Andrzej Gierak

Hydrothermal modification of silica gels (xerogels) 1. Effect of treatment temperature on their porous structure

Abstract The effect of hydrothermal treatment temperature (in the range 150–300°C) on the structural parameters of silica gels was investigated. Silica gels with different starting porous structures were used for modification. The specific surface areas of the modified sorbents varied from 830 to 12 m2 g−1. Simple mathematical equations describing the changes in specific surface area and pore diameter with treatment temperature are proposed. From the mathematical and/or graphical dependences it is possible to program the hydrothermal treatment process to obtain silica gels with desired porous structures.

Carbon adsorbents as materials for chromatography I. Gas chromatography

Abstract In this paper, the application of various types of carbon adsorbents (active carbons, carbon molecular sieves, graphitized carbon black and others) in the analytical gas chromatography are discussed. Their general surface characteristics, methods of preparation, surface property, modification and applicability have been studied.

Topography and morphology of the carbon deposit obtained by pyrolysis of methylene chloride on a silica gel surface

Abstract Structural and morphological studies of carbons produced by the thermal pyrolysis of methylene chloride on the surface of Merck Si 100 silica gel were carried out by transmission electron microscopy. The results suggest that during this pyrolysis the carbon is deposited in the form of globules of various size (100–7500 A). The dimensions of these globules depend on the temperature of pyrolysis and the reaction conditions. This globular character of the deposit was confirmed by the use of the globular model of Karnaukhov.

Hydrothermal modification of silica gels (xerogels) 2. Effect of the duration of treatment on their porous structure

Abstract The effect of the duration of hydrothermal treatment (in the range 1–20 h) on the parameters of the pore structure of silica xerogels was investigated. Silica gels with different initial porous structure parameters were used. The specific surface area (S) of the adsorbents changed from 830 to 38 m2 g−1. A simple mathematical equation describing the dependence of the specific surface area S on the autoclaving treatment time is proposed. The main changes in the geometrical structure of silica gel occur during the first 6 h of treatment. A long-term (over 6 h) hydrothermal treatment can be recommended to improve the homogeneity of the pore structure of silica gels.

EFfect of the parameters of methylens chlorids carbonization on silica gel surface on the properties of an adsorbent (carbosil)

Abstract The properties of carbon-silica adsorbents (carbosils) prepared under dynamic conditions, using special apparatus, were investigated. On the surface of silica gel of Merck Si 100, methylene chloride was thermally decomposed. The influence of such factors as pyrolysis temperature, amount of CH 2 Cl 2 used, its concentration in carrier gas and flow rate of carrier gas on the properties of carbosils has been defined. Adsorbents were characterized by: heats and isotherms of adsorption of n-hexane, benzene and chloroform, differential distribution of adsorption energy of n-hexane and specific surface area. Studies of the effect of the amount of carbon deposited on the silica surface area were carried out.

Effect of silica gel quantity on the course of hydrothermal treatment in an autoclave

Abstract An incompetently performed process of hydrothermal treatment may be a source of heterogeneous adsorbents. Silica gel samples modified hydrothermally in the temperature range of 150–250 °C were investigated. An effect of mass sorbent on the course of hydrothermal treatment as well as on the homogeneity of obtained materials was verified. The height of the material layer and, to a smaller degree, the ratio of SiO2 to H2O mass were the main parameters. Experiments have shown that a complete capillary condensation of water vapour in the adsorbent pores did not take place. Under such conditions the rate of the process of hydrothermal treatment is determined mainly by the thickness of the water film formed by the water molecules adsorbed on the silica surface.

Preparation, characterization and adsorption application of spherical carbon adsorbents obtained from sulfonated polymers

Abstract Investigations of the production and steam activation of synthetic spherical carbon adsorbents from an ion-exchange resin have been carried out. The spherical adsorbents are obtained by carbonization of a polysulfonated cation-exchange resin containing different amounts of the sulfonic group on its surface. As a secondary process, steam activation of the carbons obtained was used, which created the possibility of forming adsorbents with the desired surface area and pore structure. In some activation processes a cation catalyst was used. Heat treatment and/or partial activation in the temperature range 873–1173 K caused burning of 10–70% of the initial weight of the carbon adsorbent, which leads to formation of an active carbon adsorbent with a surface area of 400–1100 m 2 g −1 . The adsorbent obtained was used for adsorption of various volatile organic compounds from air or from aqueous solution by the dynamic head space technique, and then the adsorbed compounds were recovered directly by column thermodesorption (on-line) in order to be determined chromatographically. For some of the compounds investigated, the test of sorbent effectiveness in removing chlorinated organic compounds (i.e., trihalomethanes (THM) and chlorophenols) from aqueous solution was performed using solid phase extraction (SPE).

Effect of zirconium on preparation and sorption properties of complex carbon-mineral adsorbents

Abstract The use of complex carbon-mineral adsorbents is expected to grow and investigations will develop on their applications, not only in adsorption or in chromatography. Their complex structure determines their unique surface properties and the possibility of using them in wide area applications. In this paper, preparation, structure and sorption properties of complex adsorbents, i.e . zirconia-silica gel (ZrO 2 /SiO 2 ), carbon-silica gel (C/SiO 2 ), carbon-zirconia (C/ZrO 2 ) and carbon-zirconia-silica gel (CZrO 2 /SiO 2 ) have been presented. Complex carbon-mineral adsorbents were obtained by carburization of the starting adsorbents with methylene chloride, n -octanol, methylnaphthalene or α-methyl benzyl alcohol in an autoclave. The obtained adsorbents have been characterized by low-temperature nitrogen adsorption, diffractogram measurements, differential thermal analysis and imaging of the adsorbent surfaces with a scanning electron microscope. Values of the adsorption head and the adsorption isotherm of n -hexane, benzene and chloroform as well as the energy distribution function of chloroform were calculated. From these data, results show that the most interesting materials are adsorbents of the type ZrO 2 /SiO 2 and C/ZrO 2 /SiO 2 . Zirconium ions present on a silica surface cause an increase in the degree of conversion of organic substances to carbon, a more uniform distribution on the surface and a growth of thermal endurance.

Properties of hydrothermally modified silica gels — Effect of the parameters of porous structure on the course of the modification process

Abstract The effect of the parameters of globular structure (diameter of globules and number of their contact points) on the course of a texture modification process has been examined. Modification of silica gels was carried out in an autoclave at elevated pressure in the temperature range 110–300°C and in a water vapour atmosphere. Silica gels having specific surface area of 64–830 m2/g and predominant pore radii of 40–1000 A were modified. The mechanism of hydrothermal treatment is relatively complex. It depends, among other things on different parameters of globular texture, the temperature of a process and on the presence of different admixtures in the adsorbent being modified. Above 200°C the course of hydrothermal modification depends only slightly on the nature of the silica gel (narrow or wide porous).

Properties of thin polyethylene glycol layers on the surface of silica gel and pyrocarbon/silica gel. Effect of topography and morphology of carbon deposit

The structural and energetic heterogeneity of silica gel and pyrocarbon/silica gel adsorbents was studied by means of the adsorption methods. The properties of polyethylene glycols (PEG 4000, PEG 1500) in a mixture with the solid adsorbents have been investigated using different probe molecules for gas chromatography. The carbosils investigated possessed a mosaic structure due to carbon deposit distribution features. The mechanism of formation of polyethylene glycol layers on the carbosils surface depends on the chemical structure and amount of pyrocarbon. Chemical and structural heterogeneity of the adsorbent surface is of great importance in this case. From the practical point of view, it was interesting to study the effect of such heterogeneity on the selectivity of the chromatographic columns in relation to the model mixtures.