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  • 1.
    Hedlund, Jonas
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Schoeman, Brian
    Luleå tekniska universitet.
    Sterte, Johan
    Ultrathin oriented zeolite LTA films1997In: Chemical Communications, ISSN 1359-7345, E-ISSN 1364-548X, no 13, p. 1193-1194Article in journal (Refereed)
    Abstract [en]

    Ultrathin oriented films of zeolite LTA are prepared on single-crystal alumina supports by a method including adsorption of LTA seeds on the support followed by hydrothermal film crystallization.

  • 2.
    Mintova, S.
    et al.
    Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences.
    Schoeman, Brian J.
    Valtchev, V.
    Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences.
    Sterte, Johan
    Mo, S.
    Department of Chemistry, Purdue University, West Lafayette.
    Bein, T.
    Department of Chemistry, Purdue University, West Lafayette.
    Growth of silicalite films on pre-assembled layers of nanoscale seed crystals on piezoelectric chemical sensors1997In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 9, no 7, p. 585-589Article in journal (Refereed)
  • 3.
    Mintova, Svetlana
    et al.
    Luleå tekniska universitet.
    Hedlund, Jonas
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Schoeman, Brian
    Luleå tekniska universitet.
    Valtchev, Valentin
    Luleå tekniska universitet.
    Sterte, Johan
    Continuous films of zeolite ZSM-5 on modified gold surfaces1997In: Chemical Communications, ISSN 1359-7345, E-ISSN 1364-548X, no 1, p. 15-16Article in journal (Refereed)
    Abstract [en]

    Thin continuous films of zeolite ZSM-5 on gold surfaces are prepared by a novel method including silanization, charge modification and seeding of the surface prior to the hydrothermal crystallization of the zeolite.

  • 4.
    Mintova, Svetlana
    et al.
    Luleå tekniska universitet.
    Hedlund, Jonas
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Valtchev, Valentin
    Luleå tekniska universitet.
    Schoeman, Brian
    Luleå tekniska universitet.
    Sterte, Johan
    Preparation of ZSM-5 films from template free precursors1997In: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 7, no 12, p. 2341-2342Article in journal (Refereed)
    Abstract [en]

    Thin films of zeolite ZSM-5 on quartz substrates have been prepared in the absence of organic templates by growth of adsorbed seed crystals attached to a polymer-modified substrate surface.

  • 5.
    Mintova, Svetlana
    et al.
    Luleå tekniska universitet.
    Hedlund, Jonas
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Valtchev, Valentin
    Luleå tekniska universitet.
    Schoeman, Brian
    Luleå tekniska universitet.
    Sterte, Johan
    ZSM-5 films prepared from template free precursors1998In: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 8, no 10, p. 2217-2221Article in journal (Refereed)
    Abstract [en]

    Thin continuous films of zeolite ZSM-5 were synthesized on quartz substrates. The substrates were first surface modified and covered by a monolayer of colloidal silicalite-1 seed crystals. These crystals were grown into continuous films with thicknesses in the range 230-3500 nm by hydrothermal treatment in a synthesis gel free from organic templates. The preferential orientation of the crystals constituting the film was initially one with thec-axis close to parallel to the substrate surface. During the course of crystallization this orientation changed to one with most of the crystals having the c-axes directed approximately 35° from perpendicular to the substrate surface. A mechanism explaining this behavior is proposed. The final thickness of the film was controlled by the synthesis time but also by the addition of seed crystals to the synthesis gel. Films prepared according to this method may be of great value for the development of zeolite based membranes.

