| 1. |
- Sjöstrom, Karin, et al.
(författare)
-
Changes within the modified kraft process aiming to improve TCF bleachability
- 1999
-
Ingår i: Journal of Pulp and Paper Science (JPPS). - 0826-6220. ; 25:5, s. 176-182
-
Tidskriftsartikel (refereegranskat)abstract
- the investigated process modifications were: presulphonation, postsulphonation, polysulphide pretreatment and polysulphide pretreatment combined with a postsulphonation. The influence of the degree of delignification in the cook, on the selectivity and bleachability, was also investigated. The pulps were produced from softwood chips and were oxygen delignified before bleaching according to an AZQP sequence. All the investigated process variants had better process selectivity (pulp viscosity at a given brightness) and the pulps produced with a postsulphonation showed better bleachability, than the reference pulp. The bleachability and the process selectivity was improved by interrupting the cook at a higher kappa number and instead extending the delignification in the oxygen stage.
|
|
| 2. |
- Kornmann, Xavier, et al.
(författare)
-
Nanocomposites based on montmorillonite and unsaturated polyester
- 1998
-
Ingår i: Polymer Engineering and Science. - : Wiley. - 0032-3888 .- 1548-2634. ; 38:8, s. 1351-1358
-
Tidskriftsartikel (refereegranskat)abstract
- The concept of nanoscale reinforcement provides opportunity for synthesis of new polymer materials with unique properties. Montmorilonite (MMT) was derived from bentonite, purified, activated by sodium ions and mixed with reacting unsaturated polyester (UP). X-ray and transmission electron microscopy data were in support of the formation of a partially delaminated nanocomposite material. At an MMT content of only 1.5 vol%, the fracture energy, GQ of the nanocomposite was doubled, 138 J/m2 as compared with 70 J/m2 for the pure UP.
|
|
| 3. |
- Engström, Per, et al.
(författare)
-
Sulphur dioxide interaction with NOx storage catalysts
- 1999
-
Ingår i: Applied Catalysis B: Environmental. - 0926-3373 .- 1873-3883. ; 22:4, s. L241-L248
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of SO2 on the NOx storage capacity and oxidation and reduction activities of a model Pt/Rh/BaO/Al2O3 NOx storage catalyst was investigated. Addition of 2.5, 7.5 or 25 vol. ppm SO2 to a synthetic lean exhaust gas caused deactivation of the NOx storage function, the oxidation activity and the reduction activity of the catalyst. The degree of deactivation of the NOx storage capacity was found to be proportional to the total SO2 dose that the catalyst had been exposed to. SO2 was found to be accumulated in the catalyst as sulphate.
|
|
| 4. |
- Lipnizki, Frank, et al.
(författare)
-
Pervaporation-based hybrid process : A review of process design, applications and economics
- 1999
-
Ingår i: Journal of Membrane Science. - 0376-7388. ; 153:2, s. 183-210
-
Forskningsöversikt (refereegranskat)abstract
- Pervaporation is one of the developing membrane technologies that can be used for various industrial applications but for a predefined task, the optimal process design is unlikely to consist solely of pervaporation. Often the optimised solution becomes a hybrid process combining pervaporation with one or more other separation technologies. A distinction will be made between hybrid and integrated processes. Hybrid processes are important and consequently need to be considered in process design. This paper focuses on pervaporation-based hybrid processes that have been realised on an industrial scale. Both present and future prospects of applying these process combinations will be reviewed. The emphasis of this paper is, therefore, on pervaporation combined with distillation and with chemical reactors. The economic potential of these hybrid processes is evaluated, for various applications, by cost comparisons between the pervaporation-based hybrid processes and alternative separation processes. Pervaporation-based processes for waste water treatment and biotechnology applications involve other types of pervaporation based hybrid processes and have been excluded from this review. Copyright (C) 1999 Elsevier Science B.V.
|
|
| 5. |
|
|
| 6. |
|
|
| 7. |
- Mihailova, B., et al.
(författare)
-
Infrared spectroscopic study of a γ-mercaptopropyltrimethoxysilane monolayer on a gold surface
- 1999
-
Ingår i: Journal of Materials Chemistry. - 0959-9428 .- 1364-5501. ; 9:7, s. 1507-1510
-
Tidskriftsartikel (refereegranskat)abstract
- Reflectance-absorption infrared spectra of ?-mercaptopropyltrimethoxysilane (?-MPT) adsorbed on Au (111)-oriented films are analyzed on the basis of the calculated vibrational modes of a monomer, a dimer and n-membered rings of SiO3R (R=CH2CH2CH2SAu). The results show that the ?-MPT molecules adsorbed on the Au surface are partially hydrolyzed and that the orientation of the methyl groups and propyl chains are near perpendicular to the surface. After a hydrolysis procedure, the SiO3 groups in ?-MPT form a quasi two-dimensional system built predominantly of 6-membered puckered Si-O rings. Additional treatment of the ?-MPT monolayer with a surface-charge-reversing cationic polymer disrupts the connectivity and increases the topological disorder in the silicate system.
|
|
| 8. |
- Mihailova, B., et al.
