- Email: [email protected]
Water-improved heterogeneous transfer hydrogenation using methanol as hydrogen donor over Pd-based catalyst
Applied Catalysis A: General 375 (2010) 289–294
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Water-improved heterogeneous transfer hydrogenation using methanol as hydrogen donor over Pd-based catalyst Yizhi Xiang, Xiaonian Li *, Chunshan Lu, Lei Ma, Qunfeng Zhang State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Institute of Industrial Catalysis, Zhejiang University of Technology, Hangzhou, P.O. Box 310014, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 August 2009 Received in revised form 19 December 2009 Accepted 6 January 2010 Available online 11 January 2010
The heterogeneous catalytic transfer hydrogenation (CTH) of styrene and nitrobenzene over Pd-based catalyst using methanol as hydrogen donor was investigated in a fixed-bed reactor. With the increase of the molar ratio of water to methanol from 0 to 1, the conversions of styrene and nitrobenzene are increased from 26.3% and 7.1% to 100% and 31.9%, respectively, and the selectivity of aniline is increased from 22.0 to 94.5 mol%. The presence of water improves the hydrogen transfer from methanol to styrene or nitrobenzene through the quick reaction of water with formaldehyde, formed from the dehydrogenation of methanol, into formic acid, which is an excellent hydrogen donor for the CTH of unsaturated organics. In the presence of water, methanol is a better hydrogen donor than isopropanol, npropanol and ethanol, because water cannot easily react with acetone, propionaldehyde, and acetaldehyde formed from isopropanol, n-propanol, and ethanol, respectively. Additionally, the hydrogen atom utilization of the methanol donor in the presence of water is higher than the other donors, hydrogen atom in a part of water can also be utilized for the reduction of unsaturated organics. ß 2010 Elsevier B.V. All rights reserved.
Keywords: Water Transfer hydrogenation Methanol Heterogeneous catalysis
1. Introduction Reduction of unsaturated organics by catalytic transfer hydrogenation (CTH) using hydrogen containing compounds as hydrogen donor is an important synthetical methodology in both laboratory and industry [1–5]. In contrast to the catalytic hydrogenation using H2, the CTH using hydrogen donor has the advantages of convenience and safety. Furthermore, the CTH could also potentially afford enhanced activity and/or selectivity in reduction because the properties of hydrogen donor could affect the reductions highly [1]. Palladium has usually been regarded as the most active catalyst [2], and hydrazine, formic acid or formates, and cyclohexene, etc., are normally the effective hydrogen donor for the heterogeneous CTH. Although the alcohols can also be served as the hydrogen donor [1], the primary alcohol is generally less active than the secondary alcohol due to the smaller electronreleasing inductive effect of one alkyl group as against two. Therefore, the primary alcohols (in particular methanol) were rarely used as the hydrogen donor for the heterogeneous CTH [1]. Dumesic and coworkers have introduced a catalytic pathway for hydrogen production from biomass-derivated hydrocarbons, such as methanol, ethylene glycol, and glycerol, by aqueous-phase reforming (APR) over a Pd-, Pt-, and Ni-based catalyst [6–10].
* Corresponding author. Tel.: +86 571 88320409; fax: +86 571 88320409. E-mail address: [email protected] (X. Li). 0926-860X/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.01.004
Additionally, methanol is also a widely used solvent for many reactions including the catalytic hydrogenation of unsaturated organics. Recently, we reported a novel catalytic in situ hydrogenation system by coupling the endothermic APR of alcohols (methanol, etc.) for hydrogen production with the exothermic liquid phase hydrogenation of unsaturated organics [11]. However, the reduction of organics by the catalytic in situ hydrogenation method cannot be realized at low temperature, because the APR of alcohols for hydrogen production is kinetically inhibited due to the high activation barrier [10]. Therefore, the reduction of unsaturated organics by the CTH using methanol as hydrogen donor is quite important [12–14]. However, the CTH of unsaturated organics using methanol as the hydrogen donor is difficult to achieve, because the bond energies of the a-C–H (401.9 kJ mol1) and O–H (437.7 kJ mol1) in methanol are the highest among the aliphatic alcohols [15], and the dehydrogenation of methanol to formaldehyde and hydrogen is thermodynamically inhibited because the reaction is a highly endothermic reversible process [16]. In this paper, we reported that the heterogeneous CTH of unsaturated organics (e.g. nitrobenzene and styrene) using methanol as hydrogen donor can be efficiently achieved in the presence of water under moderate conditions. Pd–Ba/Al2O3 and Pd–Fe/Al2O3 were used as the catalysts to investigate the effect of the presence of water on the CTH of styrene and nitrobenzene using methanol as hydrogen donor, because the two catalysts were found to be more active than the Pd/Al2O3 catalyst for the catalytic in situ hydrogenation of phenol into cyclohexanone [17].
Y. Xiang et al. / Applied Catalysis A: General 375 (2010) 289–294
290
Additionally, the reaction mechanism has been proposed based on the intermediates observed in the reaction and the deuterium tracer experiments. 2. Experimental The Pd–Fe/Al2O3 and Pd–Ba/Al2O3 catalysts were prepared by step-wise incipient wetness impregnation. The commercial gAl2O3 (80–120 mesh, Zibo Boyang Chemical Co. Ltd.) was impregnated with an aqueous solution of Fe(NO3)39H2O (>98.5 wt%) or Ba(NO3)2 (>99 wt%). The impregnation was followed by drying at 383 K for 5 h, and then calcining at 623 K
for 5 h in air. BaO and Fe2O3 were formed during the calcination. The nominal loading of Fe2O3 and BaO are 8 and 2 wt%, respectively. A second impregnation was then carried out on the modified g-Al2O3 support with an aqueous solution of H2PdCl4 (0.05 g/ml, Pd nominal loading 3 wt%), followed by drying at 383 K for 5 h and calcining at 533 K for 5 h in air. The morphology and Pd particle size distribution of the reduced Pd–Fe/Al2O3 and Pd–Ba/Al2O3 catalysts were determined by a JEM2100F TEM at an operating voltage of 200 kV. The BET surface areas of the catalysts were determined by nitrogen physical adsorption at 77 K under vacuum condition by using a NOVA 1000e surface area analyzer (Quantachrome Instruments Corp.).
Fig. 1. TEM micrographs of (A) Pd–Ba/Al2O3 and (B) Pd–Fe/Al2O3 catalysts and (A0 and B0 ) Pd particle size distribution, (C) HRTEM micrographs of Pd–Ba/Al2O3.
