A New Synthetic Route to Microporous Silica with Well‐Defined Pores by Replication of a Metal–Organic Framework

Microporous amorphous hydrophobic silica materials with well‐defined pores were synthesized by replication of the metal–organic framework (MOF) [Cu₃(1,3,5‐benzenetricarboxylate)₂] (HKUST‐1). The silica replicas were obtained by using tetramethoxysilane or tetraethoxysilane as silica precursors and have a micro–meso binary pore system. The BET surface area, the micropore volume, and the mesopore volume of the silica replica, obtained by means of hydrothermal treatment at 423 K with tetraethoxysilane, are 620 m²g^?1, 0.18 mL g^?1, and 0.55 mL g^?1, respectively. Interestingly, the silica has micropores with a pore size of 0.55 nm that corresponds to the pore‐wall thickness of the template MOF. The silica replica is hydrophobic, as confirmed by adsorption analyses, although the replica has a certain amount of silanol groups. This hydrophobicity is due to the unique condensation environment of the silica precursors in the template MOF.     

Synthesis and properties of porous silica obtained by the template method

Micromesoporous samples of SiO₂ were synthesized by the sol-gel method using tetraethoxysilane as a starting reagent and 1–5 wt % cetylpyridinium chloride as a template under the conditions of preadsorption of colloid silica by polyethyleneglycol macromolecules. The adsorption and texture of the samples were studied by the low-temperature nitrogen adsorption-desorption technique. Preadsorption of silica sol was shown to affect the adsorption and capillary-condensation properties of silica. The surface area and the volume of mesopores increased at cetylpyridinium concentrations higher than 1 wt %. The micropore volume increased to a maximum. The capillary-condensation hysteresis loop of H4 type transformed into an H3 loop according to the IUPAC classification.     

The nature of lyophobic coating and the adsorption of organic molecules and water on modified silicas

The protective properties of lyophobic layers of various natures chemically grafted to a silica carrier were comparatively studied. The modifiers were silanes with the compositions CF₃(CH₂)₂Si(CH₃)₂Cl (CF₃), C₈H₁₇Si(CH₃)₂Cl (C₈H₁₇), and ClSi(CH₃)₂[OSi(CH₃)₂]₂Cl (OMS). The differences between the surface properties of chemically modified silicas observed in adsorption, chromatographic, and IR spectroscopic measurements were shown to be related to a nonuniform electron density distribution in CF₃ grafted radicals and the special features of the structure of CF₃ and OMS grafted layers caused by the possibility of interaction between the terminal groups of these radicals and the surface of the carrier. Modified silicas possessed low surface energy and were superhydrophobic materials. The sample with grafted octyl groups C₈H₁₇ had the highest stability with respect to water.     

General routes to porous metal oxides via inorganic and organic templates

The generation of porous metal oxides by removal of template molecules from inorganic polymers formed by sol-gel type hydrolysis and condensation of metal alkoxides is described. The template molecules include organic polymers, copper (II) ions in hybrid copper oxide/silica sols and copper (II) bis(hexafluorocetylacetonate) (hfac). Neutron scattering experiments on the system in which polyacrylic acid (Mw=2,000 Daltons) is used as an organic template to generate microporous tin oxide show that removal of the template generates skeletal voids. In a second series of experiments, mixed copper/silicon oxide xerogels were prepared by hydrolysis of mixtures of Si(OEt)₄ and Cu(OCH₂CH(CH₃)N(CH₃)H)₂ in the ratios of Si:Cu=2:1, 4:1, 9:1. Selective removal (etching) of the copper component generates porous silica. Neutron scattering data and BET surface area measurements are consistent with the creation of pores with molecular dimensions (micropores, 10 Å or less). In the third strategy, Si(OEt)₄ is hydrolyzed in the presence of Cu(hfac)₂, a volatile, inert inorganic template, in a 4 to 1 molar ratio. Removal of the template from the xerogel at 100°C in vacuo affords microporous silica.     

