Bromide-free TEMPO-mediated oxidation of cellulose by hypochlorite - fundamental and practical aspects

TEMPO-mediated oxidation of pulp is a pretreatment procedure which is applied to produce TEMPO-oxidized cellulose nanofibers (TOCN). TOCN is proposed to be a raw material for many applications such as electro-optical films, microelectronics, gas-barrier films, cosmetics, flame-resistant materials and other high-tech applications. The drawback in the traditional procedure, which is effective enough for industrial purposes, is the use of bromide in high quantities. Thus, bromide is considered to be highly undesired in the waste water due to the environmental and corrosion concerns. This thesis focused on the development of an industrially feasible bromide-free TEMPO-mediated oxidation procedure of birch kraft pulp which can be applied with the production of the TOCN. TEMPO conversion to TEMPO+ is an underestimated and uncharted process step in the conventional TEMPO-oxidation processes. Here, two separate TEMPO activation steps were developed. TEMPO was activated with ClO2 or NaOCl in the closed vessel prior to the addition in the reactor. Then, the bromide-free TEMPO-mediated oxidations of bleached birch kraft pulp were conducted with the industrially feasible reaction rate. Finally, the oxidized pulps with the adequate carboxylate content were fluidized to TOCN, which is typically a transparent gel. Later, an effective alkali pretreatment of bleached birch kraft pulp was developed to enhance the reaction rate of bromide-free TEMPO-mediated oxidation of the pulp. The TEMPO-oxidation time of alkali treated birch pulp reduced drastically from few hours to half an hour when an adequate hypochlorite addition was used with TEMPO-oxidation. The reached carboxylate content of the TEMPO-oxidized pulp by a high hypochlorite addition was clearly higher with the alkali treated pulps than with the untreated pulps. Atomic force microscopy topography and rheological characterization was applied to study alkali treated TOCN gels as a function of decreased xylan content. An iodometric titration method was developed to monitor TEMPO+ content of catalyst and pulp solutions. This novel analytical method was applied with studying the reaction mechanisms and kinetics of TEMPO-mediated oxidation. The kinetics of the TEMPO conversion to TEMPO+ with Cl(+1) and Br(+1) was examined with the titration method. The importance of the separate TEMPO activation step was found to be an essential part of bromide-free TEMPO-oxidation. Once TEMPO is converted to TEMPO+, which can be conducted with Cl(+1), ClO2 or Br(+1), the reaction rate is dependent on the reaction rate with TEMPO+ and primary hydroxyls or reoxidation of the reduced catalyst (TEMPOH) to TEMPO+. The results of the thesis facilitates the development of industrially feasible TOCN production.