Y. Xiang et al. / Applied Catalysis A: General 375 (2010) 289–294
The catalytic transfer hydrogenation (CTH) of unsaturated compounds (styrene, etc.) using methanol as hydrogen donor was carried out in a fixed-bed reactor (a stainless-steel tube with an inner diameter of 8 mm). The typical procedure of the CTH of styrene was described as follows. 1 g of Pd–Fe/Al2O3 catalyst was loaded in the isothermal region of the reactor, and then was reduced in H2 (99.999 vol%) stream with a flow rate of 30 ml min1 at 553 K for 2 h. The reactor pressure was then increased by adding Ar (99.999 vol%) to 2.0 MPa to maintain reaction in liquid phase at 373 or 393 K. The mixed solution of styrene (0.35 mol L1) and methanol (with or without water) at 0.1 ml min1 was co-fed into the reactor using a high-performance liquid chromatography pump (PK564AN-TG10-A2). The gases and condensates were separated in a stainless-steel vessel (about 40 ml) at the system pressure. The liquid effluent was collected and analyzed by a GC– MS instrument (Agilent-6890 GC-5973 MS equipped with 30 m HP-5 capillary) with the external standard method. The gas effluent was analyzed by an on-line GC instrument (Fuli 9790, Porapak Q & 13X molecular sieves columns) equipped with thermo conductive detector (TCD). 3. Results and discussion 3.1. Catalysts characterization The BET surface areas of the Pd–Ba/Al2O3 and Pd–Fe/Al2O3 catalysts are 254.5 and 199.4 m2 g1, respectively. The corresponding pore volumes and pore diameters are 0.29 and 0.25 ml g1, and 4.96 and 4.50 nm, respectively. The TEM micrographs of the two catalysts are shown in Fig. 1. The Pd particle sizes of the Pd–Ba/ Al2O3 and Pd–Fe/Al2O3 catalysts are mainly distributed in 1–3 nm (Fig. 1A0 ) and 2–5 nm (Fig. 1B0 ), respectively. The HRTEM of the Pd–Ba/Al2O3 catalyst exhibit that the interplanar spacing of the Pd particle is 0.225 nm (consistent with the interplanar distance of the Pd (1 1 1) plane), indicating the presence of metallic Pd instead of PdOx on support. 3.2. Effect of water on the CTH using methanol as hydrogen donor The effect of the presence of water on the CTH of styrene over the Pd–Fe/Al2O3 catalyst using methanol as hydrogen donor was investigated at 373 K and 2.0 MPa. The experimental results are shown in the entries 1–5 of Table 1. With the increase of molar ratio of water to methanol from 0 to 0.25, the conversion of styrene raises from 26.3% to 100% in the maintenance of 100 mol% selectivity to ethylbenzene. But when the molar ratio of water to methanol is greater than 0.5, the selectivity of ethylbenzene is decreased slightly. As shown in the entries 6 and 7 of Table 1, the improvement of water on the CTH of styrene has not been observed
291
Table 2 Effect of water on the CTH of nitrobenzene into aniline using methanol as hydrogen donor. Selectivitya (mol%)
Entry
Water/ methanol (mol/mol)
Nitrobenzene conversion (%)
Aniline
NMA
NMEA
NFA
1 2 3 4 5
0 0.1 0.25 0.5 1
7.1 13.6 22.9 25.8 31.9
22.0 75.1 86.7 93.8 94.5
3.4 2.9 1.5 1.0 0.6
74.6 22.0 11.8 4.7 0.7
0.0 0.0 0.0 0.5 4.2
Reactions were carried out over the Pd-Ba/Al2O3 (1 g) at 393 K while other conditions were the same as Table 1. a NMA: N-methylaniline, NMEA: N-methyleneaniline, and NFA: N-formylaniline.
using isopropanol as hydrogen donor instead of methanol. These results suggest that methanol becomes a more effective hydrogen donor than isopropanol for the CTH of styrene in the presence of water, although the reactivity of isopropanol is generally considered to be higher than methanol for the CTH in the anhydrous system [1]. The effect of water on the CTH of nitrobenzene using methanol as hydrogen donor was also investigated over the Pd– Ba/Al2O3 catalyst at 393 K and 2.0 MPa. The experimental results are shown in Table 2. With the increase of the molar ratio of water to methanol from 0 to 1, the conversion of nitrobenzene and the selectivity of aniline are increased from 7.1% to 31.9% and 22.0 to 94.5 mol%, respectively, and the selectivity of Nmethyleneaniline produced from the condensation of formaldehyde and aniline is decreased from 74.6 to 0.7 mol%. The experimental results indicate that formation of by-products, such as N-methyleneaniline and N-methylaniline, can be effectively inhibited with the presence of water, because the reaction of formaldehyde with water to form CO2 will take place easily over the catalyst (the conversions of nitrobenzene and styrene as a function of the yield of CO2 are shown in Fig. 2). Otherwise, small amounts of N-formylaniline (from the reaction of formic acid with aniline) were detected in the CTH of nitrobenzene using methanol as hydrogen donor with the presence of water, which indicates the presence of formic acid, as an intermediate, in the system. To extend the application of the current idea, the CTH of other unsaturated organics like acetophenone, benzaldehyde, and cyclohexanone using methanol (with or without water) as hydrogen donor were also investigated. The experimental results
Table 1 Effect of water on the CTH of styrene into ethylbenzene using methanol as hydrogen donor. Entry
Water/ methanol (mol/mol)
Styrene conversion (%)
Ethylbenzene selectivity (mol%)
CO2 yield (mmol min1)
1 2 3 4 5 6a 7a
0 0.1 0.25 0.5 1 0 0.1
26.3 74.4 100 100 100 57.4 56.6
100 100 100 100 97.6 100 100
0 5.1 7.5 7.2 6.7 0 0
Reactions were carried out in a fixed-bed reactor over the Pd–Fe/Al2O3 (1 g) at 373 K and 2.0 MPa with a liquid flow rate of 0.1 ml min1 (LHSV = 4 h1), the styrene concentration was 0.35 mol L1. a Isopropanol instead of methanol was used as the donor.
Fig. 2. Effect of styrene or nitrobenzene conversion on the yield of CO2 in the gas phase. (&) Nitrobenzene and (*) styrene.
