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were produced in the obtained carbons, which may serve as
the voids for the encapsulation of metal SNCs. According to
the Kirkendall effect, the newly formed defects tended to
coalesce into micro-, meso- and eventually macropores
accompanied with the outwards migration of carbon atoms,
which was largely boosted in the liquid-phase molten salt
FFA oxidation. It can be seen from Figure 3a that a Cu:Pd
molar ratio of 1:1 was optimal to achieve both high activity
and high MA selectivity.
The FFA conversion and product selectivity as a function
of time over the Cu-Pd@HPC are depicted in Figure 3d. In
view of the reaction results, a plausible reaction pathway for
medium. Subsequently, with a drop-in temperature, the KCl– the transformation of FFA to MA over Cu-Pd@HPC is
KBr molten salt was precipitated and removed by washing,
leaving the Cu SNCs homogeneously encapsulated within the
hierarchical carbons consisting of abundant micro-, meso- and
macropores. Finally, the nanoconfined Cu-Pd, Cu-Pt and Cu-
Ru SNCs were formed upon the galvanic replacement under
ultrasonic irradiations. Whereas, in the absence of KCl–KBr,
the coalescence process would be too slow to form macro-
pores. On the other hand, the structure of Cu/C might shrink
in the subsequent temperature-drop process, which could
possibly cause the collapse of the as-formed porous nano-
structure. As a result, the as-prepared Cu/C composite
featured irregular porous nanostructures without obvious
macropores (Figure S9).
proposed (Figure 3 f). Initially, the FFA molecule undergoes
a fast decarbonization-oxidation process to form furan-2(5H)-
one (FAO), a highly active compound, which is easily
converted into 5-hydroxy-2(5H)-furanone (HFAO). The
generated HFAO is oxidized on its -OH group into maleic
anhydride (MAN) and finally hydrolyzed to produce MA in
the presence of H2O. Another possible reaction path involves
the conversion of FFA to ’FAO, i.e., the isomer of FAO, and
the subsequent transformation of ’FAO into succinic acid
(SA). Under the investigated reaction conditions, FAO and
HFAO were both detected as the intermediates during FFA
oxidation over the Cu-Pd@HPC catalyst, whereas the SA
yield was only 2.1%.
The distinctive nano-architectures of Cu-M@HPC stim-
ulate us to investigate their catalytic performances in
challenging liquid-phase reactions to obtain value-added
chemicals. As we know, maleic acid (MA) is an important
reaction intermediate and extensively employed in the
manufacture of unsaturated polyester resins, pharmaceuticals
and food additives.[22] Unfortunately, the current MA pro-
duction relies heavily on petroleum-based feedstock.[23] In this
regard, the direct oxidation of furfural (FFA), a key platform
molecule industrially produced from cellulosic biomass,
represents a sustainable route to prepare MA.[24] Thus, in
recent years, a variety of catalytic systems have been
developed for the oxidation of FFA to MA. However, it
remains a big challenge to simultaneously achieve the goals of
both high activity and selectivity.[25]
The reactions were carried out at 508C and 1 atm air with
the presence of H2O2. Only 6.5% MA yield was attained at
32.6% FFA conversion within 4 h in the absence of any
catalyst. For comparison, the Cu/C and Cu-Pd/C materials
derived from Cu-BDC were also prepared and employed as
the catalysts (Figure S9), which both showed moderate MA
yields of 37.6% and 45.9% at 67.4% and 85.5% FFA
conversions, respectively (Figure 3a). Besides, a series of
metallic Cu, Pd and Cu-Pd catalysts on different carbonous
supports (e.g., active carbon and graphene, Figures S15,16)
were also prepared and employed in this reaction. These
catalysts all showed unsatisfied catalytic performances with
less than 55% FFA conversions and 16% MA yields
(Table S4). To our delight, all the Cu-M@HPC composites
were highly active for FFA transformation with high turnover
frequencies (TOF) (Figure 3c). Among them, Cu-Pd@HPC
exhibited the best catalytic performance with an up to 97.8%
selectivity towards MA at complete conversion (TOF =
20.1 hÀ1), outperforming all the previously reported hetero-
geneous catalysts under milder or similar reaction conditions
(Table S3). To examine the effect of Cu/Pd molar ratios on
their catalytic performances, a series of Cu-Pd@HPCÀx
(where x represents the molar ratio of Cu/Pd, x = 0:5, 1:4,
4:1, and 5:0) catalysts were prepared and also applied in the
Density functional theory (DFT) calculation was con-
ducted to simulate the reaction process and provide theoret-
ical insights to the reaction mechanisms. The densities of state
of Cu@HPC, Cu-Pd@HPC, Cu-Pt@HPC, and Cu-Ru@HPC
concentrating on the states of d orbitals of Cu were simulated
to investigate their electronic properties (Figure 3e). The Cu-
Pd SNCs in Cu-Pd@HPC were electron-rich centers possess-
ing the up-shifted Cu d orbital, which is much closer to the
Fermi level in comparison with that of the other three
counterparts (Cu-Pt, Cu-Ru and pristine Cu). According to
the d band center theory, the Cu-Pd@HPC has much
strengthened adsorption towards FFA, which largely contrib-
utes to its high catalytic activity.[26] In order to further
interpret the excellent MA selectivity of the Cu-Pd@HPC
catalyst, periodic DFT calculations were carried out to
simulate the adsorption of FAO and ’FAO molecules over
the (111) surfaces of Cu-Pd SNCs (Figures 3 f,g and S10, S11).
The positive adsorption energies of FAO and ’FAO on Cu
(111) facets (0.485 eV and 0.544 eV, respectively) suggest the
thermodynamical infeasibility and therefore difficulty in their
adsorption under the investigated conditions. In comparison,
both FAO and ’FAO exhibited negative adsorption energies
over the Cu-Pd (111) facets (À0.236 and À0.105 eV, respec-
tively), implying their spontaneous adsorption on Cu-Pd
SNCs. The relatively low adsorption energy of FAO suggests
its stronger adsorption over the Cu-Pd (111) facets, which
therefore boosted its subsequent transformation into HFAO
and finally MA. In view of the facts that the Cu-Pd SNCs
possess a suitable adsorption energy towards FAO rather than
’FAO, and they also show a suitable adsorption energy for the
intermediate HFAO, it could be understandable that the Cu-
Pd SNCs achieved excellent activity and MA selectivity in
FFA conversion.
The stability and recyclability of a heterogeneous catalyst
are of vital importance for potential applications. Thus, a hot
filtration experiment was performed to examine the possible
metal leaching during the reaction. As shown in Figure S12a,
a slight increment in conversion was observed after filtration,
which could be attributed to the fact that oxidation of FFA
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Angew. Chem. Int. Ed. 2021, 60, 10842 –10849