Postgraduate Innovation Project of Jiangsu Province (KYLX15_1119), Advanced Catalysis and Green Manufacturing Collaborative Innovation Center (ACGM2016-06-28), and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. References: [1]

I. Dan, Enache, J.K. Edwards, L. Philip, S.E. Benjamin, A.F. Carley, A.A. Herzing,

W. Masashi, C.J. Kiely, D.W. Knight, G.J. Hutchings, Science 311 (2006) 362-365. [2]

L. Rout, P. Nath, T. Punniyamurthy, Adv. Synth. Catal. 349 (2007) 846-848.

[3]

F.-Z. Su, Y.-M. Liu, L.-C. Wang, Y. Cao, H.-Y. He, K.-N. Fan, Angew. Chem. Int.

Ed. 47 (2008) 334–337. [4]

J. Zhu, J.L. Faria, J.L. Figueiredo, A. Thomas, Chem. -Eur. J. 17 (2011) 7112-

7117. [5]

V.R. Choudhary, A. Dhar, P. Jana, R. Jha, B.S. Uphade, Green Chem. 7 (2005)

768-770. [6]

A. Tanaka, K. Hashimoto, H. Kominami, J. Am. Chem. Soc. 134 (2012) 14526-

14533. [7]

A. Abad, P. Concepción, A. Corma, H. García, Angew. Chem. Int. Ed. 44 (2005)

4066-4069. [8]

M.J. Schultz, C.C. Park, M.S. Sigman, Chem. Commun. (2002) 3034-3035.

19

[9]

K. Mori, T. Hara, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am. Chem. Soc. 126

(2004) 10657-10666. [10]

V.M. Shinde, E. Skupien, M. Makkee, Catal. Sci. Technol. 5 (2015) 4144-4153.

[11]

M.S. Ide, D.D. Falcone, R.J. Davis, J. Catal. 311 (2014) 295-305.

[12]

F. Wang, S. Shao, C. Liu, C. Xu, R. Yang, W. Dong, Chem. Eng. J. 264 (2015)

336-343. [13]

S.E. Davis, M.S. Ide, R.J. Davis, Green Chem. 15 (2013) 17-45.

[14]

M. Sankar, E. Nowicka, R. Tiruvalam, Q. He, S.H. Taylor, C.J. Kiely, D. Bethell,

D.W. Knight, G.J. Hutchings, Chem. -Eur. J. 17 (2011) 6524-6532. [15]

P. Xiao, Y. Zhao, T. Wang, Y. Zhan, H. Wang, J. Li, A. Thomas, J. Zhu, Chem. -

Eur. J. 20 (2014) 2872-2878. [16]

Z. Li, J. Liu, C. Xia, F. Li, ACS Catal. 3 (2013) 2440-2448.

[17]

G. Feng, P. Chen, H. Lou, Catal. Sci. Technol. 5 (2015) 2300-2304.

[18]

Y. Wang, X. Wang, M. Antonietti, Angew. Chem. Int. Ed. 51 (2012) 68-89.

[19]

J. Zhu, P. Xiao, H. Li, S.A.C. Carabineiro, ACS Appl. Mater. Inter. 6 (2014)

16449-16465. [20]

K.S. Lakhi, D. Park, K. Albahily, W. Cha, B. Viswanathan, J. Choy, A. Vinu,

Chem. Soc. Rev. 46 (2016) 72-101. [21]

X. Wang, K. Maeda, A. Thomas, K. Takanabe, Nat. Mater. 8 (2009) 76-80.

[22]

F. Su, S.C. Mathew, G. Lipner, X. Fu, M. Antonietti, S. Blechert, X. Wang, J. Am.

Chem. Soc. 132 (2010) 16299-16301. [23]

X. Wang, S. Blechert, M. Antonietti, ACS Catal. 2 (2012) 1596-1606.

[24]

M. Kim, S. Hwang, J.-S. Yu, J. Mater. Chem. 17 (2007) 1656-1659.

20

[25]

Y. Zheng, J. Liu, J. Liang, M. Jaroniec, S.Z. Qiao, Energy Environ. Sci. 5 (2012)

6717-6731. [26]

S.S. Park, S.-W. Chu, C. Xue, D. Zhao, C.-S. Ha, J. Mater. Chem. 21 (2011)

10801-10807. [27]

Q. Li, J. Yang, D. Feng, Z. Wu, Q. Wu, S.S. Park, C.-S. Ha, D. Zhao, Nano Res. 3

(2010) 632-642. [28]

J. Zhang, F. Guo, X. Wang, Adv. Funct. Mater. 23 (2013) 3008-3014.

[29]

X. Jin, V.V. Balasubramanian, S.T. Selvan, D.P. Sawant, M.A. Chari, G.Q. Lu, A.

Vinu, Angew. Chem. Int. Ed. 48 (2009) 7884-7887. [30]

J. Xu, T. Chen, Q. Jiang, Y.-X. Li, Chem. -Asian J. 9 (2014) 3269-3277.

[31]

