TG/DTG profile in terms of component degradation
The TG/DTG curves measured for the holocellulose samples prepared from cedar and beech woods are illustrated in Fig. 2. Delignification lowered the temperature range in which weight loss occurred for both woods. In particular, the DTG peak temperatures were substantially lowered from 379 °C to 341 °C for cedar and from 381 °C to 353 °C for beech. This indicates that the cellulose in wood becomes very reactive when lignin is removed, because crystalline cellulose was more thermally stable than amorphous hemicellulose and degraded around the DTG peak temperature [1]. The shapes of the DTG curves for cedar and beech woods are different because of the different degradation behaviors of cellulose and hemicellulose [1]; these components degrade together in cedar but independently in beech. Removing lignin changed the shape of the DTG curve for cedar to that for beech: a shoulder can clearly be observed below the peak temperature.
To explain the TG/DTG profiles, the thermal degradation reactivities of hemicellulose and cellulose in holocellulose were determined according to the recovery rates of hydrolyzable sugars from heat-treated samples and compared with those of wood samples in previous reports [1, 2]. The reactivities of xylan and glucomannan were directly determined from the recovery rates of xylose and mannose, respectively, because these are the characteristic constituent sugars of these hemicelluloses. However, glucose is produced from cellulose and glucomannan, so the cellulose reactivity was determined according to the cellulose-derived glucose. This was estimated by subtracting the amount of glucose formed from glucomannan under the assumption that the mannose:glucose molar ratio in glucomannan is 3:1 [30, 31] and that both units have the same thermal degradation reactivity.
The amounts of cellulose, xylan, and glucomannan remaining in the pyrolyzed holocellulose were estimated based on the recovery data of the hydrolyzable sugars and their contents in each wood type; the corresponding results are plotted against the pyrolysis temperature in Fig. 3. By comparison with the DTG curves, the weight loss behavior during the heating process can be explained in terms of the degradation of cellulose and hemicellulose; this is because the same heating rate as that in the TG analysis was used with no heating time at a constant temperature.
As indicated by the TG analysis, cellulose reactivity increased when lignin was removed. Although wood cellulose degraded continuously and gradually as the pyrolysis temperature was increased, the cellulose degradation in holocellulose was divided into two modes depending on the pyrolysis temperature. Degradation started at 260 °C–280 °C, and the reactivity increased sharply above 320 °C. In the low-temperature degradation mode (260 °C–280 °C), hemicellulose degraded together with cellulose. Consequently, the polysaccharide components that degraded at the DTG shoulder and peak temperatures could not be clearly separated into hemicellulose and cellulose. Most hemicellulose and 20%–25% of cellulose decomposed below the DTG shoulder temperature (around 320 °C) for both wood types, but the remaining cellulose degraded around the peak temperatures (above 320 °C) after the hemicellulose degraded.
Reactivities of isolated and wood polysaccharides
Figure 4 shows the recovery rates of hydrolyzable sugars plotted against the pyrolysis temperature, with holocellulose normalized as 100%. The results for isolated hemicellulose [1] and Whatman cellulose are also included for comparison. These plots can be used to discuss the cellulose and hemicellulose reactivities in holocellulose as compared with those of the original wood and isolated samples.
Whatman cellulose is an example of pure cellulose; it withstood heating up to 320 °C and then degraded rapidly at higher temperatures. The low-temperature cellulose degradation mode as observed for holocellulose at 260 °C–320 °C was not detected. Accordingly, holocellulose is characterized by this low-temperature cellulose degradation, which may be caused by matrix degradation as discussed later. This phenomenon appears when lignin is removed from wood. It should be noted that the degradation behaviors of cellulose were different in cedar and beech woods but similar in their holocellulose samples.
As described in the previous paper [1], the hemicellulose reactivity in wood was different from those of isolated hemicelluloses, and the reactivity varied depending on the type of hemicellulose and wood. Xylan in wood was less reactive than isolated xylan, but glucomannan in beech was more reactive than isolated glucomannan. The high reactivity of glucomannan was explained by the presence of 4-O-MeGlcA near glucomannan in beech [5]. Removing lignin changed their reactivities to be similar to those of isolated xylan and glucomannan. A similar trend was observed for 4-O-MeGlcA, as shown in Fig. 5. Thus, lignin plays an important role in determining the thermal reactivities of hemicellulose and cellulose in wood; this role is probably due to the restraints of the specific locations of xylan, glucomannan, and 4-O-MeGlcA in the cell walls as discussed later. Removing lignin increases the mobility of these components in holocellulose.