Y. Xiang et al. / Applied Catalysis A: General 375 (2010) 289–294
292
Table 3 Experimental results of the CTH of organics using methanol or other alcohols (with or without water) as hydrogen donor. Substrates
Products
Alcohols
Water/Alcohols (mol/mol)
MeOH
0
Yield (mol%) 2.56
MeOH MeOH
0.5 0
12.88 5.07
MeOH MeOH
0.5 0
20.12 1.31
MeOH MeOH
MeOH EtOH EtOH PrOH PrOH
0.5 0
0.5 0 0.5 0 0.5
5.07
the hydrogenation of phenol and its derivatives [19]. Additionally, Xiao and coworkers have done a lot of works on the homogeneous CTH using formates as hydrogen donor in water solvent [20–22]. Water for the improved heterogeneous CTH of nitroarenes [23], C5 5C double bond [24], and aryl halides [25] using formates as hydrogen donor was also proposed early. Second, the reaction of water with formaldehyde (generated from the dehydrogenation of methanol during the CTH reaction) will form formic acid as excellent hydrogen donor and finally to form CO2, which could overcome the thermodynamic limitation during the CTH of organics using methanol as hydrogen donor. In our experimental, CO2 was detected in the gaseous products, and the presence of Nmethyleneaniline and N-formylaniline in the liquid products indicates the formation of formaldehyde and formic acid, respectively. The yield of CO2 as a function of the conversions of nitrobenzene and styrene is increased linearly with the slopes of 0.4 and 0.1, respectively (Fig. 2).
4.65
3.3. Reaction mechanism
51.45 5.10 12.50 6.66 14.43
The stoichiometric reactions and the schematic representation of the CTH of nitrobenzene using methanol as hydrogen donor are shown in Schemes 1 and 2, respectively. First, the hydrogen transfer from methanol to nitrobenzene causes the formation of formaldehyde and aniline, respectively (Scheme 1 rec. 1a, Scheme 2 step I: this reaction is both kinetically and thermodynamically inhibited as mentioned above). In the presence of water, the formaldehyde is converted into formic acid over the catalyst quickly (Scheme 1 rec. 2, Scheme 2 step III), which accelerates the CTH using methanol as the donor, and additionally, the formic acid is also an effective hydrogen donor for the CTH (Scheme 1 rec. 1b, Scheme 2 step IV) [1]. Moreover, the condensation of aniline and formaldehyde, which leads to the formation of by-products Nmethyleneaniline (Scheme 2 step II), is also highly suppressed with the presence of water. The conversion of formaldehyde into formic acid is supported by the fact that the steam and aqueous-phase reforming of methanol for H2 production are generally taking place according to the following mechanism: CH3OH ! CH3O ! CH2O ! HCOOH ! CO2 + H2 [26,27]. Additionally, N-formylaniline is produced from the dehydration of aniline and formic acid (formed from formaldehyde), Scheme 2 step V. Furthermore, this mechanism is supported by the experimental observation of CO2 (Fig. 2), N-methyleneaniline, and N-formylaniline. To confirm the proposed mechanism, the decomposition of methanol, and the APR of methanol/water mixture for hydrogen production over the Pd–Fe/Al2O3 catalyst under 393 K and 2.0 MPa were investigated. The results suggest that these two reactions could not occur under those conditions, because H2, CO,
Reactions were carried out over the Pd–Fe/Al2O3 (1 g) at 393 K and 2.0 MPa with a liquid flow rate of 0.1 ml min1, the concentration of the substrate was 0.35 mol L1.
are shown in Table 3. The improvements of water (water/ methanol = 0.5 mol/mol) on the CTH of those unsaturated organics using methanol as hydrogen donor have been observed. The ideal yields of the desired products should be realized through the modification of the catalyst and optimization of the reaction conditions in the future. The effect of water on the CTH of nitrobenzene using other primary aliphatic alcohols, such as ethanol and n-propanol, as hydrogen donors was also investigated. The yield of aniline is also increased in the presence of water. But methanol is a more active hydrogen donor than ethanol and npropanol in the CTH of nitrobenzene in the presence of water. The main reasons for the improvement of water on the activity and/or selectivity of the heterogeneous CTH of unsaturated organics by using methanol as hydrogen donor are as follows. First, the presence of water in methanol could favor the hydrogen transfer from methanol to unsaturated organics (i.e. improved the dehydrogenation of methanol into hydrogen and formaldehyde) due to the hydrophobic and aqueous solvation effects [18]. Recently, we also proposed that an aqueous system could improve
Scheme 1. Stoichiometric reactions of the water improved catalytic transfer hydrogenation of nitrobenzene using methanol as hydrogen donor.
Y. Xiang et al. / Applied Catalysis A: General 375 (2010) 289–294
293
Scheme 2. Schematic representation of the catalytic transfer hydrogenation of nitrobenzene using methanol as hydrogen donor.
Scheme 3. Stoichiometric reaction and experimental results for the reaction of nitrobenzene with formaldehyde and water.
and CO2 were not detected in the gaseous products (determined by gas-chromatographs), and CO2 was also not observed in the liquid effluent (determined by clarification limewater, Ksp (CaCO3) = 2.9 109); additionally, liquid products like formaldehyde and formic acid were also not observed. These results indicate that the catalytic in situ hydrogenation process could not occur over the Pd-based catalyst under the conditions of 393 K and 2.0 MPa. The deuterium tracer experiments were performed to gain an insight into the CTH of styrene using methanol as hydrogen donor improved by D2O. The results indicate that the H/D exchange was existed between D2O and methanol at the hydroxyl position (reaction 1) as detected by GC–MS. Fig. 3 shows the mass spectrum of ethylbenzene produced from the CTH of styrene using methanol as hydrogen donor with the presence of D2O. As shown in Fig. 3, the deuterium was observed in the molecules of Ph–C2H3D2 (m/z = 108) and Ph– C2H4D (m/z = 107). The Ph–C2H4D (m/z = 107) was produced from the CTH of styrene using CH3OD as the hydrogen donor (reaction 2), while Ph–C2H3D2 (m/z = 108) was generated through the successive occurrence of reactions 3 and 4. These results also suggest that the H/D exchange occurs between D2O and methanol. Additionally, the reaction of formaldehyde with water to form formic acid (Scheme 1 rec. 2, Scheme 2 step III) is further confirmed. Therefore, hydrogen atom in molecular water can be utilized for the reduction of organics.
