Water-Assisted Selective Hydrodeoxygenation of Guaiacol to

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Research Article pubs.acs.org/journal/ascecg

Water-Assisted Selective Hydrodeoxygenation of Guaiacol to Cyclohexanol over Supported Ni and Co Bimetallic Catalysts Minghao Zhou,† Jun Ye,‡ Peng Liu,† Junming Xu,*,‡ and Jianchun Jiang*,† †

Jiangsu Province Biomass Energy and Materials Laboratory, Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry (CAF), Nanjing 210042, China ‡ Institute of New Technology of Forestry, CAF, Beijing 100091, China S Supporting Information *

ABSTRACT: Hydrodeoxygenation (HDO) of guaiacol, a typical ligninderived phenolic compound, at relatively mild conditions was studied over γAl2O3 and ZSM-5 supported catalysts with Ni and/or Co as active metal. Among various catalysts, NiCo/γ-Al2O3 catalysts exhibited better guaiacol conversion up to 96.1% with cyclohexanol as the main product in aqueous, due to the proper acidity and interaction between metal particles and support. The effects of process parameters on guaiacol conversion and product distribution were investigated in detail associated with solvent effect. The cleavage of C−O bonds in guaiacol was investigated over NiCo/γ-Al2O3 catalysts in aqueous phase. Phenol was found as the main intermediate with 1-methyl-1,2cyclohexanediol as another intermediate instead of 2-methoxy-cyclohexanol. The demethoxylation first happened to form phenol, and then, the aromatic ring was hydrogenated to give cyclohexanol after further hydrogenation of cyclohexanone. KEYWORDS: Guaiacol, Hydrodeoxygenation, Ni-based bimetallic catalysts, Reaction pathway, Bio-oil upgrading

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INTRODUCTION Biomass has gained increasing attention as an important renewable source, which can be converted to either transportation fuel (bio-oil) or fine chemicals.1−4 However, the wide application of bio-oil is limited due to those deleterious properties such as high viscosity, low heating value, corrosivity, and thermal instability, which are mainly caused by the presence of oxygen-containing compounds including aldehydes, acids, and phenols.5,6 Lignin-derived phenolic compounds are of relatively large fraction in bio-oil,7−9 which can be converted to either aromatic or naphthenic hydrocarbon by hydrodeoxygenation.3 Catalytic hydrodeoxygenation of bio-oil is an effective method for the upgrading of bio-oil as it can reduce the O/C ratio and increase the H/C ratio.10−12 As the complex composition of raw bio-oil, model compounds that have similar properties including phenol, guaiacol, and anisole, are chosen for the initial screening of catalysts for the following upgrading of bio-oil.13−16 Among which, guaiacol is regarded as a good model compound representing phenols derived from lignin in biomass, as there exists both −OH group and −OCH3 group in guaiacol.17−19 Hydrodeoxygenation of guaiacol has been mainly studied using two kinds of catalysts including conventional hydrotreating sulfide catalysts and noble metal catalysts,10,20−26 which could give either partially hydrogenated oxygen-containing compounds including phenol, catechol, cylcohexanol or totally hydrogenated hydrocarbons including benzene and cyclohexane. Nakagawa et al. converted guaiacol to cyclohexanol © 2017 American Chemical Society

and methanol by the combination of Ru/C and MgO, indicating that MgO promoted the demethoxylation during the HDO process.20 Sels et al. investigated Ru/C, Pd/C, Ni/ CeO2, and Ni/SiO2−Al2O3 for conversion of alkylmethoxylphenols into alkylcyclohexanols, and possible reaction paths were also studied while Ni/SiO2−Al2O3 exhibited best catalytic activity.22,23 Selective conversion of guaiacol to cyclohexanol was also studied by Tomishige et al. and Fu et al. through Rubased catalysts with satisfactory selectivity.21,26 Fu et al. reported that guaiacol converted to cyclohexanol through two parallel ways: demethoxylation followed with hydrogenation (path 1) and the saturation of benzene ring followed with the demethoxylation (path 2).26 However, sulfide catalysts deactivated gradually during the HDO reaction for the loss of sulfur and the oxidation of active sulfide phase, although the catalysts were capable for the decrease of O/C ratio and increase of the H/C ratio.18 And the application of noble metal based catalysts would inevitably add to the cost for the HDO reaction, although they exhibited excellent catalytic activity and stability. Therefore, it is in urgency to develop alternatives for those sulfide catalysts and noble metal based catalysts for HDO of phenolic compounds and upgrading of bio-oil. Researchers have developed different catalysts including transition metal phosphides, carbides, nitrides, and rhenium-based cataReceived: May 23, 2017 Revised: July 24, 2017 Published: September 8, 2017 8824

DOI: 10.1021/acssuschemeng.7b01615 ACS Sustainable Chem. Eng. 2017, 5, 8824−8835

Research Article

ACS Sustainable Chemistry & Engineering Table 1. Chemical and Physical Properties of γ-Al2O3 and ZSM-5 Supported Catalysts composition (wt %)a

a

catalysts

Ni

Co

SBETb (m2/g)

Vpb (cm3/g)

average metal sizec(nm)

average metal sized(nm)

