Lightweight and highly hydrophobic silica aerogels dried in ambient pressure for an efficient oil/organic solvent adsorption

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Highlights

  • Silica aerogels were synthesized in ambient pressure by two-step sol-gel method.

  • Silylation agents TMCS, MTMS, MTES, MEMO and GLYMO were selected for modification.

  • Si-NMR analysis supports hydrophobic characteristics of the samples S-TMCS, S-MTMS and S-MEMO.

  • Adsorption properties of the hydrophobic samples were investigated for various oils and organic solvent pollutants.

  • S-TMCS and S-MTMS has very high adsorption capacities especially for kerosene and diesel oil.

Abstract

Silica aerogels are ultra-porous materials with three-dimensional cage-like morphology that makes these materials ideal for separation applications. However, their hydrophilic behavior and shrinkage of the porous network during drying makes them impractical for such applications. Therefore, conducting a proper modification strategy is crucial both in imparting a hydrophobic behavior to aerogels and in preserving the porous network during drying. This study evaluated the performance of silica aerogels silylated with mono (TMCS), tri (MTMS, MTES), or organofunctional silanes (MEMO, GLYMO) as potential adsorbing materials for oil pollution remediation. Silica aerogels were prepared by the sol-gel method under ambient conditions and were characterized by conducting Si-NMR, BET, TGA, and contact angle measurements. Among the samples, silica aerogels modified with TMCS and MTMS exhibited good hydrophobicity (θ > 140°), well-constructed solid network with mesoporous structure, high porosity (94%, 89%), and low density (0.13 g/cm3 and 0.24 g/cm3). These samples also can selectively separate oil or organic solvents from water and the adsorption capacity can reach 12.5 g/g and 8.7 g/g for S-TMCS and S-MTMS, respectively. They displayed enduring adsorption property for organic solvents after 7 cycles, which shows that silica aerogels modified with TMCS and MTMS can be promising candidates for oil/organic solvent clean up practices.

Introduction

Silica aerogels are highly porous sol-gel derived materials usually obtained by ambient pressure or supercritical drying. Due to their unique porous structure, they have many exceptional properties such as low density (3−500 kg/m3), nano-scaled pore size (1−100 nm), high specific surface area (≥500 m2/g) very high porosity (> 80%), ultra-low thermal conductivity (0.01−0.03 W/mK). All these characteristics allow silica aerogels to be a special class of porous materials and are responsible for the continued interest of these materials in various industrial applications such as thermal insulation, catalyst supports, drug delivery systems, chemical sensors, aeronautics, and aerospace applications. (Dorcheh and Abbasi, 2007, Hilonga et al., 2009, Mazraeh-Shahi et al., 2013, Parale et al., 2018a, Rao and Rao, 2009, Zhang et al., 2018). On the other hand, controlling the hydrophobicity of silica aerogels and satisfying a superhydrophobic behavior during synthesis can allow these materials to take efficiently part in many environmental applications such as oil spill remediation, adsorption of organic liquids and so on (Abolghasemi Mahani et al., 2018, Ding et al., 2020, Júlio and Ilharco, 2017, Maleki, 2016, Parale et al., 2012). To include silica aerogel in such applications, tuning of the surface free energy of the silica aerogels in a wide range is essential since it directly affects the materials wettability and interfacial surface tension between the solid/liquid interfaces (Mahadik et al., 2011). In the case of oil spill clean-up applications, the apparent surface free energy of the silica aerogels should be reduced as possible by performing an effective surface modification through the synthesis.

