360
extraordinary high surface area of the MOF improve mass transfer due to enhanced
accessibility of active sites, thus increasing the catalytic activity. In particular, the
exposed Zr sites exhibit excellent catalytic behavior which offers an advantage of
efficiently catalyzing various acid-catalyzed reactions [93]. UiO-66,35, MOFs
exhibited robust mechanical, thermal, and chemical stabilities. In addition, versatile
synthesis methods and ease of functionalization are other key favorable factors of
these MOFs.
Literature is available on use of UiO-66-(COOH) (Zr-based MOF) as a heterogeneous catalyst for levulinic acid esterification with ethanol [94]. The authors have
mentioned that, the excellent activity of this MOF, for this reaction stem from synergistic effect between the Lewis acidic Zr clusters and Brønsted acidity due to
organic functionality (–COOH groups) within the framework. Such proven Brønsted
acidity enables this material to be used as active acid catalytic material for the esterification of biomass-derived acids with alcohols [72, 73, 91, 95]. The authors have
mentioned, based on computational studies, that the Zr sites form coordination
bonds with carbonyl groups of levulinic acid, while the free carboxylic group forms
H-bond with the substrate, facilitating the removal of –OH group from levulinic
acid. The author confirmed reusability of catalyst across five runs [94].
Further, the scope exists for applications of MOFs in upgradation of lignocellulosic biomass and biomass-derived platform chemicals, and this area holds much
potential for further development. However, there are issues associated with control
of orientation of building blocks in the solid state and prediction of resulting framework structure [96]. Due to weak coordination bonding, MOFs are expected to be
less thermally stable. Thermal stability can be improved by appropriate combination of ligand and metals, and an introduction of chelate effect can also be beneficial
to enhance stability of porous MOFs.
6.2 Ion-Exchange Resins
Resins are organo-polymeric materials that exhibit high surface area, high ionexchange capacities, and characteristic functional groups which determine the
nature of resultant resins [76, 77, 97, 98]. Several types of organic transformations
are catalyzed by acid or base, which can be preferentially carried out with ionexchange resins/solids. Easy separation, ease of continuous operations, loss of valuable reactant, reusability of resin, non-corrosiveness to the reactor, and elimination
of removal acid/base from the product mixture and liquid effluent are some of the
merits of using ion-exchange solids/resins [99]. One can design tailor-made resins
to get selective product with proper choice of monomers and manipulate the degree
of cross-linking. However, one has to look into thermodynamic aspects of sorption
of reactants and products as it also affects the selectivity problem to a larger extent.
The degree of cross-linking, stiffness, swelling, functionality type, etc. are significant as they largely affect the activity of resins. The swelling property of resin is
associated with the flexibility and stiffness of resin which in turn depends on the
K. C. Maheria et al.
extraordinary high surface area of the MOF improve mass transfer due to enhanced
accessibility of active sites, thus increasing the catalytic activity. In particular, the
exposed Zr sites exhibit excellent catalytic behavior which offers an advantage of
efficiently catalyzing various acid-catalyzed reactions [93]. UiO-66,35, MOFs
exhibited robust mechanical, thermal, and chemical stabilities. In addition, versatile
synthesis methods and ease of functionalization are other key favorable factors of
these MOFs.
Literature is available on use of UiO-66-(COOH) (Zr-based MOF) as a heterogeneous catalyst for levulinic acid esterification with ethanol [94]. The authors have
mentioned that, the excellent activity of this MOF, for this reaction stem from synergistic effect between the Lewis acidic Zr clusters and Brønsted acidity due to
organic functionality (–COOH groups) within the framework. Such proven Brønsted
acidity enables this material to be used as active acid catalytic material for the esterification of biomass-derived acids with alcohols [72, 73, 91, 95]. The authors have
mentioned, based on computational studies, that the Zr sites form coordination
bonds with carbonyl groups of levulinic acid, while the free carboxylic group forms
H-bond with the substrate, facilitating the removal of –OH group from levulinic
acid. The author confirmed reusability of catalyst across five runs [94].
Further, the scope exists for applications of MOFs in upgradation of lignocellulosic biomass and biomass-derived platform chemicals, and this area holds much
potential for further development. However, there are issues associated with control
of orientation of building blocks in the solid state and prediction of resulting framework structure [96]. Due to weak coordination bonding, MOFs are expected to be
less thermally stable. Thermal stability can be improved by appropriate combination of ligand and metals, and an introduction of chelate effect can also be beneficial
to enhance stability of porous MOFs.
6.2 Ion-Exchange Resins
Resins are organo-polymeric materials that exhibit high surface area, high ionexchange capacities, and characteristic functional groups which determine the
nature of resultant resins [76, 77, 97, 98]. Several types of organic transformations
are catalyzed by acid or base, which can be preferentially carried out with ionexchange resins/solids. Easy separation, ease of continuous operations, loss of valuable reactant, reusability of resin, non-corrosiveness to the reactor, and elimination
of removal acid/base from the product mixture and liquid effluent are some of the
merits of using ion-exchange solids/resins [99]. One can design tailor-made resins
to get selective product with proper choice of monomers and manipulate the degree
of cross-linking. However, one has to look into thermodynamic aspects of sorption
of reactants and products as it also affects the selectivity problem to a larger extent.
The degree of cross-linking, stiffness, swelling, functionality type, etc. are significant as they largely affect the activity of resins. The swelling property of resin is
associated with the flexibility and stiffness of resin which in turn depends on the
K. C. Maheria et al.