Isolated lignins

Solution Properties. Lignin in wood behaves as an insoluble, three-dimensional network. Isolated lignins (milled wood, kraft, or organosolv lignins) exhibit maximum solubiUty in solvents having a Hildebrand s solubiUty parameter, 5, of 20.5 — 22.5(J/cm ) (10 — ll(cal/cm ) > and A// in excess of 0.14 micrometer where A]1 is the infrared shift in the O—D bond when the solvents are mixed with CH OD. Solvents meeting these requirements include dioxane, acetone, methyl ceUosolve, pyridine, and dimethyl sulfoxide. 

The peracetic initiator has been successfully applied also for grafting onto rayon fibers and cellophane22. The peracetic initiator has been further developed by Hatakeyama2 who combined the peracetic acid with Fe2+ or Fe + ions to a redox system found to be an effective initiator for grafting to pulps and isolated lignin. The reactions... 

Attempts to isolate lignin by degrading wood with brown rot fungi afforded only degraded lignins of low molecular weight (<1000) [cf. 33, 94, 132)] similar to Brauns soluble lignin. This is probably due... 

Lignin-derived compounds have been observed in pyrolysis products of low rank coals and associate woody tissue FI- 61. Mycke and Michaelis [2] isolated lignin-derived methoxyphenols from a Miocene coal by catalytic hydrogenolysis. Sigleo [S] reported the presence of phenolic compounds derived from iteration of lignin in pyrolysis products of silicified woody tissue as old as Triassic age. However,... 

It therefore follows that when isolated lignins (and suberins) are examined and subsequent structural representations are proposed, critical information on native structure has already been lost, e.g., as regards the extent of polymer modification during removal from the cell wall, and the effect of mixing polymers from the various cell wall layers from which they originated. For these reasons, all current representations of native lignin (and suberin) structure should be viewed with caution until such questions are satisfactorily resolved. 

At this point, it must be emphasized that, based on previous structural analysis of isolated lignins (27), appropriate lignin model compounds and synthetic dehydrogenatively polymerized (DHP) lignin preparations (28-... 

The use of fuming hydrochloric acid has received more attention. Willstatter and Zechmeister found that cellulose dissolved readily in 42-45% hydrochloric acid. Their discovery was made the basis for the Willstatter method for isolating lignin. Wohl and Krull obtained a 60.9% yield of apparent D-glucose when dry pine shavings were dissolved in fuming hydrochloric acid and allowed to stand for five hours at 20 C. 

Given the sensitivity of colligative methods to the presence of low molecular weight impurities, particular care was taken to isolate lignin samples free of such foreign contaminants. Thus, for example, the extraction... 

Data concerning the elemental composition and methods for isolating lignin from the wood structure, as well as the mean molar masses and distributions of the obtained fragments, are given in Table II (1,5). 

Hydrolysis of lignin in acidic and basic media has received attention due to the rather few and simple degradation products obtained. Acid-catalyzed hydrolysis reactions applied to isolated lignin have been studied by a number of workers. Lundquist (1), for example, subjected Bjorkman lignin to acidolysis and obtained significant yields of monomeric products. A review of the work prior to 1971 has been made by Wallis (2). 

The view that the lignin molecule was made up of building stones with a guaiacyl-syringyl propane skeleton was supported by the brilliant work of Hibbert on ethanolysis and that of Adkins and Harris on the pressure hydrogenation of wood and isolated lignin preparations. 

Infrared spectra of isolated lignins exhibit an absorption band in the region 1700-1720 cm.-1 While doubt still exists as to precisely which groups in dioxane or milled wood lignins are responsible for this absorption, clearly 16) there is at least some contribution from nonconjugated keto groups in the 0-position of the phenylpropane side chain. 

Lignocellulose or isolated lignin has been oxidized, using various reagents, and many different oxidation products have been reported. To compare previous work with our results, Tables I and II present a survey of oxidation products reported in the literature in which the oxidants we studied—namely, nitrobenzene, oxygen, and nitric acid—were used. The review has been limited to these oxidants but includes certain products from related oxidations such as hydrogen peroxide or ozone, or from nitrating nitric acid solutions. The products isolated from the different oxida-... 

Under alkaline conditions isolating lignin degradation products which are essentially of a phenyl.ethyl rather than a phenylpropyl nature is structurally important and requires a lignin structure by which the 7-carbon may be removed as a result of a 0-7 carbon-carbon cleavage reaction, either by direct alkaline hydrolysis or alkali-catalyzed hydrogenolysis. 

We now wish to report our results on isolating lignin-derived dimers by alkaline hydrogenation and to propose mechanisms for their formation. In addition, we will present some information on the (NMR) spectra of these and related compounds this technique has been useful for analyzing the structure of lignin degradation products and related synthetic materials. 

Distillation from a brush-type molecular still separated the isolated lignins into three fractions containing alkyl-substituted monomeric phenols in the most volatile fraction, a mixture of monomeric phenolic alcohols and alkyl-substituted dimeric phenols in the intermediate fraction, and nonvolatiles which presumably contained dimeric phenolic alcohols and higher molecular weight material. Unfortunately, separation of monomeric alcohols and alkyl-sbustituted dimers was not complete, and this greatly hampered separation of the alkyl-substituted dimers in later chromatographic separations. 

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