Article pubs.acs.org/IECR
Carbon Nanotubes and Activated Carbons Supported Catalysts for Phenol in Situ Hydrogenation: Hydrophobic/Hydrophilic Effect Yizhi Xiang,†,‡ Lingniao Kong,†,§ Pengyang Xie,† Tieyong Xu,† Jianguo Wang,†,* and Xiaonian Li†,* †
Industrial Catalysis Institute of Zhejiang University of Technology, State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, Hangzhou 310014, People’s Republic of China ‡ Chemical Physics of Materials (Catalysis-Tribology), Université Libre de Bruxelles, Campus Plaine, CP 243, 1050 Brussels, Belgium § Zhejiang Zanyu Technology Company Ltd., Hangzhou 310014, People’s Republic of China ABSTRACT: Carbon nanotube (CNTs) and activated carbon (AC) supported Pd and Ni catalysts were prepared for the (in situ) hydrogenation of phenol to cyclohexanone and cyclohexanol. The hydrophobic/hydrophilic properties of the catalysts were tailored by pretreating the carbonaceous support with HNO3 at various conditions and characterized by X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and transmission electron microscopy (TEM). The catalytic results suggested that Pd and Ni supported on CNTs show significantly higher activity than that supported on ACs. Pretreating the CNTs with HNO3 increases the local hydrophilicity of the active phase (by introducing oxygenated groups), which result in an increase in the cyclohexanone selectivity and strongly decrease the phenol conversion. The first-principles density functional theory calculation suggested that the adsorption/desorption behaviors of phenol, methanol, H2O, and cyclohexanone on the catalysts might be influenced highly by the hydrophobic/hydrophilic properties. The hydrophilic catalysts show high selectivity in cyclohexanone by lower conversion in phenol or vice versa.
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INTRODUCTION Tailoring heterogeneous catalysts for industrial application requires a proper understanding of the properties of the catalyst and the (surface) interactions that occurred during the reaction. Although the properties (size, shape, and so on) of the active phase, the effects of promoters, and the properties of supports (Brunauer−Emmett−Teller (BET) surface area, acidity/ basicity) have been extensively studied in the literature, the hydrophobic/hydrophilic properties of the catalysts have been of less concern. It is known that the hydrophobicity/ hydrophilicity would affect highly the adsorption/desorption behavior of molecules on the surface of the catalyst.1 Such properties are particularly important in designing catalysts for those reactions using an aqueous solvent or those reactions that produce water as the byproduct (hydrogenation of -NO2, for example).2−4 Selective hydrogenation of phenol to cyclohexanone or cyclohexanol is an important industrial process in manufacturing Nylons 6 and 66.5−8 A recent study by Matos and Corma has found that the hydrogenation of phenol produces cyclohexanone over a polar TiO2−C supported Pd catalyst while it produces cyclohexanol over a hydrophobic TiO2−C supported catalyst.2 Prior to the work of Matos and Corma, Makowski et al.3 had reported a method for the preparation of hydrophilic carbon supported Pd catalysts through hydrothermal carbonization of furfural. The catalyst was employed for phenol hydrogenation and has shown very high selectivity in cyclohexanone. These references further prove that the hydrophilicity of the catalyst could affect highly the catalytic performance in phenol hydrogenation. In order to investigate systematically the effect of hydrophobicity/hydrophilicity on the catalytic performance of phenol hydrogenation, it requires that the only tuning parameter is the hydrophobicity/ © 2014 American Chemical Society
hydrophilicity of the same support. However, Makowski et al.3 used hydrophilic monodisperse carbon spheres and commercial carbon as the support for Pd catalysts. In fact, two such kinds of carbon materials have different geometric structures, such as pore size and distribution. Matos and Corma tuned the hydrophilicity of their catalyst by functionalization of the TiO2−C hybrid support,2 which is too complicated for industrial application and analysis of the nature of the support. Indeed both studies are mainly focused on the preparation of materials (catalysts); for example, Makowski et al.3 reported a method for the preparation of hydrophilic carbon supported Pd catalysts, while Matos and Corma2 focused on the catalytic performance of TiO2−C hybrid materials by presenting only a brief discussion on the influence of hydrophilicity and hydrophobicity. It is seen that the study of the relationships between the hydrophilicity and catalytic performance is still lacking. Carbon materials, such as carbon nanotubes (CNTs) and activated carbons (ACs), have been considered as good supports for heterogeneous catalysts.9,10 Generally, the asobtained carbonaceous materials have to be pretreated by HNO3 or other oxidants to remove the impurities and increase the number of defects for anchoring the metal particles.11−13 The effects of HNO3 or hydrothermal pretreatment on the Pd particle size distribution on ACs have been reported in our previous studies.13,14 It is observed that such pretreatments not only changed the size of the Pd particle but also altered strongly the hydrophilicity (increased the oxygen content) of the Received: Revised: Accepted: Published: 2197