The reaction of formaldehyde, water, and nitrobenzene over the Pd–Fe/Al2O3 catalyst under 393 K and 2.0 MPa was also investigated to make sure the proposed mechanism. In this reaction (Scheme 3), the conversion of nitrobenzene was 100%, and the selectivity of N,N-dimethylaniline, N,N-dimethyl-4methylaniline, N-formylaniline, N-formyl-N-methylaniline, and benzenamine, 4,40 -methylenebis[N,N-dimethyl-] were 41.0, 40.6, 6.9, 2.0, and 9.5 mol%, respectively. Additionally, the yield of CO2 on the gaseous products is 54 mmol min1. The observation of N-formylaniline and CO2 indicate the occurrence of the steps III, IV, and V in Scheme 2, i.e. the nitrobenzene can be reduced into aniline by formaldehyde and water through the conversion of formaldehyde with water into formic acid as the reducing reagent. 3.4. Comparison between methanol and the other hydrogen donor In comparison with the CTH using other primary aliphatic alcohols, such as isopropanol, as hydrogen donor, the use of methanol as hydrogen donor for the CTH has several advantages. Water is used as the promoter for the CTH using methanol as hydrogen donor. The CTH using methanol as hydrogen donor has a
Reaction 1.
Reaction 2.
Reaction 3.
Reaction 4.
Fig. 3. Mass spectrum of ethylbenzene produced from the CTH of styrene using methanol as hydrogen donor improved by D2O.
294
Y. Xiang et al. / Applied Catalysis A: General 375 (2010) 289–294
higher hydrogen atom utilization than the CTH using isopropanol as the donor. All hydrogen atoms in methanol molecular can be used for the reduction of unsaturated organics, but only 25% of hydrogen atoms in isopropanol molecular can be utilized. Additionally, hydrogen atom in a part of water can be utilized for the reduction of unsaturated organics, 2 mol hydrogen atoms in water can be utilized with the consumption of 1 mol methanol. Meanwhile, the CTH using methanol as hydrogen source in the presence of water also has higher hydrogen atom utilization than our previous reported catalytic in situ hydrogenation process with hydrogen from the APR of methanol [11], because at high temperature, hydrogen generated from the APR of methanol cannot be fully used for the hydrogenation of unsaturated organics, but molecular H2 was not detected in the gaseous products in the CTH using methanol as hydrogen donor. 4. Conclusions The heterogeneous CTH of unsaturated organics over Pd-based catalyst using methanol as hydrogen donor can be achieved efficiently in the presence of water. Appropriate amounts of water in methanol could increase the conversions of styrene and nitrobenzene and/or selectivity of the desired products in the CTH using methanol as hydrogen donor highly. The deuterium tracer experiments and the mechanistic discussion suggest that the presence of water could improve the CTH using methanol as hydrogen donor through the fast conversion of formaldehyde into formic acid. The hydrogen atom utilization of the CTH using methanol as hydrogen donor in the presence of water is as high as 100%, which is significantly higher than the CTH using other hydrogen donors. Additionally, 2 mol hydrogen atoms in molecular water can also be utilized for the reduction of unsaturated organics with the consumption of 1 mol methanol.
Acknowledgements This work was supported by the Nation Nature Science Foundation of China (NSFC-20976164), the Specialized Research Fund for The Doctoral Program of High Education of China (SRFDP20060337001). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
R.A.W. Johnstone, A.H. Wilby, I.D. Entwistle, Chem. Rev. 85 (1985) 129–170. G. Brieger, T.J. Nestrick, Chem. Rev. 74 (1974) 567–580. F.Z. Su, L. He, J. Ni, Y. Cao, H.Y. He, K.N. Fan, Chem. Commun. (2008) 3531–3533. J. Li, Y.M. Zhang, D.F. Han, G.Q. Jia, J.B. Gao, L. Zhong, C. Li, Green Chem. 10 (2008) 608–611. P. Selvam, S.K. Mohapatra, S.U. Sonavane, R.V. Jayaram, Appl. Catal. B: Environ. 49 (2004) 251–255. R.R. Davda, J.W. Shabaker, G.W. Huber, R.D. Cortright, J.A. Dumesic, Appl. Catal. B: Environ. 56 (2005) 171–186. R.D. Cortright, R.R. Davda, J.A. Dumesic, Nature 418 (2002) 964–967. G.W. Huber, J.W. Shabaker, J.A. Dumesic, Science 300 (2003) 2075–2077. R.R. Davda, J.A. Dumesic, Angew. Chem. Int. Ed. 42 (2003) 4068–4071. J.W. Shabaker, J.A. Dumesic, Ind. Eng. Chem. Res. 43 (2004) 3105–3112. X.N. Li, Y.Z. Xiang, Sci. China B: Chem. 50 (2007) 746–753. H. Itatani, J.C. Bailar, J. Am. Chem. Soc. 89 (1967) 1600–1602. J.C. Bailar, H. Itatani, J. Am. Chem. Soc. 89 (1967) 1592–1599. K. Tani, A. Iseki, T. Yamagata, Chem. Commun. (1999) 1821–1822. Y.R. Luo, Hand Book of Bond Dissociation Energies in Organic Compounds, CRC Press, 2005, 22 & 189. S. Ruf, A. May, G. Emig, Appl. Catal. A: Gen. 213 (2001) 203–215. H.J. Wang, Y.Z. Xiang, T.Y. Xu, H.J. Zhou, L. Ma, X.N. Li, Chin. J. Catal. 30 (2009) 933–938. M.C. Pirrung, Chem. Eur. J. 12 (2006) 1312–1317. Y.Z. Xiang, L. Ma, C.S. Lu, Q.F. Zhang, X.N. Li, Green Chem. 10 (2008) 939–943. X.F. Wu, X.G. Li, W. Hems, F. King, J. Xiao, Org. Biomol. Chem. 2 (2004) 1818–1821. X.F. Wu, X.G. Li, F. King, J. Xiao, Angew. Chem. Int. Ed. 44 (2005) 3407–3411. X.F. Wu, D. Vinci, T. Ikariya, J. Xiao, Chem. Commun. (2005) 4447–4449. H. Wiener, J. Blum, Y. Sasson, J. Org. Chem. 56 (1991) 4481–4486. G. Cavinato, L. Toniolo, J. Mol. Catal. A: Chem. 106 (1996) 25–30. H. Wiener, J. Blum, Y. Sasson, J. Org. Chem. 56 (1991) 6145–6148. E. Jeroro, J.M. Vohs, J. Am. Chem. Soc. 130 (2008) 10199–10207. N. Takezawa, N. Iwasa, Catal. Today 36 (1997) 45–56.
