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Cite this: Chem. Commun., 2014, 50, 9679 Received 11th April 2014, Accepted 1st July 2014
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Platinum nanoparticles supported on Ca(Mg)-zeolites for efficient room-temperature alcohol oxidation under aqueous conditions† Yejiang Hong,‡a Xiaoqing Yan,‡b Xiaofeng Liao,a Renhong Li,*b Shaodan Xu,a Liping Xiaoa and Jie Fan*a
DOI: 10.1039/c4cc02685c www.rsc.org/chemcomm
Platinum nanoparticles supported on Ca(Mg)-ZSM-5 is an efficient, highly selective and stable catalyst for room-temperature oxidation of alcohols in water. Based on in situ EPR measurement and the radical trapping technique, we propose that the generation of OH radicals by cleavage of the O–O bond in the H2O2 intermediate is the rate determining step, which participated in the abstraction of H from the a-C–H bond of alcohol molecules to produce aldehydes/ketones.
Selective oxidation of alcohols into aldehydes/ketones is of great importance for many laboratory and commercial procedures.1,2 Metal nanoparticle based catalysts such as gold,3,4 palladium,5 platinum,6–8 ruthenium,9 and silver10 are able to oxidize alcohols. In particular, platinum nanoparticles (PtNPs) have been theoretically and experimentally demonstrated to be effective catalysts for oxidation of alcohols with high selectivity and stability.4,6–8 However, in most cases, base additives (e.g., NaOH or K2CO3) are often indispensable to achieve high catalytic activity, which will inevitably lead to severe problems such as undesirable side reactions, catalysts corrosion and extra waste base treatment. To address this issue, a concept of ‘‘bifunctional catalysis’’ has been recently proposed for base-free homogeneous dehydrogenation of alcohols.11,12 Considering that homogeneous catalysis usually suffers from drawbacks such as the difficulty in the separation and reuse of the catalysts, the exploration of heterogeneous Pt-based nanocatalysts is very demanding. In principle, the catalytic performance of certain catalysts can be radically enhanced by using proper supports due to the strong electronic/chemical interactions of metal particles with their supports. For the alcohol oxidation a
Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou, China. E-mail: [email protected]; Fax: +86 571 87952338; Tel: +86 571 87952338 b Key Lab of Advanced Textile Materials and Manufacturing Technology, Ministry of Education of China, Zhejiang Sci-Tech University, Hangzhou 310018, China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Catalyst synthesis, XRD analysis, Arrhenius plots, recycling test and EPR spin trapping results. See DOI: 10.1039/c4cc02685c ‡ These authors contributed equally to this work.
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process catalyzed by PtNPs, various supports ranging from carbon materials such as hollow porous carbon shells and carbon nanotubes,13,14 polymers,13 to inorganic oxides (e.g., Al2O37 and Bi2O36) have been employed and systematically studied. However, fundamental information on the support–activity relationship, the nature of the key intermediate species and the reaction mechanism remains obscure due to the difficulty in the verification of reaction intermediates. Zeolites, for example, ZSM-5, have been frequently used as support materials for catalysis. Herein, we show that the assembly of PtNPs on basic Ca(Mg)-ZSM-5 is a promising heterogeneous catalyst for the oxidative dehydrogenation of alcohols (benzyl alcohol and cyclohexanol). Several merits that meet the standards of ‘‘green chemistry’’ are involved in this specific reaction: (i) water was used as the solvent and molecular O2 as the oxidant, (ii) very low reaction temperature (i.e., room temperature, 25 1C), and (iii) high yield of target products. More importantly, the direct observation of reaction intermediate species using in situ electron paramagnetic resonance spectroscopy (EPR) and the spin trapping technique allows us, for the first time, to propose a cooperative reaction mechanism by taking into account the roles of the solvent, the catalytic Pt sites, the support and molecular O2 during alcohol oxidation. PtNP and zeolite supports were prepared separately and a colloid deposition method was followed for the loading process (see ESI† for details). XRD patterns show that the synthetic zeolites are highly crystalline (Fig. S1, ESI†). TEM images reveal that PtNPs with a mean particle diameter of B2.9 nm are evenly dispersed on Ca-ZSM-5 (Fig. 1a) as well as Mg-ZSM-5 (Fig. 1b). HADDF-STEM characterization further demonstrates the homogeneous distribution of PtNPs (Fig. 1c). The high resolution TEM (HRTEM) image (Fig. 1d) exhibits lattice fringes with an interplanar spacing of 0.221 nm, corresponding to (111) planes of fcc Pt. The oxidation of alcohols over PtNPs on different supports was carried out in aqueous solution at room temperature (see Experimental section in ESI†). The primary results are summarized in Table 1. Three striking features are observed relative to the catalytic properties of supported PtNPs. First of all, we found that
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Fig. 1 TEM images of PtNPs loaded on Ca-ZSM-5 and the corresponding size distribution of PtNPs (a) and PtNPs on Mg-ZSM-5 supports (b), HADDF-STEM image of Pt/Ca-ZSM-5 (c), and HRTEM image of a single PtNP on Ca-ZSM-5 (d). The scale bar is 100 nm for figures (a) to (c).
