Reaction Performance of Hydrogen from Aqueous-Phase Reforming

Feb 20, 2011 - David Gonzalez-Galvez , Patricia Lara , Orestes Rivada-Wheelaghan , Salvador Conejero , Bruno Chaudret , Karine Philippot , Piet W. N. ...
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Reaction Performance of Hydrogen from Aqueous-Phase Reforming of Methanol or Ethanol in Hydrogenation of Phenol Yizhi Xiang, Xiaonian Li,* Chunshan Lu, Lei Ma, Junfeng Yuan, and Feng Feng Institute of Industrial Catalysis, Zhejiang University of Technology, State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, Hangzhou, 310014, P. R. China ABSTRACT: Reaction performances of hydrogen generated from aqueous-phase reforming (APR) of methanol or ethanol were investigated in hydrogenation of phenol, o-cresol and p-tert-butylphenol, over a Raney Ni or Pd/Al2O3 catalyst, respectively. The properties of the adsorbed hydrogen from aqueous-methanol/-ethanol, and H2-gas were studied by means of temperatureprogrammed surface reaction in liquid phase and temperature-programmed desorption. The results shown that the selectivities of cyclohexanone are 20.5% and 96.1%, 27.4% and 92.4%, and 12.6% and 71.1% in the hydrogenation of phenol with hydrogen from APR of methanol, APR of ethanol, and H2-gas over the Raney Ni and Pd/Al2O3 catalyst, respectively. The limited amount of adsorbed hydrogen from the APR of methanol or ethanol favors the formation of cyclohexanone, but excessive amounts of adsorbed hydrogen from H2-gas favors the formation of cyclohexanol in hydrogenation of phenol.

’ INTRODUCTION Cyclohexanol or alkyl-substituted cyclohexanols, and cyclohexanone are important intermediates in the synthesis of nylons 6 and 66,1 pharmaceuticals, fine chemicals, etc.2 These compounds can be synthesized from the hydrogenation of phenol and alkyl-substituted phenols,3 besides the catalytic oxidation of cyclohexane.4 Generally, the hydrogenation of phenol and its derivatives (phenol, o-cresol, and p-tert-butylphenol) are carried out in the presence of H2 in gaseous5 or gas-liquid phase6 over supported Pd,5,7 Pt,8 Rh,3 or Ni6 catalyst. The adsorption performances of phenolics and hydrogen affect the hydrogenation of phenol highly.6,9 Recently, we achieved the liquid phase hydrogenation of organics (nitrobenzene, acetophenone, phenol, etc.) using aqueous-methanol as hydrogen source through in situ generation of hydrogen from aqueous-phase reforming (APR) or dehydrogenation of methanol.10-14 As an extension, the direct synthesis of imines from nitroarenes and carbonyl compounds using methanol as hydrogen source was also proposed.15 In these previous studies, we observed that the reaction performance of hydrogen generated from aqueous-methanol is different from that of H2-gas in the catalytic hydrogenation of organics. The selectivity of cyclohexanol and cyclohexanone in total for the hydrogenation of phenol with hydrogen from the APR of methanol is higher than that with H2-gas. However, the differences between the properties of the adsorbed hydrogen from the APR of methanol or ethanol and from H2-gas, and their corresponding reaction performances have not been investigated in the hydrogenation of phenol. In this study, the hydrogenation of phenol, o-cresol and p-tertbutylphenol with hydrogen generated in situ from the APR of methanol or ethanol, or H2-gas was investigated over a Raney Ni and Pd/Al2O3 catalyst, respectively. The properties of the adsorbed hydrogen from the APR of methanol or ethanol, or r 2011 American Chemical Society

dissociation of H2-gas, and their reaction performances in the catalytic hydrogenation of phenol were studied by means of temperature-programmed desorption (TPD) and temperatureprogrammed surface reaction (TPSR) in liquid phase. Additionally, the hydrogenation mechanisms of phenol, with hydrogen from the APR of methanol or ethanol, or H2-gas, into cyclohexanone and cyclohexanol, respectively, were also discussed.

