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“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 [3]. 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 [4], 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 [5]. 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 [6]. 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 relationships.

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 [7] and inorganic supports [8]. After the preliminary test on the cyclopropanation reaction, other applications are currently under development.

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” [9]. 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 [10].

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 [11].

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 [12] but further improvements are being currently done.

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 new properties.

Acknowledgements

Financial support for this work was provided by the Spanish CICYT (projects MAT99-1176, PPQ2000-0322-P4 and PPQ2002-04012) and the DGA.

References

  • [1] C. Cativiela, J. M. Fraile, J. I. García, B. Lázaro, J. A. Mayoral, A. Pallarés, Appl. Catal. A 2002, 224 , 153.
  • [2] C. Cativiela, J. M. Fraile, J. I. García, B. Lázaro, J. A. Mayoral, A. Pallarés, Green Chem. 2003, 5 , 275.
  • [3] J. M. Fraile, J. I. García, J. A. Mayoral, S. Sebti, R. Tahir, Green Chem . 2001, 3 , 271.
  • [4] S. Sebti, A. Solhy, R. Tahir, S. Abdelatif, S. Boulaajaj, J. A. Mayoral, J. I. García, J. M. Fraile, A. Kossir, H. Oumimoun, J. Catal . 2003, 213 , 1.
  • [5] J. M. Fraile, J. I. García, J. A. Mayoral, L. Salvatella, E. Vispe, D. R. Brown, G. Fuller, J. Phys. Chem. B 2003, 107 , 519.
  • [6] J. M. Fraile, J. I. García, J. A. Mayoral, E. Vispe, Appl. Catal. A 2003, 245 , 363.
  • [7] A. Cornejo, J. M. Fraile, J. I. García, E. García-Verdugo, M. J. Gil, G. Legarreta, S. V. Luis, V. Martínez-Merino, J. A. Mayoral, Org. Lett . 2002, 4 , 3927.
  • [8] A. Cornejo, J. M. Fraile, J. I. García, M. J. Gil, V. Martínez-Merino, J. A. Mayoral, Molecular Diversity 2003, 6 , 93.
  • [9] M. I. Burguete, E. Díez-Barra, J. M. Fraile, J. I. García, E. García-Verdugo, R. González, C. I. Herrerías, S. V. Luis, J. A. Mayoral, Bioorg. Med. Chem. Lett. 2002, 12 , 1821.
  • [10] E. Díez-Barra, J. M. Fraile, J. I. García, E. García-Verdugo, C. I. Herrerías, S. V. Luis, J. A. Mayoral, P. Sánchez-Verdú, J. Tolosa, Tetrahedron: Asymmetry 2003, 14 , 773.
  • [11] J. M. Fraile, J. I. García, M. A. Harmer, C. I. Herrerías, J. A. Mayoral, O. Reiser, H. Werner, J. Mater. Chem . 2002, 12 , 3290.
  • [12] A. Cornejo, J. M. Fraile, J. I. García, M. J. Gil, C. I. Herrerías, G. Legarreta, V. Martínez-Merino, J. A. Mayoral, J. Mol. Catal . A 2003, 196 , 101.

 

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©2017 Instituto de Ciencia de Materiales de Aragón | Tfno: 976 761 231 - Fax: 976 762 453
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