Study of the State of Benzene Molecules Adsorbed on the Surfaces of Organic Derivatives of Kaolinite and Silica

Values and heats of benzene adsorption on modified kaolinite and silica samples are measured by adsorption calorimetry. Entropy diagrams are calculated for benzene adsorbed on these samples. Obtained results are compared to entropy losses theoretically calculated for benzene molecules using the models of ideal localized and nonlocalized adsorption. It is demonstrated that, depending on the type of an inorganic matrix, the nature of a modifier, and the degree of modification, the adsorption of benzene molecules on studied sorbents is described by the models of a supermobile nonlocalized adsorption or a nonlocalized adsorption accompanied by the loss of some rotational degrees of freedom. For a rare case of the supermobile adsorption of benzene on the modified silica, the vibration frequency of its molecules with respect to the surface is calculated.     

Synthesis and Characterization of Microporous Silica Prepared with Sodium Silicate and Organosilane Compounds

Microporous silica gels were prepared in the pH range of 3–4 using sodium silicate as a silica source. Surface polarity of these gels was modified by grafting hydrophobic groups into the silica gel matrix with the help of hydrophilic solvents (acetone, acetonitrile, ethanol and methanol) and alkoxysilane compounds containing nonhydrolyzable alkyl groups. The porous framework and hydrophobicity of the silica gels were evaluated using nitrogen adsorption/desorption and water adsorption measurement techniques. All the measured isotherms were found to be type I which is indicative of microporosity. The surface area and microporosity of these samples were estimated by analyzing the measured nitrogen adsorption/desorption data using BET, Langmuir and Dubinin-Radushkevich (D-R) adsorption isotherms. The micropore size distribution was determined from their nitrogen adsorption isotherms using the slit-pore model of the Horvath-Kawazoe equation. Silica gels with high surface area (over 500 m²/g) as well as high microporosity (over 0.2 cc/g) were obtained at gelation pH of 3.50 from the water-solvent system.     

Effect of pore structure on surface characteristics of zirconium phosphate-modified silica

Three samples of silica of different pore structure-predominantly microporous, S1; mesoporous, S2; and nonporous, S3-were modified with zirconium phosphate and examined. Pore structure analysis showed that modification had taken place in wider pores of S1 leaving a totally microporous sample, and in large pores of S2 giving a mesoporous sample of narrower pore size distribution. The modification of the nonporous sample decreased the surface area and pore volume to a lower extent than in the other two samples, but resulted in a surface of lower energy toward N2. The different distribution of surface silanol groups on the surfaces of different porosity may result in variable pictures on the modified surfaces as reflected in the differences observed in Br?nsted acidity of modified surfaces. The use of these modified silica samples for amino acid adsorption (L-glutamic acid and L-alanine) indicated that both the isoelectric point of the amino acid and the distribution of surface groups on modified solids are controlling the adsorption process.     

Morphology and mechanical response of S–B star block copolymer – Layered silicate nanocomposites

The influence on the mechanical response by incorporation of oligostyrene-modified montmorillonite (MMT) and oligostyrene-modified bentonite (BET) into star shaped styrene–butadiene block copolymer has been investigated. The modified silicates are highly intercalated with a gallery distance of more than 9 nm and partly exfoliated. The array of tactoids consisting of 1–8 layers showing uniform state of distribution as revealed from TEM. The layers of the modified MMT and BET are observed to undergo nano-confinement, i.e. restricted to the PS-domains of the styrene–butadiene star block copolymer with impinging/bridging effects of silicate layers through the SB soft phase. DMA studies have showed an appreciable shift of the glass transition temperatures of PB- and PS-rich phases towards higher temperatures in addition to an increase of the storage modulus due to nanoclay reinforcement. Generally the Young’s modulus and yield stress was strongly increased with the incorporation of modified nanoparticles whereas at the same time the strain at break reduces slightly. The elastic–plastic hysteresis–stress and the hysteresis–work are largely increased due to effective interfacial effect; an effect that is largely attributed to the presence of highly intercalated and partially exfoliated silicate layers. The extent of increase was more in the modified MMT than in the modified BET based nanocomposites. The stress-decay and the strain-recovery aspects have also been critically analyzed in relation to their micro-structural attributes. Our study fundamentally demonstrates two critical aspects related to mechanical properties and particularly with regard to elastic–plastic hysteresis response. Firstly, partial confinement of the silicate layers is promoted by PS-aided surface modification facilitating enhancement in mechanical properties and secondly, the nano-confinement of modified MMT seems to be more effective in improving the hysteresis performance when compared to BET with higher charge density.     

