Recent Advances in Enabling Green Manufacture of Functional Nanomaterials: A Case Study of Bioinspired Silica

Abstract

Global specialty silica production is over 3 million tonnes per annum with diverse applications across sectors and an increasing demand for more complex material structures and surface chemistries. Commercial manufacturing of high-value silica nanomaterials is energy and resource intensive. In order to meet market needs and mitigate environmental impacts, new synthesis methods for these porous materials are required. The development of the bioinspired silica (BIS) product system, which is the focus of this review, provides a potential solution to this challenge. BIS is a versatile and greener route with the prospect of good scalability, attractive process economics and well controlled product materials. The potential of the system lies not only in its provision of specific lead materials but also, as itself, a rich design-space for the flexible and potentially predictive design of diverse sustainable silica nanomaterials. Realizing the potential of this design space, requires an integrative mind-set, which enables parallel and responsive progression of multiple and dependent research strands, according to need, opportunities, and emergent knowledge. Specifically, this requires development of detailed understanding of (i) the pathways and extent of material diversity and control, (ii) the influences and mechanisms of scale-up, and (iii) performance, economic and environmental characteristics and sensitivities. Crucially, these need to be developed for the system overall, which sits in contrast to a more traditional research approach, which focuses initially on the discovery of specific material leads at the laboratory scale, leaving scale-up, commercialization, and, potentially, pathway understanding to be considered as distinctly separate concerns. The intention of this review is to present important recent advances made in the field of BIS. Specifically, advances made along three research themes will be discussed: (a) particle formation pathways, (b) product design, and (c) scale-up and manufacture. These advances include first quantitative investigation of synthesis-product relationships, first structured investigation of mixing effects, preparation of a broad range of functionalized and encapsulated silica materials and continued industrial engagement and market research. We identify future challenges and provide an important foundation for the development of new research avenues. These include the need to develop comprehensive and predictive product design models, to understand markets in terms of product cost, performance and environmental considerations, and to develop capabilities enabling rapid prototyping and scale-up of desired nanomaterials.

Figure 1

Bioinspired silica development pipeline. Image reproduced with permission from Patwardhan, S. V.; Staniland, S. S.

Figure 2

System-based product design research framework.

Figure 3

Overview of BIS synthesis, formation pathways, their compositional subtypes, and particle structures. (a) The chemical formation of bioinspired silica (image reproduced with permission from Dewulf et al.) and (b) the physical formation of bioinspired silica. (c) Three compositional subtypes of bioinspired silica. (d) Multiscale particle structure of bioinspired silica showing a representative SEM image (left) and TEM image (middle) of as-made BIS. Right schematic shows the primary particles and the additives aggregated. Image adapted with permission from Entwistle et al.

Figure 4

(a–d) Porosity of bioinspired silica when using different additives (a–c). (d) Removal of the additives/purification via Calcination (C), acid elution (A) or acid elution followed by calcination (A+C). Images reproduced with permission from Routoula (a–c) and Manning et al. (d).^, Note that the difference between samples denoted C and A (or A+C) in Figure 4d is from the method, not the extent of removal (which is ∼100% for both). Calcination is known to create local “explosions” with the additives burning and leading to fracturing some pore walls/damaging pores, hence higher specific surface area.

Figure 5

Selected characteristics mapped to a broad depiction of the BIS material design space.

Figure 6

Schematic showing control of the properties of biosinspired silica with an example of ethyleneamine additives. Properties in red colored text can be tuned by varying parameters in blue text. ^an denotes the number of repeat units in the ethyleneamine additives used. ^bRoom temperature acid elution removes additives (fully for smaller and partially for larger additives, for details, see refs ( and 26)). ^cCalcination is used for fully removing larger additives. *Refers to internal porosity and excludes external porosity arising from interparticle pores.

Figure 7

(Left) Process flow diagram and (right) photo of larger scale BIS production apparatus operated in a 5 L continuous stirred tank reactor. The units are marked as follows. A, B - feedstock tanks (TK-01, TK-02); C, E - pumps (P-01, P-02); D - reactor (R-01); F - filtration (S-01, S-02); G - waste filtrate tank (TK-03); H - overflow tank; H-01 - drying oven. Images reproduced with permission from Manning.

Figure 8

Three-dimensional response surfaces for (a) the silica yield and (b) the Brunauer, Emmett, and Teller (BET) surface area. (c) Overlaid contour plot of the model for silica yield (blue) and silica BET surface area (red) for optimization of both responses simultaneously. The gray region enables to synthesize silica with the constraints that the yield should exceed 60 mol % and the BET surface area should exceed 100 m²/g. Figures reproduced with permission from Dewulf et al.

Figure 9

(a) Schematic representation of mixing and reaction kinetics change with scales. (b) Sensitivity of bioinspired silica products to scale-up.

Figure 10

Discrete sol–gel silica product families. Using a variety of additives, most commonly amine-based organic molecules, several families of silica materials developed are shown with controlled particle and pore morphology on multiple length scales. Image reproduced with permission from Manning et al.

Figure 11

Schematic showing (from left to right) how the knowledge gained from BIS, when applied to discrete sol–gel silica materials, helped unify them into a single sol–gel silica family with clarity on overlaps and intersections. Image in the right is reproduced with permission from Manning et al.

Figure 12

Bioinspired silica nanomaterials: selected material properties, corresponding target applications, and market outlook.

Figure 13

Bioinspired silica nanomaterials: recent advances and systematic research challenges. The text is color coded to match the colors of the boxes.