In this study, we report the design, fabrication, and characterization of 3D-printed zeolite RHO-based monoliths for selective CO2/CH4 separation, with the aim of advancing adsorptive separation technologies relevant to biogas upgrading. Zeolite RHO, a well-known aluminosilicate framework with excellent CO2 adsorption properties1, was combined with a photopolymer resin and shaped into gyroidal monolithic structures using digital light processing (DLP)-based 3D-printing2. A precursor formulation containing 33 wt% zeolite RHO was optimized for printability and structural fidelity3. Post-processing of the printed bodies was performed through two distinct routes: thermal calcination and pyrolysis. Calcination was carried out under air at 580 °C (3DZR-580) and 800 °C (3DZR-800), while pyrolysis was performed at 580 °C under nitrogen with (3DZRKP) and without KOH (3DZRWKP) as a porogenic agent. Thermogravimetric analysis (TGA) guided the debinding profiles, showing complete resin degradation near 580 °C, which justified the selection of sintering temperatures. Optical microscopy and SEM analyses demonstrated that the gyroid geometry and mechanical integrity of the monoliths were preserved, particularly for the pyrolyzed sample (3DZRWKP), where carbonized polymer served as a structural support embedding zeolite crystals. CO2 adsorption isotherms at 25°C revealed that the parent zeolite RHO powder exhibited a capacity of 3.96 mmol/g at 1 bar, while 3DZR-580 and 3DZRWKP showed slightly reduced capacities of 2.6 and 2.58 mmol/g, respectively. However, when normalized to the active adsorbent content, the pyrolyzed sample demonstrated a significantly improved utilization of the zeolite phase, attributed to additional adsorption by the carbon matrix. At high pressure (12 bar), the zeolite powder achieved a CO2 uptake of 5.84 mmol/g, saturating around 9 bar. In comparison, 3DZR-580 showed a CO2 uptake of 4.75 mmol/g and a low CH4 uptake of 0.13 mmol/g, resulting in exceptional CO2/CH4 selectivity ranging from 101 at 75 mbar to 36 at 12 bar. On the other hand, 3DZRWKP demonstrated higher CH4 uptake (0.96 mmol/g at 12 bar), resulting in lower CO2/CH4 selectivity values from 16 to 3.45 across the same pressure range. The combined structural effectiveness and performance metrics of the 3D-printed monoliths highlight the potential of geometrically optimized adsorbent structures for targeted gas separations. Ongoing work includes breakthrough experiments and extended high-pressure adsorption studies to further assess separation performance under dynamic conditions.
Acknowledgement
The authors acknowledge financial support from Khalifa University through the Center for Catalysis and Separation (CeCaS, RC2-2018-024). GNK acknowledges also funding from the programme “MEDICUS” (grant # 82246) of the University of Patras. Support by the Advanced Digital & Additive Manufacturing Center of Khalifa University (ADAM Center, Award No. RCII-2019-003) is also greatly appreciated. The authors acknowledge the Electron Microscopy Service of the Universitat Politècnica de València for assistance in SEM, TEM and XRM characterization.
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