(434e) Acid-Catalysed Reflux-Free Silanisation at Room Temperature: Technical and Environmental Benefits

Silica nanoparticles are versatile materials, since their surface chemistry can be functionalised for specific applications by tuning the type and grafting density of surface modifying reagents [1]. A common technique to functionalise silica nanoparticles is silanisation, conventionally conducted by refluxing silica nanoparticles in organic solvents. However, reflux-based silanisation features disadvantages including prolonged reaction times, usage of organic solvents and energy need for refluxing, and excessive consumption of surface modifying reagents [2]. Hydrophobic and superhydrophobic silica nanoparticles have particular applications in catalysis, chromatography, and rubber industry [3], [4]. These surfaces are defined by their water contact angle (WCA), with the respective ranges of 90-150° for hydrophobicity and >150° for superhydrophobicity. Hydrophobicity is usually achieved by utilising alkyl-organosilanes as surface modifying reagents [5]. However, silane reagents with long alkyl side changes are associated with technical challenges, including finding suitable solvents and reaction inefficiencies driven by steric hindrances [1].

In attainment of hydrophobically-functionalised surface chemistry, this work presents the use of liquid acids to catalyse silanisation of silica nanoparticles at room temperature in toluene and cyclohexane, demonstrating reduced reaction times, energy usage, and silane consumption compared to the conventional refluxing method. By increasing acidic strength, acetic acid (AA) and formic acid (FA) were tested as weak acids, and dichloroacetic acid (DCA), trifluoroacetic acid (TFA), and methanesulfonic acid (MSA) were tested as strong acids to catalyse the reaction. Propyltrimethoxysilane (PTMS), octyltrimethoxysilane (OTMS), and octadecyltrimethoxysilane (ODTMS) were utilised as surface modifying reagents to attain hydrophobicity. Performance of the developed method was compared with the conventional refluxing-based method. Complementing the outcomes, environmental advantages of the developed method were analysed via process modelling and life cycle assessment (LCA).

Regarding the experimental methodology, initially, silica nanoparticles were obtained via preparing 200 ml mixtures comprised of 90% ethanol, 5% deionised water, and 5% tetraethyl orthosilicate. Then, 8ml ammonia is slowly added into the mixture, and the mixture was given 3h for reaction. After the reaction, produced silica nanoparticles were recovered via centrifugation, washed with deionised water, and dried under vacuum to finalise the process. Regarding the subsequent silanisation step, 0.5g of obtained silica nanoparticles were mixed with 50ml organic solvent (i.e., toluene or cyclohexane) and 0.9 mmol modifying reagent (i.e., PTMS, OTMS or ODTMS). Then, 0.8 mmol of acid catalyst (i.e., AA, FA, DCA, TFA or MSA) was added to the mixture, and the mixture was given 1h for reaction. After the reaction, produced functionalised silica nanoparticles were recovered via centrifugation, washed with v/10v ammonia/acetone solution, and dried under vacuum to finalise the process. To compare with the novel production method, silanisation was also conducted via conventional reflux-based method, involving 20h refluxing reaction of 0.5g produced silica nanoparticles with 1.9 mmol OTMS in 50ml solvent without the acid addition, followed by the same purification steps. To evaluate the performance of produced nanoparticles, thermogravimetric analysis (TGA) was used to quantify grafting densities (chains/nm²), and WCA was measured to compare hydrophobicities.

Environmental impacts of the developed processes were quantified via LCA. Inventory for the LCA was estimated based on the amounts of silica material, respective acid catalyst, solvent usage, silanisation reagent, and energy use utilised in lab-scale experiments. Inventory data from Ecoinvent 3.6 [6] were referred to model background processes, and ReCiPE 2016 hierarchical midpoint method was used for environmental impact characterisation [7]. System boundaries were set from cradle to end of the silanisation reaction stage. LCA was conducted for the cases with OTMS as reagent; AA, FA, TFA and MSA as acid catalyst (since impact data for DCA was not found); and both toluene and cyclohexane as organic solvent. Environmental impacts of OTMS were estimated via designing an industrial process in Aspen software. Results regarding the climate change (CC) impact (kgCO₂eq/kg) were provided per reference flow of 1kg functionalised silica nanoparticles.