October 18, 2013 January 23, 2014 January 24, 2014 January 24, 2014 dx.doi.org/10.1021/ie4035253 | Ind. Eng. Chem. Res. 2014, 53, 2197−2203
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were collected from 0 to 1000 eV at a pass energy of 160 eV so as to fast quantify the oxygen content in the carbon supports. In order to qualitatively analyze the types of oxygenates, highresolution scans for C 1s and O 1s were performed over the ranges of 0−1100 eV at a pass energy of 40 eV. The XPS data analysis was performed with the XPSPeak4.1 program. The background was subtracted use a Shirley function. The C 1s and O 1s XPS signals were fitted with mixed Lorentzian− Gaussian (L-G) curves; the L-G parameter was set as 20%. The morphology and Pd particle size distribution of the Pd/CNTs and Pd/ACs catalysts were determined by a Tecnai G2 F30 STwin transmission electron microscope at an operating voltage of 300 kV. Phenol Hydrogenation and H2 Production. The in situ hydrogenation of phenol was carried out in an 8 mm i.d. stainless steel tubular reactor at 493 K and 3.5 MPa (Ar pressure). The catalyst (0.5 g) was loaded in the isothermal region of the reactor. A mixed solution of phenol (Hangzhou Shuanglin Chemical Reagents Factory, Cphenol = 0.2 mol·L−1) and aqueous methanol (Quzhou Juhua Reagents Co. Ltd.; the molar ration of CH3OH to H2O is 1:8) was fed at 0.1 mL· min−1 (LHSV = 3.87 h−1) into the reactor using a HPLC pump (PK564AN-TG10-A2). The gases and condensates were separated in a stainless steel vessel (about 60 mL). The liquid effluent (containing water, methanol, phenol, cyclohexanol, cyclohexanone, and sometimes a negligible amount of o-cresol and cyclohexane) of the reactor was sampled and analyzed by gas chromatrography−mass spectrometry (GC-MS, Agilent6890 GC-5973 mass spectrometer equipped with a 30 m HP-5 capillary) with the external standard method. The gas effluent (containing H2 and CO2) was analyzed by an online GC (Fuli 9790, Porapak Q and 13X molecular sieves columns) equipped with a thermoconductive detector (TCD). Aqueous-phase re-forming (APR) of methanol for H2 production and hydrogenation of phenol with gaseous H2 were carried out in the same tubular reactor under identical conditions. A 10 wt % aqueous solution of methanol (for APR), or a mixed solution of methanol and phenol (Cphenol = 0.2 mol· L−1) and 10 mL·min−1 H2 (for phenol hydrogenation with gaseous H2), was fed into the reactor, and the reaction was carried outaccording to the same procedure as previously described. DFT Calculation. The first-principles DFT calculations were performed using the DMol3 module in Materials Studio.15,16 The generalized gradient approximation (GGA) with PW91 functional17 is used to describe the exchangecorrelation (XC) effects. The double numerical plus polarization (DNP) basis set is used in expanded electronic wave functional. In this study, the periodic supercells of (5, 5) CNTs were adopted with optimized lengths of a, b, and c lattices of 25.00, 20.00, and 9.76 Ǻ . The length of c is four times of that in the peridodicity of (5, 5) CNTs. The minimum distance between opposing sidewalls of neighboring CNTs is bigger than 9.30 Ǻ , which can avoid interaction among repeating supercells. In order to describe the hydrophilic properties of CNTs, several typical oxygenated groups (for example OH and O) are adsorbed on part of the side walls of the CNTs. The Brillouin zone is sampled by 1 × 1 × 2 k points using the Monkhorst−Pack scheme for CNTs. All of the atoms were fully relaxed during the geometry optimization. For all of the calculations, the convergence in energy and force was set to 10−5 eV and 2 × 10−3 eV·Å.
catalyst. So, it is our intention to correlate the hydrophobic/ hydrophilic effect with the macro catalytic performance to understand the science behind it. In this study, we present the effect of hydrophobicity/ hydrophilicity of the CNTs and ACs supported Pd and Ni catalysts in the in situ hydrogenation of phenol to cyclohexanone and cyclohexanol. The hydrophilicity of the CNTs was tuned by simply pretreating with HNO3 at various conditions and characterized by temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The firstprinciples density functional theory (DFT) calculation was employed to simulate the effect of hydrophilicity/hydrophobicity on the adsorption/desorption behaviors of the reactants (phenol, methanol, and H2O) and product (cyclohexanone) on the surface of the catalysts.
