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Hydrodeoxygenation of lignin-derived phenoic compounds to hydrocarbon fuel over supported Ni-based catalysts
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Hydrodeoxygenation of lignin-derived phenoic compounds to hydrocarbon fuel over supported Ni-based catalysts ⁎
Xinghua Zhang , Wenwu Tang, Qi Zhang, Tiejun Wang, Longlong Ma
⁎
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China CAS Key Laboratory of Renewable Energy, Guangzhou 510640, People’s Republic of China Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, People’s Republic of China
H I G H L I G H T S lignin-derived phenolics were converted into hydrocarbons over Ni/SiO . • Diverse of aromatic ring preferentially occurs over Ni/SiO in the phenol HDO. • Hydrogenation exhibits excellent repeatability in the HDO mixed phenolic compounds. • Ni/SiO • Acid-catalyzed repolymerization tends to occur under thermal environment. 2
2
2
A R T I C L E I N F O
A B S T R A C T
Keywords: Lignin Phenolic compounds Hydrodeoxygenation Ni/SiO2 Hydrocarbon
Ni-based catalysts supported on γ-Al2O3 and SiO2 were prepared by impregnation. Catalyst characterization was performed using XRD, NH3-TPD, H2-TPR and chemisorption. Effects of supports on catalytic performance were tested using the hydrodeoxygenation (HDO) of phenolic compounds as a model reaction. Experiment result shows that single phenolic compounds can be converted via HDO reaction over Ni/SiO2 and Ni/γ-Al2O3 catalysts at 300 °C. The hydrocarbon yields are in the range of 60–90%. The effect of supports on the reaction mechanism was also explored. It is found that hydrogenation of the aromatic ring preferentially occurs over Ni/SiO2 catalyst while the cleavage of CAReO bond preferentially occurs over Ni/γ-Al2O3 catalyst in the HDO of phenol. Compared to Ni/γ-Al2O3 catalyst, Ni/SiO2 catalyst exhibits better repeatability and higher catalytic activity for hydrocarbon yield when mixed phenolic compounds were used as feedstock in the HDO reaction, and the carbon deposited on the surface of Ni/SiO2 catalyst is lower.
1. Introduction
graded fuels from lignin-derived phenolic compounds [3,11]. Hydrodeoxygenation (HDO) is an effective method for the removal of oxygen from lignin-derived phenolics [11–14]. Bifunctional catalyst comprised by active metal and solid acid exhibits excellent activity for HDO reaction. For example, guaiacol can be efficiently transformed into hydrocarbons over the bifunctional catalysts Ni/SiO2-ZrO2 [11,15], Ni/ HZSM-5 [16], Pt/HZSM-5 [17], Ru/HBeta [13] and Pt/HBeta [18]. Normally, it is considered that the metal acts as hydrogenation activity center while the support provides acid sites for the HDO reaction. Apart from the active metal, support material is also the key factor determining the catalytic performance. In the past, γ-Al2O3 was widely used as catalyst support for HDO catalyst due to its cheap cost, excellent texture and suitable acidity. It is well known that γ-Al2O3 supported sulfided NiMo and CoMo catalysts exhibited excellent activity in the HDO of phenolic compounds [19].
Conversion of biomass into renewable fuels has attracted considerable attention [1–4]. So far, cellulose and hemicelluloses could be converted efficiently to liquid fuel and various chemicals via hydrolysis and followed treatment technologies while lignin was usually discharged as a waste [5,6]. As we know, lignin is a three-dimensional amorphous polymer consisting of methoxylated phenylpropane structures, and can be converted into phenolic compounds by catalytic depolymerization [7–9]. However, these depolymerized phenolic products were mixtures with high oxygen content. The high content of oxygen usually leads to many deteriorated properties, such as high viscosity, thermal and chemical instability, corrosiveness, poor heating value and immiscibility with hydrocarbon fuels [3,9,10]. Hence, the removal of oxygen is necessary to obtained hydrocarbons as high-
⁎
Corresponding authors at: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China. E-mail addresses: [email protected] (X. Zhang), [email protected] (L. Ma).
http://dx.doi.org/10.1016/j.apenergy.2017.08.078 Received 17 January 2017; Received in revised form 29 June 2017; Accepted 11 August 2017 0306-2619/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Zhang, X., Applied Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.08.078
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NH3-TPD (NH3-Temperature Programmed Desorption) studies were carried out on an Automatic Chemical Adsorption Instrument (CBP-1, Quantachrome Instruments) equipped with a thermal conductivity detector (TCD). 100 mg of catalyst sample was pretreated in a flow of helium (30 mL/min) at 400 °C for 1 h, and after cooling to 100 °C, ammonia adsorption was carried out. Subsequently, excessive physisorbed ammonia was removed by purging with helium flow rate of 30 mL/min. H2-TPR (H2-Temperature Programmed Reduction) measurements were carried out on an Automatic Chemical Adsorption Instrument (CBP-1, Quantachrome Instruments) equipped with a thermal conductivity detector (TCD). Hydrogen consumption (from 300 to 750 °C) was calculated by an external standard method using H2-TPR of CuO as the standard. The amount of reduced NiO was estimated based on the hydrogen consumption, and the reduction degree was obtained by calculation [11,21]. Ni dispersion was determined by H2 pulse adsorption methods. Fourier transform infrared spectroscopy (FT-IR) of the catalysts were acquired on a Nicolet 6700 FTIR spectrometer equipped with a DTGS detector. The catalyst sample was activated in the IR cell by heating from room temperature to 200 °C under vacuum (1 × 10−4 Pa) and evacuated at this temperature for 1 h. Pyridine was introduced in the IR cell at room temperature. Desorption was carried out at 40 °C, and the catalyst was evacuated for 10 min prior to recording the IR spectra. Similar procedure was described in the literature [26].
However, coke formation tend to occur because of the induction of acid sites during HDO reaction of phenolic compounds. Catalyst activity usually disappeared swiftly because large amount of coke formed during the HDO reaction process [18]. Worse, part of γ-Al2O3 can be transformed into boehmite under hydrothermal conditions because alumina is known to be metastable under hydrothermal conditions, which usually lead to a decrease for catalytic activity [20]. To overcome these drawbacks, one of the major challenges in HDO reaction are to find a suitable catalyst support. Recently, numerous support materials such as active carbon [21], ZrO2 [22] and TiO2 [23] were explored. And the results suggested that catalyst supported on non-acidic support or weak acid support can also catalyze the HDO of phenolic compounds. SiO2 was an inert material with excellent hydrothermal stability, and had been used as catalyst support in the HDO of phenolic compounds. For example, Pt catalyst supported on SiO2 was used in the HDO of cresol and guaiacol, obtaining high hydrocarbon yields [24,25]. However, the high yield of hydrocarbons might be ascribed to the excellent hydrogenation and hydrogenolysis of noble metal. Furthermore, since HDO of bio-fuel is expected to be a large scale process, employment of noble metal-based catalysts could significantly raise production costs. Thus, it seems more reasonable to use Ni-based catalysts for biofuel production via HDO owing to its lower cost. To overcome the disadvantages mentioned above, an inexpensive non-sulfided HDO catalyst was prepared by impregnation using the pristine transition metal Ni as the active component and the common SiO2 as support. The principal aim is to convert diverse lignin-derived phenolic compounds into hydrocarbons via HDO. In the HDO process, the catalytic properties of Ni/SiO2 were investigated carefully. It is interesting that high catalytic activity for hydrogenation and hydrogenolysis were exhibited in the HDO of phenolics. Currently, few open literatures about the HDO reaction with Ni/SiO2 catalyst can be found. In this work, it is desirable to use phenolic model compounds for initial screening, while leaving the investigation of the real phenolic compounds obtained by depolymerization of lignin and optimization of reaction parameters as the subjects of further research. In addition, in order to facilitate comparison, the catalyst Ni/γ-Al2O3 was also tested in this work.
