Types Of Silanol Groups Research Articles

高碳含量新型亚微米无孔二氧化硅材料的修饰方法及其在反相加压毛细管电色谱平台上的应用

亚微米无孔二氧化硅(NPS)材料具有小粒径及表面光滑形状规整等特点,是一种性能优异的色谱材料,但其存在比表面积小、修饰效率低的问题。针对此设计了一种具有高碳含量的修饰方法:以3-缩水甘油基氧基丙基三甲氧基硅烷(GPTS)作为硅烷偶联剂,聚乙烯亚胺(PEI)作为聚合物包覆层,并以硬脂酰氯修饰得到一种氨基包覆的具有C18碳链结构的新型亚微米无孔二氧化硅材料(C18-NH2-GPTS-SiO2)。利用元素分析、傅里叶变换红外光谱、Zeta电势等进行表征,证明C18-NH2-GPTS-SiO2固定相的成功制备。该修饰方法将NPS的碳含量从0.55%提高到了8.29%,解决了过往NPS材料采用十八烷基氯硅烷等传统C18修饰方法时碳含量较低的问题。此外,29Si固体核磁显示:NPS与多孔二氧化硅(PS)微球相比不仅存在孔结构与比表面积区别,且表面硅羟基种类也不同。16%的PS微球硅原子带有一个硅羟基(孤立硅羟基,Q3)、19%带有两个硅羟基(偕硅羟基,Q2);而NPS微球不存在偕硅羟基,仅有30%硅原子处于孤立硅羟基状态。实验发现NPS微球存在硅羟基数量低且缺少偕硅羟基的特点,导致NPS材料表面修饰活性低,难以通过简单一步反应获得高碳含量。采用不同疏水物质如苯系物、多环芳烃对色谱性能及保留机理进行研究,结果表明C18-NH2-GPTS-SiO2色谱柱符合反相作用机理。氨基的包覆改变硅球表面电性,提高了NPS材料运用于加压毛细管电色谱平台(pCEC)时的电渗流大小,施加+15 kv时,显示出良好的分离能力,证实了C18-NH2-GPTS-SiO2材料通过多步反应提高碳含量的修饰方法在pCEC平台上应用的优异性。

Infrared Study of Silanol Groups on Dealuminated High Silica MFI Zeolite to Correlate Different Types of Silanol Groups with Activity for Conversion of 1-Butene to Propene

Infrared spectra of dealuminated high silica MFI zeolites were measured to elucidate the structure of silanol groups active for conversion of 1-butene to propene. The OH groups were replaced by OD groups to have clear vibrational spectra. Two sharp bands and one broad band were observed; isolated silanol at 2746 cm−1, terminal silanol at 2725 cm−1, and hydrogen-bonded silanols centered at 2570 cm−1 composed of more than one components. Based on the response of these bands to Na poisoning and pyridine adsorption together with the correlation with catalytic results, we propose that the active sites for propene formation are one of the terminal silanol groups and one of the hydrogen-bonded silanol groups in a silanol nest. The active hydrogen-bonded silanol groups are the one whose O atom is hydrogen-bonded to adjacent two H atoms in the silanol nest.

Modeling of a Hybrid Langmuir Adsorption Isotherm for Describing Interactions Between Drug Molecules and Silica Surfaces

The interaction between disulfiram (Antabus®) and silica was studied experimentally by adsorption from apolar solvent onto highly porous silica material (Santa Barbara amorphous material-3) with large surface area. The adsorption isotherm was fitted to the Langmuir model by accounting 2 different affinities contributing to the overall behavior, which were attributed to 2 different types of silanol groups (i.e., geminal and vicinal) present on amorphous silica surfaces. This assumption was supported by theoretical calculations. In addition, the model could describe the adsorption of ibuprofen to the carrier material, indicating that the model bears big potential for describing the interactions between silica surfaces and drug molecules.

