||“Greening” the Chemistry Through Catalysis
J.M. Fraile, J. I. García, J. A. Mayoral
Although Chemistry is perceived as a pollutant activity, many
chemists are involved in the development of cleaner products and processes,
which allow the sustainability and the economical growth without compromising
the environment, in the field known as “green chemistry”. Catalysis is probably
one of the most valuable tools for this purpose, because it allows a better
use of the starting materials (higher conversion), lower energy consumption
and a lower waste generation due to the higher selectivity of the process. The
use of heterogeneous catalysis results additionally advantageous because of
the easiness of separation, recycle and reuse.
As base of this philosophy, “green chemistry” must be economically
attractive for the industry, given that all the objectives have an impact on
lowering the costs of production. In fact, several industries are involved in
this type of research [1,2], trying for example the substitution of strong mineral
acids, such as sulfuric or hydrochloric acids, by solid acids which minimize
the corrosion problems, allowing at the same time an easier storage and handling.
One of the main pollution sources in the chemical industry
is the generation of by-products, inherent to the use of specific reagents.
One clear example is the oxidation reactions. The use of potassium dichromate
brings inherent the production of chromium salts and oxides. When epoxides are
considered, the most popular oxidants are peracids, such as meta -chloroperbenzoic
acid, but the corresponding acid is produced as stoichiometric by-product. Because
of that the most suitable oxidants are molecular oxygen and hydrogen peroxide.
As oxygen usually requires a sacrificial reductant, such as an aldehyde, hydrogen
peroxide remains as one of the most promising alternative, given that the only
by-product generated is water. However, a catalyst is necessary in order to
activate hydrogen peroxide and the requirements of this catalyst are quite exigent,
as it must work in the presence of water. This is not especially difficult in
the case of electrophilic alkenes, as the activation of hydrogen peroxide is
carried out with a base . Within the research line devoted to the development
of basic solid catalyst, the use of natural phosphate from Morocco has shown
a high potential interest, due to its low cost and high activity in several
reactions , with the additional advantage of low solvent requirements.
However the epoxidation of nucleophilic alkenes requires the
activation of hydrogen peroxide with a Lewis acid, a not easy task in the presence
of water. In contrast with previous ideas, our group was able to activate diluted
(30%) hydrogen peroxide with easily prepared silica-supported titanium catalysts
(Figure 1). These catalysts are highly modular and are prepared by reaction
of the silica support with Ti(O i Pr) 4 as a titanium precursor,
under soft conditions. The nature of the support plays an important role, together
with the titanium dispersion.
Figure 1: Synthesis of silica-supported titanium catalysts.
Additional modifications can be introduced on the environment
of titanium by substitution of the remaining isopropoxide groups by diols, aminoalcohols
and diamines . The modification of the Lewis acidity of the titanium centres,
as shown by experimental NH 3 desorption and theoretical calculations,
does not only modify the epoxidation activity, but also the selectivity to epoxide
due to changes in the hydrolysis activity and the participation of the radical
allylic oxidation mechanism.
Figure 2: Mechanisms of epoxidation with H 2 O 2 .
In spite of all the improvements introduced by the catalyst
design, the most important step towards the practical application of this method
is the optimization of the reaction conditions . The slow addition of hydrogen
peroxide drastically reduces the rate of decomposition, and as a consequence
the molecular oxygen concentration in the reaction medium. In this way the radical
allylic oxidation is minimized (Figure 2) and the epoxide hydrolysis remains
as the only problem to be solved.
Methods for the characterization of the titanium sites are
currently under development, together with the study of the structure-activity
The easiness of recovery of heterogeneous catalysts is especially
interesting in the case of high cost catalysts, as asymmetric ones. The chiral
ligands are usually expensive or even no commercially available, and then the
recycle of the catalyst, with the improvement in productivity, represents an
important saving in investment. The covalent grafting, which ensures the chiral
ligand recovery, is probably the best established immobilization method for
asymmetric catalysts. The development of methodologies for grafting of ligands
of general application are highly interesting, given that they open the way
to a large variety of chiral catalysts. This is the case of pyridinebis(oxazolines),
tridentate ligands able to coordinate metals such as ruthenium, rhodium or lanthanides.
The introduction of spacers in the position 4 of the pyridine ring (Figure 3)
allows the easy grafting on organic  and inorganic supports . After the
preliminary test on the cyclopropanation reaction, other applications are currently
Figure 3: Immobilization of pyridinebis(oxazoline).
In this type of immobilization, the accessibility of the ligand
is crucial for an efficient complexation with the metal precursor. When this
complexation is difficult, an important part of the expensive ligand remains
useless. Thus, even in the case of a high productivity per metal site, the productivity
per chiral ligand is low, a term we called “ligand economy” . In this regard,
the morphology of the support plays a decisive role. In the case of polymerized
chiral ligands, the composition of the monomeric mixture and the polymerization
solvent are the most important factors. We demonstrated that dendrimers can
act as more efficient cross-linking agents, conferring to the polymer a more
open structure which allows a better ligand economy .
In spite of the cited advantages, the covalent grafting of
chiral ligands requires a (sometimes) hard synthetic work in the modification
of the ligand. Moreover, the functionalization of the chiral ligand with a very
bulky substituent, as it is the solid support, leads in many cases to a drastic
reduction of the enantioselectivity. Trying to prevent this problem, our group
has been working in the immobilization of cationic complexes by electrostatic
interactions with anionic supports. In such case, the metal carrying the positive
charge is strongly hold by the support, but we detected some ligand leaching
due to competitive complexation with reaction products and by-products. This
problem is currently be solved by using analogue ligands which bind more strongly
the metal centre .
Although it is considered that the support-ligand steric interaction
is weaker in the case of catalysts immobilized through electrostatic interactions,
this is not the case when layered materials, such as clays, are used as supports.
The dielectric constant of the reaction solvent modifies the relative position
of the complex and the clay surface, and hence the steric interaction between
them. Thus it is possible to design chiral ligands specifically to be used under
such conditions, taking advantage of this interaction. In a preliminary work
this possibility has been demonstrated  but further improvements are being
In conclusion, the application of heterogeneous catalysts to
the fine chemicals synthesis allows the development of cleaner processes, through
substitution of harmful homogeneous acids and bases, the use of cleaner reagents
and the efficient recovery and reuse of the catalyst. In some cases the heterogeneous
character even modifies the selectivity of the catalyst, leading to interesting
Financial support for this work was provided by the Spanish CICYT (projects MAT99-1176, PPQ2000-0322-P4 and PPQ2002-04012) and the DGA.
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