Enhancing the quality of bio-oil from catalytic pyrolysis of kraft black liquor lignin

Abstract

Black liquor is an attractive option for the generation of biofuel and fine chemical intermediates. In this work, kraft lignin was extracted from black liquor and its potential was evaluated in the production of high quality bio-oil via catalytic pyrolysis under two different reaction regimes (N₂ and 10% H₂ in N₂). The 2D HSQC NMR result showed that the lignin obtained directly through acidic precipitation with CO₂ mainly consisted of syringyl lignin units and cross-linked C–C bond substructures. The thermal decomposition of the sample mainly occurred at 200–500 °C and the char yield was 44.46% at 700 °C. The catalytic pyrolysis experiment indicated that the addition of catalysts (NiO, MoO₂, and Co₃O₄) could inhibit char formation and enhance the properties of bio-oil by removing oxygenated compounds via releasing CO and CO₂ under a N₂ atmosphere. The addition of the catalysts could prompt lignin degradation and increase the yield of bio-oil in the presence of H₂. The yield of bio-oil increased by 26.38% with Co₃O₄ catalytic pyrolysis under a H₂/N₂ atmosphere.

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Introduction

Biomass fuel is one of the renewable, sustainable, and clean energy sources. Among the different available biomass, the lignin from the black liquor are of present research interest for the production of fuel.¹ Black liquor is a major by-product from the pulp and paper making industry, and the production of this resource can reach about 240 million tons per year globally.^2,3 Black liquor is being considered as an important and concentrated biomass resource. Generally, black liquor contains about 40% of recyclable pulping chemicals (mostly alkaline salts) and 60% of organic compounds (mainly lignin along with some cellulose and hemicellulose).^4,5 Currently, black liquor is burnt in recovery boilers to release combustion heat for electrical generation and recover pulping chemicals.^4,6 However, this method for black liquor treatment could only regenerate process chemicals and supply process heat during chemical pulping. It is far from exploring the potential of high value utilization of black liquor. Currently, precipitation of the kraft lignin via acidification of the black liquor with mixed carbon dioxide and then sulfuric acid has been commercialized.⁶ Thereby, there is an incentive for kraft pulping mills to exploit applications for surplus lignin extracted from the black liquor.

Fast pyrolysis is one of the most promising thermochemical conversion strategies to convert lignocellulosic biomass to available energy (e.g. bio-fuel or gases) and fine chemical intermediates.^7,8 Lignin, as the most abundant natural aromatic polymer, has highly branched three dimensional phenolic structure with high heating value as fuel.² The lignin structure is based on three different cinnamyl alcohols as precursors: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol compounds.⁹ Thus, kraft lignin derived from black liquor is highly suitable for value-added aromatic chemicals production and renewable energy generation via pyrolysis.¹⁰ Farag et al.¹¹ have accomplished a series of experiments for the pyrolysis of kraft lignin. The obtained yields of the aqueous phase, oil phase, noncondensable gas, and solid were 17–21%, 15–20%, 21–27%, and 32–40%, respectively. Fu et al.¹² extracted the phenols from kraft lignin microwave-pyrolysis oil indicating that guaiacol and 4-methylguaiacol were the most abundant phenols in the bio-oil.

Research has confirmed that catalytic pyrolysis could contribute to acquiring drop-in fuels and petrochemical commodities from lignocellulosic biomass.^13,14 Park et al.¹⁵ tested the lignin pyrolysis with large pore mesoporous materials (SBA-15) catalysts and unilamellar mesoporous MFI nanosheets via Py-GC/MS. The quality of bio-oil was improved and the amount of aromatics and lighter phenolics was increased via dehydration, decarbonylation, decarboxylation and cracking on the acid sites of the catalyst. Currently, zeolites (e.g. H-ZMS5 and ZSM-5) were the common used catalysts for the catalytic pyrolysis conversion of wood. However, the products of bio-oil from lignin pyrolysis seemed to exhibit low selectivity or form carbon deposits.^16–18 In recent years, nano metal oxides have draw extensive attention to various catalytic processes due to their unique properties, yet they have not been used widely in catalytic fast pyrolysis of biomass.¹⁹ Moreover, the catalytic pyrolysis effects were significantly affected by the ambient atmosphere. Especially, hydrogen is generally co-fed into the pyrolysis system which could increase hydrogenation with great accessibility to the catalyst.²⁰

In this study, kraft lignin was extracted from black liquor by acidification with dissolved carbon dioxide (CO₂). The lignin properties and structures were characterized by element analysis, Fourier translation infrared spectroscopy (FTIR) and 2D HSQC NMR, to explore the relationship between chemical properties and thermal decomposition behavior. In addition, the potential of the nano-scale catalysts (NiO, Co₃O₄, and MoO₂) were investigated in enhancing the quality of bio-oil via fast pyrolysis under different reaction regimes (H₂ and H₂/N₂). The information obtained from this study provides new insights into the possibilities of generating high quality bio-oil through catalytic fast pyrolysis of the kraft lignin precipitated from black liquor.

