364
R. Kumar et al.
Since the discovery of carbon nanotubes (CNTs) by Iijima (Tersoff and Ruoff
1994; Iijima and Ichihashi 1993; Iijima 1991), it has made supreme impacts on nanotechnology and nanoscience in the reference of electrical, thermal, and mechanical
properties of the materials (Tersoff and Ruoff 1994; Kholmanov et al. 2015). Mostly
CNTs are categorized in the single wall carbon nanotubes (SWCNTs), double wall
carbon nanotubes (DWCNTs) and multiwall carbon nanotubes (MWCNTs). The arc
discharge is one of the oldest and worthy techniques to produce the CNTs. According
to the physical and chemical properties of CNTs, these have a potential application
in the gas separation. There is a large variation in the ratio of length and diameter of
the CNTs. Many types of CNTs composites are used for gas separation applications.
Structure and orientation of CNTs are key parameters for the conductivity as well
as the permeability and selectivity of the gases. In the field of hydrogen separation,
many researchers used the aligned orientation of CNTs to improve the selectivity of
gases (Swain et al. 2017; Babu et al. 2013). It is expected that the alignment of CNTs
in polymer matrix provides more number of channels and dissolution opportunities
in comparison to randomly distributed MWCNTs. Also, the separation application
is based on the functionalization of the CNTs (Sanip et al. 2011; Ma et al. 2010).
Functionalization of the CNTs is useful for the attachment of gas-sensitive nanomaterials which helps to enhance the selectivity of a specific gas. Based on the diameter
of the CNT in the composite membrane permeation, the gases can be combined as
an effect of solution diffusion and molecular sieving model. These features make
CNTs highly useful in reinforcement nano filter, probes, energy storage, gas filters,
bio applications, and various electronic and thermal devices.
Graphene also has a great opportunity for membrane-based gas separation applications because of critical thickness, chemical stability, flexibility, and mechanical
strength. Molecular Dynamic (MD) simulation and Density Functional Theory (DFT)
calculation shows that the graphene-based membranes are one of the good choices
for the H 2 separation application (Tao et al. 2014; Wei et al. 2018). H 2 has a significant difference in its kinetic diameter (2.9 Å for H 2 ) compared with other gases
molecules (3.64 Å for N 2 , 3.8 Å for CH 4 and 3.76 Å for CO, etc.) (Robeson 1991).
DFT calculation explained that the pore size in graphene is nearly the molecular
diameter of H 2 . So there are many possibilities that H 2 molecule will separate from
other gas molecules. Because H 2 is a candidate having nearly equal pore size and
other gas molecules will block due to bigger kinetic diameter. The approach with
graphene polymer composite membrane can enhance the H 2 selectivity over other
gases multiple times. However, the drawback is that due to smaller pore size the
permeability decreases.
Functionalized graphene (graphene oxide) can be used with a polymer matrix to
reduce the lower permeability problem (Sun and Li 2018). GO can also be utilized as
a unique material for gas separation applications. Functionalization of the graphene
can enhance the H 2 attachment probabilities. Separation mechanism for the GO
composite membranes follows the molecular sieving as explained in the previous
part. Nanoparticles, nanowires, and other nanostructures can interact with the GO
lay and helpful to expand the gaps (Ebrahimi et al. 2016; Wang et al. 2017; Zhang
et al. 2019). This explanation offers fast permeation of the gas molecules through the
R. Kumar et al.
Since the discovery of carbon nanotubes (CNTs) by Iijima (Tersoff and Ruoff
1994; Iijima and Ichihashi 1993; Iijima 1991), it has made supreme impacts on nanotechnology and nanoscience in the reference of electrical, thermal, and mechanical
properties of the materials (Tersoff and Ruoff 1994; Kholmanov et al. 2015). Mostly
CNTs are categorized in the single wall carbon nanotubes (SWCNTs), double wall
carbon nanotubes (DWCNTs) and multiwall carbon nanotubes (MWCNTs). The arc
discharge is one of the oldest and worthy techniques to produce the CNTs. According
to the physical and chemical properties of CNTs, these have a potential application
in the gas separation. There is a large variation in the ratio of length and diameter of
the CNTs. Many types of CNTs composites are used for gas separation applications.
Structure and orientation of CNTs are key parameters for the conductivity as well
as the permeability and selectivity of the gases. In the field of hydrogen separation,
many researchers used the aligned orientation of CNTs to improve the selectivity of
gases (Swain et al. 2017; Babu et al. 2013). It is expected that the alignment of CNTs
in polymer matrix provides more number of channels and dissolution opportunities
in comparison to randomly distributed MWCNTs. Also, the separation application
is based on the functionalization of the CNTs (Sanip et al. 2011; Ma et al. 2010).
Functionalization of the CNTs is useful for the attachment of gas-sensitive nanomaterials which helps to enhance the selectivity of a specific gas. Based on the diameter
of the CNT in the composite membrane permeation, the gases can be combined as
an effect of solution diffusion and molecular sieving model. These features make
CNTs highly useful in reinforcement nano filter, probes, energy storage, gas filters,
bio applications, and various electronic and thermal devices.
Graphene also has a great opportunity for membrane-based gas separation applications because of critical thickness, chemical stability, flexibility, and mechanical
strength. Molecular Dynamic (MD) simulation and Density Functional Theory (DFT)
calculation shows that the graphene-based membranes are one of the good choices
for the H 2 separation application (Tao et al. 2014; Wei et al. 2018). H 2 has a significant difference in its kinetic diameter (2.9 Å for H 2 ) compared with other gases
molecules (3.64 Å for N 2 , 3.8 Å for CH 4 and 3.76 Å for CO, etc.) (Robeson 1991).
DFT calculation explained that the pore size in graphene is nearly the molecular
diameter of H 2 . So there are many possibilities that H 2 molecule will separate from
other gas molecules. Because H 2 is a candidate having nearly equal pore size and
other gas molecules will block due to bigger kinetic diameter. The approach with
graphene polymer composite membrane can enhance the H 2 selectivity over other
gases multiple times. However, the drawback is that due to smaller pore size the
permeability decreases.
Functionalized graphene (graphene oxide) can be used with a polymer matrix to
reduce the lower permeability problem (Sun and Li 2018). GO can also be utilized as
a unique material for gas separation applications. Functionalization of the graphene
can enhance the H 2 attachment probabilities. Separation mechanism for the GO
composite membranes follows the molecular sieving as explained in the previous
part. Nanoparticles, nanowires, and other nanostructures can interact with the GO
lay and helpful to expand the gaps (Ebrahimi et al. 2016; Wang et al. 2017; Zhang
et al. 2019). This explanation offers fast permeation of the gas molecules through the