Complexity of CO2 adsorption on nanoporous sulfur-doped carbons – Is surface chemistry an important factor?

Nanoporous S-doped carbon and its composites with graphite oxide were tested as adsorbents of CO₂ (1 MPa at 0 °C) after degassing either at 120 °C or 350 °C. The adsorption capacities were over 4 mmol/g at ambient pressure and 8 mmol/g at 0.9 MPa in spite of a low volume of micropores. The nitrogen adsorption experiments showed an increase in porosity upon an increase in the degassing temperature. The extent of this effect depends on the stability of surface groups. Interestingly, the CO₂ adsorption, especially at low pressure, was not affected. The good performance is due to the presence of ultramicropores similar in sizes to CO₂ molecule and to sulfur in various functionalities. Sulfur incorporated to aromatic rings enhances CO₂ adsorption via acid–base interactions in micropores. Moreover, sulfonic acids, sulfoxides and sulfones attract CO₂ via polar interactions. Hydrogen bonding of CO₂ with acidic groups on the surface can also play an important role in the CO₂ retention. These carbons have high potential for application as CO₂ removal media owing to their high degree of pore space utilization. The results obtained also show that high degassing temperatures might result in the decomposition of surface groups and thus in changes in surface interactions.     

Highly selective CO2 capture by S-doped microporous carbon materials

S-doped microporous carbon materials were synthesized by the chemical activation of a reduced-graphene-oxide/poly-thiophene material. The material displayed a large CO₂ adsorption capacity of 4.5 mmol g⁻¹ at 298 K and 1 atm, as well as an impressive CO₂ adsorption selectivity over N₂, CH₄ and H₂. The material was shown to exhibit a stable recycling adsorption capacity of 4.0 mmol g⁻¹. The synthesized material showed a maximum specific surface area of 1567 m² g⁻¹ and an optimal CO₂ adsorption pore size of 0.6 nm. The microporosity, surface area and oxidized S content of the material were found to be the determining factors for CO₂ adsorption. These properties show that the as synthesized S-doped microporous carbon material can be more effective than similarly prepared N-doped microporous carbons in CO₂ capture.     

Potassium salt-assisted synthesis of highly microporous carbon spheres for CO2 adsorption

Highly microporous carbon spheres for CO₂ adsorption were prepared by using a slightly modified one-pot Stöber synthesis in the presence of potassium oxalate. Formaldehyde and resorcinol were used as carbon precursors, ammonia as a catalyst, and potassium oxalate as an activating agent. The resulting potassium salt-containing phenolic resin spheres were simultaneously carbonized and activated at 800 °C in flowing nitrogen. Carbonization of the aforementioned polymeric spheres was accompanied by their activation, which resulted in almost five-time higher specific surface area and total pore volume, and almost four-time higher micropore volume as compared to analogous properties of the carbon sample prepared without the salt. The proposed synthesis resulted in microporous carbon spheres having the surface area of 2130 m² g⁻¹, total pore volume of 1.10 cm³ g⁻¹, and the micropore volume of 0.78 cm³ g⁻¹, and led to the substantial enlargement of microporosity in these spheres, especially in relation to fine micropores (pores below 1 nm), which enhance CO₂ adsorption. These carbon spheres showed three-time higher volume of fine micropores, which resulted in the CO₂ adsorption of 6.6 mmol g⁻¹ at 0 °C and 1 atm.     

Phase behavior of 1,3,5-tri-tert-butylbenzene–carbon dioxide binary system

1,3,5-tri-tert-butylbenzene (TTBB) is solid at ambient conditions, and has substantial solubility in liquid and supercritical carbon dioxide. We present the phase behavior of TTBB–CO₂ binary system at temperatures between 298 and 328 K and at pressures up to 20 MPa. Phase diagrams showing the liquid–vapor, solid–liquid and solid–vapor equilibrium envelopes are constructed by pressure–volume–temperature measurements in a variable-volume sapphire cell. TTBB is highly soluble in CO₂ over a wide range of compositions. Single-phase states are achieved at moderate pressures, even with very high TTBB concentrations. For example, at 328 K, a binary system containing TTBB at a concentration of 95% by weight forms a single-phase above 2.04 MPa. TTBB exhibits a significant melting-point depression in the presence of CO₂, 45 K at 3.11 MPa, where the normal melting point of 343 K is reduced to 298 K. With its high solubility in carbon dioxide, TTBB has potential uses as a binder or template in materials forming processes using dense carbon dioxide.     