(författare)
-
Vibrational spectroscopy study of the structure of silicalite-1 films on a gold surface
- 1999
-
Ingår i: Microporous and Mesoporous Materials. - 1387-1811 .- 1873-3093. ; 32:3, s. 297-304
-
Tidskriftsartikel (refereegranskat)abstract
- The structure of silicalite-1 films grown on seeded gold surfaces is investigated by modelling the observed changes in the infrared reflection absorption (IRRA) spectra of samples treated for different times in the synthesis solution. The results show that a gradual deformation of the five-membered silicon-oxygen rings occurs during the first 11 h of the hydrothermal treatment which leads to breaking of Si---O---Si linkages and to formation of linear defects along the c-axis. Interactions between the dislocations and the grain boundaries during the further growth of the film provoke the appearance of void spaces in the grain boundary interface which may cause incipient cracking in the silicalite-1 films on seeded gold surfaces. The range 1000-1300 cm-1 in the IRRA spectra is found to be appropriate for estimating the quality of silicalite-1 films grown on metal surfaces.
|
|
| 9. |
- Lipnizki, Frank, et al.
(författare)
-
Organophilic pervaporation : Prospects and performance
- 1999
-
Ingår i: Chemical Engineering Journal. - 1385-8947. ; 73:2, s. 113-129
-
Tidskriftsartikel (refereegranskat)abstract
- Among the different membrane processes, organophilic pervaporation is one of the most promising technologies for environmental applications. The aim of this paper is to give a thorough introduction to organophilic pervaporation in the context of wastewater treatment. The emerging process of organophilic pervaporation is introduced together with other membrane processes relevant for environmental applications. With regard to the performance of pervaporation, an engineering model is presented which will enable ready assessment of process and module design. Sorption and coupled diffusion are covered in the model. Selection criteria for membranes and transport resistances for the mass transport as key process engineering parameters are included. The influences of permeate pressure and temperature upon performance are discussed and a description of commercial pervaporation modules given. Following a brief description of the hierarchy of waste management practice, guidelines for applying and integrating pervaporation into a process are proposed. The importance of considering hybrid processes is highlighted. A case study for phenol recovery with water treatment to 5 ppm is considered. Finally, present restrictions on the use of pervaporation in wastewater treatment such as (a) the unavailability of appropriate membranes and (b) fouling of the membrane are discussed and approaches to overcome the restrictions are presented.
|
|
| 10. |
- Wik, Torsten, 1968
(författare)
-
Dynamic Modeling of Nitrifying Trickling Filters
- 1996
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Trickling filters, one kind of biofilm reactors, for removal of nitrogen in municipal wastewater are presently increasing in number in Sweden. For improved control and operation of wastewater treatment plants, where such reactors are used, dynamic models describing their behavior are necessary. In this thesis a physical dynamic model of cross-flow nitrifying trickling filters (NTFs), based on a general multi-species biofilm model, is presented. The model predicts effluent concentrations of ammonium, nitrite, nitrate, and alkalinity as functions of the corresponding influent concentrations, the water temperature, the flow, and the present state of the distribution of the nitrifying bacteria {\it Nitrosomonas} and {\it Nitrobacter} in the biofilm of the NTF. Efficient methods to solve the model equations, also in steady state, are presented. Experimental data achieved on a large pilot scale trickling filter are compared with model simulations, and model assumptions are experimentally validated. From the modeling and experiments it is concluded that the dynamics of the NTF can be divided into two modes: One fast mode that can be assumed to depend only on the mixing in the bulk, and one slow mode that depends on the bacterial growth and decay in the biofilm. The settling times of the two modes are separated by a factor of order 1000, which considerably simplifies model simulations. Comparisons between experimental data and a simplified version of the model show that it takes less than a few minutes for the nitrifying bacteria in the biofilm to change their substrate uptake rate after changes in substrate bulk concentrations.From pulse experiments and laminar flow theory analysis it is shown that the flow through the NTF is turbulent and, hence, significant mixing occurs inside the NTF. The residence time distribution can be approximated by a model of continuously stirred tanks in series.Comparisons between simulations and semi-stationary data show that the substrate flux into the biofilm is enhanced by an increase in flow, probably due to increased turbulence in the bulk. Analysis and simulation of steady-state multi-species biofilms in general indicate that bacterial coexistence is not only dependent on the bulk water substrate concentrations, but also on the biofilm thickness, which means that control of the biofilm thickness may be a way of controlling the bacterial composition in biofilms.
|
|
| 11. |
- Sandström, Olof, et al.
