One-step ethanolysis of lignin into small-molecular aromatic hydrocarbons over nano-SiC catalyst

Accepted Manuscript One-Step Ethanolysis of L ignin into Small-M olecular Ar omatic Hydr ocar bons over Nano-SiC Catalyst Yigang Chen, Fang Wang, Yingjie Jia, Nan Yang, Xianming Zhang PII: DOI: Reference:

S0960-8524(16)31659-5 http://dx.doi.org/10.1016/j.biortech.2016.12.008 BITE 17384

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Bioresource Technology

Received Date: Revised Date: Accepted Date:

14 October 2016 1 December 2016 2 December 2016

Please cite this article as: Chen, Y., Wang, F., Jia, Y., Yang, N., Zhang, X., One-Step Ethanolysis of L ignin into Small-M olecular Ar omatic Hydr ocar bons over Nano-SiC Catalyst, Bioresource Technology (2016), doi: http:// dx.doi.org/10.1016/j.biortech.2016.12.008

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One-Step Ethanolysis of Lignin into Small-Molecular Aromatic Hydrocarbons over Nano-SiC Catalyst Yigang Chena, *, Fang Wangb, Yingjie Jiaa, Nan Yanga, Xianming Zhang a a

School of Chemistry and Materials Science, Shanxi Normal University, Linfen 041004, China Department of Chemistry and Environmental Engineering, Anyang Institute of Technology, Anyang 455000, China b

Abstract: Catalytic depolymerization of lignin for preparation of aromatic hydrocarbons without external hydrogen was first carried out over nano-SiC catalyst in supercritical ethanol. Mixture of the catalyst and lignin was innovatively suspended in a closed reactor and small-molecular aromatic hydrocarbons were successfully achieved at 500 °C. Results revealed that not only did conversion of lignin increase sharply under the nano-SiC catalyst, but also phenols were not detected. The increase of residence time under the Fe-SiC catalyst did not change distribution of the liquid products besides the yield improvement, suggesting that the catalyst was suitable and selective towards formation of small-molecular benzenes, especially C6-C8 benzenes. Preliminary studies found that lignin depolymerization and deoxygenation were successfully fulfilled during the reactions, which provided a very effective route to conversion of lignin into high added-value molecules as transportation fuel additives.

Keywords: Lignin; Nano-SiC; Aromatic hydrocarbons; Benzenes

*

Corresponding author; Tel.: +86 357 2058033; Fax: +86 357 2051402 E-mail addresses: [email protected]

1. Introduction Alternative bioenergy obtained from biomass has been widely explored owing to exhaustion of fossil fuel and global warming problems (Vassilev et al., 2015). In our previous studies (Chen et al., 2010; 2011; 2013), biomass was transformed into petroleum-like oil mainly consisting of benzenes, phenols and alkanes by deoxy-liquefaction; however, the oils contained a varying number of phenols depending on species of biomass. Since the phenols of fuels are implicated in poor storage stability and thermal oxidative deposit formation, phenols cause a deleterious effect on fuel quality (Kolbe et al., 2009). Phenols of the bio-oils originate from degradation of lignin (Kleinert & Barth, 2008), so the selective conversion of lignin is the key issue. Lignin with weight content of 15-30% in lignocellulosic biomass is considered as the most abundant renewable aromatic resources in nature (Zakzeski et al., 2010). In traditional wood-pulp and modern bioethanol-fuel industry, lots of lignin as a waste byproduct is usually burnt in the furnace of chemical recovery plants. Developing lignin-processing technologies for conversion of lignin into transportation fuel additives is a very effective route to utilization of biomass. Lignin is an irregular and three-dimensional polymer composed of coumaryl, coniferyl and sinapyl alcohol subunits by the linkages of aromatic ether and C-C bonds (Ragauskas et al., 2014), and thereby lignin can be decomposed into phenols by catalytic pyrolysis (Bu et al., 2012; Zhang et al., 2014). However, to be utilized as transportation fuel additives, lignin must be deoxygenated to hydrocarbons like aromatic hydrocarbons. Up to now,