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EXPERIMENTAL SECTION Catalysts Preparation. CNTs were (provided by Prof. W. Fei from Tsinghua University) prepared using CH4 as carbon sources over a Fe based catalyst. ACs (prepared from coconut shell) were purchased from Shaowu Xinsen Chemical Industry Co. Ltd. The preparation of CNTs and ACs supported Pd or Ni catalysts has been reported in our previous work.15 An aqueous solution of H2PdCl4 or Ni(NO3)2·6H2O was added into an aqueous slurry of the carbonaceous support (CNTs or ACs as obtained or pretreated by HNO3). The ratio of support to water was 1 g:10 mL. The nominal metal loading is 3 wt %. The slurry was vigorously stirred at room temperature for 2 h before the pH value was adjusted slowly to 8−10 by 10 wt % sodium hydroxide, under which condition complete precipitation of the metal ions was realized. Finally, the slurry was washed by distilled water and dried under vacuum at 383 K for 10 h. The pretreatments of the CNTs or ACs were carried out in a round-bottom flask with HNO3 at various concentrations (3%, 10%, and 30%). The aqueous slurry was stirred (∼500 rpm) at 363 K (or 333 K) for 6 h (or 3 h). The samples were then washed with distilled water until pH = 7, and dried under vacuum at 393 K overnight. Untreated CNTs have been denoted as “ut-CNTs”, and HNO3 pretreated CNTs have been denoted as “pt-CNTs” or “CNTs-x%”, where x% represent the concentrations of HNO3. The same notation was also applied to ACs. Catalysts Characterization. TPD study was carried out in an 5 mm i.d. quartz tubular reactor equipped with an online mass spectrometer (OmnistarTM). Typically, 0.1 g of sample was loaded at the isothermal region of the reactor and was heated to 1123 at 10 K min−1 in Ar at 30 mL·min−1. The outlet flow of the reactor was detected by the mass spectrometer; desorption of carbon species (CO and CO2 m/z = 28 and 44) were of concern. CO uptake was measured on the same setup for the TPD through a pulse chemisorption method at ambient temperature and pressure. A 0.5 mL aliquot of CO was injected into the reactor by a six-way valve using 30 mL of He as the carry gas. The injection was repeated for at least 10 times until a constant CO signal height (peak area) was reached. The CO peak area was quantified to calculate the amount of CO chemisorption. XPS study was performed on a Kratos AXIS Ultra DLD instrument at room temperature and 3 × 10−9 mbar using Mono Al Kα radiation. The energy scale was calibrated versus adventitious carbon (C1s peak at 284.8 eV). Fast survey scans 2198
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Figure 1. Temperature-programmed desorption profiles of the Pd/AC and Pd/CNTs catalysts with or without HNO3 pretreatment: (left panel) m/ z = 44 (CO2); (right panel) m/z = 28 (CO + CO2). CNTs (10%) and ACs (10%) mean the support was pretreated with 10% HNO3.
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RESULTS AND DISCUSSION Characterization of the Carbonaceous Supports and Catalysts. The physicochemical properties of the Pd/CNTs and Pd/ACs catalysts have been shown in our previous work.15 Quite generally, the mesoporous CNTs supported Pd catalyst shows higher pore diameter but lower BET surface area than the microporous AC supported Pd catalyst. However, it must be mentioned that such a difference in textural structure should not highly affect the catalytic performance due to the diffusion limitation since almost all of the Pd particles should be anchored outside of the pores for both supports.15 Figure 1 shows the TPD results of the untreated and 10% HNO3 pretreated CNTs and ACs supported Pd catalysts. It is clearly seen that desorption of CO2 and CO occurs during the TPD at different temperatures. As expected, desorption of CO2 and CO must be originated from the thermo-decomposition of the oxygenated groups, such as -COOH, -CO, and -C−OH. For example, decomposition of the carboxylic group probably results in a desorption of CO2. As shown in Figure 1, desorption of CO was only observed at high temperatures of 773−1123 K, which indicated that decomposition of the carbonyl groups (to form CO) requires high activation energy. Desorption of CO2 was observed at three different temperature ranges, i.e., 373−523 573−723, and 773−923 K, which could be explained by the thermo-decomposition of different oxygenated groups. At low temperatures (373−523 K) such decomposition may be induced by the presence of Pd. Moreover, the relative amounts of CO2 and CO desorbed during the TPD represent the oxygen content of the catalysts. So, it can be concluded that the Pd/ut-ACs possess more oxygenated groups than the Pd/ut-CNTs, and pretreating the carbonaceous supports with HNO3 increases the oxygen content highly. Since the ideal graphitic carbon structure was hydrophobic, the oxygenated groups in the defects of the CNTs and ACs can be considered as local hydrophilic sites. It is also expected that the hydrophilicity of these sites should be tunable through pretreating the carbonaceous supports with HNO3 at various conditions (by controlling the surface oxygen content). Now we proceed to XPS studies of the carbonaceous supports (pretreated with HNO3 at various conditions) to provide a clear view on the changes of the total oxygen content with the pretreatment. Figure 2 shows the example of O 1s spectra, which are simply deconvoluted into two peaks, namely, CO (binding energy (BE) at 531.6 eV) for quinones, ketones, and aldehydes and C−O (BE at 533.2 eV) for ethers
Figure 2. XPS of O 1S for CNTs-30% and ACs-10% samples.
