Selective Hydrodeoxygenation of Lignin-Derived Phenols to

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Selective hydrodeoxygenation of lignin-derived phenols to cyclohexanols over Co-based catalysts Xiaohao Liu, Wenda Jia, Guangyue Xu, Ying Zhang, and Yao Fu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01047 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 20, 2017

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ACS Sustainable Chemistry & Engineering

Selective hydrodeoxygenation of lignin-derived phenols to cyclohexanols over Co-based catalysts Xiaohao Liu,†,‡ Wenda Jia,†,‡ Guangyue Xu,† Ying Zhang,*† 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. * E-mail: [email protected] ‡

These authors equally contributed to the work.

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

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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. Based on 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

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 chemicals1-4. Cyclohexanols, the important feedstock in industry, are widely used as the intermediates in preparation polymers, spices and medicines5-6. Although cyclohexanols can be obtained from the hydrogenation of corresponding phenols, the preparation of phenols in industry requires multi-step 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, etc7-13. Thereby, it is sustainable and environmentally friendly to use the lignin-derived phenols to produce cyclohexanols. Nevertheless, these phenolic compounds always contain methoxy group, such as guaiacol (2-methoxyphenol), alkylguaiacols, eugenol, etc. Therefore, selective HDO


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is necessary to remove the redundant methoxy group in the lignin-derived phenols. In fact, many efforts have been made to convert lignin-derived phenols to cycloalkanes or cyclohexanols in recent years14-19. There are mainly two kinds of the reaction pathways in producing cyclohexanols20-21: the saturation of aromatic ring through hydrogenation and then the cleavage of Calkyl–OCH3 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 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 bond26. Recently, they further developed Ru-MnOx/C catalyst for HDO of guaiacol27. 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,



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which contributed to pathway 220. Schutyser et al. studied the HDO of 4-akylguaiacols 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 223. 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 228. 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 H229. 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 condition29. 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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Scheme 1. Reaction pathways for producing cyclohexanols from lignin-derived phenols.

EXPERIMENT 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 4-propylcyclohexanol 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 method30. 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-bottom flask and stirred



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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 polyvinyl pyrrolidone (PVP) was added into 200 mL ethanol in a round-bottom flask and stirred at room temperature. 3.4 g Co(OAC)2·4H2O was dissolved in 30 mL 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 stirring 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)


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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 homebuilt 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 homebuilt 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



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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:

reaction rate = × ()

(1)

Catalyst Test In a typical experiment, eugenol (1mmol), 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 pre-set 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


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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 (< 3 nm) of cobalt particles were observed. SiO2 and Al2O3 supports were less than 20 nm, which could limit the size of cobalt particles. The nitrogen adsorption-desorption were conducted to test the surface area of the catalysts and the results were shown in Table S1. XRD analyses on the catalysts were carried out and the results were shown in Figure 2. According to the standard JCPDS files of Co0 (JCPDS Powder Diffraction File No. 15-0806), the diffraction peaks of (111), (200), (220) plane could be observed at 44.2°, 51.5°, 75.9°, respectively. However, it was difficult to identify these peaks from XRD patterns (Figure 2a) due to the weak intensity. Therefore, partial XRD



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patterns from 43.5° to 45.0° were analyzed. As shown in Figure 2b, in addition to SiO2 and Al2O3, small diffraction peaks were observed at about 44.2°, which could be the (111) plane of Co0. The intensity of Co0 (200) and (220) planes were too weak to identify even in partial XRD analysis. Other diffraction peaks in Figure 2a were corresponding to the supports and no peak of cobalt oxide species was observed. For Co/TiO2 catalyst, the TiO2 in the catalyst was rutile phase. H2-TPR analyses were also carried out. Typically, the reduction process of Co3O4 has two steps (Co3O4→CoO and CoO→Co，respectively) with two reduction peaks31. As shown in Figure 3, the different supports interacted with cobalt led to different reducibility of Co-based catalysts. Two or three reduction peaks with wide range of reduction temperature from 200 °C to 800 °C were observed for the series catalysts. The Co/SiO2 showed the highest reduction temperature and the first reduction peak appeared at around 700 °C. The high reduction temperature could be due to the cobalt silicates or CoOx—SiO2 species that formed under the high calcination temperature31. For Co/Al2O3 and Co/HZSM-5, more than two reduction peaks appeared. The new peaks in high reduction temperature could be ascribed to the reduction of more stable cobalt aluminates or cobalt silicates 32-33. The Co/TiO2 had two obvious peaks and the reduction peaks disappeared at about 600 °C. The Co/ZrO2 and Co/CeO2 had similar reduction temperature, but a wide peak from 400 °C to 750 °C could be observed in Co/CeO2. Base on previous research34 and the H2-TPR of pure CeO2 (Figure S2), the wide peak could be the overlap of the reduction of cobalt oxides and CeO2 support. XPS analyses were performed to explore the chemical properties of Co-based


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catalysts. Deconvolution of the Co 2p regions by peak fitting for cobalt-based catalysts was shown in Figure 4. Three distinct peaks were observed in the Co2p3/2 of Co/Al2O3, Co/HZSM-5, Co/ZrO2, Co/CeO2 and Co black. The binding energies at around 778, 781 and 786 eV were corresponding to Co0, CoOx (Co2+, Co3+) and the satellite peak of CoOx 35-37. However, for Co/TiO2 and Co/SiO2, only two distinct peaks of CoOx and CoOx sat were observed, while the peak of Co0 was not observed. According to the analysis of H2-TPR, probably cobalt species in Co/SiO2 just began to be reduced at 600 °C or the reduced cobalt species was oxidized when it exposed in the air. For Co/TiO2, the surface cobalt species in the catalyst could be fully reduced, but still no Co0 peak was observed in XPS. According to the catalytic activity test (Table 1), maybe the highly active Co/TiO2 could react 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% 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