Although traditional silica aerogels are generally synthesized via supercritical drying, producing them in ambient conditions have become much more attractive considering both cost and safety issues nowadays. During the ambient pressure drying, applying a surface modification step is crucial. Unless the surface of the wet gel is not treated, condensation reactions unavoidably occur between surface silanol groups of untreated silica gels and cause structural collapse as a result of irreversible shrinkage and lead to permanent densification during drying (Bhagat and Rao, 2006, Scherer and Smith, 1995, Smith et al., 1995, Smith et al., 1995). To prevent pore collapse, to control the capillary tension that develops during solvent evaporation and to satisfy hydrophobic behavior of the material, the surface of the wet gel should be modified by silylation. During silylation reactions, the replacement of silanol polar groups by non-polar radicals, usually coming from silanes, are encouraged. Thus, selecting proper silanes for silylation is of great importance in terms of effective modification. Up to now, several silylating agents have been used for this purpose in the literature and among them, monofunctional silanes like trimethylchlorosilane (TMCS), hexamethyldisilazane (HMDZ) in which trimethylsilyl groups grafted on silica surface through siloxane bridge, were reputed as the most effective ones (Júlio and Ilharco, 2017, Mahadik et al., 2011, Parale et al., 2012, Rao et al., 2007, Shewale et al., 2008, Torres et al., 2019). Besides, tri-functional silanes like methyltrimethoxysilane (MTMS) or methyltriethoxysilane (MTES) were also investigated in several studies. Many of them, however, have reported that silica aerogels silylated with these trifunctional silanes have appeared in denser morphology with a lower surface area and lower degree of hydrophobicity compared to that of mono-functional agents (Rao et al., 2007, Stojanovic et al., 2019, Torres et al., 2019). There are only a few studies consider organofunctional silanes that have R groups such as acrylates, amines, or epoxies to attach the silica network and they have mostly used them as co-precursors, not as silylating reagents in these studies (Hüsing et al., 1995, Parale et al., 2018b, Schwertfeger et al., 1992).

Conducting a proper surface modification makes it possible to produce ambient pressure dried silica aerogels with strikingly similar microstructural and physicochemical properties compared to those achieved by supercritical drying. In fact, we have recently reported that in addition to conventionally used mono-functional silylating agents, incorporating tri-functional silanes or organo-functional silanes like methyltriethoxysilane (MTES, MTMS) or 3-Methacryloxypropyltrimethoxysilane (MEMO) can also help to obtain equivalently qualified aerogels at ambient pressure with proper modification.

As mentioned above, in a very recent study, our research group has focused on the synthesis of the porous and/or the superhydrophobic silica aerogels by ambient pressure drying and modifying the silica surfaces with different mono/tri/organofunctional silylating agents (Çok and Gizli, 2020). Based on our previous experiences, in this study, we have focused on revealing the capability of modified silica aerogels for separating oils/organic liquids from water in an easy, rapid manner for practical applications and further elaborating the investigation about the effect of surface modification and silylation mechanisms on chemical and morphological structures of silica aerogels with detailed analyses. For this purpose, characterization of the silica aerogels has been performed, including solid-state 29Si MAS NMR analysis, pore analysis, contact angle measurements, thermal stability analysis. Batch sorption studies have also been conducted to determine the adsorption capacities of synthesized silica aerogels for the removal of various types of oil pollutants and organic solvents.

Section snippets

Materials

Tetraethylorthosilicate (TEOS, 98%) was selected as a silica precursor and supplied from Sigma Aldrich. Ethanol and n-hexane were utilized as solvents and provided from Sigma Aldrich. 0.01 M HCl and 10 M NH4OH were used as acid and base catalysis. Trimethylchlorosilane (TMCS, 98%), methyltrimethoxysilane (MTMS, 95%), methyltriethoxysilane (MTES, 99%) or 3 methacryloxypropyltrimethoxysilane (MEMO, 98%), 3-glycidyloxypropyltrimethoxysilane (GLYMO, 98%) were selected as surface modification agents

Chemical properties

29Si MAS NMR spectra for the silica aerogels modified with different silanes were displayed in Fig. 2. In Fig. 2, the signals falling in the range of −70 ppm to −120 ppm are usually attributed to the presence of a 29Si nucleus in a tetrahedral oxygen environment which is generally indicated by Qn notation and means that a Si atom is coordinated by n bridging oxygen atoms (BO) and (4-n) non-bridging oxygen atoms (NBO) (Li et al., 2012, Stojanovic et al., 2019). The total range of the 29Si

Conclusion

In this study, silica aerogels were prepared in ambient conditions by two-step sol-gel method and functionalized by mono (TMCS), tri (MTMS, MTES), or organofunctional (MEMO, GLYMO) silanes during surface modification. The samples that displayed good hydrophobicity were then tested in the adsorption experiments of oil/organic solvents from wastewater. After several characterizations, some major outcomes were summarized as follows:

  • Si-NMR analysis indicated a high degree of polymerization and

CRediT authorship contribution statement

Selay Sert Çok: Investigation, Writing - original draft. Fatoş Koç: Investigation, Visualization. Nilay Gizli: Conceptualization, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The study was financially supported by Ege University Scientific Research Foundation, Turkey under contract No: 18MÜH019 and 16MÜH122.

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