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Cosolvent-free synthesis of macroporous silica gels and monolithic silica glasses from tetraalkoxysilane–water binary systems: comparison between tetramethoxysilane and tetraethoxysilane

  • Invited Paper: Fundamentals of sol-gel and hybrid materials processing
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Abstract

Cosolvent-free (solventless) acid-catalyzed partial hydrolysis of tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS) followed by neutralization with imidazole (pKa = 7.0) was employed to form macroporous silica gels via polycondensation-induced phase separation and concurrent gelation. The reactions of TEOS at 20 °C and subsequent drying in air yielded crack-free opaque macroporous xerogels. In contrast, cooling was necessary to derive opaque gels from TMOS, and the resulting xerogels obtained by reactions at 5 °C broke into pieces during drying due to the too small size of the pores. A macroporous silica xerogel prepared from 50 mmol (10.4 g) of TEOS was dried within 30 h in air, and subsequent sintering in a helium atmosphere at 1300 °C yielded a monolithic silica glass. The yield of silica glasses was higher than 99%.

Photographs of two opaque macroporous xerogels prepared by cosolvent-free acid-catalyzed partial hydrolysis of TEOS followed by neutralization-induced gelation in parallel with phase separation, and silica glasses derived from these gels. The amount of TEOS used was 50 mmol for the gel and glass in the left column, and 100 mmol for those in the center and right columns.

Highlights

  • Formation of monolithic macroporous silica gels was examined by a cosolvent-free (solventless) sol–gel method using tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS) as silicon sources.

  • Acid-catalyzed partial hydrolysis of TEOS and subsequent neutralization with imidazole (pKa = 7.0) at 20 °C caused gelation accompanied by phase separation and formed opaque macroporous silica gels, which were easily dried in air without fracture and sintered into monolithic silica glasses with high yields exceeding 99%.

  • Cooling was necessary to form opaque gels from TMOS, whereas the resulting xerogels were fractured because of the too small size of the pores.

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Acknowledgements

We wish to thank Mr. Yuki Nishioka and Ms. Rena Asami of Tokyo Metropolitan University for assistance with SEM observations.

Funding

This work was partially supported by JSPS KAKENHI Grant Number 19H02802.

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Authors and Affiliations

  1. Department of Applied Chemistry for Environment, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo, 192-0397, Japan

    Koichi Kajihara & Mioko Goto

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Correspondence to Koichi Kajihara.

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Kajihara, K., Goto, M. Cosolvent-free synthesis of macroporous silica gels and monolithic silica glasses from tetraalkoxysilane–water binary systems: comparison between tetramethoxysilane and tetraethoxysilane. J Sol-Gel Sci Technol 104, 497–502 (2022). https://doi.org/10.1007/s10971-022-05799-9

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  • DOI: https://doi.org/10.1007/s10971-022-05799-9

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