and phenols.18 The C 1s spectrum (not shown) shows mainly CC and C−C with a certain amount of C−O and CO, which also proves the presence of -COOH and -C−OH groups. Since oxygen atoms in esters, carboxyls, and lactonics have both single bonds and double bonds with carbon atom, the oxygen atoms in these groups contribute to both of the abovementioned two peaks. The quantified ratios of CO to C−O for both CNTs and ACs are similar, which indicates that both samples contain similar types of oxygenated groups after the pretreatment by HNO3. Moreover, the carboxyl, lactonic, and phenol groups were identified by Boehm titration method, which is in accordance with our previous studies.13,14 The total oxygen content, as shown in Table 1, was increased with the increases of HNO3 concentration and pretreating temperature, and prolonged pretreating time. The ut-ACs possesses much higher oxygen content (6.82%) than the ut-CNTs (2.02%). Therefore, the Pd/ut-CNTs can be considered as a mainly hydrophobic catalyst, while the Pd/ut-ACs is hydrophilic. Additionally, the local hydrophilicity of the catalysts might be tunable through the HNO3 pretreatment at various conditions. A reprehensive TEM micrograph of the Pd/ACs, Pd/utCNTs, Pd/CNTs-3%, and Pd/CNTs-30% catalysts are shown in Figure 3. As have been discussed previously,15 the CNTs are multiwalled, curved, and twisted together. The structure of CNTs was not destroyed during the pretreatment by 30% HNO3 (Figure 3C). The Pd particle size was mainly in the 2199
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In order to study the effect of hydrophilicity/hydrophobicity on the phenol in situ hydrogenation, the catalytic performances of ut-CNTs (hydrophobic), CNTs-3% (lower hydrophilic), CNTs-30% (medium hydrophilic), and ACs (strong hydrophilic) supported Pd and Ni catalysts were investigated. The metal particle size effect on the reaction could be eliminated since these four catalysts showed similar metal particle size as seen from the TEM micrographs. The ACs and CNTs pretreated by HNO3 at other conditions were not studied for the catalytic reaction. As shown in Figure 4, pretreating the
Table 1. Oxygen Content of the CNTs and ACs Pretreated by HNO3 at Various Conditions, Determined by XPS samples
treating temp (°C)
treating time (h)
oxygen content (wt %)
ut-CNTs CNTs-3% CNTs-10% CNTs-30% CNTs-10% CNTs-10% ACs-10% ut-ACs
90 90 90 90 60 90
6 6 6 3 6 6
2.02 4.35 4.53 4.76 3.11 3.23 14.22 6.82
Figure 4. Effect of HNO3 pretreatment on CNTs supported Pd and Ni catalysts for phenol in situ hydrogenation (reaction conditions: T = 493 K; P = 3.5 MPa (Ar pressure); LHSV = 3.87 h −1 ; n(CH3OH):n(H2O) = 1:8; Cphenol = 0.2 mol·L−1).
CNTs support by HNO3 (3% or 30%) increases the selectivity of cyclohexanone but decreases the phenol conversion for both Pd and Ni catalysts. The Pd/CNTs catalyst shows higher activity and cyclohexanone selectivity than the Ni/CNTs catalyst, which indicates that the Pd is more effective than the Ni for the hydrogenation of phenol to cyclohexanone.22 The different catalytic performance among the ut-CNTs, CNTs-3%, and CNTs-30% supported Pd and Ni catalysts might be explained by the active metal local hydrophilicity of these catalysts, which will be further discussed later. The results of the phenol in situ hydrogenation over the utCNTs (hydrophobic) and ut-ACs (hydrophilic) supported Pd and Ni catalysts are shown in Figure 5. The phenol conversion on the Pd/CNTs and Ni/CNTs catalysts are 60.1% and 37.3%,
Figure 3. Typical TEM micrographs of the (A) Pd/ACs, (B) Pd/ CNTs, (C) Pd/CNTs-30%, and (D) Pd/CNTs-3% catalysts.
