5 research outputs found
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 CO<sub>2</sub> 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
[RāNHĀ(Cī»O)ĀS<sup>ā</sup> <sup>+</sup>H<sub>3</sub>NāR] under dry conditions. In addition, a small amount of
another chemisorbed species formed slowly and irreversibly. Nevertheless,
the CO<sub>2</sub> 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
Ca<sup>2+</sup> to solutions of CO<sub>3</sub><sup>2ā</sup> 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 cm<sup>3</sup>/g, a pore width of ā¼10 nm, and a specific surface
area of ā¼25ā30 m<sup>2</sup>/ 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 CO<sub>3</sub><sup>2ā</sup> 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<sup>+</sup> Exchanged Zeolite ZKā4 as a Highly Selective Sorbent for CO<sub>2</sub>
Adsorbents
with high capacity and selectivity for adsorption of CO<sub>2</sub> are currently being investigated for applications in adsorption-driven
separation of CO<sub>2</sub> from flue gas. An adsorbent with a particularly
high CO<sub>2</sub>-over-N<sub>2</sub> selectivity and high capacity
was tested here. Zeolite ZK-4 (Si:Al ā¼ 1.3:1), which had the
same structure as zeolite A (LTA), showed a high CO<sub>2</sub> capacity
of 4.85 mmol/g (273 K, 101 kPa) in its Na<sup>+</sup> form. When approximately
26 at. % of the extraframework cations were exchanged for K<sup>+</sup> (NaK-ZK-4), the material still adsorbed a large amount of CO<sub>2</sub> (4.35 mmol/g, 273 K, 101 kPa), but the N<sub>2</sub> uptake
became negligible (<0.03 mmol/g, 273 K, 101 kPa). The majority
of the CO<sub>2</sub> was physisorbed on zeolite ZK-4 as quantified
by consecutive volumetric adsorption measurements. The rate of physisorption
of CO<sub>2</sub> was fast, even for the highly selective sample.
The molecular details of the sorption of CO<sub>2</sub> 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<sup>+</sup> exchanged zeolite K-ZK-4 upon adsorption
of CO<sub>2</sub> and N<sub>2</sub> for Si:Al ratios up to 4:1. Zeolite
K-ZK-4 with Si:Al ratios below 2.5:1 restricted the diffusion of CO<sub>2</sub> and N<sub>2</sub> across the cages. These simulations could
not probe the delicate details of the molecular sieving of CO<sub>2</sub> over N<sub>2</sub>. Still, this study indicates that zeolites
NaK-ZK-4 and K-ZK-4 could be appealing adsorbents with high CO<sub>2</sub> uptake (ā¼4 mmol/g, 101 kPa, 273 K) and a kinetically
enhanced CO<sub>2</sub>-over-N<sub>2</sub> selectivity
Mechanisms and Kinetics for Sorption of CO<sub>2</sub> 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 CO<sub>2</sub> than did amine-modified MCM-48. By in situ FTIR spectroscopy, we showed that the amine groups reacted with CO<sub>2</sub> 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<sub>2</sub> 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<sub>2</sub> under dry conditions. Significant amounts of physisorbed, linear CO<sub>2</sub> were detected at relatively high partial pressures of CO<sub>2</sub>, such that they could adsorb only after the reactive amine groups were consumed