(författare)
-
Miljöpåverkan av skogsindustriutsläpp
- 1997
-
Ingår i: Naturvårdsverkets rapportserie. Rapport 4695. ; , s. 1-89
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
|
|
| 12. |
- Bjorklund, Robert B., et al.
(författare)
-
Vapor adsorption in thin silicalite-1 films studied by spectroscopic ellipsometry
- 1998
-
Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 102:12, s. 2245-2250
-
Tidskriftsartikel (refereegranskat)abstract
- Thin films of silicalite-1 grown on silicon substrates were studied by spectroscopic ellipsometry. Analysis of spectra using an optical model consisting of a single porous layer on silicon yielded average film thicknesses of 84 and 223 nm for films synthesized for 10 and 30 h. Void fraction for the films was 0.32-0.33. Vapor adsorption from a nitrogen carrier gas at room temperature was monitored by ellipsometry. Isotherms for different adsorbates were obtained by analysis of spectra taken at different vapor concentrations using an optical model where the void volume was filled with both nitrogen and condensed vapors. Quantification of the condensed vapor amount was based on the changes in refractive index when adsorbates replaced nitrogen in the pores. Adsorbate volumes for water, toluene, 1-propanol, and hexane were 0.12, 0.12, 0.15, and 0.17 cm3 liquid g-1 film, respectively.
|
|
| 13. |
- Björklund, Robert B., et al.
(författare)
-
Ellipsometric study of oxide removal from steel surfaces in hydrochloric acid solutions
- 1999
-
Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 15:2, s. 494-499
-
Tidskriftsartikel (refereegranskat)abstract
- Iron oxide films formed on three different steel surfaces by thermal oxidation were removed in hydrochloric acid solutions at 20 C. The oxide removal process in flowing solutions was followed in situ by ellipsometry. Two different removal mechanisms were observed in 0.5-2 M HCl, one where undermining of the film resulted in large intact pieces of the oxide leaving the substrate surface at the end of the removal period, and one where the film scaled off the surface in small pieces during the entire removal process. Oxide films which exhibited the undermining mechanism were found to contain about 10% hematite (Fe2O3) and 90% magnetite (Fe3O4). The scaling off mechanism was observed for films which were nearly pure magnetite. Optical models were constructed using data from scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements for the thicknesses and compositions of films after different immersion times in HCl solutions. Spectra calculated from the models agreed well with the experimental spectra.
|
|
| 14. |
- Creaser, Derek
(författare)
-
Crystal population balance model for nucleation and growth of colloidal TPA-silicalite-1
- 1999
-
Ingår i: Porous materials in environmentally friendly processes. - Amsterdam : Elsevier. - 0444502440 ; , s. 117-124
-
Konferensbidrag (refereegranskat)abstract
- A model describing the crystallization of colloidal TPA-silicalite-1 is developed and tested. The nucleation rate is approximated from experimental data. Crystal growth is surface-reaction limited and the Gibbs-Thomson effect suppresses growth of small crystals, resulting in an induction period. Slow crystal growth during the nucleation period results in the narrow crystal size distribution typically observed for colloidal TPA-silicalite-1 syntheses.
|
|
| 15. |
- Creaser, Derek, et al.
(författare)
-
Cyclic operation of the oxidative dehydrogenation of propane
- 1999
-
Ingår i: Chemical Engineering Science. - 0009-2509 .- 1873-4405. ; 54:20, s. 4437-4448
-
Tidskriftsartikel (refereegranskat)abstract
- The cyclic operation of the oxidative dehydrogenation of propane over a V-Mg-O catalyst by the alternate feeding of propane and oxygen gas mixtures was investigated. Generally, the response of the reaction products following a step-change in composition determined the cyclic conditions that resulted in an improved yield of propene. By alternating propane and oxygen with a 1 : 1 cycle split, time-average propene yields higher than steady state could be obtained. The optimal cycle period was about 60 s. This same period provided high propane conversion, as well as high propene selectivity. The cycle split could be varied by shortening the oxygen-feed half of the cycle with little effect on the time-average results. The oxygen half-cycle needed only to be long enough to reoxidize the catalyst. The propane feed scheme was varied by feeding increasing amounts of propane in the oxygen half-cycle. The change in feed scheme caused an increase in the time-average propane conversion that resulted in some further improvement in the propene yield despite some loss in propene selectivity.
|
|
| 16. |
- Creaser, D., et al.