Hydrogen storage in CO2-activated amorphous nanofibers and their monoliths

Amorphous carbon nanofibers (CNFs), produced by the polymer blend technique, are activated by CO₂ (ACNFs). Monoliths are synthesized from the precursor and from some ACNFs. Morphology and textural properties of these materials are studied. When compared with other activating agents (steam and alkaline hydroxides), CO₂ activation renders suitable yields and, contrarily to most other precursors, turns out to be advantageous for developing and controlling their narrow microporosity (<0.7 nm), V_DR(CO₂). The obtained ACNFs have a high compressibility and, consequently, a high packing density under mechanical pressure which can also be maintained upon monolith synthesis. H₂ adsorption is measured at two different conditions (77 K/0.11 MPa, and 298 K/20 MPa) and compared with other activated carbons. Under both conditions, H₂ uptake depends on the narrow microporosity of the prepared ACNFs. Interestingly, at room temperature these ACNFs perform better than other activated carbons, despite their lower porosity developments. At 298 K they reach a H₂ adsorption capacity as high as 1.3 wt.%, and a remarkable value of 1 wt.% in its mechanically resistant monolith form.     

In situ FTIR micro-spectroscopy to investigate polymeric fibers under supercritical carbon dioxide: CO2 sorption and swelling measurements

An original experimental set-up combining a FTIR (Fourier Transformed InfraRed) microscope with a high pressure cell has been built in order to analyze in situ and simultaneously the CO₂ sorption and the polymer swelling of microscopic polymer samples, such as fibers, subjected to supercritical carbon dioxide. Thanks to this experimental set-up, we have determined as a function of the CO₂ pressure (from 2 to 15 MPa) the CO₂ sorption and the polymer swelling at T = 40 °C of four polymer samples, namely PEO (polyethylene oxide), PLLA (poly-l-lactide acid), PET (polyethylene terephtalate) and PP (polypropylene). The quantity of CO₂ sorbed in all the studied polymers increases with pressure. PEO and PLLA display a significant level of CO₂ sorption (20 and 25% respectively, at P = 15 MPa). However, we observe that a lower quantity of CO₂ can be sorbed into PP and PET (7 and 8% respectively, at P = 15 MPa). Comparing their thermodynamic behaviors and their intrinsic properties, we emphasize that a high CO₂ sorption can be reach if on one hand, the polymer is able to form specific interaction with CO₂ in order to thermodynamically favor the presence of CO₂ molecules inside the polymer and on the other, displays high chains mobility in the amorphous region. PLLA and PEO fulfilled these two requirements whereas only one property is fulfilled by PET (specific interaction with CO₂) and PP (high chains mobility). Finally, we have found that for a given CO₂ sorption, the resulting swelling of the polymer depends mainly on its crystallinity.     

Preparation of sulfur-doped microporous carbons for the storage of hydrogen and carbon dioxide

Structurally well ordered, sulfur-doped microporous carbon materials have been successfully prepared by a nanocasting method using zeolite EMC-2 as a hard template. The carbon materials exhibited well-resolved diffraction peaks in powder XRD patterns and ordered micropore channels in TEM images. Adjusting the synthesis conditions, carbons possess a tunable sulfur content in the range of 1.3–6.6 wt.%, a surface area of 729–1627 m² g^?1 and a pore volume of 0.60–0.90 cm³ g^?1. A significant proportion of the porosity in the carbons (up to 82% and 63% for surface area and pore volume, respectively) is contributed by micropores. The sulfur-doped microporous carbons exhibit isosteric heat of hydrogen adsorption up to 9.2 kJ mol^?1 and a high hydrogen uptake density of 14.3 × 10^?3 mmol m^?2 at ?196 °C and 20 bar, one of the highest ever observed for nanoporous carbons. They also show a high CO₂ adsorption energy up to 59 kJ mol^?1 at lower coverages (with 22 kJ mol^?1 at higher CO₂ coverages), the highest ever reported for any porous carbon materials and one of the highest amongst all the porous materials. These findings suggest that S-doped microporous carbons are potential promising adsorbents for hydrogen and CO₂.     