The influence of the Si(OC2H5)4/(CH3O)3Si(CH2)3SH ratio on the structure-adsorption characteristics of xerogels formed and accessibility of functional groups in their surface layers

It was shown for the example of the Si(OC₂H₅)₄/(CH₃O)₃Si(CH₂)₃SH system that successively increasing the fraction of tetraethoxysilane in it (from 1: 1 to 5: 1 (mol)) successively decreased the content of 3-mercaptopropyl groups in xerogels synthesized by the sol-gel method (in the presence of methanol as a solvent and fluorine ions as a catalyst) from 5.0 to 1.9 mmol/g, whereas the specific surface area of such xerogels simultaneously increased from 13 to 631 m²/g. The sorption volume of pores also increased, their mean diameter varying insignificantly. The mean diameter of pores (2.2–2.5 nm) was close to the boundary between meso-and micropores, which was in agreement with the form of nitrogen adsorption isotherms (type I according to the IUPAC classification). It was shown by scanning electron microscopy that virtually nonporous xerogels formed at a 1: 1 ratio between alkoxysilanes consisted of spherical partially united particles 2.5–3 μm in diameter. All the 3-mercaptopropyl groups of this and other samples were, however, accessible to silver(I) ions. It follows that these groups are situated in the surface layer of xerogels. The number of thiol groups per 1 nm² of the surface of nonporous xerogels was 1.7–7.0 groups/nm² and depended on the ratio between reacting alkoxysilanes and s _sp.     

Adsorption properties and surface chemistry of MgO

The adsorption properties of MgO, which is used as a sorbent and catalyst support, were studied using gas chromatography. The test absorbents used were n-alkanes (which show only nonspecific dispersion interactions when physisorbed on any adsorbent) and adsorbates whose molecules are capable of specific interactions with the surface reactive sites of MgO. Adsorption isotherms were measured for CHCl₃, CH₃NO₂, CH₃CN, (CH₃)₂CO, CH₃COOC₂H₅, and (C₂H₅)₂O on MgO at 50–100°C. Differential molar enthalpy changes (?ΔH), equal to molar heats of adsorption, were determined. For polar adsorbates, contributions from dispersive and specific interactions into ?ΔH were determined. The electron-acceptor and electron-donor abilities of the MgO surface were estimated.     

Carbenium ion pairs on silica: UV-spectroscopic and electrokinetic measurements of triarylmethylium/halide-aerosil in 1,2-dichloroethane

The interaction of carbenium ion pairs with silica is studied by means of UV-spectroscopy, electrokinetic and adsorption measurements using triphenylmethyl derivatives, (RC₆H₄)₃CX, (X=F, Cl, Br, SCN, OH; R=Cl, I, H, CH₃, C(CH₃)₃, OCH₃, N(CH₃)₂) in interaction with silica particles suspended in 1,2-dichloroethane. The adsorption of triarylmethyl-halides onto silica is accompanied by the heterolytic dissociation of the tertiary carbonhalogen bond. The degree of ionization depends on the basicity of the counter anion and acidity of the cation, respectively. The influences of both concentration and structure of triarylmethyl halides on the zeta-potential are discussed with regard to steric and electronic factor. The zeta-potential values of the adsorbates decrease significantly, as compared with the free silica surface, from a certain triarylmethyl halide concentration where ionization takes place, to a constant level which is characteristic of the carbenium ion pair. These constant zeta-potential values of the adsorbates depend on the basicity of the counter ion and the _p Hammett constants of the ring substitutents, with exclusion of sterically hindered substituents and salt derivatives, e.g., crystal violet and malachite green.     

An examination of the product-catalyzed reaction of trimethylgallium with arsine

The reaction of (CH₃)₃Ga with AsH₃ at 203°C and 259°C has been examined over the product surfaces which were (CH₃)₃- GaAsH₃-x where the average values of x were 1.1 and 2.2 203°C and 259°C respectively. The surface reaction (catalyzed by the product surface) forming (CH₃)₂GaAsH₂ occurred on the surface between adsorbed molecules of (CH₃)₃Ga and ASH₃. The surface coverages of the reactants (gas pressures between 18 and 36 mmHg) were clearly less than monomolecular and for AsH₃ possibly as low as 0.01. For AsH₃ at a surface coverage of 0.12, adsorption data were consistent with AsH ₃ bound to the surface as a mobile film. The formation of GaAs via CH₄ elimination from (CH₃)₂GaAsH₂ or CH₃GaAsH was hindered by deposition of films of (CH₃)₃-x GaAsH₃-x even at 420°C. This was most significant for formation of GaAs (or even CH₃GaAsH) from (CH₃)₂-GaAsH₂ formed at 203°C and then heated at 420°C. The product surfaces also served as a catalyst for decomposition of AsH₃ to form H₂ and decomposition of (CH₃)₃Ga to form CH₄.     

