A Comparison of Environmental Impact of Various Silicas Using a Green Chemistry Evaluator

Abstract

To answer questions surrounding the sustainability of silica production, MilliporeSigma's DOZN 2.0 Green Chemistry Evaluator was employed as it provides quantitative values based on the 12 principles of Green Chemistry. As a first study using DOZN 2.0 to evaluate the greenness of nanomaterials, a range of silica types were considered and their greenness scores compared. These included low- and high-value silicas, both commercial and emerging, such as precipitated, gel, fumed, colloidal, mesoporous, and bioinspired silicas. When surveying these different types of silicas, it became clear that while low value silicas have excellent greenness scores, high-value silicas perform poorly on this scale. This highlighted the tension between high-value silicas that are desired for emerging markets and the sustainability of their synthesis. The calculations were able to quantify the issues pertaining to the energy-intensive reactions and subsequent removal of soft templates for the sol-gel processes. The importance of avoiding problematic solvents during processes and particularly releasing them as waste was identified. The calculations were also able to compare the amount of waste generated as well as their hazardous nature. The effects of synthesis conditions on greenness scores were also investigated in order to better understand the relationship between the production process and their sustainability.

Figure 1

Metric hierarchy used by DOZN 2.0 Green Chemistry Evaluator. The right-hand side shows the 12 principles of Green Chemistry.

Figure 2

Typical inputs for DOZN 2.0 and the workflow.

Figure 3

Comparison of overall scores for selected silicas calculated using DOZN 2.0.

Figure 4

Comparison of major industrial silicas showing the 12 principle scores. The principle numbers correspond to their conventional allocation, as shown in Figure 1 (right-hand side).

Figure 5

Group scores for selected silicas showcasing the three major aspects of improved processes and products, calculated using DOZN 2.0: (a) group 1, resource efficiency; (b) group 2, energy efficiency; and (c) group 3, hazard prevention. The scores are composed of individual principle scores as denoted by the different color bars (the principle numbers correspond to Figure 1).

Figure 6

Comparative evaluation showing the effect of (a) synthesis step duration and temperature during MCM-41 formation and (b) principle scores for two purification methods used for HMS synthesis (calcination and ethanol reflux). Only those scores are included in b which presented significant variation as a result of the changes to the purification method.

Figure 7

Comparison of purification methods for bioinspired silica based on their principle scores. Acid elution was carried out at pH 2 for 5 min, while calcination was performed at 550 °C for 4 h.

Figure 8

Sustainability considerations for the optimization of bioinspired silica synthesis. (a) The effect of improving yield on overall score. (b) Comparison of principle 7 (renewable feedstock), group 1 (resource efficiency), and overall score when water is recycled rather than wasted. (c) Effect of using different amines on the overall score. (d) Comparison of different drying conditions on energy efficiency for bioinspired silica.