In the field of pulping, the challenge of effectively removing hemicellulose while preserving cellulose has emerged as a critical research issue. To facilitate the extensive development and utilization of Acacia wood as a potential biomass resource, the microwave hydrothermal method was implemented. The results demonstrated that subjecting the material to a 170°C pretreatment for 60 min led to a notable enhancement in hemicellulose dissolution. The total concentration of xylose in the solution reached 15.69 g/L, a value that was considerably higher than that observed in the conventional hydrothermal treatment solution. The regression model constructed using the least squares method is an effective means of predicting the dissolution of composite components under a range of microwave hydrothermal treatment conditions. Furthermore, it was discovered that the pulping process could be optimized by subjecting Acacia wood to lower temperatures (120 ~ 140°C). This resulted in an increase in pulp yield and improvement in paper quality, with an enhancement of 10–20%. In summary, the approach provides new insights into the degradation rule of the composite components in Acacia wood, contributing to the enhancement of subsequent paper-based composite material production, and paving the way for an energy-efficient, sustainable, and environmentally friendly evolution of the pulp and paper industry.
Research Article
Effect of microwave hydrothermal pretreatment on dissolution of composite components in Acacia wood and subsequent pulping performance
https://doi.org/10.21203/rs.3.rs-5009853/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 22 Oct, 2024
Read the published version in Advanced Composites and Hybrid Materials →
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Acacia wood
Microwave hydrothermal pretreatment
Least squares method
paper-based composite material
The contemporary global context is characterized by two interrelated challenges: the depletion of fossil fuels and the degradation of the environment. This makes the sustainable and efficient utilization of biomass resources an increasingly important issue [1]. Lignocellulosic biomass is the most abundant renewable resource [2–4], consisting mainly of cellulose, hemicellulose, and lignin [5–15]. It is a promising alternative to fossil fuels with the potential for the production of biofuels, chemicals, and materials [16–23].
Acacia wood, classified as a lignocellulosic biomass, is a highly suitable evergreen hardwood for short-rotation mountain afforestation within the genus Acacia, which belongs to the Leguminosae family. It exhibits remarkable adaptability to various soil types and a relatively short growth cycle. Additionally, it is a prominent lignocellulosic biomass feedstock in the biorefinery industry [24, 25]. Due to its high annual growth rate (22.8–32.1 m3/hm2) [26], rich cellulose content, low fiber coarseness, and sustainability, Acacia wood is a key raw material for pulp production [27]. It is important to note, however, that the cellulose, hemicellulose, and lignin in the cell wall of Acacia wood are interconnected through glycosidic, ether or other chemical bonds to form a dense and difficult-to-degrade complex structure. The natural resistance of lignocellulosic biomass to chemicals hinders the separation of Acacia wood composite components and the preparation of high-purity paper-based composite materials [28–30]. Consequently, there is a great challenge in the effective separation and utilization of these composite components represents a significant challenge. The conventional pulping process entails the extraction of lignin and hemicellulose from the waste stream, which results in the loss of valuable components and an increase in treatment costs for the company [30, 31]. It is therefore necessary to subject the material to a pretreatment process prior to pulping. This allows the extraction of the hemicellulose, which can then be converted into high-value chemicals such as bioethanol and furfural [32]. In addition, the process loosens the wood chips and increases their porosity, which facilitates the penetration of chemicals during subsequent pulping processes and enhances the quality of paper-based composite materials [33, 34].
The principal pretreatment methods are currently classified as physical, physicochemical and biological methods [31–34]. Physical pretreatment methods include mechanical crushing, ultrasonication, and other techniques. However, these methods have the disadvantage of requiring expensive equipment and high energy consumption [35]. Chemical methods, including the use of acidic, alkaline, and organic solvents [36], have been demonstrated to be effective in the removal of hemicellulose and lignin from plant cell walls. However, these methods often result in the recondensation of lignin and carbohydrates, which can lead to corrosion of equipment [37]. In this regard, hydrothermal pretreatment (also known as autohydrolysis) represents a green and sustainable method for the hydrolysis of lignocellulosic fibers under high-temperature conditions using water as the sole solvent, and has been widely employed in practical production. For example, Yuan et al. employed hydrothermal pretreatment of wood chips to investigate the impact of the optimal combined severity factor (CSF) on subsequent enzymatic saccharification and lignin structure [38]. However, hydrothermal pretreatment has the disadvantages of high temperature and pressure, as well as significant energy loss. In light of these considerations, the search for an efficient, energy-saving and simple alternative heating technology to optimize the hydrothermal pretreatment method has attracted the attention of a wide range of researchers.
