Abstract The use of biomass as a sustainable feedstock for the production of
chemicals has become more and more important in recent years. Homogeneous
iridium catalysis offers great opportunities for the conversion of bio-derived platform molecules and even biomass components such as cellulose or lignin, due to the
air, water, and acid stability of many iridium complexes. In this chapter, we review
the application of iridium catalysts to the transformations of carbohydrate-derived
compounds, fatty acids, and lignin.
Keywords Biomass conversion · Dehydrogenation · Fatty acids · Glycerol · HMF ·
Homogeneous catalysis · Hydrogenation · Hydrogen borrowing · Iridium catalysis ·
Levulinic acid · Lignin · Platform chemicals
1 Introduction
The diminishing supply of fossil resources, as well as the climate effects caused by
the cumulative release of CO 2 from fossil energy carriers into the atmosphere, forces
us to look for more sustainable alternatives. Thus, the conversion of biomass into
useful chemicals and fuels has become a very important field of research. Whereas
initially most work was focused on the conversion of easily accessible and chemically pure compounds such as sugars and fatty acids, nowadays also direct routes
from lignocellulose to some platform chemicals have been established.
Lignocellulosic biomass is generally converted by acid hydrolysis (or in combination with enzymes) into its monomers (sugars from the cellulose and hemicellulose fractions), which are then further converted into a range of small platform
molecules (Fig. 1) [1, 2]. Here, there is scope for the use of homogenous catalysis
in the further transformation of these molecules into new and known compounds
[3]. The great advantage of iridium catalysts in this regard is their high stability
towards water, air, and acidic conditions, i.e. the exact conditions present in biomass
processing. This opens up the possibility of performing several steps in one pot or at
least without extensive and energy intensive purification of intermediates or drying
of starting materials. Additionally, many biomass derived compounds are poorly
soluble in solvents other than water.
In the following we will discuss the recent work on homogeneous iridium
catalysis in the field of biomass conversion. As most current research focusses on
the conversion of a few available platform chemicals, the chapter is divided
according to the substrates. Most of these are derived from the cellulose/hemicellulose fraction, namely sugars, bio-derived alcohols, furanics (compounds derived
from furfural or 5-hydroxymethylfurfural), and small organic acids. Another section
deals with iridium-catalyzed transformations of fatty acids, which are derived,
together with glycerol, from vegetable oils. Iridium-catalyzed conversion of lignin
is discussed in Sect. 6.
342
S. Kirchhecker et al.
chemicals has become more and more important in recent years. Homogeneous
iridium catalysis offers great opportunities for the conversion of bio-derived platform molecules and even biomass components such as cellulose or lignin, due to the
air, water, and acid stability of many iridium complexes. In this chapter, we review
the application of iridium catalysts to the transformations of carbohydrate-derived
compounds, fatty acids, and lignin.
Keywords Biomass conversion · Dehydrogenation · Fatty acids · Glycerol · HMF ·
Homogeneous catalysis · Hydrogenation · Hydrogen borrowing · Iridium catalysis ·
Levulinic acid · Lignin · Platform chemicals
1 Introduction
The diminishing supply of fossil resources, as well as the climate effects caused by
the cumulative release of CO 2 from fossil energy carriers into the atmosphere, forces
us to look for more sustainable alternatives. Thus, the conversion of biomass into
useful chemicals and fuels has become a very important field of research. Whereas
initially most work was focused on the conversion of easily accessible and chemically pure compounds such as sugars and fatty acids, nowadays also direct routes
from lignocellulose to some platform chemicals have been established.
Lignocellulosic biomass is generally converted by acid hydrolysis (or in combination with enzymes) into its monomers (sugars from the cellulose and hemicellulose fractions), which are then further converted into a range of small platform
molecules (Fig. 1) [1, 2]. Here, there is scope for the use of homogenous catalysis
in the further transformation of these molecules into new and known compounds
[3]. The great advantage of iridium catalysts in this regard is their high stability
towards water, air, and acidic conditions, i.e. the exact conditions present in biomass
processing. This opens up the possibility of performing several steps in one pot or at
least without extensive and energy intensive purification of intermediates or drying
of starting materials. Additionally, many biomass derived compounds are poorly
soluble in solvents other than water.
In the following we will discuss the recent work on homogeneous iridium
catalysis in the field of biomass conversion. As most current research focusses on
the conversion of a few available platform chemicals, the chapter is divided
according to the substrates. Most of these are derived from the cellulose/hemicellulose fraction, namely sugars, bio-derived alcohols, furanics (compounds derived
from furfural or 5-hydroxymethylfurfural), and small organic acids. Another section
deals with iridium-catalyzed transformations of fatty acids, which are derived,
together with glycerol, from vegetable oils. Iridium-catalyzed conversion of lignin
is discussed in Sect. 6.
342
S. Kirchhecker et al.