Wormhole Framework Structures Research Articles

Synthesis and Characterization of Ti-HMS and CoMo/Ti-HMS Oxide Materials with Varying Ti Content

Hexagonal mesoporous materials with wormhole framework structures having Si/Ti molar ratios of 80, 40, and 20 (Ti-HMS) were used as supports of CoMo HDS catalysts. The supports and catalysts were characterized by N2 adsorption−desorption (SBET), X-ray diffraction (XRD), UV−vis diffuse reflectance spectroscopy (DRS UV−vis), X-ray photoelectron spectroscopy (XPS), 29Si nuclear magnetic resonance (29Si NMR), FT-IR of adsorbed NO, and reactivity. The catalysts were tested in the reaction of hydrodesulfurization (HDS) of dibenzothiophene (DBT). The incorporation of titanium into the framework of HMS did not change its mesoporous character but modified the surface properties. The presence of Ti impedes the formation of Co2SiO4 and of β-CoMoO4 phases and increases the amount of octahedral Co species. The incorporation of titanium into the HMS framework led to an increase in the catalytic activity in comparison to that of the Ti-free catalyst. All Ti-containing catalysts manifest higher activity than that of a conventional CoMo/Al2O3 catalyst. The effect of Ti is not linear, the highest activity was observed for the sample with Si/Ti = 40 atomic ratio. This catalyst demonstrated to be 30% more active than the conventional industrial catalyst and 50% more active than the Ti-free catalyst. It is suggested that the higher activity of this catalyst is related to the higher value of the Co/Mo ratio in the sulfide state of the catalysts, to the larger amount of exposed Co species and to the specific degree of sulfidation of the Co species, characterized by specific electronic properties, which promote both the DDS and the HYD routes of HDS of DBT. The large pore diameter of this sample should also contribute to the elimination of possible diffusion effects.

Mercury Binding Sites in Thiol-Functionalized Mesostructured Silica

Thiol-functionalized mesostructured silica with anhydrous compositions of (SiO(2))(1)(-)(x)()(LSiO(1.5))(x)(), where L is a mercaptopropyl group and x is the fraction of functionalized framework silicon centers, are effective trapping agents for the removal of mercuric(II) ions from water. In the present work, we investigate the mercury-binding mechanism for representative thiol-functionalized mesostructures by atomic pair distribution function (PDF) analysis of synchrotron X-ray powder diffraction data and by Raman spectroscopy. The mesostructures with wormhole framework structures and compositions corresponding to x = 0.30 and 0.50 were prepared by direct assembly methods in the presence of a structure-directing amine porogen. PDF analyses of five mercury-loaded compositions with Hg/S ratios of 0.50-1.30 provided evidence for the bridging of thiolate sulfur atoms to two metal ion centers and the formation of chain structures on the pore surfaces. We find no evidence for Hg-O bonds and can rule out oxygen coordination of the mercury at greater than the 10% level. The relative intensities of the PDF peaks corresponding to Hg-S and Hg-Hg atomic pairs indicate that the mercury centers cluster on the functionalized surfaces by virtue of thiolate bridging, regardless of the overall mercury loading. However, the Raman results indicate that the complexation of mercury centers by thiolate depends on the mercury loading. At low mercury loadings (Hg/S < or = 0.5), the dominant species is an electrically neutral complex in which mercury most likely is tetrahedrally coordinated to bridging thiolate ligands, as in Hg(SBu(t))(2). At higher loadings (Hg/S 1.0-1.3), mercury complex cations predominate, as evidenced by the presence of charge-balancing anions (nitrate) on the surface. This cationic form of bound mercury is assigned a linear coordination to two bridging thiolate ligands.

Assembly of Large-Pore Silica Mesophases with Wormhole Framework Structures from α,ω-Diamine Porogens

Large-pore mesoporous silicas with wormhole framework structures have been assembled through hydrogen-bonding pathways from sodium silicate or tetraethyl orthosilicate (TEOS) as the silica source and amine-terminated Jeffamine surfactants of the type H2NCH(CH3)CH2[OCH2CH(CH3)]xNH2 as the structure-directing porogen. Depending on the molecular weight of the α,ω-diamine surfactant (x ∼ 33 and 68 for Jeffamine D2000 and D4000, respectively) and the synthesis temperature (25−65 °C), the mean pore size distributions of the mesostructured silicas (denoted MSU-J) were centered between 4.9 and 14.3 nm, making these materials comparable to hexagonal SBA-15 mesostructures in average framework pore size. In addition to the BET surface areas of MSU-J silicas (408−1127 m2/g) being comparable to those of SBA-15 (630−1040 m2/g), MSU-J silicas exhibit larger pore volumes (1.37−2.29 cm3/g) than SBA-15 silicas (0.56−1.23 cm3/g) prepared in the absence of mesitylene. MSU-J wormhole mesostructures represent the largest pore sizes observed to date for a fully three-dimensional mesoporous framework assembled from a single micellar porogen. Only mesostructured micellar foam structures exhibit larger pore sizes, but the preparation of foam structures requires the use of more complex microemulsions or latex polymers as porogens.

