Hexagonal Mesoporous Materials Research Articles

Nitrogen Adsorption and Characteristics of Iron, Cobalt, and Nickel Oxides Impregnated on SBA-15 Mesoporous Silica

Hexagonal SBA-15 mesoporous material was used as a catalytic template for impregnation, with the transition metals Fe, Co, and Ni as catalysts for chemical transformation. Nitrogen adsorption/desorption isotherms, scanning electron microscopy, and transmission electron microscopy were conducted to better understand the physicochemical properties of the metal oxide-impregnated SBA-15. The specific surface area of the original SBA-15 was approximately 680 m2/g, and the abundances of the catalysts impregnated ranged from 2 to 8%, corresponding to specific surface areas of 560-470 m2/g for Fe-SBA-15, 440-340 m2/g for Ni-SBA-15, and 410-340 m2/g for Co-SBA-15. The increase in impregnated metal loadings filled the pores and collapsed the silica walls during the metal oxides impregnation on SBA-15 and calcination procedures, resulting in a decrease in the specific surface area and pore volume of the templates. The results showed that the order of nitrogen adsorbed was SBA-15 > Fe-SBA-15 > Ni-SBA-15 > Co-SBA-15 when the metal loading was 5%. In addition, the metal oxides on SBA-15 increased the wall thickness compared with raw SBA-15. Based on the XRD spectrum analysis, Fe2O3, Co3O4, and NiO were the stable crystals on the Fe-SBA-15, Co-SBA-15, and Ni-SBA-15, respectively. The sequence of the average grain size of metal oxides on SBA-15 was Co-SBA-15 > Fe-SBA-15 > Ni-SBA-15, according to XRD spectra and Scherrer's equation. Isopropanol could be decomposed by metal oxide-impregnated SBA-15 to form carbon filament materials. Therefore, these materials have the potential to be employed for pollutant removal, catalytic reactions for organic solvent and bio-oil/biomass reforming, and recycling waste into high-value materials.

Selective Styrene Oxidation Catalyzed by Phosphate Modified Mesoporous Titanium Silicate

Selective oxidation of organics over an efficient heterogeneous catalyst under mild liquid phase conditions is a very demanding chemical reaction. Herein, we first report the modification of the surface of mesoporous silica MCM-41 material by phosphate for the efficient incorporation of Ti(IV) in the silica framework to obtain highly ordered 2D hexagonal mesoporous material STP-1. STP-1 has been synthesized by using tetraethyl orthosilicate, triethyl phosphate, and titanium isopropoxide as Si, P, and Ti precursors, respectively, in the presence of cationic surfactant cetyltrimethylammonium bromide (CTAB) under hydrothermal conditions. The observed specific surface area and pore volume of STP-1 were 878 m2g−1 and 0.75 ccg−1, respectively. Mesoporous STP-1 has been thoroughly characterized by XRD, FT-IR, Raman spectroscopy, SEM, and TEM analyses. Titanium incorporation (Ti/Si = 0.006) was confirmed from the EDX analysis. This mesoporous STP-1 was used as a heterogeneous catalyst for the selective oxidation of styrene into benzaldehye in the presence of dilute aqueous H2O2 as an oxidizing agent. Various reaction parameters such as the reaction time, the reaction temperature, and the styrene/H2O2 molar ratio were systematically studied in this article. Under optimized reaction conditions, the selectivity of benzaldehyde could reach up to 93.8% from styrene over STP-1. Further, the importance of both titanium and phosphate in the synthesis of STP-1 for selective styrene oxidation was examined by comparing the catalytic result with only a phosphate-modified mesoporous silica material, and it suggests that both titanium and phosphate synergistically play an important role in the high selectivity of benzaldehyde in the liquid phase oxidation of styrene.

