Wide Mesopores Research Articles

Copper Benzene Tricarboxylate Metal–Organic Framework with Wide Permanent Mesopores Stabilized by Keggin Polyoxometallate Ions

Porous solids with organized multiple porosity are of scientific and technological importance for broadening the application range from traditional areas of catalysis and adsorption/separation to drug release and biomedical imaging. Synthesis of crystalline porous materials offering a network of uniform micro- and mesopores remains a major scientific challenge. One strategy is based on variation of synthesis parameters of microporous networks, such as, for example, zeolites or metal-organic frameworks (MOFs). Here, we show the rational development of an hierarchical variant of the microporous cubic Cu(3)(BTC)(2) (BTC = 1,3,5-benzenetricarboxylate) HKUST-1 MOF having strictly repetitive 5 nm wide mesopores separated by uniform microporous walls in a single crystal structure. This new material coined COK-15 (COK = Centrum voor Oppervlaktechemie en Katalyse) was synthesized via a dual-templating approach. Stability was enhanced by Keggin type phosphotungstate (HPW) systematically occluded in the cavities constituting the walls between the mesopores.

Adsorption properties of phenolic resin-based mesoporous carbons obtained by using mixed templates of Pluronic F127 and Brij 58 or Brij 78 polymers

Phenolic resin-based mesoporous carbons were synthesized by using mixed templates of Pluronic F127 and Brij 58 or Brij 78. For the purpose of comparison three samples of nanoporous carbons were prepared by using single templates of Pluronic F127, Brij 58 and Brij 78 polymers, respectively. Adsorption properties of the aforementioned carbons were studied by nitrogen adsorption at −196 °C. The resulting carbons featured high specific surface areas ranging from 641 m2/g for the sample obtained in the presence of Brij 58 polymer to 825 m2/g for the carbon prepared by using a mixed template containing 23% of Pluronic F127 and 77% of Brij 58 and from 588 m2/g for the sample obtained in the presence of Brij 78 polymer to 813 m2/g for the carbon prepared by using Pluronic F127 only. It was shown that the width of mesopores increases with increasing amount of Brij 58 or Brij 78 in the mixture of one of these polymers with Pluronic F127, suggesting that Brij polymers act also as micelle expanders.

Mesoporous silica synthesis in sub- and supercritical carbon dioxide

Mesoporous silicas were synthesized from sodium silicate (Na2Si3O7) and tetraethylorthosilicate (TEOS) with Pluronic F127 (polyethylene oxide-polypropylene oxide-polyethylene oxide, EO106PO70EO106) triblock copolymer using sub- and supercritical carbon dioxide (SubCO2 and SCO2) respectively, as solvents. Templates were removed using liquid carbon dioxide (LCO2) and SCO2. The most efficient template removal was achieved by LCO2 - 92.7% (w/w), followed by LCO2 with ethanol entrainer - 85.6% (w/w), and by methanol - 78.8% (w/w). The best efficiency of template removal by SCO2 was 50.7%. Values of specific surface areas, ABET, were increased by 10% with the in- crease of an ageing time from 6 to 24 hours for Na2Si3O7-based silicas at aqueous synthesis conditions, whereas the use of SCO2 reduced this value by 19.4%. For TEOS-based silicas synthesized using SCO2, ABET values increased by 3.8 times. Application of SCO2 for synthesis of TEOS-based silicas resulted in higher mesopore volumes of 0.719 and 1.241 mL/g with an average mesopore width varying from 3.4 to 3.9 nm. Although Na2Si3O7-based silicas have almost similar mesopore width range, their mesopore volumes were 7 times less than those for TEOS-based silicas. Formation of mesopores in Na2Si3O7- and TEOS-based silicas was at the expense of micropores when synthesized in SCO2.

