catalytic depolymerization/degradation of alkali lignin by...

Journal of Bioresources and Bioproducts. 2018, 3(1): 18-24 Peer-Reviewed

www.Bioresources-Bioproducts.com 18

Catalytic depolymerization/degradation of alkali lignin by dual-component catalysts

in supercritical ethanol Cheng Zoua,b, Haizhu Maa, Yunpu Guoa, Daliang Guoa,c,*and Guoxin Xuea,*

a) Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou310018, China

b) Tianjin Key Laboratory of Pulp & Paper, Tianjin University of Science & Technology, Tianjin 300457, Chinac) State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China

*Corresponding author: [email protected]*Co-corresponding author: [email protected]

ABSTRACT

Depolymerization of lignin is an important step to obtain lignin monomer for the synthesis of functional bio-polymers. In this paper, catalytic degradation/depolymerization of an alkali lignin was investigated in a supercritical ethanol system. The process conditions were optimized in terms of lignin monomer yield, and the liquid products and solid residue were characterized. Results show that the conversion rate of the alkali lignin was improved in both the Ni7Au3 catalyzed and Nickel-catalyzed systems with supercritical ethanol as the solvent. The maximum lignin conversion rate was 69.57% and 68% respectively for the Ni7Au3 and Nickel-based catalysis systems. Gas chromatography/mass spectroscopy (GC/MS) analysis indicated that the catalytic depolymerization products of alkali lignin were mainly monomeric phenolic compounds such as 2-methoxyphenol. The highest yield of 2-methoxyphenol (84.72%) was achieved with Ni7Au3 as the catalyst.

Keywords: Bi-metallic catalysis; Alkali lignin; Supercritical ethanol; 2-Methoxyphenol

1. INTRODUCTION

With the rapid development of industries, the problemsof fossil fuel shortage and excessive emissions of greenhouse gases are becoming increasingly serious in the world. Production of fuels and chemicals through biomass conversion is considered to be an effective way to mitigate these problems. Lignin, as the richest natural and renewable aromatic polymer in nature, is considered to be the most promising alternative to fossil resources for the production of phenolic chemicals.1,2 Therefore, lignin is regarded as a great potential resource as raw material for all kinds of fine chemicals and bio-fuels, especial for aromatic compounds. These phenolic compounds not only can be hydrogenated to aromatic hydrocarbon but also can be used as highly value-added chemicals.3-5 However, it is a major challenge to achieve the efficient degradation of lignin and improve the contents of the target phenolic products, due to the complex, stable, and irregular chemical structure of lignin.6,7 A growing number of researchers have been making efforts to find effective ways for the degradation of lignin to obtain mono-phenol chemicals and bio-fuels.8-10

There are plenty of studies in the literature showing that lignin can be degraded to a wide variety of phenolic compounds by different pathways, such as hydrogenolysis, pyrolysis, enzymolysis and so on.11-14 An earlier study shows that pyrolysis was a convenient method to obtain phenolic chemicals from lignin, but at the same time a large amount of char was also produced.15 In recent years, researchers have been paying more attention to liquefaction

of lignin or lignocellulose in sub/supercritical solvents such as water, methanol, and ethanol to produce phenolic chemicals. Supercritical fluid with high density, low viscosity and good solubilizing capability can accelerate the process of liquefaction reaction.16-18 Researchers found that solvent had impact on the condensation reaction of lignin degradation products, as well as on the yield of liquid oil in systems with supercritical solvents.16,19 Huang found that supercritical ethanol was significantly more effective than methanol in yielding lignin monomers and reducing char production, with CuMgAlOx as the catalyst at 300 ºC for 4 h. Hidajat reported that the monomeric yield of lignindegradation varied from 17.9wt% to 21.5wt% in a supercritical fluid at 330 ºC for 30 min.18,20Riaz also reported that a high conversion rate of 92% and a high bio-oil yield of 85 wt% were achieved in the depolymerization treatment at 350 ºC for 30 min in supercritical ethanol.21

In addition, catalytic degradation of lignin has also been considered as an effective way to transform lignin into phenolic products. Chen found that a high yield (38.7 wt%) of phenolic alcohols could be obtained from woody biomass, with a Ni/C catalyst in a methanol-water co-solvent system. Kou discovered that 46wt% monomeric phenols were yielded in the hydrogenation of lignin with noble metal catalysts such as Pt/C, Ru/C, and Pd/C.22,23 In general, noble metal catalysts are more effective than no-noble metal catalysts in promoting monomeric phenols yield in lignin degradation processes. However, the cost of noble metal catalysts is much higher than other catalysts. To