  • 6.
    Mintova, Svetlana
    et al.
    Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences.
    Valtchev, Valentin
    Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences.
    Schoeman, Brian
    Luleå tekniska universitet.
    Sterte, Johan
    Preparation of zeolite Y-vegetal fiber composite materials1996In: Journal of porous materials, ISSN 1380-2224, E-ISSN 1573-4854, Vol. 3, no 3, p. 143-150Article in journal (Refereed)
    Abstract [en]

    Zeolite Y crystallization on chemi thermo mechanical pulp (CTMP) fluff, linen and cotton substrates using two different approaches for the deposition was studied. In the first approach a seed-film method was applied. This method involves surface charge modification of the substrates, adsorption of a monolayer of colloidal zeolite Y crystals onto the surfaces and subsequent growth of these colloidal zeolite crystals into a continuous zeolite film. Application of this method resulted in continuous films of zeolite Y on all substrates investigated. The second approach involved direct synthesis on substrates subjected to chemical or mechanical pretreatment in order to modify their surface properties. Chemical treatment was found to improve the crystallization on cotton and linen fibers but not so for the CTMP. Mechanical pretreatment enhanced the zeolite crystallization on the linen fibers but not on the other substrates. The zeolite Y-vegetal fiber composites were characterized by SEM, XRD, and thermal analysis.

  • 7.
    Schoeman, Brian
    et al.
    Luleå tekniska universitet.
    Erdem-Senatalar, Ayse
    Hedlund, Jonas
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Sterte, Johan
    The growth of sub-micron films of TPA-silicalite-1 on single crystal silicon wafers from low-temperature clear solutions1997In: Zeolites, ISSN 0144-2449, Vol. 19, no 1, p. 21-28Article in journal (Refereed)
    Abstract [en]

    The direct synthesis of thin films of crystalline silicalite-1 upon single crystal silicon wafers at a crystallization temperature of 100°C has been investigated by varying the composition of the clear tetrapropylammonium (TPA) silicate synthesis solutions. Synthesis mixture compositions known to yield monodisperse colloidal crystals of TPA-silicalite-1 upon hydrothermal treatment as well as those reported to yield silicalite-1 films at higher temperatures have been found not suitable for the preparation of silicalite-1 films at 100°C. Lower crystal growth rates and smaller thicknesses of the gel film that forms on the wafer at this temperature decrease the tolerance to alkalinity, resulting in etching via the consumption of the gel layer before the growing crystals succeed in forming a closed film followed by the removal of the protective silicon oxide film on the wafer. Thin oriented silicalite-1 films with thicknesses in the range of 180 nm to 1 μm have been obtained by varying the alkalinity and water, the TPA, and the silica contents of the reaction mixture. Lower alkalinities and higher silica concentrations favor the formation of a thicker amorphous gel layer. Although increased TPA+ concentrations at constant alkalinity increase the number of nuclei that form on this layer, higher TPA+ concentrations have been observed to be required at higher alkalinities to achieve similar rates of nucleation. Rinsing the wafer surfaces initially with a 0.025 M TPAOH solution before rinsing with water and acetone produces cleaner surfaces free of post-treatment artifacts

  • 8.
    Schoeman, Brian
    et al.
    Dow Chemical Company.
    Higberg, K.
    Dow Chemical Company.
    Sterte, Johan
    Nanoparticles of microporous materials1999In: Nanostructured materials, ISSN 0965-9773, E-ISSN 1872-9150, Vol. 12, no 1, p. 49-54Article in journal (Refereed)
    Abstract [en]

    The hydrolysis and condensation of tetraethoxy silane in the presence of structure directing agents, such as the tetraalkylammonium cations, results in the formation of microporous siliceous particles with average sizes in the range 2 to 5 nm, the size being a function of the hydrolysis conditions and the composition of the mixture. Increased dilution favors larger particles while increased pH results in smaller particles. The option of isomorphous substitution of silicon for either alumina or titania is also shown to result in nano-sized particles were the substituted metals exist in the tetrahedral coordination

  • 9.
    Schoeman, Brian J.
    Luleå tekniska universitet.
    A high temperature in situ laser light-scattering study of the initial stage in the crystallization of TPA-silicalite-11997In: Zeolites, ISSN 0144-2449, Vol. 18, no 2-3, p. 97-105Article in journal (Refereed)
    Abstract [en]

