Abstract

Materials

Whatman filter paper (No. 2) was used as cellulose substrate, cut into 0.5 by 0.5-cm pieces. P. radiata bark was provided by Sawmilling Co. Ltd. located in Christchurch, New Zealand. The bark was dried in a convection oven at 60°C ± 1°C for 48 hours and ground in a Wiley mill, and the 20- to 80-mesh-size bark powder collected was used for this study. All chemicals were reagent grades obtained from commercial suppliers.

Preparation of HWE of pine bark

The bark powder (600 g, ovendried [o.d.] weight basis) was extracted with 6 liters of deionized (DI) water at 100°C by occasional stirring for 1 hour, followed by filtering through a cartridge with a 3-μm-pore-size filter. The filtrate was evaporated on a large-scale rotary vacuum evaporator (N-12; Eyela, Tokyo, Japan) at 60°C to an approximately 2-liter volume, followed by freeze-drying, and then the residues were dried in a vacuum desiccator over P₂O₅, yielding 120 g of reddish brown powder, HWE.

Acid-catalyzed phenol liquefaction procedure of cellulose and HWE mixtures

The model liquefaction procedure was carried out basically following our previously reported acid-catalyzed phenol liquefaction procedure of P. radiata bark, wherein 2.5 g of bark and 5 g of phenol, 2.67 mmol PTSA or 1.33 mmol SA, were reacted at 140°C to 150°C (Mun et al. 2006). Because the cellulose and HWE contents in P. radiata bark were approximately 18 and 30 percent, respectively (Ku et al. 2007), the representative substrate mix in this work was chosen to have this ratio, i.e., 0.45 g of cellulose and 0.75 g of HWE based on reactions of 2.5 g of P. radiata bark. Other mix ratio samples were 0 to 0.75 g of HWE and 0.45 g of cellulose.

Five grams of phenol and acid catalysts (1.33 mmol SA or 2.67 mmol PTSA) was added into a 50-mL round-bottom flask that was then immersed in an oil bath (PEG #400) preheated to a given liquefaction temperature. When the liquefaction chemicals dissolved, 0.45 g of cellulose and 0 to 0.75 g of HWE were added into the flask and the starting time of liquefaction began. When the liquefaction reaction was ended, the resulting reaction mixture was cooled and diluted with 70 mL of the dioxane-water mixture (80/20, vol/vol), and the insoluble residues (liquefaction residues) were separated by filtration with a 1G4 glass filter. The residues were dried in a convection oven overnight at 105°C. Liquefaction yields were calculated as follows:

Most of the liquefaction yields presented in this article were calculated using the above equation, but the liquefaction yields in Figure 1 were based on cellulose only.

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The liquefaction products in dioxane-water filtrates, and the liquefaction residues, were analyzed further for combined phenol content, cellulose content, and acid-soluble and acid-insoluble polyphenol contents as described in the next sections. It is noted that the same phenol liquefaction reactions and same analytical procedures were carried out for raw P. radiata bark in our previous work (Mun et al. 2006), and part of the results were incorporated into the results of the present work to obtain a more complete comparison.

Analysis procedures of dioxane-water filtrates and liquefaction residues

Determination of acid-insoluble and acid-soluble polyphenols in liquefaction residues

The dioxane-insoluble liquefaction residues (100 mg) from the liquefaction reactions described above were treated with 1.5 mL of 72 percent SA at room temperature for 4 hours by stirring once every 30 minutes and then the mixture was diluted to 3 percent SA by adding 55 mL of DI water. The diluted solution was further hydrolyzed in an autoclave (JS-AC60; Johnsam, Buchun, Kyungkido, South Korea) at 120°C for 1 hour. The hydrolyzate was filtered through a 0.45-μm-pore-size membrane, and the acid-insoluble residues were vacuum dried overnight and weighed. These acid-insoluble residues were defined as the acid-insoluble polyphenols in percentages:

The acid-soluble polyphenols were then determined first by diluting the hydrolyzate removed of the acid-insoluble polyphenols to 100 mL with DI water and then analyzing the solution for polyphenol contents using an UV/Vis spectrophotometer (8452A; Hewlett-Packard, Palo Alto, California) at 290-nm wavelength. The content of polyphenols in the solution was calculated from the standard curve obtained from the various concentrations of HWE at 290 nm. The results were expressed as HWE equivalents/liquefaction residue (g).

Influence of HWE on the acid-catalyzed phenol liquefaction of cellulose

To investigate the influence of the bark polyphenol components on the acid-catalyzed phenol liquefaction of the bark, HWE of bark was used as bark polyphenol substitutes. The liquefaction reactions were carried out at 140°C for 20 minutes, which is milder than that of previous bark liquefaction reactions (Mun et al. 2006) in order to verify the influence of HWE and to analyze the liquefaction residues available from the milder reactions.

The liquefaction yields are plotted in Figure 1 against the amounts of HWE-to-cellulose ratios of samples for both SA and PTSA catalysts. The yields decreased with the increasing amounts of HWE, indicating that polyphenols seriously interfere with the acid-catalyzed phenol liquefaction of cellulose. The decreasing trend of liquefaction yields was more pronounced for SA catalyst than for PTSA catalyst; i.e., liquefaction yields for SA catalyst were more negatively affected by HWE than yields for PTSA catalyst. The colors of the liquefaction residues for SA catalyst were gradually changed from white to dark brown with increasing amounts of HWE (reddish brown by itself), but those obtained for PTSA changed from white to slightly pinkish even with the highest amount of HWE. This deep brown color of liquefaction residues observed for SA catalyst also appears to indicate that condensation reactions between cellulose and polyphenols occur more extensively with SA than for PTSA catalyst.

