Amorphous Mesoporous Silica Research Articles

Exploring the Effect of Porous Structure on Thermal Conductivity in Templated Mesoporous Silica Films

This work elucidates the effect of porous structure on thermal conductivity of mesoporous amorphous silica. Sol–gel and nanoparticle-based mesoporous amorphous silica thin films were synthesized by...

Preparation of Sodium Silicate Solutions and Silica Nanoparticles from South African Coal Fly Ash

The production of amorphous mesoporous silica nanoparticles can be achieved using sodium silicate (Na2SiO3) solutions prepared from South African coal fly ash waste. The first part of this study compared two processes for the preparation of Na2SiO3 solutions. The first process, hereafter called sequential acid-alkaline leaching (SAAL), is a two-stage process, which involves (i) a H2SO4 leaching step for the preferential extraction of reactive aluminium over silicon, followed by (ii) the preferential extraction of silicon over aluminium from the resulting residues using NaOH. The second process is a direct alkaline leaching (DAL) process, which consists of a single-stage elemental extraction from ash using NaOH, i.e. without the preceding acid leaching step used in SAAL. The two processes generated Na2SiO3 solutions with identical pH (11.8), similar silicon (10.2–10.3 g/L), iron (ca. 200 mg/L) and potassium (ca. 800 mg/L) content, and low calcium concentrations (≤ 29 mg/L). However, the inclusion of the acid leaching step in the SAAL process yielded a Na2SiO3 solution with significantly lower aluminium content (166 mg/L vs. 1158 mg/L). The Na2SiO3 solutions obtained from the SAAL and DAL processes were then used as silica precursors to synthesise silica nanoparticles via a sol–gel method using polyethylene glycol (PEG) as surfactant and sulphuric acid as catalyst. All samples of synthesised silica nanoparticles were characterised by a high level of purity (up to 99.3 wt % SiO2). The insight gained is now being used to improve existing processes for the production of high-grade ultra-pure silica nanoparticles (i.e. ≥ 99.9 wt % SiO2) for catalyst support applications.

Hierarchical ZSM-5 beads composed of zeolite nanosheets obtained by pseudomorphic transformation

Abstract The concept of pseudomorphic transformation was used to transform amorphous mesoporous silica beads with different size (20, 50 and 75 μm) into hierarchical MFI-type zeolite beads composed of ZSM-5 nanosheets. The beads were synthesized under hydrothermal conditions with and without mechanical stirring at different temperatures and treatment times. The influence of the different synthesis parameters was investigated by X-Ray Diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, X-Ray Fluorescence, Nitrogen adsorption-desorption measurements, 27 Al solid-state Nuclear Magnetic Resonance, Energy-Dispersive X-Ray analysis and Thermogravimetric analysis. Well-crystallized 20 μm ZSM-5 nanosheets beads similar in size and shape to the original mesoporous amorphous silica beads were obtained after a hydrothermal treatment at 150 °C for 5 days in a tumbling oven and 2 days in static conditions at 120 °C. The influence of the bead size on the pseudomorphic transformation was studied in order to prepare well-crystallized ZSM-5 nanosheets beads of 50 and 75 μm. Results showed that the required time of static treatment at 120 °C increases when the size of the parent silica spheres increases. This is explained by the fact that crystallization starts from the outer bead surface toward the center.

Organic-inorganic hybrid mesoporous titanium silica material as bi-functional heterogeneous catalyst for the CO2 cycloaddition

A kind of mesoporous titanium silica material containing polydiallyldimethylammonium chloride (PDDA) is synthesized by a facile hydrothermal method. The as-synthesized materials are determined by XRD, SEM, UV–Vis, FT-IR, TGA, CO2-TPD, organic elemental analysis and N2 adsorption-desorption. The N2 adsorption-desorption isotherms of the amorphous titanium silica materials are type IV curves, suggesting that materials possess mesoporous structure. Moreover, the PDDA not only is used as a “porogen” but also takes the function of basic active center and activates carbon dioxide (CO2). The CO2-TPD analysis is used to characterize the existence of basic sites and calculate the amount of accessible basic sites in the samples. The calculation results show that the basicity of the material increases with the enhanced amount of PDDA. The mesoporous titanium silica material contains two kinds of active centers simultaneously, which is highly active in fixation reaction of CO2 without co-catalyst. Finally, the epichlorohydrin (ECH) conversion could reach 96.5% and the cyclic carbonate selectivity is 95.0% under optimal conditions. It is worth noting that the amorphous mesoporous titanium silica material shows a superior catalytic performance comparing to Ti-mordenite with regular crystal structure and morphology. Meanwhile, the hybrid mesoporous catalyst shows good reusability and stability in the cycloaddition reaction.

