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Sökning: L773:1387 1811 OR L773:1873 3093 > (2005-2009)

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1.
  • Gao, Chuanbo, 1981- (författare)
  • Formation of mesoporous Co3O4 replicas of different mesostructureswith different pore sizes
  • 2009
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier. - 1387-1811 .- 1873-3093. ; 123, s. 314-323
  • Tidskriftsartikel (refereegranskat)abstract
    • Mesoporous metal oxides Co3O4 are prepared via hard templating synthesis method by using various mesoporous silicas with different pore size as templates. The pore size of the mesoporous silicas with the symmetry of two-dimensional (2d)-hexagonal p6mm, bicontinuous cubic Ia-3d and Pn-3m have been controlled in the range of 6.6–10.7, 4.2–7.5 and 5.1–6.7 nm, respectively, by choosing different surfactants and co-surfactants and by adjusting either the aging temperature or the ionization degree of the surfactant. The pore size of the silica template has been considered to be an important factor that determines the mesostructure of the resulting metal oxides. It has been found that for p6mm, it is easier to replicate the mesoporous symmetry at large size of mesopores. For Ia-3d, at large-pore size two sets of bicontinuous meso-channels are replicated into mesoporous Co3O4, while small-pore Ia-3d leads to replication of both one set and two sets of meso-channels. Co3O4 can replicate both one set and two sets of bicontinuous Pn-3m meso-channels at all pore sizes that can be obtained (5.1–6.7 nm), indicating the existence of ordered complementary micropores within the silica walls.
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2.
  • Gao, Chuanbo, et al. (författare)
  • Mesostructured silica based delivery system for a drug with a peptide as a cell-penetrating vector
  • 2009
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 122:1-3, s. 201-207
  • Tidskriftsartikel (refereegranskat)abstract
    • A drug delivery system using mesostructured silica as a reservoir has been developed for the storage and controlled release of a drug with a cell-penetrating peptide (CPP) as a vector. We use fluorescein isothiocyanate (FITC) as the drug model and octaarginine (R8) as a vector to endow the drug with cell-penetrating property. The mesostructured silica reservoir system was prepared by using a one-pot liquid?crystal templating method, which is suitable for the encapsulation of intact FITC-R8 conjugates and sustained release of drugs without hampering their properties. The hydrophobic poly(propyl oxide) (PPO) shell of the pore-filling Pluronic F127 and the electrostatic interaction between R8 and siloxide ions on the pore walls act as the diffusion-limiting factors of the FITC-R8 conjugate. A sigmoidal in vitro release of FITC-R8 from mesostructured silica into phosphate buffered saline (PBS, pH 7.4) was observed and the typical release duration was 5 days at 37 ‹C. Release from the reservoir yielded significant elongation in duration of the FITC signals in DU145 cells by confocal microscopic analysis, compared with a single administration of FITC-R8.  
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3.
  • Grahn, Mattias, et al. (författare)
  • Silicalite-1 coated ATR elements as sensitive chemical sensor probes
  • 2005
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 81:1-3, s. 357-363
  • Tidskriftsartikel (refereegranskat)abstract
    • A novel sensitive chemical sensor probe has been fabricated. The sensor principle is based on silicalite-1 coated ATR (attenuated total reflection) elements and FTIR spectroscopy. The microporous silicalite-1 film enriches the analyte to the probe surface, thus increasing the sensitivity. At a relative pressure of n-hexane in helium of 6 × 10−5 the sensitivity of the probe is approximately 85 times higher for the silicalite-1 coated element compared to a 10 cm transmission gas cell and ca. 180 times higher compared to an uncoated element. The performance of the probe is illustrated by determination of an adsorption isotherm for n-hexane in silicalite-1.
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4.