’ EXPERIMENTAL SECTION Catalyst Preparation. The Pd/Al2O3 catalyst was prepared by incipient wetness impregnation of commercial γ-Al2O3 (250 m2.g-1, 80-120 mesh, Zibo Boyang Chemical Co. Ltd.) with an aqueous solution of H2PdCl4 (0.05 g/mL, Pd nominal loading 3 wt %). The impregnation was followed by drying at 383 K for 5 h and calcining at 573 K for 5 h in air. Prior to the catalytic activity test, the catalyst was reduced in situ at 553 K (a temperature ramp of 1 K min-1) for 2 h in flowing of H2 (99.999%) at a rate of 30 mL min-1. The Raney Ni catalyst was prepared by leaching Al from a metallic alloy powder of Ni and Al (mass ratio = 9:11) with a NaOH solution (20%) until the content of Al less than 6 wt %, which is followed by washing with distilled water at 343-353 K until the pH value reached at about 7-8.10 Characterization. The TPSR in liquid phase was carried out in an Φ 5 mm stainless steel tubular reactor. The Raney Ni catalyst (1 g) was loaded in the isothermal region of the reactor, then swept with water (0.05 mL min-1) in Ar (20 mL min-1) at 500 K overnight to remove the adsorbed hydrogen on the surface of the Raney Ni catalyst formed during the leaching of Al by Received: July 2, 2010 Revised: December 25, 2010 Accepted: February 2, 2011 Published: February 20, 2011 3139

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Table 1. Hydrogenation Performance of Phenol and Alkyl-Phenol by H2-Gas and Hydrogen from the Aqueous-Phase Reforming of Methanol or Ethanol over the Raney Ni or Pd/Al2O3 Catalystsa C-yieldb

selectivity (%) catalyst

substrate

hydrogen source

conversion (%)

alcohols

ketones

othersc

CH4

CO2

25.6

43.6

Raney Ni

phenol

MeOH

53.1

79.2

20.5

0.3

Raney Ni

phenol

H2

82.3

74.1

12.6

13.3

Pd/Al2O3 Pd/Al2O3

phenol phenol

MeOH H2

18.1 82.5

3.2 28.6

96.1 71.1

0.7 0.3

Raney Ni

phenol

EtOH

21.2

68.4

27.4

4.2

138.5

61.1

Pd/Al2O3

phenol

EtOH

40.4

1.4

92.4

6.2

21.7

23.4

Raney Ni

o-cresol

MeOH

34.1

74.1

25.7

0.2

24.1

Pd/Al2O3

o-cresol

MeOH

8.7

3.0

97.0

0

Raney Ni

p-tert-butylphenol

MeOH

2.5

95.4

4.5

Pd/Al2O3

p-tert-butylphenol

MeOH

0.65

5.4

94.6

0.1 0

16.4

42.0 15.7

22.5

40.2 15.3

a Reaction conditions: T = 490 K, P = 3.5 MPa, LHSV = 3.5 h-1, substrate/alcohol/water (molar ratio) = 1:20:80. b Carbon (CH4 and CO2) yield in the gaseous phase (μmol.gcat-1 min-1). c Others including benzene, cyclohexane, and cresols.