The advantages of microwave hydrothermal treatment include uniform internal heating, a fast reaction speed, high conversion efficiency and low energy consumption. As a result, it has become a more efficient alternative method [39–42]. This approach demonstrated considerable potential in biomass conversion, particularly in the extraction of natural polysaccharides, phenolics, and the production of bioethanol or high-value-added materials [43, 44]. For instance, Li et al. prepared furfural from corn kernels using microwave hydrothermal pretreatment and non-homogeneous catalytic conversion, finding that controlling hemicellulose depolymerization is crucial for optimizing furfural yields [45]. However, the majority of research has concentrated on optimizing extraction parameters, rather than developing a dissolution model for composite components based on pretreatment outcomes. Furthermore, the application of microwave hydrothermal pretreatment within the pulp and paper industry remains largely uncharted territory. In this study, the effects of treatment temperature and time on the dissolution of wood fractions were investigated by using microwave hydrothermal method to pretreat Acacia wood, with the results compared to those obtained through conventional hydrothermal treatment (Fig. 1). In this process, the lignin dissolution model was established using the least squares method, which enables the accurate prediction of the optimal conditions for the dissolution of composite components in wood chips. Moreover, the impact of varying pretreatment temperatures on the physical characteristics of paper was investigated. This research is anticipated to lay a theoretical foundation and provide technical support for the pulp and paper industry, promoting the comprehensive utilization of biomass components and marking a significant step towards sustainable production and the advancement of composite material development.
2.1. Materials
Acacia wood chips containing 42.76 wt% cellulose, 21.48 wt% hemicellulose, 27.01 wt% lignin, and 2.26 wt% other extractives were produced by PacifiCorp (Shandong) Pulp and Paper Co. Sodium hydroxide, sodium sulfide, and sulfuric acid were purchased from Beijing Chemical Factory (Beijing, China).
2.2. Microwave hydrothermal treatment
The microwave hydrothermal pretreatment of Acacia wood chips was conducted using a microwave hydrothermal synthesizer (MKX-H1G1A, MAKEWAVE, China). A solid-liquid ratio of 1:5 was employed, with 40 g of completely desiccated Acacia wood chips were combined with 200 mL of deionized water. The mixture was sealed within a polytetrafluoroethylene jar and subjected to microwave hydrothermal treatment. The temperature was maintained at 120 to 190°C for a period of 20 to 100 min at a fixed microwave power of 500 W. After the reaction is complete and the temperature inside the reactor has dropped to room temperature, a Büchner funnel is used with a filter paper to filter the contents of the reaction vessel, thus separating the solids from the liquids. The liquids were bottled and stored at 4°C for subsequent analysis, while the residual solids were rinsed with deionized water to achieve a neutral pH, air-dried, and weighed to calculate the solid yield. The experimental samples were labeled according to the temperature (T) and time (t) parameters; for instance, T120t20 denotes 20 min at 120°C.
2.3 Conventional hydrothermal treatment
Mixed the Acacia wood chips with deionized water in a pressure-resistant bottle at a solid-to-liquid ratio of 1:5 (w: v). The reactor was heated in an oil bath with dimethyl silicone oil, and hydrothermal treatment was performed at 170°C for 60 min. Subsequently, the liquid was separated from the wood chips by filtration.
2.4 Kraft pulping
The objective of this study was to investigate the influence of Kraft pulping of wood chips, both prior to and following microwave hydrothermal pretreatment, on the properties of paper. The resulting unbleached pulp was employed in the production of paper. First, 100 g of pretreated wood chips and 240 mL of deionized water were boiled in a 1 L stainless steel reactor under the following conditions: 15% alkali (Na2O), 30% sulfation, heated for 90 min and held at 165°C for another 150 min. Once the experiment was complete, the pulp was washed to neutral pH and screened using a flat sieve. Prior to sheet formation, the pulp consistency was adjusted to 10% using a PFI pulper, after which paper sheets were produced on a paper machine at a weight of 100 g/m2.
2.5 Analysis of the chemical composition of pretreated liquids
The liquids were subjected to analysis for monosaccharides and degradation products in accordance with the methodologies established by the National Renewable Energy Laboratory (NREL) [46]. Firstly, dilute the sample with ultrapure water by a factor of 50 to 500 times, then inject it into the sample vial through a 0.22µm water-soluble filter. The monosaccharides (glucose, xylose, arabinose, galactose, and mannose) were identified through the use of an ion chromatograph (ICS-5000+, Thermo Fisher Scientific, America) equipped with a CarboPacPA200 series column (3 mm×150 mm) and a guard column (3 mm×30 mm) [47, 48]. The chromatographic conditions are as follows: an electrochemical (EC) detector is used, with an injection volume of 25 µL, and the column temperature is set at 30°C. The mobile phase consists of ultrapure water (A), 1 M sodium acetate (B), 250 mM sodium hydroxide (C), and 50 mM sodium hydroxide (D). The degradation products, including acetic acid, 5-HMF, and furfural, were identified and quantified using a high-performance liquid chromatography (HPLC) system [49, 50]. The concentration of oligosaccharides was determined by hydrolysis (acid hydrolysis at 121°C for 60 min under 4 wt% H2SO4) following filtration [51]. The concentration of oligosaccharides was calculated from the difference in the concentration of monosaccharides, which were measured before and after hydrolysis and expressed as monosaccharide content. The absorbance of the solution was determined using a UV-Vis spectrophotometer [52], and the concentration of acid-soluble lignin was calculated according to the following equation:
where C represents the concentration of lignin; A denotes the absorbance value at 205 nm; D is the dilution factor, and 110 is the absorbance coefficient (L·g− 1·cm− 1).
