Adsorption of sodium dodecylsulfate on modified carbon adsorbents

The adsorption of anionic surfactants on carbon adsorbents modified with water-soluble derivatives of natural polymers, cellulose and chitin, is considered with sodium dodecylsulfate taken as an example. It is shown that such modification leads to changes in the adsorption structural characteristics and the particle size distribution of carbon-water suspensions of the original adsorbent, and to the emergence of new functional groups on its surface that are able to interact selectively with adsorbate molecules. It is assumed that adsorption of anionic surfactant on carbon adsorbents under equilibrium conditions proceeds via stepwise filling of the carbon??s porous structure: we first observe volume filling of micropores according to their sizes, and then the formation of a surfactant??s monolayer in mesopores and on the outer surface of the adsorbate. It is established by thermal analysis that the thermal stability of carbon adsorbents is enhanced through the preferential localization of anionic surfactants in micropores. The filling of mesopores and the outer carbon surface by surfactant molecules leads to a regular decrease in thermal stability and an increase in the adsorbent surface??s hydrophilicity.     

Kinetics of salicylic acid adsorption on activated carbon

The adsorption and desorption of salicylic acid from water solutions was investigated in HPLC microcolumns packed with activated carbon. The adsorption isotherm was obtained by the step-up frontal analysis method in a concentration range of 0-400 mg/L and was well fitted with the Langmuir equation. The investigation of rate aspects of salicylic acid adsorption was based on adsorption/desorption column experiments where different inlet concentrations of salicylic acid were applied in the adsorption phase and desorption was conducted with pure water. The concentration profiles of individual adsorption/desorption cycles data were fitted using several single-parameter models of the fixed-bed adsorption to assess the influence of different phenomena on the column behavior. It was found that the effects of axial dispersion and extraparticle mass transfer were negligible. A rate-determining factor of fixed-bed column dynamics was the kinetics of pore surface adsorption. A bimodal kinetic model reflecting the heterogeneous character of adsorbent pores was verified by a simultaneous fit of the column outlet concentration in four adsorption/desorption cycles. The fitted parameters were the fraction of mesopores and the adsorption rate constants in micropores and mesopores, respectively. It was shown that the former rate constant was an intrinsic one whereas the latter one was an apparent value due to the effects of pore blocking and diffusional hindrances in the micropores.     

Effect of chemical modification on the adsorption and chromatographic properties of aluminas

The effect of chemical modification by sodium hydrogen carbonate on the adsorption and chromatographic properties of aluminas was studied by low-temperature nitrogen adsorption followed by thermal desorption, IR spectrometry, and gas chromatography. It was found that, at a decrease in the volume of micropores, adsorption and desorption proceed in mesopores. It was shown that, at the higher chemical homogeneity of the alumina surface, the symmetry of the chromatographic peaks and the efficiency of the column are considerably improved.     

Synthesis of mesoporous zeolite a by resorcinol-formaldehyde aerogel templating

Mesoporous zeolite A has been synthesized by using a template method with resorcinol-formaldehyde aerogels having three-dimensional mesopores. It was characterized with X-ray powder diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, field emission scanning electron microscopy, thermogravimetric analysis, and nitrogen adsorption/desorption. The pore size distribution calculated from the nitrogen adsorption isotherm is a bimodal distribution with micropores and mesopores. Field emission scanning electron micrograph observations confirm the presence of mesopores.     

Availability and interconnectivity of pores in mesostructured ZSM-5 zeolites by the adsorption and diffusion of mesitylene

Probing the mesopore architecture in mesoporous zeolites is of importance for large scale applications of the materials. In this work, the adsorption and diffusion of mesitylene with larger molecule size in mesoporous ZSM-5 zeolites were carried out, in order to acquaint the availability and interconnectivity of mesopores in zeolite crystals. The comparisons of the shape of adsorption isotherms and the mesopore volume calculated from mesitylene and N₂ adsorption in mesoporous ZSM-5 zeolites with different mesoporosities can be used to discriminate two cases of mesopores: accessible mesopores connected to external surface of the zeolite crystals and non-accessible meso-voids that are occluded in the microporous matrix. Furthermore, the effective diffusivity and activation energy of mesitylene in mesoporous ZSM-5 extracted from ZLC desorption curves as a function of mesopore volume calculated from mesitylene adsorption reveal the enhancement of mesopore interconnectivity to molecule diffusion in zeolite crystals.     

