331
of FDCA at relatively milder reaction conditions (preferably low O 2 /air pressure),
base-free and availability of multiple Ru species such as Ru(0), oxide and hydroxide. Moreover, the price of Ru is comparatively less than other noble metals.
Important findings for the HMF oxidation to FDCA using Ru metal-supported catalysts preferably under aqueous base-free medium are covered in the following
section.
3.2.2.1 Preparation of FDCA from Selective Oxidation of HMF over
Supported Ru Catalysts and its activity comparison with other metal
catalysts
Gorbanev et al. studied Ru(OH) x catalyst on different oxide supports such as TiO 2 ,
Al 2 O 3 , Fe 3 O 4 , ZrO 2 , CeO 2 , MgO, La 2 O 3 , hydrotalcite, hydroxyapatite and
MgO. La 2 O 3 for conversion of HMF into FDCA in aqueous medium, under basefree condition with low to moderate oxygen pressure [164]. ZrO 2 - and Al 2 O 3 -
supported catalysts induced the formation of formic acid as side product. Fe 3 O 4 and
hydroxyapatite supports showed good selectivity of the desired product; however, in
both cases, the observed mass balance of the reaction was low, due to the formation
of solid humins in the course of the reaction. Even though good selectivity of FDCA
was achieved using basic Mg oxide supports such as MgO, MgO.La 2 O 3 and hydrotalcite, the authors found leaching of Mg in the reaction medium. Compared with
TiO 2 , CeO 2 support showed high conversion and selectivity; at 140 °C, 2.5 bar O 2
for 18 h, Ru(OH) x /CeO 2 afforded 60% yield of FDCA and 10% of 5-hydroxymethyl- 2furan-carboxylic acid (Table 11, entry 1), and no other side products (levulinic acid
and formic acid) were detected using CeO 2 support. Recyclability of Ru(OH) x /CeO 2
system was also demonstrated for three cycles by the authors.
Palkovits research group introduced Ru supported on covalent triazine framework (CTF) catalysts for this oxidation. The catalytic activity of Ru/CTF was compared with conventional catalysts such as Ru/C, Pt/C, Ru/γ-Al 2 O 3 and Pd/C under
base-free aqueous medium using synthetic air as the oxidant (20 bar) (Table 11,
entries 2–6) [165]. Ru/γ-Al 2 O 3 and Pd/C exhibited low conversion of HMF and no
discernable yield of FDCA. Pt/C catalyst was very active for FDCA formation
(56%) but less mass balance was reported for the reaction; formation of polymeric/
oligomeric products of FDCA could be the reason behind. In the case of Ru-based
catalysts, mass balance is generally good, and side products DFF and FFCA are
formed in addition to FDCA. Ru/CTF showed 41.4% yield of FDCA which is
higher than Ru/C (23.7%) catalyst under similar reaction conditions. High surface
area (2071 m
2
/g) and high polarity related to hydrophilicity seem to have positive
effects in this conversion. Interestingly, the authors observed high yield of FDCA
(77%) by treating Ru/CTF catalyst with DMSO before reaction with a reaction time
of 3 h (Table 11, entry 7). Ru/CTF has good reusability and was found to be stable
compared to conventional Ru/C.
Yi et al. investigated HMF to FDCA over commercially available Ru/C catalyst
[166]. Initially, the reaction was conducted with strong and weak base additives, and
Levulinic Acid- and Furan-Based Multifunctional Materials: Opportunities…
of FDCA at relatively milder reaction conditions (preferably low O 2 /air pressure),
base-free and availability of multiple Ru species such as Ru(0), oxide and hydroxide. Moreover, the price of Ru is comparatively less than other noble metals.
Important findings for the HMF oxidation to FDCA using Ru metal-supported catalysts preferably under aqueous base-free medium are covered in the following
section.
3.2.2.1 Preparation of FDCA from Selective Oxidation of HMF over
Supported Ru Catalysts and its activity comparison with other metal
catalysts
Gorbanev et al. studied Ru(OH) x catalyst on different oxide supports such as TiO 2 ,
Al 2 O 3 , Fe 3 O 4 , ZrO 2 , CeO 2 , MgO, La 2 O 3 , hydrotalcite, hydroxyapatite and
MgO. La 2 O 3 for conversion of HMF into FDCA in aqueous medium, under basefree condition with low to moderate oxygen pressure [164]. ZrO 2 - and Al 2 O 3 -
supported catalysts induced the formation of formic acid as side product. Fe 3 O 4 and
hydroxyapatite supports showed good selectivity of the desired product; however, in
both cases, the observed mass balance of the reaction was low, due to the formation
of solid humins in the course of the reaction. Even though good selectivity of FDCA
was achieved using basic Mg oxide supports such as MgO, MgO.La 2 O 3 and hydrotalcite, the authors found leaching of Mg in the reaction medium. Compared with
TiO 2 , CeO 2 support showed high conversion and selectivity; at 140 °C, 2.5 bar O 2
for 18 h, Ru(OH) x /CeO 2 afforded 60% yield of FDCA and 10% of 5-hydroxymethyl- 2furan-carboxylic acid (Table 11, entry 1), and no other side products (levulinic acid
and formic acid) were detected using CeO 2 support. Recyclability of Ru(OH) x /CeO 2
system was also demonstrated for three cycles by the authors.
Palkovits research group introduced Ru supported on covalent triazine framework (CTF) catalysts for this oxidation. The catalytic activity of Ru/CTF was compared with conventional catalysts such as Ru/C, Pt/C, Ru/γ-Al 2 O 3 and Pd/C under
base-free aqueous medium using synthetic air as the oxidant (20 bar) (Table 11,
entries 2–6) [165]. Ru/γ-Al 2 O 3 and Pd/C exhibited low conversion of HMF and no
discernable yield of FDCA. Pt/C catalyst was very active for FDCA formation
(56%) but less mass balance was reported for the reaction; formation of polymeric/
oligomeric products of FDCA could be the reason behind. In the case of Ru-based
catalysts, mass balance is generally good, and side products DFF and FFCA are
formed in addition to FDCA. Ru/CTF showed 41.4% yield of FDCA which is
higher than Ru/C (23.7%) catalyst under similar reaction conditions. High surface
area (2071 m
2
/g) and high polarity related to hydrophilicity seem to have positive
effects in this conversion. Interestingly, the authors observed high yield of FDCA
(77%) by treating Ru/CTF catalyst with DMSO before reaction with a reaction time
of 3 h (Table 11, entry 7). Ru/CTF has good reusability and was found to be stable
compared to conventional Ru/C.
Yi et al. investigated HMF to FDCA over commercially available Ru/C catalyst
[166]. Initially, the reaction was conducted with strong and weak base additives, and
Levulinic Acid- and Furan-Based Multifunctional Materials: Opportunities…