Selective Hydrodeoxygenation of Lignin-Derived Phenols to

Research Article pubs.acs.org/journal/ascecg

Selective Hydrodeoxygenation of Lignin-Derived Phenols to Cyclohexanols over Co-Based Catalysts Xiaohao Liu,† Wenda Jia,† Guangyue Xu, Ying Zhang,* and Yao Fu iChEM, CAS Key Laboratory of Urban Pollutant Conversion, Anhui Province Key Laboratory for Biomass Clean Energy and Department of Chemistry, University of Science and Technology of China, No. 96, JinZhai Road, Hefei, Ahhui 230026, P. R. China S Supporting Information *

ABSTRACT: Cyclohexanols are important feedstock for polymers, spices, and medicines production in industry. In this work, a series of cobalt-based catalysts with different supports were prepared and used to catalyze lignin-derived phenols to cyclohexanols. Among the catalysts, Co/TiO2 showed the best hydrodeoxygenation (HDO) activity. An equivalent of propylcyclohexanol (>99.9%) was achieved under 1 MPa H2, 200 °C for 2 h. According to the characterization results of transmission electron microscopy (TEM), Brunauer−Emmett−Teller (BET) surface area analysis, powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR), hydrogen temperature-programmed desorption (H2-TPD) and NH3-TPD, the particle size and dispersion of Co could have important influence on catalytic activity. For Co/ TiO2, the SMSI effect may significantly affect the catalytic activity. The influences of different temperature, H2 pressure and reaction time on the eugenol conversion by Co/TiO2 were explored. 99% yield of propylcyclohexanol could even be obtained under 0.4 MPa H2, 180 °C for 8 h. This should be the mildest condition that has been reported for HDO of eugenol to propylcyclohexanol catalyzed by non-noble metal catalyst. On the basis the mechanism and substrates extension studies, all the Co-based catalysts selected in this study showed high activity to cleave the Caryl−OCH3 bond before the hydrogenation of the aromatic ring when the −OCH3 group substituted at ortho-position. KEYWORDS: Biomass, Hydrodeoxygenation, Cobalt, Phenols, Cyclohexanols

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INTRODUCTION Biomass, as the solely renewable organic carbon resource in nature, has a great potential to substitute the traditional fossil resource in producing fuels and bulk chemicals.1−4 Cyclohexanols, the important feedstock in industry, are widely used as the intermediates in preparation polymers, spices and medicines.5,6 Although cyclohexanols can be obtained from the hydrogenation of corresponding phenols, the preparation of phenols in industry requires multistep processes. In fact, lignin as an important part of lignocellulose (15−30% by weight and 40% by energy) could be converted to various phenolic compounds through different methods like pyrolysis, hydrogenolysis and biological methods, etc.7−13 Thereby, it is sustainable and environmentally friendly to use the ligninderived phenols to produce cyclohexanols. Nevertheless, these phenolic compounds always contain methoxy group, such as guaiacol (2-methoxyphenol), alkylguaiacols, eugenol, etc. Therefore, selective HDO is necessary to remove the redundant methoxy group in the lignin-derived phenols. In fact, many efforts have been made to convert ligninderived phenols to cycloalkanes or cyclohexanols in recent years.14−19 There are mainly two kinds of the reaction pathways in producing cyclohexanols:20,21 the saturation of aromatic ring through hydrogenation and then the cleavage of Calkyl−OCH3 © 2017 American Chemical Society

bond through hydrogenolysis (pathway 1, Scheme 1); the cleavage of Caryl−OCH3 bond through hydrogenolysis and then the saturation of aromatic ring through hydrogenation (pathway 2, Scheme 1). Generally, the fully hydrogenated products (B) are more stable than the corresponding phenolic Scheme 1. Reaction Pathways for Producing Cyclohexanols from Lignin-Derived Phenols