20% Ni/γ-Al2O3 (15 + 5)% NiCo/γ-Al2O3 (10 + 10)% NiCo/γ-Al2O3 (5 + 15)% NiCo/γ-Al2O3 20% Co/γ-Al2O3 (15 + 5)% NiCo/ZSM-5(25) (10 + 10)% NiCo/ZSM-5(25) (10 + 10)% NiCo/ZSM-5(38)

19.48 14.68 9.78 4.76 / 14.72 9.88 9.83

/ 4.88 9.84 15.05 19.52 4.90 9.76 9.80

172.3 179.6 185.8 188.3 178.8 185.1 200.5 195.7

0.62 0.69 0.76 0.79 0.68 0.32 0.29 0.27

19.68 14.26 13.85 13.68 16.12 25.21 23.31 23.10

19.27 14.42 13.96 13.49 16.33 25.18 23.45 23.18

Measured by AAS analysis. bEvaluated from N2 adsorption−desorption isotherms. cEstimated by XRD. dMeasured by TEM.

lysts,27−34 in order to decrease the cost for the catalytic hydrodeoxygenation and conversion of bio-oil to transportation fuels or fine chemicals. Among which, Ni and Fe based catalysts were reported as those very few active non-noble metal catalysts and exhibited good catalytic activity for the conversion of different kinds of phenols. However, recent research mainly focused on monometallic Ni-based catalysts and the HDO reaction was mainly carried at relatively hard conditions (reaction temperature > 300 °C, H2 pressure > 8 MPa).32−34 While Ni was reported as an active metal for ring opening during the hydrogenation of guaiacol, Co would add to the improvement of the deoxygenation activity.35 Thus, it was imperative to develop Ni-based catalysts with high catalytic activity for the hydrodeoxygenation of phenols and bio-oil at relatively lower temperature and hydrogen pressure. In this work, guaiacol was selected as a model compound because it contains both major functional groups of ligninderived phenolic compounds such as hydroxyl (−OH) and methoxy (−OCH3). The catalytic activity of γ-Al2O3 and ZSM5 (Si/Al = 25, 38) supported non-noble metal (Co and/or Ni) catalysts was investigated for the hydrodeoxygenation of guaiacol to give cyclohexanol; it was observed that Copromoted Ni/γ-Al2O3 catalysts (NiCo/γ-Al2O3) exhibited better catalytic activity, and the addition of Co was much preferable for the conversion of guaiacol to cyclohexanol. The synergistic effect of active metal particles and support was studied, and the catalysts developed in this study were well characterized by different techniques. The products distribution was discussed in detail considering the different process parameters. Furthermore, reaction pathways were discussed based on the intermediates detected by GC-MS, and then possible reaction pathways of guaiacol over NiCo/γ-Al2O3 catalysts were proposed based on the product distribution.

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Scientific, USA) spectrometer with Al Kα (1486.6 eV) irradiation source. The elemental peak positions were determined with a C(1s) peak at 284.6 eV as a calibration standard. The H2-TPR studies were carried out in a quartz tube reactor. The NH3-TPD studies were carried out in a quartz tube reactor with a thermal conductivity detector. Elemental analysis of the spent catalyst was performed with a True space CHNS analyzer. Catalytic Hydrodeoxygenation of Guaiacol in Aqueous. All the aqueous-phase catalytic hydrodeoxygenation of guaiacol was carried out in a 100 mL stainless autoclave equipped with an electromagnetic driven stirrer. For each run, 5 g guaiacol, 45 g water, and 2 wt % of catalyst were added to a reactor vessel, then the hydrodeoxygenation reaction was carried out in the temperature range of 160−220 °C to evaluate the effect of reaction temperature, for a reaction time of 2−10 h to evaluate the effect of reaction time. In addition, initial hydrogen pressure was also taken into consideration. After displacing air, the hydrogen pressure was raised to a certain value. Then the reactor was heated to the desired temperature and the stirring speed fixed to 800 rpm. Finally, the reactor was quickly cooled down, and the reaction products were separated from the catalysts by centrifugation. The liquid reaction products were extracted by ethyl acetate and then analyzed by GC (Ouhua GC 9160, SE-54 capillary column, 30 m × 0.32 mm × 0.5 μm) and GS-MS (Agilent 7890A, HP-5 capillary column, 30 m × 0.32 mm × 0.5 μm) to identify those possible intermediates and final products using pre-established criteria for data analysis (MS libraries NIST 08). The product yield and selectivity are calculated and defined as follows: XGUA =

Yi =

Si =

0 nGUA − nGUA

ni 0 nGUA

0 nGUA

× 100%

Yi × 100% XGUA

× 100%

(1)

(2)

(3)

0 Where nGUA and nGUA depict the amounts of guaiacol before and after reaction, respectively, in mol; and ni is the amount of reaction product, in moles. The mass balance of carbon was evaluated by the sum of reaction products and unreacted materials in comparison with raw materials, which was approximately 93−95%. The formation of gaseous product (such as CO2, etc.) and/or cokes could be attributed to the mass loss. All the experiments were carried out at least two times, and the results agreed within 5%. The effective hydrogen carbon ratio (H/Ceff) and oxygen carbon ratio (O/Ceff) were defined and calculated according to previous study.36