ORIGINAL PAPER DOI: 10.21967/jbb.v3i1.161

http://www.bioresources-bioproducts.com/



reduce cost, researchers are studying bi-metallic catalysts containing both noble and no-noble metals for lignin degradation processes. The results showed that the combination of Ni with a noble metal (Ru, Rh, or Pd) as the catalyst had the desired catalytic effect for lignin degradation, while the catalyst cost was significantly lower.24 Zhang also used NiAg as a catalyst for the degradation reaction of a β-O-4 lignin model compound and the results showed that NiAg bimetallic catalysis was indeed superior to monometallic Ni in lignin hydrogenolysis, with a 72.7% conversion rate and a 65.6% yield of target monomer compounds. In addition, Ni7Au3 was recently identified as a promising catalysis for converting lignin into phenolic products.25,26 Based on these researches, degradation of lignin in supercritical ethanol with a metal catalyst could be as an efficient method to convert lignin into aromatic platform products. However, little has been reported in the literature on the analyses of the liquid products in lignin degradation.

The aim of this work was to investigate the effectiveness of dual metal catalysts on the degradation of an alkali lignin in supercritical ethanol solvent system, by characterizing the reaction products. The chemical composition of the liquid phase was analyzed by gas chromatography/mass spectroscopy (GC/MS); the morphology of the solid char was characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectrometer (FT-IR) spectra. 2.MATERIALS AND METHODS 2.1 Materials

Alkali lignin was purchased from Aladdin (Shanghai, China). Polyvinylpyrrolidone (M.W.=58000), nickel(II) chloride hexahydrate (>98%), chloroauric acid, nickel-based catalysts, isopropanol of HPLC grade and anhydrous ethanol (95%) were purchased from Hangzhou Mike Chemical Instrument Co., Ltd. Sodium borohydride (>95%) was provided by Zhejiang Sci-Tech University (Hangzhou, China). 2.2 Catalyst preparation

According to the procedures described in the references, Ni7Au3 catalyst was prepared by mixing 14 mL of 0.011M (0.07 eq., 0.154 mmol) NiCl2·6H2O stock solution with 6 mL of a 0.011 M (0.03 eq., 0.066 mmol) HAuCl4·4H2O (50%Au) stock solution, and then 489 mg (2 eq., 4.4 mmol) of PVP was added and dissolved. Then 41.6 mg (0.5 eq., 1.1 mmol) of NaBH4 were dissolved in 10 ml anhydrous ethanol and then added quickly to the mixed solution under vigorous stirring (1000 rpm) at room temperature (25 ºC). Reduction reaction was complete in about 30 seconds when the mixture turned to brown/black color.26, 27 Then the prepared catalyst solution was immediately transferred to an

autoclave batch reactor and used for lignin degradation reaction. 2.3 lignin characterization The thermal gravity analysis (TGA) and differential thermal gravity analysis (DTGA) of alkali lignin were conducted with a thermal gravity analyzer (PYRIS 1, Perkinelmer, America). 10 mg lignin was placed on an aluminum crucible and heated up to 700 °C at a heating rate of 10 °C/min in nitrogen atmosphere (20 mL/min). 2.4 Catalytic degradation of alkali lignin in supercritical

ethanol

The alkali lignin degradation process was performed in a 100-mL Senlong parallel high pressure autoclave batch reactor (Beijing Century Senlong Experimental Apparatus CO., Ltd., Beijing, China). Typically, 1.0 g of alkali lignin and 30 mL of freshly prepared ethanol solution containing 0.22 mmol of Ni7Au3 catalyst and 20 mL of anhydrous ethanol solution was added to the reactor. The sealed reactor was heated to a reaction temperature of 210 ºC or 240 ºC, and the reaction time was varied from 1 h to 8 h. By the end or reaction, reactor was cooled down to room temperature, and the content was transferred to a flask. The solid residue was separated from the reaction mixture by filtration with a membrane filter of 0.45 µm pore size, and then dried at 50 ºC for 12 h. The dried solid product was labeled as char. The lignin conversion rate was calculated using the following equation:

Lignin conversion rate (wt%) =

100lignin freeash dry ofweight

char solid of Weight -lignin freeash dry ofWeight × (1)

The main product components of the liquid phase were

identified by a gas chromatography–mass spectrometer (GC/MS, Agilent 6890 gas chromatography (GC) equipped with a 5973I MSD using a 30 m × 0.25 mm × 0.25 μm DB-5ms column) based on the NIST 08 library. The temperature profile of GC/MS analysis was as follows. The oven temperature of was initially 40 ºC with 5 min of hold time, and ramped up to 200 ºC at a rate of 5 ºC min-1, and then increased to 280 ºC for 5 min. The highest purity helium was used as carrier gas with a flow rate of 0.8 mL/min. The relative concentrations of components were calculated by normalized peak areas.

The surface morphology of the solid char was examined with a scanning electron microscope (VLTRA55, Carl Zeiss SMT Pte. Ltd, Jena, Germany). The chemical bonds of solid char were analyzed by Fourier transform infrared spectroscopy (NICOLET5700, Madison, USA) in the range of 400 cm-1 to 4000 cm-1.




3. RESULTS AND DISCUSSION 3.1 Thermal gravity analysis of alkali lignin

Fig.1. shows the mass loss of alkali lignin occurred over a wide temperature range from 30 to 700 ºC, and was consisted of three stages: dehydration stage, fast degradation stage and slow degradation stage. The dehydration stage was in the range 30-200 ºC, and was due to the evaporation of moisture and organic solvent which caused a low weight loss of 1.89 %. The fast degradation stage corresponded to 200-450 ºC, was mainly due to decomposition of main structure of alkali lignin which caused 30.58 % of the mass of the alkali lignin transformed into volatiles.22 In this stage, the release of volatiles was divided into two stages: the initial stage corresponded to the release of a large amount of volatiles, especially phenols, while the second stage was related to increased release of low-molecular-weight volatiles such as CO, CO2, and CH4 due to the secondary cracking of higher molecular weight volatiles.28 These results show that the main degradation reaction of the alkali lignin occurred at a temperature range from 200 to 450 ºC. This was confirmed in the degradation experiments of alkali lignin. Especially, lignin conversion rate obtained from alkali lignin degraded in supercritical ethanol (240 ºC, 4 h, 7 Mpa) reached 84.72%. At the slow degradation stage occurred after 450 ºC, it was dominant that residue of lignin decomposition began to slow decomposition and turned into ash and coke eventually.

20

40

60

80

100

Wei

ght/%

TG

DTG

100 200 300 400 500 600 700-23

-22

-21

-20

-19

-18

Der

ivat

ive

Wei

ght /

%

Temperature/℃

110 ℃

314 ℃

Figure 1.Mass loss of alkali lignin at different temperature.

3.2 Catalytic depolymerization of alkali lignin in supercritical ethanol

Figure 2 shows the effect of catalyst, reaction temperature and time on the conversion rate of lignin in the degradation process with supercritical ethanol as the solvent. The conversion rate of lignin increased with increasing reaction temperature. For example, when the reaction temperature increased from 210 to 240ºC, the conversion rate of the alkali lignin increased from 63.28%, 62.08% and 38.40% to 69.57%, 68% and 44.81%, respectively, for the three different catalysis system, i.e. Ni7Au3, Raney Ni and

none catalysis. The catalysis of both Ni7Au3 and Raney Ni improved the conversion rate of the alkali lignin significantly. The maximum conversion rate of 69.57% for the alkali lignin was observed at 240 ºC of reaction temperature for 8 h of reaction time with Ni7Au3 as the catalyst. Previous research29 have shown that higher reaction temperature (e.g. 240 ºC) had higher heat transfer efficiency and a higher concentration of hydrogen radical in the supercritical ethanol system, promoting the depolymerization reaction and inhibiting the repolymerization reaction. Therefore, 240 ºC was a suitable reaction temperature for the degradation of the lignin.

