Investigation of inorganic material growth processes

In situ investigation of inorganic material growth processes. Synthetic nanoporous silicate based materials contribute substantially to the development of a sustainable society. Silica gels, ordered mesoporous silica and zeolites are essential to catalysis and molecular separation in chemical production and environmental protection, conversion and storage of energy, detergent builders and daily, new applications are being discovered. The constant need for new porous materials motivated this researcher to try to deepen the fundamental understanding of the molecular mechanisms driving the formation of these often very complicated porous structures. Such improved insight ultimately will facilitate design and tailoring of materials for specific purposes. The complicated heterogeneous nature of the synthesis media, the complicated reaction pathways and the fast kinetics hinder the analytical accessibility explaining why in spite of significant efforts, the research community still is puzzled with the chemistry involved. The sol-gel approach enables the synthesis of a large variety of silica materials including sols, gels, porous glasses, films and fibers. Synthesis in acid media and sub-stoichiometric quantities of water are known to lead toward microporous materials with narrow pore size distribution. Investigation of the pore architecture of gels prepared via acid catalysis starting from tetraethylorthosilicate (TEOS) and tetramethylorthosilicate (TMOS) revealed significant differences. TEOS preferentially polymerizes into microporous materials with a narrow pore size distribution. While TMOS directs synthesis, under similar conditions, toward larger pore sizes. The development of mesoporosity occurs quite frequently when departing from TMOS. Additional small changes in the synthesis conditions, like temperature or aging time, induced important structural differences. Intrigued by both the different pore architecture in gels obtained departing from TEOS or TMOS and the potential to form mesoporous materials without the need for sacrificial templates, the sol-gel process was analyzed using in situ 29Si-NMR and UV-Raman spectroscopy in combination with molecular modeling. The use of sub-stoichiometric H2O : TEOS molar ratios slowed the silicate oligomerization reactions and allowed monitoring of the initial steps of condensation. TEOS speciation revealed to be quite restricted and mainly contained short linear chains and small cyclosilicates. The main reaction pathways were chain extensions and ring formation via cyclo-dimerization. Afterward, branching was introduced through dimer and trimer attachment. A gel network of small cyclosilicates interconnected by short chains was observed. Molecular modeling confirmed Raman signal and 29Si-NMR shift assignments, leading to accurate assignment of several 29Si NMR and UV-Raman signals which were previously not documented in literature. The combination of experiment and modeling allowed an analysis of the reaction kinetics. The derived kinetic model and the experimental observation both revealed that the H2O : TEOS molar ratio had a strong influence on the reaction kinetics, but not on the reaction pathways.A study on silica polymerization departing from TMOS using similar approaches revealed significant differences to exist already in the early oligomerization steps. Different oligomeric structures were observed depending on the use of TMOS versus TEOS. The main mechanistic differences are situated at the level of branching and introduction of cyclic units. TMOS was converted into short chains and condensation proceeded by selective reactions involving hydrolyzed central groups in chains, leading to larger oligomers containing several branches quite early in the condensation processes. Rings were formed later through internal condensation of sizeable silicate molecules. Both silicon sources also exhibited remarkable differences among hydrolysis reactions. While in the TEOS based system hydrolysis occurs according to the sequence monomer, dimer and end-groups, in the TMOS based system hydrolysis occurs preferentially on Q2. A tentative explanation is a balance between electronic and steric effects. While departing from TEOS, a limited set of cyclic oligomeric building units are obtained. TMOS builds networks of branched chains lacking small rings. The presence of a repetitive motive of branched species enables the building of open gel networks. Even subtle changes in the way these species interconnect, will create different local structural features, leading toward broader pore distributions. The observed differences in oligomerization pathways of TEOS and TMOS, adds substantially to acid catalyzed silica sol gel chemistry. This new insight into the nature of the oligomers will enable material scientists to master the development of precursor oligomers and finally, the pore architecture. A related field of research activity is the nucleation and growth of zeolites. Zeolites are porous silicates which are widely used in ion-exchange, adsorption and separation processes and catalysis. Commonly, they are synthesized from an aluminosilicate gel under hydrothermal conditions. While the synthetic procedure of zeolites is quite simple, the chemistry involved in zeolite growth has intrigued generations of researchers. Despite these efforts, only for a limited number of zeolites some insight into the molecular mechanisms related to nucleation and crystal growth has been gained. The complexity of zeolite crystallization, classifies the process among the more demanding research fields which depend on the development of ever better research instruments.Seeking to unravel this complexity, we challenged ourselves by personally designing and developing an instrument which could offer in situ and simultaneous monitoring of the complex and fast transitions by both UV-Raman and X-ray diffraction. During development, we redesigned a wide angle X-ray diffractometer and personally combined it with a selection of Raman components. Wefully designed and integrated the entire optical path. We implemented four available Raman laser lines. Two are in the UV region to take advantage of Resonance Raman effects and to avoid fluorescence interference. We have foreseen three different X-ray geometries i.e. Bragg-Brentano geometry (reflection mode), Transmission (Debye Scherrer for capillaries) and High Flux (focusing the beam on the sample). To our knowledge, the developed machine is unique in the world, offering both a UV and VIS Raman source. With success, we validated the instrument in the investigation of zeolite nucleation and growth of two closely related topologies (LTA and FAU). The machine achieved an elegant monitoring of the transition from amorphous to crystalline state by XRD in combination with detection of the early formed oligomeric units in solution by Raman. The results quickly revealed novel scientific insights. The initial amorphous gel apparently first reorganized and then acted as reservoir for silicate species resulting in construction of the final framework. A remarkable difference was observed in the early formed building units for zeolite A and X with related topologies. Zeolite A was confirmed to entirely assemble from 4-ring species like 4R or D4R. This ruled out the sodalite cage as common primary building unit for both topologies. The designed simultaneous approach also confirmed the crucial role of the charge balancing cation during the framework assembly. The control over sol-gel porosity, may lead to industrial alternatives for the well-known solid-state reactions, which create porous silica materials at much higher temperatures, often involving sacrificial templates. In the future, the new oligomerization schemes may inspire to design completely new structures. The relevance of the combined in situ UV-Raman and XRD instrument, coined with the name DiffRam, is not limited to crystal growth processes. It may be used in other research fields as well. At first, it may serve to understand phenomena situated at the transition from molecular to particle behavior, as, for example, in zeolite templated carbon nanotube growth. Moreover, the technique fulfills all necessary requests for thorough investigation of the state of adsorbed molecules. While Raman reveals the vibrations of the confined molecules, XRD may pinpoint their systematic position within the confined structure. The combination of UV- and VIS-Raman is ideally suited for Resonance Raman studies and mandatory in case of fluorescence interference. Additional experiments are accessible thanks to the implementation of four different laser lines. The different sources may be used for selective probing of different compounds within complex mixtures or scanning of materials over different depths.

Jaar van publicatie:2011

Toegankelijkheid:Closed