Al2O3 Catalyst Promoted by

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Hydrodeoxygenation of p–cresol over Pt/Al2O3 catalyst promoted by ZrO2, CeO2 and CeO2–ZrO2 Weiyan Wang, Kui Wu, Pengli Liu, Lu Li, Yunquan Yang, and Yong Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00515 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 19, 2016

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Industrial & Engineering Chemistry Research

Hydrodeoxygenation of p–cresol over Pt/Al2O3 catalyst promoted by ZrO2, CeO2 and CeO2–ZrO2

Weiyan Wanga b, Kui Wua, Pengli Liua, Lu Lia, Yunquan Yanga *, Yong Wangb c *

a

School of Chemical Engineering, Xiangtan University, Xiangtan, Hunan, 411105, PR

China

b

Voiland School of Chemical Engineering and Bioengineering, Washington State

University, Pullman, Washington 99163, United States

c

Pacific Northwest National Laboratory, P.O. Box 999, 902 Battelle Boulevard, Richland,

Washington 99352, United States

* To whom correspondence should be addressed.

E-mail: [email protected] (Y. Yang); [email protected] (Y. Wang) Tel: 509-371-6273

1 ACS Paragon Plus Environment


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Abstract: Al2O3, ZrO2–Al2O3 and CeO2–Al2O3 were prepared by a co–precipitation method and selected as supports for Pt catalysts. The effects of CeO2 and ZrO2 on the surface area and Brønsted acidity of Pt/Al2O3 were studied. In the hydrodeoxygenation (HDO) of p–cresol, the addition of ZrO2 promoted the direct deoxygenation activity on Pt/ZrO2–Al2O3 via Caromatic–O bond scission without benzene ring saturation. Pt/CeO2–Al2O3 exhibited higher deoxygenation extent than Pt/Al2O3 due to the fact that Brønsted acid sites on the catalyst surface favored the adsorption of p–cresol. Considering the advantages of CeO2 and ZrO2, CeO2–ZrO2–Al2O3

was

prepared,

leading

to

the

highest

HDO

activity

of

Pt/CeO2–ZrO2–Al2O3. The deoxygenation extent on Pt/CeO2–ZrO2–Al2O3 was 48.4% and 14.5% higher than that on Pt/ZrO2–Al2O3 and Pt/CeO2–Al2O3, respectively.


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1. Introduction At the present time, fossil fuels still contribute to a majority of energy to our society, but its utilization is causing problems such as global warming effects and environment pollutions. This has motivated the society to explore new renewable energy sources that could supplement fossil fuels.1 Compared with the alternative energies such as solar, geothermal, wind and hydroelectric, biomass is promising because it could be converted to diversified products, e.g., liquid hydrocarbon transportation fuels, chemicals and other carbon–based materials.2 For that reason, liquefaction such as pyrolysis of biomass into bio–oil has been extensively investigated by researchers worldwide.3 However, compared with fossil fuel, the main shortcoming of bio–oil is the high oxygen content, resulting in its low heating value, immiscibility with gasoil and thermal instability.4 Oxygen in bio–oil could be selectively removed via catalytic hydrodeoxygenation (HDO) and the deoxygenation rate depends on the selected catalysts.5, 6 Among oxygen–containing compounds in the bio–oil, the delocalization of oxygen lone–pair orbital that caused by the direct connection with aromatic ring leaded to its more difficult cleavage for C−O σ–bond in phenols than other C−O bonds.4 Many efforts have been devoted to elucidating the roles of various catalyst functions and seeking highly active catalysts for removing oxygen from phenols.5, 7-14 Amorphous Ni based borides and noble metals show high hydrogenation activity. The main reaction route in the HDO of phenols on these catalysts was hydrogenation-dehydration, which increases the oxygen removal activity and lowers reaction temperature. But amorphous catalysts can easily crystallize and then deactivate. Consequently, noble metal catalysts was usually used as good candidates



