Characterization and TG analysis of ball-milled wood
The intensity of the XRD signals originating from cellulose crystallites decreased with increasing milling time for both cedar and beech wood samples (Additional file 1: Fig. S1); this phenomenon has also been reported previously [15,16,17, 28]. The crystallinity index calculated from the X-ray diffractograms using the 002 lattice diffraction decreased from 68 to 5% after ball milling for 1–2 h (Fig. 2).
Although there is less information regarding the effect of ball milling on lignin than on cellulose, some literature reports [12, 14] determined that ball milling reduced the degree of polymerization of lignin and increased the phenolic structure by cleaving the ether linkages. Therefore, the β-ether structures remaining in the ball-milled wood samples were quantified using the thioacidolysis method to evaluate the effect of ball-milling time on the cleavage of the β-ether bonds (the most abundant type of linkage in lignin). Thioacidolysis involves ethanethiol-assisted, acid-catalyzed solvolysis and leads to the formation of trithioethyl monomeric products through the cleavage of β-ether bonds. The derivatization followed by reductive cleavage (DFRC) method has also been used for quantitative analysis of the β-ether linkages, but Holtman et al. [29] reported that thioacidolysis was a better strategy for evaluating the β-ether linkages in ball-milled wood.
The relative yields of thioacidolysis products, namely guaiacyl (G)-type in cedar and G- and syringyl (S)-types in beech, are plotted as a function of the milling time in Fig. 3. These yields (normalized relative to 100% for unmilled wood) decreased as the milling time increased, and were about 20% after 4 h for both woods. This indicates that the β-ether linkages were cleaved during the ball-milling process, and that the efficiency was similar for cedar and beech woods, but slightly higher for cedar. No significant difference was observed between the S and G types in beech wood.
The TG and DTG profiles obtained for the ball-milled cedar and beech woods are presented in Figs. 4 and 5, respectively. The TG/DTG curves of the ball-milled woods for the 10 min and 4 h experiments are shown in Fig. 6 to represent typical examples and for comparison with the unmilled wood. As the ball-milling time increased, the TG curve shifted toward lower temperatures and tended to level off after 4 h of milling. The characteristics of the DTG curves of cedar and beech were largely unchanged; the beech DTG curve has a distinct shoulder, but cedar has one wide peak.
After a very short (10 min) milling time, the temperature ranges in which the TG curves changed were different for cedar versus beech samples; the change occurred near the DTG shoulder temperature for beech, but near the DTG peak temperature for cedar. Accordingly, the DTG peak tended to shift more for cedar, whereas the DTG shoulder shifted for beech. Specifically, the DTG peak temperature shifted from 374 to 365 °C (cedar), or 367 to 362 °C (beech), and the DTG shoulder temperature of beech shifted from 320 to 302 °C. These observations indicate that some heat-resistant cellulose in the cedar wood became reactive after short-term ball milling, while the hemicellulose was influenced to a greater extent in the beech wood.
When the milling time was extended to 4 h, the TG curves of both woods shifted toward lower temperatures, within the wider temperature range of 250–400 °C. The DTG peak temperatures decreased more significantly and became similar (cedar = 354 °C and beech = 355 °C). This result indicated that their cellulose reactivities became similar after 4 h of milling, although unmilled cedar cellulose was more stable. The ball-milled wood also exhibited improved weight-loss rates in the temperature range near the DTG shoulder in beech, thus changing the shape of DTG curve.
Polysaccharide reactivity
The amount of unreacted hemicellulose and cellulose remaining in 4-h-ball-milled wood after pyrolysis was evaluated based on the hydrolyzable sugars, and the results are plotted against the pyrolysis temperature in Fig. 7. The recovery rates are shown as normalized values relative to the contents in unmilled wood [30]. The quantities of xylan and glucomannan were determined based on the yields of methyl xyloside and methyl mannoside, respectively, obtained after methanolysis. The amount of cellulose-derived glucose in the hydrolysate was calculated by subtracting the yield of glucose derived from glucomannan, which was obtained from the methyl mannoside yield by assuming that (i) the mannose:glucose ratio in glucomannan was 3:1 [31, 32] and (ii) their thermal reactivities were the same.
The recovery data are compared with the DTG profiles in Fig. 7. Because the heating conditions applied in the pyrolysis experiments were similar to those used for TG analysis, the results from these two types of experiments can be directly compared. Previously reported results for the unmilled woods [3] are also included for comparison.
