@article{venica_chen_gratzl_2008, title={Soda-AQ delignification of poplar wood. Part 1: Reaction mechanism and pulp properties}, volume={62}, ISSN={["1437-434X"]}, DOI={10.1515/HF.2008.118}, abstractNote={Abstract
               Soda and soda-anthraquinone (AQ) pulpings of poplar (Populus deltoides) wood performed in a flow-through reactor gives rise to the formation of coniferyl alcohol (1) sinapyl alcohol (2) and other low molecular weight (LMW) compounds in different stages of delignification. During the heating-up period, the formation of these compounds increases until the maximum pulping temperature (t
                  max, 170°C) is reached. Afterward, their concentration in the spent liquor decreases sharply. This effect is accompanied by solubilization of high molecular weight (HMW) lignin fractions. Compared with soda pulping, the soda-AQ pulping shows a higher production of LMW compounds because of the presence of AQ-anthrahydroquinone (AHQ) redox system. During the heating-up period, the formation of 1 and 2 – apparently originated from non-etherified β-aryl-ether moieties in the lignin – is accompanied by solubilization of lignin fragments with relative LMW. Lignin cores, having β-aryl-ether bonds as major hydrolyzable inter-unit linkages, remain in the fiber, and they give rise to solubilized lignin fragments with HMW at later delignification stages. The dissolved fragments, in particular those formed during heating-up period, undergo further degradation in the liquor. The degradation occurs via oligomers at the end of the pulping, while the residual lignin cores are more resistant. Pulps with low kappa number can be produced by maintaining higher alkali concentrations than the usual in the last delignification phase. When the pulping liquor is replaced in this stage with liquors of higher alkali concentration, extended delignification will be the result. As a consequence, less degraded lignin fragments will be deposited on fibers.}, number={6}, journal={HOLZFORSCHUNG}, author={Venica, Alberto D. and Chen, Chen-Loung and Gratzl, Josef S.}, year={2008}, month={Nov}, pages={627–636} }
 @article{venica_chen_gratzl_2008, title={Soda-AQ delignification of poplar wood. Part 2: Further degradation of initially dissolved lignins}, volume={62}, ISSN={["1437-434X"]}, DOI={10.1515/HF.2008.119}, abstractNote={Abstract
               Degraded lignin fragments dissolved in pulping liquor during the soda AQ pulping of poplar (Populus deltoides) wood with a flow through reactor were divided in two fractions. The first one (LF-1) was collected during the heating-up period (100°C until the final temperature t
                  max=170°C); the second one (LF-2) was collected at t
                  max with 60 min cooking time. These two liquors were then treated at 170°C for 70 min (leading to a liquor LF-1A) and 10 min (leading to a liquor LF-2A). By these post-treatments, pulping liquors of batch pulpings were simulated. From the four liquors, lignins were isolated and after purification the chemical and physical properties were characterized by Gel Permeation Chromatography (GPC: to obtain their molecular weight distribution), nitrobenzene oxidation, permanganate oxidation, and 13C NMR spectroscopy. By these means, the reaction mechanisms of dissolved and degraded lignin fragments in pulping liquors was elucidated and insight was gained concerning the effectiveness of the anthraquinone-anthrahydroquinone (AQ-AHQ) redox system using the corresponding batch lignin preparation as reference. The simulated batch fractions LF-1A, LF-2A, and true batch lignin preparations are similar. They have a high content of condensed arylpropane units. Among them, syringylpropane units are most severely degraded, in particular, etherified β-aryl ether bonds are cleaved to a great extent after dissolution in pulping liquor. The structure of the remaining degraded lignin fragments was changed seriously, particularly on their side chains. Oxidation and disproportionation reactions are typical. In the presence of fibers, xylans tend preferentially to simple deposition on fibers instead of grafting with the degraded lignin fragments. Dissolution of carbohydrates is not affected by higher alkali concentration at the end of the pulping, and the yield losses are caused by a better lignin removal.}, number={6}, journal={HOLZFORSCHUNG}, author={Venica, Albert D. and Chen, Chen-Loung and Gratzl, Josef S.}, year={2008}, month={Nov}, pages={637–644} }