21EngBio: Engineering Biology for Molecular Precursor Production

Alkoxysilanes are silicon-containing compounds that are used in the production of a huge variety of consumer and industrial products including machine parts, electronic components, adhesives, abrasives and ceramics. For example, a type of ceramic called silicon carbide has received much interest recently as it is used in several high-technology applications. This material can conduct electricity at very high voltages and currents, and hence it is used in the power transmission components of wind turbines and electric vehicles. Its extreme hardness and heat resistance means that it has also found uses in the aerospace industry for rocket nozzles and heat shielding, as well as in personal and vehicular armour plating in the defence and security sector.

However, the production of these alkoxysilanes is currently very energy intensive, relatively inefficient and environmentally unfriendly. Specifically, their production employs refined silicon, which is itself produced by the smelting of quartz rock with coal at very high temperatures in a furnace. This silicon is then reacted with chlorine (a very toxic and corrosive gas) and an alcohol to finally produce the desired alkoxysilanes. An additional issue with the current process is that quartz of suitable quality is mainly available in two places on the globe, in Norway and the central USA. It is anticipated that it may be difficult for these sources to meet the growing demand of the manufacturing sector in the coming years. There is therefore a need to develop alternative methods of producing alkoxysilanes that are more eco-friendly and are not reliant on limited quartz reserves.

In fact, silicon is an abundant element in nature and is found in most minerals. Many plants also absorb silicon from the soil. Though there have been several attempts in the past 40 years to produce alkoxysilanes directly from these crude minerals or plant matter, they have been hampered by the difficulty in removing unwanted metal contaminants from these raw materials.

It has been known for many years that some species of marine sponges incorporate silicon into their skeletons by condensing water soluble forms of silicon that are found in seawater. This process is facilitated by an enzyme termed 'silicatein', which enables the formation of silicon-oxygen chemical bonds under mild biological conditions. Notably, more recent research has shown that the enzyme can be used to produce simple alkoxysilanes under laboratory conditions. This project will expand on these early findings to develop a biologically-inspired route to the production of alkoxysilanes that will be cheap, widely available and environmentally sustainable.
Specifically, this project will genetically modify a harmless bacterium so that it produces modified versions of silicatein enzymes that have greater ability to produce industrially relevant alkoxysilanes. The host bacteria will then further genetically modified, so that it presents these enzymes on their surface. In doing so, the bacteria can themselves perform the reaction without needing to extract the enzyme from the bacteria. The work will also focus on the isolation of these alkoxysilanes with a high degree of purity (free from trace metals) by harnessing the enzyme's natural preference for reacting only with silicon. Finally, a process will be developed that combines fermentation of sugars by yeasts to produce alcohol, with the engineered bacteria, so that by only adding crude minerals and sugar, the desired alkoxysilanes can be produced in a single process.

By demonstrating how enzymes and bacteria can be used like a chemical factory, this project will demonstrate how biology can be harnessed to address what is essentially an engineering challenge - the improved production of an industrial chemical of high economic value.

Tetraalkoxysilanes (TROS) are a family of compounds containing a silicon atom that have a tremendous range of applications. However, many advanced applications in the energy, aerospace and defence sectors demand very high purity silanes that should be free of trace metal contaminants, which degrade the performance of the materials. To produce such high purity feedstocks, a series of energy intensive steps are employed: (1) the mining and refining of quartz into metallic silicon; (2) the reaction of silicon with chlorine to give tetrachlorosilane and finally; (3) alcoholysis of tetrachlorosilane to give the desired product.

It is well-known that rice plants uptake soluble silicates and deposit them in their tissues as silica nanoparticles, which can comprise as much as 20% of its dry weight. Thus, such plant-based silicates could be used as renewable resource for the production of high purity alkoxysilanes. It has also been known for many years that some marine sponges incorporate silicon into their skeletons by polymerising dissolved silicates into inorganic silica. This polymerisation is catalysed by a family of enzymes termed the silicateins, which the applicant has shown can catalyse the condensation of alcohols and silanols to produce simple alkoxysilanes under laboratory conditions.

This project will expand on these early findings to develop a biologically-inspired route to the production of alkoxysilanes that will be cheap and environmentally sustainable. Thus, this project will: (1) recombinantly engineer silicateins with improved activity towards tetraethoxysilane, as an exemplar TROS; (2) harness the selectivity of the silicateins to enable the use of crude silicate sources (e.g. rice husks); (3) engineer E. coli to display silicatein on its surface and demonstrate whole-cell biocatalysis; and (4) demonstrate the use of membrane compartmentalisation for the integrated production of TEOS using ethanol produced from the in situ fermentation of glucose.