In the 4-h ball-milled samples, some of the cellulose degraded at temperatures lower than 320 °C, whereas the cellulose in unmilled cedar and beech woods was relatively more stable in this temperature range. Therefore, the cellulose in ball-milled wood tended to degrade in two different temperature ranges. A similar trend was reported for holocellulose [6], but the degradation in the lower temperature range was greater for ball-milled wood. In this work, it was determined that 39% and 38% of the cellulose in ball-milled cedar and beech woods, respectively, degraded below 320 °C; the remaining cellulose degraded at higher temperatures around the DTG peaks. This result is consistent with the characteristics of the DTG curve of ball-milled beech wood, i.e., the shoulder intensity increased significantly. Although the DTG shoulder was not clearly observed for ball-milled cedar wood, the weight-loss rate at temperatures below 320 °C increased.
It is interesting to note that ball milling and delignification have similar effects on cellulose reactivity in wood, although the ball-milling process does not remove any components from the wood. About 80% of the β-ether linkages were cleaved after ball milling for 4 h (Fig. 3), suggesting that the enhanced thermal reactivity of cellulose may be related to the cleavage of lignin ether linkages, rather than the formation of pores in the cell wall matrices due to lignin removal. Loosening the cell wall structure by cleaving lignin chains is considered a potential reason for this observation, and this relationship is discussed further below.
The recovery rates of hydrolyzable sugars derived from cellulose, xylan, and glucomannan in ball-milled wood (after 10 min and 4 h) were compared with those of pure cellulose (Whatman CF-11), isolated xylan, and isolated glucomannan, respectively, to better understand the polysaccharide reactivity in ball-milled wood (Fig. 8). The results of 4-h-ball-milled pure cellulose are also included for comparison. The recovery data are summarized for each ball-milled wood in Fig. 9.
In the case of cedar wood, even after a short milling time of 10 min, the recovery rates of hydrolyzable sugars from xylan and glucomannan decreased greatly, although their contributions to the weight loss were small (Fig. 6). Interestingly, clear discontinuities were observed at 300 °C for mannose and xylose in the cedar wood, indicating that xylan and glucomannan in ball-milled cedar wood are divided into two parts with different reactivities; 62% of xylan and 57% of glucomannan degraded at temperatures < 300 °C, and the remaining portions degraded at higher temperatures. A similar trend was observed for the thermal degradation of cellulose in ball-milled cedar wood (4 h), as described earlier, although the discontinuous temperature observed for xylan and glucomannan (300 °C) was slightly lower than that of cellulose (320 °C). This trend is clearly visible in Fig. 9.
The aforementioned features were only observed for cedar samples, so they may be related to the thermal degradation characteristics specific to cedar wood, in which cellulose degrades together with hemicellulose. The close assembly of xylan, glucomannan, and cellulose microfibrils (surface molecules) was demonstrated based on the conversion of metal salts to free 4-O-MeGlcA [5]. In general, the thermal degradation behaviors of cellulose and hemicellulose presented in the current study support such assembly.
The crystallinity of cellulose in cedar wood decreased from 68 to 51% following 10 min of milling, and some of the lignin β-ether linkages were cleaved (Figs. 2 and 3). Such modifications improved the mobility of the assembled xylan, glucomannan, and cellulose in cedar wood, resulting in enhanced thermal degradation and weight reduction at temperatures around the DTG peak where cellulose degraded (~ 370 °C, Fig. 6). The recovery data presented in Fig. 8 confirm the improved cellulose reactivity in this temperature range.
The recovery data for the 10-min-ball-milled beech wood (Fig. 8) indicate that the reactivities of xylan and cellulose increased. This is consistent with the changes in the TG/DTG profiles shown in Fig. 6, in which the TG/DTG curves shifted in the wide temperature range between 250 and 400 °C. In contrast, the glucomannan reactivity decreased (Fig. 8). This is also reasonably explained by the improved mobility of the matrix. The high glucomannan reactivity observed in beech wood was explained based on the 4-O-MeGlcA located near glucomannan [5], although it is bound to the xylose chain in xylan. The improved mobility of the cell wall matrix may diminish this effect.
When the milling time was increased to 4 h, the reactivity of xylan in beech wood increased, and xylan and glucomannan in both woods degraded in a similar temperature range, which was much lower than that of isolated glucomannan (Figs. 8 and 9). This is interesting, because isolated xylan bearing 4-O-MeGlcA moieties (salts or free carboxyls) was more reactive than isolated glucomannan because of the base or acid catalysis of 4-O-MeGlcA [4]. Therefore, ball milling should make the matrix components more homogeneous, thus allowing 4-O-MeGlcA to influence most of the hemicellulose components equally. This is an aspect of ball-milled wood that was not observed for holocellulose pyrolysis, where the thermal degradation reactivities of xylan and glucomannan were similar to those of isolated xylan and isolated glucomannan, respectively. This is likely because the delignification process did not allow the matrix components to mix efficiently.
