Surface Hydroxyl Groups Of Silica Research Articles

Gas Adsorption and Diffusion Behaviors in Interfacial Systems Composed of a Polymer of Intrinsic Microporosity and Amorphous Silica: A Molecular Simulation Study.

We investigate the adsorption and diffusion behaviors of CO2, CH4, and N2 in interfacial systems composed of a polymer of intrinsic microporosity (PIM-1) and amorphous silica using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. We build model systems of mixed matrix membranes (MMMs) with PIM-1 chains sandwiched between silica surfaces. Gas adsorption analysis using GCMC simulations shows that gas molecules are preferentially adsorbed in microcavities distributed near silica surfaces, resulting in an increase in the solubility coefficients of CO2, CH4, and N2 compared to bulk PIM-1. In contrast, diffusion coefficients obtained from MD simulations and then calibrated using the dual-mode sorption model show different tendencies depending on gas species: CO2 diffusivity decreases in MMMs compared to PIM-1, whereas CH4 and N2 diffusivities increase. These differences are attributed to competing effects of silica surfaces: the emergence of larger pores as a result of chain packing disruption, which enhances gas diffusion, and a quadrupole-dipole interaction between gas molecules and silica surface hydroxyl groups, which retards gas diffusion. The former has a greater impact on CH4 and N2 diffusivities, whereas the latter has a greater impact on CO2 diffusivity due to the strong quadrupole-dipole interaction between CO2 and surface hydroxyls. These findings add to our understanding of gas adsorption and diffusion behaviors in the vicinity of PIM-1/silica interfaces, which are unobtainable in experimental studies.

Reinforcement Mechanism of Silica Surface Hydroxyl: Janus Effects

Reinforcement Mechanism of Silica Surface Hydroxyl: Janus Effects

Size-dependent dissociation of surface hydroxyl groups of silica in aqueous solution

The size-dependence of surface hydroxyl dissociation of solid nanoparticles dispersed in aqueous solutions is a basic colloidal interface phenomenon, but there is no model to describe this phenomenon. In this work, nine amorphous silica samples with different radii (r, 6–196 nm) were chosen as model particles, and their effective surface hydroxyl density (Nseff) and dissociation equilibrium constants (Ka) were determined using acid-base titration, showing an obvious size-dependence. With the decrease of r, the Nseff (mol/m2) and pKa all decrease. The decrease of pKa indicates an enhanced dissociation tendency of surface hydroxyls. Two size-dependent models relating Nseff and Ka separately with r were developed, predicting that both lnNseff and pKa decrease linearly with an increase in 1/r. The model equations can well describe the size-dependent data of Nseff and Ka of silica particles. The characteristic parameters Nseff, apparent pKa, and intrinsic pKa corresponding to flat surfaces (r → ∞) for silica particles are estimated to be 9.12 μmol/m2 (or 5.49 sites/nm2), 9.85, and 8.99, respectively. The size-effect on Nseff and Ka becomes obvious when r < 40 nm. This work deepens the understanding of the size-dependent solid/liquid interface phenomena.

Evolution mechanism of surface hydroxyl groups of silica during heat treatment

The surface hydroxyl groups of nano-silica have a great influence on its application properties. In this paper, the changes of the number of various hydroxyl groups after dehydroxylation and rehydration progress of silica were quantitatively analyzed by thermogravimetric (TG) analysis and 29Si Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) measurements. According to the different stability of Si-O-Si bond formed after dehydroxylation, we propose two types of hydroxyl groups depending on the relative positions of them: chain adjacent hydroxyl groups (C-OHs) and spatially adjacent hydroxyl groups (S-OHs). DFT calculations indicate that two C-OHs dehydroxylation form a four-atoms ring structure, and the resulting OSiO bond angle deviation from optimal value is up to 19.56°, and the average bond length of the SiO bond increase more than 4.66%. Further more, its rehydration reaction activation energy (Ear) is very low, even only 0.21 kJ/mol, so it is easy to re-open and return to hydroxyl groups under environmental conditions. Conversely, the S-OHs dehydroxylation form a stable multi-atoms ring. The DFT calculations are a good illustration of the phenomenon in which some unstable siloxane bonds in a dehydroxylated sample are restored to hydroxyl groups in TG analysis and 29Si MAS NMR measurements.