The effects of the polyphenol components on the phenol liquefaction of cellulose in the presence of acid catalyst were investigated further by carrying out reactions at 143°C and 150°C for 5 to 60 minutes. Because the contents of cellulose and HWE in P. radiata bark were reported to be about 18 and 30 percent, respectively (Ku et al. 2007), the corresponding ratio sample (0.45 g of cellulose/0.75 g of HWE) was used for liquefaction. The liquefaction yield results of these experiments shown in Figure 2 indicate that liquefaction yields rapidly increase with increasing liquefaction time for both SA and PTSA catalysts. However, the liquefaction yields increased with reaction times more markedly for PTSA catalyst than for SA catalyst, the difference becoming greater as the liquefaction time increased from 10 to 60 minutes at 143°C. The liquefaction yields at 150°C were all higher for PTSA than for SA catalyst at all liquefaction times. These results are comparable to those reported on the phenol liquefaction of raw bark (Mun et al. 2006), as shown in Figure 3. Overall, it is concluded that the PTSA catalyst causes fewer condensations between cellulose and polyphenols in the phenol liquefaction reactions than the SA catalyst.

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Combined phenol contents in liquefied products

Because the phenol used as liquefying solvent can react with decomposition products of cellulose (Lin et al. 2004, Mun and Jang 2009), the combined phenol content in liquefaction products provides an important index of efficiency for acid-catalyzed phenol liquefaction reactions. The combined phenol contents from liquefaction of all cellulose–HWE mixture samples and bark, determined by GC analyses shown in Figure 4, show that they increase with increasing liquefaction times for both SA and PTSA catalysts. However, at the same liquefaction times, the combined phenol contents for PTSA catalyst were always higher than those obtained for SA catalyst. The results also agreed with the results obtained with raw bark (Mun et al. 2006).

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The combined phenol contents are also plotted against liquefaction yields in Figure 5 at 150°C of reaction temperature, where the two values are shown to linearly increase for both SA and PTSA catalysts. However, in the case of the bark, the combined phenol contents for SA catalyst were always 4 to 5 percent higher than those for PTSA catalyst at the same liquefaction yields. In the case of cellulose–HWE mixture samples, the combined phenol contents for SA catalyst at liquefaction yields above 83 percent were always higher than those for PTSA catalyst as well.

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The higher combined phenol contents at the same liquefaction yield were also observed at the phenol liquefaction of cellulose (Mun and Jang 2009). In the SA-catalyzed phenol liquefaction of cellulose, the combined phenol content was 50 percent higher than that for PTSA catalyst. This indicates that SA catalyst leads to a higher phenolation and liquefaction yield than PTSA catalyst during the phenol liquefaction of cellulose. However, the SA-catalyzed phenol liquefaction reaction was disturbed when the bark and cellulose with bark polyphenols were used as liquefaction substrates in spite of a higher combined phenol content than the PTSA catalyst. This result shows that phenolation is a necessary reaction in liquefaction, but an excessive phenolation during liquefaction of bark and cellulose with bark polyphenols can slow down the liquefaction reaction.

Chemical composition of the residues from cellulose–HWE phenol liquefaction reactions

The unliquefied residues in the phenol liquefaction reactions could give useful information on the condensation reactions between cellulose and polyphenol components. The residues were acid hydrolyzed to analyze cellulose and acid-soluble and acid-insoluble polyphenols. The analysis results are shown in Table 1.

Table 1. Analysis results of the liquefaction residues obtained from phenol liquefaction of cellulose–hot-water extract (HWE) mixtures in the presence of p-toluene sulfonic acid (PTSA) and sulfuric acid (SA) catalysts.^a

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Past research indicates that most polyphenols in the bark are condensed and precipitated as acid insoluble material with lignin when lignin in the bark is determined by using 72 percent SA (Ku et al. 2007), but some of the polyphenols are expected to dissolve in the acid. The UV spectral results of HWE, phenol, and acid-soluble fraction of liquefaction residues are shown in Figure 6. Because more than 80 percent of HWE by weight is proanthocyanidins (polyphenols), HWE shows the typical UV absorption maxima at 280 nm (Ku and Mun 2007, Ku et al. 2011). The acid-soluble fractions also show similar UV spectra and absorption maxima with those of HWE, while phenol shows a slightly different maximum at 270 nm. Further, the absorption maxima difference is only 10 nm, and the phenol peak is broad and tends to reach out to 290 nm, overlapping the spectral region of the acid-soluble fractions. Therefore, the determination of the acid-soluble polyphenol contents was carried out by using the 290-nm absorption values to minimize the influence of phenol that might be produced from the combined phenol of liquefaction residues through acid hydrolysis.

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The results of acid-soluble polyphenol contents obtained from this UV analysis shown in Table 1 indicate relatively constant values in the range of 2.4 to 2.7 percent for SA catalyst for all liquefaction times. In the case of PTSA catalyst, the acid-soluble contents were lower in the range of 1.7 to 2.1 percent. Furthermore, the acid-insoluble polyphenol contents also show similar trends, but they were all about three times greater than the acid-soluble polyphenol contents for both catalysts.

On the other hand, the cellulose contents in the acid-insoluble residues for PTSA catalyst were always higher than those for SA catalyst at the corresponding liquefaction times. Consequently, it is clear that polyphenol components cause retardations of acid-catalyzed phenol liquefaction reactions of cellulose–HWE mixtures because of their condensation with the cellulose components and also that PTSA catalyst reduce the condensation reactions and thus more efficiently catalyze phenol liquefaction reactions.