Pore structure alteration of porous carbon by catalytic coke deposition

The feasibility of preparing Carbon Molecular Sieve (CMS) by tailoring pore structure of activated carbon under catalytic cracking of benzene has been examined. In this method, benzene vapour was cracked over metal-impregnated activated carbon particles at 523–773 K. Among the metal catalysts tested, only cobalt exhibited significant cracking activity toward benzene. In this range of temperature coke was originated on the metal surface only, therefore an excessive coke deposition as indicated in non-catalytic process was not observed. The amount of coke and the site of deposition in the pore network were determined to some extent by the metal loading as well as the rate of benzene cracking. Raman spectra indicated that the coke produced was less amorphous than those produced in non-catalytic processes. Only a small loss in micropore volume and surface area was observed after the coke deposition process. The CMS produced was tested for its adsorption characteristics of carbon dioxide and methane. The improvement in the CO₂/CH₄ kinetic selectivity was observed.     

Production of carbon molecular sieves from palm shell based activated carbon by pore sizes modification with benzene for methane selective separation

Palm shell based activated carbon prepared by K₂CO₃ activation is used as precursor in the production of carbon molecular sieve by chemical vapor deposition (CVD) method using benzene as depositing agent. The influences of deposition temperature, time, and flow rate of benzene on pore development of carbon molecular sieve (CMS) and methane (CH₄) adsorption capacity were investigated. The parameters that varied are the deposition temperature range of 600 to 1000 °C, time from 5.0 to 60 min, and benzene flow rate from 3.0 to 15 mL/min. The results show that in all cases, increasing the deposition temperature, time, and flow rate of benzene result in a decrease in adsorption capacity of N₂, pore volume and pore diameter of CMS. The BET surface area of CMS (approximately 1065 m²/g) and the adsorption capacity of CH₄ were at a maximum value at a deposition temperature of 800 °C, time of 20 min and benzene flow rate of 6 mL/min. The product has a good selectivity for separating CH₄ from carbon dioxide (CO₂), nitrogen (N₂), and oxygen (O₂).     

Synthesizing activated carbons from resins by chemical activation with K2CO3

We prepared activated carbons from phenol-formaldehyde (PF) and urea-formaldehyde (UF) resins by chemical activation with K₂CO₃ with impregnation during the synthesis of the resins. The influence of carbonization temperature (773-1173 K) on the pore structure (specific surface area and pore volume) and the temperature range at which K₂CO₃ worked effectively as an activation reagent, were investigated. The specific surface area and micropore volume of PF-AC and UF-AC increased with an increase of carbonization temperature in the range of 773-1173 K. We prepared activated carbon with well-developed micropores from PF, and activated carbon with high specific surface area (>3000 m²/g) and large meso-pore volume from UF. We deduced the activation mechanism with thermogravimetry and X-ray diffraction. In preparing activated carbon from PF, K₂CO₃ was reduced by carbon in the PF char. The carbon was removed as CO gas resulting in increased specific surface area and pore volume above 1000 K. In preparing AC from UF, above 900 K the carbon in UF char was consumed during the K₂CO₃ reduction step.     

Formation of porous structures in activated brown-coal chars using O2, CO2 and H2O as activating agents

Two brown coals, xylitic and earthy, carbonized at 1173 K were activated with water vapour, carbon dioxide and oxygen, each producing a different distribution of porosity. In the xylitic coke, activated in the range of burn-offs from 1 to 70%, the action of water vapour results in the development of pores of all dimensions. At the highest burn-off the product has an effective surface area of 920 m² g^?1 and a total sorptive pore volume of 0.83 cm³ g^?1, 33% of which is in micropores. Carbon dioxide creates, from the xylitic coke at the burn-off of 70%, a highly microporous adsorbent with about the same surface area (890 m² g^?1) as the corresponding water-vapour activated product. The pore volume of the carbon dioxide sample is lower (0.49 cm³ g^?1) but these contain 63% of micropores, which amounts to a contribution of 92% of these pores to the effective total surface area. The activation of the xylitic coke with oxygen leads to a high development of porosity at low burn-offs, but becomes ineffective on continuation of the process to medium and high burn-offs. This is thought to be due to a blocking of the entrances of the micropores by surface oxygen complexes formed on the surface of the coke. Oxygen gives, at a high burn-off, a product with the lowest total adsorptive volume (0.45 cm³ g^?1) and surface area (650 m² g^?1). All the activated products obtained from the xylitic coke can be regarded, when effective surface areas are considered, as microporous adsorbents. With the earthy coke a total adsorptive pore volume (consisting mainly of wide mesopores) is developed which is higher than with the corresponding xylitic coke, but this result is difficult to reproduce, because the earthy coke samples are easily influenced by temperature in the process of activation, especially that by oxygen.     

