In situ synthesis of molybdenum oxide@N-doped carbon from biomass for selective vapor phase hydrodeoxygenation of lignin-derived phenols under H₂ atmosphere

Abstract

The vapor phase hydrodeoxygenation (HDO) of lignin-derived phenols under H₂ atmosphere has great significance for producing high-quality fuels and commodity chemicals. Herein, we reported a simple, green method to prepare molybdenum oxide@N-doped carbon (MoO_x@NC) via in situ pyrolysis of molybdenum precursor preloaded cellulose and demonstrated its catalytic performance for vapor phase HDO of lignin-derived phenols. When the pyrolysis temperature was at 600 °C, the catalyst (MoO_x@NC-600) exhibited the best catalytic performance in vapor phase HDO of guaiacol. Through systematically investigating the parameters, such as: reaction temperature, WHSV, residence time, and concentration, the optimal reaction conditions for vapor phase HDO of guaiacol were 450 °C and 1 h⁻¹ with atmospheric H₂. The concentration of the feed was 20% in mesitylene, and the residence time was about 3.3 s. The carbon yield of aromatic hydrocarbons was 83.3%, with 65.7% benzene, 15.5% toluene and 2.1% alkylbenzenes. In addition, other lignin-derived phenols were also investigated and desired results were achieved with the MoO_x@NC-600 catalyst. Furthermore, MoO_x@NC-600 showed good stability due to the N-doped carbon formed on the surface of the MoO_x particles. The catalysts were characterized using elemental analysis, AAS, BET, XRD, XPS, TEM, and EDS mapping. The high catalytic performance of MoO_x@NC-600 toward lignin-derived phenols HDO can be attributed to the synergistic effect of the carbon supports and Mo⁵⁺ (molybdenum oxynitrides), Mo^δ+ (Mo₂N) and Mo⁴⁺ on the surface of the MoO_x particles.

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1. Introduction

With the huge consumption of fossil resources and the emission of greenhouse gases, the study of alternative energy sources has received much attention. Lignocellulosic biomass is an attractive renewable feedstock that can be converted to transportation fuels and chemicals.¹ Lignin, which comprises up to 30 wt% of lignocellulosic biomass, is a natural polymer consisting of methoxylated phenylpropane units featuring numerous ether linkages (C–O–C), as well as hydroxyl (–OH), and methoxyl (–OMe) side groups.² Fast pyrolysis is an effective process that can convert lignin to generate a mixture of non-condensable liquid oils, gases, and solids.³ However, mono-phenols (such as phenol, syringol, guaiacol, and catechol), and other polysubstituted phenols in the lignin pyrolysis bio-oil lead to high instability, viscosity, corrosiveness, and polarity.^4,5 Therefore, the oxygen must be removed before the lignin pyrolysis oil can be used as a substitute for diesel, gasoline or aromatic chemicals.

Hydrodeoxygenation (HDO) is the most promising route to improve the effective H/C ratio of pyrolysis bio-oils and produce hydrocarbons either as final fuel components (e.g., gasoline and diesel) or as fuel intermediates (small olefins and alkanes).^6,7 The key challenge faced by HDO processes is achieving a high degree of oxygen removal, while reducing hydrogen consumption.⁸ Conventional hydrodesulphurization (HDS)/hydrodenitrogenation (HDN) catalysts exhibit promising activity in HDO of phenolic compounds such as phenol, anisole, and guaiacol.^9–13 However, the metal-sulfide catalysts suffer from deactivation in the presence of high water content and the continuous addition of sulfur is required in the reactant stream to maintain the catalysts in the sulfide form. In addition, some supported noble metals such as Ru, Rh, Pd, Pt, and Re, as well as base metals, such as Cu, Ni, Fe and their heterometallic alloys, are also active for hydrogenation/hydrogenolysis reactions, but the HDO process over these catalysts required high H₂ pressure, which also results in aromatic ring saturation.^14–18 The high pressure process would lead to high operational costs.¹⁹ Therefore, it is necessary to develop an HDO process employing low cost catalysts with high stability and low H₂ pressure.

Recently, many studies on vapor phase HDO of lignin-derived phenols via C–O bond cleavage without hydrogenating the aromatic ring under H₂ atmosphere were reported.^6,20 Olcese and co-workers showed that Fe/SiO₂ could be used as a catalyst in vapor phase HDO of guaiacol, while the yield of aromatic hydrocarbons was only 38%.^21,22 Wang’s group synthesized carbon-supported bimetallic Pd–Fe catalysts which have a good HDO activity of guaiacol and m-cresol with a good yield of aromatic hydrocarbons.^23,24 Nie and co-workers employed the bimetallic catalyst Ni–Fe/SiO₂ for catalytic conversion of m-cresol into toluene via a deoxygenation reaction.²⁵ Metal phosphides, such as Ni₂P, also demonstrate high HDO activity for conversion of lignin-derived phenols.²⁶ Wu and co-workers conducted atmospheric hydrodeoxygenation of guaiacol over nickel phosphide with different supports, finding that Ni₂P/SiO₂ prefers to produce aromatic hydrocarbons.²⁷ Besides the above catalysts, molybdenum-based catalysts, such as Mo₂C, Mo₂N and MoO₃, have shown good activity and selectivity in the vapor phase HDO of lignin-derived phenols. Ghampson and co-workers used Mo₂N and Mo₂N supported on activated carbon, Al₂O₃ and SBA to catalyze guaiacol HDO, observing a high activity and a significant conversion of guaiacol to phenol. They found that the active sites for catalyzing guaiacol conversion are the Mo₂N and Mo oxynitride on the catalyst surface. Furthermore, their experiments also showed that the bimetallic nitride catalyst CoMoN gave higher yields of deoxygenated products than the monometallic nitride catalyst, but the overall activity of the monometallic nitride catalyst was higher than that of the bimetallic nitride.^28–30 Lee and co-workers conducted selective vapor phase HDO of anisole to benzene over MoC₂, and the selectivity for benzene could reach about 90%.^31,32 MoC₂ was also used to catalyze HDO of lignin-derived phenolic compound mixtures containing m-cresol, anisole, 1,2-dimethoxybenzene, and guaiacol, and showed good catalytic activity at ambient pressure.³³ However, the carbide catalyst showed fast deactivation by oxidation with water, which is a challenge for the catalytic system.³⁴ In addition, MoO_x was employed to catalyze HDO of lignin-derived phenols under H₂ atmosphere. Prasomsri and co-workers found that MoO₃ is active and selective for direct C–O bond cleavage of guaiacol in vapor phase over a packed-bed flow reactor, producing phenol and hydrocarbons with a selectivity of 29.3% and 53.5%, respectively.^35–38