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Water-improved heterogeneous transfer hydrogenation using methanol as hydrogen donor over Pd-based catalyst Yizhi Xiang, Xiaonian Li *, Chunshan Lu, Lei Ma, Qunfeng Zhang State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Institute of Industrial Catalysis, Zhejiang University of Technology, Hangzhou, P.O. Box 310014, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 August 2009 Received in revised form 19 December 2009 Accepted 6 January 2010 Available online 11 January 2010
The heterogeneous catalytic transfer hydrogenation (CTH) of styrene and nitrobenzene over Pd-based catalyst using methanol as hydrogen donor was investigated in a fixed-bed reactor. With the increase of the molar ratio of water to methanol from 0 to 1, the conversions of styrene and nitrobenzene are increased from 26.3% and 7.1% to 100% and 31.9%, respectively, and the selectivity of aniline is increased from 22.0 to 94.5 mol%. The presence of water improves the hydrogen transfer from methanol to styrene or nitrobenzene through the quick reaction of water with formaldehyde, formed from the dehydrogenation of methanol, into formic acid, which is an excellent hydrogen donor for the CTH of unsaturated organics. In the presence of water, methanol is a better hydrogen donor than isopropanol, npropanol and ethanol, because water cannot easily react with acetone, propionaldehyde, and acetaldehyde formed from isopropanol, n-propanol, and ethanol, respectively. Additionally, the hydrogen atom utilization of the methanol donor in the presence of water is higher than the other donors, hydrogen atom in a part of water can also be utilized for the reduction of unsaturated organics. ß 2010 Elsevier B.V. All rights reserved.
Keywords: Water Transfer hydrogenation Methanol Heterogeneous catalysis
1. Introduction Reduction of unsaturated organics by catalytic transfer hydrogenation (CTH) using hydrogen containing compounds as hydrogen donor is an important synthetical methodology in both laboratory and industry [1–5]. In contrast to the catalytic hydrogenation using H2, the CTH using hydrogen donor has the advantages of convenience and safety. Furthermore, the CTH could also potentially afford enhanced activity and/or selectivity in reduction because the properties of hydrogen donor could affect the reductions highly [1]. Palladium has usually been regarded as the most active catalyst [2], and hydrazine, formic acid or formates, and cyclohexene, etc., are normally the effective hydrogen donor for the heterogeneous CTH. Although the alcohols can also be served as the hydrogen donor [1], the primary alcohol is generally less active than the secondary alcohol due to the smaller electronreleasing inductive effect of one alkyl group as against two. Therefore, the primary alcohols (in particular methanol) were rarely used as the hydrogen donor for the heterogeneous CTH [1]. Dumesic and coworkers have introduced a catalytic pathway for hydrogen production from biomass-derivated hydrocarbons, such as methanol, ethylene glycol, and glycerol, by aqueous-phase reforming (APR) over a Pd-, Pt-, and Ni-based catalyst [6–10].
* Corresponding author. Tel.: +86 571 88320409; fax: +86 571 88320409. E-mail address: [email protected] (X. Li). 0926-860X/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.01.004
Additionally, methanol is also a widely used solvent for many reactions including the catalytic hydrogenation of unsaturated organics. Recently, we reported a novel catalytic in situ hydrogenation system by coupling the endothermic APR of alcohols (methanol, etc.) for hydrogen production with the exothermic liquid phase hydrogenation of unsaturated organics [11]. However, the reduction of organics by the catalytic in situ hydrogenation method cannot be realized at low temperature, because the APR of alcohols for hydrogen production is kinetically inhibited due to the high activation barrier [10]. Therefore, the reduction of unsaturated organics by the CTH using methanol as hydrogen donor is quite important [12–14]. However, the CTH of unsaturated organics using methanol as the hydrogen donor is difficult to achieve, because the bond energies of the a-C–H (401.9 kJ mol1) and O–H (437.7 kJ mol1) in methanol are the highest among the aliphatic alcohols [15], and the dehydrogenation of methanol to formaldehyde and hydrogen is thermodynamically inhibited because the reaction is a highly endothermic reversible process [16]. In this paper, we reported that the heterogeneous CTH of unsaturated organics (e.g. nitrobenzene and styrene) using methanol as hydrogen donor can be efficiently achieved in the presence of water under moderate conditions. Pd–Ba/Al2O3 and Pd–Fe/Al2O3 were used as the catalysts to investigate the effect of the presence of water on the CTH of styrene and nitrobenzene using methanol as hydrogen donor, because the two catalysts were found to be more active than the Pd/Al2O3 catalyst for the catalytic in situ hydrogenation of phenol into cyclohexanone [17].
Y. Xiang et al. / Applied Catalysis A: General 375 (2010) 289–294
290
Additionally, the reaction mechanism has been proposed based on the intermediates observed in the reaction and the deuterium tracer experiments. 2. Experimental The Pd–Fe/Al2O3 and Pd–Ba/Al2O3 catalysts were prepared by step-wise incipient wetness impregnation. The commercial gAl2O3 (80–120 mesh, Zibo Boyang Chemical Co. Ltd.) was impregnated with an aqueous solution of Fe(NO3)39H2O (>98.5 wt%) or Ba(NO3)2 (>99 wt%). The impregnation was followed by drying at 383 K for 5 h, and then calcining at 623 K
for 5 h in air. BaO and Fe2O3 were formed during the calcination. The nominal loading of Fe2O3 and BaO are 8 and 2 wt%, respectively. A second impregnation was then carried out on the modified g-Al2O3 support with an aqueous solution of H2PdCl4 (0.05 g/ml, Pd nominal loading 3 wt%), followed by drying at 383 K for 5 h and calcining at 533 K for 5 h in air. The morphology and Pd particle size distribution of the reduced Pd–Fe/Al2O3 and Pd–Ba/Al2O3 catalysts were determined by a JEM2100F TEM at an operating voltage of 200 kV. The BET surface areas of the catalysts were determined by nitrogen physical adsorption at 77 K under vacuum condition by using a NOVA 1000e surface area analyzer (Quantachrome Instruments Corp.).
Fig. 1. TEM micrographs of (A) Pd–Ba/Al2O3 and (B) Pd–Fe/Al2O3 catalysts and (A0 and B0 ) Pd particle size distribution, (C) HRTEM micrographs of Pd–Ba/Al2O3.
