Hexagonal Mesoporous Silicate Research Articles

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.

Adsorption of BSA onto hexagonal mesoporous silicate loaded by APTES and tannin: Isotherm, thermodynamic and kinetic studies

In the present study, hexagonal mesoporous silica (HMS) was synthesized and modified by tannic acid as a natural poly-phenol and amine (TA-A-HMS) and was applied for the adsorption of bovine serum albumin (BSA) from aqueous media. To investigate the structure of HMS and TA-A-HMS, SEM, TEM, XRD, BET and FTIR analysis were applied. The effects of pH, adsorbent dosage, contact time and temperature on the BSA adsorption were studied. After modification, BET surface area of HMS was reduced from 885 m2/g to 51 m2/g which confirms the presence of tannin and amine groups that inhibit the adsorption of nitrogen molecules. According to the results of equilibrium data, it is shown that Langmuir isotherm with maximum adsorption capacity of 1000 mg/g is the predominant model and adsorption is mono-layer. Kinetic and thermodynamic studies also reveal that adsorption kinetic followed by pseudo-second order model and the adsorption process is exothermic.

Surface modification of mesoporous silicate by tannin for immobilization of TiO2 nanoparticles: Study of photocatalytic performance

Photocatalytic performance of TiO2 supported on hexagonal mesoporous silicate (HMS) loaded by different concentrations of oak gall tannin (OGT) for degradation of direct yellow 86 (DY86) was studied (TiO2-OGTx-HMS, x = 0.3, 0.6,1 wt%). The results were also compared with the performance of TiO2 (p-25) and TiO2 supported on HMS. Photocatalysts were characterized by XRD, SEM, EDX and TEM analysis. The analysis showed that tannin was immobilized on the surface and TiO2 nanoparticles were distributed well on the surface of OGT-HMS without aggregation. It was concluded that photocatalytic reaction follows first-order kinetic and up to 120 mg/l, Langmuir-Hinshelwood (L-H) kinetic model is acceptable.

Preparation of titanium dioxide nanoparticles supported on hexagonal mesoporous silicate (HMS) modified by oak gall tannin and its photocatalytic performance in degradation of azo dye

In this study, a new type of photocatalyst, TiO2 based on hexagonal mesoporous silicate (HMS) loaded by different concentrations of natural polyphenol oak gall tannin,OGT, (TiO2–OGTx-HMS, x=0.3, 0.6, 1wt.%) was synthesized and used in batch photocatalytic experiments. The synthesized catalysts were characterized using XRD, SEM, EDX and TEM analysis. The analysis showed that tannin was immobilized on the surface of HMS and TiO2 nanoparticles were distributed well and uniform on the surface of OGT-HMS without aggregation, while TiO2 nanoprticles aggregated on the surface of tannin free HMS. The performance of TiO2–OGTx-HMS phtocatalysts for degredation of anionic dye (i.e., Direct yellow 86) in aqueous solution were also compared with the performance of TiO2 (P-25) and TiO2–HMS. TiO2–OGT0.6%-HMS Photocatalyst exhibited higher performance than that of other photocatalysts with the adsorption and degradation efficiencies of 36.7% and 92.2%, respectively. The relative high value of activation energy, 24.8kJ/mol, confirms the effect of temperature to accelerate the photocatalytic reaction. It was also shown that photocatalytic reaction follows first order kinetic and up to 120mg/l of dye solution, Langmuir–Hinshelwood (L–H) kinetic model is acceptable.