The recovery rate of 4-O-MeGlcA is shown in Fig. 10. The thermal degradation reactivity of 4-O-MeGlcA in holocellulose was improved to a level similar to that of isolated xylan [6]; however, the improvement was not very significant in ball-milled wood, particularly for beech. This may be related to the ester linkages formed with lignin, which may be resistant to the ball-milling process, but further investigations are necessary to better explain this enhanced stability.
As discussed earlier, cellulose in wood degraded in two stages (some at temperatures below 320 °C and some at higher temperatures). In contrast, ball-milled Whatman cellulose degraded only in one mode corresponding to the high-temperature degradation of ball-milled wood. Therefore, the low-temperature degradation is characteristic of cellulose in wood. The-low-temperature-mode degradation would be explained by matrix-induced degradation as discussed later.
Role of ball milling on thermal reactivity of wood polysaccharides
The impact of ball milling on the polysaccharide reactivity of wood cell walls is discussed here using a schematic image of the interface between cellulose microfibril and hemicellulose–lignin matrix depicted in Fig. 11. Cellulose microfibrils play an important role owing to their crystalline nature. Cellulose molecules inside the crystallites are more stable than the surface molecules because of the packing in the crystallites [33], so the reactivity of the surface molecules is a critical factor for initiating the thermal degradation of bulk cellulose [34,35,36]. The difference between cellulose in wood and pure cellulose lies in this feature. Therefore, the reactivity of the matrix and its influence on the surface cellulose molecules must be considered to comprehensively understand the thermal degradation of cellulose in wood cell walls.
Our group’s previous work [3] indicated that the xylan and glucomannan reactivities are significantly influenced by the matrix construction. The arrangement of these molecules (including 4-O-MeGlcA, which has a catalytic effect) is precisely determined in wood cell walls, and it is different in cedar (softwood) versus beech (hardwood) [5]. Because these characteristic reactivities disappeared following the removal of lignin, the physical restraining effect of lignification during cell wall biosynthesis is considered to be the main reason for these distinctions [6]. Overall, the matrix is considered to be rigid and tightly associated with cellulose microfibrils in unmilled wood cell walls. Due to this tightly coagulated structure, hemicellulose and surface cellulose molecules in wood are stable for thermal degradation. The mobility must be sufficient to rearrange these polysaccharides into the transition state of thermal degradation reaction.
After the ball-milling process, the crystallinity index of cellulose in wood decreased significantly, indicating that the matrix and the interface with the cellulose microfibrils was disturbed in such a way to improve the mobility. Cleavage of the lignin β-ether linkages diminishes the physical restraining effect and should help further improve the mobility of the matrix components. These modifications would improve the thermal degradation reactivity of xylan and glucomannan. In addition to the improved mobility, the ball-milling process created a more homogeneous distribution of 4-O-MeGlcA, which improves the reactivity of glucomannan to similar levels of xylan degradation due to the catalytic effect of 4-O-MeGlcA. This feature is not the case for holocellulose, in which xylan and glucomannan have the same reactivity as isolated xylan and glucomannan. Therefore, xylan and glucomannan in both cedar and beech holocelluloses exist without affecting each other.
Unlike beech wood, short (10-min) ball milling significantly improved the reactivity of xylan and glucomannan, and the reactivity of cellulose was also improved. These results may correlate with the characteristic thermal degradation of cedar wood; cellulose, xylan and glucomannan degrade together like one ingredient [3]. Xylan and glucomannan may strongly coagulate with the surface molecules of cellulose microfibrils. This is also supported by the fact that delignification and the subsequent removal of xylan are necessary to isolate glucomannan from softwood [37]. Due to the rigid nature of crystalline cellulose microfibrils, ball milling disrupts the interface more efficiently.
Through loosening the matrix and the interface by ball milling, the surface cellulose molecules of cellulose microfibrils may become reactive and degrade in the low-temperature range in two modes of cellulose degradation that is characteristic of cellulose in holocellulose and ball-milled wood. The 4-O-MeGlcA groups and other matrix components and their thermal degradation products may induce the cellulose degradation in this temperature range through the action on the surface cellulose molecules. This proposed mechanism provides a reasonable explanation for why the thermal degradation of cellulose proceeds in two stages.