Mesoporous Silicate Materials Research Articles

Simple synthesis and characterization of nanoporous materials from talc

AbstractSynthetic siliceous mesoporous materials are of great value in many different applications, including nanotechnology, biotechnology, information technology, and medical fields, but historically the resource materials used in their synthesis have been expensive. Recent efforts have focused on indirect synthesis methods which utilize less expensive silicate minerals as a resource material. The purpose of the present study was to investigate talc, a natural silicate mineral, as one such resource. It was used as raw material to prepare two advanced materials: porous silica (PS) and ordered mesoporous silica (MCM-41). The PS, with a specific surface area of 260 m2/g and bimodal pore-size distribution of 1.2 nm and 3.7 nm, was prepared by grinding and subsequent acid leaching. The MCM-41, with a large surface area of 974 m2/g and a narrow pore-size distribution of 2.8 nm, was obtained using a surfactant, cetyltrimethylammonium bromide (CTAB), by hydrothermal treatment using the as-prepared PS as a source of Si. The two resultant materials were characterized by small angle X-ray diffraction (SAXRD) and wide-angle X-ray diffraction (WAXRD), high-resolution transmission electron microscopy (HRTEM), solid-state magic-angle-spinning nuclear magnetic resonance (MAS NMR), Fourier transform infrared spectroscopy (FTIR), and N2 adsorption-desorption measurements. Based on these measurements, possible processes of transformation of PS from talc, upon acid treatment, and the formation of MCM-41 were investigated systemically. Acid leaching induced the transformation of a rigid layered structure to a nearly amorphous one, with micropores formed by a residual layered structure and mesopores formed from a condensed framework. The MCM-41 was a mixture of silanol groups (Si(SiO)3(OH)) and a condensed Q4 framework structure (Si(SiO)4), with a small amount of remaining Q3 layered structure (Si(SiO)3OMg). The increased Q4/Q3 value confirmed greater polymerization of MCM-41 than of PS. At the low CTAB concentration used (2 wt.%), the highly charged silicate species controlled the surfactant geometry. Charge-density matching, together with the degree of polymerization of the silicates, determined the resultant mesophase.

A Merit Synthesis of Well-Ordered Two-Dimensional Mesoporous Niobium Silicate Materials with Enhanced Hydrothermal Stability and Catalytic Activity

An environmentally friendly and efficient hydrothermal method has been used for the synthesis of mesoporous NbSBA-15 materials. To find better hydrothermal stability, the synthesized NbSBA-15 materials have been treated in boiling water and water steam at different temperatures with various times. The NbSBA-15 materials have been characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), N2 adsorption, UV-visible diffuse reflectance spectroscopy (UV−vis DRS), field emission-scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). To investigate the effect of structural and textural properties with a higher Nb incorporation, NbSBA-15 has also been synthesized at different pH in sol−gel solution. The results of ICP-AES, TEM-EDS, and XPS show that a nSi/nNb ratio up to 6.7 is successfully prepared by changing the nSi/nNb ratio in the synthesis gel at pH 2.2. For all calcined NbSBA-15 materials, XRD reflection peaks shift to lower angle with increasing Nb content, resulting in the increase of the unit cell parameter. With increase in the incorporation of Nb in the framework of SBA-15, nitrogen adsorption measurement shows that the pore diameter of NbSBA-15 increases from 71.7 to 75.0 Å, the pore volume increases from 1.22 to 1.24 cm3/g with increasing surface area from 788 to 945 m2/g, and pore wall thickness increases from 32.3 to 35.13 Å. UV−vis DRS and XPS measurements show that not only the synthesized NbSBA-15(2.2pH) has a superior hydrothermal stability but also a higher amount of tetrahedral Nb5+ species is incorporated into SBA-15 with well-ordered mesoporous structure. TEM and FE-SEM images show the uniform pore diameter and ropelike hexagonal mesoporous structure of NbSBA-15. Furthermore, the NbSBA-15 materials have been used as the catalysts for epoxidation of cis-cyclooctene, and in this study, NbSBA-15(2.2pH) has a higher catalytic activity.