  • Jareman, Fredrik, et al. (författare)
  • Permeation of H2, N2, He and SF6 in real MFI membranes
  • 2005
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 83:1-3, s. 326-332
  • Tidskriftsartikel (refereegranskat)abstract
    • This work shows that a previously developed model for single gas permeation in real MFI membranes is applicable to an arbitrary MFI membrane with a different film thickness and defect distribution. The model can predict the flow of H2, N2 and He resonably. Deviations in SF6 flux for thick and oriented films were observed and attributed to a lower diffusion coefficient for the narrower pores in the a-direction of the MFI crystals. By guidance from the model, variations in previously reported single gas permeance ratios for selected membranes can now be attributed to variations in feed pressure, film thickness preferred orientation and defect distribution. It was found that high feed pressures and thick oriented films resulted in large single gas permeance ratios with SF6 in the denominator, even though these membranes were more defective than thinner membranes with more randomly oriented crystals. In general, single gas permeance ratios are strongly dependent on material properties and experimental conditions. These ratios can only be used for comparison of membranes with similar morphology and the ratios must be measured under identical conditions.
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5.
  • Jareman, Fredrik, et al. (författare)
  • Single gas permeance ratios in MFI membranes: Effects of material properties and experimental conditions
  • 2005
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 82:1-2, s. 201-207
  • Tidskriftsartikel (refereegranskat)abstract
    • A previously developed mathematical model with parameters fitted to experimental data was used to study effects of material properties and experimental conditions on single gas permeance ratios of MFI membranes. The model showed that single gas permeance ratios are highly dependent on substrate morphology, feed pressure, crystallographic orientation and defects in the film. It was found that the pore size and the thickness of the substrate affected permeance ratios, due to mass transfer resistance in the substrate. The applied feed pressure also had a significant effect on the permeance ratios. This is due to differences in mass transfer resistance of the substrate and adsorption characteristics with varying feed pressures. The crystallographic orientation of the zeolite film also affected permeance ratios due to changes in diffusivity with varying orientation of the crystals in the film. Finally, the effect of defects was investigated. As expected, it was found that the permeance ratios decreased when more defects were added in the model. However, if the membrane is not very defective, the permeance ratio is much more affected by the substrate and by variation in pressure drop than by defects. The results in the present work show that single gas permeance ratios cannot be used directly as a benchmark of membrane quality unless all other parameters are kept constant.
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6.
  • Jareman, Fredrik, et al. (författare)
  • The influence of the calcination rate on silicalite-1 membranes
  • 2005
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 79:1-3, s. 1-5
  • Tidskriftsartikel (refereegranskat)abstract
    • Silicalite-1 films with a thickness of 500 nm on asymmetric α-alumina micro filtration filters were calcined at 500 °C with heating and cooling rates varying between 0.2 °C/min and 5.0 °C/min. The membranes were characterized with single gas permeation, porosimetry, and xylene isomer separation experiments. It was found that the quality of the prepared membranes was independent of the heating/cooling rate according to the single gas permeation and porosimetry characterization. Xylene isomer separation data was found to vary between the samples, but none of the variations could be attributed to the heating/cooling rate during calcination since the variations did not follow a trend but occurred randomly. It is thus concluded that the calcination rate does not influence the quality of these membranes.
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7.
  • Lassinantti Gualtieri, Magdalena, et al. (författare)
  • The influence of heating rate on template removal in silicalite-1: An in situ HT-XRPD study
  • 2006
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 89:1-3, s. 1-8
  • Tidskriftsartikel (refereegranskat)abstract
    • The effect of heating rate on thermal behavior of TPA-silicalite-1 during calcination and the reaction kinetics for TPA decomposition were investigated. The cell parameters of the TPA-silicalite-1 during the heating cycles were determined with the aid of high temperature X-ray diffraction data and the Rietveld method. The template decomposition is accompanied by a large contraction of the unit cell. The unit cell dimensions during template removal are not affected significantly by the heating rate. Consequently, the rate of contraction is approximately proportional to the heating rate. The intensity of some diffraction peaks changes during heating, especially the 101/011 and the 200/020 peaks. The intensity change of those peaks shows the same dependence with temperature as the TPA occupancy, indicating that these parameters are related. An analysis of the kinetics for TPA decomposition based on the intensity change of the 101/011 and the 200/020 peaks was performed. The apparent activation energy (Ea) of the template decomposition in silicalite-1 determined with the Kissinger and the Flynn–Wall–Ozawa methods was 138 (±25) and 138 (±29) kJ mol−1, respectively. The reaction order, determined with the method of Kennedy and Clark, was close to 0.5 indicating that the rate-limiting step is mono-dimensional diffusion. Ea was 140 (±30) kJ mol−1, in good agreement with the results obtained with the other methods.With the results presented here, it is possible to discuss possible effects of the heating rate on the crack formation frequently observed in zeolite membranes during calcination.
