High Surface Area Silica Research Articles

Determining surface composition of mixed oxides with pH

In this study we have developed a 2-surface model with the EPHL (“Equilibrium pH at high oxide loading”) method for mixed and composite oxides. Oxide charging parameters, namely the protonation and deprotonation constants and the hydroxyl surface densities, can be established from measurements of the pure oxides and used in the 2-surface model to predict the point of zero charges (PZC) of mixed and composite oxides. The accuracy of these predictions has been demonstrated with diverse control samples of known surface composition (physical mixtures of silica and alumina of high and low surface area). The EPHL method has been extended to composite materials (bound catalysts) and can explain correlations of reactivity with catalyst surface composition. The “apparent surface coverage” (ASC) of a particular mixed or composite oxide sample may then be determined by comparing the PZC of the material to either the 2-surface model results, using parameters of the individual oxides, or a calibration curve of EPHL measurements of physical mixtures of the individual ingredients.

Disrupting admicelle formation and preventing surfactant adsorption on metal oxide surfaces using sacrificial polyelectrolytes.

The adsorption of anionic, cationic, and nonionic surfactants was measured on high-surface area silica and alumina nanoparticles when in the presence of the proposed polyelectrolyte sacrificial agents. Surfactant adsorption was characterized using two types of adsorption isotherms: one with constant polymer concentration and varying surfactant concentration, and another with a varying polymer concentration and constant surfactant concentration. Polystyrenesulfonate and Polydiallyl dimethylammonium chloride were tested as potential sacrificial agents on alumina and silica, respectively. Each surfactant/polymer system was allowed to reach equilibrium and supernatant surfactant concentrations were measured. This information was then plotted in order to determine what, if any, effect the proposed sacrificial agent had on the equilibrium adsorption. Results indicate that both of these polymers can have a large effect on total surfactant adsorption at a variety of surfactant concentrations.

Efficient fatty acid esterification using silica supported Brønsted acidic ionic liquid catalyst: Experimental study and DFT modeling

A task-specific Bronsted acidic ionic liquid (BAIL), 1-benzyl-3-methylimidazolium hydrogensulfate ([BnMIm]HSO4), was prepared and confined onto the high surface area silica gel. The catalyst has shown extraordinary activity in comparison of homogeneous ionic liquid (IL) system for Fischer esterification of fatty acid with ethanol. The ester derivatives were synthesized in high yields and catalyst was easily separated from the reaction mixture by simple filtration. The structure of the catalyst is theoretically modelized to investigate the unusual catalytic activity. Hence, a hydroxylated cage-like nano-cluster, H6Si8O12(OH)2, was selected to mimic the surface of silica gel. The interactions between the IL and this cluster have been studied using density functional theory (DFT) calculations. DFT modeling indicates that the acidity of the supported [BnMIm]HSO4 is higher than the homogeneous BAIL system.

Creating polymer templates and their use in the in-situ synthesis of biodegradable composite networks

A biodegradable polyethyleneoxide–polycaprolactone–polyethyleneoxide (PEO–PCL–PEO or PECE) triblock polymer was synthesized as a structure-directing agent for high-surface area silica formation. Systematic end-group functionalization of the triblock polymer, prior to its use as a structure-directing agent for high-surface area silica growth, imparts an additional reactive function that is exploited to grow a continuous organic phase within and around the silica particles. Hence, the biodegradable triblock also acts as a macromer whose dual function leads to in-situ generation of intimately mixed biopolymer/nanoporous–silica composite networks. This composite can be both biodegradable and biocompatible, and the modulus is comparable to other non-biodegradable materials.

