Investigation of porosity of carbon materials and related effects on gas separation properties

Carbon molecular sieving membranes are chemically robust materials with tailorable gas transport properties for O₂/N₂, CO₂/CH₄ and C₃H₆/C₃H₈ separations. Such carbon materials were formed in this study by the pyrolysis of polyimide precursors. The final pyrolysis temperature was varied to alter the carbon structure, which changed the average pore size. Characterization of the porosity of these materials and how this feature changes when pyrolysis conditions are varied could guide the systematic control of these materials. However, the carbon is an amorphous, microporous material, which makes it difficult to characterize compared to crystalline materials. From separation studies of penetrants on these materials it appears that these materials have both ultramicropores (<7 Å) and larger micropores. The ultramicropores are believed to be mainly responsible for molecular sieving while the micropores provide negligible resistance to diffusion but provide high capacity sorption sites for penetrants. Techniques such as wide angle X-ray diffraction and the analysis of carbon dioxide adsorption isotherms using density functional theory were employed to characterize the microporosity of the material. The small dimensions of the key ultramicropores make accurate determination of their pore size distribution difficult. Therefore, to effectively discuss the differences in transport properties when different pyrolysis temperatures are used as well as penetrants with different dimensions, a hypothetical ultramicropore size distribution was used as a tool to discuss and interpret a combination of parameter effects and trends of separation properties.     

气体分离用管状炭分子筛膜

以无机陶瓷管为支撑体、热塑性酚醛树脂为原料,经高温炭化制备了炭分子筛膜。用低温N2吸附的方法测定了炭分子筛膜的比表面积,用扫描电子显微镜对膜的形貌和厚度进行了表征。考察了膜的气体透过率以及气体的理想选择性随温度的变化关系:H2、CO2、O2、N2和CH4的透过率随温度的升高而增大;理想选择性α(H2/N2)、α(CO2/N2)、α(CO2/CH4)随温度的升高而减小,而α(O2/N2)随温度的升高先增大后减小,在90℃左右气体选择性达到最大。最后由阿累尼乌斯公式计算了气体透过炭分子筛膜的活化能,进一步说明气体透过机理为活化扩散。     

Non-equilibrium molecular dynamics simulation of gas separation in a microporous carbon membrane

We have carried out non-equilibrium molecular dynamics simulations of gas separation in a “selective surface flow” membrane. The gas mixture studied is hydrogen/methane, which is relevant to hydrogen purification in refineries. The simulations give insight into the separation mechanism, which is based on the transport of the more strongly adsorbing species (methane) in a dense layer near the pore wall, with the less strongly adsorbed species (hydrogen) diffusing through a less dense region close to the centre of the pore. Good agreement is obtained with experimental selectivity data. This work is also relevant to the study of the combined effects of adsorption and diffusion in microporous carbon adsorbents.     

Controlling the textural parameters of mesoporous carbon materials

The mesoporous carbon materials prepared by inorganic templating technique using mesoporous silica, SBA-15 as a template and sucrose as a carbon source, have been systematically investigated as a function of sucrose to mesoporous silica composition, with a special focus on controlling the mesoporous structure, surface morphology and the textural parameters such as specific surface area, specific pore volume and pore size distribution. All the materials have been unambiguously characterized by XRD, N₂ adsorption–desorption isotherms, high-resolution transmission electron microscopy, high-resolution field emission scanning electron microscopy, and Raman spectroscopy. It has been found that the porous structure, morphology and the textural parameters of the mesoporous carbons materials, CMK-3-x where x represent the sucrose to silica weight ratio, can be easily controlled by the simple adjustment of concentration of sucrose molecules. It has also been found that the specific surface area of the mesoporous carbon materials systematically increases with decreasing the sucrose to silica weight ratio. Moreover, the specific pore volume of the materials increases from 0.57 to 1.31 cm³/g with decreasing the sucrose to silica weight ratio from 5 to 1.25 and then decreases to 1.23 cm³/g for CMK-3-0.8. HRTEM and HR-FESEM also show a highly ordered pore structure and better surface morphology for CMK-3-1.25 as compared to other materials prepared in this study. Thus, it can be concluded that the sucrose to silica weight ratio of 1.25 is the best condition to prepare well ordered mesoporous carbon materials with good textural parameters, pore structure and narrow pore size distribution.     

