Silica-templated melamine-formaldehyde resin derived adsorbents for CO2 capture

Adsorption on porous solids is an emerging alternative for CO₂ capture that seeks to reduce the costs associated to the capture step. The enhancement of a specific adsorption capacity may be carried out by increasing the affinity of the adsorbent surface to CO₂. Nitrogen enrichment is reported to be effective in introducing basic functionalties that enhances the specific adsorbent-adsorbate interaction for CO₂. In this work a templating technique was used to produce highly porous nitrogen enriched carbons from melamine-formaldehyde resins. Nitrogen incorporated into the polymer matrix results in the greater stability of the adsorbents in terms of volatile and thermal loss of nitrogen. CO₂ capture performances were evaluated between 25 °C and 75 °C in a thermobalance. CO₂ adsorption capacities up to 2.25 mmol g⁻¹ of CO₂ at 25 °C were achieved. Both texture and surface chemistry influence the CO₂ capture performance of the adsorbents. The carbonisation temperature used during the synthesis step controls the nitrogen functional groups present, as determined by XPS, with the loss of triazine nitrogen with increasing carbonisation temperature proposed to account for the decreased CO₂ affinity.     

Amine modified mesocellular siliceous foam (MCF) as a sorbent for CO2

Adsorption is considered a promising method for carbon capture. CO₂ adsorbents take a variety of forms - but one approach is to fill mesoporous substrates with a polymeric CO₂ selective sorbent. SBA-15 and mesocellular siliceous foam (MCF) are high pore volume, high surface area ordered mesoporous materials for which modification with amine should result in high capacity, highly selective adsorbents. SBA-15 and MCF were separately loaded with approximately one pore volume equivalent of linear polyethyleneimine (PEI) (M_w = 2500) or branched PEI (M_n = 1200). CO₂ adsorption/desorption isotherms under dry CO₂ were obtained at 75, 105 and 115 °C. The CO₂ adsorption/desorption kinetics were improved with temperature, though the CO₂ capacities generally decreased. The adsorption capacity for MCF loaded with branched PEI at 105 and 115 °C were 151 and 133 mg/g adsorbent, respectively (in 50% CO₂/Ar, 20 min adsorption time). These are significantly higher than the adsorption capacity observed for SBA-15 loaded with branched PEI under same conditions, which were 107 and 83 mg/g adsorbent, respectively. Thus the results indicate that, on a unit mass basis, amine modified MCF's are potentially better adsorbents than amine modified SBA-15 for CO₂ capture at modestly elevated temperature in a vacuum swing adsorption process.     

Controlling carbon microporosity: the structure of carbons obtained from different phenolic resin precursors

Carbons were prepared from resins synthesised using the phenolic precursors phenol, para methylphenol, para ethylphenol, para n-propylphenol, para isopropylphenol and 3,5-dimethylphenol. Loss of phenolic OH from these materials was followed using solid-state nuclear magnetic resonance. The surface areas of the carbons were determined using N₂ and CO₂ adsorption. No significant differences in the loss of phenolic OH were observed. Under the same carbonisation conditions, the para alkyl phenols gave carbons with wide micropores, while the phenol and 3,5-dimethylphenol gave carbons with narrow micropores. Grinding the cured resins prior to carbonisation was found to significantly increase the surface area of the carbons obtained, with the microporous surface area increasing rapidly with a fall in particle size, without a significant increase in burn-off. Higher carbonisation temperatures widened the micropore size distribution, as shown by fitting the CO₂ adsorption isotherm with the Dubinin-Astakhov equation. The ability to change the carbon micropore structure obtained from a simple, well defined precursor, has many potential applications in carbon molecular sieves, catalyst supports and the investigation of adsorption processes.     