conversion of lignin into aromatic hydrocarbons by catalytic pyrolysis is carried out at high temperature such as at 500 °C (Rezaei et al., 2016) and above 600 °C (Kim et al., 2015; Ma et al., 2012; Shen et al., 2015). But the processes cause phenols in the majority and benzenes in the minority. Also, solvolysis approach, especially supercritical, is viewed as an attractive strategy (Ma et al., 2015). Phenols, oligomers, or/and benzyl alcohols are usually the main products during supercritical processes (Narani et al., 2015; Warner et al., 2014; Rui Ma, 2014). Conversion of lignin into small-molecular aromatic hydrocarbons is still faced with the immense challenges. Our goal is to achieve small-molecular aromatic hydrocarbons by catalytic solvolysis of lignin without external hydrogen. So, cleavage of ether and C-C bonds in lignin interunit linkages and preserving aromatic structure are crucial factors. In supercritical ethanol (above 243.1 °C and 6.4 MPa), depolymerization and further reduction are fulfilled by solvent reforming. As widely used zeolites are apt to cause low reactivity of phenols and rapid deactivation, depolymerization mainly remains at this stage of phenols. Silicon carbon (SiC) with high thermal conductivity, chemical inertness, oxidative resistance, and mechanical strength has all the physical qualities as the catalyst (de Tymowski et al., 2012). Nano-SiC with good dispersivity was first used to catalyze lignin breakdown. Based on the ideas, lignin and nano-SiC were placed in a self-made high-pressure reactor. Numerous experiments indicated that phenols still accounted for 10%-20% of the total products, and conversion of lignin was less than 25% at 500 °C; compared to the results with no catalyst, products were no marked changes, meaning that the catalyst did not work. Further research found

that the catalyst could not well contact with the lignin (at room temperature and pressure, dealkaline lignin as raw material is insoluble in ethanol), which was determined to be a significant factor in conversion of lignin (McVeigh et al., 2016; Ye et al., 2012). Then, the mixture of nano-SiC and lignin was innovatively wrapped up like Chinese dumpling, and the “dumpling” was suspended in the reactor. Not only did conversion of lignin increase sharply, but also lignin was converted into aromatic hydrocarbons, while phenols were not detected. In this work, conversion of lignin with/without the catalyst was investigated at 500 °C and the effect of the residence time on the products was analyzed. To enhance the self-hydrogenation reaction, metal powder Fe was added and its influence was researched. The reaction routes were probed.

2. Experimental 2.1. Materials and experimental setup Dealkaline lignin (98%) with dark brown powder was from Tokyo Chemical Industry Co. Ltd. Its compositions were analyzed on Vario MACRO Cube Elemental Analyzer. Contents (wt.%) of C, H, N, and O (by difference) were 51.50 %, 4.75 %, 0.38 %, and 43.37 %, respectively; the empirical formula was C4.29H4.75O2.71N0.03. Nano-SiC (40 nm, 99.9%) and reduced iron powder (Fe, 100 mesh, AR) were from Aladdin Industrial Corporation. All the chemicals were not further treated. Tubular reactor made of hastelloy C-276 with a size of Ф 35 mm × 90 mm was heated in electric furnace (graphical abstract). A temperature controller (Xiamen Yudian Automation Technology) controlled temperature and heating rate by a

thermocouple inserted in the reactor. System pressure was showed by a pressure gauge. In the typical run, lignin and catalyst were evenly mixed and the mixture was wrapped up using double-layer nets of 600 mesh made of 304 stainless steel, like Chinese dumpling, and the “dumpling” was hung in the reactor. The ethanol of 23 ml (99.7%, Tianjin Kemiou) was poured into the reactor purged at least three times with argon gas. The sealed reactor was heated to 500 °C at a rate of 10 °C/min and kept for the specified residence time. The maximum pressure reached 19–28 MPa depending on the reaction conditions. After the reaction, the reactor was pulled out, quenched in cold water and then opened. The gas was collected. The “dumpling” was taken out and the solid was obtained. The liquid with no solid residue was poured out. Specifically, lignin of 0.50 g was mixed with nano-SiC of 0.50 g, Fe powder of 0.50 g, respectively; nano-SiC of 0.50 g and Fe powder of 0.50 g (Fe-SiC catalyst) were evenly mixed, and the Fe-SiC catalyst was again mixed with the lignin of 0.50 g. The lignin without catalyst was used to comparative experiments. The four parallel tests were kept for 30 min at 500 °C. The reaction conditions were optimal. Corresponding liquid products were marked as OLSiC, OLFe, OLFe-SiC, and OL. Blank experiments were fulfilled by ethanol and catalyst under the same conditions. The residence time under Fe-SiC catalyst was set to 50, 80, 100, and 120 min, respectively. Each experiment was conducted at least three times and the mean values (the standard deviations under the same conditions were within 7%) were listed. 2.2. Product analysis Liquid products were analyzed using Trace GC Ultra/Polaris Q (GC-MS, Thermo