Contribution to the external surface of a titanium-rich sand (Abou-Khashaba, Egypt) in the uranium uptake processes

The present study reports the U(VI) uptake from solution on a Ti-rich Egyptian black sand, showing that the crystal surfaces exhibit a marginal influence to the total process of U(VI) uptake onto Ti-containing micro- and mesoporous silicate materials. The results were evaluated using Lagergren first order equation and the apparent thermodynamic parameters of uranium uptake onto the considered materials were calculated.

Solvation Forces Between Silica Bodies in Supercritical Carbon Dioxide

We report Monte Carlo simulations of the solvation pressure between two planar surfaces, which represent the interface of spherical silica nanoparticles in supercritical carbon dioxide. Carbon dioxide (CO2) was modeled as an atomistic dumbbell or a spherical Lennard-Jones particle. The interaction between CO2 molecules and silica surfaces was characterized by the standard Steele potential with energetic heterogeneities representing the hydrogen bonds. The parameters for the solid-fluid interaction potentials were obtained by fitting our simulations to the experimental isotherms of CO2 sorption on mesoporous siliceous materials. We studied the dependence of the solvation force on the distance between planar silica surfaces at T = 318 K, at equilibrium bulk pressures p(bulk) ranging from 69 to 200 atm. At 69 atm, we observed a long-range attraction between the two surfaces, and it vanished when the pressure was increased to 102 and then 200 atm. The results obtained with different fluid models were consistent with each other. According to our observations, energetic heterogeneities of the surface have negligible influence on the solvation pressure. Using the Derjaguin approximation, we calculated the solvation forces between spherical silica nanoparticles in supercritical CO2 from the solvation pressures between the planar surfaces.

Micron-Sized Phenyl-PMO Materials with a Crystalline Wall

The preparation of pure siliceous mesoporous materials with an atomic-scale crystalline wall is an illusive task even though the mesoporous material researches flourish recently because of their various intriguing applications. Periodic mesoporous organosilica (PMO) materials are nanoporous hybrid materials containing organic and inorganic moieties on the pore wall simultaneously. Single crystal-like PMO consisted of phenyl (–C6H4–) moieties was firstly reported by Inagaki et al. However, it was claimed that very strict reaction conditions such as an alkaline condition and a narrow range of molar ratio of the reactants were mandatory to produce single crystal-like phenyl-PMO materials. This rather strict synthetic condition hampers further studies to control the particle morphology and size variation of the PMO materials with intrinsically hydrophobic interior because of the constituent phenyl moieties on the wall. For example, most phenyl-PMO derivatives were prepared via quite similar reaction conditions originally proposed by Inagaki et al. Therefore, we have been interested in finding a reliable synthetic method to fine-tune their particle size and shape suitable for various advanced applications such as controlled drug delivery system, host materials for hydrophobic biomolecule encapsulation, and robust catalyst supports. We speculated that the π...π stacking interaction between two adjacent benzene moieties of different 1,4-bis(triethoxysilyl)benzene (BTEB) precursors might be strong enough to induce the single crystalline wall structure in a variety of reaction conditions unlike the original report. Therefore, we investigated various synthetic conditions such as different reactant molar ratios and precursor concentrations to prepare phenyl-PMOs having different particle size and morphology with an aforementioned single crystalline nature of the pore wall. We were particularly interested in the size control of the particles without perturbing the crystallinity of the pore wall. Herein, we describe a preliminary result of the preparation of size-controlled phenyl-PMO material with molecularly ordered pore walls. Our attempts closely relate to the literature method of preparing shape-controlled organic-functionalized micronsize MCM-41 type of hybrid silica materials. For example, an optimized reactant molar ratio is C18TMABr:BTEB: NaOH:H2O = 1.0:1.2:6.1:5644 where C18TMABr is an octadecyltrimethylammonium bromide, CH3(CH2)17N(CH3)3Br. The low relative molar ratio of C18TMABr to water was crucial to obtain small micron-size particles. The C18TMA templates of the as-made material were easily removed by an acid extraction in ethanol solution at 60 C. TEM images of the template-removed samples are shown in Figure 1. Under the low C18TMABr concentration, the products were formed as micron-size particles (average length or diameter < 3 μm) with the parallel pore direction relative to the longitudinal axis of the particle as shown in Figure 1. The mesopore orientation of the previously reported phenylPMO materials normally exhibited a perpendicular pore direction relative to the flat surface of the thin flake. In our case, however, the low surfactant concentration as well as a short period of stirring after the addition of BTEB precursors might prevent surfactant-organosilica micelles from aggregating to form large flakes. As a result, small particles with a parallel pore orientation as well as a crystal-like wall structure have been successfully obtained. Interestingly, individual nanotube-like uniform mesochannels were clearly discerned from the TEM images of Figures 1(b) and 1(c). SEM images of Figure 1 indicate the shape of particles and their size distribution. The powder XRD pattern of the template-free material, top trace of Figure 2, exhibited higher angle diffraction peaks at 2θ = 11.70, 23.36, and 35.48 which correspond to

