Adsorption properties and microporous structures of carbonaceous adsorbents

The theory underlying the absorption behavior of carbons is discussed as a basis for quantitative analysis of their adsorption properties and microporous structure. The equations of the theory of volume filling of micropores for homogeneous and inhomogeneous microporous structures and a rational method for determining the specific surface area of micropores are considered. Four principal parameters are proposed which describe the physical vapor adsorption with good accuracy and quantitatively characterize the microporous structure of carbonaceous adsorbents.     

Water vapor adsorption and the microporous structures of carbonaceous adsorbents

The principal role in adsorption of almost all vapors organic and inorganic substances on nonporous and microporous carbonaceous adsorbents is played by dispersion interactions. They are characterized by a considerable increase in adsorption potentials as a result of superposition of the fields of the opposite pore walls. This effect determines the entire specifics of adsorption in micropores and, in particular, the substantial increase in adsorbability. A theoretical estimate of the adsorption potentials of benzene and water in adsorption on graphite, and a comparison of the differential heats of adsorption of water vapors on a non-porous carbon black previously heated in a vacuum at 950°C and on an active carbon show that water adsorption is due to the formation of hydrogen bonds both between the oxygen complexes on the surface of carbonaceous adsorbents and between the adsorbed molecules themselves. Dispersion interactions are weak and can be neglected to a first approximation. It has been shown for microporous structures and the slitlike model that one can calculate, from the parameters W₀ and E₀ of the adsorption equation of the theory of volume filling of micropores (determined from the adsorption isotherm of a standard vapor, benzene) the volume of the micropores, their halfwidth, and the specific surface area of the micropore walls. The latter are in good agreement with the specific surface areas of the micropores, as estimated by the independent method of similarity of the adsorption isotherms of water in micropores and on the surface of a nonporous carbonaceous adsorbent. The application of the BET and t-methods to microporous carbonaceous adsorbents is physically unsubstantiated.     

Microporous structures of carbonaceous adsorbents

The simplest model of the porous structure of carbonaceous adsorbents is proposed, in which the microporous zones are formed by the totality of contacting and merged carbon crystallites. The micropores limited in extent resulted from the burnout on activation of several atomic layers of carbon in the crystallites by gaseous oxidizers (intercrystallite micropores) and from some development of the spaces between contacting crystallites (intercrystallite micropores). The micropore zones are shaped into regular cubes. The slit-like spaces between part of the micropore zones represent the micropore volume, while the total surface area of the walls of such pores is the mesopore surface. The parameters of the model for typical samples of microporous carbonaceous adsorbents are calculated and discussed. The formation of intercrystallite slit-like micropores as a result of the burnout of carbon atomic layers in crystallites on avtivation is considered. It is shown that only a wide micropore distribution leading to the formation of the supermicropores practically results in deviation from the applicability of the one-term adsorption equation of the theory of volume filling of micropores.     

Microporous structures and absorption properties of carbonaceous adsorbents

Two new independent methods for determining the surface area of micropore walls (the geometrical surface of micropores) of carbonaceous adsorbents have been proposed. Their satisfactory agreement has been substantiated experimentally. A comparative analysis of adsorption isotherms of benzene at 293 K on model carbonaceous micro- and supermicroporous adsorbents and of isotherms on mesoporous carbonaceous adsorbents with equal geometrical surface areas has been carried out. A quantitative estimate, under comparable conditions, of the increase in adsorption values in micro- and supermicropores as compared with nonporous or mesoporous adsorbents has been given for the first time.The formal applicability of the BET equation to adsorption isotherms of nitrogen at 77 K and benzene at 293 K on model carbonaceous microporous adsorbents, whose micropore geometrical surface area changed almost four-fold, has been shown. The determined monolayer capacities were close to the preassigned constant limiting adsorption values for the filled micropores. As a result the specific surface areas according to BET for the adsorbents under consideration were nearly equal.     

New applications for carbonaceous adsorbents

The most important adsorption properties of carbonaceous adsorbents are based on the microporous structure which can be determined by the mechanism of pore filling at equilibrium and by the diffusion behaviour of adsorbent molecules from the external surface into the grain. New processes of application of granular activated carbon in the gaseous phase besides the solvent recovery have been developed for the adsorption of sulfur containing and radioactive gases and in the aqueous phase for the purification of drinking and wastewater. New carbon molecular sieves are the base for gas separation processes, e.g. O₂- or N₂-recovery from air or H₂-recovery from H₂ containing gases.     

On the characterization of microporous adsorbents

Two exponential adsorption isotherms are discussed with regard to adsorption on microporous adsorbents at low and moderate surface coverages. One of them may be treated as a certain generalization of Dubinin isotherm. However, the other equation discussed is the exponential virial isotherm. Both isotherms may be used for characterization of the adsorbents showing a very differentiated microporous structure, because they predict universal expressions for the distribution function of adsorption potential. Adsorption data of hydrocarbons on molecular sieving carbon measured at different temperatures are interpreted in terms of the theory presented. Special cases of the exponential virial isotherm are discussed in detail.     