EXPERIMENTAL SECTION

Chemicals. All of the reagents were of analytic purity grade and were purchased from local Sinopharm Chemical Reagent, which were directly used without further treatment. Catalyst Preparation and Characterization. γ-Al2O3 and ZSM-5 supported catalysts were prepared in a similar way by the impregnation method reported in a previous study.36 All catalysts were loaded in a tubular reactor and reduced under H2 before use. The prepared γ-Al2O3 and ZSM-5 supported catalysts were characterized by XRD on a Rigaku D/max-A instrument with a Cu Kα radiation at 50 kV and 30 mA with a scan speed of 0.02°/min. The spectrometer microstructure of the catalyst was examined with TEM (Tecnai G2 20). The BET surface area of the catalysts were measured by the N2 adsorption−desorption method at liquid nitrogen temperature using a Beishide instrument (3H-2000PS1). The XPS measurements were performed on an ESCALAB-250 (Thermo-VG

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RESULTS AND DISCUSSION Catalyst Characterization. Physicochemical Properties of the Catalysts. The physicochemical properties of γ-Al2O3 and ZSM-5 supported catalysts with Ni and/or Co as active metal were exhibited in Table 1. In comparison, γ-Al2O3 supported 8825

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52.1°, and 76.6° could be ascribed to the diffraction of (111), (004), and (220) planes of metallic Co. Additionally, peaks at 2θ = 38.2°, 45.3° ascribed to the NiO and CoO species were also observed in the XRD spectrum, which was mainly caused by the oxidation due to the exposure in the air before and during the XRD detection. It could be seen in Figure 1-a-1 that, the intensity of the diffraction peaks of metallic Ni and/or Co decreased with the addition of Co, which suggested that the introduction of Co would lead to the better dispersibility of NiCo/γ-Al2O3 catalysts. A similar behavior was observed on Co-promoted Ni/CNT catalysts used for the hydrodeoxygenation of guaiacol.36 The average particle size (presented in Table 1) remarkably decreased from 19.68 nm for 20% Ni/ Al2O3 to about 13.68 nm in NiCo/Al2O3 catalysts with varied Co-loading amount, confirming that the introduction of Co would improve the dispersibility of metal particle in the catalysts, which was in good accordance with the XRD results presented in Figure 1-a-1. For those ZSM-5 supported bimetallic catalysts, both metal composition and Si/Al ratio had great effect on the catalyst dispersibility. It could be seen in Figure 1-a-2 that (10 + 10)% NiCo/ZSM-5 (Si/Al = 25) exhibited better dispersibility compared with the other two catalysts, as (10 + 10)% NiCo/ZSM-5 (Si/Al = 25) showed decreased intensity of the diffraction peaks of metallic Ni and/ or Co. Meanwhile, those ZSM-5 supported catalysts exhibited larger average metal size compared with Al2O3 supported catalysts, which were also confirmed from the TEM results in Figure 1. The TEM of (10 + 10)% NiCo/γ-Al2O3 and (10 + 10)% NiCo/ZSM-5(25) are presented in Figure 1, as can be seen in Figure 1b and c that, NiCo/γ-Al2O3 had much smaller mean metal particle size with better dispersibility, indicating that there existed stronger interaction between Ni and/or Co with γ-Al2O3 support, which was also confirmed in the following characterizations. Reducibility and Surface Acidic Properties of the Catalysts. The H2-TPR studies were carried out for the investigation of the reducibility of catalysts and the specific interaction between metal species and support, and the H2-TPR results of γ-Al2O3 and ZSM-5 supported Ni and/or Co catalysts were presented in Figure 2. The TPR spectra of 20% Ni/γ-Al2O3 consisted of two reduction peaks: one occurred at about 280 °C and the other occurred at about 400 °C. The reduction peak at relatively low temperature ascribed to the reduction of superficial metal oxide (herein NiO) with larger particle size had no or very weak interaction with γ-Al2O3 support; and the reduction peak at relatively high temperature ascribed to the reduction of metal oxide which was highly dispersed with smaller size and strongly interacted with the support.38,39 The TPR spectra of 20% Co/γ-Al2O3 consisted only one reduction peak at about 350 °C, moving to relatively higher temperature compared with Ni/γ-Al2O3, which indicated that metallic Co had much stronger interaction with γ-Al2O3 in Co/γ-Al2O3 catalyst with much smaller particle size than Ni/γ-Al2O3 catalysts. In comparison with Ni/γ-Al2O3, there were two reduction peaks at about 340 and 500 °C, and one peak at 340 °C moved to relatively lower temperature compared with Ni/γAl2O3, which indicated that the introduction of Co would contribute to the improvement of reducibility of catalysts. On the other hand, one reduction peak in NiCo/γ-Al2O3 moved to relatively higher temperature at about 500 °C compared with Co/γ-Al2O3 catalyst, which indicated that the addition of Co would also enhance the interaction between metal species (either Ni or Co) and γ-Al2O3, then reduce the metal particle