among the screened supports, basic zeolites (Mg-ZSM-5 and Ca-ZSM-5) showed the optimal performance during benzyl alcohol oxidation, achieving 495% conversion with selectivity approaching 99% at a carbon balance of 98% (entries 1 and 2). The catalytic property of PtNPs is more or less dependent on the size, for example, PtNPs/Ca-ZSM-5 prepared by the impregnation method with a Pt mean size of B5.8 nm can only transform 89% benzyl alcohol during 20 h of reaction (entry 2). Other supports even with high basicity, such as CaO and MgO, in contrast, greatly suppressed the oxidation process (entries 6 and 7). Moreover, although PtNPs on inert supports such as silica gel exhibited some activity (entry 5), the ultimate conversion of benzyl alcohol using these catalysts cannot exceed 70% even with much prolonged reaction time (e.g., 460 h). Second, no benzaldehyde was formed when AuNPs or PdNPs instead of PtNPs were used as the main catalysts under otherwise identical experimental conditions (entries 8 and 9), suggesting the unique catalytic property of PtNPs. Third, only negligible conversion of benzyl alcohol was observed when water was replaced by other
Table 1
The catalytic property of PtNPs on different supportsa
Entry
Catalyst
Substrateb
Solvent
Conv. (%)
Sel. (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Pt/Ca-ZSM-5 Pt/Mg-ZSM-5 Pt/Ca-ZSM-5c Pt/HZSM-5 Pt/SiO2 Pt/CaO Pt/MgO Au/Ca-ZSM-5 Pd/Ca-ZSM-5 Pt/Mg-ZSM-5 Pt/Ca-ZSM-5 Pt/SiO2 Pt/Ca-ZSM-5 Pt/Ca-ZSM-5 Pt/Ca-ZSM-5
BA BA BA BA BA BA BA BA BA CHA CHA CHA BA BA BA
H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O MeCN EtOH CCl4
98.8 95.5 89 65 64 0 0 0 0 20.7 21.8 5.1 o1 o1 o1
99 98 96 87 87 — — — — 85.5 82 90 — — —
a
Experimental details: a mixture of 50 mg catalyst with a unified 1.0 wt% Pt and 20 mM alcohol in various solvents was stirred at room temperature (25 1C) in open air for 20 h. b BA = benzyl alcohol; CHA = cyclohexanol. c Impregnation method was used to prepare the catalyst.
9680 | Chem. Commun., 2014, 50, 9679--9682
solvents (entries 13–15), indicating that water played an important role in the PtNP catalyzed alcohol oxidation process. In addition, the fact that no reaction occurred without molecular O2 suggested that an oxidative dehydrogenation rather than direct dehydrogenation process dominated the oxidation chemistry here. The kinetic studies were conducted at different reaction temperatures. According to the turnover frequency (TOF) values in the case of Pt/Ca-ZSM-5 catalysts, the apparent activation energy of the oxidation reaction is calculated to be as low as 21.2 kJ mol 1 (Fig. S2, ESI†). Furthermore, Pt/Ca-ZSM-5 and Pt/Mg-ZSM-5 catalysts also exhibited promising catalytic activities for cyclohexanol selective oxidation. This oxidation process is commercially important as the product cyclohexanone is highly valuable for the production of nylon 6,6 and nylon 6. However, the cyclohexanol to cyclohexanone transformation is characteristic of low selectivity and low activity due to the difficulty in dissociation of a-C–H bonds. After 20 h of reaction at room temperature, Pt/Mg-ZSM-5 and Pt/Ca-ZSM-5 catalysts gave conversion values of 20.7% at selectivity = 85.5% (entry 10) and 21.8% at selectivity = 82% (entry 11), respectively. This performance is superior to earlier studies using Pt as the main catalyst for alcohol oxidation without the addition of any promoters.1 The reuse and stability are two major concerns for a catalyst to be useful in practical applications.1 In this case, we carried out life cycle tests to assess the stability and durability of the Pt/Ca-ZSM-5 catalyst during benzyl alcohol oxidation (Table S1, ESI†). The catalytic activity remained steady after 4 reaction cycles, and more importantly, no Pt leaching and less than 1% Ca leaching were observed according to the inductively coupled plasma mass spectrometry (ICP-MS) analysis within experimental error. Thus, it is concluded that Pt/Ca-ZSM-5 may serve as a stable and efficient catalyst for practical alcohol oxidation purposes. The special catalytic performance of PtNPs on basic zeolites prompted us to elucidate the reaction intermediates and the mechanism. For this purpose, we carried out the in situ EPR experiments using 5,5-dimethylpyrroline N-oxide (DMPO) as a typical spin-trapping reagent at room temperature. During the EPR examination, a suspension containing the Pt/Ca-ZSM-5 catalyst, H2O solvent, benzyl alcohol and DMPO was injected into a capillary tube using a micro-syringe, and was then placed vertically in the ERP testing chamber. As shown in Fig. 2, a group of complex signals emerged as soon as the EPR started to record, indicating that the benzyl alcohol oxidation proceeded via a radical mechanism. The pronounced ninefold signal labeled by the gray disc was characteristic of the DMPO–H radical (hyperfine splitting constants: aN = 16.57 G; aH(1) = aH(2) = 22.58 G), which has been frequently described in the Au or Cu catalyzed alcohol dehydrogenation processes.3,15–17 Since control experiments showed that no H adduct was formed without alcohols or catalysts, the observed H spin adduct cannot be formed via hydride transfer from alcohol to the spin trap followed by oxidation. We thus suggested that the origin of hydride species is most likely due to the O–H bond cleavage on the surface of PtNPs based on previous reports.3,17,18 Generally, free hydrogen atoms are prone to form HOO and eventually H2O as a final product in the presence of molecular O2.