炭纤维基复合吸附材料的制备及其气体分离性能研究

活性炭纤维基复合炭分子筛材料作为一种新型的吸附材料,具有丰富而发达的微孔结构、大的比表面积和优异的导电性能,这些性质使其具有独特的气体分离性能。本文以沥青基活性炭纤维和酚醛树脂为原料制备复合炭分子筛材料,用扫描电镜和氮吸附等温线法研究其外观型态和孔隙结构,在变电吸附工艺条件下以CO2为探针考察了此类复合吸附材料的分离性能。     

生物质热解特性研究

以竹材、稻壳、木屑为原料,通过常规热解结合快速热解研究生物质热解特性。结果表明,生物质常规热解的液体得率较低,相比而言竹材最高,稻壳最低,且热解温度是影响竹材和木屑热解的主要因素,其液体得率随温度的升高呈先增后减的变化规律;快速热解方面,利用居里点裂解仪和GC—MS在线分析竹材热解的液相组成,其组成以糠醛和酚类物质为主,它们分别来源于纤维素、半纤维素和木质素的热解。     

Molecular dynamics simulations of transport and separation of supercritical carbon dioxide-alkane mixtures in supported membranes

The results of extensive nonequilibrium molecular dynamics (MD) simulations of flow and transport of several binary mixtures of CO₂ and an n-alkane chain, from CH₄ to C₄H₁₀, through a model porous membrane composed of three pores in series with significantly different sizes and in the presence of an external pressure gradient, are reported. The technique that we use for the simulations is a combination of the configurational-bias Monte Carlo method (used for efficient generation of molecular models of n-alkane chains) and the dual control-volume grand-canonical MD method. The selectivity of the membrane changes qualitatively as the length of the alkane chain increases, resulting in high separation factors in favor of the alkanes. Moreover, we find that, under supercritical conditions, unusual phenomena occur that give rise to direction- and pressure-dependent permeabilities for the fluids. The results, which are also in agreement with a continuum formulation of the problem, indicate that the composite nature of the membrane gives rise to the direction-dependent permeabilities. Hence, modeling flow and transport of supercritical fluid mixtures in porous materials with the type of morphology considered in this paper (such as supported porous membranes) would require using effective permeabilities that depend on both the external pressure drop and the direction along which it is applied to the materials.     

Fabrication of suspended carbon microstructures by e-beam writer and pyrolysis

A fabrication process has been developed to create suspended carbon microelectromechanical system (C-MEMS) structures. SU-8, a negative photoresist, was used as the starting material and was converted to the desired carbon microstructures using pyrolysis in an inert atmosphere. Suspended carbon-micro and nano electromechanical system (C-MEMS/NEMS) structures with feature sizes down to 500 nm were fabricated by ultra violet/electron beam (EB) lithography and pyrolysis. The problem of charging of the non-conductive SU-8 surface was solved by partially masking a thin metal layer to prevent the repulsion of negative charged electrons before EB writing. Complex suspended C-MEMS structures, such as bridges and networks have been formed. This fabrication method can accurately and reproducibly produce various suspended C-MEMS structures which have applications in microelectronics and biosensing.     

The investigation of the effects of two different polymers and three catalysts on pyrolysis of hazelnut shell

This study covers an investigation of three catalyst candidates namely; calcium carbonate (CaCO₃), Perlite and potassium dichromate (K₂Cr₂O₇) on pyrolysis of mixtures of hazelnut shell (HS) with ultra-high molecular weight polyethylene (UHMWPE) and polyethylene oxide (PEO) by measuring percentages of solid, liquid and gas products amounts gravimetrically. Pyrolysis processes were executed in a batch type tubular reactor at 500 and 650 °C for fifteen minutes. Catalysts have behaved differently on compounded mixtures. Especially the obtained liquid products from pyrolysis of mixtures shows smooth distribution compared to anomalous results of pure raw materials. The effect of catalysts on the pyrolysis of HS becomes more obvious in the presence of polymers which produces more gaseous product for K₂Cr₂O₇ case. The other catalysts (CaCO₃, Perlite) are more effective without addition of polymers which produce more liquid products. The ratio of HS should not be beyond the 0.5 for production of commercial materials and in their possible recycling process such as pyrolysis for acquisition of combustible liquids. These results may allow some clues for compounding of environmentally safe commercial materials, and easy and efficient recycling technologies.     

