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Preparation and Characterization of Covalent Organic Polymer Supported Palladium Catalysts for Oxidation of CO and Benzyl Alcohol You Zhou,† Zhonghua Xiang,§ Dapeng Cao,§ and Chang-jun Liu*,†,‡ †

Tianjin Co-Innovation Center of Chemical Science & Engineering, Tianjin University, Tianjin 300072, China Advanced Nanotechnology Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China § Division of Molecular and Materials Simulation, State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China ‡

ABSTRACT: A novel covalent organic polymer, COP-4, with high thermal stability and large surface area, was used as the support to prepare Pd/COP-4 catalyst using conventional impregnation and hydrogen reduction. The obtained catalyst shows excellent activity for oxidation reactions in gas-phase (CO oxidation) and liquid-phase (oxidation of benzyl alcohol to benzaldehyde). The solvent-free aerobic oxidation of benzyl alcohol to benzaldehyde without any base was examined for the first time. The catalyst characterization indicates that the solvent used in the impregnation has an influence on the interaction between the active metal particle and the support. Different solvent results in different catalytic property. For Pd/COP-4 prepared with distilled water, it tends to physically adsorb many more CO molecules on the surface. This would be the reason that this catalyst has better activity for CO oxidation. The Pd/COP-4 catalyst prepared in DMF has a much larger surface area, smaller average particle size, and broader size distribution. It shows better activity and worse selectivity for selective oxidation of benzyl alcohol to benzaldehyde. topic. Various supporting materials,32−36 like Al2O3, TiO2, FeOx, graphene, and SBA-15, have been used to load palladium species. In this work, we attempted to load palladium (Pd) on COP-4, a new COP material, to prepare Pd catalyst with DMF or distilled water as the precursor solvent. The conventional impregnation and hydrogen reduction were applied to make COP-4 supported Pd catalysts. The performance of the obtained catalysts was tested using CO oxidation (gas phase) and solvent-free aerobic oxidation of benzyl alcohol to benzaldehyde without any base (liquid phase). We confirm that a different precursor solvent has a significant influence on the particle distribution, the metal−support interaction, and thus the catalytic performance. The COP-4 proves to be an excellent support for Pd catalysts used in oxidation reactions.

1. INTRODUCTION Hybrid porous solids, such as metal−organic frameworks (MOFs) and covalent-organic frameworks (COFs), have recently attracted more and more attention.1−5 For example, with high surface area,a tunable nature, and permanent microporosity, MOFs have emerged as promising new materials for gas storage, gas purification, sensors, drug delivery, heterogeneous catalysis, and many other applications.6−16 Recently, a new type of coordination polymer, covalent organic polymers (COPs), an analogue of MOFs, has been synthesized.17,18 The microporous COPs have exceptional hydrothermal stability, due to the composition of relatively stable covalent C−C, C−H and C−N bonds. Furthermore, powder-like COPs are more suitable for some practical applications such as separation and catalysis owing to the less severe mass transfer limitations. For instance, their application as catalyst support has proved promising.19 To do so, loading of metallic active species is very necessary. The oxidation of benzyl alcohol to benzaldehyde is one of the most important organic transformations, not only to fundamental study but also to the fine chemical production. For its clean and cheap properties, molecular oxygen is used as the oxidant, replacing the conventional stoichiometric oxidants which are usually toxic and expensive, to realize the so-called green oxidation process.20 In recent years, numerous supported metal catalyst systems have been used for the selective oxidation of benzyl alcohol, such as Mn catalysts,21 Ru catalysts,22,23 Au catalysts,24−26 and Pd catalysts,27−31 among which Pd catalysts have attracted more attention for their excellent catalytic performance. In addition, as one of the most extensively studied reaction in the history of heterogeneous catalysis, CO oxidation over supported Pd catalysts is also a very hot © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Preparation of Supported Pd Catalysts. The COP4 material, which was used as the catalyst support, was synthesized by subsequently adding 1,5-cyclooctadiene and 2,4,6tris-(4-bromo-phenyl)-[1,3,5] triazine to a solution of bis(1,5cyclooctadiene) nickel(0) and 2,2′-bipyridyl in dry DMF and reacting at 105 °C overnight under a nitrogen atmosphere. COP-4 was then obtained by filtrating, washing, drying, and purification. The details of the process have been reported before.18 Palladium species were introduced to the support by Received: Revised: Accepted: Published: 1359