Received: April 6, 2017 Revised: July 29, 2017 Published: August 16, 2017 8594

DOI: 10.1021/acssuschemeng.7b01047 ACS Sustainable Chem. Eng. 2017, 5, 8594−8601

Research Article

ACS Sustainable Chemistry & Engineering

bottom flask and stirred at 45 °C. A calculated amount of Co(OAC)2· 4H2O was dissolved in deionized water and then added to the above suspension. After stirring for 24 h, the solvent was removed by rotary evaporation and dried at 105 °C overnight. After that, the catalyst was calcinated in air at 600 °C (5 °C/min) for 2 h and then reduced in H2 flow (100 mL/min) at 600 °C (1 °C/min) for 2 h. When the temperature was cooling down to room temperature, the catalyst was purged with N2 for 2 h. The catalysts were stored in dodecane before use. The Co3O4 was prepared by precipitation method. The preparation procedure was as follows: 10 g polyvinylpyrrolidone (PVP) was added into 200 mL of ethanol in a round-bottom flask and stirred at room temperature. 3.4 g Co(OAC)2·4H2O was dissolved in 30 mL of deionized water and then added to the above flask. After stirring for 2 h, 1 M NaOH solution was added into the mixture to adjust the solution pH to 9−10. After the mixture was stirred for another 1 h, the solid was filtered, washed, and dried at 105 °C overnight. Then the solid was calcinated in air at 600 °C (5 °C/min) for 2 h. The Co black was obtained by reducing Co3O4 in H2 flow (100 mL/min) at 600 °C (1 °C/min) for 2 h. When the temperature was cooling down to room temperature, the catalyst was purged with N2 for 2 h. Catalyst Characterization. Transmission electron microscopy (TEM) images were taken with a JEOL Model JEM-2010 LaB6 TEM system. Powder X-ray diffraction (XRD) patterns were conducted on a TTR-III X-ray diffractometer (Japan) using Cu Kα radiation (λ = 1.54056 Å). 2θ ranges were 20°−80°. X-ray photoelectron spectroscopy (XPS) analyses were conducted on an X-ray photoelectron spectrometer (ESCALAB250). The nitrogen adsorption−desorption were measured using a Micromeritics TriStar II system (TriStar II 3020 V1.03). The surface area was determined by the Brunauer− Emmett−Teller (BET) method. H2 temperature-programmed reduction (H2-TPR) analyses were conducted on a home-built reactor system coupled to a gas chromatograph. Before TPR tests, the samples (containing about 8 mg cobalt metal) were pretreated in Ar flow at 500 °C for 1 h. A TPR run was performed in a 5% H2/Ar mixture gas flow from 40 °C and the heating rate was 10 °C/min. The ice-salt bath removed the moisture from the effluent stream to avoid it entering the thermal conductivity detector (TCD). Temperature-programmed desorption of H2 (H2-TPD) analyses were also carried out in a home-built reactor system coupled to a gas chromatograph. Before TPD tests, the reduced samples (100 mg) were pretreated in pure H2 flow at 550 °C for 1 h to reduce the surface cobalt oxides, which generated when the samples exposed in air. After cooling down to 40 °C, the samples were held at 40 °C for 1 h. Then, the samples were purged with flowing Ar to remove the weakly bound physisorbed H2. Subsequently, the temperature was increased to 600 °C at a heating rate of 10 °C/min in Ar flow. The signal of H2 desorption was recorded by TCD. Temperature-programmed desorption of NH3 (NH3-TPD) analyses were conducted on the same equipment as H2-TPD. Before NH3-TPD tests, the samples (100 mg) were pretreated in Ar flow at 500 °C for 1 h. After cooling down to 40 °C, NH3 adsorption was carried out at 40 °C for 1 h. Then the physisorbed ammonia was removed by purging with Ar at 40 °C for 1 h. Subsequently, the temperature was increased to 600 °C at a heating rate of 10 °C/min in Ar flow. Atomic absorption spectroscopy (AAS) was performed on a PerkinElmer Corporation Analyst 800 instrument. The sample handling process was as follows: 4 mL of aqua regia was added into a 10 mL round-bottom flask with 10 mg of catalyst and stirred at 80 °C for 24 h. The mixture then was diluted to 25 mL in a volumetric flask. The computation formula of the reaction rate is as follows:

monomers (A), because of the steric hindrance and electronic effect that restrain the cleavage of Calkyl−OCH3 bond. Therefore, selectively going through the pathway 2 is conducive to reducing the reaction conditions.22,23 However, for previously reported catalysts (such as Pd, Ru, and Ni), the reaction for HDO of methoxyphenols always goes through pathway 1 or in parallel and requires harsh reaction conditions (4−5 MPa H2, 250 °C).18,24,25 To solve this problem, Nakagawa et al. used Ru/C with MgO as catalyst to catalyze guaiacol to cyclohexanol with 80% yield at 160 °C and 1.5 MPa H2. The addition of MgO could promote the cleavage of Caryl− OCH3 bond.26 Recently, they further developed Ru−MnOx/C catalyst for HDO of guaiacol.27 MnOx could suppress the saturation of the aromatic ring so that the pathway 2 was dominant. Long et al. employed Ni/MgO to catalyze guaiacol to cyclohexanol at 160 °C and 3 MPa H2. The conversion was 97.74% and the cyclohexanol selectivity was 100%. As the Lewis base, the MgO support could attract the phenolic hydroxyl group and weaken the Caryl−OCH3 bond, which contributed to pathway 2.20 Schutyser et al. studied the HDO of 4akylguaiacols to 4-akylcyclohexanols over commercial Ru-, Pd-, and Ni-based catalyst. The Ni/SiO2−Al2O3 showed the best performance and the low H2 pressure increased the selectivity of pathway 2.23 Recently, our group developed Ru/ ZrO2−La(OH)3 catalyst to convert guaiacol to cyclohexanol, and the reaction pathway in this system contained both pathway 1 and pathway 2.28 Subsequently, we developed CoNx@NC-650 catalyst, which showed high activity to selectively cleave Caryl−OCH3 bond with 100% eugenol conversion and 99.9% propylcyclohexanol yield at 200 °C and 2 MPa H2.29 The reaction was only carried out via pathway 2. In spite of promising progresses, the research on development of catalysts to selectively cleave Caryl−OCH3 bond is still inadequate. In our previous work, the cobalt nitride has demonstrated good activity in cleaving Caryl−OCH3 bond, and the N-doped carbon support was important for the catalytic activity. However, the preparation of the catalyst required large amounts of NH3 and the cobalt nitride was unstable under reaction condition.29 Cobalt with special support may also has a high activity in cleaving Caryl−OCH3 bond. Therefore, herein, a series of cobalt-based catalysts with different supports were prepared, characterized and screened. The HDO of eugenol with different reaction temperature, H2 pressure and reaction time was carried out. The reaction pathway was also proposed. Besides eugenol, the HDO of other lignin-derived phenols was studied and the stability of the catalyst was investigated.

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EXPERIMENTAL SECTION

Reagents. Eugenol, guaiacol, 3-methoxy phenol, 4-methoxy phenol, 2,6-dimethoxy phenol, and 4-propylphenol were purchased from Aladdin Chemistry Co., Ltd.; 2-(Benzyloxy)phenol and 4propylcyclohexanol were purchased from TCI; cobalt acetate tetrahydrate (Co(OAC)2·4H2O), acetone, and ethyl acetate were purchased from Sinopharm Chemical Reagent Co., Ltd. Dihydroeugenol was prepared in accordance with previously reported method.30 TiO2 was purchased from Sigma-Aldrich Co.; ZrO2, SiO2, Al2O3, and CeO2 were purchased from Aladdin Chemistry Co.; HZSM-5 (Si/ Al = 25) was purchased from the Catalyst Plant of Nankai University. Catalyst Preparation. All of the catalysts were prepared by wetness impregnation method and the cobalt loadings were 10 wt %. Before preparation, all of the supports were calcinated at 750 °C (4 °C/min) for 4 h. The typical preparation procedure was described as follows: a certain amount of carrier was added into acetone in a round-

reaction rate number of moles of propylcyclohexanol = number of moles of Co metal × reaction time t (h) 8595

(1)


Research Article


Figure 1. TEM images of various supported Co catalysts: (a) Co/TiO2, (b) Co/HZSM-5, (c) Co/ZrO2, (d) Co/CeO2, (e) Co/SiO2, and (f) Co/ Al2O3.

Figure 2. (a) XRD patterns and (b) partial XRD patterns of cobalt-based catalysts.

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Catalyst Test. In a typical experiment, eugenol (1 mmol), 10 wt % cobalt-based catalyst (65 mg), solvent (10 mL) were added into a 25 mL Parr reactor. After purging with H2 for 5 times, the reactor was pressurized with 1 MPa H2 at room temperature. Then, the reaction was carried out at preset temperature with magnetic stirring. After the reaction, the reaction liquid was diluted with ethyl acetate. The products were identified via a GC-MS (Agilent, Model 5975C) and quantified via a GC (Kexiao, Model 1690) with HP-INNOWAX capillary column. Bicyclohexane was employed as internal standard. The detecting condition was as follows: the injection port temperature was 280 °C; the detector (FID) temperature was 280 °C; the initial column temperature was 40 °C, and then heated to 250 °C at a heating rate of 10 °C/min. Catalyst Recycle. After reaction, Co/TiO2 was attracted on the magnetic bar on the bottom of the reactor due to the magnetism of the Co metal. Therefore, it was very easy to separate the catalyst by removing the reaction liquid from reactor. Thereafter, the fresh reactant and solvent were directly added for the next run.