Materials and methods

Materials

Kraft black liquor derived from eucalyptus was supplied by Chenming Pulping Mill (Zhanjiang, Guangdong, China). According to the previous study and our group research, the feedstock eucalyptus belongs to hard wood with 43–45% content of cellulose, 19–21% hemicellulose, 26–29% acid-insoluble lignin and 2–4% extractives.^21,22 The characterization of black liquor was: pH 12.5, viscosity 10.4 mPa s, total solids content 334.5 g L⁻¹. Nano-scale catalysts including NiO, Co₃O₄, and MoO₂ were purchased from Aladdin Industrial Corporation. The diluted sulphuric acid was prepared by H₂SO₄ and adjusted the pH to 2.0.

Kraft lignin preparation and purification

The kraft lignin extracted from black liquor was acidic precipitated by CO₂ directly under magnetic stirring condition at room temperature. The flow rate of CO₂ was 0.5 L min⁻¹ and the final pH was controlled to the value of 8.5. The lignin suspension was centrifuged at 5000 rpm; the solid precipitation was recovered and freeze dried. And then the crude lignin was obtained.

The crude lignin was purified by mild acid hydrolysis method. About 5 g crude lignin was dispersed into 100 mL acidic aqueous solution (1,4-dioxane/diluted sulphuric acid, 9/1, pH = 2) for extraction and purification at 90 °C. And then the liquid was vacuum distilled at 50 °C for the solvent removal. The concentrated liquid was dropped into 100 times volume diluted sulphuric acid (pH = 2) with mild stirring. The lignin was precipitated again. After 8 hours static sedimentation, using centrifugation to remove the supernatant fraction. The solid precipitation was washed with diluted sulphuric acid (pH = 2) for two times. After freeze dried the purified lignin was ground to the average particle size of less than 0.1 mm for further study.

Characterization of kraft lignin

Quantities of C, H, N, S of the sample was determined by an elemental analyzer (Vario EL, Elementar, Germany). The weight-average (M_w) and number-average (M_n) molecular weights of the lignin sample were determined by gel permeation chromatography (GPC). The chemical structures of the sample were detected by FTIR spectra via the KBr pellet technique and characterized by 2D HSQC NMR technique. The thermal characteristic analysis of the lignin was conducted on a thermal gravimetric instrument (TGA, TGA Q500). With a heating rate of 20 °C min⁻¹, the thermogravity of the sample was tested from room temperature to 110 °C holding for 10 min and then heating to 700 °C under the inert atmosphere conditions (high purity N₂ with a gas flow rate of 25 mL min⁻¹).

Experimental protocol for catalytic fast pyrolysis

The catalytic fast pyrolysis of lignin was investigated in a quartz U-tube, tubular reactor²³ at 550 °C under two reaction regimes (pyrolysis in N₂ and pyrolysis in 10% H₂ in N₂). Herein, three respective nano-scale catalysts (NiO, Co₃O₄, and MoO₂) were used to evaluate the potential of catalysts pyrolysis for intermediate chemicals and high quality bio-oil generation. According to previous research, catalyst additive ratio of 10% (wt/wt) was selected for this experiment.²⁴ Catalyst was mixed with lignin sample prior to the fast pyrolysis experiment. Approximately 0.1 g of the mixed sample was loaded into the quartz U-tube. The pyrolysis volatiles (bio-oil) leaving the reactor were absorbed by methanol (25 mL) and then determined via gas chromatography-mass spectroscopy (GC/MS). The non-condensable gases were collected in a gasbag and analyzed by gas chromatography (GC). Further details about the standard quantification gases can be found in our previous study.²³ The yield of the pyrolysis products (gas, bio-oil, and char) was calculated according to the previous study.²⁵

