Effect Of Hydrothermal Modification
Effect of Hydrothermal Modification
on Thermal Properties of CarbosilS
J. Skubiszewska-Zięba1, B. Charmas1,
R. Leboda1, P. Staszczuk1, P. Kowalczyk2, P. Oleszczuk3
1Faculty of Chemistry, Maria Curie-Sklodowska University, Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland
2Military Institute of Chemistry and Radiometry, 00-910 Warsaw, Poland
3Institute of Soil Science and Environmental Management, Agricultural University of Lublin, 20-069 Lublin, Poland
Hydrothermal treatment is one of the methods used for modification of porous solids structure. Scientific and patent literature on this method is abundant. Hydrothermal method of adsorbents and catalysts modification consists of the reaction of liquid water of its vapour on the solid at the temperature over 100° C under the pressure higher than the atmospheric one. Under these conditions water changes its properties due to disintegration of its primary tetrahedral structure: its density decreases, other physico-chemical properties, among others, thermal capacity, thermal conductivity, viscosity, dielectric constant also change. As a result, its solid dissolution capacity increases.
Physico-chemical changes in the modified adsorbents during hydrothermal treatment results from the change of solid chemical potential due to introducing still another component-water into the system.
It is known that sorbents and catalysts are often exploited under the conditions close to hydrothermal ones in which they should possess high hydrothermal stability as well as basic exploitation characteristics stability i.e. preserving stable ion-exchange capacity, phase composition, mechanical resistance of granules and specific surface area size. Therefore knowledge of changes of physico-chemical properties of the catalysts under the conditions of their exploitation is very important. It can be supposed that sorbents and catalysts are prepared under hydrothermal conditions which are generally much stricter than those applied in the technological practice. They possess greater resistance to water vapour at high temperature than those which were not subjected to such modification.
The aim of the paper is the study of the effect of coking and hydrothermal modification on structural and thermal properties of carbon-silica (carbosil) adsorbents. Therefore a series of model carbon-silica adsorbents was prepared based on silica gel Si-60. Dichloromethane (CH2Cl2) were used as the carbon producing substance to prepare carbon deposit. Such obtained adsorbents was subjected to hydrothermal modification (HTM) in the autoclave at 200° C.
For characterization of properties of the studied adsorbents the following methods were used:
– Low temperature adsorption/desorption of nitrogen using the Micromeritics ASAP 2405N adsorption analyser (Norcross, GA, USA);
– Thermal analysis (TG, DTG, DTA) in the range of 20-1000° C with the linear conditions of heating using the Derivarograph C (Paulik & Paulik, Hungary).
Based on the investigations there were drawn the following conclusions:
– Silica gel particle coking by means of pyrolysis of CH2Cl2 leads to preparation of complex carbon-silica (carbosil) adsorbents of different porous structure parameters (smaller specific surface area, smaller pore volume) compared with the initial silica matrix.
– Carbon deposit content in individual carbosils depends directly proportional on time of CH2Cl2 pyrolysis.
– Hydrothermal treatment (HTT) of carbon-silica adsorbents causes some changes of porous structure parameters both of silica matrix and carbon deposit.
– Increase of carbon deposit content on the carbosil surface results in smaller effect of silica matrix hydrothermal treatment.
– As follows from the derivatographic data, in the studied carbosil samples, the greatest changes take place in the temperature ranges 20-200° C and 400-800° C. The changes in the first range result from evaporation of physically adsorbed water (moisture) but in the second one from carbon deposit combustion.
– In all cases for the initial carbosils there is an increase in temperatures corresponding to the ends of exothermic peak ranges. A similar tendency can be observed for the temperatures corresponding to the maxima of these peaks.
– In the case of hydrothermal modified carbosils it can be generally stated that for the samples with small carbon deposit content the changes under discussion are insignificant but in the case of higher ones the increase in temperature is observed in the exothermic peak maximum.
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