Conspectus
Organonitrogen chemicals are essential in many
aspects of modern
life. Over 80% of the top 200 prescribed pharmaceutical products contain
at least one nitrogen atom in the molecule, while all top 10 agrochemicals
contain nitrogen, just to name a few. At present, the prevailing industrial
processes for manufacturing organonitrogen chemicals start from nonrenewable
fossil resources, but eventually we have to make these chemicals in
a more sustainable manner. Biomass represents the largest renewable
carbon resource on earth, which is inexpensive and widely available.
Integrating biomass into the organonitrogen chemical supply chain
will mitigate the carbon footprint, diversify the product stream,
and enhance the economic competitiveness of biorefinery. Short-cut
synthesis routes can be created for oxygen-containing organonitrogen
compounds by exploiting the inherent oxygen functionalities in the
biomass resources. Moreover, for nitrogen-containing biomass components
such as chitin, a unique opportunity to make organonitrogen chemicals
bypassing the energy-intensive Haber–Bosch ammonia synthesis
process arises. Estimated at 100 billion tons of annual production
in the world, chitin captures more nitrogen than the Haber–Bosch
process in the form of amide functional groups in its polymer side
chain.
In this Account, we intend to summarize our efforts to
establish
new reaction routes to synthesize valuable organonitrogen chemicals
from renewable resources. Enabled by tailor-designed catalytic systems,
diverse nitrogen-containing products including amines, amino acids,
nitriles, and N-heterocycles have been obtained from
a range of biomass feedstock either directly or via intermediate platform
compounds. Two strategies to produce organonitrogen chemicals are
presented. For platform chemicals derived from cellulose, hemicellulose,
lignin, and lipids, which are enriched with oxygen functionalities,
in particular, hydroxyl groups, the key chemistry to be developed
is the catalytic transformation of hydroxyl groups into nitrogen-containing
groups using NH3 as the nitrogen source. Along this line,
Ru- and Ni-based heterogeneous catalysts are developed to convert
alcohols to amines and/or nitriles via a thermal catalytic pathway,
while CdS nanomaterials are explored to promote −OH to −NH2 conversion under visible-light irradiation. Metal–zeolite
multifunctional systems are further established to enable the synthesis
of N-heterocycles from O-heterocycles.
The second strategy involves the use of chitin and chitin derivatives
as the starting materials. Under the concept of shell biorefinery,
distinctive protocols have been established to chemically transform
chitin as the sole feedstock to amino sugars, amino alcohols, furanic
amides, and N-heterocycles. By combining mechanochemistry
with biotransformation, an integrated process to convert shrimp shell
waste to complex, high-value, chiral compounds including tyrosine
and l-DOPA is also demonstrated.
