High Temperature TEMPO Oxidation in a Heterogeneous Reaction System: An Investigation of Reaction Kinetics, Pulp Properties, and Disintegration Behavior

Fredrik W. Brodin, Hans Theliander
2013 BioResources  
TEMPO oxidation was performed on never-dried bleached softwood kraft pulp fibres to study the influence of reaction temperature and the dosage of sodium hypochlorite in the oxidation treatment. It was found that oxidation at a high temperature shortened oxidation time, but it also resulted in more extensive degradation of the pulp. Harsh reaction conditions (high temperature and high dosage of sodium hypochlorite) enabled the rapid disintegration of pulp fibres with a low energy demand. The
more » ... rgy demand. The freeze-dried disintegrated pulp fibres had a high absorption capacity of saline liquid, 40 to 80 g/g, and retained 9 to 14 g saline liquid/g material in a standard centrifuge retention test. Four different models were investigated to describe the kinetics of TEMPO oxidation of pulp fibres. However, none of the models could adequately describe all the mechanisms involved in this reaction system. The results showed that the diffusion of hypochlorite ions was sufficiently fast compared to the chemical reactions. In contrast, the concentration of the catalytic compounds, NaBr and TEMPO, both had a strong influence on the reaction rate. Results show that the reactions have different phases, possibly with different rate-determining steps. It remains to be determined which reaction steps correspond to the different phases. PEER-REVIEWED ARTICLE bioresources.com Brodin and Theliander (2013). "TEMPO ox. kinetics," BioResources 8(4), 5925-5946. 5926 type of modification also causes a substantial decrease in the degree of polymerization (DP) (Saito and Isogai 2004), i.e., the molecular weight of the cellulose polymers decreases. Saito et al. (2006) found that TEMPO-oxidized pulp fibres could be disintegrated into microfibrillar cellulose (MFC) through treatment in a Waring blender. Furthermore, they reported that the disintegration of TEMPO-oxidized sulfite pulp was facilitated by a higher dosage of NaClO. Dang et al. (2007) reported that a greater amount of negatively charged carboxylate groups, after TEMPO oxidation, enhanced the water retention value (WRV), thus indicating enhanced fibre swelling. In contrast to this, Kitaoka et al. (1999) found that the WRV was almost unaffected, even though the amount of carboxylate groups was increased from 0.06 to 0.47 mmol/g. Under alkaline conditions, TEMPO oxidation has been tested at low and moderate temperatures up to room temperature level. Isogai and Kato (1998) studied the influence of TEMPO oxidation conditions and found that higher temperature and longer oxidation time resulted in a lower DP in the oxidized pulps. Kitaoka et al. (1999) reported that the DP decreased rapidly when the pulp was subjected to TEMPO oxidation, but at a higher dosage of sodium hypochlorite charge, the DP leveled out at about 200. TEMPO oxidation can also be performed at near neutral conditions (Saito et al. 2009 ), using a TEMPO/NaClO/NaClO 2 reaction system. The resulting pulp has almost the same DP as the starting pulp. However, there are also applications and processes where a high DP is a disadvantage or is of minor importance for the performance of the material, e.g., when the oxidized pulp is disintegrated and dried for usage in absorbent applications (Brodin and Theliander 2012). In such a material, there are other properties, such as the pore size distribution of the material and the available surface area, that have a greater influence on the absorption and retention properties of the material. The kinetics of TEMPO oxidation has been studied in a homogeneous reaction system (TEMPO/NaClO) with methyl α-D glucopyranoside as the substrate (Bragd et al. 2000) . The experiments in the kinetics study were conducted at 20 ˚C and with a 10% excess amount of hypochlorite ions relative to the amount of primary hydroxyl groups. It was found that the reaction could be described as a first-order reaction in TEMPO and the substrate. Furthermore, the authors suggested that the reaction between the nitrosium ion and the primary alcohol was the rate-determining step under the given experimental conditions. In an earlier study, de Nooy et al. (1995) found in experiments with TEMPO/NaBr/NaClO and methyl α-D glucopyranoside that the rate of carboxylate group formation was much faster (about seven times faster) than the rate of aldehyde group formation. TEMPO oxidation kinetics has also been studied for regenerated cellulose, e.g., rayon fibres (Sun et al. 2005) , and for cotton linters (Dai et al. 2011) , where the kinetics was modeled using a homogeneous reaction model. Sun et al. (2005) found that the experimental data could be well fitted to a first-order reaction expression with respect to the primary oxidant (NaClO). For cotton linters, Dai et al. (2011) found that the kinetics could be described by two first-order expressions, i.e., one for the rapid initial reaction and one for the remaining part of the reaction. These studies have been conducted with model compounds or with fibres; however, all of the studies assumed that mass-transfer resistance could be neglected. In TEMPO oxidation of fibrous substrates, the reactants must be transported from the liquid bulk to the reaction site, and dissolved reaction products must be transported from the reaction site in the fibre wall to the bulk liquid. This implies that mass transport phenomena, along with the geometry of the fibres, may influence the overall kinetics. To our knowledge, no study has approached the PEER-REVIEWED ARTICLE bioresources.com Brodin and Theliander (2013). "TEMPO ox. kinetics," BioResources 8(4), 5925-5946. 5927
doi:10.15376/biores.8.4.5925-5946 fatcat:arknrfvqerbhjow4r3wtt6uggi