Electron spin resonance study of radicals generated in cellulose/N‐methylmorpholine solutions after flash photolysis at 77 K

Abstract

The processes of radical formation in N-methylmorpholine-N-oxide monohydrate (NMMO) and cellulose/NMMO solutions were studied by ESR at 77 K under high-power UV (lambda = 248 nm) excimer laser flash photolysis. Radicals mainly generated were attributed to the nitroxide type radicals -CH2-NO*-CH2- and -CH2-NO*-CH3 at the first step and methyl *CH3 and formyl *CHO radicals at the second step of the photoreaction. Kinetic studies of these radicals revealed that formation and recombination rates of the radicals depend on the cellulose concentration in cellulose/NMMO solutions and the concentration of additional ingredients, e.g. Fe(II) and propyl gallate. Even at frozen state temperature, acceleration or quenching of radical reaction processes was found. The proposed scheme of UV light-induced NMMO degradation during irradiation based on ESR data correlates well with independently obtained results based on high-performance liquid chromatography (HPLC). The analysis of degradation products by HPLC, e.g. aminoethanol and acetaldehyde, supports the assumption concerning a radical-initiated ring opening of NMMO.

Degradation processes in the cellulose/N-methylmorpholine-N-oxide system studied by HPLC and ESR. Radical formation/recombination kinetics under UV photolysis at 77 K

“…As a consequence, the onset temperature (Ton) as the first thermal activity decreases. Additionally, as shown in a preliminary study (Konkin et al 2006a), comparing the amounts regard to the composition of the solution it is obvious that Fe(II) initiates reactions generating the highest concentration of degradation products. The stabilized solutions show reduced concentrations compared to the unstabilised and NMMO.…”

“…Spectra of the first group are practically unresolved at 77 K because of strong overlapping of all radical spectra which are included in the one broad line (see dot line in Fig. 2 and reference Konkin et al (2006a)). …”

“…According to the spectroscopic results a probable photochemical reaction pathway of radical formation and degradation (Scheme 2) under high quantum energy UV irradiation at 77 K for NMMO is suggested (Konkin et al 2006a). The starting position of the supposed reaction mechanism scheme was the analysis of ESR spectra concerning the subsequently generated radicals R 2 and R 3 attributed to • CH 3 [A H = 2.3 ± 0.005 mT (Ayscough and Thomson 1962;Fessenden and Schuler 1965) On the other hand it can not be only abstracted because of its immediately stabilization at 77 K and straightaway detecting by ESR in amounts equal to nitroxyl radicals.…”

“…The highly reactive carbenium-iminium cation enters into subsequent reactions, possibly autocatalytic ones (Rosenau et al 2001;Wendler 2005a). Moreover, ringopening reactions by the attack of the oxygen diradical were suggested (Rosenau et al 2001 Scheme 1 Main degradation reactions in the system cellulose/N-methylmorpholine-N-oxide reproduced with permission from Taeger et al (1985) and Rosenau et al (2001) The preliminary ESR study of NMMO and cellulose/NMMO solutions under high-power pulse UV irradiation by excimer laser at the temperature interval (77-120 K) were registered recently (Konkin et al 2006a). Radicals were detected directly in the cellulose/NMMO solution.…”

“…Therefore, all ESR spectra were simulated as well as fitted in some cases and discussed in detail in reference Konkin et al (2006a). In view of the comparative kinetic analysis the radical accumulation (formation/degradation) processes depend on additional ingredients in cellulose/NMMO solutions and hence, its analysis may be a fruitful preliminary step studying degradation processes.…”