Electron). The column was TRACE TR-5MS (30 m × 0.25 mm × 0.25 µm). The GC oven temperature was held at 45 °C for 3 min and raised to 180 °C at rate of 5 °C/min and then programmed to 280 °C for 2 min at rate of 20 °C/min. The compositions were identified by comparing the mass spectra with NIST II data, together with a series of benzenes containing C2 to C5 branched chains as reference compounds. FTIR spectra were obtained from KBr pellets in the range 4000-400 cm-1 on a Nicolet 5DX spectrometer. Gas products were analyzed by gas chromatograph (GC122, Shanghai) with thermal conductivity detector and carbon molecular sieve column TDX-01 (1.5 m × 2.0 mm) for H2, CO2, CO, and CH4 at 100 °C. Solid residues were dried at 120 °C for 12 h and analyzed. The higher heating value (HHV) was gained by Dulong’s formula (Rui Ma, 2014). Because of the solvent reactivity, gas was negligible, and yields of the liquid product (Yliquid, wt %) and solid residue (Ysolid =100- Yliquid) were calculated by the equation: Yliquid= 100 %× (weight of (lignin+catalyst)-weight of solid residue)/weight of lignin.

3. Results and discussion Yields ranged from 45% to 80% (Fig. 1.). Compared to the yield with no catalyst (45%), the yield with Fe was 50.5%, whereas the yield with SiC apparently increased to 58%, suggesting the SiC catalytic decomposition. The yield with Fe-SiC is further raised to 63%, exhibiting the synergistic effect of Fe and SiC. The increase of residence time resulted in the yield from 63% to 80%. When the residence time reached 100 and 120 min, respectively, the yield (78% and 80%, respectively) tended to be steady, which was confirmed by the elemental analysis of the solid residues (C:

74.23% and 75.31%, H: 3.19% and 3.07%, N: 2.45% and 2.41%, O: 20.13% and 19.21% for 100 and 120 min, respectively). Major components in Table 1 were divided into benzenes, hydrocarbon including alkanes and alkenes, and oxygen-containing compounds (O-compounds). The O-compounds (three species: alchols, acetal, and ketones) varied between 51.03% and 38.71%. One kind of phenols (only 0.49%) with no catalyst was detected in contrast to the overwhelming phenols and hydroxyphenyl compounds (Lee et al., 2016; Yang et al., 2016). The O-compounds obtained in blank experiments were considered to be from ethanol, and thereby the ethanol was reactant. The O-compounds were reduced according to the sequence of no catalyst, Fe, SiC, and Fe-SiC. Yields of benzenes ranged from 31.78% to 43.75%, and that of no catalyst was the lowest (31.78%) while with SiC, it was the highest (43.75%, Fe-SiC: 42.32%). Benzenes were basically alkylated, and C6-C8 benzenes accounted for over 50% of the total benzenes except for that of no catalyst (14.33%). And the remaining were C9-C12 benzenes. So, the SiC catalyst was active and selective towards small-molecular benzenes, especially C6-C8 benzenes. Two kinds of naphthalences (2.87% and 4.09%, respectively) were detected under SiC, compared to one kind under Fe or Fe-SiC (0.78% or 1.58%). Hydrocarbon was only generated under the catalysts, and chain hydrocarbon was detected in blank experiments. Compared to very small amount (0.49%) under SiC, cyclic hydrocarbon with Fe or Fe-SiC increased (1.9% or 2.03%), suggesting Fe catalytic hydrogenation. Taking conversion of lignin into account, the most effective catalyst was Fe-SiC. The constituents under different residence times kept unchanged (Table 2), whereas