NaOH. Subsequently, the catalyst was treated with H2 (10 mL min-1) or aqueous-methanol/-ethanol (10 wt.%, 0.2 mL.min-1) at 433 K and 1.5 MPa for 1 h, and then cooled down to 323 K. Finally, the catalyst was increased from 323 to 503 K at a heating rate of 1 K min-1 in an aqueous solution of phenol (0.01 mol L-1) with a flow rate of 0.2 mL min-1. The system pressure was maintained at 3.0 MPa by Ar (99.999%) to make the occurrence of the TPSR in the liquid phase. The liquid effluent from the reactor was analyzed once every 5 or 10 min by a GC (Japan Shimadzu GC-14B) equipped with 30mHP-5 capillary and FID detector. The TPD experiments of the Raney Ni catalyst pretreated with H2, APR of methanol or ethanol, separately, were carried out in an Φ 5 mm quartz tubular reactor equipped with a mass spectrometer (QMS 200 Omnistar); 0.5 g of pretreated catalyst (with H2 (10 mL min-1) or hydrogen from the APR of methanol or ethanol (10 wt.%, 0.2 mL min-1) at 433 K and 1.5 MPa for 1 h) was first transferred from the stainless steel tubular reactor into the isothermal region of the quartz tubular reactor. Then the sample was dried at 373 K in He (30 mL min-1) for 12 h and finally heated from 303 to 1073 K at a rate of 10 K min-1. The signals of H2 (m/z = 2), CH4 (m/z = 16), and COx (m/z = 44 and 28) were collected simultaneously by the online mass spectrometer. Hydrogenation of Phenol and Its Derivatives. The hydrogenation of phenol, o-cresol, or p-tert-butylphenol with hydrogen from the APR of methanol or ethanol was carried out in an Φ 8 mm stainless steel tubular reactor at 490 K and 3.5 MPa in Ar (99.999%) atmospheric maintained by a back pressure valve. The Pd/Al2O3 (1.0 g) or Raney Ni (1.25 g dry mass) catalyst was loaded in the isothermal region of the reactor. A mixture solution of phenol, o-cresol, or p-tert-butylphenol (Hangzhou Shuanglin Chemical Reagents Factory) and water and alcohols (methanol or ethanol, Quzhou Juhua Reagents Co. Ltd.) was fed into the reactor at 0.1 mL.min-1 using a HPLC pump (PK564AN-TG10-A2). The reactor effluent was separated in a stainless steel vessel (about 40 mL) at the system pressure. The liquid components from the reactor were analyzed once every hour by a GC-MS instrument (Agilent-6890 GC-5973 MS equipped with 30 m HP5 capillary) with the external standard method. The gas components were analyzed by an online GC instrument (Fuli 9790, Porapak Q and 13 molecular sieves columns) equipped with thermo conductive detector (TCD).

The hydrogenation of phenol with H2-gas was also carried out in the Φ 8 mm stainless steel tubular reactor at 490 K and 3.5 MPa Ar (99.999%) atmospheric. A H2 (99.999%) was cofed at 10 mL min-1 with the mixed solution into the reactor (partial pressure of H2 out of the reactor is similar with that using methanol as hydrogen source). The separation and analysis of the reactor effluent was the same as have been mentioned above.

’ RESULTS AND DISCUSSION Hydrogenation of Phenol and Its Derivatives with Different Hydrogen Sources. As shown in Table 1, the conversion of

phenol is 53.1% or 21.2%, and the selectivity of cyclohexanone and cyclohexanol in total is 99.7% or 95.8% in the hydrogenation of phenol with hydrogen generated in situ from the APR of methanol or ethanol over the Raney Ni catalyst. However, the conversion of phenol and selectivity of cyclohexanone and cyclohexanol in total are 82.3% and 86.7%, respectively, in the hydrogenation of phenol with H2-gas. Additionally, the selectivities of cyclohexanone are 20.5% and 96.1%, 27.4% and 92.4%, and 12.6% and 71.1% for the hydrogenation of phenol with hydrogen from APR of methanol, APR of ethanol, and H2-gas over the Raney Ni and Pd/Al2O3 catalyst, respectively. Otherwise, the Pd/Al2O3 catalyst is less active than the Raney Ni for the in situ hydrogenation of phenol with hydrogen from the APR of methanol, but the Pd/Al2O3 is more active than the Raney Ni with hydrogen from APR of ethanol. These results indicated that the reaction performance of the hydrogen generated in situ from the APR of methanol and ethanol should be different from that of H2-gas in the hydrogenation of phenol. During the hydrogenation of phenol using aqueous-methanol/ -ethanol as hydrogen source, the production of gaseous carboncontaining products (C-yield, that is, the yield of CH4 and CO2) proves the occurrence of the APR of methanol or ethanol. The hydrogenation of phenol with hydrogen from the APR of methanol on the Raney Ni catalyst shows higher conversion than that on the Pd/Al2O3 catalyst. Correspondingly, higher amounts of CO2 and CH4 were detected on the Raney Ni catalyst (43.6 and 25.6 μmol.gcat-1 min-1, respectively) than on the Pd/ Al2O3 catalyst (0 and 16.4 μmol gcat-1 min-1). The hydrogenation of phenol with hydrogen from the APR of ethanol, however, is quite different from that with hydrogen from the APR of 3140