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8.
  • Li, Huachun, et al. (författare)
  • Mesoporous silicalite-1 zeolite crystals with unique pore shapes analogous to the morphology
  • 2007
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 106:03-jan, s. 174-179
  • Tidskriftsartikel (refereegranskat)abstract
    • Thermal/hydrothermal stable silicalite-1 zeolite with high-porosity mesopores analogous to the crystal morphology has been synthesized by using large porous carbon (10-20 nm) as hard template originated from colloidal silica.
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9.
  • Li, Qinghua, et al. (författare)
  • Synthesis and characterization of colloidal zoned MFI crystals
  • 2005
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 78:1, s. 1-10
  • Tidskriftsartikel (refereegranskat)abstract
    • Colloidal zoned MFI crystals, i.e., continuous crystals with a compositional gradient resulting in a ZSM-5 core covered with a silicalite-1 shell, were synthesized by the addition of ZSM-5 seeds to a silicalite-1 synthesis solution. The effect of the surface aluminum content of the ZSM-5 crystals on the synthesis of the zoned MFI crystals was investigated using SEM, TEM, XPS and XRD. An acid treatment of the ZSM-5 seeds, which removed some of the aluminum at the surface, proved favorable for the synthesis of zoned MFI crystals.
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10.
  • Mosca, Alessandra, et al. (författare)
  • NO2 and N2 sorption in MFI films with varying Si/Al and Na/Al ratios
  • 2009
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 120:3, s. 195-205
  • Tidskriftsartikel (refereegranskat)abstract
    • MFI crystals or films with controlled thicknesses and different Si/Al ratios were grown on seeded cordierite monoliths using a clear synthesis mixture with template or a template-free gel. The materials were analyzed by scanning electron microscopy, X-ray diffraction, inductively coupled plasma-atomic emission spectrometry, X-ray photoelectron spectroscopy, thermogravimetric analysis and sorption experiments using N 2 or NO 2 adsorbates. The films were uniformly distributed over the support surface. As expected, the specific monolayer N 2 adsorption capacity (mol/g zeolite ) was constant and independent of film thickness. The specific molar NO 2 adsorption capacity was significantly lower than the specific molar monolayer N 2 adsorption capacity, indicating that NO 2 is adsorbed at specific sites rather than evenly distributed in a monolayer. A number of NO 2 adsorption sites with varying strengths were observed by TPD experiments. At 30 °C, the amount of adsorbed NO 2 in the MFI films increased with increasing Al and Na content as opposed to the N 2 adsorption capacity, which was independent of these parameters. At 200 °C, the adsorbed amount of NO 2 was lower than at 30 °C and apparently independent on Al concentration in the Na-MFI films. These results indicate that different mechanisms are involved in NO 2 adsorption. NO 2 may adsorb weakly on Na + cations and also react with silanol groups and residual water in the zeolite, the latter two results in more strongly bound species. Upon NO 2 adsorption, formation of NO was observed. This work represents the first systematic study of the effects of Al and Na content on NO 2 adsorption in MFI films. © 2008 Elsevier Inc. All rights reserved.
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11.
