333
size of the ruthenium particles is crucial for the catalytic performance; larger surface area leads to the formation of small Ru particles (0.8 nm for Ru/ZrO 2 H- aero,
1.0 nm for Ru/ZrO 2 H-SBA ) which were found to be more active than bigger Ru particles observed in other synthesized and commercial (monoclinic and tertragonal)
ZrO 2 supports. Ru/ZrO 2 H- aero catalyst was tested for reusability studies, and a
decrease in activity was observed with increasing reaction cycles, but no loss in the
carbon balance and no leaching of Ru metal was observed.
Antonyraj et al. studied different oxide supports such as MgO, MgAlO, La 2 O 3 ,
CeO 2 and ZrO 2 [170]. Among these supports, MgAlO (prepared by co-precipitation
under low supersaturation followed by calcination)-supported Ru catalyst showed
better activity and afforded 99% yield of FDCA which is higher as compared with
other supported Ru catalysts (Table 11, entries 29–33). Because of the high surface
area (200 m
2
/g) and appropriate basic property (6.09 mmol/g) of MgAlO, it was
found to be highly active. In the case of other supports including MgO, their
observed surface area was less than 100 m
2
/g. Even though La 2 O 3 support has strong
basic sites, the catalyst showed poor conversion due to the less surface area (13 m
2
/g)
and surface acidity. In the case of CeO 2 and ZrO 2 , less surface area and no basic
property resulted in poor selectivity towards FDCA. The authors also studied the
reusability of both catalysts, i.e. Ru/MgO and Ru/MgAlO supports were found to be
dissolved during the reaction, but Ru/MgO showed good catalytic activity/stability
for five reaction cycles.
Gao et al. studied highly dispersed ruthenium nanoparticles on hydroxyapatite
(HAP) solid support for the aerobic oxidation of HMF to FDCA in aqueous medium
under base-free condition [171]. A systematic study done by the authors includes
the effect of HMF/Ru molar ratio, oxygen pressure, reaction temperature, kinetic
study, catalyst stability upon reuse, surface acidity and basicity, morphology and
dispersion and valence state of surface Ru. At optimized condition (HMF/Ru molar
ratio: 25, 120 °C, 10 bar O 2 for 1 h), Ru/HAP showed >99% yield of FDCA
(Table 11, entry 34). The temperature was varied from 60 to 120 °C. At 60 °C, only
FFCA intermediate was observed, which clearly showed that Ru/HAP catalyst is
capable of converting other oxidized intermediates such as DFF and HMFCA to
FFCA at this temperature. Upon increasing the temperature to 120 °C, >99 yield of
FDCA from FFCA intermediate was reported. The authors disclosed that welldispersed Ru
0
and acidic-basic sites located in the HAP support were essential to
drive this base-free oxidation process. The catalyst was reusable up to five cycles,
and after five cycles, slight deactivation was observed which can be reversed by
reducing the catalyst under H 2 at 350 °C.
Sharma et al. demonstrated metal (Pt/Pd/Ru/Co/Ni)-functionalized carbon nanotubes (CNT) for base-free aerobic oxidation of HMF to FDCA in aqueous medium
[172]. The noble metal-based catalysts such as Pt/CNT, Pd/CNT and Ru/CNT
showed high yield, i.e. 98, 96 and 95%, respectively, of FDCA under relatively mild
reaction condition, i.e. 100 °C, 30 bar O 2 for 12 h (Table 11, entries 35–37). The
screened non-noble metals such as Ni and Co-loaded CNT were selective towards
DFF (Table 11, entries 38 and 39). In the absence of metal, simple-functionalized
CNT material gave 86% yield of FFCA (Table 11, entry 40). In this article, the
Levulinic Acid- and Furan-Based Multifunctional Materials: Opportunities…
size of the ruthenium particles is crucial for the catalytic performance; larger surface area leads to the formation of small Ru particles (0.8 nm for Ru/ZrO 2 H- aero,
1.0 nm for Ru/ZrO 2 H-SBA ) which were found to be more active than bigger Ru particles observed in other synthesized and commercial (monoclinic and tertragonal)
ZrO 2 supports. Ru/ZrO 2 H- aero catalyst was tested for reusability studies, and a
decrease in activity was observed with increasing reaction cycles, but no loss in the
carbon balance and no leaching of Ru metal was observed.
Antonyraj et al. studied different oxide supports such as MgO, MgAlO, La 2 O 3 ,
CeO 2 and ZrO 2 [170]. Among these supports, MgAlO (prepared by co-precipitation
under low supersaturation followed by calcination)-supported Ru catalyst showed
better activity and afforded 99% yield of FDCA which is higher as compared with
other supported Ru catalysts (Table 11, entries 29–33). Because of the high surface
area (200 m
2
/g) and appropriate basic property (6.09 mmol/g) of MgAlO, it was
found to be highly active. In the case of other supports including MgO, their
observed surface area was less than 100 m
2
/g. Even though La 2 O 3 support has strong
basic sites, the catalyst showed poor conversion due to the less surface area (13 m
2
/g)
and surface acidity. In the case of CeO 2 and ZrO 2 , less surface area and no basic
property resulted in poor selectivity towards FDCA. The authors also studied the
reusability of both catalysts, i.e. Ru/MgO and Ru/MgAlO supports were found to be
dissolved during the reaction, but Ru/MgO showed good catalytic activity/stability
for five reaction cycles.
Gao et al. studied highly dispersed ruthenium nanoparticles on hydroxyapatite
(HAP) solid support for the aerobic oxidation of HMF to FDCA in aqueous medium
under base-free condition [171]. A systematic study done by the authors includes
the effect of HMF/Ru molar ratio, oxygen pressure, reaction temperature, kinetic
study, catalyst stability upon reuse, surface acidity and basicity, morphology and
dispersion and valence state of surface Ru. At optimized condition (HMF/Ru molar
ratio: 25, 120 °C, 10 bar O 2 for 1 h), Ru/HAP showed >99% yield of FDCA
(Table 11, entry 34). The temperature was varied from 60 to 120 °C. At 60 °C, only
FFCA intermediate was observed, which clearly showed that Ru/HAP catalyst is
capable of converting other oxidized intermediates such as DFF and HMFCA to
FFCA at this temperature. Upon increasing the temperature to 120 °C, >99 yield of
FDCA from FFCA intermediate was reported. The authors disclosed that welldispersed Ru
0
and acidic-basic sites located in the HAP support were essential to
drive this base-free oxidation process. The catalyst was reusable up to five cycles,
and after five cycles, slight deactivation was observed which can be reversed by
reducing the catalyst under H 2 at 350 °C.
Sharma et al. demonstrated metal (Pt/Pd/Ru/Co/Ni)-functionalized carbon nanotubes (CNT) for base-free aerobic oxidation of HMF to FDCA in aqueous medium
[172]. The noble metal-based catalysts such as Pt/CNT, Pd/CNT and Ru/CNT
showed high yield, i.e. 98, 96 and 95%, respectively, of FDCA under relatively mild
reaction condition, i.e. 100 °C, 30 bar O 2 for 12 h (Table 11, entries 35–37). The
screened non-noble metals such as Ni and Co-loaded CNT were selective towards
DFF (Table 11, entries 38 and 39). In the absence of metal, simple-functionalized
CNT material gave 86% yield of FFCA (Table 11, entry 40). In this article, the
Levulinic Acid- and Furan-Based Multifunctional Materials: Opportunities…