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Fig. 2 In situ room temperature EPR spectra of the DMPO adducts recorded in the reaction system containing DMPO spin trapping reagent, benzyl alcohol, water solvent and Pt/Ca-ZSM-5 catalyst from the beginning to the end of 2 min reaction time.
Therefore, it is reasonable that the concomitant irregular signal labeled by the asterisk is attributed to the HOO radicals (aN = 13.2 G; aHb = 15.25 G; aHg = 1.17 G) owing to the direct combination of H with adsorbed O2 molecules. In addition to the former two spin trapping radicals, a prominent 1 : 2 : 2 : 1 DMPO– OH adduct (aN = aH = 14.59 G) was also present, as indicated by the gray triangle. As aforementioned, the yield of benzyl alcohol oxidation can reach a high value (B97.8%) when Ca-ZSM-5 is used as the support, which conflicts the conventional radical-based catalysis especially when the non-selective oxidant, OH radical, participated in the catalytic system. For this reason, a possible pathway that leads to its formation would be the homolytic O–O bond fission of H2O2, which was derived from the reaction between HOO and H radicals. This hypothesis was further demonstrated by the fact that the EPR intensity of the HOO and H radicals was weakened simultaneously, but the OH intensity remained almost unchanged as a function of reaction time. To unambiguously determine whether the obtained radicals are unique for benzyl alcohol oxidation or not, we carried out the same EPR study using cyclohexanol as another target substrate. In Pt/Ca-ZSM-5 catalyzed oxidation of cyclohexanol, identical DMPO–H , DMPO–HOO and DMPO– OH spin adducts were detected and the trend of signal intensity evolution was also the same from the beginning to the end of 2 min reaction time (Fig. S3, ESI†); the only difference is that the EPR intensity of these adducts is a little weaker than the case of benzyl alcohol oxidation. The fact that the hydride and oxygen-centered spin adducts were both observed in reactions of aromatic and alicyclic alcohols with Pt catalysts suggests a common oxidative dehydrogenation mechanism during selective alcohol oxidation. More importantly, we compared the DMPO spin-trapping signals between nondeuterated (C6H5OH) and deuterated cyclohexanol (C6D5OH) molecules, and found that the two sets of signals are almost the same within 2 min of reaction (Fig. S4, ESI†), further demonstrating that the surface hydride species was indeed from the hydroxyl groups rather than a-C–H. According to the catalysis results, the catalytic activity of PtNPs is largely dependent on their supports. Thus we further carried out EPR experiments to elucidate the effect of supports.
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Fig. 3 In situ room temperature EPR spectra of the DMPO adducts: support effect (recorded in the reaction system containing DMPO spin trapping reagent, benzyl alcohol, water solvent and Pt-based catalysts): from the beginning (a) to the end of 2 min reaction time (b), and solvent effect (recorded in the reaction system containing DMPO spin trapping reagent, benzyl alcohol, solvents and Pt/Ca-ZSM-5 catalyst): from the beginning (c) to the end of 2 min reaction time (d).