2.3. Catalytic activity test Catalytic activity test was conducted in a 50 mL stainless steel autoclave equipped with electric mechanical agitator. For each run, catalyst (0.5 g), solvent n-octane (25 mL) and reactant (phenolic compounds) were loaded into the autoclave. 5.0 MPa H2 was pressured into the reactor after displacing the air. The reactor was heated to a desired reaction temperature while the reagents were stirred at a rate of 800 rpm. Liquid samples were withdrawn from the reactor on line for subsequent off-line analysis. Solid residues were collected for subsequent analysis when the reaction was completed. 2.4. Products analysis
2. Experimental Liquid products obtained from the HDO of phenolic compounds were analyzed by gas chromatography (Shimadzu GC-2010 with a FID detector and a DB-5 column) and GC–MS (Agilent 7890A-5975C with DB-FFAP capillary column). The carrier gas was He (99.995% purity), and the oven temperature program increased from 50 °C (holding for 1 min) to 260 °C (holding for 10 min) at a rate of 10 °C/min. Conversion of phenolic compounds (Conv.%), hydrocarbon yield (Y %) and product selectivity (SCp%) were determined according to the following equations:
2.1. Catalyst preparation SiO2 was prepared by chemical precipitation using NH4NO3 and Na2SiO3 as materials. The precipitate was dried at 120 °C overnight and then calcinated at 500 °C for 4 h. γ-Al2O3 were purchased from Aladdin Industrial Co. Ltd. The catalysts Ni/γ-Al2O3 and Ni/SiO2 with 10 wt% Ni loadings were prepared by wet impregnation using Ni(NO3)2·6H2O as nickel precursor. The precipitate was dried at 120 °C overnight and followed by calcinations at 500 °C for 4 h. The prepared catalysts were reduced at 550 °C for 5 h in a continuous flow of H2 before using.
m (phenolics)initial−m (phenolics)end × 100% m (phenolics)initial
Conv . % =
Y% =
2.2. Catalyst characterization BET surface area (SBET), average pore diameter, and pore volume of catalysts were measured by N2 isothermal (−196 °C) physisorption on Autosorb-iQ-2 (Qudrasorb SI, Quantachrome Instruments). The catalyst was degassed for 22 h at 250 °C under vacuum condition before N2 adsorption. XRD (X-ray diffraction) analysis was carried out on an equipment (PANalytical, Netherlands) with Cu Kα (λ = 0.154 nm) radiation. TG (Thermogravimetry) studies of used catalysts were carried out under an air flow rate of 50 mL min−1 with a thermal analyzer (TGA Q50, US) using 10–15 mg sample and a 10 K min−1 temperature increasing.
mole of product × 100% mole (phenolics)initial
SCp % =
mCp
∑
(1)
(2)
× 100%
mCP
(3)
Cp represents the content of products obtained from the HDO reaction. 3. Results and discussions 3.1. Catalyst characterization Texture and structure of the catalysts were shown in Table 1 and 2
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Table 1 Texture and structure of the supported Ni catalysts. Sample
SBET (m2/g)
Vtotal (cm3/g)
Dp (nm)
Dm (nm)
DNiO (nm)
Ni/SiO2 Ni/γ-Al2O3 SiO2 γ-Al2O3
213.1 133.3 212.6 151.9
1.131 0.662 1.255 0.733
21.23 19.87 23.61 19.30
17.2 17.1 17.5 20.3
17.2 30.2 – –
Dp: average pore diameter, determined by the formula: Vtotal × 4/SBET. Dm: the most probable pore diameter. DNiO: the Ni crystal size of the catalysts, calculated based on the Scherrer equation.
Fig. 3. H2-TPR profiles of the Ni/SiO2 and Ni/γ-Al2O3 catalysts.
For the catalyst Ni/γ-Al2O3, the characteristic peaks of NiO and Al2O3 were observed as expected. In addition, the phase of NiAl2O4 (2θ = 37.3°, 43.3°, and 62.9°) was detected. This suggests that the reaction occurred between NiO and support γ-Al2O3 in the calcination process. In addition, the Ni crystal size of the catalysts, calculated based on the Scherrer equation, were also exhibited in Table 1. It is observed that the Ni crystal size of Ni/SiO2 catalyst is less than that of Ni/γ-Al2O3 catalyst, suggesting a better dispersion for the metal Ni on the surface of SiO2. This conclusion is well agreement with the results of texture and structure analysis determined by N2 isothermal physisorption. H2-TPR profiles of catalysts were gathered in Fig. 3. For the Ni/SiO2 catalyst, the peak positioned at lower reduction temperature (345 °C) was ascribed to the superficial NiO, which can be easily reduced to Ni0. The peak at higher temperature (455 °C) was associated with the reduction of bulk NiO, which weakly interacted with support material. A reduction peak between 650 and 750 °C was also observed. This was likely due to the formation of nickel silicate species, which strongly interacted with the support and hard to reduce [27]. In the H2-TPR profiles of Ni/γ-Al2O3, two reduction peaks were observed positioned at the temperature of 405 and 655 °C. The low temperature reduction peak can be assigned to the reduction of NiO, which weakly interacted with support material. And the high temperature reduction peak can be assigned to the formation of amorphous Ni aluminates [27]. This species had been determined by the XRD technique. In addition, as shown in Table 2, the reduced degree of Ni/SiO2 is obviously higher than that of Ni/γ-Al2O3 catalyst, suggesting more metal active sites for the Ni/ SiO2 catalyst. The dispersion degree of metal Ni was determined by H2 pulse adsorption methods. As shown in Table 2, the Ni dispersion of Ni/SiO2 is higher than that of Ni/γ-Al2O3 catalyst. It can be found that this trend is related to the texture structure of catalysts. It is plausible that the deteriorated texture structure gives rise in the aggregation of metal Ni, which leading to the lower Ni dispersion. On the contrary, the metal Ni dispersed well on the porous SiO2 due to its excellent texture structure. This conclusion is a good agreement with that of H2-TPR analysis.
Fig. 1. Pore diameter distributions of the Ni/SiO2 and Ni/γ-Al2O3 catalysts.
Fig. 1. It is found that these catalysts were mesoporous and macroporous materials. Their average pores diameter and the most probable pore diameter are very close. Compared to Ni/γ-Al2O3 catalyst, the catalyst Ni/SiO2 exhibited higher BET surfaces area and broader pore diameter distribution. In addition, the texture and structure of the Ni/ SiO2 catalyst is close to that of SiO2, suggesting a well dispersion of loaded Ni. XRD patterns of different catalysts were gathered in Fig. 2. The characteristic peaks centered at the 2θ of 37.3°, 43.3°, and 62.9° were ascribed to the crystal planes of (1 1 1), (2 0 0) and (2 2 0) of NiO phase. Therein, the characteristic peaks of NiO can be clearly seen in the XRD patterns of Ni/SiO2 sample. A broad peak between 18 and 22o was also observed, which was assigned to feature of amorphous SiO2.