Highly ordered mesoporous carbon arrays from natural wood materials as counter electrode for dye-sensitized solar cells

Highly ordered mesoporous carbon arrays are prepared by a facile carbonization of the natural bamboo and oak wood in argon atmosphere. The as-prepared oak mesoporous carbon arrays have good electrocatalytic activity and high conductivity, based on their well connected framework, highly ordered microtexture, wider mesopores and larger surface area. Consequentially, the photovoltaic performance of the DSSC with the oak mesoporous carbon array film as counter electrode is excellent and comparable to that of the DSSC with FTO/Pt counter electrode. In addition, it is a simple method for a mass production of mesoporous carbon arrays with natural wood materials.

Hydrothermal Stability of Ordered Mesoporous Titanosilicate Materials Prepared at Room Temperature

A study of hydrothermal stability, performed in boiling water under static conditions, of MCM-41 materials containing different titanium content, prepared by direct synthesis at ambient temperature and pressure, using tetraethoxysilane, titanium ethoxide and octadecyltrimethyl- ammonium bromide is presented. The behaviour is compared with pure silica grades prepared by a similar procedure. The samples were characterised by X-ray diffraction, adsorption of nitrogen at 77K and diffuse reflectance UV–Vis spectroscopy. It was found that the stability improves as the amount of titanium increases and that Ti-MCM-41 samples prepared with Si/Ti≤50 are significantly stable. After 12h in boiling water the pore size uniformity was practically maintained and only a small decrease in pore volume (5-9%), total surface area (2-7%) and mesopore width (3%) and a slight increase in pore wall thickness (3-6%) occurred. In contrast, some degradation of the MCM-41 structure for the pure silica and the lower Ti content (Si/Ti=100) samples was observed with the effect being less pronounced for the latter. The higher hydrothermal stability of titanium substituted samples is probably correlated with a higher degree of polymerisation of the pore walls and with the presence of extra framework titanium.

Study of the sorption-desorption properties of pine wood at the initial stage of decay by wood-rot fungi 10th EWLP, Stockholm, Sweden, August 25–28, 2008

Abstract Hot water extraction of pine wood was carried out after 10 days of exposure to brown-rot fungi (Gloeophyllum trabeum, Coniophora puteana and Poria placenta) and a white-rot fungus (Coriolus versicolor). The microstructure of the wood cell wall was analysed by the water vapour sorption method. The content and composition of monosaccharides in the extracts were determined by high performance liquid chromatography. Despite the absence of essential mass losses at the initial stage of the contact with the fungi, slight changes in the cell wall microstructure can be observed, namely, the hydrophobisation and the change in the character of porosity in the region of 1–10 nm pore sizes. After hot water treatment, the mass decreases considerably, i.e., to 25% for the control and brown-rot treated samples and more than 30% for the white-rot treated sample. The sorption-desorption isotherms for the washed control sample and brown-rot samples were not changed practically, but the isotherm was changed clearly in the case of the white-rot treated sample. The microstructure of the white-rot sample changed after hot water washing radically and was accompanied with the formation of wide mesopores in the range of 5–9 nm width.

Fabrication of mesoporous SiO 2–C–Fe 3O 4/γ–Fe 2O 3 and SiO 2–C–Fe magnetic composites

A synthetic method for the fabrication of silica-based mesoporous magnetic (Fe or iron oxide spinel) nanocomposites with enhanced adsorption and magnetic capabilities is presented. The successful in situ synthesis of magnetic nanoparticles is a consequence of the incorporation of a small amount of carbon into the pores of the silica, this step being essential for the generation of relatively large iron oxide magnetic nanocrystals (∼10 ± 3 nm) and for the formation of iron nanoparticles. These composites combine good magnetic properties (superparamagnetic behaviour in the case of SiO 2–C–Fe 3O 4/γ–Fe 2O 3 samples) with a large and accessible porosity made up of wide mesopores (>9 nm). In the present work, we have demonstrated the usefulness of this kind of composite for the adsorption of a globular protein (hemoglobin). The results obtained show that a significant amount of hemoglobin can be immobilized within the pores of these materials (up to 180 mg g −1 for some of the samples). Moreover, we have proved that the composite loaded with hemoglobin can be easily manipulated by means of an external magnetic field.