Morphology and Texture of Silica Prepared by Sol–Gel Synthesis on the Surface of Fibrous Carbon Materials

Silica materials are synthesized by the sol–gel method including the deposition of tetraethoxysilane on various micro- and nanocarbon fibers. The use of nanofibrous carbon as a template makes it possible to prepare thermally stable mesoporous SiO₂ samples with unusually high surface areas (up to 1255 m²/g) and high porosity (up to 5.6 cm³/g). These silica materials and aerogels prepared by supercritical drying have comparable pore volumes. It is found by high-resolution electron microscopy that a thin-wall matrix permeated by channels is a prevailing structure of silica materials. When some catalytic fibrous carbons are used as templates, silica nanotubes can be prepared.     

Synthesis of Polysiloxane Xerogels Using Tetraethoxysilane/(Diethylphosphatoethyl)triethoxysilane System

The xerogels, containing phosphonic acid groups ≡Si(CH₂)₂P(O)(OC₂H₅)₂ in the surface layer of their particles, are synthesized by the sol-gel method (ethanol as a solvent and fluoride ion as a catalyst). It is shown that, when 2 : 1, 3 : 1, and 4 : 1 tetraethoxysilane/(diethylphosphatoethyl)triethoxysilane ratios are used, nonporous substances are formed, whereas, at 6 : 1, 8 : 1, and 10 : 1 ratios, the xerogels with highly porous structures are produced (the specific surface area is 505–534 m²/g, the sorption pore volume is 0.34–0.53 cm³/g, and the pore diameter is 3.6–4.6 nm).     

Sol–gel synthesis of Zn-thiourea-SiO2 thin films from (EtO)3Si(CH2)3NHC(S)NHPh as molecular precursor

In order to study new and convenient sol–gel syntheses to homogeneous nanocomposites systems based on zinc sulphide clusters embedded in films of silica glass, a thiourea-functionalized silane ((EtO)₃Si(CH₂)₃NHC(S)NHPh, SilTu) was investigated as starting precursor. Electrospray ionization mass spectrometry (ESI-MS), FT-IR and NMR spectroscopies clearly evidence that SilTu coordinates Zn²⁺ ions in solution through the thiourea function. Both zinc acetate (Zn(CH₃COO)₂) and zinc chloride (ZnCl₂) were used as zinc source. The SilTuZn²⁺ complex is also stable under sol–gel conditions. Alcoholic solutions of SilTu, zinc acetate and tetraethoxysilane Si(OEt)₄ (TEOS) were used to prepare the thin films by dip-coating. Transparent, homogeneous and crack-free layers were obtained under annealing up to 600 °C. The samples were investigated by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). Beside ZnS formation, the presence of oxygenated zinc species was observed inside the silica matrix.     

One-Pot Synthesis and Formation Mechanism of Hollow ZSM-5

Coffin-shaped hollow ZSM-5 zeolite (HZZ) particles with shell thickness of about 200 nm and hollow diameter of approximately 1.5 μm were synthesized in one pot by using tetrapropylammonium bromide (TPABr), aluminum triisopropoxide Al[OCH(CH₃)₂]₃ and tetraethoxysilane (TEOS) as the structure-directing agent (SDA), aluminum and silica source, respectively. The appropriate molar ratios of TPABr/SiO₂ and Si/Al as well as suitable crystallization temperature are the key factors for the formation of HZZ. The formation of the HZZ can be attributed to the existence of intrinsic density variation inside the initial amorphous aggregates and the Al zoning in the outer surface of the ZSM-5 particles. Amorphous silica with low crystallinity formed at early stages and low Al concentration, which has been subsequently dissolved and recrystallized on the ZSM-5 particle surface through Ostwald ripening, leading to the formation of HZZ. This approach, which uses a high concentration of SDA, will provide new possibilities and insight into the prospective fabrication of hollow zeolites.     

Role of the concentration and nature of grafted groups in the adsorption of hydrocarbon vapors on silica modified by monofunctional polyfluoroalkylsilanes

The role of the grafting density of monofunctional polyfluoroalkylsilanes of the C _n F_{2n − 1}(CH₂) _m Si(CH₃)₂Cl general formula (where n = 3, 4, and 6; and m = 2 and 3) and their composition in intermolecular interactions of the molecules of saturated and aromatic hydrocarbons with a surface of chemically modified silica is studied by means of IR spectroscopy and adsorption-static and gas chromatography. It is shown that the higher the concentration and the shorter the length of the grafted chain, the greater (by a factor of 2 to 25) the drop in the adsorption values of hydrocarbons as a result of modifications, due to an increase in the degree of oleophobization of surface upon the formation of polyorganofluorine coatings. The high specificity of the surface with respect to benzene, which is due to the active participation of the polar fragment of a grafted chain in adsorption process, is related to the features of a relatively low-density sample with a concentration of grafted perfluorobutyl groups of 1.7 nm⁻². It is shown that the thermodestruction of polyfluoroalkyl silica remains virtually unobserved upon heating to 523 K in an argon flow.     