Benzene Adsorption Isotherms on MCM-41 and their Use for Pore Size Analysis

This work is focused on the elaboration of methodology for adsorption characterization of porous silicas by using benzene adsorption isotherms measured on good quality MCM-41 materials. Three MCM-41 samples were synthesized by using tetraethyl orthosilicate (TEOS) as silica source and surfactants, octyltrimethylammonium (C8), decyltrimethylammonium (C10) and cetyltrimethylammonium (C16) bromides as templates. A characteristic feature of this synthesis was relatively long hydrothermal treatment (5 days) at 373 K, which gave well ordered samples as evidenced by powder XRD analysis. Benzene adsorption isotherms measured on these MCM-41 samples were used to evaluate such standard quantities as the BET specific surface area, total pore volume, external surface area and the volume of ordered mesopores, and to obtain the statistical film thickness (t-curve) as well as the Kelvin-type relation, which describes the dependence between pore width and condensation pressure for benzene on silica at 298 K. The latter relations were incorporated into the Barrett-Joyner-Halenda algorithm to extend its applicability to calculate the pore size distributions from benzene adsorption data.     

Thermodynamics of adsorption of organic liquids on Ni-modified solid obtained from collodal silica

Summary In order to reduce the well known silica surface inhomogeneity and to get a material suitable for selective chromatographic separations, a modified variety of the material was prepared by a controlled sorption of Ni²⁺ ions at the solid surface; its adsorption properties were then investigated using the GSC method. Since the starting material was of very high purity, it was possible to follow the influence of Ni²⁺ ion on the adsorption of various organic liquids, including n-alkanes, cyclohexane, benzene and halomethanes. Adsorption experiments, spanning zero surface coverage and the finite concentration region, were performed in the temperature range from 333 to 443 K. Both the surface activity change resulting from the modification and the possible interactions between the organic substances and the prepared adsorbent were examined using the standard free energy change of adsorption in zero coverage conditions. The adsorption isotherms of all the adsorbates listed were obtained using the Elution by Characteristic Point (ECP) technique and the data were fitted to the BET equation. To get a better insight into the adsorption process in the finite concentration region, the thermodynamic parameters of adsorption were determined and are discussed in terms of surface coverage.     

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.     

Preparation and characterization of functionalized precipitated silica SYLOID® 244 using ionic liquids as modifiers

Studies were performed on the chemical modification of SYLOID^® 244 (Grace) precipitated silica using ionic liquids. The unmodified and ionic‐liquid‐functionalized silica were subjected to a comprehensive physicochemical and dispersive evaluation. Particle size, polydispersity and specific surface area (BET) and porous structure were examined. Moreover, stability of the colloids obtained was evaluated by estimation of isoelectric points. The extent of modification and its type were defined using fourier transform infrared spectroscopy and ²⁹Si‐cross‐polarization/magic angle spinning/nuclear magnetic resonance techniques. The modification was not observed to deteriorate the dispersive properties of the silica. The silicas obtained exhibited a restricted tendency to form larger accumulations of particles, i.e. aggregates and agglomerates. Moreover, application of ionic liquids with a heptyloxymethyl or octyloxymethyl substituent resulted in complete hydrophobization of the silica surface. Tested powders manifested the highest stability within a pH range of 10–12. Copyright © 2004 John Wiley & Sons, Ltd.     

Effect of preparation pH on pore structure of silica gels

Surface characterization of silica gels prepared at different gelation pH from water glass and sulphuric acid were made by argon adsorption at 77 K using continuous volumetric method. While microporous silica gels prepared in the pH range of 1–3 had BET surface areas of 504–571 m² g^–1, total pore volumes of 0.26–0.31 cm³ g^–1 and micropore volumes of 0.16–0.23 cm³ g^–1, mesoporous silica gels prepared in the pH range of 3.36–0.65 had BET surface areas of 374–530 m² g^–1 and pore volumes of 0.61–0.79 cm³ g^–1.     

Efficient Removal of Pb(II) from Aqueous Medium Using Chemically Modified Silica Monolith

The adsorptive removal of lead (II) from aqueous medium was carried out by chemically modified silica monolith particles. Porous silica monolith particles were prepared by the sol-gel method and their surface modification was carried out using trimethoxy silyl propyl urea (TSPU) to prepare inorganic–organic hybrid adsorbent. The resultant adsorbent was evaluated for the removal of lead (Pb) from aqueous medium. The effect of pH, adsorbent dose, metal ion concentration and adsorption time was determined. It was found that the optimum conditions for adsorption of lead (Pb) were pH 5, adsorbent dose of 0.4 g/L, Pb(II) ions concentration of 500 mg/L and adsorption time of 1 h. The adsorbent chemically modified SM was characterized by scanning electron microscopy (SEM), BET/BJH and thermo gravimetric analysis (TGA). The percent adsorption of Pb(II) onto chemically modified silica monolith particles was 98%. An isotherm study showed that the adsorption data of Pb(II) onto chemically modified SM was fully fitted with the Freundlich and Langmuir isotherm models. It was found from kinetic study that the adsorption of Pb(II) followed a pseudo second-order model. Moreover, thermodynamic study suggests that the adsorption of Pb(II) is spontaneous and exothermic. The adsorption capacity of chemically modified SM for Pb(II) ions was 792 mg/g which is quite high as compared to the traditional adsorbents. The adsorbent chemically modified SM was regenerated, used again three times for the adsorption of Pb(II) ions and it was found that the adsorption capacity of the regenerated adsorbent was only dropped by 7%. Due to high adsorption capacity chemically modified silica monolith particles could be used as an effective adsorbent for the removal of heavy metals from wastewater.     