Role of silanol groups on silica gel on adsorption of benzothiophene and naphthalene

The adsorptive removal of sulfur-containing compounds such as thiophene derivatives from transportation fuels has received extensive attention because of its economic and environmental advantages compared to the hydrodesulfurization process. In our previous study, we suggested that the adsorption sites for benzothiophene and naphthalene on the silica surface consist of different types of silanol groups, and the removal of vicinal silanols from the silica surface should enable silica materials to adsorb benzothiophene selectively. In this study, an attempt is made to prove this hypothesis for the development of novel desulfurization sorbents. Heat treatment of silica gel was performed at different temperatures to remove vicinal silanol groups, and the adsorption capacities of benzothiophene and naphthalene were evaluated. The amount of naphthalene adsorbed on heat-treated silica gel decreased with increasing heat-treatment temperature, whereas the amount of benzothiophene adsorbed on silica gel treated at 873 and 1073 K showed only a slight decrease. Considering the heat treatment might lead to the removal of vicinal silanols due to dehydration of surface silanols, the results indicate that vicinal silanol groups were selective adsorption sites for naphthalene. Moreover, for binary-component systems, the adsorption capacities of benzothiophene and naphthalene were almost identical because of multilayer adsorption of naphthalene on adsorbed benzothiophene, and the adsorption capacities of benzothiophene for single- and binary-component systems were also identical. This suggests that the heat treatment of silica enables the selective removal of benzothiophene.

The elusive silica/water interface: isolated silanols under water as revealed by vibrational sum frequency spectroscopy.

It has been long recognized that the surface chemistry of silica, and in particular the type and relative amount of surface bound silanol groups, plays a critical role in many of the properties associated with the material, where a typical example is the discrepant adsorption behavior observed depending on the pretreatment history of the surface. However, in spite of its importance, the direct probing of specific surface silanol groups under water has been hampered by instrumental limitations. Here we make use of vibrational sum frequency spectroscopy (VSFS) to first, identify under water the OH stretch of isolated surface silanols, and second, explore its acid/base behavior and dependence on the surface pretreatment method. The properties of other types of silanol groups (i.e. hydrogen bonded/geminal) are also inferred from the data. The ability to directly probe these functional groups under water represents a crucial step to further improving our understanding of this widely used mineral oxide.

Density functional theory study of adsorption of benzothiophene and naphthalene on silica gel

The adsorptive removal of sulfur-containing compounds such as thiophene and dibenzothiophene derivatives from transportation fuels has received extensive attention because of the economical and environmental advantages of adsorptive removal compared to the hydrodesulfurization process. Metal-ion-exchanged zeolites have been known to exhibit high desulfurization performance. However, zeolites induce side reactions because of their strong acidity. Accordingly, silica-based adsorbents showing lower acidity have potential for the desulfurization adsorbent. In this study, to facilitate the development of efficient desulfurization adsorbents, the mechanisms of benzothiophene and naphthalene adsorption onto silica surface were investigated using density functional theory (DFT) in conjunction with experimental approaches. Both the computational and the experimental results suggested that the sites at which benzothiophene and naphthalene adsorb onto silica gel are different types of silanol groups. The adsorption sites of benzothiophene at the initial stage were vicinal-type and/or geminal-type silanols and the final adsorption sites were isolated-type silanols. In addition, the results of DFT calculations suggest that multilayer adsorption might occur. In contrast, the adsorption sites of naphthalene at the initial stage were vicinal-type silanols. Afterward, multilayer adsorption through a T-shape interaction might occur successively on the silica surface.

Systematic investigation of the pore structure and surface properties of SBA-15 by water vapor physisorption

In order to establish a structure-property correlation in water vapor sorption, various sets of the ordered mesoporous silica SBA-15 with systematically varying properties were synthesized and characterized. General trends concerning the surface polarity, aging temperature during synthesis and micropore volume were observed. Calcining the materials with increasing temperatures of 300–800 °C decreases the performance in water vapor sorption which correlates mainly with the amount and type of silanol groups on the silica surface. Furthermore, the aging temperature during synthesis influences the water vapor sorption capacity of SBA-15 at low relative pressure. The best results in terms of an increased uptake of water over a broad range of relative pressure can be obtained for materials with high micropore volume and high surface polarity. They can be tailor-made by using a routine to remove the template, i.e., a combination of extraction with ethanol and subsequent calcination at 300 °C.

The Silica-Water Interface: How the Silanols Determine the Surface Acidity and Modulate the Water Properties.