Effect of surface hydroxyl group of ultra-small silica on the chemical states of copper catalyst for dimethyl oxalate hydrogenation

Abstract Cu/SiO2 catalyst prepared by ammonia evaporation method was reported to perform a great catalytic activity and selectivity in dimethyl oxalate (DMO) hydrogenation, which is one of the key steps in ethylene glycol (EG) synthesis from syngas. In recent years, significant advances have achieved on the nature of active sites and catalytic mechanism. However, the fabrication of Cu/SiO2 catalyst with controllable copper species remains challenging. Here, we reported a facile and effective approach to manipulate the surface hydroxyls of silica, which was revealed to be important factor for the formation of active species. An excellent linear correlation between surface hydroxyl groups of silica and the amount of Cu+ species was demonstrated, indicating that the formation of copper phyllosilicate can be kinetically favoured with increasing surface hydroxyls during preparation. Furthermore, as the copper phyllosilicate formation is enhanced, the specific surface area is significantly increased and the polymerization of copper hydroxide may be slow down, resulting in a highly improved dispersion of metallic copper as well. The enlarged surface areas of Cu0 and Cu+ species greatly enhanced the catalytic performance of Cu/SiO2 in DMO hydrogenation to EG. These understandings on the relationship between surface hydroxyl groups and chemical states of copper catalyst may lead to new possibilities in rational design of catalysts.

Investigation of the Isolated Cr(VI) Species in Cr/MCM‐41 Catalysts and its Effect on Catalytic Activity for Dehydrogenation of Propane

AbstractCr‐modified MCM‐41 with different loadings were prepared and tested in the propane dehydrogenation (PDH) reaction. As for Cr/MCM‐41 system, the general knowledge was that high Cr loadings resulted in the reduction of silica surface hydroxyl groups, thus leading to the formation of inactive Cr(III) species, as clusters and/or crystalline Cr2O3. However, in the present work, isolated Cr(VI) states were identified in the Cr/MCM‐41 catalysts, instead of Cr(III) species, using UV‐vis and H2‐TPR. These isolated Cr(VI) species are difficult to reduce, causing the decreased activity for propane conversion and low selectivity towards C3H6 in the PDH process. In contrast, a good dispersion of the Cr species was obtained using a chromium column adsorption process, and the absence of the isolated state of Cr(VI) species on this sample contributed to high activity toward PDH process. The investigation of the isolated Cr(VI) species is very important for the Cr‐based PDH catalyst system.

In situ Surface Functionalization of Hydrophilic Silica Nanoparticles via Flame Spray Process

Hydrophobic silica nanoparticles grafted with a high amount of organic molecules were successfully prepared by an in situ functionalization method in flame spray pyrolysis (FSP) process. Hydrophilic SiO2 nanoparticles were converted into hydrophobic ones by silylation between 3-methacryloxypropyltrimethoxyl silane (MPS) and silica's surface hydroxyl groups. The freshly formed silica nanoparticles in flame were continuously functionalized by a fine spray of 3-methacryloxypropyltrimethoxyl silane (MPS) solution at a preferred temperature. The functionalization extent, morphology structure and size of silica nanoparticles were characterized by transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectronic spectroscopy (XPS). The influence of concentration, pH value and pre-activation of organic silane solution on the surface grafting density was investigated in detail. The obtained silica nanoparticles had a higher MPS functional content of 15.0 wt% (an average density of 2.7 MPS molecule/nm2) than that of the silica modified by wet chemistry route, showing an excellent, stable hydrophobic property. The results have demonstrated that the in situ FSP functionalization process is a simple, effective and promising route for the scalable preparation of advanced, hydrophobic nanomaterials.