Meanwhile, as the reaction time prolonged from 1 h to 8 h, the conversion rate for the alkali lignin improved for both the Ni7Au3 and Nickel-based catalysis processes. For instance, for the Ni7Au3 catalyzed system at 240 ºC, the lignin conversion rate increased from 53.83% to 62.5% and 69.57% respectively, when the reaction time was increased from 1 h to 4 h and 8 h. However, for the none catalysis process, the lignin conversion rate first increased and then decreased with increasing reaction time from 1 hour to 8 hours. as it has been reported earlier that excessive reaction time caused re-polymerization and carbonization of degraded products.30 Nevertheless, in the Ni7Au3 and Raney Ni catalyzed systems, hydrogenation of intermediate products was promoted and the repolymerization reaction was suppressed.26 Therefore, Ni7Au3 and Raney Ni are effective in catalyzing the lignin degradation process to improve lignin conversion rate.

40

50

60

70

1 h 4 h 8 h(a) 210 ℃

40

50

60

70

NoneRaney NiNi7Au3

NoneRaney Ni

240 ℃(b)

Catalyst type

Con

vers

ion/

%

Ni7Au3

Figure 2. Effect of reaction temperature (a. 210°C; b. 240°C),

time and catalysis on the conversion rate of alkali lignin. 3.3 Effect of reaction temperature (a. 210°C; b. 240°C), time and catalysis on the conversion rate of alkali lignin

Table 1 summarizes the results of GC-MS analyses of the chemical compounds in the liquid phase from the lignin degradation reactions in supercritical ethanol systems. The main lignin degradation products from a catalyzed process commonly include phenols, esters, alcohols, ketones and ethers.31-33 Results in Table 1 shows that there were fewer types of degradation products in the catalyzed systems than in the non-catalyzed system. For the Ni7Au3 catalyzed




system, 2-Methoxyphenol was the only main product, and others compounds detected were only in small quantities, such as 4-ethyl-2-methoxyphenol, 2-methoxy-4-methlphenol, 4-hydroxy-3-methoxy benzaldehyde. However, for the non-catalyzed system, quite a few main degradation products were found in the liquid phase, including 3,4-Dihydroxyacetophenone, 1,4-Dimethoxy-2,3-dimethylbenzene, 4-hydroxy-3-tertbutylbenzylether, 4'-Hydroxy-3'-methoxyacetophenone, 4-Methoxyphenol, 2-methoxy-4-propylphenol. The difference in the number of main degradation products between the non-catalyzed and Ni7Au3 catalyzed systems suggested that the addition of Ni7Au3 in the lignin depolymerization process inhibited the side reactions and promote the generation of phenols,

especially 2-methoxyphenol. The relative amount of 2-methoxyphenol found in the non-catalyzed system was 55.50%, while this value for the Ni7Au3 catalyzed system was 84.72%.

For Raney Ni catalyzed system, the relative amount of 2-methoxyphenol was 54.29%, similar to the number for the non-catalyzed system, indicating that the Raney Ni catalyst had no effect on 2-methoxyphenol production. Moreover, the compounds found in the Raney Ni catalyzed system as well as in the non-catalyzed system were much more complex than those in the Ni7Au3 catalyzed system. Therefore, Ni7Au3 was effective in catalyzing the main degradation reaction of lignin and reducing the distribution of degradation products.

Table 1.Degradation products of alkali lignin in the liquid phase of three different catalysis systems in supercritical ethanol

Products name Molecular formula Content w/%, 240 ºC 4 h

None catalysis Nickel-based catalysis Ni7Au3catalysis

3,4-Dihydroxyacetophenone

OHHO

HOO

23.04 -* -

Dimethoxy-2,3-dimethylbenzene O O

3.33 - -

4-hydroxy-3-tertbutylbenzylether OH

HO

9.92 - -

9-Ethylfluorene

2.85 - -

2-Methoxyphenol O

OH

55.50 54.29 84.72

2-Methoxy-4-methylphenol O

OH

- - 2.92

2-Methoxy-5-methylphenol O

OH

- 5.07 -

2-methoxy-4-propylphenol O

OH

- 2.71 -

4-Methoxyphenol O OH - 15.45 -

4-Ethyl-2-methoxyphenol O

OH

5.36 22.48 12.37

*: - not found.

