Tailoring the Hydrophobic Character of Mesoporous Silica by Silylation for VOC Removal

MCM-41 and non-ordered mesoporous silica were modified using hexamethyl-disilazane (HMDS). The hydrophobic and hydrophilic properties of grafted materials were studied and compared to a purely all silica BEA zeolite by using competitive and noncompetitive water toluene adsorption. A linear correlation between the silylation degree and the hydrophobicity measurements has been found for MCM-41 materials. Even if highly silylated MCM-41 material have more hydrophilic sites (silanol groups) than all silica zeolite, water molecule condensation is not observed because these sites are isolated. Thus, the highly silylated MCM-41 sample exhibits not only hydrophobicity 2.3 times higher than all silica BEA zeolite but also possesses a storage capacity for toluene and chlorobenzene 3 times higher than this zeolite. In competition with water, the organic molecule (toluene or chlorobenzene) adsorption is always favored even if water adsorption is enhanced by chlorobenzene polarity.     

TiO2 photocatalyst for degradation of organic compounds in water and air supported on highly hydrophobic FAU zeolite: Structural,sorptive, and photocatalytic studies

Preparation of highly hydrophobic FAU zeolite has been achieved by a two-step preparation method that comprises predealumination treatment using concentrated mineral acids and subsequent calcination treatment. The hydrophilic/hydrophobic character of the zeolites was estimated by detailed adsorption measurements using water and toluene molecules as adsorbates, which showed the strict hydrophobic nature of the prepared FAU zeolites. By means of TG, FT-IR, and ²⁹Si MAS NMR analyses, it was concluded that the enhancement in hydrophobicity originates from the healing of silanol defect sites and the formation of a refined silica surface with fewer adsorption sites. Thus-prepared FAU zeolite, with high crystallinity, structural and thermal stability, and hydrophobicity, significantly improved the photocatalytic degradation rates of 2-propanol in water and acetaldehyde in air on TiO₂ when it was used as a catalyst support. A distinct correlation was found between the hydrophobicity of zeolites and photocatalytic activity of the supported TiO₂.     

On the Limited Selectivity of Silica-Based Epoxidation Catalysts

Epoxidation of 2-cyclohexen-1-ol with TBHP and TiO₂–SiO₂, Fe₂O₃–SiO₂ and SiO₂ aerogels has been studied. The influence of surface silanol groups and transition metal impurities in titania–silica on the product distribution has been analyzed. The results, supported by literature data, indicate that high surface area silica cannot generally be regarded as an inert matrix in epoxidation catalysts. Contribution of Ti-free silica limits the selectivity of Ti- and Si-containing catalysts in demanding epoxidation reactions. The activity of silica – together with the background oxidation of the olefin – may lead to overestimated intrinsic activities when the reaction rate is related to the Ti content of the catalyst.     

Modification and Evaluation of Non‐Polar Sorbents for Solid‐Phase Extraction

Abstract

Modification of packings for solid‐phase extraction through high‐frequency low‐temperature plasma treatment is a new method to obtain a wide variety of sorbents. This method is still in the process of investigation and application. In this study, several commercial sorbents have been modified in high‐frequency low‐temperature plasma with the mixed hydrogen/benzene system. The solvation parameter model is used to characterize the sorption properties of commercially available and plasma treated sorbents for solid‐phase extraction. Chromatographic test mixtures from the literature were used to probe hydrophobicity, free silanol interactions and shape selectivity of the sorbents. The physical and retention properties of the plasma treated sorbents indicate their suitability to be used in solid‐phase extraction. Some suggestions concerning the possible practical application of the newly obtained sorbents have been made.     

Hierarchically porous monoliths of carbon and metal oxides with ordered mesopores

Hierarchically porous carbon and metal oxide materials offer great benefits in separations, catalysis and renewable energy. We have here used hierarchically porous silica monoliths with ordered mesopores as hard templates to produce nanocast carbon, Co₃O₄, and NiO monoliths with similar structures. Besides providing the materials with more well-defined physicochemical properties, the ordered mesopore structure also offers an excellent model system for investigating the nanocasting process in detail. The mesopores of the silica monoliths were first infiltrated with furfuryl alcohol or metal nitrate precursor solutions, which subsequently could be thermally converted to carbon or the corresponding metal oxides. After the silica scaffolds have been removed by etching in base solutions, the resulting replica monoliths display macroscopic morphology and macropore structure similar to the original silica template. However, while the carbon and Co₃O₄ materials both display a well-organized nanowire structure, giving rise to high surface area and narrow pore size distribution, the NiO monoliths exhibit a significantly lower surface area and less well-defined mesopore structure implying that only part of the silica mesopores has been replicated. We believe this apparent difference between the two metal oxides is a consequence of differences in mass transport.     