Synthesis of high surface area silica gel templated carbon for hydrogen storage application

In this study silica gel templated carbons were synthesized by pyrolysis using furfuryl alcohol as carbon precursor. Effects of temperatures (650, 750 and 850°C) and dwelling times (1, 2, 3 and 4h) on physical properties of templated carbons were investigated. The synthesized templated carbons were studied for hydrogen uptake at atmospheric pressure and subzero temperatures (−50 and −100°C). The surface area and pore structure of the templated carbons varied significantly with synthesis parameters. Templated carbon with highest surface area (1975m2/g) and pore volume (3.07ml/g) was obtained at pyrolysis temperature of 650°C and 3h dwelling time. For all synthesized carbons, 90–95% of the pores were in the range of 2–20nm and 38–67% of the pores were in the range of 2–6nm. Highest hydrogen uptake of 0.16wt.% was observed at adsorption temperature of −100°C for templated carbon synthesized at 650°C and dwelling time of 3h.

Highly Porous Silica Glasses and Aerogels Made Easy: The Hypersaline Route

Through the addition of inorganic salts as porogens during an alcohol-based sol-gel process high surface area silica glasses and aerogels with varying pore sizes are synthesized. In the presence of ZnCl2 and CaCl2 respectively, tetraethyl orthosilicate (TEOS) dissolved in ethanol hydrolyzes also in the absence of any acid or base catalysts, eventually forming high surface area monolithic silica networks. The porosity can be controlled by the gel ageing step as well as the salt nature. Two different gel drying conditions were investigated, i.e., room temperature (RT) air drying at ambient pressure and supercritical (sc) CO2 drying. While a reference solution which only contains ethanol and TEOS does not gel at all, the addition of ZnCl2 leads to mesoporous aerogels with high surface areas of 1400 m2 g−1 after sc CO2 drying. Interestingly, the utilization of CaCl2 however, produces aerogel-like materials of high porosity for both drying methods under preservation of the nano-and macroscopic features. Thereby, it is shown that inorganic salts can influence the structure formation during the sol-gel process where the type of salt controls the framework connectivity/stability and tunes the porosity towards smaller pore sizes than usually obtained. At the same time, the herein presented hypersaline aerogel-synthesis is very simple, fast and in the case of CaCl2 drying can even be performed in air

Aerosol synthesis of macroporous silica adsorbents with high performance in paclitaxel purification from plant cell cultures

Macroporous spherical silica particles having high performance for the purification of paclitaxel were synthesized by spray pyrolysis using polystyrene nanoparticles as a template. In terms of increasing the purity and yield of paclitaxel, the synthesized macroporous silica showed the better performance than mesoporous silica of high surface area as well as commercial sylopute. The generation of macroporous with the sacrifice of losing surface area was proved to be helpful for enhancing the performance of adsorbents for paclitaxel purification. As a result, the high purity (67.3%) and yield (80.0%) of paclitaxel was achieved by using macroporous silica prepared by spray pyrolysis.

Quantitative Surface Coverage Calculations via Solid-State NMR for Thin Film Depositions: A Case Study for Silica and a Gallium Amidinate

For the interrogation of precursor nucleation for vapor deposition processes like atomic layer deposition (ALD) and chemical vapor deposition (CVD), a modified method for quantitative analysis of surface coverage was undertaken via NMR. The initial chemisorption of a new gallium(III) alkyl amidinate compound was investigated on high-surface area silica. N,N′-Diisopropylacetamidinatediethylgallium(III) (2) was found to have excellent volatility with no decomposition during a ramped thermogravimetric analysis experiment. Stepped-isotherm experiments showed a 1 Torr vapor pressure at 64 °C. Compound 2 was exposed to a pretreated high-surface area silica substrate at 100, 200°, and 300 °C and was found to exhibit stable, persistent chemisorbed surface species at all three temperatures. Substrates were analyzed by 29Si and 13C solid-state nuclear magnetic resonance spectroscopy (SS-NMR) and 1H high-resolution NMR. At 100 and 200 °C the reactivity of compound 2 to geminal and lone hydroxyl surface sites varied slightly eliminating either one or both ethyl groups to produce an alkylated (or nonalkylated) gallium acetamidinate on the silica surface and producing fractional coverages of 0.087–0.088. At 300 °C there was a larger degree of reactivity producing a minor amount of the same surface species as at 100 and 200 °C but also producing additional chemisorbed products likely arising from the decomposition of the ligand framework but ultimately giving a fractional coverage of 0.232 on hydroxyl-terminated silica.