Four typical supports ranging from acidic (HZSM-5), to neutral (silica gel) and then to basic (MgO and Mg-ZSM-5) were selected. As shown in Fig. 3a, in the case of MgO, very intensive DMPO–H and DMPO–HOO spin adducts were generated as soon as the EPR began to record, indicating that benzyl alcohol underwent prompt O–H cleavage similar to the case of Ca-ZSM-5, and the O–H cleavage seemed to be favorable on the Pt surface because hydride species formed whatever supports were used, irrespective of the signal intensity. However, during the entire reaction, only a very weak OH signal was observed for MgO (Fig. 3b), suggesting that the cleavage of the O–O bond is greatly inhibited when MgO was used as its support. Importantly, considering that Pt/MgO was efficient in the dehydrogenation of the O–H bond of alcohol, but showed no catalytic activity during the whole oxidation process, we supposed that the rate determining step would be the homolytic cleavage of the O–O bond to generate OH radicals, which subsequently act as the oxidative species to abstract the H atom from a-C–H bonds. Therefore, basic Ca(Mg)-ZSM-5 may provide stability for Pt species by interfacial metal–support interactions so favoring the adsorption of O2 and the subsequent cleavage of the O–O bond; meanwhile, the specific pore size of zeolites may also offer promising basic sites and an alkaline microenvironment (pH E 8) for abstracting the proton from the hydroxyl group in alcohol as well as the diffusion and long-term existence of the main oxidant, OH radicals. For the other two supports, HZSM-5 and silica gel, although the same DMPO–HOO and DMPO– OH adducts were observed, their intensity was lower than that of Ca-ZSM-5, which accounts for their lower catalytic activity. Solvent is also a crucial factor to the success of the oxidation process. According to the EPR experiments (Fig. 3c and d), we found that no significant EPR signals were produced when CCl4 and MeCN were used as the solvents throughout the entire reaction. This is consistent with the catalysis results because no benzyl alcohol transformation occurred in the presence of these two solvents. In the case of ethanol, a prominent six-fold peak
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was generated (red discs), which is possibly derived from the DMPO– OC2H5 spin adducts (aN = 15.22 G; aH = 1.16 G). The appearance of the ethyoxyl group indicated that ethanol experienced adsorption and subsequent O–H bond cleavage. Obviously, the participation of ethanol will greatly prevent the adsorption and activation of benzyl alcohol molecules on the catalyst surface. Although water is not an applicable solvent for benzyl alcohol because of its relatively low solubility, it is still found to be the optimal one. In this case, we considered that the presence of water solvent may not only increase the residence time of O2 on the catalyst surface but also reduce the energy barrier of O2 hydrogenation.18 In addition, water can also advantageously mediate for the existence and migration of OH radicals from the catalyst surface to the reactants. On the basis of the EPR and experimental results, we proposed the following overall mechanism for the Pt/Ca(Mg)-ZSM-5 catalyzed alcohol oxidation process. After the adsorption of alcohols and molecular O2 on the Pt catalyst surface, the breaking of the hydroxyl bond of alcohol occurred, leading to the formation of Pt–H species, which was subsequently trapped by the pre-adsorbed O2 to produce HOO radicals. The resulting HOO radical may abstract H from another neighboring alcohol molecule, giving rise to a H2O2 intermediate. Next, the homolytic cleavage of H2O2 led to the generation of OH radicals, most likely occurring on the interface between PtNPs and zeolites since individual PtNPs or zeolites are unable to activate oxygen as well as cleave the O–O bond in H2O2. Importantly, this step may be the rate-limiting process for the overall alcohol oxidation, since the breaking of the a-C–H bond of alcohols became rather easy under the attack of OH radicals. In conclusion, we demonstrated that well dispersed PtNPs on Ca(Mg)-ZSM-5 are stable and promising catalysts for selective oxidation of alcohols in water at room temperature. The generation of OH radicals by the cleavage of the O–O bond in the H2O2 intermediate is supposed to be the rate determining step, which took part in the abstraction of H from the a-C–H bond of alcohol molecules to produce aldehydes/ketones. On the basis of this study, future developments may also be possible in the field of C–H activation and transformation
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using supported PtNPs as the catalysts, molecular O2 as the oxidant and water as the solvent. We are grateful for financial support from the National Science Foundation of China (21222307 and 21003106), Fok Ying Tung Education Foundation (131015), the Fundamental Research Funds for the Central Universities (2014XZZX003-02), China Postdoctoral Science Foundation (2014M550333) and the Young Researchers Foundation of Key Lab of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University (2013QN04).
