Green Chemistry Principles - Process Intensification for Green Chemistry - page 47

Table 2.2 US DOE-identified key platform molecules [52].

Glycerol Levulinic acid

3-hydroxypropionic acid Succinic acid

Xylitol/arabinitol Glutamic acid

3-hydroxybutrolactone Itaconic acid

Sorbitol Fumaric acid

Glucaric acid Malic acid

2,5-furan-dicarboxylic acid Aspartic acid

meeting the demands of existing and new industries. Initial conversion of renewable
feedstocks into chemicals tends to operate by one of two main routes - thermochemical
and enzymatic (biocatalysis). Both routes deconstruct the biopolymer structure of the
biomass into smaller molecules. Fermentation or biochemical routes tend to rely upon
enzymatic (or pre-treatment) routes to sugars, followed by biocatalysis into useful small
molecules. Thermochemical routes such as gasification work by near-total destruction of
the biomass into very small fragments such as carbon monoxide, hydrogen and methane,
followed by reformation via Fischer-Tropsch [50] and other upgrading techniques into
useful molecules and fuels. Thermochemical treatment such as pyrolysis, which can be
achieved by direct heating or microwave heating, makes a bio-oil consisting of a wide
range of decomposition products such as sugars and phenols and rearrangement products
such as furans and syngas. Typically, the challenge is to upgrade this bio-oil into
chemicals and/or fuels for use in a wide range of techniques and combinations of
techniques. This upgrading process has some challenges and issues associated with it
[51].
Platform molecules are molecules obtained from the deconstruction of biomass that can
be used as the building blocks for the manufacture of more complex chemicals. Acknowl-
edging the wide variety of bio-derived molecules available compared to the limited number
of chemicals derived from fossil fuel sources, the US Department of Energy (DOE)
screened over 300 compounds for their suitability as platform molecules [52]. Table 2.2
shows a list of suitable molecules identified by this work, which took into account the
availability and ease of generation of these compounds, as well as their suitability,
flexibility and ease of conversion into more complex chemical products.
A major focus of biorefinery and green chemical research is now on finding methods to
generate large quantities of these molecules in high purity with low energy usage and
developing synthetic methodologies by which they can be converted into useful molecules
and polymers for the chemical-using industries.

2.5.1 Case Study: Wheat Straw Biorefinery

Recently, work at the University of York demonstrated the use of wheat straw as a
biorefinery feedstock [46]. Using wheat straw, an abundant, relatively low-value agricul-
tural residue found in Europe, a wide range of products were obtained. As shown in
Figure 2.8, first the high-value waxes which coat the outside of the straw were removed
using scCO 2 . This technique was found to be more selective than conventional solvent
extractions and capable of splitting the waxes into various fractions with a variety of