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Materials Science in Additive Manufacturing Emerging 3D-printed zeolitic gas adsorbents

Table 2. Factors that influence the adsorption of gases on zeolites [58,59] .
Factors Influence on the adsorption of gases on zeolites
Basicity Zeolites with higher basicity exhibit a greater capacity for gas adsorption due to the enhanced electron density of the
framework oxygen.
Exchangeable cations • Polarizing power: Zeolites with smaller exchangeable cations have higher polarizing power and stronger interactions with
polar gas molecules such as CO . 2
• Distribution: The distribution of exchangeable cations within zeolites contributes to the heterogeneous character of the
adsorption process. Different sites within the zeolite structure result in the selectivity of the gas adsorption.
• Size: The energy of interaction between the gas molecules and exchangeable cations is inversely proportional to the size of
cations, influencing the overall adsorption capacity and selectivity.
• Number: A higher number of exchangeable cations can provide more sites for interaction with gas molecules, leading to an
increase in the gas adsorption capacity and selectivity of zeolites
Silica-alumina ratio A lower silica-alumina ratio increases the electric field within the zeolite pores, enhancing the adsorption of polar molecules
and resulting in improved gas adsorption properties.
Size of pores The size of pores within zeolites determines the accessibility and penetration of gas molecules into the structure. The appropriate
pore size allows for effective adsorption, while larger or smaller pore sizes may limit the adsorption capacity and rate.
Polarity of adsorbates The polarity of adsorbates influences their interaction with the electric field within zeolites, which in turn affects gas
adsorption. Adsorbates with a greater polarity tend to have stronger interactions with the electric field of zeolites, leading to
enhanced adsorption capabilities.
Dimension of adsorbates The dimensions of adsorbates, such as the size and shape of molecules, play a role in their adsorption on zeolites. The
porosity of zeolite cavities is a selective factor for adsorbed molecules, with specific dimensions being more favorable for
effective adsorption.
Pressure At low pressures, the amount of gas adsorbed is directly proportional to the cationic density in the zeolite pores. In contrast,
at high pressures, the volume of the pores becomes more significant.
Temperature An increase in temperature can decrease the adsorbent-adsorbate interactions, leading to a decrease in gas adsorption of zeolites.

support surfaces. The slurry coating is often applied for can experience a significant pressure drop as the gas flows
the fabrication of zeolite monoliths because it is easy to through the bed. This pressure drop can affect the overall
apply the wash coat. Zeolite crystals and precursors of the efficiency of the adsorption process and may require
binder are carried by the wash-coat solutions. The bonding additional energy for gas flow. Furthermore, there is a
between the crystallites and support is then obtained possibility that the gas preferentially flows through certain
through the calcination of coatings . In contrast to slurry pathways or channels, bypassing some of the adsorbent
[69]
coating, zeolite crystals are directly coated on the support material. This can result in reduced contact between the gas
surfaces through in situ coating. Such a coating process and the adsorbent, leading to lower adsorption efficiency.
requires the support to be immersed in a clear solution Die-based extrusion lacks the ability to fabricate zeolite
such as the synthesis gel. The nucleation and growth of absorbents with very complex geometries rather than
zeolite crystals are intended to occur on the support than continuous channels or other simple structures. Stuecker
in the solution, leading to the fabrication of monoliths et al. performed a simulation study on the flow velocities
with higher mechanical strength than those produced by of gas passing through an extruded honeycomb-structured
slurry coating. In this way, dense and homogeneous zeolite zeolite monolith and a 3D-printed one . As shown in
[72]
coatings can be achieved for the fabrication of monolith . Figure 3, the maximum gas flow velocity achieved by the
[70]
Zeolite membrane can also be made through the growth
of zeolite crystals over the support surfaces to form straight continuous channel system is found to be over
polycrystalline film . 4 times less than that offered by the interlocking channel
[71]
system fabricated through 3D printing. Such a complex
2.2.4. Drawbacks of conventional methods interlocking channel system cannot be manufactured by
extrusion. This is because the nature of this conventional
Pellets and granules are easier to produce and have a fabrication method is to create a continuous shape with a
faster production rate, but they have some disadvantages consistent cross-section.
compared to extruded structures, such as honeycombs.
These drawbacks include millimeter-sized particles, Coating of structured zeolitic gas adsorbents involves
leading to a reduced external surface area. Therefore, applying a thin layer of zeolite materials onto a support
pellets and granules are packed into a fixed bed to increase structure, which can be fabricated into complex interlocking
the bed voidage for gas adsorption. However, the fixed bed channel systems through other fabrication approaches,

Volume 2 Issue 4 (2023) 5 https://doi.org/10.36922/msam.1880