Notes and references 1 T. Mallat and A. Baiker, Chem. Rev., 2004, 104, 3037–3058. 2 G. Centi and M. Misono, Catal. Today, 1998, 41, 287–296. 3 A. Maldotti, A. Molinari, R. Juarez and H. Garcia, Chem. Sci., 2011, 2, 1831–1834. 4 B. N. Zope, D. D. Hibbitts, M. Neurock and R. J. Davis, Science, 2010, 330, 74–78. 5 K. Mori, T. Hara, T. Mizugaki, K. Ebitani and K. Kaneda, J. Am. Chem. Soc., 2004, 126, 10657–10666. 6 T. Lu, Z. Du, J. Liu, H. Ma and J. Xu, Green Chem., 2013, 15, 2215–2221. 7 K. Kon, S. M. A. H. Siddiki and K.-i. Shimizu, J. Catal., 2013, 304, 63–71. 8 T. Wang, C.-X. Xiao, L. Yan, L. Xu, J. Luo, H. Shou, Y. Kou and H. Liu, Chem. Commun., 2007, 4375–4377. 9 B. Feng, C. Chen, H. Yang, X. Zhao, L. Hua, Y. Yu, T. Cao, Y. Shi and Z. Hou, Adv. Synth. Catal., 2012, 354, 1559–1565. 10 T. Mitsudome, Y. Mikami, H. Funai, T. Mizugaki, K. Jitsukawa and K. Kaneda, Angew. Chem., Int. Ed., 2008, 47, 138–141. 11 M. Nielsen, A. Kammer, D. Cozzula, H. Junge, S. Gladiali and M. Beller, Angew. Chem., Int. Ed., 2011, 50, 9593–9597. 12 R. Kawahara, K.-i. Fujita and R. Yamaguchi, J. Am. Chem. Soc., 2012, 134, 3643–3646. 13 H. Miyamura, M. Shiramizu, R. Matsubara and S. Kobayashi, Chem. Lett., 2008, 37, 360–361. 14 D. Liang, J. Gao, H. Sun, P. Chen, Z. Hou and X. Zheng, Appl. Catal., B, 2011, 106, 423–432. 15 J. Fan, Y. Dai, Y. Li, N. Zheng, J. Guo, X. Yan and G. D. Stucky, J. Am. Chem. Soc., 2009, 131, 15568–15569. 16 Y. Dai, X. Yan, Y. Tang, X. Liu, L. Xiao and J. Fan, ChemCatChem, 2012, 4, 1603–1610. 17 M. Conte, H. Miyamura, S. Kobayashi and V. Chechik, J. Am. Chem. Soc., 2009, 131, 7189–7196. 18 C. Shang and Z.-P. Liu, J. Am. Chem. Soc., 2011, 133, 9938–9947.
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Cite this: Chem. Commun., 2014, 50, 9679 Received 11th April 2014, Accepted 1st July 2014
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Platinum nanoparticles supported on Ca(Mg)-zeolites for efficient room-temperature alcohol oxidation under aqueous conditions† Yejiang Hong,‡a Xiaoqing Yan,‡b Xiaofeng Liao,a Renhong Li,*b Shaodan Xu,a Liping Xiaoa and Jie Fan*a
DOI: 10.1039/c4cc02685c www.rsc.org/chemcomm
Platinum nanoparticles supported on Ca(Mg)-ZSM-5 is an efficient, highly selective and stable catalyst for room-temperature oxidation of alcohols in water. Based on in situ EPR measurement and the radical trapping technique, we propose that the generation of OH radicals by cleavage of the O–O bond in the H2O2 intermediate is the rate determining step, which participated in the abstraction of H from the a-C–H bond of alcohol molecules to produce aldehydes/ketones.
Selective oxidation of alcohols into aldehydes/ketones is of great importance for many laboratory and commercial procedures.1,2 Metal nanoparticle based catalysts such as gold,3,4 palladium,5 platinum,6–8 ruthenium,9 and silver10 are able to oxidize alcohols. In particular, platinum nanoparticles (PtNPs) have been theoretically and experimentally demonstrated to be effective catalysts for oxidation of alcohols with high selectivity and stability.4,6–8 However, in most cases, base additives (e.g., NaOH or K2CO3) are often indispensable to achieve high catalytic activity, which will inevitably lead to severe problems such as undesirable side reactions, catalysts corrosion and extra waste base treatment. To address this issue, a concept of ‘‘bifunctional catalysis’’ has been recently proposed for base-free homogeneous dehydrogenation of alcohols.11,12 Considering that homogeneous catalysis usually suffers from drawbacks such as the difficulty in the separation and reuse of the catalysts, the exploration of heterogeneous Pt-based nanocatalysts is very demanding. In principle, the catalytic performance of certain catalysts can be radically enhanced by using proper supports due to the strong electronic/chemical interactions of metal particles with their supports. For the alcohol oxidation a
Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou, China. E-mail: [email protected]; Fax: +86 571 87952338; Tel: +86 571 87952338 b Key Lab of Advanced Textile Materials and Manufacturing Technology, Ministry of Education of China, Zhejiang Sci-Tech University, Hangzhou 310018, China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Catalyst synthesis, XRD analysis, Arrhenius plots, recycling test and EPR spin trapping results. See DOI: 10.1039/c4cc02685c ‡ These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2014
