Evidence for selectivity of absorption of volatile organic compounds by a polydimethylsiloxane solid-phase microextraction fibre

Solid-phase microextraction using a 30 microns polydimethylsiloxane fibre has been used to sample the volatile organic compounds from standard mixtures and from mixtures produced by the decomposition of organic compounds. This method of sampling has been compared with the direct injection of an aliquot of headspace gas and shows an enrichment factor of approximately 100 over a 1 ml gas injection for organosulphur gases such as dimethyldisulphide. The performance of the fibre has been evaluated with respect to accuracy and precision at several concentrations in representing the composition of multicomponent mixtures. It was found that the presence of a second component in a gas sample reduced the capacity of the fibre to absorb the primary component. The selectivity of the fibre for various volatile compounds with differing functionality was also studied. It was found that the non-polar polydimethylsiloxane fibre preferentially absorbed the non-polar components of a mixture, e.g. nonane and, correspondingly, under reported the more polar components, e.g. ethanol. Hence, the fibre discriminates in favour of non-polar and against polar components in a mixture in comparison with direct analysis of a headspace sample. Thus, quantitation of a component in a multi-component mixture is liable to error from competitive interference from other components. A major advantage of the technique, however, is that it does not absorb, and therefore introduce, water into the analytical system.     

活性炭纤维吸附脱除硫化氢研究进展

从活性炭纤维(ACF)的结构特点、未改性ACF吸附H2S和改性ACF吸附H2S三方面综述了国内外对ACF吸附H2S的研究进展。归纳了ACF的改性方法、ACF吸附H2S的反应机理和动力学研究。总结了相对湿度、原料气组成、生成物和改性方法等工艺条件对ACF吸附H2S性能的影响。展望了改性ACF吸附脱除H2S的研究前景。     

Multicomponent gas separation by selective surface flow (SSF) and poly-trimethylsilylpropyne (PTMSP) membranes

A selective surface flow (SSF) membrane consisting of a thin layer of a nanoporous carbon was produced in a tubular form using a macroporous alumina support. The membrane was tested for hydrogen enrichment applications. Simulated waste gases from a petrochemical refinery and a hydrogen pressure swing adsorption unit were used as the feed gas to the membrane. Very high rejections of C₁C₃ hydrocarbons (saturated and unsaturated) and carbon dioxide over hydrogen were exhibited by the membrane at low feed gas pressures. The hydrogen enriched stream was produced at the feed gas pressure.The separation characteristics of a polymeric poly-trimethylsilylpropyne (PTMSP) membrane in a tubular form was also tested for the same applications using identical conditions of operation. This membrane also selectively rejected heavier components of the feed gas mixture over hydrogen and produced the hydrogen enriched stream at the feed gas pressure. The SSF membrane exhibited much higher hydrogen recovery and hydrocarbon rejections than the PTMSP membrane for these applications under identical conditions of operations using identical support materials.     

Preparation and Characterization of Adsorption-Selective Carbon Membranes for Gas Separation

The preparation and characterisation of adsorption-selective carbon membranes (ASCMs) is described. ASCMs can separate the components of a gas mixture depending on their adsorption strength. These membranes allow the separation of non-adsorbable or weakly adsorbable components (e.g. N₂, H₂, O₂, etc) from the more strongly adsorbable components (e.g. hydrocarbons) in a gas mixture. They are prepared from the deposition of a thin film of a phenolic resin on the inner face of an alumina tube. Air oxidative treatment at temperatures in the range of 300–400°C, prior to carbonisation (pre-oxidation) or after carbonisation (under vacuum at 700°C) (post-oxidation) gives rise to an adsorption-selective carbon membrane. This membrane shows a high permeability and selectivity towards the separation of gas mixtures formed by hydrocarbons and N₂. Taking into account the permeation and separation properties of the membranes, post-oxidation treatment is shown to be more effective than pre-oxidation. The separation characteristics of the carbon membranes are dependent on the composition of the gas mixture (i.e. proportion of more strongly adsorbable components) and temperature.     

Preparation and characterisation of SrCe0.95Yb0.05O2.975 hollow fibre membranes

A mixed proton and electronic conducting hollow fibre membrane, SrCe_0.95Yb_0.05O_2.975 (SCYb), has been prepared by spinning a polymer solution containing suspended SCYb particles to a hollow fibre precursor, which is then sintered at elevated temperatures. The SCYb powders having a sub-micron size, i.e. an essential size for fabrication of the hollow fibre with good mechanical strength, were synthesised through a polymerised water-soluble complex method. By controlling weight ratio of the SCYb ceramic powder to the polymer binder and sintering temperatures, the SCYb ceramic hollow fibres with gas-tight properties can be prepared. Some primary factors affecting microstructures, gas tightness and mechanical strength of the mixed conducting hollow fibre membranes were studied in details.     

