A structural study of host-guest chemistry in zeolites

Thesis

Petrochemical products have played an inarguably key role in the activity of mankind since the turn of the last century. The conversions of fossil fuels such as crude gas and oil into olefins and aromatics are very important catalytic reactions, producing platform chemicals used in the syntheses of numerous commodity chemicals and polymer materials. Zeolites are one of the most commonly used catalysts for these conversions. The recent development in the catalytic conversions of biomass to petrochemical products and environmental applications over zeolites is of great significance. Due to the inherent acidity in zeolites that is caused by the Brønsted and Lewis acid sites, zeolites catalyse many hydrocarbon reactions, such as isomerisation, oligomerisation, alkylation and cracking. In addition, the combination of the intrinsic shape and size selectivity of internal channels and pore mouths along with mass transport effects makes zeolites a great candidate in selective molecular sorption and for catalysis reactions.

Despite the extensive applications of zeolites, the catalysis chemistry in zeolites is in fact not well understood from the structural perspective. This is largely due to the fact that most spectroscopic and thermal methods provide limited structural information about the local atomic information of the molecular adsorbates with respect to the acid sites. Despite many theoretical calculations of zeolites have studied the acid properties and the mechanistic study of catalysis reactions, there are however various application limitations. In fact, the recent progressive advancement in the diffraction techniques has allowed the structural study of organic molecules in different microporous materials.

In this thesis, using high resolution synchrotron X-ray diffraction (SXRD) as the primary characterisation technique, we propose a novel methodology in understanding the molecular chemistry in zeolites (with H-ZSM-5 as an illustration) from the structural perspective. The following topics are discussed in a systematic manner:

(i) the acid-base interactions with functional molecules,

(ii) the locations of the substituted Al sites (the Brønsted acid sites),

(iii) the structural chemistry of various organic adsorbates in the framework,

(iv) the potential storage capability for functional gases,

(v) the mechanistic study of a novel biomass reforming reaction, and

(vi) the mechanistic study of the initial step of the industrially important methanol-to-hydrocarbons process.

It is also envisaged that this methodology can be broadly applied to the study of other zeolites or crystalline microporous materials.

Authors: Lo, T

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford