The kinetics of the spontaneous, proton- and AlIII-catalysed hydrolyses of the C1O4′ bond in 1,5-anhydrocellobiitol have been measured at elevated temperatures (125.0220.0 °C). Data for the first two processes extrapolate to the expression k = (8.6 ± 2.1 × 1016) + (1.4 ± 0.2 × 109-pH) s1 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 AlIII was observed at 150.0 °C in 0.1 mol/L succinate buffers of room temperature pH 3.05 and 3.35 (k2 = 8.1 ± 0.4 × 103 and 4.2 ± 0.2 × 103 (mol/L) 1 s1, respectively). The apparent activation energy of the AlIII-catalysed process was 31 ± 4 kJ mol-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 AlIII, directly acting as a Lewis acid, not the AlIII hydration sphere as a Brønsted 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.
1,5-脱
水赤霉糖醇的自发、质子和Al
III催化
水解的动力学在高温(125.0-220.0°C)下被测量。前两个过程的数据在25°C下外推到表达式k=(8.6±2.1×10
-16) + (1.4±0.2×10
-9-pH) s
-1。这些室温数据被用来模拟通过af Ekenstam方程进行的
纤维素降解。自发过程过于缓慢,不会对纸张老化时的强度损失产生贡献,即使是酸催化过程也只在pH 4.0以下显著。然而,自发
水解很容易解释碱性(例如,卡夫)制浆过程中
纤维素聚合度(DP)的降低。在室温pH 3.05和3.35的0.1 mol/L
琥珀酸盐缓冲液中,Al
III在150.0°C时表现出高效的电泳催化作用(分别为k
2=8.1±0.4×10
-3和4.2±0.2×10
-3(mol/L)
-1s
-1)。Al
III催化过程的表观活化能为31±4 kJ mol
-1,低于质子催化途径的表观活化能,这表明随着温度接近环境温度,电泳催化作用的重要性增加。因此,看来“
松香明胶”大小纸张不稳定的罪魁祸首是Al
III,直接作为
路易斯酸,而不是Al
III水合球作为布朗斯特酸。保护措施应该针对这一点或是通用的(例如,低温存储)。关键词:
纤维素、
水解、卡夫制浆、纸张保护、
松香明胶大小。