Pure and binary adsorption of CO2, H2, and N2 on activated carbon

A new developing field of application for pressure swing adsorption (PSA) processes is the capture of CO₂ to mitigate climate change, especially the separation of CO₂ and H₂ in a pre-combustion context. In this process scheme the conditions of the feed to the separation step, namely a pressure of 3.5 to 4.5 MPa and a CO₂ fraction of around 40% are favorable for an adsorption based separation process and make PSA a promising technology. Among the commercial adsorbent materials, activated carbon is most suitable for this application. To evaluate the potential, to benchmark new materials, and for process development a sound basis of the activated carbon thermodynamic data is required, namely equilibrium adsorption isotherms of the relevant pure components and mixtures, Henry’s constants and isosteric heats.     

Facile Synthesis of Polyethylenimine and Nano-TiO<Subscript>2</Subscript> Particles Functionalized PolyHIPE Beads for CO<Subscript>2</Subscript> Capture

Polyethylenimine (PEI) and titanium dioxide nanoparticles (nano-TiO₂) functionalized poly- HIPE beads were synthesized by suspension polymerization of styrene/divinylbenzene high internal phase emulsion (HIPE) containing PEI and nano-TiO₂ particles in inner phase. The products are uniform and spherical beads with average diameter of 1 mm. Characterization results showed good thermal stability and desired mechanical strength. CO₂ adsorption tests were performed with CO₂/H₂O/N₂ (1 : 1 : 8) gas mixture. Nano-TiO₂ particles distinctly improved the CO₂ adsorption performance of the polyHIPE beads, resulting in enhanced CO₂ adsorption capacity and fast adsorption/desorption kinetics. Besides, the functionalized polyHIPE beads exhibited remarkable cycle stability.     

Application of thermogravimetric analysis to the evaluation of aminated solid sorbents for CO<Subscript>2</Subscript> capture

In this work a series of solid sorbents were synthesized by immobilizing liquid amines on the surface of a mesoporous alumina. The samples were chemically characterized and BET surface areas calculated from the N₂ adsorption isotherms at 77 K. The CO₂ capture performance of the sorbents and their thermal stability was studied by thermogravimetric methods. The effect of amine loading on the CO₂ capture performance of the prepared sorbents was also evaluated. Analysis of TG-DTG curves showed that thermal stabilization of the amines is significantly improved by immobilizing them on an inorganic support. Temperature-programmed CO₂ adsorption tests from 298 K up to 373 K at atmospheric pressure, proved to be a useful technique for assessing the capacity of sorbents for CO₂ capture. Alumina impregnated with diethylenetriamine presented the highest CO₂ adsorption capacities throughout the tested temperature range.     

Pressure swing adsorption for CO<Subscript>2</Subscript> capture in Fischer-Tropsch fuels production from biomass

Environmental concerns and oil price rises and dependency promoted strong research in alternative fuel sources and vectors. Fischer-Tropsch products are considered a valid alternative to oil derivatives having the advantage of being able to share current infrastructures. As a renewable source of energy, synthesis gas obtained from biomass gasification presents itself as a sustainable alternative. However, prior to hydrocarbon conversion, the bio-syngas must be conditioned, which includes the removal of carbon dioxide for subsequent sequestration and capture. A pressure swing adsorption cycle was developed for the removal and concentration of CO₂ from the bio-syngas stream. Activated carbon was chosen as adsorbent. The simulation results showed that it was possible to produce a (H₂ + CO) product with a H₂/CO stoichiometric ratio of 2.14 (suitable as feed stream for the Fischer-Tropsch reactor) and a CO₂ product with a purity of 95.18%. A CO₂ recovery of 90.3% was obtained. A power consumption of 3.36 MW was achieved, which represents a reduction of about 28% when compared to a Rectisol process with the same recovery.     

FT-IR study on CO<Subscript>2</Subscript> adsorbed species of CO<Subscript>2</Subscript> sorbents

The stability of amine-functionalized silica sorbents prepared through the incipient wetness technique with primary, secondary, and tertiary amino organosilanes was investigated. The prepared sorbents were exposed to different gaseous streams including CO₂/N₂, dry CO₂/air with varying concentration, and humid CO₂/air mixtures to demonstrate the effect of the gas conditions on the CO₂ adsorption capacity and the stability of the different amine structures. The primary and secondary amine-functionalized adsorbents exhibited CO₂ sorption capacity, while tertiary amine adsorbent hardly adsorbed any CO₂. The secondary amine adsorbent showed better stability than the primary amine sorbent in all the gas conditions, especially dry conditions. Deactivation species were evaluated using FT-IR spectra, and the presence of urea was confirmed to be the main deactivation product of the primary amine adsorbent under dry condition. Furthermore, it was found that the CO₂ concentration can affect the CO₂ sorption capacity as well as the extent of degradation of sorbents.     