Table 2 Results of the H2-TPR and H2-TPD of various catalysts. Catalysts
Ni/SiO2 Ni/γ-Al2O3 Fig. 2. XRD patterns of the supported Ni catalysts.
a
3
Peak position (°C) θ1
θ2
θ3
345 405
455 650
710
Reduced degreea (%)
Ni dispersion (%)
93.6 27.9
7.84 6.36
Calculated from 200 °C to 550 °C based on the H2-TPR profiles.
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1445–1460 cm−1 are ascribed to the signal of Lewis acid sites [29]. As shown in Fig. 4(B), the peaks at about 1445 and 1453 cm−1 are observed on all catalysts while only a very weak peak at around 1540 cm−1 is observed on the Ni/γ-Al2O3 catalyst, indicating Lewis acid sites are dominated for all samples. The amounts of Lewis acid sites and BrØnsted acid sites calculated based on the peak area of the band at around 1450 cm−1 and 1540 cm−1 are summarized in Table 3. As given in Table 3, the density of Lewis acid sites on the catalyst surface increases in the order of Ni/SiO2 < Ru/γ-Al2O3. In addition, two catalysts exhibit the peak at 1492 cm−1 which represents both types of acid sites [30]. Bands around 1600 cm−1are ascribed to pyridine adsorbed on Lewis acid sites and can be taken as a measure of the Lewis acid strength [29,31]. In this work, bands at around 1614 cm−1, which are observed on all catalysts, may characterize the strong Lewis acid sites. Bands at around 1598 cm−1 may be due to medium-strong Lewis acid sites, and bands at around 1592 cm−1 may be arisen from the hydrogen-bonded pyridine or the weak-medium Lewis acid sites [29,31]. This suggests that the strength of Lewis acid on the two catalysts is approximately equal. It is clear that the surface acidity characterized from the NH3-TPD and Py-FTIR is similar and consistent.
3.2. HDO of phenol Catalytic performances of the supported Ni catalysts and their corresponding supports were investigated in HDO reaction of phenol. As shown in Fig. 5, the results show that the catalytic activity of these supports is rather low. Particularly, the conversion of phenol was negligible when SiO2 was used as catalyst in the HDO reaction. Benzene was the major product when γ-Al2O3 was used as catalyst in HDO reaction of phenol. It is speculated that benzene was originated from cracking of the CeO bond over the acid sites of γ-Al2O3. Strong acidity favors adsorption and activation of CeO bond [32,33], results in the formation of benzene via the CeO bond cleavage due to the heating effect. The principle of this process is similar to the catalytic cracking of bio-oil over ZSM-5 [34]. It had been reported that the phenolic compounds could be converted over the support of γ-Al2O3 alone [35]. In addition, biphenol (included in “others” in Fig. 5) was observed in the products, although minor. This suggests that acid-catalyzed intermolecular repolymerization occurred under thermal environment. Also as shown in Fig. 5, the experimental results indicate that phenol can be efficiently converted over Ni/γ-Al2O3 and Ni/SiO2 catalyst with high hydrocarbon yield. Cyclohexane is dominant among the detected HDO products. In addition, a small amount of benzene and cyclohexene can also be observed in reaction products. Ni/γ-Al2O3 is a common bifunctional HDO catalyst with active metal and solid acid.
Fig. 4. NH3-TPD (A) and IR spectra of pyridine (B) of the supported Ni catalysts.
Table 3 Acid sites distribution for several supported Ni catalysts. Catalyst
Ni/SiO2 Ni/γ-Al2O3
Total acid sites (mmol NH3g−1)a
1.1 15.6
Acid sites (μmolPyg-1)b Lewis
BrØnsted
10.7 312.8
0 4.76
a
Amount of desorbed ammonia was determined by NH3-TPD. Amount of pyridine was determined by IR spectra of pyridine adsorbed and outgassed at 40 °C. b
Acid sites on the catalyst surface were investigated by NH3-TPD and pyridine-FTIR because gas-phase characterization of acid sites could be used to predict catalytic activity in the liquid phase [28]. As shown in Fig. 4(A), a broad NH3 desorption band ranged from 190 °C to 580 °C was observed in the pattern of Ni/γ-Al2O3 catalyst. This indicates that Ni/γ-Al2O3 catalyst has considerable weak, medium and strong acid sites on the surface. The Ni/SiO2 catalyst has a weak NH3 desorption band between 220 °C and 580 °C. The amounts of acid sites based on NH3 desorption are summarized in Table 3. It can be seen that the amount of acid sites on the Ni/γ-Al2O3 is much higher than that of Ni/ SiO2 catalyst. Infrared experiments of pyridine adsorption were performed to determine the type of acid sites (BrØnsted or Lewis) and their relative proportion. Generally, bands around 1540–1548 cm−1 are assigned to the signal of BrØnsted acid sites while the bands around
Fig. 5. Phenol conversion and distribution of main products over different catalysts. Reaction condition: catalyst: 0.5 g; anisole: 2.5 g; solvent: octane, 21.5 mL; PH2 = 5.0 MPa; T = 300 °C; t = 16 h.
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result had been reported in our previous work [38], which showed that Ni/SiO2 catalyst could promote the hydrogenation of phenol in the formation of cyclohexanol with high yield. Similar to Ni/SiO2 catalyst, phenol conversion and cyclohexane selectivity over Ni/γ-Al2O3 also increased gradually with reaction time. However, as shown in Fig. 6(B), the selectivity of benzene is relative high catalyst at the beginning of the reaction, and then decreases gradually with reaction time lengthened. It can be inferred that the HDO reaction of phenol over Ni/γ-Al2O3 occurred via intermediate benzene. More specifically, as shown in the route of (b) in Fig. 7, Lewis acid sites are distributed on the surface of Ni/γ-Al2O3 with strong acidity. The lone pair electrons of oxygen would be adsorbed strongly on the Lewis acid sites [32], resulting in the activation of CAReO bond. The activated CAReO bond cleaved heterolytically, and then the positively charged phenyl group could combine the active hydrogen originated from the dissociation of hydrogen promoted by Ni sites, forming the intermediate benzene. Subsequently, benzene could be converted into cyclohexane via hydrogenation. From Fig. 6(B), it can also be observed that the selectivity of cyclohexanol is 5.6% at the reaction time of 2 h, and then decreases gradually as the reaction was prolonged. This result suggests that the phenol HDO reaction also occurred via intermediate cyclohexanol (route of (a) in Fig. 7) over Ni/γ-Al2O3 catalyst. Summarizing the above discussions, it can be said that two reaction pathways might be parallel competing pathways during the course of phenol HDO over Ni/γ-Al2O3 catalyst. 3.3. HDO of other phenolic compounds Apart from phenol, HDO reactions for more phenolic compounds (anisole, guaiacol, eugenol, cresol and vanillin) were also investigated over the supported Ni catalysts at 300 °C. The results were presented in Table 4. Apart from vanillin, it can be seen that cresol, guaiacol, and eugenol were all converted with high conversion over the Ni/SiO2 and Ni/γ-Al2O3 catalysts. However, lower conversions were observed in the HDO reaction of vanillin with Ni/SiO2 and Ni/γ-Al2O3 catalysts. A possible reason is related to the carbonyl group contained in vanillin. It was reported that the carbonyl group could suppress the HDO of phenolic compounds because the carbonyl group preferred to be adsorbed on the metal sites [39]. The HDO of mixtures was also explored with two catalysts. For the Ni/SiO2 catalyst, most of the phenolic compounds were converted. The main products were cyclohexane, methylcyclohexane and propyl-cyclohexane. These results were similar to the results in the HDO of single phenolic compound. The octane numbers of these hydrocarbon products are quite high. Their carbon atom number is in the range of gasoline [11]. Thus, with the Ni/SiO2 catalyst, lignin-derived phenolic compounds can be converted into hydrocarbons. This is meaningful to produce high-grade liquid fuels from lignin. However, for the catalyst Ni/γ-Al2O3, the yield of hydrocarbons drastically decreased compared with the results of HDO of single phenolic compounds under the same conditions. This implies that obvious interactions between components exist in the reaction system using Ni/ γ-Al2O3 as catalyst, which has negative impact on the HDO reaction. The possible reason is related to the acidity of the catalyst. Generally, guaiacols (guaiacol, eugenol, and vanillin contained in mixtures) are often adsorbed on the Lewis acidic sites distributed on the surface of the Ni/γ-Al2O3 catalyst. The ArOeCH3 bond can be activated and broken, forming the intermediates catechol and its derivatives. During the course of HDO reaction, these intermediates catechol and its derivatives tend to produce high molecular weight polymers via polymerization, which is the precursor of coke [33]. That is to say, part of phenolic compounds was converted to undesired by-products over Ni/γ-Al2O3 catalyst during the HDO process. The repeatability of Ni/SiO2 and Ni/γ-Al2O3 was also employed using phenolic mixtures as reactant. As shown in Table 4, it was found
Fig. 6. Conversion and product selectivity for phenol HDO at different reaction time. (A) Ni/SiO2 catalyst; (B) Ni/γ-Al2O3 catalyst.
