256
Bifunctionalized MOFs
MOFs are tailorable and offer the opportunity to introduce acid, base, or both functionality and metal-active sites. This property of MOFs makes them an ideal material to be used as a bifunctional catalyst. For example, palladium supported on
amine-functionalized MOF (Pd/MIL-101(Al)-NH 2 ) is used for hydrogenation of
5-HMF to 2,5-hydroxymethyl-tetrahydrofuran (DHMTHF). The metallic site and
free amine moiety are observed to be the controlling factors in the conversion of
5-HMF to DHMTHF. The amine moiety in the MOF is crucial in uniform dispersion
of Pd nanoparticles on amine-functionalized support. When 5-HMF is fully
converted, a 96% yield of DHMTHF is obtained under optimal reaction conditions.
The reaction was carried out at a low temperature of 30 °C in an aqueous medium.
The MOF support could stabilize the Pd nanoparticles efficiently as only a marginal
(0.03%) amount of Pd was leached after five cycles (60 h) of operation. However,
the selectivity for DHMTHF was reduced from 96% to 80% [63].
An important platform chemical is γ-valerolactone (GVL), which is used for the
synthesis of useful chemicals. GVL is synthesized by hydrogenation of levulinic
acid and further converted to ethyl valerate (EV yield 83%) in the presence of
ethanol via hydrodeoxygenation (HDO) over a bimetallic catalyst (Pd/MIL-101SO 3 H) [64]. GVL can also be produced from methyl levulinate (ML) by using Ru
nanoparticles deposited on the Zr-based metal-organic framework (SO 3 H-UiO-66).
The catalyst showed duel functionality, and about 100% yield of GVL was obtained.
Alternately, if GVL is produced by a two-step method, i.e., hydrogenation of methyl
levulinate over Ru/C catalyst to produce intermediate (4-hydroxypentanoic acid
methyl ester) followed by conversion of this intermediate in the absence of metal
catalyst, the yield was very limited [65].
Sorbitol is an important chemical and widely used as a sweetener in the food
industry as a moisture controller in medical and cosmetic applications and feedstock
for hydrogen and alkane production. Sorbitol can be obtained by hydrogenation of
cellulose and also by hydrolysis of cellobiose. Both the reactions can be catalyzed
by the bifunctional catalyst supported on MOF, i.e., Ru-PTA/MIL-100(Cr). The
important factor for cellulose and cellobiose conversion was the ratio of active acid
sites (n A ) to the number of Ru surface atoms (n Ru ). The optimum ratio of two catalytic
activities for maximum conversion was 8.84 < (n A /n Ru ) < 12.90. Ruthenium (Ru)
promoted hydrogenation reaction, and PTA/MIL-100(Cr) was active for hydrolysis
reactions. Though the leaching of metal was not significant (0.006%), the catalyst
activity decreased significantly. The yield of sorbitol was 95.1% for fresh catalysts
and decreased to 8.5% when the catalyst was reused for the reaction. The loss in
activity was attributed to the catalyst poisoning by insoluble substrates [66]. It is,
therefore, expected that the MOF catalysts could serve as novel catalysts for biomass
upgrading by metal/acid dual functionality but need improvement in catalyst
stability. The synchronized effect among the metal and acid functionality in the
presence of an environmentally benign solvent will be the key to the catalytic
process.
R. Bhoi et al.
Bifunctionalized MOFs
MOFs are tailorable and offer the opportunity to introduce acid, base, or both functionality and metal-active sites. This property of MOFs makes them an ideal material to be used as a bifunctional catalyst. For example, palladium supported on
amine-functionalized MOF (Pd/MIL-101(Al)-NH 2 ) is used for hydrogenation of
5-HMF to 2,5-hydroxymethyl-tetrahydrofuran (DHMTHF). The metallic site and
free amine moiety are observed to be the controlling factors in the conversion of
5-HMF to DHMTHF. The amine moiety in the MOF is crucial in uniform dispersion
of Pd nanoparticles on amine-functionalized support. When 5-HMF is fully
converted, a 96% yield of DHMTHF is obtained under optimal reaction conditions.
The reaction was carried out at a low temperature of 30 °C in an aqueous medium.
The MOF support could stabilize the Pd nanoparticles efficiently as only a marginal
(0.03%) amount of Pd was leached after five cycles (60 h) of operation. However,
the selectivity for DHMTHF was reduced from 96% to 80% [63].
An important platform chemical is γ-valerolactone (GVL), which is used for the
synthesis of useful chemicals. GVL is synthesized by hydrogenation of levulinic
acid and further converted to ethyl valerate (EV yield 83%) in the presence of
ethanol via hydrodeoxygenation (HDO) over a bimetallic catalyst (Pd/MIL-101SO 3 H) [64]. GVL can also be produced from methyl levulinate (ML) by using Ru
nanoparticles deposited on the Zr-based metal-organic framework (SO 3 H-UiO-66).
The catalyst showed duel functionality, and about 100% yield of GVL was obtained.
Alternately, if GVL is produced by a two-step method, i.e., hydrogenation of methyl
levulinate over Ru/C catalyst to produce intermediate (4-hydroxypentanoic acid
methyl ester) followed by conversion of this intermediate in the absence of metal
catalyst, the yield was very limited [65].
Sorbitol is an important chemical and widely used as a sweetener in the food
industry as a moisture controller in medical and cosmetic applications and feedstock
for hydrogen and alkane production. Sorbitol can be obtained by hydrogenation of
cellulose and also by hydrolysis of cellobiose. Both the reactions can be catalyzed
by the bifunctional catalyst supported on MOF, i.e., Ru-PTA/MIL-100(Cr). The
important factor for cellulose and cellobiose conversion was the ratio of active acid
sites (n A ) to the number of Ru surface atoms (n Ru ). The optimum ratio of two catalytic
activities for maximum conversion was 8.84 < (n A /n Ru ) < 12.90. Ruthenium (Ru)
promoted hydrogenation reaction, and PTA/MIL-100(Cr) was active for hydrolysis
reactions. Though the leaching of metal was not significant (0.006%), the catalyst
activity decreased significantly. The yield of sorbitol was 95.1% for fresh catalysts
and decreased to 8.5% when the catalyst was reused for the reaction. The loss in
activity was attributed to the catalyst poisoning by insoluble substrates [66]. It is,
therefore, expected that the MOF catalysts could serve as novel catalysts for biomass
upgrading by metal/acid dual functionality but need improvement in catalyst
stability. The synchronized effect among the metal and acid functionality in the
presence of an environmentally benign solvent will be the key to the catalytic
process.
R. Bhoi et al.