Moisture effect on the separation of CO2/CH4 mixtures with amine-functionalised porous silicas

The effect of minor amounts of water on the CO2 and CH4 adsorption on primary and secondary amine-functionalised mesoporous silicas (APTES@SBA-15 and DEAPTES@SBA15, respectively) was studied with a combination of high-pressure gas adsorption, solid state NMR of labelled (CO2)-C-13 and density functional theory (DFT) calculations. Known amounts of water were pre-adsorbed on the materials (0.047 to 0.157 mmol center dot g(-1)) and the adsorption performance for CO2 and CH4 was compared to the performance of the dry samples. We observed that even when only minor amounts of water are present, the tertiary amine-functionalised material revealed a significant enhancement of the selectivity for CO2 (from ca. 5.8 to 208) while the one with primary amine maintained the same adsorption properties. This is related to the change in the adsorption mechanism in DEAPTES@SBA15 when water is present since water participates in the reaction of the tertiary amine with CO2 to produce bicarbonate (confirmed by NMR and DFT results). Such species was not observed in APTES@SBA-15 with water. From a broader perspective, the results presented in this work are relevant to confirm the suitability of this type of hybrid adsorbents in industrial applications related with CO2 adsorption, where minor amounts of water may be present in the process streams, such as during bio, landfill, or natural gas upgrading, or also in carbon capture applications.

Automated summary

The effect of minor amounts of water on CO2 and CH4 adsorption on amine-functionalised materials was studied, and it was found that the presence of water significantly enhances the selectivity for CO2 on tertiary amine-functionalised materials. However, the presence of water has no effect on the adsorption properties of primary amine-functionalised materials.