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Title: Photocatalytic oxidation of glucose and cellobiose using TiO2 supported bimetallic nanoparticles
Author: Da Via, Luigi
ISNI:       0000 0004 6058 6621
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2016
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There is a growing interest in developing new photocatalytic routes for biomass upgrade to chemicals and fuels, but despite of some recent progress this remains an area in infancy. The ultimate aim is to convert hemicellulose, cellulose and lignin into valuable platform chemicals using green processes, there is a need to find model compounds to mimic their properties and obtain fundamental understanding of the chemistry. For this reason, cellobiose is used as model compound for cellulose because of its dimer structure, whilst glucose is cheap, easily sourced, and very abundant as the monomeric unit of cellulose, the most abundant polymer on earth, which presents great potential for commercial applications and its upgrade via selective oxidation reactions is highly desirable. However, for oxidation processes, photocatalysis is typically associated with total mineralisation reactions. In this PhD thesis will be presented for the first time the visible light mediated selective photo-catalytic oxidation of glucose and cellobiose with +98% selectivity to partial oxidation products and a near total suppression of the mineralization pathway. In this study three different experimental set-ups were used. A photoreactor equipped with UVA lamps for a total power of 112 W, a 300 and 1000 W Xenon lamps equipped with visible light filters with a cut-off wavelength of 420 nm. Typically, 40 mg of catalyst were suspended in H2O and exposed to visible light for different reaction times from 2-24h at room temperature. TiO2-P25 displayed significant activity under visible light when glucose and cellobiose were used as substrates. Nonetheless, the decoration of the surface of the support with the metal nanoparticles enhances the overall activity of the catalyst achieving 100% cellobiose conversion in under UVA light in under 2 hours at room temperature. With the addition of the 1 wt% AuAg nanoparticles with different molar ratios to the surface of the TiO2 support, cellobiose conversions of up to 30% were observed. Under UVA light the AuAg catalysts displayed a 100% cellobiose conversion with the presence of the partial oxidation products and the oxidation products coming from the hydrolysis of cellobiose. The reaction conditions were optimized by controlling the catalyst to substrate ratio, the light source power and the wavelength range. The catalysts were recycled 3 times with no significant loss of activity or changes in the product distribution values and were characterized with ICP, XRD, XPS, TEM and solid UV-Vis. This study attempts to apply a systematic approach towards the understanding of photocatalytic reactions, specifically focusing on the possibility of upgrading glucose and cellobiose to valuable chemicals. The reactivity of sugars when exposed to UV light and metal supported catalysts has been reported in the literature, but for the first time, this work tries to combine the production of the gaseous products (H2 and CO2) along with the presence of partially oxidised products in the aqueous phase with a new global reaction mechanism which takes into account what are believed to be the two main competitive reaction pathways, the Ruff degradation, and the alpha scission of sugars. This study can be transferred to other bio-derived molecules to get a better understanding on how to optimize and exploit photoactive materials to upgrade biomass under mild conditions and emphasises the crucial role of the Au and Ag metal nanoparticles present on the surface towards extending the activity range in the visible part of the spectrum.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: QD Chemistry