Catalytic Epoxidation of Cyclohexene with Tert-butylhydroperoxide Using an Immobilized Molybdenum Catalyst

This work describes the synthesis of molybdenum complexes immobilized on a silica support and their performance in the epoxidation reaction of cyclohexene using tert-butylhydroperoxide as the oxidizing agent. The catalyst synthesis included solubilization of variable amounts of bis-oxomolybdenum (VI) acetylacetonate precursor in different solvents such as tetrahydrofuran (THF), ethanol, THF/water and ethanol/water mixtures and contact with the silica support. Characterization techniques demonstrated that the nature of the incorporated molybdenum species depends markedly on the solvent employed. If the solvent employed is an ethanol:water mixture, physical adsorption of the Mo-species onto the support surface occurs; however, when THF is used as the solvent (THF catalyst series), molybdenum is grafted on the silica surface via chemical bonding with the surface hydroxyl groups of silica. Specifically, these latter catalysts show similar performance to that of the homogeneous catalyst, although long-term experiments showed deactivation by leaching of the active phase.

Investigating Lysine Adsorption on Fumed Silica Nanoparticles

The adsorption of amino acids on silica surfaces has attracted considerable interest because it has a broad range of applications in various fields such as drug delivery, solid-phase peptide synthesis, and biocompatible materials synthesis. In this work, we systematically study lysine adsorption on fumed silica nanoparticles with thermal analysis and solid-state NMR. Thermogravimetric analysis results show that the adsorption behavior of lysine in low-concentration aqueous solutions is well-described by the Langmuir isotherm. With ultrafast magic-angle-spinning 1H NMR and multinuclear and multidimensional 13C and 15N solid-state NMR, we successfully determine the protonation state of bulk lysine and find that lysine is adsorbed on silica nanoparticle surfaces through the side-chain amine groups. Density functional theory calculations carried out on lysine and lysine–silanol complex structures further confirm that the side-chain amine groups interact with the silica surface hydroxyl groups via strong hydro...

Dielectric and thermal studies of segmental dynamics in silica/PDMS and silica/titania/PDMS nanocomposites

ABSTRACTEffects of silica and silica/titania nanoparticles on glass transition and segmental dynamics of poly(dimethylsiloxane) (PDMS) were studied for composites of a core–shell type using differential scanning calorimetry, thermally stimulated depolarization current, and dielectric relaxation spectroscopy techniques. Strong interactions between the filler and the polymer suppress crystallinity (Tc, Xc) and affect significantly the evolution of the glass transition in the nanocomposites. The segmental relaxation associated with the glass transition consists of three contributions, arising, in the order of decreasing mobility, from the bulk (unaffected) amorphous polymer fraction (α relaxation), from polymer chains restricted between condensed crystal regions (αc relaxation), and from the semi‐bound polymers in an interfacial layer with strongly reduced mobility due to interactions with surface hydroxyls of silica and silica/titania nanoparticles (α′ relaxation). The evolution of surface affected CH3 groups, as well as the degree of interaction of PDMS molecules with surface hydroxyl groups as a function of treatment temperature, was assessed by Fourier transform infrared spectroscopy, thermogravimetry and differential thermal analysis. The effectiveness of silica/PDMS and silica/titania/PDMS nanocomposites as hydrophobic coatings was investigated by static contact angle measurements. It was shown that the presence of titania nanoparticles and adsorbed PDMS promotes the hydrophobic properties of the PDMS coating after treatment in the 80–650°C range. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41154.

Effect of the temperature on surface modification of silica and properties of modified silica filled rubber composites

The modified silica at different temperature (MSaDT) with bis(3-triethoxysilylpropyl)tetrasulfide (TESPT), and MSaDT filled solution styrene butadiene rubber (SSBR) composites were prepared to investigate the effect of temperature on surface modification of silica. The results showed that TESPT was successfully bonded on the surface of silica by chemical bonds. The grafting degree (K) of MSaDT of 50°C was 62.2% and higher than that at the other temperatures. The thermal weight loss and the size distribution of MSaDT showed that the silanol of TESPT hydrolysates reacted with the surface hydroxyl groups of silica, decreasing the average size and agglomeration of modified silica. For 50°C modified silica/SSBR composite, the static mechanical properties and rubber–filler interaction of the composite were better than those of the others. As far as dynamic mechanical properties are concerned, the 50°C modified silica/SSBR composite owned a best combination of low rolling resistance and high wet skid resistance.