process catalyzed by PtNPs, various supports ranging from carbon materials such as hollow porous carbon shells and carbon nanotubes,13,14 polymers,13 to inorganic oxides (e.g., Al2O37 and Bi2O36) have been employed and systematically studied. However, fundamental information on the support–activity relationship, the nature of the key intermediate species and the reaction mechanism remains obscure due to the difficulty in the verification of reaction intermediates. Zeolites, for example, ZSM-5, have been frequently used as support materials for catalysis. Herein, we show that the assembly of PtNPs on basic Ca(Mg)-ZSM-5 is a promising heterogeneous catalyst for the oxidative dehydrogenation of alcohols (benzyl alcohol and cyclohexanol). Several merits that meet the standards of ‘‘green chemistry’’ are involved in this specific reaction: (i) water was used as the solvent and molecular O2 as the oxidant, (ii) very low reaction temperature (i.e., room temperature, 25 1C), and (iii) high yield of target products. More importantly, the direct observation of reaction intermediate species using in situ electron paramagnetic resonance spectroscopy (EPR) and the spin trapping technique allows us, for the first time, to propose a cooperative reaction mechanism by taking into account the roles of the solvent, the catalytic Pt sites, the support and molecular O2 during alcohol oxidation. PtNP and zeolite supports were prepared separately and a colloid deposition method was followed for the loading process (see ESI† for details). XRD patterns show that the synthetic zeolites are highly crystalline (Fig. S1, ESI†). TEM images reveal that PtNPs with a mean particle diameter of B2.9 nm are evenly dispersed on Ca-ZSM-5 (Fig. 1a) as well as Mg-ZSM-5 (Fig. 1b). HADDF-STEM characterization further demonstrates the homogeneous distribution of PtNPs (Fig. 1c). The high resolution TEM (HRTEM) image (Fig. 1d) exhibits lattice fringes with an interplanar spacing of 0.221 nm, corresponding to (111) planes of fcc Pt. The oxidation of alcohols over PtNPs on different supports was carried out in aqueous solution at room temperature (see Experimental section in ESI†). The primary results are summarized in Table 1. Three striking features are observed relative to the catalytic properties of supported PtNPs. First of all, we found that
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Fig. 1 TEM images of PtNPs loaded on Ca-ZSM-5 and the corresponding size distribution of PtNPs (a) and PtNPs on Mg-ZSM-5 supports (b), HADDF-STEM image of Pt/Ca-ZSM-5 (c), and HRTEM image of a single PtNP on Ca-ZSM-5 (d). The scale bar is 100 nm for figures (a) to (c).
among the screened supports, basic zeolites (Mg-ZSM-5 and Ca-ZSM-5) showed the optimal performance during benzyl alcohol oxidation, achieving 495% conversion with selectivity approaching 99% at a carbon balance of 98% (entries 1 and 2). The catalytic property of PtNPs is more or less dependent on the size, for example, PtNPs/Ca-ZSM-5 prepared by the impregnation method with a Pt mean size of B5.8 nm can only transform 89% benzyl alcohol during 20 h of reaction (entry 2). Other supports even with high basicity, such as CaO and MgO, in contrast, greatly suppressed the oxidation process (entries 6 and 7). Moreover, although PtNPs on inert supports such as silica gel exhibited some activity (entry 5), the ultimate conversion of benzyl alcohol using these catalysts cannot exceed 70% even with much prolonged reaction time (e.g., 460 h). Second, no benzaldehyde was formed when AuNPs or PdNPs instead of PtNPs were used as the main catalysts under otherwise identical experimental conditions (entries 8 and 9), suggesting the unique catalytic property of PtNPs. Third, only negligible conversion of benzyl alcohol was observed when water was replaced by other
Table 1
The catalytic property of PtNPs on different supportsa
Entry
Catalyst
Substrateb
Solvent
Conv. (%)
Sel. (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Pt/Ca-ZSM-5 Pt/Mg-ZSM-5 Pt/Ca-ZSM-5c Pt/HZSM-5 Pt/SiO2 Pt/CaO Pt/MgO Au/Ca-ZSM-5 Pd/Ca-ZSM-5 Pt/Mg-ZSM-5 Pt/Ca-ZSM-5 Pt/SiO2 Pt/Ca-ZSM-5 Pt/Ca-ZSM-5 Pt/Ca-ZSM-5
BA BA BA BA BA BA BA BA BA CHA CHA CHA BA BA BA
H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O MeCN EtOH CCl4
98.8 95.5 89 65 64 0 0 0 0 20.7 21.8 5.1 o1 o1 o1
99 98 96 87 87 — — — — 85.5 82 90 — — —
a
Experimental details: a mixture of 50 mg catalyst with a unified 1.0 wt% Pt and 20 mM alcohol in various solvents was stirred at room temperature (25 1C) in open air for 20 h. b BA = benzyl alcohol; CHA = cyclohexanol. c Impregnation method was used to prepare the catalyst.
