Mesopore Size Distribution Research Articles

Improvement of Mesopore Structure of Resorcinol and Formaldehyde Carbon Cryogels by Acid Solution

Resorcinol and formaldehyde (RF) carbon cryogels are prepared by sol-gel polycondensation by using sodium carbonate (C) as a catalyst with water (W) as solvent, immersed in acid solution, hydrochloric acid (HCl), dried by freeze-drying technique and then carbonized under inert atmosphere, respectively. Compared with their carbon precursors, both Vmes and mesopore size distributions of all carbon cryogels can be developed since the pore shrinkage is the crucial role in the change of pore structure of carbon gels. HCl may promote the strength of small pore structure and increase the pore shrinkage of large pore structure. Consequently, the treatment of HCl does not only enhance the partial collapse of large pore structure, the treatment also increases the strength of small pore structure to inhibit the shrinkage of this structure during carbonization as well.

Characterization of Al, Cr-pillared clays and CO oxidation

Bentonite from Turkey was used in pillared clay synthesis by use of single and mixed oxide pillars of aluminium and chromium. X-ray photoelectron spectroscopy (XPS) confirmed that chromium was in two different oxidation forms, while energy dispersive spectroscopy (EDS) indicated the dispersion of chromium on the surface of the pillars and the clay mineral layers. The basal spacing of montmorillonite, and as a result, the specific surface area values were increased by pillaring. The microsurface area and micropore volume values occupied a considerable part of the total values. Nitrogen adsorption/desorption studies and scanning electron microscopy (SEM) images confirmed that the micropore and mesopore size distributions were not affected by synthesis conditions. Fourier transform infrared spectroscopy (FTIR) showed the presence of Brønsted and Lewis acid sites. The maximum conversion of carbon monoxide was observed with the use of a pillaring solution having a Cr/Al ratio of 0.33.

The effect of dopant Cu, Fe, Ni or La on the structures and properties of mesoporous Co-Ce-O compound catalysts

Abstract A series of Co-Ce-O mesoporous catalysts doped with Cu, Fe, Ni or La and the undoped one were synthesized by using tri-block copolymer P-123 as the template. These catalysts show wormhole-like structures, high surface areas (144–167 m 2 /g) and uniform meso-pore size distributions (4.0–4.8 nm) after calcination at 500 °C. The activity for low-temperature CO oxidation and the thermal stability of the mesoporous Co-Ce-O catalyst are largely modified by the dopant Cu, Fe, Ni or La in different ways. It is revealed by in situ diffuse reflectance infrared spectroscopy that CO oxidation over all the samples except the Ni-doped one undergoes carbonates pathway. In this case, the oxidation activities of the catalysts are mainly determined by the mobility of surface lattice oxygen species, which is indicated by the temperature-programmed reduction and desorption results. Doping with Cu greatly enhances the oxidation activity of Co-Ce-O catalyst at the calcination temperatures of 500 °C and 650 °C, and doping with La significantly improves its activity at the calcination temperature of 800 °C. However, doping with Fe always decreases the activity of Co-Ce-O catalyst regardless of the calcination temperature. Largely different from other dopants, the addition of Ni induces a change of the mechanism for CO oxidation and results in a remarkable decrease in the activity.

Textural characterization of native and n-alky-bonded silica monoliths by mercury intrusion/extrusion, inverse size exclusion chromatography and nitrogen adsorption