Changes in the pore character of silicas caused by surface bonding and polymer coating

The pore character of packings for liquid chromatography, especially reversed-phase (RPLC) packings, has been studied by means of the nitrogen adsorption method (BET method). Micro-spherical silica gels with 9, 12, 30, 40 and 50 nm average pore diameter have been used as carriers. These silica gels have been modified with monochlorodimethyloctadecylsilane and several polymers (polyoctadecylmethacrylate-methylmethacrylate co-polymer, polyacrylamide gel, polyvinyl alcohol, poly-2-hydroxyethyl-methacrylate). A larger decrease in the specific surface area values was observed in the case of the polymer coating with the polyoctadecylmethacrylate-methylmethacrylate co-polymer compared with the derivatization by silanes or the modification with polymers without C18-groups. A new approach has been suggested to explain some questions concerning the interpretation of the data obtained during the measurements of the pore characteristics of the derivatized packings. An attempt has been made to reveal peculiarities connecting values of the measured surface of RP-packings with the specific surface area values of the initial silica, as well as with the chromatographically accessible surface.     

Sol–gel synthesis of mesoporous silicas containing albumin and guanidine polymers and its application to the bilirubin adsorption

The organic–inorganic composite materials based on mesoporous silica were synthesized using sol–gel method. The surface area of silicas was modified by bovine serum albumin (BSA) and guanidine polymers: polyacrylate guanidine (PAG) and polymethacrylate guanidine. The mesoporous silicas were characterized by nitrogen adsorption–desorption analysis, Fourier transform infrared spectroscopy, transmission electron microscopy. The obtained materials were used as adsorbents for selective bilirubin removal. It was shown that the structural properties and surface area of modified materials depend on the nature of polymers. Incorporation of polymers in silica gel matrix during sol–gel process leads to the formation of mesoporous structure with high pore diameter and a BET surface area equals to 346 m²/g for SiO₂/BSA and 160 m²/g for SiO₂/PAG. Analysis of adsorption isotherms showed that modification of silica by BSA and guanidine polymers increases its adsorption ability to bilirubin molecules. According to Langmuir model, the maximum bilirubin adsorption capacity was 1.18 mg/g.     

KIT-1介孔分子筛的化学修饰及吸附性能

介孔分子筛MCM-41具有较大、可调的孔径和较高的比表面积，其骨架组成具有较强的可调变性．为该材料的应用提供了很大的空间。但全硅MCM-41分子筛在潮湿空气中，即使在室温条件下也会发生水解反应，使其介孔结构遭到破坏。前人已针对MCM-41分子筛稳定性不足的问题相继合成了稳定性较高的介孔分子筛KIT-1,     

Morphology and surface properties of fumed silicas

Several series of fumed silicas and mixed fumed oxides produced and treated under different conditions were studied in gaseous and liquid media using nitrogen and water adsorption-desorption, mass spectrometry, FTIR, NMR, thermally stimulated depolarization current (TSDC), photon correlation spectroscopy (PCS), zeta potential, potentiometric titration, and Auger electron spectroscopy methods. Aggregation of primary particles and adsorption capacity (Vp) decrease and hysteresis loops of nitrogen adsorption-desorption isotherms becomes shorter with decreasing specific surface area (S(BET)). However, the shape of nitrogen adsorption-desorption isotherms can be assigned to the same type independent of S(BET) value. The main maximum of pore size distribution (gaps between primary nonporous particles in aggregates and agglomerates) shifts toward larger pore size and its intensity decreases with decreasing S(BET) value. The water adsorption increases with increasing S(BET) value; however, the opposite effect is observed for the content of surface hydroxyls (in mmol/m2). Associative desorption of water (2(SiOH)-->SiOSi+H2O) depends on both the morphology and synthesis conditions of fumed silica. The silica dissolution rate increases with increasing S(BET) and pH values. However, surface charge density and the modulus of zeta-potential increase with decreasing S(BET) value. The PCS, 1H NMR, and TSDC spectra demonstrate rearrangement of the fumed silica dispersion depending on the S(BET) value and the silica concentration (C(SiO2)) in the aqueous suspensions. A specific state of the dispersion is observed at the C(SiO2) values corresponding to the bulk density of the initial silica powder.     