J. Xu, K.-Z. Long, T. Chen, B. Xue, Y.-X. Li, Y. Cao, Catal. Sci. Technol. 3 (2013)

3192-3199. [32]

S.N. Talapaneni, S. Anandan, G.P. Mane, C. Anand, D.S. Dhawale, S. Varghese,

A. Mano, T. Mori, A. Vinu, J. Mater. Chem. 22 (2012) 9831-9840. [33]

J. Xu, Y. Wang, J.-K. Shang, Q. Jiang, Y.-X. Li, Catal. Sci. Technol. 6 (2016) 4192-

4200. [34]

J. Xu, F. Wu, H.-T. Wu, B. Xue, Y.-X. Li, Y. Cao, Micropor. Mesopor. Mater. 198

(2014) 223-229. [35]

M.B. Ansari, B.-H. Min, Y.-H. Mo, S.-E. Park, Green Chem. 13 (2011) 1416-

1421. [36]

Z. Huang, F. Li, B. Chen, T. Lu, Y. Yuan, G. Yuan, Appl. Catal. B 136 (2013) 269-

277. [37]

Q. Su, J. Sun, J. Wang, Z. Yang, W. Cheng, S. Zhang, Catal. Sci. Technol. 4

(2014) 1556-1562.

21

[38]

K.K.R. Datta, B.V.S. Reddy, K. Ariga, A. Vinu, Angew. Chem. Int. Ed. 49 (2010)

5961-5965. [39]

Y. Wang, J. Yao, H. Li, D. Su, M. Antonietti, J. Am. Chem. Soc. 133 (2011) 2362-

2365. [40]

Y. Li, X. Xu, P. Zhang, Y. Gong, H. Li, Y. Wang, RSC Adv. 3 (2013) 10973-10982.

[41]

J.A. Singh, N.J. Dudney, M. Li, S.H. Overbury, G.M. Veith, ACS Catal. 2 (2012)

1138-1146. [42]

Y. Gong, M. Li, H. Li, Y. Wang, Green Chem. 17 (2015) 715-736.

[43]

J. Fan, C. Yu, F. Gao, J. Lei, B. Tian, L. Wang, Q. Luo, B. Tu, W. Zhou, D. Zhao,

Angew. Chem. Int. Ed. 115 (2003) 3146-3150. [44]

L. Huang, X. Yan, M. Kruk, Langmuir 26 (2010) 14871-14878.

[45]

P. Yuan, J. Yang, X. Bao, D. Zhao, J. Zou, C. Yu, Langmuir 28 (2012) 16382-

16392. [46]

Y. Gong, P. Zhang, X. Xu, Y. Li, H. Li, Y. Wang, J. Catal. 297 (2013) 272-280.

[47]

A. Vinu, P. Srinivasu, D.P. Sawant, T. Mori, K. Ariga, J.-S. Chang, S.-H. Jhung,

V.V. Balasubramanian, Y.K. Hwang, Chem. Mater. 19 (2007) 4367-4372. [48]

A. Vinu, Adv. Funct. Mater. 18 (2008) 816-827.

[49]

A. Vinu, K. Ariga, T. Mori, T. Nakanishi, S. Hishita, D. Golberg, Y. Bando, Adv.

Mater. 17 (2005) 1648-1652. [50]

J. Xu, K. Shen, B. Xue, Y.-X. Li, Y. Cao, Catal. Lett. 143 (2013) 600-609.

[51]

A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J.-O. Müller, R. Schlögl,

J.M. Carlsson, J. Mater. Chem. 18 (2008) 4893-4908. [52]

S. Aditya, C.E. Chan-Thaw, R. Ilenia, V. Alberto, P. Laura, ChemCatChem 6

(2014) 3464-3473.

22

[53]

T. Mallat, A. Baiker, Chem. Rev. 104 (2004) 3037-3058.

[54]

D. Ferri, C. Mondelli, F. Krumeich, A. Baiker, J. Phys. Chem. B 110 (2006)

22982-22986. [55]

Q. Zhang, W. Fang, Y. Wang, H. Wan, Chin. J. Catal. 31 (2010) 1061-1070.

[56]

B. Karimi, H. Behzadnia, M. Bostina, H. Vali, Chem. -Eur. J. 18 (2012) 8634-

8640. [57]

Y. Wang, Q. Jiang, J.-K. Shang, J. Xu, Y.-X. Li, Acta Phys. -Chim. Sin. 32 (2016)

1913-1928. [58]

X. Chen, Y.-S. Jun, K. Takanabe, K. Maeda, K. Domen, X. Fu, M. Antonietti, X.

Wang, Chem. Mater. 21 (2009) 4093-4095. [59]

Y. Li, Y. Gong, X. Xu, P. Zhang, H. Li, Y. Wang, Catal. Commun. 28 (2012) 9-12.

[60]

B. Long, Z. Ding, X. Wang, ChemSusChem 6 (2013) 2074-2078.

[61]

F. Su, M. Antonietti, X. Wang, Catal. Sci. Technol. 2 (2012) 1005-1009.

[62]

J. Xu, K.-Z. Long, Y. Wang, B. Xue, Y.-X. Li, Appl. Catal. A 496 (2015) 1-8.

[63]

P. Zhang, Y. Gong, H. Li, Z. Chen, Y. Wang, Nat. Commun. 4 (2013) 1593.

[64]

H. Watanabe, S. Asano, S. Fujita, H. Yoshida, M. Arai, ACS Catal. 5 (2015)

2886-2894.

