Gas phase catalytic tandem processes for sustainable chemical production
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This PhD thesis has investigated a series of alternative sustainable routes for the production of commercially important chemicals (i.e. multifunctional ketones/aldehydes, alkene, alcohols and chalcones) via continuous gas phase (P = 1 atm; 453 K ≤ T ≤ 623 K) selective tandem (dehydrogenation-hydrogenation and dehydrogenation-hydrogenation-condensation) processes over unsupported (SiO2, ZnO, ZrO2, CeO2 and MgO) and supported mono- (Au, Cu) and bi-metallic (Au-Cu) catalysts. The role of copper oxidation state was considered in the case of benzyl alcohol dehydrogenation coupled with phenylacetylene hydrogenation over a series of CeO2 supported catalysts. Cu+ /Cu0 molar ratio (confirmed by XPS analysis) is shown to impact on catalytic activity, selectivity and hydrogen utilisation efficiency. The continuous hydrogenation of biomass-derived furaldehydes (e.g. furfural and 5-hydroxymethyl-2-furaldehyde) coupled with alcohol (e.g. 2-butanol and cyclohexanol) dehydrogenation was investigated over oxide supported gold/copper catalysts. Physical mixtures of Au/CeO2 + Cu supported on a range of oxides (Al2O3, ZrO2, SiO2, TiO2 and CeO2) served to prove a correlation between support basicity and H2 generation where hydrogen transfer was favoured by coke. The use of a bimetallic Au-Cu/CeO2 (based on XPS and HRSTEM-EDX) was adopted as a strategy to improve hydrogen transfer. The feasibility of continuous gas phase catalytic condensation of ketones and aldehydes to valuable chalcones was also examined. Under explicit catalytic control, it was demonstrated that conversion of acetophenone + benzaldehyde → benzylidenacetophenone over commercial MgO obeys a Langmuir-Hinshelwood type model where the catalyst shows long term stability. Benzylidenacetophenone production rate exhibits a positive dependence on Lewis basicity (quantified by CO2 chemisorption/TPD) for a series of thermally and chemically (Li- and Cs-promoted) magnesium oxide catalysts, where the caesium-promoted system delivered the highest activity. The potential of tandem dehydrogenation-hydrogenation condensation of benzyl alcohol + 2ˈ-nitroacetophenone → 2ˈ-aminochalcone over physical mixtures of supported gold (Au/CeO2, Au/MgO and Au/TiO2) and copper catalysts (Cu/CeO2 and Cu/MgO) was demonstrated, where product selectivity is sensitive to the Au oxidation state. The results gathered in this thesis demonstrate feasible innovative tandem processes with enhanced activity/selectivity and orders of magnitude improved H2 utilisation efficiency relative to standard catalytic routes.