Full Papers
the autoclave to the desired temperature of 808C in approxi-
mately 10 min. Surprisingly, full conversion is already achieved
after 3 h for Ru/CTF-a. In contrast, 6 h is required until full con-
version with Ru/C. At approximately 84% conversion, Ru/CTF-
active species into solution. The conversion did not increase if
the filtered reaction mixture was used under the reaction con-
ditions. Nevertheless, the current reactivation procedure relies
on rather harsh conditions. The agglomeration of metal nano-
particles together with an associated loss of external metal sur-
face area could cause the observed decrease in conversion and
À3
À1 À1
a achieves a productivity of 25.4ꢁ10 molg h , which is ap-
proximately seven times higher than the productivity of 3.3ꢁ
À3
À1 À1
10
molg
h
for Ru/C. DFF yields remain limited to approxi-
DFF yield. Furthermore, N physisorption studies on the Ru/
2
mately 78%, which corresponds to our earlier observations. As
no significant byproduct formation occurred and the reaction
solutions remained colorless, we suggest the adsorption of
HMF and DFF on the catalyst surface. Nevertheless, the forma-
tion of polymeric byproducts that adsorb on the catalysts
cannot be excluded. For further comparison, the effect of the
temperature on the activity and selectivity has been studied
for Ru/CTF-a. After 1 h at 908C, a conversion of 92.3% and
a 73.4% DFF yield were obtained. A further increase of the
temperature to 1008C led to 99.2% conversion and 77.9% DFF
yield. Interestingly, after washing the catalyst with 15 mL of
acetone and considering the extracted compounds in the
CTF-a catalyst as prepared and after five cycles reveal a signifi-
cant loss of pore volume and surface area (Figure S1 and
Table S1). This effect emphasizes a strong substrate and prod-
uct adsorption as mentioned previously together with poten-
tial polymer formation. Consequently, further optimization of
catalyst preparation and regeneration has to aim for reduced
adsorption properties and suitable means to facilitate the com-
plete removal of polymeric deposits. Therefore, future studies
will aim to optimize recycling and reactivation conditions to-
gether with a continuous operation for HMF oxidation.
mass balance, DFF yields could be further increased to 86.8 Conclusions
908C, conversion (X)=90.5%) and 92.0% (1008C, X=99.1%),
(
An efficient catalyst system based on Ru supported on cova-
lent triazine frameworks (CTFs) was developed for the selective
oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfur-
an (DFF) under aerobic conditions. The bimodal and mesopo-
rous Ru/CTF catalysts showed high conversions and DFF yields
at low temperatures using air as the sole oxidant and methyl t-
butyl ether as an easily separable and reusable solvent. The
catalytic activity depends strongly on the structural parameters
of the CTF materials, such as specific surface area and total
pore volume. These parameters can be controlled by the
choice of the linker as well as the synthesis time for the frame-
work. High conversions of 97.3% and DFF yields of 72.7%
could be obtained after only 3 h at 808C using 20 bar of air. At
which leads to a nearly closed mass balance. This gives further
evidence for the strong adsorption of both product and sub-
strate as suggested before. At the same time, the conversion
decreases slightly, as both DFF and HMF are adsorbed on the
catalyst surface.
Ru/CTF-a and Ru/C have been recycled to investigate their
stability (Figure 7). Both catalysts showed a strong loss in activ-
ity if simply washed with an organic solvent, dried under
84% HMF conversion, the productivity of Ru/CTF-a was nearly
seven times higher than that of Ru/C. The recycling of these
Ru-based catalysts is still challenging. Nevertheless, Ru/CTF-
a exhibits only minor deactivation if a reactivation procedure is
applied under H flow. Therefore, this concept paves the way
2
for an environmentally benign continuous production of bio-
mass-derived chemicals that does not depend on diminishing
fossil resources and enables sustainable industrial processing.
Figure 7. Recycling study of a) Ru/C and b) Ru/CTF-a. Reaction conditions:
1
h, 808C, 20 bar of air, 15 mL MTBE, 500 rpm stirring speed, HMF/metal
molar ratio 40:1. Catalysts were reactivated after each run.
Experimental Section
vacuum, and reused without further treatment. Our findings
suggest that not only polymeric surface species but also the
surface oxidation of the supported metal nanoparticles cause
the observed loss of activity. Therefore, catalysts were reacti-
Catalyst preparation
For the synthesis of CTF-a, 1,3-dicyanobenzene (0.621 g,
4.85 mmol, 1 equiv.) and ZnCl (3.305 g, 24.25 mmol, 5 equiv.) were
2
vated at 3508C for 3 h under H flow. With this strategy, a sig-
mixed and ground together, transferred into a quartz ampoule,
and dried under vacuum for at least 3 h. The ampoule was then
flame-sealed and placed inside a furnace for 10 h of heat treatment
2
nificantly reduced deactivation could be achieved together
with the stable catalytic activity of Ru/CTF-a after two recycling
steps. Overall, Ru/CTF-a exhibits not only superior activity but
also minor deactivation compared to Ru/C. We assign this ob-
servation to the N functionalities of the support that provide
a stabilizing effect to the Ru species and hinder agglomeration
À1
at 4008C and a further 10 h at 6008C (heating rate: 10 Kmin ).
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 thoroughly with
water and dilute HCl (0.1m). The solid material was then ground in
a ball mill (Fritsch Pulverisette23, 5 min, 30 Hz) to obtain a black
powder, which was washed successively with water, dilute HCl,
[
24,25]
and leaching of metal species.
Additionally, hot filtration
tests were performed to exclude the leaching of catalytically
&
ChemSusChem 0000, 00, 0 – 0
6
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!