  • Naydenov, Valeri, et al. (författare)
  • Meso/macroporous AlPO-5 spherical macrostructures tailored by resin templating
  • 2005
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 78:2-3, s. 181-188
  • Tidskriftsartikel (refereegranskat)abstract
    • A multi-step procedure for the preparation of meso/macroporous AlPO-5 spherical macrostructures using cation exchange resin beads as macrotemplates is presented. Firstly, aluminum species were introduced into the resin beads by ion exchange resulting in a resin-aluminum composite. Thereafter, the resin-aluminum composite was mixed with TEAOH, H3PO4 and distilled water and hydrothermally treated at 150 °C to yield resin-AlPO-5 composite. Finally, the resin was removed by calcination leaving behind self-bonded AlPO-5 spheres. The product AlPO-5 macrostructures were thoroughly characterized by SEM, XRD, nitrogen adsorption measurements, 31P and 27Al solid state NMR spectroscopy. The influence of various components of the synthesis mixture on the crystallinity, phase purity and stability of the AlPO-5 spheres was systematically studied. Samples prepared for different treatment times using the initial synthesis composition that gives spheres of the highest quality were used to study the crystallization process within the resin.
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12.
  • Ng, Jovice Boon Sing, 1975-, et al. (författare)
  • The radial dependence of the spatial mesostructure of monodisperse mesoporous silica spheres
  • 2008
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 112, s. 589-596
  • Tidskriftsartikel (refereegranskat)abstract
    • The pore structure of monodisperse mesostructured silica spheres has been investigated by a combination of X-ray diffraction, nitrogen sorption isotherms and transmission electron microscopy (TEM). The radial dependence of the mesostructure has been obtained by TEM analysis of thin microtomed slices taken at different distance from the particle core. A structural model is proposed based on the TEM observations where bundles of the hexagonally ordered cylindrical channels bend and twist, without any preferred direction, close to the particle core and attain a radially extended structure closer to the particle surface. Thermal calcination results in a pore shrinkage of about 16% and some loss of the long-range mesostructure while radiative disintegration and partial removal of the organic template by an UV/Ozone treatment had a negligible effect on the inorganic framework in terms of pore size or spatial pore arrangement.
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13.
  • Ren, Tie-Zhen, et al. (författare)
  • Single crystal manganese oxide hexagonal plates with regulated mesoporous structures
  • 2008
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 112:03-jan, s. 467-473
  • Tidskriftsartikel (refereegranskat)abstract
    • Porous hexagonal plates of manganese oxides are prepared by a simple hydrothermal method from metal Mn foils/powders and manganese acetate precursors tinder basic conditions. The oriented aggregation-based assembly of the nanoparticles of mixed feitknechtite and hausmannite phases gave rise to well-defined hexagonal nanoplates with irregular mesopores of similar to 5 nm in size. The hexagonal plate-like morphology was retained after calcination, and mesoporous single-crystalline Mn5O8 and alpha-Mn2O3 plates were obtained after calcinations at 400 and 700 degrees C, respectively. The interior mesopores in each plate were enlarged with the increase of calcination temperature, and turned out to be regular-shaped, i.e. polyhedral or even well-defined rectangular. Furthermore, 700 degrees C-calcination led to the fusion of nanocrystals into structurally uniform alpha-Mn2O3 crystals of hexagonal plates with regular-shaped mesoporous structure. X-ray diffraction and electron diffraction, scanning and transmission electron microscopy, X-ray energy dispersive spectroscopy, and nitrogen sorption analysis were employed to characterize these porous nanoplates. Such mesoporous hexagonal nanoplates of manganese oxides with high crystallinity may have many applications.
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14.
  • Ren, Tie-zhen, et al. (författare)
  • SU-57 – An aluminosilicogermanate with a DFT topology and variable compositions
  • 2009
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 117:1-2, s. 285-291
  • Tidskriftsartikel (refereegranskat)abstract
    • An aluminosilicogermanate with a DFT zeotype framework (denoted as SU-57) was synthesized for the first time by ethanol-assisted hydrothermal synthesis at 160 and 170 °C. The compound was characterized by single crystal and powder X-ray diffraction, scanning and transmission electron microscopy, energy dispersive spectroscopy (EDS), thermogravimetry (TG) and solid state nuclear magnetic resonance (NMR) spectroscopy. SU-57 crystallizes in space group P42/n with estimated a ≈ 10.38–10.46 Å, c ≈ 8.88–8.91 Å, V ≈ 957–975 Å3. It has variable Al–Si–Ge composition with an approximate formula |C2H10N2| [Al(SixGe1−x)O4]2 (x ≈ 0.3–0.9), which results in a super-structure originated from different cation occupancies of the two unique tetrahedral (T) sites. Single crystal X-ray structure refinements, together with results from X-ray powder diffraction (XRPD) and EDS analysis, showed that (i) the AlO4 and (Si, Ge)O4 tetrahedra are only partially ordered over the DFT framework and do not follow a strict alternating manner. (ii) Al resides predominantly on one T site and Si and Ge predominantly on the other. (iii) The Al cation concentration (Al/(Al + Si + Ge)) is nearly constant and slightly less than 50 at%, while the Si and Ge cation concentrations vary over a large range. (iv) Al and Ge occupy both T sites. The cation disorder was confirmed by 27Al and 29Si solid state NMR. TG analysis and in situ XRPD showed that SU-57 was stable up to 375 °C in N2 atmosphere.
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15.
  • Rezai, Seyed Alireza Sadat, et al. (författare)
  • Water/hydrogen/hexane multicomponent selectivity of thin MFI membranes with different Si/Al ratios
  • 2008
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 108:1-3, s. 136-142
  • Tidskriftsartikel (refereegranskat)abstract
    • MFI films with a thickness of about 550 nm were prepared on α-alumina substrates. The surface Si/Al ratios (XPS) were 157 and 62 for silicalite-1 and ZSM-5 films, respectively, and in accordance, XRD data indicated lower ratios for ZSM-5 films. Higher ratios were observed by ICP-AES for crystals grown in the bulk of the synthesis mixtures. Six membranes of each type were prepared. Porosimetry measurements showed that all membranes were of high and similar quality. Single gas permeances for H2, N2, He, CO2 and SF6 at 25 °C were very similar within each type of membranes. However, the average hydrogen permeance was 27% lower and the average H2/SF6 single gas permeance ratio was 67% higher for ZSM-5 membranes. These differences are attributed to a narrower effective pore diameter for the ZSM-5 membranes due to the sodium counter ions. Separation of mixtures of H2O, H2 and n-hexane (helium balance) was investigated in the temperature range 25-350 °C. The highest separation factors α-H2O/H2 were observed at 25 °C and were 14.3 and 19.7 for silicalite-1 and ZSM-5, respectively. The membranes were selective also at 100 °C and the separation factors were about 3.2 and 6 for silicalite-1 and ZSM-5, respectively. However, the selectivity decreased at elevated temperatures and the separation factor approached 1 at temperatures above 180 °C for both membrane types. The observed water selectivity was attributed to weak adsorption of water on polar sites. A low (1.5-3) α-H2O/n-C6 separation factor was observed for both membrane types for the entire investigated temperature range.
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16.
  • Serra, Elias, et al. (författare)
  • Immobilization of lipase in ordered mesoporous materials : Effect of textural and structural parameters
  • 2008
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 114:03-jan, s. 201-213
  • Tidskriftsartikel (refereegranskat)abstract
    • A systematic study dealing with the influence of several parameters oil the immobilization of lipase in ordered mesoporous materials (OMM) is presented here. In a first step, a series of OMM have been synthesized trying to cover the most relevant structures. The aim is to get variation in the key properties susceptible of influencing their behavior as lipase supports, such as the structure (cubic or hexagonal), the nature of the pores (channel-like or cage-like), the connectivity of the porous network and the pore size. Also, by following the co-condensation technique, 5-10%-methylated analogues of the pure-silica materials have been prepared. All the samples have been fully characterized with XRD, TEM (including 3D reconstruction), SEM, TGA and N-2 isotherms, and the incorporation of the organic function has been demonstrated by Si-29 NMR. All of them have been tested as supports in the immobilization of Candida antarctica Lipase B (CaLB) and the leaching of the enzyme in aqueous media evaluated. With such a systematic approach, valuable information on the influence of the textural properties and the nature of the porous network oil the yields of immobilization and enzyme desorption have been stated. Very interestingly, leaching of the enzyme can be diminished until it practically disappears without being covalently bonded to the wall, which places the ordered mesoporous materials at the starting point of a new scenario in enzyme immobilization on preexisting supports. 