catalysts exhibited better dispersibility with smaller metal particle size than ZSM-5 supported catalysts, although those γ-Al2O3 supported had relatively smaller BET surface area. It also could be seen that, the BET surface area of γ-Al2O3 supported Co-containing was relatively larger than Ni/γ-Al2O3 with larger pore volume. Additionally, the addition of Co to Ni/γ-Al2O3 could also lead to the decrease of the average metal particle size, the above results were in line with Martin and Ma’s research results, which also confirmed that the presence of Co would help to decrease the particle size.54,55 The above results meant that the introduction of Co in Ni/γ-Al2O3 would help to increase the dispersibility of metal particles, and contribute to restraining the aggregation of metal particles during the thermal treatment, which might be due to the strong interaction between Co and/or Ni species and support. Similar behaviors were also reported elsewhere in NiCo/CNT and NiCu/γ-Al2O3 catalysts used for the hydrodeoxygenation of guaiacol.36,37 These changes would inevitably lead to significant improvement on the catalytic activity. Structural Properties and Morphology of the Catalysts. The XRD patterns of γ-Al2O3 supported catalysts after calcination and reduction are presented in Figure 1-a-1. The peaks at 46.5° and 68.7° could be ascribed to the diffraction peaks of the (002), (003) planes of γ-Al2O3. The peaks at 2θ = 44.6°, 52.1°, and 76.3° assigned to the diffraction of (111), (200), and (220) planes of metallic Ni confirming the presence of metallic Ni in the reduced catalysts.36,38 The peaks at 44.7°,

Figure 1. (a) XRD spectra of γ-Al2O3 and ZSM-5 supported catalysts. (b) TEM spectra and particle size distribution of (10 + 10)% NiCo/γAl2O3. (c) TEM spectra and particle size distribution of (10 + 10)% NiCo/ZSM-5(25) 8826

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Figure 2. H2-TPR spectra of γ-Al2O3 and ZSM-5 supported catalysts.

size. A similar observation was presented in CNT supported Ni and/or Co catalysts.36,40,41 As exhibited in Figure 2b, the H2-TPR profiles of ZSM-5(25) supported catalysts showed two similar reduction peaks at about 250 and 350 °C, which moved to relatively lower temperature, indicating the decreased interaction between metal species and ZSM-5 support compared with γ-Al2O3 supported catalysts. The increased interaction between Ni and/or Co species and γ-Al2O3 support led to the relatively reduce of metal particle size and increase of the dispersibility and specific surface area of catalysts, with the introduction of Co to Ni/γ-Al2O3 catalysts, which could help to inhibit the thermal transmigration and sintering of metal particles during calcination and reduction.42 These improvements in catalysts properties would inevitably contribute to the improvement in catalytic activity, which was confirmed in the hydrodeoxygenation of guaiacol. Herein, the acidity of γ-Al2O3 supported Ni and/or Co catalysts and (10 + 10)% NiCo/ZSM-5(25) were measured by NH3-TPD, as (10 + 10)% NiCo/ZSM-5(25) exhibited better catalytic activity among ZSM-5 supported catalysts (in the section Effect of Metal Composition and Support ). According to the temperature of the NH3 desorption peak, the acid sites could be divided into three kinds, weak acid (150−250 °C), medium acid (250−420 °C), and strong acid (420−750 °C) strength.38,43 As can be seen from Figure 3, the (10 + 10)% NiCo/ZSM-5(25) catalyst exhibited a broad NH3 desorption peak at 100−250 °C and a NH3 desorption peak at 300−350 °C, suggesting the presence of weak acid sites and medium acid sites in NiCo/ZSM-5(25). It also could be seen that, only weak acid sites were found in 20% Ni/γ-Al2O3, and both weak acid sites and medium acid sites existed in 20% Co/γ-Al2O3. Additionally, the NH3 desorption peak in NiCo/γ-Al2O3 gradually shifted to relatively higher temperature, medium acid sites began to appear, and the intensity of NH3 desorption peak increased obviously, suggesting the presence of both weak and medium acid sites, which indicated that the addition of Co would help to improvement of the catalysts acidity and was similar to Xiao’s research.36 However, the (10 + 10)% NiCo/ ZSM-5(25) catalyst showed stronger acidity compared with γAl2O3 supported on either monometallic or bimetallic catalysts. In comparison with the H2-TPR results of γ-Al2O3 and ZSM5 supported catalysts, it could be found that support and metal

Figure 3. NH3-TPD spectra of γ-Al2O3 and ZSM-5 supported catalysts.

composition had great effect on both catalyst reducibility and acidity, as γ-Al2O3 supported catalysts exhibited relatively stronger interaction between metal species and support and better metal particle size dispersibility and ZSM-5 supported catalysts processed much stronger acidity. Additionally, it was found that the addition of Co favored the formation of a highly dispersed catalyst with smaller average metal size which strongly interacted with γ-Al2O3. The specific acidity amount of γ-Al2O3 and ZSM-5 supported catalysts were presented in Table S1 in the Supporting Information, and the NH3-TPD results showed that γ-Al2O3 supported catalysts possessed relatively weaker acidity than ZSM-5 supported catalysts; the order of catalyst acidity was (10 + 10)% NiCo/ZSM-5(25) > (10 + 10)% NiCo/γ-Al2O3 > (5 + 15)% NiCo/γ-Al2O3 > (15 + 5)% NiCo/γ-Al2O3 > 20%Co/γ-Al2O3 > 20% Ni/γ-Al2O3. These improvements in catalysts properties would inevitably contribute to the improvement in catalytic activity, which was confirmed in the hydrodeoxygenation of guaiacol in the following section. Surface Element Compositions of the Catalysts. The surface element compositions of NiCo/γ-Al2O3 catalysts after reduction were analyzed by XPS, herein XPS results of either Ni/γ-Al2O3 or Co/γ-Al2O3 were not listed as the spectra were 8827

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Figure 4. XPS pattern of NiCo/γ-Al2O: (a) Ni2p analysis; (b) Co2p analysis.

