Abstract
A graphene/hemin (H-GN) catalyst for lignin degradation was prepared by a wet-chemistry method with graphene oxide and hemin. Hemin was absorbed onto the graphene surface through π-π interaction. Graphene served as a supporting material for hemin, providing a large contact area between the active molecules of catalyst and substrate, as well as protecting hemin from self-oxidation and maintaining its active molecules. The H-GN catalyst showed high catalytic efficiency in the degradation of alkaline lignin under gentle conditions. At pH 13.0, the degradation rate was 49.7% with H-GN and H2O2 (mass ratio of H2O2 to lignin of 10:1) under 60 °C, which was higher than 34.9% for non-catalyst degradation. At pH 13.2, it was as high as 92.9 wt.% at 100 °C. The lignin was decomposed into small molecules with styrene as the main final product below pH 13 and with the major products of 4-hydroxy-4-methyl-2-pentanone and bis(2-ethylhexyl) phthalate at pH 13.2.
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Graphene/Hemin Hybrid Material as a Catalyst for Degradation of Alkaline Lignin with Hydrogen Peroxide
Suzhuang Yu, Li Chen, and Zongcheng Yan *
A graphene/hemin (H-GN) catalyst for lignin degradation was prepared by a wet-chemistry method with graphene oxide and hemin. Hemin was absorbed onto the graphene surface through π-π interaction. Graphene served as a supporting material for hemin, providing a large contact area between the active molecules of catalyst and substrate, as well as protecting hemin from self-oxidation and maintaining its active molecules. The H-GN catalyst showed high catalytic efficiency in the degradation of alkaline lignin under gentle conditions. At pH 13.0, the degradation rate was 49.7% with H-GN and H2O2 (mass ratio of H2O2 to lignin of 10:1) under 60 °C, which was higher than 34.9% for non-catalyst degradation. At pH 13.2, it was as high as 92.9 wt.% at 100 °C. The lignin was decomposed into small molecules with styrene as the main final product below pH 13 and with the major products of 4-hydroxy-4-methyl-2-pentanone and bis(2-ethylhexyl) phthalate at pH 13.2.
Keywords: Graphene/hemin catalyst; Wet-chemistry method; π-π-Interaction; Lignin; Degradation
Contact information: School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, P.R. China; *Corresponding author: zcyan@scut.edu.cn
INTRODUCTION
Lignin is a multifunctional phenolic polymer containing hydroxyl, carbonyl, and carboxyl groups. It is an important aromatic biopolymer composed of 30% non-fossil organic carbon (Nguyen et al. 2014), and it is the second most abundant naturally occurring polymer on earth. There has been extensive industrial and academic research on lignin depolymerisation, although it is difficult to degrade using either chemical or biological methods (Wu et al. 2005). Various methods, including thermochemical, ionic liquid, hygrogenolysis (Zhang et al. 2015b), photocatalysis (Li et al. 2015), microwave assisted technology (Dong et al. 2014; Ouyang et al. 2015a), and catalytic reduction/degradation have been explored to make lignin a value-added fuel (Zakzeski et al. 2010; Snowdon et al. 2014). Catalytic reduction/degradation methods are expected to make lignin a high-grade biofuel under gentle and environment friendly conditions (Bouxin et al. 2015). Additionally, catalytic processes with oxygen, hydrogen peroxide, or ozone as oxidants have been developed to degrade lignin.
Hemin, the active centre of most haemoglobin, is a natural metalloporphyrin with a simple structure (Lee et al. 2009). The advantages of hemin over inorganic metal catalysts include higher selectivity, lower cost, and higher surface activity (D’Souza and Muller 2007). Its activity is similar to that of the peroxidase enzyme (Guo et al. 2011), and it exhibits good electro catalysis based on the reversible redox of Fe(III)/Fe(II) (Kong et al. 2015). However, it is unstable and tends to self-oxidise, easily forming catalytic inactive dimers in oxidative media (Xue et al. 2012), thereby reducing catalytic activity and limiting widespread application (Zhang et al. 2013b). Recent research has attempted to protect the activity of hemin by modifying the hemin construct or applying suitably biocompatible support materials (Fruk and Niemeyer 2005).
Graphene is a standard, two-dimensional, one-atom-thick sheet material consisting of sp2–hydridised carbons (Novoselov et al. 2004), an extended honeycomb network, and long-range π-conjugation (Liu et al. 2012). Its unique construction and properties have led to its wide use in energy conversion and storage, nano-catalysts, and other applications. Combining hemin with graphene by π-π bonds is possible; graphene/hemin hybrid materials (H-GN) have high catalytic activity and maintain the special properties of both hemin and graphene (Xue et al. 2012; Zhang et al. 2012; Li et al. 2013).