On the adsorption of toluene on amorphous mesoporous silicas with tunable sorption characteristics.

The development of novel adsorbents for the purification of natural gas from aromatic hydrocarbons and the optimization of adsorption processes represent some of the most crucial environmental challenges. In this work, two amorphous mesoporous silica (AMS) samples with different sorption characteristics were prepared by modifying the synthesis method of amorphous mesoporous silica-aluminas, and tested as adsorbents of aromatic molecules for the purification of natural gas. The physico-chemical properties of the obtained materials were finely characterized by means of different experimental techniques (including FTIR and solid-state NMR) with the aim of determining their sorption and surface features. The adsorption capacity of the produced solids towards toluene, chosen as the reference of aromatic molecules, was determined by using FTIR, solid-state NMR spectroscopy and microgravimetric analysis. Finally, in view of applications under more realistic conditions, the adsorption properties of the AMS materials were also investigated after prolonged treatments in water.

Design and characterization of nanocarriers loaded with Levofloxacin for enhanced antimicrobial activity; physicochemical properties, in vitro release and oral acute toxicity

Inorganic and carbon based nanomaterials are widely used against several diseases, such as cancer, autoimmune diseases as well as fungi and bacteria colonization. In this work, Santa Barbara Amorphous mesoporous silica (SBA), Halloysite Nanotubes (HNTs) and Multiwalled Carbon Nanotubes (CNTs) were loaded with fluoroquinolone Levofloxacin (LVF) to be applied as antimicrobial agents. The prepared via adsorption nanocarriers were characterized by Fourier-Transformed Spectroscopy, Scanning Electron Microscopy as well as High Pressure liquid Chromatography. In vitro release studies were carried out using Simulated Body Fluid at 37o C and data analyzed by various kinetic models showing slow dissolution over 12-24 hours. Antimicrobial studies showed improved antibacterial activity against Escherichia coli, Enterococcus faecalis, Listeria monocytogenes, Staphylococcus aureus, and Staphylococcus epidermidis compared to neat nanomaterials. CNTs were found to be the most promising candidates for LVF delivery and they were chosen to be further studied for their acute oral toxicity and histopathological examination using C57/Black mice. Histological examination depicted that drug loading did not affect mice organs morphology as well as hepatocyte degeneration, central vein degeneration and parenchymal necrosis scores. To conclude, the prepared nanomaterials present significant characteristics and can act as antimicrobial drug carriers; CNTs found to be safe candidates when orally fed to mice.

On the Structure of Amorphous Mesoporous Silica Nanoparticles by Aberration-Corrected STEM.

Mesoporous silica materials have demonstrated a vast spectrum of applications, stimulating an intensive field of study due to their potential use as nanocarriers. Nonetheless, when produced at the nanoscale, their structural characterization is hindered due to the re-arrangement of the pores. To address this issue, this work combines molecular dynamics simulations with electron microscopy computer simulations and experimental results to provide an insight into the structure of amorphous mesoporous silica nanoparticles. The amorphous silica model is prepared using a simple melt-quench molecular dynamics method, while the reconstruction of the mesoporous nanoparticles is carried out using a methodology to avoid false symmetry in the final model. Simulated scanning transmission electron microscopy images are compared with experimental images, revealing the existence of structural domains, created by the misalignment of the pores to compensate the surface tension of these spherical nanoparticles.

Ferrosilicate-Based Heterogeneous Fenton Catalysts: Influence of Crystallinity, Porosity, and Iron Speciation

Different ferrosilicate samples have been prepared with varying degrees of crystallinity, porous texture, and speciation of the Fe sites by both hydrothermal and sol–gel procedures: Fe-silicalite-1 with microcrystals (2–10 µm) and nanocrystals (180 nm), Fe-containing composite material consisting of silicalite-1 and amorphous silica, and two samples of mesoporous Fe-containing amorphous silica Fe–SiO2. The resulting solids have been tested for their potential as organic pollutants removal under Fenton-like conditions in heterogeneous catalytic wet peroxide oxidation of phenol and clarithromycin lactobionate. Our results indicate that the three aforementioned parameters show a strong interplay towards the abatement of pollutants in liquid phase. Thus, samples with high crystallinity show an improved performance in the oxidation of organic contaminants over amorphous samples in which the Fe speciation is very well controlled.

