Synthesis and characterization by FTIR spectroscopy of silica aerogels prepared using several Si(OR)4 and R′′Si(OR′)3 precursors

doi:10.1016/j.molstruc.2008.08.025

Journal of Molecular Structure

Volume 919, Issues 1–3, 17 February 2009, Pages 140-145

https://doi.org/10.1016/j.molstruc.2008.08.025 Get rights and content

Abstract

We report the synthesis and Fourier Transform Infrared spectroscopy characterization results dealing with the surface modification of silica aerogels obtained via a two-step sol–gel process where various silicon precursors and co-precursors were used. The hydrolysis and poly-condensation steps were followed by carbon dioxide supercritical drying (T_c = 31.1 °C; P_c = 73.7 bar). The silicon precursors contain four identical hydrolysable alkoxy groups (methoxy or ethoxy), while in the co-precursors, one of the alkoxy groups is substituted by a non-hydrolysable alkyl group (methyl, ethyl, n-propyl, iso-butyl, n-octyl, vinyl or phenyl). Identically, surface-functionalized silica aerogels were obtained from various silicon precursor/co-precursor combinations and their chemical structures were compared. The infrared spectroscopy revealed the existence of chemically comparable solid networks with some differences due to the nature of the silicon precursors.

Introduction

Silica aerogels are exceptionally porous solids showing high specific surface areas (up to 1000 m²g⁻¹), high optical transmission (80–90% in the visible range), low densities (0.003–0.35 g cm⁻³) and low thermal conductivities (0.014 W m⁻¹ K⁻¹) [1], [2]. Consequently, these materials are promising for several scientific and technological applications [3], [4], [5], [6], [7], [8], [9], [10]. Despite having exciting and attractive properties, these silica aerogels are unstable for long-time applications [11] due to their fragility and their ability to adsorb atmospheric moisture [12]. This constraint owes to the existence of polar surface silanol groups in the silica network which are the main cause of the hydrophilic character of these solids [13]. Several studies were carried out showing how the surfaces of these aerogels could be made hydrophobic. Various techniques were used for this aim, of which is the use of silicon precursors having one organic groups in addition to three alkoxy groups R′′Si(OR′)₃ (R′′ and R′ are organic groups) as co-precursors during the alcogel synthesis [11], [13], [14], [15], [16], [17]. The structure of the obtained silica aerogels was characterized by various structural techniques of which is the Fourier Transform Infrared spectroscopy [14]. This technique reveals the energies of possible stretching and deformation vibrations within the solid network. The assignment of these vibrations to specific chemical groups allows the qualitative and quantitative identification of the chemical components of the solid network and therefore a better comprehension of its behavior in different environments. In the present work, we report the synthesis and FTIR characterization of various silica aerogels obtained by the use of two different silicon precursors: R′′Si(OR′)₃ and Si(OR)₄ where R and R′ are methyl or ethyl groups while R′′ are methyl, ethyl, n-propyl, iso-butyl, n-octyl, vinyl or phenyl groups. This study is the first to our knowledge where identically surface-modified silica aerogels are synthesized from various silicon precursors/co-precursors combinations and characterized by FTIR spectroscopy.

Section snippets

Materials

The chemicals were used in this study as received and without further purification. Tetramethoxysilane (TMOS – Si(OCH₃)₄) was provided from Merck. Tetraethoxysilane (TEOS – Si(OC₂H₅)₄), Methyltrimethoxysilane (MTMS – CH₃Si(OCH₃)₃), Methyltriethoxysilane (MTES – CH₃Si(OC₂H₅)₃), Ethyltriethoxysilane (ETES – C₂H₅Si(OC₂H₅)₃), Propyltrimethoxysilane (PrTMS – C₃H₇Si(OCH₃)₃), Isobutyltriethoxysilane (isoBTES – (CH₃)₃CSi(OC₂H₅)₃), Vinyltriethoxysilane (VTES – CH₂ double bond CHSi(OC₂H₅)₃) were purchased from Fluka.

Silica sol–gel synthesis

The followed synthesis procedure conducted to the formation of hard and low-density white non-transparent silica aerogels. In the first step, the H⁺ ions of the hydrochloric acid solution play the role of a catalyst in the hydrolysis of the alkoxy groups of the silicon precursors leading to the formation of silanol groups (Si single bond OH).

In the other hand, the non-hydrolysable silicon precursors R′′ groups remain unmodified.

The silicon precursors are expected to be totally hydrolyzed after 24 h of

Conclusion

Surface organo-modified silica aerogels were obtained from combinations of diverse silicon precursors and co-precursors and dried under supercritical conditions. The synthesized sol–gel materials were studied by FTIR spectroscopy that revealed the existence of different chemical groups corresponding to the silicon precursors and co-precursors that the aerogels were made of, as well as groups achieved after the reaction of the precursor’s molecules with water (hydrolysis) and with other

Acknowledgements

The authors acknowledge the American University of Beirut Research Board for funding this research work.

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Synthesis and characterization by FTIR spectroscopy of silica aerogels prepared using several Si(OR)4 and R′′Si(OR′)3 precursors

Abstract

Introduction

Section snippets

Materials

Silica sol–gel synthesis

Conclusion

Acknowledgements

J. Supercrit. Fluids

J. Non-Cryst. Solids

J. Nucl. Instrum. Methods

J. Non-Cryst. Solids

Appl. Catal.

J. Non-Cryst. Solids

J. Non-Cryst. Solids

Micro. Meso. Mater.

Appl. Surf. Sci.

Acta Part A

J. Non-Cryst. Solids

J. Non-Cryst. Solids

J. Non-Cryst. Solids

Polym. Degrad. Stab.

Synthesis and characterization by FTIR spectroscopy of silica aerogels prepared using several Si(OR)₄ and R′′Si(OR′)₃ precursors