(359g) Preparation of Silica By Sol-Gel Methods: How Sustainable Are Current Research Methods?

Silica-based nano-materials (SBNMs) have been adopted in a wide range of engineering applications including separation technologies, filler materials, and novel biomedical applications (Singh et al., 2014). This is primarily due to the maturity of methods available for controlling the size, porosity, and surface chemistry of SBNMs. Importantly, a significant portion of SBNMs are manufactured according to a sol-gel process.

The sol-gel process is a “wet” production method which relies on the hydrolysis and condensation of an orthosilicate reagents in aqueous solution (Singh et al., 2014; Minju et al., 2021). For example, this includes the use of the sodium silicates in commercial settings due to their low reagent cost, and alkoxide-silicates such as tetraethyl-orthosilicate (TEOS) in academia - due to the greater control of SBNM properties that they afford. However, our previous work (Errington et al., 2023) and other research (Baccile et al., 2009) suggested that the use of SBNMs derived from alkoxide-silicates may carry environmental burdens that are significantly larger than sodium silicate-derived SBNMs.

This work investigates the extent to which use of alkoxide-silicate reagents may be affecting environmental burdens associated with SBNMs being produced in literature. By performing Life Cycle Assessment, the current study aims to:

• address the knowledge gaps and create Life Cycle Inventory (LCI) models for SBNMs derived from alkoxide precursors using TEOS-derived SBNMs as an example,
• estimate the environmental impacts associated with TEOS-derived SBNMs based on the newly produced LCI models,
• compare the environmental impact of TEOS-derived and TMOS-derived silica with that of sodium silicate-derived silica - as detailed modelled previously by the authors (Errington et al., 2023).

The functional unit used is one kilogram of SBNM product and impact assessment is carried out according to the ReCiPe 2016 Hierarchical method (Huijbregts et al., 2016).

Following a review of patent and academic literature, the LCI model for TEOS-derived SBNMs is developed based on consideration of a total of total of 70 unique implementations reported across 9 separate sources. For fairness of comparison, this is done according to an inventory averaging approach in which special consideration is made for the material efficiency of each implementation would make it a fair representation of any commercial process.

Our LCA results indicate that the environmental impact of TEOS-derived SBNMs may be significantly larger than that of a sodium silicate-derived equivalent SBNM (depending on the impact category). Global warming potential of TEOS-derived silica is at least 4 times greater than that of the sodium-silicate alternative considered (see Figure 1). Importantly, the source of the higher impact is discussed from the perspective of the low atom economy of the TEOS reagent when compared to sodium silicate (29% vs. 42–59% (Errington et al., 2023)) and additional insight is provided via a contribution analysis which identifies the need extent of organic solvents use as also being significant.

Our presented case study shows that alkoxide-derived SBNMs may be significantly more impactful than sodium silicate-derived alternates. These results form a new contribution to the literature by providing the first quantitative assessment of the environmental impact of alkoxide-derived SBNMs. A discussion is opened surrounding the social responsibility of investigating alkoxide-derived SBNMs given their high environmental impacts (shown here).

References

Errington, E., Guo, M. and Heng, J.Y., 2023. Synthetic amorphous silica: environmental impacts of current industry and the benefit of biomass-derived silica. Green Chemistry.

The functional unit used is one kilogram of SBNM product and impact assessment is carried out according to the ReCiPe 2016 Hierarchical method (Huijbregts et al., 2016).

Minju, N., Nair, B.N. and Savithri, S., 2021. Sodium silicate-derived aerogels: effect of processing parameters on their applications. RSC advances, 11(25), pp.15301-15322.

Singh, L.P., Bhattacharyya, S.K., Kumar, R., Mishra, G., Sharma, U., Singh, G. and Ahalawat, S., 2014. Sol-Gel processing of silica nanoparticles and their applications. Advances in colloid and interface science, 214, pp.17-37.