Surface functionalization of ordered mesoporous carbons--a comparative study

Hexagonally structured mesoporous carbons C15 and CMK-5 and cubically structured carbon C48 were synthesized using ordered silica SBA-15 and MCM-48 as templates and carbon precursors of different structures. The surfaces of these ordered carbons were chemically functionalized by employing an approach, in which the selected diazonium compounds were in situ generated and reacted with the carbon frameworks of the mesoporous carbons. The aromatic organic molecules containing chlorine, ester, and alkyl groups were covalently attached to the surface of these ordered mesoporous carbons. The presence of functional groups on the modified carbons was confirmed with Fourier transform infrared spectroscopy, thermogravimetric analysis, and nitrogen adsorption. The BET-specific surface area and the pore width of ordered carbons were significantly reduced, whereas the primary structure of these ordered carbons and their unit cells were intact. Basically, the density of grafted functional groups is related to the specific surface area of the sample, particularly the surface area of mesopores. The surface functionalization reaction takes place only on the external surface of carbon C15, while it occurs on both of the internal and external surface of CMK-5 carbon with the nanopipe structure. The presence of the micropores in CMK-5 carbon should be responsible for its lower grafting density because the small micropores are inaccessible in the reaction. It was also proposed that the preferred adsorption/reaction in C48 may be related to the observed unsymmetrical degradation of the XRD patterns for the functionalized C48 samples. The chemical modification process considerably reduced the primary mesopores in these ordered carbons by approximately 1-1.5 nm, affording carbons with micropores in the cases of C15 and C48, and mixed micropores and small mesopores in the case of CMK-5. A grafting density of approximately 0.9-1.5 micromol/m(2) was achieved under current research.     

高层间距氧化铝柱层状钛酸盐的制备和表征

层往化合物因其具有独特的大分子吸附和催化等性能而受到人们的高度重视．至今，人们对层柱粘土和四价金属磷酸盐的制备和表征已作了比较详细和深入的研究k一句，但对一系列以八面体骨架结构为基；出的层状金属氧化物的“插x化学”（intercalationcheAnstry）的研究相对较少，且大部分。作都局限在用有机胺来支撑此类层状金属氧化物［‘－’］由于层间的有机物不耐高温，此类层柱材料在各方面（尤其是催化方面）的应用就受到了很大的限制．合成高热稳定性和大层间距的层柱金属氧化物一直是该领域的研究重。点之一．同层状粘土不同，这类层…     

Effect of surface heterogeneity on adsorption on solid surfaces. Application of inverse gas chromatography in the studies of energetic heterogeneity of adsorbents

The paper presents a literature review of the chromatographic methods used for investigations of the heterogeneity of solid surfaces. Special attention is paid to inverse gas chromatography (IGC). Quantitative characteristics of heterogeneity of real solid surfaces including extreme models on adsorption centre topography of the "patch-wise" and "random" types are described. Analytical and numerical methods used for calculating the adsorption energy distribution function as a quantitative measure of surface heterogeneity are presented. Special attention is paid to the condensation approximation as well as to other approximations based on this assumption. IGC is presented as a quick, precise and effective method to characterise physicochemical properties of different kinds of adsorbents. Advantages of IGC over traditional methods of gas and vapour adsorption are shown.     

Silver nanoparticle growth in 3D-hexagonal mesoporous silica films

The 3D-hexagonal mesoporous films are used as templates to grow uniform silver nanoparticles. The grafting of hydrophobic groups at the pore surface, significantly slows down the silver ion diffusion, anchoring small silver clusters in micropores and leading to organized domains of silver particles in mesopores with a narrow size distribution.     

Thermodynamic and neutron scattering study of hydrogen adsorption in two mesoporous ordered carbons

Two mesoporous ordered carbon materials (MOCs) have been synthesized from silica templates by using sucrose as the carbon precursor. The textural characterization using Ar, N2, and CO2 adsorption combined with neutron diffraction showed that the two samples exhibit a significant microporous volume close to 0.5 cm3/g and an ordered network of mesopores. For both MCM48 and SBA15 templated carbons, adsorption first proceeds with the filling of micropores and then by the filling of mesopores with an adsorption energy close to the enthalpy of vaporization of bulk hydrogen. The hydrogen isosteric heat of adsorption in the micropores (6-8 kJ/mol) is significantly larger than that on the graphite surface (approximately 4 kJ/mol) but still too small for a reasonable use of these MOCs as hydrogen adsorbents for storage at room temperature. The neutron scattering study showed that the structure at 10 K of the adsorbed deuterium phase is poorly organized; it exhibits short and medium range orders of about 13 angstroms in micropores and about 20 angstroms in mesopores, respectively. The average distance between adsorbed molecules decreases with coverage by about 10%. In the mesopores, the diffracted line is consistent with a pseudohexagonal packing.     