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Figure 1. TPD profiles of the Raney Ni catalyst pretreated with H2-gas (1), aqueous-ethanol (2), and aqueous-methanol (3), respectively. (a) Desorption of H2 and (b) desorption of COx and CH4.

methanol. In addition, the occurrence of the catalytic transfer hydrogenation of phenol using ethanol as hydrogen donor could also be taken into consideration. TPD Study. The TPD profiles of the adsorbed hydrogen on the Raney Ni catalyst from H2-gas, and APR of methanol or ethanol are shown in Figure 1. There are three types of H2 (m/z = 2) desorption peak at 550, 700, and 850 K, respectively, for the adsorbed hydrogen from H2-gas (Figure 1-(1a)). The peak at 550 K corresponds to the hydrogen weakly adsorbed on the Ni surface. The peak at 700 K could originate from much strongly adsorbed hydrogen, probably located at the interface of the Raney Ni catalyst as spillover species.16-18 The peak at 850 K could come from the decomposition of the metal hydride.19 However, one more H2 desorption peak, from the reformation and/or decomposition of the adsorbed alcoholic species (methanol or ethanol), was observed at 450-500 K for the

adsorbed hydrogen from the APR of ethanol or methanol (Figure 1-(2a) and -(2b)). Chen and Falconer20 observed that H2, CO, and CH4 were generated simultaneously during the TPD of methanol, ethanol, and L-propanol on the Al2O3 and Ni/ Al2O5 catalysts. The desorption peaks of COx (CO2 and CO) and CH4, in accordance with the H2 desorption peak at 450-500 K, were also observed simultaneously at 500 K (show in Figure 1-(2b) and -(3b)). The formation of these carbon species confirms the occurrence of the reformation or decomposition of the alcoholic species (methanol or ethanol) (CH3OH þ H2O f 3H2 þ CO2, CH3OH f 2H2 þ CO, 3H2 þ CO f CH4 þ H2O, H2O þ CO f H2 þ CO2, and 4H2 þ CO2 f CH4 þ 2H2O) adsorbed on the Raney Ni catalyst during the TPD.16 On the basis of the desorption peak areas of H2 and carbon containing species (Table 2), it can be concluded that the amount of hydrogen 3141

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Figure 2. Liquid-phase TPSR profiles of phenol with adsorbed hydrogen from H2-gas, and aqueous-phase reforming of methanol or ethanol, respectively, over the Raney Ni catalyst. (4) Cyclohexanol selectivity, (g) cyclohexanone selectivity, (0) phenol conversion, (O) phenol converted into cyclohexanol and cyclohexanone with hydrogen generated in situ from the aqueous-phase reforming of phenol.

adsorbed on the catalyst is highly dependent upon the hydrogen sources. Additionally, the aqueous-phase reforming of methanol and ethanol shows different hydrogen selectivity since the ratio of CH4/COx is varied. By the way, the small desorption peaks of COx and CH4 at 720 K could come from the strongly adsorbed alcoholic species.16 The desorption of COx and CH4 for the Raney Ni pretreated by H2-gas (Figure 1-(1b)) could originate from the preparation process of the Raney Ni catalyst. TPSR in Liquid Phase. Reaction performances of the ad* ) and the APR of ethanol sorbed hydrogen, from H2-gas (HH (H*) E or methanol (H* M), respectively, in the hydrogenation of phenol were investigated by TPSR in liquid phase. As shown in Figure 2, the initial temperatures of phenol hydrogenation are * , HE*, and HH *, 373, 413, and 323 K in the TPSR of phenol with HM respectively. The conversion of phenol is enhanced gradually during the TPSR of phenol with the adsorbed hydrogen of H*E and HE* (Figure 2-(1) and 2-(2)). However, the conversions of phenol show two peak values at 423 and 483 K (Figure 2-(3)), respectively, in the TPSR of phenol with the adsorbed hydrogen of H*H. The conversion of phenol was decreased at the temperature range of 420-460 K, because the amounts of adsorbed hydrogen on the catalyst continuously decreased during the TPSR. The peak value at 483 K could be attributed to the hydrogenation of phenol with the hydrogen generated in situ from the APR of phenol,21,22 because cyclohexanol and cyclohexanone are also observed at 460 K in the TPSR of aqueousphenol over the Raney Ni catalyst without preadsorbed hydrogen. The peak value of phenol conversion was not observed in * the TPSR of phenol with the adsorbed hydrogen of HE* and HM because phenol could also be partially hydrogenated by the hydrogen generated in situ from the adsorbed alcoholic species during the TPSR at the temperature above 440 K. The selectivities of cyclohexanone are range from 20-30%, 20-100%, and 0-20% for the TPSR of phenol with the adsorbed hydrogen of H*, E H* M, and H* H, respectively, over the Raney Ni catalyst. However, the formation of cyclohexanone was * , which is observed only at 450-493 K in the TPSR with HH correspondingly with the formation of cyclohexanol and cyclohexanone from phenol with hydrogen generated in situ from the

