Lignin alkaline conditions

Electrophiles other than protons were indeed shown to react with lignin model compounds at the 2- and 6-positions in acidic media Kratzl and Wagner investigated the reaction of paraphenolic benzyl alcohols in alkaline and acidic solutions (10). When 4-hydroxy-3-methoxybenzyl alcohol 4 was reacted with the 4-alkyl substituted phenol 5 under alkaline conditions, the expected ortho linked product (6) was isolated. However, under acidic conditions the methylene linkage formed meta to the phenolic hydroxy group (Fig. 2). 

Figure 6. Suggested mechanism for the reaction of ortho-quinonoid structures in lignin derivatives with hydrogen peroxide under alkaline conditions (according to Gierer, ref. 3).

It is obvious that several options exist for the synthesis of lignin-based epoxide resins. The use of ECH under alkaline conditions seems to be favored. The degree of epoxidation is controlled by the amount of catalyst... 

Thus, similarities in color formation exist between the treatment of hardwood meal with alkali and the treatment of syringyl alcohol with alkali. In either instance, when oxygen is absent or suppressed, color formation is minimized. It may be concluded then, that oxygen is necessary for color formation when wood is pulped under alkaline conditions, and that units of the lignin component can be responsible for the color. 

Freudenberg (77, 15, 76, 17) studied the formation of formaldehyde from lignins under acidic and alkaline conditions and suggested that cohydroxymethyl groups may be the source of formaldehyde. Kratzl 24)... 

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. 

Table III shows the results of such a study. The variation in the nature of the lignin degradation products with initial pH follows the same general pattern as observed using aspen wood—i.e., an increased percentage of the phenylpropane over the phenylpropanol derivative in acid medium and the almost sole production of phenylethane products under alkaline conditions. The total yield of identified products, based on original Klason lignin, is significantly less than that for aspen (Table I).

Coniferyl alcohol and coniferaldehyde structures. It has been estimated that spruce lignin contains about 6% of coniferyl alcohol units as end groups (28). In previous work, the behaviour of coniferyl alcohol structures during a mild sulfite treatment simulating the chemical pretreatment in a CTMP process was studied (75). It was found that coniferyl alcohol is liberated from spruce wood more rapidly under acidic than under neutral or alkaline conditions. The treatment time needed for complete liberation of coniferyl alcohol structures from wood lignin was, however, found to be more than 100 minutes irrespective of treatment conditions. Considering the short treatment time (5-15 min.) employed in the commercial process, it can... 

The pre-extraction of wood or pulp is routinely performed to remove extraneous substances which would otherwise interfere with the determination. However, as emphasized by Lai and Sarkanen (1971), any advantage gained through inclusion of a pre-extraction step may be offset as a result of extractants such as hot water and ethanol removing a portion of the lignin. According to Brauns (1952), the hot water extraction may be omitted in the case of chemical pulps and the ethanol-benzene extraction may be disregarded for pulps produced under alkaline conditions. 

Another factor which may influence the yield of acids obtained from lignin in wood is the presence of easily hydrolyzable aryl ether linkages such as those represented by phenylpropane-a-aryl ether structures In phenolic units, such structures are easily cleaved even under very mild alkaline conditions (Adler et al 1968) Therefore, it can be assumed that, in the alkylation step of the oxidative degradation procedure, a partial hydrolysis of such structures may occur, thereby creating new phenolic end groups that may affect the analysis... 

Pepper JM, Rahman MD (1987) Lignin and related compounds XI Selected degradation of aspen poplar lignin by catalytic hydrogenolysis Cellul Chem Technol 21 233-239 Rahman RD, Pepper JM (1988) Lignin and related compounds XII Catalytic degradation of proto and isolated aspen lignins under initially alkaline conditions J Wood Chem Technol... 

Mechanical pulps are bleached by procedures that preserve lignin, but eliminate most coloured groups. Most mechanical pulps can be bleached to brightness levels of 75-80%, that are suitable for uncoated and coated papers. The principal bleaching procedures for mechanical pulps are reductive bleaching with sodium dithionite (hydrosulphite), and oxidative bleaching with peroxide under alkaline conditions. 

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