The chemistry of side reactions and byproduct formation in the system NMMO/cellulose (Lyocell process)

Abstract

N-Methylmorpholine-N-oxide monohydrate (NMMO) is used as a solvent for direct dissolution of cellulose in industrial fiber-making (Lyocell process). Ideally, Lyocell fiber production per se should be an entirely physical process that does not cause chemical changes in pulp or solvent. However, there are several side reactions and considerable byproduct formation in the system cellulose/NMMO/water which can cause detrimental effects, such as degradation of cellulose, temporary or permanent discoloration of the resulting fibers, decreased product performance, pronounced decomposition of NMMO, increased consumption of stabilizers, or even thermal runaway reactions.The present paper will focus on chemical aspects of the system NMMO/cellulose. After a short section on Lyocell fiber production, the analytical techniques to monitor side reactions in the Lyocell system will be discussed. In the main part, the side reactions of NMMO in the Lyocell process have been divided into homolytic (radical) and heterolytic (non-radical) processes in a systematic investigation.In all homolytic reactions of NMMO, cleavage of the N–O bond with formation of an aminium (aminyl) radical is the first step. Formation, properties and subsequent reactions of this primary radical species will be summarized. In the absence of oxygen, the radical undergoes disproportionation or other redox processes that finally produce N-methylmorpholine, or morpholine and HCHO, respectively. In contrast, reactions of C-centered tautomers of the radical with dioxygen dominate in the presence of oxygen. Also the effects of transition metal ions, which are potent inducers of homolytic reactions of NMMO, are described.Heterolytic reactions in the Lyocell system proceed according to three major pathways. First, reductive deoxygenation of NMMO produces N-methylmorpholine with concomitant oxidation of available reductants, e.g. cellulose or carbohydrate model compounds. Second, Polonowski type reactions, which are intramolecular redox processes, finally generate morpholine and formaldehyde. In a third pathway, an autocatalytic process induced by carbonium–iminium ions can cause quantitative decomposition of NMMO. The formation and determination of reactive intermediates and heterolytic degradation products as well as their role in the system is analyzed.Furthermore, thermal degradation reactions under controlled conditions or under conditions leading to uncontrolled thermal degradation, i.e. explosions or ‘exothermic events’, are considered. Especially the latter processes are important from the viewpoint of system safety since they can be induced by several of the discussed radical and non-radical reactions.The homolytic, heterolytic and thermal degradation reactions in the system cellulose/NMMO/water have been reviewed in terms of reaction mechanisms, byproduct formation, and negative effects on the Lyocell system. Precise knowledge of reaction mechanisms as well as the role of harmful intermediates and products allows for the safe and economical production of Lyocell fibers, and a rational design of chemical stabilizers for the Lyocell system.