The Investigations Of The Adsorption And Porosity Properties Of Wide-pore Silica Gel Surfaces Using Quasi-isothermal Thermogravimetry
P. Staszczuk, D. Hawryluk, R. LebodaFaculty of Chemistry, Maria Curie-Sklodowska University,
Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland
Among numerous silica gels the wide-pore gels are especially useful as adsorbents because of their high degree of chemical purity, homogeneity and mechanical resistance. Water similarly as other liquids (n-butanol, benzene, n-octane) adsorbed on solid surfaces, especially on silica gel surfaces, induces the changes of physicochemical properties of interface. Owing to interactions of active centers change of the properties of adsorption layers in comparison to those of bulk liquid. Water also changes the physicochemical properties of silica gel surface which is due to the blocking of active centers by molecules.
The measurements of thermodesorption of the liquid were made in quasi-isothermal conditions. As a result of performed desorption measurements the Q-TG and Q-DTG curves as a function of temperature and time were obtained. On the basis of above curves the adsorption properties and porosity of tested silica materials in relation to individual liquids were studied. For precise determination of inflection points on Q-TG curves the Q-DTG curves registered as a function of temperature and time were utilized. The measurements were performed on the following silica materials: wide-pore Si-100 silica gel, 2MCVD silica and Tullanox 500 (T-500 silica). In experiments the water, n-butanol, benzene and n-octane liquids have been used. The adsorption and porosity properties of tested silica materials were also calculated from measurements of low-temperature adsorption of gas nitrogen at temperature of liquid nitrogen using Sorptomatic 1800 apparatus (Carlo Erba Milano, Italy). The Q-1500 derivatograph (MOM Hungary) consisting of furnace, controlling system (permitting to keep the constant rate of heating of the sample) and registrator was used in our experiments. This apparatus was connected via interface with computer equipped appropriate Derivat program serving to registration of mass loss Q-TG and differential Q-DTG curves in
relation to temperature and time. Before the measurement, the samples of tested materials were dried at 200° C in order to remove water from their surface and then were saturated with vapors of selected liquids in vacuum desiccator at p/po = 1. The prepared samples were placed in platinum crucible and then the measurements of programmed thermodsorption were started. The samples were heated with the rate of 6 o/min (program II) at balance sensitivity equal to 200 mg. During the measurements the mass loss Q-TG curves were registered as a function of temperature or time.
For calculation of number of statistical monolayers of the liquid, number of hydroxyl groups and coverage degree of the surface of tested silica materials we utilized the values of specific surface area of the samples calculated on the basis of BET adsorption isotherm. The values of specific surface area are contained in the range 300-500 m2/g. Such values of specific area are typical for wide-pore adsorbents. Mean pore radii and pore volume are typical for mesopores. On the basis of thermodesorption measurements the adsorption capacity of samples, number of statistical monolayers of individual liquids adsorbed on the surface, volume of mesopores, number of surface hydroxyl groups and surface coverage degree have been calculated and correlated with analogous one from sorptometric method. The values of mesopore volume calculated from thermodesorption of individual liquids from the surface of tested samples and those calculated on the basis of the BET adsorption isotherms are comparable.
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