23

B

3

3

1

1

1

Quantity adsorbed (cm g , STP)

dV/dD Pore volume (cm g nm )

A

100 0.0

0.2

0.4

0.6

0.8

0.02

1.0

1

10

p/p0

100

Pore size (nm)



Fig. 1 N2 adsorption–desorption isotherms (A) and pore size distributions (B) of CN (■) , 2Pd/CN ( ●) , 4Pd/CN (▲) , and 6Pd/CN (▼) materials.

111

Ln (I)

311

a

b c d e

1

0.5

1.0

1.5

2.0

1

q (nm )

Fig. 2 SAXS patterns of FDU-12 (a), CN (b), 2Pd/CN (c), 4Pd/CN (d), and 6Pd/CN (e) materials.

24

Fig. 3 TEM images of 2Pd/CN (A), 4Pd/CN (B), and 6Pd/CN (C) materials and the corresponding histograms of particle sizes.

25

Intensity (a.u.)

3000

f e d c b a 10

20

30

40

50

60

2(°)

Fig. 4 XRD patterns of CN (a), 2Pd/CN (b), 4Pd/CN (c), 6Pd/CN (d), 4Pd/AC (e), and 4Pd/CNTs (f).

Transmittance (%)

a

b c d

4000

3500

3000

2500

2000

1500

1000

500

1

Wave number (cm )

Fig. 5 FT-IR spectra of CN (a), 2Pd/CN (b), 4Pd/CN (c), and 6Pd/CN (d) materials.

26

C

B

A

407

405

2500

2500

2500

403

401

399

397

395 407

405

403

401

399

397

395 407

405

Binding energy (eV)

Binding energy (eV)

403

401

399

397

395

Binding energy (eV)



Fig. 6 N 1s spectra of CN (A), 2Pd/CN (B), and 4Pd/CN (C) materials.

Pd

2+

Pd

0

A 1000

350

348

346

344

342

340

338

336

334

332

346

344

342

340

338

336

334

332

B 1000

350

348

Binding energy (eV)



Fig. 7 Pd 3d spectra of 2Pd/CN (A), and 4Pd/CN (B) materials.

27

213

O2 desorbed (a.u.)

1 216

c 240

b

a 100

150

200

250

300

350

400

450

Temperature (°C)

Fig. 8 O2-TPD profiles of CN (a), 2Pd/CN (b), and 4Pd/CN (c) materials.

100

Conversion (%)

80

60

40

20

0 1

2

3

4

5

Running cycles



Fig. 9 Catalytic performance of 4Pd/CN for selective oxidation of benzyl alcohol during five catalytic runs. Reaction conditions: VBZA= 1 mL, Vtoluene = 10 mL, T = 80 °C, t = 3 h, and Wcatal. = 25 mg.

28

CO2 desorbed (a.u.)

0.5

d

c b a 100

150

200

250

300

350

400

Temperature (°C)

Fig. 10 CO2-TPD profiles of CN (a), 2Pd/CN (b), 4Pd/CN (c), and 4Pd/mpg-C3N4 (d).

Table 1 Surface areas, pore sizes, and volumes of various materials. SBET (m2 g−1)

Pore size (nm) a

Pore volume (cm3 g−1)

CN

591

7.7

1.34

2Pd/CN

535

6.9

1.17

4Pd/CN

501

6.7

1.06

6Pd/CN

425

6.5

0.87

4Pd/AC

738

4.0

0.40

4Pd/CNTs

143

2.7

0.74

Sample

Table 2 Catalytic performances of Pd/CN catalysts in selective oxidation of benzyl alcohol a. Sample

Conv.(%)

Sel.(%)

–

<1

100

29

CN

2±1

100

1Pd/CN

7±2

100

2Pd/CN

19±2

100

3Pd/CN

45±3

100

4Pd/CN

94±4

100

5Pd/CN

94±3

100

6Pd/CN

96±3

100

Table 4 Comparison of catalytic performance of various supported Pd catalysts in selective oxidation of benzyl alcohol a. Sample

Conv.(%)

Sel.(%)

TOF(h−1)

4Au/CN

10

100

–

4Pt/CN

4

100

–

4Pd/CN b

33

100

672

4Pd/mpg-C3N4

2

100

7

4Pd/AC

12

100

41

4Pd/CNTs c

25

100

170

Table 5 A tandem reaction of benzyl alcohol and methyl cyanoacetate catalyzed by 4Pd/CN a

30

Time (h)

Conv.(%) benzyl alcohol

methyl cyanoacetate

1

47±2

9±1

2

67±2

16±2

3

79±3

21±2

4

90±4

29±3

5

93±3

33±2

31