Underlying Processes Driving the Evolution of Nanoporous Silica in Water and Electrolyte Solutions

In this study, we related the alteration of mesoporous materials in aqueous solutions to the properties of confined water in the presence of ions, that is, its structure and its dynamics. To reach this goal, we have determined and quantified the evolution of the morphology of hexagonal mesoporous silica-based materials, that is, MCM-41 (mean pore size of 2.9 nm and dense pore wall) and SBA-15 (mean pore size of 6.8 nm and microporous pore wall), during their alteration in water and electrolyte solutions with ions presenting various kosmotropic properties ([XCl2] = 1 M and X = Ba, Ca, Mg) at 50 °C by in situ small and wide angle X-ray scattering. The experimental results and the calculated small-angle X-ray scattering spectra have demonstrated that the alteration behavior is strongly dependent on the type of silica and the water properties (water dynamics at a picosecond time scale and water structure) related to the confinement size, the nature of the cations, and their surface ion excesses. For MCM-41 silica, having reduced water dynamics, the alteration behavior is mainly driven by silica hydrolysis—recondensation/precipitation processes leading to a pore deformation and a possible precipitation of XCl2 metastable phases partially clogging the pores. In case of SBA-15, its alteration behavior in aqueous solutions starts with an initial increase of the pore size and the formation of an alteration layer within its microporosity which grows following a diffusive process of the solution. Inside the microporosity, similar alteration processes to MCM-41 silica occur. These recondensation/precipitation processes may continue until a thermodynamically stable silica phase is formed. The results obtained within the scope of this work revealed the importance of the influence of the dynamics and the structure of water in chemical reactions taking place in confined media filled with water and ions.

Highly dispersed metal incorporated hexagonal mesoporous silicates for catalytic cyclohexanone oxidation to adipic acid

Abstract Adipic acid is a dicarboxylic acid of great industrial importance, mainly used in the production of nylon-6,6 and polyurethane. The use of nitric acid as an oxidant in the industrial production of adipic acid poses significant carbon footprint to the environment. Clean adipic acid synthesis methods using a heterogeneous catalyst with H2O2 as oxidant and water as solvent have potential advantages of low catalyst cost, easy synthesis and recovery, cleanness and environmental protection. In this work, hexagonal mesoporous silicate materials were synthesized by a sol–gel method and evaluated for cyclohexanol/cyclohexanone oxidation to adipic acid. The physical and chemical properties of Fe-HMS were characterized by XRD, HR-TEM, BET and UV–Vis. The experimental results showed that Fe-HMS materials show pore sizes ranging from 2–3 nm. W- and Mo-based polyoxometalates were also evaluated and compared to the Fe-based HMS catalysts. To improve the adipic acid yield, the influence of the transition metal as well as the effect of metal loading, reaction temperature and catalyst amount on the catalytic performances of Fe-HMS have been investigated in details. When Si/Fe atomic ratio = 100, Fe-HMS catalyst shows the highest activity, with a cyclohexanone conversion of 92.3% and adipic acid selectivity of 29.4%. The reaction pathway of cyclohexanone oxidation was further proposed based on experimental data.

Immobilization of the enzyme invertase in SBA-15 with surfaces functionalized by different organic compounds

The search for appropriate supports for enzyme immobilization has attracted increasing interest in the field of biocatalysis. Immobilized enzymes are catalysts of enormous industrial interest because they combine the advantages of heterogeneous catalysis with the high selectivity and mild operational conditions of enzymes. This study evaluated the use of different organic molecules as functionalizing agents anchored on the silanol groups present on the surface of mesoporous materials such as SBA-15. This modification aims to increase the interaction of the enzyme invertase to the support, allowing for a greater amount of this biocatalyst to be immobilized. The hexagonal mesoporous material SBA-15, which has pores in the range of 40–60 A, was synthesized and modified by the following organic compounds: trimethoxyphenylsilane, vinyltrimethoxysilane and 3-aminopropyltriethoxysilane (APTES). APTES showed the best results in increasing the anchoring of enzymes on the material’s surface, providing a specific amount of enzyme loaded onto the surface of approximately 80% and, thus, showing successful functionalization.

Hexagonal sheet-like mesoporous titanium phosphate for highly efficient removal of lead ion from water

Hexagonal sheet-like mesoporous titanium phosphate nanostructured materials synthesized via a simple hydrothermal method showed highly efficient removal of Pb(ii) ions from water.

