INSIGHTS | PERSPECTIVES242 15 APRIL 2022 • VOL 376 ISSUE 6590GRAPHIC: N. CARY/SCIENCEscience.org SCIENCEBy Edmond Gravel and Eric DorisC
atalysis plays a central role in mod-
ern synthetic chemistry by reducing
the activation energy needed for a
reaction to take place. Among metal
catalysts, copper (Cu) is an inexpen-
sive and highly versatile catalyst
with applications ( 1 ) such as in hydrogena-
tion reactions used in the bulk production
of ethylene glycol from dimethyl oxalate
(DMO). During the DMO-to-ethylene gly-
col transformation, high pressure of hydro-
gen gas (H 2 ) is usually needed. This can be
problematic from an engineering and safety
perspective, and can also lead to catalyst de-activation. On page 288 of this issue, Zheng
et al. ( 2 ) provide a solution to the pressure
critical point by associating a conventional
Cu catalyst with fullerenes, which are mol-
ecules made of carbon atoms organized in
a spherical structure such as in C 60. The
method stabilizes the catalytic species
and enables the hydrogenation of DMO
into ethylene glycol under mild conditions
of pressure.Ethylene glycol is a commodity chemical
with applications in many areas of everyday
life. It is used as a solvent, a coolant in auto-
motive radiators, a deicing fluid for aircraft,
and a building block in the synthesis of poly-
ester fibers and resins ( 3 ). The current global
ethylene glycol production capacity is ap-
proximately 42 million metric tons per year
and is forecast to exceed 70 million metric
tons by 2025 ( 4 ). The industrial synthesis of
ethylene glycol relies mostly on the oxida-
tion of ethylene followed by thermal hydra-
tion of the resulting ethylene oxide (see the
figure), a process established in the 1930s by
the Union Carbide Corporation. However,
ethylene is a petroleum-based chemical, andother routes have recently emerged from
more sustainable feedstocks. The path from
synthesis gas (syngas), a gasification product
of coal or biomass ( 5 ) that yields primarily a
mixture of hydrogen and carbon monoxide
(CO), is of particular interest. This two-step
process involves converting CO into DMO by
means of oxidative coupling with methanol,
followed by hydrogenation of DMO ( 6 ).
The most challenging part of this syn-
thetic sequence is the hydrogenation of DMO
to ethylene glycol because high pressures of
hydrogen gas at elevated temperatures are
required for the reaction to be effective ( 7 ).
Metals such as Cu catalyze the hydrogena-
tion of DMO ( 8 ), but the drastic conditionsrequired for the reaction to occur can lead to
catalyst deactivation ( 9 ) as well as safety and
environmental issues.
To stabilize Cu and make the catalytic
hydrogenation of DMO more robust, Zheng
et al. came up with the idea of associating
Cu to an electron-buffer fullerene unit to
keep the right balance between the catalyti-
cally active forms of the metal throughout
the hydrogenation process. Cu can exist in
various oxidation states, ranging from the
elemental Cu^0 to the ion Cu4+, but only Cu^0
and Cu+ are useful for the hydrogenation
reaction of DMO to ethylene glycol. It is
thought that Cu^0 and Cu+ act in a synergistic
fashion, with Cu^0 promoting the dissocia-
tion of H 2 , and Cu+ promoting the addition
of H to DMO ( 10 ). Fullerenes, specifically
the C 60 “buckyballs” ( 11 ), are electronically
active and can accept and, to some extent,
give back electrons. This electron-buffering
property was exploited by Zheng et al. to de-
sign a hybrid catalyst that associates Cu and
C 60 on an inert silica platform: C 60 –Cu/SiO 2.
In this setup, the fullerenes electronically
interact with Cu to protect the unstable Cu+
from oxidation and reduction and maintain
a catalytically active Cu^0 /Cu+ ratio. Evidence
of the buffering effect of fullerenes on Cu
was obtained through various techniques,
including cyclic voltammetry, and further
supported with theoretical calculations.
Zheng et al. demonstrated that C 60 could act
sequentially as a single-electron acceptor
(from Cu^0 ) and a donor (to Cu2+) to prevent
the active Cu+ catalyst payload from chang-
ing oxidation state.
When compared with a conventional
Cu/SiO 2 catalyst, the fullerene-buffered Cu
catalyst C 60 –Cu/SiO 2 showed strong perfor-
mances in the vapor phase conversion of
DMO. And in the case of C 60 –Cu/SiO 2 , the
transformation could be carried out under
ambient pressure of H 2 gas, whereas until
now, standard catalysts required high hy-
drogen pressures of more than 20 bar. At
ambient pressure, the C 60 –Cu/SiO 2 catalyst
leads to a 10-fold increase in ethylene gly-
col yield (98%) compared with that of Cu/
SiO 2 and is more selective because it gener-
ates fewer by-products. The use of 12 g of
C 60 –Cu/SiO 2 permitted the flow production
of ethylene glycol on a multikilogram scale
with no alteration of the catalyst over 1000
hours of operating time at ~180°C. After the
reaction was complete, the catalyst couldCATALYSISFullerenes make copper catalysis better
Ethylene glycol can be reliably produced by mild hydrogenation of dimethyl oxalate
Université Paris-Saclay, CEA, Institut National de
Recherche pour l’Agriculture, l’Alimentation et
l’Environnement (INRAE), Département Médicaments et
Technologies pour la Santé (DMTS), Service de Chimie
Bioorganique et de Marquage (SCBM), 91191 Gif-sur-Yvette,
France. Email: [email protected]SyngasOilCarbon
monoxide
CO20 bar of
HydrogenCatalystDimethyl
oxalate
(CO 2 CH 3 ) 21 bar of
HydrogenHigh yield
and
high
selectivityEthylene
C 2 H 4Ethylene oxide
C 2 H 4 OEthylene glycolCopper-fullerene
(on silica)Copper
(on silica)C 2 H 6 O 2Toward a more sustainable ethylene glycol production
Ethylene glycol is classically produced from petroleum-based ethylene (top). A more sustainable production
method uses dimethyl oxalate obtained through oxidative dimerization of carbon monoxide from syngas
but requires a hydrogen pressure of >20 bar (middle). Zhang et al. designed a copper-fullerene catalyst that
eliminates the pressure requirement (bottom).