Ab initio cluster calculations of silica surface sites

Silica surface sites, which can be formed in cleavage processes, and their hydrolyzed counterparts are investigated with ab initio cluster calculations. Natural Bond Orbital (NBO) theory is used to characterize bonding around silica surface sites. Higher energy lone pairs of electrons on oxygen atoms either hyperconjugate to vicinal silanol/siloxane antibonding orbitals or backdonate electron density via donor–acceptor π-type bonding with participation of pd or p hybrids on silicon atoms. Upon substitution of hydroxyl groups of orthosilicic acid with silica monomers the strength of siloxane and silanol Si–O bonding increases as energies of bonding orbitals and contributions from p-orbitals decrease. Silanone sites and a complementary pair of silyl/siloxy radical sites are found to be the most stable geminal and single non-hydrolyzed sites, respectively. Atomic charges based on natural wavefunctions and on fitting to electrostatic potential, and characteristic bands of IR spectra associated with siloxane and silanol stretching vibrations of silica surface sites are reported.     

Metathesis activity and properties of Mo-alkylidene sites differently located on silica. A density functional theory study

Ethene metathesis proceeding on Mo-methylidene centers on silica is investigated with density functional theory, applying the cluster approach. Three different locations of the active sites are considered, in which the Mo center replaces a pair of geminal silanols, two silanols from adjacent geminal pairs and two single silanols, respectively. It is shown that metathesis activity of the Mo-methylidene sites strongly depends on their location on silica. Different reactivity of the centers toward alkene is explained by differences in their geometrical and electronic structure parameters. The calculated C-H stretching vibrations of the proposed Mo-methylidene, Mo-ethylidene, and molybdacyclobutane surface complexes are well consistent with the reported IR spectra for the corresponding species generated on real molybdena-silica catalysts. On the basis of the obtained results it is proposed that among the studied cases, the Mo centers replacing two silanols from adjacent geminal pairs of silica surface are the most adequate models of the real active sites.     

Interaction between Globular Proteins and Silica Surfaces

The adsorption of bovine serum albumin on silica of three different types, i.e., aerosil, macroporous silica gel, and ground natural quartz, was studied. The IR spectra of this protein adsorbed on aerosil were measured and analyzed. It was shown that the carbonyl groups of albumin macromolecules interact with vicinal hydroxyl groups while imido groups, with individual hydroxyl groups of silica surface. The geminal hydroxyl groups of the surface behaved as single adsorption sites with respect to albumin. The number or such sites on quartz surface was estimated. The IR spectra indicated that the adsorption of albumin macromolecules caused the dehydration of aerosil surface and the appearance of a small amount (to 10%) of unfolded -regions in the secondary structure of the adsorbed protein, while the -helical macromolecular structure remains preserved as a whole. Changes in the tertiary structure of the protein resulted from the adsorption were discussed. Protein macromolecules folded into globules were shown to be tilted with respect to the adsorbent surface.     

The infrared and Raman spectra of water in sodium hexafluorophosphate monohydrate, NaPF6·H2O

The O---H and O---D stretching modes in partially deuterated NaPF₆·H₂O occur at what seem to be the highest frequencies so far recorded for HDO groups in crystalline hydrates, showing that the hydrogen bonds in this compound are very weak.     

Piperidine adsorption on hydrated alpha-alumina (0001) surface studied by vibrational sum frequency generation spectroscopy

The adsorption of piperidine vapor on the hydrated alumina (alpha-Al2O3, corundum) (0001) surface was investigated using vibrational broad bandwidth and scanning sum frequency generation (SFG) spectroscopy. The interfacial vibrational signature in the C-H stretching region of piperidine at the alumina (0001) surface is shown to be a sensitive spectroscopic probe revealing the adsorption mechanism. The neat piperidine surface, aqueous piperidine surface, and aqueous piperidium chloride surface were also investigated in the C-H stretching region by SFG to establish vibrational reference frequencies. After piperidine adsorption, piperidine vapor was removed and piperidine was found to be chemisorbed onto the alumina (0001) surface through protonation by surface hydroxyl groups. The O-H stretching region of the alumina surface before and after piperidine adsorption was also investigated, and the results revealed the decrease of the surface number density of alumina surface hydroxyl groups.     

