Influence of the acid concentration on the morphology of micropores and mesopores in porous glasses

Abstract

The polymodal microporosity and mesoporosity of porous glasses prepared by through leaching of phase-separated alkali borosilicate glasses in hydrochloric acid solutions of different concentrations are investigated by two new methods based on an analysis of the kinetic and equilibrium isotherms of gas desorption at a temperature of 77.5 K under low partial pressures, as well as by a modified classical adsorption method under medium and high pressures. It is established that an increase in the concentration of hydrochloric acid in the leaching solution brings about transformations in the microporous and mesoporous structures of the porous glasses. The structural transformations can be summarized as follows: (i) a considerable decrease in the specific surface of mesopores and the corresponding increase in the diameter of secondary silica globules, (ii) a decrease in the mesopore volume and the corresponding increase in the coordination number of the packing of secondary silica globules in channels formed upon phase separation, (iii) an increase in the diameter of mesopores arising from gaps between secondary silica globules in channels formed upon phase separation, (iv) an increase in the diameter of regions in these channels that are partially or completely free from secondary silica globules, and (v) the formation of ultramicropores with a molecular size of 0.4-0.5 nm.

Effect of leaching concentration and time on the morphology of pores in porous glasses

“…6e), at a higher HCl concentration of 3 M, the specific mesopore surface area decreases, and this would indicate that an aggregation of the silica particles and then an increase in the diameter of secondary silica globules occurs [5]. However, these "larger" silica particles could also block the porous channel [11].…”

“…The second phase is almost pure silica with a low percentage of sodium oxide. The pores are formed during the phase separation process in a wide range of sizes, from 0.3 to 1000 nm in diameter, depending on the glass composition, leaching conditions and the time and temperature of phase separation [5][6][7]. After leaching, some colloidal silica deposits may be formed within the mesopores that are generated from the dissolution of the soluble, sodium-rich borate phase.…”

“…In addition, the leaching conditions (i.e., acidic concentration, time of leaching and addition of basic solution) have also been investigated [5,6,9,[12][13][14][15][16]. It has been established that the hydrochloric acid (HCl) concentration and the leaching treatment time leads to the transformation of the micro-and mesoporous structure of porous glasses.…”

“…It has been established that the hydrochloric acid (HCl) concentration and the leaching treatment time leads to the transformation of the micro-and mesoporous structure of porous glasses. Recently, Kreisberg et al [5] have shown structural transformation of the porous structure and, especially, the silica particle morphologies inside the pores by varying the acid concentration from 1 to 3 M. Moreover, former studies have shown that a leaching temperature of 50°C leads to the formation of a more homogeneous glass with less silica precipitate compared to those leached at 1000°C [12].…”

Production and properties of phase separated porous glass

“…The silicate domains/regions of the dissolved alkali-borate phase may remain in the leaching solution as colloidal silica, coagulating to form silica clusters that cannot be dissolved in the acidic solution [6,8,9]. Accordingly, hierarchically porous artifacts with pore sizes around 7-54 nm due to liquation channels (interconnected network) [6,8], and around 2-5 nm due to spaces in between silica clusters (and the gaps that is in between those with channel walls) [6,8,9] together with intra-particle porosity (0.4-0.7 nm) [10,11] are generally observed. The silica clusters remaining inside the channels can be removed by basic (alkaline) solutions which may further enlarge the channel diameter as well [12].…”