    The events occurring in a clear TPA-silicalite-1 precursor solution, hydrothermally treated at 70°C, were monitored in situ by dynamic light scattering using a high-effect laser light source. Subcolloidal silicate particles with an average diameter of 3.3 nm are detected in the greater part of the crystallization. An initial decrease in the scattered light intensity upon hydrothermal treatment indicates a reduction in the particle number concentration at the expense of an increase in the particle size. This is interpreted as being due to an Ostwald ripening mechanism. A second discrete particle population (which coexists with the subcolloidal particles) is detected after ca. 9.5 h of hydrothermal treatment, at which time the average particle size is ca. 12 nm. The two particle populations (subcolloidal particles and the growing crystals) present in suspension before sampling at 9.5 h cannot be distinguished due to the resolution limitations inherent in the light-scattering technique. Deconvolution of intensity data collected during the time interval, corresponding to particle sizes of ca. 3 to 10 nm, indicates that the large-sized particles (crystalline silicalite-1) have as their origin subcolloidal particles initially present in solution before hydrothermal treatment. There are indications therefore that certain subcolloidal particles may possess a short-range structure such that they increase in size upon hydrothermal treatment and may thus be termed zeolitic nuclei. The initial growth of the growing crystals may be described as being a non-linear function of time until an average particle size of ca. 20 nm is attained, after which linear growth at a rate of 0.72 nm/h was recorded.

  • 10.
    Schoeman, Brian J.
    Luleå tekniska universitet.
    A study of the initial stage in the crystallization of TPA-silicalite-11996In: Zeolites, ISSN 0144-2449, Vol. 17, no 5-6, p. 447-456Article in journal (Refereed)
    Abstract [en]

    The initial stage in the crystallization of discrete colloidal crystals of tetrapropylammonium (TPA)-silicalite-1 has been studied with high effect laser light scattering and cryo-transmission electron microscopy (cryo-TEM). The apparently clear TPA-silicate precursor solutions contain discrete subcolloidal particles prior to hydrothermal treatment with an average size of 3.8 nm measured by dynamic light scattering (DLS) and about 5 nm when observed with cryo-TEM. These subcolloidal particles are present in the hydrothermally treated sol throughout the course of crystallization. Following the onset of hydrothermal treatment, the average DLS particle size of the subcolloidal fraction increases from 3.8 to 5.0 nm because of redistribution of silica via an Ostwald ripening mechanism. The first sampling point was at 1.5 h at which time the particle size distribution is monomodal. The apparent absence of growing particles at this point is thought to be a result of the inability of the light scattering technique to resolve two particle populations of similar average size. The first indication of a second growing particle fraction, shown to be TPA-silicalite-1, is after 2.5 h, the second sampling time, at which stage the crystal size is 26 nm. The crystal size increase as a function of time, calculated as 18.5 nm/h, is linear from this time onward. The number of subcolloidal particles prior to hydrothermal treatment is estimated to be of the order of 1017/g of sol, whereas the number of growing crystals is only 4.6 · 1010/g of sol. The possible role of the subcolloidal particles is discussed

  • 11.
    Schoeman, Brian J.
    Luleå tekniska universitet.
    Analysis of the nucleation and growth of TPA-silicalite-1 at elevated temperatures with the emphasis on colloidal stability1998In: Microporous and Mesoporous Materials, ISSN 1387-1811, E-ISSN 1873-3093, Vol. 22, no 1-3, p. 9-22Article in journal (Refereed)
    Abstract [en]