range of 4.8−6.5 nm for these three Pd/CNTs catalysts (estimated from at least three different TEM micrographs for the same sample), which is in accordance with the CO chemisorption results.15 Pretreating CNTs (with 3% or 30% HNO3) shows little effect on Pd particle size. Additionally, as mentioned in the Experimental Section, complete precipitation of all metal ions was realized by tuning the pH value. We assume that the metallic content of the catalysts should be the same as the nominal content. Therefore, the only major factor affects the catalytic performance for those catalysts should be hydrophilic (oxygen content). It should be valid to use these carbonaceous supports to discuss the hydrophobic/hydrophilic effect on the (in situ) hydrogenation of phenol to cyclohexanone and cyclohexanol. In Situ Hydrogenation of Phenol. The in situ hydrogenation of phenol was achieved through the coupling of H2 production and phenol hydrogenation.19−22 The activated “H” in situ generated from the aqueous-phase re-forming (APR) or dehydrogenation of methanol was used directly for the hydrogenation of phenol. The limited availability of H results in an inhomogeneous distribution, which favored the formation of cyclohexanone.22
Figure 5. In situ hydrogenation of phenol over the CNTs and ACs supported Pd and Ni catalysts (reaction conditions identical to those shown in Figure 4). 2200
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of methanol over the Pd/CNTs and Pd/ACs catalysts. Obviously, the activity of the Pd/CNTs catalyst is significantly higher than the Pd/ACs (13.7 h−1 vs 0.16 h−1 for the time of flight (TOF) of methanol). The yield (formation rate) of H2 over the Pd/CNTs and Pd/ACs catalysts are 41.4 and 0.9 μmol·gcat−1·min−1, respectively. The results for the hydrogenation of phenol with H2-gas are shown in Table 3. It is also seen that the Pd/CNTs show significantly higher activity than the Pd/ACs. The conversions of phenol over the Pd/CNTs and Pd/ACs are 99.7% and 71%, respectively, at 493 K and 2 MPa (H2 pressure). The phenol conversion is 87.8% when the reaction temperature and H2 pressure were decreased to 423 K and 1 MPa. The cyclohexanone selectivity on the Pd/ACs and Pd/CNTs catalysts during the hydrogenation with H2 gas is lower than the in situ hydrogenation process.22 Other byproducts, such as o-cresol and cyclohexane were also produced over the Pd/ CNTs at higher temperature and H2 pressure with negligible amount. First-Principles DFT Calculations and Discussion. It is seen from the above results that the hydrophobicity/hydrophilicity of the catalysts could affect highly the catalytic performance of phenol (in situ) hydrogenation. To understand intrinsic reasons behind such an effect, the adsorption of methanol, water, phenol and cyclohexanone on the CNTs with various hydrophilicity were studied by first-principles DFT calculation. As shown in Figure 7, three ideal regions, namely, oxygenated-groups-rich region (site 1, strongly hydrophilic), close to oxygenated-groups region (site 2, medium hydrophilic), and far-from-oxygenated-groups region (site 3, hydrophobic), respectively, were selected for the adsorption study. Obviously, adsorption of more polar molecular (water) on the hydrophilic region (site 1 in Figure 7a) is stronger than the other molecules (Figure 7c). The adsorption of both methanol and phenol will be inhibited highly in the presence of H2O in this region (site 1). However, on the hydrophobic region (site 3 in Figure 7a), the adsorption energies for methanol, water, phenol, and cyclohexanone are similar, which indicate that the hydrophobic surface adsorbs all of the reactants simultaneously. Since the oxygenated groups on the carbon supports are of many different types (as have been found with TPD, XPS, and Boehm titration), the most possible oxygenated groups on the carbon materials include -COOH, -CO, and -C−OH, etc.13,14 However, it is not necessary to study the adsorption behavior on all of these groups, separately, by DFT. Actually, according to the comparison studies between the -CO (Figure 7a,c) and -C−OH (Figure 7b,d) groups, one might be expected that the trends of the adsorption behavior of the investigated molecules will not be affected by the types of oxygenated groups. Although the above DFT studies are focused on the adsorption of reactants and products on the CNTs supports without anchoring the active metal particles, it is still valid to discuss the correlation between hydrophilicity and catalytic performance based on such simplified DFT calculations. The gap between the ideal CNTs supports and real catalysts are filled by the fact that active metal particles (Pd, for example) are preferentially anchored on the defects (contain oxygenated groups) of the carbon supports.11 Therefore, the local hydrophilicity of the metal particles is related highly with the oxygen content at the defects of the support. Based on our and other theoretical studies,11,23 the adsorption of oxygenated aromatics molecules on Pd surfaces and clusters is mainly by
respectively, which are significantly higher than that over the Pd/ACs and Ni/ACs catalysts (2.9% and 3.2%, respectively). The selectivities of cyclohexanone over the Pd/ACs and Ni/ ACs catalysts are higher than that over the Pd/CNTs and Ni/ CNTs catalysts (95.6% and 41% vs 81.6% and 24.3%). These results also indicated that the hydrophobicity/hydrophilicity of the catalysts may affect the activity and cyclohexanone selectivity although CNTs and ACs are two different kinds of materials. Further information was observed when the cyclohexanone selectivity was plotted as a function of phenol conversion (see Figure 6). All points above the dotted line represent a good
Figure 6. Cyclohexanone selectivity as a function of phenol conversion over various catalysts.
