Graphitic mesoporous carbons synthesised through mesostructured silica templates

In this paper the fabrication and characterization of graphitizable and graphitized porous carbons with a well-developed mesoporosity is described. The synthetic route used to prepare the graphitizable carbons was: (a) the infiltration of the porosity of mesoporous silica with a solution containing the carbon precursor (i.e. poly-vinyl chloride, PVC), (b) the carbonisation of the silica–PVC composite and (c) the removal of the silica skeletal. Carbons obtained in this way have a certain graphitic order and a good electrical conductivity (0.3 S cm⁻¹), which is two orders larger than that of a non-graphitizable carbon. In addition, these materials have a high BET surface area (>900 m² g⁻¹), a large pore volume (>1 cm³ g⁻¹) and a bimodal porosity made up of mesopores. The pore structure of these carbons can be tailored as a function of the type of silica selected as template. Thus, whereas a graphitizable carbon with a well-ordered porosity is obtained from SBA-15 silica, a carbon with a wormhole pore structure results when MSU-1 silica is used as template. The heat treatment of a graphitizable carbon at a high temperature (2300 °C) allows it to be converted into a graphitized porous carbon with a relatively high BET surface area (260 m² g⁻¹) and a porosity made up of mesopores in the 2–15 nm range.     

Easy synthesis of an ordered mesoporous carbon with a hexagonally packed tubular structure

Ordered mesoporous carbon with hexagonal arrays of tubes (CMK-5) was successfully synthesized via a nanocasting process by directly using SBA-15, instead of AlSBA-15, as a template, furfuryl alcohol as a carbon source and oxalic acid as the catalyst. The time consuming impregnation of SBA-15 with aluminum could be saved. The as-synthesized CMK-5 exhibits a tubular structure with double pore system. The loading amount of carbon precursor on the pore walls of SBA-15 is the key factor for the formation of the CMK-5 structure with two-dimensional hexagonal arrays of tubes, and the pore diameter can be adjusted by varying the loading amount of the carbon precursor. The CMK-5 carbon exhibits high apparent surface area up to ∼2500 m²/g and high pore volume reaching ∼2 cm³/g, which is due to the unique structure of CMK-5. The characterization results confirmed that carbonization under argon atmosphere instead of vacuum is sufficient for the structural formation of CMK-5 carbons, and can be used as an alternative pathway to prepare tubular-type carbons.     

Preparation and pore control of highly mesoporous carbon from defluorinated PTFE

Porous carbon having more than 2000 m²/g of BET specific surface area was synthesized by defluorination of polytetrafluoroethylene (PTFE) at 473 K using sodium metal. The porous carbon as-prepared had a large amount of narrow mesopores 2-3 nm in pore width, together with micropores. Control of the pore structure was attempted by simple heat-treatment of the carbon in nitrogen, and change of the porous structures was characterized by nitrogen adsorption techniques. As a result, it was found that the ratio between micro- and mesopores was easily varied. Electric double layer capacitance was measured as one of the applications for the mesoporous carbon with specific porosity, and the effect of pore control on capacitance was investigated.     

Synthesis methods for preparing microporous carbons with a structural regularity of zeolite Y

As reported in previous communications, novel porous carbons were synthesized by using zeolite Y as a template. The carbons possess a periodic ordering structure and high BET surface area with large micropore volume. In this work, the details of the synthesis methods for preparing the ordered microporous carbons were examined. It was found that the following two-step process, the filling of carbon into zeolite channels by impregnation of furfuryl alcohol and then chemical vapor deposition (CVD) of propylene, was indispensable for preparing carbon with highly periodic ordering. In addition, low-temperature CVD and the further heat treatment of zeolite/carbon composite after the CVD are key points for the appearance of both good long-range periodicity and very high BET surface area with almost no mesoporosity in the carbons.     

Synthesis and characterization of microporous carbons templated by ammonium-form zeolite Y

Emerging applications such as gas storage require porous carbon materials with tailored structural and surface properties. Template synthesis approach to porous carbons offers opportunities for tailoring these properties. In this study, ammonium-form zeolite Y (NH₄Y) was used as a template and furfuryl alcohol (FA) was employed as a carbon precursor to prepare microporous carbons by simple impregnation method. The effects of synthesis conditions such as carbonization temperatures and heating rates on the pore structure of the microporous carbons were investigated. The thermal behaviors of FA-NH₄Y mixtures and zeolite/carbon composites were studied by thermogravimetric analysis (TGA). The physical, structural, and surface properties of the microporous carbons were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM), elemental analysis, and physical adsorption of nitrogen. Microporous carbons with high surface areas, pore volumes and nitrogen-containing surface functional groups can be readily synthesized.     

Carbonaceous adsorbents from sewage sludge and their application in a combined activated sludge-powdered activated carbon (AS-PAC) treatment

In this study, waste biological sludge is converted to an adsorbent by chemical activation with sulphuric acid. The adsorbent obtained is then applied to the aerated vessel of an activated sludge process treating glucose and phenol to improve the quality of the treated effluent. The sludge-based carbonaceous adsorbent was found to be mesoporous in nature, with a good adsorption capacity for large molecular weight compounds and limited removal efficiency for smaller molecules such as phenol. The addition of carbon, either sludge-based or commercial, enhanced phenol removal from 58% to 98.7% and from 87% to 93% the organic matter removal as measured by the chemical oxygen demand (COD) when operated with feed concentrations of 100 mg phenol/l and 2500 mg COD/l. No differences were found between the activated sludge-activated carbon bench scale continuous reactors operating with either commercial or sludge-based adsorbents in spite of the higher adsorption capacity of the former. It is suggested that powdered adsorbent bioregeneration in the combined AS-PAC system may be impaired by the obstruction of pores due to bacterial growth, the effect being more important for the commercial activated carbon with a narrower pore size distribution.     