Y. Xiang et al. / Applied Catalysis A: General 375 (2010) 289–294
The catalytic transfer hydrogenation (CTH) of unsaturated compounds (styrene, etc.) using methanol as hydrogen donor was carried out in a fixed-bed reactor (a stainless-steel tube with an inner diameter of 8 mm). The typical procedure of the CTH of styrene was described as follows. 1 g of Pd–Fe/Al2O3 catalyst was loaded in the isothermal region of the reactor, and then was reduced in H2 (99.999 vol%) stream with a flow rate of 30 ml min1 at 553 K for 2 h. The reactor pressure was then increased by adding Ar (99.999 vol%) to 2.0 MPa to maintain reaction in liquid phase at 373 or 393 K. The mixed solution of styrene (0.35 mol L1) and methanol (with or without water) at 0.1 ml min1 was co-fed into the reactor using a high-performance liquid chromatography pump (PK564AN-TG10-A2). The gases and condensates were separated in a stainless-steel vessel (about 40 ml) at the system pressure. The liquid effluent was collected and analyzed by a GC– MS instrument (Agilent-6890 GC-5973 MS equipped with 30 m HP-5 capillary) with the external standard method. The gas effluent was analyzed by an on-line GC instrument (Fuli 9790, Porapak Q & 13X molecular sieves columns) equipped with thermo conductive detector (TCD). 3. Results and discussion 3.1. Catalysts characterization The BET surface areas of the Pd–Ba/Al2O3 and Pd–Fe/Al2O3 catalysts are 254.5 and 199.4 m2 g1, respectively. The corresponding pore volumes and pore diameters are 0.29 and 0.25 ml g1, and 4.96 and 4.50 nm, respectively. The TEM micrographs of the two catalysts are shown in Fig. 1. The Pd particle sizes of the Pd–Ba/ Al2O3 and Pd–Fe/Al2O3 catalysts are mainly distributed in 1–3 nm (Fig. 1A0 ) and 2–5 nm (Fig. 1B0 ), respectively. The HRTEM of the Pd–Ba/Al2O3 catalyst exhibit that the interplanar spacing of the Pd particle is 0.225 nm (consistent with the interplanar distance of the Pd (1 1 1) plane), indicating the presence of metallic Pd instead of PdOx on support. 3.2. Effect of water on the CTH using methanol as hydrogen donor The effect of the presence of water on the CTH of styrene over the Pd–Fe/Al2O3 catalyst using methanol as hydrogen donor was investigated at 373 K and 2.0 MPa. The experimental results are shown in the entries 1–5 of Table 1. With the increase of molar ratio of water to methanol from 0 to 0.25, the conversion of styrene raises from 26.3% to 100% in the maintenance of 100 mol% selectivity to ethylbenzene. But when the molar ratio of water to methanol is greater than 0.5, the selectivity of ethylbenzene is decreased slightly. As shown in the entries 6 and 7 of Table 1, the improvement of water on the CTH of styrene has not been observed
291
Table 2 Effect of water on the CTH of nitrobenzene into aniline using methanol as hydrogen donor. Selectivitya (mol%)
Entry
Water/ methanol (mol/mol)
Nitrobenzene conversion (%)
Aniline
NMA
NMEA
NFA
1 2 3 4 5
0 0.1 0.25 0.5 1
7.1 13.6 22.9 25.8 31.9
22.0 75.1 86.7 93.8 94.5
3.4 2.9 1.5 1.0 0.6
74.6 22.0 11.8 4.7 0.7
0.0 0.0 0.0 0.5 4.2
Reactions were carried out over the Pd-Ba/Al2O3 (1 g) at 393 K while other conditions were the same as Table 1. a NMA: N-methylaniline, NMEA: N-methyleneaniline, and NFA: N-formylaniline.
using isopropanol as hydrogen donor instead of methanol. These results suggest that methanol becomes a more effective hydrogen donor than isopropanol for the CTH of styrene in the presence of water, although the reactivity of isopropanol is generally considered to be higher than methanol for the CTH in the anhydrous system [1]. The effect of water on the CTH of nitrobenzene using methanol as hydrogen donor was also investigated over the Pd– Ba/Al2O3 catalyst at 393 K and 2.0 MPa. The experimental results are shown in Table 2. With the increase of the molar ratio of water to methanol from 0 to 1, the conversion of nitrobenzene and the selectivity of aniline are increased from 7.1% to 31.9% and 22.0 to 94.5 mol%, respectively, and the selectivity of Nmethyleneaniline produced from the condensation of formaldehyde and aniline is decreased from 74.6 to 0.7 mol%. The experimental results indicate that formation of by-products, such as N-methyleneaniline and N-methylaniline, can be effectively inhibited with the presence of water, because the reaction of formaldehyde with water to form CO2 will take place easily over the catalyst (the conversions of nitrobenzene and styrene as a function of the yield of CO2 are shown in Fig. 2). Otherwise, small amounts of N-formylaniline (from the reaction of formic acid with aniline) were detected in the CTH of nitrobenzene using methanol as hydrogen donor with the presence of water, which indicates the presence of formic acid, as an intermediate, in the system. To extend the application of the current idea, the CTH of other unsaturated organics like acetophenone, benzaldehyde, and cyclohexanone using methanol (with or without water) as hydrogen donor were also investigated. The experimental results
Table 1 Effect of water on the CTH of styrene into ethylbenzene using methanol as hydrogen donor. Entry
Water/ methanol (mol/mol)
Styrene conversion (%)
Ethylbenzene selectivity (mol%)
CO2 yield (mmol min1)
1 2 3 4 5 6a 7a
0 0.1 0.25 0.5 1 0 0.1
26.3 74.4 100 100 100 57.4 56.6
100 100 100 100 97.6 100 100
0 5.1 7.5 7.2 6.7 0 0
Reactions were carried out in a fixed-bed reactor over the Pd–Fe/Al2O3 (1 g) at 373 K and 2.0 MPa with a liquid flow rate of 0.1 ml min1 (LHSV = 4 h1), the styrene concentration was 0.35 mol L1. a Isopropanol instead of methanol was used as the donor.
Fig. 2. Effect of styrene or nitrobenzene conversion on the yield of CO2 in the gas phase. (&) Nitrobenzene and (*) styrene.