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17.
  • Sörensen, MH, et al. (författare)
  • Control of internal (2D and 3D hexagonal) mesostructure and particle morphology of spherical mesoporous silica particles using the emulsion and solvent evaporation (ESE) method
  • 2009
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 120:3, s. 359-367
  • Tidskriftsartikel (refereegranskat)abstract
    • In this study we demonstrate the opportunities controlling internal structure as well as exterior morphology of surfactant templated mesostructured materials through the newly developed emulsion and solvent evaporation (ESE) method. In particular, we consider the control of synthesis temperature and map the influence upon both internal structure and surface morphology of particles templated by the temperature sensitive Pluronic block copolymer F127. Furthermore, we vary compositions, by adding poly(propylene glycol) acting as a swelling agent, as well as by controlling the moisture content. Both of these are having an impact on the internal mesostructure as well as the pore size. Apart from probing internal structure by scattering techniques, the accessibility of the mesoscopic pores of these materials are investigated by measuring the adsorption of a cationic dye, Janus Green B, into the materials. This method shows that accessibility varies dramatically with internal structure. Further, by carefully controlling the moisture content when using the cationic surfactant C16TAB as template, a well ordered 3D hexagonal closed packed (P63/mmc) material with large surface area as well as pore volume was prepared. This further indicates the versatility of the new preparation technique.
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18.
  • Sörensen, MH, et al. (författare)
  • Expansion of the F127-templated mesostructure in aerosol-generated particles by using polypropylene glycol as a swelling agent
  • 2008
  • Ingår i: Microporous and Mesoporous Materials. - 1387-1811 .- 1873-3093. ; 113, s. 1-13
  • Tidskriftsartikel (refereegranskat)abstract
    • Expansion of the mesostructure in aerosol generated particles was performed through incorporation of polypropylene glycol (PPG), a non-volatile swelling agent. TEOS was used as silica source and the Pluronic block copolymer, F127, as template. The ratio of TEOS to F127 was kept constant during synthesis, while varying the weight ratio of PPG to F127 systematically. The impact of the PPG on the expansion of the structure was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and nitrogen adsorption. Different methods were used to calculate the pore size distributions, the BJH, the BdB-FHH, the KJS and the NLDFT method. Simple geometrical models of the expansion were derived to interpret the experimental data and establish their accuracy. Experimental data showed a roughly linear expansion of the unit cell and pore size, consistent with that expected by modelling the swelling of a hexagonal (p6mm) structure assuming constant wall thickness. The expansion is increasing as a function of increasing PPG/F127 ratio by about 25 Å. An expression of the density of the silica wall was calculated from the models resulting in a density of 1.95±0.2 g/cm3. At a PPG/F127 ratio of approximately 0.31, the p6mm structure (found at lower PPG/F127 ratios) transforms to a microemulsion-templated foam structure. At an even higher PPG/F127 ratio (0.63-1.56), phase separation of the oil from the swollen template occurred, yielding a two-phase system of coexisting foam and large vesicles.
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19.
