Experimental and modelling studies of carbon dioxide capture onto pristine, nitrogen-doped, and activated ordered mesoporous carbons

The search for suitable materials for carbon dioxide capture and storage has attracted the attention of the scientific community in view of the increased global CO₂ levels and its after-effects. Among the different materials under research, porous carbons and their doped analogues are extensively debated for their ability to store carbon dioxide at high pressures. The present paper examined high-pressure carbon dioxide storage studies of 1-D hexagonal and 3-D cubic ordered mesoporous pristine and N-doped carbons prepared using the nano-casting method. Excess carbon dioxide sorption isotherms were obtained using the volumetric technique and were fitted using the Toth model. Various parameters that influence CO₂ storage on metal-free ordered mesoporous carbons, such as the effect of pore size, pore dimension, pyrolysis temperature, the impact of nitrogen substitution, and the effect of ammonia activation are discussed. It was observed that the carbon dioxide storage capacity has an inverse relation to the total nitrogen doped, the amount of pyridinic nitrogen functionality, and the pyrolysis temperature, whereas the pore size seems to have a linear relationship. On the other hand, the presence of oxygen has a positive effect on the sorption capacity. Among the prepared ordered mesoporous carbons, the ammonia-treated one has shown the highest adsorption capacity of 37.8 mmol g^-1 at 34 bar and 0 °C.

Enhanced CO2 Adsorption on Nitrogen-Doped Carbon Materials by Salt and Base Co-Activation Method

Nitrogen-doped carbon materials with enhanced CO₂ adsorption were prepared by the salt and base co-activation method. First, resorcinol-formaldehyde resin was synthesized with a certain salt as an additive and used as a precursor. Next, the resulting precursor was mixed with KOH and subsequently carbonized under ammonia flow to finally obtain the nitrogen-doped carbon materials. A series of samples, with and without the addition of different salts, were prepared, characterized by XRD (X-ray powder diffraction), elemental analysis, BET (N₂-adsorption-desorption analysis), XPS (X-ray photoelectron spectroscopy) and SEM (Scanning electron microscopy) and tested for CO₂ adsorption. The results showed that the salt and base co-activation method has a remarkable enhancing effect on the CO₂ capture capacity. The combination of KCl and KOH was proved to be the best combination, and 167.15 mg CO₂ could be adsorbed with 1 g nitrogen-doped carbon at 30 °C under 1 atm pressure. The materials characterizations revealed that the introduction of the base and salt could greatly increase the content of doped nitrogen, the surface area and the amount of formed micropore, which led to enhanced CO₂ absorption of the carbon materials.

On the Structure-Property Relationships of Cation-Exchanged ZK-5 Zeolites for CO2 Adsorption

The CO₂ adsorption properties of cation-exchanged Li-, Na-, K-, and Mg-ZK-5 zeolites were correlated to the molecular structures determined by Rietveld refinements of synchrotron powder X-ray diffraction patterns. Li-, K-, and Na-ZK-5 all exhibited high isosteric heats of adsorption (Q_st ) at low CO₂ coverage, with Na-ZK-5 having the highest Q_st (ca. 49 kJ mol^-1 ). Mg²⁺ was located at the center of the zeolite hexagonal prism with the cation inaccessible to CO₂ , leading to a much lower Q_st (ca. 30 kJ mol^-1 ) and lower overall uptake capacity. Multiple CO₂ adsorption sites were identified at a given CO₂ loading amount for all four cation-exchanged ZK-5 adsorbents. Site A at the flat eight-membered ring windows and site B/B* in the γ-cages were the primary adsorption sites in Li- and Na-ZK-5 zeolites. Relatively strong dual-cation adsorption sites contributed significantly to an enhanced electrostatic interaction for CO₂ in all ZK-5 samples. This interaction gives rise to a migration of Li⁺ and Mg²⁺ cations from their original locations at the center of the hexagonal prisms toward the α-cages, in which they interact more strongly with the adsorbed CO₂ .

Aminosilanes grafted to basic alumina as CO2 adsorbents--role of grafting conditions on CO2 adsorption properties

Solid oxide-supported amine sorbents for CO2 capture are amongst the most rapidly developing classes of sorbent materials for CO2 capture. Herein, basic γ supports are used as hosts for amine sites through the grafting of 3-aminopropyltrimethoxysilane to the alumina surface under a variety of conditions, yielding the expected surface-grafted alkylamine groups, as demonstrated by FTIR spectroscopy and (29)Si and (13)C cross-polarization magic-angle spinning (CPMAS NMR) spectroscopy. Grafting amine sites on the surface in the presence of water leads to a high density of amine sites on the surface whereas simultaneously creating a unique type of aluminum species on the surface, as demonstrated by both 1D and 2D (27)Al MAS NMR spectroscopy. The thus prepared sorbents result in higher CO2 adsorption capacities and amine efficiencies compared to sorbents prepared in the absence of water or similar amine loading sorbents prepared using silica supports. In situ FTIR spectra of the sorbents exposed to CO2 at various pressures show no distinct difference in the nature of the adsorbed CO2 species on alumina- versus silica-supported amines, whereas water adsorption isotherms show that the improved performance of the amine-grafted alumina support is not a consequence of retained water on the more hydrophobic aminoalumina materials. The findings demonstrate that amine-grafted, basic alumina materials can be tuned to be more efficient than the corresponding silica-supported materials at comparable amine loadings, further demonstrating that the properties of amine sites can be tuned by controlling or adjusting the support surface properties.

Activated carbon spheres for CO2 adsorption

A series of carbon spheres (CS) was prepared by carbonization of phenolic resin spheres obtained by the one-pot modified Stöber method. Activated CS (ACS), having diameters from 200 to 420 nm, high surface area (from 730 to 2930 m(2)/g), narrow micropores (<1 nm) and, importantly, high volume of these micropores (from 0.28 to 1.12 cm(3)/g), were obtained by CO2 activation of the aforementioned CS. The remarkably high CO2 adsorption capacities, 4.55 and 8.05 mmol/g, were measured on these AC spheres at 1 bar and two temperatures, 25 and 0 °C, respectively.

Carbon dioxide and nitrogen adsorption on cation-exchanged SSZ-13 zeolites

Samples of high-silica SSZ-13, ion exchanged with protons and alkali-metal cations Li(+), Na(+), and K(+), were investigated using adsorption isotherms of CO(2) and N(2). The results show that Li-, Na-SSZ-13 have excellent CO(2) capacity at ambient temperature and pressure; in general, Li-SSZ-13 shows the highest capacity for N(2), CO(2) particularly in the low-pressure region. The effect of cation type and Si/Al ratio (6 and 12) on the adsorption properties was investigated through analysis of adsorption isotherms and heats of adsorption. The separation of CO(2) in a flue gas mixture was evaluated for these adsorbents in the pressure swing adsorption and vacuum pressure adsorption processes.