Influence of preparation method on supported Cu–Ni alloys and their catalytic properties in high pressure CO hydrogenation

Silica supported Cu–Ni (20 wt% Cu + Ni on silica, molar ratio of Cu/Ni = 2) alloys are prepared via impregnation, coprecipitation, and deposition–coprecipitation methods. The approach to co-precipitate the SiO2 from Na2SiO3 together with metal precursors is found to be an efficient way to prepare high surface area silica supported catalysts (BET surface area up to 322 m2 g−1, and metal area calculated from X-ray diffraction particle size up to 29 m2 g−1). The formation of bimetallic Cu–Ni alloy nanoparticles has been studied during reduction using in situ X-ray diffraction. Compared to impregnation, the coprecipitation and deposition–coprecipitation methods are more efficient for preparation of small and homogeneous Cu–Ni alloy nanoparticles. In order to examine the stability of Cu–Ni alloys in high pressure synthesis gas conversion, they have been tested for high pressure CO hydrogenation (50 bar CO and 50 bar H2). These alloy catalysts are highly selective (more than 99 mol%) and active for methanol synthesis; however, loss of Ni caused by nickel carbonyl formation is found to be a serious issue. The Ni carbonyl formation should be considered, if Ni-containing catalysts (even in alloyed form) are used under conditions with high partial pressure of CO.

Fabrication of Liquid and Vapor Protective Cotton Fabrics

Through microwave-assisted techniques, cotton textiles treated with heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane in the presence of high surface area silica nanoparticles create a material capable of repelling bulk liquid challenges while simultaneously adsorbing organic vapors from bulk liquid droplets. Characterizing the contradictory behavior of adsorption of vapors and repellency of liquids is the primary focus of this article. These procedures reveal a quick and simple method for a one-step deposition of a vapor-sorptive, liquid-repellent, Cassie-Baxter surface onto textiles. Packed column breakthrough and single swatch permeation experiments showed that treated materials possess a high affinity for 3-hepten-2-one vapor, while goniometry revealed contact angles in excess of 120° for surface-deposited, 5 μL droplets of several test liquids. Scanning electron micrograph images confirm a lotus-like, nanorough surface, while ATR-FTIR spectra confirm surface fluorocarbon moieties. The performance of so-treated materials lends itself to the application of chemical protective apparel, while the simplicity of the treatment bodes well for potential commercialization.

Systematic synthesis of high surface area silica nanoparticles in the sol–gel condition by using the central composite design (CCD) method

The sol–gel method is employed for producing high surface area silica nanoparticles from a cheap precursor, that is water glass (sodium silicate). To design the experiments systematically, the response surface method (RSM) combined with the central composite design (CCD) approach is used. Four major factors including the concentration of sodium silicate solution, solution pH, reaction temperature and reaction time are identified as the major controlling parameters and the particle surface area is considered as the response or the output parameter. A total of 31 experiments are designated by the CCD. The experiments are conducted at a centre point chosen based on experience and at its vicinity to investigate how the response changes as the factors change. Nanoparticles with a surface area as high as 630 m2/g and a particle size as low as 8 nm are produced at the optimum parameters of sodium silicate concentration of 1.5 × 10−4 g/L, pH of 4, a reaction temperature of 25°C and a reaction time of 1.5 h. A regression analysis is performed on the experimental data and a correlation is obtained that may be used to predict the particle surface area and investigate the effect of varying the factors on the response. Within the range of parameters studied here, it is found that the solution pH and then the process temperature have a profound effect on the surface area and concentration and reaction time have a moderate effect. An increase of the solution pH from 4 to higher values results in a rapid drop in the particle surface area.