Gas sorption studies on a highly-thermostable microporous Zn(II) coordination polymer constructed from 2D honeycomb layers

By combining experiment with molecular simulation, the CO₂ sorption performance of a 2D honeycomb layered coordination polymer, {[Zn₂(bpydb)₂(H₂O)₂](DMA)₃(H₂O)}_n (1) (bpydb = 4,4′-(4,4′-bipyridine-2,6-diyl) dibenzoate) was systematically investigated. The desolvated 1 not only shows high CO₂ capacity (72.5 cm³/g at 273 K and 42.9 cm³/g at 298 K) with a moderate high zero-coverage adsorption enthalpy (29.7 kJ/mol), but also exhibits excellent CO₂/N₂ selectivity around room temperature. To better understand the adsorption behaviors and adsorption sites for CO₂ in 1, GCMC simulations were carried out, which indicate that the open metal sites between two adjacent layers account for the high CO₂ sorption capacity and strong CO₂ binding ability. Moreover, the thermal stability of 1 was further confirmed by the TGA and VT-PXRD, which indicate that 1 could be thermally stable up to 400 °C.     

Solubility and diffusion coefficient of supercritical-CO2 in polycarbonate and CO2 induced crystallization of polycarbonate

The solubility and diffusion coefficient of supercritical CO₂ in polycarbonate (PC) were measured using a magnetic suspension balance at sorption temperatures that ranged from 75 to 175 °C and at sorption pressures as high as 20 MPa. Above certain threshold pressures, the solubility of CO₂ decreased with time after showing a maximum value at a constant sorption temperature and pressure. This phenomenon indicated the crystallization of PC due to the plasticization effect of dissolved CO₂. A thorough investigation into the dependence of sorption temperature and pressure on the crystallinity of PC showed that the crystallization of PC occurred when the difference between the sorption temperature and the depressed glass transition temperature exceeded 40 °C (T − T_g ≥ 40 °C). Furthermore, the crystallization rate of PC was determined according to Avrami's equation. The crystallization rate increased with the sorption pressure and was at its maximum at a certain temperature under a constant pressure.     

SBA-15 grafted with 3-aminopropyl triethoxysilane in supercritical propane for CO2 capture

Supercritical propane (SC-propane) was found to be a promising solvent for grafting (3-aminopropyl)triethoxysilane (APS) onto synthesized SBA-15 for CO₂ capture. The influence of operating conditions in SC-propane for CO₂ adsorption at different pressures (8.3–13.8 MPa), temperatures (85–120 °C), and periods of time (4–16 h) were evaluated. The CO₂ adsorption conditions under different partial pressures, temperatures and moisture were evaluated. The results showed a reduction in pore characteristics and an increased amount of grafted APS with increasing pressure and temperature after grafting. After grafting in SC-propane at 11.0 MPa and various temperatures for 16 h, a 3–20% increase in the amount of grafted APS and a 6–49% increase in the CO₂ adsorption capacity over the toluene refluxing was observed. The time required for grafting in SC-propane could be reduced while maintaining higher nitrogen content and CO₂ adsorption capacity compared with grafting in toluene refluxing.     

Effects of temperature on water adsorption on controlled microporous and mesoporous carbonaceous solids

Water adsorption isotherms in porous carbons were measured at temperatures in the range 263 and 298 K. To investigate the role of porous structure, we used RF-50, RF-100, RF-200, derived from resorcinol–formaldehyde cryogels, and an activated carbon (B-AC) derived from bamboo. RF-50 is microporous while RF-100 and RF-200 are micro- and mesoporous with non-overlapping distributions of micropores and mesopores. The isotherms of RF-50 exhibit only one step at all temperatures, and they are independent of temperature when plotted against the reduced pressure. On the other hand, the isotherms of RF-100 and RF-200 have two-step uptake at 298 K, while they exhibit only one-step uptake at 263 K; the first step of the 298 K-isotherms is practically the same as the 263 K-isotherm. We attributed the first step to adsorption in micropores and the second one to mesopores. The isotherms of B-AC, showing a single broad hysteresis loop, could be viewed as the merging of the two steps that we observed with RF-100 and RF-200 because of the broad pore size distribution of B-AC. From the analysis of all isotherms we developed a universal mechanism for water adsorption in porous carbon, which is based on the temperature dependence of the clustering of water molecules.     