Table 2. Binding Energies and Atomic Ratios of Reduced Catalysts Obtained from XPS Analysis binding energy (eV) 0

2+

0

2+

0

atomic ratio 2+

0

2+

2+

catalysts

Ni 2p3/2

Ni 2p3/2

Ni 2p1/2

Ni 2p1/2

Co 2p3/2

Co 2p3/2

Co 2p1/2

Co 2p1/2

Ni /Ni0

Co2+/Co0

20% Ni/γ-Al2O3 (15 + 5)% NiCo/γ-Al2O3 (10 + 10)% NiCo/γ-Al2O3 (5 + 15)% NiCo/γ-Al2O3

852.4 852.4 852.4 852.5

857.2 857.3 857.2 857.3

870.0 870.0 870.1 870.1

874.8 874.8 874.9 874.8

778.5 778.3 778.4

779.8 779.9 779.8

793.2 793.3 793.1

796.9 797.0 796.8

0.268 0.209 0.198 0.190

0.284 0.280 0.282

nation of guaiacol was carried out over Ni-based catalysts with γ-Al2O3 and ZSM-5 (Si/Al = 25, 38) as support. The guaiacol conversion and the products distribution were presented in Figure 5. It can be found that both guaiacol conversion and products distribution varied greatly with the change in active metal composition and support. It could be concluded that support had great effect on both guaiacol conversion and products selectivity; when γ-Al2O3 was used as support, the guaiacol conversion were much higher than ZSM-5 support.

similar. The Ni 2p and Co 2p XPS spectra and corresponding binding energies of the catalysts were exhibited in Figure 4 and Table 2, respectively. As can be seen in Figure 2a, NiCo/γAl2O3 catalysts exhibited binding energies at 852.4 and 857.3 eV were corresponding to Ni0(2p3/2) and Ni2+(2p3/2), respectively; and the binding energies at 870.1 and 874.8 eV attributed to the main line of Ni0(2p1/2) and Ni2+(2p1/2),38 suggesting the presence of both metallic Ni and NiO in the surface of support. It was shown in Figure 2b that binding energies at 778.5 and 779.8 eV were corresponding to Co0(2p3/2) and Co2+(2p3/2), respectively, and binding energies at 793.2 and 796.9 eV corresponded to the main line of Co0(2p1/2) and Co2+(2p1/2),42 indicating the presence of both metallic Co and CoO. It can be found that, no obvious Ni and/ or Co satellite peaks were observed in the XPS spectra of all NiCo/γ-Al2O3 catalysts. It could be seen in Table 2 that, the binding energies of Ni02p1/2 and Ni02p3/2 moved to relatively higher binding energies with the improvement of Co content, indicating that the electron donor−acceptor interaction between Ni, Co, and/or Al in γ-Al2O3 might occur on the catalyst surface.44,45 In comparison with XPS results exhibited in Table 2, the Ni2+/Ni0 ratio decreased when Co was added in Ni/γ-Al2O3 catalysts, which indicated that the introduction of Co would be partially beneficial for the reduction of NiO species and/or CoO species to metallic Ni and/or Co in comparison with Ni/γ-Al2O3. The H2-TPR results of Ni/γAl2O3, Co/γ-Al2O3, and NiCo/γ-Al2O3 catalysts in Figure 2 also confirmed that the reducibility partially improved as the reduction peak (at about 340 °C) in NiCo/γ-Al2O3 catalysts moved to relatively lower temperature compared with Ni/γAl2O3 (mainly at 400 °C), which was in good accordance with the XPS results. Catalytic Hydrodeoxygenation of Guaiacol. Effect of Metal Composition and Support. Herein the hydrodeoxyge-

Figure 5. Guaiacol conversion and product distribution over different catalysts. Reaction conditions: T = 200 °C, p = 5 MPa, t = 8 h; catalyst (A) (15 + 5)% NiCo/ZSM-5(25), (B) (15 + 5)% NiCo/ZSM-5(38), (C) (10 + 10)% NiCo/ZSM-5(25), (D) 20% Ni/γ-Al2O3, (E) 20% Co/γ-Al2O3, (F) (15 + 5)% NiCo/γ-Al2O3, (G) (10 + 10)% NiCo/γAl2O3, (H) (5 + 15)% NiCo/γ-Al2O3. 8828

