Xingyan Huang · Feng Li · Jiulong Xie · Cornelis F. De Hoop ·Xiaopeng Peng · Jinqiu Qi · Yuzhu Chen · Hui Xiao

Abstract Eucalyptus grandis W. Hill ex Maiden bark was liquefied in glycerol with two types of catalysts. The chemical components of the residues with respect to temperature were examined to investigate the liquefaction behavior of bark. The results reveal that sulfuric acid was more efficient in converting bark into fragments in glycerol at low temperatures ≤433.15 K, equivalent to 160 °C than phosphoric acid. The liquefaction order of chemical components was lignin, hemicelluloses, and cellulose. The decrease of liquefaction yields at high temperatures(＞453.15 K) catalyzed by sulfuric acid was possibly a result of the recondensation of lignin and/or hemicelluloses.

Keywords Chemical components · Eucalyptus grandis bark · Liquefaction · Liquefied residue

Introduction

Utilization of lignocellulosic biomass has received considerable attention in recent years due to the abundance of renewable chemicals (i.e., cellulose, hemicelluloses, and lignin) and positive environmental benefits. As an important renewable biomass, bark has potential as a raw material for synthesizing bio-based resins (Lee and Liu 2003; Sankar and Yan 2014), bio-polyols (D’Souza and Yan 2013), and polyurethane foams (Cruz-Lopes et al.2016). However, bark is currently an underutilized resource available in large volumes suitable for industrial applications (D’Souza et al. 2016).

Liquefaction is an effective approach to convert lignocellulosic biomass into liquid (Gollakota et al. 2018).Polyhydric alcohols, such as glycerol, are commonly used as solvents. Acids, for example sulfuric acid, normally serve as catalysts to boost the liquefaction reaction. Zhang et al. (2012) reported that glycerol reduced the surface tension of the liquefaction solvent at high temperatures to enhance the penetration of the catalyst into biomass samples. Furthermore, it could result in a uniform distribution of the reagent within the biomass by accelerating the diffusion of liquefied biomass into the solvent. Sulfuric acid can provide highly reactive protons (H+ions) able to promote the hydrolytic reactions of glycosidic bonds,resulting in the rapid decomposition of biomass. However,a high concentration sulfuric acid load may lead to recondensation during liquefaction (Xu et al. 2016).Therefore, the catalytic action of a weak acid, i.e., phosphoric acid, is evaluated in this study. The liquefaction products can be used for the further synthesis of polymer materials such as polyurethane foams (Xie et al. 2015a),phenolic resins (Roslan et al. 2014), and liquid biofuels(Yin 2012).

Previous research on the characterization of liquefied wood residues under different reaction conditions provided a new approach to better understand the liquefaction of wood (Pan et al. 2007). Reaction temperatures and catalyst types are two primary factors that significantly affect the liquefaction process (Xu et al. 2016). Increasing temperatures could boost yields, whereas excessive temperatures would have a negative effect.

Cellulose, hemicelluloses, and lignin are the main cell wall components in biomass and remarkably different between wood and bark. Zhang et al. (2012) demonstrated that the main cell wall components of wood have different stabilities with response to liquefaction. However, there is no quantitative analysis on changes of the cell wall components of bark during liquefaction. Although the decomposition of biomass chemical components through a liquefaction process is the main purpose for the integrated utilization of biomass, a recondensation reaction among the liquefied fragments under critical conditions also occurs,which will reduce liquefaction yields (Xie et al. 2014).

To acquire a basic understanding of the liquefaction behavior of bark, Eucalyptus grandis bark was subjected to a liquefaction process in an oil bath catalyzed by sulfuric acid and phosphoric acid at different temperatures. The main components of the bark as well as the composition of the liquefied residues were quantitatively investigated using wet chemistry analysis. The results in this study could provide fundamental information for the integrated utilization of bark via liquefaction.

Materials and methods

The bark of 6-year-old Eucalyptus grandis trees was collected from a plantation near Ya’an, Sichuan, China. The bark particles were screened through a 40-mesh sieve and retained on a 60-mesh sieve, and dried to a constant weight in an oven at 353.15 K (80 °C). The particles were stored in polyethylene bags and used without further treatment.All chemicals, sulfuric acid (H2SO4), phosphoric acid(H3PO4), methanol and glycerol, were analytical grade,commercially available and used without further purification.

