Alfonso Cornejo

Surface-mediated improvement of enantioselectivity with clay-immobilized copper catalysts

The clay surface plays a role in the enantioselectivity of the cyclopropanation reaction, in the case of copper catalysts immobilized on laponite by electrostatic interactions. The effect is different depending on the type of chiral ligand, bis(oxazoline) or pyridineoxazoline. In the first case the clay promotes a reversal of the stereochemical course of the reaction. The pyridineoxazoline-copper complexes lead to very low enantioselectivity in homogeneous phase. However, the asymmetric induction increases when the complexes are immobilized on laponite, showing that it is possible to design good chiral ligands specific for the use in immobilized catalysts.

Efficient enhancement of copper-pyridineoxazoline catalysts through immobilization and process design

Copper-pyridineoxazoline (Cu-pyox) complexes are poor homogeneous catalysts for asymmetric cyclopropanation reactions. Pyox ligands have been immobilized by polymerization of monomers possessing a vinyl group directly attached to position 6 with styrene and divinylbenzene. The corresponding heterogeneous catalysts show a significant enhancement in enantioselectivity, up to 7-fold that of the analogous homogeneous Cu-pyox catalysts. This effect is due to a synergic effect between the proximity of the polymeric backbone and the presence of a bulky substituent in the chiral oxazoline ring around copper. The obtained values of enantioselectivity are similar to those found with supported C2-symmetric bis(oxazolines), but with only half the chiral information given the presence of only one oxazoline ring in pyox. Besides, the co-polymerization in the presence of the right porogen inside a column allows the preparation of monolithic mini-flow reactors. Continuous flow processes contribute to further improve the catalytic efficiency in both classical solvents (dichloromethane) and neoteric greener ones, such as supercritical CO2. The use of scCO2 as solvent yields the same selectivities obtained in batch processes in combination with higher productivity avoiding the use of VOC.

Immobilizing a single pybox ligand onto a library of solid supports

Chiral pyridinebis(oxazoline) (pybox) ligands can be efficiently immobilized onto silica through position 4 of the pyridine ring. The crucial intermediate in this strategy is 4-chloropybox, which is prepared in good yield from chelidamic acid. 4-Chloropybox reacts with p-hydroxybenzaldehyde and p-aminophenol to give two intermediates (pybox-CHO and pybox-NH2) that allow to introduce the formyl and amino groups able to link to spacers with triethoxysilylgroups. The modified ligands and their ruthenium complexes are immobilized by grafting onto pre-formed silicas or, alternatively, the support is created by sol-gel synthesis using the functionalizedchiral ligand as a silica monomer. In this way it is possible to create a library where the variation involves the support rather than the catalyst. The aim of this approach is to study the influence of different parameters, such as the textural properties of the support and the immobilization method, on the functionalization and catalytic performance. Some of the immobilized complexes are compared as heterogeneous catalysts in the cyclopropanation reaction of styrene with ethyl diazoacetate.

Biomass Sources for Hydrogen Production

Biomass is a promising feedstock for the production of hydrogen. Within this chapter, current and future biomass resource potential are evaluated. The composition and properties of the most commonly used biomass feedstocks are also described. The required pretreatments of biomass prior to the conversion to hydrogen are discussed as a function of the biomass structure, composition and properties as well as the technology used for its transformation to hydrogen. A revision of the most common currently available methodologies for the production of hydrogen from biomass is also envisaged. Finally, life cycle assessment and costs will be considered for the currently developed technologies.

Biological Hydrogen Production

In spite of the intense research developed over last decades about the hydrogen (H 2 ) production by microorganisms, we still remain far from thermochemical efficiency and a sustainable bioproduction of hydrogen. We revisit the state of the art of the biological hydrogen production (dark and photofermentation, syngas processing through biological water–gas shift reaction, direct and indirect water biophotolysis, as well as combined methods) using a thermodynamic and mechanistic approach. Main reactors types as well as adequate substrates and operational parameters are also discussed. Finally, life cycle assessment will be considered for the current developed technologies.