GC-MS analysis of molecular products

The GC/MS analysis of the pyrolysate was conducted with an Agilent 7890N gas chromatography equipped with a 5975N mass selective detector (MSD). The temperature of GC/MS interface was 250 °C. The GC column HP-Innowax polyethylene glycol (30 m × 0.25 mm, i.e., film thickness 0.25 μm) was used for the molecular products determination. The flow rate of carrier gas (He) was 1.00 mL min⁻¹ and the split ratio was 1 : 10. The temperature programming was typically: 50 °C initial temperature holding for 3 min → heating rate of 8 °C min⁻¹ → 250 °C, with a final hold time of 3 min. The mass scan range was 33–500 amu. For quantification of the yield of selected phenolic compounds (2-methoxy-phenol, 2-methoxy-4-methyl-phenol, 4-ethyl-2-methoxy-phenol, and 2,6-dimethoxy-phenol), an external calibration method was used. The yield of the selected phenolic compounds was based on the raw material. The yield of phenolic compound was calculated by the following equation:

The m_(k) represents the mass of selected phenolic compounds (k varies with 4-ethyl-2-methoxy-phenol, 2-methoxy-4-methyl-phenol, 2-methoxy-phenol, and 2,6-dimethoxy-phenol in this experiment). The m_o denotes the mass of lignin for catalytic pyrolysis.

Results and discussion

Composition analysis of kraft lignin sample

The element contents of kraft lignin were shown in Table 1. The content of C, H, and O was 63.42%, 6.06%, and 28.60%, respectively. The molar ratios of H/C and O/C are related to the degree of aromaticity and polarity. The H/C and O/C ratio of lignin was 1.15 and 0.34, respectively. The kraft lignin could be calculated by the empirical formula [C₁₀H_11.47O_3.38N_0.01S_0.11]_n. The lignin sample contained a little bit of sulfur (1.82%), which was probably attributed to the kraft pulping process. However, no negative effect about it has been found on the pyrolysis products according to the fast pyrolysis experiment. The lignin sample had a high molecular weight (9119 g mol⁻¹). This may due to the high pH (8.5) of the black liquor was adjusted during the acidic precipitation process, which led to the lignin with large molecular weights easier to precipitate. Moreover, it had a wide polydispersity and the M_w/M_n was much higher than 1.5. It indicated that there were more lignin fractions with different molecular weight co-existing in this lignin.²⁶

Table 1 Chemical characteristics of the kraft lignin sample

Sample	Ultimate analysis (wt% dry)					H/C^b	O/C^b	HHV (MJ kg^−1)	Mw	Mn
	C	H	O^a	N	S
a Calculated by difference.b Atomic ratio.
Kraft lignin	63.42	6.06	28.60	0.10	1.82	1.15	0.34	25.16	9119	5304

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Chemical structure analysis of FTIR

The FTIR spectrum of lignin sample was presented in Fig. 1. The representative peaks for this sample were assigned according to the previous research.^26,27 A strong absorbance at 3448 cm⁻¹ assigned to –OH stretching vibrations was caused by the presence of alcoholic and phenolic hydroxyl groups involved in hydrogen bonds. Signals with great intensity at 2939 cm⁻¹ and 2842 cm⁻¹ were represented C–H of methyl and methylene units, indicating that the lignin had abundant side-chain structures. Absorption bands located at around 1611 cm⁻¹, 1517 cm⁻¹, and 1425 cm⁻¹ were assigned to vibrations of aromatic rings, suggesting that the intact aromatic rings were presented in lignin and the basic structure of lignin was not changed appreciably during the pulping process.²⁶ The band at 834 cm⁻¹ represented the deformation vibrations of C–H bonds in the aromatic rings. In particular, peaks at 1328 cm⁻¹, 1217 cm⁻¹, and 1114 cm⁻¹ were assigned to the vibrations of syringyl rings and guaiacyl rings. Moreover, there were many active functional groups including C=O, –OCH₃ and ether in lignin.

Fig. 1 FT-IR spectrum of the kraft lignin sample.