Styrene, a bulk raw material in basic organic chemical industry, is mainly used in manufacture of polystyrene resin, ion-exchange resin, synthetic resin coatings, unsaturated polyester resin, and insulation materials. Diacetone alcohol, widely used in chemical industry, is in great demand. Bis(2-ethylhexyl) phthalate is an important general-purpose plasticizer, mainly used in PVC resin processing, polyester resin processing, acetate resin, ABS resin and rubber polymer.
An H-GN catalyst system is reusable and far less toxic than an organic solvent reaction system. It shows great potential as a green reaction system for biomass treatment. Thus, in this work, an H-GN hybrid material was synthesised via a wet-chemical method, and was investigated as an effective catalyst for lignin degradation. It was reported for the first time that styrene, diacetone alcohol and bis(2-ethyhexyl) phthalate can be converted from lignin in the presence of H-GN catalyst.
EXPERIMENTAL
Materials
Lignin (Chenming Group, Shandong, China) was recovered from soda pulping liquor by acid precipitation. Graphene oxide was purchased from Chengdu Organic Chemicals Co. Ltd. Hemin, NaBH4, 30 wt.% H2O2 solutions were obtained from Aladdin Reagent Co., Ltd. (Shanghai, China).
Methods
Synthesis
The synthesis of H-GN hybrid materials by simple wet-chemical method (Zhang et al. 2013a) is shown in Fig. 1. In this process, 100 mL of a 0.5 g/L aqueous graphene oxide (GO) solution was transferred to ethanol and mixed with 100 mL of a 0.5 g/L hemin ethanol solution under ultrasonic treatment. The mixture was placed in an oil bath and heated to 65° C for 6 h after NaBH4 was added.
The H-GN dispersion in ethanol was washed three times by centrifugation with ethanol to eliminate unattached hemin residue. The clean H-GN was stored in a sealed vial at room temperature. To characterise the H-GN sample, Raman spectra was carried out by LabRAM Aramis spectra (H.J.Y, France). X-ray photoelectron spectra (XPS) was determined using a ThermoFisher K-Alpha spectrophotometer (Waltham, MA, USA). UV-visible spectra was recorded on a U-3900H UV-visible spectrophotometer (Hitachi, Tokyo, Japan).
Fig. 1. H-GN hybrid material synthesis process and catalytic reaction
Degradation of alkaline lignin
A mixture containing 0.4 g of alkaline lignin, 0.01 g of H-GN catalyst, and an appropriate amount of 30 wt. % H2O2 was added to a 45 mL pressure seal tube. The pH of the mixture was adjusted to alkaline with 0.4 M NaOH measured by an Ohaus, starter2100 pH meter (Shanghai, China). The mixture reacted under mild conditions, and HCl was added to recover the non-degradation components, followed by centrifugal treatment and drying. The recovered H-GN catalyst was rinsed with DW, dried at 60 °C, and used to assess catalyst stability. The degradation rate (Li et al. 2015) was calculated according to Eq.1,
(1)
where m0 is the initial mass and md is the mass after degradation under different experimental conditions.
The products were characterised by UV/vis spectroscopy, Fourier transform infrared (FTIR) spectroscopy, gas chromatography mass spectrometry (GCMS), and high resolution mass spectrometry (HRMS). FTIR spectra were collected on a VERTEX 33 spectrophotometer (Munich, Germany) with KBr pellets. All liquid products were analysed by GCMS on a GCMS–QP2010 system (Shimadzu, Kyoto, Japan). During analysis, the column temperature was first held at 60 °C for 2 min before ramping up to 280 °C at 20 °C/min and a final hold at 280 °C for 10 min. The products were identified by the mass spectra database, and by comparing their retention times with those computer library (Ehara et al. 2005). HRMS spectra were detected by a maXis impact (Bruker, Germany) to further identify the degradation products.
RESULTS AND DISCUSSION
Material Characterization and Analysis
Graphene oxide (GO), hemin, and H-GN samples were characterised by Raman spectroscopy (Fig. 2). Compared with GO and hemin samples, the G and D peaks in the H-GN sample underwent a bathochromic shift to 1587 cm-1 (G peak) and 1328 cm-1 (D peak), and the ratio of ID/IG increased from 1.1062 to 1.2228 because of the increased small sp2 domains and reduction average size, which indicated GN formation. Subsequently, XPS spectroscopy (Fig. 3a) exhibited N1s and Fe2p signals (Li et al. 2013) at 400 eV and 710 eV, respectively.