Multi-stage activation of brown-coal chars with oxygen

Activation of xylitic brown-coal coke XBC 900 with water vapour and carbon dioxide, when modified by partial replacement of the basic activating agent with 10% oxygen at a lower temperature, results in products with an increased microporosity. Thus, oxygen as activating agent for xylitic coke develops, preferentially, micropores, and this property is more strongly pronounced for oxygen than for the carbon dioxide and water vapour. A drawback to the process of activation with oxygen, i.e. blockage of initially formed micropores by chemisorbed oxygen, can be eliminated by removal of the chemisorbed oxygen by heat treatment in argon (multi-stage oxygen activation). This increases the micropore volume of the xylitic brown-coal coke XBC 900 activated with oxygen to 70% total burnoff, from about 0.2 cm³ g^?1 to almost 0.5 cm³ g^?1. The increase of the total adsorptive volume (micropores and mesopores) of these samples is from 0.45 cm³ g^?1 to over 0.6 cm³ g^?1 and the surface area S_BET in benzene increases from 650 m² g^?1 to over 1200 m² g^?1. These last values are close to the limiting conditions for 70% activation obtainable for this material. Temperature of carbonization of the brown-coal char has a strong effect on the possibility of pore development through further activation. Multi-stage oxygen activation of xylitic brown-coal semicoke XBC 500 produces a material with a smaller micropore volume and a lower surface area than that of xylitic brown-coal coke XBC 900 similarly activated.     

Production of activated carbons from coffee endocarp by CO2 and steam activation

In this work the use of coffee endocarp as precursor for the production of activated carbons by steam and CO₂ was studied. Activation by both methods produces activated carbons with small external areas and microporous structures having very similar mean pore widths. The activation produces mainly primary micropores and only a small volume of larger micropores. The CO₂ activation leads to samples with higher BET surface areas and pore volumes when compared with samples produced by steam activation and with similar burn-off value. All the activated carbons produced have basic characteristics with point of zero charge between 10 and 12. By FTIR it was possible to identify the formation on the activated carbon's surface of several functional groups, namely ether, quinones, lactones, ketones, hydroxyls (free and phenol); pyrones and Si–H bonds.     

Molecular probe technique for the assessment of the carbon molecular sieve structure

Nitrogen adsorption at 77 K is the most common technique for defining the surface area and pore volume of a porous material. However it is not adequate to assess the microporosity of carbon molecular sieves (CMS), because of activated diffusion effects. In this paper, a molecular probe technique was used to defining the pore size of CMS materials. Adsorption of gases (vapors) with different molecular sizes, were measured by a gravimetric method using a spring balance. The amount adsorbed at room temperature was recorded over a 24-h period. The following molecular probes were chosen: CO₂ (0.33 nm), C₂H₆ (0.4 nm), n-C₄H₁₀ (0.43 nm), i-C₅H₁₁₂ (0.5 nm), and CCl₄ (0.6 nm). The micropore volumes were estimated by the Dubinin-Raduhkevich (DR) equation. Assuming that the diameters of the micropores are larger than those of the adsorbed molecules, the micropore volume distribution of each sample was estimated. The results demonstrated that the main pore size of the studied CMSs are less than 0.5 nm. One of the samples had a narrow pore size distribution in the range of 0.33–0.43 nm, which is the critical pore size for kinetic separation of oxygen from nitrogen. It is concluded that the molecular probe technique is an effective mean to assess the CMS adsorbents structure, which is not currently possible using conventional approaches with a single adsorbate, such as nitrogen or argon.     