The role of water in the reusability of aminated silica catalysts for aldol reactions

The reusability of methylaminopropyl active sites grafted on mesoporous amorphous silica, either with cooperative silanol groups or trimethylsilated, was assessed in the aldol reaction of acetone with 4-nitrobenzaldehyde. Raman, 13C NMR, and UV–Vis spectroscopy demonstrated the presence of stable enamines on the spent catalysts. These enamines are produced as a side product from iminium intermediates in the catalytic cycle. Water co-feeding enhances the desorption of the iminium intermediates and, hence, suppresses the formation of these stable enamine species. The reusability of the cooperative catalyst increased to 70% with co-feeding 0.69 wt% water, while an almost complete reusability was achieved for the trimethylsilated catalyst. Continuous-flow experimentation showed that the cooperative effect of the silanol groups was lost during the first 7 h on-stream, yet activity losses continued, most likely due to silica hydrolysis. Activity losses persisted on the more hydrophobic trimethylsilated catalyst, but were significantly less pronounced.

In situ polymerization of poly(vinylimidazole) into the pores of ‎hierarchical MFI zeolite as an acid–base bifunctional catalyst for one-pot ‎C–C bond cascade reactions

Poly(vinylimidazole)/hierarchical ZSM-5 has been prepared as a novel and efficient acid–base bifunctional catalyst by a simple method. First, the hierarchical ZSM-5 zeolite was synthesized by an indirect method from KIT-6 as a silica source. By this method, control of the zeolite crystallization was achieved due to the adjustment transformation processing of amorphous mesoporous silica to zeolite crystal. Then, vinylimidazole as a basic part was polymerized by an in situ method into the zeolite’s pores. This acid–base bifunctional heterogeneous catalyst was characterized by FT-IR, TG-DTG, N2 adsorption–desorption, TEM, SEM, NH3-TPD, and XRD. The catalyst has been applied to one-pot C–C bond formation tandem reactions including deacetalization–Henry reaction and deacetalization–Aldol condensation by the simple method at low temperature. Due to the uniform distribution of poly(vinylimidazole) in the micro-meso pores of the hierarchical zeolite reactants, the products easily pass through the catalytic active sites. The co-existence of acidic and basic sites in the structure of the catalyst has a crucial role in the superior activity of this catalyst. Moreover, the catalyst showed excellent recyclability and high activity even after 7 runs with ‎only a 10% reduction in activity being detected.‎

5-Hydroxymethylfurfural production from dehydration of fructose catalyzed by Aquivion@silica solid acid

5-Hydroxymethylfurfural (HMF) production from catalytic dehydration of fructose was studied over mesoporous silica heterogenized perfluorosulfonic acid resin (Aquivion@silica) catalyst. Amphiphilic Aquivion PFSA was used as both acidic and template agent for the sol–gel synthesis of amorphous mesoporous silica which was found very appropriate to effectively immobilize Aquivion resin by using an impregnation method. Various characterizations including elemental analysis, acid-base titration, XRD, FTIR, Raman, N2 adsorption–desorption, SEM and TEM revealed that Aquivion@silica catalyst presented large surface area, sufficient porosity and afforded highly accessible sulfonic acid groups. Efficient and productive HMF production from the dehydration of fructose was achieved in the polar dimethyl sulfoxide (DMSO) solvent at low temperature of 90 °C, i.e. 85% yield of HMF and 43 molHMF molH+ productivity of HMF. The influence of reaction time, reaction temperature, Aquivion loading, catalyst dosage and solvent on the dehydration process was systematically investigated. The catalyst could be regenerated through a simple ion-exchange method and a stable HMF yield was maintained after being used four times.

Separation of trace amounts of palladium from water and wastewater samples using MPTMS-SBA-15 mesoporous silica sorbents

ABSTRACTThis study describes the application of functionalized Santa Barbara Amorphous mesoporous silica as an easily prepared and stable solid sorbent for the separation of trace amounts of palladium (Pd) ions in aqueous solutions. The mesoporous silica was functionalized by 3-mercaptopropyltrimetoxysilane, and its application as solid sorbents for the preconcentration of Pd ion was studied. Different experimental conditions such as pH, type and concentration of eluent solution were optimized. The influence of various interferences ions on the recovery of Pd(II) was investigated. In the optimized condition, the recovery was greater than 98.3%. The maximum adsorption capacity was obtained 146.2 mg g−1 for the modified sorbent. The proposed procedure was applied for the separation and preconcentration of Pd in water and wastewater samples.

