T. M. Roshchina

Adsorption study of alkyl-silicas and methylsiloxy-silicas

We report the synthesis and adsorption study of the lyophobic porous silicas. Four adsorbents were prepared and tested: (1) octyl-silica, (2) hexadecyl-silica, (3) bis(trimethylsiloxy)-silica, and (4) oligo(dimethylsiloxane)-silica. Octyl- and hexadecyl-silicas were prepared via the reaction of silica with (CH3)2NSi(CH3)2CnH(2n+1) (n=8 and 16), the reactions were carried under the optimized conditions yielding high bonding densities of alkyl groups approximately 2.9-3.0 groups/nm2 and highly uniform non-polar adsorbents. Bis(trimethylsiloxy)-silica was prepared via the reaction silica with ClSi(CH3)2(CH2)10Si(CH3)[OSi(CH3)3]2. Oligo(dimethylsiloxane)-silica was prepared via the reaction of silica with ClSi(CH3)2-[OSi(CH3)2]2-Cl. Adsorption of small organic compounds (n-alkanes, alkylbenzene, benzene, diethyl ether) was investigated using two methods, classical static adsorption and gas chromatography. Thermodynamic parameters (heat, Gibbs energy, and entropy) of the adsorption of organic compounds were studied as a function of the nature of adsorbate and of the nature of the bonded layer as well. The results obtained suggest penetration of the adsorbate molecules into the bonded layer and the importance of this process in the retention mechanism in gas chromatography. Energy of the dispersion interactions with the surface decreases in the following order: n-C16H33(CH3)2Si- > n-C8H17(CH3)2Si- > [(CH3)3SiO]2Si(CH3)-(CH2)10(CH3)2Si- > -[[(CH3)2SiO]2]x-(CH3)2Si-. Energy of the electrostatic and hydrogen bonding interactions with the surface, as assessed from the adsorption of benzene and diethyl ether molecules, decreases in the opposite direction, indicating that alkyl-silicas are less polar adsorbents than methylsiloxy-silicas.

Surface Properties of Silicas with Chemically Bonded Polyfluoroalkyl Groups

Adsorption isotherms of hexane and benzene were measured on silicas with chemically bonded polyfluoroalkyl groups and the thermodynamic characteristics of the adsorption of n-alkanes, benzene and alkylbenzenes on the same materials studied at zero coverage by gas chromatography (GC). The prepared polyfluoroalkylsilicas were also investigated by IR spectroscopy over the spectral range 6000–2000 cm−1 when it was shown that the residual SiOH groups on the silica surface do not take part in the adsorption process as the attached organic groups effectively screen them. The polyfluoroalkylsilicas exhibited an extremely low adsorption potential and high polarity relative to unmodified alkylsilicas. The specific surface areas of the chemically modified silicas could be evaluated by comparison of their GC entropy values with those calculated from an ideal two-dimensional gas model.

Adsorption of water, diethyl ether, and acetonitrile on silicas with grafted perfluorohexyl coatings

The effect of the functionality of modifiers on the adsorption of vapors of compounds capable of strong specific interactions, including water, diethyl ether, and acetonitrile, on silicas with chemically grafted perfluorohexylsilanes (RSi(CH3)3 −mClm, where m (functionality) is 1, 2, 3 and R is polyfluoroalkyl) was studied by adsorption under static conditions, gas chromatography, and IR spectroscopy. It was found that modification with monofunctional silane results in the most hydrophobic coating and a comparatively low-polarity surface with a strongly decreased contribution of specific interactions compared with other modifiers.

The Adsorption of Saturated and Aromatic Hydrocarbon Vapors on Silicas with Chemically Grafted Perfluorohexyl Groups

Regardless of the nature of the modifier (mono-, bi-, and trichlorosilanes with perfluorohexyl groups or monochlorooctylsilane), the modification of silica decreased retention volumes and adsorption values of both n-alkanes and aromatic hydrocarbons. The entropy factor can play a key role in adsorption intermolecular interactions on the surface of chemically modified silicas. The modification of the surface with bi- and trichloroperfluorohexylsilanes caused the appearance of new centers more active compared with those of the initial carrier. The most oleophobic and nonpolar coatings were obtained using monochlorosilane with perfluorohexyl groups as a modifier.

Interaction of nitrogen and hydrocarbon vapors with mesoporous silicas modified with perfluorohexylsilane

Using the static method and gas chromatography, the adsorption of nitrogen and hydrocarbons by SBA-15 silicas and commercially available silica gel (SG), as well as changes in the surface properties of these samples as a result of chemical modification with multifunctional perfluorohexylsilane ClSi(CH3)2(CH2)2(n-C6F13), are studied. The main factor that causes the high values of thermodynamic characteristics of adsorption on SBA-15 and its modified form is a relatively small pore size. The differences in the polarities of the samples testify to a high concentration of silanol groups on the initial and modified SG.

Combined nitrogen, hexane, and benzene adsorption characterization of pores and surfaces of lyophobic mesoporous silicas.