Native and n-alkyl-bonded ( n-octadecyl) monolithic silica rods with mesopores in the range between 10 and 25 nm and macropores in the range between 1.8 and 6.0 μm were examined by mercury intrusion/extrusion, inverse size exclusion chromatography (ISEC) and nitrogen sorption. Our results reveal very good agreement for the mesopore size distribution obtained from nitrogen adsorption (in combination with an advanced NLDFT analysis) and ISEC. Our studies highlight the importance of mercury porosimetry for the assessment of the macropore size distribution and show that mercury porosimetry is the only method which allows obtaining a combined and comprehensive structural characterization of macroporous/mesoporous silica monoliths. Our data clearly confirm that mercury porosimetry hysteresis and entrapment have different origin, and indicate the intrinsic nature of mercury porosimetry hysteresis in these silica monoliths. Within this context some silica monoliths show the remarkable result of no entrapment of mercury after extrusion from the mesopore system (i.e. for the first intrusion/extrusion cycle). The results of a systematic study of the mercury intrusion/extrusion behavior into native silica monoliths and monoliths with bonded n-alkyl groups reveals that the macro (through) pore structure, which controls the mass transfer to and from the mesopores, here mainly controls the entrapment behavior. Our data suggest that mercury intrusion/extrusion porosimetry does not only allow to obtain a comprehensive pore structure analysis, but can also serve as a tool to estimate the mass transport properties of silica monoliths to be employed in liquid-phase separation processes.

Improvement of mesoporosity of carbon cryogels by acid treatment of hydrogels

Wattle tannin–furfural (TFu) gels are synthesized by the sol–gel polycondensation of wattle tannin with furfural by using sodium hydroxide as a catalyst and cured at 363 K. After cured, the TFu hydrogels are treated in hydrochloric acid (HCl) solutions with various variables as follows: aging temperature, HCl concentration and pH. TFu cryogels aged are then freeze-dried and pyrolyzed under an inert atmosphere to obtain TFu carbon cryogels. The TFu and carbon cryogels were characterized by N 2 adsorption and scanning electron microscope. The high HCl concentration yields the increase of mesopore volume almost twice especially at high temperature and the mesopore size distribution can be greatly developed sharper and narrower when the high concentration of HCl or/and the high temperature are used. Moreover, this method is the versatile approach for developing the mesopore structure of the low and high porous TFu hydrogels and has no serious problem of weight loss after treatment.

Alkoxy-derived multiscale porous TiO2 gels probed by ultra-small-angle X-ray scattering and small-angle X-ray scattering

Titania (TiO2) monoliths with well-defined bicontinuous macropores and gel skeletons were prepared through the alkoxy-derived sol–gel process accompanied by spinodal decomposition, and the structural evolution during evaporation drying and heat treatment was probed by a combination of ultra-small-angle X-ray scattering and small-angle X-ray scattering. X-ray scattering profiles of wet and dried gels revealed that microporous structures related to the existence of primary particles are present in the gel skeletons at the wet stage and are preserved during drying. Additionally, it is found that the primary particles swollen in the wet condition are dried to compact aggregates to produce the smooth surface of gel skeletons. Upon heating at 400 °C, the particle–particle correlation associated with regularity of mesostructures is enhanced. From nitrogen adsorption–desorption measurements, the average pore size is less than 1 nm in the dried gel and increases to 3.1 nm by the heat treatment. Homogeneous growth of primary particles due to interparticle-polycondensation reaction is responsible for the increased size and uniform distribution of mesopores in the heat-treated gel.

Characterization of polymer-based monolithic capillary columns by inverse size-exclusion chromatography and mercury-intrusion porosimetry

Organic-polymer monolithic capillary columns were prepared in fused-silica capillaries by a radical copolymerization reaction of butyl methacrylate and ethylene dimethacrylate monomers in the presence of 1,4-butanediol and 1-propanol as porogen solvents and azobisisobutyronitrile as the initiator. The porous properties could be influenced by changing the ratio of porogen solvents, while keeping the monomer content constant, or by changing the ratio of monomers to porogen in the polymerization mixture. The resulting chromatographic properties, such as porosity, permeability were determined under high-pressure liquid chromatography (HPLC) conditions. The mesopore size distributions of the monolithic materials determined with inverse size-exclusion chromatography (ISEC) were compared with those measured by mercury-intrusion porosimetry. Both techniques are complementary. While mercury-porosimetry measures the entire range of pore sizes and provides more physical information on the monoliths, ISEC is very suitable for determining the size of mesopores in the swollen monoliths.

