Endwise Degradation Research Articles

Further Insight into Carbohydrate Degradation and Dissolution Behavior during Kraft Cooking under Elevated Alkalinity without and in the Presence of Anthraquinone

The polysaccharide degradation and dissolution behavior during high liquor-to-wood ratio (200:1) kraft cooking of Scots pine wood meal was studied at high (1.55 M) and moderate (0.50 M) hydroxide ion concentrations at a constant sulfidity of 33%. Both alkalinity levels were studied in and without the presence of anthraquinone (AQ) (0.05, 0.15, and 0.25 g AQ/L). High alkalinity experiments without AQ at 130–160 °C clearly confirmed significant galactoglucomannan stabilization (in respect to lignin content) throughout initial and bulk delignification phases. Additionally, at high alkali compared to moderate alkali concentration, lower amounts of low molecular weight carboxylic acids originating from the degradation of carbohydrates were detected in spent black liquor. The presence of AQ provided significant hemicellulose stabilization against endwise degradation reactions, being more pronounced at moderate 0.50 M concentration than at 1.55 M hydroxyl ion concentration. In all cases, higher alkalinity promot...

Degradation of cellulose under alkaline conditions

A review of the important area of cellulose degradation under alkaline conditions is presented; it focuses on its relevance to the possible disposal of radioactive wastes in an underground repository in which cement-based waste encapsulation grouts and backfill may be employed. An overview of the alkaline degradation pathways of monosaccharides and substituted monosaccharides is initially presented, before progressing to the reactions involved in the alkaline degradation of cellulose, namely end-wise degradation, termination, alkaline scission, and oxidative alkaline degradation. Physical factors affecting reaction rates and the alkaline degradation of hemicellulose are also discussed. A review of the identity of the commonly detected alkaline degradation products (and their numerous synonyms) is presented, along with discussion of the rates of degradation of cellulose.

Time-dependent light scattering from singly and multiply stranded linear polymers undergoing random and endwise scission

The time-dependent light scattering signatures for singly and multiply stranded polymers of arbitrary shape undergoing random and endwise degradation are developed. For the case of random scission, useful generalizations at low scattering angle are found for linear polymers of arbitrary conformation and any initial polydispersity, which permit deductions to be made concerning the degree of association of the polymer (single, double, triple strand, mixtures, etc.), in addition to determination of absolute polydispersity-independent depolymerization rates. For the particular case of random scission of multiply stranded random coils the form factor is shown to be the Debye function whose usual argument is simply translated by a function dependent on the average number of cuts per original strand and on the number of strands and monomers per strand. The contrasting case of end-wise cleavage (e.g., ‘‘exocydic’’ enzymatic cleavage) is also presented.

Structural changes of LODP cellulose in the original and mercerized state during enzymatic hydrolysis

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Effect of anthraquinones on endwise degradation of hydrocellulose in relation to alkaline pulping

Anthraquinones promote early delignification and stabilisation of cellulose during alkaline pulping. Alkali-catalysed endwise depolymerisation of hydrocellulose was studied at 97°C under nitrogen. In the presence of sodium anthraquinone-2-sulphonate (AQS), the rates of both the chain-propagated unzipping reaction and the competing termination reaction were lowered. Since the former reaction was inhibited to a greater extent than the latter, less dissolution of cellulose occurred at this temperature. These findings are inconsistent with a mechanism that hypothesises coupling of an oxidative stabilisation of cellulosic chain-ends with chemical reduction of an anthraquinone catalyst. The inhibitory action of AQS at 97°C is ascribed to its possible action of diminishing the degree of ionisation of cellulosic reducing chain-ends. As a possible mechanism for the accelerated dissolution of cellulose at pulping temperatures, it is suggested that anthraquinones promote intrachain scissions in cellulose, enhancing the dissolution of chain fragments before unzipping. An anomalously high rate of peeling near neutrality is attributed to an uncatalysed reaction occurring simultaneously with the base-catalysed degradation.

End-Wise Degradation of Hydrocellulose in Mildly Alkaline Solutions and Its Retardation By Ammonia

Abstract At pH 11 and 140°C, the rate of end-wise degradation (“peeling”) of hydrocellulose is approximately 27 percent of that in 1 N NaOH. As the pH is reduced further, the rate of peeling declines by a factor of 0.34 per pH-unit. This decline is less than expected on the basis of end-group ionization. Consequently end-wise degradation can be a significant reaction even at neutral pH. The peeling rate is strongly retarded at pH 11 and below by the presence of ammonia and, to a lesser degree, by borate. The retardation is believed to be caused by the formation of Shiff's bases in case of ammonia.

