An eco-friendly, tunable and scalable method for producing porous functional nanomaterials designed using molecular interactions

Despite significant improvements in the synthesis of templated silica materials, post-synthesis purification remains highly expensive and renders the materials industrially unviable. In this study we address this issue for porous bioinspired silica, developing a rapid room-temperature solution method for complete extraction of organic additives. Using elemental analysis and N2 and CO2 adsorption, we demonstrate the ability to both purify and controllably tailor the composition, porosity and surface chemistry of bioinspired silica in a single step. For the first time we have modelled the extraction using molecular dynamics, revealing the removal mechanism is dominated by surface-charge interactions. We extend this to other additive chemistry, leading to a wider applicability of the method to other materials. Finally we estimate the environmental benefits of our new method compared with previous purification techniques, demonstrating significant improvements in sustainability

Unified mechanistic interpretation of amine-assisted silica synthesis methods to enable design of more complex materials

The design of porous sol–gel silica materials is a thriving research field, owing to silica's diversity of properties and potential applications. Using a variety of additives, most commonly amine-based organic molecules, several families of silica materials have been developed including silica nanospheres, zeolites, mesoporous silicas, and bioinspired silicas with controlled particle and pore morphology on multiple length scales. Despite the wide range of study into these materials, and similarity in terms of reagents and additive compounds, none can recreate the features and complexity present within naturally occurring biosilica materials. This is due in part to a lack of ‘joined-up’ thinking during research into silica synthesis strategies and methodology. Specifically, mechanistic insights gained for one set of conditions or additive structures (i.e. material types) are not translated to other material types. In order to improve the structural complexity available in synthetic silica materials, as well as to improve both understanding and synthesis methods for all silica types, a unified approach to mechanistic understanding of formation in amine-assisted silica synthesis is required. Accordingly, in this review we analyse contemporary investigations into silica synthesis mechanism as a function of (amine) additive structure, analysing how they imprint varying levels of order into the eventual silica structure. We identify four fundamental driving forces through which additives control silica structure during synthesis: (i) controlling rates of silica precursor hydrolysis and condensation; (ii) forming charge-matched adducts with silicate ions in solution; (iii) self-assembling into mesophases to physically template pores; and (iv) confining the location of synthesis into specifically shaped vesicles. We analyse how each of these effects can be controlled as a function of additive structure, and highlight recent developments where multiple effects have been harnessed to form synthetic silica materials with further structural complexity than what was previously possible. Finally, we suggest further avenues of research which will lead to greater understanding of the structure–function relationship between amine additives and final materials, hence leading to more complex and high-value silica and other materials

Mimicking bio-sintering: the identification of highly condensed surfaces in bioinspired silica materials

Interfacial interactions between inorganic surfaces and organic additives are vital to develop new complex nanomaterials. Learning from biosilica materials, composite nanostructures have been developed, which exploit the strength and directionality of specific polyamine additive-silica surface interactions. Previous interpretations of these interactions are almost universally based on interfacial charge matching and/or hydrogen bonding. In this study, we analyzed the surface chemistry of bioinspired silica (BIS) materials using solid-state nuclear magnetic resonance (NMR) spectroscopy as a function of the organic additive concentration. We found significant additional association between the additives and fully condensed (Q4) silicon species compared to industrial silica materials, leading to more overall Q4 concentration and higher hydrothermal stability, despite BIS having a shorter synthesis time. We posit that the polyfunctionality and catalytic activity of additives in the BIS synthesis lead to both of these surface phenomena, contrasting previous studies on monofunctional surfactants used in most other artificial templated silica syntheses. From this, we propose that additive polyfunctionality can be used to generate tailored artificial surfaces in situ and provide insights into the process of biosintering in biosilica systems, highlighting the need for more in-depth simulations on interfacial interactions at silica surfaces

Inside Cover: An Eco-Friendly, Tunable and Scalable Method for Producing Porous Functional Nanomaterials Designed Using Molecular Interactions (ChemSusChem 8/2017)

The Inside Cover picture shows the purification of silica using a new room‐temperature “greener” route compared to the traditional calcination approach. The new route reduces energy cost by 95 %, recovers 100 % of the template, yet completely purifies silica within minutes. Below, the mechanism for this greener approach is shown, with template molecules (red) being controllably removed from silica (grey). More details can be found in the Full Paper by Manning et al. on page 1683 in Issue 8, 2017 (DOI: 10.1002/cssc.201700027)

Quality-by-design approach to process intensification of bioinspired silica synthesis

Characterizing nanomaterials is challenging due to their macromolecular nature, requiring suites of physicochemical analysis to fully resolve their structure. As such, their synthesis and scale-up are notoriously complex, especially when compared to small molecules or bulk crystalline materials, which can be provided a unique fingerprint from nuclear magnetic resonance (NMR) or X-ray diffraction (XRD) alone. In this study, we address this challenge by adopting a three-step quality-by-design (QbD) approach to the scale-up of bioinspired silica nanomaterials, demonstrating its utility toward synthesis scale-up and intensification for this class of materials in general. First, we identified material-specific surface area, pore-size distribution, and reaction yield as critical quality attributes (CQAs) that could be precisely measured and controlled by changing reaction conditions. We then identified the critical process parameters (CPPs) controlling bioinspired synthesis properties, exploring different process routes, incorporating commercial reagents, and optimizing reagent ratios, comparing silica properties against original CQA values to identify acceptable limits to each CPP. Finally, we intensified the synthesis by increasing reagent concentration while simultaneously incorporating the optimized CPPs, thereby modifying the bioinspired silica synthesis to make it compatible with existing manufacturing methods. We increased the specific yield from ca. 1.1 to 38 g/L and reduced the additive intensity from ca. 1 to 0.04 g/g product, greatly reducing both synthesis cost and waste production. These results identify a need for mapping the effects of critical process parameters on material formation pathways and CQAs to enable accelerated scale-up and transition from the lab to the market