High Mesopore Volume Research Articles

Synthesis and Properties of Phloroglucinol−Phenol−Formaldehyde Carbon Aerogels and Xerogels

Carbon aerogels and xerogels were successfully prepared from phloroglucinol-phenol mixtures and characterized by different techniques to determine their potential. We examined the influence of the phloroglucinol/phenol ratio, reactant concentration, cure conditions, and drying method on the morphology and porosity of the samples. The gelation time was found to be independent of the phloroglucinol/phenol ratio in spite of the different reactivities of both monomers. In general, carbon aerogels have a high volume of mesopores and of micropores without diffusion restrictions. Carbon xerogels are denser materials without mesopores but with a well-developed microporosity that shows a strong molecular sieve effect. Therefore, while micro-/mesoporous carbon aerogels can be used as catalyst supports or VOC adsorbents, the microporous carbon xerogel could offer high selectivity in the separation of small molecules from gaseous mixtures.

Removal of hydrogen sulphide on sewage sludge/industrial sludge based carbonaceous adsorbents

The adsorbents were prepared for sewage sludge, waste oil sludge and metal oil sludge. On the materials obtained dynamic adsorption of H2S was measured. The initial and exhausted adsorbents were characterised using sorption of nitrogen, thermal analysis, and XRF, XRD and surface pH measurements. The adsorbents have high capacity for H2S (10% wt) The high volume of mesopores contributes significantly to reactive adsorption and provides space to store sulphur as the oxidation product. The highly dispersed mineral-like phase formed during pyrolysis provides basicity and catalytic centres for hydrogen sulphide dissociation and its oxidation to sulphur.

Effects of pore characters of mesoporous resorcinol–formaldehyde carbon gels on enzyme immobilization

This paper demonstrates, for the first time, the use of resorcinol–formaldehyde carbon gels (RFCs) as enzyme carriers. The immobilization behavior of Bacillus licheniformis serine protease in RFCs of different pore characters was investigated. RFCs derived with (RF1) and without (RF2) cationic surfactant (trimethylstearylammonium chloride; C18) resulted in predominantly microporous, and mesoporous characters, respectively. It was found that support pore size and volume were key parameters in determining immobilized enzyme loading, specific activity, and stability. RF2, with higher mesopore volume ( V mes: RF1 = 0.21 cm 3/g; RF2 = 0.81 cm 3/g) and mesopore size radius (RF1 = 1.7–3.8 nm; RF2 = 7.01 nm), accommodated approximately fourfold more enzyme than RF1. Serine protease loading in RF2 could reach as high as 21.05 unit/g support. In addition, RF2 was found to be a better support in terms of serine protease operation and storage stability. Suitable mesopore size likely helped preventing immobilized enzyme from structural denaturation due to external forces and heat. However, immobilized enzyme in RF1 gave 12.8-fold higher specific activity than in RF2, and 2.1-fold higher than soluble enzyme. Enzyme leaching was found to be problematic in both supports, nonetheless, higher desorption was observed in RF2. Enhancement of interaction between serine protease and RFCs as well as pore size adjustment will be necessary for repeated use of the enzyme and further process development.

Novel method to synthesize ordered mesoporous silica with high surface areas

SBA-15 type ordered mesoporous silicas with high surface areas and mesopore volumes have been obtained using H 3PO 4 as catalyst during the synthesis process. Moreover, the surfactant removal process has been optimized to enhance the textural properties of the resulting materials. Therefore, silica-based ordered mesoporous materials exhibiting surface areas more than twice larger than those of conventional SBA-15 are reported. Additionally, the incorporation of phosphorous centers into the silica framework has been achieved using high amounts of H 3PO 4 as catalyst during the synthesis process. This textural properties' enhancement makes these materials excellent candidates to be used in several scientific and technological research areas, where high surface areas and mesopore volumes are required.