CO2 capture using some fly ash-derived carbon materials

Adsorption is considered to be one of the more promising technologies for capturing CO₂ from flue gases. For post-combustion capture, the success of such an approach is however dependent on the development of an adsorbent that can operate competitively at relatively high temperatures. In this work, low cost carbon materials derived from fly ash, are presented as effective CO₂ sorbents through impregnation these with organic bases, for example, polyethylenimine aided by polyethylene glycol. The results show that for samples derived from a fly ash carbon concentrate, the CO₂ adsorption capacities were relatively high (up to 4.5 wt%) especially at high temperatures (75 °C), where commercial active carbons relying on physi-sorption have low capacities. The addition of PEG improves the adsorption capacity and reduces the time taken for the sample to reach the equilibrium. No CO₂ seems to remain after desorption, suggesting that the process is fully reversible.     

Surface chemistry of phosphorus-containing carbons of lignocellulosic origin

Activated carbon adsorbents were prepared by phosphoric acid activation of fruit stones in an argon atmosphere at various temperatures in the 400-1000 °C range and at different acid/precursor impregnation ratios (0.63-1.02). The surface chemistry of the carbons was investigated by elemental analysis, cation exchange capacity (CEC, measured by neutralization of NaOH with acidic surface groups), infrared spectroscopy and potentiometric titration. Porous structure was derived from adsorption isotherms (N₂ at −196 °C and CO₂ at 0 °C). It was demonstrated that all carbons show considerable cation exchange capacity, the maximum (CEC = 2.2 mmol g⁻¹) being attained at 800 °C, which coincides with the maximum contents of phosphorus and oxygen. The cation exchange properties of phosphoric acid activated carbons from fruit stones are chemically stable in very acidic and basic solutions. Proton affinity distributions of all carbons show the presence of three types of surface groups with pK at 2.0-3.3, 4.6-5.9 and 7.6-9.1. These pK ranges were ascribed primarily to: (a) phosphorus-containing and carboxylic groups; (b) lactonic groups, and (c) phenolic groups, respectively. Phosphoric acid activated carbons are microporous with a relatively small contribution of mesopores. A maximum BET surface area of 1740 m² g⁻¹ was attained at 400 °C.     

On the preparation and characterization of chars and activated carbons from orange skin

Activated carbons were obtained by carbonization of orange skin waste and partial gasification with CO₂. The orange skin contains a significant amount of inorganic matter mainly potassium, calcium and phosphorus. CO₂ gasification is catalyzed by potassium and calcium, resulting in carbons with a microporous structure. Thermal treatment up to 900 °C applied to orange skin-derived activated carbons yields carbons with a highly developed porous structure, and a significant contribution of mesopores, due to the activation effect of potassium compounds. This porous structure is initially blocked by the inorganic matter that is removed by a subsequent acid wash, opening the porous structure of the final carbon; an activated carbon with a very wide porous structure and a specific surface area of around 1200 m²/g was obtained. The activated carbon with high potassium content shows relatively high NO adsorption capacities in the presence of oxygen at 120 °C, probably due to the catalytic effect of potassium on the oxidation of NO. The breakthrough times of the NO adsorption in the presence of oxygen at 120 °C were predicted by the Bohart and Adams model with a relevant agreement between the calculated and the experimental times.     

Chemical modification of carbon powders with aminophenyl and aryl-aliphatic amine groups by reduction of in situ generated diazonium cations: Applicability of the grafted powder towards CO2 capture

Aminophenyl, p-aminobenzyl and p-aminoethylphenyl groups were grafted at the surface of carbon Vulcan XC72R by spontaneous reduction of the in situ generated diazonium cations from the corresponding amine. X-ray photoelectron spectroscopy and elemental analysis confirmed an amine loading of about 1 mmol/g. The grafting of amine functionalities leads to a decrease of specific surface area from 223 to about 110 m²/g with a drastic loss of microporosity. Acid-base properties of the surface are also affected by the modification. Aminophenyl grafted groups make the surface more acidic while aryl-aliphatic amines groups tends to render it more basic. The grafted layer shows in each case a good thermal stability up to 250 °C. The affinity of the modified powder towards CO₂ and N₂ has been evaluated by thermal swing adsorption. The maximum adsorption capacity of CO₂ of modified carbons is lower than the unmodified carbon but the presence of the amine functionalities involves a better selectivity of the material towards CO₂ adsorption in comparison of N₂ adsorption.     