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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

2-methoxy-4-propylcyclohexanol

was

detected

2-methoxy-4-propylcyclohexanol,

trace

amounts

trace

(Figures of

amounts S9-14).

of

Besides

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 S4). 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


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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 S1, a), 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 Due to 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



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160 °C, the yield of propylcyclohexanol 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 h-1 to 2.44 h-1 when the reaction temperature increased from 160 °C to 170 °C, indicating that the HDO of eugenol to propylcyclohexanol was a temperature-sensitive reaction, which was consistent with our previous research29. 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 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 for HDO of eugenol to propylcyclohexanol catalyzed by non-noble metal catalyst. Mechanism From

Table

1

and

Table

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


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the cobalt catalysts are almost solely pathway 2. In order 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 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 2-methoxy-4-propylcyclohexanol could not be the intermediate for this reaction. Based on 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.



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Scheme 2. Proposed reaction pathway for HDO of eugenol in the presence of Co/TiO2 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 (Entry 5 and 7). When 2-methoxyphenol as the substrate, phenol was the intermediate (Entry 1). No phenol but

considerable

amount

of

methoxycyclohexanol

was

detected

when

3-methoxyphenol and 4-methoxyphenol 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

4-methoxyphenol, the main product was still 4-methoxycyclohexanol even after 8 h.


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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 lignin-derived phenols are almost ortho-substituted phenols3, the Co/TiO2 is still a highly active catalyst for HDO of lignin-derived phenols. Recyclability of the Catalyst The recyclability of Co/TiO2 was investigated. As shown in Figure 7, after three runs, 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 nm to 16 nm after four runs. AAS analysis indicated that the Co content slightly decreased from 10.4 wt% 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. CONCLUSION A series of cobalt-based catalysts with different supports were prepared and



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

characterized. The Co/TiO2 showed the best performance for HDO of eugenol among the catalysts. Based on 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 ortho-position.

ASSOCIATED CONTENT Supporting Information Enlarged TEM images of various supported Co catalysts; NH3-TPD and H2-TPD profiles of cobalt catalysts; XPS, XRD, TEM of Co/TiO2 before and after four runs; XRD of Co3O4; 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.

AUTHOR INFORMATION


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Corresponding Author * Email: [email protected] (Y. Zhang) ORCID Ying Zhang: 0000-0003-2519-7359 Yao Fu: 0000-0003-2282-4839 Author Contributions † These

authors equally contributed to the work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS 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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Figures Captions Figure 1. TEM images of various supported Co catalysts. Figure 2. a) XRD patterns, b) partial XRD patterns of cobalt-based catalysts Figure 3. H2-TPR profiles for Co-based catalysts. Figure 4. XPS spectra of Co2p3/2 in Co-based catalysts. Figure 5. Effect of H2 pressure. Reaction condition. Figure 6. Product distributions as a function of reaction time. Figure 7. Recycle of the catalyst.


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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, f) Co/Al2O3.

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



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Figure

3.

H2-TPR

profiles

for

Figure 4. XPS spectra of Co2p3/2 in Co-based catalysts.


Co-based

Page 28 of 35

catalysts.

Page 29 of 35

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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.

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



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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.


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Tables Captions Table 1. HDO of eugenol over different cobalt-based catalysts Table 2. HDO of eugenol with different reaction temperatures Table 3. HDO of other phenolic compounds



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Table 1. HDO of eugenol over different cobalt-based catalystsa

Yield % Entry

a

Catalysts

Conversion (%) 1

2

3

1c

Co/CeO2

100

14.1

16.3

69.1

2c

Co/ZrO2

100

56.9

25.9

16.5

3cd

Co/HZSM-5

100

62.2

25.1

12.2

4c

Co/Al2O3

100

92.6

6.4

0.7

5d

Co/SiO2

100

96.9

1.4

0

6

Co/TiO2

100

>99.9

0

0

7b

Co/TiO2

100

0

0

>99.9

8b

Co/SiO2

100

0

0

>99.9

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. b The catalysts were not reduced by H2. c Trace amount of 2-methoxy-4-propylcyclohexanol was detected. propylcyclohexane was detected.


d

Trace amount of

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Table 2. HDO of eugenol with different reaction temperatures a Yield % Entry

a

Temperature (°C) Conversion (%) 1

2

3

1

150

100

0.2

1.4

98.1

2

160

100

3.9

3.1

92.2

3

170

100

53.8

12.5

33.6

4

180

100

97.9

1.5

0.5

5

190

100

99.5

0.4

0

6

200

100

>99.9

0

0

7

210

100

>99.9

0

0

8b

180

100

99.0

0.5

0.4

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. b The H2 pressure was 0.4 MPa and the reaction time was 8 h. The products of 1-3 were consistent with Table 1.



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Table 3. HDO of other phenolic compounds a Conversion Entry

Substrates

Time/h

Products and Yields (%) (%)

1

2

93.7

92.3

2

3

100

98

3

6

100

99.9

1.0

71.5 4

b

4

100

99.9 28.3

5

2

100

65.7

6

4

100

98

7

2

100

4.6

95

8

8

100

6.5

93

a

34

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. b The dosage of the substrate was 0.5 mmol.


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TOC/Synopsis

Synopsis: Co-based catalysts were prepared to catalyze lignin-derived phenols to cyclohexanols under mild condition through selectively cleave Caryl–OCH3 bond.


Selective Hydrodeoxygenation of Lignin-Derived Phenols to

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