Communications
have not yet been detected in the vapor phase upon thermal
reconstitution, a 1:1 mixture of TMG and propionic acid
decomposition of the neat ionic liquids.
([TMGH][CO Et] (1)) was distilled using a Bꢂchi kugelrohr
2
Pathways b and c have been observed to contribute to the
thermal decomposition of a large range of basic dialkyl
short-path distillation apparatus (3 g, 100–2008C over 30 min,
1.0 mmHg). This distillation process is presented in Figure 1
along with an illustration of the temperature-dependent,
conjugated–unconjugated acid–base equilibrium.
[
8]
imidazolium salts. A recent publication by Rinaldi and co-
[8a]
workers demonstrates that pathway b occurs even at low
temperatures (100–1608C), whereas at temperatures above
1
608C, pathway c dominates. The temperatures described for
pathway b may well be within process and drying conditions
for the use of these ionic liquids in bioprocessing and other
applications. This mode of decomposition is not compatible
with maintaining the sustainability of these processes.
Although this method of decomposition has been described
as ꢀsluggishꢁ in the Rinaldi report, formation of volatile
alkenes at these temperatures prevents equilibration and
enables a gradual acidification of the dissolution media by
buildup of the protonated alkyl imidazolium salt. Pathway c
generates alkyl halides and pseudo-halides, which can be
hydrolyzed or alcoholized, in the presence of water or
lignocellulosic solutes, respectively. This process will also
result in formation of acid, which would catalyze decompo-
sition of dissolved biopolymers. From a recyclability point of
view, pathway c also effectively allows for dissociation by the
reverse Menshutkin reaction. This characteristic can facilitate
the recovery by distillation of the starting materials required
for reformation of the starting ionic liquid by the forward
Menshutkin reaction. This process, however, is again likely to
yield low-purity and potentially corrosive ionic liquids.
Unfortunately, to date there are no reports describing the
decomposition, distillation, and reformation of [emim][OAc]
as a key structure, or any other basic imidazolium-based ionic
liquid, for that matter. Reports do exist, however, describing
the distillation of some ionic liquids through vaporization of
Figure 1. Distillation of compound 1 using a Bꢁchi kugelrohr short-
path distillation apparatus.
[TMGH][CO Et] (1) was distilled with 99.4% recovery of
2
the original mass. There was a clear polymeric residue
amounting to 0.6% of the original mass left in the first
kugelrohr bulb. This material was insoluble in water and
organic solvents, although it became swollen and opaque
when treated with sulfuric acid. The purity of the distilled 1
was determined to be greater than 99% by NMR spectro-
scopic analysis, which is a significant improvement on the
value of greater than 95% quoted in the BASF patent for the
[
9a]
[9b]
[9c]
ion pairs and dissociated neutral species.
BASF details essentially the short-path distillation of ionic
liquids such as [emim][OAc] and [emim][Me PO ], although
A patent by
distillation of [emim][OAc].
[
9c]
To further investigate the recyclability and dissolution
efficiency of these types of ionic liquids, 1:1 mixtures of TGA
2
4
thorough analyses, product recoveries, and purity of fractions
are not reported. In general, detailed reports into the efficient
recycling of ionic liquids capable of dissolving cellulose are
not forthcoming.
with formic acid ([TMGH][CO H] (2)), acetic acid ([TMGH]-
2
[OAc] (3)), propionic acid ([TMGH][CO Et] (1)), butanoic
2
acid ([TMGH][CO nPr] (4)), valeric acid ([TMGH][CO nBu]
2
2
(5)), hexanoic acid ([TMGH][CO nAm] (6)), trifluoroacetic
2
The concept of distillable ionic liquids for bioprocessing is
not unknown in the literature, however. N,N-Dimethylam-
monium-N’,N’-dimethylcarbamate (DIMCARB), formed
from the addition of dimethylamine to carbon dioxide, has
been reported by the MacFarlane group to efficiently extract
tannins from certain plant species. The residual ionic liquid
can then be distilled, thus allowing for the recovery of the
hydrolyzable polyphenolic compounds. This process repre-
sents one of the more successful direct applications of
functional ionic liquids in the area of bioprocessing, beyond
the imidazolium series.
acid ([TMGH][CO CF ] (7)), and triflic acid ([TMGH][OTf]
(8)) were prepared. Additionally, 1,1,3,3-tetramethylguanidi-
2
3
nium bistriflimide ([TMGH][NTf ] (9)) was prepared by the
2
addition of one equivalent of trimethylsilyl chloride (TMSCl)
to one equivalent of TMG in ether. What was assumed to be
the hydrolytically unstable [TMSTMG]Cl product was then
directly treated with one equivalent of lithium bistriflimide
[10]
(LiNTf ) in aqueous solution, allowing for crystallization of 9
2
in high yield (Scheme 2). As hydrogen-bond basic media
enable the breakage of intra- and intermolecular hydrogen
bonds in crystalline cellulose, this selection of ionic liquids 1–9
allowed for a comparison of the properties of structures with a
range of hydrogen-bond basicities; starting with the carbox-
ylates 1–6 as the most basic followed by the trifluoroacetate 7,
then the triflate 8, and finally the bistriflimide 9 as the least
basic structures.
Within the present research, we have developed a new
generation of ionic liquid structures based upon the conjuga-
tion of the organic superbase 1,1,3,3-tetramethylguanidine
(
TMG) with carboxylic acids such as formic, acetic, and
propionic acids. This method produces ionic liquids that both
rapidly dissolve cellulose to high concentration and are
recyclable by distillation with recoveries and purities over
X-ray crystal structures were obtained for compounds 1
and 9 (Figure 2). The crystal structure for 1 shows a repeating
dimeric ion-pair conformation in the crystal lattice, with
9
9%. To demonstrate this dissociation, distillation, and
6
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 6301 –6305
King, Alistair W. T.