3.4 Characterization of the solid products from the catalytic depolymerization of alkali lignin in supercritical ethanol

Figure 3 shows the char residue after thermal degradation of the alkali lignin in the supercritical ethanol systems. The char residue from the Ni7Au3 catalysis system at 240 ºC appeared to be inter-connected micro spheres with smooth surface (Fig.3a). In the thermal degradation process, the alkali lignin particles first softened, melted, and then fused into a cluster of micro-sized under the high reaction

temperature. A previous study also found that the char residue from the thermal degradation of alkali lignin particles had irregular polygonal structure with smooth surface.34 For the non-catalysis degradation process, the char residue also had the shape of micro-sized spheres, but many of the spheres had holes in their wall structure (Fig.3b). The holes were probably formed when volatile gases such as CO and CO2 evolved from the inside of the lignin granules, due to more severe side reactions in the non-catalysis process. Similar phenomenon has been observed by other researchers.34, 35




Figure 3. The char SEM of alkali lignin degraded at 240 ºC for 4 h : (a) with Ni7Au3 catalysis and (b) without catalysis

4000 3500 3000 2500 2000 1500 1000 5000.0

0.2

0.4

0.6

0.8

1.0

1.2

b

d

c

Tr

ansm

ittan

ce/%

Wavenumber/cm-1

34231587 1510

11121035

1450

2939a

Figure 4. FTIR spectra of the char residue from thermal degradation of the alkali lignin degraded in different catalysis systems (a. Ni7Au3 catalysis; b. non-catalysis; c. Raney Ni

catalysis; d. alkali lignin)

The FT-IR spectra of the alkali lignin and char residues after thermal degradation in supercritical ethanol at 240 ºC for 4 hours with or without catalysis are presented in Fig. 4. The FT-IR adsorption profiles of the char residues were different from that for the alkali lignin, with some of the adsorption peaks weakened significantly in the former, indicating that some bonds of the alkali lignin were destroyed in the thermal reactions. All the char residues from the three processes had similar IR absorption profiles, suggesting similar chemical structures and functional groups in the char residues. Table 2 lists the main absorption peaks found in the FT-IR spectra for the char residues from the thermal degradation of the alkali lignin in supercritical ethanol. The bands at 3423 cm-1 could be assigned to stretching vibrations of aromatic O-H groups, the intensity of absorption peak decreased markedly, indicating the decrease of the aromatic O-H groups in the char residues.36-38 Compared with the spectrum for the alkali lignin, the absorption peak at 1112-1035 cm-1 for the char residues decreased markedly, showing that the C-O linkage was broken under the reaction conditions. The C-O linkage of methoxyl groups and ether bond were easy to break in a supercritical fluid system.39The peak at 1035 cm-1 in the spectra for the char residues almost disappeared, suggesting that the structures with methoxyl groups were

largely removed in the degradation process. This is consistent with the results of GC-MS analysis that a large amount of products with guaiacyl structure such as 2-methoxyphenol were found in the liquid phase from the lignin degradation reactions. The absorption bands at 1450 to 1589 cm-1 for the aryl groups were much weaker in the spectra for the char residues compared with those for the alkali lignin, indication that the aromatic skeleton in the alkali lignin were largely degraded.

Table 2.Main absorption peaks found in the FT-IR spectra for

the char residues from the thermal degradation of the alkali lignin in supercritical ethanol

Wavenumber/cm-1 Functional group Vibration type

3423-3550 O-H Stretching 2860-3100 C-H Stretching 1300-1400 O-H Stretching

1589

Skeleton 1450-1589

Stretching 1035-1112 C-O Stretching

4. CONCLUSIONS

Both the Raney Ni and Ni7Au3 catalysts increased the conversion rate of the thermal degradation of alkali lignin in supercritical ethanol. The maximum conversion rate was 69.57% obtained at 240 ºC for 8 h, with Ni7Au3 as the catalyst. The catalysis of Ni7Au3 improved the yield of 2-methoxypheonl product. The maximum relative content of 2-methoxyphenol in the degradation products reached 84.72% for Ni7Au3 catalysis system under the reaction conditions of 240 ºC and 4 h.

ACKNOWLEDGMENTS

This work was supported by the National Key Research

and Development Program of China (Grant 2016YFE0125800), the National Natural Science Foundation of China (Grant 31500492), China Postdoctoral Science Foundation (Grant 2017M612035), Zhejiang Provincial Natural Science Foundation of China (Grant LY16C160005), the Foundation (Grant 201601) of Tianjin Key Laboratory of Pulp & Paper (Tianjin University of Science & Technology), the open fund of State Key Laboratory of Pulp and Paper Engineering (Grant No. 201605) and the Science Foundation of Zhejiang Sci-Tech University (Grant No. 14012079-Y).




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