Titanium and cerium-containing mesoporous silicate materials as catalysts for oxidative cleavage of cyclohexene with H2O2: A comparative study of catalytic activity and stability

Abstract Catalytic properties of transition-metal-containing silicate materials with hexagonally packed mesopore-channels, Ti-MMM-2 and Ce-SBA-15, have been studied in oxidative cleavage of cyclohexene (CyH) to adipic acid using aqueous H2O2 as oxidant. In a solvent-free system, the yield of adipic acid reached 10–15% at H2O2/CyH 3.6 mol mol−1 and 80 °C after 72 h. The stepwise addition of the oxidant to the reaction mixture allowed increasing the yield of the target product up to 33 and 18% for Ti-MMM-2 and Ce-SBA-15, respectively. For both catalysts, the yield of adipic acid decreased significantly in the second run despite CyH conversion attained 100%. The catalyst stability and the reasons of the deactivation have been explored. For both catalytic materials, partial decrease of the mesopore surface area and mesopore volume was observed after the CyH oxidation run. DRS–UV–vis study revealed irreversible formation of oligomerized TiO2-like species on the Ti-MMM-2 surface due to interaction of Ti centres with the oxidant and the reaction products. The elemental analysis data showed that adipic acid causes leaching of titanium from Ti-MMM-2 into solution. Ce-SBA-15 was found to be more stable with respect to the active metal leaching but prone to deactivation due to agglomeration of cerium ions to form cerium oxide nanocrystallites; partial reduction of Ce(IV) to Ce(III) also occurred.

Mesoporous SBA-15 Supported Iron Oxide: A Potent Catalyst for Hydrogen Sulfide Removal

A novel silicate mesoporous material, SBA-15 supported Fe2O3, was synthesized by post-synthesis method via ultrasonic-assisted route. The desulfurization test from a gas mixture containing 0.1 vol% H2S was carried out over SBA-15 supported Fe2O3 in a fixed-bed system at atmospheric pressure and room temperature. The effects of the chemical nature of Fe2O3 and the textural properties of the material on desulfurization capacity were studied. Materials before and after the desulfurization test were characterized using nitrogen adsorption, XRD, TEM, FTIR, XPS, ICP and other standard methods. The characterization results suggest that modification process does not change the two-dimensional hexagonal mesostructure of SBA-15. Iron species disperses inside channels and the outside surface in the crystalline phase of iron oxide. The material with iron content of 31.3 wt% presented highest H2S uptake capacity. Structural properties of the material also play important roles in desulfurization performance besides the catalytic effects of iron oxide. The basic feature of material and enough oxygen supply are benefit for the reaction. SBA-15 supported Fe2O3 can be an effective alternative to capture H2S from gas streams.

Chromatographic determination of the differential isosteric adsorption enthalpies and differential entropies on ordered silica materials

The adsorption properties of the ordered mesoporous siliceous materials: MCM-41C16 (denoted as C16), MCM-41C16-SH and MCM-41C16-NH 2 (known as MCMs) having different surface functionalities were studied by inverse gas chromatography to assess their suitability for adsorption of analytes from gas and liquid phases. Polar and non-polar adsorbates were employed. The differential isosteric enthalpies, −Δ H ads, and differential entropies, −Δ S ads, of adsorption of different ‘molecular probes’ were determined chromatographically. A mathematical link between the −Δ H ads, and −Δ S ads magnitudes and experimental data was derived through an Antoine-type equation. The present studies have been entirely restricted to the region of low adsorbate concentration. The problem of the interrelationship between the −Δ H ads, and −Δ S ads values, known as the ‘thermodynamic compensation effect’, and interpretation of chromatographic data for the adsorption of different adsorbates on the MCMs have been considered in the light of both experimental data obtained in the present studies and the data available in the literature for siliceous adsorbents with randomly ordered structures. It was shown chromatographically that there is substantial parallelism between the magnitudes of the differential isosteric enthalpy and differential entropy for some ‘molecular probes’ chromatographed on C16 and its derivatives. Complementary information was obtained by atomic-force microscopy (AFM), X-ray photoelectron spectroscopy and X-ray diffraction (XRD) spectroscopy.