N-doped carbon (NC), as a fascinating kind of material, has attracted worldwide attention recently.^39,40 Because the incorporation of nitrogen atoms in the carbon architecture can enhance the chemical, electrical, and functional properties, it has been widely applied in flexible electronics, energy conversion/storage devices, and catalyst supports.^41–46 Xu and co-workers synthesized Pd nanoparticles supported on N-doped carbon and catalyzed bio-oil upgrading.⁴⁷ Li and co-workers synthesized Pd/N-doped carbon for catalytic hydrogenation of phenol, and found N-doped carbon could strongly promote the chemoselective reduction of phenol.^48,49

In this study, a new molybdenum-based catalyst (MoO_x@NC) for selective HDO of lignin-derived phenols was synthesized. MoO_x@NC catalysts were synthesized via co-pyrolysis of biomass and (NH₄)₆Mo₇O₂₄·4H₂O in one step. Catalysts prepared at different pyrolysis temperatures were obtained. Guaiacol was served as the model compound to test the catalytic activity. The factors which may affect the catalytic performance of the MoO_x@NC catalysts in the HDO process, including catalysts (C-600, NC-600, MoO_x@C, and MoO_x@NC-T), reaction temperature, WHSV, residence time and the content of guaiacol in the mixture, were investigated systematically. Different lignin-derived phenols and dimers were tested to produce aromatic hydrocarbons in this study. Furthermore, the lifetime of the MoO_x@NC catalyst was measured. In addition, the catalysts were characterized using elemental analysis, scanning electron microscope (SEM), Brunauer–Emmett–Teller surface area (BET), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS).

2. Experimental section

2.1 Chemicals

Anisole (AR), benzaldehyde (AR), phenol (AR), m-cresol (AR), benzene (AR), toluene (AR), and xylene (AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. Eugenol (AR), and n-propylbenzene (AR) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Guaiacol (AR), diphenyl ether (AR), 1,2-diphenylethane (AR), and mesitylene (AR) were purchased from Tokyo Chemical Industry Co. Ltd. (NH₄)₆Mo₇O₂₄·4H₂O was purchased from Alfa Aesar Co. Ltd. All these chemicals were used as received without any further purification. N₂ (99.999%), H₂ (99.999%), Ar (99.999%), NH₃ (≥99.5%), and He (99.999%) were purchased from Nanjing Special Gases Factory.

2.2 Catalyst preparation

For the synthesis of MoO_x@NC: the MoO_x@NC catalysts were synthesized by direct pyrolysis of (NH₄)₆Mo₇O₂₄·4H₂O and microcrystalline cellulose under ammonia conditions, which was conducted in a pyrolysis reactor. The tubular quartz pyrolysis reactor was placed in an electrical furnace with a PID temperature controller and the temperature of the experimental system was monitored using a K-type thermocouple. Before the pyrolysis process (Fig. S1†), 10.0 g of cellulose and a certain amount of (NH₄)₆Mo₇O₂₄·4H₂O solution were mixed in a flask and shaken at a constant temperature for 12 hours. Thereafter, the water in the mixture was evaporated under reduced pressure. Finally, the solid residue was dried again at 110 °C to remove the moisture and the Mo-preloaded cellulose was obtained. Then, the Mo-preloaded cellulose was filled into the feed pipe. 200 ml min⁻¹ of NH₃ flowed through the pyrolysis system to remove air. Until the temperature reached the pre-set value, the mixture was fed into the tubular quartz pyrolysis reactor through a piston. The volatiles produced during pyrolysis were condensed using cold water to obtain bio-oil. After the fast pyrolysis process, the solid residue was kept in the reaction zone for another 2 hours for further carbonization. Then, the reactor was cooled under N₂ flow (200 ml min⁻¹) to room temperature. At last the MoO_x@NC-T (T: pyrolysis temperature) catalyst was obtained and ground using a glass mortar. For synthesis of NC-600: cellulose was pyrolysized in the presence of ammonia at 600 °C for 2 hours (NC-600). For synthesis of C-600: cellulose was pyrolysized in the presence of argon at 600 °C for 2 hours (C-600). For synthesis of MoO_x@C: the pyrolysis process was similar to that of MoO_x@NC-T, except that the carrier gas was changed to argon during pyrolysis.

2.3 Catalyst test

As shown in Fig. S2,† a bench-top continuous flow reactor consisting of a quartz tube reactor heated by a furnace and a condensation tube bathed in liquid nitrogen was used for these experiments. The catalyst bed supported by quartz wool was built up in the heating zone of the reactor. A certain concentration of phenol solution was fed into the reactor with a peristaltic pump under a certain flow rate and purged with H₂. Volatile products were trapped in the condensation tube cooled with liquid N₂. The gas product was collected with a gas bag. The detailed method for analyzing the products and processing data are provided in the ESI.†

The conversion of phenols, the yield of aromatic hydrocarbons, phenols, and gases, and the selectivity of different aromatic hydrocarbons, were calculated from eqn (1) to (5).