Solid state NMR investigation of γ-irradiated composite siloxanes: Probing the silica/polysiloxane interface

We employ silicon-proton (29Si{1H}) cross polarization magic-angle spinning (CP-MAS) to probe directly the interface of a silica-filled polysiloxane elastomer subjected to gamma radiation. Using a traditional spin-lock CP experiment in conjunction with a silicon-edited proton rotating-frame longitudinal relaxation sequence, the full suite of CP build-up parameters are extracted for six resolvable silicon chemical shifts. The data, represented by three parameters, THX, T1ρH, and M∞, are interpreted by means of a model where the simultaneous effects of silica surface hydroxyl modification, interfacial water content, and radiation-induced chemistries are considered responsible for the observed macroscopic behavior of these engineering materials.

Synthesis gas production from partial oxidation of methane over highly dispersed Pd/SiO2 catalyst

Pd/SiO2 catalyst prepared by impregnation method using (NH4)2PdCl4, Pd(NO3)2 and Pd(acac)2 as precursors was used for the synthesis gas production by partial oxidation of methane (POM). Compared to the catalysts prepared from other two precursors, the catalyst prepared from Pd(acac)2 demonstrated better activity and stability for the reaction at 700°C. Methane conversion above 90% with H2 yield higher than 80% was achieved during 7h on stream. The catalysts were characterized by N2 physical adsorption, CO chemisorption, Transmission electron microscopy and in situ X-ray diffraction techniques. The Pd/SiO2 catalyst prepared from Pd(acac)2 was found to have a superior performance against sintering during air-calcination at 800°C. The size of Pd particles remained at 4–5nm even after the POM reaction at 700°C for 7h. The results of mass spectroscopy characterization on the thermal decomposition of a SiO2 supported Pd(acac)2 sample under air suggest that formation of highly dispersed (SiOs)2Pd intermediate through the H-transfer from silica surface hydroxyl groups to the acac ligand of Pd(acac)2 followed by a fast reduction of the (SiOs)2Pd at ∼200°C by acacH species plays an important role in the formation of highly dispersed Pd particles on SiO2 with excellent thermal stability.

Sensing behavior of silica-coated Au nanoparticles towards nitrobenzene

In the present work, we report silica-stabilized gold nanoparticles (SiO2/Au NPs) as a wide-range sensitive sensing material towards nitrobenzene (NB). Surface hydroxyl groups of silica selectively form Meisenheimer complex with electron-deficient aromatic ring of NB and facilitate its immobilization and subsequent catalytic reduction by Au cores. Silica-coated Au NPs were synthesized and characterized for their chemical, morphological, structural, and optical properties. SiO2/Au NPs-modified electrodes were characterized with impedometric and cyclic voltammetric electrochemical techniques. SiO2/Au NPs are found to have a higher optical detection window of range, 0.1 M to 1 μM and a lower electrochemical detection window of range, 10−4 to 2.5 × 10−2 mM with a detection limit of 12.3 ppb. A significant enhancement in cathodic peak current, C1, and sensitivity (102 μA/mM) was observed with modified electrode relative to bare and silica-modified electrodes. The IP was found to be linearly co-related to NB concentration (R2 = 0.985). The interference of cationic and anionic species on sensor sensitivity was also studied. Selectivity in the present sensing system may be further improved by modifying silica with specific functional moieties.

