REACH compliant epoxides used in the synthesis of Fe(III)-based aerogel monoliths for target fabrication Download: 579次
1 Introduction
Aerogel materials have found many potential applications due to their very specific properties[1]: thermal barriers, catalytic surfaces, lightweight optics, range finders, speakers, energy absorbers and capacitors. Aerogel materials have also been used as a component in many designs for high-energy density (HED) physics targets. For example, aerogels offer the ability to have low density, with the potential for uniform porosity, optically thick components in HED experiments, without the problem of relatively large pore sizes as found in pure metal foams.
Metal oxide aerogels have been more challenging to synthesize and are less well understood than the silica type[2], especially when these aerogels have been formed via catalyzed hydrolysis and condensation reaction of metal alkoxide precursors[3, 4]. Unfortunately, the lack of suitable metal alkoxides and their associated handling issues has restricted the production of metal oxide aerogels. However, the epoxide-assisted gelation approach expanded the production of metal oxide aerogels and the range of possible salt precursors that could be used[2]. From the literature, propylene oxide (PO) has been well documented as the prime epoxide precursor used to create these metal oxide aerogels[5].
Although the evidence is good and functional aerogel structures have been reported when using PO, the precursor is recognized as a ‘substance of very high concern’ (SVHC) under the European Union (EU) Registration, Evaluation, Authorization and Restriction of Chemicals[6, 7] (REACH) legislation. The remit of REACH is to provide a high level of protection to human health and the environment, regulate the production and movement of chemicals and enhance innovation in the EU chemical industry. Within the REACH legislation there is the SVHC subsection which controls the use of chemicals that are deemed to have serious or irreversible effects on human health and the environment. As such, sanctions on an SVHC-listed chemical can range from limiting general access to a total ban on usage within the EU. Particular emphasis is placed on organizations finding suitable replacements for listed materials, which is why finding a suitable epoxide replacement for PO is of interest.
Creating a sol–gel is usually the first step in the process of aerogel synthesis and involves the crosslinking of particles dispersed in a solvent, for example ethanol, to form a nanostructured (1–100 nm) wet gel[8]. More precisely, the gel is a monolithic structure based on covalent bonding forming clusters and crosslinks between clusters. Once the wet gel has formed it is allowed to ‘age’, often over a period of hours or days, while submerged in a solvent; this allows for all chemical reactions to complete and also for ‘clean’ solvent to defuse within the gel matrix. This process allows for the removal of any unreacted salts and contaminants, for example water, from the structure. To create the final aerogel, the liquid component is displaced from the wet gel by undertaking solvent exchange with supercritical
The type of salt used, chloride or nitrate, has an effect on gelation time where chlorides tend to gelate faster than nitrates[9]. The salt used can also affect the structure of the resulting aerogel, a good example being the
Fig. 1. SEMs showing the microstructure of Fe(III)-based aerogels created using nitrate salts, ethanol and the epoxides (a) PO; (b) EB; (c) TO.
2 Methodology
Absolute ethanol (200 proof, Arcos Organic), methanol (SLR, Fisher Chemical) and distilled water were used as solvents. The epoxides were PO (99.5%, Arcos), trimethylene oxide (TO), (98%, Fisher Chemical), 1,2-epoxybutane (EB), (99%, Sigma-Aldrich) and cyclohexene oxide, (98%, Sigma-Aldrich). The iron salts were iron(III) nitrate nonahydrate (98%, Alfa Aesar) and iron(III) chloride hexahydrate (98%, Sigma-Aldrich).
The relevant quantities of salt (11:1 molar ratio epoxide to salt) were dissolved in the required solvent in a flat bottom flask using a magnetic mixer at ambient temperature. Once all the salt dissolved, it was transferred to a mould. The candidate epoxide was mixed with the appropriate quantity of solvent and then added to the salt solution in the mould using a pipette. The reaction process was left to continue until a gel had formed, at which point it was left to age for 12–24 h with frequent solvent exchange taking place using ethanol. The aged gels were removed from the moulds and placed in to open top polytetrafluoroethylene (PTFE) pots containing ethanol. One formulation used as part of this paper is shown in Table
Table 1. Example formulation used for formation of Fe(III)-based aerogels.