the contents varied. The O-compounds were further reduced to 24.55 % (120 min) from 30.86 % (50 min). Benzenes increased to 60.17 % (120 min) from 47.9 % (50 min). C6-C8 and C9-C12 benzenes increased with the increase in residence time. Naphthalences (1.58 %-2.43 %) had no marked increase. It indicated that distribution of the carbon number did not shift towards the heavier hydrocarbons with increasing residence time, and thus further condensation did not happen. Based on the listed constituents with Fe-SiC catalyst, HHV was estimated with Dulong’s formula to be as high as 41.5, 42.6, 42.4, 43.1, and 42.6 MJ kg-1 for 30, 50, 80, 100, and 120 min, respectively, comparable to 30.7 MJ kg-1 for ethanol fuel. At room temperature and atmospheric pressure, the liquid products with Fe-SiC catalyst had little change in color (light yellow like gasoline) after 6 h and 18 h storage, respectively, which preliminarily confirmed that the product had good stability. FT-IR analysis of Fe-SiC products suggested that the peaks between 3400 and 3500 cm-1 were attributed to vibration of O-H. Intense peaks of Ar-O from 1260-1180 cm-1 were absent, indicating that phenols were not detected. The bands between 2960 and 2850 cm-1 were assigned to stretching of C–H. The peaks at 1640 cm-1 and near 1120 cm-1 were derived from C= O group and vibration of C–O-C bonds, respectively. To achieve benzenes, suppressing re-condensation during lignin cleavage and protecting reaction intermediates were crucial. It could be deduced that the ethanol stabilized the intermediates. Owing to hydrogen deficiency of lignin (C4.29H4.75O2.71N0.03), solvent reforming provided hydrogen (McVeigh et al., 2016). Gas analysis showed that apart from CO2, CH4, and CO, H2 was detected with/without

the catalyst. By combining the reactant suspension mode, the routes were laid out. First, lignin was degraded into aromatic monomers. The monomers broke away from named Chinese dumpling and were merged into supercritical fluid. Second, the monomers were unstable, and to stabilize the monomers, the ethanol acted as a capping-agent (Kang et al., 2013; Huang et al., 2015). Finally, hydrodeoxygenation and removal of the methoxy groups occurred, and then alkylation, isomerization, transalkylation, and hydrogen transfer arose to achieve benzene and alkylated benzenes. Excess hydrogenation of aromatic rings caused cyclic alkanes. Further mechanism research was ongoing.

4. Conclusion Ethanolysis of lignin without external hydrogen was first explored over nano-SiC catalyst at 500 °C. The reactant was innovatively hung in a closed reactor and lignin was successfully converted into small-molecular benzenes rather than phenols. Results indicated that the catalyst had high activity and selectivity towards small-molecular benzenes, especially C6-C8 benzenes. Apart from an apparent improvement in the yield, the increase of residence time did not change distribution of liquid products under the Fe-SiC catalyst. Preliminary results indicated that lignin depolymerization and deoxygenation were successfully fulfilled during the reactions.

Acknowledgements We are grateful for financial support from the National Natural Science Foundation of China (Grant No. 21406134), and Shanxi Province Science Foundation for Youths (2014021014-4).

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Table 1 Relative content (mol %) of the main compounds without /with catalysts Table 2 Relative content (mol %) of the main compounds at different residence times Fig. 1. Conversion (wt %) of liquid products at different catalysts and residence times (min)

Table 1 Relative content (mol %) of the main compounds without /with catalysts RT 2.07 2.22 2.43 2.54 2.73 2.88 3.13 3.28 3.48 3.98 4.16 4.34 4.71 4.81 4.95 5.06 5.48 5.69 6.55 6.79 7.47 7.8 9.65 10.17 10.64 11.54 11.98 12.61 13.2 13.46 14.08 15.15 15.38 15.74 16.46 16.55 16.77 17.2 17.82 18.24 20.03