RESULTS AND DISCUSSION Characterization of the Catalysts. In this work, a series of cobalt-based catalysts with different supports were prepared and characterized. The morphologies of the catalysts were characterized by TEM. As shown in Figure 1, the Co particles were well dispersed on TiO2, HZSM-5, ZrO2 and CeO2 supports. The average particle size of the catalysts were 13− 20 nm (Figure 1a-d). It was difficult to identify Co particles in Co/SiO2 and Co/Al2O3 from Figure 1e and f, however, in the enlarged TEM images (Figure S1e and f), small size (99.9 0 0

16.3 25.9 25.1 6.4 1.4 0 0 0

69.1 16.5 12.2 0.7 0 0 >99.9 >99.9

a

Reaction condition: 164 mg (1 mmol) of eugenol, 65 mg of catalyst, 10 mL of n-dodecane at 200 °C and 1 MPa H2 for 2 h. bThe catalysts were not reduced by H 2 . c Trace amount of 2-methoxy-4propylcyclohexanol was detected. dTrace amount of propylcyclohexane was detected.

with the oxygen and the surface of Co could be oxidized quickly when the catalyst exposed in the air. For other catalysts with lower activity, only partial Co on surface was oxidized and Co0 was detected by XPS. Catalytic Activity Test and Discussion. The catalytic activities of Co-based catalysts were tested by HDO of eugenol at 200 °C and 1 MPa H2 for 2 h. From Table 1, the catalysts with different supports showed different catalytic activity. The eugenol conversions for all of the catalysts were 100%. The yield of propylcyclohexanol was only 14.5% when Co/CeO2 was used as the catalyst. Besides propylcyclohexanol, 16.9% 8597


Research Article

ACS Sustainable Chemistry & Engineering yield of propylphenol, and 70.9% yield of dihydroeugenol, which could be the intermediates, were detected. When ZrO2 and HZSM-5 were used as the supports, 59.6% and 62.2% yield of propylcyclohexanol was obtained under the same reaction conditions, respectively. High yields of propylcyclohexanol (>92%) were achieved when SiO2, Al2O3, and TiO2 were used as the supports. Moreover, Co/TiO2 showed the best performance in HDO of eugenol and the propylcyclohexanol yield was >99.9%. No 2-methoxy-4-propylcyclohexanol was detected when Co/TiO2 and Co/SiO2 were used as catalysts. For other catalysts, only trace amounts of 2-methoxy-4propylcyclohexanol was detected (Figures S9−14). Besides 2methoxy-4-propylcyclohexanol, trace amounts of propylcyclohexane were generated when Co/HZSM-5 and Co/SiO2 were used as catalysts, which could be due to the acidity of the catalysts (Figure S3). It should be noted that no propylcyclohexanol was generated when the Co/TiO2 and Co/SiO2 was not reduced by H2, indicating the reduction treatment of the catalysts were necessary for catalytic activity. .After prolonging the reaction time to 6 h, high yield of propylcyclohexanol was obtained over Co/TiO2, Co/Al2O3, Co/ZrO2, and Co/CeO2. Only trace amount of propylcyclohexane was detected, indicating that the propylcyclohexanol was stable under these reaction system (Table S2). Due to the acidity which promotes dehydration of propylcyclohexanol, 72.2% and 16.4% yield of propylcyclohexane was obtained when Co/HZSM-5 and Co/ SiO2 were uses as the catalysts, respectively. There are many factors can affect the catalytic activity such as particle size, dispersion, acid/base properties, chemical properties and support, etc. NH3-TPD analyses were carried out (Figure S3). However, there was no correlativity between catalytic activity and acid properties. Then H2-TPD analyses were also carried out and the cobalt dispersions were estimated (Figure S4 and Table S1). The Co/SiO2 and Co/Al2O3 with small particle size (Figure 1) have higher cobalt dispersion. For other catalysts with larger particle size, the dispersion values were low. Apart from Co/TiO2, it seems that the catalysts with high cobalt dispersion have high catalytic activity. It indicated that the cobalt dispersion could be a major factor that affect the catalytic activity. However, for Co/TiO2, there must be other factors. In fact, much research has been done to explore the strong metal−support interaction (SMSI) effect between TiO2 support and metal, which could greatly enhanced the selective hydrogenation and hydrolysis activity,.38−40 Lee et al. has verified that TiO2 could migrate onto Co particles forming TiOy layer under high temperature calcination and reduction conditions (SMSI effect). This effect could promote the Co/ TiO2 to selectively cleave C−O bond in furan ring, thereby producing 1,5-pentanediol from furfuryl alcohol.40 In this work, we also found the TiOy layer in Co/TiO2 (Figure S1a), indicating the presence of SMSI effect. This may be able to explain why the cobalt dispersion of Co/TiO2 was lower than that on most of other supports but showed better performance. We speculated that the SMSI effect may also contribute to the cleavage of phenolic Caryl−OCH3 bond, and this may be the reason for the good performance of Co/TiO2. Effect of the Reaction Temperature and H2 Pressure. Because of the high activity for HDO of eugenol, Co/TiO2 was selected to explore the optimal reaction conditions. Reactions with different temperatures were carried out (Table 2). When the reaction was 150 °C, only 0.2% yield of propylcyclohexanol was obtained and the main product was dihydroeugenol. When the temperature reached to 160 °C, the yield of propylcyclo-