The surface of ordered mesoporous benzene-silica hybrid material: an infrared and ab initio molecular modeling study

Joint IR and computational results allow a detailed characterization of the surface properties of a mesoporous benzene-silica hybrid material with crystal-like wall structure. After outgassing at 450 degrees C, hydroxyl species mainly consist of noninteracting silanols, with both O-H and Si-O stretching modes at lower frequencies than those of SiOH in silica. Interaction with several probe molecules, followed both by experiment and calculus, shows that the aryl group in the coordination sphere of Si imparts a lesser acidity with respect to the isolated silanol in silica. In contrast, adsorption isotherms indicate that the interaction with acetone is stronger with benzene-silica than with silica: this is interpreted in terms of secondary interactions taking place between the slightly acidic CH in acetone and the electronic cloud in benzene-like rings. This suggests that both the inorganic component and the organic one play a role in dictating the surface behavior.     

FTIR investigation of 2-chlorophenol chemisorption on a silica surface from 200 to 500 degrees C

The time-dependent chemisorption of 2-chlorophenol on a fumed silica surface was studied in situ from 200 to 500 degrees C using a temperature-controlled dosing cell and FTIR absorption spectroscopy. 2-Chlorophenol was found to chemisorb at isolated and geminal surface hydroxyl sites. 2-Chlorophenol chemisorption and subsequent surface oxidation resulted in a mixture of chlorophenolate and partial oxidation products, such as formates and acetates. The rates of chemisorption were measured, and the activation energy of adsorption was found to be 15 +/- 4 kJ mol(-1) for a fast, initial reaction and 22 +/- 2 kJ mol(-1) for a slower reaction at higher surface coverage. This work was motivated by the observation that combustion-generated fly ash mediates the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) at temperatures between 250 and 450 degrees C. Although transition metals such as copper are known to catalyze or mediate this reaction, silica is the major component of fly ash and chemisorption at higher concentration surface sites of silica must have a significant impact on the surface-mediated PCDD/F formation on fly ash surfaces.     

Inaccessible hydroxyl groups on silica are accessible in supercritical CO2

The three main types of hydroxyl groups on a silica surface are classified as isolated, hydrogen bonded, and inaccessible. The isolated and hydrogen bonded groups are the most important as these readily exchange with D(2)O and thus are exposed to reactant molecules. However, it has generally been accepted that the inaccessible groups do not participate in surface reactions as only a small fraction of these groups exchange with D(2)O. It is shown that the inaccessible hydroxyl groups on nonporous fumed silica and mesoporous MCM-48 silica powders and films fully exchange with D(2)O and are reactive with octadecylydimethylchlorosilane when supercritical CO(2) is used as the solvent. Furthermore, it is found that the CO(2) penetrating the regions containing the inaccessible groups is not removed by simple evacuation but rather slowly diffuses from the silica over periods of months.     

The structure of modifying layers and identification of residual hydrophilic groups on modified silica surface by the adsorption and chromatographic measurements

Amorphous silica Silochrom-80 is modified in a liquid phase in two stages with poly(ethylhydrosiloxane) and hexamethyldisilazane. Adsorption studies suggest that the modified silicas have a porous structure. Hexamethyldisilazane is found to have an active role in the modification of the secondary porous structure of the samples. The thickness of modifying layers on the surface of the samples is calculated. Residual hydrophilic sites are identified on the modified silica surface, and their concentration is determined using chromatography data and the parameters of the Langmuir equation. It is shown that the two-stage modification of silica with the selected reagents makes it possible to entirely deactivate the most active vicinal hydroxyl groups on the sorbent surface and, to a substantial extent, its isolated hydroxyl groups.     

环氧天然橡胶接枝高分散白炭黑增强天然橡胶复合材料的制备及表征

基于白炭黑表面硅羟基与环氧基团的可反应性,利用Haake流变仪的高温高剪切作用,在170℃下,实现了环氧天然橡胶(ENR)对白炭黑的固态原位接枝,制备出一种高分散疏水型白炭黑.探讨了白炭黑和ENR的反应配比对增强性能的影响,确定合适的反应比例为3∶1.FTIR、TGA和TEM的分析结果证实了ENR被接枝到白炭黑表面上.对比研究了接枝前、后白炭黑对增强天然橡胶(NR)复合材料性能的影响,测试结果表明接枝白炭黑在天然橡胶中具有良好的分散性并能明显改善对天然橡胶的增强效果;接枝于白炭黑表面上的环氧天然橡胶分子玻璃化转变向高温偏移,使该复合材料在常温下具备优异力学性能的同时也体现出了高动态滞后的特点.     