Thus, it is plausible that excellent catalytic activity was exhibited in the HDO reaction of phenol over Ni/γ-Al2O3. It is astonished that Ni/SiO2 catalyst also exhibited excellent catalytic activity in the HDO reaction of phenol, though SiO2 is inert material and almost no catalytic activity for the phenol HDO. This can be explained as follow: reduced Ni particles (Ni0) accept the delocalized aromatic ring electron, resulting in the activation of the ring, which allows the nucleophilic addition reaction occurring through the adsorbed H attacking the aromatic rings, completing the conversion of phenol to cyclohexanol [36]. And then, the cyclohexanol is converted to cyclohexane via dehydration over acid sites or hydrogenolysis/deoxygenation over active metal Ni. There is evidenced that the deoxygenation of cyclohexanol takes place by direct adsorption of the alcohol on the nickel particle [37]. As discussed in H2TPR analysis, the catalyst Ni/SiO2 possesses a large amount of metal nickel active sites, which make the Ni/SiO2 catalyst a high catalytic activity for hydrodeoxygenation. Variation of conversion and product distribution with reaction time was presented in Fig. 6. Over the catalyst Ni/SiO2, phenol conversion increased from 34.6% to 99.1% in the range of 2–16 h. To go along with this, the selectivity of cyclohexane also increased gradually with reaction time as shown in Fig. 6(A). It should be noted that the selectivity for cyclohexanol is relatively high at the beginning of the reaction, and then decreases gradually with reaction time lengthened. This result directly evidences the aforementioned inference that phenol is firstly hydrogenated to form cyclohexanol, and then the cyclohexanol is converted to cyclohexane as shown in Fig. 7 (route of (a)). Similar 5
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Fig. 7. Possible reaction pathway for the phenol HDO reaction over different Ni-based catalysts.
phenolic compounds dropped significantly with lower yield of hydrocarbons when the Ni/γ-Al2O3 was reused, suggesting that the catalyst had become deactivated during the first HDO experiment. To find the possible reason for the decline of catalytic activity, thermogravimetry (TG) analysis of the reused catalyst was performed. As shown in Fig. 8, only a slight weight loss (< 2 wt%) was observed after the catalyst Ni/ SiO2 was used. However, about 9 wt% weight loss for the Ni/γ-Al2O3 was observed, which can be assigned to the combustion of coke or/and residual polymer deposited on the catalyst. It is plausible that most of the activate sites of the catalyst have been covered by coke after the catalyst Ni/γ-Al2O3 was used in the HDO reaction of mixture, resulting in a deteriorated repeatability. Therefore, it can be deduced that catalyst Ni/SiO2 is superior to Ni/γ-Al2O3 in the conversion of diverse phenolic compounds to hydrocarbons via hydrodeoxygenation due to its excellent repeatability.
Table 4 HDO of other lignin-derived phenolic compounds over the supported Ni catalysts.* Substrates
Cresol Anisole Guaiacol Eugenol Vanillin Mixturea Mixturec
Ni/SiO2
Ni/γ-Al2O3
Conv.b (%)
Hydrocarbons yield (%)
Conv. (%)
Hydrocarbons yield (%)
97.6 99.4 98.1 98.5 79.5 83.8 80.2
85.6 79.9 81.4 80.6 62.6 64.8 61.1
98.7 99.6 99.3 98.5 81.2 86.9 46.6
89.8 73.7 78.4 76.6 53.6 45.3 19.0
* Reaction conditions: catalyst: 0.5 g; reactant: 2.5 g; solvent: octane, 21.5 mL; PH2 = 5.0 MPa; T = 300 °C; t = 16 h. a The mixture were consisted of phenol (0.5 g), anisole (0.5 g), cresol (0.5 g), guaiacol (0.5 g), eugenol (0.25 g), and vanillin (0.25 g). b Conversion of mixture was calculated by the mass of reduced mixture divided by mass of mixture before reaction. c HDO experiment of the mixture with the reused catalyst without any treatment.
4. Conclusions Effects of supports γ-Al2O3 and SiO2 on catalytic performance of Nibased catalysts were investigated in the HDO of phenolic compounds to hydrocarbons. Apart from vanillin, single phenolic compounds can be efficiently converted with high hydrocarbon yield over Ni/SiO2 and Ni/ γ-Al2O3 catalysts. It is found that hydrogenation of the aromatic ring preferentially occurs over Ni/SiO2 catalyst while the cleavage of CAReO bond preferentially occurs over Ni/γ-Al2O3 catalyst in the process of phenol HDO. In addition, in HDO reaction of mixed phenolic compounds, Ni/SiO2 catalyst exhibits high catalytic activity for hydrocarbons yield and excellent repeatability than that of Ni/γ-Al2O3 catalyst. Coke, caused by Lewis acid sites of Ni/γ-Al2O3 catalyst, is assigned to the cause of deteriorated catalytic activity and repeatability. Acknowledgements This work was funded by the National Natural Science Foundation of China – China (Nos. 51536009 & 51576198), the National Key Technology R & D Program – China (2015BAD15B06), the STS program of Chinese Academy of Sciences – China (KFJ-EW-STS-138), the Science and Technology Planning Project of Guangdong Province – China (2014A01016020) and the Youth Innovation Promotion Association of Chinese Academy of Sciences – China (No. 2015288).
Fig. 8. Weight loss of the used catalysts Ni/SiO2 and Ni/γ-Al2O3. Reaction conditions: catalyst: 0.5 g; reactant: mixed phenolics: 2.5 g; octane: 21.5 mL; PH2 = 5.0 MPa; T = 300 °C; t = 16 h.