The Characterization of the Micropore Structures of Some Activated Carbons of Plant Origin by N2 and CO2 Adsorptions

Abstract

In this study, active carbons prepared from almond and hazelnut shells under various experimental conditions were investigated. Merck-2514 and Merck-2184 active carbons were used for comparison. N₂ (77 K) gas and CO₂ (273 and 195 K) gas adsorptions were determined as comparison criteria. Regarding the specific surface area and micropore volume results obtained from these adsorption data, it is concluded that N₂ (77 K) adsorption by itself is inadequate in the characterization of active carbons which are low-sized microporous dominated. In addition, it is concluded that it would be useful to investigate CO₂ (195 and 273 K) adsorption. The iodine and methylene blue tests at 298 K were also applied for the characterization of the carbon adsorbents mentioned. From these data it was seen that the iodine test can be applied as a total porosity indicator and that the methylene blue test can be used as a developed microporosity indicator. These results indicate that the best adsorbents were those prepared from hazelnut shells. Depending on the preparation conditions, the physically activated carbon has an activation time up to 4 hours and has adsorption properties on the level of Merck commercial carbons.     

Synthetic carbons activated with phosphoric acid III. Carbons prepared in air

Synthetic activated carbons were prepared by phosphoric acid activation of a styrene-divinylbenzene copolymer in an air atmosphere at various temperatures in the 400-900 °C interval. The carbons were characterized by elemental analysis, cation-exchange capacity measurement, infrared spectroscopy, potentiometric titration, copper adsorption from solution and physical adsorption of N₂ at −196 °C and CO₂ at 0 °C. It was shown that, similarly to synthetic phosphoric acid activated carbons obtained in argon, the synthetic carbons activated with phosphoric acid in air possess an acidic character and show considerable cation-exchange properties. The contribution of oxygen-containing surface groups along with phosphorus-containing groups to CEC is higher for carbons obtained in air. Three types of surface groups were identified on carbons prepared at temperatures up to 600 °C, and four types on carbons prepared at higher temperatures. These groups were assigned to ‘super-acidic’ (pK<0), phosphorus-containing (pK=1.1-1.2), carboxylic (pK=4.7-6.0) and phenolic (pK=8.1-9.4) groups. The cation-exchange capacity was at a maximum for the carbon prepared at 800 °C. Copper adsorption by synthetic phosphoric acid activated carbons obtained in air at temperatures lower than 800 °C is higher than for similar carbons obtained in argon. The increase is due to additional formation of oxygen-containing surface groups. Calculated copper binding constants revealed the importance of phosphorus-containing and carboxylic groups for adsorption of copper from aqueous solution. All carbons show a multimodal pore size distribution including simultaneously micropores and mesopores, but the porous texture is not a prime factor in determining the cation-exchange capacities of these carbons. Synthetic phosphoric acid activated carbons show a greater development of porosity when obtained in air as compared to carbons carbonized in argon.     