DFT study of the adsorption of microsolvated glycine on a hydrophilic amorphous silica surface

Density functional theory (DFT) periodic ab initio molecular dynamics calculations are used to study the adsorption of gaseous and microsolvated glycine on a hydroxylated, hydrophilic silica surface. The silica model is presented and the interaction of water with surface silanols is studied. The heat of interaction of water is higher with the associated silanols (be they terminal or geminal ones) studied here than with isolated silanols presented in past works. Glycine is stabilized in a parallel mode on the hydroxylated surface. Terminal silanols do not allow the stabilization of the zwitterionic form, whereas geminal silanols do. Molecular dynamics (MD) first-principle calculations show that microsolvated zwitterion glycine directly binds through the carboxylate function to a surface silanol rather than through water molecules. The adsorption mode, whether with or without additional water molecules, is parallel to the surface. The ammonium function does not interact directly with the silanol groups but rather through water molecules. Thus, the carboxylate and ammonium functions exhibit two different reactivities towards silanols. The calculated free energies, taking into account the chemical potentials of water and glycine in the gas phase, suggest the existence of a thermodynamic domain in which the glycine is present in the gas phase as well as strongly adsorbed on specific sites of the surface.     

A Geometrical Model for Chemically Bonded TMS and PDS Phases

Abstract

A geometrical model is developed for monomeric TMS and PDS phases that are chemically bonded to silica. Using experimental data for maximum surface coverage and considering amorphous silica as a collection of distorted crystals, we calculate that each nm² of the silica surface contains 2.3 modified hydroxyl groups, 1.3 free hydroxyl. groups and 0.6 pairs of bonded hydroxyl groups, respectively. From the dimensions of the silane molecule it is concluded that for maximum coverage the TMS and PDS molecules are rigidly attached to the silica surface with an Si-O-Si bond angle between 120 and 140 degrees. The unreacted hydroxyl groups are not completely screened but will be quite inaccessible on either phase. Very little free surface area remains on the silica surface.     

Interaction of Globular Albumins with the Silica Surface

Bovine serum albumin macromolecules interact with vicinal hydroxyl groups of the silica surface at carbonyl groups and with isolated surface hydroxyl groups at the imide groups. Geminal surface hydroxyl groups behave as single adsorption sites relative to albumin adsorption. The concentration of such groups on a quartz surface was evaluated. The secondary structure of globular albumins is altered as the result of interaction in the silica–globular albumin system. Extended -segments appear on the surface and the concentration of -spirals decreases. This alteration leads to change in the tertiary globular structure of the albumin. The adsorption of albumin macromolecules on the silica surface is inclined.     

Vibrational mode frequencies of silica species in SiO2-H2O liquids and glasses from ab initio molecular dynamics

Vibrational spectroscopy techniques are commonly used to probe the atomic-scale structure of silica species in aqueous solution and hydrous silica glasses. However, unequivocal assignment of individual spectroscopic features to specific vibrational modes is challenging. In this contribution, we establish a connection between experimentally observed vibrational bands and ab initio molecular dynamics (MD) of silica species in solution and in hydrous silica glass. Using the mode-projection approach, we decompose the vibrations of silica species into subspectra resulting from several fundamental structural subunits: The SiO(4) tetrahedron of symmetry T(d), the bridging oxygen (BO) Si-O-Si of symmetry C(2v), the geminal oxygen O-Si-O of symmetry C(2v), the individual Si-OH stretching, and the specific ethane-like symmetric stretching contribution of the H(6)Si(2)O(7) dimer. This allows us to study relevant vibrations of these subunits in any degree of polymerization, from the Q(0) monomer up to the fully polymerized Q(4) tetrahedra. Demonstrating the potential of this approach for supplementing the interpretation of experimental spectra, we compare the calculated frequencies to those extracted from experimental Raman spectra of hydrous silica glasses and silica species in aqueous solution. We discuss observed features such as the double-peaked contribution of the Q(2) tetrahedral symmetric stretch, the individual Si-OH stretching vibrations, the origin of the experimentally observed band at 970 cm(-1) and the ethane-like vibrational contribution of the H(6)Si(2)O(7) dimer at 870 cm(-1).     