Thin-Layer Chromatography of Metal Ions in Formic Acid Medium on Impregnated and Unimpregnated Silica Gel G: Semiquantitative Determination of Fe3+, Cd2+, Th4+, Al3+, Zn2+, UO2+ 2, VO2+, Ce4+, and Ni2+

Abstract

The adsorption behavior of metal ions in the formic acid-sodium formate system using unimpregnated as well as impregnated silica gel G as thin layers has been studied. A remarkable result of this study is the dramatic selectivity of impregnated silica gel G thin layers when compared to the corresponding unimpregnated silica gel G thin layers. This impregnation effect provides us with a new adsorbent phase which is sufficiently stable in the formic acid-sodium formate medium. Some important ternary separations have been achieved on silica gel G thin layers. Al³⁺, Ni²⁺, Fe³⁺, and Pb²⁺ have been qualitatively separated from mixtures of other ions. Cu²⁺ has been selectively separated on impregnated silica gel G thin layers. The semi-quantitative determination of nine metal ions on impregnated silica gel G layers has also been attempted.     

Hydrolysis,polycondensation, and catalytic properties of Ru(II) complex of 3‐4,5‐dihydroimidazol‐1‐yl‐propyltriethoxysilane

The preparation and measurements of some properties of organic–inorganic hybrid materials derived from Ru(II)‐3‐4,5‐dihyroimidazol‐1‐yl‐propyltriethoxysilane inside a polysiloxane network have been achieved. The hydrolysis and polycondensation of Ru(II)‐3‐4,5‐dihyroimidazol‐1‐yl‐propyltriethoxysilane were performed in different experimental conditions, producing a new organic–inorganic silica. The alkoxysilyl groups available were used for the construction of inorganic backbone by the sol‐gel process, and the imidazole group was found suitable for incorporating Ru(II) by coordination. The coordination of metal complex is retained because there is no leaching from the metal complex containing gels. To ensure sufficient catalytic properties, a series of hybrid materials from tetraethoxysilane was prepared. These materials were identified and catalytic activities were tested for cyclization of (Z)‐3‐methylpent‐2‐en‐4‐yn‐1‐ol to 2,3‐dimethylfuran. Heterogeneous Ru(II) catalyst can also be recycled and reused without significant loss of selectivity or activity. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1329–1334, 2001     

Esterification of the Silanol Groups in the Mesoporous Silica Derived from Kanemite

Silanol groups in the mesoporous silica derived from a layered silicate, kanemite (NaHSi1₂O₅ 3H₂O), were esterified with alcohols (C_nH_2n+1OH n=2 , 4, and 8). Most of the silanol groups in the mesopores were modified with ethanol though some silanol groups remained after the reactions with 1-butanol and 1-octanol. With the increase in the alkyl chain lengths of the alcohols, the pore sizes of the esterified products decreased and the hydrophobicity increased.     

Synthesis and characterization of novel saponified guar‐graft‐poly(acrylonitrile)/silica nanocomposite materials

The combination of carbohydrates with silicon‐based ceramic materials offers attractive means of production for high performance materials. Present article describes the synthesis of novel nanocomposites out of SiO₂ and saponified guar‐graft‐poly(acrylonitrile) (SG). Tetraethoxysilane was used as the precursor for silica and growth of SiO₂ phase was allowed concurrently in the presence of SG. The material so obtained was thermally treated at 80°C, 160°C, 500°C, and 900°C to study the effect of thermal curing on its properties. During the curing process, silanol surface groups of silica globules reacted to create the reinforced SiO₂‐SG substance. It was observed that at 900°C, the SiO₂ phase crystallized out in tetragonal shape (similar to Cristobalite form of silica) in presence of SG. The chemical, structural and textural characteristics of the composites were determined by FTIR, XRD, TGA‐DTA, SEM and BET studies. The materials were also evaluated as efficient Zn²⁺ metal binder. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 536–544, 2007     

