Porous Silica Gel Research Articles

ChemInform Abstract: Multiphase Reaction Media Involving Water and Carbon Dioxide for Selective Hydrogenation

AbstractReview: 43 refs.

CH4 recovery and CO2 sequestration using flue gas in natural gas hydrates as revealed by a micro-differential scanning calorimeter

Abstract The CH 4 –flue gas replacement in naturally occurring gas hydrates has attracted significant attention due to its potential as a method of exploitation of clean energy and sequestration of CO 2 . In the replacement process, the thermodynamic and structural properties of the mixed gas hydrates are critical factors to predict the heat flow in the hydrate-bearing sediments and the heat required for hydrate dissociation, and to evaluate the CO 2 storage capacity of hydrate reservoirs. In this study, the 13 C NMR and gas composition analyses confirmed that the preferential enclathration of N 2 molecules in small 5 12 cages of structure I hydrates improved the extent of the CH 4 recovery. A high pressure micro-differential scanning calorimeter (HP μ -DSC) provided reliable hydrate stability conditions and heat of dissociation values in the porous silica gels after the replacement, which confirmed that CH 4 in the hydrates was successfully replaced with flue gas. A heat flow change associated with the dissociation and formation of hydrates was not noticeable during the CH 4 –flue gas replacement. Therefore, this study reveals that CH 4 –flue gas swapping occurs without structural transitions and significant hydrate dissociations.

Desulfurization Performance of Ether-Functionalized Imidazolium-Based Ionic Liquids Supported on Porous Silica Gel

Ether-functionalized imidazolium-based ionic liquids (EFIILs) [C3O1Mim][H3CSO3], [C5O2Mim][H3CSO3], and [C7O3Mim][H3CSO3] were supported onto porous silica gel particles via a facile impregnation method. The synthesized EFIILs/SiO2 were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and Brunauer–Emmett–Teller (BET). Results show that the EFIILs/SiO2 have good thermal stability, high porosity, and large specific surface area. The ability of both of the EFIILs and EFIILs/SiO2 to reversibly absorb gaseous sulfur dioxide (SO2) was experimentally investigated, and high capacity and rate for SO2 sorption were confirmed. The capacities reached 2.621 mol of SO2/mol of [C3O1Mim][H3CSO3], 3.106 mol of SO2/mol of [C5O2Mim][H3CSO3], and 3.453 mol of SO2/mol of [C7O3Mim][H3CSO3], which indicate that the SO2 sorption capacity increases with the increase of the ether chain length in the cation at 25 °C. The EFIILs/SiO2 have better ad...

Synthesis of nanosized platinum based catalyst using sol-gel process

The nano-sized platinum based catalysts using high surface area silica support have been prepared by sol-gel method. Tetramethoxysilane (TMOS) diluted in methanol was hydrolyzed to form a porous silica gel. Platinum (2%) was loaded at sol state using platinum chloride solution. After gelation, the solvent from the gel pores was extracted at ambient temperature which resulted in porous silica matrix incorporated with nanosized platinum. X-ray diffraction studies indicated the presence of elemental platinum in the silica-platinum composites. Transmission electron microscopy of the platinum -silica composites revealed that nanosized platinum particles of about 5-10 nm are homogeneously dispersed in silica matrix. Chemisorptions studies showed high dispersion (more than 50%) of platinum on silica support with specific surface area of 400 m2/g which puts them as promising candidates as catalyst in heterogeneous reactions.

Amine impregnated porous silica gel sorbents synthesized from water–glass precursors for CO2 capturing

In this work, porous silica gel-solid beads have been made from economically affordable water–glass precursors via sol–gel nano casting technique. A stable nanometric silica sol was prepared first from water glass and studied for surface potential and sol to gel transition. A free-flow, injectable gel was obtained upon aging the sol which was then assembled into spherical silica beads in a chemical bath. A surface area of 304.7m2g−1 was obtained for water glass derived silica gel beads. These gel beads were impregnated with 3-aminopropyltrimethoxysilane (APTMS) and polyethylenimine (PEI) active functional groups at different percentages for turning the gel beads as sorbents for CO2 gas adsorption. The effect of amine loading on the thermal stability, morphology as well as porosity was studied and was correlated with CO2 adsorption values. Depending upon the amount of amine loaded in the gel support CO2 uptake was found varied. These amine modified silica gel porous adsorbents showed CO2 adsorption capacity at temperatures as low as 100°C; samples modified with 15wt% PEI had CO2 adsorption capacity of 1.16mmolg−1 at 50°C.