Borane-Induced Dehydration of Silica and the Ensuing Water-Catalyzed Grafting of B(C6F5)3 To Give a Supported, Single-Site Lewis Acid, ≡SiOB(C6F5)2

A supported, single-site Lewis acid, ≡SiOB(C(6)F(5))(2), was prepared by water-catalyzed grafting of B(C(6)F(5))(3) onto the surface of amorphous silica, and its subsequent use as a cocatalyst for heterogeneous olefin polymerization was explored. Although B(C(6)F(5))(3) has been reported to be unreactive toward silica in the absence of a Brønsted base, we find that it can be grafted even at room temperature, albeit slowly. The mechanism was investigated by (1)H and (19)F NMR, in both the solution and solid states. In the presence of a trace amount of H(2)O, either added intentionally or formed in situ by borane-induced dehydration of silanol pairs, the adduct (C(6)F(5))(3)B·OH(2) hydrolyzes to afford C(6)F(5)H and (C(6)F(5))(2)BOH. The latter reacts with the surface hydroxyl groups of silica to yield ≡SiOB(C(6)F(5))(2) sites and regenerate H(2)O. When B(C(6)F(5))(3) is present in excess, the resulting grafted boranes appear to be completely dry, due to the eventual formation of [(C(6)F(5))(2)B](2)O. The immobilized, tri-coordinate Lewis acid sites were characterized by solid-state (11)B and (19)F NMR, IR, elemental analysis, and C(5)H(5)N-TPD. Their ability to activate two molecular C(2)H(4) polymerization catalysts, Cp(2)ZrMe(2) and an (α-iminocarboxamidato)nickel(II) complex, was explored.

Fly ash-supported cerium triflate as an active recyclable solid acid catalyst for Friedel–Crafts acylation reaction

An efficient solid Lewis acid, has been synthesized by loading cerium triflate (7 wt%) on the acid activated fly ash with high silica content (81%). The physico-chemical properties of synthesized fly ash-supported cerium triflate catalyst (CFT) were monitored by XRD, FT-IR spectroscopy, FT-IR spectroscopy of the ammonia adsorbed catalyst, SEM-EDAX, TEM, Flame Atomic Absorption Spectrophotometer and TG-DTA study. The increased concentration of silica surface hydroxyl groups on activated fly ash have a major influence on the loading of cerium triflate. The catalytic activity of the catalyst CFT was tested in the acylation of veratrole using acetic anhydride as the acylating agent. The proposed model structure of CFT shows that the triflate species withdraws the electron density from the surface cerium making it electron deficit and generate Lewis acidity on the surface of fly ash as confirmed by NH 3 adsorbed FT-IR spectrum. The activity data indicate that this heterogeneous catalyst is very active, corresponding to high conversion (88%) of veratrole to 3,4-dimethoxyacetophenone. The catalyst could be easily recovered and reused giving similar conversion up to three reaction cycles indicating its stability under experimental conditions. Thus fly ash-supported cerium triflate is a novel and efficient catalyst and is a promising way of bulk utilization of waste fly ash by developing cost effective catalyst system for industrially important Friedel–Crafts acylation reactions.

Study of Carbon Dioxide Adsorption on Mesoporous Aminopropylsilane-Functionalized Silica and Titania Combining Microcalorimetry and in Situ Infrared Spectroscopy

Two calcined mesoporous supports, silica and titania, were functionalized with aminopropylsilane (APS). The samples were characterized using ATR (attenuated total reflectance), nitrogen sorption at 77 K, and thermogravimetric analysis. The functionalized silica and titania are mesoporous and were grafted with 1.4 and 1.6 molecules of APS per nm2, respectively. Infrared measurements propose that the properties of the amine sites are affected by the chemical properties of the support. For example, the NH2 bending vibration δ(NH) was shifted to lower wavenumbers from 1597 to 1575 cm−1 from the silica to the titania grafted sample, respectively. This could be explained by different interactions with the surface hydroxyl groups of silica and titania. The grafted samples were investigated for carbon dioxide adsorption by combining microcalorimetry and in situ FTIR spectroscopy. Their CO2 adsorption properties are presented in comparison to the nongrafted support materials. Microcalorimetric measurements show important enthalpies of adsorption at low CO2 coverage (more than −80 kJ mol−1) for the APS-grafted materials, indicating a strong reactivity between carbon dioxide and the amine sites. In situ infrared spectroscopy was used to study this reactivity. The formation of three products (carbamate, carbamic acid, and bidentate carbonate) is proposed.

