The replacement of fossil resources that currently provide more than 90% of our energy needs and feedstocks of the chemical industry in combination with reduced emission of carbon dioxide is one of the most pressing challenges of mankind. Biomass as a globally available resource has been proposed as an alternative feedstock for production of basic building blocks, which could partially or even fully replace the currently utilized fossil-based ones in well-established chemical processes. The destruction of lignocellulosic feed followed by oxygen removal from its cellulose and hemicellulose content by catalytic processes results in the formation of initial platform chemicals (IPCs). However, their sustainable production strongly depends on the availability of resources, their efficient or even industrially viable conversion processes, and replenishment time of feedstocks. Herein, we overview recent advances and developments in catalytic transformations of the carbohydrate content of lignocellulosic biomass to IPCs (i.e., ethanol, 3-hydroxypropionic acid, isoprene, succinic and levulinic acids, furfural, and 5-hydroxymethylfurfural). The mechanistic aspects, development of new catalysts, different efficiency indicators (yield and selectivity), and conversion conditions of their production are presented and compared. The potential biochemical production routes utilizing recently engineered microorganisms are reviewed, as well. The sustainability metrics that could be applied to the chemical industry (individual set of sustainability indicators, composite indices methods, material and energy flow analysis-based metrics, and ethanol equivalents) are also overviewed as well as an outlook is provided to highlight challenges and opportunities associated with this huge research area.
The
application of biomass-based resources for the production of
chemicals could slow the depletion rate of fossil reserves and enable
the development of a sustainable chemical industry. Three sustainability
metrics, the sustainability value of resource replacement (SVrep), the sustainability value of the fate
of waste (SVwaste), and the sustainability
indicator (SUSind), were defined for biomass-based
carbon chemicals by using the ethanol equivalent (EE) as a common
currency. These sustainability metrics were calculated for ethylene,
propylene, toluene, p-xylene, styrene, and ethylene
oxide in the U.S.A. for 2008 and 2014. Our calculations are based
on the initial chemical dehydration of corn-ethanol to ethylene followed
by its conversion by existing chemical processes. These basic chemicals
cannot be produced sustainably at this time primarily due to the limited
availability of bioethanol. Consequently, bioethanol-based carbon
products should only be labeled “sustainable” when the
necessary biomass is available to produce the required bioethanol,
independently of social and economic changes. The waste management
of the processes shows much better sustainability values than the
resource management, due to the successful greening of petrochemical
processes.
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