Structured Nanoporous Materials for Biogas Upgrading

Porous materials have shown potential for solving one of the significant challenges to sequestering CO2 gas from point sources. Microporous materials, zeolites and metal-organic frameworks (MOFs) are porous materials with tailorable adsorptive properties to develop higher performance for an energy-efficient CO2 separation. One of the potential applications of these materials is the removal of CO2 from biogas (biogas upgrading) to achieve biomethane, which can be utilised to produce heat, electricity and vehicle fuel. Biogas upgrading is a key technology to utilise biogas as a high-value fuel and contribute towards the transition from fossil fuels to carbon-neutral fuel. The aim of the thesis is to enhance the adsorptive properties and mass transfer kinetics of the hierarchically structured microporous materials by novel approaches to achieve efficient biogas upgrading. In this thesis, zeolites and metal-organic frameworks were tailored and specifically structured to develop multimodal porous adsorbents. The commercial CaA and NaX binderless granules were optimised via a partial ion-exchange process using potassium and cesium cations to achieve high IAST (Ideal Adsorbed Solution Theory) CO2-over-CH4 selectivity of 1775 and 525, respectively, than their corresponding zeolites. Moreover, the optimised ion-exchanged CaA and NaX binderless granules displayed high CO2 adsorption capacities of 4.3 mmol/g and 5.1 mmol/g at 298 K and 100 kPa, repectively. The breakthrough experiment showed that the NaK4.5Cs0.3X binderless granules retained 97% of their CO2 adsorption capacity after five adsorption-desorption cycles with the mass transfer coefficient of 0.41 m/s, suggesting a high methane recovery in biogas upgrading cycles. To design complex structures at nanoscale, an electrospinning technique was used to process a hierarchical porous ZSM-5 nanofiber composite for enhanced CO2 adsorptive properties. ZSM-5 nanofiber composite showed an increase of 30.4% in the BET surface area after the post-thermal processing compared to pure ZSM-5 nanopowder with a significant increment of 34 % in the CO2 uptake capacity, 2.15 mmol/g at 293 K and 100 kPa. Furthermore, the ZSM-5 composite nanofiber was structured into mechanically strong pellets with a maximum tensile strength of 6.5 MPa to withstand the rapid pressure swings. The ZSM-5 pellets displayed stable adsorption-desorption cycles (up to 5) in the breakthrough experiment, signifying negligible chemisorbed CO2 with a high mass transfer coefficient of 1.24 m/s. Another novel structuring approach with the ability to control pore morphology was applied to NaX and CaA zeolite powder using a low-cost freeze granulation technique. The freeze granulation procedure was optimised to fabricate homogeneous NaX and CaA freeze granules, 2-3 mm in diameter. The ice templated pores formed during the freeze-drying provided a high degree of additional macroporosity of 77.9% for NaX and 68.6% for CaA freeze granules in order to achieve rapid diffusion to the adsorption sites in the granules. The zeolite NaX freeze granules showed a sharp breakthrough curve, implying a low mass transfer resistance with a mass transfer coefficient of 1.3 m/s while keeping the high equilibrium CO2 uptake capacities of 5.8 mmol/g at 273 K and 1 bar, respectively. Further on in the studies, we optimised the synthesis parameters to obtain hierarchical Cu-MOF nanocrystals to introduce mesoporosity to enhance the CO2 mass transfer kinetics. The volumetric CO2 capacity of hierarchical Cu-MOF was 2.58 mmol/g at 293 K, 100 kPa with low isosteric heat of adsorption, 28 kJ/mol, and they displayed a high specific BET surface area, 627.4 m2/g. To investigate the CO2 separation performance, the hierarchical Cu-MOF powders were mixed with a polymer binder for structuring into pellets to impart additional mechanical stability of 1.4 MPa. The hierarchical Cu-MOF crystals displayed a high CO2 gravimetric uptake of 160 mg/g. In the breakthrough experiments, the hierarchical Cu-MOF pellets achieved stability after the first cycle and showed high mass transfer kinetics, 1.8 m/s, implying rapid CO2 separation with reduced cycle time