Design of (ordered) mesoporous aluminosilicates

What will be the future feedstock of the chemical industry? Will renewable carbon sources one day replace fossil sources? What will be the surface temperature of the earth by the time that happens? These are all very important questions which will drive scientific research in the near future. Whatever the answers might be, heterogeneous catalysts will play a crucial role in the sustainable and selective conversion of these sources to useful products by minimizing the energy input and the amount of unwanted by-products. Porous amorphous aluminosilicates are one of the most important materials in heterogeneous acid catalysis. Nevertheless the catalytic potential of these materials is poorly investigated. A literature study in this work on the acidic properties of existing amorphous aluminosilicates showed that the total number acid sites is typically rather low. Moreover only very few aluminum atoms in the silica network effectively give rise to an acid site. Previous works generally only incorporated more aluminum to counter these effects, however this decreased the porosity of the catalyst in many cases.

This PhD research is centered around the question whether more acid sites can be created in amorphous mesoporous aluminosilicates, without detrimental effects on the porous properties. Rather than incorporating an increasing amount of aluminum, a more fundamental research was done to determine a more strategic synthesis method for the acid sites. This led to a post-synthesis alumination procedure for porous silica in which a key factor was the pH development from alkaline to neutral. By using initially alkaline solutions, the aluminum in solution is present as tetrahedrally coordinated aluminate species, which can be incorporated into the silica material. The tetrahedral coordination of aluminum in a silica network is crucial for the creation of acid sites. In this manuscript the synthesis, the characterization of porosity, morphology, composition and acidity and the catalytic testing are all thoroughly discussed. The underlying synthesis mechanisms were studied as well.

Ordered mesoporous silica with and without Al have been used as model materials for amorphous aluminosilicates because of the easy synthesis and characterization. In Al-containing silica the use of an alkaline activation led to the transformation of unwanted extra-structure Al to tetrahedrally coordinated Al. This caused an increased acidity and catalytic activity. and an increased pore size. The use of an initially alkaline alumination protocol on pure silica materials did not alter the porous structure and generated a similar amounts of acid sites as the best available (commercial) methods. The materials in this work however contained considerably fewer Al, indicating the effectiveness of the developed procedure. Moreover the catalytic properties of these aluminated samples was unprecedented. The value of the alkaline alumination protocol was validated by the application on commercial silica. Despite only being a first extrapolation of the procedures for ordered materials, these samples showed higher or at least equal catalytic activity to commercially available amorphous silica-alumina in a broad range of acid catalyzed reactions. The optimization of the alkaline alumination procedure from this work for commercially available porous silica, can open the door to an affordable and simple synthesis technique for amorphous porous aluminosilicates with a high acidity and high catalytic activity.

Furthermore the incorporation of Al into hierarchical mesoporous materials was elaborated in this work as an example of a device with macroscopic ordering of accessible mesopores. Herein the use of Al-iso-propoxide as an Al source is exploited as the dissociated propanol decreases the formation of undesired mesopore orientations by a swelling of the surfactant micelles during the synthesis. This acid functionalization can generate new opportunities for applications.