Y. Xiang et al. / Applied Catalysis A: General 375 (2010) 289–294
292
Table 3 Experimental results of the CTH of organics using methanol or other alcohols (with or without water) as hydrogen donor. Substrates
Products
Alcohols
Water/Alcohols (mol/mol)
MeOH
0
Yield (mol%) 2.56
MeOH MeOH
0.5 0
12.88 5.07
MeOH MeOH
0.5 0
20.12 1.31
MeOH MeOH
MeOH EtOH EtOH PrOH PrOH
0.5 0
0.5 0 0.5 0 0.5
5.07
the hydrogenation of phenol and its derivatives [19]. Additionally, Xiao and coworkers have done a lot of works on the homogeneous CTH using formates as hydrogen donor in water solvent [20–22]. Water for the improved heterogeneous CTH of nitroarenes [23], C5 5C double bond [24], and aryl halides [25] using formates as hydrogen donor was also proposed early. Second, the reaction of water with formaldehyde (generated from the dehydrogenation of methanol during the CTH reaction) will form formic acid as excellent hydrogen donor and finally to form CO2, which could overcome the thermodynamic limitation during the CTH of organics using methanol as hydrogen donor. In our experimental, CO2 was detected in the gaseous products, and the presence of Nmethyleneaniline and N-formylaniline in the liquid products indicates the formation of formaldehyde and formic acid, respectively. The yield of CO2 as a function of the conversions of nitrobenzene and styrene is increased linearly with the slopes of 0.4 and 0.1, respectively (Fig. 2).
4.65
3.3. Reaction mechanism
51.45 5.10 12.50 6.66 14.43
The stoichiometric reactions and the schematic representation of the CTH of nitrobenzene using methanol as hydrogen donor are shown in Schemes 1 and 2, respectively. First, the hydrogen transfer from methanol to nitrobenzene causes the formation of formaldehyde and aniline, respectively (Scheme 1 rec. 1a, Scheme 2 step I: this reaction is both kinetically and thermodynamically inhibited as mentioned above). In the presence of water, the formaldehyde is converted into formic acid over the catalyst quickly (Scheme 1 rec. 2, Scheme 2 step III), which accelerates the CTH using methanol as the donor, and additionally, the formic acid is also an effective hydrogen donor for the CTH (Scheme 1 rec. 1b, Scheme 2 step IV) [1]. Moreover, the condensation of aniline and formaldehyde, which leads to the formation of by-products Nmethyleneaniline (Scheme 2 step II), is also highly suppressed with the presence of water. The conversion of formaldehyde into formic acid is supported by the fact that the steam and aqueous-phase reforming of methanol for H2 production are generally taking place according to the following mechanism: CH3OH ! CH3O ! CH2O ! HCOOH ! CO2 + H2 [26,27]. Additionally, N-formylaniline is produced from the dehydration of aniline and formic acid (formed from formaldehyde), Scheme 2 step V. Furthermore, this mechanism is supported by the experimental observation of CO2 (Fig. 2), N-methyleneaniline, and N-formylaniline. To confirm the proposed mechanism, the decomposition of methanol, and the APR of methanol/water mixture for hydrogen production over the Pd–Fe/Al2O3 catalyst under 393 K and 2.0 MPa were investigated. The results suggest that these two reactions could not occur under those conditions, because H2, CO,
Reactions were carried out over the Pd–Fe/Al2O3 (1 g) at 393 K and 2.0 MPa with a liquid flow rate of 0.1 ml min1, the concentration of the substrate was 0.35 mol L1.
are shown in Table 3. The improvements of water (water/ methanol = 0.5 mol/mol) on the CTH of those unsaturated organics using methanol as hydrogen donor have been observed. The ideal yields of the desired products should be realized through the modification of the catalyst and optimization of the reaction conditions in the future. The effect of water on the CTH of nitrobenzene using other primary aliphatic alcohols, such as ethanol and n-propanol, as hydrogen donors was also investigated. The yield of aniline is also increased in the presence of water. But methanol is a more active hydrogen donor than ethanol and npropanol in the CTH of nitrobenzene in the presence of water. The main reasons for the improvement of water on the activity and/or selectivity of the heterogeneous CTH of unsaturated organics by using methanol as hydrogen donor are as follows. First, the presence of water in methanol could favor the hydrogen transfer from methanol to unsaturated organics (i.e. improved the dehydrogenation of methanol into hydrogen and formaldehyde) due to the hydrophobic and aqueous solvation effects [18]. Recently, we also proposed that an aqueous system could improve
Scheme 1. Stoichiometric reactions of the water improved catalytic transfer hydrogenation of nitrobenzene using methanol as hydrogen donor.
Y. Xiang et al. / Applied Catalysis A: General 375 (2010) 289–294
293
Scheme 2. Schematic representation of the catalytic transfer hydrogenation of nitrobenzene using methanol as hydrogen donor.
Scheme 3. Stoichiometric reaction and experimental results for the reaction of nitrobenzene with formaldehyde and water.
and CO2 were not detected in the gaseous products (determined by gas-chromatographs), and CO2 was also not observed in the liquid effluent (determined by clarification limewater, Ksp (CaCO3) = 2.9 109); additionally, liquid products like formaldehyde and formic acid were also not observed. These results indicate that the catalytic in situ hydrogenation process could not occur over the Pd-based catalyst under the conditions of 393 K and 2.0 MPa. The deuterium tracer experiments were performed to gain an insight into the CTH of styrene using methanol as hydrogen donor improved by D2O. The results indicate that the H/D exchange was existed between D2O and methanol at the hydroxyl position (reaction 1) as detected by GC–MS. Fig. 3 shows the mass spectrum of ethylbenzene produced from the CTH of styrene using methanol as hydrogen donor with the presence of D2O. As shown in Fig. 3, the deuterium was observed in the molecules of Ph–C2H3D2 (m/z = 108) and Ph– C2H4D (m/z = 107). The Ph–C2H4D (m/z = 107) was produced from the CTH of styrene using CH3OD as the hydrogen donor (reaction 2), while Ph–C2H3D2 (m/z = 108) was generated through the successive occurrence of reactions 3 and 4. These results also suggest that the H/D exchange occurs between D2O and methanol. Additionally, the reaction of formaldehyde with water to form formic acid (Scheme 1 rec. 2, Scheme 2 step III) is further confirmed. Therefore, hydrogen atom in molecular water can be utilized for the reduction of organics.