High-pressure sorption isotherms and sorption kinetics of CH4 and CO2 on coals

Using a manometric experimental setup, high-pressure sorption measurements with CH₄ and CO₂ were performed on three Chinese coal samples of different rank (VR_r = 0.53%, 1.20%, and 3.86%). The experiments were conducted at 35, 45, and 55 °C with pressures up to 25 MPa on the 0.354-1 mm particle fraction in the dry state. The objective of this study was to explore the accuracy and reproducibility of the manometric method in the pressure and temperature range relevant for potential coalbed methane (CBM) and CO₂-enhanced CBM (CO₂-ECBM) activities (P > 8 MPa, T > 35 °C). Maximum experimental errors were estimated using the Gauss error propagation theorem, and reproducibility tests of the high-pressure sorption measurements for CH₄ and CO₂ were performed. Further, the experimental data presented here was used to explicitly study the CO₂ sorption behaviour of Chinese coal samples in the elevated pressure range (up to 25 MPa) and the effects of temperature on supercritical CO₂ sorption isotherms.The experiments provided characteristic excess sorption isotherms which, in the case of CO₂ exhibit a maximum around the critical pressure and then decline and level out towards a constant value. The results of these manometric tests are consistent with those of previous gravimetric sorption studies and corroborate a crossover of the 35, 45, and 55 °C CO₂ excess sorption isotherms in the high-pressure range. The measurement range could be extended, however, to significantly higher pressures. The excess sorption isotherms tend to converge, indicating that the temperature dependence of CO₂ excess sorption on coals at high-pressures (>20 MPa) becomes marginal. Further, all CO₂ high-pressure isotherms measured in this study were approximated by a three-parameter excess sorption function with special consideration of the density ratio of the “free” phase and the sorbed phase. This function provided a good representation of the experimental data.The maximum excess sorption capacity of the three coal samples for methane ranged from 0.8 to 1.6 mmol/g (dry, ash-free) and increased from medium volatile bituminous to subbituminous to anthracite. The medium volatile bituminous coal also exhibited the lowest overall excess sorption capacity for CO₂. However, the subbituminous coal was found to have the highest CO₂ sorption capacity of the three samples. The mass fraction of adsorbed substance as a function of time recorded during the first pressure step was used to analyze the kinetics of CH₄ and CO₂ sorption on the coal samples. CO₂ sorption proceeds more rapidly than CH₄ sorption on the anthracite and the medium volatile bituminous coal. For the subbituminous coal, methane sorption is initially faster, but during the final stage of the measurement CO₂ sorption approaches the equilibrium value more rapidly than methane.     

Activation of carbon nanofibres for hydrogen storage

The present study was aimed to investigate different methods of activation of carbon nanofibres, CNF, in order to determine the beneficial effect on the hydrogen sorption capacities of increasing the surface area. Two activation systems were used: physical activation with CO₂ and chemical activation with KOH. A range of potential adsorbents were thus prepared by varying the temperature and time of activation. The structure of the CNF proved more suitable to activation by KOH than by CO₂, with the former yielding higher surface area carbons (up to 1000 m² g⁻¹). The increased surface area, however, did not correspond directly with a proportional increase in hydrogen adsorption capacity. Although high surface areas are important for hydrogen storage by adsorption on solids, it would appear that it is essential that not only the physical, but also the chemical, properties of the adsorbents have to be considered in the quest for carbon based materials, with high hydrogen storage capacities.     

Nitrogen-containing carbons from phenol-formaldehyde resins and their catalytic activity in NO reduction with NH3

Porous carbons with controlled nitrogen contents were prepared from phenol-formaldehyde resins impregnated with different amounts of m-phenylenediamine. The chemical compositions of the resin precursors and the resulting carbons were characterized. Comparison of results from X-ray photoelectron spectroscopic and elemental analysis showed that the nitrogen functional groups in the carbons are more numerous in the internal part and are mainly of the pyridine type. The catalytic activity of the carbons in NO reduction with NH₃ increased upon nitrogen impregnation. The activity showed a clear correlation with the nitrogen content obtained using X-ray photoelectron spectroscopy, indicating that the reaction mainly occurred at the external part of the carbon particles. Under a low temperature regime (<140 °C), the reaction was dominated by the equilibrium adsorption of the reactants, which rendered a negative apparent activation energy, while chemical interactions on the surface controlled the reaction at higher temperatures. The effects of the impregnated nitrogen atoms were suggested, due to the introduction of extra electron to the aromatic rings in carbon, to promote the adsorption of NO, as well as to activate the adsorbed species.     