SYNTHESIS OF HIGHLY ORDERED TITANIUM-CONTAINING SBA-15 MESOPOROUS SILICAS FOR CATALYTIC ECO-FRIENDLY OXIDATIONS

Mesoporous Ti-SBA-15 materials have been synthesized up to a Si/Ti ratio of 20 using a two-step pH adjusting method. Low angle X-ray diffraction and diffuse reflectance UV-Vis spectroscopy revealed that the optimal pH value to synthesize highly ordered mesoporous solids having Ti atoms primarily in tetrahedral coordination environments is around 7.5. The nitrogen physisorption and TEM indicated also ordered mesostructures having relatively large surface areas and uniform mesopore size distribution. It was found that the amount of Ti (IV) increases with the decrease in the Si/Ti ratio. High titanium to silicon ratios (>20) lead to the formation of nanosized anatase phases. DR UV-vis results demonstrated the formation of Ti-OOH active functionalities at the Ti (IV) sites of Ti-SBA-15 upon exposure to H2O2 aqueous solution.

Low‐Temperature Fabrication of Porous β‐SiC Ceramics in Sodium Vapor

β‐silicon carbide (SiC) porous ceramics were synthesized from pelletized powder mixtures of silicon (Si) and fullerene or Si and amorphous carbon (carbon black) at 1000 K for 24 h in sodium (Na) vapor. The relative density of the ceramics was 29%–34% of the theoretical density of SiC. Scanning electron microscopic observation of the fracture surface showed that the ceramics prepared with fullerene were agglomerates of submicrometer‐sized SiC particles and open spaces. The samples prepared with carbon black had a smooth fracture surface with cavities and voids. Using transmission electron microscopy, grains of over 250 nm and a diffuse electron diffraction ring pattern of β‐SiC were observed for the sample prepared with fullerene, and grains of 10–20 nm with a β‐SiC spot ring pattern for the sample prepared with carbon black. A surface area of 11–17 m2/g and a mesopore size distribution in the range of 2–10 nm were shown by a nitrogen adsorption technique. Energy‐dispersive X‐ray analysis detected 1–5 at.% of Na against Si on the fracture surface of the ceramics.

Mesoporous and nanostructured CeO 2 as supports of nano-sized gold catalysts for low-temperature water-gas shift reaction

Mesoporous particles and 1D nanorods of cerium oxides have been prepared by modifying the hydrothermal route of a surfactant-assisted controllable synthesis. Mesoporous cerias were obtained in a sealed glass vessel under continuous stirring, while ceria nanorods were obtained in a Teflon-lined autoclave without stirring. The mesoporous cerias did not show long-range mesoscopic organization, exhibiting a broad mesopore size distribution in the region 8–15 nm. A BET surface area of 100 m 2/g with a total pore volume of 0.33 cm 3/g is obtained for as-synthesized mesoporous ceria. The ceria nanorods exhibit a cubic crystalline structure after calcination, having the lengths in the range of 150–300 nm and diameters in the range of 10–25 nm. The growth direction of ceria nanorods is along [1 1 0]. A surface area of above 50 m 2/g is obtained in the calcined nanorods. These synthesized ceria materials were used as supports of nano-sized gold catalysts, prepared by deposition–precipitation method. Their catalytic activity was evaluated by the low-temperature water-gas shift reaction. The gold/mesoporous ceria catalytic system exhibited higher catalytic activity than gold/ceria nanorods. It is revealed that the mesoporous and nanostructured cerias are of much interest as potential supports for gold-based catalysts that are effective for low-temperature water-gas shift reaction.