References that mixed phenolic compounds were efficiently converted with high hydrocarbons yield when the catalyst Ni/SiO2 was repeatedly used. This result implied that the catalyst Ni/SiO2 had a stable catalytic activity for the HDO of phenolic compounds. However, the conversion of
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Hydrodeoxygenation of lignin-derived phenoic compounds to hydrocarbon fuel over supported Ni-based catalysts ⁎
Xinghua Zhang , Wenwu Tang, Qi Zhang, Tiejun Wang, Longlong Ma
⁎
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China CAS Key Laboratory of Renewable Energy, Guangzhou 510640, People’s Republic of China Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, People’s Republic of China
H I G H L I G H T S lignin-derived phenolics were converted into hydrocarbons over Ni/SiO . • Diverse of aromatic ring preferentially occurs over Ni/SiO in the phenol HDO. • Hydrogenation exhibits excellent repeatability in the HDO mixed phenolic compounds. • Ni/SiO • Acid-catalyzed repolymerization tends to occur under thermal environment. 2
2
2
A R T I C L E I N F O
A B S T R A C T
Keywords: Lignin Phenolic compounds Hydrodeoxygenation Ni/SiO2 Hydrocarbon
Ni-based catalysts supported on γ-Al2O3 and SiO2 were prepared by impregnation. Catalyst characterization was performed using XRD, NH3-TPD, H2-TPR and chemisorption. Effects of supports on catalytic performance were tested using the hydrodeoxygenation (HDO) of phenolic compounds as a model reaction. Experiment result shows that single phenolic compounds can be converted via HDO reaction over Ni/SiO2 and Ni/γ-Al2O3 catalysts at 300 °C. The hydrocarbon yields are in the range of 60–90%. The effect of supports on the reaction mechanism was also explored. It is found that hydrogenation of the aromatic ring preferentially occurs over Ni/SiO2 catalyst while the cleavage of CAReO bond preferentially occurs over Ni/γ-Al2O3 catalyst in the HDO of phenol. Compared to Ni/γ-Al2O3 catalyst, Ni/SiO2 catalyst exhibits better repeatability and higher catalytic activity for hydrocarbon yield when mixed phenolic compounds were used as feedstock in the HDO reaction, and the carbon deposited on the surface of Ni/SiO2 catalyst is lower.
1. Introduction
graded fuels from lignin-derived phenolic compounds [3,11]. Hydrodeoxygenation (HDO) is an effective method for the removal of oxygen from lignin-derived phenolics [11–14]. Bifunctional catalyst comprised by active metal and solid acid exhibits excellent activity for HDO reaction. For example, guaiacol can be efficiently transformed into hydrocarbons over the bifunctional catalysts Ni/SiO2-ZrO2 [11,15], Ni/ HZSM-5 [16], Pt/HZSM-5 [17], Ru/HBeta [13] and Pt/HBeta [18]. Normally, it is considered that the metal acts as hydrogenation activity center while the support provides acid sites for the HDO reaction. Apart from the active metal, support material is also the key factor determining the catalytic performance. In the past, γ-Al2O3 was widely used as catalyst support for HDO catalyst due to its cheap cost, excellent texture and suitable acidity. It is well known that γ-Al2O3 supported sulfided NiMo and CoMo catalysts exhibited excellent activity in the HDO of phenolic compounds [19].
Conversion of biomass into renewable fuels has attracted considerable attention [1–4]. So far, cellulose and hemicelluloses could be converted efficiently to liquid fuel and various chemicals via hydrolysis and followed treatment technologies while lignin was usually discharged as a waste [5,6]. As we know, lignin is a three-dimensional amorphous polymer consisting of methoxylated phenylpropane structures, and can be converted into phenolic compounds by catalytic depolymerization [7–9]. However, these depolymerized phenolic products were mixtures with high oxygen content. The high content of oxygen usually leads to many deteriorated properties, such as high viscosity, thermal and chemical instability, corrosiveness, poor heating value and immiscibility with hydrocarbon fuels [3,9,10]. Hence, the removal of oxygen is necessary to obtained hydrocarbons as high-
⁎
Corresponding authors at: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China. E-mail addresses: [email protected] (X. Zhang), [email protected] (L. Ma).
http://dx.doi.org/10.1016/j.apenergy.2017.08.078 Received 17 January 2017; Received in revised form 29 June 2017; Accepted 11 August 2017 0306-2619/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Zhang, X., Applied Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.08.078
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NH3-TPD (NH3-Temperature Programmed Desorption) studies were carried out on an Automatic Chemical Adsorption Instrument (CBP-1, Quantachrome Instruments) equipped with a thermal conductivity detector (TCD). 100 mg of catalyst sample was pretreated in a flow of helium (30 mL/min) at 400 °C for 1 h, and after cooling to 100 °C, ammonia adsorption was carried out. Subsequently, excessive physisorbed ammonia was removed by purging with helium flow rate of 30 mL/min. H2-TPR (H2-Temperature Programmed Reduction) measurements were carried out on an Automatic Chemical Adsorption Instrument (CBP-1, Quantachrome Instruments) equipped with a thermal conductivity detector (TCD). Hydrogen consumption (from 300 to 750 °C) was calculated by an external standard method using H2-TPR of CuO as the standard. The amount of reduced NiO was estimated based on the hydrogen consumption, and the reduction degree was obtained by calculation [11,21]. Ni dispersion was determined by H2 pulse adsorption methods. Fourier transform infrared spectroscopy (FT-IR) of the catalysts were acquired on a Nicolet 6700 FTIR spectrometer equipped with a DTGS detector. The catalyst sample was activated in the IR cell by heating from room temperature to 200 °C under vacuum (1 × 10−4 Pa) and evacuated at this temperature for 1 h. Pyridine was introduced in the IR cell at room temperature. Desorption was carried out at 40 °C, and the catalyst was evacuated for 10 min prior to recording the IR spectra. Similar procedure was described in the literature [26].
However, coke formation tend to occur because of the induction of acid sites during HDO reaction of phenolic compounds. Catalyst activity usually disappeared swiftly because large amount of coke formed during the HDO reaction process [18]. Worse, part of γ-Al2O3 can be transformed into boehmite under hydrothermal conditions because alumina is known to be metastable under hydrothermal conditions, which usually lead to a decrease for catalytic activity [20]. To overcome these drawbacks, one of the major challenges in HDO reaction are to find a suitable catalyst support. Recently, numerous support materials such as active carbon [21], ZrO2 [22] and TiO2 [23] were explored. And the results suggested that catalyst supported on non-acidic support or weak acid support can also catalyze the HDO of phenolic compounds. SiO2 was an inert material with excellent hydrothermal stability, and had been used as catalyst support in the HDO of phenolic compounds. For example, Pt catalyst supported on SiO2 was used in the HDO of cresol and guaiacol, obtaining high hydrocarbon yields [24,25]. However, the high yield of hydrocarbons might be ascribed to the excellent hydrogenation and hydrogenolysis of noble metal. Furthermore, since HDO of bio-fuel is expected to be a large scale process, employment of noble metal-based catalysts could significantly raise production costs. Thus, it seems more reasonable to use Ni-based catalysts for biofuel production via HDO owing to its lower cost. To overcome the disadvantages mentioned above, an inexpensive non-sulfided HDO catalyst was prepared by impregnation using the pristine transition metal Ni as the active component and the common SiO2 as support. The principal aim is to convert diverse lignin-derived phenolic compounds into hydrocarbons via HDO. In the HDO process, the catalytic properties of Ni/SiO2 were investigated carefully. It is interesting that high catalytic activity for hydrogenation and hydrogenolysis were exhibited in the HDO of phenolics. Currently, few open literatures about the HDO reaction with Ni/SiO2 catalyst can be found. In this work, it is desirable to use phenolic model compounds for initial screening, while leaving the investigation of the real phenolic compounds obtained by depolymerization of lignin and optimization of reaction parameters as the subjects of further research. In addition, in order to facilitate comparison, the catalyst Ni/γ-Al2O3 was also tested in this work.