Synthetic carbons activated with phosphoric acid: II. Porous structure

Synthetic activated carbons were prepared by H₃PO₄ activation of a chloromethylated and sulfonated copolymer of styrene and divinylbenzene, using an impregnation weight ratio of 0.75 and carbonization temperatures in the 400-1000 °C range. Other impregnation ratios (0.93 and 1.11) were also used at a carbonization temperature of 800 °C. The porous texture of the resulting carbons was characterized by N₂ adsorption at −196 °C and CO₂ adsorption at 0 °C. All carbons exhibited a multimodal pore size distribution with maxima in the micropore and meso/macropore regions. Maxima in pore volume were attained at 900 °C for micropores and at 500 and 900 °C for mesopores. The mesopore volume was less sensitive than the micropore volume to changes in the impregnation ratio. It is concluded that the porous texture is not a prime factor in determining the outstanding cation exchange capacities of these carbons.     

Preparation of a highly microporous carbon from a carpet material and its application as CO2 sorbent

In order to increase the use of carpet wastes (pre- and/or post-consumer wastes), this work studies for the first time the preparation and characterisation of a microporous material from a commercial carpet (pile fiber content: 80% wool/20% nylon; primary and secondary backings: woven polypropylene; binder: polyethylene) and its application for CO₂ capture. The porous material was prepared from an entire carpet material using a standard chemical activation with KOH and then, characterised in terms of their porous structure and surface functional groups. Adsorption of CO₂ was studied using a thermogravimetric analyser at several temperatures (25-100 °C) and under different CO₂ partial pressures (i.e. pure CO₂ flow and a ternary mixture of 15% CO₂, 5% O₂ and 80% N₂). In order to examine the adsorbent regenerability, multiple CO₂ adsorption/desorption cycles were also carried out. The surface area and micropore volume of the porous adsorbent were found to be 1910.17 m² g^− 1 and 0.85 cm³ g^− 1, respectively. The CO₂ adsorption profiles illustrate that the maximum CO₂ capture on the sample was reached in less than 10 min. CO₂ adsorption capacities up to 8.41 wt.% and 3.37 wt.% were achieved at 25 and 70 °C, respectively. Thermal swing regeneration studies showed that the prepared adsorbent has good cyclic regeneration capacities.     

Importance of activated carbon's oxygen surface functional groups on elemental mercury adsorption

The effect of varying physical and chemical properties of activated carbons on adsorption of elemental mercury (Hg⁰) was studied by treating two activated carbons to modify their surface functional groups and pore structures. Heat treatment (1200 K) in nitrogen (N₂), air oxidation (693 K), and nitric acid (6N HNO₃) treatment of two activated carbons (BPL, WPL) were conducted to vary their surface oxygen functional groups. Adsorption experiments of Hg⁰ by the activated carbons were conducted using a fixed-bed reactor at a temperature of 398 K and under N₂ atmosphere. The pore structures of the samples were characterized by N₂ and carbon dioxide (CO₂) adsorption. Temperature-programmed desorption (TPD) and base-acid titration experiments were conducted to determine the chemical characteristics of the carbon samples. Characterization of the physical and chemical properties of activated carbons in relation to their Hg⁰ adsorption capacity provides important mechanistic information on Hg⁰ adsorption. Results suggest that oxygen surface complexes, possibly lactone and carbonyl groups, are the active sites for Hg⁰ capture. The carbons that have a lower carbon monoxide (CO)/CO₂ ratio and a low phenol group concentration tend to have a higher Hg⁰ adsorption capacity, suggesting that phenol groups may inhibit Hg⁰ adsorption. The high Hg⁰ adsorption capacity of a carbon sample is also found to be associated with a low ratio of the phenol/carbonyl groups. A possible Hg⁰ adsorption mechanism, which is likely to involve an electron transfer process during Hg⁰ adsorption in which the carbon surfaces may act as an electrode for Hg⁰ oxidation, is also discussed.     

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.     