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cyclohexanol selectivity increased obviously compared with ZSM-5 supported catalysts. It also could be seen that, the cyclohexane selectivity increased when ZSM-5 was used as support due to the stronger acidity, although its total yield was still very low. In this study, ZSM-5 supported catalysts with stronger acidity did not exhibit better catalytic activity for the HDO of guaiacol in comparison with γ-Al2O3 supported catalysts, because the catalytic activity always associated with several factors, such as metal composition, metal dispersity, acidity. As Chen et al. indicated that the formation of metal particles on the support surface would lead to the pore blockage and the reduction of amount of active acid sites in catalysts.47 It was presented in Table 1 and Figure 1 that γ-Al2O3 supported catalysts showed much smaller particle size and better dispersity than ZSM-5 supported catalysts, which would attribute to the decrease of catalytic activity of ZSM-5 supported catalysts. On the other hand, the cyclohexane yield improved a little over ZSM-5 supported catalysts, as increased acidity was favorable for the dehydration of cyclohexanol to cyclohexane, which was in accordance with the Chen study.47 As (10 + 10)% NiCo/γAl2O3 exhibited better catalytic activity among others, so herein, (10 + 10)% NiCo/γ-Al2O3 was chosen for the further studies for the HDO of guaiacol. Effect of Solvent on Guaiacol Conversion and Product Distribution. It was reported that obvious solvent effects were observed in previous studies.23,48,49 In this study, the effect of different solvents on the HDO of guaiacol over (10 + 10)% NiCo/γ-Al2O3 was investigated at 200 °C with an initial hydrogen pressure of 5 MPa. Water, methanol, and ethanol were respectively used as solvent during the HDO of guaiacol (see Table 3, entry 1−3), and obvious solvent effects were

Additional, cyclohexanol turned out to be the main product, which was of much higher H/Ceff in comparison with other compounds, such as phenol, 1,2-cyclohexandiol. As the catalytic activity always associated with several factors, such as metal composition, support, metal dispersity, acidity. It was reported that higher metal loading amount and better dispersity would lead to the improved catalytic activity and larger metal particles due to the particle size effect.22,46 Herein, although the same total metal loading amount was used with varied metal composition in this study, a particle size effect was also observed in both average metal size presented Table 1 and reaction results in Figure 5. It could be seen in Table 1 that the order of the average particle size of different catalysts was as below: (15 + 5)% NiCo/ZSM-5(25) > (10 + 10)% NiCo/ ZSM-5(25) > 20% Ni/γ-Al2O3 > 20% Co/γ-Al2O3 > (15 + 5)% NiCo/γ-Al2O3 > (10 + 10)% NiCo/γ-Al2O3 > (5 + 15)% NiCo/γ-Al2O3. The results indicated that γ-Al2O3 supported catalysts showed smaller particle size and dispersity than ZSM-5 supported catalysts, which was also presented in Figure 1, and the introduction of Co obviously decreased the average metal particle size in comparison with those γ-Al2O3 supported catalysts in Table 1. The catalytic activity of different catalysts in Figure 5 indicated that better dispersity with smaller average metal particle size would lead to the improvement in HDO activity, and less stronger acidity was needed considering both guaiacol conversion and cyclohexanol selectivity, as Copromoted catalysts exhibited smaller particle size and better reactivity for the conversion of guaiacol to cyclohexanol. The results were in good accordance with the research results of Martin and Sels.22,46 As exhibited in Figure 2, metal species had stronger interaction with γ-Al2O3 support and showed better dispersity than ZSM-5 supported catalysts, and in γ-Al2O3 supported catalysts, the reduction peaks moved to relatively higher temperature, indicating that the introduction of Co would contribute to the formation of highly dispersed catalyst with stronger interaction between metal species and γ-Al2O3. In addition to the particle size effect, the catalyst acidity also has great effect on the catalytic activity. As presented in Figure 3, γ-Al2O3 supported catalysts exhibited relatively weaker acidity than ZSM-5 supported catalysts, as γ-Al2O3 supported catalysts possessed less number of acid sites and weaker acid strength. Detailed acidity of different catalysts were presented in Table S1, and the order of catalysts acidity was (10 + 10)% NiCo/ZSM-5(25) > (10 + 10)% NiCo/γ-Al2O3 > (5 + 15)% NiCo/γ-Al2O3 > (15 + 5)% NiCo/γ-Al2O3 > 20% Co/γ-Al2O3 > 20% Ni/γ-Al2O3. The HDO activities did not correspond with the order of those catalysts, although Chen et al. reported that Pd/HZSM-5, Ru/HZSM-5, and Ru/γ-Al2O3 exhibited varied guaiacol conversion and selectivity toward cyclohexanol and cyclohexane due to different active metal composition and catalyst acidity.47 Additionally, Chen et al. also reported that more acid sites were favorable for the dehydration of cyclohexanol during the hydrodeoxygenation process. As can be seen in Figure 5, when the reaction was carried out over either (15 + 5)% NiCo/ZSM-5(25), (15 + 5)% NiCo/ZSM5(38), or (10 + 10)% NiCo/ZSM-5(25), the reaction gave obviously lower cyclohexanol selectivity during the HDO reaction. In comparison, the cyclohexanol selectivity improved evidently when the HDO reaction was conducted over γ-Al2O3 supported either monometallic catalysts (herein Ni/γ-Al2O3 or Co/γ-Al2O3) or bimetallic catalysts (NiCo/γ-Al2O3 with varied metal composition). As for Ni/γ-Al2O3 or Co/γ-Al2O3 and NiCo/γ-Al2O3 catalysts, both guaiacol conversion and the

Table 3. Effect of Solvent on Guaiacol Conversion and Product Distributiona selectivity entry

catalysts

guaiacol con (%)