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for HDO reactions. The high cost of noble metals can be offset by supporting the precious metals on a high surface area carrier. Until now, supported Pt catalysts was widely applied in the HDO reaction,15-21 but their activity is dependent on the type of supports to a certain extent. For example, Y. Wang et al.22 had reported the essential effect of Pt supported catalyst with a strong acidity and the low adsorption and diffusion resistance for the catalyst with large pores. C. Liang et al.23 had concluded that the deposited Al2O3 or ZrO2 on the surface of mesoporous SiO2 was very beneficial to the dispersion of Pt nanoparticles and the relatively strong acidic sites on the catalyst surface promoted the deoxygenation. CeO2 and ZrO2 were good candidates as supports for HDO catalysts.24-28 Bui et al.29 had reported that ZrO2 appeared to be a promising carrier because its exposed surface hydroxyl groups acted as adsorption sites. Yakovlev et al.30 had reported that CeO2 and ZrO2 played a crucial role in the HDO reactions. But both ZrO2 and CeO2 possessed relatively low pore volume and specific surface area. They usually appeared as composite compounds such as ZrO2–Al2O3, CeO2–Al2O3, ZrO2–SiO2, etc.31-34 In this case, the intrinsic properties of individual metal oxide support remained, and might even present some other properties. For example, C. Sievers et al.35 had found that CeO2–ZrO2 could dissociate hydrogen and create oxygen vacancy sites for activating the C–O bonds for the HDO reactions. However, the effects of ZrO2 and CeO2 on the HDO activity of Pt/Al2O3 are still unclear. In addition, ZrO2–Al2O3 and CeO2–Al2O3 supports prepared by traditional method had small pore diameter.31, 32 Therefore, in this study, Al2O3, CeO2–Al2O3, ZrO2–Al2O3, CeO2–ZrO2–Al2O3 were synthesized by a modified method and used as carries for supported Pt catalysts. Their activities were investigated in the HDO of p–cresol.


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2. Experimental The supports were synthesized by a modified precipitation method. In a typical experiment for CeO2–ZrO2–Al2O3, 5.7 g Al2(SO4)3, 2.7 g Ce(NO3)3 and 1.5 g ZrOCl2 were dissolved in a 500 mL water. Under the conditions of vigorous agitation and ultrasonic environment and 40 °C, the precipitant NH4HCO3 solution (10.0 wt%) was added to the above aqueous solution to adjust pH to 8.5. After reaction, the white precipitate was filtered and washed and aged at 90 °C for 2 h, and then filtered again and washed with ethanol. Finally, the white precipitate was dried at 120 °C in an oven for 4 h and calcined at 600 °C for 5 h. CeO2–Al2O3, ZrO2–Al2O3, CeO2–ZrO2–Al2O3 with a (CeO2+ZrO2)/Al2O3 molar ratio of 1:1 and single Al2O3 were also prepared by the above procedure. Pt supported catalysts were prepared by impregnating with chloroplatinic acid solution following reducing by hydrogen. Temperature programmed reduction results (seen in Figure S1) showed that Pt oxides had been reduced after treatment at 400 °C in hydrogen environment for 6 h. The structural properties and activities of Pt supported catalysts were determined according to the methods appeared in previous literatures.36-38 The detailed preparation procedure, characterization methods and active test were showed in the Supporting Information. 3. Results and discussion 3.1. Structural and morphological features of supported Pt catalysts The XRD patterns of the synthesized Pt/Al2O3, Pt/CeO2–Al2O3, Pt/ZrO2–Al2O3 and Pt/CeO2–ZrO2–Al2O3 are shown Figure 1. Due to the high dispersion and the low loading of Pt on the supports, no peak corresponding to metallic Pt or its oxide was observed in all the XRD patterns. The diffraction peaks at 2θ=36.6°, 39.7°, 46.1°, 60.8° and 66.6° were



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assigned to γ–Al2O3.39 Some weak peaks appeared at about 2θ=30.4° and 50.5° were attributed to tetragonal ZrO2 phase while the peaks at about 2θ=28.6°, 33.2°, 47.6° and 56.4° corresponded to cubic CeO2.40, 41 Compared with Pt/Al2O3, all the peaks attributed to γ–Al2O3 in the XRD patterns of Pt/CeO2–Al2O3 and Pt/ZrO2–Al2O3 became very weak or disappeared, which resulted from the uniform mixture of γ–Al2O3 with ZrO2 or CeO2 because the strong chemical and mechanical effects of ultrasound wave dispersed the precipitates immediately.42 Figure 1 also presented that all the diffraction peaks for Pt/CeO2–ZrO2–Al2O3 shifted to the right compared with that of Pt/CeO2–Al2O3, indicating that CeO2 and ZrO2 in CeO2–ZrO2–Al2O3 support were not a simple physical mixture but formed an interaction,43 which would produce some new properties and enhance the catalytic activity. All samples showed a type IV isotherm with H2 hysteresis (Seen in Figure S2),44 suggesting a typical mesoporous structure for the prepared catalysts. Because of the uniform dispersion of metal oxides in the composite support, some Al2O3 pores was filled with CeO2 or ZrO2,32 presenting much lower volume adsorbed for Pt/CeO2–Al2O3, Pt/ZrO2–Al2O3 and Pt/CeO2–ZrO2–Al2O3 than Pt/Al2O3. The surface area, and pore volume are summarized in Table 1. During the preparation of supports, the precipitation of hydroxide was washed by anhydrous ethanol to remove the physical adsorbed water before calcination. This method prevented shrinkage of the Al2O3 skeleton and maintained its original pore. Hence, Pt/Al2O3 had a surface area of 282.0 m2/g with a pore volume of 1.8 cm3/g. But the surface area decreased to 150.0 m2/g and 91.4 m2/g, and the pore volume reduced to 0.4 cm3/g and 0.5 cm3/g after doping ZrO2 and CeO2, respectively. The surface


