Adsorption kinetics for CO_2 on highly selective zeolites NaKA and nano-NaKA

摘要

Carbon dioxide removal from flue gas via swing adsorption processes requires adsorbents with a high CO_2 selectivity and capacity. These properties are particularly valuable to reduce the cost of carbon capture and storage (CCS). Zeolite NaKA was studied for its ability to selectively adsorb CO_2 from flue gas, as we previously observed that zeolite NaKA, with a K~+/(K~+ + Na~+) ratio of 17 atomic%, was highly selective towards CO_2 over N~2 adsorption by tuning the size of the pore window apertures [1]. The reduced pore apertures may, however, retard the adsorption rate of CO_2. Here, we studied the kinetics of CO_2 adsorption on regularly sized zeolite NaKA and on nano-sized zeolite NaKA. We used in situ infrared (IR) spec-troscopy and observed that CO_2 physisorbed relatively rapidly. Density functional theory (DFT) was used for quantum chemical calculations, and the results indicated that CO_2 molecules bridged across two or three Na~+ ions in the samples with no or very small amount of K~+. When more K~+ ions are present the CO_2 molecules no longer bridged across multiple metal ions and adopted an end-on configuration. The calculation showed a shift in the stretching vibration frequency of physisorbed CO_2 as observed by IR spectroscopy. Nano-sized zeolite NaKA were synthesised and studied to improve the rate of CO_2 adsorption, as the diffusion rate typically increases quadratically with decreasing particle size. Still, the CO_2 adsorption rate on nano-sized zeolites NaA and NaKA did not increase significantly. For nano-sized zeolite NaA, we speculate that the absence of such an increased rate is an effect from a skin layer that had formed on the nano-sized zeolite NaA, a layer that was possibly related to intergrowths with extremely small crystals on the surface. The apparently slow adsorption kinetics of CO_2 on nano-sized zeolite NaKA was more difficult to explain because it could relate to imperfections within the small crystals, remaining water, or other effects. Overall, the CO_2 adsorption rates on zeolite NaKA crystals of different sizes were fast and relevant for the time scales required for adsorption based CCS processes, such as vacuum and temperature swing adsorption (VSA/TSA).