The template synthesis of mesoporous silicas with the thiourea functional group

Mono- and bifunctional mesoporous silicas containing the thiourea group ≡Si(CH₂)₃NHC(S)NHC₂H₅ or thiourea and various nitrogen-containing groups [≡Si(CH₂)₃NH₂, ≡Si(CH₂)₃NH(CH₂)₂NH₂, or {≡Si(CH₂)₃}₂NH], respectively, in the surface layer were prepared by template syntheses with cetylpyridinium chloride as a template. The synthesized samples had well-developed porous structures (S _sp = 750–1150 m²/g, V _c = 0.51–0.72 cm³/g, and d = 2.4–3.5 nm) and high functional group contents (1.0–2.0 mmol/g). Hydrothermal treatment of the mesophases in a mother liquor at 80°C (24 h) improved the structure-adsorption characteristics of the functionalized mesoporous silicas.     

Influence of activated carbon in zeolite X/activated carbon composites on CH<Subscript>4</Subscript>/N<Subscript>2</Subscript> adsorption separation ability

A series of zeolite X/activated carbon composites with different ratio of zeolite X and activated carbon were prepared, which were adjusted by adding solid pitch powder and silicon dioxide as additional carbonaceous and silica source, respectively. The corresponding modified samples were obtained by treatment with the ammonium chloride solution. CH₄ and N₂ adsorption isotherms on all composites were determined within the pressure of 0–100 kPa at 298 K, and fitted with Henry model and Freundlich model. The results showed the adsorption separation abilities for CH₄ and N₂ were strongly influenced by activated carbon content, micropore structure and surface properties. The increase of activated carbon content increased the BET surface area, micropore surface area and micropore volume, leading to an enhanced CH₄ adsorption capacity and CH₄/N₂ adsorption selectivity. Compared with the unmodified composites, the modified composites showed higher CH₄/N₂ adsorption selectivity, and CH₄ adsorption capacity decreased slightly, which can be attributed to the reduction of the micropore structure parameters, the surface basic amount and basic strength. Furthermore, the modified composite HAX-3 presented the highest CH₄/N₂ selectivity of 3.4, and high CH₄ adsorption capacities, which is favorable for application in pressure swing adsorption processes.     

Synthesis and characterization of cationic silsesquioxane hybrids by hydrolytic condensation of triethoxysilane derived from 2-(dimethylamino)ethyl acrylate

A new family of silsesquioxane hybrids was synthesized by hydrolytic condensation of a triethoxysilane precursor, R-Si(OCH₂CH₃)₃, R = -CH₂CH₂CH₂N[CH₂CH₂COOCH₂CH₂N(CH₃)₂]₂, derived from 2-(dimethylamino)ethyl acrylate. Condensation of the triethoxysilane precursor proceeded as a homogeneous system in methanol in the presence of aqueous HF solution (3.2?%) to afford the water-soluble silsesquioxane hybrid having a high density of chemically bonded peripheral tertiary amino groups on the outermost surface, as confirmed by nuclear magnetic resonance and Fourier transform infrared analyses. The relatively low polydispersity (M _w/M _n ?=?1.33) and a reasonable molecular weight (M _n ?=?2700), corresponding to species having 6–12 silicon atoms, were confirmed by size exclusion chromatography. The size of the silsesquioxane hybrid (1.7?nm) was also determined by X-ray diffraction. Co-condensation of tetraethoxysilane (TEOS) with the triethoxysilane precursor was carried out under different feed ratios, and water-soluble products were obtained in the cases of TEOS molar ratio up to 40?%. Quaternization reaction of the tertiary amine-containing hybrids with methyl iodide led to cationic silsesquioxane hybrids containing quaternized amine functionalities, which showed good solubility in polar solvents. Scanning force microscopy measurements indicated the formation of the cationic silsesquioxane hybrids having relatively narrow size distribution with average particle diameter (about 2.0?nm) without aggregation.     

