Analysis of the evolution of the pore size distribution and the pore network connectivity of a porous carbon during activation

The development of the microporosity, pore size distribution and pore network connectivity has been studied in the production of activated carbons from lignite char. Grand Canonical Monte Carlo simulation of adsorption was applied to the characterisation of a set of activated carbons produced at a sequence of times. The pore size distributions obtained from nitrogen at 77 K and ethane at 264 K were used as inputs to a method based on percolation theory to study the changing connectivity of the system. The incorporation of percolation concepts in the study of the porosity development gives an insight into the processes involved. The analysis is applied to a particular environmental application, the adsorption of phenanthrene.     

氮气吸附法和压汞法测定Al2O3载体孔结构

作为催化剂载体的活性氧化铝,其孔径分布主要集中在中孔范围内,压汞法和氮气吸附法是测定多孔材料比表面积、孔径及其分布的经典方法,采甩压汞仪和比表面积、孔径测定仪对活性氧化铝载体孔结构进行测定。氮气吸附法采用BET原理进行比表面积测定,采用BJH原理进行孔径分布和孔容的测定,压汞法采用Wasburn公式测定比表面、孔径分布和孔容。对两种方法实验结果进行了讨论,认为氮气吸附法更适合氧化铝载体孔结构的测定。     

Quenched solid density functional theory and pore size analysis of micro-mesoporous carbons

We present a new model of adsorption on micro-mesoporous carbons based on the quenched solid density functional theory (QSDFT). QSDFT quantitatively accounts for the surface geometrical inhomogeneity in terms of the roughness parameter. We developed the QSDFT models for pore size distribution calculations in the range of pore widths from 0.4 to 35 nm from nitrogen at 77.4 K and argon at 87.3 K adsorption isotherms. The QSDFT model improves significantly the method of adsorption porosimetry: the pore size distribution (PSD) functions do not possess gaps in the regions of ∼1 nm and ∼2 nm, which are typical artifacts of the standard non-local density functional theory (NLDFT) model that treats the pore walls as homogeneous graphite-like plane surfaces. The advantages of the QSDFT method are demonstrated on various carbons, including activated carbons fibers, coal based granular carbon, water purification adsorbents, and mirco-mesoporous carbon CMK-1 templated on MCM-48 silica. The results of PSD calculations from nitrogen and argon are consistent, however, argon adsorption provides a better resolution of micropore sizes at low vapor pressures than nitrogen adsorption.     

Quenched solid density functional theory method for characterization of mesoporous carbons by nitrogen adsorption

Quenched solid density functional theory (QSDFT) model for characterization of mesoporous carbons using nitrogen adsorption is extended to cylindrical and spherical pore geometries. The kernels of theoretical isotherms in the range from 0.4 to 50 nm are constructed accounting for different possible variations of the pore shapes in micropore and mesopore regions. The results of QSDFT method are illustrated with experimental data on adsorption on novel CMK-3 and 3DOm carbons. The proposed method is recommended for pore size distribution calculations for micro–mesoporous carbons obtained through various templating mechanisms.     

溶胶凝胶法N,N-二甲基甲酰胺对多孔炭孔隙结构的影响

以正硅酸乙酯(TEOS)为模板硅源,蔗糖为碳前体,添加N,N-二甲基甲酰胺(DMF)作为控制干燥化学助剂(DCCA),运用溶胶凝胶(Sol-Gel)法制备多孔炭材料。通过SEM和低温N2等温吸脱附等手段对材料的结构进行了测试与表征,结果表明:在优选工艺条件后,成功地制得了无龟裂混合干凝胶,溶硅去模后多孔炭材料孔径主要集中分布在2～7nm。     

Removal of copper (II) and phenol from aqueous solution using porous carbons derived from hydrothermal chars

The feasibility of hydrothermal char (HTC), a byproduct from biomass hydrothermal liquefaction for bio-oil production, as raw material for preparation of porous carbons was investigated in the present study. The resultant HTC-derived porous carbons were characterized and utilized as adsorbents for copper (II) and phenol removal from aqueous solution. Compared with porous carbons using pyrolytic char as precursor, the HTC-derived porous carbons exhibited unique textural features, e.g., narrow pore size distribution, high surface area and large pore size. In addition, FT-IR analysis confirmed that substantial amount of ketene groups existed on the surface of the HTC-derived porous carbons. As the adsorbents, the copper (II) adsorption onto HTC-derived carbons was strongly affected by the pH value of the solution in comparison with phenol adsorption. The carbons derived from pinewood and rice husk HTC exhibited high adsorption capacity of 83.88 and 39.30 mg/g for phenol and 25.18 and 22.62 mg/g for copper (II), respectively. The adsorption data for copper (II) and phenol onto the carbon adsorbents could be well described by Langmuir and Freundlich models. In comparison with pinewood sawdust HTC-derived carbon, the adsorption onto rice husk HTC-derived carbon preferentially followed Freundlich model due to the presence of silica on the surface.     