Compound

Molecular Formula

No Catalyst

SiC

Fe

Fe-SiC

2-Butanone 2-Butanol 1-Methyl-cyclopentene Benzene 3-Penten-1-ol 3-Methyl-2-butanol 2-Pentanol 3,4-Dimethyl-2-pentene 1,1-Diethoxy-ethane, 3-Methyl-2-pentanone Toluene 1,4-Dimethyl-cyclohexane 2-Heptene 2-Heptanone 3-Methyl-2-heptene 6-Methyl-5-heptene-2-ol 4-Methyl-1,3-heptadiene Ethyl-cyclohexane p-Xylene 1,3-Dimethyl-benzene Ethylbenzene 1-Ethyl-4-methyl-cyclohexane 1-Ethyl-4-methyl-benzene 1-Ethyl-2-methyl-benzene 1,2,4-Trimethyl-benzene 3-Ethyl-1,5-dimethyl-benzene 1,4-Diethyl-benzene 2-Ethyl-1,4-dimethyl-benzene 3-Ethyl-1,4-dimethyl-benzene 1-Ethenyl-4-ethyl-benzene 1-Propyl-4-ethyl-benzene 1,3,5-Trimethyl-2-ethyl-benzene 1-Ethyl-4-methylethyl-benzene 1-Methyl-3,5-diethyl-benzene 3-Phenyl-hexane, Naphthalene 1-Methyl-4-(3-amyl)-benzene 1-Methyl-5-isopropyl-phenol 1-Propyl-4-isopropyl-benzene 1,2,4-triethyl -benzene 1-Methylnaphthalene

C4H8O C4H10O C6H10 C6H6 C5H10O C5H12O C5H12O C7H14 C6H14O2 C6H12O C7H8 C8H16 C7H14 C7H14O C8H16 C8H16O C8H14 C8H16 C8H10 C8H10 C8H10 C9H18 C9H12 C9H12 C9H12 C10H14 C10H14 C10H14 C10H14 C10H12 C11H16 C11H16 C11H16 C11H16 C12H18 C10H8 C12H18 C10H14O C12H18 C12H18 C11H10

4.89 22.94 – 1.79 3.4 4.26 4.34 – 9.08 0.66 4.85 – – 0.6 0.41 0.86 – – 2.3 3.31 2.08 – 2.33 0.8 0.67 0.46 1.0 0.89 0.49 1.7 0.37 1.24 0.88 0.91 0.77 – 2.0 0.49 0.92 0.59 0.94

4.1 9.12 0.17 3.19 6.61 3.5 3.27 0.22 13 – 10.6 – 0.12 – – 0.5 – – 3.86 5.26 1.75 0.32 2.76 0.89 0.53 0.25 1.66 1.1 0.26 1.02 0.25 1.24 1.31 0.33 – 2.87 – – 0.36 0.17 4.09

9.63 11.65 0.6 4.14 4.12 4.82 3.92 0.4 4.96 0.83 9.82 0.3 1.78 1.45 0.66 1.08 0.35 0.5 – 4.48 3.39 0.5 3.51 1.17 0.96 0.82 1.2 1.06 0.53 1.72 0.36 0.87 0.47 0.61 0.72 – 1.28 – 0.3 0.49 0.78

6.75 8.77 0.6 6.74 4.69 4.83 3.5 0.84 4.34 1.57 10.36 0.55 1.78 2 0.66 2.26 0.38 0.38 2.77 3.29 3.41 0.5 2.76 0.59 0.54 0.56 0.72 1.77 0.69 1.93 0.42 0.63 0.35 0.42 0.47 – 1.43 – 0.4 0.49 1.58

Table 2 Relative content (mol %) of the main compounds at different residence times RT 2.07 2.22 2.43 2.54 2.73 2.88 3.13 3.28 3.48 3.98 4.16 4.34 4.71 4.81 4.95 5.48 5.69 6.55 6.79 7.47 7.8 9.35 9.65 10.17 10.64 11.54 11.98 12.61 13.2 13.46 14.08 15.15 15.38 15.74 15.92 16.46 16.77 17.82 18.24 20.03