2.6 Properties of residual wood chips after pretreatment
The surface morphology of the wood chips was examined using a scanning electron microscope (SEM) (TM4000Plus, Hitachi, Japan) with an accelerating voltage of 10–15 KV. High-resolution three-dimensional imaging of the internal structure of the wood chips was conducted using micro-CT (SkyScan 2211, Bruker, Germany), with an X-ray source operating at 45 kV and 80 µA [53].
The alterations in the functional groups of the Acacia wood raw material and the residual solid fractions resulting from disparate pretreatments were examined through the use of Fourier transform infrared spectroscopy (FTIR) (Invenios, Bruker, Germany). The wood powder was milled, mixed with KBr, compressed into tablets, and scanned 32 times at a resolution of 4 cm− 1 over a scanning wavelength range of 4000 to 400 cm− 1.
Prior to and following the application of the aforementioned pretreatment, the acacia wood powder, which was finer than 100 mesh, was prepared and subsequently analyzed using an X-ray diffractometer (XRD, D8-ADVANCE, Bruker, Germany). Scans were conducted at 0.02° intervals at a rate of 20°/min from 5° to 50°, and the crystallinity of the cellulose was analyzed based on the XRD patterns. The crystallinity index (CrI) was calculated as follows [54]:
where I002 is the diffraction intensity in the crystalline region, and Iam is the diffraction intensity in the amorphous region.
2.7 Construction of the model
In light of the intricate nature of the microwave hydrothermal pretreatment mechanism for wood chips and the pivotal role of component dissolution in pulping, the least squares method [55] was selected to construct a model that correlates component dissolution with experimental variables during the pretreatment phase. The algorithm was then implemented in MATLAB to align the nonlinear curve. It is postulated that the regression equation for the theoretical lignin concentration Y in the treatment solution can be expressed as follows:
where XT is temperature, Xt is time, a1, a2, a3, a4, a5, and b are coefficients to be determined.
2.8 Paper physical property testing
The tensile index, tear index, breakage resistance index, and ring crush index of the paper were evaluated in accordance with the national standards GB/T12914-2018, GB/T 455–2002, GB/T 454–2002, and GB/T 2679.8–2016.
3.1 Recovery analysis of pretreated wood chips
The yield of pretreated wood chips is contingent upon the manner in which the treatment affects the feedstock. As shown in Fig. 2a, the efficacy of microwave hydrothermal pretreatment was significantly enhanced when the temperature was increased from 120°C to 190°C with a 60 min holding time. The recovery of wood chips exhibited a notable decline, decreasing from 99.37–79.18%. Similarly, the pH of the hydrolysate exhibited a significant reduction, dropping from 4.58 to 3.08. The elevated temperatures in this process accelerated the chemical reaction, causing hemicellulose and some lignin in Acacia wood chips to degrade and dissolve into the pretreatment solution, which led to a decrease in the yield of wood chips.
In addition, the acetyl groups in hemicellulose were hydrolyzed into acetic acid during the pretreatment, which resulted in the creation of an acidic environment and a subsequent decrease in the pH of the hydrolysis solution [56]. As shown in Fig. 2b, at 170°C, the yield of wood chips exhibited a notable decline, from 93.11–82.65%, while the pH value decreased from 3.4 to 3.01 when the holding time was extended from 20 min to 100 min. In compared to the treatment temperature, the holding time had a relatively minor effect on wood chip recovery. The wood chip yield and liquid pH value after microwave hydrothermal pretreatment are significantly lower than those after conventional hydrothermal pretreatment under identical treatment conditions. A comparison of all microwave hydrothermal pretreatment samples revealed that the T140t60 conditions were similar to the conventional hydrothermal pretreatment used in factories (T170t60*), with minimal differences in wood chip yield and liquid pH value (Fig. 2c). A comparison of all microwave hydrothermal pretreatment samples revealed that the T140t60 treatment conditions were analogous to the conventional hydrothermal pretreatment conditions commonly employed in mills (T170t60*), with comparable outcomes for wood chip yield and liquid pH, indicating that the microwave hydrothermal pretreatment can effectively reduce the treatment temperature, lower energy consumption, and prevent the degradation of cellulose, thereby facilitating subsequent pulping and enhancing pulp quality.