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condenser to recover the vaporized mixture. In each reaction run, the mixture was heated to 160 °C under vigorous stirring (stirring rate 1000 rpm). Oxygen was bubbled into the mixture at a constant flow rate of 30 mL/min to initiate the reaction. After the allowed reaction time, the liquid organic products were analyzed using an Agilent 4890 gas chromatograph equipped with an HP-5 capillary column and a flame ionization detector (FID). The conversion, selectivity, and turnover frequency (TOF) are defined as follows:

wet impregnation with Pd(NO3)2 as a precursor. Two 5 wt % Pd/COP-4 samples were prepared as follows. The nonpretreated COP-4 was first impregnated with a DMF/aqueous solution of palladium nitrate for about 12 h. Then the samples were dried at 120 °C for another 6 h in vacuum. Before being used as a catalyst, the samples were reduced by flowing pure H2 at 300 °C for 1 h with a flow rate of 20 mL/min. They were denoted as Pd/COP-4-DH (DDMF, H = hydrogen reduction) and Pd/COP-4-WH (W = water), respectively. For comparison, 1 wt % Pd/MOF-5 using DMF as the solvent was prepared in the same way, and denoted as 1% Pd/MOF-5-DH. The MOF-5 material was synthesized according to Huang’s work,37 with a surface area of 974 m2/g. The material sacrificed some surface area, to gain better hydrothermal stability. Different from the typical oxide-supported catalysts, all samples here were not calcined, for the special thermal property of coordination polymers.11,12 2.2. Characterization of the Supported Pd Catalysts. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to estimate the metal dispersion. The XRD patterns were recorded with a Rigaku D/MAX-2500 V/PC using Cu radiation (40 kV and 200 mA). The TEM observations were performed using a Philips TECNAI G2F20 system. The mean particle diameter was calculated from the mean frequency distribution by counting ca. 200 particles. Inductively coupled plasma mass spectrometry (ICP-MS) was performed using an Agilent 7700x instrument. The loading content of Pd decided by ICP-MS is 4.9% for Pd/COP-4-DH, 4.7% for Pd/ COP-4-WH, and 1.0% for the 1% Pd/MOF-5-DH sample, respectively. N2-physisorption for determining the specific surface area and pore size distribution was performed at liquid nitrogen temperature using an Autosorb-1 analyzer from Quantachrome Instruments (AUTOSORB-1-C). The samples were degassed at 150 °C for 6 h under vacuum before analysis. The Langmuir and BET methods were employed to calculate the specific surface areas. The pore size distributions were derived from the isotherms using the HK model. Thermogravimetric analysis (TGA) was performed to determine the thermal stability of the samples. It was carried out under a N2 atmosphere (total flow: 30 mL/min) at a constant rate of 10 °C/min, using a Netzsch STA 449 F3 system. The IR spectra were obtained on a BRUKE Tensor-27 spectrometer equipped with a diffuse reflectance accessory. For diffuse reflectance Fourier transform infrared (DRIFT) spectra of adsorbed CO, the catalyst samples were purged by He (20 mL/min) at 150 °C for 1 h and then exposed to 20 mL/min CO (1.11 vol. %)/helium at 25 °C for 30 min. After the cell was flushed with He for another 30 min, the DRIFT spectra were recorded at a resolution of 4 cm−1 and 64 scans. To get the FT-IR spectra, 0.5 mg of the sample was mixed thoroughly with 200 mg of homogenized porcelain-milled KBr (FT-IR grade), and pressed into a wafer. Then the wafer was put into the sample holder and FT-IR spectra were recorded at a resolution of 4 cm−1 and 64 scans. 2.3. Catalytic Activity. The solvent-free aerobic oxidation of benzyl alcohol using molecular O2 without any base was carried out in a batch-type reactor operated under atmospheric conditions. A three-necked glass flask (capacity 50 mL), precharged with benzyl alcohol and catalyst, was used to conduct the experiments. The mixture was stirred using a magnetic stirrer and heated in a silicon oil bath. The system was equipped with a thermocouple to control the temperature, and a reflux