Sugarcane Bagasse as a Renewable Source of Silica to Synthesize Santa Barbara Amorphous-15 (SBA-15)

Sugarcane bagasse is an agricultural waste that is potentially used as natural silica resources. Natural silica claimed to be safe in handling, cheap and can be generate from cheap resource. In this study, Santa Barbara Amorphous-15 (SBA-15) was synthesized using sodium silicate that is extracted from Sugarcane bagasse ash (SCBA). Post treatment method using hydrochloric acid (HCl) was introduced during the silica extraction process to improve its purity. The result indicated that the amount of SiO2 present in the raw sugarcane bagasse ash was 53.10% while the silica composition in acid treatment sample was 88.13%. The FTIR pattern of extracted sodium silicate shows a similar spectrum with the commercial pattern of sodium silicate. While the Inductive Coupled Plasma (ICP) result shown that the concentration of silica in extracted sodium silicate was 4873ppm. Then the extracted sodium silicate produce was used as a silica precursor in the synthesis of mesoporous silica, Santa Barbara Amorphous-15 (SBA-15). The formation of SBA-15 was characterized using X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The result indicated that the synthesis SBA-15 have present peaks 100, 110 and 200 which represent the hexagonal mesoporous material associated with p6mm symmetry.

Predicting Sizes of Hexagonal and Gyroid Metal Nanostructures from Liquid Crystal Templating.

We describe a method to predict and control the lattice parameters of hexagonal and gyroid mesoporous materials formed by liquid crystal templating. In the first part, we describe a geometric model with which the lattice parameters of different liquid crystal mesophases can be predicted as a function of their water/surfactant/oil volume fractions, based on certain geometric parameters relating to the constituent surfactant molecules. We demonstrate the application of this model to the lamellar (Lα), hexagonal (H1), and gyroid bicontinuous cubic (V1) mesophases formed by the binary Brij-56 (C16EO10)/water system and the ternary Brij-56/hexadecane/water system. In this way, we demonstrate predictable and independent control over the size of the cylinders (with hexadecane) and their spacing (with water). In the second part, we produce mesoporous platinum using as templates hexagonal and gyroid phases with different compositions and show that in each case the symmetry and lattice parameter of the metal nanostructure faithfully replicate those of the liquid crystal template, which is itself in agreement with the model. This demonstrates a rational control over the geometry, size, and spacing of pores in a mesoporous metal.

Measuring the specific surface area of mesoporous silica using x-ray scattering

The development of porous silica or carbon material with high specific surface area raises a high interest in the field of materials science given their potential interest in a wide range of applications including catalysis, water treatment or drug delivery. Among these mesoporous structures, those consisting of one-dimensional pores aligned along a compact hexagonal packing are of prime importance and can be referred to as "hexagonal mesoporous materials" (HMPM). The most famous silica structures of this kind are MCM-41 and SBA-15. The same symmetry can be found in carbon mesoporous materials, for example in FDU-15 structures. The precise characterization of HMPM is necessary for most of the applications envisioned for these materials (pore size, pore density, specific surface and sometimes thickness of the functionalization layer). Small angle X-ray scattering techniques offer the opportunity to determine the mean structural parameters of HMPM. Although different approaches can be found in the literature in order to numerically reproduce the experimental data obtained on HMPM or hexagonal liquid crystals, when the sample is a powder, fitting the experimental data in absolute scale with numerical models becomes necessary. However, with a large scattering contribution of grain at low q vector as well as short range correlation contribution at large q, the analysis is not so simple. In this paper, we propose a comprehensive study [1] devoted to the quantitative interpretation of small-angle scattering patterns of HMPM in terms of structure and specific surface estimation based on the formalism proposed by Spalla et al. [2]. In the case of two real samples, namely a SBA-15 and a MCM-41 powder, the specific surface area of the mesopores is estimated and is discussed in the light of gas adsorption measurements.