Adsorption of heavy metals by pirymidine-derivated mesoporous hybrid material

This paper describes the synthesis of pyrimidine-derivated hybrid mesoporous material via chemical bonding. This has been done by modifying the silanol groups of the support with a synthesised pyrimidine propyl triethoxysilane. The hybrid material has been characterized by X-ray diffraction, nitrogen adsorption techniques, FT-IR, ²⁹Si and ¹³C MAS NMR. The potential of the material of removing heavy metal as copper, nickel and cobalt from water has been tested. Effects of pH, velocity of adsorption, selectivity, maximum adsorption capacity and preconcentration capacity have been determined.     

The role of silanol groups on the reaction of SiO2 and anhydrous hydrogen fluoride gas

It is essential to etch SiO₂ for producing silica glass components, semiconductor devices, and so on. Although wet-etching with hydrogen fluoride (HF) solutions is usually employed for this purpose, it faces a drawback that microstructures stick during the drying of the solution. To overcome this problem, we have developed a dry-etching technique with gaseous HF at high temperatures. In the present study, an interesting phenomenon was found that silicon thermal oxides were much less etched than vitreous silica by gaseous HF. Such difference had not been found in wet- or humid HF gas etching. Because their bulk chemical formulae are the same (SiO₂), it was suggested that the surface species affected the reaction rate. In fact, preprocessing with water vapor plasma remarkably increased the etching rate on the thermal oxides layer, and vacuum heating almost completely suppressed the reaction on the vitreous silica and the plasma-treated thermal oxides. These results indicate that the surface silanol groups enhance the reaction between SiO₂ and gaseous HF. Based on the results, a model of chain reaction for SiO₂ and gaseous HF was proposed, where the surface silanol groups act as the reaction center.     

Influence of β-Si3N4 whiskers on sintering and crystallization of fused silica ceramics

Fused silica ceramics are widely applied for radome materials, crucibles, and vanes, but the mechanical properties were deteriorated due to the cristobalite crystallization. The fused silica ceramics added with by β-Si₃N₄ whiskers were prepared by a slip-casting method to retard the cristobalite crystallization. The influences of the sintering environments and the β-Si₃N₄ whiskers on the microstructure and phase structure were investigated. The silanol (Si-(OH)_n) and oxygen vacancies (V_O) in the fused silica in formed in different conditions were studied by Fourier Transform Infra-Red (FT-IR) and X-ray photoelectron spectroscopy (XPS), and the results indicated that the ball-milled produced a large amount of the silanol groups onto the surface of the fused silica particles. The fused silica heated in the vacuum created the maximum oxygen vacancies (24.2%) on the surfaces. Silanol groups reacted with the β-Si₃N₄ whiskers, and the O atoms in the silanol groups were fixed into the bulk materials. And the crystallization kinetics and the activation energy of Si₃N_4w/SiO₂ ceramics at the temperature ranging from 1200 to 1400°C were calculated based on the JMA(Johnson-Mehl-Avrami) model. The activation energy of the fused silica ceramics with the addition of the β-Si₃N₄ is 506.2 kJ/mol, increased by 23.6% than that of the pure fused silica ceramic.     

Influence of operation parameters on the separation of mixtures by pervaporation and vapor permeation with inorganic membranes. Part 1: Dehydration of solvents