Chemisorption of hydrogen sulphide on zinc oxide modified aluminum-substituted SBA-15

Aluminum silicate mesoporous material, ZnO/Al–SBA-15, was synthesized by post-synthesis and immobilization method via microwave-assisted route. Desulphurization tests from a gas mixture with low content H 2S were carried out as the probing reaction on these materials. Chemical effects and the nature of the ZnO additive and textural properties on desulphurization capacity were studied over this material. Material was characterized using N 2 adsorption, XRD, TEM, FTIR, XPS, ICP and other techniques. The analysis suggests that the as-synthesized material had well-ordered hexagonal mesopores and was abundant in micropores. ZnO nanoparticles dispersed well and anchored both in the channel and the wall of mesoporous silica. The material with 2.1 wt.% zinc loading presented the highest H 2S uptake capacity. Both micropores and mesopores are active sites for H 2S capture, especially micropores. The enhancement of H 2S removal capacity was attributed to the integration of the pore structure of mesoporous material and the promising desulphurization properties of ZnO nanoparticles. ZnO/Al–SBA-15 could be an effective alternative to remove H 2S from gaseous streams and it also extends the research of mesoporous material.

Conversion of Ethene to Propene on Nickel Ion-loaded Mesoporous Silica Prepared by the Template Ion Exchange Method

Although surface of amorphous silica is widely accepted to be neutral or slightly acidic, siliceous mesoporous materials have been found to act as excellent acid catalysts by us and other groups. On the other hand the increment in propene production is one of the significant objectives to be achieved in the present petroleum chemistry. Especially the one-path conversion of ethene to propene (ETP reaction, 3C2H4 → 2C3H6) is the most desired process. In our efforts, the specific acid catalysis of mesoporous silica was combined with the catalysis of nickel ion, which could turn the new type of ETP reaction into reality. The one-path conversion of ethene was 68% and the propene selectivity was 48% with 43% of butenes in a continuous gas-flow system at 673 K and an atmospheric pressure. The reactivity of lower olefins and the dependences of the ETP reaction on the contact time and the partial pressure of ethene followed the reaction mechanism consisting of dimerization of ethene to 1-butene, isomerization of 1-butene to 2-, and metathesis of 2-butene and ETP. The Ni-loaded mesoporous silica catalysts were characterized mainly by EXAFS and TPR techniques. The local structures of the nickel species active for the ETP reaction were very similar to that of layered nickel silicate, while those on the inert catalysts were the same as that of NiO particles.

Influence of the acidity of nanostructured CoMo/P/Ti-HMS catalysts on the HDS of 4,6-DMDBT reaction pathways

The influence of catalyst acidity on the reaction mechanism of the 4,6-dimethyldibenzothiophene (4,6-DMDBT) hydrodesulfurization (HDS) over CoMo hydrotreating catalysts was studied using a batch autoclave and fixed-bed reactor (T=598K and P=5.5MPa). P-free Ti-HMS and P-containing P/Ti-HMS mesoporous siliceous materials were synthesized and used as supports. The effect of the catalyst preparation method (successive vs. simultaneous impregnation) and phosphorous addition on catalyst acidity was studied by TPD-NH3 and DRIFT-NH3 techniques. For all synthesized catalysts, the reaction proceeds via dealkylation (DA) and isomerization (ISO) pathways, with the later being the main reaction route. Incorporation of Co and Mo phases by simultaneous impregnation was found to be the best method for catalyst preparation, whereas P-addition promote the isomerization route of 4,6-DMDBT transformation to a greater extent than the dealkylation route. On the contrary, the reaction on a CoMoP/γ-Al2O3 reference catalyst proceeds via HYD and DDS reaction pathways, the later being the main reaction route. The catalyst acidity–activity correlation indicates that both activity and selectivity depend largely on the presence of Brønsted acid sites as well as on the total amount of Brønsted and Lewis acid sites. By correlating HRTEM-activity data, the enhancement of activity and isomerization observed with the catalyst prepared by simultaneous impregnation and modified with P was related to the cumulative effects of the lower size of MoS2 slabs and their higher surface density.