conversion (%) = selectivity (%) =

mol of reactant converted × 100% mol of reactant in feed mol of product formed × 100% mol of reactant converted

mol of reactant converted (mol of active sites)(reaction time) NrX rM m ≈ mCat X mDmt

TOF(h‐1) =

where Xr is the conversion of benzyl alcohol at certain reaction temperature; Nr is the initial amount of benzyl alcohol in unit of mol; mCat is the amount of catalyst; Xm is the noble metal loading percent in the catalyst; Dm is the dispersion of the noble metal, estimated from the TEM mean particle diameter with the assumption that the particles are cubic;38,39 Mm is the molar weight of the noble metal (106.42 g/mol for Pd); t is the reaction time in unit of hour. The catalytic oxidation of CO was carried out in a quartztube (i.d. 4 mm) fixed-bed reactor. The catalyst (10 mg, powder) was pretreated at 300 °C in a flow of 20 mL/min pure H2 for 1 h. After evacuating H2 with Ar and cooling to the room temperature, the gas mixture (total flow =20 mL/min, 1.0 vol.% CO/20 vol.% O2/balance N2) was fed into the reactor. For each temperature point, there was duration of 40 min. The effluent was analyzed using an online gas chromatograph (Agilent 6890) equipped with a Porapak Q column and a thermal conductivity detector (TCD). CO conversion(%) mol of COin − mol of COout = × 100% mol of COin TOF(h‐1) = ≈

mol of reactant converted (mol of active sites)(reaction time)

FrX rM m mCat X mDm

where Xr is the conversion of CO at certain reaction temperature, Fr is the amount of CO at inlet in unit of mol/s, and other definitions are the same as those for TOF of the benzyl alcohol oxidation.

3. RESULTS AND DISCUSSION 3.1. Characterization of Supported Pd Catalysts. X-ray diffraction was carried out to identify the palladium phases in the samples. The patterns of the support and the samples are shown in Figure 1, which have been reported in Figure S2 in our previous work.19 A broad diffraction peak at ca. 25°, observed for all samples especially the COP-4 support, can be ascribed to the amorphous framework of COP-4. The reflection intensity weakens a lot after the loading of Pd species. Just as 1360

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More information concerning the dispersion of Pd species was obtained by TEM characterization. The images of the support and two samples are shown in Figure 2, which have been mentioned in Figure S1 in our former work.19 The particles of the Pd/COP-4-WH sample highly disperse on the layered support with a uniform size of about 9.9 nm, as shown in Figure 2d−f. By contrast, for the Pd/COP-4-DH sample, some particles are smaller than 5 nm, while others form inhomogenous agglomeration with much larger sizes. Figure 2g−i obviously reveals the nonuniform of the particle size and dispersion on the Pd/COP-4-DH. The mean size is 7.7 ± 1.8 nm. In the high-resolution transmission electron microscopy (HRTEM) images, some particles in the Pd/COP-4-WH sample show distinct lattice fringes. As shown in Figure 2c, the lattice fringes with d = 0.224 nm could be attributed to the Pd(111) planes, which implies the Pd species on the Pd/COP4-WH sample are metallic palladium. Comparing the images of the support before and after the loading of Pd species, we can find a minor change in the structure, which is obvious for the Pd/COP-4-DH sample. However, for both two Pd contained samples, the layered property does not change, indicating the basic structure of the support stays intact. The result of TEM confirms that of XRD, demonstrating that the solvents will influence the property of the supported catalysts prepared by an impregnation method. The initial state of Pd species can be determined by the DRIFT spectra in Figure 3. Before the analysis, all samples were dried at 150 °C for 6 h in vacuum to remove the residual solution and moisture. A30 and D30 mean absorbing CO for 30 min and desorbing CO for another 30 min after absorbing, respectively. The broad bands appear only in the A30 curve for all samples centered at 2174 and 2118 cm−1 and are attributed to the gaseous CO adsorption, which disappear after removal of the physisorptive CO. For Pd/COP-4-WH, the band centered