Effect of various structure directing agents on the physicochemical properties of the silica aerogels prepared at an ambient pressure

We studied the effects of various surfactants on the textural properties (BET surface area, pore size, and pore volume) of the silica aerogels prepared at an ambient pressure. A simple surface modification of silica gel prepared at an ambient pressure through hydrolysis and polycondensation of TEOS as a silica precursor was conducted using various structure directing agents. The treatment was found to induce a significant difference in the porosity of the silica aerogel. Highly porous silica aerogels with bimodal porous structures were prepared by modifying the surface of the silica wet-gel (alcogel) with trimethylchlorosilane (TMCS) in order to preserve its porosity. The samples were analyzed by small-angle X-ray scattering and nitrogen adsorption. In this work, a possible new type of highly porous hydrophobic silica aerogel with a bimodal porous structure is presented. A hydrophilic extremely porous (high surface area and large pore volume) silica aerogel was obtained by heating the as-synthesized hydrophobic silica aerogel at 400°C for 1h. There was a significant effect of structure directing agent on the textural properties, such as specific surface area, pore size distribution and cumulative pore volume of the silica aerogels.

Titration of Free Hydroxyl and Strained Siloxane Sites on Silicon Dioxide with Fluorescent Probes

A technique enabling the detection and quantification of low density sites on planar SiO2 surfaces is demonstrated. Fluorescent probes are used to titrate free hydroxyl and strained siloxane sites on the surface of amorphous SiO2 substrates in vacuum. The titration of free hydroxyl sites was performed to validate the method and to provide a reference for the measurement of the strained siloxane site density. Perylene derivatives with different functional groups are chemisorbed onto the surface sites, enabling in situ photoluminescence (PL) measurements of the bound fluorophores. An amine functional group is used to selectively titrate strained siloxane sites, while an alcohol group is used for the titration of free hydroxyl sites. Emission intensity was found to be nonlinear with coverage for bound fluorophore densities greater than 0.1 nm(-2), necessitating the removal of molecules from the surface into a solution to obtain accurate density measurements. For lower densities, the coverage of bound fluorophores can be estimated directly from in situ PL measurements. The measured areal densities of bound fluorophores after titrating free hydroxyl sites are in good agreement with literature values for the densities of such sites on high surface area silica. PL measurements of SiO2 surfaces titrated with an amine derivative of perylene indicate that strained siloxane sites exist for vacuum pretreatment temperatures of 300 °C and increase with increasing pretreatment temperature. Densities of strained siloxane sites on the silica surface are estimated at 0.004-0.02 nm(-2) for pretreatment temperatures of 300-700 °C, demonstrating the sensitivity of this technique.

Synchronous ATR infrared and NIR-spectroscopy investigation of sepiolite upon drying

A new environmental cell allowing for the independent synchronous collection of the near- and mid-infrared spectra (12,000–600cm−1) in the diffuse reflection and attenuated total reflection (ATR) modes, respectively, is reported. The cell is employed to study in real time the dehydration of the phyllosilicate mineral sepiolite, Mg8Si12O30(OH)4(OH2)4·wH2O, in both its natural form and after in situ deuteration at ambient. The spectra are obtained under dynamic purging with dry N2 and compared to those of the same material conditioned over saturated salt solutions. Sepiolite is an important industrial mineral with a modulated structure of alternating tunnels and ribbons. Its mild drying is associated with pronounced vibrational spectral changes due to the removal of surface and zeolitic H2O and the concomitant structural relaxation of the ribbons. Detailed assignments are provided for the fundamental, combination and overtone spectrum of H2O confined in the tunnels of sepiolite, SiOH groups on the external surface of the particles, and Mg3OH groups in the 2:1 ribbons. The spectra are discussed in comparison to those of palygorskite (modulated phyllosilicate with narrower ribbons and tunnels), talc (trioctahedral magnesian phyllosilicate without modulation) and high-surface area silica. It is demonstrated that sepiolite exhibits three discrete states of zeolitic hydration at ambient temperature: Besides the previously known hydrated (w=7–8) and dry (w=0–1) states which dominate the spectra above 30% and below 3% relative humidity, respectively, a hitherto unknown intermediate (w=4–5) is found in the 3–10% range. The new state is most conveniently identified in the near-infrared by a ν02 Mg3O-H stretching mode at 7205cm−1 (ν01=3686cm−1, X=83.5cm−1) and a characteristic H2O combination band at 5271cm−1 (D2O: 3908cm−1).