Bimodal mesoporous TiO2–P25 composite thick films with high photocatalytic activity and improved structural integrity

Bimodal mesoporous TiO2–P25 composite thick films with improved structural integrity (less crack formation) were successfully synthesized by employing an environmental friendly and highly viscous nonionic surfactant Tween 20 as a tri-functional template, which can simultaneously improve film texture, bimodal mesopore structure and structural integrity in the films. It was found that increasing Tween 20 loading from 10% to 50% (v/v) could simultaneously increase both volumes of small mesopore and secondary large mesopore. Degussa P25 nanoparticle can function as a kind of pore wall contributing to the formation of secondary large mesopore. Optimizing the loading of Tween 20 could result in optimum film thickness, even with only one dip-coating cycle. The final film at optimum Tween 20 loading (50%, v/v) exhibited large porosity (50.6%) and bimodal mesoporous size distribution with small mesopore size of ∼5nm and large mesopore size of ∼34nm. Compared with mono-modal mesoporous TiO2 film (no Degussa P25) at optimum preparation conditions (Tween 20, 50% (v/v), six dip-coating layers), the TiO2–P25 composite films prepared with the same Tween 20 loading (one dip-coating layer) or with 25% (v/v) Tween 20 loading (two dip-coating layers) demonstrated higher photocatalytic activities in the degradation of creatinine, an important human metabolite present in human urine. These results prove that bimodal mesoporous TiO2–P25 composite films have promising properties for photocatalytic water treatment.

Direct synthesis of mesoporous silica presenting large and tunable pores using BAB triblock copolymers: Influence of each copolymer block on the porous structure

Mesoporous silicas with large and tunable (from 4 to 40 nm) accessible pores have been successfully synthesized using laboratory-made BAB (A: hydrophilic block, B: hydrophobic block) type triblock copolymers as templates. A direct synthesis is carried out in the absence of swelling agent and without any further treatment. The polystyrene- b–poly(ethylene oxide)- b–polystyrene (PS- b–PEO- b–PS) copolymers were synthesized using living/controlled radical polymerization. The porous structure (mesopore size, size distribution and microporous volume) was characterized using electron transmission microscopy and nitrogen sorption measurements. This study is focused on the control of the porous structure as a function of the hydrophobic PS block length and the hydrophilic block length (whilst keeping the other block length constant). It was found that when the PS block length increases: (i) the mesopore size increases linearly with PS block length which thus proffers a fine tuning of the pore diameter from 4 to 40 nm, (ii) the micropore volume decreases. When the PEO block length increases, (i) the mesopore size decreases (ii) the micropore volume increases. All these results were explained considering (i) that both PS and PEO block lengths have an influence on the degree of stretching of the PS chains, thus on the micellar core radius (ii) that the mesopore size is directly connected to the micellar core radius, i.e. that PEO chains do not directly participate to mesoporosity (due to its particular loop configuration) but only to microporosity.

Carbon-coated mesoporous silica with hydrophobicity and electrical conductivity

The pore surface of mesoporous silica SBA-15 was coated with 2,3-dihydroxynaphthalene (DN) through a dehydration reaction between the surface silanol groups in SBA-15 and the hydroxyl groups of the DN molecules. By the carbonization of DN in the SBA-15 pores, the pore surface was uniformly covered with an extremely thin carbon layer, which comprised only 1–2 graphene sheets. The resulting carbon-coated SBA-15 still possessed the characteristics of the original SBA-15—large surface area and pore volume, long-range ordered structure, and sharp mesopore size distribution. In addition, the carbon-coated SBA-15 showed marked hydrophobicity and high electrical conductivity, both of which are not intrinsic properties of SBA-15. The appearance of these features can be explained from the almost perfect carbon coating on the pore surface. Newly developed graphene coating technique can donate characteristic carbon properties to mesoporous silica.

Control of mesoporous properties of carbon cryogels prepared from wattle tannin and furfural

Wattle tannin–furfural (TFu) carbon cryogels are synthesized by sol–gel polycondensation of wattle tannin with furfural by using sodium hydroxide (NaOH) as a catalyst, dried by freeze-drying technique and then pyrolyzed under inert atmosphere, respectively. The amounts of wattle tannin (T), furfural (Fu), NaOH (C) and distilled water (W) are changed for preparing the mesoporous TFu carbon cryogels. The mole ratio of tannin to catalyst T/C plays a crucial role for the synthesis of TFu organic and carbon cryogels. The results suggest that the T/C ratio should be above 0.25 but <1.0 to prepare the mesoporous and homogeneous cryogels. Although TFu carbon cryogels have the broad mesopore size distribution, the mesoporous structure is controllable by the synthesis conditions. The carbon cryogels possess the mesopore volume less than 0.56 cm3/g and the BET surface area less than 600 m2/g. Moreover, the ratio of catalyst to water C/W can be used to prepare the homogeneous and mesoporous carbon cryogels, and to control the mesopore radius of carbon cryogels in the range of 1.6–9.6 nm.