Porous texture evolution in activated carbon fibers prepared from poly ( p-phenylene benzobisoxazole) by carbon dioxide activation

Activated carbon fibers (ACFs) were prepared from poly ( p-phenylene benzobisoxazole) (PBO) in two varieties, as spun (AS) and high-modulus (HM). The precursor polymer was carbonized at 1123 K followed by physical activation with carbon dioxide at 1073 K to different burn-off (BO) degrees. The porous texture of the obtained materials was characterized by physical adsorption of nitrogen at 77 K and carbon dioxide at 273 K. Following CO 2 activation, the surface area and pore volume of the obtained ACFs progressively increased with increasing BO. The obtained adsorbents were principally supermicroporous, with a certain contribution from narrow mesopores. The pore size distributions became increasingly heterogeneous as the activation degree increased. The most significant difference between the two varieties of PBO used as precursors was a greater abundance of wide mesopores in the adsorbents prepared from the high-modulus variety.

Effect of organosilane/polymer ratio on adsorption properties of periodic mesoporous ethane-silica

Remarkable achievements in the area of pure siliceous mesoporous molecular sieves such as MCM-41, SBA-15, SBA-16 and FDU-1 paved the way for the discovery of periodic mesoporous organosilicas (PMOs). This work reports the effect of organosilane/polymer ratio on the synthesis of PMO with ethane bridging groups inside the SBA-15-type framework. This ethane-silica was obtained by self-assembly of 1,2-bis(triethoxysilyl)ethane (BTESE) precursor and poly(ethylene oxide)- block-poly(propylene oxide)- block-poly(ethylene oxide) triblock copolymer Pluronic P123 (EO 20PO 70EO 20) as structure directing agent under low acidic conditions. The amount of triblock copolymer P123 was increased progressively from 0.5 and 3 g, while the mass of BTESE in the synthesis gel mixture was kept constant. The polymeric template was removed via extraction with acidified ethanolic solution. The resulting PMO materials were characterized by powder X-ray diffraction, thermogravimetry and nitrogen adsorption at −196 °C. The latter was used to evaluate the specific surface area, mesopore width, total pore volume and the volume of complementary pores. The polymer/BTESE weight ratios of 0.73 and 1.46 favoured the formation of additional amorphous silica inside channel-like mesopores, which resulted in plugged hexagonal templated silicas (PHTS) with low mesopore volume, small mesopore size, wide pore size distribution (PSD), thick pore walls and low mesostructural ordering. A further increase in the polymer/BTESE ratio (2.2) led to a material with large pore diameter, narrow PSD, uniform pore openings and high structural ordering. On the other hand, high polymer/BTESE ratios (from 2.93 to 4.39) afforded PMOs with very broad PSD and high non-uniformity of pore entrances as well as low ordering. Furthermore, a complete removal of ethane bridging groups at elevated temperatures in air did not lead to the disappearance of mesostructural ordering in the OMS studied.

Removal of cyanocobalamine from aqueous solution using mesoporous activated carbon

The adsorption of cyanocobalamine was studied using coal-based mesoporous activated carbon (AC). The ACs tested showed a comparable pore volume, around 0.5 cm 3 g −1, but different contribution of mesopores ranging from 0.53 to 0.82 cm 3 g −1. The adsorption of cyanocobalamine was carried out in slightly alkaline solution in static conditions. Three kinetic models including a first-order Lagergren model, a pseudo-second-order model, and an intraparticle diffusion model were applied to describe the kinetics and mechanism of adsorption. The adsorption of cyanocobalamine on mesoporous carbons followed the pseudo-second-order model. The diffusion of cyanocobalamine molecule within smaller mesopores was identified to be the rate-limiting step. The analysis of adsorption equilibrium data indicates that the adsorption of cyanocobalamine better fits the Langmuir equation than the Freundlich equation. The Langmuir adsorption capacity of the carbon is strongly related to the degree of mesoporosity development. Among ACs tested, the carbon with the highest mesopore volume and mesopore width of 10–50 nm shows the greater ability to remove cyanocobalamine from aqueous solution. The effect of pore-size distribution on the kinetics and mechanism of adsorption has been discussed.