Preparation and characterization of gold nanoparticle-loaded silica-gel films for localized surface plasmon resonance sensing

Gold nanoparticles (AuNPs) were investigated as a sensing material because their absorption wavelength due to localized surface plasmon resonance (LSPR) shifts depending on the surrounding environment. For accurate sensing, the immobilization of dispersed AuNPs on a substrate is desirable. However, attaching AuNPs to smooth surfaces is difficult because the adhesion forces between AuNPs and most substrates are very weak. In this study, thick and porous silica-gel films were prepared via a dip-coating method using polyvinylpyrrolidone-containing silica sols. The AuNPs were precipitated in the silica-gel films by immersion in HAuCl4 solution followed by heating. The effects of the silica-gel film calcination temperature and the silica-sol aging conditions on sample structures, LSPR peak wavelengths, and LSPR sensing abilities against the change in refractive index were investigated. When the calcination temperature decreased, the intensity of LSPR absorption increased significantly because the silica gel calcined at low temperatures had a sparse structure and a significant quantity of AuNPs in the silica gel pores. The aging conditions of the silica sol also affected the structures of the AuNP-loaded silica gel films and their LSPR sensing abilities. The sample prepared from a silica sol aged at 25 °C contained much larger AuNPs and exhibited a higher sensing ability than the sample prepared from a silica sol aged at 40 °C. The high sensing ability of the sample prepared from the silica sol aged at 25 °C was considered to result from both the large size of the AuNPs and the low silica gel density around them.

Kinetics of methane hydrate replacement with carbon dioxide and nitrogen gas mixture using in situ NMR spectroscopy.

In this study, the kinetics of methane replacement with carbon dioxide and nitrogen gas in methane gas hydrate prepared in porous silica gel matrices has been studied by in situ (1)H and (13)C NMR spectroscopy. The replacement process was monitored by in situ (1)H NMR spectra, where about 42 mol % of the methane in the hydrate cages was replaced in 65 h. Large amounts of free water were not observed during the replacement process, indicating a spontaneous replacement reaction upon exposing methane hydrate to carbon dioxide and nitrogen gas mixture. From in situ (13)C NMR spectra, we confirmed that the replacement ratio was slightly higher in small cages, but due to the composition of structure I hydrate, the amount of methane evolved from the large cages was larger than that of the small cages. Compositional analysis of vapor and hydrate phases was also carried out after the replacement reaction ceased. Notably, the composition changes in hydrate phases after the replacement reaction would be affected by the difference in the chemical potential between the vapor phase and hydrate surface rather than a pore size effect. These results suggest that the replacement technique provides methane recovery as well as stabilization of the resulting carbon dioxide hydrate phase without melting.

Thermal characterization of static and dynamical properties of the confined molecular systems interacting through dispersion force

We investigated the thermal properties of liquid methylcyclohexane and racemic sec-butylcyclohexane, as representatives of a molecular system with only dispersion-force intermolecular interactions, confined in the pores (thickness/diameter d = 12, 6, 1.1 nm) of silica gels by adiabatic calorimetry. The results imply a heterogeneous picture for molecular aggregate under confinement consisting of an interfacial region and an inner pore one. In the vicinity of a glass-transition temperature Tg,bulk of bulk liquid, two distinguishable relaxation phenomena were observed for the confined systems and their origins were attributed to the devitrification, namely glass transition, processes of (1) a layer of interfacial molecules adjacent to the pore walls and (2) the molecules located in the middle of the pore. A third glass-transition phenomenon was observed at lower temperatures and ascribed to a secondary relaxation process. The glass transition of the interfacial-layer molecules was found to proceed at temperatures rather above Tg,bulk, whereas that of the molecules located in the inner pore region occurred at temperatures below Tg,bulk. We discuss the reason why the molecules located in different places in the pores reveal the respectively different dynamical properties.