The reaction of formaldehyde, water, and nitrobenzene over the Pd–Fe/Al2O3 catalyst under 393 K and 2.0 MPa was also investigated to make sure the proposed mechanism. In this reaction (Scheme 3), the conversion of nitrobenzene was 100%, and the selectivity of N,N-dimethylaniline, N,N-dimethyl-4methylaniline, N-formylaniline, N-formyl-N-methylaniline, and benzenamine, 4,40 -methylenebis[N,N-dimethyl-] were 41.0, 40.6, 6.9, 2.0, and 9.5 mol%, respectively. Additionally, the yield of CO2 on the gaseous products is 54 mmol min1. The observation of N-formylaniline and CO2 indicate the occurrence of the steps III, IV, and V in Scheme 2, i.e. the nitrobenzene can be reduced into aniline by formaldehyde and water through the conversion of formaldehyde with water into formic acid as the reducing reagent. 3.4. Comparison between methanol and the other hydrogen donor In comparison with the CTH using other primary aliphatic alcohols, such as isopropanol, as hydrogen donor, the use of methanol as hydrogen donor for the CTH has several advantages. Water is used as the promoter for the CTH using methanol as hydrogen donor. The CTH using methanol as hydrogen donor has a
Reaction 1.
Reaction 2.
Reaction 3.
Reaction 4.
Fig. 3. Mass spectrum of ethylbenzene produced from the CTH of styrene using methanol as hydrogen donor improved by D2O.
294
Y. Xiang et al. / Applied Catalysis A: General 375 (2010) 289–294
higher hydrogen atom utilization than the CTH using isopropanol as the donor. All hydrogen atoms in methanol molecular can be used for the reduction of unsaturated organics, but only 25% of hydrogen atoms in isopropanol molecular can be utilized. Additionally, hydrogen atom in a part of water can be utilized for the reduction of unsaturated organics, 2 mol hydrogen atoms in water can be utilized with the consumption of 1 mol methanol. Meanwhile, the CTH using methanol as hydrogen source in the presence of water also has higher hydrogen atom utilization than our previous reported catalytic in situ hydrogenation process with hydrogen from the APR of methanol [11], because at high temperature, hydrogen generated from the APR of methanol cannot be fully used for the hydrogenation of unsaturated organics, but molecular H2 was not detected in the gaseous products in the CTH using methanol as hydrogen donor. 4. Conclusions The heterogeneous CTH of unsaturated organics over Pd-based catalyst using methanol as hydrogen donor can be achieved efficiently in the presence of water. Appropriate amounts of water in methanol could increase the conversions of styrene and nitrobenzene and/or selectivity of the desired products in the CTH using methanol as hydrogen donor highly. The deuterium tracer experiments and the mechanistic discussion suggest that the presence of water could improve the CTH using methanol as hydrogen donor through the fast conversion of formaldehyde into formic acid. The hydrogen atom utilization of the CTH using methanol as hydrogen donor in the presence of water is as high as 100%, which is significantly higher than the CTH using other hydrogen donors. Additionally, 2 mol hydrogen atoms in molecular water can also be utilized for the reduction of unsaturated organics with the consumption of 1 mol methanol.
Acknowledgements This work was supported by the Nation Nature Science Foundation of China (NSFC-20976164), the Specialized Research Fund for The Doctoral Program of High Education of China (SRFDP20060337001). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
R.A.W. Johnstone, A.H. Wilby, I.D. Entwistle, Chem. Rev. 85 (1985) 129–170. G. Brieger, T.J. Nestrick, Chem. Rev. 74 (1974) 567–580. F.Z. Su, L. He, J. Ni, Y. Cao, H.Y. He, K.N. Fan, Chem. Commun. (2008) 3531–3533. J. Li, Y.M. Zhang, D.F. Han, G.Q. Jia, J.B. Gao, L. Zhong, C. Li, Green Chem. 10 (2008) 608–611. P. Selvam, S.K. Mohapatra, S.U. Sonavane, R.V. Jayaram, Appl. Catal. B: Environ. 49 (2004) 251–255. R.R. Davda, J.W. Shabaker, G.W. Huber, R.D. Cortright, J.A. Dumesic, Appl. Catal. B: Environ. 56 (2005) 171–186. R.D. Cortright, R.R. Davda, J.A. Dumesic, Nature 418 (2002) 964–967. G.W. Huber, J.W. Shabaker, J.A. Dumesic, Science 300 (2003) 2075–2077. R.R. Davda, J.A. Dumesic, Angew. Chem. Int. Ed. 42 (2003) 4068–4071. J.W. Shabaker, J.A. Dumesic, Ind. Eng. Chem. Res. 43 (2004) 3105–3112. X.N. Li, Y.Z. Xiang, Sci. China B: Chem. 50 (2007) 746–753. H. Itatani, J.C. Bailar, J. Am. Chem. Soc. 89 (1967) 1600–1602. J.C. Bailar, H. Itatani, J. Am. Chem. Soc. 89 (1967) 1592–1599. K. Tani, A. Iseki, T. Yamagata, Chem. Commun. (1999) 1821–1822. Y.R. Luo, Hand Book of Bond Dissociation Energies in Organic Compounds, CRC Press, 2005, 22 & 189. S. Ruf, A. May, G. Emig, Appl. Catal. A: Gen. 213 (2001) 203–215. H.J. Wang, Y.Z. Xiang, T.Y. Xu, H.J. Zhou, L. Ma, X.N. Li, Chin. J. Catal. 30 (2009) 933–938. M.C. Pirrung, Chem. Eur. J. 12 (2006) 1312–1317. Y.Z. Xiang, L. Ma, C.S. Lu, Q.F. Zhang, X.N. Li, Green Chem. 10 (2008) 939–943. X.F. Wu, X.G. Li, W. Hems, F. King, J. Xiao, Org. Biomol. Chem. 2 (2004) 1818–1821. X.F. Wu, X.G. Li, F. King, J. Xiao, Angew. Chem. Int. Ed. 44 (2005) 3407–3411. X.F. Wu, D. Vinci, T. Ikariya, J. Xiao, Chem. Commun. (2005) 4447–4449. H. Wiener, J. Blum, Y. Sasson, J. Org. Chem. 56 (1991) 4481–4486. G. Cavinato, L. Toniolo, J. Mol. Catal. A: Chem. 106 (1996) 25–30. H. Wiener, J. Blum, Y. Sasson, J. Org. Chem. 56 (1991) 6145–6148. E. Jeroro, J.M. Vohs, J. Am. Chem. Soc. 130 (2008) 10199–10207. N. Takezawa, N. Iwasa, Catal. Today 36 (1997) 45–56.