3.2 Chemical Composition in Pretreatment Solution
It has been demonstrated that microwave hydrothermal pretreatment results in the hydrolysis of hemicellulose in Acacia wood, which in turn causes sugar to dissolve in the pretreatment solution. During hydrolysis, acetyl groups in the hemicellulose side chain are initially released, forming acetic acid. This acid can then catalyze the depolymerization and degradation of the hemicellulose into long-chain oligosaccharides [38]. In consequence, the concentration of oligosaccharides is consistently higher than that of monosaccharides under mild treatment conditions. The alterations in the oligosaccharide and monosaccharide concentrations within the hydrolysate, as a consequence of varying treatment conditions are shown in Fig. 3. The findings indicated that both treatment temperature and time had a notable impact on the sugar content in the pretreatment solution. The concentrations of oligosaccharides and monosaccharides exhibited a tendency to increase and then decrease as the treatment temperature increased under the same holding time. The concentration of oligosaccharides in the pretreatment solution reached a maximum of 15.90 g/L under T170t60, while the highest concentration of monosaccharides was observed at 12.27 g/L under T180t60. Additionally, notable discrepancies in the total sugar content of the hydrolyzed solutions were observed when under different pretreatment conditions were employed. The highest concentration of total sugars in the pretreatment solution was 20.34 g/L at reaction condition T170t60 (Fig. 3a). As the pretreatment temperature increased from 170°C to 190°C, the concentration of oligosaccharides decreased from 15.90 g/L to 0.07 g/L, indicating an accelerated hydrolysis rate of hemicellulose with increasing temperature. However, concurrently, the elevated temperature also intensifies the further decomposition of oligosaccharides and promotes the occurrence of side reactions. Consequently, the degradation rate of oligosaccharides surpasses their formation rate, leading to a significant reduction in concentration. At a constant treatment temperature, the concentration of monosaccharides exhibited a gradual increase, whereas the concentration of oligosaccharides demonstrated a biphasic pattern, initially rising and subsequently declining with an extended holding time. The highest concentration of oligosaccharides was observed in the pretreatment solution under the aforementioned condition of T170t60, with a value of 15.90 g/L (Fig. 3b). In other words, the hemicellulose removed by mild microwave hydrothermal pretreatment was primarily hydrolyzed to oligosaccharides and dissolved in the pretreatment solution, which is also consistent with the results previous work [53]. Specifically, the reduction in the total sugar content of the pretreatment solution may be attributed to the ionization of water at elevated temperatures, which produces hydrogen ions (H+) that act as catalysts in acidic hydrolysis reactions, enhancing their intensity [57, 58]. Therefore, it promotes the further degradation of the dissolved monosaccharides is promoted to furfural and 5-HMF, which results in a reduction of the total sugar content. This phenomenon also explains the decline in xylose content. As can be seen from the above conditions, it is more appropriate to pretreat Acacia wood chips at a treatment temperature in the range of 160–170°C. In this range, the contents of both monosaccharides and oligosaccharides in the pretreatment solution were higher.
The hemicellulose present in Acacia wood is primarily composed of D-xylose [53, 59], with minor quantities of glucose, mannose, galactose, and arabinose. The extent of the degradation degree and dissolution content of these monosaccharides will vary during microwave hydrothermal pretreatment. It is therefore necessary to explore the influence of different treatment conditions on the changes in the various monosaccharide components present in the pretreatment solution. As shown in Fig. 4a, the concentrations of xylose, arabinose, galactose and mannose in the pretreatment solution exhibited a tendency to increase and then decrease. The maximum concentrations of arabinose, galactose and mannose were all below 1.2 g/L, and the temperatures corresponding to the maximum amount of the dissolution of these sugars was also differed. In comparison to other monosaccharides, arabinose demonstrated a higher degradation and solubilization rate at lower temperatures, indicating that it was more readily solubilized than other sugars. The hydrolysis temperature of 180°C is suboptimal for the extraction of monosaccharides. The concentration of various monosaccharides in the pretreatment liquid decreases markedly under T180t60 conditions. This is primarily due to the fact that the rate of monosaccharide decomposition is much higher than the rate at which hemicellulose is hydrolyzed into monosaccharides. As shown in Fig. 4b, the contents of arabinose, xylose, and galactose contents decreased when the holding time exceeded 80 min for the same treatment time. This suggests that under high temperature conditions, an extended holding time is not conducive to the extraction of monosaccharides.
In comparison to conventional hydrothermal pretreatment, the microwave hydrothermal pretreatment demonstrated a more pronounced degradation of hemicellulose and a comparatively reduced impact on cellulose. This can be attributed to the fact that the crystalline regions of cellulose exhibit a high degree of polymerization and an absence of branched-chain structure. In contrast, hemicellulose exhibits a lower degree of polymerization, a greater number of side chains, and reduced stability. It is noteworthy that the xylose content of the pretreatment solution decreased under extreme treatment conditions such as T190t60 and T170t100. Pretreatment conditions that are elevated facilitate the degradation of xylose, but they also enhance the production of inhibitors. However, the glucose content in the pretreatment solution continued to increase, indicating that the majority of the non-cellulosic glucose and a small amount of cellulosic glucose were degraded and dissolved in the pretreatment solution with increasing pretreatment conditions, suggesting that the degradation of cellulose would occur at higher treatment conditions [60].