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area of Pt/CeO2–ZrO2–Al2O3 was 97.0 m2/g, between that of Pt/CeO2–Al2O3 and Pt/ZrO2–Al2O3. This indicated that the addition of ZrO2 or CeO2 into Al2O3 decreased the surface area. Figure 2 presents the IR spectra of adsorbed pyridine on Pt/Al2O3, Pt/ZrO2–Al2O3, Pt/CeO2–Al2O3 and Pt/CeO2–ZrO2–Al2O3 in the region of 1700–1400 cm–1. Pt/Al2O3 presented two bands at 1448 cm–1 and 1620 cm–1, and a very weak band around 1540 cm–1 demonstrated that Pt/Al2O3 possessed a low Brønsted acidity but high Lewis acidity.45 Figure 2 (b) showed that the intensity of band at 1448 cm–1 was increased but the band to Brønsted acidity almost disappeared in the spectra of Pt/ZrO2–Al2O3, suggesting that adding ZrO2 could enhance the Lewis acidity but lower the Brønsted acidity of support. A noticeable band around 1540 cm–1 was observed in Figure 2 (c) and (d), and this signal in Figure 2 (c) was stronger than that Figure 2 (d). According to the composition of the supports, it was obvious that the Brønsted acid sites of supported Pt catalyst were formed by adding CeO2, which was affected by the amounts of CeO2. 3.2. HDO of p–cresol on supported Pt catalysts The study on the changes of reactant and product concentrations versus reaction time was carried to determine the HDO routes on supported Pt catalysts, as shown in Figure 3. In the HDO of p–cresol on Pt/Al2O3, only methylcyclohexane but no aromatic compound and oxygen–containing compound were detected. Toluene could be hydrogenated into methylcyclohexane on supported Pt catalyst at the presence of hydrogen because of its high hydrogenation activity. Hence, it was hard to determine whether the direct deoxygenation (DDO) route occurred or not on Pt/Al2O3, but HYD was the predominant reaction route



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according to its high hydrogenation activity and the high bond dissociation energy of C–O in p–cresol. When adding ZrO2 in Pt/Al2O3, besides methylcyclohexane, there appeared toluene and 4–methylcyclohexanone in the product, indicating that the presence of ZrO2 was beneficial to enhance the DDO route. Similar results had been observed in the HDO reaction on Ni–Cu and Mo based sulfide supported catalysts.29, 30 Compared with Pt/Al2O3, p–cresol conversion on Pt/CeO2–Al2O3 increased remarkably, and 4–methylcyclohexanol and 4–methylcyclohexanone were detected during the reaction. Figure 3 (c) and (d) showed that 4–methylcyclohexanone concentration increased first and then decreased with the reaction time, giving the evidence for hydrogenation–dehydration (HYD) route in p–cresol HDO, as shown in Figure 4. As shown in Table 2, the conversion on Pt/Al2O3 was 49.6% with a deoxygenation degree of 47.2% after running reaction at 275 °C for 6 h. In general, more active sites expose in the catalyst with a larger surface area, which enhances the catalytic activity. Pt/CeO2–Al2O3 presented 3–fold smaller surface area than Pt/Al2O3, but the conversion and deoxygenation degree on Pt/CeO2–Al2O3 were 86.8% and 80.6%, respectively, being much higher than that on Pt/Al2O3. This indicated that adding CeO2 into Pt/Al2O3 was a good strategy for promoting its activity in the HDO of p–cresol and the surface area was not a decisive factor. Associated with pyridine absorption results, it was obviously that the Brønsted acidity of supported Pt catalyst had a great effect on their HDO activity, which was consistent with previous investigations.46, 47 It had been proposed that phenols adsorbed on the catalyst surface via two ways: vertical adsorption for C–O hydrogenolysis and co–plane adsorption for hydrogenation, deciding the HDO products’ selectivity.48 In addition, the dehydration