Phosphonic Acid Functionalized Ordered Mesoporous Material: A New and Ecofriendly Catalyst for One-Pot Multicomponent Biginelli Reaction under Solvent-Free Conditions

We report a new ordered 2D hexagonal mesoporous organosilica material (PAFMS-1) bearing phosphonic acid functionality at the surface. This hybrid material showed high Brunauer-Emmett-Teller surface area (565 m(2) g(-1)) and ordered assembly of mesoporoes with an average pore diameter of ca. 2.1 nm. This novel hybrid mesoporous material has been synthesized via cocondensation of (triethoxysilyl)(propyliminomethyl)biphenylmethyl phosphoester (PEFOS) and tetraethyl orthosilicate (TEOS) in the presence of cationic surfactant cetyltrimethylammonium bromide (CTAB) at 373 K. The phosphoester-functionalized organosilane (PEFOS) precursor has been synthesized for the first time by a simple SN2 reaction followed by Suzuki coupling and a Mannich reaction. The material has been characterized by powder X-ray diffraction, N2 sorption, and transmission electron microscopy image analysis, whereas the presence of organic moieties (an aromatic biphenyl ring and an aliphatic side chain), phosphrous, and silicon in the pore wall of the material have been characterized by solid-state magic-angle-spinning NMR, X-ray photoelectron, and Fourier transform infrared (FT-IR) spectroscopic tools. Further, the surface acid strength of the hybrid material has been determined by FT-IR analysis of the samples via temperature-programmed pyridine adsorption studies. The material has been utilized as a reusable heterogeneous catalyst for the synthesis of biologically important and value added multifunctionalized 3,4-dihydropyridin-2-1H-(ones)/3,4-dihydropyridin-2-1H-(thiones) (DHPMs) through a multicomponent Biginelli condensation reaction under solvent-free conditions at 333 K. The phosphonic acid functionalized 2D hexagonal mesoporous material showed much higher catalytic activity in this multicomponent condensation reaction over sulfonic acid functionalized mesoporous silica (MCM-41-SO3H) bearing an aliphatic chain in the hybrid framework.

Fabrication of Organized Mesoporous TiO<sub>2</sub> Films Assembled via Different Template and Characterization of their Photocatalytic Activity

Hexagonal and cubic mesoporous TiO2 materials were synthesized via a sol-gel route modified by evaporation–induced self–assembly process with different templates. XRD and TEM results confirmed that the mesoporous films were highly organized. Furthermore, the hexagonal mesoporous TiO2 films were formed in the P123-templated films, whereas the cubic structure ones were found in the F127-templated films. Both the hexagonal and cubic mesoporous films showed photocatalytic activity in decomposing methyl orange solution under UV region, among which the cubic mesostructure presented superior potocatalytic activity because of larger surface area, more open framework and less obstructed diffusion paths of guest molecules.

Activity of amino-functionalised mesoporous solid bases in microwave-assisted condensation reactions

Abstract Aminopropylated functionalised hexagonal mesoporous silicas (HMS) and SBA-15 materials with different amino-loadings (5–30 wt.% NH2) were synthesized, characterised and their catalytic activities were subsequently investigated in the microwave-assisted Knoevenagel condensation of cyclohexanone and ethyl cyanoacetate as well as in the Michael reaction between 2-cyclohexen-1-one and nitromethane. The effects of the quantity of the catalyst in the reaction as well as a variety of microwave parameters including the power, temperature and time of microwave irradiation were optimised. High activities and selectivities to the condensation product could be achieved at short times of microwave irradiation for both base-catalysed processes. The low loaded HMS-5%NH2 and higher loaded SBA-15-20%NH2 were found to give the best activities in the reactions. This observation seems to be related to the significant deterioration observed in textural properties of HMS materials at amino-loadings larger than 10%.