2.4 Catalyst characterization

The elemental content of the catalysts was measured using atomic absorption spectroscopy (ICP/AAS, Atomscan Advantage, Thermo Jarrell Ash Corporation, USA). The N₂ adsorption/desorption isotherms of the catalysts were measured at −196 °C using a COULTER SA 3100 analyzer to do the Brunauer–Emmett–Teller (BET) surface area analyses. Scanning electron micrographs of the MoO_x@NC were obtained using a scanning electron microscope (SEM, Sirion 200, FEI Electron Optics Company, USA). Transmission electron microscopy (TEM) investigations were performed on a JEM-2100F instrument (JEOL, Japan). Powder X-ray diffraction (XRD) analysis of the catalyst was carried out on a theta rotating anode X-ray diffractometer (TTR-III, Rigaku, Japan) with Cu Kα radiation (30 kV/160 mA, λ = 1.54056 Å) with a scan rate (2θ) of 0.05° s⁻¹. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCALAB250 instrument (Thermo-VG Scientific, UK). The intensity of the XPS peaks was recorded as counts per second (CPS) and deconvoluted into subcomponents using a Gaussian (80%)–Lorentzian (20%) curve-fitting program (XPSPEAK 4.1 software), with Shirley type background.

3. Results

3.1 Effect of catalyst preparation conditions

Herein, a simple and green method was developed to prepare molybdenum oxide@N-doped carbon (MoO_x@NC) via in situ pyrolysis of molybdenum precursor preloaded cellulose. Table 1 shows the effect of catalyst preparation conditions on the catalytic activity of the MoO_x@NC catalyst. Seven kinds of catalyst were prepared under different conditions. As shown in entry 1 and 2, we firstly tested the catalytic activity of carbon (C-600) and N-doped carbon (NC-600) on the vapor phase HDO of guaiacol, and found that both carbon and N-doped carbon could catalyze HDO of guaiacol to form aromatic hydrocarbons and selectively cleaved the C–O bond to form phenols. When C-600 served as the catalyst, the conversion of guaiacol was 35.6%, and the carbon yields of aromatic hydrocarbons and phenols were 10.3% and 18.9%, respectively. When the N-doped carbon (NC-600) served as the catalyst, the conversion of guaiacol was 53.4%, and the carbon yields of aromatic hydrocarbons and phenols were 16.7% and 26.3%, which were much higher than those of C-600. Therefore, N-doped carbon (NC-600) showed better catalytic performance for vapor phase HDO of guaiacol. In addition, the molybdenum oxide supported on C-600 (MoO_x@C-600) and NC-600 (MoO_x@NC-600) catalysts were also tested. As shown in entry 3 and entry 4, MoO_x@NC-600 showed better catalytic performance than MoO_x@C-600 did. When MoO_x@NC-600 served as the catalyst, the conversion of guaiacol was 100%, and the carbon yields of aromatic hydrocarbons and phenols were 68.8% and 6.7%, respectively. Meanwhile, when MoO_x@C-600 served as the catalyst, the conversion of guaiacol was 97.5%, and the carbon yields of aromatic hydrocarbons and phenols were 59.8% and 14.7%, respectively. Compared with the catalytic performance of the catalysts prepared under N₂ atmosphere, the catalyst prepared under ammonia atmosphere could produce more aromatic hydrocarbons and fewer phenols. Therefore, the catalyst prepared under ammonia atmosphere could enhance the HDO performance.

Table 1 The effect of the catalyst preparation conditions on the guaiacol HDO process^a

a The catalytic test conditions: solvent: mesitylene, guaiacol concentration = 50%, T = 400 °C, WHSVmixture = 1 h^−1, residence time = 2.2 s, PH2 = 1 atm, each time-on-stream is 1 h.b The yield in this study is carbon yield.
Entry	1	2	3	4	5	6	7
Catalyst	C-600	NC-600	MoOx@C-600	MoOx@NC-600	MoOx@NC-500	MoOx@NC-550	MoOx@NC-650
Conversion (%)	36.5	53.4	97.5	100	100	100	100
Overall yield^b	32.9	47.1	95.2	95.1	92.7	89.2	92
Gases	—	—	8.8	9.3	8.0	9.7	14.5
Coke	3.7	4.2	11.9	10.3	11.7	10.9	11.4
Aromatic hydrocarbons	10.3	16.7	59.8	68.8	46.5	49.7	58.2
Phenols	18.9	26.2	14.7	6.7	26.5	18.9	7.9
Aromatic hydrocarbons
Benzene	8.4	13.0	45.9	46.1	35.2	35.6	37.1
Toluene	1.9	3.7	13.6	19.8	10.0	12.4	18.6
Other alkylbenzenes	0	0	0.3	2.9	1.3	1.7	2.5
Phenols
Phenol	1.5	0.9	9.2	3.3	17.9	12.1	1.8
Anisole	17.4	24.5	1.5	1.7	4.1	3.6	3.4
Cresol	0	0.8	2.7	1.4	3.7	2.0	1.1
Other alkylphenols	0	0	1.3	0.3	0.8	1.2	1.6

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Because MoO_x@NC catalysts were prepared via an in situ pyrolysis process, the pyrolysis temperature was an important parameter to affect the catalyst performance. Entry 4 to entry 7 in Table 1 shows the guaiacol conversion and the detailed product distribution catalyzed by MoO_x@NC catalysts prepared at different pyrolysis temperatures. Fig. 1 shows the detailed carbon yield of different aromatic hydrocarbons and phenols. As shown in Table 1, under the selected conditions, all the guaiacol conversions were 100%. The carbon yield of coke did not change significantly, and remained at about 11%. The catalyst preparation temperature affected the carbon yield of aromatic hydrocarbons and phenols significantly. When the MoO_x@NC-500 served as the catalyst, the carbon yields of aromatic hydrocarbons and phenols were 46.5% and 26.5%, respectively. With the catalyst preparation temperature increasing to 600 °C, the carbon yield of aromatic hydrocarbons reached 68.8% and the carbon yield of phenols was only 6.7%. If the catalyst preparation temperature further increased to 650 °C, the carbon yield of aromatic hydrocarbons decreased, while the carbon yield of phenols increased. As shown in Fig. 1, the carbon yield of phenol and m-cresol decreased with the catalyst preparation temperature increasing. When it was 600 °C, the highest carbon yield of benzene (46.1%) and toluene (19.8%) was obtained. Therefore, 600 °C was the suitable pyrolysis temperature for preparing the HDO catalyst, and MoO_x@NC-600 was the suitable catalyst for catalytic vapor phase HDO of lignin-derived phenols.

