Pillared clays: preparation and characterization of clay minerals and aluminum-based pillaring agents

After an extensive introductory chapter (Chapter I), in which the background and the aim of the research is dealt with, the hydrothermal synthesis and the characterization of Na-beidellite is discussed in Chapter II and III. The conditions of temperature, water pressure, and sodium activity under which Na-beidellite can be synthesized in the chemical system Na₂0-AI₂0₃-Si0₂-H₂0 are investigated in Chapter IV. The stability field is limited at low temperatures by the formation of kaolinite and at high temperatures by that of paragonite and quartz. In Chapter V solid-state magic-angle spinning 23Na NMR combined with XRD and TGA is used to study the interlayer collapse and the migration of sodium present at the interlayer during dehydration of synthetic Na-beidellite. It is shown that the Na+ surrounded by two water molecules is relocated in the hexagonal cavities of the tetrahedral sheet. The low-temperature synthesis of ammonium-saponites is considered in Chapter VI, while solid-state 27AI NMR combined with 29Si I\lMR provides evidence for the presence of AI at the octahedral and the interlayer sites as dealt with in Chapter VII. In Chapter VIII a model is developed for the crystallization of saponite based on the hydrothermal synthesis of ammonium-saponite at increasing periods of aging time. Chapter IX discusses the synthesis of Mg-saponites using gels containing different competing cations. Since the preparation of AI pillaring agents is highly important in the preparation of pillared clays, much work has been devoted to the study of the forced hydrolysis of AI³⁺. Chapter X to XV are dealing with NMR studies of the aqueous chemistry of AI³⁺. Chapter X to XIII concentrate on the formation, thermal stability, and aging of monomeric, oligomeric, and tridecameric A1³⁺ species during forced hydrolysis. In Chapter XIII direct evidence based on NMR line broadening data for the existence of the [AI(OH)₂] + species is provided. The aging of the tridecameric AI complex in partly hydrolyzed AI-sec-butoxide solutions and the reaction to fibrous boehmite are considered in Chapter XIV. Chapter XV and XVI deal with the characterization, thermal stability, and NMR properties of basic AI sulfate resulting from addition of sodium sulfate solution to partly hydrolyzed AI solutions containing the tridecameric AI species, such as described in Chapter X to XIII. Pillaring and thermal treatment of synthetic beidellite and natural montmorillonite is described in Chapter XVII and the catalytic conversion of thiophene over pillared clays into which nickel has been applied in Chapter XVIII. Finally in Chapter XIX some concluding remarks concerning the results obtained in this study and the implications for the applications of (synthetic) clay minerals in catalytic reactions are made