Adsorption properties of lamellar silica

Isotherms and heats of adsorption of water, n-heptane, trimethylamine, and methanol (at 303 K) vapors and isotherms of adsorption of nitrogen (at 77 K) on the lamellar silica prepared by removing metals from natural mineral vermiculate were measured. The surface concentration of adsorption sites and their energetic characteristics were found to be similar to those of ordinary silicas. The distinction of the lamellar silica from ordinary silicas manifests itself through extended desorption branches, a feature that makes it possible to classify the lamellar silica under study as a limitedly swellable sorbent.     

Adsorption properties and synthesis of silica aerogel-hollow silica microsphere hybrid (sandwich) structure

Journal of Sol-Gel Science and Technology - Silica aerogels have the potential use in many applications due to their large specific surface area and high porosity. One of these applications can be...     

Anodic alumina supported dual-layer microporous silica membranes

We present a new processing scheme for the deposition of microporous, sol–gel derived silica membranes on inexpensive, commercially available anodic alumina (Anodisk™) supports. In a first step, a surfactant-templated mesoporous silica sublayer (pore size 2–6 nm) is deposited on the Anodisk support by dip-coating, in order to provide a smooth transition from the pore size of the support (20 or 100 nm) to that of the membrane (3–4 Å). Subsequently, the microporous gas separation membrane layer is deposited by spin-coating, resulting in a defect-free dual-layer micro-/mesoporous silica membrane exhibiting high permeance and high selectivity for size selective gas separations. For example, in the case of CO₂:N₂ separation, the CO₂ permeance reached 3.0 MPU (1 MPU = 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹) coupled with a CO₂:N₂ separation factor in excess of 80 at 25 °C. This processing scheme can be utilized for laboratory-scale development of other types of microporous or dense inorganic membranes, taking advantage of the availability, low cost and low permeation resistance of anodic alumina (or other metal oxide) meso- and macroporous supports.     

Adsorption properties of highly dispersed silica with a partially hydrophobic surface

Highly dispersed silica samples with different degrees of substitution of silanol groups by trimethylsilyl groups are obtained by the gas-phase modification method. The adsorption of vitamins B1, B6, and E on initial and modified silica samples is studied. It is shown that the modified adsorbents with degrees of substitution of silanol groups up to 40% exhibit higher affinity to vitamin molecules; in this case, adsorption properties of the modified silica samples depend on the nature of the adsorbate and the number of grafted groups.     

Long-term pervaporation performance of microporous methylated silica membranes

The incorporation of methyl groups in microporous silica membranes was proven to enhance the service time in the dehydration of a butanol-water mixture at 95 degrees C from a few weeks to more than 18 months with a water flux of about 4 kg m(-2) h(-1) and a selectivity between 500 and 20 000.     

Adsorption of carbon dioxide on microporous carbon adsorbents

Adsorption isotherms of carbon dioxide on microporous carbon adsorbents prepared by activation with potassium sulfide in water vapor were measured. The measurements were carried out in the pressure interval from 1 Pa to 0.1 MPa at temperatures from 216.2 to 293.15 K. Based on the theory of volumetric filling of micropores, the main structural and energetic parameters of the microporous carbon adsorbents were calculated. The adsorption isosters of carbon dioxide were calculated from the adsorption isotherms in the same pressure and temperature ranges and approximated by linear dependences. The plots of the differential mole isosteric heats of adsorption vs amount adsorbed were constructed by using the adsorption isosters.     

Adsorption studies of a microporous phthalocyanine network polymer

The adsorption/desorption of N2 at 77 K and the adsorption from aqueous solution at 298 K of four organic probe molecules of different sizes (phenol, 4-nitrophenol, orange II, naphthol green B) were studied for a phthalocyanine network polymer of intrinsic microporosity (PIM) and for an activated carbon (Darco 20-40 mesh). N2 sorption analysis gave similar surface areas for the PIM and the carbon (610 and 545 m2 g(-1), respectively) but showed differences in pore size distribution, the PIM being essentially microporous (pore size < 2 nm), with a high proportion of ultramicropores (<0.7 nm), while the carbon had a broader pore size distribution, extending into the mesopore region. The carbon acted as an adsorbent for all the organic probe molecules studied, while the PIM was more selective, adsorbing the smaller molecules but rejecting the large dye naphthol green B. The PIM offers selectivity combined with a well-defined chemical structure incorporating catalytic sites.     