Guest-host encapsulation of microporous zeolites in ordered mesoporous materials by molecular simulations

The present work provides the first study of ordered mesoporous materials SBA-15 coated with microporous zeolites ZSM-5 using molecular simulations. Several model structures with characteristics such as periodic arrangement of mesopores, randomly arranged micropores, surface hydroxyls and bulk deformations of SBA-15 were used. Cartesian coordinates of ZSM-5 unit lattice were obtained from the literature and the 100 face of H-ZSM-5 unit cell was then placed on the surface of SBA-15 and the entire structure was equilibrated to obtain final configuration. The resulting structure was characterized using simulated small angle and wide angle X-ray diffraction, Connolly surface area (to compare BET area), accessible pore volume for nitrogen molecules (to compare with t-plot volume of micro and mesopores) and methane adsorption at 303 K. The orientation of ZSM-5 on the SBA-15 had no effect on the surface area, pore volume or adsorption capacity. In order to find out if the addition of microporous ZSM-5 should increase the total methane adsorption capacity due to addition of micropores, we studied adsorption on bare and coated SBA-15. However, total adsorption capacity was found to decrease, while the number of methane molecules adsorbed per unit cell of the SBA-15 structure increased. An existing experimental method (J. Am. Chem. Soc., 2004, 126, 14324) of the synthesizing hybrid ZSM-5/SBA-15 structure was studied using accessible micropore volume (by t-plot). It was found that the procedure made all the micropores inaccessible. A modification of the method or use of other host materials is suggested to use the benefits of narrow micropore distribution in ZSM-5.     

Effect of pore characteristics on electrochemical capacitance of activated carbons

Activated carbons (ACs) for electric double layer capacitors (EDLCs) were fabricated from waste tea leaves, activated with the pore-forming substances ZnCl₂ then, carbonized at high-temperature in N₂ atmosphere. The surface texture and porosity of the ACs were determined using transmission electron micros-copy and N₂ adsorption/desorption studies. The surface area of the 20 wt % ZnCl₂ treated sample was found to be 1029 m²g^?1 and had a distribution of micropores and mesopores. The electrochemical properties of the ACs were evaluated by using cyclic voltammetry and galvanostatic charge-discharge studies. ACs from waste tea leaves exhibited excellent specific capacitance as high as 196 F g^?1 in the 0.1 M Na₂SO₄ neutral electrolyte, with rectangular-like cyclic voltammetry curves at a cell potential of 1.5 V and good cyclability with a capacitance retention of 95% at a high current density of 100 mA g^?1 for 2000 cycles. The results show that the pore texture properties and specific surface area of ACs are dominated by changing carbonization temperature and the amount of activating agent ZnCl₂. The electrochemical performance is influenced mainly by surface area, but the pore size distribution becomes a dominating factor for specific capacitance of a carbon electrode material when the pore structure is in range of micropores/mesopores.     

The effective surface energy of heterogeneous solids measured by inverse gas chromatography at infinite dilution

Inverse gas chromatography (IGC) at infinite dilution has been widely used to access the nonspecific surface free energy of solid materials. Since most practical surfaces are heterogeneous, the effective surface energy given by IGC at infinite dilution is somehow averaged over the whole sample surface, but the rule of averaging has thus far not been established. To address this problem, infinite dilution IGC analysis was carried out on mixtures of known heterogeneity. These materials are obtained by mixing two types of solid particles with significantly different surface energies as characterized individually with IGC, and results are obtained for binary combinations in varying proportions. It is found that when all surface components have the same accessibility by probe molecules, the effective surface energy of such a heterogeneous surface is related to the surface energy distribution by a square root linear relationship, square root sigma(eff)(LW)= summation operator (i)phi(i) square root sigma(i)(LW), where sigma(i)(LW) refers to the nonspecific (Lifshitz-van der Waals) surface energy of patches i, and phi(i) to their area fraction.     