Compound

Molecular Formula

50 min

80 min

100 min

2-Butanone 2-Butanol 1-Methyl-cyclopentene, Benzene 3-Penten-1-ol 3-Methyl-2-butanol, 2-Pentanol 3,4-Dimethyl-2-pentene 1,1-Diethoxy-ethane 3-Methyl-2-pentanone Toluene 1,4-Dimethyl-cyclohexane 2-Heptene 2-Heptanone 3-Methyl-2-heptene 4-Methyl-1,3-heptadiene Ethyl-cyclohexane p-Xylene 1,3-Dimethyl-benzene Ethylbenzene 1-Ethyl-4-methyl-cyclohexane Propyl-benzene 1-Ethyl-4-methyl-benzene 1-Ethyl-2-methyl-benzene 1,2,4-Trimethyl-benzene 3-Ethyl-1,5-dimethyl-benzene 1,4-Diethyl-benzene 2-Ethyl-1,4-dimethyl-benzene 3-Ethyl-1,4-dimethyl-benzene 1-Ethenyl-4-ethyl-benzene 1-Propyl-4-ethyl-benzene 1,3,5-Trimethyl-2-ethyl-benzene 1-Ethyl-4-methylethyl-benzene 1-Methyl-3,5-diethyl-benzene (2,3-Dimethyl propyl)-benzene 3-Phenyl-hexane 1-Methyl-4-(3-amyl)-benzene 1-Propyl-4-isopropyl-benzene 1,3,5-Triethyl- benzene 1-Methylnaphthalene

C4H8O C4H10O C6H10 C6H6 C5H10O C5H12O C5H12O C7H14 C6H14O2 C6H12O C7H8 C8H16 C7H14 C7H14O C8H16 C8H14 C8H16 C8H10 C8H10 C8H10 C9H18 C9H12 C9H12 C9H12 C9H12 C10H14 C10H14 C10H14 C10H14 C10H12 C11H16 C11H16 C11H16 C11H16 C11H16 C12H18 C12H18 C12H18 C12H18 C11H10

6.56 9.35 0.65 3.65 0.68 4.87 3.8 3.34 3.43 0.62 9.38 0.6 2.39 1.55 0.79 0.67 0.46 3.52 4.66 2.92 0.78 1.1 2.91 0.78 0.7 0.62 1.33 0.93 0.84 1.36 0.75 0.47 1.3 0.96 0.33 2.79 0.73 0.44 3.42 2.01

4.69 11.72 0.78 6.49 3.38 3.86 1.99 1.63 3.46 – 13.92 1.41 1.54 0.67 1.33 0.21 0.39 4.58 5.28 2.78 0.3 0.79 3.8 1 0.82 0.65 1.39 1.29 0.89 2.18 0.3 1.09 0.52 0.67 0.28 0.92 2.83 0.52 0.64 1.58

4.81 11.16 0.7 4.5 1.74 3.55 1.77 4.05 1.32 0.77 10.11 0.64 2.41 1.28 0.67 0.65 0.64 3.38 5.26 3.33 0.42 1.18 3.79 0.94 1.02 1.11 1.53 1.19 0.99 1.66 0.85 1.50 1.64 1.01 0.53 1.6 2.14 0.79 2.89 2.48

120 min 5.9 9.3 0.72 6.8 1.64 3.55 1.22 1.41 1.78 0.68 16.57 0.7 1.29 0.48 0.4 – 0.43 5.2 6.0 2.69 0.7 0.78 3.76 1.06 0.74 0.55 1.49 1.33 0.55 2.08 0.47 1.34 0.7 0.55 0.45 0.92 2.29 0.36 1.06 2.43

Fig. 1. Conversion (wt %) of liquid products at different catalysts and residence times (min)

80

Conversion (wt %)

70

60

50

40

30

20

OL

OLFe

OLSiC

OLFe-SiC 50

80

100

120

Ethanolysis of lignin by the reactant suspension was carried out over nano-SiC catalyst; Small-molecular aromatic hydrocarbons rather than phenols were achieved; The increase of residence time did not change distribution of liquid products; Lignin depolymerization and deoxygenation were successfully fulfilled.