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solvents (entries 13–15), indicating that water played an important role in the PtNP catalyzed alcohol oxidation process. In addition, the fact that no reaction occurred without molecular O2 suggested that an oxidative dehydrogenation rather than direct dehydrogenation process dominated the oxidation chemistry here. The kinetic studies were conducted at different reaction temperatures. According to the turnover frequency (TOF) values in the case of Pt/Ca-ZSM-5 catalysts, the apparent activation energy of the oxidation reaction is calculated to be as low as 21.2 kJ mol 1 (Fig. S2, ESI†). Furthermore, Pt/Ca-ZSM-5 and Pt/Mg-ZSM-5 catalysts also exhibited promising catalytic activities for cyclohexanol selective oxidation. This oxidation process is commercially important as the product cyclohexanone is highly valuable for the production of nylon 6,6 and nylon 6. However, the cyclohexanol to cyclohexanone transformation is characteristic of low selectivity and low activity due to the difficulty in dissociation of a-C–H bonds. After 20 h of reaction at room temperature, Pt/Mg-ZSM-5 and Pt/Ca-ZSM-5 catalysts gave conversion values of 20.7% at selectivity = 85.5% (entry 10) and 21.8% at selectivity = 82% (entry 11), respectively. This performance is superior to earlier studies using Pt as the main catalyst for alcohol oxidation without the addition of any promoters.1 The reuse and stability are two major concerns for a catalyst to be useful in practical applications.1 In this case, we carried out life cycle tests to assess the stability and durability of the Pt/Ca-ZSM-5 catalyst during benzyl alcohol oxidation (Table S1, ESI†). The catalytic activity remained steady after 4 reaction cycles, and more importantly, no Pt leaching and less than 1% Ca leaching were observed according to the inductively coupled plasma mass spectrometry (ICP-MS) analysis within experimental error. Thus, it is concluded that Pt/Ca-ZSM-5 may serve as a stable and efficient catalyst for practical alcohol oxidation purposes. The special catalytic performance of PtNPs on basic zeolites prompted us to elucidate the reaction intermediates and the mechanism. For this purpose, we carried out the in situ EPR experiments using 5,5-dimethylpyrroline N-oxide (DMPO) as a typical spin-trapping reagent at room temperature. During the EPR examination, a suspension containing the Pt/Ca-ZSM-5 catalyst, H2O solvent, benzyl alcohol and DMPO was injected into a capillary tube using a micro-syringe, and was then placed vertically in the ERP testing chamber. As shown in Fig. 2, a group of complex signals emerged as soon as the EPR started to record, indicating that the benzyl alcohol oxidation proceeded via a radical mechanism. The pronounced ninefold signal labeled by the gray disc was characteristic of the DMPO–H radical (hyperfine splitting constants: aN = 16.57 G; aH(1) = aH(2) = 22.58 G), which has been frequently described in the Au or Cu catalyzed alcohol dehydrogenation processes.3,15–17 Since control experiments showed that no H adduct was formed without alcohols or catalysts, the observed H spin adduct cannot be formed via hydride transfer from alcohol to the spin trap followed by oxidation. We thus suggested that the origin of hydride species is most likely due to the O–H bond cleavage on the surface of PtNPs based on previous reports.3,17,18 Generally, free hydrogen atoms are prone to form HOO and eventually H2O as a final product in the presence of molecular O2.
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Fig. 2 In situ room temperature EPR spectra of the DMPO adducts recorded in the reaction system containing DMPO spin trapping reagent, benzyl alcohol, water solvent and Pt/Ca-ZSM-5 catalyst from the beginning to the end of 2 min reaction time.
Therefore, it is reasonable that the concomitant irregular signal labeled by the asterisk is attributed to the HOO radicals (aN = 13.2 G; aHb = 15.25 G; aHg = 1.17 G) owing to the direct combination of H with adsorbed O2 molecules. In addition to the former two spin trapping radicals, a prominent 1 : 2 : 2 : 1 DMPO– OH adduct (aN = aH = 14.59 G) was also present, as indicated by the gray triangle. As aforementioned, the yield of benzyl alcohol oxidation can reach a high value (B97.8%) when Ca-ZSM-5 is used as the support, which conflicts the conventional radical-based catalysis especially when the non-selective oxidant, OH radical, participated in the catalytic system. For this reason, a possible pathway that leads to its formation would be the homolytic O–O bond fission of H2O2, which was derived from the reaction between HOO and H radicals. This hypothesis was further demonstrated by the fact that the EPR intensity of the HOO and H radicals was weakened simultaneously, but the OH intensity remained almost unchanged as a function of reaction time. To unambiguously determine whether the obtained radicals are unique for benzyl alcohol oxidation or not, we carried out the same EPR study using cyclohexanol as another target substrate. In Pt/Ca-ZSM-5 catalyzed oxidation of cyclohexanol, identical DMPO–H , DMPO–HOO and DMPO– OH spin adducts were detected and the trend of signal intensity evolution was also the same from the beginning to the end of 2 min reaction time (Fig. S3, ESI†); the only difference is that the EPR intensity of these adducts is a little weaker than the case of benzyl alcohol oxidation. The fact that the hydride and oxygen-centered spin adducts were both observed in reactions of aromatic and alicyclic alcohols with Pt catalysts suggests a common oxidative dehydrogenation mechanism during selective alcohol oxidation. More importantly, we compared the DMPO spin-trapping signals between nondeuterated (C6H5OH) and deuterated cyclohexanol (C6D5OH) molecules, and found that the two sets of signals are almost the same within 2 min of reaction (Fig. S4, ESI†), further demonstrating that the surface hydride species was indeed from the hydroxyl groups rather than a-C–H. According to the catalysis results, the catalytic activity of PtNPs is largely dependent on their supports. Thus we further carried out EPR experiments to elucidate the effect of supports.