Effect of silica type and grafting method on the reactivity of tetraneopentylchromium(IV) towards and on silica

The grafting of tetraneopentylchromium(IV) onto the surface hydroxyls of silica has been investigated by varying both the method of deposition and the nature of the silica support. Reaction of the volatile organometallic with the silica surface in vacuo is compared with reaction in solution. A faster reaction but a lower ultimate Cr loading was obtained using the solution deposition technique. The reactions of the organometallic compound with Aerosil 200, a fumed silica often used to model catalyst supports, and Sylopol 952, a silica gel used as a carrier for chromium-based ethylene polymerization catalysts, are compared. After thermal pretreatment at 200 °C, grafting on both supports yields bis(neopentyl)chromium(IV) fragments regardless of the grafting method used, suggesting that a paired arrangement of the surface hydroxyl groups exists on both types of silicas. The higher Cr loading achieved on the silica gel is attributed to its higher surface area. Thermally-induced neopentane elimination from the grafted bis(neopentyl)chromium(IV) fragments occurs at the same rate and with the same stoichiometry for both Aerosil- and Sylopol-supported materials. Consequently, interactions of the grafted organometallic fragments with nearby siloxanes appear to be unimportant in the early stages of the transformation of bis(alkyl)chromium(IV) to the alkylidene.

Interactions of biopolymers with silica surfaces: Force measurements and electronic structure calculation studies

Pull-off forces were measured between a silica colloid attached to an atomic force microscope (AFM) cantilever and three homopolymer surfaces representing constituents of extracellular polymeric substances (EPS). The pull-off forces were −0.84 (±0.16), −0.68 (±0.15), and −2.37 (±0.31) nN as measured in water for dextran, phosphorylated dextran, and poly- l-lysine, respectively. Molecular orbital and density functional theory methods (DFT) were applied to analyze the measured pull-off forces using dimer clusters representing interactions between the three polymers and silica surfaces. Binding energies for each dimer were calculated with basis set superposition error (BSSE) and interpolated using corrections for silica surface hydroxyl density and silica charge density. The binding energies were compared with the normalized pull-off forces with the effective silica surface area contacting the polymer surfaces. The predicted binding energies at a −0.064 C/m 2 silica surface charge density corresponding to circum-neutral pH were −0.055, −0.029, and −0.338 × 10 −18 J/nm 2 for the dimers corresponding to the silica surface with dextran, phosphorylated dextran, and poly- l-lysine, respectively. Polarizable continuum model (PCM) calculations with different solvents, silanol vibrational frequency calculations, and orbital interaction analysis based on natural bonding orbital (NBO) showed that phosphate groups formed stronger H-bonds with neutral silanols than hydroxyl and amino functional groups of polymers, implying that phosphate containing polymers would play important roles in EPS binding to silica surfaces.

Interaction between abrasive particles and films during chemical-mechanical polishing of copper and tantalum

Chemical-Mechanical Polishing (CMP) of copper is a critical step for the formation of interconnect on wafer during the manufacturing of advanced microelectronic devices. One of the key performance metrics of a copper CMP slurry is the removal rate selectivity between copper and its corresponding adhesion/barrier layer such as Ta or TaN. In order to create a slurry that has the desirable and/or tunable removal rate selectivity, it is critical to understand the influential factors that control the removal rates. In this study, such influential factors are elucidated based on the relative importance of interaction between abrasive particle and the film to be polished. More specifically, particles with varying bulk and surface properties are used to obtain the removal rate selectivity for copper and Ta. It is found that the Ta material removal rate has a direct correlation with the effective hydroxyl content on the surface of abrasive particles. The material removal rate and surface quality of copper, on other hand, is heavily influenced by the hardness of the abrasive particles and the presence of oxidizer/complexing agent. The reliance on the interaction with abrasive surface hydroxyl groups for Ta removal is further confirmed with a set of experiments in which the silica surface hydroxyl content are significantly reduced via alkylation of the hydroxyl groups. As expected, the surface treatment that remove some of the surface hydroxyl groups also reduce the Ta material removal rate. The difference in removal mechanism for copper and tantalum is also explained with the distribution of the abraded materials between spent slurry and pad.