Preparation and characterization of vanadia catalysts supported on non-porous, microporous and mesoporous silicates for oxidative dehydrogenation of propane (ODP)

The present study focuses on the synthesis and characterization of vanadia catalysts supported on non-porous (SiO 2), microporous (silicalite), mesoporous (MCM-41, HMS, SBA-15) and ultra-large mesoporous (MCF) silicate materials. The structure and porosity characteristics of the above mesoporous supports were verified by means of XRD, N 2 porosimetry and HR-TEM studies. Vanadium was introduced in the porous materials via wet impregnation of vanadium oxalate complex in strongly acidic aqueous media. No crystalline V 2O 5 phases were found on the surface of all the above mesoporous supports for V loadings as high as 8 wt.%, in contrast to microporous zeolite silicalite and non-porous SiO 2 on which V 2O 5 crystals were formed as evidenced by XRD. Increasing addition of vanadium under the strongly acidic impregnation conditions followed by typical calcination of the catalyst, resulted in partial structure breakdown and loss of surface area of the mesoporous supports which was more pronounced in the case of SBA-15 and MCF materials. The most active catalysts for the formation of propene in the oxidative dehydrogenation of propane (ODP) at reaction temperatures 500–600 °C were those supported on HMS and MCM-41 mesoporous materials showing high propene yields up to 19% at about 600 °C. The vanadia catalysts based on SBA-15 and MCF were less active in the production of propene (yields up to 13%) compared to the MCM-41 and HMS based catalysts. The SiO 2 and silicalite based catalysts showed much lower yields of propene (<7%).

Synthesis and characterization of P-modified mesoporous CoMo/HMS–Ti catalysts

A hexagonal mesoporous siliceous material with a wormhole framework structure incorporating Ti (HMS-Ti; Si/Ti atomic ratio of 40) was modified with variable amounts of phosphorous and used as support for CoMo phases. The catalysts were prepared by successive impregnation, with Mo being introduced first. The supports and catalysts were characterized by N 2 adsorption–desorption, High-resolution transmission electron microscopy, X-ray diffraction, FT-IR study of the framework vibrations, DRIFT spectra in the OH region, 1H NMR, FT-IR spectra of adsorbed NO, micro-Raman spectroscopy and X-ray photoelectron spectroscopy. The catalysts were tested in the reaction of hydrodesulfurization (HDS) of dibenzothiophene (DBT) and their activity compared with that of a commercial P-containing CoMo/γ-Al 2O 3 catalyst. The physical and chemical characterization of the P-modified HMS–Ti substrates shows that the presence of P 2O 5 on the support surface does not change its mesoporous character, but modifies its surface properties. In addition, characterization data of the oxide catalysts show that phosphate favors the dispersion of the active phases and increases the population of octahedral Co 2+ ions associated to Mo species. As a result, HDS activity was strongly enhanced upon P-loading, which reached a maximum of 0.64 wt%. This catalyst is 3.7 times more active than the commercial one and 2.4 times more active than its P-free counterpart. The highest activity of this catalyst was explained in terms of the specific electronic properties of its active phases and the largest Mo surface exposure on the support.

Direct propylene epoxidation over barium-promoted Au/Ti-TUD catalysts with H 2 and O 2: Effect of Au particle size