(författare)
-
Oxygen partial pressure effects on the oxidative dehydrogenation of propane
- 1999
-
Ingår i: Chemical Engineering Science. - 0009-2509 .- 1873-4405. ; 54:20, s. 4365-4370
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of the gas-phase oxygen partial pressure for the oxidative dehydrogenation of propane over a V-Mg-O catalyst was investigated to explore reasons for improved propene yields when dehydrogenation was carried out under periodic operation. Steady-state experiments were performed in which the oxygen partial pressure was varied over a large range at constant propane partial pressures. In addition, the catalyst mass was varied to control the propane conversion so that selectivity and yield could be compared at a constant propane conversion. At the same propane conversion, propene selectivity increased as the partial pressure of oxygen decreased. Thus, propene yields can be improved at steady state by employing low oxygen partial pressures. It is likely that this is the reason for the higher propene yields observed under unsteady-state or periodic operation. It appears that the reaction mechanism must consist of more than one pathway for production of carbon oxides
|
|
| 17. |
- Creaser, Derek, et al.
(författare)
-
Transient kinetic analysis of the oxidative dehydrogenation of propane
- 1999
-
Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 182:1, s. 264-269
-
Tidskriftsartikel (refereegranskat)abstract
- Oxidative dehydrogenation of propane was studied using various transient techniques. Results support a redox mechanism in which propane reduces the catalyst, which is reoxidized by gas-phase oxygen. Only lattice oxygen participates in propene formation. Desorbable oxygen is a major source of poor selectivity, although lattice oxygen also causes total oxidation. Consequently, propene selectivity in the absence of gas-phase O2is superior to co-feed, steady-state selectivity at the same propane conversion. Propene selectivity is further improved by increasing the degree of catalyst reduction.
|
|
| 18. |
- Creaser, Derek, et al.
(författare)
-
Transient study of oxidative dehydrogenation of propane
- 1999
-
Ingår i: Applied Catalysis A. - 0926-860X .- 1873-3875. ; 187:1, s. 147-160
-
Tidskriftsartikel (refereegranskat)abstract
- Kinetics and the mechanism of the oxidative dehydrogenation of propane were investigated using various transient techniques. Results support a redox reaction mechanism in which propane and intermediate products react with lattice oxygen, reducing the catalyst surface, which is reoxidized by gas-phase O2. Partial reduction of the catalyst occurs during the start-up to a steady state. Successive pulsing with C3H8 reduced V5+ in the magnesium ortho-vanadate phase to V2+. Carbon-containing species were observed upon interruption of the reaction, although only minute amounts were formed. Cycling increases the amount of the carbon deposited, but this carbon is reactive and most of it is oxidized in the succeeding O2 pulse. Temperature-programmed oxidation (TPO) experiments on the catalyst used in steady-state operation revealed mainly strongly bound carbonaceous matter on the catalyst, but this carbon deposition did not affect catalyst activity. Thus, adsorbed oxygen is an important source of total combustion. Our experiments show, however, that lattice oxygen also produces total oxidation. Propene selectivity of the reaction in the absence of gas-phase O2 was superior to steady-state selectivity, at the same propane conversion. Propene selectivity could be further improved by increasing the degree of reduction of the catalyst
|
|
| 19. |
- Dyhr, Kurt, et al.
(författare)
-
Effects of zeolite addition on the manganese catalyzed decomposition of hydrogen peroxide
- 1999
-
Ingår i: Journal of porous materials. - 1380-2224 .- 1573-4854. ; 6:4, s. 275-282
-
Tidskriftsartikel (refereegranskat)abstract
- Manganese catalyzed decomposition of hydrogen peroxide in the presence of various zeolites was investigated at different pH values, Mn concentrations and zeolite: Mn ratios. Depending upon the level of these parameters the zeolites studied displayed both catalytic and inhibitive effects on the rate of decomposition. At a high pH (10.8) Mordenite 10A in a twofold excess on an ion-exchange capacity basis acted as an inhibitor whereas the other zeolites had little or no effect on the reaction rate. At a lower pH (9.0) zeolite A when added in large excess resulted in a remarkable inhibitive effect whereas the same zeolite resulted in a promotion of the catalytic action of manganese at an intermediate pH (9.9). The complex catalytic nature of zeolite-peroxide-Mn systems is discussed.
|
|
| 20. |
- Dyhr, Kurt, et al.
