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Title: Gas-phase photocatalytic oxidation of alkenes using nano-structured heterogeneous semiconductor materials
Author: Kamba, Emmanuel Alhassan
ISNI:       0000 0004 8503 3660
Awarding Body: University of Sussex
Current Institution: University of Sussex
Date of Award: 2019
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This work focuses on the development of a flow-type photocatalytic reactor and its applications in both photodecomposition of organic air pollutants and photocatalytic conversion of alkenes into industrially important epoxides. Particular importance was given to commercial viability of the developed technologies. Nanostructured semiconductor photocatalysts including TiO2, Bi2WO6, TS-1 and CeO2 were synthesised through hydrothermal and sol-gel methods. In addition, several modifications of the materials including doping and coupling with metal ions and metal oxides were performed to enhance their photocatalytic activities. The powders obtained from these syntheses were coated on glass beads through a novel technique with high adherence efficiency. These coated glass beads filled the entire space in the photo-reactor which afforded high packing density. The streamline design of the reactor made it possible for all the reacting species to be in contact simultaneously, while the transparency of the glass beads provided an excellent UV light penetration to even the innermost part of the reactor. The functionality of the reactor was first tested by performing photodecomposition of acetone in gas phase. Using commercially available P25, acetone was successfully mineralised in to CO2 and H2O. Photo-epoxidation of propylene was achieved using the designed reactor with increased light intensity. Typical reaction mixture of propylene:oxygen:nitrogen corresponding to the ratio 1:1:15, afforded propylene oxide (PO) in addition to other products such as acetone, acetaldehyde and propanal, observed by Fourier transformed infrared spectroscopy (FTIR) in tandem with gas chromatography mass spectroscopy (GCMS). It was established from the results that coupled Bi2WO6-TiO2 photo-catalysts were preferable for selectivity of PO peaking at 64%. This was achieved in a typical flow reaction for 1hr at the temperature of 345 K at atmospheric pressure under UVA illumination. An interesting colour change in the synthesised Bi2WO6 nanoflowers was observed during the series of experiments. As such, further study was performed on the photochromic property of the catalyst. Upon exposure to UV irradiation, light-induced photochromism was observed to be highly dependent on the amount of coating on the glass beads; and the colour change from pale yellow to black is reversible. Further test on the effect of photochromic behaviour of Bi2WO6 on the photocatalytic epoxidation of propylene was performed. The results revealed enhancement of PO selectivity by 17 %. The kinetics of the epoxidation of cyclohexene in gas-phase using the product of reaction with a mixture of H2 and O2 was investigated. The results showed that the gas-phase reaction mechanism follows the Eley-Rideal mechanism. The physisorbed cyclohexene reacted directly with the intermediate formed through reaction between Ti base and the OOH species, to produce the cyclohexene oxide. Reaction activation energy was measured to be approximately 31 kJmol-1. Further experiments focused on in-situ reaction of the partial photo-oxidation products of photo-epoxidation of 1-hexene were performed. The exhaust of the photo-reactor was passed directly through methanol which acted as a nucleophile under acidic condition for several hours. This made it possible to establish the features of some of the reactive species formed during the partial photo-oxidation of 1-hexene process, ultimately demonstrating the high versatility of the developed photo-reactor. Finally, a "pseudo" in-situ photo-oxidation of styrene in liquid-phase using NMR spectroscopy was developed. Using molecular oxygen as the oxidant, Co-doped CeO2 showed the highest conversion of 48% with a selectivity of styrene oxide of 78%. As revealed by the kinetic study in this work, the photo-oxidation reaction proceeded according to Langmuir-Hinshelwood model.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry ; TP0155 Chemical engineering