Full Papers
evolved from expensive metals and ligands, to base metals.[10]
Although most reports are still based on Ru[11] and Ir, recent
efforts have focused on base metals such as Fe,[12,13,14] Mn[15] and
Co.[16,17]
ally, more work is needed in order to pinpoint the active phase
and the relative role of Cu and Ni in the mechanism of the
reaction, calling for an extensive characterization study detail-
ing the nanostructure of the catalyst before and after reaction
Thus, developing waste-efficient synthesis of well-defined
NPs on a big scale is imperative for far-reaching catalytic
applications. Furthermore, despite their attractiveness, hetero-
geneous base metal catalysts (Cu, Ni, Mn, Fe) still require high
loadings to offset their lower activity. For instance, up to 38 mol
% Ni loading were required in Shi’s report. A careful nano-
structure design could minimize the metal amounts. To obtain
crystalline and well-defined NPs, high temperatures are re-
quired to decompose NP precursor and allow ordered growth.
In liquid phase though, ligand-free NP agglomeration is difficult
to avoid, driving the search for solid-state[41] or gas phase
processes.[42] Taking these considerations into account, we
turned to induction plasma as a technique for the synthesis of
Ni/Cu-containing NPs. Temperatures as high as ~6.000 K and
~10.000 K can be reached in plasma reaction chambers,
allowing in situ annealing and precise phase control.[43] Through
a strict control of precursor residence time and growth
quenching, plasma induction allows the single-step production
of monodisperse and crystalline NPs in high quantities (up to
30 g/h for 50 kW units) that is easily scalable with higher energy
torches and larger reactors.[44] Radio-frequency (RF) plasma
induction has already been used for the synthesis of ferric oxide
nanoparticles from their corresponding metal nitrate salts, such
as ZnFe2O4[45] and NiFe2O4.[46] Herein we report the first example
of nanocatalysts for alcohol amination, using (Ni0.5Cu0.5)Fe2O4
NPs made by plasma induction. The catalyst was easily
recyclable by magnetic separation and it was able to catalyze
the reaction at low metal loadings comparable to that of
homogeneous catalysts (<2 mol% of metal). To the best of our
knowledge, no mixed ferrite synthesis from a RF plasma reactor
has been reported so far, nor was the use of plasma-made
catalysts in liquid-phase organic reactions.
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In an effort to overcome recycling and separation issues
associated with these expensive metal complexes, heteroge-
neous catalysts have been extensively explored.[18] Both Ru NPs
[19]
supported on Fe3O4
and Ag NPs have shown a strong
potential.[20,21,22] Bimetallic catalysts have proven particularly
attractive, enabling synergistic effects (AuÀ Pd)[23] and also
allowing “dilution” of the precious metal with a cheaper one
(AgÀ Cu,[20] PtÀ Sn,[24] Cu-Au[25], Pd-Zn[26], Co-Rh[27]). From our
perspective though, abundant metals should be favored to
exploit the full potential of heterogeneous catalysis towards
eco-friendlier processes. Some have explored Fe3O4 NPs,[28] Cu
NPs,[29] zeolites,[30] Cu-Al[31], and even metal-free graphene oxide
sheets[32] as catalysts for alcohol amination. In most examples
though, a base (KOH, K2CO3) is required to promote proton
removal in the alcohol dehydrogenation step. Yet, the presence
of a base is undesirable during the reaction as it may also favor
side reactions typical for alcohols and transient carbonyls
(Cannizzaro condensation, alcohol dehydration, and aldol
condensation). Heterogeneous Ni has been widely explored
since its first report in 1932 by Winans et al., requiring though
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100 bar H2 at 200 C for the alkylation of cyclohexylamine with
EtOH.[33] Ni has also been shown by Shimizu and others to be
highly competent for BH when supported on θ-Al2O3,[34] and
can even perform ammonia alkylation in flow synthesis.[35,36]
However, Ni is a carcinogenic and allergenic metal, driving the
search towards partial substitution with other metals, for
instance Cu.[37] The Ni/Cu pair is indeed active for alcohol
amination: Cu acts as a dehydrogenation catalyst for the
starting alcohol, while Ni performs the hydrogenation of the
imine.[38] Cu and Ni stearates with various stabilizers such as Ba
stearate have been reported the production of long alkyl chains
N,N-dimethylamines.[39] The Shi group reported a NiCuFeOx
catalyst (3.6/1.1/1 weight ratio) through the calcination of metal
carbonates then reduction under a H2 flow.[40] Their alcohol
amination scope was extensive, with 113 examples (up to 98% Results and Discussion
yield, Scheme 2).
In all these examples, NP synthesis are produced using well-
established synthesis methods such as coprecipitation or sol-
gel. These methods are known for their good control over the
particle size and composition. However, the synthesis steps can
be time-consuming and suffer from batch-to-batch reproduci-
bility issues. These drawbacks become problematic for a scaled
up, industrial production for which the catalyst‘s properties
need to be reproducible from one batch to the next. Addition-
First, (Ni0.5Cu0.5)Fe2O4 NPs were synthesized in an induction
thermal plasma reaction chamber, following a procedure al-
ready reported previously by us for the synthesis of NiFeO2
NPs[46] and using a plasma setup described elsewhere (Fig-
ure 1).[47] The synthesis consisted in the coaxial injection
aqueous solution of metal precursors (Fe, Ni and Cu nitrates in
a 4/1/1 molar ratio) with a carrier gas (Ar) into an inductively
coupled thermal plasma torch. A sheath gas of Ar/O2 was used
to control the trajectory of the NPs and provide them with
oxygen atoms. The tip of the atomization probe coincides with
the center of an induction thermal plasma torch connected to a
3 MHz RF power supply. As shown in Figure 1, the plasma torch
connects into the top of a water-cooled cylindrical chamber
(Main reactor).
At its bottom, the main reactor is connected to another
water-cooled cylindrical chamber (Auxiliary chamber) which
contains 4 microporous filters connected to a vacuum pump.
Scheme 2. Former example of Ni/Cu for alcohol amination.
ChemCatChem 2019, 11, 1–15
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