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The method used for the synthesis is as follows
Mix the 2.5 mL of ethanol with the salt in the beaker, stir until complete dissolution and add the 0.108 mL of water. Cover to avoid ethanol evaporation.Add 2.5 mL of ethanol to the syringe and then add 1.431 mL of epoxide, stir together in the syringe.Dropwise (drop/2 S) add solution of salt and ethanol to the solution of epoxide and ethanol.Quickly cover to avoid ethanol evaporation.Gelation will occur in no longer than 24 h. Store the syringe in a place avoiding vibration.After gelation age the gel for 48 h.Unmould the gel into a PTFE beaker.After unmoulding, add ethanol.Beaker with gel and ethanol should be supercritically dried, in
Aerogel drying was undertaken using supercritical
3 Results and Discussion
SEMs of Fe(III)-based aerogels with nitrate precursor using the PO, EB and TO epoxides are shown for comparison in Figure
Table 2. Average density, pore size and particle size for each epoxides used.
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Fig. 2. Fe (III)-based aerogel synthesized using PO (a) monolith, (b) SEM showing the larger clusters are made up of nanoparticles in the 70–100 nm range.
Compared to the PO aerogel, the EB and TO samples, did not suffer from the same level of clustering and all had a lower average density and particle size as can be seen in Table
Fig. 3. SEM microstructure of cyclohexene oxide aerogel, created using chloride salt and methanol.
Table 3. Average Gelation times for epoxides used across the breath of salts used.
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There was also a noticeable difference in aerogel structure when either nitrate or chloride salts were used. A good example of this can be seen in Figure
Fig. 4. SEM micrograph of an Fe (III)-based aerogel using iron(III) chloride hexahydrate and TO.
One of the parameters that were investigated during the synthesis process was how the ratio of solvent used with the epoxide to that in the initial salt solution affected the quality of the final aerogel. It should also be noted that some of the exothermic reactions were lessened when the solvent was distributed between the salt and epoxide solutions. Table
Table 4. Ratio of ethanol solvent used in epoxide/salt solution during EB synthesis of Fe(III)-based aerogel.
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Another important parameter is the ratio of epoxide to salt, showing linkage to a variation in pore size. An example of this can be seen for the EB aerogel (Figure
Fig. 6. Comparison between EB Fe(III)-based aerogels with different ratios of epoxide to 0.808 g of iron nitrate salt. (a) 2 and (b) 3 mL.
Table 5. EB Fe (III)-based aerogels gelation time and gel quality comparison.
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From these investigations it can be seen that there is a direct relation between the final aerogel microstructure and the type of epoxide used in synthesis. With an average pore size of 190 nm across all variations the TO produced a highly porous aerogel while the EB and CO gave structures which had higher average densities, explained by the fact that the pore size was much smaller than the TO. The monoliths produced using the TO were also structurally stronger than those produced by the other epoxides, due to the fibrous nature of the resulting aerogel as opposed to the amorphous particles seen in the others.
4 Conclusion
This work shows that viable alternatives to PO can be used to create monolithic Fe(III)-based aerogels via an epoxide-assisted gelation route, and the identified epoxides do not class as SVHC chemicals under REACH legislation. The combination of the precursor epoxide and iron salt with the solvent system, has been shown to play an important role in affecting the internal morphological structure of the aerogel and the subsequent mechanical strength of the resultant monolith. The best monolithic structures have been achieved by an optimized dilution of the epoxide with solvent before addition to the dissolved salt, which has delivered structures with greater average sized porosity while maintaining mechanical strength. It has not only been seen that the dilution of the epoxide aids the mixing of the precursors, but that also a relatively rapid gelation time is required to achieve the best monoliths. The work presented here builds on our understanding of the use of alternative epoxides[10–12] by demonstrating the production of monolithic samples over a greater range of reaction conditions.
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Alberto Valls Arrufat, Magdalena Budziszewska, Clement Lopez, Aymeric Nguyen, Jakub Sitek, Paul Jones, Chris Shaw, Ian Hayes, Gareth Cairns, Glenn Leighton. REACH compliant epoxides used in the synthesis of Fe(III)-based aerogel monoliths for target fabrication[J]. High Power Laser Science and Engineering, 2017, 5(4): 04000e24.