2.3. Catalytic activity test Catalytic activity test was conducted in a 50 mL stainless steel autoclave equipped with electric mechanical agitator. For each run, catalyst (0.5 g), solvent n-octane (25 mL) and reactant (phenolic compounds) were loaded into the autoclave. 5.0 MPa H2 was pressured into the reactor after displacing the air. The reactor was heated to a desired reaction temperature while the reagents were stirred at a rate of 800 rpm. Liquid samples were withdrawn from the reactor on line for subsequent off-line analysis. Solid residues were collected for subsequent analysis when the reaction was completed. 2.4. Products analysis
2. Experimental Liquid products obtained from the HDO of phenolic compounds were analyzed by gas chromatography (Shimadzu GC-2010 with a FID detector and a DB-5 column) and GC–MS (Agilent 7890A-5975C with DB-FFAP capillary column). The carrier gas was He (99.995% purity), and the oven temperature program increased from 50 °C (holding for 1 min) to 260 °C (holding for 10 min) at a rate of 10 °C/min. Conversion of phenolic compounds (Conv.%), hydrocarbon yield (Y %) and product selectivity (SCp%) were determined according to the following equations:
2.1. Catalyst preparation SiO2 was prepared by chemical precipitation using NH4NO3 and Na2SiO3 as materials. The precipitate was dried at 120 °C overnight and then calcinated at 500 °C for 4 h. γ-Al2O3 were purchased from Aladdin Industrial Co. Ltd. The catalysts Ni/γ-Al2O3 and Ni/SiO2 with 10 wt% Ni loadings were prepared by wet impregnation using Ni(NO3)2·6H2O as nickel precursor. The precipitate was dried at 120 °C overnight and followed by calcinations at 500 °C for 4 h. The prepared catalysts were reduced at 550 °C for 5 h in a continuous flow of H2 before using.
m (phenolics)initial−m (phenolics)end × 100% m (phenolics)initial
Conv . % =
Y% =
2.2. Catalyst characterization BET surface area (SBET), average pore diameter, and pore volume of catalysts were measured by N2 isothermal (−196 °C) physisorption on Autosorb-iQ-2 (Qudrasorb SI, Quantachrome Instruments). The catalyst was degassed for 22 h at 250 °C under vacuum condition before N2 adsorption. XRD (X-ray diffraction) analysis was carried out on an equipment (PANalytical, Netherlands) with Cu Kα (λ = 0.154 nm) radiation. TG (Thermogravimetry) studies of used catalysts were carried out under an air flow rate of 50 mL min−1 with a thermal analyzer (TGA Q50, US) using 10–15 mg sample and a 10 K min−1 temperature increasing.
mole of product × 100% mole (phenolics)initial
SCp % =
mCp
∑
(1)
(2)
× 100%
mCP
(3)
Cp represents the content of products obtained from the HDO reaction. 3. Results and discussions 3.1. Catalyst characterization Texture and structure of the catalysts were shown in Table 1 and 2
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Table 1 Texture and structure of the supported Ni catalysts. Sample
SBET (m2/g)
Vtotal (cm3/g)
Dp (nm)
Dm (nm)
DNiO (nm)
Ni/SiO2 Ni/γ-Al2O3 SiO2 γ-Al2O3
213.1 133.3 212.6 151.9
1.131 0.662 1.255 0.733
21.23 19.87 23.61 19.30
17.2 17.1 17.5 20.3
17.2 30.2 – –
Dp: average pore diameter, determined by the formula: Vtotal × 4/SBET. Dm: the most probable pore diameter. DNiO: the Ni crystal size of the catalysts, calculated based on the Scherrer equation.
Fig. 3. H2-TPR profiles of the Ni/SiO2 and Ni/γ-Al2O3 catalysts.
For the catalyst Ni/γ-Al2O3, the characteristic peaks of NiO and Al2O3 were observed as expected. In addition, the phase of NiAl2O4 (2θ = 37.3°, 43.3°, and 62.9°) was detected. This suggests that the reaction occurred between NiO and support γ-Al2O3 in the calcination process. In addition, the Ni crystal size of the catalysts, calculated based on the Scherrer equation, were also exhibited in Table 1. It is observed that the Ni crystal size of Ni/SiO2 catalyst is less than that of Ni/γ-Al2O3 catalyst, suggesting a better dispersion for the metal Ni on the surface of SiO2. This conclusion is well agreement with the results of texture and structure analysis determined by N2 isothermal physisorption. H2-TPR profiles of catalysts were gathered in Fig. 3. For the Ni/SiO2 catalyst, the peak positioned at lower reduction temperature (345 °C) was ascribed to the superficial NiO, which can be easily reduced to Ni0. The peak at higher temperature (455 °C) was associated with the reduction of bulk NiO, which weakly interacted with support material. A reduction peak between 650 and 750 °C was also observed. This was likely due to the formation of nickel silicate species, which strongly interacted with the support and hard to reduce [27]. In the H2-TPR profiles of Ni/γ-Al2O3, two reduction peaks were observed positioned at the temperature of 405 and 655 °C. The low temperature reduction peak can be assigned to the reduction of NiO, which weakly interacted with support material. And the high temperature reduction peak can be assigned to the formation of amorphous Ni aluminates [27]. This species had been determined by the XRD technique. In addition, as shown in Table 2, the reduced degree of Ni/SiO2 is obviously higher than that of Ni/γ-Al2O3 catalyst, suggesting more metal active sites for the Ni/ SiO2 catalyst. The dispersion degree of metal Ni was determined by H2 pulse adsorption methods. As shown in Table 2, the Ni dispersion of Ni/SiO2 is higher than that of Ni/γ-Al2O3 catalyst. It can be found that this trend is related to the texture structure of catalysts. It is plausible that the deteriorated texture structure gives rise in the aggregation of metal Ni, which leading to the lower Ni dispersion. On the contrary, the metal Ni dispersed well on the porous SiO2 due to its excellent texture structure. This conclusion is a good agreement with that of H2-TPR analysis.
Fig. 1. Pore diameter distributions of the Ni/SiO2 and Ni/γ-Al2O3 catalysts.
Fig. 1. It is found that these catalysts were mesoporous and macroporous materials. Their average pores diameter and the most probable pore diameter are very close. Compared to Ni/γ-Al2O3 catalyst, the catalyst Ni/SiO2 exhibited higher BET surfaces area and broader pore diameter distribution. In addition, the texture and structure of the Ni/ SiO2 catalyst is close to that of SiO2, suggesting a well dispersion of loaded Ni. XRD patterns of different catalysts were gathered in Fig. 2. The characteristic peaks centered at the 2θ of 37.3°, 43.3°, and 62.9° were ascribed to the crystal planes of (1 1 1), (2 0 0) and (2 2 0) of NiO phase. Therein, the characteristic peaks of NiO can be clearly seen in the XRD patterns of Ni/SiO2 sample. A broad peak between 18 and 22o was also observed, which was assigned to feature of amorphous SiO2.