Fig. 1 Effect of the catalyst preparation temperature on the detailed carbon yields of different aromatic hydrocarbons and phenols.

3.2 Effect of reaction conditions on the vapor phase HDO of guaiacol

3.2.1 HDO reaction temperature. Based on the above study, we found that MoO_x@NC-600 was the optimal catalyst for catalytic HDO of guaiacol under H₂ atmosphere. However, the carbon yield of aromatic hydrocarbons was only about 68.8%, and there was still some phenols producing via partial HDO process. Herein, we further investigated the reaction conditions on the vapor phase HDO, which could also affect the product distribution and yield. Firstly, the effect of the HDO reaction temperature was investigated in the range of 300 °C and 600 °C over the MoO_x@NC-600 catalyst. Fig. 2 shows the guaiacol conversion and the overall yields of coke, gases, aromatic hydrocarbons and phenols (a); the detailed carbon yield of different aromatic hydrocarbons and phenols (b) at different HDO reaction temperatures. The detailed product distributions at different temperatures are given in ESI Table S3.† As shown in Fig. 2, the guaiacol conversion and the carbon yield of coke, gases, aromatic hydrocarbons and phenols were sensitive to the HDO temperature. With the temperature increasing from 300 to 600 °C, the carbon yield of gases increased from 4.2% to 16.2%. In contrast, the carbon yield of coke decreased from 19.5 to 4.8%. This could be caused by the higher temperature promoting phenol deep cracking to form non-condensable gas products and preventing coke formation. When the HDO reaction was below 400 °C, the guaiacol conversion could not reach 100%. Meanwhile, the main detected products in the liquid were not aromatic hydrocarbons but phenols, which were produced via a partial HDO process. Phenol was the main product in the phenols. When the HDO reaction temperature was at 350 °C, the carbon yield of phenols reached 63.9% (Fig. 2a), and the carbon yield of phenol was 50.9% (Fig. 2b). The lower reaction temperature prevented the catalytic activity, and led to a partial deoxygenation reaction. At 400 °C, with full guaiacol conversion, the product distributions also changed dramatically. Aromatic hydrocarbons became the main products. The carbon yield of phenols decreased from 63.9% to 6.7%, while the carbon yield of aromatic hydrocarbons increased from 6.1% to 68.8%. Benzene and toluene were the main products in the aromatic hydrocarbons. The carbon yield of other alkylbenzenes (including xylenes) was very low. When the HDO reaction temperature increased to 450 °C, the carbon yield of aromatic hydrocarbons reached the maximum (70.2%) and the carbon yields of benzene and toluene were 51.8% and 15.0%, respectively (Fig. 2b). Conversely, the carbon yield of phenols reached the minimum (only 1.8%). If the reaction temperature further increased (500 °C, and 600 °C), the carbon yield of aromatic hydrocarbons decreased, while the carbon yield of phenols increased. The higher reaction temperature may also cause the catalyst deactivation, and lead to the partial deoxygenation to form phenols, which could be the reason that the carbon yield of phenols increased with the reaction temperature further increasing. Thus, 450 °C was the optimal reaction temperature for HDO of guaiacol over MoO_x@NC-600 under H₂ atmosphere.

Fig. 2 Effect of HDO temperature on HDO of guaiacol over MoO_x@NC-600: (a) overall yield; and (b) detailed carbon yield of different aromatic hydrocarbons and phenols (reaction condition: catalyst = MoO_x@NC-600, solvent: mesitylene, guaiacol concentration = 50%, WHSV_mixture = 1 h⁻¹, residence time = 2.2 s, P_H2 = 1 atm.).

3.2.2 WHSV. In addition to HDO reaction temperature, the effect of WHSV was also investigated in this study. Table 2 shows the detailed product distributions at different WHSV. The WHSV was defined as the ratio of the mass flow rate of the guaiacol/mesitylene mixture to the mass of catalyst used in the reactor. During the experiments, the mass flow rate of guaiacol ranged from 0.5–2 g h⁻¹ while the mass of catalyst was kept constant at 1 g. The HDO reaction temperature was kept at 450 °C. As shown in Table 2, all the guaiacol conversions were 100% in the range of 0.5 to 1.5 h⁻¹. When the WHSV increased to 2 h⁻¹, the conversion decreased to 96.3%. Meanwhile, the carbon yield of gas and coke decreased from 13.8% to 10.7% with the WHSV increasing from 0.5 to 2 h⁻¹. The carbon yield of coke remained at about 7.5%. The carbon yield of aromatic hydrocarbons and phenols was affected by WHSV significantly. When the WHSV was at 0.5 h⁻¹, no phenols were detected, and the carbon yield of aromatic hydrocarbons was about 60.9%. With the WHSV increased from 0.5 to 1 h⁻¹, the carbon yields of aromatic hydrocarbons and phenols increased from 60.9% and 0% to 70.2% and 1.8%, respectively. If the WHSV further increased to 1.5 h⁻¹ and 2 h⁻¹, the carbon yield of aromatic hydrocarbons started to decrease. Meanwhile, the carbon yield of phenols increased with the WHSV further increasing. Thus, the optimal WHSV was 1 h⁻¹ for catalytic HDO of guaiacol over MoO_x@NC-600 under H₂ atmosphere.