Synthesis of carbon molecular sieves from palm shell by carbon vapor deposition

A series of experiments were conducted to produce carbon molecular sieves (CMS) through carbon deposition from a locally available palm shell of Tenera type for separating gaseous mixtures. The process involves three stages; carbonization, physical activation with steam, and carbon deposition by using benzene cracking technique. Carbonization of the dried palm shells was occurred at 900°C for duration of 1 h followed by steam activation at 830°C for 30–420 min to obtain activated carbons with different degree of burn-offs. The highest micropore volume of activated carbon obtained at 53.2% burn-off was used as a precursor for CMS production. Subsequent carbon deposition of the activated sample at temperature range from 600 to 900°C for 30 min has resulted in a series of CMSs with different selectivities of CO₂/CH₄ and O₂/N₂. The kinetic adsorption isotherm of CO₂, CH₄, O₂ and N₂ at room temperature also presented in this work.     

Adsorption properties of carbon molecular sieves prepared from an activated carbon by pitch pyrolysis

In this study a heat-treatment process using an activated carbon and coal-tar pitch was developed to prepare carbon molecular sieves (CMSs) for CH₄/CO₂ separation. This process results in a partial blockage of the pores of the activated carbon precursor, so that a reduction in the pore size takes place. Equilibrium CO₂ adsorption measurements at different temperatures, and CO₂ and CH₄ kinetic measurements at different temperatures and feed pressures were carried out using the TEOM technique for a carbon molecular sieve (CMS) prepared by this process (sample CB3) and a commercial CMS (Takeda 3A, sampleT3A). The overall diffusion for CO₂ in sample CB3 was faster than that in T3A and a slightly higher CO₂ adsorption capacity of CB3 was obtained. The transient uptake profiles in both samples at different temperatures and different CO₂ partial pressures were described in some cases by a micropore diffusion model, and in other cases by a dual resistance model. Both equilibrium and kinetic results demonstrate a better CO₂/CH₄ separation performance for the CMS prepared in the present study (CB3) than for the commercial CMS (Takeda 3A), due to the existence of slightly wider pore-mouth openings in sample CB3. This study demonstrates that the process used in this work is an interesting and reproducible approach to prepare CMS for CO₂/CH₄ separation.     

Effects of chemical modification of petroleum cokes on the properties of the resulting activated carbon

Activated carbons have been prepared from petroleum cokes by the combination of a chemical treatment with HClO₄ or H₂O₂ and a chemical activation with KOH at a constant KOH/coke ratio of 3/1. The influence of different chemical treatments on the properties of the activated carbon precursors and final carbons activated with KOH was invested by using XRD, FTIR, and BET techniques. XRD results indicated that the value of interplanar distance d₀₀₂ increased by chemical treatment and the disappearance of the peak corresponding to 0 0 2 faces correlated to high specific surface area. FTIR studies showed that chemical modification promoted the formation of surface oxygen functionalities. Significant effects on BET surface area, pore texture and iodine adsorption capacity were evidenced. The results show that chemical modification prior to activation dramatically increased the BET surface area and total pore volume of the resulting activated carbon. Modified petroleum coke based activated carbon with chemical activation had higher specific surface area (2336 m²/g) and better iodine adsorption value (1998 mg/g).     

The Pore Mouth Tailoring of Coal and Coconut Char Through Acid Treatment Followed by Coke Deposition

Carbon Molecular Sieves (CMS) obtained by coke deposition through deep cracking of hydrocarbons on the wide pore mouths of coal and coconut char are important adsorbents for separation of, difficult to separate, gaseous as well as liquid mixtures. The adsorption studies on these CMS show a high selectivity towards the adsorption of one or the other component from its mixture. In this work, CMS is prepared from pre treated raw materials like bituminous coal and coconut shell. The product samples are characterized in terms of kinetic adsorption and equilibrium adsorption of various gas adsorbents. It is observed that, all these samples are very good for CO₂ removal from mixtures containing CH₄ or H₂ in it. The CMS prepared from coconut shell showed an uptake ratio 4, for adsorption of O₂ and N₂, indicating that separation of nitrogen from air is viable by choosing suitable conditions in Pressure Swing Adsorption (PSA) Technique.     