Synthesis of oak gall tannin-immobilized hexagonal mesoporous silicate (OGT-HMS) as a new super adsorbent for the removal of anionic dye from aqueous solution

Tannin as a natural biomass has been extracted from oak gall and was immobilized on hexagonal mesoporous silicate (HMS) in order to remove direct yellow 86 (DY86) from aqueous solution. The characteristics of the oak gall tannin-HMS adsorbent were substantiated by different techniques such as FTIR, TEM, TGA, BET, and XRD. From the BET analysis, a decline of 75.61% for the surface area of HMS nanoparticles was observed and indicates the proper coating through tannin. In order to analyze the adsorption data, five different isotherms were utilized and nonlinear regression scheme was employed using MATLAB software. It was concluded from the Langmuir isotherm that the maximum monolayer adsorption capacity was 435.5 mg g−1. The thermodynamic analysis of this study also revealed that the adsorption was feasible, since the process was spontaneous and endothermic. The findings also indicate that the adsorption of DY86 follows the pseudo-second-order kinetic model.

Mechanistic study of diclofenac and carbamazepine adsorption on functionalized silica-based porous materials

Adsorption of diclofenac and carbamazepine by hexagonal mesoporous silicate (HMS), and the amine- and mercapto-functionalized HMS derivatives (A-HMS and M-HMS, respectively), were evaluated in comparison to that for the mesoporous silicates SBA-15 and MCM-41 plus powdered activated carbon, so as to evaluate the adsorption mechanism, effects of surface functional groups, pH and temperature on the adsorption capacity of these adsorbents. The adsorption of diclofenac and carbamazepine onto all adsorbents was found to decrease rapidly in the first 30min and reach equilibrium within 4h. The adsorption kinetics and isotherm data obtained were best described by a pseudo-second-order kinetic rate model and linear isotherm model, respectively. Comparison of the adsorption capacities of HMS, A-HMS and M-HMS suggested that the addition of a mercapto functional group to HMS, increased its adsorption capacity, most likely through increased hydrogen bonding and hydrophobic interactions. The adsorption capacities of these adsorbents were dependent on the pH, being highest at pH 5 due to the optimal electrostatic interactions and hydrogen bonding at this pH. Thermodynamic analysis indicated that the adsorption process was endothermic for all adsorbents. The adsorption capacity of the adsorbent for each solute might be related to the molecular size of the adsorbate.

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.

Synthesis of the well-ordered hexagonal mesoporous silicate incorporated with phosphotungstic acid through a novel method and its catalytic performance on the oxidative desulfurization reaction

Well-ordered hexagonal mesoporous silicate (HMS) materials with various contents of phosphotungstic acid (HPW) were synthesized via an original direct synthesis method. The characterization results show that the samples have regular fingerprint-like mesopores and spherical morphology. Compared with the conventional wet impregnation method, the samples obtained by the original direct synthesis method have better HPW dispersion and larger specific surface area. Finally, the catalytic performance of these samples was tested through the oxidative desulfurization (ODS) of DBT in the model oil. The results indicate that the direct synthesized samples exhibit better catalytic performance than the impregnated samples.

Adsorption characteristics of haloacetonitriles on functionalized silica-based porous materials in aqueous solution

The effect of the surface functional group on the removal and mechanism of dichloroacetonitrile (DCAN) adsorption over silica-based porous materials was evaluated in comparison with powdered activated carbon (PAC). Hexagonal mesoporous silicate (HMS) was synthesized and functionalized by three different types of organosilanes (3-aminopropyltriethoxysilane, 3-mercaptopropyltrimethoxysilane and n-octyldimethysilane). Adsorption kinetics and isotherm models were used to determine the adsorption mechanism. The selective adsorption of five haloacetonitriles (HANs) in the single and mixed solute systems was also studied. The experiments revealed that the surface functional groups of the adsorbents largely affected the DCAN adsorption capacities. 3-Mercaptopropyl-grafted HMS had a high DCAN adsorption capacity compared to PAC. The adsorption mechanism is believed to occur via an ion–dipole electrostatic interaction in which water interference is inevitable at low concentrations of DCAN. In addition, the adsorption of DCAN strongly depended on the pH of the solution as this related to the charge density of the adsorbents. The selective adsorption of the five HANs over PAC was not observed, while the molecular structure of different HANs obviously influenced the adsorption capacity and selectivity over 3-mercaptopropyl-grafted HMS.