An investigation of the high gas permeability of poly (1-Trimethylsilyl-1-Propyne)

The permeability of poly (1–trimethylsilyl–1−propyne), PMSP, to light gases is higher than that of any other nonporous synthetic polymer at ambient temperatures. PMSP is in the glassy polymer state at these temperatures. Permeability, diffusion, and solubility coefficients were determined for N₂, O₂, CH₄, and CO₂ in PMSP, and are compared with values reported for these gases in poly (dimethyl siloxane). The higher gas permeability of PMSP results primarily from a substantial gas solubility, which appears to be due, in turn, to a large “excess” free volume in the unrelaxed (Langmuir) domains of this glassy polymer. The structure of PMSP, which consists of relatively rigid backbone chains separated by bulky trimethylsilyl side groups, probably is responsible for this large free volume.     

Ceramic asymmetric hollow fibre membranes—One step fabrication process

Ceramic hollow fibre membranes which have an asymmetric structure have been prepared in one step, using an immersion induced phase inversion technique. With this method, membranes with a high surface area per unit volume ratio can be produced, while production cost is dramatically reduced. Yttria-stabilised zirconia (YSZ) is selected as a membrane material, as it is relatively inexpensive and has superior mechanical strength as well as oxygen ion conducting properties. Therefore, both the porous and non-porous membranes prepared from the YSZ have potential applications. For example, the porous YSZ membranes can be used for fluid separations in harsh environments where normal polymeric membranes cannot be sustained, while the non-porous YSZ membranes can be applied as a solid electrolyte in electrochemical devices such as solid oxide fuel cells, oxygen pumps and chemical gas sensors.Gas permeation analysis suggests that non-porous YSZ hollow fibre membranes can be prepared at sintering temperature of 1400 °C or greater, below which the membrane contains pores. Pore sizes of the YSZ porous membrane prepared fall into the pore size range of ultrafiltration membranes. However, the surface porosities of the membranes prepared from two-population sized particles at sintering temperatures of 1200 °C and 1400 °C are around 5000 m⁻¹ and 300 m⁻¹, respectively. The former is comparable to polymeric membranes, while the latter is an order of the magnitude smaller.     

Gas mixture flow in nanoporous media in the presence of surface forces. The dusty-gas model

The dusty-gas model has been generalized to the case of gas mixture flow in nanoporous media under the conditions of the action of surface forces. A basic set of transport equations has been derived proceeding from kinetic equations for a gas mixture and dust particles. To take into account the surface forces, the interaction between a gas and dust particles has been represented as a sum of a long-range potential, which reflects the surface forces, and a short-range potential, which describes gas molecule scattering on the surface of pore walls. The contribution of the long-range component has been taken into account in the self-consistent approximation, while the short-range component has been considered in the standard manner. The surface forces have been shown to have a substantial effect on the transfer of mixed gases through porous bodies; in particular, it becomes possible to separate mixture components due to different potentials of the interaction of their molecules with pore surface.     

Thermal curing of mesophase pitch: An alternative to oxidative stabilisation for the development of carbon–carbon composites

Thermal curing of mesophase pitch was studied as an alternative to oxidative stabilisation for the development of carbon–carbon composites, with the aim of avoiding several problems associated to the oxidative process such as thickness limitations or gradients in the properties of the resultant materials. Carbon fibre preforms densified with the mesophase were submitted to thermal treatments at temperatures between 400 and 475 °C for different periods in order to promote polymerisation and thus avoid exudation of the matrix precursor during carbonisation. Changes induced in the matrix precursor were monitored by thermogravimetric analysis and infrared spectroscopy. The effectiveness of the treatments was evaluated from the porosity of the resultant materials after carbonisation. The highest degree of polymerisation of the matrix precursor was achieved with treatments at 400 °C for 24 h or 475 °C for 5 h, the resultant materials having similar porosity values to those obtained by oxidative stabilisation.     

Effect of the preparation conditions on the formation of asymmetric poly(vinylidene fluoride) hollow fibre membranes with a dense skin

Integrally skinned asymmetric poly(vinylidene fluoride) hollow fibre membranes were prepared and characterized. The effects of phase inversion methods (dry-wet or wet) and spinning conditions, such as the type of solvent (NMP, DMAc), the concentration of polymer in dope solution, temperature of the external coagulation bath and the composition of the inner coagulant on the morphology and on the formation of a dense skin layer were investigated. The structure of the membranes was analyzed by scanning electron microscopy and the gas permeation properties with six different gases (He, H₂, N₂, O₂, CH₄ and CO₂) were measured at 25 °C to confirm the integrity of the selective skin layer. Under the proper conditions highly selective and permeable PVDF hollow fibre membranes were thus obtained by dry-wet spinning of a 30 wt.% PVDF solution in DMAc, using hot water (50 °C) as the external coagulant and a bore fluid of pure water as the internal coagulant. The best membrane had a selective outer skin with an effective thickness of approximately 0.2 μm. The ideal selectivity of the hollow fibres approached or even exceeded the intrinsic ideal selectivity of a dense PVDF film, for instance the selectivity for He over N₂ was 86.2 for the hollow fibre, whereas it was 83.5 for a dense PVDF reference film. DSC and FT-IR/ATR analysis indicated a higher fraction of the β-crystal phase in the selective skin and a high overall crystallinity than in the melt-processed film. The latter explains the relatively high selectivity and low permeability of the membranes. Intrinsic polymer properties make the membranes also suitable for vapour transport than for gas separation.     