Preparation of carbon nanotubes/phenolic-resin-derived activated carbon spheres for the removal of middle molecular weight toxins

A novel composite-derived activated carbon spheres, i.e., carbon nanotubes/phenolic-resin-derived activated carbon spheres (CNTs/P-ACSs) were prepared by carbonizing and activating spheres of CNTs/phenolic-resin. The activated carbon spheres produced have excellent mechanical strength, high-mesoporous volume and efficient adsorption to middle molecular weight toxins. The breakage force and adsorption capacity of 30% CNTs doped composite spheres are both several times of that of P-ACSs and commercial macroporous resin, showing great potential to be efficient haemoadsorbent for the removal of middle molecular weight toxins in haemoperfusion.

Silica, carbon and boron nitride monoliths with hierarchical porosity prepared by spark plasma sintering process

Silica SBA-15, carbon CMK-3, boron nitride (BN), the latter synthesized from the first two compounds as templates, are mesoporous materials in the form of powders. They have a high specific surface area and an important mesoporous volume. The porosity is organized with the hexagonal symmetric space group p6mm. For selected applications, it could be interesting to preserve these characteristics with materials in a well-defined shape at a macroscopic scale (few millimeters to centimeter). Spark plasma sintering (SPS) is a well-known technique which allows to prepare monoliths with relatively mild conditions. The SPS technique has been used on these mesoporous powders without charge or with a uniaxial charge and at temperatures of 600 °C, 800 °C for silica, 1100 °C, 1300 °C for carbon and 1600 °C, 1700 °C for boron nitride during 1–5 min. The nitrogen adsorption/desorption isotherms reveal that the obtained monoliths present high specific surface area (300–500 m 2/g) and important mesoporous volume. The coexistence of interconnected mesoporosity and macroporosity (with volume’s close value) was observed by SEM and TEM, while the XRD and TEM characterization show that the mesoporosity organization is partially preserved.

Preparation and characterization of RuO2 · xH2O/carbon aerogel composites for supercapacitors

A series of RuO2 · xH2O/carbon aerogel (CA) composite electrode materials was prepared by a chemical precipitation method. Ultrasonication was used to accelerate the chemical reaction and improve the dispersion of RuO2 · xH2O particles on the surface and the pores of the aerogel. The structure and morphology of the as-prepared composite were characterized by N2 adsorption isotherm, X-ray diffraction (XRD), and field emission-scanning electron microscopy (FE-SEM). The results showed that the CA had a pearly network structure and the composites had a relatively high specific surface area and mesopore volume. The electrochemical performance of the composite electrodes was studied by cyclic voltammetry, galvanostatic charge/discharge measurements and electrochemical impedance measurements. The results indicated a substantial increase in the specific capacitance of the composite. Moreover, the utilization efficiency of RuO2 · xH2O was greatly improved by loading it on the conductive and porous CA due to a significant improvement in the inter-particle electronic conductivity and the extensive mesoporous network of the composites.

Template-Derived Mesoporous Carbons with Highly Dispersed Transition Metals as Media for the Reactive Adsorption of Dibenzothiophene

Mesoporous carbons with highly dispersed copper, cobalt, and iron were obtained from an organic polymer within amorphous silica powder, alumina, and zeolite 13X. The materials were characterized using the adsorption of nitrogen, potentiometric titration, and elemental analysis. The small metal content (less than 1%) and its chelation in the precursor polymers ensure a high dispersion of metallic centers. The materials obtained are mainly mesoporous but differ significantly in their porosity and surface chemistry, which is linked to the effect of template constraints and chemistry and the kind of metal and is related to the differences in the carbonization mechanism. On the carbon obtained, the adsorption of dibenzothiophene (DBT) from hexane was carried out. The high capacities (up to 130 mg S/g) obtained were linked to the high volume of mesopores and specific interactions of DBT with surface acidic groups and strong interactions of metals with dibenzothiophene via S-M sigma bonds or, in the case of copper, via interaction of metals with disturbed pi electrons of aromatic rings of DBT.

