Effect of combined primary and secondary amine loadings on the adsorption mechanism of CO2 and CH4 in biogas

Biomethane, produced by biogas upgrading, is a promising energy source that can play a key role towards net-zero emissions targets. The incorporation of amine functionalities into adsorbents for biogas upgrading can facilitate the selective adsorption of CO2, but their effect has not been comprehensively studied within the context of CH4 mixtures. In this work, the effectiveness of amine functionalities in selectively separating CO2 from biogas, is investigated. Primary, diamine, and triamine organo-silanes grafted at various loadings on SBA-15 were used to study the adsorption mechanisms associated with amine functionalities for CO2:CH4 gas mixtures. The successful incorporation of amines was confirmed with thermogravimetric analysis (TGA), Fourier Transform Infrared (FTIR), and elemental analysis (EA). The different amine reagents and loadings resulted in an alteration of adsorption mechanism that provided key information on the developing relationship between adsorption capacity, selectivity, and energy efficiency. Diamine with an amine loading of 2.5 mmol/g and a moderate silane coverage of 1.54 molecules/nm2 was found to provide the best balance of an enhanced CO2 adsorption capacity (1.12 mmol/g), a superior selectivity to densely grafted primary amines, and the lowest isosteric heat of adsorption of ∼25 kJ/mol at 1.12 mmol/g compared to ∼41 kJ/mol for primary and triamine materials. Amongst all the samples, a lower amine loading on the bare adsorbent enhanced CO2 adsorption capacity and selectivity while minimising the heat duty associated with adsorbent regeneration. Moreover, under isothermal desorption conditions at 25 °C, some samples achieved working capacities comparable to higher amine loaded materials