The Catalyst Review February 2020 - 13

EXPERIMENTAL
Straightforward Path to Linear
Alkylbenzenes Identified...

New Catalyst Recycles Greenhouse Gases Into Fuel and
Hydrogen Gas...

John Hartwig of the University of
California, Berkeley; Yoshiaki Nakao
of Kyoto University; and coworkers
have coupled select arenes with an
unconjugated terminal alkene to form
linear alkylbenzenes in one step with
85-96% yields and high selectivity. The
current acylation and reduction methods
to produce these compounds is through
a Friedel-Crafts acylation, which requires
multiple steps. The new reaction is
the first to be done in one step, with
approximately 50:1 linear to branched
selectivity, Hartwig says. In addition, the
catalyst, a nickel cycloocta-1,5-diene
compound, could turn over 280 times and
still give an 85% yield. This turnover is
more than 10-fold higher than reactions
involving catalytic hydroarylation of
benzene, Hartwig says.

Scientists have taken a major step toward a circular carbon economy by developing
a long-lasting, economical catalyst that recycles greenhouse gases into ingredients
that can be used in fuel, hydrogen gas, and other chemicals. The results could be
revolutionary in the effort to reverse global warming, according to the researchers.
"We set out to develop an effective catalyst that can convert large amounts of the
greenhouse gases carbon dioxide and methane without failure," said Cafer T. Yavuz,
paper author and associate professor of chemical and biomolecular engineering and of
chemistry at the Korea Advanced Institute of Science and Technology (KAIST).

Normally in these reactions, the alkenes
isomerize between internal and terminal
alkenes, Hartwig says. If the reaction
is not selective for terminal alkenes, it
will either give a mixture of linear and
branched alkylbenzenes, or more or less
stop. This new reaction only occurs with
the terminal alkene, which gets around
the isomerization problem. Hartwig and
coworkers found that the reaction goes
through an alkyl nickel-aryl intermediate
and involves an unexpected hydrogen
transfer between catalyst ligands followed
by reductive elimination. Further analysis
of the mechanism revealed that the bulk
of the N-heterocyclic carbene ligands did
not affect the activity. The reactivity has
more to do with noncovalent attractive
interactions between ligands, instead of
repulsive ones, Hartwig says.
If the catalyst activity were high enough,
this reaction would be easy to scale up to
the industrial level, Hartwig says, because
it's a simple addition reaction, and there
are no side products. However, it's not
perfect. "The biggest drawback of this
system, in addition to catalyst lifetime,
is its low tolerance for a lot of functional
groups," such as esters and nitriles, he
says. Source: Chemical & Engineering
News, 2/17/2020, p. 8.

The catalyst, made from inexpensive
and abundant nickel, magnesium,
and molybdenum, initiates and
speeds up the rate of reaction
that converts carbon dioxide and
methane into hydrogen gas. It can
work efficiently for more than a
month. This conversion is called
"dry reforming," where harmful
gases, such as carbon dioxide, are
processed to produce more useful
chemicals that could be refined
for use in fuel, plastics, or even
pharmaceuticals. It is an effective
process, but it previously required
rare and expensive metals such as
platinum and rhodium to induce a
brief and inefficient chemical
reaction.

Newly developed catalyst that recycles greenhouse
gases into ingredients that can be used in fuel, hydrogen
gas and other chemicals. Credit: The Korea Advanced
Institute of Science and Technology (KAIST)

Other researchers had previously proposed nickel as a more economical solution, but
carbon byproducts would build up and the surface nanoparticles would bind together
on the cheaper metal, fundamentally changing the composition and geometry of the
catalyst and rendering it useless. "The difficulty arises from the lack of control on scores
of active sites over the bulky catalysts surfaces because any refinement procedures
attempted also change the nature of the catalyst itself," Yavuz said.
The researchers produced nickel-molybdenum nanoparticles under a reductive
environment in the presence of a single crystalline magnesium oxide. As the ingredients
were heated under reactive gas, the nanoparticles moved on the pristine crystal surface
seeking anchoring points. The resulting activated catalyst sealed its own high-energy
active sites and permanently fixed the location of the nanoparticles-meaning that the
nickel-based catalyst will not have a carbon build up, nor will the surface particles bind
to one another.
The researchers dubbed the catalyst Nanocatalysts on Single Crystal Edges (NOSCE).
The magnesium-oxide nanopowder comes from a finely structured form of magnesium
oxide, where the molecules bind continuously to the edge. There are no breaks or
defects in the surface, allowing for uniform and predictable reactions. "Our study solves
a number of challenges the catalyst community faces," Yavuz said. "We believe the
NOSCE mechanism will improve other inefficient catalytic reactions and provide even
further savings of greenhouse gas emissions." Source: PhysOrg, 2/17/2020.

The Catalyst Review

February 2020

The Catalyst Review February 2020

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