  • Sörensen, Malin H., et al. (författare)
  • Expansion of the F127-templated mesostructure in aerosolgeneratedparticles by using polypropylene glycol as a swelling agent
  • 2008
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 113, s. 1-13
  • Tidskriftsartikel (refereegranskat)abstract
    • Expansion of the mesostructure in aerosol-generated particles was performed through incorporation of polypropylene glycol (PPG), a non-volatile swelling agent. TEOS was used as silica source and the Pluronic block copolymer, F127, as template. The ratio of TEOS to F127 was kept constant during synthesis, while varying the weight ratio of PPG to F127 systematically. The impact of the PPG on the expansion of the structure was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and nitrogen adsorption. Different methods were used to calculate the pore size distributions, the BJH, the BdB-FHH, the KJS and the NLDFT method. Simple geometrical models of the expansion were derived to interpret the experimental data and establish their accuracy. Experimental data showed a roughly linear expansion of the unit cell and pore size, consistent with that expected by modelling the swelling of a hexagonal (p6mm) structure assuming constant wall thickness. The expansion is increasing as a function of increasing PPG/F127 ratio by about 25 Å. An expression of the density of the silica wall was calculated from the models resulting in a density of 1.95 ± 0.2 g/cm3. At a PPG/F127 ratio of approximately 0.31, the p6mm structure (found at lower PPG/F127 ratios) transforms to a microemulsion-templated foam structure. At an even higher PPG/F127 ratio (0.63–1.56), phase separation of the oil from the swollen template occurred, yielding a two-phase system of coexisting foam and large vesicles.
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20.
  • Urbonaite, Sigita, et al. (författare)
  • Porosity development along the synthesis of carbon from metal carbides
  • 2008
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 113:1-3, s. 14-21
  • Tidskriftsartikel (refereegranskat)abstract
    • Carbide derived carbons (CDCs) are materials containing a wide range of pore size, their properties and applications being a function of the pore structure. For this reason it is important to find the relationship between synthesis parameters, such as initial carbide and temperature, and resulting structure. In this work, CDCs were prepared from VC, TiC, WC, TaC, NbC, HfC and ZrC at different temperatures and they have been characterized by means of gas adsorption. Nitrogen at −196 °C and carbon dioxide at 0 °C adsorptions were used to obtain structural information such as apparent surface area (SBET), micropore volume (Vmicro) and pore size distribution (PSD). These studies show that these parameters vary strongly depending on synthesis temperature (700–1200 °C) and initial carbide. Since the size of micropores is near the range suggested as being ideal for methane (CH4) storage, the high-pressure methane adsorption at 25 °C was performed to check the possible use of CDCs as storage material. The CH4 adsorption capacity of CDCs can compete with other carbons on a gravimetric basis, though the volumetric one is far from desired due to their low packing density.
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21.
  • Wang, Zheng, et al. (författare)
  • Oriented films of epitaxial MFI overgrowths
  • 2006
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 95:1-3, s. 86-91
  • Tidskriftsartikel (refereegranskat)abstract
    • Growth of oriented films of epitaxial MFI overgrowths by the in situ method was studied by SEM, TEM and powder XRD. By using a short hydrothermal treatment, it was possible to grow well-defined, b-oriented ZSM-5 films on polished quartz substrates. It was found that these b-oriented precursor ZSM-5 films grew along the b-axis in an aluminum free silicalite-1 synthesis mixture. Simultaneously, 90° rotational intergrowths formed on top of the b-oriented crystals. Upon further growth in several short synthesis steps, the 90° rotational intergrowths formed an a-oriented silicalite-1 film on top of the first b-oriented film. Only very weak reflections representing other crystallographic planes than (h 0 0) and (0 k 0) are observed by XRD in these samples, which shows that all crystals are a- or b-oriented. Continuous crystals, extending from the support to the top surface of the film was observed by electron microscopy.
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22.
  • Wang, Zheng, et al. (författare)
  • Polycrystalline films of epitaxial MFI overgrowths
  • 2006
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 97:1-3, s. 27-33
  • Tidskriftsartikel (refereegranskat)abstract
    • Films of epitaxial MFI overgrowths, i.e. ZSM-5 films covered with silicalite-1, with crystals extending from the substrate to the top surface, were synthesized using a seeding method with acid leaching between the synthesis of the MFI layers. The films were characterized by SEM, TEM, XRD and XPS. When acid leaching was omitted, secondary nucleation of silicalite-1 on the ZSM-5 film resulted in a sandwich film with discontinuous crystals. When the precursor ZSM-5 film was acid leached and the surface Si/Al ratio increased from 23 to 47, the crystals in the precursor film grew epitaxially upon treatment in a silicalite-1 synthesis mixture. It was thus revealed that reduction of the aluminum content at the external surface of ZSM-5 may be important for the successful synthesis of epitaxial MFI overgrowths. Alternatively, acid leaching may provide a cleaner surface, and facilitate epitaxial growth.