CTAB-assisted fabrication of mesoporous composite consisting of wormlike aluminosilicate shell and ordered MSU-S core

Novel mesoporous composites comprised of aluminosilicate shell with wormhole framework structure and well-ordered MSU-S core have been firstly prepared via treatment of MSU-S with NaAlO 2 solution in the presence of cationic surfactant cetyltrimethylammonium bromide (CTAB). The obtained products were thoroughly characterized by XRD, HRTEM, N 2 sorption isotherm, TG-DSC, 27Al MAS NMR and 29Si MAS NMR, etc. Characterization results based on these techniques revealed that the introduction of CTAB during the treatment process played a crucial role in the formation of mesoporous composite, otherwise the parent ordered MSU-S would be completely destroyed. Furthermore, aluminum content in the final product determined by ICP and EDAX was significantly increased compared to the parent MSU-S, thus assuming that CTAB added in this modification process led a key role in the introduction of Al into the final solid via self-assembly with additionally added AlO 2 − and partly dissolved silicate species from the parent MSU-S particles. In addition, acidity and catalytic performance of the prepared mesoporous composite were also substantially improved in comparison to the parent MSU-S sample.

XAS study of mercury(II) ions trapped in mercaptan-functionalized mesostructured silicate with a wormhole framework structure.

Directly assembled wormhole mesostructures with high level functionalized mercaptan (MP-HMS) have been shown to be effective mercury(II) (Hg2+) trapping agents. Sorption of Hg2+ onto MP-HMS was investigated using X-ray absorption spectroscopy (XAS) to identify the structural coordination of the adsorbed Hg. Samples with different fractions of mercaptan functionalized groups (i.e., x = 0.1 and 0.5) with various Hg/S molar ratios ranging from 0.05 to 1.4 were investigated. XAS analysis indicates that adsorbed Hg first coordination shell is best fitted with an Hg-O path and an Hg-S path. The Hg-S atomic distance (R(Hg-S)) remained relatively constant while the Hg-S coordination numbers (CN) decreased as Hg/S loading increased. For the Hg-O path, both the CN and the R(Hg-O) increased with increasing Hg loading. XAS results suggest that at low Hg loadings, adsorbed Hg2+ forms mostly monodentate sulfur complexes (-S-Hg-OH) with the sulfur functional groups on the MP-HMS surfaces. At high Hg loadings, the Hg coordination environment is consistent with the formation of a double-layer structure of Hg attached to sulfur binding sites (-S-Hg-O-Hg-OH).

Assembly of wormhole aluminosilicate mesostructures from zeolite seeds

Aluminosilicate mesostructures (Si/Al = 1.6–49) with wormhole framework structures were assembled from protozeolitic nanoclusters (zeolite seeds) that normally nucleate the crystallization of faujasite (FAU), zeolite ZSM-5 (MFI) and zeolite Beta (BEA). The cationic surfactant cetyltrimethylammonium, the electrically neutral surfactant dodecylamine and the small molecule triethanolamine all are effective structure directors for the assembly of wormhole frameworks from zeolite seeds. The resulting aluminosilicate mesostructures exhibit high textural porosity, hydrothermal stability toward boiling water and 20% steam at 800 °C, and acidity for catalytic cumene cracking. That is, the amine and onium ion structure directors used to template a wormhole framework structure do not compromise the benefits provided by the zeolite seeds.

A versatile pathway for the direct assembly of organo-functional mesostructures from sodium silicate.

Silica mesostructures with well-expressed hexagonal and wormhole framework structures and up to 50% organo-functionalization of the framework silicon sites have been directly assembled from sodium silicate.

