Flow-injection determination of sodium and potassium by separationon a silica column and extraction-spectrophotometry with benzo-18-crown-6 and tetrabromophenolphthalein ethyl ester

Sodium and potassium ions in waters are determined by flow-injection extraction-spectrophotometry. The ion-association complexes formed between the metal/crown ether cations and the tetrabromophenolphthalein ethyl ester anion (TBPE^-) are extracted into chlorobenzene/benzene (1:3) and the absorbance of the organic phase is measured after phase separation with a porous membrane. Sodium and potassium are separated on-line with a column (1 mm i.d.×30 cm) packed with silica gel (100-200 mesh). The manifold comprises four streams, each at 0.8 ml min^?1. The sample is injected into a water stream and mixed with a reagent stream containing lithium acetate and benzo-18-crown-6 before entering the silica gel column; after the separation, the stream is mixed with EDTA (trilithium salt) and lithium hydroxide, and then with the extraction solution containing TBPE.H. Extraction proceeds in a 2-m coil; the absorbance of the organic phase is measured at 620 nm. CAlibration graphs are linear inthe ranges 0–×10^?3 M sodium and 0–2×10^?4 M potassium. The sample throughput is 15 h^?1. The procedure is applicable to river and tap waters.     

Synthesis of new silica gel adsorbent anchored with macrocyclic receptors for specific recognition of cesium cation

In this article, a new class of silica gel adsorbent functionalized with macrocyclic receptors was developed for cesium recognition. A calixcrown molecule, with strong affinity to cesium cation, was decorated precisely at the 1,3-alternate benzene rings with reactive amino substituents, followed by the anchoring to the silica matrix to obtain a high functionalization degree. Structural characterization of the monomers and the organosilicas was carried out by ¹H/¹³C NMR, ²⁹Si/¹³C solid-state NMR, and FT-IR spectra. Besides, XPS survey, BET, TGA and ESEM were employed to investigate the surface property, thermal stability and micro-morphology of the organosilicas. Due to the host–guest interaction between the calixcrown receptor and cesium cation, efficient separation of cesium in the presence of competing alkali metals including sodium and potassium was realized. Mechanism regarding the recognition effect was discussed. The calixcrown-grafted organosilica material possesses the potential to be applied for the separation of cesium in radioactive liquid waste.     

Sodium and potassium silicate-based catalysts prepared using sand silica concerning biodiesel production from waste oil

Heterogeneous catalysts, named SPS (sodium potassium silicates), were synthesized with an alternative silica (MPI silica) obtained from beach sand. In this work, the MPI was modified with NaOH and KOH producing silicate-based catalyst for biodiesel synthesis from waste cooking oil (WCO). The obtained catalyst was characterized by XRD, CO₂-TPD, the Hammett basicity test, XRF, FESEM, EDX, FTIR and TG/DTG. The results confirmed the presence of K₂O/Na₂O oxides and their silicates, the main active sites responsible for the catalytic action. CO₂-TPD and the Hammett basicity data suggested the presence of weak, medium and strong basic sites. Biodiesel yield was about 92% and the SPS catalyst was reused for five cycles. The biodiesel conversion by NMR ¹H was about 93.89%. The DTG deconvolution revealed the decomposition of four typical biodiesel compounds (R² = 0.9987). The method applied for the WCO biodiesel production using SPS catalyst represents an environmentally friendly process, based on low-cost material and reuse of waste biomass.     

Diphenylphosphino functionalization of mesoporous silica using tripodal linker units

The immobilization of diphenyl phosphine onto ordered mesoporous silicas using a tripodal linker unit possessing one bromopropyl group and three anchoring silicon atoms was investigated. Solid-state ³¹P, ²⁹Si, and ¹³C cross-polarization/magic angle spinning (CP/MAS) NMR spectroscopic studies as well as isothermal nitrogen adsorption/desorption measurements revealed that grafting the tripodal linker unit and a subsequent reaction with potassium diphenylphosphide (the “bottom-up” method) successfully realized diphenylphosphino functionalization of silica while maintaining the mesoporous structure. In contrast, directly grafting tripodal diphenylphosphino ligands pre-synthesized from the tripodal linker unit onto silica (the “top-down” method) was unsuccessful.     