Using DFT analysis of adsorption data of multiple gases including H2 for the comprehensive characterization of microporous carbons

Hydrogen and nitrogen adsorption isotherms at cryogenic temperatures (77 and 87 K) were used to characterize the microporosity of a series of activated carbons, representing various pore size distributions (PSD). The PSD for each carbon was calculated by simultaneous fitting of the DFT model isotherms to their experimental counterparts. The resulting PSD represents robust characteristics of the carbon structure that is consistent with all the data used in the analysis. The range of pore size analysis in this method is extended to smaller pore sizes compared to the standard nitrogen adsorption analysis. In addition, it is shown that this approach allows to detect and exclude experimental points that are not fully equilibrated due to diffusion problems in narrow micropores. The results of the analysis of a series of carbons activated with systematically increasing burn-off show that the presented approach is a useful tool for a comprehensive characterization of microporous carbons, and for obtaining detailed and reliable carbon PSDs.     

New adsorption-condensation theory for adsorption of vapors on porous activated carbons

A new analytical model to describe equilibrium adsorption of condensable vapors on porous adsorbents is developed. It accounts for the heterogeneous pore structure of the adsorbent, adsorption in the micropores by a pore filling mechanism and adsorption and condensation in the macropores. A gamma-type pore size distribution function is used. Langmuir-type adsorption equations are used to describe both micropore filling and adsorption on the macropore walls. The vapor condensation in the pores is described by the Kelvin equation. The model is successfully tested using isotherm data for adsorption of various condensable vapors on different porous activated carbons and charcoals. All three types (I, IV and V) of adsorption isotherms by the Brunauer classification which are depicted by the porous adsorbents can be described by the model.     

The significance of physical factors on the adsorption of polyaromatic compounds by activated carbons

The adsorption of three synthetic organic compounds (SOCs) (i.e., phenanthrene, biphenyl, and 2-chlorobiphenyl), with similar physicochemical properties but different molecular conformations (i.e., planar and nonplanar), by an activated carbon and an activated carbon fiber was investigated. The physical characteristics of the carbons were obtained from low temperature nitrogen adsorption isotherms using BET, DR, and DFT models. Their surface chemistry was characterized by water vapor adsorption, pH of the point of zero charge, acid/base uptakes, and elemental analysis. The results indicated that adsorbent pore structure characteristics and adsorbate molecular conformation are important in the adsorption of SOCs by porous carbonaceous adsorbents. To predict the adsorption of SOCs by activated carbons, accurate characterization of pore shape and size distribution in primary micropores is important. The results indicated that adsorbate molecules can access and fill more efficiently the slit-shape pores than ellipsoidal pores, whereas the ellipsoidal pores create higher adsorption potential than slit-shape pores. Both molecular conformation and dimensions of adsorbate affect the adsorption. Planar molecules appear to access and pack in slit-shape pores more efficiently as compared to nonplanar molecules. Nonplanar molecular conformation weakens the interactions between adsorbate molecules and carbon surfaces.     

Increased H2 gravimetric storage capacity in truncated carbon slit pores modeled by Grand Canonical Monte Carlo

Hydrogen adsorption in slit shaped pores built up from truncated graphene fragments has been simulated using Grand Canonical Monte Carlo technique and the influence of pore wall edges on hydrogen storage by physisorption has been analyzed. We show that due to the additional gas adsorption at the pore edges the adsorbed gravimetric amount significantly increases (by a factor of two) with respect to models of pores with infinite graphene walls. The contribution of the edges’ adsorption to the total hydrogen uptake is independent of the pore wall shape but it depends on its surface. We also show that the maximum of the excess adsorption shifts towards higher pressures when the edge contribution increases. This information can be used to characterize experimentally structures of porous adsorbents and complement pore size distribution analysis usually performed with gases others than hydrogen. We suggest that porous carbons built from polycyclic hydrocarbons can achieve storage performances required for practical applications.     