  •  
23.
  • Öhrman, Olov, et al. (författare)
  • Synthesis and catalytic evaluation of zoned MFI films
  • 2006
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 91:1-3, s. 312-320
  • Tidskriftsartikel (refereegranskat)abstract
    • Alumina and quartz supports were coated with well-defined precursor ZSM-5 films ranging from 350 nm to 2300 nm in thickness. The precursor samples were subsequently treated with hydrochloric acid and hydrothermally treated in a silicalite-1 synthesis solution in order to obtain zoned MFI films. A dense, but rougher, silicalite-1 film was formed on top of the smooth precursor ZSM-5 film after two hydrothermal treatments. Defects such as open grain boundaries and cracks were observed by SEM and in concert, mesopores were detected by gas adsorption in the precursor and zoned films. The mesopore volume per gram film increased after acid treatment and zoning, probably due to that a small fraction of the zeolite film dissolved in this process. As expected, the aluminum concentration at the surface of the un-calcined zoned film was lower compared to that at the surface of the calcined precursor ZSM-5 film. However, after calcination of the zoned film, the concentration of aluminum at the surface increased, probably due to aluminum migration from the precursor film to the surface of the zoned film during calcination. The aluminum concentration at the surface of the calcined zoned film was even higher than at the surface of the calcined precursor ZSM-5 film. Consequently, the triisopropylbenzene cracking rate constant did not decrease as expected after zoning, probably due to the increased amount of defects and/or aluminum migration. The p-xylene diffusivity increased after zoning, probably due to the formation of defects, whereas the xylene isomerization rate constants were unaffected, as expected.
  •  
24.
  • Öhrman, Olov, et al. (författare)
  • Thin ZSM-5 film catalysts on quartz and alumina supports
  • 2005
  • Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 78:2-3, s. 199-208
  • Tidskriftsartikel (refereegranskat)abstract
    • Alumina beads, quartz and soda glass were coated with ZSM-5 films using a seeding method. Films with a thickness of 150, 350, 800 and 2300 nm were prepared. The catalysts were tested by para-xylene (pX) isomerization and triisopropylbenzene (TiPB) cracking. Model parameters, i.e. rate constants and diffusivities, were fitted to experimental results. The films on quartz glass were much more active for pX isomerization and for cracking of TiPB, which was also reflected by the model parameters. Low and zero rate constants for alumina and soda glass supported catalysts, respectively, were attributed to poisoning of the ZSM-5 film due to impurities in these supports. Larger diffusivity in quartz supported catalysts and thick films was attributed to more defects in the film.
  •  
25.
  • Adhikar, Subhra, et al. (författare)
  • Thermodynamic analysis of gas phase chemistry in hot wire chemical vapor deposition of a-Si:H and μc-Si:H
  • 2006
  • Ingår i: Journal of Non-Crystalline Solids. - : Elsevier BV. - 0022-3093 .- 1873-4812. ; 362:9-20 Spec Iss., s. 928-932
  • Tidskriftsartikel (refereegranskat)abstract
    • Gas phase reactions amongst filament-generated radicals play a crucial role in growth and properties of films deposited by hot wire chemical vapor deposition (HWCVD) technology. Gas phase species of interest are SiH4, H2, Si, H, SiH3, SiH2 and SiH. Partial pressures of these species for different sets of deposition conditions have been determined from the standard Gibbs free energy data. Equilibrium concentrations of the film forming precursors have been determined. The effect of the various process parameters on the equilibrium concentration of the precursors has been studied. H, Si and SiH are found to be the dominant species in gas phase above a filament temperature of 2300 K. However SiH3 and SiH2 concentration peaks are between 1900 and 2300 K, of the filament temperature
  •  
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