Modification of silica gel with diethylamino group by novel reaction and its characterization

1,3,5,7-Tetramethylcyclotetrasiloxane (H₄) was deposited on silica gel at 80°C by utilizing a chemical vapor deposition (CVD) method, where it was catalytically polymerized to form a surface coating of polymethylsiloxane (PMS). Treated silica gel (PMS-Si) increased in weight up to a plateau level, and there was no further increase with increasing reaction time. The film of PMS was partially cross linked; typical values of crosslinking ratio and film thickness were 2% and 0.6 nm, respectively. An anionic ion exchanger containing diethylamino groups was synthesized from PMS-Si by hydrosilylation of allyl glycidyl ether followed by treatment with diethylamine. Its structure was confirmed by¹³C and²⁹Si CP/MAS NMR spectroscopy and FT-IR spectrophotometry. Characterization of silica gel (DEA-Si) modified with diethylamino group was evaluated by a packing of the column for liquid chromatography. As a mixture of five nucleotides was completely separated, it was recognized that DEA-Si was operated by ion exchange action. Because the surface of the silica gel was covered with hydrophobic PMS, the peak heights and retention times did not change after washing of the column with alkaline solution.     

Synthesis and properties of crosslinked polyvinylformamide and polyvinylamine hydrogels in conjunction with silica particles

Polyvinylamine hydrogels with silica particles encapsulated (PVAm/silica) were produced by a two‐step synthesis. In the first step, polyvinylformamide/silica (PVFA/silica) hybrids were synthesized from vinylformamide (VFA) and 1,3‐divinylimidazolidin‐2‐one (1,3‐bisvinylethyleneurea, BVU), as the crosslinker, by radical copolymerization in silica/water suspensions using different compositions of VFA/BVU. The target product PVAm/silica was obtained by acidic hydrolysis of the PVFA/silica hydrogels in a second step. The chemical structures of both hydrogels, PVFA/silica and PVAm/silica, respectively, were revealed by solid‐state ¹³C(¹H) cross‐polarity/magic‐angle spinning NMR spectroscopy. Both hydrogels swelled significantly in water. The swelling capacity of the two systems was characterized by the correlation length ξ (or hydrodynamic blob size) of the network meshes with small‐angle neutron scattering experiments. ξ is significantly larger for PVAm/silica than for PVFA/silica, which corresponds to the observed higher swelling capacity of this polyelectrolyte material. Furthermore, the swelling behavior of the hybrid hydrogels was quantitatively described in terms of free swell capacity, centrifuge‐retention capacity, adsorption against pressure, and free swell rate as compared with values of the corresponding copolymer hydrogels. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3144–3152, 2002     

Formation and characterization of mesostructured silica nanotubes

The multi-walled mesoporous silica nanotubes are prepared using cetyltrimethylammonium bromize (CTAB) as the surfactant micellar template and tetraethylorthosilicate (TEOS) as the silica precursor via a one-step wet chemical approach. The synthesized tubes are found to be double/triple walled and of amorphous nature. Their diameter and the length are about 100 nm to 1 μm and about 0.1–20 μm, respectively. The specific surface area approaches 1,488 m²/g. Based on the transmission electron microscopy analysis, it is inferred that the formation of the double/triple walled silica nanotubes is associated with the lamellar curling mechanism. A striking photoluminescence effect is detected in the mesostructured silica nanotubes. These nanotubes are expected to be a promising material for various applications such as gas storage, catalyst, or catalyst supports.     

Synthesis and spectroscopy of tpps-doped silica gels by the sol-gel process

Tetraphenylporphinetetrasulfonic acid (TPPS), which is well known as a photochemical hole-burning (PHB) dye, was incorporated in silica gels obtained by the sol-gel process from tetramethoxysilane (TMOS). The form of TPPS must be free base (H₂P), which exists in basic condition, to be active in PHB. To obtain transparent and higher density silica gels doped with free-base TPPS, two-step hydrolysis processes using solutions containing NaOH were developed, and the gels having bulk density of about 1.5 g/cm³ were synthesized. The form of TPPS in the gels was investigated by measuring the absorption and luminescence spectra, and it was found that in the silica gels almost all the TPPS retained free-base form at the molar ratio of NaOH/TMOS above 10^–3.     

Hierarchically macro/mesoporous silica monoliths constructed with interconnecting micrometer-sized unit rods

Under typical dilute reactant compositions (3 ~ 5 wt% of surfactant template concentration) and conventional hydrothermal conditions for mesoporous materials synthesis, successful preparation of hierarchically macro/mesoporous silica monoliths was reported in this paper. The resultant materials were characterized by a series of techniques including powder X-ray diffraction, N₂ adsorption–desorption, SEM, TEM/EDS, and Hg porosimetry. A new kind of stable and hierarchically porous pure silica monoliths was confirmed, which are featured with highly ordered mesoporous structures, rod-shaped unit particles, large specific surface area of 492 m²/g, continuous macropores of about 4.0 μm in size and high macropore volume of about 13.1 cm³/g. Moreover, using the resultant silica monoliths as hard templates, carbon monoliths have been successfully replicated, which inherit the structural characters of parent silica materials. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Porous texture of silica aerogels made with ionic liquids as gelation catalysts