Synthesis and characterization of microporous carbons templated by ammonium-form zeolite Y

Emerging applications such as gas storage require porous carbon materials with tailored structural and surface properties. Template synthesis approach to porous carbons offers opportunities for tailoring these properties. In this study, ammonium-form zeolite Y (NH₄Y) was used as a template and furfuryl alcohol (FA) was employed as a carbon precursor to prepare microporous carbons by simple impregnation method. The effects of synthesis conditions such as carbonization temperatures and heating rates on the pore structure of the microporous carbons were investigated. The thermal behaviors of FA-NH₄Y mixtures and zeolite/carbon composites were studied by thermogravimetric analysis (TGA). The physical, structural, and surface properties of the microporous carbons were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM), elemental analysis, and physical adsorption of nitrogen. Microporous carbons with high surface areas, pore volumes and nitrogen-containing surface functional groups can be readily synthesized.     

The complete pore structure analysis of fine porous solids

A new method is presented for the determination of the distribution of pore volume and surface in micropores. The analysis facilitates obtaining typical parameters of micropore structure from the adsorption isotherm. The calculated characteristic micropore radii and the micropore surface area for a model of cylindrical pores agree with the data from small-angle X-ray scattering. Micropore cumulative distribution curves for two typical active carbons were joined together with a high pressure mercury porosimetry data. The overall porosity cumulative distribution curve agrees with the Dubinin model of the porous structure of active carbons.     

Characterization of microporous activated carbons using molecular probe method

Accurate knowledge of an adsorbent’s porosity is fundamental for scientific and industrial applications of adsorption technology. Over the last decades many approaches have been established to assess porosity of adsorbent materials by analyzing their nitrogen uptake at 77 K with volumetric measurement devices. Despite using highly sophisticated physical models, all approaches make assumptions on pore shape as well as on the interactions between adsorbent and adsorptive molecules. Subsequently, significant differences in pore size distributions are observed depending on which modeling parameters were used. The molecular probe method presented in this paper therefore restrains to a minimum of approximations by measuring isotherms of chemically similar substances of increasing molecular size. Differences in pore volume can be reduced to sterical limitations in micropores below the size of adsorptive, leading to a high-resolution pore size distribution below 0.7 nm where only few comparable methods exist. The analytical procedure was customized to take account of the amorphous and heterogeneous pore structure of activated carbon. By measuring adsorption isotherms of N₂, n-hexane, iso-octane and cyclohexane on various activated carbons, it is shown that differences in pore accessibility of tested adsorptives are specific for each adsorbent. Using molecular probe molecules hence appears to be a promising method for a complementary porosity analysis of activated carbons.     

NaOH活化法制备煤基活性炭的研究

以焦作无烟煤为原料,NaOH为活化剂,采用化学活化法制备煤基活性炭,分别考察了碱炭比、活化温度和活化时间等工艺参数对活性炭吸附性能和收率的影响;利用低温N2吸附法对活性炭的比表面积、总孔容及孔径分布进行了表征.结果表明,在碱炭比为4,活化温度为750℃和活化时间为1 h的条件下,可以制得比表面积为2 483 m2/g,总孔容为1.41 cm3/g,碘吸附值为2 530 mg/g,亚甲蓝吸附值为418 mg/g的煤基活性炭.     

溶胶凝胶法多孔炭材料的制备及其表征

以十六烷基三甲基溴化胺(CTAB)稳定过的商业硅溶胶为模板硅源、蔗糖为炭前体、运用溶胶凝胶法制备了多孔炭材料。并采用低温N2等温吸脱附、X射线衍射等对材料的结构进行了测试与表征。结果表明:CTAB的加入使所得的多孔炭孔径分布更加集中,由于炭化温度较低,所得的炭材料仍为无定形结构。     

Comparisons of pore properties and adsorption performance of KOH-activated and steam-activated carbons

