Application of differential scanning calorimetry to study porous structure of hydrothermally modified silicas

Abstract

The paper presents the studies on determination of physicochemical and structural properties of hydrothermally treated (HTT) silica gel Si-60, using the differential scanning calorimetry (DSC) and N 2 adsorption methods. Modification was performed in overheated water vapours or under the liquid water layer using various sources of thermal energy (in the classical autoclave or in the high-pressure microwave reactor). The characteristics of porous structure were determined on the basis of low-temperature nitrogen adsorption/desorption isotherms and using the calorimetric data (DSC) of thermal effects of phase transition of water introduced inside pores of the studied materials. Great compatibility of the characteristics obtained by means of the above-mentioned methods was proved. As follows from the SEM analysis, during HTT silica is rebuilt resulting in great changes of surface morphology and porous structure of the materials. The range of these changes depends on both the applied system (water vapour/liquid water) and the kind of used energy (classical/microwave).

Desiccant Films Made of Low-Density Polyethylene with Dispersed Silica Gel—Water Vapor Absorption, Permeability (H2O, N2, O2, CO2), and Mechanical Properties

“…Bühlmann reported that silica gel in contact with water vapor at increased temperatures of up to 200 °C (melt temperature in this study was up to 210 °C) caused decrease of surface area and increase of pore diameters (less smaller pores) in silica gel [103]. Hydrothermal changes of the silica gel structure, such as lower surface area and higher pore diameters already at 200 °C, were observed by Leboda et al and Charmas [104,105,106,107].…”

Characterization of Multimodal Silicas Using TG/DTG/DTA, Q-TG, and DSC Methods

“…The introduction of additives into the reaction mixture was made in different ways: in the sol stage at 20 • C in air (samples 1, 2, and 3), the sol-gel transition at 200 • C in the autoclave (HTT, samples 4, 5, 6, and 7), or in the wet gel stage during MChT at 300 rpm (samples 8,9,10) or 500 rpm (sample 11). MChT was performed using the ball mill Pulverisette-7 (Fritsch, Germany).…”

“…The differential f S (R) functions were used to estimate the deviation (∆w) of the pore shape [21]. The f V (R) functions were also used to calculate the contribution of micropores (V micro , S micro for R p < 1 nm), mesopores (V meso , S meso for 1 nm < R p < 25 nm), and macropores (V macro , S macro for R p > 25 nm) [10,22,23]. The obtained results are summarized in Table 1.…”

“…This was due to the essence of hydrothermal modification during which dissolution of small silica molecules and their deposition on larger molecules take place. Such mass transport results in significant redevelopment of the structure [10,32,33].…”

“…Many modern research methods are used to assess the characteristics of silica materials. These include, among others, adsorption of different gases and liquids, thermal analysis under dynamic or quasi-isothermal conditions [8][9][10][11][12][13][14][15], FTIR and NMR spectroscopies [8,9,14,[16][17][18], ESR [8], SEM or TEM imaging [8,9], and many other methods.…”