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Aqueous behaviour of characteristic molecules in the synthesis of porous silica materials : - an atomistic description

Nilsson, Emelie LU (2016)
Abstract (Swedish)
Kisel är ett vanligt förekommande grundämne som gärna bildar en oxid, nämligen kiseldioxid. Kiseldioxid träffar man i hög utsträckning på i det dagliga livet. Det är nämligen huvudbeståndsdelen i sand, glasvaror, kvarts, många keramiska material men också i den färgskiftande ädelstenen opal. Det finns också flera grupper av porösa kiseldioxidmaterial, till exempel så kallade zeoliter som har mycket små porer och så kallade mesoporösa material med något större porer. Zeoliter träffar man ofta på i tvättmedel där de fungerar som jonbytare och gör hårt vatten mjukt. Zeoliter används också som katalysatorer och absorptionsmaterial. Mesoporösa material kan användas för bland annat sortering av molekyler, eller som läkemedelsbärare då... (More)
Kisel är ett vanligt förekommande grundämne som gärna bildar en oxid, nämligen kiseldioxid. Kiseldioxid träffar man i hög utsträckning på i det dagliga livet. Det är nämligen huvudbeståndsdelen i sand, glasvaror, kvarts, många keramiska material men också i den färgskiftande ädelstenen opal. Det finns också flera grupper av porösa kiseldioxidmaterial, till exempel så kallade zeoliter som har mycket små porer och så kallade mesoporösa material med något större porer. Zeoliter träffar man ofta på i tvättmedel där de fungerar som jonbytare och gör hårt vatten mjukt. Zeoliter används också som katalysatorer och absorptionsmaterial. Mesoporösa material kan användas för bland annat sortering av molekyler, eller som läkemedelsbärare då porstorleken och dess struktur kan påverka tiden för läkemedelsutsöndring.

För att kunna anpassa dessa material efter det ständigt ökande kravet på nya applikationer, hade det varit önskvärt att kunna kartlägga bildandet av dessa ämnen. Hade vi haft total förståelse för hur dessa material bildas så hade vi kunnat, mycket enklare, designa materialen utifrån efterfrågan på deras egenskaper.

Den här studien har syftat på att försöka förstå hur porösa kiseldioxid material byggs upp; hur de olika byggstenarna sätts samman och hur de påverkar varandra. Modelsystem är därför skapat för att likna materialens syntes, men som är förenklade och innehåller bara ett fåtal komponenter. Själva reaktionen går fort och i modellsystemen är det som om vi "stannat tiden" för att få en ögonblicksbild av synteslösningen. Modelleringen, av dessa system, ger oss många ögonblicksbilder vilket ger en översikt av var molekylerna finns i lösning. Med denna förståelse för molekylerna och deras beteende kan vi förhoppningsvis "styra" dears beteende och bli bättre på att designa materialen. (Less)
Abstract
The formation process of porous silica materials is an intricate process, involving a number of components that interact to form a highly organized material. Both zeolites and mesoporous silica materials typically require the presence of organic structure director agents (SDAs) around which the silica network polymerizes. For zeolites the SDAs are molecular and for mesoporous silica the SDAs assemble into aggregates, i.e. micelles. The aim of this thesis is to further the understanding of the initial stages of the formation of porous silica materials. This has been done by investigating aqueous model systems of components critical for the formation of porous silica materials using neutron scattering coupled with empirical potential... (More)
The formation process of porous silica materials is an intricate process, involving a number of components that interact to form a highly organized material. Both zeolites and mesoporous silica materials typically require the presence of organic structure director agents (SDAs) around which the silica network polymerizes. For zeolites the SDAs are molecular and for mesoporous silica the SDAs assemble into aggregates, i.e. micelles. The aim of this thesis is to further the understanding of the initial stages of the formation of porous silica materials. This has been done by investigating aqueous model systems of components critical for the formation of porous silica materials using neutron scattering coupled with empirical potential structure refinement (EPSR) to arrive at atomistic descriptions of the systems. The systems are based on aqueous solutions of single components under different conditions as well as a system in which two components are mixed and allowed to interact. Molecular SDAs, tetramethylammonium bromide and tetrapropylammonium bromide respectively, were found to have different association behavior; small clusters of tetrapropylammonium ions were formed whereas tetramethylammonium only formed pairs. Micellar SDA aggregates of decyltrimethylammonium ions with different counterions reveal a strong dependence on the identity of the counterions. The effect of the counterions appears to originate from a subtle balance between electrostatic interactions and ion-specific effects.

