Mechanisms and Kinetics for Sorption of CO2 on Bicontinuous Mesoporous Silica Modified with n-Propylamine

We studied equilibrium adsorption and uptake kinetics and identified molecular species that formed during sorption of carbon dioxide on amine-modified silica. Bicontinuous silicas (AMS-6 and MCM-48) were postsynthetically modified with (3-aminopropyl)triethoxysilane or (3-aminopropyl)methyldiethoxysilane, and amine-modified AMS-6 adsorbed more CO(2) than did amine-modified MCM-48. By in situ FTIR spectroscopy, we showed that the amine groups reacted with CO(2) and formed ammonium carbamate ion pairs as well as carbamic acids under both dry and moist conditions. The carbamic acid was stabilized by hydrogen bonds, and ammonium carbamate ion pairs formed preferably on sorbents with high densities of amine groups. Under dry conditions, silylpropylcarbamate formed, slowly, by condensing carbamic acid and silanol groups. The ratio of ammonium carbamate ion pairs to silylpropylcarbamate was higher for samples with high amine contents than samples with low amine contents. Bicarbonates or carbonates did not form under dry or moist conditions. The uptake of CO(2) was enhanced in the presence of water, which was rationalized by the observed release of additional amine groups under these conditions and related formation of ammonium carbamate ion pairs. Distinct evidence for a fourth and irreversibly formed moiety was observed under sorption of CO(2) under dry conditions. Significant amounts of physisorbed, linear CO(2) were detected at relatively high partial pressures of CO(2), such that they could adsorb only after the reactive amine groups were consumed.authorCount :7</p

Adsorption of Carbonyl Sulfide on Propylamine Tethered to Porous Silica

Carbonyl sulfide (COS) reacts slowly with amines in the aqueous solutions used to absorb CO₂ from natural gas and flue gas and can also deactivate certain aqueous amines. The effects of COS on amines tethered to porous silica, however, have not been investigated before. Hence, the adsorption of COS on aminopropyl groups tethered to porous silica was studied using in situ IR spectroscopy. COS chemisorbed mainly and reversibly as propylammonium propylthiocarbamate ion pairs [R–NH­(CO)­S^– ⁺H₃N–R] under dry conditions. In addition, a small amount of another chemisorbed species formed slowly and irreversibly. Nevertheless, the CO₂ capacities of the adsorbents were fully retained after COS was desorbed

Spherical and Porous Particles of Calcium Carbonate Synthesized with Food Friendly Polymer Additives

Porous calcium carbonate particles were synthesized by adding solutions of Ca²⁺ to solutions of CO₃^2– containing polymeric additives. Under optimized conditions well-defined aggregates of the anhydrous polymorph vaterite formed. A typical sample of these micrometer-sized aggregates had a pore volume of 0.1 cm³/g, a pore width of ∼10 nm, and a specific surface area of ∼25–30 m²/ g. Only one mixing order (calcium to carbonate) allowed the formation of vaterite, which was ascribed to the buffering capacity and relatively high pH of the CO₃^2– solution. Rapid addition of the calcium chloride solution and rapid stirring promoted the formation of vaterite, due to the high supersaturation levels achieved. With xanthan gum, porous and micrometer-sized vaterite aggregates could be synthesized over a wide range of synthetic conditions. For the other food grade polymers, hydroxypropyl methylcellulose (HPMC), methylcellulose (MC), and sodium carboxyl methylcellulose, several intensive and extensive synthetic parameters had to be optimized to obtain pure vaterite and porous aggregates. HPMC and MC allowed well-defined spherical micrometer-sized particles to form. We expect that these spherical and porous particles of vaterite could be relevant to model studies as well as a controlled delivery of particularly large molecules

K⁺ Exchanged Zeolite ZK‑4 as a Highly Selective Sorbent for CO₂

Adsorbents with high capacity and selectivity for adsorption of CO₂ are currently being investigated for applications in adsorption-driven separation of CO₂ from flue gas. An adsorbent with a particularly high CO₂-over-N₂ selectivity and high capacity was tested here. Zeolite ZK-4 (Si:Al ∼ 1.3:1), which had the same structure as zeolite A (LTA), showed a high CO₂ capacity of 4.85 mmol/g (273 K, 101 kPa) in its Na⁺ form. When approximately 26 at. % of the extraframework cations were exchanged for K⁺ (NaK-ZK-4), the material still adsorbed a large amount of CO₂ (4.35 mmol/g, 273 K, 101 kPa), but the N₂ uptake became negligible (<0.03 mmol/g, 273 K, 101 kPa). The majority of the CO₂ was physisorbed on zeolite ZK-4 as quantified by consecutive volumetric adsorption measurements. The rate of physisorption of CO₂ was fast, even for the highly selective sample. The molecular details of the sorption of CO₂ were revealed as well. Computer modeling (Monte Carlo, molecular dynamics simulations, and quantum chemical calculations) allowed us to partly predict the behavior of fully K⁺ exchanged zeolite K-ZK-4 upon adsorption of CO₂ and N₂ for Si:Al ratios up to 4:1. Zeolite K-ZK-4 with Si:Al ratios below 2.5:1 restricted the diffusion of CO₂ and N₂ across the cages. These simulations could not probe the delicate details of the molecular sieving of CO₂ over N₂. Still, this study indicates that zeolites NaK-ZK-4 and K-ZK-4 could be appealing adsorbents with high CO₂ uptake (∼4 mmol/g, 101 kPa, 273 K) and a kinetically enhanced CO₂-over-N₂ selectivity

Mechanisms and Kinetics for Sorption of CO₂ on Bicontinuous Mesoporous Silica Modified with <i>n</i>-Propylamine

We studied equilibrium adsorption and uptake kinetics and identified molecular species that formed during sorption of carbon dioxide on amine-modified silica. Bicontinuous silicas (AMS-6 and MCM-48) were postsynthetically modified with (3-aminopropyl)triethoxysilane or (3-aminopropyl)methyldiethoxysilane, and amine-modified AMS-6 adsorbed more CO₂ than did amine-modified MCM-48. By in situ FTIR spectroscopy, we showed that the amine groups reacted with CO₂ and formed ammonium carbamate ion pairs as well as carbamic acids under both dry and moist conditions. The carbamic acid was stabilized by hydrogen bonds, and ammonium carbamate ion pairs formed preferably on sorbents with high densities of amine groups. Under dry conditions, silylpropylcarbamate formed, slowly, by condensing carbamic acid and silanol groups. The ratio of ammonium carbamate ion pairs to silylpropylcarbamate was higher for samples with high amine contents than samples with low amine contents. Bicarbonates or carbonates did not form under dry or moist conditions. The uptake of CO₂ was enhanced in the presence of water, which was rationalized by the observed release of additional amine groups under these conditions and related formation of ammonium carbamate ion pairs. Distinct evidence for a fourth and irreversibly formed moiety was observed under sorption of CO₂ under dry conditions. Significant amounts of physisorbed, linear CO₂ were detected at relatively high partial pressures of CO₂, such that they could adsorb only after the reactive amine groups were consumed