Figure 1. XRD patterns of (a) COP-4, (b) Pd/COP-4-WH, and (c) Pd/COP-4-DH.19

the results that have been well studied for zeolites and mesoporous silica materials,40,41 the decrease of the reflection intensity with respect to the parent, empty COP-4 is a consequence of the inclusion of guest molecules in the framework. In both Pd-contained samples, the cubic form of Pd could be identified by the reflections at 2θ = 40.1° for the Pd (111) plane, 46.7° for the Pd (200) plane, and 68.1° for the Pd (220) plane. However, the Pd peaks are more intense for the Pd/COP-4WH sample and narrower for the Pd/COP-4-DH sample, reflecting the presence of a higher percentage of crystalline Pd particles in the former sample and bigger ones in the latter one. The XRD results indicate that the use of different dipping solutions directly influences the crystalline structure of the Pd species and the size of the particles.

Figure 2. TEM images of COP-4 (a and b), Pd/COP-4-WH (c−f), and Pd/COP-4-DH (g−i).19 1361

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more adsorption sites for CO than Pd/COP-4-DH. The result further confirms the influence of the solvent on the property of the catalysts. Thermal stability is one of the most important properties of the samples. Then thermogravimetric analysis (TGA) was performed to check that. Before the analysis, all samples were dried at 150 °C for 6 h in vacuum. As shown in Figure 4, COP4 is stable in inert gas up to 600 °C. After Pd species are loaded and reduced, the curves present different trends. For Pd/COP4-WH, there is a little larger weight loss between 300 and 650 °C than that of COP-4, but the two curves meet after 750 °C. It indicates that the structure of the support in Pd/ COP-4-WH has negligible change. The earlier weight loss process should be promoted by the Pd particles attached on the support. The weight loss step between 600 and 750 °C results from the decomposition of the COP-4 material. However, for Pd/COP-4-DH, the curve is much different from that of COP-4. The weight of the sample keeps decreasing as the temperature rises. The weight loss process between 150 and 600 °C may be attributed to the removal of the guest molecules (DMF mainly) which interact with the support and Pd particles with different strength, as well as minor structure change induced by the Pd particles. The weight loss step between 600 and 750 °C, just as that emerges in the other two curves, is attributed to the decomposition of the support COP-4. The DSC curves also show that there is a broad exothermic peak between 300 and 600 °C just in the curve of the Pd/COP-4-DH sample, which may be triggered by the minor structure change. The endothermic peak before 200 °C in the curve of Pd/COP-4-WH may be due to the evaporation of the moisture. The exothermic peak that appears at 740 °C for all samples results from the decomposition of the COP-4 support. The results imply that the structure of the support has no big change after the loading of Pd species and H2 reduction. However, the use of different solvents in the process of impregnation influences the interaction between the Pd species and the support and as a consequence the thermal stability of the sample. Obviously, the residual of the Pd/COP-4-DH is active at high temperature; hence, the weight of the sample keeps decreasing in the tested range. We estimate that DMF has a strong interaction with Pd species and the COP-4 material, while distilled water just plays the role of carrier for Pd ions. The interaction changes the stability of the COP-4 material in Pd/COP-4-DH.

