Methane Activation and Functionalisation using Zinc-Modified Zeolites

Abstract

This thesis has investigated methane activation and subsequent functionalisation over zinc-modified zeolites, predominantly where the zinc has been introduced by vapour deposition. Particular focus was paid to the role of the zeolite framework and the nature of the zinc active site on methane activation.
Selective methane activation to form [ZnII-CH3] species was confirmed by 13C MAS NMR spectroscopy for three zeolite frameworks with substantially different micropore topologies (ZSM-5 (15), FER (10) and MOR (10)). For Zn(VD)/MOR, two signals were observed by 13C NMR spectroscopy, likely resulting from two distinct [ZnII-CH3] species present in the 12 MR and 8 MR side pockets, as supported by additional NMR experiments. The percentage of active zinc sites, measured through quantitative NMR spectroscopic studies, varied with the zeolite framework and was found to be ZSM‐5 (5.7 %), MOR (1.2 %) and FER (0.5 %). Furthermore, the vapour deposition sample Zn(VD)/CH4/ZSM-5 produced significantly higher levels of active zinc sites compared to the Zn(IE)/CH4/ZSM-5 ion exchanged sample (5.7% compared with 0.9%) highlighting that both zeolite framework and method of zinc introduction influence methane activation. The observed products of oxidation of the [ZnII-CH3] species are also shown to depend on the zeolite framework type and the oxidative conditions used.
XANES analysis of the vapour deposition (VD) samples, Zn(VD)/ZSM-5 and Zn(VD)/MOR, highlighted the presence of a low energy band tentatively assigned to a [ZnI-ZnI] dimeric species. Samples prepared by ion exchange or zinc vapour deposition samples heated to 600 °C, which are no longer able to activate methane, do not show this lower energy band. EXAFS analysis suggested the VD samples can be fitted using a model containing both Zn-O and [ZnI-ZnI] in the first co-ordination sphere whilst IE and 600 °C sample can only be fitted using Zn-O. The XAS evidence supported by futher experiments led us to conclude that the [ZnI-ZnI] dimer is the active species in vapour deposition samples.
Zinc catalysts prepared through zinc vapour deposition, ion exchange and impregnation using four different zeolite frameworks (ZSM-5 (12.5), BETA (12.5), MOR (10) and FER(10)) were tested for activity in the methane dehydroaromatisation (MDA) reaction. Overall, zeolite framework plays a key role in the MDA reaction with ZSM-5 proving to be the best host framework, in line with results presented in the literature for Mo catalysts. The method of zinc introduction, and therefore the active zinc species, was also crucial as zinc introduction by vapour deposition proved superior to both ion exchange and impregnation.