The effect of alkyl chain length on the structure of pillared carbons prepared by the silylation of graphite oxide with alkyltrichlorosilanes

Pillared carbon was obtained from graphite oxide silylated with alkyltrichlorosilanes with various alkyl chain lengths. The interlayer spacings of silylated graphite oxides became maximal when 1.1-1.5 molecules of bound silyl groups were attached per C₈ graphite oxide unit. Pillared carbon with an interlayer spacing of 1.27 nm was obtained only when graphite oxide silylated with methyltrichlorosilane was heated under vacuum above 300 °C. Infrared absorption measurements indicated that the pillars possessed the ladder type silsesquioxane structure. The adjacent graphite oxide layers were connected to each other even before pyrolysis, judging from the observation that n-hexadecylamine was not intercalated. The formation of a ladder type pillar was completed after the oxygen-containing groups were removed from the graphite oxide layers. When the alkyl chain length was longer, the connection of adjacent layers became difficult and pillared carbons were not formed. The surface area of the pillared carbon was very small because the distance between pillars was too small for nitrogen molecules to pass, but it was estimated based on the composition of the pillared carbon.     

Preparation of pillared carbon thin films from the reduction of silylated graphite oxide by UV light irradiation and their size selective electrical response to various molecules

Silylated graphite oxide thin films were reduced by UV light irradiation using a super high pressure Hg lamp at 95 °C. The reduction of silylated graphite oxide was completed within 24 h and a new pillared carbon with an interlayer spacing of 0.81 nm was obtained. Pillared carbons with larger interlayer spacings of about 1.13 nm were also obtained from graphite oxide silylated with octyltrichlorosilane and then with 3-aminopropyltriethoxysilane for 1.5–6 h, when they were reduced by UV light irradiation and heating at 200 °C under vacuum. Selective electrical response during exposure to gaseous vinylene carbonate, acetonitrile, ozone and hydrogen molecules has been achieved by changing the pillar density in the resulting pillared carbon films.     

Introduction of amino groups into the interlayer space of graphite oxide using 3-aminopropylethoxysilanes

Graphite oxide was silylated by 3-aminopropylethoxysilanes under various reaction conditions. Two types of layered materials were obtained when 3-aminopropyltriethoxysilane was reacted with graphite oxide. One obtained at lower temperatures was 3-aminopropyltriethoxysilane-intercalated graphite oxide with an interlayer spacing of 1.27 nm, in which amino groups of 3-aminopropyltriethoxysilane were bonded to hydroxyl groups of graphite oxide. At higher temperatures, silylation of graphite oxide occurred, giving a phase with larger interlayer spacing of 1.37 nm. Chemical reduction of graphite oxide to disordered carbon by amino groups occurred at the same time. Similar reactions occurred when 3-aminopropyldiethoxymethylsilane and 3-aminopropylethoxydimethylsilane were reacted and they were introduced into the interlayer space of graphite oxide. Large amounts of silicon, and accordingly amino groups, were introduced in graphite oxide when it was silylated by 3-aminopropyldiethoxymethylsilane because its reactivity for chemical reduction of graphite oxide was relatively lower than that of 3-aminopropyltriethoxysilane and additional silylation was possible. The amount of amino groups available for chemical adsorption of formaldehyde reached a very high value of 3.8 mmol/g for graphite oxide silylated by 3-aminopropyldiethoxymethylsilane.     

Preparation of carbon-based transparent and conductive thin films by pyrolysis of silylated graphite oxides

Thin films of silylated graphite oxide were obtained from a chloroform/cyclohexane dispersion of n-hexadecylamine-intercalated silylated graphite oxide by a casting method at a low temperature. Carbon-based thin films were obtained from the pyrolysis of the resulting films under a reduced pressure at 500 °C or higher temperatures. The resulting samples were well adhered to the substrate because of the presence of silicon containing species as a “glue”. The resistivity decreased with an increase in the film thickness or a decrease in the transparency. Based on the data obtained for the samples prepared from graphite with different particle sizes and graphite oxide with different oxygen contents, the conduction of the electrons within each carbon sheet seemed important for large film thickness and conduction through the boundary seemed important when the film thickness was small. A low sheet resistance of 3.7 kΩ/sq for 80% of transmittance was achieved, when graphite oxide with a lower oxygen content was prepared from graphite with smaller particle sizes and the precursor film was heated at 500 °C. At 900 °C, it further decreased to a value of 700 Ω/sq.     