Effect of gas environment on properties of particles prepared by spray pyrolysis of metal nitrates

Particles in different states of oxidation were prepared by spray pyrolyses of aqueous solutions of silver and copper nitrates under different gas environments: air, nitrogen or mixture gas of 10 vol%H₂-N₂, respectively. Silver nitrate was converted to phase-pure silver at temperatures below 500 °C whose densification and crystallization were completed around 500 °C, irrespective of the gas environment. On the other hand, phase-pure copper(II) oxide was formed from copper nitrate up to 1,000 °C with air, but below 800 °C with nitrogen, above which copper(I) oxide was produced. Phase-pure copper particles were obtained with the mixture gas at temperatures above 400 °C. Copper(I) oxide was sintered and crystallized more easily than copper(II) oxide. The rates of the metallization, sintering and crystallization of copper were between those of silver and nickel.     

气体膜分离混合气中二氧化碳的研究进展

气体膜分离技术作为碳捕获方案被国际社会认为是最有发展潜力的脱碳方法之一.综述介绍了中空纤维膜接触器、膜结构、系统工艺和吸收剂的研究现状.相对于水和碳酸盐类,醇胺具有的二氧化碳吸收率高、反应热低、反应速度快以及容易再生等优点,在研究与工业过程中是应用最广泛的吸收剂之一.     

负载铁SBA-15分子筛催化热解甲醇制备碳纳米管

采用水热合成法制备了SBA-15,利用液相浸渍法合成负载铁SBA-15介孔分子筛,在常压下用所合成的含铁SBA-15介孔分子筛为催化剂通过CVD法成功制备了碳纳米管。采用X射线粉末衍射、扫描电子显微镜、拉曼光谱等方法对合成的样品进行了表征。且合成的碳纳米管质量好,表面没有无定形碳粒子被发现。     

Carbon molecular sieve gas separation membranes-I. Preparation and characterization based on polyimide precursors

Carbon molecular sieving membranes are new, high-performance materials for gas separations. The selectivities of these membranes are much higher than those typically found with polymeric materials and the selectivities are achieved without sacrificing productivity. Ultramicroporous carbon membranes can be produced by pyrolyzing various thermosetting polymeric materials under a variety of pyrolysis conditions. The membranes described in this paper were produced from the pyrolysis of a hollow-fiber polyimide precursor under conditions found to yield membranes with good air separation properties. Membranes were produced by two different temperature protocols, and were evaluated with mixed gas feeds at pressures ranging up to 200 psig (1.48 MPa). The lower temperature protocol yielded membranes with selectivities ranging from 8.5 to 11.5, and a higher temperature pyrolysis yielded membranes with selectivities ranging from 11.0 to 14.0. These membranes were found to be quite stable over time periods of several days with high-purity, dry feeds. Limited studies also showed that these membranes were highly effective for the separation of other mixed gas pairs, including

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Hollow porous carbon microspheres obtained by the pyrolysis of TiO2/poly(furfuryl alcohol) composite precursors

Disordered carbon materials with high porosity were prepared through the pyrolysis of TiO₂/poly(furfuryl alcohol) composites, obtained by the sol-gel method. The composites were prepared starting from titanium tetra-isopropoxide (TTIP) and furfuryl alcohol (FA) as precursors. Two different synthetic procedures for our composites were carried out, based on the addition of furfuryl alcohol (FA) before or after the TiO₂ nanoparticles formation. Also, different TTIP/FA ratio was tested. The hybrid materials obtained by both synthetic routes were pyrolyzed, under argon flow, at 900 °C producing novel TiO₂/carbon composites. All samples were characterized by XRD, FT-IR, DR-FTIR, Raman spectroscopy and TEM. Results indicated the effective FA polymerization on TiO₂ (anatase) nanoparticles, and polymer conversion to disordered carbon after the pyrolysis, simultaneously with TiO₂ anatase-rutile phase transition. The resulting TiO₂/carbon composites were treated with HF solution aiming the oxide dissolution, yielding an extremely porous carbon material as insoluble fraction. The morphology of these porous carbon materials is strongly dependent on the synthetic route adopted for the composite precursor, varying from carbon foam to highly ordered hollow microspheres.     