Liquefaction of bark

Bark liquefaction was performed using a conventional oil heating method. Sulfuric acid (98 wt%) and phosphoric acid (85 wt%) were used as catalysts. A typical run of 10 g of bark particles, 50 g of glycerol, and 1.5 g of catalyst was loaded in a flask with a magnetic stirring bar, the flask maintained at a desired temperature for 1 h. After the reaction, the resultant was dissolved in 750 ml of methanol under constant stirring for 4 h. The solutions were vacuumfiltered through Whatman No. 4 filter paper, and the residue retained on the filter paper was oven-dried at 378.15 K(105 °C) over 24 h until completely dry. The residue content was calculated as follow:

Chemical analysis

Holocelluloses, α-cellulose, lignin content, hot-water extracts, alcohol-toluene extractives, 1% NaOH solubility,and ash content were determined according to ASTM D1104-56 (1971), ASTM D1103-60 (1971), ASTM D1106-96 (1996), ASTM D1110-96 (1996), ASTM D1107-96 (1996), ASTM D1109-84 (2001), and ASTM D1102-84 (2001), respectively. Hemicelluloses content was established by the differences between holocellulose and α-cellulose.

Results and discussion

Chemical analysis of Eucalyptus grandis bark

The chemical components provide fundamental information for further analysis. The hot-water extractives,toluene-alcohol extractives, and 1% NaOH solubility were 7.0%, 10.3%, 23.7%, respectively (Table 1). As reported by Guo et al. (2005), hot-water extractives, toluene-alcohol extractives, and 1% NaOH solubility for Eucalyptus grandis wood were 4.7%, 1.2%, 15.6%, respectively. In comparison, bark has higher extractive contents and 1%NaOHsubstancesthanthewood.Cellulose,hemicelluloses, and lignin contents in bark were 35.7%,21.7%, and 43.6%, respectively. The wood had remarkably higher cellulose content compared to the bark, i.e., the cellulose content for wood was 50.5%. However, the lignin content for bark was 1.55 times that for wood. With regards to the ash content, the bark contents were greater than the wood. Based on these results, remarkable differences in chemical components of bark and wood were observed. In terms of biomass liquefaction, the properties, including structures and chemical components of lignocellulosic biomass, were closely related to its solvolysis decomposition/degradation behavior in an organic solvent with a catalyst (Wang et al. 2012; Xiao et al. 2013; Xie et al.2016). Although liquefaction of Eucalyptus wood has been carried out for renewable chemicals (Fu et al. 2011), liquefaction of Eucalyptus bark still needs to be determined because of remarkable differences in chemical components.

Table 1 Chemical composition of Eucalyptus grandis bark

Liquefaction of Eucalyptus grandis bark

Liquefaction was carried out in glycerol with catalysts of sulfuric acid and phosphoric acid. Since high catalyst loading would result in a negative effect on the liquefaction yield (Xie et al. 2015b), 3% of the mass ratio of the solvent of the catalysts was used to study liquefaction of the bark with respect to temperature (Fig. 1).

Fig. 1 Liquefaction residue contents of Eucalyptus grandis bark with different catalysts

The residue of the bark liquefied with sulfuric acid and phosphoric acid dramatically decreased with increasing temperature from 353.15 K (80 °C) to 453.15 K (120 °C),revealing that the increase in temperatures could significantly improve liquefaction yields. The bark residue liquefied with sulfuric acid was much lower than with phosphoric acid. The average difference in residue content was about 37.8%. This result revealed that sulfuric acid was more effective than phosphoric acid in converting Eucalyptus grandis bark into liquids via glycerol liquefaction. At temperatures of 353.15 K (80 °C) to 393.15 K(120 °C), the reaction rate of sulfuric acid liquefaction,indicated by the relative larger scope rate of the curve, was higher compared to that of phosphoric acid liquefaction.Compared with phosphoric acid, sulfuric acid has a higher boiling point and stronger acidity, which contributed to its higher efficiency as a catalyst. It provides highly reactive protons (H+ions) that promote hydrolytic reactions of glycosidic bonds, resulting in the dissolution of bark (Xu et al. 2016). Moreover, the residue content of sulfuric acid liquefaction leveled off at temperatures of 433.15 K(160 °C) to 453.15 K (180 °C), indicating that recondensation took place. Previous studies have indicated that recondensation was ascribed to reactions of hemicelluloses/lignin derivatives (Xie et al. 2014). Furthermore, the sidereaction of acid-catalyzed hydrolysis of cellulose into levulinic acid could also contribute to the recondensation reaction (Girisuta et al. 2007).