Supercritical hydrogen adsorption in nanostructured solids with hydrogen density variation in pores

Experimental excess isotherms for the adsorption of gases in porous solids may be represented by mathematical models that incorporate the total amount of gas within a pore, a quantity which cannot easily be found experimentally but which is important for calculations for many applications, including adsorptive storage. A model that is currently used for hydrogen adsorption in porous solids has been improved to include a more realistic density profile of the gas within the pore, and allows calculation of the total amount of adsorbent. A comparison has been made between different Type I isotherm equations embedded in the model, by examining the quality of the fits to hydrogen isotherms for six different nanoporous materials. A new Type I isotherm equation which has not previously been reported in the literature, the Unilan-b equation, has been derived and has also been included in this comparison study. These results indicate that while some Type I isotherm equations fit certain types of materials better than others, the T?th equation produces the best overall quality of fit and also provides realistic parameter values when used to analyse hydrogen sorption data for a model carbon adsorbent.     

Adsorption of Acid Orange II from Aqueous Solution by Plasma Modified Activated Carbon Fibers

As a main composition of dye wastewater, organic pollutant which has a negative effect on the environment can be effectively removed by active carbon adsorption. In the present work, activated carbon fiber (ACF) was modified by a novel modification technology, gilding arc discharge, while its adsorption capacity was studied with the acid orange II (AO II) solution selected as the target wastewater. Several factors, such as air flow rate, distance between samples and the discharge area, pH of the solution and plasma treating time, were investigated with respect to their effects on properties of the plasma-treated ACF, in terms of texture characteristic, surface chemical compositions and adsorption capacities. The results showed that the specific surface area and pore volume of ACF decreased after the plasma treatment, while the amounts of oxygen-containing functional groups on the surface of ACF increased compared with the raw ACF. Moreover, it was observed that the adsorption capacity of the modified ACF was improved by nearly 20.9 %, which was beneficial to the industrial application.     

Large-Scale and Controllable Syntheses of Covalently-Crosslinked Poly(ionic liquid) Nanoporous Membranes

Herein, we report an exciting synthetic procedure for the scalable and controllable fabrication of covalently crosslinked poly(ionic liquid) (PIL) nanoporous membranes (CPILMs) in water solution under ambient conditions. We found that the pore sizes, flexibility and compositions of freestanding CPILMs can be finely tailored by a rational structural choice of PIL, diketone and aldehyde. Studies on the CPILM formation mechanism revealed that hydrogen bonding-induced phase separation of amino-functionalized homo-PIL between its polar and apolar domains coupled with structural rearrangements due to the Debus Radsizewski reaction-triggered ambient covalent crosslinking process created a stable three-dimensionally interconnected pore system in water solution. Employing structurally stable CPILMs in ion sieving devices resulted in an excellent Li⁺/Mg²⁺ separation efficiency due to the positively charged nature and “Donann” effects. This green, facile yet versatile approach to the production of CPILMs is a conceptually distinct and commercially interesting strategy for making useful nanoporous functional polyelectrolyte membranes.     

Application of novel activated carbon fiber solid-phase, microextraction to the analysis of chlorinated hydrocarbons in water by gas chromatography-mass spectrometry

This paper presents a study on the performance of activated carbon fiber (ACF) used as extraction fiber for solid-phase microextraction (SPME) and its application for analysis of chlorinated hydrocarbons in water. By means of evaluating scanning electron microscope (SEM) images, specific surface area, pore volume, pore distribution, and properties of adsorption and desorption, the optimal active concentration of phosphoric acid has been determined. Coupled with gas chromatograph-mass spectrometry (GC-MS), ACF-SPME is suitable for determination chlorinated hydrocarbons in water with headspace. Experimental parameters such as adsorption and desorption conditions were studied. The optimized method has an acceptable linearity, good precision, with R.S.D. values <10% for each compound. Compared with commercial fibers, ACF has many advantages such as better resistance to organic solvents, better endurance to high temperature and longer lifetime.     

