The kinetics of the spontaneous, proton- and [Al.sup.III]-catalysed hydrolysis of 1,5-anhydrocellobiitol--models for cellulose depolymerization in paper aging and alkaline pulping, and a benchmark for cellulase efficiency (1)

The kinetics of the spontaneous, proton- and [Al.sup.III]-catalysed hydrolyses of the C1--O4' bond in 1,5-anhydro-cellobiitol have been measured at elevated temperatures (125.0-220.0°C). Data for the first two processes extrapolate to the expression k = (8.6 ± 2.1 x [10.sup.-16]) + (1.4 ± 0.2 x [10.sup.-9-pH]) [s.sup.-1] at 25°C. These room-temperature figures were used to model cellulose depolymerization by the af Ekenstam equation. The spontaneous process is too slow to contribute to loss of paper strength on aging, and even the acid-catalysed process is significant only below ~pH 4.0. However, the spontaneous hydrolysis readily accounts for the reduction of cellulose degree of polymerization (DP) during alkaline (e.g., kraft) pulping of cellulose fibres. Efficient electrophilic catalysis by [Al.sup.III] was observed at 150.0°C in 0.1 mol/L succinate buffers of room temperature pH 3.05 and 3.35 ([k.sub.2] = 8.1 ± 0.4 x [10.sup.-3] and 4.2 ± 0.2 x [10.sup.-3] (mol/L)-1 [s.sup.-1], respectively). The apparent activation energy of the [Al.sup.III]-catalysed process was 31 ± 4 kJ [mol.sup.-1], lower than that of the proton-catalysed path, suggesting the electrophilic catalysis increases in importance as the temperature approaches ambient. Consequently, it appears that the culprit in the impermanence of 'rosin-alum'-sized paper is [Al.sup.III], directly acting as a Lewis acid, not the [Al.sup.III] hydration sphere as a Bronsted acid Conservation measures should either address this or be generic (e.g., low-temperature storage). Key words: cellulose, hydrolysis, kraft pulping, paper conservation, rosin-alum sizing. Resume : Operant a des temperatures elevees (125,0 a 220,0°C), on a mesure les vitesses des reactions d'hydrolyse, spontanee et catalysees par le proton ou l'aluminium(III), de la liaison C1--O4' du 1,5-anhydrocellobiitol. Les donnees pour les deux premiers processus conduisent a l'expression k = (8,6 ± 2,1 x [10.sup.-16]) + (1,4 ± 0,2 x [10.sup.-9-pH]) [s.sup.-1] extrapolee a 25°C. On a utilise ces valeurs a la temperature ambiante comine modele pour la depolymerisation de la cellulose par l'equation de af Ekenstam. Le processus spontane est trop lent pour contribuer a une perte de la force du papier avec l'age; le processus catalyse par un acide n'est significatif qu'a des pH inferieurs a environ 4,0. Toutefois, l'hydrolyse spontanee permet d'expliquer facilement la reduction du degre de polymerisation (DP) de la cellulose durant le traitement alcalin (kraft) des fibres de cellulose. On a observe une catalyse electrophile efficace par l'aluminium(III) a 150,0°C, dans des tampons de succinate, a la temperature ambiante et a des pH de 3,05 et 3,35 ([k.sub.2] = 8,1 ± 0,4 x [10.sup.-3] et 4,2 ± 0,2 x [10.sup.-3] [(mol/L).sup.-1] [s.sup.-1] respectivement). L'energie d'activation apparente pour le processus catalyse par l'aluminium(III) est de 31 ± 4 kJ [mol.sup.-1], inferieure a celle du processus catalyse par le proton, ce qui suggere que la catalyse electrophile augmente en importance lorsque la temperature tend vers la temperature ambiante. En consequence, il semble que le coupable de la non permanence du papier collage a la rosine et a l'alun serait l'aluminium(III) qui agirait directement comine acide de Lewis et non pas la sphere d'hydratation de l'aluminium(III) qui agirait comine un acide de Bronsted. Les mesures de conservation devraient tenir compte de ces conclusions ou etre generiques, tel qu'un entreposage a basse temperature. Mots cles : cellulose, hydrolyse, pulpage alcalin, conservation du papier, traitement a la rosine et a l'alun. [Traduit par la Redaction]
Introduction The loss of mechanical strength of cellulose fibres with age is an important limiting factor in the preservation of paper documents and, to a lesser extent, textiles. The degradation [...]