Intercalation of various organic molecules into pillared carbon

Intercalation of various organic molecules into pillared carbon prepared from the pyrolysis of graphite oxide silylated with methyltrichlorosilane for three times was investigated. Liquid n-alkylamine molecules with various alkyl chain lengths were intercalated into the resulting pillared carbon and the interlayer spacing increased with increasing in the alkyl chain length, until it became a constant value of 2.24 nm for chains having more than six carbon atoms. This suggests that the length of the pillar is 1.9 nm. Various organic molecules including non-polar xylene isomers and alcohol molecules can also penetrate into pillared carbon. The n-hexadecylamine molecules with a higher melting point than room temperature were intercalated into pillared carbon simply by mixing them with pillared carbons in hexane, though the interlayer spacing was smaller. The space between the layers of pillared carbon was saturated with n-hexadecylamine molecules when 1.8 molecules per 40 carbon atoms were added. The n-hexadecylamine molecules occupied 51% of the micropore volume of pillared carbon. For the intercalation of organic molecules into pillared carbon, the shorter axis of them should be smaller than 0.87 nm.     

Removal of formaldehyde from gas phase by silylated graphite oxide containing amino groups

Graphite oxide silylated by 3-aminopropylmethyldiethoxysilane was used an adsorbent of formaldehyde from gas phase at low concentration. The amount of formaldehyde adsorbed on this adsorbent was much higher than that on activated carbon, even when water molecules are preadsorbed. The utility of the amino groups seemed to be higher for the sample with lower contents of organic component between the graphite oxide layers and larger interlayer spacings.     

Synthetic carbons activated with phosphoric acid III. Carbons prepared in air

Synthetic activated carbons were prepared by phosphoric acid activation of a styrene-divinylbenzene copolymer in an air atmosphere at various temperatures in the 400-900 °C interval. The carbons were characterized by elemental analysis, cation-exchange capacity measurement, infrared spectroscopy, potentiometric titration, copper adsorption from solution and physical adsorption of N₂ at −196 °C and CO₂ at 0 °C. It was shown that, similarly to synthetic phosphoric acid activated carbons obtained in argon, the synthetic carbons activated with phosphoric acid in air possess an acidic character and show considerable cation-exchange properties. The contribution of oxygen-containing surface groups along with phosphorus-containing groups to CEC is higher for carbons obtained in air. Three types of surface groups were identified on carbons prepared at temperatures up to 600 °C, and four types on carbons prepared at higher temperatures. These groups were assigned to ‘super-acidic’ (pK<0), phosphorus-containing (pK=1.1-1.2), carboxylic (pK=4.7-6.0) and phenolic (pK=8.1-9.4) groups. The cation-exchange capacity was at a maximum for the carbon prepared at 800 °C. Copper adsorption by synthetic phosphoric acid activated carbons obtained in air at temperatures lower than 800 °C is higher than for similar carbons obtained in argon. The increase is due to additional formation of oxygen-containing surface groups. Calculated copper binding constants revealed the importance of phosphorus-containing and carboxylic groups for adsorption of copper from aqueous solution. All carbons show a multimodal pore size distribution including simultaneously micropores and mesopores, but the porous texture is not a prime factor in determining the cation-exchange capacities of these carbons. Synthetic phosphoric acid activated carbons show a greater development of porosity when obtained in air as compared to carbons carbonized in argon.     

Preparation of pillared carbon thin films and their size selective electrical response to organic vapor

Thin films of graphite oxide silylated with octyltrichlorosilane were first prepared and they were further silylated with 3-aminopropyltriethoxysilane. Pillared carbon thin films with an interlayer spacing of 1.7 nm were prepared from the pyrolysis of the resulting thin films. Larger propylene carbonate molecules were not intercalated into them. They showed selective electrical response to electron donating and smaller organic molecules. The increase in the electrical resistance upon exposure to organic molecule seems to be larger for those with larger donor numbers.     

Effect of nanocarbon sources on microstructure and mechanical properties of MgO–C refractories

A study of microstructural evolution, mechanical and thermo-mechanical properties of MgO–C refractories, based on graphite oxide nanosheets (GONs), carbon nanotubes (CNTs) and carbon black (CB), was carried out by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), three-point bending and thermal shock tests. Meanwhile, these results were compared to the conventional MgO–C refractory containing 10 wt% flaky graphite prepared under the same conditions. The results showed that higher cold modulus of rupture was obtained for the composition containing GONs, and the composition containing CNTs exhibited larger displacement after coking at 1000 °C and 1400 °C. Also, the addition of nanocarbons led to an improvement of the thermal shock resistance; in particular, both compositions containing CNTs and CB had higher residual strength ratio, approaching the thermal shock resistance of the reference composition containing 10 wt% flaky graphite, as it was associated with the presence of nanocarbons and in-situ formation of ceramic phases in the matrix.     