Ethylene/ethane separation by CuCl/AC adsorbent prepared using CuCl<Subscript>2</Subscript> as a precursor

Activated carbon (AC) supported CuCl (CuCl/AC) for ethylene/ethane separation has been prepared with CuCl₂ as precursor by a solid-state dispersion method. The samples are characterized by inductively coupled plasma optical emission spectrometry, X-ray diffraction, N₂ adsorption/desorption and X-ray photoelectron spectroscopy, and investigated for ethylene (C₂H₄) and ethane (C₂H₆) adsorptions. The characterization results reveal that CuCl₂ supported on AC can be highly dispersed on the surfaces of AC support and completely converted to CuCl after activation at 543 K in N₂. The resultant adsorbent displays high ethylene adsorption capacity, high C₂H₄/C₂H₆ adsorption selectivity and excellent reversibility. The adsorption isotherms of ethylene and ethane on CuCl/AC at temperatures up to 333 K can be well fitted by the Sips models, and the corresponding isosteric heats of adsorption are calculated from the Clausius–Clapeyron equation. The value of isosteric heat of adsorption suggests that the interaction of ethylene with CuCl/AC is between physisorption and chemisorption.     

Equilibrium isotherms and isosteric heat for CO<Subscript>2</Subscript> adsorption on nanoporous carbons from polymers

Four nanoporous carbons obtained from different polymers: polypyrrole, polyvinylidene fluoride, sulfonated styrene–divinylbenzene resin, and phenol–formaldehyde resin, were investigated as potential adsorbents for carbon dioxide. CO₂ adsorption isotherms measured at eight temperatures between 0 and 60 °C were used to study adsorption properties of these polymer-derived carbons, especially CO₂ uptakes at ambient pressure and different temperatures, working capacity, and isosteric heat of adsorption. The specific surface areas and the volumes of micropores and ultramicropores estimated for these materials by using the density functional theory-based software for pore size analysis ranged from 840 to 1990 m² g^?1, from 0.22 to 1.47 cm³ g^?1, and from 0.18 to 0.64 cm³ g^?1, respectively. The observed differences in the nanoporosity of these carbons had a pronounced effect on the CO₂ adsorption properties. The highest CO₂ uptakes, 6.92 mmol g^?1 (0 °C, 1 atm) and 1.89 mmol g^?1 (60 °C, 1 atm), were obtained for the polypyrrole-derived activated carbon prepared through a single carbonization-KOH activation step. The working capacity for this adsorbent was estimated to be 3.70 mmol g^?1. Depending on the adsorbent, the CO₂ isosteric heats of adsorption varied from 32.9 to 16.3 kJ mol^?1 in 0–2.5 mmol g^?1 range. Overall, the carbons studied showed well-developed microporosity and exceptional CO₂ adsorption, which make them viable candidates for CO₂ capture, and for other adsorption and environmental-related applications.     

Adsorption behavior of <Superscript>99</Superscript>Tc on Fe,Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> and Fe<Subscript>2</Subscript>O<Subscript>4</Subscript>

Summary The adsorption of ⁹⁹Tc on the adsorbers Fe, Fe₂O₃ and Fe₃O₄ was studied by batch experiments under aerobic and anoxic conditions. The effects of pH and CO₃^2- concentration of the simulated ground water on the adsorption ratios were also investigated, and the valences of Tc in solution after the adsorption equilibrium were studied by solvent extraction. The adsorption isotherms of TcO₄- on the adsorbers Fe, Fe₂O₃ and Fe₃O₄ were determined. Experimental results have shown that the adsorption ratio of Tc on Fe decreases with the increase of pH in the range of 5-12 and increases with the decrease of the CO₃^2- concentration in the range of 10^-8M-10^-2M. Under aerobic conditions, the adsorption ratios of ⁹⁹Tc on Fe₂O₃ and Fe₃O₄ were not influenced by pH and CO₃^2-concentration. When Fe was used as adsorbent, Tc existed mainly in the form of Tc(IV) after equilibrium and in the form of Tc(VII) when the adsorbent was Fe₂O₃ or Fe₃O₄ under aerobic conditions. The adsorption ratios of Tc on Fe, Fe₂O₃ and Fe₃O₄ decreased with the increase of pH in the range of 5-12 and increased with the decrease of the CO₃^2- concentration in the range of 10^-8M-10^-2M under anoxic conditions. Tc existed mainly in the form of Tc(IV) after equilibrium when Fe, Fe₂O₃ and Fe₃O₄ was the adsorbent under anoxic conditions. The adsorption isotherms of TcO_4- on the adsorbers Fe, Fe₂O₃ and Fe₃O₄ are fairly in agreement with the Freundlich’s equation under both aerobic and anoxic conditions.     