In C1s XPS spectra of GO (Fig. 3b), specific peaks were observed at 284.3, 286.4, 287.9, and 286.9 eV, corresponding to C-C, C-O, O-C=O, and C=O, respectively (Bismarck et al. 1997). After reduction, the intensity of the peaks corresponding to O-C=O and C=O species clearly decreased, as shown in Fig. 3c, mainly because of the removal of oxygen-containing functional groups (Zu et al. 2011) and double bond groups.
The N1s XPS spectra of H-GN (Fig. 3d) revealed two dominant peaks at 397.5 eV and 399.5 eV, which corresponded to C-N and Fe-N, respectively. This result confirms that most hemin molecules remain in monomer form on GN.
In the UV/Vis spectra of the H-GN dispersion after reduction (Fig. 4), the sample turned black, and the spectrogram changed, with a broad peak at 268 nm showing the formation of GN. The absorption peak at 408 nm showed that the Soret band of hemin had undergone a bathochromic shift of 9 nm, which was due to the π-π interaction between GN and hemin molecules. In sum, the results confirmed that GO was reduced to GN, and hemin was loaded on the surface of GN by the wet-chemical method.
Fig. 2. Raman spectra of GO\Hemin\H-GN
Fig. 3. XPS deconvolution of GO and H-GN. (a) Survey of H-GN; (b) deconvolution of C1s in the GO sample; (c) deconvolution of C1s in the H-GN sample; (d) deconvolution of N1s in the H-GN sample
Fig. 4. UV/Vis spectra of GO\Hemin\H-GN
Catalytic Activity Result for Alkaline Lignin
Without a catalyst, the degradation rate was only 32.1% and 34.9% after a reaction time of 1 h and 5 h, respectively. Additionally, with hemin, the mixture could not react normally because of over-abundant oxygen, which caused it to bubble and overflow. Figure 5 shows the influence of reaction time and H2O2 dosage on lignin degradation. The degradation rate increased to 41.8% and 48.0% at 1 h and 5 h, respectively, with the addition of the H-GN catalyst. Clearly, the H-GN catalyst efficiently increased the degradation speed and degradation rate, enhancing them by 34.27%.
Fig. 5. Influence of reaction time and H2O2 dosage on lignin degradation rate. A 10:1 mass ratio of H2O2 to lignin and pH 13 were used when time was varied. A reaction time of 5 h and pH 13 were used when the H2O2-to-lignin mass ratio was varied
Effect of 30 wt. % H2O2 dosage on lignin degradation
The effect of 30 wt.% hydrogen peroxide dosage on the degradation of lignin was investigated by changing the mass ratio of H2O2 to lignin from 4:1 to 12:1 under normal conditions at 60 °C for 5 h. Figure 5 shows that increasing H2O2-to-lignin ratio from 4:1 to 10:1 increased the degradation rate from 37.31 wt.% to 47.99 wt.%. Further increasing the H2O2-to-lignin ratio to 12:1 had little effect on degradation. As a strong oxidant, H2O2 produces hydroxyl radicals to form oxoiron (Fe4+=O) and accelerates lignin degradation. However, excessive hydroxyl radicals formed from excess H2O2 may result in re-condensation of degraded lignin.
Effect of initial solution pH on lignin degradation
Figure 6 showed the effects of pH and temperature in the tested lignin solution. The effect of initial solution pH on the degradation of lignin under 60 °C for 5 h was determined. The degradation rate at pH 4, 10, 11, 12, 13, and 13.2 was 28.4%, 27.6%, 30.8%, 34.1%, 48.0%, and 73.6%, respectively. The degradation increased from 28.4% at pH 4 to 73.6% at pH 13.2, corresponding to 159% increase in activity. This observation is in agreement with the result from Li et al. (2015). This effect is probably associated with the acid-base equilibrium of the adsorbed hydroxyl group; higher pH favors the generation of hydroxyl radicals. The results reveal that the H-GN catalyst is a suitable for the lignin degradation and is a new feasible way to deal with lignin degradation and reclamation.
Effect of reaction temperature on lignin degradation
The effect of reaction temperature from 40 °C to 100 °C on lignin degradation was evaluated (Fig. 6). The degradation increased with increasing temperature, and the highest lignin degradation of 92.9% was achieved at 100 °C with 0.4 M NaOH.
Fig. 6. Influence of pH and T (°C) on degradation rate of lignin. The mass ratio of H2O2 to lignin, time, and T (°C) were 10:1, 5 h, and 60 °C, respectively, when the pH was varied. The mass ratio of H2O2 to lignin, time, and pH were 10:1, 5 h and 14 in when the T (°C) was varied.
Characterization of lignin before and after degradation
To further research the effects of reaction time for lignin degradation, the raw and degraded lignin were characterised by elemental analysis, UV/Vis, FTIR, GCMS, and HRMS. Elementary analysis results are shown in Fig. 7. The decrease of C and H contents and increase of O content along the degradation pattern may be explained by incorporation of oxygen from hydrogen peroxide during degradation (Wiermans et al. 2015).