Table 2 Effect of WHSV^a

a The catalytic test condition: solvent: mesitylene, guaiacol concentration = 50%, T = 450 °C, residence time = 2.2 s; PH2 = 1 atm, each time-on-stream is 1 h.b The yield in this study is carbon yield.
Entry	1	2	3	4
WHSV (h^−1)	0.5	1.0	1.5	2.0
Conversion (%)	100	100	100	96.3
Overall yield^b	82.9	92.3	91.1	94.1
Gases	13.8	13.4	12.5	10.7
Coke	8.2	6.9	7.6	8.1
Aromatic hydrocarbons	60.9	70.2	65.2	64.1
Phenols	0	1.8	5.8	11.2
Aromatic hydrocarbons
Benzene	41.9	51.8	46.3	43.9
Toluene	14.8	15.0	15.2	16.3
Other alkylbenzenes	4.2	3.4	3.7	3.9
Phenols
Phenol	0	0.7	1.1	4.3
Anisole	0	0.6	0.8	1.2
Cresol	0	0.5	2.9	5.0
Other alkylphenols	0	0	1.0	0.7

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3.2.3 Residence time. To further optimize the reaction conditions of catalytic HDO of guaiacol over MoO_x@NC-600 under H₂ atmosphere, the effect of residence time was also investigated via changing the catalyst usage from 0.5 g to 2 g and fixing the reaction temperature, H₂ flow rate, and WHSV at 450 °C, 70 ml min⁻¹, and 1 h⁻¹. Herein, the residence time was in the range of 1.1 s and 4.4 s. Table 3 shows the detailed product distributions at different residence times. The guaiacol conversion, and the carbon yield of coke and gas were not affected by the residence time significantly. With the residence increasing from 1.1 s to 4.4 s, all the guaiacol conversions were 100%, and the carbon yields of coke and gas kept at about 7% and 13%, respectively. However, the carbon yield of aromatic hydrocarbons and phenols was very sensitive to the residence time. When the residence time was at 1.1 s, the carbon yields of aromatic hydrocarbons and phenols were 54.8% and 22.1%, respectively. Then, with the residence time increasing from 1.1 s to 3.3 s, the carbon yield of aromatic hydrocarbons increased to 76.8%, while the carbon yield of phenols decreased to 0.7%. If the residence time further increased to 4.4 s, no phenols were detected. However, the carbon yield of aromatic hydrocarbons also decreased. Therefore, the optimal residence time was 3.3 s, and the catalyst usage was 1.5 g. Meanwhile, the carbon yields of benzene and toluene reached 53.9% and 18.1%, respectively.

Table 3 Effect of residence time^a

a The catalytic test condition: solvent: mesitylene, guaiacol concentration = 50%, T = 450 °C, WHSVmixture = 1 h^−1, PH2 = 1 atm, each time-on-stream is 1 h.b The yield in this study is carbon yield.
Entry	1	2	3	4
Catalyst usage (g)	0.5	1.0	1.5	2.0
Residence time (s)	1.1	2.2	3.3	4.4
Conversion (%)	100	100	100	100
Overall yield^b	93.8	92.3	96.5	94.3
Gases	10.7	13.4	12.7	13.2
Coke	6.2	6.9	6.3	7.8
Aromatic hydrocarbons	54.8	70.2	76.8	73.3
Phenols	22.1	1.8	0.7	0
Aromatic hydrocarbons
Benzene	39.3	51.8	53.9	50.3
Toluene	13.8	15.0	18.1	16.6
Other alkylbenzenes	1.7	3.4	4.8	6.4
Phenols
Phenol	11.2	0.7	0.5	0
Anisole	4.6	0.6	0.2	0
Cresol	5.3	0.5	0	0
Other alkylphenols	1.0	0	0	0

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3.2.4 Guaiacol concentration. Besides the effects of reaction temperature, WHSV, and residence time, the effect of guaiacol concentration in mesitylene was investigated in the range of 20% to 100%. Table 4 shows the detailed product distributions of catalytic HDO of guaiacol with different concentrations. As shown in Table 4, the guaiacol conversion and carbon yield of gas were not affected by guaiacol concentration significantly. The guaiacol conversion remained at 100%, and the carbon yield of gases was about 12%. However, the carbon yield of coke, aromatic hydrocarbons and phenols was affected by the guaiacol concentration. With the guaiacol concentration increasing from 20% to 100%, the carbon yields of coke and phenols increased from 2.7% and 0% to 12.4% and 33.3%, respectively. Meanwhile, the carbon yield of aromatic hydrocarbons decreased from 83.3% to 39.8%. The lower the guaiacol concentration, the lesser the coke formation, and the greater the aromatic hydrocarbons production. Therefore, the optimal guaiacol concentration was 20%. Meantime, the carbon yields of benzene and toluene were 65.7% and 15.5%.

Table 4 Effect of guaiacol concentration in mesitylene^a

a The catalytic test condition: T = 450 °C, residence time = 3.3 s, WHSVmixture = 1 h^−1, PH2 = 1 atm, each time-on-stream is 1 h. The yield in this study is carbon yield.
Entry	1	2	3	4
Concentration	20%	50%	80%	100%
Conversion (%)	100	100	100	100
Gases	11.6	12.7	11.0	11.8
Coke	2.7	6.3	8.9	12.4
Aromatic hydrocarbons	83.3	76.8	50.0	39.8
Phenols	0	0.7	23.2	33.3
Aromatic hydrocarbons
Benzene	65.7	53.9	37.1	28.2
Toluene	15.5	18.1	9.9	8.1
Other alkylbenzenes	2.1	4.8	3.0	3.5
Phenols
Phenol	0	0.5	17.3	23.4
Anisole	0	0.2	1.9	3.6
Cresol	0	0	3.8	6.0
Other alkylphenols	0	0	0.2	0.3

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Through systematically investigating the parameters (such as: reaction temperature, WHSV, residence time, and concentration) in the vapor phase HDO of guaiacol under H₂ atmosphere, the optimal reaction conditions for vapor phase HDO of guaiacol were at 450 °C, 1 h⁻¹. The concentration of feed was 20%, and the residence time was about 3.3 s. The carbon yield of aromatic hydrocarbons was 83.3%, and no phenols were detected. The carbon yields of benzene, toluene and alkylbenzenes were 65.7%, 15.5%, and 2.1%, respectively.