Chemical structure analysis of 2D HSQC NMR

2D HSQC NMR techniques could identify the ¹³C–¹H correlations for lignin and provide the important internal structural information about side-chain linkages and aromatic units of lignin. HSQC spectra of aromatic (δ_C/δ_H 100–135/5.5–8.5) and aliphatic (δ_C/δ_H 50–90/2.5–6.0) regions of lignin sample were presented in Fig. 2. The assignment of these signals were recognized and listed in ESI Table 1† according to the previous literatures.^26,28–31 The main signals in the aliphatic region of lignin were contained –OCH₃, β-O-4′, β–β′, and β-5′. The result showed that β-O-4′ substructures accounted for 50.4%, 33.0% and 11.6%, respectively. The contents of these linkages in lignin sample could be calculated by semi-quantitative based on per 100 aromatic ring.^28,29 However, the semi-quantitative results implied that only 3.8 (A), 2.5 (B) and 1.3 (C) per 100 aromatic ring. The lignin sample was mainly cross-linked by C–C bond. Compared with the raw lignin of eucalyptus, the amount of β-O-4′ substructures of the kraft lignin researched in our study was much lower.³² It suggested that the ether bond structure was easier to be degraded than C–C bond in kraft pulping process. Thereby, considering catalytic pyrolysis under hydrogen-rich atmosphere, it may have a positive effect on lignin degradation.

Fig. 2 2D HSQC NMR spectra of the kraft lignin sample: aliphatic region (left column) and aromatic region (middle column); (A) β-O-4′ aryl ether linkages with a free –OH at the γ-carbon; (A′) β-O-4′ aryl ether linkages with a acetylated –OH at α-carbon; (B) resinol substructures formed by β–β′, α-O-γ′, and γ-O-α′ linkages; (C) phenylcoumarane substructures formed by β-5′ and α-O-4′ linkages; (F) stilbenes units; (G) guaiacyl units; (H) p-hydroxyphenyl units; (S) syringyl units; (S′) oxidized syringyl units with a C_α ketone.

Moreover, signals from carbohydrates in the HSQC spectra demonstrated that xylan was the major carbohydrate attached to lignin side chains and the lignin carbohydrate complex linkages were partly retained after kraft treatment. With respect to the different lignin unit types, it showed a predominance of syringyl lignin unit (S and S′, 61% and 11%), followed by guaiacyl lignin unit (G, 26%) and lower amounts of p-hydroxyphenyl lignin unit (H, 2%). These data were consistent with the lignin structure of eucalyptus (S–G). S/G ratio was considered as an important parameter for delignification evaluation in the pulping process.²⁹ A high S/G (2.76) of kraft lignin was recorded. It suggested that syringyl lignin unit was easier to be explored in the pulping process due to the lower condensation degree than guaiacyl lignin unit by its additional –OCH₃ unit.

Thermal decomposition characteristics

Thermogravimetric (TG) and differential thermogravimetric (DTG) profiles of lignin thermal decomposition were depicted in Fig. 3. Generally, the thermal degradation process of the kraft lignin could be divided into three distinct weight loss stages.²³ The first stage was the dehydration process ranging from room temperature to 200 °C. The second stage was the active pyrolysis process within the temperature range of 200–500 °C. Most of organic matters were degraded in this period and the maximum devolatilization rate was observed at 366 °C. Compared with cellulose and hemicellulose, the active pyrolysis zone of lignin was broader and the shoulder of the maximum devolatilization rate evolved to higher temperature.^33,34 A explanation for this result was that lignin was abundant in the inherent recalcitrant polymeric structure of phenyl propane structure units, while cellulose and hemicellulose were mainly consisted of glycosyl units. Moreover, the 2D HSQC NMR resulted indicated that the linkage of kraft lignin sample was mainly constituted of C–C bond, whereas the carbohydrate was mainly of consisted of glycoside bonds. The bond-cleavage reactions of lignin need more bond energies than that of cellulose and hemicellulose. The third stage (500–700 °C) was dominated by tailing carbonization. The char yield of kraft lignin was 44.46% at final temperature 700 °C.

Fig. 3 Thermogravimetric curve (TG) and differential thermogravimetric curve (DTG) of kraft lignin at 20 °C min⁻¹ under N₂ atmosphere.

Products derived from catalytic fast pyrolysis

The primary compounds detected by GC/MS and their relative distributions were presented in Table 2. 2,6-Dimethoxy-phenol and 4-hydroxy-3-methoxy-benzoic acid were the most abundant products of lignin pyrolysis contributing over 45% of the total compounds analyzed. The second most abundant products were 2-methoxy-4-methyl-phenol, 1,2,3-trimethoxy-5-methyl-benzene, 4-ethyl-2-methoxy-phenol, and 2-methoxy-phenol. S-Phenols (syringyl units), G-phenols (guaiacyl units) were the major products of lignin pyrolysis, and the total yields of them reached around 95%. This results was consisted with the pyrolysis products of Chinese fir lignin (softwood) and maple lignin (hardwood) that guaiacol-type and syringol-type compounds were predominated.³⁵ Only a minor amount of H-phenols (p-hydroxyphenyl unit) was detected, including phenol, 2-methyl-phenol, 3-methyl-phenol, 4-methyl-phenol. This result was consistent with the chemical structure of the kraft lignin sample that contained only about 2% p-hydroxyphenyl unit (cf. Fig. 2). HSQC analysis indicated that the lignin sample was rich in syringyl lignin units (72%). However, the pyrolysis products identified by GC/MS showed that more G-phenols were obtained than S-phenols in our pyrolysis experiment. It suggested that the demethoxy reaction occurred during the lignin pyrolysis process. Moreover, previous study indicated that the more content of C–C bond in lignin, the demethoxy reaction would be dominant in the pyrolysis process when competed with the internal linkages cleavage.²¹ For clarity, methane was the major gas products (cf. Table 3), which was generated from demethoxy reaction.³⁶