The separation performance of two different commercially available tubular inorganic membranes was studied for solvent dehydration. The separation layers consisted of A-type zeolite and microporous silica. The membrane characteristics were determined as function of operating conditions such as feed composition, temperature, and permeate pressure in pervaporation and vapor permeation. Among different membranes of the same batch, flux and selectivity were reproducible within 10%. The partial flux of water as the preferentially permeating component increases linearly with the water vapor pressure difference between feed and permeate and depends only marginally (viscosity influence) upon the properties of the organic component. The flux of the organic (retained) component is low and can best be described by assuming a substance and membrane specific permeance (flux over partial pressure difference) that is independent of composition. At very low water concentration in the feed one would expect a strong increase in permeability of the retained component through non-zeolite pores and larger silica pores as predicted by pure component measurements. However, this effect was not observed in mixtures within the concentration range studied here. A temperature rise improves flux rates exponentially while selectivity remains high. Thus, higher module cost in comparison to polymeric membranes can be compensated by reduced membrane area if a higher operating temperature can be chosen. Flux and selectivity decline as a function of permeate pressure with decreasing driving force. In vapor permeation with inorganic membranes superheating of the vaporous feed improves their performance while for polymeric materials a steep flux decline is observed. High flux and selectivity are obtained in the separation of water from alcohols. The normalized flux values of the A-type zeolite membrane are roughly 10 kg/m² h bar with a mixture selectivity of 2000 for methanol, 4000 for ethanol and 8000 for n-butanol. The average permeance of the amorphous silica membrane lies above 12 kg/m² h bar with mixture selectivity of 50 for methanol, 500 for ethanol and 2000 for n-butanol. The separation mechanism is mainly based on adsorption and diffusion enhanced by shape selectivity and size exclusion in some cases. The transport characteristics could be described with a simple transport model based on normalized permeate fluxes. With regard to the operation stability of the membranes, no deterioration of the performance was observed for the A-type zeolite in solvent dehydration or in separation of water from reaction mixtures. The silica membrane showed an initial conditioning effect involving a rearrangement of Si-OH groups with an increase in selectivity and decrease in flux of about 30%. After a few hours the performance stabilized and remained constant during further operation.     

Studies on functionalized mesoporous materials—Part I: characterization of silylized mesoporous material of type MCM-41

The synthesis of organic–inorganic composite materials was performed by the surface modification of mesoporous material type MCM-41 by chlorodimethylphenylsilane and dichloromethylphenylsilane. Applying IR spectroscopy, low temperature nitrogen adsorption/desorption (isotherms BET), thermogravimetric measurements and technique of competitive adsorption of toluene and water it was shown that the degree of silylation, hydrophobicity, surface and volume properties (pore size distribution, pore volume) strongly depends on the nature of silylation agent and the ratio of calculated amount of silanol groups to the modifier. Two types of condensation reaction take place: (1) the reaction of the modifier with surface silanol groups, and (2) an inter-molecular condensation of the modifier, resulting in additional pore blocking. Only 24–36 % of the surface silanol groups react with modifier agent. The materials are stable up to temperatures of about 170 °C that is higher than the corresponding polymeric resins. The TG/DTA data allowed concluding that the degree of grafting depends on the ratio silylation agent to SiOH groups. As shown by Fourier transform diffuse reflectance mode spectroscopy only free silanol groups react with modifier.     

硅羟基对二氧化硅固载有机锡酯交换催化性能的影响

采用新方法制备了二氧化硅固载有机锡。以无机SnCl₄为锡源,在合成有机锡的同时制备有机锡硅烷试剂,再将其固载到二氧化硅上,用于碳酸二甲酯与苯酚酯交换合成碳酸二苯酯的反应。通过不同的制备方法和处理方法,制得不同硅羟基含量的二氧化硅载体;研究硅羟基对锡固载量及固载有机锡催化性能的影响。TG与ICP表征显示,锡固载量与二氧化硅载体的硅羟基含量正相关。²⁹Si MAS NMR表征发现,制备的固载有机锡中含有较多的T²和T³物种,利于有机锡的稳定固载。二氧化硅硅羟基含量越高,催化剂的锡固载量越高,活性越好。介孔二氧化硅为载体制备的Sn-MSiO₂表现出最好的催化活性：用量1.0 g,在150～180℃反应9 h,苯酚转化率达50.4%,酯交换选择性高于99.9%;用0.5 g考察重复使用性能,重复使用5次,苯酚转化率由41.2%降低到35.0%。锡流失是活性下降的主要原因。     

Preparation of silica xerogel with high silanol content from sodium silicate and its application as CO2 adsorbent

In this study, silica xerogels with high silanol content were synthesized by using sodium silicate as low-cost silica source in the presence of hydrochloric acid and acetic acid via sol–gel process for CO₂ adsorption purpose. The effect of amount of acetic acid on the surface and structural properties of silica xerogel was investigated. Fourier transform infrared spectroscopy (FTIR) and thermal gravimetric analysis (TGA) revealed that the silanol groups existed on the surface of silica xerogel products and their concentration increased with increasing the amount of acetic acid. The BET surface area and total pore volume of the silica xerogel prepared using 6 mL of acetic acid (MMS-6) were found to be 1021 m²/g and 0.72 cm³/g, respectively. The pore structure of silica xerogel products consisted of the interparticle voids between the nanoparticles aggregates, and the interconnected wormhole microporous structure. The latter pore structure was more uniform on increasing the amount of acetic acid. CO₂ adsorption/desorption measurements were carried out using TGA unit with high purity CO₂ (99.999%). The highest CO₂ sorption capacity (83.6 mg_CO2/g_sorbent) was obtained with MMS-6 product. Thermal swing adsorption studies showed that the silica xerogel products exhibited strongly reversible CO₂ adsorption capacity and stable during 5-repeated cycle of adsorption/desorption experiment at 35 °C.     