Removal of Cationic and Ionic Dyes on Industrial−Municipal Sludge Based Composite Adsorbents

The new class of reactive adsorbents obtained by pyrolysis of individual and mixed industrial sludges were used as media for the removal of cationic (Basic Fuchsin) and anionic (Acid Red 1) dyes. Materials were characterized using adsorption of nitrogen, thermal analysis (TA), potentiometric titration, X-ray diffraction (XRD), and scanning eletron microscopy (SEM). Kinetic measurements showed a fast decolorization process. Dye adsorption isotherms were fitted to the Langmuir−Freundlich model isotherm. The limiting capacities for Acid Red were between 35 and 73 mg/g, whereas the capacities for Basic Fuchsin removal ranged between 70 and 127 mg/g. While the Acid Red removal capacity was comparable to that on commercial activated carbon, the sludge-derived materials adsorb more Basic Fuchsin than carbon. The high efficiency of adsorption for both cationic and ionic dyes was linked to surface chemical heterogeneity and a high volume of mesopores. The diversity in surface chemistry, which leads to ion exchange processes, is a result of the presence of minerals, which are formed during pyrolysis.

Activated carbon catalyst for selective oxidation of hydrogen sulphide: On the influence of pore structure, surface characteristics, and catalytically-active nitrogen

The catalytic oxidation of hydrogen sulphide (H 2S) on various activated carbon materials was studied. The effects of pore structure, surface characteristics, and nitrogen content on the activity and selectivity of the carbons towards oxidation of H 2S were investigated. It was found that a high volume of both micropores and small mesopores, in combination with a relatively narrow pore size distribution, were crucial for the retention of sulphur dioxide (SO 2), a by-product of H 2S oxidation. For the retention of carbonyl sulphide (COS), another H 2S oxidation by-product, high surface reactivity with a significant amount of basic groups were found to be important. The only carbon with all these characteristics, and consequently the carbon that was able to retain both H 2S and COS for an extended period of time, was an experimental product, “WSC”. This carbon was found to be superior to the other carbons studied, exhibiting high activity and selectivity for oxidation of H 2S to sulphur. H 2S breakthrough capacities and selectivity values of the carbons were found to be dependent on the nitrogen content of the carbons. In a hydrogen stream, carbons possessing the highest nitrogen contents exhibited the greatest H 2S breakthrough capacities but, at the same time, the lowest selectivity with respect to sulphur formation. In reformate streams, the maximum breakthrough capacity and greatest selectivity were exhibited by carbons with a nitrogen content of about 1–1.5 wt%.

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.

Municipal Sludge−Industrial Sludge Composite Desulfurization Adsorbents: Synergy Enhancing the Catalytic Properties

Mixtures of sewage sludge, waste oil sludge, and metal oil sludge were prepared and carbonized at 950 degrees C in an inert atmosphere. Dynamic adsorption of H2S was measured on the materials obtained, and the breakthrough capacity was calculated. The initial and exhausted adsorbents after the breakthrough tests were characterized using sorption of nitrogen, thermal analysis, and XRF, XRD, and surface pH measurements. Mixing sludges leads to very high capacity adsorbents on which hydrogen sulfide is oxidized to elemental sulfur. Although the micropore volume of the adsorbents obtained is not high, their high volume of mesopores contributes significantly to reactive adsorption and provides space to store the oxidation products. The H2S breakthrough capacity on the new materials reaches 10 wt %. These adsorbents work until all active pores are filled and the catalytic centers are exhausted. The reason for such high capacity is in the formation of catalytically active mineral like phases during pyrolysis in the presence of nitrogen and carbon. This highly dispersed phase provides basicity and catalytic centers for hydrogen sulfide dissociation and its oxidation to sulfur.

Thermal stability of high surface area silicon carbide materials

The synthesis of mesoporous silicon carbide by chemical vapor infiltration of dimethyl dichlorosilane into mesoporous silica SBA-15 and subsequent dissolution of the silica matrix with HF was investigated. The influence of the synthesis parameters of the composite material (SiC/SBA-15) on the final product (mesoporous SiC) was determined. Depending on the preparation conditions, materials with specific surface areas from 410 to 830 m 2 g −1 and pore sizes between 2 and 10 nm with high mesopore volume (0.31–0.96 cm 3 g −1) were prepared. Additionally, the thermal stability of mesoporous silicon carbide at 1573 K in an inert atmosphere (argon) was investigated, and compared to that of SBA-15 and ordered mesoporous carbon (CMK-1). Mesoporous SiC has a much higher thermal textural stability as compared to SBA-15, but a lower stability than ordered mesoporous carbon CMK-1.