Table 2. HDO of Eugenol with Different Reaction Temperaturesa yield (%) entry

temperature (°C)

conversion (%)

1

2

3

1 2 3 4 5 6 7 8b

150 160 170 180 190 200 210 180

100 100 100 100 100 100 100 100

0.2 3.9 53.8 97.9 99.5 >99.9 >99.9 99.0

1.4 3.1 12.5 1.5 0.4 0 0 0.5

98.1 92.2 33.6 0.5 0 0 0 0.4

a

Reaction condition: 164 mg (1 mmol) of eugenol, 65 mg of 10% Co/ TiO2, 10 mL of n-dodecane, 1 MPa H2 for 2 h. bThe H2 pressure was 0.4 MPa and the reaction time was 8 h. The products of 1−3 were consistent with Table 1.

hexanol increased (3.9%), but the main product was still dihydroeugenol (92.2%). However, a sharp increase of propylcyclohexanol yield (53.8%) was observed when the temperature was 170 °C and the yield increased to 97.9% when the temperature further increased to 180 °C. The reaction rate (Table S3) accelerated greatly from 0.18 to 2.44 h−1 when the reaction temperature increased from 160 to 170 °C, indicating that the HDO of eugenol to propylcyclohexanol was a temperature-sensitive reaction, which was consistent with our previous research.29 When the reaction temperature reached 200 °C, an equivalent amount of propylcyclohexanol (>99.9%) was achieved. The effect of H2 pressure was also studied (Figure 5). It was interesting to find that the H2 pressure was not a key factor for

Figure 5. Effect of H2 pressure. Reaction condition: 164 mg (1 mmol) of eugenol, 65 mg of 10% Co/TiO2, 10 mL of n-dodecane, 200 °C for 2 h.

HDO of eugenol. The propylcyclohexanol yield reached 92.3% when H2 pressure was only 0.4 MPa H2 (after reaction, the pressure in reactor was ∼0.1 MPa). When the pressure increased to 0.6 MPa, the product yield (95.4%) slightly increased. The propylcyclohexanol yield reached 99.9% when the H2 pressure was equal or greater than 1 MPa. Milder condition was carried out with 0.4 MPa H2 and 180 °C, and the propylcyclohexanol yield reached 99% after 8 h (Table 2, entry 8). This should be the mildest condition that has been reported 8598


Research Article

ACS Sustainable Chemistry & Engineering for HDO of eugenol to propylcyclohexanol catalyzed by nonnoble metal catalyst. Mechanism. From Tables 1 and S2, no or only trace amounts of 2-methoxy-4-propylcyclohexanol but high yield of propylphenol were detected, indicating that the reaction pathway of catalyzing eugenol to propylcyclohexanol by the cobalt catalysts are almost solely pathway 2. To further prove the reaction pathway, the distribution of the products catalyzed by Co/TiO2 at different reaction times was tracked and analyzed (Figure 6). Because of the allyl group was easily

Scheme 2. Proposed Reaction Pathway for HDO of Eugenol in the Presence of Co/TiO2