Photocatalytic Degradation of Toluene on SiO2/TiO2 Photocatalyst in a Fluidized Bed Reactor

Silica supported titania photocatalyst, SiO2/TiO2, is prepared by the sol–gel doping method using titanium tetra isopropoxide as the precursor and nitride acid as the acid catalyst. The structure and physical properties of SiO2/TiO2 particles are characterized by SEM/EDS, XRD, and BET specific surface area measurement. When a small amount of TiO2 coated onto the silica gel surfaces, the BET specific surface area of the SiO2/TiO2 particles increases. However, when the ratio of SiO2/TiO2 precursor decreases, TiO2 fills the pores of silica gel and the BET specific surface area of SiO2/TiO2 particle decreases. The as-prepared SiO2/TiO2 particles are used as the fluidizing media in a fluidized bed photoreactor and toluene is degraded therein. The results show that the contaminated stream containing toluene with the concentration as high as 1000ppm can be continuously degraded in the fluidized bed photoreactor. The continuous toluene removal efficiency is around 30%-40% at steady state.

Diffusion Mechanism and Quantitative Measurement of Moisture Transport in Porous Silica Gel Filter Using Dynamic Neutron Radiography Technique

Neutron radiography (NR) can investigate the diffusion mechanisms of moisture transport within the silica gel material. Dynamic neutron radiography (DNR) possesses the possibility to investigate the moisture profile inside the silica gel filter. The technique explained the relationship between moisture movement and time. This study investigated the diffusion mechanisms (absorption and infiltration) of moisture in the silica gel filter by state-of-the-art dynamic neutron radiography (DNR) technique and advanced software. This study also measured the ability of the DNR technique to find out the diffusion coefficients of a silica gel filter material as a function of temperature and time.

Interfacial interactions and complex segmental dynamics in systems based on silica-polydimethylsiloxane core–shell nanoparticles: Dielectric and thermal study

Effects of two types of silica particles, with different characteristics of porosity, on segmental mobility of poly(dimethylsiloxane), PDMS, in core–shell nanocomposites with two contents of polymer, 40 and 80 wt%, were studied by employing thermal (DSC) and dielectric (DRS, TSDC) techniques, under different annealing procedures. Filler morphology and polymer–particle interactions were found to affect PDMS crystallinity and segmental mobility. The dielectric techniques allowed the detection of four contributions to the segmental dynamics associated with the glass transition arising, in the order of decreasing mobility, from confined polymer chains in the pores of silica gel (αp relaxation), from the bulk (unaffected) amorphous polymer fraction (α relaxation), from polymer chains restricted between condensed crystal regions (αc) and from semibound polymer in an interfacial layer close to the nanoparticle surface (α′). For the core–shell nanocomposites with high PDMS content (80 wt%) the interfacial α′ relaxation process is found to have similar characteristics with that in conventional PDMS/silica nanocomposites, for lower PDMS content (40 wt%), however, the interfacial relaxation process exhibits distinct differences. The experimental findings are discussed in terms of models proposed for the description of conformations of polymers adsorbed onto a solid surface.

Оценка влагоемкости силикагеля для обеспечения безопасной эксплуатации пневматических систем

An analysis of main methods for silica gel water capacity determining has been presented. A new method of silica gel water
 capacity’s experimental evaluation in pneumatic systems’ operating conditions has been developed. Bases of fundamental
 method for constant humidity assurance of compressed air used for silica gel water capacity evaluations have been
 revealed. A description of a test bench for this fundamental method practical realization has been presented. A technique
 for experimentation with proposed method’s different modifications has been described. Measurement error estimation
 has been performed. Experimental results of finely porous silica gel water capacity evaluation in static conditions and in
 absorption process’s different parameters while using this method have been presented. These results’ correlation between
 similar data obtained by other methods has been carried out. Analyses results’ statistical processing has been carried out
 and it has been shown that a packed density of finely porous silica gel can be considered only as the indirect and qualitative
 characteristic of its water capacity which isn´t allowing to provide the necessary accuracy of quantitative estimates for
 pneumatic systems’ operation safety assurance.