One‐stage Template‐free KOH Activation for Mesopore‐enriched Carbons and Their Application in CO2 Capture

Activated carbons with a high mesoporous structure were prepared by a one‐stage KOH activation process without the assistance of templates and further used as adsorbents for CO₂ capture. The physical and chemical properties as well as the pore structures of the resulting mesoporous carbons were characterized by N₂ adsorption isotherms, scanning electron microscopy (SEM ), X‐ray diffraction (XRD ), Raman spectroscopy, and Fourier transform infrared (FTIR ) spectroscopy. The activated carbon showed greater specific surface area and mesopore volume as the activation temperature was increased up to 600°C, showing a uniform pore structure, great surface area (up to ~815 m²/g), and high mesopore ratio (~55%). The activated sample exhibited competitive CO₂ adsorption capacities at 1 atm pressure, reaching 2.29 and 3.4 mmol/g at 25 and 0°C, respectively. This study highlights the potential of well‐designed mesoporous carbon as an adsorbent for CO₂ removal and widespread gas adsorption applications.     

Adsorption equilibria of CO<Subscript>2</Subscript> and CH<Subscript>4</Subscript> in cation-exchanged zeolites 13X

Ion-exchange with different cations (Na⁺, NH₄ ⁺, Li⁺, Ba²⁺ and Fe³⁺) was performed in binderless 13X zeolite pellets. Original and cation-exchanged samples were characterized by thermogravimetric analysis coupled with mass spectrometry (inert atmosphere), X-ray powder diffraction and N₂ adsorption/desorption isotherms at 77 K. Despite the presence of other cations than Na (as revealed in TG-MS), crystalline structure and textural properties were not significantly altered upon ion-exchange. Single component equilibrium adsorption isotherms of carbon dioxide (CO₂) and methane (CH₄) were measured for all samples up to 10 bar at 298 and 348 K using a magnetic suspension balance. All of these isotherms are type Ia and maximum adsorption capacities decrease in the order Li > Na > NH₄–Ba > Fe for CO₂ and NH₄–Na > Li > Ba for CH₄. In addition to that, equilibrium adsorption data were measured for CO₂/CH₄ mixtures for representative compositions of biogas (50 % each gas, in vol.) and natural gas (30 %/70 %, in vol.) in order to assess CO₂ selectivity in such scenarios. The application of the Extended Sips Model for samples BaX and NaX led to an overall better agreement with experimental data of binary gas adsorption as compared to the Extended Langmuir Model. Fresh sample LiX show promise to be a better adsorption than NaX for pressure swing separation (CO₂/CH₄), due to its higher working capacity, selectivity and lower adsorption enthalpy. Nevertheless, cation stability for both this samples and NH₄X should be further investigated.     

Preparation and characterization of waste ion-exchange resin-based activated carbon for CO<Subscript>2</Subscript> capture

Waste ion-exchange resin was utilized as precursor to produce activated carbon by KOH chemical activation, on which the effects of different activation temperatures, activation times and impregnation ratios were studied in this paper. The CO₂ adsorption of the produced activated carbon was tested by TGA at 30 °C and environment pressure. Furthermore, the effects of preparation parameters on CO₂ adsorption were investigated. Experimental results show that the produced activated carbons are microporous carbons, which are suitable for CO₂ adsorption. The CO₂ adsorption capacity increases firstly and then decreases with the increase of activation temperature, activation time and impregnation rate. The maximum adsorption capacity is 81.24 mg/g under the condition of 30 °C and pure CO₂. The results also suggest that waste ion-exchange resin-based activated carbons possess great potential as adsorbents for post-combustion CO₂ capture.     

