Lignins hydroxymethylation

Figure 2. Strength and wood failures obtained on beech strips glued with lignin hydroxymethylated for different times at pH 13 or 12.

Figure 8. DSC thermograms of lignin hydroxymethylated under different sodium to lignin ratios a) 1.0, b) 0.75, c) 0.5, and d) 0.25.

There is a possibiUty that (hydroxymethyl)phosphines might be catalyzing hydration of activated olefinic moieties in lignin. The Michael addition reaction shown in eq. (6a) is catalyzed by 5% THP in water at ambient conditions, with 70% conversion of the acrylonitrile no such reaction is seen with aciyhc acid or the methyl ester, but analogous hydromethoxylation of these compounds is seen in MeOH (42) (eq. (6b), R = H or Me). There is a report on similar catalytic use of tiialkylphosphines, which, like THP, are strong nucleophiles (43). 

As can be seen from Fig 2b, the solid state C-13 nmr spectrum of T. aestivum also shows sets of enhanced resonances at 61 ppm and 169.6-174.9 ppm respectively (24). However, their relative intensities are very different from that observed for L. leucocephala. Indeed, it can immediately be seen that very little reduction of the administered precursor to hydroxymethyl analogues (at 61 ppm) has occurred. On the other hand, the dominant resonances at 169.6 and 174.9 ppm are coincident with bound hydroxycin-namic acids (e.g. ferulic 5a) and its esters (31). Subsequent analysis of its isolated acetal lignin derivative (32) indicated that much of the lignin contained hydroxycinnamate residues (33). 

The etherified hardwood lignin model II reacted at a similar rate as the phenolic model indicating the etherification of the phenolic group has a small effect on the reaction rate. When this reaction was repeated at 55°C with an excess formaldehyde, some mefa-hydroxymethylated products were obtained (Fig. 4B). 

In the above paragraph it was shown that the lignin Cg units can be polymerized at the 2- and 6-positions with formaldehyde in acidic aqueous dioxane. The polymerization reaction proceeds presumably via a hydrox-ymethylated intermediate which was isolated in low yields only when large excesses of formaldehyde were employed. If the hydroxymethylation of the meta position can be achieved in high yield it would clearly afford a very... 

The three industrial lignins were subsequently reacted with formaldehyde under conditions where mainly mono-mefa-hydroxymethylation is expected. In 10% aqueous dioxane containing 0,4N hydrochloric acid (Fig. 7), the lignin reacted very fast and consumed about half of the theoretical amount of formaldehyde in one hour. 

Formaldehyde reacts with lignin at the 2- and 6-positions of the phenyl-propane medium in 10 to 20% aqueous dioxane containing 0.4 to IN hydrochloric acid, to afford meta hydroxymethylated intermediates. This results in a reactive mefa-hydroxymethylated lignin intermediate capable of being modified with various reagents or by reacting with itself. 

From the results presented it is evident that the 2- and 6-positions of the phenylpropanoid nuclei can be used for the modification of alkali lignin. The positions can be used for the controlled polymerization of alkali lignin. Secondly, the positions were also used to introduce hydroxymethyl groups. The hydroxymethyl groups are reactive towards nucleophiles such as phenol and resorcinol. The modification of the 2- and 6-positions of the C9 units holds tremendous potential for the utilization of alkali lignin in polymeric applications. 

Van der Klashorst, G. H. The Modification of Lignin at the 2- and 6-Positions of the Phenylpropanoid Nuclei. Part III. Hydroxymethylation of Industrial Alkali Lignin. J. Wood Chem. Technol., in press. 

B). Lignin contains carbonyl groups in various positions of the side chain. Hydroxymethyl groups can be introduced into the neighboring positions by substituting their activated hydrogen atoms (Tollens reaction). 

Table III. Evidence for Hydroxymethylation of Kraft Pine Lignin...

Hydroxymethylated lignin, when subjected to thermal cure, becomes insoluble in alkali. If we compare the DTA curves (P) of kraft lignin and methylol kraft lignin (Figure 5) we observe that in the latter a highly exothermic reaction started around 125°C., the material became stabilized over 190°C., and did not melt. Untreated kraft lignin is quite stable only... 

As shown by thioacidolysis of coniferyl alcohol, compounds 22 and 23 are formed from coniferyl alcohol end groups. They are recovered in larger amounts, relative to compound pair 11, from kraft lignin than from than from milled wood lignin. Compound 24 originates from Q.C-, alkyl aryl ether structures with a-carbonyl substituents via elimination of the Cy hydroxymethyl group (Lapierre 1986, Lapierre et al. 1987). 

Fig. 6.7.4. fl-5 and fi- lignin models, ozonation to 2-hydroxy-3-hydroxymethyl-butanedioic acid (Habu et al. 1990, Tsutsumi et al. 1990)... 

Lignin was obtained from an industrial soda bagasse spent liquor as before (5). The lignin was initially evaluated as a thermosetting adhesive by the beech strip test. Prior to its application as adhesive, the lignin was reacted with formaldehyde in alkali at temperatures below 60 °C to afford a hydroxymethylated lignin (fi). The hydroxymethylation reaction was done at pH 12 and 13, and samples of the reaction mixtures were evaluated on beech strips with overlaps of 25 x 25 mm, cured for 4 hours at 90 °C and 12% equilibrium moisture content. 

The final density of the boards was approximately 700 kg/m3. Different ratios of commercial PF and UF resins were added to hydroxymethyl at ed lignin. 

Table III. Beech Strip Strengths of Hydroxymethylated Bagasse Lignin Adhesives Crosslinked with Various Fortifiers...

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