Metal-loaded carbonaceous adsorbents templated from porous clay heterostructures

Carbonaceous adsorbents with small amounts of highly dispersed metals were synthesized within various porous clay heterostructures (PCHs) serving as templates. To increase chemical heterogeneity, PCHs that contained iron, zinc, or copper were used and sucrose served as a carbon precursor. The samples were carbonized at 900 °C. The inorganic templates were removed using HF and HCl treatment. The carbons obtained were highly porous, with surface areas of 500–800 m 2/g and pore volumes of 0.500–0.800 cm 3/g. The pore size distributions indicated the heterogeneity of the structure with micropore sizes similar in size to those in the PCH material. Thermal analysis showed that even though the final materials consist mainly of carbon, minor amounts of PCH still remained. The content of metals decreased gradually after HF and HCl washing. Elemental analysis indicated that of all the metal-loaded carbons, only the one with iron contains a significant amount of metal, above 1%. This is due to hydrolysis of iron and formation of insoluble hydr(oxides) which are difficult to remove using HCl or HF treatment. XRD measurements indicated that some short-range order exists in the carbons obtained, suggesting the memory effect of the layered PCHs structure. The high volume of mesopores is due to metals migration and metal reduction during pyrolysis, which resulted in the consumption of carbon and formation of channels/imperfections connecting the stacked-together layers of carbons. Moreover, TEM analysis indicated the presence of tubular entities that could be inherited from the PCH templates. Long-range order in these materials was not detected. The carbons can find applications in catalysis where the presence of mesopores is required along with high dispersion of metals. The results indicate that the content of metals/inorganic phase can be tailored by extent of acid washing.

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.

Synthesis of Mesoporous Carbons Using Ordered and Disordered Mesoporous Silica Templates and Polyacrylonitrile as Carbon Precursor

Mesoporous carbons were synthesized from polyacrylonitrile (PAN) using ordered and disordered mesoporous silica templates and were characterized using transmission electron microscopy (TEM), powder X-ray diffraction, nitrogen adsorption, and thermogravimetry. The pores of the silica templates were infiltrated with carbon precursor (PAN) via polymerization of acrylonitrile from initiation sites chemically bonded to the silica surface. This polymerization method is expected to allow for a uniform filling of the template with PAN and to minimize the introduction of nontemplated PAN, thus mitigating the formation of nontemplated carbon. PAN was stabilized by heating to 573 K under air and carbonized under N2 at 1073 K. The resulting carbons exhibited high total pore volumes (1.5-1.8 cm3 g(-1)), with a primary contribution of the mesopore volume and with relatively low microporosity. The carbons synthesized using mesoporous templates with a 2-dimensional hexagonal structure (SBA-15 silica) and a face-centered cubic structure (FDU-1 silica) exhibited narrow pore size distributions (PSDs), whereas the carbon synthesized using disordered silica gel template had broader PSD. TEM showed that the SBA-15-templated carbon was composed of arrays of long, straight, or curved nanorods aligned in 2-D hexagonal arrays. The carbon replica of FDU-1 silica appeared to be composed of ordered arrays of spheres. XRD provided evidence of some degree of ordering of graphene sheets in the carbon frameworks. Elemental analysis showed that the carbons contain an appreciable amount of nitrogen. The use of our novel infiltration method and PAN as a carbon precursor allowed us to obtain ordered mesoporous carbons (OMCs) with (i) very high mesopore volume, (ii) low microporosity, (iii) low secondary mesoporosity, (iv) large pore diameter (8-12 nm), and (v) semi-graphitic framework, which represent a desirable combination of features that has not been realized before for OMCs.