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was closely related to the Brønsted acidity of the catalyst. Fig 3 presented that toluene concentration on Pt/ZrO2–Al2O3 and methylcyclohexane concentration on Pt/CeO2–Al2O3 were increased obviously compared with that on Pt/Al2O3. Therefore, it could be speculated that adding CeO2 into Pt/Al2O3 enhanced the adsorption of p–cresol and the dehydration of 4–methylcyclohexanol while adding ZrO2 favored the vertical adsorption of p–cresol. To further demonstrate the effect of Brønsted acid sites, other supported Pt catalysts with different loading (1.0 wt.%, 1.5 wt.% and 2.5 wt.%) were prepared. The total Pt amount remained unchanged but varied the total catalyst weight, as shown in Figure 5 (a), (b) and (c). After reaction for 10 h, the deoxygenation degree on all catalysts reached nearly to 100%,

but

the

intermediate

product

concentrations

changed

with

catalysts.

4–Methylcyclohexanol concentration at the same reaction time increased with the decrease of catalyst weight, suggesting that the dehydration was inhibited, which was resulted from the decrease of Brønsted acid sites on the catalyst’ surface. Figure 5(d) showed the HDO of p–cresol on 0.45 g 1.0 wt.% Pt/CeO2–Al2O3. The corresponding reaction time was 0.5–fold shorter than that in Figure 5 (a) when the deoxygenation degree reached to 100%. These demonstrated that the support had a significant effect on the HDO activity of supported Pt catalyst. Table 1 showed that adding ZrO2 or CeO2 into Pt/Al2O3 could enhance the DDO route and deoxygenation degree for p–cresol HDO, respectively. Consequently, Pt/CeO2–ZrO2–Al2O3 was prepared. After reaction at 275 °C for 6 h, the deoxygenation degree reached to 95.1%, which increased by 14.5% and 48.4% compared with that on Pt/CeO2–Al2O3 and Pt/ZrO2–Al2O3, respectively. These suggested that some new properties were formed in



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CeO2–ZrO2–Al2O3. C. Sievers et al.35 had proposed that the incorporation of ZrO2 into CeO2 facilitates the formation of oxygen vacancies. These vacancies were available to the activation of the C–O bonds. Associated with the dissociation hydrogen on Pt, the deoxygenation

degree

was

increased.

Therefore,

the

high

HDO

activity

of

Pt/CeO2–ZrO2–Al2O3 was attributed to its Brønsted acidity and the created oxygen vacancies. 4. Conclusion In comparison with Pt/Al2O3, the addition of CeO2 and ZrO2 decreased the surface area. In the HDO of p–cresol, due to the increased Brønsted acid sites on Pt/CeO2-Al2O3, the adsorption of reactant molecules was enhanced, leading that both the conversion and deoxygenation degree were increased. The presence of ZrO2 in Pt/Al2O3 promoted its DDO activity as evidenced by the increased toluene selectivity. The dehydration rate and the deoxygenation degree were found to be dependent on the Brønsted acid sites. Adopting the advantages of CeO2 and ZrO2, Pt/CeO2–ZrO2–Al2O3 was prepared which showed higher HDO activity. The deoxygenation extent reached to 95.1% at 275 °C at 6 h time-on-stem, which was attributed to the presence of Brønsted acidity and the oxygen vacancies in CeO2–ZrO2 composite oxide. Increasing Brønsted acid sites and optimizing CeO2/ZrO2 molar ratio could further enhance the HDO activity of supported Pt catalyst. Associated content Supporting Information Detailed experimental processes of the preparation, XRD analysis, BET analysis, pyridine-FTIR analysis and HDO activity test for Pt supported catalysts. TPR profiles and


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Nitrogen adsorption–desorption isotherms of Pt supported catalysts. Acknowledgements We thank the financial support from the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. We also acknowledge the financial support from the China Scholarship Council.



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#

#

#

#

# # Pt/Al2O3

Intensity (a.u.)