Figure 3. Schematic representation of the hydrogenation of phenol into cyclohexanone and cyclohexanol. H* represent the adsorbed hydrogen on the catalyst.

APR of phenol (symbol (O) in Figure 2-(3)). These results indicated that the selectivity of cyclohexanone should be highly dependent upon the properties of the adsorbed hydrogen in the hydrogenation of phenol. Mechanisms and Discussion. Equations 1-4 and Figure 3 schematically represent the hydrogenation of phenol with hydrogen generated in situ from the APR of methanol (eq 1) or ethanol (eq 2), or dissociated from H2-gas (eq 3). The adsorbed hydrogen (H*) was first produced from the eqs 1-4 on the metal of the catalyst, which is followed by the hydrogenation of phenol into either cyclohexanone or cyclohexanol. As shown in Figure 3-A, cyclohexanone could be produced when the orthoposition of phenol was first attacked by an adsorbed hydrogen atom (H*) to produce intermediate (1), which is then followed by the quick occurrence of the enol-tautomerism into intermediate (2). However, as shown in Figure 3-B, phenol will be hydrogenated into cyclohexanol when the six carbon atom of phenol was attacked by six adsorbed hydrogen atoms, simultaneously. Cyclohexanone could also be hydrogenated into cyclohexanol, as shown in Figure 3-C.

3142

CH3 OH þ H2 O þ 6σ f CO2 þ 6Hσ

ð1Þ

CH3 CH2 OH þ 3H2 O þ 12σ f 2CO2 þ 12Hσ

ð2Þ

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Figure 4. Catalytic mechanisms of the hydrogenation of phenol (1) into cyclohexanone with aqueous-methanol/-ethanol as hydrogen source, and (2) into cyclohexanol with H2-gas.

Table 2. Desorption Peak Areas of H2, CH4, and COx during the TPD of the Raney Ni Pretreated by Different Sources hydrogen sources

H2 1.28  10

CH4 -8

CH4/COx -10

3.13  10

0.61

aqueous-methanol 1.83  10-9 1.85  10-9

5.19  10-9

0.36

1.56  10-9 4.74  10-9

8.49  10-9

0.56

H2-gas aqueous-ethanol

1.91  10

COx -10

H2 þ 2σ T 2Hσ

ð3Þ

CH3 CH2 OH þ 2σ f CH3 CHO þ 2Hσ

ð4Þ

The hydrogenation of phenol are highly associated with the types of catalyst (Raney Ni or Pd/Al2O3) and the sources of hydrogen (H2-gas, aqueous-methanol, and aqueous-ethanol). The selectivity of cyclohexanone over the Pd/Al2O3 catalyst is significantly higher than that over the Raney Ni catalyst because phenol was non-coplanarly adsorbed on the Pd/ Al2O323 but coplanarly adsorbed on the Raney Ni24 that favors the occurrence of the routes in Figure 3-A and Figure 3-B, respectively. Although the coplanarly adsorbed phenol on the Raney Ni catalyst favors the formation of cyclohexanol, cyclohexanone could still be produced if only the ortho-position of phenol was surrounded by the adsorbed hydrogen atom (H*), as shown in Figure 4-(1). In the hydrogenation of phenol with hydrogen generated in situ from the APR of methanol or ethanol, limited amount of adsorbed hydrogen (H*) could be formed on the catalyst, because the H2 desorption peak areas in the TPD from the three types of hydrogen source are decreased according to the order of H2-gas > aqueous-methanol > aqueous-ethanol (Table 2). Therefore, hydrogen might be inhomogeneously distributed near the adsorbed phenolic species (Figure 4-(1)), and cyclohexanone could be produced when the H* was adsorbed at the ortho-position of phenol. In the hydrogenation of phenol with H2-gas, however, excessive amounts of hydrogen could be homogeneously adsorbed on the catalyst, i.e., adsorbed phenol was equally surrounded by H* (Figure 4-(2)), which could favor the formation of cyclohexanol according to Figure 3B. Additionally, the presence of limited amount of hydrogen on the catalyst also inhibited the hydrogenation of cyclohexanone into cyclohexanol. During the TPSR of phenol with adsorbed hydrogen of H*H in liquid phase, cyclohexanone could not be produced at the temperature of 320-460 K, because phenol and H* was adsorbed on the surface of the catalyst as the model shown in Figure 4-(2), but at the temperature of 460-493 K, only limited amount of hydrogen was maintained remained on the surface of the catalyst, which thus increased the selectivity of cyclohexanone from 0 to 20%. As shown in Figure 4-(1), the TPSR of