Figure 3. DRIFT spectra of CO absorbed on (a) COP-4, (b) Pd/ COP-4-WH, and (c) Pd/COP-4-DH at 25 °C (down: absorbing for 30 min; up: desorbing for 30 min).

at 2051 cm−1 changes into a broad and low one after removal of the weak-adsorbed CO, which can be attributed to linearly bonded CO on Pd0. Furthermore, the intensive peak centered at 1958 cm−1 and the should at 1921 cm−1 shift to 1937 and 1894 cm−1 after desorbing CO for 30 min. The two peaks are attributed to bridge-bonded CO on Pd (111) and 3-fold hollow-bonded CO on Pd (111), respectively.31,42 The red shift is due to the removal of gaseous CO which brings a stronger interaction between the residual CO and the Pd species. As for Pd/COP-4-DH, the broad and low band centered at 2051 cm−1 in the D30 curve could also be attributed to linearly bonded CO on Pd0. The band centered at 1937 cm−1 in the D30 curve, resulting from the red shift of the band at 1950 cm−1 in the A30 curve, is attributed to bridge-bonded CO on Pd (111).31,42 The results of DRIFT spectra indicates that the initial state of the Pd species on both the Pd/COP-4-WH and Pd/COP-4-DH is Pd0. It coincides well with the results of XRD and TEM. However, the different intensity and position of the peaks for the two samples indicate that there are different types and quantity of defect sites on the two samples. It should originate from the different metal−support interaction in the two samples. It is obvious that the Pd/COP-4-WH sample can supply much

Figure 4. TGA (left) and DSC (right) curves of (a) COP-4 (black), (b) Pd/COP-4-WH (red), and (c) Pd/COP-4-DH (blue). 1362

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The −CH2− group should originate from the trace amount of DMF which remains in the Pd/COP-4-DH and interacts with Pd particles. We can also recognize the peaks with much weaker intensity at the same positions in Pd/COP-4-WH. The trace DMF contained in Pd/COP-4-WH comes from the preparation process of the COP-4 material, during which DMF is used as the solvent. Though the bare COP-4 material may also contain trace DMF in the pores, no interaction between them can be detected by FT-IR before the loading of Pd species, which should be the reason for the absence of the peaks in the bare COP-4 material. Figure 6 shows the N2 adsorption−desorption isotherms and pore size distributions of the samples. The specific surface areas of the samples are summarized in Table 1. The a little lower surface area of COP-4 shown in this work compared with the previous published results may arise from the different measurement equipment and its initial kinetically covalent reaction, which irreversibly form covalent bonds.18 The hysteresis loops in the isotherms for all the samples are ascribed to the swelling effect of soft porous organic materials. The isotherms of the two Pd/COP-4 samples have similar shape with that of COP-4. What’s more, they have nearly the same pore size distribution calculated by HK method. It implies that the pore structure of COP-4 has not changed a lot after Pd species are loaded and reduced by H2. However, the specific surface areas calculated by Langmuir method, have decreased from 1987 m2/g for COP-4 to 757.9 m2/g for Pd/COP-4-WH and increased to 2205 m2/g for Pd/COP-4-DH. The average particle size of Pd/COP-4WH is about 10 nm, which is much larger than the pore diameter of COP-4 about 1.2 nm. This implies that the Pd species are on the surface of Pd/COP-4-WH and may block the pores. As for Pd/COP-4-DH, the reason for the increase of surface area may be due to the interaction among DMF, Pd particles, and the support. The interaction brings a minor structure change to the support, just as the results of TGA and FTIR show. However, the basic pore structure does not change. Especially, COP-4 is a kind of lipophilic but hydrophobe material. The solvents, distilled water and DMF, will have different interactions with it. After H2 reduction, the dispersion of the particles and the surface areas are different from those reported in our previous work with the room temperature electron reduction.8,19 Obviously, the higher temperature (300 °C) and hydrogen have important influences. For Pd/COP-4-DH, a high temperature in H2 reduction may bring DMF as well as the particles out of the pores again. H2 may break the interaction among the support, solvent and Pd particles. On the other hand, for Pd/COP-4-WH, a high temperature under H2 atmosphere just leads to larger Pd particles. The results agree well with those of the former characterizations, further proving that different solvents for impregnation will bring distinct properties for the samples. All characterizations indicate that the roles of DMF and distilled water in the preparation of the catalysts are very different. Distilled water leads the palladium ions onto the surface of COP-4, while DMF not only leads the ions into the pores of the support but also they have interaction with the palladium species and the support. Taking the results in our previous work in consideration,19 we conclude that the minor structure change in Pd/COP-4-DH is due to the H2 reduction process. During the high temperature of 300 °C, H2 may change the interaction between DMF and palladium species and then lead the Pd particles to the surface of the support gathering into different sizes.