Preparation and characterization of helical carbon/silica nanofibers with lamellar mesopores on the surfaces

Helical 1,2-ethylene-silica nanofibers with lamellar mesopores on the surfaces and twisted rod-like mesopores inside were prepared according to literature procedures. After carbonization, helical carbon/ silica nanofibers with lamellar mesopores on the surfaces and twisted rod-like micropores inside were obtained. The morphologies and pore architectures of the carbon]silica nanofibers were characterized using transmission electron microscopy, field-emission scanning electron microscopy, powder X-ray diffraction and N2 sorptions. Although the mesopores inside shrank into micropores, the helical nanostructure remained. Moreover, several carbon/silica nanofibers with lamellar mesopores on the surfaces and concentric circular micropores inside were also obtained. After being calcined in air, helical silica nanofibers with lamellar mesopores on the surfaces and twisted rod-like micropores inside were produced as well.     

Adsorption of n-alkanes on plasma-oxidized high-strength carbon fibers

The objective of this work was to characterize the degree of heterogeneity brought about by oxygen plasma treatment of carbon fibers by studying its effects on the adsorption of n-alkanes. Untreated and unsized high-strength carbon fibers were subjected to oxygen plasma treatments with different degrees of severity. A sample of the same material oxidized following a standard industrial method was also studied for comparison. Adsorption of C5-C10n-alkanes at 303-353 K was measured by inverse gas chromatography (IGC). Elution peaks were symmetrical for the fresh and industrially oxidized samples; however, a large extent of asymmetry was observed for the plasma-treated fibers. Differences in surface heterogeneity were quantified in terms of several adsorption thermodynamic magnitudes. Differential heats of adsorption exhibited values similar to those corresponding to the probe-basal plane interaction. The dispersive component of the surface tension of the solids increased clearly upon plasma oxidation, the increase being systematic according to the severity of plasma treatment. It can be concluded that plasma oxidation generates high-surface-energy sites responsible for trapping of n-alkane molecules, this effect being more marked as the chain length increases. The possibility of this effect being associated to creation of micropores was ruled out on the basis of volumetric CO2 adsorption experiments and IGC measurements at finite dilution. Scanning tunneling microscopy observations allowed us to establish a possible connection between fiber surface nanostructure and IGC results. The sites accessible to n-alkane molecules in the industrially oxidized sample seem to be highly disordered, thus leading to a weaker interaction with the adsorbate.     

Catalytic cracking of polyethylene over nanocrystalline HZSM-5: Catalyst deactivation and regeneration study

In this work, catalytic cracking of low density polyethylene (LDPE) over nanocrystalline HZSM-5 zeolite in a batch reactor at 340 °C was carried out with the aim of study both the catalyst resistance to deactivation and the catalyst regeneration process. For looking into the catalyst deactivation, consecutive polyethylene cracking experiments were carried out. Subsequently, the same reactions has been also performed but by regenerating the zeolite after each experiment by promoting coke combustion at 550 °C under air flow. Zeolites used were two samples of nanocrystalline HZSM-5 with very different textural properties (sample A, with most of its surface corresponding to zeolitic micropores and sample B with a considerable amount of external surface, mesopores and super-micropores) in order to study the effect of their textural properties in the deactivation and regeneration experiments. Although conversion and activity values reached over both samples are quite high taking into account the mild conditions used, sample B shows a very much higher initial activity than A due to its improved textural properties. In both samples two different deactivation mechanisms occur: a reversible deactivation by coke deposition removable by the regeneration procedure, and an irreversible or permanent deactivation by other factors. Deactivation of sample B occurs faster due to the higher amount of coke deposited on this material. However, the improved textural properties of this catalyst supposes an advantage for the regeneration process, since coke deposited either on the external surface or on the super-micropores and mesopores, makes easier the regeneration procedure.     

Temperature effects on the composition and microstructure of spray-dried nanocomposite powders

Porous composite powders, prepared by spray drying of silica and polybromostyrene nanoparticles, were calcined at various temperatures up to 750 degrees C. The structure in these powders are quantitatively investigated by ultra small-angle X-ray scattering, thermogravimetric analysis, and nuclear magnetic resonance measurements. It has been found that the polybromostyrene latex is efficient in templating mesopores. However, polybromostyrene remains almost completely in the interstitial micropores in the grain after the spray-drying process. A post thermal treatment of the powders has been applied from 250 up to 750 degrees C. We found that the hydrocarbon part of the polybromostyrene is decomposed and leaves the micropores at around 350 degrees C. However, it is demonstrated that a significant amount of bromine remains in the interstitial micropores between the silica particles. At around 600 degrees C, the silica nanoparticles start to fuse with each other and a coalescence of the micropores takes place. At still higher temperature, around 750 degrees C, the micropore network totally disappears, and the growth in pore size occurs due to the coalescence of the mesopores with a significant decrease of the total porosity. During this process, the silica network densification is accompanied by a lowering of the specific surface area.     

