Environmental Engineering Reference
In-Depth Information
In addition, all these routes require huge amounts of hydrogen that has to be produced
separately (from biomass or otherwise).
Example 18.3 Greenness of HMF conversion to C15 alkane
How would you assess the greenness of the route for the production of a C15
alkane, shown in Figure 18.7?
Solution
When studying the reactions in Figure 18.7, one learns that this route is in contrast
with at least four of the twelve green chemistry principles: (1) a multistep process is
required; (2) aldol condensation is a noncatalytic step (requiring a base and there-
with lots of salt waste), which is generally hampered by low yields; and (3) because
of the additional hydrogen required, a large amount of energy is required for its
production. (4) For all these steps, substantial amounts of solvents are needed.
Therefore, without addressing the exact E-factor, one can safely say that this is
not a green process.
18.4.2 Furfural
Wheat bran and straw available from agricultural residues are rich in xylan-type
hemicellulose mainly consisting of linear chains of xylose, combined with arabinose
(
33 wt%). These so-called hemicelluloses are easily depolymerized through hydrol-
ysis in dilute acid. Furfural is one of the molecules that can be obtained by removal of
three molecules of water fromC5 sugars, as depicted in Figure 18.8. Yields in this case
are higher than for HMF (Figure 18.5). In the literature, yields of up to 87% (with 95%
selectivity) of furfural from xylose have been reported under optimized laboratory
reactor conditions (Marcotullio and De Jong, 2011). The advantage compared to
HMF is that furfural contains less oxygen. A disadvantage is that it stems from
C5 sugars, rather than C6 sugars, which are less abundant.
In a recent publication by researchers from Shell (Lange et al., 2012), the use of
furfural as platform molecule for biofuels has been evaluated. From a range of can-
didates, methylfuran (MF) and ethylfurfuryl ether (EFE) apparently had the most
favorable properties. Figure 18.9 depicts the conversion of furfural to MF and
EFE. In both cases, hydrogen is required to upgrade furfural. MF was positively
evaluated in road tests in 10 vol.% blends with gasoline.
18.4.3 Levulinic Acid/gVL/Valeric Acid as C5 Platform
Cellulosic streams can be directly converted into LA/formaldehyde mixtures through
acid hydrolysis, as shown in Figure 18.10 (Alonso et al., 2011; Wright and Palkovits,
2012). This is another C5 platform, next to furfural, which leads to favorable C/O and
C/H ratios and which has the advantage of having a linear structure. In Figure 18.10,
the possible reactions for LA are shown. Hydrogenation leads to gVL and further
hydrogenation to valeric acid (VA).
Search WWH ::
Custom Search