(författare)
-
Use of zeolites in hydrogen peroxide bleaching of pulp
- 1998
-
Ingår i: Nordic Pulp & Paper Research Journal. - : Walter de Gruyter GmbH. - 0283-2631 .- 2000-0669. ; 13:4, s. 257-262
-
Tidskriftsartikel (refereegranskat)abstract
- The use of zeolites for inhibition of transition metal catalyzed decomposition of hydrogen peroxide was evaluated. A number of zeolite types (A, P, Y, and mordenite 10A), with different crystal structures and ion-exchange characteristics, were tested as additives in bleaching experiments using a mechanical (TMP) pulp and a chemical (kraft) pulp. For the mechanical pulp, addition of zeolites A and P at a moderate alkali charge resulted in a significant improvement of the bleaching performance as signified by an increase in both brightness and residual peroxide content of the bleaching liquor. The other zeolite types studied did not give any positive effects at the bleaching conditions used. For the chemical pulp, no significant improvements were obtained at any alkali charge using zeolite A as an additive. At high alkali charges, addition of zeolite A resulted in a substantially lower brightness than that reached in a zeolite free reference experiment. The effects of zeolite addition upon the bleaching of pulp are complex and dependent upon a number of factors. Parameters of great importance are the bleaching pH, consistency and the zeolite type and charge
|
|
| 21. |
|
|
| 22. |
- Engström, Vania
(författare)
-
A vibrational spectroscopy study of the growth of silicate-1 films on noble metal surfaces
- 1999
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main objective of this thesis was to acquire a better understanding of the mechanisms involved in the preparation of Silicalite-1 films in general and on gold surfaces in particular. Molecular sieve films on gold surfaces were prepared using the seed film method. The deficiency of negative charge on a gold surface in aqueous solution was overcome by a coupling agent (gamma-mercaptopropyltrimethoxysilane or sodium hydrosulfide). The resulting negatively charged surface was modified by a cationic polymer, facilitating the adsorption of negatively charged colloidal silicalite-1 seed crystals. The seed crystals were grown into a continuous molecular sieve film during hydrothermal treatment in synthesis solution. Reflection absorption infrared spectroscopy was found to be an excellent method to gain insight about the structures formed after each step in this preparation procedure. The recorded vibrational spectra were compared with vibrational modes calculated from the dynamic matrix of the clusters concidered. These comparisons showed that the organic coupling agent was oriented with the methylene chain perpendicular to the gold surface, the cationic polymer introduces disorder in the silane layer, the silanol groups predominantly form 6-membered puckered Si-O rings upon condensation and that linear defects in the films are formed during the hydrothermal treatment. The effects of seed size and hydrothermal treatment time on the formation of linear defects were investigated.
|
|
| 23. |
- Hedlund, Jonas, et al.
(författare)
-
Controlling the preferred orientation in silicalite-1 films synthesized by seeding
- 1999
-
Ingår i: Microporous and Mesoporous Materials. - 1387-1811 .- 1873-3093. ; 28:1, s. 185-194
-
Tidskriftsartikel (refereegranskat)abstract
- Single crystal silicon (100) wafers were seeded with colloidal silicalite-1 crystals and hydrothermally treated in a precursor solution to grow thin silicalite-1 films. A total of 28 experiments in eight series were investigated with SEM and XRD to evaluate the preferred orientation of the crystals constituting the films. The investigated parameters in the film formation process were seed crystal size, amount of adsorbed seed crystals and film thickness after hydrothermal treatment of the seeded substrates. In thin films, most of the crystalline material is oriented with the b-axis perpendicular to the substrate surface. In thick films, most of the crystalline material is oriented with the a-axis perpendicular to the substrate surface. The change in preferred orientation with film thickness is faster when small seeds are used. The amount of adsorbed seeds has a larger influence on the preferred orientation when large seeds are used. A mechanism explaining these trends is proposed. The choice of size and coverage of seeds can be used to control the preferred orientation of the crystals in a film of given thickness within certain limitations.
|
|
| 24. |
|
|
| 25. |
- Hedlund, Jonas, et al.
(författare)
-
Synthesis of thin molecular sieve films
- 1999
-
Ingår i: Nanostructured materials. - 0965-9773 .- 1872-9150. ; 12:1-4, s. 413-416
-
Tidskriftsartikel (refereegranskat)abstract
- A method to synthesize thin films of various molecular sieves on a number of substrates employing pre-seeding has been developed. The substrate is modified to enable adsorption of seed crystals. Substrates with an originally negative surface charge are charge reversed by adsorption of cationic polymer molecules. Nobel metal substrates are first silanized to obtain a negative surface charge. Nano seed crystals are adsorbed on the modified substrates from a sol and induced to grow in a synthesis solution to form a dense film with controlled thickness. The versatility of the method may be of great value in the development of novel applications. Films have been tested in membrane and sensor applications with successful results.
|
|
| 26. |
- Hedlund, Jonas, et al.