Rational Design of Nickel Phosphide Hydrodesulfurization Catalysts: Controlling Particle Size and Preventing Sintering

The size-dependent catalytic activity of Ni2P for hydrodesulfurization (HDS) remains unstudied because the traditional temperature programmed reduction (TPR) method used in catalyst preparation results in highly polydisperse Ni2P particles. The ability to control the Ni2P particle size in the range 5–20 nm by varying the quantity of oleylamine in solution-phase arrested precipitation reactions is reported. Particles were introduced to a high surface area silica support (Cab-O-Sil, M-7D grade, 200 m2/g) via incipient wetness, and HDS activity was probed against dibenzothiophene (DBT). All samples were less active than TPR prepared materials, and the smallest particles were the least active, contrary to expectation. This is attributed in part to particle sintering under HDS conditions. Sintering occurs independently of wt% loading of catalyst, time, incipient wetness procedure, and ionic additives, at all temperatures greater than 200 °C. Sintering is minimized by encapsulation of Ni2P nanoparticles in a me...

Catalytic Combustion of Lean Methane at Low Temperature Over Palladium on a CoO x –SiO2 Composite Support

A composite support of CoO x –SiO2 was obtained through modifying silica with cobalt of different loadings; with CoO x –SiO2 as the support, a series of Pd/CoO x –SiO2 catalysts were prepared for the combustion of lean methane at low temperature. The effects of CoO loading and palladium precursor on the catalytic performance of Pd/CoO x –SiO2 were investigated. The Pd/CoO x –SiO2 catalyst containing 20 wt% CoO and 1 wt% Pd with acetate as palladium precursor performs excellently in lean methane combustion; over it a complete conversion of methane can be achieved at 420 °C and the activity does not show any decline in a long-term test of 120 h. Various characterizations suggested that cobalt species are highly dispersed on the silica of high surface area and there exists a synergetic interaction between palladium and cobalt species, which improves the redox properties of Pd/CoO x –SiO2 and contributes to the excellent catalytic performance in lean methane combustion. Silica modified with cobalt (CoO x –SiO2) is an idea support to prepare Pd/CoO x –SiO2 catalyst of high activity in lean methane combustion; over it a complete conversion of methane can be achieved at 420 °C and the conversion does not show any decline in a long-term test of 120 h.

Copper Iminopyrrolidinates: A Study of Thermal and Surface Chemistry

Several copper(I) iminopyrrolidinates have been evaluated by thermogravimetric analysis (TGA) and solution based (1)H NMR studies to determine their thermal stability and decomposition mechanisms. Iminopyrrolidinates were used as a ligand for copper(I) to block previously identified decomposition routes of carbodiimide deinsertion and β-hydrogen abstraction. The compounds copper(I) isopropyl-iminopyrrolidinate (1) and copper(I) tert-butyl-iminopyrrolidinate (2) were synthesized for this study, and compared to the previously reported copper(I) tert-butyl-imino-2,2-dimethylpyrrolidinate (3) and the copper(I) guanidinate [Me(2)NC((i)PrN)(2)Cu](2) (4). Compounds 1 and 2 were found to be volatile yet susceptible to decomposition during TGA. At 165 °C in C(6)D(6), they had half-lives of 181.7 h and 23.7 h, respectively. The main thermolysis product of 1 and 2 was their respective protonated iminopyrrolidine ligand. β-Hydrogen abstraction was proposed for the mechanism of thermal decomposition. Since compound 3 showed no thermolysis at 165 °C, it was further studied by chemisorption on high surface area silica. It was found to eliminate an isobutene upon chemisoption at 275 °C. Annealing the sample at 350 °C showed further evidence of the decomposition of the surface species, likely eliminating ethene, and producing a surface bound methylene diamine.

