CO2 as a sustainable feedstock for the production of valuable chemicals: mechanism determination and heterogeneous catalyst development

Abstract

Concerns arising from the effects of carbon dioxide emissions on the global climate mean that CO2 is almost always portrayed in a negative light. The result of this reputation is that the potential of CO2 as a cheap, safe and green alternative to many hazardous and toxic feedstock chemicals is often overlooked. With virtually unlimited availability, CO2 has the potential to be a sustainable source of industrial carbon in a world with finite petroleum reserves.

This thesis provides an in-depth examination into several novel methods to utilize CO2 as a feedstock rather than discarding it as a byproduct. The first section of the work describes an original process for the high-yield conversion of CO2 to formic acid by reduction with potassium borohydride under ambient conditions. The mechanism of the reaction was probed through isotopic labelling using a time-resolved in situ 1H and 11B nuclear magnetic resonance (NMR) technique developed especially for this task. The formation of H2, HD and a hydroxyborohydride intermediate (BH3OH-) was directly observed and enabled the reaction mechanism to be established.

The second section of the thesis describes the development of heterogeneous mixed oxide catalysts for the cycloaddition of CO2 to propylene oxide for propylene carbonate synthesis. A custom high pressure reactor was designed and built for this task. A Ca:Zn:Al/Na mixed oxide material was developed and it was discovered that the interface at the junction between co-deposited ZnO and NaAlO2 nanoparticles is of critical importance for the promotion of the cycloaddition reaction with this material.

An alternative material in which ZnO quantum dots are homogenously distributed within an amorphous spherical silica matrix was also found to be active towards this reaction. An original synthesis method is presented whereby the ZnO quantum dots are grown in situ via a novel dissolution/re-deposition mechanism and encapsulated during the simultaneous precipitation of tetraethyl orthosilicate (TEOS) at room temperature. The mechanism by which this process occurs is explored in depth via XRD, TEM, STEM-EDS, Raman and thermogravimetric methods.