A catalyst consisting of gold supported on a Ti-containing silicate mesoporous material (TUD) with 13-nm pores was used for the epoxidation of propylene with mixtures of H 2 and O 2. The catalyst activity was enhanced by the addition of a Ba promoter. The pH of deposition was important in controlling the gold loading and particle size. A pH of 7 gave a Au loading of 2.7 wt% and an average particle size of 2.0 nm, as determined by transmission electron microscopy, whereas a pH of 9 produced a much lower Au loading of 0.11 wt% and smaller particles of size about 0.9 nm, as estimated by X-ray absorption fine structure measurements. At 423 K and 0.1 MPa total pressure, the catalyst prepared at pH 7 gave a steady-state propylene conversion of 2.1%, a propylene oxide (PO) selectivity of 79%, and a H 2 efficiency of 3.8%, whereas that prepared at pH 9 gave a conversion of 1.4%, a PO selectivity of 99%, and a H 2 efficiency of 17%. The turnover frequency for PO production based on total gold increased on going to the sample prepared at pH 9. Thus, it is concluded that very small Au particles (about 1 nm) are the most active for epoxidation, whereas larger Au particles (about 2 nm) are less active because they promote direct H 2 oxidation to H 2O. X-ray absorption near-edge spectroscopy results indicated that at reaction conditions, the small particles had partially oxidized gold but the larger particles had metallic gold, suggesting that the smaller particles had high coverage of oxygen or oxygen-derived species.

Mesoporous silicate materials as substrates for molecular machines and drug delivery

Abstract Mesoporous silica thin films and nanoparticles prepared by surfactant-templated sol–gel techniques are versatile substrates that can be easily derivatized with active molecules to create functional materials. By exploiting the chemical and physical differences that exist in different regions of the mesostructure, active molecules can be deliberately placed using one-pot techniques, or they can be tethered to the exposed surfaces post-synthetically. The methods available for functionalization have been used to design operational machines including nanoimpellers based on the dynamic photoisomerization of azobenzene, and nanovalves based on the switchable motion of supramolecular rotaxanes and pseudorotaxanes. The ability of nanoimpellers and nanovalves to control the release of molecules from the pores of mesoporous silica materials is demonstrated using luminescence spectroscopy. These machines can be designed to operate under a range of external stimuli, including light, electrical (redox) or chemical (pH, competitive binding) energy, making them useful systems for a variety of controlled release applications. Mesoporous silica nanoparticles not functionalized with molecular machines are capable of delivering water-insoluble anticancer drugs to cancer cells. Carefully designed nanoimpellers and nanovalves supported on mesoporous silica nanoparticles offer the ability to develop sophisticated drug delivery vehicles for a wide range of drug molecules.

Applications of Pore-Expanded Mesoporous Silica. 7. Adsorption of Volatile Organic Compounds

Three different varieties of mesoporous silicas were synthesized by varying the postsynthesis treatment of an as-synthesized ordered mesoporous material type MCM-41. The resulting materials consisted of a purely siliceous MCM-41, a pore-expanded MCM-41 (PE-MCM-41C), and a surfactant-laden pore-expanded MCM-41 (PE-MCM-41E) and were evaluated as adsorbents for two types of volatile organic compounds, i.e., chlorinated and aromatic hydrocarbons. Values of heat of adsorption and Henry's law constant were determined by pulse chromatography. Additionally, adsorption capacities were calculated with a dynamic method using breakthrough curves for single components in dry and humid environments. The surfactant-containing material exhibited good compatibility with chlorinated compounds in terms of heat of adsorption and efficiency in gaseous streams containing moisture. Purely siliceous mesoporous materials, i.e., MCM-41 and PE-MCM-41C, were more selective toward aromatic hydrocarbons but also gave rise to exceptionally strong adsorption.

Self-Organized Meso- and Hybridic Phases of Poly(aspartic acid) and Poly(glutamic amino acid) with Cationic Surfactants (CnTAB, n = 14, 16) and a Silica Source (TEOS)

In this work the self-organized mesophases obtained by the ionic interaction of poly(amino acids) poly(l-aspartic acid (P-l-Asp) and poly(l-glutamic acid) (P-l-Glu) with the cationic surfactants C14TAB and C16TAB, as well as the mesoporous silicate materials obtained by addition and hydrolysis of tetraethyl orthosilicate (TEOS) on the hybridic mesophases, are described. The mesophases isolated at pH 6.5−8.5 were examined by X-ray diffraction (XRD) analysis for their structure, thermogravimetry for their thermal stability, scanning electron microscopy for their structural features, and circular dichroism (CD) for the conformation of the polypeptide−CnTAB (n = 14, 16) complexes. The mesophases P-l-Asp/C14TAB isolated at pH 7.5 and 8.5 showed in XRD a single peak at d = 3.45 nm and d = 3.54 nm, respectively, corresponding to typical hexagonal packing of the micellar aggregates. The mesophases P-l-Asp/C16TAB isolated at pH 7.5 showed an XRD peak at d = 3.90 nm, corresponding to a similar hexagonal packing of ...