To elucidate the enhancing effect of microwave hydrothermal pretreatment on the degradation of hemicellulose, a comparison was conducted between the sugar component levels in the pretreatment liquor between microwave hydrothermal pretreatment and conventional hydrothermal pretreatment, both conducted at 170°C for 60 min. The results revealed that the sugar concentration in the microwave hydrothermal pretreatment solution was significantly higher than that observed in the conventional hydrothermal method. This suggests that the microwave hydrothermal approach may facilitate more efficient dissolution of hemicellulose, reduce the required reaction temperature and time, and enhance overall production efficiency.
|
Sample |
Xylose |
Glucose |
Arabinose |
Galactose |
Mannose |
Oligosaccharide |
|---|---|---|---|---|---|---|
|
g/L |
g/L |
g/L |
g/L |
g/L |
g/L |
|
|
T170t60* |
0.18 |
0.03 |
0.08 |
0.02 |
0.07 |
0.24 |
|
T170t60 |
3.24 |
0.10 |
0.36 |
0.32 |
0.42 |
5.73 |
In addition to monosaccharides and oligosaccharides, there will be other inhibitors in the pretreatment solution will contain other inhibitors produced by the microwave hydrothermal pretreatment process [58] These inhibitors will affect the conversion and utilization of the sugar components in the pretreatment solution. Therefore, it is necessary to explore the changes in the content of acetic acid, furfural, and 5-HMF under different microwave hydrothermal treatment conditions. The notable inhibitors generated by monosaccharides and their concentrations in this study are shown in Fig. 5a, b. Under mild treatment conditions, the quantities of acetic acid were relatively low. As the microwave hydrothermal treatment conditions were enhanced, the acetic acid content exhibited a notable increase, reaching levels of 0.35–16.74 mg/g and 2.45–12.99 mg/g, respectively. The findings revealed a notable elevation in the acetic acid concentration following the microwave hydrothermal pretreatment, which persisted with elevated temperatures and extended durations. This phenomenon may be attributed to the elevated rate of hemicellulose degradation resulting from the pretreatment conditions and the augmented organic acid content of organic acids in the pretreatment solution. This finding is corroborated by the pH testing results (Fig. 2). Consequently, the presence of acetic acid increases the cost of purified pretreatment solutions and impairs their subsequent application.
In the presence of elevated temperatures, the degradation of pentose and hexose, resulting from the breakdown of carbohydrates, is further accelerated under acidic conditions, leading to the production of furfural and 5-HMF. The concentration of 5-HMF and furfural increased in accordance with the intensification of treatment conditions. The generation pattern of 5-HMF is analogous to that of furfural, with the distinction that 5-HMF can be generated at lower temperatures, whereas furfural requires higher treatment conditions to start forming. Additionally, the concentration of furfural is significantly higher than that of 5-HMF. As shown in Fig. 5a, the production of furfural is negligible at temperatures below 150°C. However, at temperatures exceeding 180°C, the xylose present in the pretreatment liquid undergoes a significant degradation, leading to a rapid increase in furfural content. Despite the increase in temperature and duration of microwave hydrothermal treatment, the rise in 5-HMF was observed to be more gradual. This phenomenon may be attributed to the higher pentose content compared to hexose in Acacia wood. In summary, while intensifying the treatment conditions can facilitate the extraction of sugars and other composite components, it also leads to the generation of elevated levels of acetic acid, furfural, and 5-HMF. Therefore, it is essential to select suitable treatment conditions for the pretreatment of Acacia wood to minimize the formation of inhibitors.
During the course of the treatment, the degradation and dissolution of hemicellulose will result in the suspension or dissolution of small molecules of lignin in the pretreatment solution. The subsequent isolation and purification of the liquid in the sugar and other composite components has presented certain difficulties, and thus it is necessary to explore the effect of different microwave hydrothermal treatment conditions on the solubilization of lignin. Figure 6 shows the lignin content in the pretreatment solution under various hydrothermal pretreatment conditions. The data indicated that as the temperature and duration of microwave hydrothermal pretreatment were extended, the lignin content exhibited a progressive increase. The dissolved lignin content exhibited a notable increase, from 3.71 mg/g to 23.37 mg/g when the temperature was elevated from 120°C to 190°C. At a constant temperature, the dissolved lignin content increased from 11.62 mg/g to 21.34 mg/g with an extension of the holding time prolonged from 20 to 100 min. This phenomenon can be attributed to the microwave hydrothermal pretreatment, which resulted in the disruption of β-O-4 bonds within the lignin structure, a reduction in the molecular weight of the lignin, and the dissolution of the smaller molecular weight lignin.
