Tuning the Mesopore Network in Meso-Macroporous Silica Monoliths by Hydrothermal Treatment - A Physisorption Study

Macro-mesoporous silica monolith columns, prepared by a sol-gel procedure developed by K. Nakanishi, show beneficial flow and separation properties due to their 3D-interconnected macropores in combination with mesopores, providing a high surface area. Building on this, they are routinely used in analytical liquid chromatography. Within the synthetic process, fine-tuning of the mesopore dimension and interconnection is achieved by an etching step involving hydrothermal treatment under basic conditions, typically in the range of 80 °C-100 °C. The present study aims to unravel details of this harsh procedure by a comprehensive analysis of the resulting mesoporous network. Thus, a series of silica monoliths was prepared across a range of hydrothermal treatment temperatures (HTT) between 80 and 110 °C, thereby tuning the mesoporosity. Mercury intrusion porosimetry confirmed that enhanced HTT does not alter the macropore dimension and only affects the mesopore space. The study employed state-of-the-art physisorption analysis applying two adsorptives, Ar (87 K) and N₂ (77 K), to identify changes in the mesopore size and network connectivity as a function of HTT. Also, advanced hysteresis scanning was performed on the same materials, providing independent insights into pore network effects. These analyses indicate that increasing HTT systematically enhances the average mesopore size from 8 nm (80 °C) to approximately 25 nm (110 °C) and widens the pore size distribution, pointing to pronounced dissolution of SiO₂ at higher HTT. Surprisingly, the total mesopore volume remains constant upon increasing the HTT, implying a dissolution-reprecipitation mechanism for SiO₂, rather than mere etching. Importantly, the in-depth porosity analysis reveals an increase in the size of necks, which reduces restrictions in the mesopore network connectivity. Furthermore, the data are in line with a recently proposed spatial mesopore size gradient in monoliths, which we find to be relevant at lower HTT and to systematically diminish toward higher HTT.