For lyophobic porous surfaces, structural analysis by vapor adsorption is complicated due to weak adsorbate-adsorbent interactions and limited wetting of the pores (nonzero contact angles). To investigate further, adsorption isotherms of three distinct adsorbates (nitrogen - 77 K, n-hexane and benzene - 298 K) were studied for SBA-15 ordered mesoporous silica where the surface was functionalized with lyophobic perfluoroalkyl groups (C6F13 termini). The results demonstrated a clear advantage of the combined use of the adsorption isotherms of less surface sensitive (nitrogen) and more surface sensitive (hydrocarbons) adsorbates. The adsorption of nitrogen provided basic structural characteristics like surface area, pore volume, and pore size distribution, while the isotherms of benzene and n-hexane were used to characterize wetting (contact angles) and surface energy of the C6F13 surfaces within the pores. For the first time, the statistical film thickness for nitrogen, benzene, and n-hexane are being reported for the adsorption on fluorinated surfaces, thereby providing critical data for the pore size and the contact angle determination of the lyophobic materials.

Role of the Concentration and Nature of Grafted Groups in the Adsorption of Hydrocarbon Vapors on Silica Modified by Monofunctional Polyfluoroalkylsilanes

The role of the grafting density of monofunctional polyfluoroalkylsilanes of the CnF2n − 1(CH2)mSi(CH3)2Cl general formula (where n = 3, 4, and 6; and m = 2 and 3) and their composition in intermolecular interactions of the molecules of saturated and aromatic hydrocarbons with a surface of chemically modified silica is studied by means of IR spectroscopy and adsorption-static and gas chromatography. It is shown that the higher the concentration and the shorter the length of the grafted chain, the greater (by a factor of 2 to 25) the drop in the adsorption values of hydrocarbons as a result of modifications, due to an increase in the degree of oleophobization of surface upon the formation of polyorganofluorine coatings. The high specificity of the surface with respect to benzene, which is due to the active participation of the polar fragment of a grafted chain in adsorption process, is related to the features of a relatively low-density sample with a concentration of grafted perfluorobutyl groups of 1.7 nm−2. It is shown that the thermodestruction of polyfluoroalkyl silica remains virtually unobserved upon heating to 523 K in an argon flow.

Conversion of C2-C4 alcohols over copper-containing catalysts on carbon and fluorocarbon fibers

Carbon and fluorocarbon fibers were used as carriers for the preparation of copper catalysts from copper oxalate as precursor. The catalytic properties of catalyst were studied in the reaction of the dehydrogenation of C2-C4 alcohols by the pulsed microcatalytic method. The effect of the copper content in the catalyst, the reaction temperature on the degree of conversion, and the relation of the reaction channels were studied. The electron microphotographs were obtained, specific surfaces were measured, and X-ray pictures and infrared spectra of catalysts were taken. The activity of the catalysts on the carbon and fluorocarbon fibers in the dehydration-dehydrogenation reactions of C2-C4 alcohols was comparatively estimated. It was shown that the selectivity of the products from the dehydrogenation reaction is higher for the Cu-fluorocarbon fiber catalyst.

The nature of lyophobic coating and the adsorption of organic molecules and water on modified silicas

The protective properties of lyophobic layers of various natures chemically grafted to a silica carrier were comparatively studied. The modifiers were silanes with the compositions CF3(CH2)2Si(CH3)2Cl (CF3), C8H17Si(CH3)2Cl (C8H17), and ClSi(CH3)2[OSi(CH3)2]2Cl (OMS). The differences between the surface properties of chemically modified silicas observed in adsorption, chromatographic, and IR spectroscopic measurements were shown to be related to a nonuniform electron density distribution in CF3 grafted radicals and the special features of the structure of CF3 and OMS grafted layers caused by the possibility of interaction between the terminal groups of these radicals and the surface of the carrier. Modified silicas possessed low surface energy and were superhydrophobic materials. The sample with grafted octyl groups C8H17 had the highest stability with respect to water.

Properties of the surface of silicas modified with bi-and trifunctional perfluorohexylsilanes: Adsorption of benzene

Diffuse reflection IR spectroscopy, static adsorption, and gas chromatography were used to study the chemistry and adsorption properties of the surface of silicas modified by chemically grafted bi-(C6F13(II)) and trifunctional (C6F13(III)) perfluorohexylsilanes. It was established that the modification of silica (SiO2) results in a nearly complete screening of free silanols. At the same time, IR spectra featured absorption bands belonging to weakly perturbed OH groups capable of interacting with benzene molecules. The IR spectroscopy data on benzene adsorption were found to be in close agreement with the results of static adsorption and chromatographic measurements, according to which the specific amount of benzene adsorbed decreases in the series SiO2 > C6F13(III) > C6F13(II). It was demonstrated that the heat of adsorption of benzene on the C6F13(III) sample is higher than that on the initial carrier. Grafted layers prepared from the trifunctional modifier exhibited higher efficiency in preventing benzene molecules from reaching the carrier surface.