compromise between the cyclohexanone selectivity and phenol conversion. The relationship is valid for Pd supported on ACs (up triangle) and for the Pd supported on the CNTs (square). The treatment in HNO3 increases the selectivity in cyclohexanone while the conversion of phenol decreases. Similar phenomena were also observed for the Ni based catalyst. Correlating the hydrophilicity of the catalysts with the catalytic performance, it might be concluded that hydrophilicity favors the formation of cyclohexanone but impeded the hydrogenation rate. APR of Methanol and Hydrogenation of Phenol with H2 Gas. To simplify the reaction of the in situ hydrogenation of phenol, we divided the combined reaction into the APR of methanol (for H2 production) and the hydrogenation of phenol (with H2 gas), separately. Table 2 shows the results of the APR Table 2. H2 Production from the Aqueous-Phase Re-forming of Methanol (10 wt %) over Pd/ut-ACs and Pd/ut-CNTs Catalystsa yield (μmol·gcat−1· min−1) catalysts
H2 selectivityb (%)
H2
CO2
TOFc (h−1)
Pd/ut-ACs Pd/ut-CNTs
100 100
0.9 41.4
0.28 13.6
0.16 13.7
a
Reaction conditions: mcat = 0.5 g; T = 493 K; P = 3.5 MPa; liquid phase flow rate = 0.1 mL·min−1; LHSV = 3.87 h−1. bH2 selectivity was defined as follows: 100% − (H2 loss due to the formation of CO and CH4)/(ideal H2 production according to the stoichiometric equation) × 100%. cCalculated based on the CO chemisorption results that are shown in our previous work.15 2201
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Table 3. Hydrogenation of Phenol with H2 Gas over the Pd/ut-CNTs and Pd/ut-ACs Catalystsa selectivity (%) catalyst
T (K)
P (MPa)
phenol conv (%)
TOFb (h−1)
cyclohexanol
cyclohexanone
Pd/ut-ACs Pd/ut-CNTs Pd/ut-CNTs
493 493 423
2.0 2.0 1.0
71.0 99.7 87.8
15.3 39.7 35.0
73.2 64.2 94.7
26.8 3.4 0.3
Reaction conditions: mcat = 0.5 g; liquid phase flow rate = 0.1 mL·min−1; LHSV = 3.87 h−1; Cphenol = 0.2 mol·L−1. Byproducts such as o-cresol and cyclohexane were produced at 493 K with negligible amount. bCalculated based on the CO chemisorption results that are shown in our previous work.15 a
Figure 7. Side and top view of (5, 5) CNTs with adsorbed (a) hydroxyl and oxygen (b) hydroxyl groups. The adsorption energy of water, methanol, phenol, and cyclohexanone on the completely hydrophilic (site 1), partially hydrophilic (site 2), and completely hydrophobic (site 3) regions of (5, 5) CNTs with adsorbed (c) hydroxyl and oxygen and (d) hydroxyl groups.