Processing of mesoporous silica materials (MCM-41) from coal fly ash

Mesoporous silica is an important material which has various industrial applications. Currently, it is produced from precursors that are costly. In this paper, the successful preparation of siliceous mesoporous materials MCM-41 with pores of ordered 2-dimensional hexagonal structure by condensation–polymerization using quaternary ammonium salt as structure directing agent and supernatant of the coal fly ash (CFA) as silica precursor is described. The synthesized materials had high surface area of 732 m 2/g and median pore diameter of 2.5 nm. X-ray diffraction analyses revealed that fusion processes are important in extracting silica from quartz in CFA. SEM images of fused CFA revealed two different morphologies of agglomerated spheres attributable to the fused CFA and distorted octahedral grains to the faujasite phase in comparison to the cenospheres morphology of the as-received CFA. MCM-41 was also produced from pure silica source for comparison. The results obtained in the present work demonstrate the feasibility of employing coal fly ash as a potential source of silica to produce MCM-41 mesoporous materials.

Microporosity, pore anisotropy and surface properties of organized mesoporous silicates (OMSi) containing cobalt and cerium

Nanostructured, organized mesoporous silicate materials, containing cerium plus cobalt, were synthesized from a system based on polyacrylic acid (Pac), C16TAB and TEOS. The synthesis took place along a titration path from low pH (≈2) to high pH (≈10). Along this path the C16TAB species are attached to the Pac, and on the formed micelles, hydrolysis of TEOS (as well as of cobalt and cerium species) takes place. The initial atomic ratios of Ce(IV) : Co(II/III) were 1 : 9, 2.5 : 7.5 and 5 : 5. In each of those three cases, three different samples were isolated at pH = 5.5, 7.5 and 9.5 which, after drying and firing at 600 °C, were characterized by X-ray diffraction (XRD), nitrogen adsorption porosimetry, diffuse reflectance spectrospopy (DRS), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Cerium is already quantitatively incorporated into the solid at pH = 5.5. At this low pH value, only ≈20% of the cobalt is incorporated into the solid, but it is fully incorporated at the high pH of 9.5 The silicate species containing a low content of Ce(IV) and Co(II/III) (x : z = 1 : 9) show a self-organized morphology as previously observed in samples free of cerium. This self-organization is destroyed when the content of cerium is increased (x : z = 2.5 : 7.5 and 5 : 5). The samples with a low content of Ce(IV) and Co(II/III) possess an MCM-41 structure, which is gradually degenerated as the amount of the metals increases, and tiny crystallites of CeO2 and Co3O4 become apparent with XRD. The surface area of the solids was in the range 1230–770 m2 g−1, and it drops almost linearly as the content of the metals in the solids increases with the increase of pH. The I-point methodology was successfully applied to detect and distinguish distinct domains of microporosity and mesoporosity in the solids, as well as for the estimation of the percentage of micropore volumes (%Vmic), found in the range 15–30%, using the variation of the C parameter of the BET equation. The pore anisotropy, b, was also determined in the range 10 < b < 2300. A correlation of the form log b = log b0 − (%Vmic) was observed for the materials of the present study, in line with previous findings.

Mesoporous Aluminum Silicate Catalyst with Single-Type Active Sites: Characterization by Solid-State NMR and Studies of Reactivity for Claisen Rearrangement Reactions

A mesoporous aluminum silicate (Al-MS) material has been successfully synthesized via a base-catalyzed co-condensation reaction. Solid-state NMR studies showed that the aluminum atoms are tetrahedrally coordinated and are stable up to at least 400 °C. The double resonance 27Al-29Si NMR experiment provided first direct evidence that all Al atoms are surrounded by Si atoms forming Al−O−Si bonds in the framework. The silicon to aluminum atomic ratio at the surface (Si/Al)s is estimated at around 100, which exceeds the overall Si/Al ratio of 60. Reactivity study of the mesoporous aluminosilicate shows an excellent catalytic reactivity for the Claisen rearrangement of allyl phenyl ether despite the low surface concentration of Al.