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used for the first cycle and below 15 mg was used for the fifth
cycle. The overall high yield in all five cycles leads to higher
amounts of adsorbed species, which makes subsequent poly-
merization of FDCA more likely in the case of Ru/CTF-c and
leads to faster deactivation.
as a relatively cheap noble metal in the absence of a base. This
procedure facilitates product separation and enables environ-
mentally benign processing.
Experimental Section
As mentioned in our previous studies, physisorption indi-
cates the formation of polymeric byproducts on the catalyst
surface, which are not removed by simple washing. However,
the rather harsh reduction conditions applied in our studies
seem to be reliable to accomplish the decomposition of ad-
sorbed polymeric species and, therefore, facilitate catalyst reac-
tivation. It can be assumed that both the surface species and
oxidation of the metallic species are the most probable rea-
sons for deactivation and the leaching of the active species
plays a minor role. This could also be confirmed by inductively
coupled plasma optical emission spectroscopy (ICP-OES) of the
aqueous solution after catalysis as well as a hot-filtration test
(Table S1). In this test the filtrate after the removal of the cata-
lyst was allowed to react for 1 h at 1408C and 20 bar. This did
not result in the further conversion of HMF, although the FFCA
and FDCA yields increased slightly (1.4 and 0.1%, respectively),
which is likely to occur in the absence of catalyst. Nevertheless,
under these reduction conditions, agglomeration and the loss
of the external metal surface area cannot be excluded. Future
studies will focus on a more comprehensive understanding of
deactivation mechanisms that occur during the oxidation of
HMF over supported Ru catalysts. The discussed correlation of
activity, porosity, and surface polarity offers the first insights
with regard to the importance of tailored interactions between
the solvent, substrate, and catalyst surface.
Preparation of the catalysts
For the synthesis of CTF-a, 1,3-dicyanobenzene (0.621 g,
4.85 mmol, 1 equiv.) and ZnCl2 (3.305 g, 24.25 mmol, 5 equiv.) were
mixed and ground together, transferred into a quartz ampoule,
and dried in vacuum for at least 3 h. The ampoule was then flame-
sealed and placed inside a furnace for 10 h at 4008C and 10 h at
6008C (heating rate: 108CminÀ1). After cooling to RT, the ampoule
was broken open (CAUTION: the ampoules are under pressure,
which is released during opening), and the solid product was
ground and washed with water and diluted HCl (0.1m) thoroughly.
The solid material was then ground by using a ball mill (Fritsch Pul-
verisette23, 5 min, 30 Hz) to obtain a black powder, which was
washed successively with water, diluted HCl, diluted NaOH, water,
and THF, and was dried under vacuum for at least 12 h. Materials
based on 2,6-pyridinedicarbonitrile (CTF-b), 1,4-dicyanobenzene
(CTF-c), and 4,4’-biphenyldicarbonitrile (CTF-d) were synthesized in
the same way. For Ru impregnation, CTF (600 mg) was added to
a solution of RuCl3·xH2O (0.079 g, 0.381 mmol) in EtOH (400 mL)
heated to reflux and stirred for 6 h. After cooling to RT, the
RuIII@CTF material was then collected by filtration and washed with
EtOH to remove any uncoordinated Ru precursor. After drying
under vacuum at 608C for at least 12 h, the RuIII@CTF material was
reduced under H2 by using
a tube furnace (heating rate:
108CminÀ1, 3508C, H2 flow 100 mLminÀ1, 3 h) to obtain Ru/CTF
(for Ru loading see Table 1). Ru/C, Ru/g-Al2O3, Pd/C, and Pt/C cata-
lysts (5 wt%) were purchased from Sigma–Aldrich and were used
as received. The CTF materials were characterized by thermogravi-
metric analysis, elemental analysis, N2 and H2O vapor sorption
measurements, TEM, and XRD. N2 physisorption measurements
were conducted by using a Micromeretics ASAP 2010 measure-
ment device at À195.88C by a static volumetric method. H2O
vapor sorption measurements were performed by using an Auto-
sorb iQ2 measurement device at 19.58C by a static volumetric
method. For both sorption measurements samples were activated
at 2508C by using a FloVacDegasser for at least 15 h. The DPF was
calculated as follows [Eq. (1)]:
Conclusions
We have presented a new catalyst system based on Ru sup-
ported on covalent triazine frameworks (CTF) that catalyzes
the base-free oxidation of 5-hydroxymethylfurfural to 2,5-furan-
dicarboxilic acid in water efficiently. Bimodal micro- and meso-
porous CTF supports with large specific surface areas were ac-
cessible and contained variable amounts of N moieties. The
control of the structural parameters of the CTF tunes the cata-
lytic activity as microporous materials show only low activities
compared to mesoporous CTFs. The specific surface areas of
the materials play an important role during catalysis, and the N
content seems to influence the polarity of the material signifi-
cantly as indicated by H2O physisorption. The polarity of the
materials provided a further beneficial effect during catalysis
performed in water. High conversions and yields of furandicar-
boxylic acid could be obtained at 1408C and 20 bar initial air
pressure. Both Ru/C and Ru/CTF-c showed promising recycla-
bility. Nevertheless, to obtain satisfactory C balances, washing
with DMSO was inevitable. Reactivation of the catalyst is chal-
lenging, and rather harsh conditions (3508C, H2 flow) need to
be applied to maintain the catalyst activity. However, only
batch reactions have been performed so far, and continuous
reactions will be conducted in future studies. Furthermore, the
oxidation could be performed with air as the most abundant
and sustainable oxygen resource over a catalyst based on Ru
V
PðH2OÞ
DPF ¼
 100%
ð1Þ
VPðtotalÞ
with VP(H O) being the water uptake determined by H2O vapor phys-
2
isorption at p/p0 =0.90 and VP(total) being the total pore volume de-
termined at p/p0 =0.98.
The Ru-doped materials were analyzed by ICP-OES, SEM-EDX, TEM,
and XRD.
Selective oxidation of HMF to FDCA
Typically, a stainless-steel autoclave (75 mL) with a glass inlet was
charged with a solution of HMF (0.1261 g, 1 mmol) in H2O (15 mL).
The catalyst (0.05 g for Ru- and Pd-based catalysts, 0.10 g for Pt/C,
HMF/metal molar ratio: 40:1) was added, and the autoclave was
equipped with a stirring bar and temperature sensor. It was sealed,
pressurized to 20 bar with synthetic air (hydrocarbon free), and
heated to 1408C with stirring at 500 rpm. After a certain time, the
autoclave was cooled and depressurized. The reaction mixture was
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