(författare)
-
Synthesis of ultra thin films of molecular sieves by the seed film method
- 1997
-
Ingår i: Progress in zeolite and microporous materials. - Amsterdam : Elsevier. - 0444823441 ; , s. 2203-2210
-
Konferensbidrag (refereegranskat)abstract
- A novel and flexible technique for synthesizing continuous ultra thin (<100 nm) molecular sieve films on substrates has been developed. The technique consists of two steps. A monolayer of discrete colloidal zeolite seed crystals is adsorbed on the substrate, whereafter the crystals are allowed to grow into a continuous film upon hydrothermal treatment in a molecular sieve precursor solution. The technique is exemplified by the formation of silicalite-1 films on silicon wafers. The final film thickness in the examples presented was in the range 80 and 800 nm. The films are continuous and crystalline as confirmed by SEM, Kr-adsorption data, ellipsometry, XRD and FTIR.
|
|
| 27. |
- Hedlund, Jonas
(författare)
-
Thin films of molecular sieves : synthesis and applications
- 1998
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The seed film method for synthesis of ultra thin and thin molecular sieve films was developed. The versatility of the method was demonstrated by successful preparation of films of a number of molecular sieves (silicate-1, ZSM-5, NA) on various supports (silicon, alumina, gold, etc.). The preferred orientation of the crystalline material could be controlled. Silicate-1 films on silicon wafers were evaluated as optical gas sensors. Sorption isotherms of water, toluene, 1-propanol and n-hexane were recorded. ZSM-5 films with a thickness of 1.5 um were crystallized on a porous support and the membranes were evaluated in permeation experiments. The flux in single gas experiments decreased in the series H2O, H2, CO2, O2, N2 and CH4. The difference in flux between each molecule was about 1 order of magnitude which indicated the high quality of the membranes. The measured separation factors for binary mixtures of N2/SF6 and H2/i-C4 were 99 and 66.
|
|
| 28. |
- Hedlund, Jonas, et al.
(författare)
-
Thin zeolite NA films by the seed film method
- 1999
-
Ingår i: Proceedings of the 12th International Zeolite Conference. - Warrendale, Pa : Materials Research Society. - 1558994637 ; , s. 1857-1862
-
Konferensbidrag (refereegranskat)
|
|
| 29. |
|
|
| 30. |
- Hedlund, Jonas, et al.
(författare)
-
Ultrathin oriented zeolite LTA films
- 1997
-
Ingår i: Chemical Communications. - 1359-7345 .- 1364-548X. ; :13, s. 1193-1194
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 31. |
- Hedlund, Jonas, et al.
(författare)
-
ZSM-5 membranes synthesized without organic templates using a seeding technique
- 1999
-
Ingår i: Journal of Membrane Science. - 0376-7388 .- 1873-3123. ; 159:1-2, s. 263-273
-
Tidskriftsartikel (refereegranskat)abstract
- Porous α-alumina supports were seeded with colloidal TPA-silicalite-1 crystals and calcined. The supports were treated in a synthesis solution to grow the seed crystals into ZSM-5 films. The synthesis solution was free from organic template molecules in order to avoid the calcination step which often introduces cracks in the synthesized zeolite film. An SEM investigation indicated that the zeolite films on the supports were defect free and that the film thickness was approximately 1.5 μm. XRD data showed that the film consisted of well-crystallized ZSM-5. The permeance in single gas experiments decreased in the order H2O, H2, CO2, O2, N2 and CH4. The difference in permeance between each molecular species in the series was almost one order of magnitude which indicated that the membranes were of a high quality. Molecules larger than CH4 permeated with similar and low rates, independent of their kinetic diameters, indicating a non-selective permeation path through defects in the zeolite films. However, the permeance of these larger molecules was less than 1/10 000 of that for H2O. The highest measured separation factors for binary mixtures of N2/SF6 and H2/i-C4 were 110 and 99.
|
|
| 32. |
- Lassinantti, Magdalena, et al.
(författare)
-
Synthesis of thin zeolite Y films on polished α-alumina wafers using a seeding technique
- 1999
-
Ingår i: Porous materials in environmentally friendly processes. - Amsterdam : Elsevier. - 0444502440 - 9780444502445 ; , s. 181-187
-
Konferensbidrag (refereegranskat)abstract
- Zeolite Y films were synthesized on polished α-alumina wafers by using a seeding technique which includes surface modification, seed adsorption and growth of the seed crystals into a dense film. The film thickness was found to be a linear function of synthesis duration up to a certain point. Further treatment reduced the film thickness. Continuous and crack free films with thicknesses in the range 210 to 2670 nm were synthesized in this work.
|
|
| 33. |
- Li, Q., et al.