Table 3. HDO of Other Phenolic Compoundsa

Figure 6. Product distributions as a function of reaction time. Reaction conditions: 164 mg (1 mmol) of eugenol, 65 mg of 10% Co/TiO2, 10 mL of n-dodecane, 1 MPa H2, 200 °C.

hydrogenated, the eugenol was completely converted to dihydroeugenol during the initial heating process (0 min). 7.1% yield of propylcyclohexanol and 9.6% yield of propylphenol were generated at the first 10 min. Then, the content of dihydroeugenol decreased rapidly while the propylcyclohexanol increased rapidly and reached 76.4% yield at 20 min. The propylphenol yield reached the maximum at the same time and then decreased until to the end of the reaction. The propylcyclohexanol yield increased gradually and reached 99.4% at 60 min. The equivalent amount of propylcyclohexanol was achieved until at 120 min. No 2-methoxy-4-propylcyclohexanol was detected during the reaction, indicating the 2methoxy-4-propylcyclohexanol could not be the intermediate for this reaction. On the basis of the above results, the reaction pathway of catalyzing eugenol to propylcyclohexanol by Co/ TiO2 should only follow the pathway 2 that we mentioned in Introduction. When the total yield of propylcyclohexanol and propylphenol was 97%, about 60% yield of methanol was detected, which further verified the pathway we proposed (Scheme 2). This result shows that the Co/TiO2 has much higher activity to cleave Caryl−OCH3 bond of eugenol than saturate the benzene ring, which could be the reason why the catalyst could catalyze the eugenol under so mild condition. HDO of Other Phenolic Compounds. Besides eugenol, HDO of other substituted phenols were also tested to value the catalytic activity of Co/TiO2. From Table 3, it was easy to find that Co/TiO2 catalyst showed high activity for ortho substituted phenols. 2-methoxyphenol, 2,6-dimethoxyphenol and 2-(benzyloxy)phenol were totally converted with >98% yield of cyclohexanol after extending reaction time. However, when the −OCH3 group substituted at meta and para-position, 34% and 95% yield of methoxycyclohexanol was obtained after reaction for 2 h, respectively (entries 5 and 7). When 2-

a

Reaction condition: 164 mg (1 mmol) of eugenol, 65 mg of 10% Co/ TiO2, 10 mL of n-dodecane, 1 MPa H2, 200 °C for 2 h. bThe dosage of the substrate was 0.5 mmol.

methoxyphenol as the substrate, phenol was the intermediate (entry 1). No phenol but considerable amount of methoxycyclohexanol was detected when 3-methoxyphenol and 4methoxyphenol as the substrates, indicating that the Co/TiO2 could not cleave Caryl−OCH3 bond effectively when the −OCH3 group substituted at meta and para-position, especially for the latter. Extending the reaction time, 3-methoxyphenol could be totally converted to cyclohexanol, but for 4methoxyphenol, the main product was still 4-methoxycyclohexanol even after 8 h. According to the above results, we can conclude that the Co/TiO2 shows excellent activity to cleave Caryl−OCH3 bond only when the −OCH3 group presents in ortho-position. The reason for this phenomenon is unclear and further research need to be done in future. Since the ligninderived phenols are almost ortho-substituted phenols,3 the Co/ TiO2 is still a highly active catalyst for HDO of lignin-derived phenols. 8599



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Recyclability of the Catalyst. The recyclability of Co/ TiO2 was investigated. As shown in Figure 7, after three runs,

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01047. Enlarged TEM images of various supported Co catalysts; NH3-TPD and H2-TPD profiles of cobalt catalysts; XPS, XRD, and TEM of Co/TiO2 before and after four runs; XRD of Co 3 O 4 ; gas chromatograms of eugenol conversion; BET and cobalt dispersion of the catalysts; HDO of eugenol after prolonging the reaction time to 6 h; reaction rates at different reaction temperatures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ying Zhang: 0000-0003-2519-7359 Yao Fu: 0000-0003-2282-4839

Figure 7. Recycle of the catalyst. Reaction condition: 164 mg (1 mmol) of eugenol, 65 mg of 10% Co/TiO2, 10 mL of n-dodecane, 1 MPa H2, 200 °C for 2 h.