Incommensurate modulation in the microporous silica SSZ-24

A detailed investigation of the structure of microporous silica, SSZ-24, is presented. It is shown by X-ray powder diffraction and (29)Si MAS NMR experiments that the structure deviates from the previously proposed AlPO(4)-5-type structure. At room temperature, electron diffraction (ED) patterns exhibit extra diffraction spots, which can be attributed to an incommensurate structural modulation along the c axis. This in turn results in a pleat pattern in real space with two different intervals arranged aperiodically along the c axis, as observed with high-resolution electron microscopy (HREM). The modulated structure may easily turn into a disordered one through excessive electron irradiation or heat-treatment. In order to understand the origin of the modulation, soft phonon-modes of the ideal premodulated structure were analyzed by the use of the rigid-unit-mode model. The distribution of soft modes in reciprocal space might correspond roughly to diffuse streaks that could be observed in the diffraction patterns at higher temperatures. It was found that several phonon branches soften at specific wave vectors, which are incommensurate with respect to the original period and might be responsible for the modulation. We present a simple analytic treatment to deduce the wave vectors and associated displacement eigenvectors for the incommensurate soft-modes.     

Adsorption of perfluoropropane on the PAC microporous carbon adsorbent

The adsorption of perfluoropropane (N₃F₈) on the PAC microporous carbon adsorbent, which is close in properties to monoporous adsorbents, was studied at temperatures of 216, 243, 295, 318, and 343 K in the pressure interval from 1 to 1·10⁵ Pa. The adsorption isosters are well approximated by straight lines in the studied interval of temperatures and pressures. The dependence of the isosteric heats of adsorption on filling is described by a curve with a maximum in the region of high fillings. Such a curve is characteristic of adsorbents with a narrow pore size distribution.     

Highly hydrothermally stable microporous silica membranes for hydrogen separation

Fluorocarbon-modified silica membranes were deposited on gamma-Al2O3/alpha-Al2O3 supports by the sol-gel technique for hydrogen separation. The hydrophobic property, pore structure, gas transport and separation performance, and hydrothermal stability of the modified membranes were investigated. It is observed that the water contact angle increases from 27.2+/-1.5 degrees for the pure silica membranes to 115.0+/-1.2 degrees for the modified ones with a (trifluoropropyl)triethoxysilane (TFPTES)/tetraethyl orthosilicate (TEOS) molar ratio of 0.6. The modified membranes preserve a microporous structure with a micropore volume of 0.14 cm3/g and a pore size of approximately 0.5 nm. A single gas permeation of H2 and CO2 through the modified membranes presents small positive apparent thermal activation energies, indicating a dominant microporous membrane transport. At 200 degrees C, a single H2 permeance of 3.1x10(-6) mol m(-2) s(-1) Pa(-1) and a H2/CO2 permselectivity of 15.2 were obtained after proper correction for the support resistance and the contribution from the defects. In the gas mixture measurement, the H2 permeance and the H2/CO2 separation factor almost remain constant at 200 degrees C with a water vapor pressure of 1.2x10(4) Pa for at least 220 h, indicating that the modified membranes are hydrothermally stable, benefiting from the integrity of the microporous structure due to the fluorocarbon modification.     