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Fig. 3 In situ room temperature EPR spectra of the DMPO adducts: support effect (recorded in the reaction system containing DMPO spin trapping reagent, benzyl alcohol, water solvent and Pt-based catalysts): from the beginning (a) to the end of 2 min reaction time (b), and solvent effect (recorded in the reaction system containing DMPO spin trapping reagent, benzyl alcohol, solvents and Pt/Ca-ZSM-5 catalyst): from the beginning (c) to the end of 2 min reaction time (d).
Four typical supports ranging from acidic (HZSM-5), to neutral (silica gel) and then to basic (MgO and Mg-ZSM-5) were selected. As shown in Fig. 3a, in the case of MgO, very intensive DMPO–H and DMPO–HOO spin adducts were generated as soon as the EPR began to record, indicating that benzyl alcohol underwent prompt O–H cleavage similar to the case of Ca-ZSM-5, and the O–H cleavage seemed to be favorable on the Pt surface because hydride species formed whatever supports were used, irrespective of the signal intensity. However, during the entire reaction, only a very weak OH signal was observed for MgO (Fig. 3b), suggesting that the cleavage of the O–O bond is greatly inhibited when MgO was used as its support. Importantly, considering that Pt/MgO was efficient in the dehydrogenation of the O–H bond of alcohol, but showed no catalytic activity during the whole oxidation process, we supposed that the rate determining step would be the homolytic cleavage of the O–O bond to generate OH radicals, which subsequently act as the oxidative species to abstract the H atom from a-C–H bonds. Therefore, basic Ca(Mg)-ZSM-5 may provide stability for Pt species by interfacial metal–support interactions so favoring the adsorption of O2 and the subsequent cleavage of the O–O bond; meanwhile, the specific pore size of zeolites may also offer promising basic sites and an alkaline microenvironment (pH E 8) for abstracting the proton from the hydroxyl group in alcohol as well as the diffusion and long-term existence of the main oxidant, OH radicals. For the other two supports, HZSM-5 and silica gel, although the same DMPO–HOO and DMPO– OH adducts were observed, their intensity was lower than that of Ca-ZSM-5, which accounts for their lower catalytic activity. Solvent is also a crucial factor to the success of the oxidation process. According to the EPR experiments (Fig. 3c and d), we found that no significant EPR signals were produced when CCl4 and MeCN were used as the solvents throughout the entire reaction. This is consistent with the catalysis results because no benzyl alcohol transformation occurred in the presence of these two solvents. In the case of ethanol, a prominent six-fold peak
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was generated (red discs), which is possibly derived from the DMPO– OC2H5 spin adducts (aN = 15.22 G; aH = 1.16 G). The appearance of the ethyoxyl group indicated that ethanol experienced adsorption and subsequent O–H bond cleavage. Obviously, the participation of ethanol will greatly prevent the adsorption and activation of benzyl alcohol molecules on the catalyst surface. Although water is not an applicable solvent for benzyl alcohol because of its relatively low solubility, it is still found to be the optimal one. In this case, we considered that the presence of water solvent may not only increase the residence time of O2 on the catalyst surface but also reduce the energy barrier of O2 hydrogenation.18 In addition, water can also advantageously mediate for the existence and migration of OH radicals from the catalyst surface to the reactants. On the basis of the EPR and experimental results, we proposed the following overall mechanism for the Pt/Ca(Mg)-ZSM-5 catalyzed alcohol oxidation process. After the adsorption of alcohols and molecular O2 on the Pt catalyst surface, the breaking of the hydroxyl bond of alcohol occurred, leading to the formation of Pt–H species, which was subsequently trapped by the pre-adsorbed O2 to produce HOO radicals. The resulting HOO radical may abstract H from another neighboring alcohol molecule, giving rise to a H2O2 intermediate. Next, the homolytic cleavage of H2O2 led to the generation of OH radicals, most likely occurring on the interface between PtNPs and zeolites since individual PtNPs or zeolites are unable to activate oxygen as well as cleave the O–O bond in H2O2. Importantly, this step may be the rate-limiting process for the overall alcohol oxidation, since the breaking of the a-C–H bond of alcohols became rather easy under the attack of OH radicals. In conclusion, we demonstrated that well dispersed PtNPs on Ca(Mg)-ZSM-5 are stable and promising catalysts for selective oxidation of alcohols in water at room temperature. The generation of OH radicals by the cleavage of the O–O bond in the H2O2 intermediate is supposed to be the rate determining step, which took part in the abstraction of H from the a-C–H bond of alcohol molecules to produce aldehydes/ketones. On the basis of this study, future developments may also be possible in the field of C–H activation and transformation
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using supported PtNPs as the catalysts, molecular O2 as the oxidant and water as the solvent. We are grateful for financial support from the National Science Foundation of China (21222307 and 21003106), Fok Ying Tung Education Foundation (131015), the Fundamental Research Funds for the Central Universities (2014XZZX003-02), China Postdoctoral Science Foundation (2014M550333) and the Young Researchers Foundation of Key Lab of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University (2013QN04).
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