Preparation and characterization of HMS supported 11-molybdo-vanado-phosphoric acid for selective oxidation of propylene

The preparation of Keggin-type phosphovanadomolybdic acid H 4PMo 11VO 40 (PMo 11V) supported on a hexagonal mesoporous silicate (HMS) material was performed via three different methods: dry impregnation, wet impregnation and mechanical mixing. The state of the heteropolyacid (HPA) on the siliceous surface and the integrity of the support were characterized by several physico-chemical techniques: elemental analysis, X-ray diffraction, transmission and diffuse reflectance (DR) FT–IR study of the framework vibrations, Raman and X-ray photoelectron spectroscopies, nitrogen physisorption, thermal analysis (TG–DTA) and scanning electron microscopy (SEM). The catalytic performance of supported materials was compared to that of the bulk H 4PMo 11VO 40 in the selective oxidation of propylene by molecular oxygen in the temperature range of 200–400 °C. The physico-chemical characterization of the supported HPA showed mainly that the primary Keggin structure is intact and that a good dispersion of the heteropolyacid on the HMS surface is obtained when the impregnation methods are used. In the case of mechanical mixture, the dispersion of heteropolyacid clusters is not homogeneous. The surface area of HMS decreased when the HPA was added. The impregnation methods lead to mesopores with uniform size of approximatively 3.2 nm in diameter. The TG–DTA showed that the supported HPAs obtained by the impregnation methods have a higher thermal stability than that of bulk HPA. The catalytic results showed that H 4PMo 11VO 40/HMS are more active than the bulk H 4PMo 11VO 40 and lead to oxygenated compounds formation (acrolein, acetaldehyde and acetic acid), whereas the bulk HPA only leads to the formation of CO x .

Effect of surface functional group on adsorption of organic pollutants on hexagonal mesoporous silicate

Hexagonal mesoporous silicates (HMSs) are synthetic silicate materials that have uniform mesopores, comparatively large surface areas, and uniform surface functional groups, which lead to higher adsorption selectivity. Selective adsorption characteristics of HMSs for six types of organic pollutants (2,4-d, mecoprop, 4-chlorophenol, toluene, dichloroacetic acid, and thioflavin T) from synthetic wastewater were investigated. Five different types of HMSs were synthesized by surfactant-templating methods, and three of them were subsequently grafted with organic surface functional groups, i,e. n-octyldimethyl-, 3-aminopropyltriethoxy-, and 3-mercaptopropyl-groups. Titanium-substituted HMS was also made in the same way as pristine HMS. Increasing hydrophobicity of HMSs did not always enhance adsorption of hydrophobic adsorbates, such as toluene. Grafted organic functional groups changed surface charge, which enhanced electrostatic force between HMSs and ionic pollutants. Negatively charged contaminants, i.e. 2,4-d, mecoprop and dichloroacetic acid, were more readily adsorbed on positively charged AM-HMS by electrostatic interaction. Hydrogen bonding and van der Waals interaction between adsorbents and adsorbates, as well as combination of these forces, also enhanced the adsorption capacities of HMSs. In addition to the electrostatic interaction, a cationic dye, thioflavin T, was adsorbed on the surfaces due to hydrogen bonding and van der Waals interaction for hydrophilic surfaces and hydrophobic surfaces, respectively.

Highly Active 2D Hexagonal Mesoporous Titanium Silicate Synthesized Using a Cationic−Anionic Mixed-Surfactant Assembly

New highly ordered 2D hexagonal mesoporous titanium silicate materials have been synthesized using a mixture of anionic (sodium lauryl sulfate, SDS) and cationic (cetyltrimethylammonium bromide, CTAB) surfactants as the structure-directing agent (SDA) in the presence of tartaric acid (TA) as a mineralizer of Ti. XRD, N2 adsorption, and TEM data indicate that these mesoporous materials have very high surface areas (1136−1418 m2 g-1) and highly ordered pore arrangements. N2 adsorption studies further suggest that, in addition to uniform mesopores, small micropores with average diameters of ca. 0.58 nm also exist in the pore wall. UV−visible and FTIR spectroscopic studies indicate the incorporation of highly dispersed tetrahedral titanium(IV) species in the silica mesophase. These mesoporous titanium silicate samples showed excellent catalytic activity and selectivity in the epoxidation of styrene using dilute aqueous H2O2 as the oxidant.