Chemical analysis of the liquefied residue

The lignin, hemicelluloses, and cellulose components of the liquefied bark residues were determined (Table 2). The Klason lignin content dramatically decreased from 353.15 K (80 °C) to 433.15 K (160 °C). Further increasing the temperature to 453.15 K (180 °C), the lignin content remarkably increased. Similar trends in lignin contents with increasing temperatures were reported by Chen et al.(2012) and Xiao et al. (2013). This finding illustrates that the increase in temperatures enhanced the decomposition of lignin at a relatively low temperature range of≤433.15 K (160 °C). The Klason lignin content in liquefied bark residue from sulfuric acid liquefaction was much lower than from phosphoric acid liquefaction below 433.15 K (160 °C), while the former showed a higher lignin content at 453.15 K (180 °C), i.e., lignin contents for sulfuric acid and phosphoric acid liquefaction were 30.4%and 25.0%, respectively. This result indicates that sulfuric acid was more effective in lignin decomposition than phosphoric acid at low temperatures ≤433.15 K (160 °C).However, the initially decomposed lignin compounds in sulfuric acid also enhanced the recondensation of lignin at critical reaction conditions (high temperatures), which contributed to higher residues.

In terms of hemicelluloses content in the residue, previous research has indicated that it decreased with prolonged time or increased reaction temperatures (Chen et al.2012; Xiao et al. 2013). However, in this study, the hemicelluloses contents in the bark residues from sulfuric acid and phosphoric acid liquefactions increased in the range of 353.15 K (80 °C)-393.15 K (120 °C). Theincrease at the initial liquefaction stage (low temperature)was also observed in the liquefaction of Eucalyptus wood(Zhang et al. 2012). Although the kinetic study of the three principal cell wall components in wood demonstrated that hemicelluloses were the most susceptible component to liquefaction owing to their low activation energy compared with lignin and cellulose (Zhang et al. 2014), the decomposition rate of lignin in this study at the low temperature was higher than for hemicelluloses. This contributed to the relative increase in hemicelluloses content.

Table 2 Chemical composition of liquefied bark residues from different liquefaction conditions

The hemicelluloses contents in residues significantly decreased when temperatures increased from 393.15 K(120 °C) to 433.15 K (160 °C). For the phosphoric acid liquefaction, hemicelluloses contents gradually decreased to a low level at 453.15 K (180 °C). However, hemicelluloses contents in residues from sulfuric acid liquefaction dramatically increased from 22.4 to 30.3% as the reaction temperature increased from 433.15 K (160 °C) to 453.15 K (180 °C). The decrease in hemicelluloses was caused by their decomposition, while the increase in residue from the sulfuric acid liquefaction may be attributed to recondensation of decomposed hemicelluloses and sulfuric acid acetified glycerol, resulting in an increase in the amount of insoluble residue (Zhang et al. 2012). The difference in hemicelluloses content variations with respect to reaction temperatures between sulfuric acid and phosphoric acid residues indicates that the catalysts had significant effects on the liquefaction behavior of hemicelluloses.

It is noteworthy that the cellulose content in the liquefied bark residues first increased rapidly, and then significantly decreased with increasing temperature from 353.15 K (80 °C) to 453.15 K (160 °C). The variation pattern of the cellulose contents changed with respect to reaction temperatures, and was similar to the findings reported by Xiao et al. (2013). The increase of cellulose content in residues was due to the decomposition of lignin and hemicelluloses prior to the degradation of cellulose,and the reduction in lignin and hemicelluloses contents yielded relatively high cellulose levels. A decrease in cellulose content occurred above 393.15 K (120 °C) as the cellulose started to decompose. An interesting finding was that cellulose started to decrease at 393.15 K (120 °C) for sulfuric acid liquefaction, and at 413.15 K (140 °C) for phosphoric acid. The difference in cellulose contents of bark residues between sulfuric acid and phosphoric acid liquefactions reveal that the efficiency of the catalyst in cellulose solvolysis was significantly different.

Conclusion

Eucalyptus grandis bark contained higher contents in extractives, 1% NaOH solubility, Klason lignin and ash,while lower cellulose content than in wood. Sulfuric acid was more efficient in converting bark into fragments in glycerol at low temperatures ≤433.15 K (160 °C) than phosphoric acid. Changes in lignin, hemicelluloses and cellulose, with respect to liquefaction temperatures, indicates that the liquefaction order of the chemical components in Eucalyptus grandis bark was lignin,hemicelluloses, and cellulose. The decrease in liquefaction efficiency at high temperatures was probably caused by the recondensation of lignin and/or hemicelluloses.