Solid-state dechlorination pathway for the synthesis of few layered functionalized carbon nanosheets and their greenhouse gas adsorptivity over CO and N2

A simple solid-state dechlorination route has been demonstrated to synthesize few layered functionalized carbon nanosheets (FCNS) utilizing hexachloroethane as carbon source and copper as reducing agent under the autogenic pressure at 300 °C. The obtained FCNS possesses the sheet thickness of 6–12 nm as analyzed by transmission electron microscopy. The particle nature of the FCNS provides the excess porosity having the surface area 836 m²/g. The equilibrium gas adsorption study of FCNS for greenhouse gases (CO₂ and CH₄), toxic gas (CO) and light gas (N₂) showed the maximum adsorption capacity for CO₂ (2.95 mmol/g; at 288 K) with maximum capacity selectivity of 10.1 at 318 K. The very strong adsorbate–adsorbent interaction was observed in case of CO compare to other gases resulted in higher heat of adsorption for CO. The gas adsorption and FT-IR study showed that the interaction of CO with copper present in minute quantities in FCNS improves the CO adsorption due to π complexation. The FCNS obtained under present methodology showed the very high equilibrium selectivity for CO over N₂ (197) followed by CH₄ (61) and CO₂ (7.3) at 288 K.     

Isothermal exfoliation of graphene oxide by a new carbon dioxide pressure swing method

Graphene oxide (GO) was prepared by a modified Hummers’ method. GO was modified using a simple CO₂ pressure swing technique to obtain exfoliated GO. The microcrystalline structures and morphologies were characterized using the X-ray diffraction and scanning electron microscope/transmission electron microscope measurements. The textural properties were investigated by N₂ (77 K) adsorption/desorption isotherms. CO₂ (298 K, 30 bar) and H₂ (298 K, 100 bar) adsorption experiments were conducted to investigate their adsorption behaviors. The results indicated that the best sample had a specific surface area of 547 m²/g and total pore volume of 2.468 cm³/g. According to the results, CO₂ pressure swing method can be used to increase the efficiency of exfoliation and expansion of the graphitic interlayers in GO.     

Density and viscosity of the binary polyethylene glycol/CO2 systems

Density of CO₂ saturated solutions of polyethylene glycols (PEGs) of different molecular weight was measured in pressure range from 8.0 MPa up to 47.7 MPa at a temperature of 343 K by a volumetric method. To validate the method density of pure CO₂ was measured at different pressures and a temperature of 293 K. The results were compared to the literature data and the accuracy was better than 2%. The density was between 1.17 g/mL for PEG 1000/CO₂ at 14.5 MPa and 1.78 g/mL for the system PEG 4000/CO₂ at 35 MPa. Further, the data were compared to results, obtained by a gravimetric method using magnetic suspension balance (MSB).Viscosity of CO₂ saturated solutions of polyethylene glycols (PEGs) of different molecular weight at different pressures and at a temperature of 343 K was measured using a high pressure view cell. Also a temperature impact on the viscosity of pure PEGs was observed at ambient pressure. After saturating PEG 1500 with 10 MPa of CO₂ pressure its viscosity decreases from 76.6 mPa s to 2.24 mPa s at 333 K. Further addition of CO₂ and increasing the pressure results in even lower viscosity and the highest viscosity reduction was reached at the highest pressure; at 35 MPa viscosity of the system PEG 1500/CO₂ is only 0.665 mPa s.     

Molecular hydrogen and spiltover hydrogen storage on high surface area carbon sorbents

A series of templated carbons with various high surface areas (2033–3798 m²/g) have been prepared using various microporous zeolites as hard templates. Molecular hydrogen storage and spiltover hydrogen storage on these templated carbons were investigated and compared with superactivated carbon AX-21 and other reported porous carbon sorbents at 298 K and 100 atm. Two relationships between the surface areas of these carbons and their hydrogen capacities were obtained. The relationship between molecular hydrogen capacity and surface area showed a 0.23 wt.% H₂/1000 m²/g of carbon sorbent at 298 K and 100 atm, indicating that merely increasing surface areas of the carbon sorbents cannot achieve a significant molecular hydrogen capacity at ambient temperature. Spiltover hydrogen storage was achieved by doping Pt nanoparticles (as dissociative hydrogen source) on these carbons (spiltover hydrogen receptor). Our first result on the relationship between the spiltover hydrogen capacity and surface area showed 0.4 wt.% H₂/1000 m²/g of carbon sorbent at 298 K and 100 atm, which indicated that storage via spillover can lead to an average of 70% enhancement compared to molecular hydrogen storage.     