The effect of sulphuric acid activation on the crystallinity, surface area, porosity, surface acidity, and bleaching power of a bentonite

The Hançılı (Keskin, Ankara, Turkey) bentonite was activated with H 2SO 4 by dry method at 97 °C for 6 h to obtain optimum parameters for imparting a maximum bleaching power towards soybean oil. The H 2SO 4 content in dry bentonite-acid mixture was changed between 0% and 70%. The natural and activated samples were examined by X-ray diffraction (XRD), N 2 adsorption–desorption, and n-butylamine adsorption (from the solution in cyclohexane). The specific surface area ( S), specific micro–mesopore volume ( V), mesopore size distribution (PSD), and surface acidity ( n m) of the samples were determined. The bleaching power (BP) of each sample for alkali-refined soybean oil was determined. The S, V, n m, and BP increase after activation at various acid contents up to 40% H 2SO 4 without any considerable change in crystal structure of the smectite. The BP is controlled more by the PSD rather than other adsorptive properties of the bleaching earth. The optimum parameters for activation to obtain maximum bleaching power, are H 2SO 4% = 50–60, S = 250–230 m 2 g −1, V = 0.46–0.47 cm 3 g −1, n m = 9.0 × 10 −4–8.4 × 10 −4 mol g −1 and PSD mainly distributed between 1.4 and 6.0 nm.

Phase Separation in Alkoxy-Derived Silica System Containing Polyacrylamide

ABSTRACTBy inducing phase separation parallel to the sol-gel transition of alkoxy-derived silica systems, gels having both macroporous and mesoporous structures can be obtained. Using poly(acrylamide) (PAAm) as a phase-separation inducer, macro/mesoporous silica gels were synthesized. After solvent exchange by water, the size distribution of mesopores of wet gels was evaluated by thermoporometry using a differential scanning calorimetry (DSC). Alternatively, gels were evaporation-dried after solvent exchange by ethanol or water/ethanol, followed by heat-treatment to completely remove volatile and organic components. Characterization of the dried or heat-treated samples was carried out using a scanning electron microscope (SEM) and by nitrogen adsorption measurements. Experimental results showed that the interaction between PAAm and silica is not so strong as the case of polymers having poly(oxyethylene) chains. The contribution of the secondary phase separation within the crosslinking silica-rich phase was suggested to be responsible for the mesopore formation in the PAAm-silica system.

Direct synthesis of Cerium(III)-incorporated SBA-15 mesoporous molecular sieves by two-step synthesis method

Cerium(III)-incorporated SBA-15 (Ce-SBA-15) mesoporous molecular sieves were successfully synthesized by a two-step direct synthesis method in acid media, and characterized by XRD, UV–vis, ESR, TEM, N 2 adsorption and FT-IR. The Ce-SBA-15 samples prepared show highly ordered mesostructures with larger pore diameter, pore volume and uniform mesopore size distribution. The valence of cerium species incorporated into the frameworks of SBA-15 can be controlled by adjusting the pH value of the synthesis solution accurately. When the pH value of the solution is ⩽6.0, Ce(III)-SBA-15 can be synthesized. If the pH value is higher than 6.0, the cerium species exist in the SBA-15 sample in form of Ce(IV). Moreover, it can be found that the two-step synthesis method is a feasible, effective and simple method to expand the pore size and the pore volume of SBA-15.

