4.2 Pyrolysis of ASA pulps
4.2.3 Exploratory data analysis
4.2.3.6 Comparison of the two delignification series . 117
CHAPTER 4. DISCUSSION OF RESULTS
0 5
10 15
20 25
0.0530 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
Lignin (%)
Abundance / lignin content
series A series B
Figure 4.44: Peak areas of homovanillin marker ion m/z 164, normalized by the lignin content and plotted against the lignin content.
The β-O-4 linkages are the primarily attacked lignin structures and are de-graded steadily in the course of delignification. In contrast, homovanillin may be derived from lignin moieties involved inβ-O-4 structures. Accordingly, in figure 4.40 a steady decline of the homovanillin fraction can be noticed.
0 30 60 90 120 150 180 210 240 270 300 330 0
0.05 0.1 0.15 0.2
cooking time (min)
Abundance
series A series B
Figure 4.45: Peak areas of methanol marker ion m/z 32 plotted against cooking time.
CHAPTER 4. DISCUSSION OF RESULTS
In section 4.2.3.5 it was contemplated that homovanillin may derive from phenyl propane units under pyrolytic cleavage of the γ-carbon releasing methanol. Although methanol may derive from several sources within lig-nocellulosic biomass figure 4.41 suggests that lignin is the main source of methanol detected. When the peak areas of the marker ion (m/z 32) are plotted against the cooking time the decreasing trends of the abundance of methanol match considerably well to the trends of delignification for both series, depicted in figure 4.4 in section 4.1.3. It can be noticed that the abundances for series B are throughout higher in comparison to the corre-sponding series A stage, even at the last cooking stages. It is believed that methanol is a very good indicator for phenyl propane units present in lignin.
Extensive demethoxylation reactions are not expected to have occurred, par-ticularly not under the fairly mild cooking conditions in the first stages of series B. Here the methanol abundances were highest which indicates that pyrolytic cleavages of the γ-carbon of propane side chains are the primary source of methanol.
0 5
10 15
20 25
30 0.2 0.25 0.3 0.35
Lignin (%)
Abundance
series A series B
Figure 4.46: Peak areas of 3-hydroxybenzaldehyde marker ion m/z 121 plotted against lignin content.
In softwoods only low proportions (3 - 5 %) of p-hydroxyphenyl (H) lignin units are found. Compared to S- and G-lignin units these are known to show a higher degree of condensation and to be more resistant towards degra-dation. Because of the overall low abundance of these moieties they have been hardly considered in pulping studies on wood. In particular when
Py-GC/MS is employed for analysis care may have to be taken to choose phenol, methylphenols or dimethylphenols as markers for H-lignin, even though they are usually the most abundant H-lignin components found in pyrolysis data of pulps. It has been reported that these phenolic components may also de-rive from carbohydrates (Moldoveanu 1998). In figure 4.42 the extracted ion for 3-hydroxybenzaldehyde is plotted against the lignin content. It can be observed that the abundance stayed more or less constant in the course of the cooking series, only a slight increase is noticeable. If the product was derived from polysaccharides it would be expected to increase towards the end of the delignification.
0 5
10 15
20 25
30 0 0.02 0.04 0.06 0.08 0.1
Lignin (%)
Abundance / lignin content
series A series B
Figure 4.47: Peak areas of 3-hydroxybenzaldehyde marker ion m/z 121, normalized by the lignin content and plotted against lignin content.
When the ratio of 3-hydroxybenzaldehyde to overall lignin content is reviewed a more or less exponential increase of the H-lignin fraction becomes apparent (figure 4.43). Several H-lignin monomers, including 4-hydroxybenzaldehyde and phenol, showed a similar trend. For some H-lignin monomers, however, the trend was less pronounced. One example depicted in figure 4.44.
It could not clearly be verified whether the phenolic compounds, tentatively assumed as H-lignin markers, were derived from lignin or polysaccharides.
