Insights into Reaction Behaviours in Single Zeolite ZSM-5 Channel Orientations as Studied with Operando UV-Vis Spectroscopy

Abstract

Zeolite ZSM-5 with an MFI framework, consisting of sinusoidal (a-axis) and straight (b-axis) channels, is a heavily utilised catalysts. This drives continuous research on understanding the structure-performance relationships. However, the role of each single zeolite channel orientation during chemical reactions has yet to be understood, due to the complex structures of the conventional zeolites. In this thesis, by rationally synthesising oriented crystals and thin-films with high (only) accessibility to sinusoidal or straight channels, the channel-performance relationships during oligomerisation reactions and alcohol-to-hydrocarbons processes was studied using a series of advanced characterisation techniques, e.g. operando UV-Vis Spectroscopy, Confocal Fluorescence Microscopy (CFM), X-ray Diffraction and NH3-TPD. Firstly, a- and b-oriented zeolite ZSM-5 crystals, with high accessibility to sinusoidal and straight channels, respectively, were studied to decipher the product distribution and deactivating behaviours of the catalysts, showing faster deactivation of a-oriented crystals and a more pronounced aromatic cycle in b-oriented crystals. Furthermore, the knowledge obtained from the oriented crystals was confirmed on oriented zeolite ZSM-5 thin-films. First, larger molecules: 4-methoxystyrene and thiophene, that are also catalysed by zeolite ZSM-5 were studied, demonstrating that the a-oriented channel favours the formation of small molecules, while b-oriented channels promote the formation of extended molecules. Using operando UV-Vis diffuse-reflectance spectroscopy coupled with mass spectrometry the methanol-to-hydrocarbons process was monitored. It was demonstrated that small coke species are primarily formed in straight channels, while large coke species mainly cover the a-oriented surface. Meanwhile, toluene was observed in the effluent from the a-oriented channel, while MS data shows the absence of toluene in the effluent from the b-oriented channels. We speculate that, due to their tortuous structure, the formation of small coke species in the a-oriented channel is suppressed and toluene can diffuse out and further conjugates to large coke species on the external surface. We believe this work opens a new research approach to study heterogenous catalysts.