Metakaolin as a reagent for the assembly of mesoporous aluminosilicates with hexagonal, cubic and wormhole framework structures from proto-faujasitic nanoclusters

Metakaolin, formed through the thermal de-hydroxylation of kaolin clay, has been shown to be an effective reagent for the preparation of faujasitic zeolite nanoclusters for framework incorporation into hexagonal, cubic and wormhole aluminosilicate mesophases with Si/Al ratios in the range 5.6–1.6. Single tetrahedral aluminum sites with 27Al NMR chemical shift of 60 ppm are observed for each mesostructure, which is indicative of the incorporation of the protozeolitic nanoclusters into the framework walls. Unlike aluminosilicate mesostructures assembled from conventional precursors, no framework de-alumination was observed for the mesostructures upon calcination at 550 °C. The results suggest that metakaolin is a promising low-cost precursor for the industrial scale production of high quality aluminosilicate mesophases.

Optimizing Organic Functionality in Mesostructured Silica: Direct Assembly of Mercaptopropyl Groups in Wormhole Framework Structures

A series of mercaptopropyl-functionalized wormhole mesostructures, denoted MP-HMS, have been prepared through the S0I0 assembly of alkylamine surfactants (S0) and mixtures of 3-mercaptopropyltrimethoxysilane and tetraethyl orthosilicate as framework precursors (I0). Unprecedented levels of organo functionalization, corresponding to at least 50% of the silicon sites, were achieved while retaining well-expressed mesostructures with pore sizes, pore volumes, and surface areas as high as 2.8 nm, 0.69 cm3/g, and 1225 m2/g, respectively. Also, up to ∼90% of the framework silicon sites could be fully cross-linked, lending exceptional hydrothermal stability to the mesostructures. The key to highly functionalized MP-HMS derivatives lies in the use of long-chain alkylamine surfactants as structure directors (e.g., octadecylamine) in combination with a relatively high assembly temperature (e.g., 65 °C) and a high-polarity water−ethanol solvent. Increasing the assembly temperature increased both the framework pore size and the degree of framework cross-linking, whereas increasing the MP content lowered the pore size while improving the framework cross-linking. The effects of assembly temperature and MP loading were attributable to changes in hydration at the H-bonded S0I0 interface and concomitant changes in the surfactant packing parameter. Highly functionalized MP-HMS derivatives are promising materials for use as heavy-metal ion-trapping agents and as precursors for sulfonic-acid-functionalized mesostructured catalysts.

Non-ionic surfactant assembly of wormhole silica molecular sieves from water soluble silicates

Thermally stable mesoporous silica molecular sieves with wormhole framework structures, previously obtainable only from silicon alkoxides, have been prepared from low cost soluble silicate precursors in the presence of non-ionic surfactants as structure directors.

Textural Mesoporosity and the Catalytic Activity of Mesoporous Molecular Sieves with Wormhole Framework Structures

Three different water−alcohol cosolvent systems were used to assemble mesoporous molecular sieve silicas with wormhole framework structures (previously denoted HMS silicas) from an electrically neutral amine surfactant (S°) and a silicon alkoxide precursor (I°). The fundamental particle size and associated textural (interparticle) porosity of the disordered structures were correlated with the solubility of the surfactant in the water−alcohol cosolvents used for the S°I° assembly process. Polar cosolvents containing relatively low volume fractions of CnH2n+1OH alcohols (n = 1−3) gave heterogeneous surfactant emulsions that assembled intergrown aggregates of small primary particles with high textural pore volumes (designated HMS−HTx). Conversely, three-dimensional, monolithic particles with little or no textural porosity (designated HMS−LTx) were formed from homogeneous surfactant solutions in lower polarity cosolvents. Aluminum substituted Al-HMS−HTx analogues with high textural porosity and improved framework accessibility also were shown to be much more efficient catalysts than Al-HMS−LTx or monolithic forms of hexagonal Al-MCM-41 for the sterically demanding condensed phase alkylation of 2,4-di-tert-butylphenol with cinnamyl alcohol. Transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) studies verified the textural differences between wormhole HMS and electrostatically assembled hexagonal MCM-41 and SBA-3 molecular sieves. Power law fits to the scattering data indicated a surface fractal (Ds = 2.76) for HMS−HTx, consistent with rough surfaces. A second power law at lower-q indicated the formation of a mass fractal (Dm = 1.83) consistent with branching of small fundamental particles. Hexagonal MCM-41 and SBA-3 silicas, on the other hand, exhibited scattering properties consistent with moderately rough surfaces (Ds = 2.35 and 2.22, respectively) and large particle diameters (≫1 μm). HMS−LTx silicas showed little or no mass fractal character (Dm = 2.87), and no surface fractal scattering.