A cautionary note on thermal runaway reactions in mixtures of 1-alkyl-3-methylimidazolium ionic liquids and N-methylmorpholine-N-oxide
A cautionary note on thermal runaway reactions in mixtures of 1-alkyl-3-methylimidazolium ionic liquids and N-methylmorpholine-N-oxide
Stefan B?hmdorfer . Takashi Hosoya . Thomas R?der . Antje Potthast . Thomas Rosenau 0 1 2 3
0 T. Ro ?der Lenzing AG , Werkstr. 2, 4860 Lenzing , Austria
1 Present Address: T. Hosoya Graduate School of Life and Environmental Sciences, Kyoto Prefectural University , Shimogamo-hangi-cho 11-5, Sakyo-ku, Kyoto-shi, Kyoto , Japan
2 S. Bo ?hmdorfer ? T. Hosoya ? A. Potthast ? T. Rosenau (&) Department of Chemistry, University of Natural Resources and Life Sciences Vienna (BOKU) , Muthgasse 18, 1190 Vienna , Austria
3 T. Rosenau Johan Gadolin Process Chemistry Centre, A ? bo Akademi University , Porthansgatan 3, 20500 A ? bo/Turku , Finland
N-Methylmorpholine-N-oxide (NMMO) cannot be completely separated by extraction from mixtures with common 1,3-dialkylimidazolium ionic liquids (ILs) due to strong ionic interactions between the two components. At elevated temperatures, above approx. 90 ?C, especially under dry conditions and in the presence of acid, alkylating or acylating agents, remaining NMMO in ILs tends to undergo autocatalytic degradation. This is a highly exothermic, unstoppable process that results in explosions, flames, and complete charring of the reaction mixtures. Thus, caution must be exercised when drying or heating ILs that were in previous contact with NMMO, and the absence of amine oxide must be confirmed to avoid potential danger.
Cellulose; Ionic liquids; NMMO; Binary mixtures; Extractability; Decomposition; Degradation; Exothermicity; Work safety
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N-Methylmorpholine-N-oxide (NMMO), as the
monohydrate, is well-known as a cellulose solvent and is
used industrially in the production of Lyocell.
1-Alkyl3-methylimidazolium ionic liquids (ILs)?with
1-butyl-3-methylimidazolium and
1-ethyl-3methylimidazolium being the most prominent
representatives?are much newer, and can be seen as a first
generation of ionic liquid-based cellulose solvents.
NMMO, apart from its beneficial properties regarding
interaction with cellulose, is a strong oxidant and tends
to undergo side reactions (Rosenau et al. 2001; Bernier
et al. 2009). These might manifest themselves by
degradation of the solvent, oxidation of dissolved
cellulose (Potthast et al. 2006), discoloration during
processing, and yellowing of spun fibers (Rosenau
et al. 2005a). There are efficient stabilizers for lyocell
dopes available that reliably counteract those effects
(Wendler et al. 2008; Rosenau et al. 2005b). Most
severe and dangerous, however, is the tendency of
NMMO to undergo autocatalytic degradation, with
carbenium-iminium ions as the central, catalytically
active intermediate in this cycle (Rosenau et al. 1999).
Once this reaction has started there is no way to stop it
again, because of its high exothermicity. Eventually, it
will result in complete charring of the mixture, possibly
accompanied by explosive degradation, flames, and
strong smoke development. Literature accounts
describing these effects often use the terms ?thermal
runaway reaction? or ?exothermicity?, which are
euphemisms for explosive events that in the best case
damage lab equipment or production sites or?much
worse?might harm health and wellbeing of workers.
The degradation mechanisms of NMMO have been
comprehensively reviewed (Rosenau et al. 2001);
some of them have even been turned into procedures
useful in organic synthesis (Rosenau et al. 2004).
1-Butyl-3-methylimidazolium (BMIm) and
1-ethyl-3-methylimidazolium (EMIm), mostly as
acetate and chloride, were among the first ionic
liquids to be described as cellulose solvents
(Swatloski et al. 2002). Most 1,3-dialkylimidazolium ILs
are of the 1-alkyl-3-methyl type, so in the following
the general term XMIm will be used to described
these ILs. Initial optimistic claims of seemingly
unlimited applications of these solvents in
lignocellulose research have given way to a more realistic
view of their advantages and drawbacks. In
particular, problems with recycling and byproduct
accumulation have cast doubts on their general
usefulness in biomass fractionation or pulping, as
have literature accounts showing that these solvents
undergo reactions with cellulose (Ko?hler et al. 2007;
Liebert et al. 2009; Ebner et al. 2008; Zweckmair
et al. 2015) and thus lack the inertness that is often
claimed as one of benefits of ILs. Also thermal stress
causes slow byproduct formation in XMim ionic
liquids, mainly imidazole derivatives (Wendler et al.
2012; Liebner et al. 2010; Dorn et al. 2008).
NMMO has been used in combination with amines
or polar organic solvents to achieve cellulose
dissolution. Also, ILs have been used as the second component
in binary NMMO solvent mixtures, and in turn NMMO
has been added to ILs in attempts to further boost their
dissolution power (Ingildeev et al. 2013). Most
frequently, ILs and NMMO were combined in
biorefinery approaches, attempting to pretreat or dissolve
bi (...truncated)