Hexagonal mesoporous fluorescent hybrid materials with electron acceptor viologen units in the framework

Mesoporous MCM-41-type fluorescent hybrid materials with methylene viologen units in the framework have been synthesized via the co-hydrolysis and -condensation of dichloride of N,N′-bis(triethoxysilylmethyl)-4,4′-bipyridinium (VP) and tetraethoxysilane (TEOS). The obtained hybrid materials are characterized by the small-angle X-ray diffraction (SAXRD), high-resolution transmission electron microscope (HR-TEM), Fourier transform infrared spectrometer (FTIR), solid-state 29Si NMR spectrum, nitrogen adsorption/desorption analyse, diffuse reflectance UV–VIS (DR UV-Vis), and confocal laser scanning fluorescence microscopy (CLSFM). The results show that the VP units are covalently bonded into the silica framework to form mesoporous hybrid materials. The obtained hexagonal mesoporous hybrid materials are found to emit fluorescence at ca.380 nm and 420 nm. The fluorescent intensity enhances with increasing the amount of the VP occupied in the silica framework, and it is not affected by the hexagonal or cubic array of the pore channels. It could be prospected that such hybrid materials would present great potential applications in drug delivery and fluorescence probing for medical diagnosis and synchronous therapy.

FDU-15-Pt composites with different Pt loading and their electrocatalytic reduction to ultratrace nitroaromatic compounds

FDU-15 is a hexagonal mesoporous material with nanometer-sized, highly ordered arrays and large special surface area. In this work, FDU-15-Pt with 2.0%, 5.0% and 8.0% Pt loading were synthesised and used for electrochemical detection of trace nitroaromatic compounds (NACs). The FDU-15-Pt samples were characterised by CO Chemisorption, transmission electron microscopy (TEM) and X-ray diffraction (XRD). It has been demonstrated that FDU-15-Pt with 2.0% Pt loading has the smallest Pt particle size of 2.9 nm, highest Pt metal dispersion of 37.7% and largest Pt metal surface area of 21.36 m2 g−1. The FDU-15-Pt/PDDA modified electrode were assembled by electrostatic adsorption of Poly (diallyldimethylammonium chloride) (PDDA) and FDU-15-Pt. The 2.0% FDU-15-Pt modified sensor showed higher selectivity for NACs than those of 5.0% and 8.0% FDU-15-Pt, which were verified by electrochemical experiments. A linear response over TNT concentration ranging from 8.8 × 10−9 M to 1.2 × 10−5 M was exhibited with a low detection limit of 2.9 × 10−9 M (S/N = 3). Moreover, the proposed 2.0% FDU-15-Pt/PDDA modified sensor has been applied to the detection of NACs in spiked environmental water samples and shows promise for fast and accurate determination of trace NACs in real samples.

Improved hydrothermal stability and acidic properties of ordered mesoporous SBA-15 analogs assembled from nanosized ZSM-5 precursors

Ordered hexagonal mesoporous materials have been successfully synthesized in a two-step process from assembly of pre-formed nanoscale ZSM-5 precursors at elevated temperature by pH adjusting during synthesis. The resulting materials were characterized by small-angle X-ray scattering (SAXS), N2 adsorption and desorption (BET), transmission electron microscopy (TEM), temperature-programmed desorption of ammonia (NH3-TPD), FTIR spectroscopy of pyridine adsorption–desorption (pyr-FTIR), atomic absorption spectroscopy (AAS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). It was found out that a highly ordered mesoporous material with both improved hydrothermal stability and acidic properties can be optimally prepared at pH 3.5 and 200°C. The ordered mesostructure of such material is well preserved even after steaming at 800°C for 4h, with little loss in BET specific surface of 10.7% only, while its acidic properties are significantly enhanced in terms of both acid strength and total acidity. These improved properties can be attributed to the synergistic advantages of the use of zeolite nanoseeds, high temperature synthesis and pH adjustment.

Elaboration of nickel-impregnated over hexagonal mesoporous materials and their catalytic application

Hexagonal mesoporous silicas with different nickel contents have been synthesized and characterized by several techniques such as N2 physical adsorption, elemental analysis, XRD, TEM and temperature programmed reduction (TPR). In fact, the nickel-impregnated over hexagonal mesoporous silicas showed both high activity and high selectivity for Friedel–Crafts alkylations of benzene with benzyl chloride. The kinetics of the reaction over these catalysts have been investigated and the reaction has been extended to other substrates like toluene, p-xylene, anisole, naphthalene and methylnaphthalene.