Carbonaceous adsorbents with controllable pore sizes derived from carbonized pistachio shells (i.e., char) were prepared by the KOH activation and steam activation methods in this work. The pore properties including the BET surface area, pore volume, pore size distribution, and pore diameter of these activated carbons were characterized by the t-plot method based on N₂ adsorption isotherms. Through varying the KOH/char ratios from 0.5 to 3, the KOH-activated carbons exhibited BET surface areas ranging from 731 to 1687 m²/g with a similar micropore content (80–92%). The carbons activated by steam at 830 °C for 2 h had a BET surface area of 821 m²/g with the micropore content of 42%. The micropore/total pore volume ratio (V_micro/V_pore) and average pore size (D_pore) were independent of the KOH/char ratio, revealing that KOH activation is a powerful method in developing and controlling the number of micropores with a very similar pore size distribution. The adsorption equilibria and kinetics of methylene blue, basic brown 1, acid blue 74, 2,4-dichlorophenol, 4-chlorophenol, and phenol from water on all activated carbons at 30 °C were investigated to demonstrate the fact that adsorption of organics is not only dependent upon the BET surface area but is also determined by the relative size between pores and molecules. The adsorption isotherms were subjected to the model fitting according to Langmuir and Freudlich equations. By comparing the projected area of adsorbates, the surface coverage of phenols is about 3.6 times of that of dyes (based on unit gram of activated carbon). The Elovich equation was found to suitably describe the adsorption process of all KOH-activated carbons while the adsorption behavior on the steam-activated carbon was reasonably fitted with the intraparticle diffusion model.     

巨正则系综蒙特卡罗法研究活性炭吸附

近年来,巨正则系综蒙特卡罗计算机模拟在研究炭素材料,特别是活性炭的吸附特性以及炭材料的结构表征和新材料设计方面得到应用。对巨正则系综蒙特卡罗法,目前在活性炭吸附特征研究中所普遍采用的活性炭微孔模型和分子与原子之间相互作用的模型进行较为详细的介绍。同时还给出了作者有关活性炭吸附氮和甲烷的基本特性的研究结果。     

Preparation of porous carbon derived from mixtures of furfuryl resin and glycol with controlled pore size distribution

This paper describes the preparation and properties of porous carbon by a technique which consists of mixing a carbon precursor (furfuryl resin and furfural alcohol), a pore-forming agent and a solvent (glycol), polymerizing the resin mixture, and pyrolyzing the hybrid of resin and glycol. The properties of porous carbons have been systematically investigated as a function of composition and heat treatment, with emphasis on understanding and controlling their morphology and pore size distribution. The results seem to indicate that by varying the ratios of the constituents in the polymer system, porous carbons with a wide variation in pore size distribution and morphology can be obtained. Three types of morphologies were observed: interconnected carbon with secondary spherical pores, discrete carbon particulates, and a crosslinked carbon network. Porous carbons with a very narrow pore size distribution have been obtained and the average pore size was controlled between 5 and 0.008 μm. The microstructure of porous carbon formed as a result of phase separation of resin-rich phase and glycol-rich phase, rather than a result of the pyrolysis process. Heat treatment had little effect on the properties of the porous carbons.     

A self-consistent method to determine accessible volume,area and pore size distribution (APSD) of BPL,Norit and AX-21 activated carbon

Accessible volume, geometrical area and accessible pore size distribution are the fundamental structural parameters in the characterization of porous solids. We provide a novel “inverse” procedure, which is based on the mass balance and an MC optimization scheme, to determine these parameters from the analysis of experimental adsorption isotherms for a number of commonly used activated carbons: BPL, AX-21 and Norit. Our results, based on a kernel of model pores generated from GCMC simulations, are compared with, and shown to be different from, the results obtained from the conventional method. We show that the discrepancies arise from an incorrect evaluation of both micropore and mesopore sizes.     

纳米结构多孔固体在二氧化碳吸附分离中的应用

二氧化碳（CO2）的双重角色（温室气体及一碳化工原料）使其吸附分离研究具有重要学术及社会经济意义。本文以多孔吸附材料为主线,系统评述了多孔炭、分子筛、有机金属骨架类材料及多孔聚合物等的CO2吸附分离最新研究进展。这些吸附材料的特点：多孔炭的微观及宏观形貌可控,孔结构可调,稳定性好;分子筛的具有丰富的微孔,孔径分布集中;有机金属骨架及多孔聚合物的种类多样,代表一类新兴的CO2吸附材料。分析了多孔固体应用于CO2吸附分离所涉及的关键科学问题,即高效吸附材料立体设计及影响选择性和吸附量等重要参数。提出澄清微孔/介孔/大孔比例以及表面基团种类和数量对CO2吸附贡献的定量关系的必要性,对材料的定向合成与优化有重要指导意义。