Production of carbonaceous adsorbents by using novolac-resin and cottonseed

Mixtures of novolac resin with pressed cottonseed in different proportions after their curing as small cylinders, and also pressed cottonseed as powder were pyrolyzed until 1000°C. Some samples follow, as well as an activation process by steam between 700 and 950°C. The weight losses of the samples are much higher, up to 500°C, while small weight losses are observed over 800°C. The weight losses for the 100% pressed cottonseed during the carbonization are essentially higher than those of the specimens also containing novolac; indeed, for the total weight losses up to 1000°C an increasing linear dependence was found with increasing proportion of the cottonseed in its mixture with novolac. The shrinkage of the specimens also containing novolac follows in general a curve similar to that of the weight losses for the corresponding case. The specific surface areas of the carbonaceous materials produced show higher values with measuring by CO₂ than by N₂ due to the larger dimensions of the N₂ molecule as compared to the CO₂ molecule, so that N₂ areas are more sensitive to slight changes in aperture sizes of the porous materials produced. The specific surface areas of the materials that have also been activated indicate higher values in comparison with those without activation and also higher than activated carbon commercially available. The more increased and fine porous structure of the activated carbons as compared to those only pyrolyzed become also obvious from the corresponding photos obtained by the scanning electron microscope. These results are also in agreement with the discoloring ability of the produced adsorptive media for the methylene blue from aqueous solution, following the Freundlich equation for the adsorption.     

吸附法回收油气的研究

分别以活性炭和活性炭纤维为吸附剂吸附回收油气,比较了两者油气吸附性能的差异,研究结果表明:在相同的吸附条件下,活性炭纤维对油气的穿透吸附量为114.0mg/g,明显大于活性炭对油气的穿透吸附量(71.8 mg/g),而且活性炭纤维床层的最高温升仅为4.7℃,低于活性炭床层温升(12.0℃);活性炭纤维对油气的吸附速率快、穿透时间短,但是能处理的油气的浓度小;活性炭重复利用18次后失活,活性炭纤维利用20次后失活.     

Adsorption of associating and non-associating adsorbates on microporous carbon adsorbents

The adsorption of different chemical substances on microporous carbon adsorbents has been investigated. It has been determined that the chemical character of the molecules adsorbed as well as the adsorbent texture determine the adsorption kinetics. In the case of alcohols, one of the parameters determining the sorption mechanism is the formation of complexes. As was shown earlier, the rates of complex molecule formation for methyl and ethyl alcohols differ significantly.     

Selection of microporous adsorbents for the clean-up of manufacturing waste water

Conclusions -- A procedure has been developed for selecting microporous sorbents for the clean-up of waste water, which takes into account overall the diverse technological characteristics of adsorbers.-- The procedure has been checked out as applicable to selecting carbon sorbents for cleaning up caprolactam-containing waste water in the manufacture of polyamide fibres.Translated from Khimicheskie Volokna, No. 5, pp. 24–27, September–October, 1990.     

Disclination structures of carbonaceous mesophase spherules

The permissible disclination structures of carbonaceous mesophase spherules with tangential boundary conditions were analysed theoretically. Besides the well-known Brooks-Taylor spherules, we observed during experiment the equilibrium carbonaceous mesophase spherules with symmetric configurations of 5 and 6 disclinations which are in close agreement with theoretical results. These spherules have been found to be formed and stabilized in coalescence of two or more mesophase spherules with a Brooks-Taylor structure. A thorough study has been made of the dynamics of the coalescence of carbonaceous mesophase spherules and disclination reactions in them, including the annihilation of disclinations of different sign. The rate of change of distance r between the disclination nuclei was shown to be r² = a (t−t_c), where t_c is the time of disclination annihilation. It is known that the bulk carbonaceous mesophase contains primarily wedge disclinations of strength . We have found that the disclinations of greater strengths, namely, and +2 may also exist. The effect of small impurity particles and gaseous bubbles on the stabilization of these possible but energetically less favorable disclinations in the carbonaceous mesophase is discussed.     

Solution‐reprocessable microporous polymeric adsorbents for carbon dioxide capture

Solution‐processable microporous polymers are promising materials for CO₂ capture because of their low synthetic cost and high processability. In this work, we for the first time systematically evaluate the feasibility of two microporous polymers, namely PIM‐1 and its hydrolyzed form hPIM‐1, as adsorbent materials for postcombustion CO₂ capture. By conducting ternary CO₂/N₂/H₂O breakthrough experiments, PIM‐1 demonstrates several promising features: moderate CO₂ working capacity, low water vapor uptake capacity, good moisture resistance, and easy regeneration process. In addition, we have pioneeringly studied the multiple‐cycle CO₂ adsorption–desorption induced relaxation effect on soft PIM‐1 polymers. Through a simple dissolution–precipitation approach, PIM‐1 can restore its BET surface area, CO₂ uptake capacity, and pore‐size distribution. The solution reprocessability of PIM‐1 demonstrated in this study distinguishes it from other rigid adsorbents and thus offers a new insight for the future design of economically‐viable and facilely regenerable adsorbents. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3376–3389, 2018     

磷酸活化法制备污泥含碳吸附材料的研究

研究了磷酸活化法制备污泥含碳吸附剂的工艺条件,探讨了活化剂质量分数、活化温度、活化时间和浸渍质量比对活化效果的影响.结果表明,在磷酸质量分数为50%、活化温度450℃、活化时间1 h、浸渍质量比1.5:1的条件下,制得的含碳吸附剂碘值在210 mg/g以上,产品收率50%,应用该吸附材料处理废水效果较好.