Highly mesoporous carbons obtained using a dynamic template method

New nanoporous carbons with extremely high mesopore volumes and surface areas were obtained using mesoporous silica with a 3-D wormhole porous framework as templates. Mesoporous silica was synthesized following the literature described methods. Polystyrene sulfonic acid-based organic salts were used as carbon precursors. To evaluate the effect of sodium on porosity development silica matrices with various thicknesses of pore walls were synthesized. Prior to carbonization, in order to increase surface heterogeneity, the precursor chemistry was modified by cation exchange with catalytically active metals (i.e., copper, nickel, cobalt). Carbonization followed by HF etching of silica templates generated mesoporous carbons with large surface areas and high pore volumes, which is accompanied by high dispersion of catalytically active metals on the carbon surface. Sodium present in the carbonaceous precursor causes in the dynamic template effect via its reactions with a silica matrix during carbonization. This, along with reactive gases evolved during heating lead to the expansion of the carbonaceous structure, and thus to the unique wide mesopore size distributions of the templated carbons with pore sizes between 10 and 50 nm and their volume exceeding 2.5 cm 3 g −1.

Stepwise Removal of the Copolymer Template from Mesopores and Micropores in SBA-15

A method for the removal of the triblock copolymer template from mesoporous silica SBA-15 in two stages has been developed. The mesopores of SBA-15 are first vacated by treatment with sulfuric acid that decomposes accessible portions of the template via ether cleavage. The concentration of the acid was found to be crucial for the decomposition of the template. The micropores of SBA-15 can then be vacated by calcination at 200 °C, a low temperature that prevents the silica matrix from shrinking, so that both the mesopores and the micropores retain their original sizes and volumes. The connectivity of the mesopores was studied by imaging the inverse platinum replicas of SBA-15 materials. The acid-treated SBA-15 materials also show less shrinkage, even when subsequently calcined at 540 °C. Compared with the template removal just by calcination, the stepwise removal of the copolymer template generates materials with wider mesopores and larger micropore volumes. Furthermore, this method opens up the possibilit...

Chromatographic characterization of macroporous monolithic silica prepared via sol-gel process

A continuous porous silica monolith prepared by the sol-gel process including phase separation was aged in a basic solvent making use of hydrolysis of urea to prepare extended mesopore structures for chromatographic applications. The dissolution–reprecipitation kinetics at the interfaces between wet gel skeletons and an external solvent affected the size and volume of pores formed within the skeletons. At above 200°C, the pore size attained the macropore dimensions (>50 nm). The results of chromatography indicate that the monolithic silica column with wide mesopore could reduce the separation time compared to the conventional column packed with 5 μm silica particle.

Calcium phosphate formation on porous sol-gel-derived SiO2 and CaO-P2O5-SiO2 substrates in vitro.

Sol-gel-derived SiO2 and CaO-P2O5-SiO2 have been shown to be bioactive and bone bonding. In this study bioactive sol-gel-derived SiO2 and CaO-P2O5-SiO2 systems were tested for in in vitro bioactivity. The calcined ceramic monoliths were immersed in a simulated body fluid and analyzed to follow the hydroxyapatite formation on the ceramic surface. Apatite-forming ability was investigated in terms of structural changes by changing the composition and the preparation method. The role of Ca and P dopants in the substrate structure is complicated, and careful characterization is needed. The composition and structure together determine the in vitro bioactivity. The pore structure was analyzed using N2-adsorption/desorption isotherms. The results indicate that a great mesopore volume and a wide mesopore size distribution favor hydroxycarbonate apatite nucleation and a great surface area is not needed. The performed preparation process for silica in a basic environment provides a convenient way to prepare a mesoporous material.