3.3 Properties of residual wood chips after pretreatment
The impact of the pretreatment on the solid residues was observed by SEM. Prior to pretreatment, the surface of the chips exhibited a smooth and intact appearance, with a limited number of wrinkles (Fig. 7a). Both conventional hydrothermal and microwave hydrothermal pretreatments resulted in the disruption of the compact surface structure of Acacia wood. In comparison to the wood chips that underwent conventional hydrothermal treatment, the microwave pretreatment resulted in a more porous and looser structure of the Acacia wood chips (Fig. 7e, f). This phenomenon may be attributed to the breakdown of hemicellulose and lignin, which results in a more uneven surface on the wood chips, a greater degree of fragmentation, and the formation of cracks within the pore structure. At 190°C, the collapse of the wood chips surface became more pronounced, with the appearance of holes and the development of "droplets" on the surface. These "droplets" resulted from the acidic environment of the pretreatment affecting parts of the lignin structure. The fractured lignin is susceptible to condensation reactions, which result in the formation of deposits on the surfaces of the wood chips. These deposits have a detrimental impact on the penetration of the solution during the pulping process [61]. Therefore, it is essential to ensure that the pretreatment conditions are suitable to facilitate the desired outcome. The objective is to achieve a loose and porous wood chip structure that enhances the accessibility of the fibers, thereby facilitating the subsequent pulping process. It is important to note that excessive treatment conditions can have a counterproductive effect on the pulping reaction and potentially lead to unfavorable alterations in paper properties.
To gain further insight into the impact of pretreatment on the internal structural changes of Acacia logs, the pretreated and post-treated samples were subjected to non-destructive micro-CT scanning. The resulting data are presented in Fig. 8. The nano-microcomputed tomography images demonstrated that the raw Acacia wood exhibited a high degree of structural integrity, with the fibers displaying a high degree of connectivity. In contrast, following high-temperature microwave hydrothermal pretreatment, the edges of the wood chips exhibited significant fragmentation, with a notable reduction in overall structural integrity. The cross-sectional view clearly showed that the holes in the Acacia wood chips had enlarged, accompanied by extensive fragmentation of the edges. This structural alteration facilitates the penetration of chemicals from subsequent pulping into the wood chips.
The structural properties of the raw material and the solid resulting from different pretreatments were determined by FT-IR, and the results are shown in Fig. 9. The spectra display comparable signal profiles, yet the relative signal intensities exhibit variation, suggesting that the wood chips retain the same core structure as the feedstock across different pretreatments. A comparison of the characteristic absorption peaks in the IR spectra with those in other references reveals that the C-H stretching vibration peaks of the -CH₂- and -CH₃- groups appear at 2910 cm− 1, while the characteristic absorption peak of hemicellulose at 1740 cm− 1 corresponds to the C = O stretching vibration, primarily from the carbonyl or ester groups in xylan [62]. The aforementioned absorption peaks diminish following pretreatment due to the loss of acetyl groups from xylan, which results in the removal of hemicellulose. The aromatic ring backbone vibrational peaks of lignin's basic units and the C-H stretching vibrational peaks associated with specific lignin structures appear at 1600 cm− 1, 1510 cm− 1, 1422 cm− 1, and 1120 cm− 1 [63]. The intensities of these vibrational peaks undergo minor alterations following pretreatment, but the changes are minor, indicating that while lignin does undergo partial degradation, but the extent of solubilization is minimal. The peak at 1160 cm− 1 is the characteristic absorption peak of cellulose [64], and its intensity remains almost unchanged after pretreatment, suggesting that the degradation of cellulose by both treatments is minimal. These spectrogram results corroborate the dissolution behavior of the composite components mentioned above, confirming the significant removal of hemicellulose by both microwave hydrothermal pretreatment and conventional hydrothermal pretreatment of Acacia wood chips.
Moreover, to substantiate the influence of microwave hydrothermal heating on the physicochemical characteristics of Acacia wood, the solids prior to and following pretreatment were subjected to X-ray diffraction (XRD). Figure 10 represents the crystallinity indices of Acacia wood cellulose under different pretreatment conditions. As can be seen from Fig. 7c, all diffraction patterns exhibit distinctive peaks at 2θ = 18° (110 and 101 planes) and 22.4° (002 planes), indicating that neither the microwave hydrothermal pretreatment nor the conventional hydrothermal treatment resulted in any alteration to the crystalline structure of the cellulose. This observation is consistent with the retention of the crystal structure of cellulose I [65, 66]. As depicted in Fig. 10a, b, the untreated sample had a crystallinity index of 49.42%. The crystallinity index of cellulose in the residual solids demonstrated a notable increase with elevated microwave hydrothermal pretreatment temperature and extended pretreatment time. Specifically, the index rose from 50.22–68.87% with increasing temperature, and from 52.15–66.73% with extended pretreatment time. The pretreatment process resulted in the dissolution of the majority of the hemicellulose and a portion of the lignin, thereby exposing the cellulose and enhancing its accessibility. Conversely, the amorphous zone of cellulose is more susceptible to damage than the crystalline zone, resulting in an increase in crystallinity. In comparison to conventional hydrothermal pretreatment, microwave hydrothermal pretreatment of Acacia wood chips resulted in a higher degree of cellulose crystallinity, reaching up to 58.95% (Fig. 10c). The monosaccharide content in the pretreatment solution (Table 1) indicates that the microwave hydrothermal pretreatment resulted in greater dissolution of xylose and more extensive destruction of hemicellulose than the conventional hydrothermal pretreatment. This led to a more pronounced disruption of the amorphous zone, as evidenced by the elevated crystallinity index of the residual solids in the microwave hydrothermal pretreatment was relatively high.