Preparation and characterization of silylated graphite oxide

Graphite oxide was silylated by various alkylchlorosilanes in the presence of butylamine and toluene, and new intercalation compounds were obtained. The silylating reagents with two or three chlorine atoms at silicon in them reacted with graphite oxide, while no reaction occurred when silylating reagents with only one chlorine atom was used. The silylating reagent mainly reacted with hydroxyl group of graphite oxide, forming Si-O bonding. The role of butylamine was not only exfoliating graphite oxide layer but also scavenging HCl molecule which caused the decomposition of silylated graphite oxide. The silicon content was almost constant ≈0.6 mol/graphite oxide for the samples silylated by alkyltrichlorosilane with shorter alkyl chain lengths. It increased with the increase of alkyl chain length and reached 1.7 mol/graphite oxide. The higher silicon content could be ascribed to further silylation on hydroxyl groups formed at silicon atoms of silylating reagent bonded to graphite oxide, bridging two silylating reagents.     

Preparation and characterization of charcoals that contain dispersed aluminum oxide as adsorbents for removal of fluoride from drinking water

Charcoals adsorbents that contain dispersed aluminum and iron oxides have been synthesized by impregnating wood with salt solutions followed by carbonization at 500 °C, 650 °C or 900 °C. The adsorbents were characterized and their performance for fluoride removal from aqueous solution was evaluated. Aluminum and iron oxides were well dispersed into the porous charcoals. The carbons were amorphous and highly porous. XRD of the adsorbents showed crystalline iron oxide but did not show any form of crystalline aluminum oxides. All the adsorbents showed acidic surface properties. The efficiency of defluoridation was found to depend on the carbonization temperature, the pH of point of zero charge (pHPZC), and the co-existing ions. Substrates prepared at 650 °C with aluminum and iron oxides exhibited the best efficiency with a fluoride sorption capacity of 13.64 mg g⁻¹. More than 92% removal of fluoride was achieved within 24 h from a 10 mg L⁻¹ solution at neutral pH. Fluoride adsorption kinetic was well fitted by a pseudo-second order model. The amounts of residual Al and Fe in treated solution were pH dependant. At neutral pH, the amounts of dissolved Al and Fe were found to be 0.67 and 1.8 mg L⁻¹, respectively.     

Silica-pillared graphene sheets with iron–nitrogen units as an oxygen reduction catalyst

Graphene layers with silica pillars which widen the interlayer distance and functionalize the space as micropores have been prepared as one of the new porous carbon materials. In this study, the silica-pillared graphene with Fe–N units as a catalytic active site for cathodic oxygen reduction was formed as a new type of carbonaceous noble-metal-free oxygen reduction catalysts with the ordered pore structure. The formation of the pillared carbon was performed by introducing silylating reagents as the pillar source and also as the nitrogen source of the units in the graphite oxide interlayer spaces, introducing the Fe ions, and heat treatment in vacuum at 500 °C. The X-ray diffraction (XRD) patterns showed the ordered layered structure. The X-ray absorption fine structure (XAFS) of the Fe-K edge showed that the unit was a square planar Fe–N₄ moiety. The catalytic activity for the O₂ reduction was demonstrated by measurements using rotating disk electrodes with the pillared carbon fixed on the surface and immersed in an oxygen-saturated acidic solution. The activity was enhanced by the combination of the nitrogen containing compound to the silylating reagent and using the nitrogen-rich silylating reagent.     

Graphite blocks with high thermal conductivity derived from natural graphite flake

In this work, natural graphite flake (NG) and mesophase pitch were used as precursor carbons to prepare the graphite blocks, which were doped with Si and Ti powders. After hot-pressed at 2700 °C, we investigated the effect of mean size of NG on properties and microstructure of the graphite blocks. Results showed that both thermal conductivity and flexural strength of the graphite blocks were improved as mean size of NG in raw material increased from 50 to 246 μm. However, a decrease of thermal conductivity was observed when mean size of NG was higher than 246 μm. The density and open porosity were respectively 2.26 g/cm³ and 5.82% when mean size of NG in raw material was 246 μm. The thermal conductivity was enhanced, however, the flexural strength was reduced as hot-pressing temperature increased from 2300 to 3000 °C. The thermal conductivity and flexural strength of the graphite block were respectively 704 W/m K and 21.1 MPa when hot-pressing temperature was 3000 °C. Phase analysis demonstrated there were diffraction peaks of graphite, TiC and α-SiC crystals in the graphite block as the hot-pressing temperature was less than 2500 °C. No SiC crystals were evident when the hot-pressing temperature was 2700 °C or above.     

