Catalytic selectivity in alcohol organosolv pulping of spruce wood
Creators: name: Yawalata, Dominggus
Publisher: name: University of British Columbia
Year: 2009

description: Development of a more efficient and environmentally friendly pulping process is seen as a necessity to cope with problems currently faced by pulp and paper industries. The organosolv pulping process seems to be the right choice to deal with the problems. Although many organosolv pulping process variants have been developed and are at various stages of commercialization, most processes are incapable or struggle with pulping softwoods. Furthermore, employment of a catalyst in organosolv pulping liquor appears to be essential for softwood pulping. However, systematic studies of catalyst effectiveness, especially for softwood pulping by organosolv means, are lacking. Therefore, the objectives of this research were to investigate the effect of catalysts on pulping behavior, selectivity, pectin removal and delignification leading to fiber separation in a catalysed alcohol organosolv pulping process. Under the specified pulping conditions, it was found that not all catalysts used in the organosolv pulping of softwood were able to liberate the fibers. The pulping outcomes were different, depending on the catalyst species. Different species of cations and anions had an impact on cooking liquor pH that subsequently controlled the pulping process and its outcomes. Generally, the pH of the cooking liquor controlled the pulping selectivity and fiber separation. Furthermore, pulping selectivity and topochemistry, and not necessarily delignification alone, control both fiber liberation and fiber quality. For high fiber yield and quality, control of the final pH between 3.8 and 4.2 is a necessity. In some cases, the use of two different catalysts in the cooking liquor showed a synergistic effect on the improvement of fiber liberation. It is concluded that neither mono- nor trivalent acidic inorganic salts, nor simple organic acids in themselves, except citric acid, can be used as catalysts for effective fiber liberation in organosolv pulping, due largely to the uncontrolled drop of pH below 3.5, at which point delignification becomes significantly retarded. The mystery of the unique neutral alkali earth metal (NAEM) salt catalyst effectiveness is now fully explained and described. Investigation of NAEM catalysed pulps showed total loss of arabinose and galactose from among the carbohydrates of the pulp as if their presence controlled the process of fiber liberation. Early fiber liberation resulting in high pulp yield and viscosity can only be accomplished with a cooking liquor possessing high chemical and topochemical selectivity for lignin. In this respect, effective removal of the middle lamella compounds is seen as the key factor for fiber liberation. On the other hand, the removal of middle lamella compounds is found to be an antagonistic process, in which the removal of esterified pectin preferred less acidic pH due to the P-elimination mechanism while more acidic cooking liquor (pH &lt;4.5) was required to enhance and accelerate delignification. At low pH (&lt;3.8), however, carbohydrate degradation and removal are also enhanced due to hydrolysis. The enhancement of carbohydrate degradation and loss grows with increasing cooking liquor acidity, while delignification decreases or even stops. The removal of middle lamella compounds, to liberate the fibers, basically followed 2 different pathways: 1) the removal of pectin and some hemicelluloses, followed by delignification, as in the neutral alkali earth metal salt catalysed process, and 2) the removal of hemicelluloses followed by delignification and pectin removal, as in the citric acid catalysis. It was also found that in all organosolv cooks, fibers can not be completely separated without removing all arabinose and galactose. Extending the cooking beyond the fiber liberation point provides no benefit at all, because the extent of additional delignification is small and is attained at the expense of carbohydrate degradation and loss, and lignin re-precipitation. Therefore, highest yield is obtained if delignification is halted at the fiber liberation point. Unexpectedly, the effectiveness of citric acid as a catalyst was found to be comparable to that of the NAEM salt catalysed organosolv pulping process. In both NAEM salt and citric acid catalysed organosolv pulping with 80 % methanol, the loss of hemicelluloses comprising xylose and mannose increased when the degree of delignification exceeded 60 %. The mechanism of fiber liberation in the NAEM salt catalysed organosolv pulping process can now be described as "versatile" being able to affect both initial removal of the pectin and hemicelluloses (arabinogalactan) followed by bulk and residual delignification. NAEM salt catalysts therefore play a double role : 1) effectively promote the initial pectin removal at pH &gt;5, and 2) effectively buffer the cooking liquor against uncontrolled pH drops as more and more protons are liberated due to ion exchange with carboxyls and liberation of acetyls. The role of pectin dissolution in controlling effective fiber liberation in organosolv pulping of softwood is described for the first time. The topochemistry of pectin removal from spruce wood was followed by immunolabelling followed by fluorescent microscopy of thin cross-sections.