Mounzer, Hamza
(2009).
Heterogeneous oxidation of alcohols.
University of Birmingham.
Ph.D.
Abstract
The selective oxidation of alcohols is one of the most challenging reactions in green chemistry. While the current chemical industry uses organic and inorganic oxidants to produce carbonyl compounds, it is highly desirable to use a heterogeneous catalyst for the efficient oxidation of alcohols. The present research is focused on increasing the activity and selectivity towards the corresponding carbonyl of the heterogeneous oxidation for alcohols. The low activity of 5 wt.%Pt-1 wt. %Bi/Carbon for the oxidation of 2-octanol was investigated in a 500ml stirred tank reactor. The fast reaction rate drops dramatically from 0.23 M/hr to 0.006 M/hr after 15 minutes reaction time when heptane was used as solvent. Different possible causes such as overoxidation, leaching and poisoning were examined. It was found that the loss of high conversion rate was due to product adsorption and hence, different solvents were investigated. A mixture of 16-18% v/v dioxane in heptane was able to effectively regenerate active sites and allow a constant reaction rate of 0.07M/hr. The effects of temperature and pressure were also studied. Gas-liquid, liquid-solid and internal mass transfer effects were determined experimentally and semi-empirically. Six different Langmuir-Hinshelwood rate models were examined where a modified model based on Schuurman et al. (1992) was found to adequately describe the experimental data. The novel 2.5%Au-2.5%Pd/titania catalyst was investigated for the oxidation of benzyl alcohol. Different parameters such as catalyst oxidation state, pressure and stirrer design were studied to increase the activity and selectivity of the catalyst. The selectivity was shown to be highly dependent on the oxygen concentration. Therefore, the pressure, temperature, catalyst treatment were optimised and the reactor configuration was re-designed to enhance oxygen transport to the catalyst. While the conversion rate was unaffected by the oxygen concentration, the catalyst pre-treatment significantly increased the reaction rate. Eventually, the use of a Rushton Turbine at 20 rps with a shower disc sparger and a treated catalyst allowed the selectivity to reach 93%. The reaction could be described with a Power Law model satisfactorily. Transition Metal Oxide catalysts such as \(AgO/ SiO_2, Fe_2O_3/SiO_2, CuO/SiO_2\) and \(CuO/Al_2O_3\) were investigated as an alternative to the expensive noble metal based catalyst. However, it was shown that such catalysts are ineffective for the oxidation of different alcohols by studying the effects of different engineering parameters. The maximum conversion reached was 15% with calcined copper oxide catalyst for 1-octanol oxidation.
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