Georgeite: A rare gem for catalysis

Smith, Paul John 2015. Georgeite: A rare gem for catalysis. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

The industrial Cu/ZnO/Al2O3 heterogeneous catalyst has been in commercialisation for more than 50 years, whilst ambiguity still remains regarding its synthesis procedure. This involves a multi-step process, whereby each of these steps contributes to the resulting microstructural features of the final state catalyst. The initial step involves preparation of a crystalline hydroxycarbonate by aqueous co-precipitation using metal nitrates and sodium carbonate. This is a highly sensitive technique, whereby the retention of residual nitrates and alkali metals is highly influential on the resulting catalyst activity. These impurities are poisons and inhibit the full potential of such catalysts from being exploited. Furthermore, the manufacture of these catalyst materials requires renewed research and investment in order to be more compliant with green chemistry. The widespread utilisation of nitrate salts is primarily attributed to their low cost and significant solubility in aqueous media. However, production of nitrate waste streams places a pronounced burden and strain on our environment and has resulted in government legislation. In many studies, the mineral malachite has been identified as the optimum hydroxycarbonate phase for the role of catalyst precursor. However, whilst fine-tuning of the precipitation route has taken place over the years regarding its optimization, it has in essence remained largely unchanged during this timeframe. Consequently, the work presented in this thesis focuses on identifying, understanding and improving sustainable synthesis routes for the preparation of Cu/ZnO/Al2O3 catalysts that are devoid of nitrate and alkali-metal reagents. Two preparation routes using precipitation techniques for the production of hydroxycarbonates is reported which fulfil these requirements. This involves using supercritical CO2 as an anti-solvent precipitant, as well as utilising a modified co-precipitation methodology based on the current industrial procedure. Moreover, efforts have specifically focused on the preparation of the amorphous hydroxycarbonate termed georgeite. Thus the work neglects the large volume of literature reporting malachite as the optimum catalyst precursor. The ability to readily manufacture what was previously known to be a rare mineralogical phase could potentially open up additional applications. A series of Cu, Zn and Al containing georgeite precursor materials were produced from metal acetate salts using the semi-continuous supercritical CO2 anti-solvent (SAS) process. The addition of water to the system was found to result in a phase transformation of the resulting precipitate from a disordered mixed metal acetate to the georgeite phase. The physicochemical properties of the precipitant are also highly sensitive to the content of water used, whereby a compromise must be struck between enhancing the content of georgeite produced in the resulting precipitate and maintaining an optimum homogeneous system to operate in. Despite georgeite being a meta-stable phase, it is shown to be readily made. It is capable of incorporating both zinc and aluminium into its phase, which enhances the thermal stability of the material. This can be achieved without the production of additional by-phases, which currently implies no limit to the degree of zinc and aluminium incorporation. The choice of aluminium reagent has been investigated, whereby the aluminium acetylacetonate salt and an aluminium boehmite sol have been identified as suitable reagents. Alternatively, georgeite precursors can be readily prepared by co-precipitation using ammonium carbonate reagents. Whilst the simultaneous formation of a copper-ammine complex hinders the precipitation process, these materials can still compete against conventional malachite precursors. This is attributed to the exclusion of sodium carbonate in the preparation which renders the avoidance of catalytic poisons. However, precursors prepared by co-precipitation are still illustrated to be inferior to those prepared using supercritical technology. Georgeite derived catalysts prepared by both SAS and co-precipitation techniques are shown to be capable of competing against established industrial catalysts derived from crystalline precursors. For methanol synthesis, the georgeite catalysts have superior activity for the initial 10 hours but a severe deactivation phenomenon appears to hinder their performance and is a fundamental problem in this chemical process. It is plausible that this deactivation is correlated to the density of intermediate chemical species on the catalyst surface, which facilitates Cu sintering. However in direct contrast, these catalysts display exceptionally superior performance in comparison to the industrial standard for the LTS reaction, both in terms of catalyst activity and stability. This can be achieved without the alumina component which could eventually make it redundant in the construction of these catalysts. It is emphasised that these significant findings will permanently alter the way georgeite is perceived in the field of Cu/ZnO/Al2O3 catalysts regarding how these distinct catalysts operate under testing conditions. Evidently, the ‘structure-activity’ relationship was examined in more detail using state of the art techniques to rationalise these catalyst behaviours. This involved examination of both georgeite and malachite precursors, and their subsequent evolution into final state catalysts. This enabled a direct comparison to be made which could shed new light on their unique properties. It was determined that both precursors have a porous meso-structure, which is found to be a key attribute of an optimum catalyst. However, the formation of intimately mixed, poorly structured phases derived from high temperature carbonate species with negligible sodium loadings are also expressed as essential components of an optimum catalyst, and this is readily achieved from georgeite precursors.

Item Type:	Thesis (PhD)
Status:	Unpublished
Schools:	Chemistry
Subjects:	Q Science > QD Chemistry
Date of First Compliant Deposit:	30 March 2016
Last Modified:	04 Jun 2017 08:27
URI:	https://orca.cardiff.ac.uk/id/eprint/78550

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