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, Fourier Transform Infrared, and elemental analysis. 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.