Two-step modification of silica to obtain energetically homogeneous chromatographic sorbents

We have carried out a two-step liquid-phase modification of amorphous silica Silokhrom-80 with polyethyl hydride siloxane and hexamethyldisilazane. Using the parameters of the Langmuir equation, we have determined the concentration of residual hydrophilic sites on the surface of the modified silica, and we have identified them. We have shown that two-step modification of silica by selected reagents makes it possible to completely deactivate the most active vicinal hydroxyl groups on the sorbent surface, and to deactivate its isolated hydroxyl groups to a significant extent. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 2, pp. 125–129, March–April, 2006.     

硅胶自环己烷中吸附含氧芳香化合物的研究

测定了15℃和30℃时硅胶自环己烷中吸附苯甲醇、苯甲醚、苯甲醛和苯甲酸的吸附等温线,这些等温线皆可用Langmuir公式表示。利用Langmuir常数n_m^s和b值计算了吸附过程的△G⁰、△H⁰和△S⁰;四种化合物的△S⁰皆为正值。测定了在较大复盖度时红外光谱图上硅胶表面自由羟基峰的移动值△v_OH。结果表明△H⁰和△v_OH及△v_OH^1/2间有线性关系。文中对所得结果做了初步的解释。     

Acidic properties of [Al], [Ga] and [Fe] isomorphously substituted zeolites. Density functional model cluster study of the complexes with a probe CO molecule

The Brønsted acid strength and related characteristics of bridging hydroxyl groups in [Al]-, [Ga]- and [Fe]-framework-substituted zeolites have been studied using a model cluster density functional approach based on a gradient-corrected exchange-correlation energy functional. The acidity is found to decrease in the order Al(OH)Si > Ga(OH)Si > Fe(OH)Si in agreement with existing experimental and theoretical results. The present quantification is based on the calculated deprotonation energy of H₃Si(OH)TH₃, on the adsorption energy of a CO probe molecule as well as on the changes of the vibrational frequencies and absolute IR intensities of the O---H and of the C---O modes induced by CO adsorption. The vibrational parameters of the carbonyl adsorption complex in [Fe]-zeolites are predicted.     

Mechanism of hydroxyl radical generation from a silica surface: molecular orbital calculations

The interaction of an H(2)O molecule with cluster models of fractured silica surfaces was studied by means of quantum mechanical calculations. Two clusters representing homolytic cleavage (triple bond Si(*) and triple bond SiO(*)) and two representing heterolytic cleavage (triple bond Si(+) and triple bond Si-O(-)) of silica surfaces were modeled. Vibrational frequencies of the reactants and products of these silica surfaces reacting with H(2)O have been calculated and compare favorably with experiment. Comparisons of the Gibbs free and potential energies for the model ionic and radical states were made, and the radical pair of sites was predicted to be more stable by approximately -70 to -85 kJ/mol, depending on the computational methodology. These calculations suggest that when silica is fractured in a vacuum homolytic cleavage is favored. Reaction pathways were investigated for these four model surface sites interacting with H(2)O. The reaction of H(2)O with triple bond SiO(*) was predicted to generate OH(*). Rate constants for these reactions were also calculated and predict a rapid equibrium for the reaction triple bond SiO(*) + H(2)O --> triple bond SiOH + OH(*). Stability of a finite number of triple bond SiO(*) sites at equilibrium in the above reaction with H(2)O was also predicted, which implies a long-term ability of silica surfaces to produce OH(*) radicals if the sites of the broken bonds do not repolymerize to form siloxane groups.     

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...     

二氧化硅-硅油分散体系粘度的研究

不同结构或相同结构不同表面处理的二氧化硅,可用其硅油分散体在一定体积分数和特定剪切速率下测定的表观粘度来进行表征。从热老化前后分散体粘度变化的结果表明,二氧化硅表面羟基是引起硅氧烷降解的重要因素,硅烷化处理可改进在密闭条件下体系的热稳定性。