Silica is the most abundant metal oxide and the main component of the Earth's crust. Its behavior in contact with water plays a critical role in a variety of geochemical and environmental processes. Despite its key role, the details of the aqueous silica interface at the microscopic molecular level are still elusive. Here we provide such a detailed understanding of the molecular behavior of the silica-water interface, using density functional theory based molecular dynamics (DFTMD) simulations, where a consistent treatment of the electronic structure of solvent and surface is provided. We have calculated the acidity of the silanol groups at the interface directly from the DFTMD simulations, without any fitting of parameters to the experimental data. We find two types of silanol groups at the surface of quartz: out-of-plane silanols with a strong acidic character (pKa = 5.6), which consequently results in the formation of strong and short hydrogen bonds with water molecules at the interface, and in-plane silanols with a pKa of 8.5, forming weak hydrogen bonds with the interfacial water molecules. Our estimate of the quartz point of zero charge (1.0) is found in good agreement with the experimental value of 1.9. We have also shown how the silanols orientation and their hydrogen bond properties are responsible for an amphoteric behavior of the surface. A detailed analysis has identified two species of adsorbed water molecules at the solid-liquid interface, which using the language of vibrational spectroscopy can be identified as "liquid-like" and "ice-like" water or, in other words, water molecules forming respectively weak and strong H-bonds with the oxide surface. These two populations of water are in turn responsible for two distinct peaks in the infrared spectrum of interfacial water and thus provide a molecular explanation of the experimental sum frequency generation spectrum recorded in the literature. In the specific case of quartz, we show that the liquid-/ice-like behavior is the result of the silanol groups ability to donate or accept hydrogen bonds with different strengths, which consequently modulates the vibrational properties of the adsorbed water layer.

Effect of Hydrothermal Treatment on Adsorption Properties of Silica Gel

It has been proven by coupling near infared spectroscopy to second derivative processing of the spectra that the free and hydrogen bonded silanol groups attached to water molecules absorb at 5314 and 5270 cm−1, respectively. The same method was employed to probe the surface of the hydrothermally treated silica gel samples after pretreatment and evacuation at three different temperatures. The chemical and physical properties of silica gel depend on the nature of the surface silanol groups. Free silanol groups and hydrogen bonded silanol groups play an important role in adsorption of water molecules. Any change or variation in the silanol groups is expected to change the adsorption properties of any silica gel. Two silica gel samples with varying surface characteristics were used in this study. After heating and evacuating at 200, 750, and 1100 °C, the samples were hydrothermally treated in steel bombs and their surface functionalities were probed using near-infrared spectroscopy. The same samples were also dehydroxylated at 450 and 650 °C and analyzed by near-infrared spectroscopy. Furthermore, effectiveness of the treated samples in adsorbing water molecules was followed by monitoring the mass of water adsorbed by silica gel. These investigations form the basis for understanding the relationship between adsorption effeciveness and types of silanol groups on silica gel surface. The results clearly indicate that the hydrothermal treatment of silica gel pretreated at temperatures up to 750 °C increases the concentration of hydrogen bonded silanol groups on the silica gel surface. The sample treated at 1100 °C did not undergo any change under hydrothermal treatment. The concentration of free silanol groups increases in the dehydroxylated samples. The results also show that the adsorption rates of water molecules on the surface decrease significantly after pretreatment of these samples at a particular humidity.

A Theoretical Analysis of the Influence of Electroosmosis on the Effective Ionic Mobility in Capillary Zone Electrophoresis

A theoretical description of the influence of electroosmosis on the effective mobility of simple ions in capillary zone electrophoresis is presented. The mathematical equations derived from the space-charge model contain the pKa value and the density of the weak acid surface groups as parameters characterizing the capillary. It is shown how the zeta potential of the capillary and the effective mobility of typical anions are affected by the pH and ionic strength of the background electrolyte. Furthermore, the model is also used to analyze experimental results on the pH dependency of the electroosmotic mobility in a fused silica capillary, yielding estimates for the pKa values of two types of silanol groups and the total surface density of these groups . With these estimated values for the properties of the silanol groups, the model predictions of the effective mobility are compared with literature data.

