experimental design and gas adsorption properties of MOFs and their derivatives for
hydrogen storage applications (Campesi et al. 2010; Wang et al. 2017) and also clean
energy applications due to their excellent high surface area, permanent porosity and
pore size distributions (Tranchemontagne et al. 2008; Wang et al. 2017; Zhang et al.
2017).
10.4.2 Structure of Metal Organic Frameworks
The arrangement of MOFs structures can either be two-dimensional (2D) or threedimensional (3D) networks. These networks are assembled from organic ligands and
metal ion or cluster nodes (Jiang et al. 2013). Unsaturated metal sites or accessible
metal sites in MOFs play a major role in influencing their adsorption performance.
Transition metals such as Cu, Zn, Mn, Co are usually the ones chosen as centre
connectors and they serve as Lewis acids for an activation of the coordinated organic
ligands for organic transformation (Raoof et al. 2015; Kim et al. 2013; Nas et al.
2015). The adsorption capacity is due to partial positive charges of metal sides
(Cheetham et al. 2006; Raoof et al. 2015). The transition metals can provide variety
in geometries depend largely on their oxidation number (Tan et al. 2017; Feng et al.
2013). The linkers possess the aromatic rings in the framework, which help to
maintain the structural integrity of the complex and direct the geometry of the
framework. The pore volume and surface area of MOFs can be organized by
Modification of organic ligands influences the surface area of the MOFs. The
common used organic linkers are provided in Fig. 10.3 (Bedia et al. 2019).
Secondary Building Units (SBUs) are critical role players dictating the final
geometry of MOFs. The structural and chemical properties of the SBUs and
organic ligands result in the prediction of the schemes and synthesis of MOFs. It
has been reported that under careful selection of reaction preconditions, multidentate
linkers can form some aggregates which affect the formation of SBUs (Salunkegawali et al. 2012). Subsequently, the SBUs will join with rigid organic links to
develop MOFs with high structural stability (Yuan et al. 2016). However, the
structure of the SBU is controlled by the metal-to-ligand ratio, solvent and source
of anions used in balancing metal ions charges [Sun and Sun 2014; Wang et al.
2014). MOF structures with internal diameter of pores of up to 4.8 nm can extend the
free space and thus results in H 2 storage (Abbasi et al. 2017). However, the empty
spaces remain intact after the guest molecules have been removed. The size of the
pores on the adsorbents is responsible for possible interaction between the absorbed
gas molecules and the surface of the surrounding walls. Furthermore, it should be
close to the kinetic diameter of H 2 molecule (0.289 nm) to endorse stronger
interactions amongst H 2 molecules and the framework. The MOF (NU-100)
containing <2 nm micropores possesses storage capacity of 8 wt.% [Zhao et al.
2014; Azad et al. 2016) and 7 wt.% for MOF-5 possessing 0.77 nm micropores
(Langmi et al. 2013). Generally, the structure of MOFs with bigger pore sizes are
more susceptible to collapse, and this affects their permanent porosity. However,
268
T. C. Maponya et al.
hydrogen storage applications (Campesi et al. 2010; Wang et al. 2017) and also clean
energy applications due to their excellent high surface area, permanent porosity and
pore size distributions (Tranchemontagne et al. 2008; Wang et al. 2017; Zhang et al.
2017).
10.4.2 Structure of Metal Organic Frameworks
The arrangement of MOFs structures can either be two-dimensional (2D) or threedimensional (3D) networks. These networks are assembled from organic ligands and
metal ion or cluster nodes (Jiang et al. 2013). Unsaturated metal sites or accessible
metal sites in MOFs play a major role in influencing their adsorption performance.
Transition metals such as Cu, Zn, Mn, Co are usually the ones chosen as centre
connectors and they serve as Lewis acids for an activation of the coordinated organic
ligands for organic transformation (Raoof et al. 2015; Kim et al. 2013; Nas et al.
2015). The adsorption capacity is due to partial positive charges of metal sides
(Cheetham et al. 2006; Raoof et al. 2015). The transition metals can provide variety
in geometries depend largely on their oxidation number (Tan et al. 2017; Feng et al.
2013). The linkers possess the aromatic rings in the framework, which help to
maintain the structural integrity of the complex and direct the geometry of the
framework. The pore volume and surface area of MOFs can be organized by
Modification of organic ligands influences the surface area of the MOFs. The
common used organic linkers are provided in Fig. 10.3 (Bedia et al. 2019).
Secondary Building Units (SBUs) are critical role players dictating the final
geometry of MOFs. The structural and chemical properties of the SBUs and
organic ligands result in the prediction of the schemes and synthesis of MOFs. It
has been reported that under careful selection of reaction preconditions, multidentate
linkers can form some aggregates which affect the formation of SBUs (Salunkegawali et al. 2012). Subsequently, the SBUs will join with rigid organic links to
develop MOFs with high structural stability (Yuan et al. 2016). However, the
structure of the SBU is controlled by the metal-to-ligand ratio, solvent and source
of anions used in balancing metal ions charges [Sun and Sun 2014; Wang et al.
2014). MOF structures with internal diameter of pores of up to 4.8 nm can extend the
free space and thus results in H 2 storage (Abbasi et al. 2017). However, the empty
spaces remain intact after the guest molecules have been removed. The size of the
pores on the adsorbents is responsible for possible interaction between the absorbed
gas molecules and the surface of the surrounding walls. Furthermore, it should be
close to the kinetic diameter of H 2 molecule (0.289 nm) to endorse stronger
interactions amongst H 2 molecules and the framework. The MOF (NU-100)
containing <2 nm micropores possesses storage capacity of 8 wt.% [Zhao et al.
2014; Azad et al. 2016) and 7 wt.% for MOF-5 possessing 0.77 nm micropores
(Langmi et al. 2013). Generally, the structure of MOFs with bigger pore sizes are
more susceptible to collapse, and this affects their permanent porosity. However,
268
T. C. Maponya et al.