Table 2 Pyrolysis products identification of kraft lignin catalytic pyrolysis under different reaction regimes

Groups	Compounds	Area Pct (%)
		N2				H2/N2 = 1/9
		Blank	NiO	Co3O4	MoO2	Blank	NiO	Co3O4	MoO2
G-Phenols	Phenol, 2-methoxy-	7.48	7.33	6.57	6.19	7.03	7.78	6.82	6.76
	Phenol, 2-methoxy-3-methyl-	1.40	1.28	1.09	1.11	1.24	1.56	1.21	1.24
	Phenol, 2-methoxy-4-methyl-	8.54	8.52	7.97	7.91	8.09	8.43	8.10	8.03
	Phenol, 4-ethyl-2-methoxy-	4.79	4.69	4.20	3.98	4.24	4.62	4.27	3.97
	Phenol, 2-methoxy-4-propyl-	0.72	0.67	0.47	0.55	0.58	0.54	0.46	0.48
	2-Methoxy-4-vinylphenol	1.60	1.04	2.81	2.15	1.63	0.82	1.81	2.29
	Phenol, 2-methoxy-4-(1-propenyl)-, (E)-	1.46	1.41	2.20	1.86	1.58	1.51	1.64	1.89
	Benzoic acid, 4-hydroxy-3-methoxy-	23.81	24.80	23.92	24.68	24.28	23.25	23.01	23.69
Total		50.84	50.86	50.43	49.76	49.82	50.48	49.51	49.70
H-Phenols	Phenol, 2-methyl-	0.52	—	0.37	0.41	—	0.54	0.45	0.48
	Phenol	0.54	—	0.57	0.83	—	0.61	0.67	1.01
	Phenol, 4-methyl-	0.57	—	0.31	0.60	—	—	—	0.57
	Phenol, 3-methyl-	—	—	0.25	0.40	—	—	—	0.52
Total		1.63	—	1.50	2.23	—	1.14	1.13	2.57
S-Phenols	Phenol, 4-methoxy-3-(methoxymethyl)-	0.33	—	0.33	0.35	—	—	—	0.39
	Phenol, 2,6-dimethoxy-	27.01	27.94	24.67	25.21	26.31	26.32	24.21	25.32
	Benzene, 1,2,3-trimethoxy-5-methyl-	13.42	13.60	12.46	12.81	12.58	12.20	11.67	11.26
	Phenol, 2,6-dimethoxy-4-(2-propenyl)-	0.98	1.27	1.41	1.28	1.31	1.13	1.46	1.15
	Ethanone, 1-(2,5-dimethoxyphenyl)-	3.10	3.34	5.74	4.64	4.83	3.74	6.06	4.60
	Phenol, 2,6-dimethoxy-4-(2-propenyl)-	1.45	1.74	2.26	2.64	3.95	3.57	4.64	3.83
Total		46.29	47.89	46.88	46.93	48.97	46.96	48.04	46.55
Other	1,4-Benzenediol, 2,3,5-trimethyl-	1.24	1.25	1.19	1.08	1.21	1.42	1.32	1.18
	Acetic acid	1.04	1.13	1.20	1.33	1.14	1.96	2.18	1.35

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Table 3 Gas yields derived from kraft lignin catalytic pyrolysis under different reaction regimes

Gases	Gas content (mg g^−1)
	N2				H2/N2 = 1/9
	Blank	NiO	Co3O4	MoO2	Blank	NiO	Co3O4	MoO2
H2	0.05	0.74	0.62	0.18	—	—	—	—
CH4	5.41	13.57	13.64	12.22	8.53	9.61	4.73	9.12
CO	7.86	26.98	24.45	17.28	12.74	13.77	5.5	10.07
CO2	9.15	28.19	27.78	29.52	11.43	17.15	6.87	14.71