Electrochemistry at nanoporous interfaces: new opportunity for electrocatalysis

Physical and electrochemical features of nanoporous electrodes arising from their morphology are presented in this perspective. Although nanoporous electrodes have been used to enhance electrocatalysis for several decades, the origin of their capability was understood on the basis of enlarged surface area or crystalline facet. However, considerable attention should be paid to the fact that nano-confined space of nanoporous electrodes can significantly affect electrochemical efficiency. Molecular dynamics in nano-confined spaces is capable of offering much more chances of interaction between a redox molecule and an electrode surface. The mass transport in the nanoporous electrode depends on various pore characteristics such as size, shape, charge, connectivity, and symmetry as well as molecular properties such as size, charge, and kinetics. Moreover, when the pore size is comparable to the thickness of an electric double layer (EDL), the EDLs overlap in the porous structure so that electrochemically effective surface area is not the same as that of the real electrode surface. These unique properties come from simply nanoporous structure and suggest new opportunity to innovative electrocatalysts in the future.     

Development of a headspace-solid phase micro extraction method to monitor changes in volatile profile of rose (Rosa hybrida, cv David Austin) petals during processing

In the present study, headspace solid phase microextraction combined to capillary gas chromatography (HS-SPME-GC) has been applied for the determination of changes in the volatile profile of rose petals (Rosa hybrida, cvs David Austin) following processing (heat treatment and addition as an ingredient to a food product--for example yoghurt). Four SPME fibres at two sampling temperatures (40 and 60 degrees C) with a sampling time of 30 min were examined. Volatile profiles were detected either by FID or/and by olfactometry (ODP-II, Gerstel). Fibre testing was performed using raw rose petals for sampling temperature selection and an 18 characteristic rose volatile standard mixture in water was used to compare fibre performances at the sampling temperature of 60 degrees C. Polydimethylsiloxane-divinylbenzene (PDMS-DVB) fibre at the sampling temperature of 60 degrees C was the most suitable to sample the rose alcohols phenyl ethanol, citronellol, nerol, geraniol and eugenol, as assessed by GC-olfactometry, not only from raw petals, but also from processed rose petals and the food product. PDMS-DVB fibre also showed a desired low affinity to volatiles from yoghurt, which reduces the influence of food matrix on the volatile profile. The method was linear over two orders of magnitude and had satisfactory repeatability, with limits of detection for the rose alcohols ranging from <1 to 10 ng/ml concentration levels.     

Efficient One‐Pot Synthesis of Bis(4‐hydroxycoumarin)methanes in the Presence of Sulfonic Acid Functionalized Nanoporous Silica (SBA‐Pr‐SO3H)

An efficient and simple method for the synthesis of bis(4‐hydroxycoumarin)methanes has been achieved through a one‐pot condensation of aryl aldehydes and 4‐hydroxycoumarin in the presence of nanoporous solid acid catalyst of SBA‐Pr‐SO₃H with pore size of 6 nm. Excellent yields, short reaction times, reusability of the catalyst and simple workup are advantages of this synthetic procedure.     

Immobilization of flower-like ZnO on activated carbon fibre as recycled photocatalysts

Research on Chemical Intermediates - ZnO was grown on the surface of activated carbon fibre (ACF) by a hydrothermal method to form a recyclable photocatalyst. With an increase in hydrothermal time...     

Cavitation and pore blocking in nanoporous glasses

In gas adsorption studies, porous glasses are frequently referred to as model materials for highly disordered mesopore systems. Numerous works suggest that an accurate interpretation of physisorption isotherms requires a complete understanding of network effects upon adsorption and desorption, respectively. The present article deals with nitrogen and argon adsorption at different temperatures (77 and 87 K) performed on a series of novel nanoporous glasses (NPG) with different mean pore widths. NPG samples contain smaller mesopores and significantly higher microporosity than porous Vycor glass or controlled pore glass. Since the mean pore width of NPG can be tuned sensitively, the evolution of adsorption characteristics with respect to a broadening pore network can be investigated starting from the narrowest nanopore width. With an increasing mean pore width, a H2-type hysteresis develops gradually which finally transforms into a H1-type. In this connection, a transition from a cavitation-induced desorption toward desorption controlled by pore blocking can be observed. Furthermore, we find concrete hints for a pore size dependence of the relative pressure of cavitation in highly disordered pore systems. By comparing nitrogen and argon adsorption, a comprehensive insight into adsorption mechanisms in novel disordered materials is provided.     

The Characteristic Detection of Xenon by Photoionization Detector and the Dynamic Absorption of Xenon by Activated Carbon Fiber

Activated carbon fiber (ACF) is a new type absorber which has been developed since 1960's.ACF has many characteristics different from active charcoal such as high specific surface,narrow pore distribution, high adsorption capacity and adsorption efficiency. Photoionazition detector (PID) is a new type detector, which is applied, in chromatographic analysis in recent years, the detection limit of PID is high and the selectivity of PID is fine.