Number and Strength of Surface Acidic Sites on Porous Aluminosilicates of the MCM-41 Type Inferred from a Combined Microcalorimetric and Adsorption Study

A combined microcalorimetry and adsorption study has been used to characterize the surface acidity of two series of MCM-41 aluminosilicates (referred to as SiAlxCn, where x is the mole Si:Al ratio and n the chain length of the surfactant template). 29Si magic angle spinning NMR spectra of a selected sample (SiAl32C16) indicates the presence of siloxane groups, Si(OSi)4, and three types of silanol groups, that is, single (SiO)3−Si−OH, hydrogen-bonded (SiO)3−SiOH···HO−Si−(SiO)3, and geminal (SiO)2−Si(OH)2. It is also possible to detect the contributions from Si(3Si,1Al) and Si(2Si,2Al) sites. Generally, the width of NMR spectra is characteristic of amorphous materials and suggests a large variety of Si−O−Si and Si−O−Al bond angles and lengths. The 27Al MAS NMR study made on the hydrogen-exchanged SiAl32C14 sample (H+−SiAl32C14) and the one rich in aluminum (SiAl8C14) clearly points to the presence of extraframework six-coordinate Al in the calcined materials. The volumetric and calorimetric measurements of gas ammonia adsorption at 353 K were used to determine the number and strength of surface acidic sites. With the exception of H+−SiAl32C14 and SiAl8C14, all samples have low surface acidity. The density of acidic sites ranges between 0.04 and 0.09 μmol m-2, increasing a little with decreasing pore size. These sites are relatively homogeneous and weak. For H+−SiAl32C14, surface acidity is enhanced as a result of the formation of strongly acidic Brönsted-type sites and extraframework aluminum. The density of acidic sites increases to 0.11 μmol m-2 and the fraction of strongly acidic sites is about 60%. In the case of the SiAl8C14 sample, the density of acidic sites reaches a value of 0.3 μmol m-2. These sites are very heterogeneous: the corresponding differential enthalpy of ammonia adsorption decreases from −160 to −80 kJ mol-1. Following the pyridine-TPD study on this sample, Lewis acid sites producing surface pyridine complexes constitute the strongest acidic sites.

Water Adsorption on Pyrogenic Silica Followed by1H MAS NMR

On the surface of two commercial pyrogenic silicas (Degussa and Cabot), five resonances were identified on the basis of the chemical shift, homonuclear coupling (T2), and spin–lattice relaxation behavior (T1). In accordance with previous studies we observed three different types of silanol groups: (i) weakly coupled (longT2), water inaccessible, isolated “internal” silanols at 1.8 ppm; (ii) weakly coupled, external “free” silanols revealed upon dehydration at 2.5 ppm; and (iii) strongly coupled external hydrogen bound silanols with an unresolved broad resonance between 3 and 7 ppm. The resonance of water, whose position between 2.6 and 4.6 ppm depended on water content, corresponded to two unresolved species of slightly differentT1. By equating this resonance to the weighted average of two distinct populations of water, we were able to distinguish the first layer of strongly hydrogen bound water at 2.7 ppm from liquid-like water at 5 ppm. The first layer is complete for water relative humidity as low as 3.6% and corresponds to a surface coverage of 4.75 H2O/nm2. If we assumed a cristobalite-based surface structure, this meant a 1:1 ratio between surface hydroxyls and the first layer of physisorbed water. This ratio was the same for the two silicas regardless of surface area.

Comprehensive Study of Surface Chemistry of MCM-41 Using 29Si CP/MAS NMR, FTIR, Pyridine-TPD, and TGA

A comprehensive study was conducted on mesoporous MCM-41. Spectroscopic examinations demonstrated that three types of silanol groups, i.e., single, (SiO)3Si−OH, hydrogen-bonded, (SiO)3Si−OH-OH−Si(SiO)3, and geminal, (SiO)2Si(OH)2, can be observed. The number of silanol groups/nm2, αOH, as determined by NMR, varies between 2.5 and 3.0 depending on the template-removal methods. All these silanol groups were found to be the active sites for adsorption of pyridine with desorption energies of 91.4 and 52.2 kJ mol-1, respectively. However, only free silanol groups (involving single and geminal silanols) are highly accessible to the silylating agent, chlorotrimethylsilane. Silylation can modify both the physical and chemical properties of MCM-41.