Bio-dissolution of colloidal-size clay minerals entrapped in microporous silica gels

Four colloidal-size fractions of strongly anisotropic particles of nontronite (NAu-2) having different ratios of basal to edge surfaces were incubated in the presence of heterotrophic soil bacteria to evaluate how changes in mineral surface reactivity influence microbial dissolution rate of minerals. To avoid any particle aggregation, which could change the reactive surface area available for dissolution, NAu-2 particles were immobilized in a biocompatible TEOS-derived silica matrix. The resulting hybrid silica gels support bacterial growth with NAu-2 as the sole source of Fe and Mg. Upon incubation of the hybrid material with bacteria, between 0.3% and 7.5% of the total Fe included in the mineral lattice was released with a concomitant pH decrease. For a given pH value, the amount of released Fe varied between strains and was two to twelve-fold higher than under abiotic conditions. This indicates that complexing agents produced by bacteria play an important role in the dissolution process. However, in contrast with proton-promoted NAu-2 dissolution (abiotic incubations) that was negatively correlated with particle size, bacterial-enhanced dissolution was constant for all size fractions used. We conclude that bio-dissolution of nontronite particles under acidic conditions seems to be controlled by bacterial metabolism rather than by the surface reactivity of mineral.     

Chemical assembly of a titanium oxide layer on microporous silica

Russian Journal of General Chemistry - The chemical assembly of a titanium oxide coating on a regular microporous silica surface by the atomic layer deposition method was considered. The change in...     

Adsorption of DNA into mesoporous silica

In these experiments, double-stranded, linear DNA sequences were adsorbed into the pores of spherically shaped acid-prepared mesoporous silica (APMS). The lengths of the sequences were either 760 base pairs or 2000 base pairs. DNA adsorption into the interior of the mesoporous material was confirmed using confocal microscopy of sequences containing fluorescently labeled DNA molecules. Additional characterization with N(2) physisorption and powder X-ray diffraction supported this finding. The extent of adsorption was measured at various concentrations using UV-visible spectrophotometry to establish adsorption isotherms. APMS alone adsorbed a negligible amount of DNA; however, exchanging divalent cations such as Mg(2+) and Ca(2+) into the pores of APMS prior to DNA uptake was found to cause a significant amount of DNA to be adsorbed. Using Na(+) caused a lower amount of DNA to be adsorbed. DNA adsorption was also dependent on the pore diameter of APMS. Adsorption increased upon expansion of the pore size of the metal ion-exchanged material from 34 to 54 A; however, no additional uptake was measured by further increasing the pore size to 100 A. The amount of DNA adsorbed could also be significantly increased by using (aminopropyl)triethoxysilane to covalently link ammonium ions to the surface. Postsynthetic modification of the silica surface with aminopropyl groups increased the maximum DNA adsorption to 15.7 microg/mg silica, for materials with pore diameters of 100 A, which is 2 to 3 times more adsorbed DNA than for metal ion-exchanged material. This indicated that DNA binds more strongly in the presence of the ammonium group compared to the metal counterions. Finally, calculation and comparison of Freundlich and Langmuir constants for these adsorption processes indicate that intermolecular interactions between the DNA molecules within the pores are significant when the effective pore diameter is small, including materials with larger pores that were modified with organosilane.     

Adsorption of carbon dioxide on microporous carbon adsorbent PAU-10

Carbon dioxide adsorption on the microporous carbon adsorbent PAU-10 within the 177.8—423 K temperature and 0.1—5.13·10⁶ Pa pressure intervals was studied. The isosteres of absolute adsorption are well approximated by straight lines, which do not change their slope on going to temperatures higher than the critical temperature of CO₂. An increase in the differential molar isosteric heat of adsorption (q _st) at 0 < a < 1 mmol g^–1 is explained by the influence of the endothermic effect of adsorption expansion of the adsorbent. In the region of high pressures and nonideal gas phase, q _st is temperature-dependent.     

Adsorption of molecular hydrogen on ultrafine microporous/mesoporous materials

The comparative processing of H₂ adsorption isotherms obtained at 77 K is demonstrated to be applicable to the investigation of the microtexture of the ultrafine oxide materials MCM-41 and ZSM-5 and their mechanical mixtures. The H₂ sorption method allows the micropore volume to be determined correctly for mixed ultrafine microporous/mesoporous materials.