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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pt/ZrO2-Al2O3 Pt/CeO2-Al2O3 Pt/CeO2-ZrO2-Al2O3

10

20

30

40

50

60

70

80

90

2 Theta (degree) Figure 1 XRD patterns of Pt/Al2O3, Pt/CeO2–Al2O3, Pt/ZrO2–Al2O3 and Pt/CeO2–ZrO2–Al2O3



(a)

Transmittance

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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(b) (c) (d)

1680 1640 1600 1560 1520 1480 1440 -1

Wavenumber/cm

Figure 2 IR spectra of adsorbed pyridine on (a) Pt/Al2O3, (b) Pt/ZrO2–Al2O3, (c) Pt/CeO2–Al2O3 and (d) Pt/CeO2–ZrO2–Al2O3


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(b) 0.9 -1

0.8

Concentration (mol· L )

p-Cresol Methylcyclohexane

-1

Concentration (mol · L )

(a) 0.9 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

1

2

3

4

5

6

7

8

9

p-Cresol 4-Methylcyclohexanone Methylcyclohexane 3-Methylcyclohexene Toluene

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

10

1

2

3

4

Time (h)

0.8

-1

-1


(d) 0.9

0.8 0.7 0.6

p-Cresol 4-Methylcyclohexanol 4-Methylcyclohexanone Methylcyclohexane 3-Methylcyclohexene Toluene

0.5 0.4 0.3 0.2 0.1 0.0 0

1

2

3

4

5

6

5

6

7

8

9

10

Time (h)

(c) 0.9 Concentration (mol· L )

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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7

8

9

10

0.7 0.6


0.5 0.4 0.3 0.2 0.1 0.0 0

1

2

Time (h)

3

4

5

6

7

8

9

10

Time (h)

Figure 3 HDO of p–cresol on (a) Pt/Al2O3, (b) Pt/ZrO2–Al2O3, (c) Pt/CeO2–Al2O3 and (d) Pt/CeO2–ZrO2–Al2O3



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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4 Reaction scheme of the conversion of p-cresol on Pt supported catalysts


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0.8

-1

-1


(b) 0.9

0.8


(a) 0.9 0.7 0.6


0.5 0.4 0.3 0.2 0.1 0.0 0

1

2

3

4

5

6

7

8

9

0.7 0.6


0.5 0.4 0.3 0.2 0.1 0.0

10

0

1

2

3

4

Time (h)

6

7

8

9

10

(d) 0.9 -1

0.8


-1

5

Time (h)

(c) 0.9 Concentration (mol· L )

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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.7 0.6


0.5 0.4 0.3 0.2 0.1 0.0 0

1

2

3

4

5

6

7

8

9

10

0.8 0.7 0.6


0.5 0.4 0.3 0.2 0.1 0.0 0

1

2

Time (h)

3

4

5

6

7

8

9

10

Time (h)

Figure 5 Effect of metal loading on catalytic activity of supported Pt catalysts: (a) 0.23 g 1.0 wt.% Pt/CeO2–Al2O3, (b) 0.15 g 1.5 wt.% Pt/CeO2–Al2O3, (c) 0.09 g 2.5 wt.% Pt/CeO2–Al2O3 and (d) 0.45 g 1.0 wt.% Pt/CeO2–Al2O3



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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 1 Physical propertiesof supported Pt catalysts Catalysts

Surface area (m2/g)

Pore volume (cm3/g)

Pt/Al2O3

282.0

1.8

Pt/ZrO2–Al2O3

150.0

0.4

Pt/CeO2–Al2O3

91.4

0.5

Pt/CeO2–ZrO2–Al2O3

97.0

0.4


Page 25 of 26

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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2 Conversion, product selectivity and deoxygenation degree in the HDO of p–cresol on supported Pt catalysts at 275 °C for 6 h Pt/Al2O3

Pt/ZrO2–Al2O3

Pt/CeO2–Al2O3

Pt/CeO2–ZrO2–Al2O3

49.6

47.2

86.8

95.5

4–Methylcyclohexanol

0

0

4.5

0

4–Methylcyclohexanone

0

0

1.0

0

3–Methylcyclohexene

0

0.9

0.5

0.1

100

79.2

89.2

95.4

0

19.9

4.8

4.5

47.2

46.7

80.6

95.1

Catalysts Conversion (%) Product selectivity (%)

Methylcyclohexane Toluene D. D. (wt. %)



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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

For Table of Contents Only


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