3.3 Vapor phase HDO of different lignin-derived compounds over MoO_x@NC catalyst

Based on the above study, guaiacol could be efficiently converted to aromatic hydrocarbons via a vapor phase HDO process over an MoO_x@NC catalyst. In order to investigate the applicability of the catalyst, seven other kinds of lignin-derived compounds including phenol, anisole, m-cresol, benzaldehyde, eugenol, diphenyl ether, and phenethoxybenzene were also catalyzed by MoO_x@NC to produce aromatic hydrocarbons (Table 5). For the simple lignin-derived compounds (phenol, m-cresol, anisole and benzaldehyde), MoO_x@NC-600 showed good catalytic activity for vapor phase HDO to produce aromatic hydrocarbons. The carbon yield of aromatic hydrocarbons was above 80%. When benzaldehyde served as the raw material, the carbon yield of the main product, toluene, was 61.7%. This indicated that MoO_x@NC-600 can catalyze HDO of the aldehyde group in addition to removal of the methoxyl and phenolic hydroxyl groups. More interestingly, 1,2-diphenylethane could be detected in the products, and the carbon yield of 1,2-diphenylethane was 8.2%. 1,2-Diphenylethane could be produced via a coupling reaction of benzaldehyde catalyzed by N-doped carbon, which had some basic sites. Eugenol could also be effectively converted to aromatic hydrocarbons by the MoO_x@NC-600 catalyst. Meanwhile, the allyl group in eugenol was hydrogenated to a propyl group. Thus, propyl benzene was the main HDO product from eugenol. MoO_x@NC-600 could catalyze lignin dimer model compounds (diphenyl ether and phenethoxybenzene) HDO. When diphenyl ether served as the feed, benzene was the only aromatic hydrocarbon product, and the carbon yield of benzene reached 87.2%. When phenethoxybenzene served as the raw material, the carbon yield of benzene could reach 40.5% (theoretical carbon yield = 42.8%), indicating that MoO_x@NC-600 can effectively remove the phenolic hydroxyl groups. However, the carbon yield of ethylbenzene was only 28.3%, which means that MoO_x@NC-600 cannot effectively remove the alcohol hydroxyl group.

Table 5 Vapor phase HDO of different lignin-derived phenols over MoO_x@NC-600 under H₂ atmosphere^a

Feed	Yield (C%)	Aromatic hydrocarbons and yields^b
		Benzene	Toluene	Other alkylbenzenes
a All the feed conversions were 100%. The catalytic test conditions: guaiacol concentration = 20%, T = 450 °C, WHSV = 1 h^−1, residence time = 3.3 s, PH2 = 1 atm, each time-on-stream is 1 h.b The yield in this study is carbon yield.
	90.6	90.6	N.D	N.D
	87.3	10.3	77.0	N.D
	82.8	54.3	23.0	4.5
	82.0	12.1	61.7
	73.2	12.6	5.3
	87.2	87.2	N.D	N.D
	70.9	40.5	2.1

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3.4 Catalyst stability

The catalyst’s stability is very important in the vapor phase HDO process. Prasomsri et al. reported that molybdenum oxides showed fast deactivation in the vapor phase HDO of cresol.³⁵ When the HDO reaction temperature was at 400 °C, the cresol conversion decreased from 100% to 10% after a 4 hour reaction. Herein, the stability of MoO_x@NC was also investigated at 450 °C using guaiacol as the feed. Fig. 3 shows the guaiacol conversion and carbon yield of aromatic hydrocarbons and phenols (a), and the detailed carbon yield of different aromatic hydrocarbons and phenols (b) at different time on streams. The detailed product distributions are given in Table S4 in the ESI.† As shown in Fig. 3a, in the early 7.5 hours, the guaiacol conversion remained at 100%, and the carbon yields of aromatic hydrocarbons and phenols kept above 80% and 5%. Meanwhile, the carbon yields of benzene and toluene were about 60% and 20% (Fig. 3b), respectively. Anisole was the main component in the phenols. With the reaction time further extending to 20 hours, the guaiacol conversion still remained at 100%. However, the carbon yield of aromatic hydrocarbons kept decreasing. Meanwhile, the changing trend of benzene and toluene was consistent with that of the aromatic hydrocarbons. Conversely, the carbon yield of phenols increased. When the reaction time reached 20 hours, the carbon yield of aromatic hydrocarbons decreased to 42.7%, meanwhile, the carbon yield of phenols increased to 25.8%. To investigate the reason for the selectivity of aromatics dropping substantially after 7.5 hours and the change of the catalyst structure, the catalyst after the reaction was characterized using elemental analysis, N₂ adsorption/desorption and XPS. Table S5† shows the physicochemical properties of MoO_x@NC-600 before and after the reaction. The elemental content, surface elemental content, BET surface area and pore volume of the catalyst changed a lot after the reaction. The content of carbon on the surface increased. The content of Mo and N, and the BET surface area and pore volume decreased a lot. This indicated that coke formed after the reaction, and the coke may cover the active site of the catalyst, which caused the catalytic activity of the MoO_x@NC-600 to decrease.

Fig. 3 The catalyst stability: (a) conversion and overall yield; (b) detailed carbon yields of different aromatic hydrocarbons and phenols (reaction condition: solvent: mesitylene, guaiacol concentration = 20%, T = 450 °C, residence time = 3.3 s, WHSV_mixture = 1 h⁻¹).

