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Green Chemistry
However, most hydrogen peroxide-based epoxidation reac- according to Hodge and Sterner between 4 and 6 (4: slightly
tions catalyzed by [H]3[PW12O40] are performed in biphasic toxic, 6: relatively harmless). This information is reported in
systems using chlorinated (e.g. methylene chloride, chloro- Table 1 as well as the emission of volatile organic compounds
form) or hazardous solvents (e.g. acetonitrile, a very inflam- (VOCs) according to the European Directive 1999/13/CE. Also,
mable liquid with a flash point of 12.8 °C).9 As pointed out by if existing, the risk phrase associated with a flammability
Hill et al.: “the use of chlorocarbons largely defeats the significant hazard (R10: flammable, R11: highly flammable) is indicated.
environmental and economic advantages of using H2O2 as the Another criterion for the selection of solvents was their
oxidant”.10 In 2003, Noyori et al. developed the epoxidation of miscibility with aqueous H2O2 solution because it is used as
olefins under organic solvent- and halide-free conditions.11 an oxidizing agent in the proposed olefin epoxidation (see
This smart catalytic system seems to be ideal because it is below). The “bio-solvents”, i.e. solvents produced from renew-
economically, technically, and environmentally satisfying. able resources, are labeled as green in Table 1. For compari-
However, the reaction is performed at high temperature son, two conventional and widely used solvents (chloroform
(90 °C) in the presence of a surfactant (quaternary ammonium and acetonitrile) have also been reported and investigated for
hydrogenosulfate) and phosphonic acid in order to create POM-based catalyses (see below).4 It is noteworthy that the sol-
in situ the Venturello catalyst. In recent years, balanced catalytic vents in Table 1 have been classified according to their initial
surfactants, combining an amphiphilic quaternary ammonium turnover frequency (TOF0) observed in the catalytic cyclooctene
2−
cation and a catalytic counteranion, such as MoO42− or WO4
,
epoxidation (see below).
have been designed to obtain very efficient two- and three-
In a chemical process, the use of a solvent is dictated by its
phase microemulsion systems for the catalytic oxidation basicity/acidity, its polarity and its protic or aprotic nature. In
(i.e. dark singlet oxygenation, epoxidation, sulfide oxidation) order to estimate the physicochemical properties of solvents,
of various substrates.12 More recently, based on the same one classical approach consists of determining the Kamlet–
concept, catalytic amphiphilic POM nanoparticles resulting Taft plot.15,16 Indeed, solvents are identified by three descrip-
3−
from the electrostatic coupling of [PW12O40
]
anions and tors: α (a measure of the “proticity” or H-bond donating
alkyltrimethyl ammonium cations have been shown to stabi- ability), β (an expression of their basicity or H-bond accepting
lize the so-called Pickering emulsions.13 This new amphiphilic ability) and π* (a measure of their polarity and polarizability).
catalyst gives pH-neutral conditions (pH ≈ 6.5) preventing Classically, a 2D chart is plotted with π* on the x-axis and β on
epoxide hydrolysis.12 This catalyst works at 65 °C with competi- the y-axis (Fig. 2).
tive reaction rates.13 However, the drawback of this method is
the aromatic solvents used to generate the Pickering emul-
sion.13 In order to obtain a more sustainable catalytic process,
the use of these amphiphilic nanoparticles has been investi-
gated for the epoxidation of olefins in eighteen eco-friendly
solvents with the aim to find relevant alternatives to the aro-
matic, chlorinated and other hazardous solvents which are
commonly used to perform reactions with POM catalysts
(Table 1). The reaction media as well as the catalytic nanoparti-
cle behavior in the “green” solvents have been characterized by
transmission electron microscopy (TEM) and dynamic and
multiple light scattering experiments. Finally, the Kamlet–
Taft parameters were measured in order to correlate the
physicochemical properties of the solvents with their catalytic
activity.
Fig. 2 Kamlet–Taft plot of solvents (1–19, A and B) as a function of
their π* (polarity and polarizability), β (basicity or H-bond accepting
ability) and α (“proticity” or H-bond donating ability) values (blue large
Results and discussion
Choice of solvents and their physicochemical characteristics
sphere: α > 0.5; red small sphere: α < 0.5). Data were obtained from
ref. 15 and 16 or experimentally determined for 1, 4, 7, 14, 16, and 17.
“Green” solvents are used to minimize the environmental
impact resulting from the use of solvents during chemical pro-
cesses. Eighteen “green” or “sustainable” solvents, with a good
EHS (Environment, Health and Safety) profile and preferably
obtained from the biomass feedstock, have been selected
through the review of the technical, commercial and scientific
literature (Table 1). In particular, the choice of the “green” sol-
vents was dictated by the freezing (F.p. < 0 °C) and the boiling
In Fig. 2, the sphere size represents the protic (α > 0.5:
small sphere) or the aprotic nature of the solvent (α < 0.5:
large sphere). The data values were obtained from the litera-
ture or experimentally determined (see Table 2). Although the
investigated solvents cover a large part of the Kamlet–Taft plot,
we can distinguish different groups. The first one is composed
of solvents 1–10, 13, 16 and B. For these solvents, the β values
(B.p. > 50 °C) points and also by the toxicity index, Itox
,
270 | Green Chem., 2014, 16, 269–278
This journal is © The Royal Society of Chemistry 2014