UV/Vis spectra detected the decomposition of aromatic structures (Fig. 8). Raw lignin exhibited a strong absorbance peak at 266 nm and a shoulder peak at 280 to 286 nm, which were due to the characteristic absorbance of lignin among conjugated molecular groups, such as aromatic groups (Ouyang et al. 2015a). After lignin degradation by H-GN, the maximum absorbance was reduced and exhibited a hypochromatic shift of 14 nm. The hypochromatic shift reflected the reaction of the chromophoric group of lignin.
Fig. 7. Elemental analysis of raw lignin and degraded lignin
Fig. 8. UV/Vis spectra of raw lignin and degraded lignin
The FTIR spectra for raw and degraded lignin with different reaction times are shown in Fig. 9. The raw lignin curve exhibited major absorbance peaks at wavenumbers of 1600, 1365, 1144, 818, and 621 cm-1, which were attributed to the absorption of aromatic nucleus vibration, aromatic nucleus stretching vibration, and C=O stretching vibration. Curve b, which represented the 1 h reaction, was similar to curve a, which showed the incomplete reaction. At increased reaction time, curves c, d, e, and f changed. Compared with raw lignin, the absorbance peaks at 1600, 1365, and 621 cm-1 disappeared, and the peaks at 1144 cm-1 and 818 cm-1 decreased, reflecting the opened and broken aromatic rings.
Fig. 9. FTIR spectra for raw lignin and degraded lignin with different reaction times
Liquid production from lignin degradation at different pH
GCMS analysis was used to identify the composition of degradation products (Fig. 10). Table 1 lists the major products, retention times, names, molecular formula, content, and structure detected by GCMS at an initial pH ranging from 10.0 to 13.2 for lignin degradation. Increasing the pH from 10 to 13.2 affected the products selectivity. The main product was styrene when the pH was below 13. However, the major products at pH 13.2 were 4-hydroxy-4-methyl-2-pentanone and bis(2-ethylhexyl) phthalate. HRMS analysis was carried out to further identify the products. The HRMS spectra of lignin degradation product (C24H38O4) are shown in Fig. 11. The m/z ratio of the target product was 413.2662, which was the adduct mass of model compound with sodium ion. HRMS calculated for C24H38O4 [M+Na]+ 413.2662, found 413.2656. This mean that the molecular weight of the synthesized model compound was 390, which was equal to that of C24H38O4 (Ouyang et al. 2015b). These may result from oxoiron (Fe4+=O) species oxidizing the lignin polymer. And it was found to be highly pH-dependent (Rahikainen et al. 2013): an increase in pH changed the products. The higher pH value was, the more hydroxyl ions was contained. Different products of lignin degradation may be due to the cleavage of distinctive linkage.
Fig. 10. Major products of lignin degradation
Fig. 11. HRMS spectra of lignin degradation product [bis(2-ethylhexyl) phthalate]
Table 1. GCMS of Degraded Lignin at Different pH
According to previous reports (Guo et al. 2011), the simplified mechanism for lignin catalytic reaction can be separated into two stages, as shown in Fig. 12. Under alkaline condition, hydrogen peroxide dissociates into hydroxyl radicals and superoxide ions that may react with hydroxyl radicals, resulting in oxygen and water as final products (Lalitendu et al. 2016). With the addition of the H-GN catalyst, oxygen donors from the peroxide oxidise hemin to form oxoiron (Fe4+=O) (Li et al. 2013). Once oxoiron (Fe4+=O) species accumulate to a certain level, activated Fe=O species start to weaken and break aromatic rings.
Fig. 12. Simplified mechanism for lignin catalytic reaction process
CONCLUSIONS
- H-GN hybrid material was validated as a highly effective catalyst in lignin degradation under gentle conditions.
- The degradation rate was as high as 92.9 wt.% at pH 13.2 with H-GN and H2O2, which was superior to 34.9 wt.% for non-catalyst heating degradation.
- GCMS analysis showed that lignin was degraded into small molecules. Below pH 13, the main product was styrene; at pH 13.2, the major products were 4-hydroxy-4-methyl-2-pentanone and bis(2-ethylhexyl) phthalate.
ACKNOWLEDGEMENTS
This research was supported by National Natural Science Foundation of China (21376088). The authors also gratefully acknowledge support from the Guangdong Provincial Laboratory of Green Chemical Technology.
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Article submitted: September 22, 2016; Peer review completed: December 12, 2016; Revised version received: January 12, 2017; Accepted: January 23, 2017; Published: February 10, 2017.
DOI: 10.15376/biores.12.2.2354-2366