Frequency response of microcantilevers immersed in gaseous,liquid, and supercritical carbon dioxide

The frequency response of ferromagnetic nickel microcantilevers with lengths ranging between 200 μm and 400 μm immersed in gaseous, liquid and supercritical carbon dioxide (CO₂) was investigated. The resonant frequency and the quality factor of the cantilever oscillations in CO₂ were measured for each cantilever length in the temperature range between 298 K and 323 K and the pressure range between 0.1 MPa and 20.7 MPa. At a constant temperature, both the resonant frequency and the quality factor were found to decrease with increasing pressure as a result of the increasing CO₂ density and viscosity. Very good agreement was found between the measured cantilever resonant frequencies and predictions of a model based on simplified hydrodynamic function of a cantilever oscillating harmonically in a viscous fluid valid for Reynolds numbers in the range of [1;1000] (average deviation of 2.40%). At high pressures of CO₂, the experimental Q-factors agreed well with the predicted ones. At low CO₂ pressures, additional internal mechanisms of the cantilever oscillation damping caused lowering of the measured Q-factor with respect to the hydrodynamic model predictions.     

Dual gas analysis of microporous carbons using 2D-NLDFT heterogeneous surface model and combined adsorption data of N2 and CO2

Knowledge of the pore structure of carbon materials including micropores is crucial for applications such as double layer supercapacitors, gas separation, and other applications requiring high specific surface area materials. High surface area is always associated with fine micropores. The pore size distribution (PSD) of microporous carbons is usually evaluated from nitrogen adsorption isotherms measured at 77 K in the relative pressure range from 10⁻⁷ to 1. Due to the very slow gas diffusion into fine pores at cryogenic temperatures and low pressures, the adsorption measurements may be extremely time consuming and sometimes inaccurate when the adsorption equilibrium is difficult to achieve during the measurement. In this work, we discuss an approach in which the carbon PSD is calculated from the combined N₂ and CO₂ data measured in the pressure range from 1 to 760 Torr. Under such conditions, the diffusion into micropores is usually fast and equilibration times are short for both measurements. In the PSD calculations we use 2D-NLDFT models for carbons with heterogeneous surfaces (J. Jagiello and J.P. Olivier, Adsorption 19, 2013, 777–783). We show that both isotherms can be fitted simultaneously with their corresponding models and as a result the unified PSD can be obtained.     

Enhancement of CO2 adsorption on phenolic resin-based mesoporous carbons by KOH activation

Carbons with high surface area and large volume of ultramicropores were synthesized for CO₂ adsorption. First, mesoporous carbons were produced by soft-templating method using triblock copolymer Pluronic F127 as a structure directing agent and formaldehyde and either phloroglucinol or resorcinol as carbon precursors. The resulting carbons were mainly mesoporous with well-developed surface area, large total pore volume, and only moderate CO₂ uptake. To improve CO₂ adsorption, these carbons were subjected to KOH activation to enhance their microporosity. Activated carbons showed 2–3-fold increase in the specific surface area, resulting from substantial development of microporosity (3–5-fold increase in the micropore volume). KOH activation resulted in enhanced CO₂ adsorption at 760 mmHg pressure: 4.4 mmol g⁻¹ at 25 °C, and 7 mmol g⁻¹ at 0 °C. This substantial increase in the CO₂ uptake was achieved due to the development of ultramicroporosity, which was shown to be beneficial for CO₂ physisorption at low pressures. The resulting materials were investigated using low-temperature nitrogen physisorption, CO₂ sorption, and small-angle powder X-ray diffraction. High CO₂ uptake and good cyclability (without noticeable loss in CO₂ uptake after five runs) render ultramicroporous carbons as efficient CO₂ adsorbents at ambient conditions.     

Fabrication of polyetherimide microporous membrane using supercritical CO2 technology and its application for affinity membrane matrix

Polyetherimide (PEI) microporous membranes with uniform cellular structure, high porosity, and narrow pore size distribution were formed by supercritical CO₂ (ScCO₂) phase inversion method, and the membrane was modified to be a matrix for the preparation of affinity membrane due to its low solvent residue and appropriate porous structure. The effects of ScCO₂ temperature and pressure on the morphology and pure water flux of the membrane were investigated. The membrane prepared at 24 MPa and 45 °C with a large mean cell diameter of 6.0 μm, high porosity of 73%, narrow pore size distribution and a pure water flux of 56 L/(m² h bar) was coated with chitosan to improve its hydrophilicity and coupled with Cibacron Blue F3GA (CB) as a special ligand to form an affinity membrane (PEI-coated chitosan-CB membrane). The PEI-coated chitosan-CB membrane showed a high adsorption capacity of 33.9 mg/g membrane to bovine serum albumin and was higher than most of affinity membranes. Moreover, the tensile strength of PEI-coated chitosan-CB membrane was 11.58 MPa and was much higher than those of affinity membranes. This work demonstrates that ScCO₂ phase inversion method is a potential method to prepare an affinity matrix.