Silanisation performance with OTMS, by the used acid catalyst and organic solvent, is provided in Figure 1a. Stronger acids (DCA, TFA, MSA) yielded significantly higher grafting densities with >1.5 chains/nm², compared to the weaker acids (AA, FA) with <0.5 chains/nm². The enhanced silanisation with stronger acids was attributed to their increased release of [H⁺] cations, translating into more effective silane hydrolysis. Contrasting the acidic strength, FA resulted in a lower grafting density than AA, and MSA resulted in a lower grafting density than DCA and TFA. That is reasoned by the higher affinity of FA and MSA with the silica surface, potentially competing with the reagents during the reaction. The WCA results reflected the grafting density outcomes, with higher hydrophobicities attained with the cases of highest grafting density. Cyclohexane and toluene resulted in a similar grafting density and WCA across all the selected acids, demonstrating that either solvent can be used to achieve the corresponding performance under the tested experimental conditions. Solvent choice resulted in a more sizable impact for the conventional refluxing method (Figure 1a), by respective grafting densities of 0.9 chains/nm² for cyclohexane and 1.3 chains/nm² for toluene. This difference is reasoned by reaction temperature, that is determined by the solvent’s boiling point (BP) in the refluxing case. As the BP of toluene (111°C) is greater than cyclohexane (81°C), thermal hydrolysis of silanes is higher for the former, translating into higher silanisation performance. Performances of surface modifying reagents were compared for the case of TFA catalyst in toluene (Figure 1b). Attributed to increasing steric hinderance with the longer carbon chain length from PTMS (3) to OTMS (8) to ODTMS (18), the reagents resulted in respective grafting densities of 2.9, 2.6, and 2.1 chains/nm². Since longer carbon chain length increases hydrophobicity performance, the reagents ultimately demonstrated similar WCA results, being 145°, 159°, and 156°, respectively.

Results of the LCA are illustrated in Figure 2. The novel acid-catalysed route significantly reduced the overall CC impacts compared to the conventional refluxing process. Contributions of the acid catalyst, silane reagent, and process energy were minor for the novel route. Thus, the CC impacts were not sizably impacted by the catalyst choice. Toluene performed better than cyclohexane as organic solvent, reducing the CC impact for the novel route. With dominant process energy contributions, overall CC impacts were significantly higher for the conventional refluxing process for both solvents. Cyclohexane performed better than toluene for the conventional process, by requiring less process energy thanks to its lower boiling point. Overall, the novel process demonstrated considerable reductions for the CC impacts compared to the conventional process, across all catalyst types and both solvents. As a future direction, application-specific life cycle impacts (e.g., per adsorption capacity functional units) would describe the environmental benefits of individual catalysts and reagents for the novel process.

Briefly, a novel acid-catalysed silanisation process is presented in this work, that is operable at ambient temperature. The technique was validated by obtaining comparable grafting densities and WCA outcomes with nanoparticles functionalised through the conventional refluxing process. High acidic strength and low affinity with the original silica surface were detected to improve the catalytic performance. Longer alkyl chains of modifying reagents resulted in lower grafting densities, reasoned by steric hinderances. Toluene and cyclohexane demonstrated similar technical performance as organic solvents; however, toluene performed environmentally better, resulting in lower CC impact. Acid catalyst, silane reagent, and process energy contributed minorly to the overall CC impact for the novel process. The conventional process resulted in significantly higher CC impacts for both solvents, driven by high process energies. Altogether, the developed method is promising to mitigate the technical challenges and environmental impacts of the conventional process, by minimising reaction times, operating at ambient temperatures, removing refluxing energies, and reducing modifying reagents consumption.

References

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