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

because the polymer matrix can act as a support structure by organic molecules, creating a highly ordered network
for the zeolite particles, preventing them from collapsing with a large surface area and high porosity [112] . As shown
or agglomerating during use. Thakkar et al. fabricated the in Figure 11, zeolitic imidazolate framework-8 (ZIF-8) is
zeolite-embedded polymer monoliths through the DIW a type of commercialized MOF material that has garnered
technique [109] . The printed monoliths consisted of Torlon significant attention in recent years due to its excellent
polymer and zeolites 13X and 5A, with around 31 wt% gas adsorption properties and affordable price for large
zeolite particles incorporated into the polymer matrix. The quantities. As a type of MOFs being topologically isomorphic
composite monoliths demonstrated a compressive strength with zeolites, ZIF-8 has a regular arrangement of pores and
of approximately 210 MPa. This compressive strength was channels that allow for efficient gas transport, making it
notably higher than that of 3D-printed zeolite monoliths attractive for various gas adsorption applications [113] .
developed in the previous work, which is below 1 MPa [109] . Evans et al. adopted FDM for the 3D printing of
The improved compressive strength of the polymer-zeolite ZIF-8 in thermoplastic polymer composites. The MOF-
composite monoliths makes them highly robust structures thermoplastic polymer composites were produced by
with outstanding mechanical integrity. This enhanced incorporating ZIF-8 homogeneously into both polylactic
strength allows for better resistance to deformation and acid and thermoplastic polyurethane matrices at high
attrition during handling and adsorption processes. loadings of up to 50% by mass. The BET surface area of
In 2017, Wudy et al. investigated using the SLS the printed ZIF-8 was not high initially, but it increased
technique to manufacture zeolite-filled polypropylene after CH OH solvent exchange and evacuation, yielding
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2
composites. The properties of printed composite materials a BET surface area of 105 m /g. The specific surface area
2
are comparable to those manufactured through extrusion increased to 141 ± 27 m /g after printing the filament with
processes, with the added benefit of improved water the ZIF-8 to PLA weight ratio of 1.5 [114] .
adsorption properties. The addition of 20 wt% zeolite Bible et al. developed composite materials by
resulted in a 50% increase in water uptake compared incorporating ZIF-8 into acrylonitrile butadiene styrene
to unfilled polypropylene. It was anticipated that the (ABS) through FDM [115] . The authors previously determined
addition of thermally conductive fillers could enhance gas adsorption kinetics for 3D-printed pure ABS, which had
the thermal conductivity of the composite, which might a BET surface area of 0.6 m /g and a total pore volume of
2
have implications for vapor adsorption. Furthermore, 0.002 cm /g. In contrast, the printed ABS-ZIF-8 composite
3
SLS-manufactured composite is expected to have improved had a BET surface area of 1.5 m /g and a total pore volume
2
mechanical properties than its extruded counterpart due of 0.005 cm /g, indicating that the incorporation of ZIF-8
3
to better interfacial bonding between polymer matrix MOFs significantly increased the porosity and surface
and zeolite fillers. The authors stated that the detailed area of the composite material. The printed MOFs also
mechanical strength performance of the printed composite maintained their N adsorption capacities within the
2
would be evaluated in their future study [110] . composites even after soaking in water [115] .
Zhang et al. explored the fabrication of polymer-zeolite Lefevere et al. reported the use of the DIW technique
composite with up to 75 wt% zeolite filler contents through to synthesize ZIF-8 monoliths with open flow-through
direct laser writing (DLW), a type of photopolymerization- channels. The MOF structures were fabricated layer-by-
based 3D printing. The printed composite can absorb layer using a binder recipe containing methylcellulose and
water and expand, causing the 3D object to change shape. bentonite binder, and thermally treated to remove the organic
When the water is removed, the composite returns to binder without destroying the ZIF-8 structure. The resulting
its original shape due to the shape memory effect of the ZIF-8 monoliths exhibited high mechanical stability with
polymer, resulting in 4D behavior. The storage modulus of compressive strengths ranging from 0.6 to 1.4 MPa and
the composites increased by 10-fold compared to that of a high adsorption capacities for CO , CH , and N gases, with
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pure polymer. The tensile strength and elongation at the maximum uptakes of 1.5, 0.5, and 0.3 mmol/g, respectively [116] .
break of the composites were slightly lower than those of
the pure polymer [111] . Table 4 summarizes the composition, 3.2. Structural design
mechanical strength, and gas adsorption properties of the The design of the zeolite structures can influence the
above-mentioned polymer-zeolite composites. diffusion, adsorption, and desorption properties of
the adsorbent. Even zeolite materials with identical
3.1.3. Metal-organic frameworks-based type
compositions can display distinct structural characteristics
Metal-organic frameworks (MOFs) are a class of porous for gas adsorption. Table 5 summarizes some structures
materials consisting of metal ions or clusters linked together which are commonly adopted for the fabrication of

Volume 2 Issue 4 (2023) 11 https://doi.org/10.36922/msam.1880