Silica-based strong anion exchange media for protein purification

The main objective of our research was to develop silica-based, polymer-functionalized ion exchange materials for single-use bioprocess applications, with the ultimate goal of achieving maximal binding capacity for target proteins. Herein we report the utilization of Grace® wide pore silica gel and bonding the silica with cationic polymers. The strong anion exchange materials have been prepared by a two-step process involving initial bonding with two trimethoxysilanes and subsequent aqueous solution radical polymerization with quaternary ammonium ion containing monomers and an azo initiator. Using the binding capacities for bovine serum albumin (BSA), a model protein for the evaluation of the new materials, we optimized the processes with regard to the median pore size of the silica gel, as well as polymer composition and ratios, which were determined by reagent ratios and reaction conditions. The products were also characterized by both chemical and physical methods. It has been found that higher binding capacities are associated with lower ligand density and higher molecular weight for the attached polymers, with over 20% higher in both static and dynamic binding capacity values with the same amount of attached polymers. The advantages of a large pore size distribution and optimal median pore size for the base silica are discussed. Optimal pore size range of 500–1500Å and distribution of Span 90 for over 1.0 give the highest BSA binding capacities. Silica-based strong anion exchange materials showed excellent flow characteristics when packed into a column and were superior to commercial agarose-based strong anion exchange material with respect to dynamic binding capacity, elution of proteins, and baseline separation of a mixture of three model proteins.

Phase and surface area studies of maghemite nanoparticles dispersed in silica gel

First, the superparamagnetic maghemite nanoparticles were synthesised using Massart's procedure. Then, the nanocomposites of the synthesised maghemite nanoparticles and silica were produced by dispersing the as-synthesised maghemite nanoparticles into the silica xerogel prepared by sol–gel technique. The system was then heated for 3 days at 140°C. The phase analysis performed using X-ray diffraction confirmed that the as-synthesised nanoparticles and the nanoparticles within the silica gel were maghemite. Surface characteristic of the nanocomposite was evaluated by N2 adsorption. The ‘pure’ silica gel and maghemite nanoparticles showed high values of surface area (150–160 m2 g−1), while the surface area of nanocomposite was less than 40 m2 g−1. This was probably due to the formation of dense structures caused by incorporation of maghemite nanoparticles within the pores of silica gel. The pore width increased with increasing content of maghemite nanoparticles.

Simple Technique for Preparing Stable and Recyclable Cross-Linked Enzyme Aggregates with Crude-Pored Microspherical Silica Core

With the aim of providing a highly stable and easy recyclable biocatalyst, we prepared a new biocatalyst formulation based on cross-linked enzyme aggregate (CLEA) technology by the physical adsorption of cross-linked phenylalanine ammonia lyase on a highly porous microspherical silica gel solid support (S-CLEAs). The main factors in the preparation of S-CLEAs were optimized by response surface methodology (RSM). Analysis by scanning electron microscopy (SEM), FT-IR spectroscopy, and confocal laser scanning microscopy (CLSM) indicated that the CLEAs were successfully immobilized on the crude-pored microspherical silica gel. Compared to standard CLEAs, the S-CLEAs can be easily separated from the reaction mixture by natural precipitation without centrifugation and filtration treatments. Although the S-CLEAs revealed an excellent stability of the enzyme against high temperature and denaturants, the volumetric activity of the S-CLEAs decreased because of the presence of silica gel. In a word, the S-CLEA technology in this study might open an attractive route toward effectively improving the catalytic properties of traditional CLEAs.

Selective adsorption and stable solidification of radioactive cesium ions by porous silica gels loaded with insoluble ferrocyanides

Development of highly functional cesium selective adsorbents for the decontamination of high-activity-level water (HALW) from the Fukushima NPP-1 accident is very urgent. In order to selectively adsorb the radioactive cesium, three kinds of novel porous silica gels loaded with insoluble ferrocyanides (SLFC) were prepared using a successive impregnation/precipitation method. Based on the results of previous research, the SLFC composites have relatively large uptake ratio above 95%, distribution coefficients (K d) above 103 cm3/g, and excellent adsorption kinetics even in seawater. The solidification results also indicate that zeolites have an excellent Cs immobilization characteristic, gas-trapping and self-sintering abilities, and low leachability. We chose three kinds of SLFC composites to achieve the optimization of solidification by mixing with nine kinds of additives at high temperatures (up to 1200 °C). The Cs contents in the three composites were estimated to be below 30% of the initial contents and decreased with the three stages at calcination temperatures ranging from 25 to 1200 °C. By contrast, the Cs immobilization ratio was markedly lowered by mixing with additives: of those, allophane had the best immobilization result. By increasing the additive ratio to 50 wt%, the Cs immobilization ratio became almost 100% and no volatilization of Cs was detected even after calcination at 1200 °C. This result indicates that calcination of the mixture of SLFC composites after adsorbing Cs+ ions and specific additives under appropriate ratio is effective for stable solidification.