Table 2 Results of the H2-TPR and H2-TPD of various catalysts. Catalysts
Ni/SiO2 Ni/γ-Al2O3 Fig. 2. XRD patterns of the supported Ni catalysts.
a
3
Peak position (°C) θ1
θ2
θ3
345 405
455 650
710
Reduced degreea (%)
Ni dispersion (%)
93.6 27.9
7.84 6.36
Calculated from 200 °C to 550 °C based on the H2-TPR profiles.
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1445–1460 cm−1 are ascribed to the signal of Lewis acid sites [29]. As shown in Fig. 4(B), the peaks at about 1445 and 1453 cm−1 are observed on all catalysts while only a very weak peak at around 1540 cm−1 is observed on the Ni/γ-Al2O3 catalyst, indicating Lewis acid sites are dominated for all samples. The amounts of Lewis acid sites and BrØnsted acid sites calculated based on the peak area of the band at around 1450 cm−1 and 1540 cm−1 are summarized in Table 3. As given in Table 3, the density of Lewis acid sites on the catalyst surface increases in the order of Ni/SiO2 < Ru/γ-Al2O3. In addition, two catalysts exhibit the peak at 1492 cm−1 which represents both types of acid sites [30]. Bands around 1600 cm−1are ascribed to pyridine adsorbed on Lewis acid sites and can be taken as a measure of the Lewis acid strength [29,31]. In this work, bands at around 1614 cm−1, which are observed on all catalysts, may characterize the strong Lewis acid sites. Bands at around 1598 cm−1 may be due to medium-strong Lewis acid sites, and bands at around 1592 cm−1 may be arisen from the hydrogen-bonded pyridine or the weak-medium Lewis acid sites [29,31]. This suggests that the strength of Lewis acid on the two catalysts is approximately equal. It is clear that the surface acidity characterized from the NH3-TPD and Py-FTIR is similar and consistent.
3.2. HDO of phenol Catalytic performances of the supported Ni catalysts and their corresponding supports were investigated in HDO reaction of phenol. As shown in Fig. 5, the results show that the catalytic activity of these supports is rather low. Particularly, the conversion of phenol was negligible when SiO2 was used as catalyst in the HDO reaction. Benzene was the major product when γ-Al2O3 was used as catalyst in HDO reaction of phenol. It is speculated that benzene was originated from cracking of the CeO bond over the acid sites of γ-Al2O3. Strong acidity favors adsorption and activation of CeO bond [32,33], results in the formation of benzene via the CeO bond cleavage due to the heating effect. The principle of this process is similar to the catalytic cracking of bio-oil over ZSM-5 [34]. It had been reported that the phenolic compounds could be converted over the support of γ-Al2O3 alone [35]. In addition, biphenol (included in “others” in Fig. 5) was observed in the products, although minor. This suggests that acid-catalyzed intermolecular repolymerization occurred under thermal environment. Also as shown in Fig. 5, the experimental results indicate that phenol can be efficiently converted over Ni/γ-Al2O3 and Ni/SiO2 catalyst with high hydrocarbon yield. Cyclohexane is dominant among the detected HDO products. In addition, a small amount of benzene and cyclohexene can also be observed in reaction products. Ni/γ-Al2O3 is a common bifunctional HDO catalyst with active metal and solid acid.
Fig. 4. NH3-TPD (A) and IR spectra of pyridine (B) of the supported Ni catalysts.
Table 3 Acid sites distribution for several supported Ni catalysts. Catalyst
Ni/SiO2 Ni/γ-Al2O3
Total acid sites (mmol NH3g−1)a
1.1 15.6
Acid sites (μmolPyg-1)b Lewis
BrØnsted
10.7 312.8
0 4.76
a
Amount of desorbed ammonia was determined by NH3-TPD. Amount of pyridine was determined by IR spectra of pyridine adsorbed and outgassed at 40 °C. b
Acid sites on the catalyst surface were investigated by NH3-TPD and pyridine-FTIR because gas-phase characterization of acid sites could be used to predict catalytic activity in the liquid phase [28]. As shown in Fig. 4(A), a broad NH3 desorption band ranged from 190 °C to 580 °C was observed in the pattern of Ni/γ-Al2O3 catalyst. This indicates that Ni/γ-Al2O3 catalyst has considerable weak, medium and strong acid sites on the surface. The Ni/SiO2 catalyst has a weak NH3 desorption band between 220 °C and 580 °C. The amounts of acid sites based on NH3 desorption are summarized in Table 3. It can be seen that the amount of acid sites on the Ni/γ-Al2O3 is much higher than that of Ni/ SiO2 catalyst. Infrared experiments of pyridine adsorption were performed to determine the type of acid sites (BrØnsted or Lewis) and their relative proportion. Generally, bands around 1540–1548 cm−1 are assigned to the signal of BrØnsted acid sites while the bands around
Fig. 5. Phenol conversion and distribution of main products over different catalysts. Reaction condition: catalyst: 0.5 g; anisole: 2.5 g; solvent: octane, 21.5 mL; PH2 = 5.0 MPa; T = 300 °C; t = 16 h.
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result had been reported in our previous work [38], which showed that Ni/SiO2 catalyst could promote the hydrogenation of phenol in the formation of cyclohexanol with high yield. Similar to Ni/SiO2 catalyst, phenol conversion and cyclohexane selectivity over Ni/γ-Al2O3 also increased gradually with reaction time. However, as shown in Fig. 6(B), the selectivity of benzene is relative high catalyst at the beginning of the reaction, and then decreases gradually with reaction time lengthened. It can be inferred that the HDO reaction of phenol over Ni/γ-Al2O3 occurred via intermediate benzene. More specifically, as shown in the route of (b) in Fig. 7, Lewis acid sites are distributed on the surface of Ni/γ-Al2O3 with strong acidity. The lone pair electrons of oxygen would be adsorbed strongly on the Lewis acid sites [32], resulting in the activation of CAReO bond. The activated CAReO bond cleaved heterolytically, and then the positively charged phenyl group could combine the active hydrogen originated from the dissociation of hydrogen promoted by Ni sites, forming the intermediate benzene. Subsequently, benzene could be converted into cyclohexane via hydrogenation. From Fig. 6(B), it can also be observed that the selectivity of cyclohexanol is 5.6% at the reaction time of 2 h, and then decreases gradually as the reaction was prolonged. This result suggests that the phenol HDO reaction also occurred via intermediate cyclohexanol (route of (a) in Fig. 7) over Ni/γ-Al2O3 catalyst. Summarizing the above discussions, it can be said that two reaction pathways might be parallel competing pathways during the course of phenol HDO over Ni/γ-Al2O3 catalyst. 3.3. HDO of other phenolic compounds Apart from phenol, HDO reactions for more phenolic compounds (anisole, guaiacol, eugenol, cresol and vanillin) were also investigated over the supported Ni catalysts at 300 °C. The results were presented in Table 4. Apart from vanillin, it can be seen that cresol, guaiacol, and eugenol were all converted with high conversion over the Ni/SiO2 and Ni/γ-Al2O3 catalysts. However, lower conversions were observed in the HDO reaction of vanillin with Ni/SiO2 and Ni/γ-Al2O3 catalysts. A possible reason is related to the carbonyl group contained in vanillin. It was reported that the carbonyl group could suppress the HDO of phenolic compounds because the carbonyl group preferred to be adsorbed on the metal sites [39]. The HDO of mixtures was also explored with two catalysts. For the Ni/SiO2 catalyst, most of the phenolic compounds were converted. The main products were cyclohexane, methylcyclohexane and propyl-cyclohexane. These results were similar to the results in the HDO of single phenolic compound. The octane numbers of these hydrocarbon products are quite high. Their carbon atom number is in the range of gasoline [11]. Thus, with the Ni/SiO2 catalyst, lignin-derived phenolic compounds can be converted into hydrocarbons. This is meaningful to produce high-grade liquid fuels from lignin. However, for the catalyst Ni/γ-Al2O3, the yield of hydrocarbons drastically decreased compared with the results of HDO of single phenolic compounds under the same conditions. This implies that obvious interactions between components exist in the reaction system using Ni/ γ-Al2O3 as catalyst, which has negative impact on the HDO reaction. The possible reason is related to the acidity of the catalyst. Generally, guaiacols (guaiacol, eugenol, and vanillin contained in mixtures) are often adsorbed on the Lewis acidic sites distributed on the surface of the Ni/γ-Al2O3 catalyst. The ArOeCH3 bond can be activated and broken, forming the intermediates catechol and its derivatives. During the course of HDO reaction, these intermediates catechol and its derivatives tend to produce high molecular weight polymers via polymerization, which is the precursor of coke [33]. That is to say, part of phenolic compounds was converted to undesired by-products over Ni/γ-Al2O3 catalyst during the HDO process. The repeatability of Ni/SiO2 and Ni/γ-Al2O3 was also employed using phenolic mixtures as reactant. As shown in Table 4, it was found
Fig. 6. Conversion and product selectivity for phenol HDO at different reaction time. (A) Ni/SiO2 catalyst; (B) Ni/γ-Al2O3 catalyst.