Surface Chemistry of Nanohybrids with Fumed Silica Functionalized by Polydimethylsiloxane/Dimethyl Carbonate Studied Using 1H, 13C,and 29Si Solid-State NMR Spectroscopy

The investigation of molecular interactions between a silica surface and organic/inorganic polymers is crucial for deeper understanding of the dominant mechanisms of surface functionalization. In this work, attachment of various depolymerized polydimethylsiloxanes (PDMS) of different chain lengths, affected by dimethyl carbonate (DMC), to silica nanoparticles pretreated at different temperatures has been studied using ²⁹Si, ¹H, and ¹³C solid-state NMR spectroscopy. The results show that grafting of different modifier blends onto a preheated silica surface depends strongly on the specific surface area (SSA) linked to the silica nanoparticle size distributions affecting all textural characteristics. The pretreatment at 400 °C results in a greater degree of the modification of (i) A-150 (SSA = 150 m²/g) by PDMS-10/DMC and PDMS-1000/DMC blends; (ii) A-200 by PDMS-10/DMC and PDMS-100/DMC blends; and (iii) A-300 by PDMS-100/DMC and PDMS-1000/DMC blends. The spectral features observed using solid-state NMR spectroscopy suggest that the main surface products of the reactions of various depolymerized PDMS with pretreated nanosilica particles are the (CH₃)₃SiO-[(CH₃)₂SiO-]_x fragments. The reactions occur with the siloxane bond breakage by DMC and replacing surface hydroxyls. Changes in the chemical shifts and line widths, as shown by solid-state NMR, provide novel information on the whole structure of functionalized nanosilica particles. This study highlights the major role of solid-state NMR spectroscopy for comprehensive characterization of functionalized solid surfaces.     

Standard nitrogen adsorption data for α-alumina and their use for characterization of mesoporous alumina-based materials

Nitrogen adsorption isotherm measured at ?196 °C for a macroporous α-alumina (α-Al₂O₃) is reported. This isotherm is compared with the previously reported adsorption data measured on LiChrospher 1000 silica and with available reference isotherms measured at moderate and high relative pressures on macroporous aluminas. The isotherm reported in this work for α-Al₂O₃ and that recorded previously on LiChrospher 1000 silica were used as reference data for adsorption characterization of ordered and disordered mesoporous aluminas by α _s -plot analysis and pore size analysis. It is shown that both reference isotherms provide almost identical adsorption characteristics of the aforementioned mesoporous aluminas, indicating that the available reference data for the silica surface are also suitable for adsorption analysis of alumina-based materials.     

S-layer templated bioinspired synthesis of silica

The current understanding of the molecular mechanisms involved in the bioinspired formation of silica structures laid foundation for investigating the potential of the S-layer protein SbpA from Lysinibacillus sphaericus CCM 2177 as catalyst, template and scaffold for the generation of novel silica architectures. SbpA reassembles into monomolecular lattices with square (p4) lattice symmetry and a lattice constant of 13.1 nm. Silica layers on the S-layer lattice were formed using tetramethoxysilane (TMOS) and visualized by transmission electron microscopy. In situ quartz crystal microbalance with dissipation monitoring (QCM-D) measurements showed the adsorption of silica in dependence on the presence of phosphate in the silicate solution and on the preceding chemical modification of the S-layer. An increased amount of precipitated silica could be observed when K₂HPO₄/KH₂PO₄ was present in the solution (pH 7.2). Further on, independent of the presence of phosphate the silica deposition was higher on S-layer lattices upon activation of their carboxyl groups with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) compared to native S-layers or EDC treated S-layers when the activated carboxyl groups were blocked with ethylene diamine (EDA). Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy revealed the formation of an amorphous silica gel (SiO₂)_x·yH₂O on the S-layer. The silica surface concentrations on the S-layer was 4 × 10^?9 to 2 × 10^?8 mol cm^?2 depending on the modification of the protein layer and corresponded to 4–21 monolayers of SiO₂.     

Adsorption of organic vapor on trimethylsilylated silicas

Adsorption of various classes of organic compounds has been studied on silicas chemically modified by silanes that contain a trimethylsilyl group: (CH₃)₃Sil(TMS) and (CH₃)₃Si(CH₂)₃(CH₃)₂SiCl (C3TMS). The effect caused by the nature of the adsorbate and grafted groups on the thermodynamic parameters of adsorption has been studied. Surface modifying is shown to dramatically decrease the Henry constants of n-alkane adsorption equilibrium relative to the unmodified matrix. The contribution of specific interactions to adsorption, estimated by different methods, in the TMS-silica sample is higher than in the C3TMS-silica sample.