3.4 Results of model construction
In order to gain deeper insights into the dissolution behavior of Acacia wood composite components during microwave hydrothermal pretreatment, a mathematical model was developed using lignin dissolution content as an illustrative example. Lignin concentration (Y) was designated as the dependent variable, with temperature (XT) and time (Xt) as the independent variables. Both linear and nonlinear relationships among the variables were examined. A total of twenty-two datasets regarding lignin leaching content were compiled, and the processed experimental data are detailed in Table 2.
|
Number |
XT |
Xt |
Y |
Number |
XT |
Xt |
Y |
|---|---|---|---|---|---|---|---|
|
1 |
120 |
20 |
0.5855 |
12 |
150 |
60 |
2.1091 |
|
2 |
120 |
40 |
0.7091 |
13 |
160 |
20 |
1.5925 |
|
3 |
120 |
60 |
0.7436 |
14 |
160 |
40 |
2.2045 |
|
4 |
130 |
20 |
0.8600 |
15 |
160 |
60 |
2.8500 |
|
5 |
130 |
40 |
1.0382 |
16 |
170 |
20 |
2.6818 |
|
6 |
130 |
60 |
0.7364 |
17 |
170 |
40 |
2.8727 |
|
7 |
140 |
20 |
0.9709 |
18 |
170 |
60 |
2.9455 |
|
8 |
140 |
40 |
1.2400 |
19 |
170 |
80 |
4.2364 |
|
9 |
140 |
60 |
1.2582 |
20 |
170 |
100 |
4.3000 |
|
10 |
150 |
20 |
1.3682 |
21 |
180 |
60 |
3.9636 |
|
11 |
150 |
40 |
1.5773 |
22 |
190 |
60 |
4.7091 |
Utilizing the data from Table 2, we developed a nonlinear equation that describes the relationship between lignin concentration and time and temperature, as shown in Eq. (4). The coefficient of the interaction term (0.0003) was minimal but discernible, indicating that the interaction of temperature and time exerts a slight influence on lignin concentration.
To validate the nonlinear least squares model, performance was evaluated using the mean square error (MSE) [67] and the coefficient of determination (R²) [68]. A lower MSE value indicates a superior model fit, whereas R² quantifies the degree of correlation between the regression line and the actual data points. An R² value of 1 indicates a perfect correlation between the predicted and observed data. In addition, an F-test [69] was employed to ascertain the suitability of the regression equation, and a model assessment was conducted to evaluate its overall significance of the model.
A combined three-dimensional plot of the actual values against the fitted curve was generated using MATLAB software to obtain Fig. 11. The amount of lignin dissolved was observed to depend on both time and temperature. As temperature and time increased, the amount of dissolved lignin also rose, which is consistent with the earlier described results. New data sets were employed to verify the generalization ability of the model as shown in Table 3.
|
Number |
XT |
Xt |
y |
Y |
Relative Error |
|---|---|---|---|---|---|
|
1 |
150 |
20 |
1.3818 |
1.3767 |
0.37 |
|
2 |
150 |
60 |
2.4045 |
2.4205 |
0.66 |
|
3 |
160 |
80 |
1.8591 |
1.8533 |
0.31 |
|
4 |
160 |
100 |
2.6682 |
2.6732 |
0.19 |
|
5 |
170 |
60 |
2.4727 |
2.4711 |
0.07 |
|
6 |
170 |
60 |
3.0682 |
3.0703 |
0.07 |
The MATLAB software effectively fits the provided experimental data through a least squares model, and the model demonstrates satisfactory performance on the training data set. The coefficient of determination (R2) is 0.9806, and the mean square error (MSE) is 0.0318. Furthermore, the regression was significant at the level of 0.01 (F = 161.6355, P < 0.05). The predicted results on the new validation data set were also relatively accurate, and the lignin leaching content y, theoretical content Y and relative error were shown in Table 3. The relative error between the actual value and the theoretical value is minimal, indicating that the model exhibits robust generalization ability. In the future, the model parameters can be further optimized, or additional sample data can be incorporated to enhance the stability and accuracy of the model.
3.5 Pulp and Paper Properties Analysis
Acacia wood chips, both before and after the pretreatment stage, underwent kraft pulping under identical conditions, resulting in high-quality pulp suitable for composite paper production following screening. Figure 12a shows that the yield of pulp exhibits an initial increase followed by a decline as the pretreatment temperature rises, maintaining consistent pulping conditions. Specifically, the highest pulp yield of 51.58% was achieved at a pretreatment temperature of 130°C. In the temperature range of 140 to 180°C, the yield exhibited a gradual decline. However, at temperatures exceeding 180°C, a precipitous decline in yield was observed. In comparison to the pulp produced from raw materials that had not undergone any form of pretreatment, the yield exhibited a reduction of approximately 17.55%. Additionally, the loss of fine pulp was observed to be more significant, indicating that the degradation of carbohydrates is more pronounced when the pretreatment conditions are excessively high. Therefore, it is imperative to select appropriate pretreatment conditions prior to pulping to effectively extract hemicellulose, which will positively impact the production of high-quality composite paper.