Effect of various aminosilanes functionalized inside nanoporous silica on CO<Subscript>2</Subscript> adsorption performance

Three different aminosilanes ((3-aminopropyl)trimethoxysilane (1NS), N-[3-(trimethoxysilyl) propyl]ethylenediamine (2NS), N¹-(3-trimethoxysilylpropyl)diethylenetriamine (3NS)) were grafted covalently inside nanoporous silica (NPS-1) with a large surface area to prepare CO₂ adsorbents. The prepared CO₂ sorbents were evaluated for their CO₂ sorption capacity, kinetic behavior, temperature programmed desorption (TPD) and textural properties. Grafting efficiency of 1NS was better due to the smaller molecular size compared to 2NS and 3NS, which are difficult to react with the hydroxyl group of the silica surface due to steric hindrance. The highest adsorption capacity of 7.0 wt% was observed for the 2NS/NPS-1 adsorbent, followed by 5.2 wt% for 1NS/NPS-1, then 5.0 wt% for 3NS/NPS-1. The adsorption capacity of 2NS/NPS-1 was highest at 30 °C, and it gradually decreased as the adsorption temperature increased. TPD analysis showed that the reaction of primary amine of 2NS with CO₂ inside the nanoporous silica could form less thermally stable carbamic acid and carbamate compared to 1NS and 3NS.     

Diffusion mechanism of CO2 in 13X zeolite beads

A systematic study of the diffusion mechanism of CO₂ in commercial 13X zeolite beads is presented. In order to gain a complete understanding of the diffusion process of CO₂, kinetic measurements with a zero length column (ZLC) system and a volumetric apparatus have been carried out. The ZLC experiments were carried out on a single bead of zeolite 13X at 38 °C at a partial pressure of CO₂ of 0.1 bar, conditions representative of post-combustion capture. Experiments with different carrier gases clearly show that the diffusion process is controlled by the transport inside the macropores. Volumetric measurements using a Quantachrome Autosorb system were carried out at different concentrations. These experiments are without a carrier gas and the low pressure measurements show clearly Knudsen diffusion control in both the uptake cell and the bead macropores. At increasing CO₂ concentrations the transport mechanism shifts from Knudsen diffusion in the macropores to a completely heat limited process. Both sets of experiments are consistent with independent measurements of bead void fraction and tortuosity and confirm that under the range of conditions that are typical of a carbon capture process the system is controlled by macropore diffusion mechanisms.     

Zeolite synthesis from waste fly ash and its application in CO<Subscript>2</Subscript> capture from flue gas streams

Zeolite A and A + X mixtures were prepared from coal-fly ash procured from China by using an alkali fusion method. X-ray diffraction showed that both the materials were crystalline and reproducible. Scanning Microscopy revealed that pure zeolite A particles have cubic morphology while the mixture shows intergrowth of cubic and pyramidal crystals. The surface area for A + X mixture was around 330 m²/g which is higher than zeolite A, however, lower than typical X zeolite. CO₂ and N₂ adsorption isotherms were measured and the data was fitted by the Dual Site Langmuir equation. These zeolites were then tested for CO₂ capture at different temperatures in a process with a nine step cycle. When compared with 13X zeolites at higher temperature (∼90 °C), both the zeolite A + X mixture and zeolite A prepared from fly ash showed better performance in CO₂ capture from flue gas because they have higher selectivity of CO₂ over N₂.     

A Rational Design of Microporous Aerogel for Excellent CO<Subscript>2</Subscript> Capture and Selectivities via Co-Synergistic Effects of Electrostatic In-plane and π–π Stacking Interactions

Microporous aerogel (PEINK) with a nitrogen-containing framework, high CO₂ capture and selectivity were obtained through the condensation polymerization of 1,3,5-tris-(4-fluorobenzoyl)benzene with 5-hydroxyindol. The uptake capacity for CO₂ reached up to 17.7 wt % (1.0 bar, 273 K), and the high CO₂/N₂ = 16, CO₂/CH₄ = 29 selectivity. Density functional theory illustrated that a CO₂ molecular was captured via synergistic efforts with π–π stacking of indole group and electrostatic in-plane binding of an adjacent ether unit, thus the selectivity and adsorption of CO₂ were strengthen. The proposed co-synergistic effect on one CO₂ molecular is expected to be a new theory for the design of high CO₂ capture materials.     

Effect of amine double-functionalization on CO<Subscript>2</Subscript> adsorption behaviors of silica gel-supported adsorbents

Amine double-functionalized adsorbents were fabricated using silica gel as supports and their capabilities for CO₂ capture were examined. Aminopropyltrimethoxysilane (1N-APS), and N¹-(3-trimethoxysilylpropyl)diethylenetriamine (3N-APS) were used as grafted amine compounds, and tetraethylenepentamine and polyethyleneimine were used as impregnated species. The influence of double-functionalization method on the CO₂ adsorption performance and textural properties of adsorbents was investigated. The adsorption capacity, the amine efficiency, and the thermal stability of double-functionalized sorbents depend strongly upon molecular variables associated with two different functional states (i.e., chemically grafted and physically impregnated amines). The temperature dependence of adsorption isotherms reveals that the CO₂ adsorption behavior in the double-functionalized adsorbents follow the diffusion limitation model proposed by Xu et al. (Energy Fuels 16:1463–1469, 2002) where the CO₂ adsorption is helped by the diffusion of impregnated amines. It is also found that the adsorption isotherm in the double-functionalized sorbent system with a proper choice for grafted and impregnated amines is nearly independent of temperature, which may offer a novel means to fabricate practically useful sorbents that can be used in a wide range of temperature without loss of CO₂ adsorption capacity.     