Preparation of Mesoporous Silica from Electrolytic Manganese Slags by Using Amino-Ended Hyperbranched Polyamide as Template

Large quantities of waste slags are produced during the preparation of electrical manganese, causing serious pollution to the environment. The recycling and utilization of electrolytic manganese slags (EMS) is a serious challenge to the industry. Here, we report the utilization of EMS in preparation of high-performance mesoporous silica using amino-ended hyperbranched polyamide (AEHPA) as template. The effects of AEHPA content and molecular weight on properties of mesoporous silica, including the specific surface area, pore diameter, pore volume, size and distribution, have been investigated. Based on 0.3 wt% AEHPA-2 during preparing silica, the specific surface area, pore volume and pore diameter of the produced amorphous mesoporous silica are 451.34 m2•g-1, 0.824 cm3•g-1 and 7.09 nm, respectively, showing remarkable improvements over the silica without AEHPA with specific surface area (271.05 m2•g-1), pore volume (1.167 cm3•g-1) and pore diameter (17.43 nm). The formation mechanism of mesoporous silica ...

Propylsulfonic acid functionalized MCA cubic mesoporous and ZSM-5-MCA composite catalysts for anisole alkylation

Propylsulfonic acid functionalized mesoporous cubic Ia-3d amorphous (MCA) silica (MCA-Pr-SO3H) was synthesized by co-condensation. Different amounts of ZSM-5 seeds were added into the Ia-3d mesopore silica gel to yield ZM-x composites (where x is the wt.% loading level of ZSM-5). The structure characterization, morphology and acidity of the synthesized materials were characterized by X-ray diffraction, nitrogen adsorption-desorption, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy and 13C-NMR techniques. The composite materials were found to have properties of both mesoporous (MCA) and ZSM-5, with the ZSM-5 loading level having a major effect on the structure of the Ia-3d mesoporous composite. The MCA-Pr-SO3H catalytic activity in the anisole alkylation reaction with tert-butanol was compared to that of Amberlyst-15, ZSM-5 and ZM-30 materials. The highest yield of total tert-butylated anisole (TBA) products was obtained with MCA-Pr-SO3H, with the major products being 4-tert-butyl anisole (4-TBA), 2-tert-butyl anisole and 2, 4-di-tert-butyl anisole. Compared with the other materials, ZM-30 promoted high selectivity of para-alkylated anisole (4-TBA) and the yield of TBA products.

Effects of Void Environment and Acid Strength on Alkene Oligomerization Selectivity

The effects of channel connectivity, void environment, and acid strength on the relative rates of oligomerization, β-scission, and isomerization reactions during light alkene conversion (ethene, propene, isobutene; 2–400 kPa alkene; 473–533 K) were examined on microporous (TON, MFI, MOR, BEA, FAU) and mesoporous (amorphous silica–alumina (SiAl), MCM-41, Keggin POM) Bronsted acids with a broad range of confining voids and acid strength. Skeletal and regioisomers equilibrate under all conditions of pressure and conversion and on all catalysts, irrespective of their acid strength, void size, or framework connectivity, consistent with rapid hydride and methyl shifts of alkoxides intermediates and with their fast adsorption–desorption steps. Such equilibration is evident from detailed chemical speciation of the products and also from intramolecular isotopic scrambling in all oligomers formed from 2-13C-propene on TON, MFI, SiAl, and POM clusters. Previous claims of kinetic control of skeletal isomers in oligom...

A new method for the generation of realistic atomistic models of siliceous MCM-41

A new method is outlined for constructing realistic models of the mesoporous amorphous silica adsorbent, MCM-41. The procedure uses the melt-quench molecular dynamics technique. Previous methods are either computationally expensive or overly simplified, missing key details necessary for agreement with experimental data. Our approach enables a whole family of models spanning a range of pore widths and wall thicknesses to be efficiently developed and yet sophisticated enough to allow functionalisation of the surface – necessary for modelling systems such as self-assembled monolayers on mesoporous supports (SAMMS), used in nuclear effluent clean-up.The models were validated in two ways. The first method involved the construction of adsorption isotherms from grand canonical Monte Carlo simulations, which were in line with experimental data. The second method involved computing isosteric heats at zero coverage and Henry law coefficients for small adsorbate molecules. The values obtained for carbon dioxide gave good agreement with experimental values.We use the new method to explore the effect of increasing the preparation quench rate, pore diameter and wall thickness on low pressure adsorption. Our results show that tailoring a material to have a narrow pore diameter can enhance the physisorption of gas species to MCM-41 at low pressure.