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Further analysis of pyrolysis products revealed that NiO could inhibit the H-phenols formation under the N₂ atmosphere, whereas MoO₂ had a promotion effect on H-phenols generation. The addition of the catalyst could not improve the bio-oil yield significantly under inert atmosphere. However, the yield of gas was increased obviously, indicating that the catalyst could change the distribution of gas and char to regulate the pyrolysis products and inhibit the char formation. Moreover, the catalyst tended to contribute decarboxylation and decarbonylation in the pyrolysis process. The yield of CO and CO₂ was increased more than 250%, which suggested that it could decrease the oxygen content in the bio-oil and upgrade the bio-oil quality.³⁷

As the Table 3 depicted, the yield of CH₄, CO and CO₂ were decreased significantly during the catalytic pyrolysis process under H₂/N₂ atmosphere compared with the N₂ atmosphere. The decrease of CO and CO₂ was mainly due to the deoxygenation reaction via the removal of H₂O during the pyrolysis process with the presence of H₂. The decrease of CH₄ indicated that the competitive reaction of the C–C bond cleavage and demethoxy reaction was alleviated under H₂/N₂ atmosphere, that is, the presence of H₂ could prompt the lignin degradation. As a general rule, lignin pyrolysis is considered as a free radical reaction.⁹ The free radicals generated in the pyrolysis process would form monophenol compounds or oligomeric fragments via secondary reaction randomly without any external conditions such as catalyst or changing atmosphere. Under the H₂/N₂ atmosphere, H₂ could cap the free radicals derived from lignin pyrolysis, which means it could accelerate the lignin degradation and inhibit the secondary reaction. As the Table 3 showed that Co₃O₄ exhibited a better catalytic characteristic with the good capping effect under H₂/N₂ atmosphere and the yield of gas was much lower than that of other two kinds of catalysts NiO and MoO₂. The absence of gas product might ascribe to the Co₃O₄ could improve the hydrogen radicals transfer efficiency to cap the lignin pyrolysis primary products. Thus, it could be more effective to inhibit the secondary reaction on gas generation. However, its mechanism needs to be further studied.

Pyrolysis product distributions of kraft lignin sample (cf. Fig. 4) which supported lignin pyrolysis in presence of H₂ was conducive to increase the yield of bio-oil. Particularly, the yield of bio-oil increased 26.38% combined with Co₃O₄ catalytic pyrolysis. However, the addition of catalysts did not exhibit a significant effect on the species and selectivity of the pyrolysis products under the H₂/N₂ atmosphere (cf. Table 2). Quantitative analysis of typical monophenol products indicated that the contribution of the catalyst to the yield of monophenol compounds was not significant. This was probably due to the low content of monophenol compounds of bio-oil. Previous study implied that bio-oil was mainly consisted of oligomeric fragments.^38,39 Our study found that the presence of H₂ could increase the yield of bio-oil by capping the oligomeric fragments derived from lignin pyrolysis with nano catalysts added, but it was difficult to prompt their further degradation to monophenol compounds. In addition, a large amount of oligomeric fragments in the form of steam would be taken away rapidly from the high temperature zone by the carrier gas, which can inhibit the oligomeric fragments for further degradation. Overall, on the basis of these results, it appeared that under H₂/N₂ atmosphere, the addition of catalysts could prompt the lignin degradation and increase the yield of bio-oil.

Fig. 4 Product yields from catalytic fast pyrolysis of kraft lignin.

Conclusion

The kraft lignin extracted from black liquor directly by CO₂ was mainly consisted of syringyl lignin unit and cross-linked C–C bond substructures. The active pyrolysis stage of the lignin sample was at 200–500 °C and a high char yield (44.46%) was recorded at 700 °C. The addition of catalysts including NiO, MoO₂, and Co₃O₄ under N₂ atmosphere could inhibit the char formation and enhance the properties of bio-oil by removing oxygenated compounds via CO and CO₂. In the presence of H₂, the three kinds of catalysts could prompt the lignin degradation and increase the bio-oil yield. Co₃O₄ exhibited a positive effect on the lignin catalytic pyrolysis under H₂/N₂ atmosphere, increasing the yield of bio-oil by 26.38%. Besides, considering the deactivation and recycling of the catalyst, we will try different addition methods to lessen these problems in our future work.

Acknowledgements

This study was supported by the National Basic Research Program of China (973 Program, 2013CB228101).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18923g

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