Traditional industrial synthesis of organic chemicals predominantly proceeds
via steam-cracking of crude oil to furnish a small number of high-volume primary
platform chemicals, from which all major synthetic routes of industrial organic
chemicals are branching off. The primary intermediates are composed of:
(i) Olefins, i.e. ethylene, propene and butenes (1- and 2-butene, 1,3-butadiene,
i-butene),
(ii) aromatics (benzene, toluene, xylenes), and
(iii) one alkane (methane).
The latter have a low oxidation state of carbon (from -4 in methane to -1 in
aromatics), which renders the introduction of functional groups bearing elements
with high electronegativity (such as O, N, S, halogen, etc.) a ‘hidden oxidation’,
which is thermodynamically favored. This – together with the high efficiency of
steam-cracking (~98%) – is the main reason for the unparalleled success of traditional synthetic organic chemistry on industrial scale.
As non-renewable carbon feedstocks are slowly depleted, alternative sources are
sought with increasing intensity from biomass. So far, three main groups of
compounds have been proposed as substitute for non-renewable carbon feedstocks
for the synthesis of organic chemicals: [30]
(i) Carbohydrates In a simplistic version, they are oligomeric forms of [—CHOH—] and as such contain carbon at comparably high oxidation state (0), which
renders the introduction of functional groups thermodynamically less favorable.
Carbohydrates are anything but ideal as starting materials for synthesis, because
they are over-decorated by hydroxy-groups, which prevent direct functionalization
of the carbon framework without protecting and/or activating strategies. In addition, they are under-functionalized, because they contain only a single functional
group – the alcohol moiety – besides the occasional carbonyl group in aldoses or
ketoses, which is usually masked as (hemi)acetal. Hence, carbohydrates appear
more useful as feedstocks for fermentation or synthesis gas production than as
starting material for organic synthesis.
Advances in biotechnology have enabled us to produce complex fermentations
products, such as vitamins (e.g. B12, niacin), antibiotics (penicillins), steroids and
terpenoids (taxadiene, artemisinic acid), which are either used directly as bioactive
(pharmaceutical) ingredient, or as indispensable building block for the (semi)
synthesis of natural products for therapy of hormone disorders, cancer (taxol) or
parasites (artemisinin), which could never be achieved economically by chemical
synthesis alone. In addition to these highly complex structures, fermentation
yields a range of low molecular metabolites, which are mainly composed
of carboxylic acids (citric, succinic, fumaric, itaconic, lactic acid) or alcohols
(ethanol, 1,2- and 1,3-propanediol, 1-butanol, 1,4-butanediol). On a first glimpse,
this looks appealing, but it has to be kept in mind that fermentation is far less
efficient than steam-cracking and after all, the overall energy efficiency of the
initial step in bio-synthesis – photosynthesis – is 2%. Consequently, the use of
biofuels for the conversion of sunlight into chemical energy – in contrast to
photovoltaics – might massage our ecological ego, but is scientific nonsense [31].
412
4 State of the Art and Outlook
via steam-cracking of crude oil to furnish a small number of high-volume primary
platform chemicals, from which all major synthetic routes of industrial organic
chemicals are branching off. The primary intermediates are composed of:
(i) Olefins, i.e. ethylene, propene and butenes (1- and 2-butene, 1,3-butadiene,
i-butene),
(ii) aromatics (benzene, toluene, xylenes), and
(iii) one alkane (methane).
The latter have a low oxidation state of carbon (from -4 in methane to -1 in
aromatics), which renders the introduction of functional groups bearing elements
with high electronegativity (such as O, N, S, halogen, etc.) a ‘hidden oxidation’,
which is thermodynamically favored. This – together with the high efficiency of
steam-cracking (~98%) – is the main reason for the unparalleled success of traditional synthetic organic chemistry on industrial scale.
As non-renewable carbon feedstocks are slowly depleted, alternative sources are
sought with increasing intensity from biomass. So far, three main groups of
compounds have been proposed as substitute for non-renewable carbon feedstocks
for the synthesis of organic chemicals: [30]
(i) Carbohydrates In a simplistic version, they are oligomeric forms of [—CHOH—] and as such contain carbon at comparably high oxidation state (0), which
renders the introduction of functional groups thermodynamically less favorable.
Carbohydrates are anything but ideal as starting materials for synthesis, because
they are over-decorated by hydroxy-groups, which prevent direct functionalization
of the carbon framework without protecting and/or activating strategies. In addition, they are under-functionalized, because they contain only a single functional
group – the alcohol moiety – besides the occasional carbonyl group in aldoses or
ketoses, which is usually masked as (hemi)acetal. Hence, carbohydrates appear
more useful as feedstocks for fermentation or synthesis gas production than as
starting material for organic synthesis.
Advances in biotechnology have enabled us to produce complex fermentations
products, such as vitamins (e.g. B12, niacin), antibiotics (penicillins), steroids and
terpenoids (taxadiene, artemisinic acid), which are either used directly as bioactive
(pharmaceutical) ingredient, or as indispensable building block for the (semi)
synthesis of natural products for therapy of hormone disorders, cancer (taxol) or
parasites (artemisinin), which could never be achieved economically by chemical
synthesis alone. In addition to these highly complex structures, fermentation
yields a range of low molecular metabolites, which are mainly composed
of carboxylic acids (citric, succinic, fumaric, itaconic, lactic acid) or alcohols
(ethanol, 1,2- and 1,3-propanediol, 1-butanol, 1,4-butanediol). On a first glimpse,
this looks appealing, but it has to be kept in mind that fermentation is far less
efficient than steam-cracking and after all, the overall energy efficiency of the
initial step in bio-synthesis – photosynthesis – is 2%. Consequently, the use of
biofuels for the conversion of sunlight into chemical energy – in contrast to
photovoltaics – might massage our ecological ego, but is scientific nonsense [31].
412
4 State of the Art and Outlook