A model system for oligomeric silica species was evaluated. The system is based on cubic silsesquioxane. 29Si-NMR was used to identify and quantify the silica species that occur in solution. The silica species reveal a high dependence on additives in solution, which can stabilise the cubic silica molecule. Finally the silica model was probed in the presence of decyltrimethylammonium ions in order to probe interactions that occur in the early stages of the synthesis of MCM-41. The silica species show little affinity for the micelle surface, even though being negatively charged, however their presence in the system greatly decrease the size of the micelles compared to those in a pure aqueous solution. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Schüth, Ferdi, Max-Planck-Institut für Kohlen-forschung, Mülheim, Germany
organization
publishing date
type
Thesis
publication status
published
subject
keywords
porous silica, struture directing agents, 29Si-NMR, neutron scattering, empirical potentail structure refinement, micelles
pages
146 pages
publisher
Lund University, Faculty of Science, Department of Chemistry, Division of Physical Chemistry
defense location
The Center for chemistry and chemical engineering, lecture hall B, Naturvetarvägen 14 (former Getingevägen 60), Lund
defense date
2016-06-13 10:00
ISBN
978-91-7422-452-8
language
English
LU publication?
yes
id
ff118023-d849-4f1c-b9f6-c0f3e5f6ec26
date added to LUP
2016-05-13 11:19:34
date last changed
2016-09-19 08:45:20
@phdthesis{ff118023-d849-4f1c-b9f6-c0f3e5f6ec26,
  abstract     = {The formation process of porous silica materials is an intricate process, involving a number of components that interact to form a highly organized material. Both zeolites and mesoporous silica materials typically require the presence of organic structure director agents (SDAs) around which the silica network polymerizes. For zeolites the SDAs are molecular and for mesoporous silica the SDAs assemble into aggregates, i.e. micelles. The aim of this thesis is to further the understanding of the initial stages of the formation of porous silica materials. This has been done by investigating aqueous model systems of components critical for the formation of porous silica materials using neutron scattering coupled with empirical potential structure refinement (EPSR) to arrive at atomistic descriptions of the systems. The systems are based on aqueous solutions of single components under different conditions as well as a system in which two components are mixed and allowed to interact. Molecular SDAs, tetramethylammonium bromide and   tetrapropylammonium bromide respectively, were found to have different association behavior; small clusters of tetrapropylammonium ions were formed whereas tetramethylammonium only formed pairs. Micellar SDA aggregates of decyltrimethylammonium ions with different counterions reveal a strong dependence on the identity of the counterions. The effect of the counterions appears to originate from a subtle balance between electrostatic interactions and ion-specific effects.<br/> <br/> A model system for oligomeric silica species was evaluated. The system is based on cubic silsesquioxane. 29Si-NMR was used to identify and quantify the silica species that occur in solution. The silica species reveal a high dependence on additives in solution, which can stabilise the cubic silica molecule. Finally the silica model was probed in the presence of decyltrimethylammonium ions in order to probe interactions that occur in the early stages of the synthesis of MCM-41. The silica species show little affinity for the micelle surface, even though being negatively charged, however their presence in the system greatly decrease the size of the micelles compared to those in a pure aqueous solution. },
  author       = {Nilsson, Emelie},
  isbn         = {978-91-7422-452-8},
  keyword      = {porous silica,struture directing agents,29Si-NMR,neutron scattering,empirical potentail structure refinement,micelles},
  language     = {eng},
  pages        = {146},
  publisher    = {Lund University, Faculty of Science, Department of Chemistry, Division of Physical Chemistry},
  school       = {Lund University},
  title        = {Aqueous behaviour of characteristic molecules in the synthesis of porous silica materials : - an atomistic description},
  year         = {2016},
}