Adsorption of mercury by carbonaceous adsorbents prepared from rubber of tyre wastes

Rubber from tyre wastes has been used to prepare carbonaceous adsorbents and the products obtained have been tested as adsorbents for mercury in aqueous solution. The adsorbents have been prepared by applying thermal, chemical and combined (thermal and chemical or vice versa) treatments. Tyre rubber has been: heated at 400 or 900 °C for 2 h in N 2, chemically-treated with H 2SO 4, HNO 3 or H 2SO 4/HNO 3 solution for 24 h, and in two successive steps first heated at 400 °C for 2 h in N 2 and then treated with a H 2SO 4/HNO 3 solution for 24 h, or vice versa. Resultant products have been characterised in terms of elementary composition and textural properties. The adsorption of mercury has been studied from kinetic and equilibrium standpoints. The treatments effected to tyre rubber decrease the carbon content and the hydrogen content. The oxygen content and the nitrogen content increase for the chemically-treated products. The heat treatment of tyre rubber results in a larger development of surface area, microporosity, and mesoporosity than the chemical treatments. These treatments, however, produce a great creation of macropores. In comparison to the starting rubber, the adsorption process of mercury is faster when the material is heated or treated with the H 2SO 4, HNO 3 or 1:3 H 2SO 4/HNO 3 solution. These adsorbents are either a non-porous solid or possess a high mesopore volume or a wide pore size distribution in the macropore range. The adsorption capacity is larger for products prepared by heat, chemical and combined treatments of the rubber. A common textural characteristic of these adsorbents is their better developed microporosity. The ability to adsorb mercury is higher for the heated products than for the chemically-treated ones. The maximum adsorption of mercury is 211 mg g −1. The constant K f of the Freundlich equation is as high as 108.9 mg g −1.

Nanocomposite Structure Based on Silylated MCM-48 and Poly(vinyl acetate)

As-synthesized MCM-48 was modified via silylation with Cl−Si(CH3)3. The resulting silylated MCM-48 possesses a well-ordered long-range structure, a narrow distribution of nanosized pore diameters, a high mesoporous volume, a large specific surface area, and, most significantly, a hydrophobic surface. A nanocomposite structure based on silylated MCM-48 and poly(vinyl acetate) (PVAc) has been prepared through both in situ polymerization and incorporation of preformed polymer. Characterization by FT-IR, XRD, and N2 adsorption−desorption techniques indicates the latter method is more effective than the in situ polymerization method. The composites obtained by addition of the PVAc/silylated MCM-48 particles to a PVAc matrix have significantly enhanced mechanical performance, including toughness and elasticity, when compared with PVAc itself or with composites derived from PVAc and pure silylated MCM-48 or precipitated silica.

Synthesis of mesoporous silica foams with hierarchical trimodal pore structures

A new type of mesoporous silica foam having extremely low bulk density (<0.01 g cm−3), high mesopore volume (>2 cm3 g−1) and specific surface area (>1000 m2 g−1) was prepared by the spraying of aqueous siliceous solutions containing alkyltrimethylammonium surfactants under high pH conditions. These remarkable properties were ascribed to macrovoids and two kinds of mesopores ranging around 3 and 40 nm. The macrocellular structure produced with air bubbles was stabilized through a rapid condensation reaction during drying. The framework of the macrovoids contained mesopores with a bimodal distribution derived from the surfactant micelles and the interparticle spaces of silica nanoparticles. The hierarchical self-assembly of surfactant micelles, silica nanoparticles and air bubbles provided the specific architecture with a trimodal pore size distribution.

EFFECT OF CHELATING AGENT 1,5-DIAMINOPENTANE ON THE MICROSTRUCTURES OF SOL-GEL DERIVED ZIRCONIA MEMBRANES

Zirconia is more chemically stable than silica and γ-alumina, which are commonly used in the preparation of inorganic membranes. However, the hydrolysis of zirconia alkoxides, typical zirconia precursors, proceeds rapidly, leading to difficulties in preparing clear sols. Thus, an attempt was made to reduce the hydrolysis rate of zirconium propoxide by a ligand-exchange reaction with 1,5-diaminopentane (DAP). The availability of water was also reduced by the dropwise addition of glacial acetic acid, which produced an ester by reaction with the solvent, 1-propanol. The addition of DAP was effective in preparing zirconia with a narrow pore-size distribution and a high mesopore volume. DAP also decreased the phase transformation temperature of zirconia and had almost no effect on the micropore structures. The gas permeation properties of the prepared zirconia membranes were fundamentally controlled by the Knudsen diffusion mechanism.