To investigate the effect of different microwave hydrothermal pretreatments on the properties of paper-based composites. The unbleached pulp was prepared and subsequently manufactured into paper with a quantitative weight of 100 g/m², and evaluated for its physical properties. The whiteness, tear index, ring compression index, tensile index, and breakage index of the paper are shown in Fig. 12. The pretreatment temperature has a significant impact on the physical properties of the paper. At lower treatment temperatures (120 ~ 140°C), the strength of the paper after microwave hydrothermal pretreatment was markedly superior to that of the untreated paper. The paper exhibited the highest mechanical properties when subjected to the T130t60 pretreatment condition. The tear index was found to be 7.36 mN‧m2/g, the tensile index was 102 N‧m/g, the breakage resistance index was 6.12 kPa‧m2/g, and the ring pressure index was 14.7 N‧m/g. A specific range of temperatures (130°C ~ 170°C) was found to result in a decrease in other properties, with the exception of whiteness, as the treatment temperature increased. The results demonstrate that at milder treatment temperatures, hemicellulose removal may lead to a looser structure of the wood chips, which can enhance cellulose utilization and facilitate the production of high-quality paper. If the temperature of the treatment is excessive, it will result in the removal of excessive amounts of hemicellulose, or even complete removal, and the hydrolysis of part of the cellulose. This will have an adverse effect on the cellulose in the subsequent pulping process, ultimately leading to a decline in the performance of the paper. Table 4 indicates that under identical pretreatment conditions, the properties of the paper resulting from conventional hydrothermal treatment were superior to those resulting from microwave hydrothermal treatment, contradicting the earlier findings regarding sugar component content in the pretreatment solution. This discrepancy may be attributed to the excessive heating or degradation of composite components during microwave hydrothermal treatment, factors that are likely to have had an adverse impact on the physical properties of the paper. The sugar content in the pretreatment solution was found to be essentially the same under microwave hydrothermal treatment at 140°C for 60 min as it was under conventional hydrothermal treatment at 170°C for the same duration. However, the paper properties obtained from the microwave hydrothermal treatment were found to be significantly better than those from the conventional hydrothermal treatment.
This result may be attributed to the distinctive characteristics of the microwave heating mechanism, which facilitates more uniform penetration of the material and enhances the efficacy of chemical and physical transformations. This leads to improved paper properties, making the pulp derived from microwave hydrothermally pretreated wood chips an excellent substrate for high-quality paper-based composites. In summary, microwave hydrothermal pretreatment of wood chips not only serves as an effective method for the removal and recovery of hemicellulose but also paves the way for optimal pulp yield and superior mechanical properties of paper when the treated chips are pulped. This process is crucial for advancing pulp quality and the development of paper-based composite materials in the paper industry.
|
Sample |
Tear index |
Tensile index |
Ring pressure index |
Breakage resistance index |
|---|---|---|---|---|
|
mN·m2/g |
N·m/g |
N·m/g |
kPa·m2/g |
|
|
Raw material |
6.10 |
92.58 |
10.99 |
5.5 |
|
T170t60* |
4.22 |
53.39 |
10.70 |
4.35 |
|
T170t60 |
3.80 |
50.60 |
10.09 |
2.45 |
In this study, an innovative microwave hydrothermal pretreatment technology was successfully introduced and optimized and applied to the pretreatment of Acacia wood chips. The effects of different microwave hydrothermal pretreatment conditions on the composite fraction of Acacia wood and subsequent pulping were investigated. The results indicated that microwave hydrothermal pretreatment effectively removed hemicellulose from wood chips and achieved a total xylose concentration of 15.69 g/L in the solution, which was superior to that of conventional hydrothermal pretreatment. The mathematical model established by the least squares method can effectively predict the changes in lignin dissolution during microwave hydrothermal pretreatment. Meanwhile, the use of a moderate pretreatment temperature improved the physical properties of the paper by 10% to 20%, revealing the unique advantages of low-temperature pretreatment strategies in improving pulp quality. This method provides a new idea for the comprehensive utilization of wood biomass resources and contributes to the green transformation and sustainable development of the paper industry.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Funding
This work was supported by the National Natural Science Foundation of China (No. 32230070), Natural Science Foundation of Shandong Province of China (No. ZR2021ZD38), Jinan Innovation Team (No. 2021GXRC023, 202228044), the QUTJBZ Program (No. 2022JBZ01-05), and the Taishan Scholars Program and Taishan Industrial Experts Program.
Data availability Data will be made available on request.
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No competing interests reported.