Pore structure of activated carbons prepared by carbon dioxide and steam activation at different temperatures from extracted rockrose

The influence of the activation temperature on the pore structure of granular activated carbons prepared from rockrose (Cistus ladaniferus L.), extracted previously into petroleum ether, is comparatively studied. The preparation was carried out by pyrolysis of a char in nitrogen and its subsequent activation by carbon dioxide and steam (flow of water controlled to generate the same mol number per minute of water as well as carbon dioxide/nitrogen) at 700-950°C to 40% burn-off. The techniques applied to study the pore structure were: pycnometry (mercury, helium), adsorption (carbon dioxide, 298 K; nitrogen, 77 K), mercury porosimetry and scanning electron microscopy. The preparation by steam activation, especially at 700°C, yields activated carbons showing a total pore volume larger than those prepared by carbon dioxide activation. The pore structures present the greatest differences when the activations are carried out between 700 and 850°C and closer at higher temperatures. At high temperatures, the decrease of differences in pore development caused by carbon dioxide or steam is attributed to an external burn-off. The micropore structure of each activated carbon is mainly formed by wide micropores. At the lowest activation temperatures, especially at 700°C, steam develops the mesoporosity much more than carbon dioxide. At 950°C, a similar reduction of pore volume in the macropore range occurs.     

Adsorption of volatile organic compounds by means of activated carbon fibre-based monoliths

Activated carbon fibre monoliths (ACFMs) were prepared from the rejects of polymeric fibres (Nomex™). These were carbonised, agglomerated with a phenolic resin and steam activated at burnoff degrees between 0 and 40%. Adsorption experiments with n-butane at 30 °C show that, at high adsorbate concentrations, the amount adsorbed is a function of pore volume, but at low concentrations this mainly depends on pore size distribution. The porosity of Nomex-based ACFMs is formed by narrow micropores, which permit higher amounts of vapour to be adsorbed in low concentrations compared to monoliths prepared from different commercial activated fibres and a commercial granular activated carbon, which exhibits wider pores. The agglomeration of Nomex-fibres to form ACFMs does not cause any loss in adsorption properties with respect to non-agglomerated activated fibres. From the adsorption experiments of different vapours on a Nomex-based ACFM (40% burnoff) it was found that at high concentrations (p/p_o=1) the adsorbed volume was independent of the nature of the adsorbate and depended only on pore volume. However, at low vapor concentrations (p/p_o=0.004), the amount adsorbed depended on the adsorbate being well correlated to the molecular parachor and the polarizability of the adsorbates     

The development of porosity in heat-treated polymer carbons upon activation by carbon dioxide

A series of microporous carbons was prepared from cellulose triacetate by heat-treatment in the range 1230–2275°K and a parallel series of carbons was prepared by activation of members of the heat-treated series to 30% burn-off by reaction with carbon dioxide. The changes in porosity with heat-treatment temperature (HTT) were investigated by adsorption of carbon dioxide in the range 195–248°K and by measurement of mercury densities. By comparing porosity in unactivated and activated carbons the extents to which closed porosity can be recovered and open porosity developed by activation were investigated as a function of HTT. The predominant effect of heat-treatment was found to be conversion of open micropores to closed micropores with little change in total pore volume. Activation of 1230 and 1475°K carbons is confined almost entirely to development of micropores. With increasing HTT (meso- + macro-) pore development increases on activation while development of open micropores and opening of closed micropores become less significant.     

Preparation and properties of phenolic resin-based activated carbon spheres with controlled pore size distribution

Three kinds of phenolic resin-based activated carbon spheres (P-ACS) with different pore size distribution were prepared successfully by adding pore-forming agents to novolac-type phenolic resin. Polyethylene glycol and polyvinyl butyral, serving as pore-forming agents, evaporated during pyrolysis and left a small amount of carbon residue in the matrix of the phenolic resin-based carbon, thus changing the carbonization and activation behavior of the resin. Mesopores between 3 and 5 nm were created in the P-ACS, which possessed excellent adsorption properties for creatinine. Ferrocene has little effect on the carbonization process of the phenolic resin, but has a great impact on the activation process. Mesopores and macropores with a range from 3-5 to 10-90 nm were produced in the P-ACS, which exhibited large adsorption properties for VB₁₂, a larger molecule than creatinine. P-ACS without pore-forming agents exhibited a small specific surface area and mainly micropores, which resulted in a very small amount of creatinine and VB₁₂ adsorbed.     

Modification of pore size in activated carbon by polymer deposition and its effects on molecular sieve selectivity

The separation of air for nitrogen production can be carried out by pressure-swing-adsorption over a carbon molecular sieve. The separation is kinetically controlled, since the equilibrium adsorption of both oxygen and nitrogen is very similar, but the adsorption kinetics for oxygen is faster than for nitrogen. Several methods to prepare carbon molecular sieves are reported. In this work, we synthesized a carbon molecular sieve from a commercial activated carbon. After deposition of polyfurfuryl alcohol, these materials were subjected to carbonization at 800°C under an inert atmosphere. All the microporous materials were characterized by analysis of kinetics and equilibrium adsorption data. The molecular sieve performance was assessed by the O₂/N₂ uptake ratio. The material prepared by two depositions has characteristics similar to those of commercial CMS.     