3.5 Catalyst characterization

According to the above study, we found that MoO_x@NC-600 was the optimal catalyst for catalytic vapor phase HDO of lignin-derived phenols under H₂ atmosphere. Table 6 shows the physicochemical properties of the MoO_x@NC-600 catalyst, including BET surface area, elemental content, and the catalyst’s surface elemental content. The BET surface area of MoO_x@NC-600 was 81.0 m² g⁻¹ (the adsorption/desorption of MoO_x@NC-600 is shown in Fig. S3 in the ESI†). The content of C, H, N, and O was 47.2%, 1.7%, 4.5% and 20.5%, respectively. The metal loading of MoO_x@NC-600 was about 26.1%. For comparison, the catalyst’s surface content was also measured using XPS. The content of C, N, O, and Mo was 85.7%, 6.1%, 6.5%, and 1.7%, respectively. The content of carbon and nitrogen on the catalyst surface was much more than that of the bulk catalyst. In contrast, the content of Mo and oxygen on the surface was lower than that of the bulk catalyst. This indicated that N-doped carbon might form on the surface of the MoO_x particles, which could be beneficial for the catalyst’s stability.

Table 6 Physicochemical properties of MoO_x@NC-600

BET surface area m^2 g^−1	Elemental content (wt%)	C	H	N	Mo	O
a The content of C, H, and N was determined using an elemental analyzer. The content of molybdenum in MoOx@NC-600 was measured using atomic absorption spectroscopy (Escalab 250Xi, Thermo Fisher).b The elemental content of the catalyst surface was determined using XPS.
81.0	Catalyst elemental content^a	47.2	1.7	4.5	26.1	20.5
	Catalyst surface elemental content^b	85.7	—	6.1	1.7	6.5

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XRD was used to monitor the crystallite phase composition of MoO₂, NC-600, and MoO_x/NC-600. As shown in Fig. 4, amorphous carbon was in the NC-600. The characteristic diffraction peaks of MoO₂ patterns were at 2θ of 26.1°, 36.9°, 41.6°, 53.3°, 60.4°, 66.7°, and 78.8°. The diffraction peaks at 2θ of 26.1°, 36.9°, 41.6°, 53.3°, 60.4°, 66.7°, and 78.8° were also present in the XRD patterns of MoO_x@NC-600, indicating that the MoO_x in the MoO_x@NC-600 was MoO₂.

Fig. 4 XRD patterns of NC-600 and MoO_x@NC-600.

In order to further evaluate the electronic state of molybdenum, nitrogen and oxygen present in the catalyst, the binding energies of the relative substances were determined using XPS. Although the diffraction peaks shown in the XRD patterns can only be attributed to MoO₂, from Mo 3d in Fig. 5b, the Mo 3d doublet contained mixed chemical states with contributions from Mo^δ+, Mo⁴⁺, Mo⁵⁺, and Mo⁶⁺. In detail, the 3d_5/2 energy at 229.0 eV, corresponding to Mo with an oxidation state between +4 and +2, is assigned to the Mo₂N species.^28–30 Mo₂N is an effective catalyst for catalytic HDO of lignin-derived phenols.^28,30 The 3d_5/2 energy at 229.7 eV, corresponding to Mo⁴⁺, can be assigned to MoO₂.^35,36 The 3d_5/2 energy at the 230.8 eV peak could be assigned to molybdenum oxynitrides.^30,50–52 Molybdenum oxynitrides are similar to MoO_xC_yH_z, which is associated with the active sites and the HDO activities.^35,36 The percentage of Mo⁶⁺, Mo⁵⁺, Mo⁴⁺ and Mo^δ+ was 42.9 : 12.5 : 34.9 : 9.7. In addition, no signal corresponding to Mo metal (i.e., the Mo⁰ state) was detected. From N 1s in Fig. 5c, the catalyst displayed 4 binding energy peaks at 395.0 eV, 396.6 eV, 398.5 eV and 400.9 eV. The peak at 395.0 eV could be attributed to the molybdenum oxynitrides; the peak at 396.6 could be attributed to the Mo–N in the Mo₂N; the peak at 398.5 eV could be attributed to pyridinic-like (398.5 ± 0.2 eV) nitrogen atoms incorporated into graphitic sheets (pyridinic-N); the peak at 400.7 eV can be mainly assigned to pyrrolic/pyridone-N.^53–55 From O 1s in Fig. 5d, three peaks at 530.5 eV, 532.0 eV and 533.2 eV could be attributed to Mo–O, OH– and H₂O, respectively.^56,57

Fig. 5 XPS curves of MoO_x@NC-600. (a) Survey XPS curves of O 1s, N 1s, C 1s and Mo 3d of the catalyst. (b) X-ray photoelectron Mo 3d spectra of Mo 3d of the MoO_x@NC-600 catalyst. (c) X-ray photoelectron N 1s spectra of the MoO_x@NC-600 catalyst. (d) X-ray photoelectron O 1s spectra of MoO_x@NC-600.

Fig. 6 shows the SEM, TEM, and EDS mapping of MoO_x@NC-600. Fig. 6a shows the porous morphology on the rough carbon sheet, which is similar to the typical pyrolytic biochar reported by Liu and co-workers.^58,59 TEM images and the corresponding elemental mapping analysis of MoO_x@NC-600 (Fig. 6b–f) could further reveal the morphology of the MoO_x@NC-600 catalyst. Fig. 6b showed that the MoO_x particle was irregular, and the particle size of MoO_x reached about 200 nm. The EDS mapping (Fig. 6d–f) shows that the MoO_x had been successfully embedded on the N-doped carbon. In addition, it also indicated that N-doped carbon was deposited on the surface of the MoO_x particles, which would be beneficial for the catalyst’s stability.

Fig. 6 The SEM, TEM and EDS mapping of MoO_x@NC-600: (a) SEM analysis; (b) TEM analysis; (c) elemental mapping of Mo; (d) elemental mapping of C; (e) elemental mapping of O; (f) elemental mapping of N.