COL

COE

tMCOL

cMCOL

1 2 3

water methanol ethanol

96.0 40.1 48.1

70.9 31.9 32.6

0.36 2.2 1.1

12.9 29.4 32.8

8.1 23.9 22.4

a

COL: cyclohexanol. COE: cyclohexane. t-MCOL: 1-methyl-1,2cyclohexanediol, trans. c-MCOL: 1-methyl-1,2-cyclohexanediol, cis.

observed concerning the guaiacol conversion and products distribution. It was found that the maximum guaiacol conversion was observed when water was used as solvent (see entry 1 in Table 3), with a selectivity of 70.9% of cyclohexanol. The guaiacol conversion was only 40.1% and 48.1% when methanol or ethanol was used as solvent during the HDO of guaiacol (see entries 2 and 3 in Table 3). In comparison to those alcohol solvents, when water was used as solvent, the HDO reaction could give better guaiacol conversion. Those above results conformed well to the studies of Lercher, Rinaldi, and Lou.32,49,50 Lercher et al. reported that water could give the highest phenol hydrogenation rate, and acetal reactions would happen between methanol and intermediates (such as cyclohexanone, etc.) when methanol was used as a solvent. Rinaldi et al. previously reported that methanol and ethanol would adsorb on the active sites, hindering chemisorptions of reactant on the active sites, which would lead to low conversion rate, because the heterogeneous 8829

DOI: 10.1021/acssuschemeng.7b01615 ACS Sustainable Chem. Eng. 2017, 5, 8824−8835

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selectivity of 72.5% at 200 °C, which was almost the lowest reaction temperature in comparison with other studies concerning the HDO of guaiacol over non-noble metal based catalysts.22,23 However, the selectivity of 1-methyl-1,2-cyclohexanediol (trans and cis) increased from 28.6% to 37.6% with the increase of temperature, which indicated that much higher temperature (>200 °C) would lead to the increase of 1-methyl1,2-cyclohexanediol selectivity and lower cyclohexanol selectivity, as they underwent different conversion pathway (see pathways III and IV in Figure 9 in the section Possible Reaction Pathway of Guaiacol over NiCo/γ-Al2O3 Catalyst). For the above reasons, 200 °C was finally chosen as the optimum reaction temperature considering both guaiacol conversion and cyclohexanol selectivity. The HDO reactions were carried out at varied initial hydrogen pressure from 4 to 8 MPa at 200 °C to investigate the catalytic activity of the (10 + 10)% NiCo/γ-Al2O3 catalyst. As can be seen in Figure 7, the initial hydrogen pressure had great

catalyzed HDO reaction always included the interactions between solvent−catalyst, reactant−catalyst, and reactant− solvent.48,49 Lou et al. also confirmed that the HDO of phenol gave a low conversion rate when methanol or ethanol was used as solvent. So herein, it is reasonable that water could give better guaiacol conversion rate and cyclohexanol selectivity than alcohol solvents. From another perspective, it could be speculated that, as methanol was one of those byproducts produced during the HDO reaction of guaiacol through the demethoxylation or demethylation process, the presence of either methanol or ethanol would restrict the demethoxylation or demethylation step that happened during the HDO process, which would inevitably lead to low guaiacol conversion and cyclohexanol selectivity. As the best catalytic performance was observed when water was adopted as a solvent. Therefore, further studies were carried out over (10 + 10)% NiCo/γ-Al2O3 catalyst with water as solvent. Product Distribution Studies over NiCo/γ-Al2O3 Catalyst. The effect of reaction temperature on guaiacol conversion and product distribution over (10 + 10)% NiCo/γ-Al2O3 catalyst with an initial hydrogen pressure of 5 MPa was presented in Figure 6. It could be observed that the guaiacol conversion

Figure 7. Effect of hydrogen pressure on guaiacol conversion and product distribution over (10 + 10)% NiCo/γ-Al2O3.

effect on both guaiacol conversion and product distribution. The selectivity of cyclohexanol decreased a little with the increase of initial hydrogen pressure and achieved its maximum when the reaction was conducted at an initial hydrogen pressure 5 MPa of hydrogen. The results conformed well with the Tomishige study, which also reported that lower initial hydrogen pressure was beneficial for higher cyclohexanol selectivity over Ru/C and MgO. The improvement of cyclohexanol selectivity in lower initial hydrogen pressure (≤5 MPa) could be explained from two perspectives: for one thing, hydrogen was a reactant in the HDO reaction, and the increased hydrogen amount would be beneficial to the hydrodeoxygenation reaction due to the pushing effect of excess hydrogen on the reaction balance; for another thing, increased hydrogen pressure would contribute to the solubility of hydrogen in the solution, which would add to the opportunities of hydrogen reached on the active sites of the catalysts during the hydrodeoxygenation reaction.42,51 The cyclohexane selectivity decreased with the increase of initial hydrogen pressure. Meanwhile, it could be seen that the total selectivity of 1-methyl-1,2-cyclohexanediol (trans and cis) increased with the increase of initial hydrogen pressure, suggesting that excessive hydrogen would contribute to the generation of more 1-methyl-1,2-cyclohexanediol and the specific reaction pathways were discussed in the following

Figure 6. Effect of reaction temperature on guaiacol conversion and product distribution over (10 + 10)% NiCo/γ-Al2O3.