Importance of Micropore–Mesopore Interfaces in Carbon Dioxide Capture by Carbon‐Based Materials

Mesoporous carbonaceous materials (Starbons®) derived from low‐value/waste bio‐resources separate CO₂ from CO₂/N₂ mixtures. Compared to Norit activated charcoal (AC), Starbons® have much lower microporosities (8–32 % versus 73 %) yet adsorb up to 65 % more CO₂. The presence of interconnected micropores and mesopores is responsible for the enhanced CO₂ adsorption. The Starbons® also showed three–four times higher selectivity for CO₂ adsorption rather than N₂ adsorption compared to AC.     

New CaO-based adsorbents prepared by solution combustion and high-energy ball-milling processes for CO2 adsorption: Textural and structural influences

In the present work, new CaO-based adsorbents were obtained by a fast solution combustion method and high-energy ball-milling process to study their CO₂ capture behavior under different moderate pressure and temperature conditions. The as-prepared CaO products were characterized systematically using different analytical techniques such as X-ray diffraction, scanning electron microscopy and N₂ physisorption measurements. The results showed that the CaO prepared by solution combustion and ball-milled during 2.5 h showed the maximum CO₂ adsorption capacity of 9.31 mmol/g at 25 °C and 1 atm mainly via chemisorption with CaCO₃ formation, which was corroborated by infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy studies. In general, the obtained results revealed that the synthesized CaO nanopowders from solution combustion that were treated by high-energy ball-milling enhanced their CO₂ adsorption capacity due to improved structural and textural properties, and this CaO-based adsorbent can be used as a promising material for CO₂ capture in post-combustion CO₂ capture technologies on a large scale, under atmospheric pressure and temperature conditions.     

Adsorption behaviors of CO<Subscript>2</Subscript> and CH<Subscript>4</Subscript> on zeolites JSR and Na<Subscript>n</Subscript>JSR using the GCMC simulations

The adsorption behaviors of CO₂ and CH₄ on new siliceous zeolites JSR and Na_nJSR (n = 2, 8, 16) were simulated using the Grand Canonical Monte Carlo method. The adsorption isotherms of CO₂ became higher with an increase in the Na⁺ number at a low pressure range (<150 kPa), whereas the isotherms showed a crossover with increasing pressure and the adsorption amount became smaller at a high pressure range (>850 kPa). With an increase in Na⁺ number, the pore volume decreased as the pore space was occupied by increasing Na⁺ ions. Additionally, two energy peaks on the interaction energy curves implied that CO₂ was adsorbed on two active sites. On the other hand, the adsorption amount of CH₄ decreased with an increase in the Na⁺ number and only one energy peak was observed. Adsorption isotherms were well fitted with the Langmuir and Freundlich equations up to 1000 kPa and the adsorption affinity of CO₂ on Na₁₆JSR zeolite was highest. The adsorption capacities of CO₂ in the studied zeolites were up to 38 times higher than those of CH₄. Diffusion constants of CO₂ and CH₄ decreased with an increase in the adsorbed amount and Na⁺ number. Considering the adsorbed amount, adsorption selectivity and affinity, zeolites JSR with a low Na⁺ number (JSR and Na₂JSR) is a good candidate for a pressure swing adsorption in the separation of CO₂/CH₄ mixture whereas JSR zeolites with high Na⁺ ratios (Na₁₆JSR and Na₈JSR) may be a better selection for a vacuum swing adsorption.     

R and D Note: Separation of a Nitrogen-Carbon Dioxide Mixture by Rapid Pressure Swing Adsorption

A N₂-CO₂ mixture is separated in a rapid pressure swing adsorption apparatus, which consists of single or double adsorbent beds filled with silica gel and operates in the sequence of adsorption, backflow and desorption. Nitrogen-rich gas is produced at the top of the bed, and carbon dioxide-rich gas at the bottom. Carbon dioxide purity of 89.5% and recovery of 70% were obtained in the single-bed apparatus, while purity of 93.5% and recovery of 72.3% were obtained in the double-bed apparatus. The feed in both cases consisted of 81% N₂ and 19% CO₂.