Author Contributions †

X.L. and W.J. authors equally contributed to the work.

the yield of propylcyclohexanol remained at >99.9% without any decrease. However, after four runs, the propylcyclohexanol yield decreased to 85%. In order to explore the reasons for the deactivation of the catalyst, a series of characterizations were carried out. According to the TEM (Figure S6) images, the cobalt particles had a certain aggregation and the average particle size increased from 13 to 16 nm after four runs. AAS analysis indicated that the Co content slightly decreased from 10.4 to 10.1 wt % (Table S4). XPS of the catalyst after recycle was conducted (Figure S5). Comparing with the fresh catalyst, the binding energy of cobalt species had no change, but the peak of Co0 appeared. It demonstrated that the cobalt oxides on the surface could be reduced under reaction condition. The XRD analysis showed no apparent change (Figure S7). Therefore, the aggregation and loss of cobalt could be the main reasons for deactivation of the catalyst.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the NSFC (21572213), National Basic Research Program of China (2013CB228103), Program for Changjiang Scholars and Innovative Research Team in University of the Ministry of Education of China, and the Fundamental Research Funds for the Central Universities (wk 2060190040).

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REFERENCES

(1) Tuck, C. O.; Perez, E.; Horvath, I. T.; Sheldon, R. A.; Poliakoff, M. Valorization of biomass: deriving more value from waste. Science 2012, 337, 695−699. (2) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering. Chem. Rev. 2006, 106, 4044−4098. (3) Li, C.; Zhao, X.; Wang, A.; Huber, G. W.; Zhang, T. Catalytic Transformation of Lignin for the Production of Chemicals and Fuels. Chem. Rev. 2015, 115, 11559−11624. (4) Gallezot, P. Conversion of biomass to selected chemical products. Chem. Soc. Rev. 2012, 41, 1538−1558. (5) Sikkema, D. J.; Hoogland, P. Poly(7-Oxabicyclo[2.2.1]HeptaneCo-Tetrahydrofuran) - Polymerization, Polymer Characterization and Spinning. Polymer 1986, 27, 1443−1452. (6) Lednicer, D.; Vonvoigtlander, P. F.; Emmert, D. E. 4-Amino-4Arylcyclohexanones and Their Derivatives - a Novel Class of Analgesics 0.2. Modification of the Carbonyl Function. J. Med. Chem. 1981, 24, 404−408. (7) Chu, S.; Subrahmanyam, A. V.; Huber, G. W. The pyrolysis chemistry of a beta-O-4 type oligomeric lignin model compound. Green Chem. 2013, 15, 125−136. (8) He, J.; Zhao, C.; Lercher, J. A. Ni-catalyzed cleavage of aryl ethers in the aqueous phase. J. Am. Chem. Soc. 2012, 134, 20768−20775. (9) Sergeev, A. G.; Hartwig, J. F. Selective, nickel-catalyzed hydrogenolysis of aryl ethers. Science 2011, 332, 439−443. (10) Van den Bosch, S.; Schutyser, W.; Vanholme, R.; Driessen, T.; Koelewijn, S.-F.; Renders, T.; De Meester, B.; Huijgen, W.; Dehaen, W.; Courtin, C.; et al. Reductive lignocellulose fractionation into soluble lignin-derived phenolic monomers and dimers and processable carbohydrate pulps. Energy Environ. Sci. 2015, 8, 1748−1763.

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CONCLUSION A series of cobalt-based catalysts with different supports were prepared and characterized. The Co/TiO2 showed the best performance for HDO of eugenol among the catalysts. On the basis of the catalyst characterization, the particle size and dispersion could have important influence on catalytic activity. For Co/TiO2, the SMSI effect may play important role on the catalytic activity. Using Co/TiO2 as catalyst, an equivalent amount of propylcyclohexanol was achieved under 1 MPa H2, 200 °C for 2 h. Moreover, under 0.4 MPa H2 and 180 °C, 99% yield of propylcyclohexanol was obtained in 8 h. This should be the mildest condition that has been reported for HDO of eugenol to propylcyclohexanol catalyzed by non-noble metal catalyst. All the cobalt catalysts that we studied could efficiently cleave Caryl−OCH3 bond to form propylphenol as the intermediate in our reaction system. Especially for Co/TiO2, no 2-methoxy-4-propylcyclohexanol was generated during the reaction. The HDO of other phenolic compounds were also studied. The Co/TiO2 showed high activity to cleave Caryl− OCH3 bond when the −OCH3 group present in orthoposition. 8600


Research Article

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