(författare)
-
Nucleation period for TPA-silicalite-1 crystallization determined by a two-stage varying-temperature synthesis
- 1999
-
Ingår i: Microporous and Mesoporous Materials. - 1387-1811 .- 1873-3093. ; 31:1, s. 141-150
-
Tidskriftsartikel (refereegranskat)abstract
- A two-stage-varying-temperature synthesis procedure, which involves a rapid change in temperature at some point during the course of crystallization, was applied to the synthesis of discrete colloidal particles of TPA-silicalite-1. As the duration of the period at the initial synthesis temperature was extended, the crystal concentration and ultimate crystal size varied until they were approximately equal to those obtained for a complete synthesis at the initial temperature. At this point in the crystallization, it was concluded that the nucleation stage was completed. For syntheses performed at 60, 80 and 100°C, the duration of the nucleation period determined by this method was about 100 h, between 4 and 6 h and less than 2 h, respectively. Thus, nucleation, for this system, is a continuous process, and it was found that the rate of nucleation, which is initially high, declines, throughout the nucleation period. In all cases, nucleation occurred during an induction period when little or no crystal growth was observed, which explains why the syntheses yielded a product with a rather narrow crystal size distribution. If, for the two-stage syntheses, the temperature change was made after completion of the nucleation period, the second synthesis temperature controlled only the linear growth rate of the crystals and the final yield of silicalite-1 obtained
|
|
| 34. |
- Li, Qinghua, et al.
(författare)
-
The synthesis of small colloidal crystals of TPA-silicalite-1 with short synthesis times and high yields
- 1999
-
Ingår i: Studies in Surface Science and Catalysis. - 0167-2991. ; 125, s. 133-140
-
Tidskriftsartikel (refereegranskat)abstract
- A two-stage varying-temperature crystallization procedure can be used to synthesize particularly small colloidal crystals of TPA-silicalite-1 with reduced synthesis times and high yields. Using this procedure involves starting a synthesis at a reduced temperature which favours the nucleation of a large crystal population. The reduced temperature is maintained throughout the nucleation stage, after which an elevated temperature is used to accelerate the crystal growth and reach higher equilibrium yields favoured by higher temperatures
|
|
| 35. |
- Mintova, Svetlana, et al.
(författare)
-
Continuous films of zeolite ZSM-5 on modified gold surfaces
- 1997
-
Ingår i: Chemical Communications. - 1359-7345 .- 1364-548X. ; :1, s. 15-16
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 36. |
- Mintova, S., et al.
(författare)
-
Growth of silicalite-1 films on gold substrates
- 1997
-
Ingår i: Microporous Materials. - 0927-6513 .- 1873-3107. ; 11:3-4, s. 149-160
-
Tidskriftsartikel (refereegranskat)abstract
- The crystallization of thin continuous silicalite-1 films on modified gold surfaces prepared at 100°C in clear precursor solutions has been studied by grazing angle X-ray diffractometry, infrared spectroscopy, dynamic light scattering, scanning electron microscopy, Kr adsorption and ellipsometry. The gold surfaces were pre-treated with a silane coupling agent, gamma-mercaptopropyltrimethoxysilane, and hydrolysed in an acidic solution, pH < 4, to create a modified negatively charged interface on which positively charged colloidal silicalite-1 seed crystals were adsorbed. The colloidal seed crystals are believed to be preferentially adsorbed with their {010} face parallel to the Au surface. Continued growth of the oriented seed crystals in a following hydrothermal step results in the formation of a continuous thin oriented crystalline film. The mechanism for the formation of silicalite-1 films on Au is discussed
|
|
| 37. |
|
|
| 38. |
- Mintova, Svetlana, et al.
(författare)
-
Preparation of zeolite Y-vegetal fiber composite materials
- 1996
-
Ingår i: Journal of porous materials. - 1380-2224 .- 1573-4854. ; 3:3, s. 143-150
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 39. |
- Mintova, Svetlana, et al.
(författare)
-
Preparation of ZSM-5 films from template free precursors
- 1997
-
Ingår i: Journal of Materials Chemistry. - 0959-9428 .- 1364-5501. ; 7:12, s. 2341-2342
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 40. |
- Mintova, Svetlana, et al.
(författare)
-
ZSM-5 films prepared from template free precursors
- 1998
-
Ingår i: Journal of Materials Chemistry. - 0959-9428 .- 1364-5501. ; 8:10, s. 2217-2221
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 41. |
- Nohlgren, Ingrid
(författare)
-
Recovery of kraft black liquor using the titanate process : kinetics of the direct causticization reaction between sodium tri-titanate and sodium carbonate
- 1999
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The solid state reaction between sodium tri-titanate and sodium carbonate, forming mainly sodium penta-titanate, was investigated. Experiments were carried out in a micro-differential reactor made of quartz glass at various temperatures between 800°C and 880°C and in a pilot fluidized bed reactor operated in a semi-batch mode. In the micro-differential reactor, basic kinetic data was obtained by measuring the release of carbon dioxide during the reaction. Different kinetic models were considered to describe the conversion, such as the Valensi-Carter model for diffusion controlled reaction rates and the phase-boundary model for first-order reaction kinetics. Furthermore, a model that included both diffusion in the solid material and the chemical kinetics was derived. This model described the experimental data obtained in the micro-differential reactor very well. Finally, rating models describing the conversion obtained in a fluidized bed and in an entrained flow reactor were developed.
|
|
| 42. |
- Ravishankar, Raman, et al.