Hybrid organosiloxane material/metal oxide composite as efficient anticorrosive coatings for mild steel in a saline medium

Hybrid sol–gel materials have been found very promising anticorrosive coatings for metal substrates. In this article, the synthesis of novel hybrid organic‐inorganic sol–gel polymer; starting from tetraethyl orthosilicate, (3‐aminopropyl) trimethoxysilane, dimethoxy‐methyl‐octadecylsilane and polydimethylsiloxane, silanol terminated precursors, is reported. The hybrid polymer has been further loaded individually as well with five different metal oxides, then deposited on mild steel panels. All cured coating formulations have been characterized using thermogravimetric analysis, contact angle measurements, electrochemical impedance spectroscopy, vicker‐microhardness, surface roughness, and critical load analyses. Results have revealed that the parent coating exhibits excellent thermal stability and hydrophobic nature with minor observed changes on the two properties for the metal oxide‐loaded coatings. Electrochemical impedance and visual inspection results indicated excellent corrosion protection performance for all metal oxide composite coatings (except magnesium oxide) on steel when immersed in 3.5% NaCl solution for a prolonged time. Furthermore, the coating containing molybdenum oxide exhibited a maximum hardness, homogeneity, and adherence to the steel surface. The developed coating formulations in this study can be considered as a promising alternative to the currently‐used toxic chromate and phosphate coatings. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 134, 46718     

Variations in the Chromatographic Properties of Different Silica Gel Samples

Abstract

Silica gels for adsorption chromatography differ widely in their performance, depending upon the nature of the starting silica and its subsequent treatment. A general theory of the origin of these differences (and their control) is proposed in terms of the structure of the silica surface. Variations in the activity, selectivity, and linear capacity of different silica samples are readily understandable in terms of this theory. Details are provided for the initial evaluation, optimum treatment (activation and deactivation), and final standardization of silica adsorbents.     

Selective online solid-phase extraction of copper using p-morpholino-methylcalix[4]arene appended silica-based column

ABSTRACT

This study deals with investigation of selectivity of p-morpholino-methylcalix[4]arene appended silica from a group of metal ions having similar charge and radii. The values of distribution ratio revealed that modified silica possess higher selectivity for Cu²⁺. In addition, relative selectivity coefficients of modified silica were found to be 24.1, 13.8, 8.6 and 4.01 for Cu²⁺/Ni²⁺, Cu²⁺/Co²⁺, Cu²⁺/Cd²⁺, Cu²⁺/Pb²⁺, respectively. The maximum adsorption capacity was 1.5 mmolg^?1. The reusability data suggested no any loss of adsorption capacity of this material up to 10 cycles. The developed material was also applied for determination of copper in lake water samples with satisfactory results.     

Effect of Sublimation Treatment on the Catalytic Performance of MFI Catalysts in Pyridine and 3‐Picoline Synthesis

Mordenite framework inverted (MFI)‐structure catalysts with framework trivalent metal cations and extra‐framework metal cations were prepared by the sublimation method to synthesize pyridine and picolines. The results show that the sublimation treatment of volatile metal halides, i.e., GaCl₃ and FeCl₃, consumes the silanol groups of the parent sample. Most of the Ga cations are incorporated into the zeolite framework, which produces hydroxyl groups and dramatically enhances the proportion of Brönsted acid sites. At the same time, a large number of Fe cations exist in the extra‐framework, causing an increase in the proportion of Lewis acid sites. The increased acid amount and strength effectively improve the activity and stability of the catalyst. Moreover, the generated stronger acid sites increase the selectivity of pyridine. The Ga cations of the modified zeolite can promote the dehydrogenation step for pyridine synthesis, which markedly increases the pyridine selectivity.