Titanium-rich highly ordered mesoporous silica synthesized by using a mixed surfactant system

A new titanium-rich highly ordered 2-D hexagonal mesoporous titanium silicate has been synthesized using a mixture of cationic (cetyltrimethylammonium bromide, CTAB) and non-ionic (Brij-35, C 12H 25-(OC 2H 4) 23-OH, a polyether and aliphatic hydrocarbon chain surfactant) mixed surfactant system as the supramolecular structure directing agent (SDA) in the presence of tartaric acid (TA) as a mineralizer of Ti(IV). XRD, N 2 adsorption and TEM data suggested the presence of mesophase with hexagonal pore arrangements and the UV–visible, FT IR and XPS studies suggested the incorporation of mostly tetrahedral titanium (IV) species in the highly ordered silica network. This mesoporous titanium silicate material showed excellent catalytic activity and selectivity in the epoxidation of styrene using dilute aqueous H 2O 2 as oxidant.

Adsorption and recovery of alkylphenol polyethoxylates from synthetic wastewater using hexagonal mesoporous silicate

A large amount of alkylphenol polyethoxylate (APnEOs), one of endocrine disrupters, is disposed of directly to biological wastewater treatment plants. But microbial oxidation processes cannot completely degrade these molecules to nontoxic forms. Adsorption and recovery efficiency of APnEOs was investigated using four different types of Hexagonal Mesoporous Silicate (HMSs) and powdered activated carbon (PAC). HMSs were synthesized by surfactant-templating methods, and two of them were subsequently grafted with surface functional groups. The two types of organic functional groups grafted on the surface were n-octyldimethyl- and 3-mercaptopropyl- groups. Titanium substituted HMS was also made in the same way as HMS. Adsorption of APnEOs on synthesized HMSs was higher than that on PAC. Larger pore sizes of HMS and Ti-HMS enhanced accessibility of APnEOs to active surface sites in mesopores, which realized higher adsorption capacities and L-shape (Langmuir) adsorption isotherms. Adsorption capacities of APnEOs are influenced by water solubility of APnEOs. APnEOs adsorbed on HMSs can be completely recovered by a mixture of alcohol and water at 5:5 ratio, which is more effective than the recovery from PAC.

Quantitative TEM analysis of a hexagonal mesoporous silicate structure

TEM analysis of mesoporous materials is generally undertaken to give qualitative results. Accurate quantitative analysis is demonstrated in this study. A systematic image analysis of a powder form of a hexagonal mesoporous material known as KIT-6 is conducted using a transmission electron microscope (TEM). Three types of image contrast typically appear in this material (a hexagonal honeycomb structure, wide and narrow parallel lines). The honeycomb face is used to characterise this material in terms of a conventional 2-D hexagonal structure and the d-spacings for the (100) and (110) planes are experimentally measured in varying focus conditions. A tilting experiment is conducted to determine how the angle of tilt affects the line spacing and their visibility. Tilting has very little effect on the line spacing, whereas it affects the visibility of both the wide and narrow lines by limiting an angle range of visibility. The hexagonal lattice structure parameter determined by TEM method is found to be approximately 7% lower than that calculated by low-angle X-ray diffraction. Thus we conclude that TEM data can be used to determine the geometry and dimensions of hexagonal mesoporous silica materials, with a small error in the hexagonal lattice parameter.

