Abstract
Solid state formation from solution is one of the most fundamental and important processes of chemistry in general. The elementary mechanisms, leading to the formation of specific solid state structures, have remained to be highly elusive phenomena. This is mostly due to the size of species occurring during the prenucleation and nucleation step which lies between molecules and the final solid and the hence resulting analytical limitations.
Zeolitic materials form one of the most important classes of solid catalysts and are widely used in industrial applications such as catalysis, chemical separation and adsorption. Their properties highly depend on their framework topology and on the presence, nature and distribution of reactive intra- or extraframework centers. Hence, the ultimate goal would be to establish tailored, rationally controlled syntheses, leading to zeolitic materials with structural characteristics, most adequate for a particular application; instead of trial-and-error approaches. However, to fulfill this goal it is necessary to understand the formation of zeolitic materials on a molecular level. Therefore mass spectrometry, and especially electrospray ionization mass spectrometry (ESI-MS), were used as the major tool to analyze prenucleating and nucleating solutions of zeolitic materials. Furthermore, structural information about occurring species were obtained in MS/MS experiments.
By the analyses of prenucleating solutions, the incorporation of various heteroelements into silicate species was studied. High resolution mass spectrometry as well as experiments with isotopically labeled compounds allowed to determine the incorporation of aluminum into silicate species. Aluminum was found to be randomly built into all detected silicate species. In combination with MS/MS experiments it has been shown, that Loewenstein's rule does not necessarily have to be fulfilled. Still, aluminum containing silicate species in which Al-O-Al bonds are avoided have been found to be predominant in solution.
Gallium and germanium containing prenucleating solutions, have been studied as well. The obtained data have proven that structure directing effects results from the introduction of both heteroelements into the synthesis mixtures. This structure directing effect was found to be even stronger than the effect of the used organic templates. Furthermore, it highly depends on the character of the introduced heteroelement: Germanium on the one hand induces the presence of single 4R and D4R units in solution; containing a maximum number of three germanium atoms per D4R. Gallium on the other hand is not incorporated into cyclic or bicyclic silicate species. Rather, it is attached to or acts as a linker between those.
A further topic of this thesis was the study of heteroelement containing nucleating solutions of zeolitic materials. Depending on the reaction parameters applied, different reactor techniques have been used to enable time-dependent analysis. In these works new insight into the formation mechanisms of systems like the [Ge]ZSM-5, TS-1 (both with MFI topology), the polymorph C of zeolite Beta (BEC topology), zeolite A (LTA topology) and zeolite ITQ-21 was obtained. For the MFI type zeolites, species based on the D5R unit were found to occur in solution. Although the D5R unit does not occur in the MFI structure as such, it includes a major structural element of MFI-type zeolites, a five membered ring (5R). Such a 5R could be incorporated into a growing crystal by a mechanism determined for isolated D3R units in this thesis: Based on experiments with isotopically labeled compounds, two D3R units show a concerted exchange of complete three-ring faces, leading to the formation of a new D3R and two single 3R which can recombine as well.
All other nucleating systems studied in this thesis were synthesized with higher germanium loadings (Si/Ge = 2 or 1). Thus 4R and D4R species were observed in the nucleating solutions containing up to three germanium atoms. Larger oligomers with structural characteristics of the final zeolitic materials were found to be present immediately before nucleation as well. MS/MS experiments proved, that these oligomers are built up by 4R and D4R subunits, whereas their connectivity depends on the used organic structure directing agent (SDA).
In addition, the influence of the used mineralizing agent was studied for both systems, polymorph C of zeolite Beta (BEC topology) and zeolite ITQ-21. Although the refinement of XRD data suggests a preferred position of fluoride ions in the center of the D4R units no signals, which could be attributed to fluoride-containing species, were observed in the ESI mass spectra, and in particular no signals of D4R units containing F-. Thus, the fluoride found in the fully crystallized materials has to be incorporated later during the syntheses. However, the use of fluoride instead of hydroxide ions as mineralizer does influence the formation of both materials: Although a structure directing effect emerging from fluoride appears to be rather improbable for the studied systems, the fluoride accelerates the reaction rate, as shown by DLS experiments.
In this thesis, the use of ESI-MS and ESI-MS/MS has given new insight into the formation of different zeolitic materials. Structure directing effects resulting from both, the introduction of heteroelements and organic SDAs, highly influence the formation of the occurring silicate species. For the first time, silicates with structural imprints of the final zeolitic materials were observed already in solution, immediately before nucleation.
