Struktur-Aktivitätsbeziehungen von Zeolithen als feste Säurekatalysatoren in der Synthese von Oxymethylendimethylethern

• Structure-activity relations of zeolites as solid acid catalysts in the synthesis of oxymethylene dimethyl ethers

Fink, Anja Silke; Palkovits, Regina (Thesis advisor); Liauw, Marcel (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022

Abstract

Oxymethylene dimethyl ethers (OMEx) with a chain length of x = 3 - 5 (OME3-5) represent a promising diesel additive that can be produced from renewable resources. In this thesis, the structure-activity relations of zeolites in the OMEx synthesis were investigated starting from methanol and paraformaldehyde. The properties of zeolites can be tailored by a variety of parameters, but their interactions are controversially discussed in literature. Therefore, a fundamental understanding at the molecular level was developed. In the first part of the work, 14 commercial zeolites of four different structure types (HY, Hβ, HZSM-5, HMOR), each with different Si/Al ratios, were used in the OMEx synthesis. The results showed that zeolites with low Al content exhibited the highest catalytic activity related to a single acid site (AS). HZSM-5 114, the zeolite with the lowest Al content, possessed the highest TOF with a value of 8791 molOME3-5 molAS-1 h-1. This can be explained by the fact that the decrease in Al content leads to a decline of interactions between Al atoms and consequently to an increase in intrinsic acid strength i. e. the acid strength of a single site. It is thus confirmed that the catalysis of the OMEx synthesis requires a high acid strength. The interactions of Al atoms are also affected by the framework of the zeolite lattice. Therefore, the parameters Al content and framework type were combined in the topological Al density (TDAl). At TDAl < 2.6 ∙ 10-2, a significant increase in catalytic activity was observed, which can be attributed to the fact that all Al atoms are isolated in the zeolite lattice and hence each acid site has reached its highest acid strength. Subsequently, the model of TDAl was further developed to account for the nature of the acid. In agreement with literature, the Brønsted acid sites (BAS) were identified as active species in the OMEx synthesis. Here, Hβ 81 demonstrated the highest Brønsted catalytic activity with 20722 molOME3-5 molBAS-1 h-1. The results suggest that Lewis acid sites can increase the acid strength of neighbouring Brønsted acid sites through local interactions and therefore have a significant impact on the catalytic activity of zeolites. In the second part of the work, the acid sites of zeolite HY 35 were selectively passivated using silylation agents with different steric demands, thereby gaining knowledge about the localisation of the most active acid sites. It could be concluded that acid sites at the pore entrance possessed the highest catalytic activity, which can be explained by good accessibility and positive influences of the crystal lattice. The findings substantiate the high catalytic activity of Brønsted acid sites in zeolites, which was higher by a factor of 10 for Hβ 81 compared to the ion exchange resin Amberlyst 36. Furthermore, optimisation potentials for zeolites were revealed. On the one hand, the relevance of acid sites in the pore entrances was demonstrated. On the other hand, the results provide a basis to explore zeolites with low Al content, whose Brønsted acid density can be reduced while maintaining a certain Lewis acid density. Overall, this work establishes a foundation for the development of zeolites with a high catalytic activity in the OMEx synthesis and, as a result, contributes to the efficient use of resources in terms of a sustainable chemistry.