The data in Figs. 4 and 5 are rearranged in Fig. 6 to understand the differences depending on the wood type. Although the temperature ranges at which cellulose and hemicellulose decomposed were similar, the shapes of the graphs differ for cedar and beech holocellulose. With cedar holocellulose, the recovery–temperature relationships of mannose and xylose show similar trends. Two reflection points can be observed at 260 °C and 300 °C; this indicates that the reactivity changed at these temperatures. This tendency is not observed for beech holocellulose. Thus, these results indicate that xylan and glucomannan degraded synchronously in cedar holocellulose, and the degradation can be divided into three types depending on the reactivity: < 260 °C, 260 °C–300 °C, and > 300 °C. The recovery–temperature relationship of cellulose-derived glucose indicates that the degradations of cellulose and hemicellulose occurred synchronously in cedar. The cellulose degradation started around 260 °C, at which point approximately half of the xylan and some glucomannan decomposed. After the rapid degradation of the remaining hemicellulose around 300 °C–320 °C, cellulose quickly degraded at 320 °C–340 °C. Therefore, the characteristic thermal degradation behaviors observed for cedar were maintained when lignin was removed. Unlike for beech holocellulose, the cellulose degradation is intimately related to the hemicellulose degradation in cedar holocellulose.
Role of lignin
On the basis of the present results, the role of lignin in wood pyrolysis is discussed using a schematic of a single cellulose microfibril surrounded by a matrix (Fig. 7), although further studies are necessary to confirm the following proposal. Xylan, glucomannan, and 4-O-MeGlcA are anchored in specific locations within the matrix [1, 5]. Meanwhile, 4-O-MeGlcA and its salts act as acid and base catalysts, respectively [2], which increase the thermal degradation reactivity of adjacent components. However, previous experimental results [1, 2] indicated that the catalytic activity is not effective in wood. For example, 4-O-MeGlcA is bound to xylan, but the xylan was very stable in both wood samples. Rather, 4-O-MeGlcA influences the glucomannan degradation in beech wood. All these features disappeared in holocellulose. These results can be reasonably explained by considering the role of lignin, which physically strengthens the ultrastructure formed between hemicellulose and the cellulose interface in the matrix. The formation of LCC linkages [11,12,13] may also be involved in this process.
The thermal degradation of cellulose is known to occur nonuniformly at the crystallite level [32,33,34]; surface molecules preferentially tend to decompose as internal molecules are more stable owing to stabilization by filling the crystallites. Accordingly, the reactivity of surface molecules plays an important role in the thermal degradation of cellulose [35,36,37], and the matrix and its degradation are expected to affect the reactivity of cellulose microfibrils by affecting the surface cellulose molecules at the interface. This may induce cellulose degradation particularly in the low-temperature degradation mode of cellulose, which was observed in both types of holocellulose in the temperature range of 260 °C–320 °C (Fig. 4).
To understand the effect of low-temperature cellulose degradation (260 °C–320 °C) on the high-temperature cellulose degradation (> 320 °C), TG analysis was conducted for holocellulose samples at different heating rates of 1, 5, and 10 °C/min (Fig. 8). By decreasing the heating rate, TG and DTG curves shifted to lower temperature side. Although researchers try to explain these shifts with the time lag in temperature measurement [38], heating rates and sample weight (1 mg) used in the present TG analysis are very small to account for such a large shift (DTG peak temperature: 342 °C → 326 °C → 297 °C (10 °C/min → 5 °C/min → 1 °C/min) for cedar holocellulose, 357 °C → 338 °C → 307 °C (10 °C/min → 5 °C/min → 1 °C/min) for beech holocellulose).
By redrawing the TG/DTG curves in Fig. 8 with the unit of weight-loss rate changed from mg/min to mg/°C, the appearance of TG/DTG curves becomes very similar (Fig. 9a). This is confirmed by Fig. 9b, where TG/DTG curves are moved with respect to the temperature axis so that the peaks of the DTG curves are aligned. Surprisingly, these graphs match well. These results lead to a hypothesis; low- and high-temperature modes of cellulose degradation are closely related. Although the temperature range where thermal degradation of cellulose occurs is different depending on the heating rate, once thermal degradation of cellulose begins in low-temperature mode, this determines the subsequent cellulose degradation including high-temperature mode. This hypothesis gives insights in understanding cellulose pyrolysis, although further studied are necessary to confirm it.