Kinetic Study of the Reactions of Chlorosilanes with Porous Vycor Glass

The alternating reactions of SiCl 4 and H 2O with the surface of Vycor glass have been studied by thermogravimetric analysis at 400-600°C. The results are interpreted by postulating two types of silanol groups (Si-OH) on the Vycor surface, paired and isolated. Reaction of SiCl 4 with paired silanols proceeds by competing steps which control the total extent of silylation. Upon reaction with H 2O, all surface Si-Cl groups are hydrolyzed, reproducing Si-OH. Simultaneous silanol-silanol and silanol-surface chloride condensations eliminate H 2O or HCl to form new Si-O-Si bridges with final product silica. A kinetic model incorporating the silylation, hydrolysis, and condensation steps describes well the TGA data.

Microporous silica microballoons

Silica microballoons about 1.5 μm in diameter were prepared from sodium silicate solution by the ultrasonic spraying dry and spray dry methods. Sodium ions were removed with 0.01 N HCl solution, using 1–3 1 g −1. The prepared balloons were characterized with the aid of scanning electron microscopy (SEM), thermal analyses, electron probe X-ray microanalyses, Fourier transform-infrared, and nitrogen and water adsorptions. The SEM observation showed the formation of silica balloons about 1.5 μm in diameter whose shell thickness is about 0.1 μm. Thermogravimetry showed that microballoons prepared by these two methods have different types of silanol groups. The nitrogen adsorption isotherms were of Type II; the α s plots suggested the presence of micropores. The surface area of washed samples is greater than that of unwashed samples. However, the water adsorption isotherms of washed microballoons show a large uptake in the lower relative pressure region and were rather close to Type I. The surface area of washed samples from water adsorption measurements was much greater than that from nitrogen adsorption, indicating the presence of narrow micropores accessible only to water molecules.

FT i.r. study of the silanol groups in dealuminated HY zeolites: Nature of the extraframework debris

A detailed i.r. study has been performed in the silanol frequency range of silica, silica-alumina, and modified zeolites with different structures. Three different types of silanol groups have been found in dealuminated zeolites: (i) a first band at 3747–3749 cm −1 sensitive to pyridine (adsorption and desorption at 423 K), which corresponds to SiOH groups attached to amorphous silica-alumina debris; (ii) a second band at 3744–3746 cm −1 unsensitive to pyridine corresponding to silanol groups attached to extraframework silicon-rich debris; and (iii) a third component at 3738 cm −1 unsensitive to pyridine, assigned to terminal framework silanol groups. The silanol bands present in modified zeolites provide information on the nature of the extraframework debris and on framework structural defects. After steaming, the extraframework phase is alumina-rich, whereas after a subsequent acid leaching, it becomes richer in silica as the strength of leaching increases.

Analysis of Surface of Porous Glass

The density of surface silanol groups in three kinds of porous glasses with different pore structures was determined by using CH3Mgl, (CH3)3SiCl(TMS), (CH3)2Cl2(DMS) and CH3OH. The density ranged from 2.3 to 7.7μmol/m2. The density determined by TMS was about 3μmol/m2 regardless of the pore structures. The surface of porous glasses was boron-rich after exposure to air for a long period. But, washing with acid solution removed boron, and the surface contained only silicon. The number of silanol group of single type was dominant in all types of silanol groups and occupied 50-80% regardless of the pore structures. Geminal type comes second. But, when pore was small, the number of geminal type was almost the same as that of paired type.

Raman and infrared spectroscopic studies of the reactions of silica and glass surfaces

Different types of silanol groups on the surfaces of Cabosil and glass microspheres have been detected using Raman spectroscopy. The spectral region at 980 – 1005 cm −1 is found to be related to the SiO vibrations of the surface silanol groups. The 980 cm −1 Raman line is associated with vibrations of the strongly hydrogen-bonded silanols. The weakly hydrogen-bonded silanols are responsible for the 990 ± 2 cm −1 Raman lines. The latter type of silanol is more reactive than the former one. After ammonia and silane treatments, glass microspheres picked up more vinylsilane than did Cabosil. Reactions with the composite matrix can also be followed.

Exchange of Na + for the silanolic protons of silica

It has been shown that a single mass-action law expression is inadequate for prediction of the amount of exchange of simple metal ions for H + on a silica surface above pH 5·5. This paper examines mathematical treatments based on three different models for the exchange of cations with silanolic protons. In the first model the surface is assumed to consist initially of two types of silanol groups with widely different reactivities. In the second the apparent equilibrium constant for the exchange process is assumed to vary as the reaction progresses. The third model combines the assumptions of the first and second and appears to be the most successful.