395
• Structure type, e.g., ZMOF-n (zeolite-like metal organic framework), ZIFn (zeolitic imidazolate framework), or mesoMOF-n (mesoporous metal organic
framework)
• Detailing about the group or laboratory or institute where it was synthesized
Whereas, the trailing number roughly represents the chronological order of the
preparation of the porous solids in that series. The MOFs display explicit active
sites (isolated metal sites), and reactive functional (inorganic or organic) groups,
that enhance the feasibility of different types of reactions to occur. Therefore, they
have been exclusively studied as heterogeneous catalysts. The examples are: (1)
vanadium-based MOFs (MIL-47, MOF-48) are more active and extremely selective
in the transformation of CH 4 and CO to acetic acid, (2) ceria-based MOFs help to
catalyze water oxidation reactions and gas-phase reactions, etc. These frameworks
also offer new materials with a vast degree of crystallinity, chemical nature, and
porosity. The building blocks resulting in 3D-skeletons of MOFs are called secondary building units (SBU). A SBU consists of organic linkers bonded covalently to a
positively charged metal center. The type and size of the SBU engaged is the dominating parameter which needs to be controlled to obtain the desired porosity with
optimum openness of the framework. It is observed that larger organic linkers
depress the 3D structure or reduce the porosity through lattice self-interpenetration,
whereas the stability of MOFs is affected by various factors such as pK a values of
ligands, reduction potential, oxidation state, ionic radius of the metal ions, metalligand coordination structure, and its hydrophobicity [11]. However, metal-linker
coordination bond is the weakest spot of MOFs. Therefore, in aqueous medium, it
hydrolyzed to form a protonated linker and a de-ligated inorganic moiety. Thus,
both acidic and basic solutions hasten the disintegration of MOF structures.
Modulated synthesis, isoreticular expansion, and topology-governed layout are
various kinds of preparation mechanisms for the synthesis of stable MOFs [10].
Thus obtained well-defined crystalline structures of MOFs are characterized by
various characterization techniques such as XRD, SEM, TEM, NMR, UV-vis, and
Raman spectroscopy. Recently the MOFs crystalline structures are also determined
by computational chemistry [11] (Fig. 12).
3.2 Advantages of MOFs
MOFs usually exhibit a number of exceptional properties like higher thermal stability, perpetual porosity, and structural toughness which makes them very promising
for future applications. In comparison with zeolites, MOFs allow a proper control of
its pore size, shape, and functionalities which makes the more suitable porous materials beyond that of zeolites [18]. For example, the surface area MOF-177 and
MIL-101 found to be 5640 and 5900 m
2
/g, respectively. Since one of the major
advantages of MOFs lies within the diversity of their designing principles which
Non-conventional Catalytic Materials for Refining and Petrochemicals
• Structure type, e.g., ZMOF-n (zeolite-like metal organic framework), ZIFn (zeolitic imidazolate framework), or mesoMOF-n (mesoporous metal organic
framework)
• Detailing about the group or laboratory or institute where it was synthesized
Whereas, the trailing number roughly represents the chronological order of the
preparation of the porous solids in that series. The MOFs display explicit active
sites (isolated metal sites), and reactive functional (inorganic or organic) groups,
that enhance the feasibility of different types of reactions to occur. Therefore, they
have been exclusively studied as heterogeneous catalysts. The examples are: (1)
vanadium-based MOFs (MIL-47, MOF-48) are more active and extremely selective
in the transformation of CH 4 and CO to acetic acid, (2) ceria-based MOFs help to
catalyze water oxidation reactions and gas-phase reactions, etc. These frameworks
also offer new materials with a vast degree of crystallinity, chemical nature, and
porosity. The building blocks resulting in 3D-skeletons of MOFs are called secondary building units (SBU). A SBU consists of organic linkers bonded covalently to a
positively charged metal center. The type and size of the SBU engaged is the dominating parameter which needs to be controlled to obtain the desired porosity with
optimum openness of the framework. It is observed that larger organic linkers
depress the 3D structure or reduce the porosity through lattice self-interpenetration,
whereas the stability of MOFs is affected by various factors such as pK a values of
ligands, reduction potential, oxidation state, ionic radius of the metal ions, metalligand coordination structure, and its hydrophobicity [11]. However, metal-linker
coordination bond is the weakest spot of MOFs. Therefore, in aqueous medium, it
hydrolyzed to form a protonated linker and a de-ligated inorganic moiety. Thus,
both acidic and basic solutions hasten the disintegration of MOF structures.
Modulated synthesis, isoreticular expansion, and topology-governed layout are
various kinds of preparation mechanisms for the synthesis of stable MOFs [10].
Thus obtained well-defined crystalline structures of MOFs are characterized by
various characterization techniques such as XRD, SEM, TEM, NMR, UV-vis, and
Raman spectroscopy. Recently the MOFs crystalline structures are also determined
by computational chemistry [11] (Fig. 12).
3.2 Advantages of MOFs
MOFs usually exhibit a number of exceptional properties like higher thermal stability, perpetual porosity, and structural toughness which makes them very promising
for future applications. In comparison with zeolites, MOFs allow a proper control of
its pore size, shape, and functionalities which makes the more suitable porous materials beyond that of zeolites [18]. For example, the surface area MOF-177 and
MIL-101 found to be 5640 and 5900 m
2
/g, respectively. Since one of the major
advantages of MOFs lies within the diversity of their designing principles which
Non-conventional Catalytic Materials for Refining and Petrochemicals