Phase and morphological segregation in Ti-MCM-41

In this study Ti-MCM-41 materials have been synthesised and characterised with a view to developing Ti anchoring sites for supporting metal nanoparticles. Up to 20%Ti has been incorporated into the matrix using titanium iso-propoxide. A soft-templating approach has been followed, and the reaction is carried out under highly alkaline conditions. The N 2-sorption and XRD results indicate that a hexagonal mesoporous material is present. However, electron microscopy characterisations indicate that phase and morphological heterogeneities exist. At the atomic scale two phases of oxide particles are formed: a hexagonal mesoporous silicate phase and an amorphous porous titanosilicate phase. At the bulk scale these particles gather into aggregates (>100 μm) which are segregated into Ti-rich and Si-rich entities. Thus, the heterogeneity occurs at both atomic and bulk scales. Although clearly revealed by electron microscopy, these heterogeneities are impervious to detection by the traditional N 2-sorption and low-angle XRD approaches.

Adsorption of xylene isomers on ordered hexagonal mesoporous FDU-15 polymer and carbon materials

The oil industry has been facing the challenges of separation of xylene isomers, o-xylene, m-xylene and p-xylene or removing them from the environment. In our present work, we investigated the adsorption of the three isomers on two mesoporous materials, FDU-15-350 polymer and FDU-15-900 carbon materials. The isomer adsorption capacities are well correlated with their physical pore properties. It is found that the micropores are very crucial for the adsorption of these three isomers. The more micropore volume the adsorbent has, the better the adsorption capacity is. Henry’s constants were also calculated for the three isomers on the two adsorbents. Both on FDU-15-350 polymer and FDU-15-900, the Henry’s constants for the three isomers show the same trend o>m>p xylene which is coincidently in accordance with their polarity trend, indicating more polar adsorbate is preferred for adsorption on the two adsorbents. The isosteric heats of adsorption are correlated with the microporosity and the size of the adsorbate molecule. More microporosity and smaller molecules give higher heats of adsorption. Extracted information on pore properties of adsorbents by using the three isomers has very similar results as that resolved from nitrogen adsorption, indicating the feasibility of using the three isomers as adsorbates to extract pore information.

Monte Carlo simulations of self-assembling hexagonal and cage-like bifunctional periodic mesoporous materials

Self-assembly of lyotropic liquid crystalline mesophases formed by amphiphilic molecules was studied using computer simulations. The addition of an inorganic and two hybrid organic–inorganic precursors, one with a bridging and the other with a terminal organic functionality, lead to the formation of bifunctional hexagonally-packed mesoporous materials. These structures exhibit very ordered and uniform mesopores with the organic functional groups located in the cylindrical pores and in their walls, and are found to be stable over a relatively broad range of precursor concentrations. Hexagonal-to-lamellar and hexagonal-to-cubic phase transitions have been observed at constant surfactant concentrations by tuning the relative content of the hybrid precursors which modify the overall solvophilic character of the solvent and, as a consequence, the surfactant solubility in its surrounding environment. The long range ordered cubic phases show interconnected, roughly spherical mesocages of uniform size and an interesting distribution of the organic functionalities.

Study of catalysts comprising zirconium sulfate supported on a mesoporous molecular sieve HMS for esterification of fatty acids under solvent-free condition

Abstract New solid acid catalysts consisting of zirconium sulfate (ZS) supported on a pure hexagonal mesoporous material (HMS) have been prepared and characterized. ZS is intact on the HMS surface and form finely dispersed species. No ZS crystal phase was developed even at ZS loadings as high as 40 wt%. The interaction of ZS with the HMS material enhances the acidity and the catalytic activity of this reaction. The effects of ZS loading on structure, acidity and catalytic activity were closely monitored. The HMS-supported ZS catalyst exhibits higher catalytic activity than bulk ZS and various solid acid catalysts in liquid-phase esterification of oleic acid with n -butanol under solvent-free conditions. It is assumed that the reaction occurred mainly inside the mesopores of HMS. The reaction was found to obey Eley–Rideal (ER) mechanisms. The HMS-supported ZS is a promising solid acid catalyst for acid-type reactions, which involve large organic molecules such as fatty acids.