Performance study of composite silica gels with different pore sizes and different impregnating hygroscopic salts

Composite solid desiccant materials are fabricated by impregnating hygroscopic salts into pores of different silica gels. In this paper, sorption characteristics of composite desiccants with different pore sizes (namely, 2–3nm, 7–8nm and 9–10nm) of silica gels and different salts in terms of LiCl, LiBr and CaCl2 are tested and analyzed. Researches on nitrogen adsorption suggest that, due to impregnated salt particles, composite desiccants are different from those of pure silica gels and have smaller surface area and pore volume. Sorption isotherms and sorption kinetics with respect to different composite desiccant materials are tested and fitted with the Dubinin–Astakhov (D–A) equation and the Linear Driving Force (LDF) model, respectively. Results showed that both the pore size of silica gel substrates and categories of impregnated salts affect the sorption characteristics of composite desiccants. Microporous silica gel with pore size of 2–3nm is not fit for preparing composite desiccants due to decreased water sorption quantity and sorption rate. Type B and mesoporous silica gels can be a promising host matrix of composite desiccant owing to enhanced water sorption quantity and favorable sorption rate. In addition, composite desiccants employing LiCl have the best sorption properties, while CaCl2 and LiBr follow next.

Magnetic Field Effects on Synthesis of Porous Silica Gel and the Adsorption Property of Pb<sup>2+</sup> Ions

Under the effect of an adjustable magnetic field (0-0.6T), porous silica gel was synthesized via sol-gel process by phase separation method to investigate the effect of magnetic field on the formation time of porous silica gel and the influence of porous silica gel synthesized under the induction of magnetic field (m-SiO2) on Pb2+ adsorption property. The findings of this study show that the reaction rate can be increased and the gelation time shortened under the effect of magnetic field. Within the range of intensity of the magnetic field in this study, the 0.25T magnetic field affects the porous silica gel most significantly and can reduce the gelation time from 120min to 90min; the Pb2+ adsorption property of m-SiO2 is better than that of ordinary silica gel (SiO2); under the condition that the solid-to-liquid ratio and the pH value change, the Pb2+ equilibrium adsorption rate and equilibrium absorption capacity of silica gel synthesized with the 0.25T magnetic field, compared with ordinary silica gel (SiO2), can averagely increase by 11.32% and 11.20mg/g, respectively.

Multiphase reaction media involving water and carbon dioxide for selective hydrogenation

The features of multiphase reaction media using water (bulk water, supported thin aqueous phase) and dense phase carbon dioxide are reviewed for selective hydrogenation of carbonyl, nitro, and cyano compounds. The interface of aqueous and organic phases is important in determining the rate of hydrogenation and the product selectivity. For supported aqueous phase catalysts, the interface between the thin aqueous film and a support material (for example, porous silica gel) has also an influential impact. The pressurization of a liquid reaction mixture (organic and/or aqueous phases) with carbon dioxide shows interesting positive effects on the reaction rate, the product selectivity, and the deactivation of a conventional supported metal catalyst. In some cases the use of both water and dense phase carbon dioxide serves as an effective reaction medium. In other words, these green solvents may act as reaction promoters. Possible roles of their actions are discussed.

Anomalous thermal expansion of thin cetane layer solidified at the inner surface of confining nanoporous silica gel

Silica gel with extremely small pores of 3 nm diameter was filled with liquid cetane. Samples with various coefficients of filling k of cetane were prepared. After the solidification of confined cetane, its free volume has been investigated. The positron annihilation lifetime spectroscopy at 24–300 K and differential scanning calorimetry (DSC) at 233–303 K measurements have been performed. Decreasing the temperature, only small changes in dimensions of silica gel matrix and pronounced contraction of confined cetane have been indicated. In the case of k < 0.2, however, via cooling below 180 K, the temperature dependence changes its sign and the free volume in confined cetane increases. Decreasing the cetane content, the negative apparent expansion coefficient dramatically grows. This anomalous temperature dependence is interpreted by the cracking of thin layer of solid cetane which is in contact with the walls of silica gel pores.