Chemical Engineering April 2023 - 9

Using a 'green' oxidizer to
degrade Kraft lignin into vanillin
T
he demand for vanillin vastly outstrips
the
this
natural
flavoring
agent. A chemical
process is thus used to produce
the required large quantities of vanillin
from petroleum, which is far less expensive
than obtaining the substance from
fermented genuine vanilla pods. Another
alternative is to make vanillin from lignin,
a waste product of the wood pulping industry.
A team led by professor Siegfried
Waldvogel of Johannes Gutenberg University
Mainz (JGU; Germany; www.unimainz.de)
has now managed to further
enhance its method of electrochemical
production of vanillin from lignin by using a
" green " oxidation method for this purpose.
This degrades Kraft lignin - a supposed
worthless byproduct - to release vanillin.
" We are confident that our sustainable,
environmentally friendly process can contribute
towards an even larger-scale production
of vanillin, " Waldvogel says.
The method, described in a recent
issue of Angewandte Chemie International
Edition, combines an electrosynC
hemists
at Hokkaido University
(Sapporo, Hokkaido, Japan;
www.global.hokudai.ac.jp)
and the Institute for Chemical
Reaction Design and Discovery have
developed what is said to be the first
high-performance catalyst specifically
designed and optimized for solid-state,
mechanochemical synthesis. The new
approach was reported in the March
9 issue of the Journal of the American
Chemical Society.
Mechanochemical synthesis, in which
solid state crystals and powders are
ground together (in a ball mill, for example)
has advantages over conventional
solvent-based methods, because it reduces
the use of hazardous solvents,
and can allow reactions to proceed faster
and at lower temperatures. Such advantages
have led to a renewed interest in
mechanochemical synthesis, which was
first proposed in the 1890s (see Chem.
Eng., December 2022, pp. 26-27).
However, because solid-state reactions
occur in a very different environment than
solution-based reactions, catalysts behave
differently. For example, previous
studies found that palladium-complex
catalysts originally designed for use in
solution often did not work sufficiently in
resources of
thesis of peroxodicarbonate (C2O6
2-. The fresh C2O6
2-)
from CO3
2- oxidant is
then used in a separate continuous-flow
electrolyzer with a boron-doped diamond
(BDD) anode and stainless-steel cooling
jacket that also serves as cathode. The
synthesis of the C2O6
2- oxidant is performed
" ex-cell " because it requires a
low (below 0°C) temperature, whereas
the depolymerization of lignin occurs at
a higher (over 100°C) temperature. With
this method, the researchers demonstrated
the oxidative depolymerization of
Kraft lignin at 180°C to produce vanillin
yields of up to 6.2 wt.%, " which is an outstanding
result, " emphasized Waldvogel.
(The maximum yield achievable to date
is 13 wt.% for nitrobenzene oxidation,
which is not used commercially due to
the toxicity and hazards associated with
nitrobenzene). Moreover, the carbonate
used is a necessary make-up chemical
for the operation of cellulose plants. This
means that there are various options for a
technical solution of combining the process
with the operation of a biorefinery.
A customized catalyst for solid-state reactions
solid-state mechanochemical reactions,
and that high reaction temperatures were
required. Using the unmodified palladium
catalyst for solid-state reactions resulted
in limited efficiency due to the tendency
of palladium to aggregate into an inactive
state. The research team designed a catalyst
to overcome this mechanochemical
problem of aggregation.
" We developed an innovative solution,
linking palladium through a specially designed
phosphine ligand to a large polymer
molecule called polyethylene glycol, "
explains chemistry professor Hajime Ito.
The polyethylene glycol molecules form
a region between the solid materials that
behaves like a molecular-level fluid phase,
where mechanochemical Suzuki-Miyaura
cross-coupling reactions proceed much
more efficiently and without the problematic
aggregation of palladium. In addition
to achieving significantly higher product
yields, the reaction proceeded effectively
near room temperature - the previously
best-performing alternative required
heating to 120°C.
The researchers believe their approach
could be adapted for many other reactions,
and also for catalysts using other
elements from the transition metals of the
periodic table.
n
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
APRIL 2023
willow growth. This tackles
the common criticism of many
renewable energy projects,
which is that land is lost from
food production.
NITROGEN RINGS
An international team with researchers
from the University of
Bayreuth (www.uni-bayreuth.
de) presents a potentially
groundbreaking discovery for
nitrogen chemistry in Nature
Chemistry: For the first time, a
compound containing aromatic
rings
of
nitrogen atoms has
been synthesized. The compound,
K9N56, was produced
under extremely high pressures
and temperatures. It has a very
complex structure, but its major
building block is the planar ring
of six nitrogen atoms, which is
called hexazine anion, [N6]4−.
The arrangement of nitrogen
atoms in [N6]4− is similar to that
of carbon atoms in benzene,
and the structure conforms to
Hückel's rule for aromaticity.
Although aromaticity was initially
thought to be exclusive to
carbon cycles, it has since been
shown that numerous systems
composed of carbon heterocycles
and non-carbon cycles
can have an aromatic character.
Nitrogen aromaticity, however,
has thus far been restricted to
the [N5]- pentazolate anion.
To make the K9N56, potassium
azide (KN3) and molecular nitrogen
were compressed under a
pressure higher than 400,000
atm and heated to 2,000°C
with high-power lasers. The
structure of K9N56 is composed
of a complex arrangement of
[N6]4− and [N5]- rings, as well as
neutral nitrogen dimers.
WATER-SOLUBLE PLASTIC
The University of Alicante's
(UA; San Vicente del Raspeig,
Alicante, Spain; www.ua.es)
Waste, Energy, Environment
and Nanotechnology research
group has developed a process
for obtaining a water-soluble
plastic based on potato starch,
which will soon be introduced
on the market through the UA
technology-based company
Solublion (www.solublion.com).
This new material is said to be
compostable and biodegradable,
and suitable for use as a
flexible film, preferably in bags
and packaging.
❐
9
http://www.uni-bayreuth.de http://www.uni-bayreuth.de http://www.uni-mainz.de http://www.uni-mainz.de http://www.global.hokudai.ac.jp http://www.ua.es http://www.solublion.com http://WWW.CHEMENGONLINE.COM
Chemical Engineering April 2023

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