Capillary Condensation of N2 on Multiwall Carbon Nanotubes

The crystalline carbon nanotubes having mesopores that are open at one end, with pore width of 4 ± 0.8 nm, were characterized by XRD, XPS, and HRTEM. The N2 adsorption isotherm of multiwall carbon nanotubes was measured at 77 K. A hump was observed near P/P0 = 0.4 in the adsorption isotherm. The adsorption isotherm was deconvoluted into those on the inner and external surfaces using the separately determined total, inner, and external surface areas. The adsorption isotherm on the inner surface was without adsorption hysteresis and had a sharp uptake at the P/P0 region corresponding to the mesopore width, which was ascribed to capillary condensation in the mesopores of carbon nanotubes.

Pore structures and bulk properties of titania gels prepared from titanous chloride

A series of titanium dioxide gels has been prepared by the oxidation of precipitated hydrous Ti(III) oxide, obtained by the addition of aqueous ammonia or ammonium carbonate to aqueous solutions of titanous chloride. The gels were dried in air at 110°C and their structures and surface properties studied with the aid of DTA, X-ray diffraction, nitrogen adsorption, and spectrophotometry. Analysis of the nitrogen isotherms has revealed that the gels exhibited a wide range of different pore structures, extending from narrow micropores to wide mesopores. The development of the gel porosity appears to have been largely controlled by the mode of removal of water ligands from the hydration sphere of the cations, enlargement of the micropore volume being obtained by the replacement of water by carbonato ligands and pore widening by increase in the pH of precipitation (over the pH range 2.3–7.1). All the gels were X-ray amorphous in their original state: some were converted directly to rutile at 500°C while others underwent the more usual transformation into the intermediate anatase form. It is postulated that the rapid removal of the water ligands can result in the formation of a defect structure which controls the development of the pore structure and also the behavior of the gels on heat treatment.

Formation of porous structures in activated brown-coal chars using O 2, CO 2 and H 2O as activating agents

Two brown coals, xylitic and earthy, carbonized at 1173 K were activated with water vapour, carbon dioxide and oxygen, each producing a different distribution of porosity. In the xylitic coke, activated in the range of burn-offs from 1 to 70%, the action of water vapour results in the development of pores of all dimensions. At the highest burn-off the product has an effective surface area of 920 m 2 g −1 and a total sorptive pore volume of 0.83 cm 3 g −1, 33% of which is in micropores. Carbon dioxide creates, from the xylitic coke at the burn-off of 70%, a highly microporous adsorbent with about the same surface area (890 m 2 g −1) as the corresponding water-vapour activated product. The pore volume of the carbon dioxide sample is lower (0.49 cm 3 g −1) but these contain 63% of micropores, which amounts to a contribution of 92% of these pores to the effective total surface area. The activation of the xylitic coke with oxygen leads to a high development of porosity at low burn-offs, but becomes ineffective on continuation of the process to medium and high burn-offs. This is thought to be due to a blocking of the entrances of the micropores by surface oxygen complexes formed on the surface of the coke. Oxygen gives, at a high burn-off, a product with the lowest total adsorptive volume (0.45 cm 3 g −1) and surface area (650 m 2 g −1). All the activated products obtained from the xylitic coke can be regarded, when effective surface areas are considered, as microporous adsorbents. With the earthy coke a total adsorptive pore volume (consisting mainly of wide mesopores) is developed which is higher than with the corresponding xylitic coke, but this result is difficult to reproduce, because the earthy coke samples are easily influenced by temperature in the process of activation, especially that by oxygen.