    The question as to whether silicalite-1 grows via an aggregation of smaller particles or similar sized particles has been addressed by considering the fundamentals governing colloidal stability - the quantitative theory underlying colloidal stability being given by the extended Derjaguin-Landau and Verwey-Overbeek (DLVO) theory. Application of the extended DLVO theory to discrete colloidal particles in silicalite-1 precursor sols shows that a net repulsive interactive energy exists between the negatively charged particles. The thermal energy of the colloidal particles (1/2kT at 373 K, the crystallization temperature) is not sufficient to overcome the net repulsive energy barrier. The extended DLVO theory has been applied to two growth scenarios with similar results: growth by aggregation of particles of very different sizes and growth by aggregation of similar sized particles. The significance of these conclusions is that a proposed growth mechanism of MFI type zeolite (growth by aggregation of subcolloidal particles) is deemed not to be a reasonable description of molecular sieve growth. The conclusion that the colloidal crystals are stable with respect to aggregation is supported by experimental observations. The ideas presented in this study are based upon crystallization of molecular sieves from clear solutions in the presence of quaternary ammonium cations that play a significant role in the stabilization of the colloidal crystals. Extension of the ideas presented shows that the extended DLVO theory is equally applicable to wholly inorganic heterogeneous systems.

  • 12.
    Schoeman, Brian J.
    Luleå tekniska universitet.
    Homogeneous nature of clear TPA-silicalite-1 precursor solutions1997In: Microporous Materials, ISSN 0927-6513, E-ISSN 1873-3107, Vol. 9, no 5-6, p. 267-271Article in journal (Refereed)
    Abstract [en]

    The polymeric silica present in clear tetrapropylammonium (TPA) silicate solutions, with the molar composition 9TPAOH:25SiO2:480H2O:100ethanol and from which discrete colloidal crystals of TPA-silicalite-1 may be synthesized, has been characterized with respect to particle size using a modified method in which the reaction rate of monomeric silica with molybdic acid was recorded. The estimated particle size of 2.8 nm obtained with this method agrees well with previous studies of the sub-colloidal fraction wherein cryo-TEM and high effect laser light scattering techniques were employed.

  • 13.
    Schoeman, Brian J.
    et al.
    Department of Engineering Chemistry, Chalmers University of Technology.
    Sterte, Johan
    Otterstedt, J-E
    Department of Engineering Chemistry, Chalmers University of Technology.
    Dynamic light scattering applied to the synthesis of colloidal zeolite1995In: Journal of porous materials, ISSN 1380-2224, E-ISSN 1573-4854, Vol. 1, no 2, p. 185-198Article in journal (Refereed)
    Abstract [en]

    The use of dynamic light scattering as an analysis method within the field of zeolite synthesis has proved to be a powerful and robust tool with which the particle size and the corresponding particle size distribution can be determined. The method has been employed in the evaluation of the crystallization of several types of colloidal zeolite from 'clear homogeneous' solutions. Examples of such zeolites are zeolite N-Y, hydroxysodalite, and TPA-silicalite-1. The fact that the particle size can be determined in as-synthesized zeolite sols as a function of synthesis time enables one to follow, for example, the crystallization process in terms of particle size increase, the process of particle size tailoring as well as to obtain information on the growth mechanism in zeolite synthesis. The colloidal nature of sols following redispersion of zeolite powders and colloidal zeolite organosols has been assessed using dynamic light scattering. The advantages as well as problems associated with the use of dynamic light scattering for characterization of colloidal zeolites are discussed. © 1995 Kluwer Academic Publishers.

  • 14.
    Tantekin-Ersolmaz, B.
    et al.
    Istanbul Technical University.
    Atalay-Oral, C.
    Istanbul Technical University.
    Tather, M.
    Istanbul Technical University.
    Erdem-Senatalar, A.
    Istanbul Technical University.
    Schoeman, Brian
    Sterte, Johan
    Effect of zeolite particle size on the performance of polymer–zeolite mixed matrix membranes2000In: Journal of Membrane Science, ISSN 0376-7388, E-ISSN 1873-3123, Vol. 175, no 2, p. 285-288Article in journal (Refereed)
    Abstract [en]

    The effect of zeolite particle size on the performance of silicalite–PDMS mixed matrix membranes is investigated at two different zeolite loadings. The separation properties of the membranes prepared are characterized by permeability measurements for O2, N2 and CO2 gases. The permeabilities of the silicalite–PDMS mixed matrix membranes are determined to increase with increasing particle size. The variations occurring in the permeability values with changes made in the particle size are much more pronounced at the higher zeolite loading. The ideal selectivity values corresponding to the mixed matrix membranes, on the other hand, generally seem to be less affected by the changes made in the particle size. The permeability values corresponding to the mixed matrix membranes exceed those pertaining to the original polymer membrane only at relatively higher zeolite loadings and/or for relatively larger particle sizes. The variations occurring in the permeabilities and selectivities with changes made in the zeolite particle size may be responsible for the different values of these parameters reported in the literature for the same types of zeolite filled polymeric membranes.

