255
-NH 3 , and –NO 2 ) is used for this reaction. The catalytic activity of UiO-66 reduced
when functionalized with –NH 2 and –NO 2 for both hydrogenation and
transesterification reactions owing to the poor dispersion of Ru. On the other hand,
–SO 3 H-functionalized MOF showed increased activity (22% higher yield of GVL)
for transesterification reaction when compared with Ru/UiO-66 [59].
Acid-Functionalized MOFs
The reaction that is primarily explored using acid MOFs is the production of a
5-HMF form of glucose, which is a two-step reaction that involves isomerization
(glucose to fructose) followed by dehydration (fructose to 5-HMF). The isomerization requires Lewis acidity, and dehydration requires Brønsted acidity. It is,
therefore, beneficial if the catalyst possesses Lewis as well as Brønsted acidity for
the single-step conversion of glucose. These bifunctional characteristics can be
induced by partial modification of the metal-organic framework. UiO-66 is one
such example where the organic linker of the zirconium organic framework is
replaced with 2-monocular-benzene-1,4-dicarboxylate. The catalyst was recyclable and showed higher selectivity for 5-HMF and fructose up to 90%. Similarly,
NU-1000 (acidic zirconia modified by phosphate) was used for glucose conversion via the isomerization-dehydration mechanism with higher selectivity [7]. To
prevent the side reactions, the Lewis and Brønsted active sites were purposely
poisoned as both Lewis and Brønsted acidities were required, but at reduced level.
It was also observed through the isotope tracer studies that the isomerizationdehydration mechanism is the favored mechanism over the direct-dehydration
mechanism. The direct-dehydration mechanism utilizes phosphate-modified titania and bare niobia as catalysts [60]. The presence of water as a solvent is essential to carry out glucose to 5-HMF conversion. This was verified by conducting a
reaction with MIL-101Cr (MIL-SO 3 H) catalysts in pure tetrahydrofuran (THF) as
a solvent. No product was formed. The findings suggest that the presence of water
for glucose conversion is indispensable. Therefore, the catalyst should have tolerance for water as a large amount of water is generated during a dehydration
reaction.
Productions of levulinic acid and 5-HMF from glucose are competing reactions
and highly sensitive to the catalyst used. For example, MIL-SO 3 H favored the
formation of 5-HMF over levulinic acid (molar ratio 1:0.3). On the other hand,
Amberlyst-15 and H 2 SO 4 favored levulinic acid over 5-HMF with molar ratio of 3:1
and 10:1, respectively. The catalyst did not show good reusability, and reactivation
remains a challenge [61]. The kinetics of conversion of fructose to 5-HMF reveal
that MIL-101Cr-SO 3 H-promoted reaction follows pseudo-first-order kinetics with
an activation energy of 55 kJ/mol. The reaction was conducted in the presence of
dimethyl sulfoxide (DMSO) at 120 °C for 60 min. The high yield (90%) of 5-HMF
was obtained with the full conversion of fructose [62]. The results highlight MOFs
as promising alternative solid acid catalysts for biomass conversion.
Sustainability of the Catalytic Process for Biomass Conversion: Recent Trends and…
-NH 3 , and –NO 2 ) is used for this reaction. The catalytic activity of UiO-66 reduced
when functionalized with –NH 2 and –NO 2 for both hydrogenation and
transesterification reactions owing to the poor dispersion of Ru. On the other hand,
–SO 3 H-functionalized MOF showed increased activity (22% higher yield of GVL)
for transesterification reaction when compared with Ru/UiO-66 [59].
Acid-Functionalized MOFs
The reaction that is primarily explored using acid MOFs is the production of a
5-HMF form of glucose, which is a two-step reaction that involves isomerization
(glucose to fructose) followed by dehydration (fructose to 5-HMF). The isomerization requires Lewis acidity, and dehydration requires Brønsted acidity. It is,
therefore, beneficial if the catalyst possesses Lewis as well as Brønsted acidity for
the single-step conversion of glucose. These bifunctional characteristics can be
induced by partial modification of the metal-organic framework. UiO-66 is one
such example where the organic linker of the zirconium organic framework is
replaced with 2-monocular-benzene-1,4-dicarboxylate. The catalyst was recyclable and showed higher selectivity for 5-HMF and fructose up to 90%. Similarly,
NU-1000 (acidic zirconia modified by phosphate) was used for glucose conversion via the isomerization-dehydration mechanism with higher selectivity [7]. To
prevent the side reactions, the Lewis and Brønsted active sites were purposely
poisoned as both Lewis and Brønsted acidities were required, but at reduced level.
It was also observed through the isotope tracer studies that the isomerizationdehydration mechanism is the favored mechanism over the direct-dehydration
mechanism. The direct-dehydration mechanism utilizes phosphate-modified titania and bare niobia as catalysts [60]. The presence of water as a solvent is essential to carry out glucose to 5-HMF conversion. This was verified by conducting a
reaction with MIL-101Cr (MIL-SO 3 H) catalysts in pure tetrahydrofuran (THF) as
a solvent. No product was formed. The findings suggest that the presence of water
for glucose conversion is indispensable. Therefore, the catalyst should have tolerance for water as a large amount of water is generated during a dehydration
reaction.
Productions of levulinic acid and 5-HMF from glucose are competing reactions
and highly sensitive to the catalyst used. For example, MIL-SO 3 H favored the
formation of 5-HMF over levulinic acid (molar ratio 1:0.3). On the other hand,
Amberlyst-15 and H 2 SO 4 favored levulinic acid over 5-HMF with molar ratio of 3:1
and 10:1, respectively. The catalyst did not show good reusability, and reactivation
remains a challenge [61]. The kinetics of conversion of fructose to 5-HMF reveal
that MIL-101Cr-SO 3 H-promoted reaction follows pseudo-first-order kinetics with
an activation energy of 55 kJ/mol. The reaction was conducted in the presence of
dimethyl sulfoxide (DMSO) at 120 °C for 60 min. The high yield (90%) of 5-HMF
was obtained with the full conversion of fructose [62]. The results highlight MOFs
as promising alternative solid acid catalysts for biomass conversion.
Sustainability of the Catalytic Process for Biomass Conversion: Recent Trends and…