Adsorption of CO2 from dry gases on MCM-41 silica at ambient temperature and high pressure. 2: Adsorption of CO2/N2, CO2/CH4 and CO2/H2 binary mixtures

Adsorption equilibrium capacity of CO₂, CH₄, N₂, H₂ and O₂ on periodic mesoporous MCM-41 silica was measured gravimetrically at room temperature and pressure up to 25 bar. The ideal adsorption solution theory (IAST) was validated and used for the prediction of CO₂/N₂, CO₂/CH₄, CO₂/H₂ binary mixture adsorption equilibria on MCM-41 using single components adsorption data. In all cases, MCM-41 showed preferential CO₂ adsorption in comparison to the other gases, in agreement with CO₂/N₂, CO₂/CH₄, CO₂/H₂ selectivity determined using IAST. In comparison to well known benchmark CO₂ adsorbents like activated carbons, zeolites and metal-organic frameworks (MOFs), MCM-41 showed good CO₂ separation performances from CO₂/N₂, CO₂/CH₄ and CO₂/H₂ binary mixtures at high pressure, via pressure swing adsorption by utilizing a medium pressure desorption process (PSA-H/M). The working CO₂ capacity of MCM-41 in the aforementioned binary mixtures using PSA-H/M is generally higher than 13X zeolite and comparable to different activated carbons.     

Characterization of pistachio shell-derived carbons activated by a combination of KOH and CO2 for electric double-layer capacitors

The micropores and surface oxygen functional groups of KOH-activated carbons were respectively extended and desorbed by the gasification of CO₂ during the activation process of chars derived from pistachio shells. These activated carbons (ACs) were found to exhibit ideal capacitive performances (i.e., a rectangular shape of CVs at a wide range of scan rates, high power property, and excellent reversibility) in aqueous electrolytes for electric double-layer capacitors. Although the specific capacitance of these ACs measured at a low scan rate (25 mV s⁻¹) is decreased with reducing the density of surface functional groups, the ideal capacitive characteristics can be maintained at a much higher scan rate (300 mV s⁻¹) when the CO₂ gasification time is equal to or longer than 30 min because of the relatively high proportion of mesopores.     

Carbon adsorbents from waste ion-exchange resins

A series of activated carbons was prepared from different waste commercial ion-exchange resins and studied by means of adsorption, SEM and IR methods. Samples were additionally washed or washed/frozen. This resulted in increases in micro- and mesoporosity in comparison with initial activated carbons. For some samples, the latter treatment gives enhancement of mesoporosity but reduction of microporosity and vice versa comparing with only washed carbons due to different localization of water droplets in mesopores or micropores. Changes in the morphology of chars and activated samples depended on resin composition and history. Relatively high values of porosity (V_p ≈ 0.4 cm³/g) and specific surface area (S_BET ≈ 600 m²/g) show that activated carbons prepared from waste ion-exchange resins can be utilized for different purposes, especially after additional treatment (such as washing, impregnation by certain compounds and subsequent thermal activation).     

Oxygen and phosphorus enriched carbons from lignocellulosic material

Activated carbons were prepared by phosphoric acid activation of fruit stones in air at temperatures 400-1000 °C. The surface chemistry was investigated by elemental analysis, cation exchange capacity, infrared spectroscopy and potentiometric titration. The porous structure was analyzed from adsorption isotherms (N₂ at 77 K and CO₂ at 273 K). It was demonstrated that all carbons show considerable cation exchange capacity, the maximum (2.2 mmol g⁻¹) being attained at 700 °C, which coincides with the maximum contents of phosphorus and oxygen. The use of air instead of argon during thermal treatment increased the amount of cation exchangeable surface groups for carbons obtained at 400-700 °C. Proton affinity distributions of all carbons show the presence of three types of surface groups with pK 1.8-3.1 (carboxylic and polyphosphates), 4.8-6.3 (second dissociation of carboxylic, weak acid in polyphosphates and enol structures) and 8.1-9.7 (phenols and enol structures). Carbons obtained in air at 400-600 °C show enhanced copper adsorption from 0.001 mol L⁻¹ Cu(NO₃)₂ in acidic solutions as compared to carbons obtained in argon. Carbons obtained in air show well-developed porous structure that is equivalent or higher as compared with carbons obtained in argon; the difference being progressively increased with increasing treatment temperature.