Metal-ion pillared clays as hydrocracking catalysts (I): Catalyst preparation and assessment of performance at short contact times

A set of pillared clay catalysts based on montmorillonite (a natural clay) and laponite (a synthetic clay) have been prepared. The new catalysts have been pillared with tin, chromium and aluminium pillars as well as layered double hydroxides based on polyoxo-vanadate and -molybdate. The activities of these novel catalysts have been compared with that of a commercial supported NiMo/Al₂O₃ catalyst and with sulphided Mo(CO)₆ during short (10 min) contact runs. A coal extract sample was reacted at 440 °C in a microbomb reactor in the presence of tetralin and 19 MPa hydrogen. Products were compared by size exclusion chromatography, using NMP as eluent, and by UV-fluorescence. Boiling point distributions of hydrocracked products were determined by a TGA based method; ‘conversions’ were defined as the decrease in the fraction of material with boiling points >450 °C during the reaction. Previous work at 440 °C and 19 MPa H₂ indicates extensive thermal (pyrolytic) cracking during the first 10 min; in the absence of catalyst recombination reactions rapidly take over. Results with several of the new catalysts did not show any improvement compared to the absence of catalyst with ∼39% conversion. The highest conversion (∼70%) was obtained with the Sn laponite pillared clay. The Cr montmorillonite catalyst, pre-calcined at 500 °C, gave the greatest overall shift to smaller molecular masses even though the observed conversion of >450 °C boiling material was relatively poor.     

Transmission electron microscopy characterization of nanostructured carbon derived from Cr3C2 and Cr(C5H7O2)3

The preparation, characterization and comparison of nanostructured carbons derived by direct chlorination of Cr₃C₂ and Cr(C₅H₇O₂)₃ are reported in this work. Cr₃C₂ precursor was treated at 400 and 900 °C with a reaction time of 1 h. The nanostructure of the products has been characterized in some detail by means of transmission electron microscopy and associated techniques, such as electron energy-loss and X-ray energy dispersive spectroscopies and high-angle annular dark field imaging. Remains of Cr₃C₂ encapsulated in an amorphous carbon shell were observed at 400 °C, whereas carbon with higher ordering degree was produced at 900 °C. In the latter case, the sample can be described as a continuous variation from poorly-stacked graphene-like carbon to graphitic agglomerates. Remains of the reaction by-product, CrCl₃, are detected in the carbon particles, forming monolayers intercalated inside the graphitic agglomerates and amorphous nanoparticles. As a comparison, carbon samples derived from Cr(C₅H₇O₂)₃ were prepared at 300 and 900 °C. They mainly consist of highly disordered carbon, with local graphite-like stacking in the sample prepared at 900 °C.     

Preparation of microporous sorbents from cedar nutshells and hydrolytic lignin

Carbon nutshells and hydrolytic lignin were used as starting materials for the preparation of microporous active carbons. Optimum parameters for cedar nutshell carbonization have been selected (temperature of carbonization 700-800 °C, rate of heating less than 3 °C/min) for the preparation of microporous carbons (average pore width 0.56 nm). The textural characteristics of microporous carbons made from nutshell are similar to those of a ‘Coconut’ carbon molecular sieve, but the latter has both a higher CO₂ adsorption capacity and a higher coefficient of N₂/O₂ separation. The influence of carbonization and steam-activation parameters on the microtexture and molecular-sieve properties of granular carbons made from hydrolytic lignin was also investigated. A low rate of heating (less 3 °C/min) promotes the formation of micropores with average sizes around 0.56-0.58 nm at carbonization temperature 700 °C. At the same carbonization temperature the average sizes of micropores were 0.7-0.78 nm at rates of heating more than 3 °C/min. The activation of lignin-char with steam at 800 °C resulted in the formation of active carbons with more developed micropore volume (0.3-0.35 cm³ g⁻¹) and with micropores of widths around 0.6-0.66 nm which are able to separate He from a He-CH₄ mixture. The size of the micropores was varied as a function of burn off value.     