Formation of aldonic acid end groups during NO2/O2 treatment and alkali cooking of wood meal

The increased carbohydrate yield after NaOH cooking of wood resulting from NO2/O2 pretreatment is related to the oxidation of reducing sugar end groups to aldonic acid groups during both the pretreatment and the subsequent alkali cook. This leads to a suppressed end-wise degradation during the alkali cook, reflected in a lowered formation of 3-deoxyaldonic and 2-C-methylglyceric acid end groups in competing reactions.

Endwise degradation of hydrocellulose in bicarbonate solution

Determinations of individual terminal carboxylic acid endgroups and terminal reducing sugar moieties together with analysis of spent liquor revealed that the same reactions occur during treatment of hydrocellulose with hot sodium bicarbonate as with sodium hydroxide solution. Some fragmentation reactions, of little importance in the presence of sodium hydroxide, are favored in bicarbonate medium while benzilic acid rearrangements are less favored. Hence, the formation of 2-dieoxyerythropentonic acid endgroups is more important. Among the soluble reaction products, 3,4-dideoxypentonic acid formed via 3-deoxypentulose and cyclic compounds formed via the same precursor are much more abundant in bicarbonate medium while 3-deoxy-2-hydroxymethylpentonic (isosaccharinic) acids are less abundant.

Galacturonic acid groups in birch xylan

A large number of reducing xylose end groups are present in birch wood (Betula verrucosa) while all galacturonic acid moieties are non-reducing. A large portion of the reducing xylose end groups are removed during treatments with alkali used for the islation of xylan, while reducing galacturonic acid moieties are formed by endwise degradation during this procedure.

Formation of 1,4-Anhydro-3-deoxypentitol-2-carboxylic Acids by Alkaline Degradation of Cellulose.

An anhydroisosaccharinic acid obtained in large amounts by end-wise degradation of cellulose in alkaline media, and in small amounts in hydrolysates of cellulose, has been identified as a 1,4-anhydro-3-deoxypentitol-2-carboxylic acid (3-hydroxy-5-(hydroxymethyl)oxolane-3-carboxylic acid). Structural evidence was obtained by GC-MS studies of the compounds (as Me3Si derivatives) obtained on degradation of the acid to 1,4-anhydro-3-deoxypentitol by reduction of the methyl ester followed by periodate oxidation and borohydride reduction. In aqueous alkali, the acid is likely to be formed from a dicarbonyl precursor by benzilic acid rearrangement.

End‐wise degradation of hydrocellulose during hot alkali treatment

AbstractDeterminations of reducing and carboxylic acid end groups in hydrocellulose treated with hot alkali showed that the end‐wise degradation proceeded to such an extent that some of the molecules were brought completely into solution. The major carboxylic acid end groups formed in reactions competing with the end‐wise degradation were 3‐deoxyhexonic and 2‐C‐methylglyceric acids; but 2‐deoxypentonic, 2‐C‐methylpentonic, and minor amounts of aldonic acids were also formed. The formation of these stable end groups explains the observed increase in number average degree of polymerization during the treatment. The ratio between the number of lost glucose moieties and the number of carboxylic acid groups formed in the solid phase was about 170 and was only slightly affected by the duration of the hot alkali treatment.

Marine plant polymers : Part III. A kinetic analysis of the alkaline degradation of polysaccharides with specific reference to (1→3)-β- D-glucans

A general kinetic expression is presented for the rate of alkaline degradation of linear polysaccharides in terms of mono- and di-anionic species formed from the reducing end-groups. Specific rate constants have been determined for the end-wise depolymerization of the (1→3)-β- D-glucans, laminaran, laricinan, and pachyman, and compared with similar data for amylose degradation. The rate constants of degradative chain-propagation via the mono- and di-anion intermediates have been shown to be essentially equal. The effect of the type and concentration of base on the mechanism of end-wise degradation is described for both (1→3)-linked and (1→4)-linked polysaccharides. Inhibition of alkaline degradation is discussed in terms of chain branching and blockage of reducing end-groups.

Kinetic study on the alkaline degradation of cotton hydrocellulose

AbstractThe kinetics of alkaline degradation of cotton hydrocellulose were determined in 5% NaOH at various temperatures. An activation energy of 24 kcal./mole was found for the endwise degradation reaction, while that of termination to a stable m‐saccharinic acid endunit is 32 kcal./mole. Consequently, the DPn of the degradable chain length is highly dependent on the reaction temperature, being 1000 at 65°C. and 140 at 132°C. At lower temperatures, the majority of degrading chains terminate to a normal reducing endgroup at the crystalline–amorphous transition region. From the data, the DPn of cellulose chain segments participating in the amorphous regions of the original cotton fibril was calculated to be 143. This result strongly supports the classical micellar fibril structure over the folded model proposed by Manley.