Slow diffusion of methane in ultra-micropores of silicon carbide-derived carbon

We investigate macroscopic uptake kinetics of CH₄ in silicon carbide-derived carbon (SiC-DC). Ultra-microprosity in SiC-DC is found based on CO₂ adsorption at 273 K, but which has poor accessibility to Ar at 87 K. The adsorption kinetics of CH₄ is found to follow a bidisperse pore structure model, considering relatively rapid particle scale diffusion in large micropores, and a much slower local grain (or microparticle) scale diffusion in ultra-micropores. The grain scale activation energies are comparable with values for carbon molecular sieves, and consistent with values expected for the size range of the ultra-micropores, while the activation energies for transport in the larger particle scale micropores are comparable to those for conventional activated carbons. The particle scale diffusivities compare well with the results of equilibrium molecular dynamics simulations using a hybrid reverse Monte Carlo simulation constructed model of SiC-DC, with similar activation energy. On the other hand microscopic quasi-elastic neutron scattering measurements are found to probe only short-range barriers with lower activation energy. It is anticipated that ultra-micropores will not make a significant contribution to the transport in any membrane or adsorption-based process based on SiC-DC, due to the extremely slow transport in these ultra-micropores and their small pore volume.     

酚醛树脂基活性炭材料的制备及其电化学性能

以酚醛树脂（PF）为原料,聚乙二醇（PEG）为造孔剂,采用聚合物共混炭化及水蒸气活化法制备超级电容器电极用活性炭。通过热重（TG）分析探讨了PF、PEG及其共混物（PF-PEG）在升温过程中的热解行为,用N₂-BET法测试比表面积及其孔结构参数。通过测试恒流充放电、循环伏安和交流阻抗曲线分析其电化学性能,研究了活化温度、水蒸气流速及活化时间对活性炭孔结构及电化学性能的影响。结果表明,当活化温度为900℃、水蒸气流速为1 ml·min^-1、活化时间为2 h时制备的活性炭结构和性能相对较好,孔径主要分布在2 nm以下,比电容达到105.4 F·g^-1,具有良好的电容特性。     

Preparation of microporous activated carbon from Aegle marmelos fruit shell by KOH activation

Activated carbon (AC) is well‐known for its unique properties; hence, the search for new precursors and the investigation of new methods for the preparation of AC is still drawing attention of many researchers. In the present work, microporous AC was prepared from Aegle marmelos fruit shell (AMFS) by potassium hydroxide (KOH) activation. The effect of process parameters such as impregnation ratio, carbonisation temperature and holding time on porous characteristics was investigated. The porous characteristics of prepared AC samples were analysed by N₂ adsorption–desorption isotherms, and it was found that the isotherms obtained resemble typical microporous solids (Type‐I). The Langmuir surface area and total pore volume of the sample prepared at optimum conditions were found to be 937 m²/g and 0.33 cm³/g, respectively. The contribution of micropores to the porous characteristics of the prepared AC is very much appreciable, and about 97% of the total surface area and pore volume is attained by micropores. Pore size distribution (PSD) by Dubinin–Astakhov (DA) and micro‐pore (MP) methods confirmed the presence of micropores to a great extent with insignificant mesoporosity. © 2013 Canadian Society for Chemical Engineering     

Influence of preparation conditions on porous structures of olive stone activated by H3PO4

An olive factory residue was used as a precursor in the preparation of granular activated carbon by chemical activation with H₃PO₄. Effects of final activation temperature, time, and H₃PO₄ concentration used in the impregnation stage on the porous development were investigated. SO₂ adsorption experiments were also performed for some of the activated carbon samples to represent their adsorption performance. Activation at low temperature represented that micropores were developed first at early stages of the temperatures. Mesoporosity developed at around 250 °C, enhanced up to 400 °C, and then started to decrease due to possibly shrinking of pores. The optimum temperature for olive stone was found to be around 400 °C on the basis of total pore volume and BET surface area. It was clearly demonstrated that H₃PO₄ concentration used in the impregnation stage was not only effective for development of surface area and pore volumes but also an effective tool for tailoring the pore structure and size distribution.