Preparation and characterization of mesoporous activated carbon from waste tires

Activated carbons were produced from waste tires and their characteristics were investigated. Rubber separated from waste tires was first carbonized at 500 °C in N₂ atmosphere. Next, the obtained chars were activated with steam at 850 °C. As a result, fairly mesoporous activated carbons with mesopore volumes and BET surface areas up to 1.09 cm³/g and 737 m²/g, respectively, were obtained. To further improve the porous properties of the activated carbons, the char was treated with 1 M HCl at room temperature for 1 day prior to steam activation. This treatment increased mesopore volumes and BET surface areas of the activated carbons up to 1.62 cm³/g and 1119 m²/g, respectively. Furthermore, adsorption characteristics of phenol and a dye, Black 5, on the activated carbon prepared via acid treatment were compared with those of a commercial activated carbon in the liquid phase. Although the prepared carbon had a larger micropore volume than the commercial carbon, it showed a slightly lower phenol adsorption capacity. On the other hand, the prepared carbon showed an obviously larger dye adsorption capacity than the commercial carbon, because of its larger mesopore volume.     

X-ray absorption fine structure study on residue bromine in carbons with different degrees of graphitization

The structure of bromine residue compounds was investigated by X-ray absorption fine structure (XAFS) in order to interpret where and how bromine is present in carbons with different degrees of graphitization. The residue compounds can be classified into three groups, as obtained from X-ray absorption near edge structure (XANES) spectra and the values of the intramolecular distance r_Br–Br determined by extended X-ray absorption fine structure (EXAFS). In Group I, prepared from the host carbons heat treated at temperatures higher than 1900 °C, bromine exists in the interlayer space of graphite in the form of Br₂ molecules with interaction of the π electrons of graphite. In Group III, from carbon heat treated at 1000 °C, most of the bromine probably reacts with carbon atoms having a dangling bond or functional groups. For Group II, where the host carbons are heat treated at intermediate temperatures, it is likely that bromine exists in undeveloped defects with a unique electronic state.     

Preparation of highly crystalline carbon nanofibers from pitch/polymer blend

High crystalline carbon nanofibers were prepared by using polymer blend technique. Naphthalene-based mesophase pitch (AR pitch) was dispersed finely in polymethylpentene matrix, spun by using a melt-blown spinning machine, stabilized at 160 °C in an oxygen atmosphere and carbonized at 900 °C in a nitrogen atmosphere. Bundles of the carbon nanofibers with ca. 100 nm in diameter were obtained after removal of polymethylpentene at the carbonization process. No impurity carbon was observed. The carbon nanofibers consisted of fine carbon crystallites with preferred orientation along the fiber axis. After heating to 3000 °C, the carbon crystallites grew drastically to have an interlayer spacing of 0.3367 nm and a crystallite thickness of 56.9 nm, respectively, with remarkable improvement of the preferred orientation of the crystallites. Advantages and disadvantages of the present method were discussed briefly.     

Preparation of porous carbons from thermoplastic precursors and their performance for electric double layer capacitors

Porous carbons with high surface area were successfully prepared from thermoplastic precursors, such as poly(vinyl alcohol) (PVA), hydroxyl propyl cellulose and poly(ethylene terephthalate), by the carbonization of a mixture with MgO at 900 °C in an inert atmosphere. After carbonization the MgO was dissolved out using a diluted sulfuric acid and the carbons formed were isolated. The mixing of the PVA carbon precursor with the MgO precursors (reagent grade MgO, magnesium acetate or citrate) was done either in powder form or in an aqueous solution. The BET surface area of the carbons obtained via solution mixing could reach a very high value, such as 2000 m²/g, without any activation process. The pore structure of the resultant carbons was found to depend strongly on the mixing method; the carbons prepared via solution mixing were rich in mesopores, but those produced via powder mixing were rich in micropores. The size of mesopores was found to be almost the same as that of the MgO particles, suggesting a way of controlling the mesopore size in the resultant carbons. Measurement of capacitance was carried out in 1 mol/L H₂SO₄ electrolyte. The porous carbon with a BET surface area of 1900 m²/g prepared at 900 °C through solution mixing of Mg acetate with PVA showed a fairly high EDLC capacitance, about 250 F/g with a current density of 20 mA/g and 210 F/g with 1000 mA/g. The rate performance was closely related to the mesoporous surface area.