  • 15. Valtchev, V.
    et al.
    Hedlund, Jonas
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Schoeman, Brian
    Sterte, Johan
    Mintova, S.
    Deposition of continuous silicalite-1 films on inorganic fibers1997In: Microporous Materials, ISSN 0927-6513, E-ISSN 1873-3107, Vol. 8, no 1-2, p. 93-101Article in journal (Refereed)
    Abstract [en]

    Silicalite-fiber composites were prepared by a new method for the synthesis of molecular sieve films. The method consists of two steps: in the first one, the surface charge of the support is reversed by treatment with a cationic polymer solution and a monolayer of negatively charged colloidal molecular sieve seed crystals is adsorbed on the surface. The second step consists of hydrothermal treatment of the support in a molecular sieve precursor solution resulting in the formation of a continuous film of molecular sieve. The method was applied to carbon, ZrO2, Al2O3, mullite, ceramic and pyrex glass fibers. The crystallization of continuous silicalite-1 films on all types of fibers was confirmed by SEM, XRD, FTIR, DTA analyses and BET surface area measurements.

  • 16.
    Valtchev, Valentin
    et al.
    Luleå tekniska universitet.
    Schoeman, Brian
    Hedlund, Jonas
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Mintova, Svetlana
    Luleå tekniska universitet.
    Sterte, Johan
    Preparation and characterization of hollow fibers of silicalite-11996In: Zeolites, ISSN 0144-2449, Vol. 17, no 5-6, p. 408-415Article in journal (Refereed)
    Abstract [en]

    Hollow fibers consisting solely of silicalite-1 were prepared by a novel method consisting of four basic steps. Carbon fibers were first surface modified to facilitate adsorption of colloidal seed crystals of silicalite-1. Such particles were then adsorbed as a monolayer on the fiber surface and induced to grow into a continuous film of intergrown crystals of silicalite-1. Finally, the carbon fiber used as a template was removed by calcination in air, yielding hollow fibers of silicalite-1. Hollow fibers were prepared in two experimental series employing different synthesis conditions resulting in two different modes of growth. The first experimental series was performed with a clear homogeneous TPAOH/silica solution crystallizing at 100°C, and the second series with a NaOH-TPABr/silica gel crystallizing at 150°C. Materials produced in the first series showed a smooth and homogeneous surface and a relatively uniform fiber wall with a final thickness of 1.1 μm. In these materials the film was formed by crystals growing outward from the surface of the carbon fiber. Materials produced in the second series were less uniform with a final thickness of the fiber wall of about 3.2 μm. The crystal growth in these materials appeared to take place both in an inward and outward direction starting from the seed crystals adsorbed on the carbon fiber. The hollow fibers prepared were characterized by SEM, XRD, FT i.r., and nitrogen adsorption measurements.

  • 17.
    Zhang, Guangya
    et al.
    Luleå tekniska universitet.
    Sterte, Johan
    Schoeman, Brian
    Luleå tekniska universitet.
    Discrete colloidal crystals of titanium silicalite-11995In: Journal of the Chemical Society. Chemical communications, ISSN 0022-4936, Vol. 22, p. 2259-2260Article in journal (Refereed)
    Abstract [en]

    Stable colloidal suspensions of discrete crystals of titanium silicalite-1 with an average ultimate crystal size of less than 100 nm are prepared in the presence of ethanol at 100°C with crystallization times of less than 48 hours.

1 - 17 of 17
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