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

Table 1. Properties of selected typical zeolites for gas adsorption
Zeolite Group Silica‑alumina ratio [44‑46] Application for gas adsorption
Chabazite Natural 1 – 4 Separation of CO from natural/landfill gas; removal of NH gas [47,48]
2 3
Clinoptilolite Natural 6 Removal of CO , H S, N , and H O from natural gas; removal of SO 2
2
2
2
2
from flue gas; removal of NH3 gas [49,50]
Mordenite Natural 5.5 Adsorption of CO , SO and xylene [51,52]
2 2
LTA Synthetic 0.7 – 1.2 Storage and separation of CO ; dehydration agent [53]
2
X Synthetic 1 – 1.5 Storage and separation of CO and H [54]
2 2
Y Synthetic 1.5 – 7.9 Storage and separation of CO , H O, and CH [55]
2 2 4
in a stronger electric field when Al is present. Therefore, mixture into small, spherical particles. This is typically
zeolites with high alumina content, such as zeolite A achieved by applying pressure to the powder mixture using
and X, generally have higher adsorption capacities when a pellet press or extruder. The pressure helps to bind the
compared to zeolite Y [56,57] . particles together, forming cohesive pellets . Granulation,
[60]
Besides the structural characteristics of zeolitic gas on the other hand, involves the formation of larger,
adsorbents, the adsorbent-adsorbate interaction is also irregularly shaped particles by agglomerating powder
affected by the adsorbates. Adsorbates with a greater particles with a binder. The powder mixture is typically
polarity, such as CO , tend to have stronger interactions mixed with a liquid binder, such as water or a solvent, to
2
with the electric field within zeolites. This favors their create a wet mass. This wet mass is then subjected to drying
adsorption, as they can effectively interact with the or spray drying processes to remove the liquid and form
[61]
surface and pores of zeolites. On the other hand, less granules . These methods offer simplicity and versatility
polar adsorbates may have weaker interactions and lower in material incorporation, allowing for the inclusion of
adsorption capacities. Dimension of adsorbates also affects various components or additives in the pellet or granule
their ability to access the available pore space in zeolites. structure.
Zeolite structures usually consist of interconnected pores 2.2.2. Die-based extrusion
of specific sizes and shapes. Smaller adsorbate molecules
that are even smaller than the pore size can easily enter Compared to granulation and pelletization, extrusion
and occupy the available pore space, leading to efficient has been developed for the fabrication of structured
adsorption. In addition, environmental conditions, adsorbents with open channels [60-63] . Such open channeling
such as pressure and temperature, also influence the can reduce resistance to gas flow, resulting in an improved
adsorbent-adsorbate interaction in zeolites. Pressure gas transfer rate and more efficient adsorption. Unlike
can affect adsorption capacity by limiting the available the extrusion-based 3D printing technology, a die is
pore space, while temperature can affect the strength of required for conventional extrusion manufacturing. The
the adsorbent-adsorbate interaction by increasing the die-based extrusion usually starts with mixing zeolite
mobility of the adsorbate molecules. Table 2 summarizes with binder materials and water. The mixture is then
the relation between the above-mentioned factors and the extruded into green monoliths in different shapes using
gas adsorption performance of zeolites. corresponding dies. The final products can be obtained
after drying and firing . However, the usage of binders
[64]
2.2. Conventional shaping and structuring methods could weaken the support strength and block the pores
[65]
Zeolites, in their natural form, are typically synthesized in of zeolites . Although hydraulic extrusion is applied to
powder form. However, it is necessary to transform them fabricate binderless zeolites, a marginal amount of binding
into structured materials for their industrial applications agent is still required, leading to a zeolite loading lower
as gas adsorbents. A general introduction to conventional than 100 wt% [66-68] . Besides the direct forming of zeolite
shaping and structuring methods for zeolitic gas adsorbents monoliths, extrusion is commonly adopted for fabricating
is given in this section. microporous alumina supports where zeolite monoliths or
membranes can be grown through the coating.
2.2.1. Pelletization and granulation
2.2.3. Coating
Pelletization and granulation are traditional methods
used for adsorbent geometric processing. In pelletization, Slurry coating and in situ synthesis are two widely used
the process involves compressing and shaping a powder methods for coating zeolite monolith onto the monolith

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