4. Discussion

In the above study, we found that the pyrolysis temperature was important to the catalytic activity of the MoO_x@NC catalysts. To further illustrate the effect of pyrolysis temperature on the catalytic activity of the MoO_x@NC catalysts, these catalysts were also characterized using elemental analysis (Table S1†), XRD (Fig. 7), and XPS (Fig. 8). As shown in Table S1,† with the pyrolysis temperature increasing, the content of molybdenum increased, while the content of carbon and nitrogen decreased. However, when the pyrolysis temperature was 600 °C, the content of nitrogen started to increase, which could be due to the molybdenum nitride formation in the pyrolysis process (shown in Fig. S1†). XRD patterns (Fig. 7) showed that molybdenum existed mainly as MoO₂ when the pyrolysis temperature ≤ 600 °C. When the pyrolysis temperature increased to 650 °C, the diffraction peaks at 2θ of 41.6°, 53.3°, 60.4°, 66.7°, and 78.8° disappeared. Meanwhile, new diffraction peaks at 2θ of 43.3°, 53.5°, 62.8°, and 75.5° appeared. The new diffraction peaks could be attributed to Mo₂N.

Fig. 7 XRD patterns of MoO_x@NC-T catalysts.

Fig. 8 XPS curves of Mo in different MoO_x@NC-T catalysts. (a): MoO_x@NC-500; (b) MoO_x@NC-550; (c) MoO_x@NC-600; (d) MoO_x@NC-650.

Furthermore, XPS curves (Fig. 8) also showed that the Mo 3d doublet contained mixed chemical states attributed to Mo^δ+, Mo⁴⁺, Mo⁵⁺, and Mo⁶⁺. According to the previous studies, Mo⁵⁺ (corresponding to molybdenum oxynitrides), and Mo^δ+ (corresponding to Mo₂N) are active for catalytic HDO of lignin-derived phenols, while Mo⁶⁺ is not.^28–30 In the present system, with the pyrolysis temperature increasing, the percentage of Mo⁶⁺ remained at about 44%, while the percentage of Mo⁵⁺ (corresponding to molybdenum oxynitrides), Mo⁴⁺ (corresponding to MoO₂), and Mo^δ+ (corresponding to Mo₂N) changed significantly. When the pyrolysis temperature was at 500 °C, the Mo 3d was attributed to Mo⁴⁺, Mo⁵⁺, and Mo⁶⁺. The signal (Mo^δ+) corresponding to Mo₂N was not detected. The percentage of Mo⁶⁺, Mo⁵⁺, and Mo⁴⁺ was 43.4, 37.3 and 19.3, respectively. When the pyrolysis temperature increased to 550 °C, the Mo^δ+ assigned to Mo₂N was detected and the percentage of Mo^δ+ was 5%. Then, the percentage of Mo^δ+ increased with the temperature further increasing. When the pyrolysis temperature was at 650 °C, the percentage of Mo^δ+ was 26.5%. By contrast, the percentage of Mo⁵⁺ decreased with the pyrolysis temperature increasing. When the pyrolysis temperature was at 650 °C, the percentage of Mo⁵⁺ was 12.3%. For the Mo⁴⁺, when the pyrolysis temperature was ≤600 °C, the percentage of Mo⁴⁺ increased from 19.3% to 34.9%. If the pyrolysis temperature further increased to 650 °C, the percentage of Mo⁴⁺ decreased to 15.3%. When the pyrolysis temperature was at 600 °C, the percentage of Mo⁴⁺ was highest, and the catalyst also showed the best catalytic performance. In addition, the variation tendency of Mo⁴⁺ was consistent with the variation tendency of the catalyst’s catalytic performance. Therefore, besides the Mo⁵⁺ and Mo^δ+, the Mo⁴⁺ should also be the active site in the MoO_x@NC catalyst for catalytic HDO.

5. Conclusion

In this study, a simple and green method was developed to prepare molybdenum oxide@N-doped carbon (MoO_x@NC) via in situ pyrolysis of molybdenum preloaded cellulose. MoO_x@NC demonstrated excellent catalytic performance and stability for vapor phase HDO of lignin-derived phenols. Through the study of catalyst preparation conditions, we found that N-doped carbon could catalyze HDO of guaiacol to form aromatic hydrocarbons and selectively cleaved C–O bonds to form phenols. When the pyrolysis temperature was at 600 °C, the catalysts (MoO_x@NC-600) exhibited the best catalytic performance for vapor phase HDO of guaiacol. Through systematically investigating the parameters (such as: reaction temperature, WHSV, residence time, and concentration) in the vapor phase HDO of guaiacol under H₂ atmosphere, the optimal reaction conditions for vapor phase HDO of guaiacol were at 450 °C, and 1 h⁻¹. The concentration of guaiacol in mesitylene was 20%, and the residence time was about 3.3 s. The carbon yield of aromatic hydrocarbons was 83.3%, and no phenols were detected. The carbon yields of benzene, toluene and alkylbenzenes were 65.7%, 15.5%, and 2.1%, respectively. Other lignin-derived phenols were also investigated and achieved good results by using MoO_x@NC-600. Furthermore, the catalysts were also characterized using elemental analysis, AAS, BET, XRD, XPS, TEM, and EDS mapping. Mo⁴⁺ could be the main active site of the MoO_x@NC catalysts, and the N-doped carbon formed on the surface of the MoO_x particles could also be beneficial for the catalyst’s stability. Therefore, the high catalytic performance of MoO_x@NC-600 toward lignin-derived phenols HDO can be attributed to the synergistic effect of the carbon supports and Mo⁵⁺ (molybdenum oxynitrides), Mo^δ+ (Mo₂N) and Mo⁴⁺ on the surface of the MoO_x particles.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (21572213), the National Basic Research Program of China (2013CB228103), the Anhui Provincial Natural Science Foundation (1408085MKL04), the Program for Changjiang Scholars, the Innovative Research Team in the University of the Ministry of Education of China and Fundamental Research Funds for the Central Universities for the financial support.

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Footnotes

† Electronic supplementary information (ESI) available: pyrolysis reactor for preparing MoO_x@NC catalysts and catalyst test; N₂ adsorption–desorption isotherms of MoO_x@NC-600 catalyst; physicochemical properties of catalysts; detailed product distributions of guaiacol HDO under different catalysts, reaction temperatures, and reaction times. See DOI: 10.1039/c6ra21989f

‡ These authors equally contributed to the work.

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