Monodisperse polymer beads as packing material for high-performance liquid chromotography: Effect of divinylbenzene content on the porous and chromatographic properties of poly(styrene-co-divinylbenzene) beads prepared in presence of linear polystyrene as a porogen

The effect of concentration of divinylbenzene on pore size distribution and surface areas of micropores, mesopores, and macropores in uniformly sized porous poly(styrene-co-divinylbenzene) beads prepared in the presence of linear polystyrene as a component of the porogenic mixture has been studied. While the total specific surface area was clearly determined by the content of divinylbenzene, the sum of pore volumes for mesopores and macropores as well as their size distribution does not change within a broad range of DVB concentrations. Consequently, the size exclusion chromatography calibration curves are almost identical for all the beads prepared with different percentages of crosslinking monomer. However, the more crosslinked beads have better mechanical and hydrodynamic properties. © 1994 John Wiley & Sons, Inc.     

酸预处理对于 Fe-ZSM-5和 Fe-beta分子筛催化消除 N2O的影响

作为六大温室气体之一, N2O的增温潜能是 CO2的310倍,甲烷的21倍,目前仍然以0.80 ppb/年的速度增长,但是减排成本很低,因此对 N2O的消除具有重要意义.在工业中金属修饰的微孔分子筛因其优良的催化活性,高水热稳定性,低成本等优点而成为研究重点.但是微孔分子筛狭窄的微孔孔道限制了金属前驱体的进入,导致活性金属含量低,进而限制了活性的提高.因此采用一定的改性手段减小分子筛颗粒尺寸,缩短微孔孔道长度或者扩大微孔孔道来增加活性铁物种的含量进而提高分子筛催化活性.
　　本文选用商用的 ZSM-5和 beta分子筛作为母板分子筛,按照3 g ：50 mL比例将分子筛母板与1.0 mol/L的 HNO3在室温下混合,分别搅拌0,2和24 h,然后采用液相离子交换法负载金属铁制备得到 Fe-ZSM-5和 Fe-beta.通过 X射线衍射、N2物理吸吸脱附、电感耦合等离子原子发射光谱仪、扫描电镜、透射电镜、NH3程序升温脱附及紫外漫反射(UV-vis)等手段对不同时间处理的分子筛的形貌、酸性和铁物种等物理化学性质进行表征.对两种催化剂催化消除 N2O的反应性能进行了测试.结果显示,温和的酸处理下分子筛脱除了部分 Al,其中, ZSM-5分子筛的表现为由外向内逐层刻蚀,颗粒尺寸减小,孔道长度缩短,但是由于 MFI型分子筛较高的稳定性,酸处理对分子筛孔道大小的改变并不明显,而对于 beta分子筛,首先是其中大量无定形物种的去除,然后对孔道进行修饰,使之略微扩大,但是对颗粒尺寸的影响不大.
　　 ICP结果显示,商用 ZSM-5和 beta分子筛经过温和的酸处理改性后, Si/Al比增大,负载 Fe的含量明显增加,各催化剂催化消除 N2O的活性也出现了不同程度的提高. Fe-ZSM-5和 Fe-beta分子筛上 N2O完全转化温度分别向低温段移动了10–15和30°C. UV-vis谱图显示,分子筛中存在着不同种类的铁物种,通过分峰计算发现,孤立的 Fe3+铁离子和低聚态的 Fex3+Oy均是催化活性铁物种,其含量的增加部分也解释了活性提高的原因.     

The role of mesopores in intracrystalline transport in USY zeolite: PFG NMR diffusion study on various length scales

PFG NMR has been applied to study intracrystalline diffusion in USY zeolite as well as in the parent ammonium-ion exchanged zeolite Y used to produce the USY by zeolite steaming. The diffusion studies have been performed for a broad range of molecular displacements and with two different types of probe molecules (n-octane and 1,3,5-triisopropylbenzene) having critical molecular diameters smaller and larger than the openings of the zeolite micropores. Our experimental data unambiguously show that, in contrast to what is usually assumed in the literature, the intracrystalline mesopores do not significantly affect intracrystalline diffusion in USY. This result indicates that the intracrystalline mesopores of USY zeolite do not form a connected network, which would allow diffusion through crystals only via mesopores.