To explore the interaction among solvents, Pd species, and the support, FT-IR analysis was performed. The FT-IR spectra in the above part of Figure 5 show strong absorption bands for

Figure 5. FT-IR spectra of (a) COP-4, (b) Pd/COP-4-WH, and (c) Pd/COP-4-DH after drying at 150 °C for 6 h in vacuum.

all samples. The peaks at 3062 and 3038 cm−1 resulting from C−H stretching vibration in benzene ring, and the peaks between 1450 and 1620 cm−1 resulting from CC stretching vibration in benzene ring indicate the existence of a benzene ring in all samples. The peak centered at 1360 cm−1 in all samples can be attributed to the CN stretching vibration. No obvious peak originating from NO3− (1380 cm−1) can be found for the two Pd/COP-4 samples. The residual nitrate species are negligible. The similar shape of the samples implies that the basic structure of COP-4 has not changed after the loading of the Pd species and H2 reduction. However, compared with the bare COP-4 material, some peaks of the two Pd/COP-4 samples weaken or disappear. A minor difference can be identified in the enlarged partial spectra, as shown in the below part of Figure 5. The intensity of the peaks centered at 3062 and 3038 cm−1 weakens after the loading of Pd species and H2 reduction, especially the Pd/COP-4-DH sample. The peaks centered at 2921 and 2854 cm−1 in Pd/COP-4-DH could be ascribed to C−H stretching vibration in −CH2−. As discussed in our previous work,19 there is no −CH2− group in COP-4. 1363

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Figure 6. Isotherms (left) and HK pore size distribution (right) of (a) COP-4, (b) Pd/COP-4-WH, and (c) Pd/COP-4-DH.

dissolve in the mixture before the reaction. However, under the reaction temperature, the solubility of O2 in benzyl alcohol is very low. The sample with a larger surface area can supply more adsorption sites for the reactants, while the sample with smaller particles can supply more active sites. That is to say, COP-4 provides the adsorption sites, while the Pd species serves as the active sites. Comparing the two Pd/COP-4 samples, we can find that there is much difference. The conversion and TOF value over the Pd/COP-4-DH are much higher than those of Pd/COP-4-WH, but the selectivity toward benzaldehyde is much lower. However, the yield (conversion times selectivity) is essentially the same for both catalysts at 0.18. The Pd/COP4-DH has a much larger surface area and smaller average particle size, which may supply more adsorption sites for the reactants and more active sites. However, the particle size distribution of Pd/COP-4-DH is broad, while the particle sizes of Pd/COP-4-WH are similar. Therefore, the Pd/COP-4-DH sample has a better activity and a worse selectivity. From Sankar’s work,43 we know that the Pd species can catalyze the disproportionation reaction in which two molecules of benzyl alcohol interact to give equal amounts of benzaldehyde and toluene. Then from the reaction results, we can estimate that most of the active sites in Pd/COP-4-DH are for the disproportionation reaction. Figure 7 shows the time-dependence on benzyl alcohol conversions and benzaldehyde selectivity catalyzed by the two Pd/ COP-4 samples. It is obvious that benzyl alcohol conversions increase nearly linearly with prolonged reaction time for both samples. The conversions of benzyl alcohol have little difference for the two samples during the 8 h testing time. In contrast to the results in Table 2, we find that the dosage of the