the chemical bonding between the aromatic ring and Pd atoms. However, the adsorptions of these molecules are influenced by the presence of H2O or OH (hydrophilic groups) on Pd clsuters or support.23 Therefore, the adsorption of phenol on the interface between Pd and oxidized CNTs and ACs is much stronger than only on Pd surfaces due to both chemical bonding (aromatic ring/Pd) and hydrogen bonding (-OH groups in the phenol molecule/hydrophilic support). However, the interface of the active metals (Pd and Ni) that anchored on the hydrophilic supports (ACs and HNO3 pretreated CNTs) were preferentially surround by those more polar molecules. Then the adsorption/activation of none or less polar reactants will be inhibited. In the case of the APR of methanol (Table 2), the catalytic activity was very low because the adsorption of methanol was completely inhibited by H2O over the hydrophilic catalysts. The same explanations can be applied to the hydrogenation of phenol with H2 gas (Table 3) or in situ hydrogenation of phenol with hydrogen from the APR of methanol. The active metal (Pd and Ni) anchored on the hydrophobic support (untreated CNTs) were preferentially located on the defects containing less oxygenates. So, the active phase of this catalyst exhibits hydrophobicity. Adsorption of phenol on these catalysts is mainly at only a Pd cluster (a
hydrogen bond between -OH group and hydrophilic support does not exist). According to the DFT study, the access of all reactants (water, methanol, and phenol) to the active phase shows equal opportunity. Therefore, the Pd/ut-CNTs have shown high activity for the APR of methanol and phenol hydrogenation with both H2 gas and in situ hydrogen from APR of methanol. In terms of the cyclohexanone selectivity, the ACs supported Pd and Ni catalyst was higher than the CNTs supported one. Moreover, pretreating the CNTs with HNO3 increases the selectivity of cyclohexanone. These results suggested that the hydrophilic catalyst favors the formation of cyclohexanone, which is in accordance with the work in the literature.2,3 As mentioned, the adsorption of phenol on the interface between Pd and oxidized CNTs and ACs is much stronger than only Pd surfaces due to both chemical bonding (aromatic ring/Pd) and hydrogen bonding (-OH groups/hydrophilic support), which might affect the deprotonation of phenol and lead to the major product being cyclohexanone. On nonoxygenated CNT or active carbon support Pd catalysts, the adsorption of phenol is mainly at only the Pd sites, which leads to the major product being cyclohexanol since the hydroxyl group is away from Pd surfaces. Additionally, cyclohexanone can also be considered as 2202
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Industrial & Engineering Chemistry Research
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(5) Dodgson, I.; Griffin, K.; Barberis, G.; Piganttaro, F.; Tauszik, G. A low cost phenol to cyclohexanone process. Chem. Ind. 1989, 24, 830− 833. (6) Xiang, Y. Z.; Ma, L.; Lu, C. S.; Zhang, Q. F.; Li, X. N. Aqueous system for the improved hydrogenation of phenol and its derivatives. Green Chem. 2008, 10, 939−943. (7) Liu, H.; Jiang, T.; Han, B.; Liang, S.; Zhou, Y. Selective phenol hydrogenation to cyclohexanone over a dual supported Pd-Lewis acid catalyst. Science 2009, 326, 1250−1252. (8) Wang, Y.; Yao, J.; Li, H. R.; Su, D. S.; Antonietti, M. Highly selective hydrogenation of phenol and derivatives over a Pd@Carbon nitride catalyst in aqueous media. J. Am. Chem. Soc. 2011, 133, 2362− 2365. (9) Auer, E.; Freund, A.; Pietsch, J.; Tacke, T. Carbons as supports for industrial precious metal catalysts. Appl. Catal., A 1998, 173, 259− 271. (10) Serp, P.; Corrias, M.; Kalck, P. Carbon nanotubes and nanofibers in catalysis. Appl. Catal., A 2003, 253, 337−358. (11) Wang, J. G.; Lv, Y. A.; Li, X. N.; Dong, M. D. Point-defect mediated Bonding of Pt Clusters on (5, 5) Carbon Nanotubes. J. Phys. Chem. C 2009, 113, 890−893. (12) Boehm, H. P. Some aspects of the surface chemistry of carbon blacks and other carbons. Carbon 1994, 32, 759−769. (13) Li, J. Y.; Ma, L.; Li, X. N.; Lu, C. S.; Liu, H. Z. Effect of nitric acid pretreatment on the properties of activated carbon and supported palladium catalysts. Ind. Eng. Chem. Res. 2005, 44, 5478−5482. (14) Xu, T. Y.; Zhang, Q. F.; Yang, H. F.; Li, X. N.; Wang, J. G. Role of Phenolic Groups in the Stabilization of Palladium Nanoparticles. Ind. Eng. Chem. Res. 2013, 52, 9783−9789. (15) Xiang, Y. Z.; Lv, Y. A.; Xu, T. Y.; Li, X. N.; Wang, J. G. Selectivity difference between hydrogenation of acetophenone over CNTs and ACs supported Pd catalysts. J. Mol. Catal. A: Chem. 2011, 351, 70−75. (16) Delley, B. An All-Electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules. J. Chem. Phys. 1990, 92, 508−517. (17) Delley, B. From molecules to solids with the DMol3 approach. J. Chem. Phys. 2000, 113, 7756−7764. (18) Dongil, A. B.; Bachiller-Baeza, B.; Guerrero-Ruiz, A.; RodríguezRamos, I.; Martínez-Alonso, A.; Tascón, J. M. D. Surface chemical modifications induced on high surface area graphite and carbon nanofibers using different oxidation and functionalization treatments. J. Colloid Interface Sci. 2011, 355, 179−189. (19) Li, X. N.; Xiang, Y. Z. A novel liquid system of catalytic hydrogenation. Sci. China, Ser. B: Chem. 2007, 50, 746−753. (20) Xiang, Y. Z.; Kong, L. N.; Lu, C. S.; Ma, L.; Li, X N. Lanthanumpromoted Pd/Al2O3 catalysts for liquid phase in situ hydrogenation of phenol to cyclohexanone. React. Kinet., Mech. Catal. 2010, 100, 227− 235. (21) Wang, H. J.; Xiang, Y. Z.; Xu, T. Y.; Zhou, H. J.; Ma, L.; Li, X. N. Catalytic performance of Ba-modified Pd/Al2O3 for liquid phase in situ hydrogenation of phenol to cyclohexanone. Chin. J. Catal. 2009, 30, 933−938. (22) Xiang, Y. Z.; Li, X. N.; Lu, C. S.; Ma, L.; Yuan, J. F.; Feng, F. Reaction performance of hydrogen from aqueous-phase reforming of methanol or ethanol in hydrogenation of phenol. Ind. Eng. Chem. Res. 2011, 50, 3139−3144. (23) Yang, J.; Dauenhauer, P. J.; Ramasubramaniam, A. The role of water in the adsorption of oxygenated aromatics on Pt and Pd. J. Comput. Chem. 2013, 34, 60−66.
the intermediate for the hydrogenation of phenol to cyclohexanol; strong adsorption of cyclohexnanone on the hydrophobic catalyst was not desirable (it will result in further hydrogenation to cyclohexanol). According to the DFT study (Figure 7) the hydrophilic catalyst favors the desorption of cyclohexanone, which could explain the reason for the high selectivity of cyclohexanone obtained over hydrophilic ACs supported catalysts.
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CONCLUSIONS To summarize, we demonstrated that the hydrophilic/hydrophobic properties of the catalysts (Pd or Ni supported on carbonaceous supports of CNTs and ACs) highly affects the catalytic performance in phenol in situ hydrogenation. Such an effect has not been systematically discussed before, even though, rather impressive selectivity has been reported in phenol hydrogenation to cyclohexanone or cyclohexanol over the Pd or Ni based catalysts. The Pd or Ni supported on utCNTs show hydrophobic, which thus provide equal opportunity for the access of phenol, methanol, H2O, and cyclohexanone to the active metals. The ACs and CNTs (pretreated with HNO3) supported catalysts show local hydrophilicity, which preferentially adsorbs H2O while inhibiting others. Therefore, the high hydrophilicity is correlated with high selectivity in cyclohexanone but low conversion in phenol. Moreover, the local hydrophilicity of the catalysts can be tuned by treating the carbonaceous supports with HNO3 at various conditions. The present observation is particularly important in the designing of industrial catalyst for those reactions using a polar solvent such as H2O or producing H2O as the byproducts.
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AUTHOR INFORMATION
Corresponding Authors
*(J.G.W.) Tel.: +86-571-88871037. Fax: +86-571-88871037. Email:
[email protected]. *(X.N.L.) Tel.: +86-571-88320002. Fax: +86-571-88320259. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program; Grants 2011CB710803 and 2013CB733501) and National Natural Science Foundation of China (Grants NSFC-20976164, 21176221, 21136001, and 91334103),
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REFERENCES
(1) Li, Y.; Chen, X.; Gu, N. Computational investigation of interaction between nanoparticles and membranes: hydrophobic/ hydrophilic effect. J. Phys. Chem. B 2008, 112, 16647−16653. (2) Matos, J.; Corma, A. Selective phenol hydrogenation in aqueous phase on Pd-based catalysts supported on hybrid TiO2-carbon materials. Appl. Catal., A 2011, 404, 103−112. (3) Makowski, P.; Cakan, R. D.; Antonietti, M.; Goettmann, F.; Titirici, M. M. Selective partial hydrogenation of hydroxy aromatic derivatives with palladium nanoparticles supported on hydrophilic carbon. Chem. Commun. (Cambridge, U. K.) 2008, 8, 999−1001. (4) Nhut, J. M.; Pesant, L.; Tessonnier, J. P.; Winé, G.; Guille, J.; Pham-Huua, C.; Ledoux, M. J. Mesoporous carbon nanotubes for use as support in catalysis and as nanosized reactors for one-dimensional inorganic material synthesis. Appl. Catal., A 2003, 254, 345−363. 2203
dx.doi.org/10.1021/ie4035253 | Ind. Eng. Chem. Res. 2014, 53, 2197−2203