phenol with limited amount of adsorbed hydrogen from the APR * ) or ethanol (HE*) produces cyclohexanone with of methanol (HM the selectivity of 20-100% and 20-30%, respectively, over the Raney Ni catalyst. The hydrogenation rate of phenol could also be highly dependent upon the amounts of the H* on the surface of the catalyst. The conversion of phenol in the hydrogenation of phenol with H2-gas is higher conversion than that with hydrogen from the APR of methanol or ethanol (Table 1).

’ CONCLUSIONS Hydrogen generated from the APR of methanol or ethanol over the Raney Ni or Pd/Al2O3 catalyst can be used in situ for the hydrogenation of phenol and its derivatives (phenol, o-cresol and p-tert-butylphenol) to get higher cyclohexanone selectivity than H2-gas. The limited amount of adsorbed hydrogen from the APR of methanol or ethanol is formed, but the excessive dissociation of H2-gas is produced. Inhomogeneous distribution of the limited amount of hydrogen near the adsorbed phenolic species could inhibit the hydrogenation of cyclohexanone into cyclohexanol, which favors the formation of cyclohexanone but also decreases the hydrogenation of phenol. Oppositely, the excessive amounts of hydrogen favor the formation of cyclohexanol. Additionally, TPSR in liquid phase could be employed to study the reaction performance of adsorbed species in many reactions under liquid phase and high pressure. ’ AUTHOR INFORMATION Corresponding Author

*Tel: þ86-571-88320409. Fax: þ86-571-88320409. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (NSFC-20976164) and National Basic Research Program of China (973 Program) (2011CB710803). ’ REFERENCES (1) Dodgson, I.; Griffin, K.; Barberis, G.; Piganttaro, F.; Tauszik, G. A Low Cost Process of PhOH to Cyclohexanone. Chem. Ind. 1989, 24, 830. (2) Hiyoshi, N; Mine, E.; Rode, C. V.; Sato, O.; Ebina, T.; Shirai, M. Control of Stereoselectivity in 4-tert-butylphenol Hydrogenation over a Carbon-supported Rhodium Catalyst by Carbon Dioxide Solvent. Chem. Lett. 2006, 35, 1060. (3) Rode, C. V.; Joshi, U. D.; Sato, O.; Shirai, M. Catalytic Ring Hydrogenation of Phenol under Supercritical Carbon Dioxide. Chem. Commum. 2003, 15, 1960. (4) Li, J.; Shi, Y.; Xu, L.; Lu, G. Z. Selective Oxidation of Cyclohexane over Transition-Metal-Incorporated HMS in a Solvent-Free System. Ind. Eng. Chem. Res. 2010, 49, 5392. (5) Claus, P.; Berndt, H.; Mohr, C.; Radnik, J.; Shin, E. J.; Keane, M. A. Pd/MgO: Catalyst Characterization and Phenol Hydrogenation Activity. J. Catal. 2000, 192, 88. (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. (7) Mahata, N.; Vishwanathan, V. Influence of Palladium Precursors on Structural Properties and Phenol Hydrogenation Characteristics of Supported Palladium Catalysts. J. Catal. 2000, 196, 262. 3143

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