Characterization of mesoporous silica and its pseudomorphically transformed derivative by gas and liquid adsorption

Pseudomorphism is a term introduced by mineralogist to describe phase transformation that does not change the shape of a material. Pseudomorphic process, assisted by surfactants, for mesoporous pre-shaped silica particles allows to narrow the mesopore size distribution, to increase the surface area and the pore volume without changing the initial shape of silica particles. The textural, chemical and mechanical comparisons between commercial silica LiChrospher 60 (Merck) and its pseudomorphic transformation at constant particle size and morphology opens a unique opportunity to understand the effects of such mesoporosity transformation and judge the performance of both types of materials. On a mechanical point of view, commercial silica and its pseudomorphic transformation are stable until 40 MPa, which is enough to resist to chromatographic column packing. Liquid chromatographic tests reveal that column efficiency and mass transfer properties are maintained during the pseudomorphic transformation. Adsorption from polar probes and thermogravimetric measurements show that the surface chemistry is strongly modified by the pseudomorphic transformation, which is not pseudo-chemical. The surfactants use to generate well defined mesoporosity induct in one hand a low silanol density and in the other hand a more homogeneous silanol distribution on the silica surface, which opens the route towards a better control of surface functionalization. Moreover, grafted-mesoporous silicas exhibit an extremely high mechanical stability (>280 MPa). Pseudomorphic-silicas are remarkable candidates for applications needing morphology and surface chemistry control.

Preparation of monolithic columns with target mesopore-size distribution for potential use in size-exclusion chromatography

The main factors affecting the mesopore porosity of methacrylate-ester based monolithic columns were investigated. We prepared 40 monolithic capillary columns with porosity controlled by varying the proportions of butyl methacrylate (BMA) and ethylene dimethacrylate (EDMA) monomers and of 1,4-butanediol (BUT) and 1-propanol (PROP) as the porogen solvent in the polymerisation mixtures by thermally initiated in situ polymerisation in fused-silica capillaries. Using mixture design software, we systematically varied the composition of the polymerisation mixtures to find significant factors affecting mesopore formation. Multivariate analysis of the experimental data obtained for the fabricated columns yielded a model for prediction of the mesopore porosity in monolithic beds as a function of the composition of the polymerisation mixture used to prepare polymethacrylate monolithic capillary columns. The mean absolute deviation of predicted porosities is 0.029 for most of the columns, with only eight columns showing deviations exceeding 0.050. The main factor affecting the mesopore porosity proved to be the combination of the concentration of hydrophobic monomer (BMA) and the concentration of the less-polar solvent, 1-propanol, in the porogen mixture. The proportion of mesopores in the monolithic capillary columns increases with increasing concentration of 1-propanol and with decreasing concentration ratios of the cross-linker (EDMA) to monomer (BMA) and of BUT to PROP porogenic solvents.

Sol–gel synthesized of nanocomposite palladium–alumina ceramic membrane for H 2 permeability: Preparation and characterization

Palladium–alumina membrane with mesopore and narrow pore size distribution was prepared by the sol–gel method. Effect of the finely dispersed metal on the microstructure and the characteristic properties of the palladium–alumina membrane were investigated. Observations were made on membrane weight loss, morphology, pore structure, pore size, surface area, pore surface fractal and membrane's crystal structure. Autosorb analysis, X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Fourier Transform Infrared (FTIR) analysis were employed in the membrane characterization. Autosorb analysis found that, BET surface area decreased and pore size of the membrane increased with the increasing of calcinations temperature ( 500 – 1100 ∘ C ) and with the increasing of palladium amount in the membrane. FTIR and TG/DTA analysis show that the suitable temperature for calcinations of palladium–alumina membrane is at 700 ∘ C . Palladium metals are highly dispersed at calcinations temperature of 700 ∘ C as observed by TEM analysis. The fine crystallinity of the palladium and γ -alumina phase was obtained after calcined at 700 ∘ C . The SEM morphology shows a smooth and free crack layer of palladium–alumina membrane after repeating the process of dipping, drying and calcinations at temperature of 700 ∘ C . The membrane also successfully coated with a good adhesion on support. The thickness of the final membrane layer was estimated as 9 μ m .