The effect of four ionic liquids on the porous texture of silica aerogels synthesized from mixed tetramethoxysilane and methyltrimethoxysilane and dried by the CO₂ supercritical method, was studied. Two of these ionic liquids were composed of BF₄ ⁻ anions while the other two included Cl⁻ anions. The synthesis of gels from ionic liquids did not require another acidic catalyst for silica hydrolysis, nor a basic catalyst for silica condensation. These aerogels were compared with traditional aerogels made according to a double step catalysis, which first involved hydrolysis with HCl followed by condensation with pH 9 Tris HCl buffer. Gel mass analysis and thermogravimetric data showed that, when the initial molar of ionic liquid to Si was 1.58, only ~2% (by mass) of the initial ionic liquids consisting of BF₄ ⁻ anions and ~10% (by mass) of ionic liquids containing Cl⁻ anions, remained in the aerogels after supercritical drying. Moreover, X-ray diffraction confirmed that in ionic liquids based on BF₄ ⁻ anions, evaporation of the volatile components before supercritical CO₂ drying led to the formation of regularly ordered mesopores.     

A study of the titrimetric potassium silicofluoride method for determination of silica in aluminosilicate materials

A critical study of the titrimetric potassium silicofluoride method for determination of silica reveals that quantitative precipitation of the sificofluoride is possible in 3.5-7.5N acid containing 1% sodium fluoride and >/ 12% potassium chloride. An aqueous wash solution (pH approximately 5.3) containing 0.1% sodium fluoride and 12% potassium chloride has been found ideal for washing the precipitate without significant hydrolysis even at 35 degrees C. The interference of large amounts of aluminium (up to 160 mg of Al(2)O(3)) is eliminated by precipitating the silicofluoride in 6-7.5N acid solution, the aluminium then forming practically no fluoro-complex, because of extensive protonation of fluoride. Interference by large amounts of boron (up to 160 mg of B(2)O(3)) is eliminated by precipitating silicofluoride with the minimum permissible potassium chloride concentration (12%) and washing with the aqueous wash solution at room temperature. Nitrazene Yellow has been used as a more sensitive indicator.     

Modification of silica nanoparticles by grafting of copolymers containing organosilane and fluorine moities

The free‐radical cotelomerization of 3‐(trimethoxysilyl)propyl methacrylate (TMSPMA) with 1,1,2,2‐tetrahydroperfluorodecyl acrylate (PFDA) in the presence of 2‐mercaptoethanol was performed at 80 °C in acetonitrile. Hydroxy end‐groups of the cotelomers were reacted with 2‐isocyanatoethyl methacrylate to give macromonomers. The P(TMSPMA‐stat‐PFDA) cotelomers, containing fluoro and silane groups, were then grafted onto silica nanoparticles. Optimal grafting conditions were found with TMSPMA monomer alone in toluene at 110 °C. The structure of the modified silica was analyzed by FTIR and ²⁹Si solid‐state NMR. The amount of grafted TMSPMA or P(TMSPMA‐stat‐PFDA) was calculated by thermogravimetric and elemental analyzes. The grafting yield increased with the copolymer/silica weight ratio until a maximum value of 2.26 μmol/m². © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4617–4628, 2009     

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.     

Fabrication of flame-retardant and smoke-suppressant isocyanate-based polyimide foam modified by silica aerogel thermal insulation and flame protection layers

Because of containing urea groups, flame resistance and smoke releasing behaviors of isocyanate-based polyimide foam (IBPIF) produced using free foaming technology require further improvement. In this work, silica aerogel layers were incorporated into cells of IBPIF through an in situ growth process of silica sol (SS). Compared with silica aerogel particles directly mixed into the foaming slurry, the silica aerogel layers that firmly attached to the pores and surfaces of cells not only provided exceptional thermal insulation and flame protection, but also kept original cellular structure. With increase in ratio of SS mass to IBPIF volume, silica aerogel incorporation dosage was gradually increased. Accompanied by flame resistance was obviously improved and smoke releasing behavior was effectively suppressed. Those were indicated by the improved limiting oxygen index (LOI), decreased heat release rate (HRR), peak of HRR, and specific optical density of smoke (Ds) in trials with pilot flames. Compared with pure IBPIF, when the ratio reached to 5/15 g/cm³, it resulted in LOI increasing from 22.0% to 33.0%, peak of HRR, total smoke production (TSP), and maximum value of Ds decreasing from 174 to 72 kW/m², 1.11 to 0.37 m²/m², 45.90 to 17.45, respectively.     