But the low correlation of these compounds with well identified products
CHAPTER 4. DISCUSSION OF RESULTS
0 5
10 15
20 25
0.0330 0.04 0.05 0.06 0.07 0.08 0.09 0.1
Lignin (%)
Abundance / lignin content
series A series B
Figure 4.48: Peak areas of 4-methylphenol marker ion m/z 107, normalized by the lignin content and plotted against lignin content.
derived from polysaccharides supports the assumption that these products originate from lignin.
In figure 4.45 the trend of the overall most prominent pyrolysis product is il-lustrated. It was discussed before (4.2.3.5) that samples with similar cellulose content may show considerable differences in the abundance of levoglucosan.
From the plot it can be concluded that these differences are rather due to structural or compositional differences between the samples than to random scatter of the abundance. Only the replicates of one sample of series A showed a considerably low reproducibility of the levoglucosan (depicted by the large errorbar).
Some further details of interest could be derived from the pyrolysis data. As already discussed in section 4.1.3 the rapid decline of the pH in the cooks of series B was attributed to the almost exhaustive cleavage of the acetyl groups of the glucomannans. Figure 4.46 illustrates that Py-GC/MS is well suited to monitor the fate of the acetyl groups. Already after the first cooking stage only approx. 15 % of the initial abundance of acetic acid could be found.
All subsequent stages showed a fairly constant level of acetic acid. It can be noticed that in the first 5 cooking stages the abundances for series B
0 5
10 15
20 25
30 0 500 1000 1500
Lignin (%)
Abundance
series A series B
Figure 4.49: Peak areas of levoglucosan marker ion m/z 60 plotted against lignin content
0 30 60 90 120 150 180 210 240 270 300 330
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
cooking time (min)
Abundance
series A series B
Figure 4.50: Peak areas of acetic acid marker ion m/z 43 plotted against cooking time
CHAPTER 4. DISCUSSION OF RESULTS
were slightly higher. This may be attributed to comparably mild cooking conditions and the higher glucomannan content.
0 30 60 90 120 150 180 210 240 270 300 330
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
cooking time (min)
Abundance
series A
series B SO
2 m/z 48
Tmax
Figure 4.51: Peak areas of sulfur dioxide marker ion m/z 64 plotted against cooking time.
It has been shown by van Loon et al.(1993) that sulfur dioxide may be used as an indicator for the contents of sulfonic acid groups in lignosulfonates.
Also for pulps Py-GC/MS has proven to be very suitable to monitor the degree of sulfonation. At a pyrolysis temperature of 500◦C SO2 is the only major pyrolysis product associated with sulfonic acids. When the abundance of SO2 is plotted against cooking time the difference in the degree of sulfona-tion within the first 6 cooking stages becomes apparent (figure 4.47). These differences were already discussed in section 4.1.3.2. When the errorbars are reviewed a high reproducibility of the marker ion for SO2 can be assumed.
Another interesting observation was the detection of anthraquinone in the pulps. In figure 4.48 the abundance of the marker ion (m/z 180) is plotted against cooking time. It can be noticed that AQ was not detected in the pulps of the last 3 cooking stages. A further observation was that both series showed distinct systematic differences in the abundances of AQ. In series A the abundance of AQ increased more rapidly than in series B. The maximum abundance was reached directly after the heating-up period and rapidly declined hereafter. Additionally, the maximum abundance was nearly 2-fold higher than the maximum in series B. In series B a slower and more
0 30 60 90 120 150 180 210 240 270 300 330
−2 0 2 4 6 8 10 12 14x 10−3
cooking time (min)
Abundance
series A series B
Tmax AQ m/z 180
Figure 4.52: Peak areas of anthraquinone marker ion m/z 180 plotted against cooking time.
moderate increase and decline of AQ in pulp could be perceived. These observation may be explained by the differences of the pH profiles and the rates of delignification in the course of the two delignification series. The high NaOH concentration in series A promoted the swelling of the fibers and the penetration of the chemicals. The implication of this observation may be that the more rapid delignification in the first stages of the A series was due to both, higher NaOH concentration and quicker AQ penetration.
CHAPTER 4. DISCUSSION OF RESULTS