Thus, it is plausible that excellent catalytic activity was exhibited in the HDO reaction of phenol over Ni/γ-Al2O3. It is astonished that Ni/SiO2 catalyst also exhibited excellent catalytic activity in the HDO reaction of phenol, though SiO2 is inert material and almost no catalytic activity for the phenol HDO. This can be explained as follow: reduced Ni particles (Ni0) accept the delocalized aromatic ring electron, resulting in the activation of the ring, which allows the nucleophilic addition reaction occurring through the adsorbed H attacking the aromatic rings, completing the conversion of phenol to cyclohexanol [36]. And then, the cyclohexanol is converted to cyclohexane via dehydration over acid sites or hydrogenolysis/deoxygenation over active metal Ni. There is evidenced that the deoxygenation of cyclohexanol takes place by direct adsorption of the alcohol on the nickel particle [37]. As discussed in H2TPR analysis, the catalyst Ni/SiO2 possesses a large amount of metal nickel active sites, which make the Ni/SiO2 catalyst a high catalytic activity for hydrodeoxygenation. Variation of conversion and product distribution with reaction time was presented in Fig. 6. Over the catalyst Ni/SiO2, phenol conversion increased from 34.6% to 99.1% in the range of 2–16 h. To go along with this, the selectivity of cyclohexane also increased gradually with reaction time as shown in Fig. 6(A). It should be noted that the selectivity for cyclohexanol is relatively high at the beginning of the reaction, and then decreases gradually with reaction time lengthened. This result directly evidences the aforementioned inference that phenol is firstly hydrogenated to form cyclohexanol, and then the cyclohexanol is converted to cyclohexane as shown in Fig. 7 (route of (a)). Similar 5
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Fig. 7. Possible reaction pathway for the phenol HDO reaction over different Ni-based catalysts.
phenolic compounds dropped significantly with lower yield of hydrocarbons when the Ni/γ-Al2O3 was reused, suggesting that the catalyst had become deactivated during the first HDO experiment. To find the possible reason for the decline of catalytic activity, thermogravimetry (TG) analysis of the reused catalyst was performed. As shown in Fig. 8, only a slight weight loss (< 2 wt%) was observed after the catalyst Ni/ SiO2 was used. However, about 9 wt% weight loss for the Ni/γ-Al2O3 was observed, which can be assigned to the combustion of coke or/and residual polymer deposited on the catalyst. It is plausible that most of the activate sites of the catalyst have been covered by coke after the catalyst Ni/γ-Al2O3 was used in the HDO reaction of mixture, resulting in a deteriorated repeatability. Therefore, it can be deduced that catalyst Ni/SiO2 is superior to Ni/γ-Al2O3 in the conversion of diverse phenolic compounds to hydrocarbons via hydrodeoxygenation due to its excellent repeatability.
Table 4 HDO of other lignin-derived phenolic compounds over the supported Ni catalysts.* Substrates
Cresol Anisole Guaiacol Eugenol Vanillin Mixturea Mixturec
Ni/SiO2
Ni/γ-Al2O3
Conv.b (%)
Hydrocarbons yield (%)
Conv. (%)
Hydrocarbons yield (%)
97.6 99.4 98.1 98.5 79.5 83.8 80.2
85.6 79.9 81.4 80.6 62.6 64.8 61.1
98.7 99.6 99.3 98.5 81.2 86.9 46.6
89.8 73.7 78.4 76.6 53.6 45.3 19.0
* Reaction conditions: catalyst: 0.5 g; reactant: 2.5 g; solvent: octane, 21.5 mL; PH2 = 5.0 MPa; T = 300 °C; t = 16 h. a The mixture were consisted of phenol (0.5 g), anisole (0.5 g), cresol (0.5 g), guaiacol (0.5 g), eugenol (0.25 g), and vanillin (0.25 g). b Conversion of mixture was calculated by the mass of reduced mixture divided by mass of mixture before reaction. c HDO experiment of the mixture with the reused catalyst without any treatment.
4. Conclusions Effects of supports γ-Al2O3 and SiO2 on catalytic performance of Nibased catalysts were investigated in the HDO of phenolic compounds to hydrocarbons. Apart from vanillin, single phenolic compounds can be efficiently converted with high hydrocarbon yield over Ni/SiO2 and Ni/ γ-Al2O3 catalysts. It is found that hydrogenation of the aromatic ring preferentially occurs over Ni/SiO2 catalyst while the cleavage of CAReO bond preferentially occurs over Ni/γ-Al2O3 catalyst in the process of phenol HDO. In addition, in HDO reaction of mixed phenolic compounds, Ni/SiO2 catalyst exhibits high catalytic activity for hydrocarbons yield and excellent repeatability than that of Ni/γ-Al2O3 catalyst. Coke, caused by Lewis acid sites of Ni/γ-Al2O3 catalyst, is assigned to the cause of deteriorated catalytic activity and repeatability. Acknowledgements This work was funded by the National Natural Science Foundation of China – China (Nos. 51536009 & 51576198), the National Key Technology R & D Program – China (2015BAD15B06), the STS program of Chinese Academy of Sciences – China (KFJ-EW-STS-138), the Science and Technology Planning Project of Guangdong Province – China (2014A01016020) and the Youth Innovation Promotion Association of Chinese Academy of Sciences – China (No. 2015288).
Fig. 8. Weight loss of the used catalysts Ni/SiO2 and Ni/γ-Al2O3. Reaction conditions: catalyst: 0.5 g; reactant: mixed phenolics: 2.5 g; octane: 21.5 mL; PH2 = 5.0 MPa; T = 300 °C; t = 16 h.
References that mixed phenolic compounds were efficiently converted with high hydrocarbons yield when the catalyst Ni/SiO2 was repeatedly used. This result implied that the catalyst Ni/SiO2 had a stable catalytic activity for the HDO of phenolic compounds. However, the conversion of
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