蒸气促进干胶法合成硅胶负载的TS-1纳米晶

采用蒸气促进的干胶法, 在工业硅胶上原位合成出TS-1纳米晶. 先用TS-1合成液浸渍硅胶, 再于130 ℃下和水蒸气作用2 d, 得到硅胶上负载的TS-1纳米晶. 采用XRD, FTIR, UV-Vis和NMR谱以及N2吸附、 ICP、 SEM和TEM等对所得样品进行了表征. 结果表明, 将TS-1纳米晶负载在硅胶上, 其晶粒大小为20 nm左右, Ti进入了分子筛的骨架; 所得的TS-1/silica gel复合材料的比表面积为113.2 m2/g, 平均中孔孔径达到23.7 nm. 将其用于苯的羟基化反应, 以n(C6H6)/n(H2O2)=3进料, 环丁砜为溶剂, 于100 ℃下反应2 h, 苯的转化率达到8.7%, 苯酚的选择性超过99%, 苯的氧化速率(TOF)为11.37, 其催化活性明显高于传统水热合成的TS-1.     

Adsorption properties and porous structure of new carbon materials

The adsorption properties of the new carbon materials, sibunites, which are mesoporous samples with a developed surface of pores, were studied. The isotherms of the adsorption of benzene vapor were determined to estimate the porous structure of these materials. The principal methods for calculating the parameters of the porous structure of sibunites were analyzed. The application of the BET equation even in the presence of a small number of micropores can distort the results, therefore the most suitable method for estimating the surface of mesopores is one that is based on the Dubinin—Zaverina equation. The estimation of the surface of sibunites using water vapor adsorption is demonstrated.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1377–1380, August, 1993.     

Adsorption–Photometric Determination of Cationic Surfactant Traces

A procedure has been developed for the adsorption–photometric determination of cationic surfactants in natural water. The procedure is based on the adsorption preconcentration of the cationic surfactants on silica gel, the reaction of the concentrate with the anionic reagent bromothymol blue to form ion pairs on a solid surface, and the photometric determination of excess bromothymol blue in solution. The analytical range is (0.5–5) × 10^–5 M for a 50-mL sample.     

Hydrothermal modification of carbon adsorbents

The effects of high-pressure autoclave treatments on porous structure and surface properties were studied for a variety of activated carbons (AC, synthetic and produced from plum stones) treated with water vapour, hydrogen peroxide (10–50%) or 10% aqueous ammonia solution at relatively low temperatures (250, 350, 400^○C). Surface and structural parameters of modified ACs were determined using nitrogen, water, ammonia and benzene adsorption isotherms. It was found that the effects of AC modification resulting in changes in their porous structure and surface chemistry depend on the kind of initial ACs, modifier type and concentration of modifier and treatment temperature. At the same conditions synthetic ACs are modified to a larger extent than ACs prepared using natural raw materials. Repeated treatment of a given carbon intensifies changes in its porous structure.     

Effect of surface modification on back electron transfer dynamics of dibromo fluorescein sensitized TiO2 nanoparticles

Electron injection and back electron transfer (BET) dynamics have been carried out for dibromo fluorescein (DBF) sensitized TiO2 nanoparticles capped (modified) with sodium dodecyl benzene sulfonate using transient absorption techniques in picosecond and microsecond time domain. BET dynamics have been compared with bare (unmodified) nanoparticles for the same DBF/TiO2 system. It has been observed that BET reaction is slow on the modified surface compared to a bare surface in earlier time domain (picosecond). This observation has been explained by the fact that on surface modification the energy levels of the semiconductor nanoparticles are pushed up in energy. As a result, the free energy of reaction (-deltaG zero) for BET reaction of a dye/SM-TiO2 system increases as compared to the dye/bare TiO2 system. High exoergic BET reaction in dye-sensitized TiO2 nanoparticles surfaces fall in the Marcus inverted regime, so with increasing free energy of reaction, BET rate decreases on the modified surface. However, a reversible trend in BET dynamics has been observed for the above systems in the longer time domain (microsecond). In microsecond time domain BET reaction is faster on the modified surface as compared to on the bare surface. Modification of this surface reduces the density of deep trap states. Recombination dynamics between deep-trapped electron and parent cation is slow due to low coupling strength of BET reaction. As the density of deep-trapped electrons is high in bare particles, BET reaction is slow in longer time domain.     

Modified silica precipitated in the medium of organic solvents — an active rubber filler

Studies were performed on application of highly dispersed silicas, specially obtained in organic solvent medium, to processing of elastomers. Particular attention was paid to silica surface modification and to estimation of an extent of the surface modification. To this aim, near infrared techniques were used and heat effects associated with surface wetting with water and benzene were estimated. Application studies were also conducted on the use of modified silicas in butadiene-styrene rubber.