Selective capture and fluorescent quantification of glycoproteins using aminophenylboronic acid functionalized mesoporous silica coated CdTe quantum dots.

In this work, aminophenylboronic acid functionalized mesoporous silica coated CdTe quantum dots (APBA-coated QDs) were synthesized and applied to the selective capture and fluorescent quantification of glycoproteins. The as-prepared APBA-coated QDs, relying on the interaction between APBA and cis-diol containing structures, demonstrated excellent selectivity for the glycoproteins. Their high surface area and APBA-enriched silica matrixes give rise to faster binding kinetics and higher binding capacity for the glycoproteins, which further make them attractive for biomedical/chemical sensing applications. Based on the fluorescent properties of the particles, the as-prepared APBA-coated QDs were successfully applied to the fluorescence quantification of glycoproteins. The present study provides a facile strategy to fabricate functionalized fluorescent materials and is of great significance for isolation and detection of glycoproteins in proteomics.

Removal of BPA model compounds and related substances by means of column chromatography using Octolig®

Octolig®, a polyethylenediimine ligand covalently attached to high-surface area silica gel, was used to study the removal of phenolic compounds from aqueous samples by column chromatography. Model phenolic compounds of Bisphenol A (BPA), 4-isopropylphenol and 4-(t-butyl) phenol, were selected for this study due to their similarities in pKa and log P values. The percent removal of these compounds by Octolig® was 26 ± 2 and 22 ± 2, respectively. Furthermore, the three isomers of nitrophenol were investigated as well as additional phenolic compounds, such as amoxicillin and five phenolic dyes. These compounds have a pKa range of 2–10.2. The compounds that have pKa values less than 8.3 were able to be completely removed by Octolig®, yet compounds with pKa values of 8.3 and higher resulted in approximately 20–26% removal.

Adsorptive desulfurization of jet and diesel fuels using Ag/TiOx–Al2O3 and Ag/TiOx–SiO2 adsorbents

The objective of this work is to examine the performance of Ag/TiOx–Al2O3 and Ag/TiOx–SiO2 for adsorptive desulfurization of JP5, JP8, Off-Road Diesel (ORD), and Ultra Low Sulfur Diesel (ULSD) at ambient conditions. These adsorbents were observed to be effective for desulfurizing liquid hydrocarbon feeds that ranged from 7.5 to 1172ppmwS and comprised of diverse sulfur compositions. In fixed bed continuous adsorption tests, 4wt.%Ag/TiOx–Al2O3 demonstrated saturation capacities of 10.11, 6.11, and 7.4mgS/g adsorbent for JP5 (1172ppmwS), JP8 (630ppmwS), and ORD (452ppmwS), respectively. In equilibrium saturation experiments, the adsorbent was able to desulfurize ULSD down to less than 75ppbwS and this was achievable even when the initial concentration of non-sulfur aromatics was greater than 25,000 times higher. Dispersing TiOx onto high surface area alumina and silica substrates increased the sulfur adsorption capacities of both. This enhancement in adsorption capacity increased still further with silver addition. Higher Ag loading (up to ∼12wt.%Ag) on TiOx–Al2O3 proved beneficial to sulfur adsorption capacity when compared to TiO2 (up to ∼4wt.%Ag), indicating that the mixed oxide supports were able to host more active silver oxides. The adsorption selectivity toward different sulfur compounds present in the fuels varied during the fixed bed adsorption tests. The adsorbent had the greatest affinity for BT’s and the least for TMBT’s and DMDBT’s (the selectivity order from the strongest to weakest adsorption was: BT>MBT>DMBT>DBT≈MDBT>TMBT≈DMDBT). Ag/TiOx–Al2O3 was thermally regenerable in air for multiple cycles at temperatures ranging from 110 to 450°C. The effects of titanium loading, silver loading, and titanium precursor on sulfur adsorption capacity are also presented.