Fabrication of carbon membranes for gas separation--a review

Carbon membrane materials are becoming more important in the new era of membrane technology for gas separation due to their higher selectivity, permeability and stability in corrosive and high temperature operations. Carbon membranes can be produced by pyrolysis of a suitable polymeric precursor under controlled conditions. This paper reviews the fabrication aspects of carbon membranes, which can be divided into six steps: precursor selection, polymeric membrane preparation, pretreatment of the precursor, pyrolysis process, post-treatment of pyrolyzed membranes and module construction. The manipulation of the pretreatment variables, pyrolysis process parameters and post-treatment conditions were shown to provide an opportunity to enhance the separation performance of carbon membranes in the future. By understanding the available methods, one can choose and optimize the best technique during the fabrication of carbon membranes. Furthermore, areas of future potential in carbon membrane research for gas separation were also briefly identified.     

n-C7H16/CCl4热解处理对粘胶基碳纤维性能的影响

采用浸渍热解法,以n-C_7H_(16)/CCl_4为热解质,讨论浸渍液体积比、热解温度等对粘胶基碳纤维(RCF)拉伸性能的影响。结果表明,经热处理后纤维线密度降低,表面缺陷减少,当n-C_7H_(16)/CCl_4为2:3(体积比),热解温度900～1100℃时,碳纤维拉伸强度可提高17％。用韦氏统计理论对强度分布进行分析.证实经热解处理后,RCF强度分布更集中,均匀性更好。     

碳分子筛变压吸附空气分离活性及表征

石油焦基活性炭经Ni(NO3)2溶液室温浸渍过夜,红外灯缓慢烘干(Ni载持量5%),再经1-丁烯沉积处理,制备了催化炭沉积碳分子筛S-CMS,探讨了空气进料流量、压力和再生时间对碳分子筛空分效果的影响。结果表明,当流量为10~50 mL/min时,对空气分离效果影响很小;当吸附压力增加时,氮气浓度应该是增加的。脱附时间大于60 s时,S-CMS能够被再生。与商用T1-CMS比较,S-CMS是优秀的,在常温常压下,可获得浓度为93%的富氮气体。通过对碳分子筛表面结构的FT-IR、骨架结构-XRD及其氧化性in-situ TG/DTA的测试显示,S-CMS具有微孔分布独特的骨架网络结构,是一种潜在的优越变压吸附剂。     

Porous amorphous carbon models from periodic Gaussian chains of amorphous polymers

An algorithm has been developed to create structural models for amorphous carbons using Monte Carlo simulations in the canonical ensemble. The simulation method used follows the experimental preparation of nanoporous carbons (NPC) by pyrolysis from polyfurfuryl alcohol as a guideline. The resulting structure exhibits properties that compare favorably to those observed experimentally for real NPCs. These atomistic NPC models are approaching a realistic representation of NPCs used for gas separations and as such, are being used to study the diffusion of small gas molecules in these materials. Limitations of the method and possible improvements are discussed.     

Control of mesoporous properties of carbon cryogels prepared from wattle tannin and furfural

Wattle tannin–furfural (TFu) carbon cryogels are synthesized by sol–gel polycondensation of wattle tannin with furfural by using sodium hydroxide (NaOH) as a catalyst, dried by freeze-drying technique and then pyrolyzed under inert atmosphere, respectively. The amounts of wattle tannin (T), furfural (Fu), NaOH (C) and distilled water (W) are changed for preparing the mesoporous TFu carbon cryogels. The mole ratio of tannin to catalyst T/C plays a crucial role for the synthesis of TFu organic and carbon cryogels. The results suggest that the T/C ratio should be above 0.25 but <1.0 to prepare the mesoporous and homogeneous cryogels. Although TFu carbon cryogels have the broad mesopore size distribution, the mesoporous structure is controllable by the synthesis conditions. The carbon cryogels possess the mesopore volume less than 0.56 cm³/g and the BET surface area less than 600 m²/g. Moreover, the ratio of catalyst to water C/W can be used to prepare the homogeneous and mesoporous carbon cryogels, and to control the mesopore radius of carbon cryogels in the range of 1.6–9.6 nm.