(författare)
-
Characterization of Nanosized Material Extracted from Clear Suspensions for MFI Zeolite Synthesis
- 1999
-
Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 103:24, s. 4960-4964
-
Tidskriftsartikel (refereegranskat)abstract
- The silica species contained in an aged clear suspension, which upon heating gives rise to the crystallization of Silicalite-1, were extracted with 80% efficiency using a sequence of acidification, salting out, phase transfer into organic solvent, and freeze-drying methods. This silica powder was characterized by X-ray scattering, transmission electron microscopy, atomic force microscopy, and 29Si magic angle spinning nuclear magnetic resonance. These techniques gave evidence for the presence of a very specific morphology, corresponding to slab shaped particles, with dimensions of 1.3 × 4.0 × 4.0 nm. The nanoslabs have the MFI structure with nine channel intersections per particle, each containing a TPA cation. The identity of the extracted nanoslabs with the species in suspension is evidenced with in situ and ex situ X-ray scattering.
|
|
| 43. |
- Ravishankar, Raman, et al.
(författare)
-
Physicochemical characterization of silicalite-1 nanophase material
- 1998
-
Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 102:15, s. 2633-2639
-
Tidskriftsartikel (refereegranskat)abstract
- A silicalite-1 nanophase material with an elementary particle size of 18-100 nm is synthesized from clear solution and isolated and purified using supercentrifugation. The nanopowder is characterized in detail using scanning electron microscopy, high-resolution transmission electron microscopy, attenuated force microscopy, 29Si magic angle spinning NMR, 13C cross polarization magic angle spinning NMR, X-ray diffraction, dinitrogen physisorption, and thermogravimetric analysis and compared with micrometer-sized silicalite-1. The nanosized and micrometer-sized materials have many common properties including the refined structure and the nature and concentrations of tetrapropylammonium species incorporated during the synthesis. Unique properties of the nanophase are a splitting of the characteristic framework vibration at 550 cm-1 into a doublet at 555 and 570 cm-1, a high concentration of defect sites, and a strain in the crystallites along the "a" crystallographic direction. The nanophase exhibits a two-stage dinitrogen physisorption in the low-pressure region, ascribed to adsorptions in micropores created by the stacking of the nanoparticles in addition to adsorptions in the intracrystalline micropores.
|
|
| 44. |
- Schoeman, Brian J.
(författare)
-
A high temperature in situ laser light-scattering study of the initial stage in the crystallization of TPA-silicalite-1
- 1997
-
Ingår i: Zeolites. - 0144-2449. ; 18:2-3, s. 97-105
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 45. |
- Schoeman, Brian J., et al.
(författare)
-
A study of the initial stage in the crystallization of TPA-silicalite-1
- 1996
-
Ingår i: Zeolites. - 0144-2449. ; 17:5-6, s. 447-456
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 46. |
- Schoeman, Brian J.
(författare)
-
Analysis of the nucleation and growth of TPA-silicalite-1 at elevated temperatures with the emphasis on colloidal stability
- 1998
-
Ingår i: Microporous and Mesoporous Materials. - 1387-1811 .- 1873-3093. ; 22:1-3, s. 9-22
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 47. |
- Schoeman, Brian J., et al.
(författare)
-
Dynamic light scattering applied to the synthesis of colloidal zeolite
- 1995
-
Ingår i: Journal of porous materials. - 1380-2224 .- 1573-4854. ; 1:2, s. 185-198
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 48. |
- Schoeman, Brian J.
(författare)
-
Homogeneous nature of clear TPA-silicalite-1 precursor solutions
- 1997
-
Ingår i: Microporous Materials. - 0927-6513 .- 1873-3107. ; 9:5-6, s. 267-271
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 49. |
- Schoeman, Brian J., et al.
(författare)
-
Nanoparticles of microporous materials
- 1999
-
Ingår i: Nanostructured materials. - 0965-9773 .- 1872-9150. ; 12:1, s. 49-54
-
Tidskriftsartikel (refereegranskat)abstract
- 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
|
|
| 50. |
- Schoeman, Brian J., et al.
(författare)
-
The growth of sub-micron films of TPA-silicalite-1 on single crystal silicon wafers from low-temperature clear solutions
- 1997
-
Ingår i: Zeolites. - 0144-2449. ; 19:1, s. 21-28
-
Tidskriftsartikel (refereegranskat)abstract
- 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
|
|