Mesoporous silica derived from kaolin: Specific surface area enlargement via a new zeolite-involved template-free strategy

A novel zeolite-involved template-free strategy is proposed and demonstrated to process natural kaolin into mesoporous silica with enlarged specific surface area (SBET). Kaolin was calcined and alkali-treated into aluminosilica zeolite, and then transformed into amorphous mesoporous silica with SBET of 751.5m2/g and pore size of ~4.0nm at the maximum probability in a subsequent acid-treatment. The alkali-treatment and the formation of zeolite intermediate were found to be the key for the SBET enlargement. The mechanisms were studied by XRD, XRF, SEM, TEM and N2 adsorption/desorption. The mesoporous silica shows favorable adsorption ability towards methylene blue with a monolayer adsorption capacity of up to 756.5mg/g, indicating its promising potential in adsorption application.

Synthesis and characterization of amorphous mesoporous silica using TEMPO-functionalized amphiphilic templates

Inorganic–organic hybrid materials based on amorphous mesoporous silica containing organized nitroxide radicals within its mesopores have been prepared using the micellar self-assembly of TEOS solutions containing the nitroxide functionalized amphiphile (4-(N,N-dimethyl-N-hexadecylammonium)-2,2,6,6-tetramethyl-piperidin-N-oxyl-iodide) (CAT-16). This template has been used both in its pure form and in various mixtures with cetyl trimethylammonium bromide (CTAB). The samples have been characterized by chemical analysis, N2 sorption studies, magnetic susceptibility measurements, and various spectroscopic methods. While electron paramagnetic resonance (EPR) spectra indicate that the strength of the intermolecular spin–spin interactions can be controlled via the CAT-16/CTAB ratio, nuclear magnetic resonance (NMR) data suggest that these interactions are too weak to facilitate cooperative magnetism.

Evaluation of Ability of Hydrogen Absorption in SAB-16/Pd Nanostructure Composite

The purpose of this work was to study the hydrogen adsorption on the surface of mesoporous materials based on silica (SBA-16) modified with palladium via temperature. Since mesoporous silica materials have a high specific surface area, and the ordered mesoporous size of 2-10nm, they are suitable for adsorption and storage of hydrogen. SBA-16 is suitable for this purpose due to its cubic crystalstructure and open pores. Single-stage sol-gel method was used to produce nanostructure composite from salt of palladium (PdCl3) and mesoporous silica precursor. The aging time was selected as 12 hr at 80 C. Furthermore, the obtained materials were heated at 550 C for 6 hr to remove surfactant and to form pores. Then the materials were characterized by large angle and small angle x-ray diffraction analysis, and hydrogen absorption analysis at upto 200kPa pressure at three different temperatures of -196 C (77 K), -123 C (150 K) and 30 C (303 K). Furthermore, adsorption-desorption of nitrogen gas was studied. The surface morphology was observed by field emission scanning electron microscope (FESEM). In addition, the amount of palladium, oxygen, and silicon were measured by using energy dispersive spectroscopy (EDS). Finally, the functional groups on the surface of mesoporous silica materials were evaluated using Fourier transform infrared spectroscopy (FTIR). The results of XRD and EDS analyses confirmed the presence of palladium and palladium oxide in mesoporous amorphous silica. In addition, BET results showed that addition of palladium in SBA-16 decreased the surface area, and produced 791 and 538m/g for SBA-16 and SBA-16/Pd, respectively. Hydrogen absorption in nano structure composite was decreasing with temperatur in comparison with SAB-16. On the other hand, the maximum hydrogen absorption in the nano structure composite containing palladium was obtained at -196 C (77 K).

Isolation and immobilization of alkaline protease on mesoporous silica and mesoporous ZSM-5 zeolite materials for improved catalytic properties

Alkaline protease from brinjal leaf (Solanum melongena) having milk clotting activity has been purified to 9.44 fold to a final specific activity of 45.71U/mg. SDS-PAGE of the final preparation revealed a single protein band of approx 14kDa. Purified enzyme was characterized and was successfully immobilized into the amorphous mesoporous silica (SBA-15) and crystalline mesoporous zeolite (Nano-ZSM-5) using entrapment method. Maximum immobilization of 63.5% and 79.77% was obtained with SBA-15 and Nano-ZSM-5, respectively. This protocol serves as a novel approach for bioprocesses, mainly as milk coagulant for local dairy products and particularly, cheese making, and opens the new dimension of further research and other innovation.