Table 1. Pore Structure Parameters of the Samples

COP-4 Pd/COP-4-WH Pd/COP-4-DH

Langmuir (m2/g)

BET (m2/g)

average pore diameter (nm)

1987 757.9 2205

1256 474.9 1392

1.2 1.1 1.2

3.2. Catalytic Activities in Benzyl Alcohol Selective Oxidation. The oxidation of benzyl alcohol to benzaldehyde was carried out to estimate the activity of the Pd/COP-4 samples in liquid-phase reactions. The results of solvent-free benzyl alcohol selective oxidation with molecular O2 over the samples are listed in Table 2. We can find that, for all samples tested, benzaldehyde is the main product, with different percents of toluene as the byproducts. The support COP-4 without loading of Pd species displays even better activity than the 1% Pd/MOF-5 sample, which indicates that COP-4 should be a good catalyst support for the reaction. In the 1% Pd/ MOF-5 sample, the Pd particles may distribute in the micropores of the MOF-5 sample, which are hard to reach for the reactant molecular, resulting in the low activity of the sample. The TOF values of Pd/COP-4-DH and Pd/COP-4WH are 95 411 and 65 924 h−1, respectively, while it is 11 877 for 1% Pd/MOF-5. It indicates that both Pd/COP-4 samples show much better activity than the 1% Pd/MOF-5 sample. What’s more, the TOF values of the Pd/COP-4 samples are also much higher than those of Pd/CeO2 (17 572 h−1) and Pd/ MnOx (9526 h−1) under nearly the same reaction conditions.20 The high specific surface area of COP-4 support and uniform distribution of the Pd particles may play an important role in the excellent activity. As one of the reactants, O2 should

Table 2. Catalytic Results of Benzyl Alcohol Oxidation over Different Samplesa selectivity (%) catalyst

size (nm)

dispersion

conversion (%)

benzaldehyde

toluene

TOF (h−1)

COP-4 Pd/COP-4-DH Pd/COP-4-WH 1%Pd/MOF-5-DHb

7.7 ± 1.8 9.9 ± 0.2 2.5 ± 0.1

0.145 0.113 0.448

6.2 32.0 20.1 5.1

100 57.1 89.8 99.4

0 42.9 10.1 0.6

95 411 65 924 11 877

Reaction conditions: Catalyst, 4 mg; benzyl alcohol, 100 mmol; O2, 30 mL/min; temperature, 160 °C; reaction time, 1 h. bReaction conditions: Catalyst, 5 mg; benzyl alcohol, 50 mmol; O2, 30 mL/min; temperature, 160 °C; reaction time, 1 h. a

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Figure 7. Benzyl alcohol conversions and benzaldehyde selectivity catalyzed by the two Pd/COP-4 samples. Reaction conditions: catalyst, 2 mg; benzyl alcohol, 100 mmol; O2, 30 mL/min; temperature, 160 °C; reaction time, 8 h.