Effect of Organic Grafting Modification of Hexagonal Mesoporous Silicate on Haloacetic Acid Removal

Effect of Organic Grafting Modification of Hexagonal Mesoporous Silicate on Haloacetic Acid Removal

Zr K-Edge XAS and29Si MAS NMR Studies on Hexagonal Mesoporous Zirconium Silicate

Hexagonal mesoporous silicate (HMS) molecular sieve containing zirconium species (Zr-HMS) has been synthesized using a neutral template of hexadecylamine with ethanol and water. A disordered channel structure with a uniform diameter of 44.1 A was confirmed by powder X-ray diffraction after calcination. From N2 adsorption/desorption measurement, it is found that Zr-HMS has a very high surface area of about 910 m2/g (BET) and a mesopore with a size of 21 A (BJH). According to the X-ray absorption spectroscopy at the Zr K-edge, it is confirmed that Zr atoms are coordinated by six oxygen neighbors with a distance of 2.13 A, which is shorter than the (Zr—O) bond distance found in ZrO2 or ZrOCl2·8H2O. The 29Si MAS-NMR spectrum for Zr-HMS shows two additional peaks around −94.6 and −107.2 ppm due to the Q3Zr and Q4Zr components, respectively, besides three peaks at −91.1, −101.5, and −111.3 ppm due to the silicate network. The relation between the pore and crystal structures and the local environment around Zr is discussed in detail.

Immobilization of 12-molybdophosphoric and 12-tungstophosphoric acids on metal-substituted hexagonal mesoporous silica

Hexagonal mesoporous silicate (HMS) materials containing Ti, Zr and Al ions were prepared using the neutral template route. The effect of the cation on the immobilization of 12-molybdophosphoric and 12-tungstophosphoric acids on mesoporous materials was studied. The texture of the samples was studied by the N2 adsorption–desorption isotherms, and their structure by X-ray diffraction (XRD), UV-Vis diffuse reflectance (DRS), infrared (IR) and X-ray photoelectron spectroscopy (XPS), and 1H and 31P nuclear magnetic resonance (NMR). Incorporation of different metals into the framework of HMS led to change in the structural and surface properties of mesoporous silica. All metal-containing samples showed a lower surface area compared to that of pure silica. The introduction of Zr and Al cations led to the development of textural mesoporosity in addition to the framework mesoporosity, characteristic of HMS and Ti-containing mesoporous silica (TiHMS) materials. IR and 1H and 31P NMR spectroscopic data revealed that the species present on the surface of HMS and TiHMS mesoporous materials after impregnation with solutions of 12-molybdophosphoric and 12-tungstophosphoric acid are non-degraded heteropoly anions with a preserved Keggin unit. Partial degradation of the anion was observed for zirconium-containing silicate samples (ZrHMS)-supported acids, the extent of which was stronger for supported molybdophosphoric acid in comparison with that of tungstophosphoric acid.

NO Gas Sensor Based on Surface Photovoltage System Fabricated by Self-Ordered Hexagonal Mesoporous Silicate Film

The first reported NO gas sensor based on a surface photovoltage (SPV) semiconductor device system is fabricated with a metal/SiO2 (self-ordered hexagonal mesoporous)/Si3N4/SiO2/Si structure (MIS). A size controlled silicate mesoporous film is successfully synthesized by spin coating on a Si3N4/SiO2/Si silicon wafer using poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (Pluronic P123=EO20PO70EO20) triblock copolymers as a template. The characteristics of the mesoporous films were investigated by X-ray diffraction (XRD) and transmission electron microscope (TEM). The sensing properties of the self-ordered hexagonal mesoporous SPV system have been investigated by repeated exposure to NO gas and air. The changes in the average value and phase of the AC photocurrent (Iph) have been observed after exposure of the films to 100 ppm NO gas. The response of the alternative photocurrent results from the physical adsorption and chemical interaction between detected NO gases and the self-ordered hexagonal mesoporous film.

Structural Control of Surfactant-Templated Hexagonal, Cubic, and Lamellar Mesoporous Silicate Thin Films Prepared by Spin-Casting

Structural Control of Surfactant-Templated Hexagonal, Cubic, and Lamellar Mesoporous Silicate Thin Films Prepared by Spin-Casting