332
the authors observed that weak base such as CaCO 3 is attractive and showed high
yield (95%) of FDCA under O 2 pressure (2 bar) (Table 11, entries 8–12). Moreover,
a clear solution was obtained in the presence of weak bases, while a brownish
coloured solution was obtained for strong bases (NaOH or KOH) owing to the degradation of HMF and promotion of side reactions. Ru/C catalyst was applied in the
absence of base as well. At optimized reaction condition (120 °C, 5 bar O 2 for 10 h),
the catalyst afforded 88% yield of FDCA with complete conversion of HMF
(Table 11, entry 13). Recyclability for Ru/C was also examined, and after each reaction, the catalyst was washed with methanol before being used, that showed good
activity and stability for three reaction cycles. From the control experiments, the
authors proposed the reaction path, following the Route-A mechanism (Scheme 16)
via intermediate DFF and FFCA.
Mishra et al. discussed supported Ru catalysts for air oxidation of HMF to FDCA
in aqueous medium and absence of base. In this work, supports were chosen from
various Mg- and Co-based oxides such as MnCo 2 O 4 , CoMn 2 O 4 , MnCo 2 CO 3 , MnO 2
and CoO [167]. Among them, Ru/MnCo 2 O 4 exhibited the highest selectivity for
FDCA (99.1% yield) at relatively milder reaction conditions (120 °C, 24 bar air for
10 h) (Table 11, entries 14–18). The appropriate acidic sites in Ru/MnCo 2 O 4 catalyst was confirmed by NH 3 -TPD, Bronsted acid sites (10.7 mmol/g) and Lewis acidcoordinated NH 3 (7.3 mmol/g), i.e. a total of 18 mmol/g aided in the selective
formation of FDCA. Moreover, MnCo 2 O 4 support is a crystalline cubic system,
whereas others have distorted spinel structures (e.g. CoMn 2 O 4 is body-centred
tetragonal phase). Generally, the spinel system has more adsorption affinity of oxygen. Additionally, using MnCo 2 O 4 support had high surface area (151.1 m
2
/g) and
high Ru dispersion % (39.2) which were beneficial for its higher activity. Ru/
MnCo 2 O 4 was reused for five cycles with similar conversion and selectivity. ICP
analysis precluded even traces of leached Ru metal in the solution.
Yang et al. investigated the effect of reaction parameters and solvents towards
HMF oxidation using Ru
III
-incorporated zirconium phosphate (ZrP-Ru) catalyst
[168]. Various solvents were used for this study including H 2 O, MeCN, ethanol,
methanol, DMSO, MIBK, toluene and p-chlorotoluene. Among these, DMSO, toluene and p-chlorotoluene resulted in high selectivity of FDCA (Table 11, entries
19–25). In the case of alcohol solvents, etherified product was predominant which
could be attributed to the Lewis acidity of Zr. Addition of base and increasing the
temperature of the reaction improved the selectivity of FDCA (Table 11, entry 26).
The decrease in selectivity of FDCA may be associated with the feed pattern of O 2
pressure, as the authors flushed the reaction set-up with oxygen under atmospheric
pressure at a flow rate of 20 mL/min.
Pichler et al. demonstrated different ZrO 2 -supported ruthenium catalysts for this
conversion in aqueous basic medium using O 2 gas as the oxidant (10 bar) [169]. The
high surface area-containing catalysts such as Ru/ZrO 2 H-SBA and Ru/ZrO 2 H- aero
showed high catalytic activity and 97% and 100% conversion of HMF with 87%
and 97% selectivity of FDCA (Table 11, entries 27 and 28). The high activity of
these supports is associated with high surface area; for Ru/ZrO 2 H-SBA the surface
area is 256 m
2
/g and 239 m
2
/g for Ru/ZrO 2 H- aero. The experiments show that the
S. Gundekari et al.
the authors observed that weak base such as CaCO 3 is attractive and showed high
yield (95%) of FDCA under O 2 pressure (2 bar) (Table 11, entries 8–12). Moreover,
a clear solution was obtained in the presence of weak bases, while a brownish
coloured solution was obtained for strong bases (NaOH or KOH) owing to the degradation of HMF and promotion of side reactions. Ru/C catalyst was applied in the
absence of base as well. At optimized reaction condition (120 °C, 5 bar O 2 for 10 h),
the catalyst afforded 88% yield of FDCA with complete conversion of HMF
(Table 11, entry 13). Recyclability for Ru/C was also examined, and after each reaction, the catalyst was washed with methanol before being used, that showed good
activity and stability for three reaction cycles. From the control experiments, the
authors proposed the reaction path, following the Route-A mechanism (Scheme 16)
via intermediate DFF and FFCA.
Mishra et al. discussed supported Ru catalysts for air oxidation of HMF to FDCA
in aqueous medium and absence of base. In this work, supports were chosen from
various Mg- and Co-based oxides such as MnCo 2 O 4 , CoMn 2 O 4 , MnCo 2 CO 3 , MnO 2
and CoO [167]. Among them, Ru/MnCo 2 O 4 exhibited the highest selectivity for
FDCA (99.1% yield) at relatively milder reaction conditions (120 °C, 24 bar air for
10 h) (Table 11, entries 14–18). The appropriate acidic sites in Ru/MnCo 2 O 4 catalyst was confirmed by NH 3 -TPD, Bronsted acid sites (10.7 mmol/g) and Lewis acidcoordinated NH 3 (7.3 mmol/g), i.e. a total of 18 mmol/g aided in the selective
formation of FDCA. Moreover, MnCo 2 O 4 support is a crystalline cubic system,
whereas others have distorted spinel structures (e.g. CoMn 2 O 4 is body-centred
tetragonal phase). Generally, the spinel system has more adsorption affinity of oxygen. Additionally, using MnCo 2 O 4 support had high surface area (151.1 m
2
/g) and
high Ru dispersion % (39.2) which were beneficial for its higher activity. Ru/
MnCo 2 O 4 was reused for five cycles with similar conversion and selectivity. ICP
analysis precluded even traces of leached Ru metal in the solution.
Yang et al. investigated the effect of reaction parameters and solvents towards
HMF oxidation using Ru
III
-incorporated zirconium phosphate (ZrP-Ru) catalyst
[168]. Various solvents were used for this study including H 2 O, MeCN, ethanol,
methanol, DMSO, MIBK, toluene and p-chlorotoluene. Among these, DMSO, toluene and p-chlorotoluene resulted in high selectivity of FDCA (Table 11, entries
19–25). In the case of alcohol solvents, etherified product was predominant which
could be attributed to the Lewis acidity of Zr. Addition of base and increasing the
temperature of the reaction improved the selectivity of FDCA (Table 11, entry 26).
The decrease in selectivity of FDCA may be associated with the feed pattern of O 2
pressure, as the authors flushed the reaction set-up with oxygen under atmospheric
pressure at a flow rate of 20 mL/min.
Pichler et al. demonstrated different ZrO 2 -supported ruthenium catalysts for this
conversion in aqueous basic medium using O 2 gas as the oxidant (10 bar) [169]. The
high surface area-containing catalysts such as Ru/ZrO 2 H-SBA and Ru/ZrO 2 H- aero
showed high catalytic activity and 97% and 100% conversion of HMF with 87%
and 97% selectivity of FDCA (Table 11, entries 27 and 28). The high activity of
these supports is associated with high surface area; for Ru/ZrO 2 H-SBA the surface
area is 256 m
2
/g and 239 m
2
/g for Ru/ZrO 2 H- aero. The experiments show that the
S. Gundekari et al.