Incorporation of niobium into bridged silsesquioxane based silica networks

In this work different synthesis routes were evaluated with the aim of optimizing the incorporation of niobium within a hybrid silica matrix on an atomic scale. The fast kinetics of the hydrolysis/polycondensation of the organic Nb precursor Nb(OEt)₅ entails a segregation of the resulting material into Nb₂O₅ and a silica based network. To overcome this effect we (a) performed a prehydrolysis of 1,2-bis-triethoxy-ethane (BTESE) prior to adding niobium penta-ethoxide, or (b) attempted to reduce the availability of Nb via a complexation of Nb by either acetylacetone or 2-methoxyethanol. The network organization was evaluated from results of Fourier transform infrared as well as ¹³C, ²⁹Si and ¹⁷O MAS NMR spectroscopy. Whereas the prehydrolysis of BTESE and the addition of 2-methoxyethanol induced only moderate mixing of Nb and Si, leading to a network in which islands of Nb₂O₅ are linked to the hosting silica based matrix via Nb–O–Si bonds, the use of acetylacetonate lead to a mixing of Nb and Si on the atomic scale, forming a mixed Nb–O–Si network without any extended clusters of segregated Nb₂O₅. The Si–C–C–Si bridge from the silsesquioxane is found to survive the condensation process and is even present in the resulting materials after annealing at 200 °C.     

Immobilization of phosphomolybdic acid on the surface of mesoporous silica functionalized with alkylammonium groups

Silica containing protonated 3-aminopropyl groups has been synthesized by a template method using Pluronic 123 as a surfactant in an acidic medium at a hydrochloric acid concentration of 1.72 M. The X-ray diffraction data have indicated that the obtained sample has a highly ordered hexagonal structure, which agrees with the large specific surface area and sorption pore volume (S _sp = 880 m²/g, V _s = 1.28 cm³/g) typical of SBA-15 materials. However, transmission electron microscopy data show that this material contains two phases, with one of them being similar to SBA-15 mesoporous silica and the other belonging to socalled “honeycomb foams.” An ion-exchange reaction has been employed to immobilize phosphomolybdic acid H₃[P(Mo₃O₁₀)₄] on the pore surface, and the fact of immobilization has been confirmed by IR and TGA data. This functionalization significantly reduces the S _sp and V _s values of the sample but has no effect on isotherm pattern, thereby attesting to retaining the silica structure after the modification.     

Fluorescent sensing of nitrite at nanomolar level using functionalized mesoporous silica

Aminopyrene was covalently anchored onto mesoporous silica through serial post-grafting to obtain a fluorescent solid that can be used as a sensing material for the determination of nitrite. The latter, in acidic medium, reacts with the secondary amino groups on the material to form a non-fluorescent nitroso derivative. Based on the fluorescence quenching caused by this specific reaction, a method was developed for the determination of nitrite at nanomolar levels. The range for detection of nitrite in 1.5?mol.L^?1 HCl is linear between 1.50?nM to 0.45???M and 0.45???M to 2.22???M, the detection limit being 1.10?nM and 0.307???M respectively at an S/N of 3.

Cobalt/silica gel system as a chemisorbent for oxygen

A study has been made of oxygen chemisorption and the phase composition of cobalt/silica gel samples containing from 1.1 to 7.7% Co by weight. Reduction in hydrogen leads to the formation of metallic cobalt with a crystal size of about 50 Å. The quantity of chemisorbed O₂ at 20°C increases in proportion to the increase of [Co]. The uptake of O₂ is essentially completed at 300°C with the formation of Co₃O₄. The high adsorption capacity for O₂ is preserved unchanged in repetitive reduction-oxidation cycles. The residual content of O₂ in the gas after passage through the layer of the cobalt/silica gel chemisorbent is 3·10^–6% by volume.N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, 117913 Moscow. Translated from Izvestiya Akademii Nauk, Seriya, Khimicheskaya, No. 1, pp. 40–43, January, 1992.     

Polyamine-substituted epoxy-grafted silica for aqueous metal recovery

Glycidoxypropyltriethoxysilane (GPS) was used as a reactive silane to graft metal- complexing ligands onto silica gel in aqueous media under mild conditions. The synthesis entailed the reaction of GPS with silica gel, followed by grafting polyamine onto the epoxy functional group. GPS was added to silica gel in ethanol with 5 vol. % water and the mixture was air-dried for 24 h. Subsequently, excess amounts of polyamines: triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine were individually added to the silanised silica, followed by solvent evaporation and ovendrying at 60°C. The ligand-grafted silica gel particles showed a rapid heavy metal uptake in batch or flow-through experiments with capacities reaching 0.1 mmol g⁻¹ for copper, zinc, cadmium, or lead ions. Columns packed with the modified particles could be readily regenerated by acid-washing with only a small decrease in activity. The particles could be used for the colourimetric detection of heavy metal pollution or for pre-concentration for analytical purposes. Competition between Cu²⁺, Zn²⁺, Pb²⁺, and Cd²⁺ ions for the three synthesised silica showed that Cu²⁺ ions were adsorbed more strongly than the other metal ions. The general method developed can be applied to graft other molecules with terminal amino groups for other purposes.