not be detected by ICP-MS, and the conversions kept nearly constant in the subsequent reaction after the removal of the catalysts. 3.3. Catalytic Activities in CO Oxidation. CO oxidation was performed to assess the catalytic functionality of the Pd/ COP-4 samples for gas-phase reactions. The reaction was carried out from room to elevated temperatures. The results depicted in Figure 8 show the CO conversion as a function of the reaction temperature. Pd/COP-4-DH shows obvious activity for CO oxidation from 100 °C. The activity promotes slowly until 140 °C. The conversion of CO increases substantially after 140 °C and reach 100% at 170 °C. As for Pd/ COP-4-WH, the activity starts at 60 °C and improves gradually until 120 °C, at which the conversion of CO starts to increase significantly. The complete conversion of CO realizes at 150 °C, which is 20 °C lower than that of Pd/COP-4-DH. The result of DRIFT has shown that there are many more CO molecules physically adsorbing on the surface of Pd/COP-4WH. It should be the reason the Pd/COP-4-WH sample has better activity for CO oxidation. The bare COP-4 material shows no activity for the reaction even up to 320 °C. We have also checked the activity of 1%Pd/MOF-5-DH in the same reaction condition. To make the amount of Pd species in agreement, the dosage of 1% Pd/MOF-5-DH is 50 mg. We can find that the complete conversion of CO realizes at 185 °C over 1% Pd/MOF-5-DH, which is 35 and 15 °C higher than that over Pd/COP-4-WH and Pd/COP-4-DH, respectively. TOF values of the samples at 130 and 140 °C are shown in Table 3. At 130 °C, the TOF value of Pd/COP-4-WH is 72.9 × 10−3 s−1, nearly 5 times that of Pd/COP-4-DH and 40 times that of 1% Pd/MOF-5-DH. At 140 °C, the TOF value of Pd/COP-4-WH is 145.8 × 10−3 s−1, more than 5 times that of Pd/COP-4-DH and 45 times that of 1% Pd/MOF-5-DH. Both the two Pd/ COP-4 samples show better activity than 1%Pd/MOF-5-DH,

Figure 8. Conversion of CO over the Pd/COP-4 samples as a function of temperature.

catalyst have a stronger influence on the benzyl alcohol conversion of the samples, especially for Pd/COP-4-DH. In other words, there should be an optimal rate of catalyst dosage to the alcohol dosage. For Pd/COP-4-DH, the conversion of benzyl alcohol falls from 32.0% for 1 h to 22.2% for 2 h when the dosage reduces by half, while the selectivity toward benzaldehyde increases a lot. For both the samples, the selectivity toward benzaldehyde has the highest value in the beginning at 2 h. Then, the value decreases with the increasing reaction time. However, Pd/COP-4-WH has higher selectivity toward benzaldehyde than Pd/COP-4-DH throughout the whole testing time. It may result from the uniformity of the Pd particles on the Pd/COP-4-WH. Furthermore, after the reaction, the catalysts were removed from the reaction mixture by filtration. The leaching amounts of palladium species in the filtrate could

Table 3. Catalytic Results of CO Oxidation over Different Samples catalyst

size (nm)

dispersion

TOF130°C × 103 s−1

TOF140°C × 103s−1

Pd/COP-4-DH Pd/COP-4-WH 1%Pd/MOF-5-DH

7.7 ± 1.8 9.9 ± 0.2 2.5 ± 0.1

0.145 0.113 0.448

14.9 72.9 1.8

25.8 145.8 3.2

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indicating COP-4 is a suitable substitution of MOF material as the support used in gas-phase reaction.

4. CONCLUSIONS The present work demonstrated that the Pd/COP-4 catalysts prepared with an impregnation method and H2 reduction have excellent activity for both the solvent-free aerobic oxidation of benzyl alcohol to benzaldehyde and CO oxidation. It indicates that the Pd/COP-4 catalysts have a great potential for oxidation reactions in gas and liquid phases. For its high hydrothermal stability and the high dispersion of the particles on Pd/COP-4 samples, COP-4 proves to be a good material as the support to disperse metal particles. Our results show that the solvents used in the process of preparing the catalysts will influence the interaction between the support and the metal particles and thus the distribution of the particles and the reaction activity. DMF used in this contribution can build a bridge between the COP-4 support and the Pd particles, which will vary after the H2 reduction. Inspired by the results, we can try the Pd/COP-4 samples in many other reactions and expect good results.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 22 27406490. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the national natural science foundation (20990223). D.C is thankful to Huo Yineong Foundation (121070), National 973 Program (2011CB706900), NSF of China (21274011).

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