dramatically improved from 45.1 to 97.2% with the increase of reaction temperature from 160 to 220 °C with a reaction time of 8 h. The selectivity of cyclohexanol arrived at its maximum when HDO reaction was carried out at 200 °C, and the selectivity of cyclohexanol decreased with the continuous increase of temperature. The selectivity of cyclohexane exhibited similar tendency considering the effect of reaction temperature. The above results indicated that the conversion of guaiacol to cyclohexanol might happen on the active sites of the catalysts at relatively higher reaction temperature, which was in good accordance with the work of Sels and Tomishige.20,23 Sels et al. confirmed that higher reaction temperature was beneficial for the conversion of guaiacol to cyclohexaol, although higher temperature (250 or 300 °C) and higher Ni loading amount (65 wt %) was needed during the reaction. Tomishige et al. also reported that higher cyclohexanol selectivity could be obtained at higher reaction temperature with lower initial hydrogen pressure over Ru/C and MgO. Herein, the HDO reaction of guaiacol could give a conversion of 94.2% with a cyclohexanol 8830

DOI: 10.1021/acssuschemeng.7b01615 ACS Sustainable Chem. Eng. 2017, 5, 8824−8835

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pathways for the conversion of guaiacol to cyclohexanol: one involved the hydrogenation of guaiacol to give 2-methoxycyclohexanol, followed by demethoxylation to form cyclohexanol; the other included the demethoxylation of guaiacol to form phenol, followed by the hydrogenation to give cyclohexanol.20,22 In this study, different reaction pathways for the hydrodeoxygenation of guaiacol over NiCo/γ-Al2O3 catalyst were proposed based on the products detected by GC-MS (see Figure S1 in the Supporting Information), which was exhibited in Figure 9. It can be seen in Figure S1 that cyclohexanol, cyclohexanone, 2-methyl-cyclohexanol (cis and trans), phenol, 1,2-cyclohexanediol, 1-methyl-1,2-cyclohexanediol (trans and cis) were detected by GC-MS, and the mass spectra of those compounds are presented in Figures S2−S7. Herein, methanol and cyclohexane were not observed in Figure S1, because those peaks before/and solvent peak were all excluded during the GC-MS analysis. According to the products detected by GCMS in Figure S1 and the product distribution in Figures 5−8, we deduced that the HDO of guaiacol over NiCo/γ-Al2O3 underwent different pathways, with phenol as the main intermediate and 1-methyl-1,2-cyclohexanediol (trans and cis) instead of 2-methoxy-cyclohexanol as another intermediate. As exhibited in Figure S1, very little phenol was detected, because phenol could be quickly hydrogenated into cyclohexanol, which was already confirmed by Sels and his coworkers.23 In comparison with the amount of phenol, a relatively large amount of 1-methyl-1,2-cyclohexanediol (trans and cis) was observed and 1-methyl-1,2-cyclohexanediol might undergo the dehydration and hydrogenation process to give 2methyl-cyclohexanol (cis and trans) (pathway IV in Figure 9). However, the subtle amount of 2-methyl-cyclohexanol suggested that it was difficult to convert 1-methyl-1,2cyclohexanediol to 2-methyl-cyclohexanol during the HDO process over NiCo/γ-Al2O3 catalyst. 1-Methyl-1,2-cyclohexanediol might undergo a quite slow process to form 2-methylcyclohexanol, as there was a quite subtle amount of 2-methylcyclohexanol, which was not exhibited in Figures 5−8. Herein, 1-methyl-1,2-cyclohexanediol was detected instead of 2methoxy-cyclohexanol as another intermediate, which was in accordance with Xiao’s research, reporting the presence of 1methyl-1,2-cyclohexanediol during the hydrodeoxygenation of guaiacol.36 In comparison with previous studies,20−23 pathway IV was proposed as another different reaction pathway with 1methyl-1,2-cyclohexanediol as second kind of intermediate instead 2-methoxy-cyclohexanol. It was very interesting to find that almost no 2-methoxy-cyclohexanol and 2-methoxycyclohexanone was detected during the HDO process, while Sels reported that 2-methoxy-cyclohexanol was another possible intermediate and could undergo the demethoxylation process to give cyclohexanol under high temperatures (250 or 300 °C). Additionally, very little amounts of 1,2-cyclohexanediol were observed from the GC-MS results in Figure S1, which was reported mainly generated from 2-methoxy-cyclohexanol.53 The above results indicated the small possibility of pathway II during the HDO process over NiCo/γ-Al2O3 catalyst. From another perspective, the above results also conformed to the study of the solvent effect, the HDO reaction exhibited very low guaiacol conversion with methanol as solvent, as the presence of methanol would suppress the demethoxylation process and decrease the yield of cyclohexanol. So in this study, pathways III and IV were the favorable reaction pathways with 1-methyl-1,2-cyclohexanediol and phenol as the main intermediates over NiCo/γ-Al2O3 catalyst.

section considering the product distribution (see Figure 9). This phenomenon conformed to the findings of the Tomishige study, which indicated that higher hydrogen pressure would suppress the formation of cyclohexanol and lead to the formation of other compounds such as 2-methoxy-cyclohexanol.20 Finally, 5 MPa was selected as the optimal reaction pressure considering guaiacol conversion and product distribution based on the above results. Li et al. reported that short hydrotreating duration (e.g.,