Carbon foams prepared from polyimide using urethane foam template

Polyimide and carbon foams were successfully prepared using polyurethane foams as a template. Impregnation of polyimide precursor, poly(amide acid), followed by imidization at 200 °C gave polyurethane/polyimide (PU/PI) composite foams, which resulted in PI foams by heating above 400 °C and then carbon foams above 800 °C. Foams carbonized at 1000 °C were graphitized by the heat treatment at 3000 °C, keeping foam characteristics. Two applications of these carbon foams, i.e., an adsorbent of ambient water vapor and a substrate of photocatalyst anatase TiO₂, were experimentally confirmed. For the former application, the present foam could be characterized by prompt adsorption of ambient water vapor. Some of carbon foams prepared were floating on water, even after loading photocatalyst anatase, which might be advantageous for photodecomposition of pollutants in water in respect to the UV rays efficiency.     

Sorption of gold and silver on carbon adsorbents from thiocyanate solutions

Sorption recovery of thiocyanate gold and silver complexes on different carbon adsorbents has been studied using model solutions with concentrations 0.08-0.82 mmol/l and 0.16-1.06 mmol/l for gold and silver, respectively. The potassium thiocyanate concentration in these solutions was 0.25 mol/l and the pH of the contacting solution was ∼2. The degree of recovery exceeded 90% for gold and 80% for silver. The separate step-by-step desorption of thiocyanate gold and silver complexes was carried out by varying the initial concentration of thiocarbamide (desorption agent). The degree of recovery of noble metals can be increased up to 95% using basic thiocarbamide solutions (in 0.1-0.2 M NaOH) at the higher temperature of the process (up to 150 °C).     

Surface chemistry of ordered mesoporous carbons

Ordered mesoporous carbons (OMC) were produced by pyrolysis of hydrocarbons adsorbed in two different silica matrices (MCM-48 and SBA-15), followed by dissolution of the matrix in either hydrofluoric acid or sodium hydroxide. Some carbons were subsequently heat treated at temperatures of up to 1600 °C. The chemistry of the external surface was studied by X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectroscopy (SIMS). Information on the graphitic order of the surface of the mesopores was obtained from low-pressure nitrogen adsorption data. The external and internal surface of the OMC has a polyaromatic, graphite-like character. This character increases considerably with increasing pyrolysis and/or post-pyrolysis temperature, as expected. According to the XPS and the nitrogen adsorption data, this increase was especially pronounced for temperatures above 1100 °C. In spite of the different pore structures, only small differences in the polyaromatic character were found for OMC synthesised either in a MCM-48 or in a SBA-15 matrix. Differences exist for the non-carbon elements. When hydrofluoric acid is used for dissolution of the silica matrix, organic fluorine compounds are formed. Their concentration is higher when a MCM-48 matrix as opposed to a SBA-15 matrix was used. Dissolution of the silica matrix in sodium hydroxide yielded a less contaminated OMC as compared to dissolution in hydrofluoric acid.     

A study of nanoporous carbon obtained from ZC powders (Z=Si, Ti, and B)

Nanoporous carbon (NPC) material prepared from polycrystalline powders of SiC, TiC, or B₄C carbides, using different techniques, was studied by means of ESR, Hall and conductance measurements for three groups of each type of sample—control group, and those annealed at T=960 and 1180 °C. It is found, according to Hall measurements, that holes are the majority charge carriers in NPC. The concentration of free carriers is found to be rather high (about 10¹⁹–10²¹ cm⁻³). The ESR spectrum has the so-called ‘Dyson line shape’ with a large signal asymmetry. Two groups of carriers with strongly different electronic and magnetic properties are found. The values of the g-factor and their variability are related to the existence of two different kinds of particles: small carbon nanoclusters (1–3 nm) and large-scale carbon skeleton structural elements (up to 1000 nm) of complex shape, where the holes move along graphite-like sp² bonds. The temperature dependence of the integral intensities shows the existence of two non-separable spin subsystems—charge carriers and localised spins. A role of oxygen atoms in the electronic system of NPC is studied.     

Evaluation of slitlike porosity of carbon adsorbents

Deviations of a pore shape from a slitlike one were analyzed for a variety of carbon adsorbents on the basis of calculations of pore size distributions (PSDs) with respect to the pore volume (f_V(x)) used for estimation of the PSDs related to the specific surface area (f_S(x)) applying several models of pores: individual slitlike pores and mixtures of slitlike and cylindrical pores and gaps between spherical particles. The use of complex pore models allows us to diminish the difference between S_BET and the specific surface area (S_sum) calculated from f_S(x).     

Templating synthesis of ordered mesoporous carbon particles

This research reports the synthesis of spherical ordered mesoporous carbon particles using mesoporous silica particles as a template. The silica particles with ordered cubic and lamellar mesostructures were synthesized from an aerosol-assisted self-assembly process using block copolymer surfactants as the structural directing agent. Infiltrating the pores of the silica particles with a sucrose solution followed by carbonization, and silica removal results in mesoporous carbon particles with replicated mesostructures. The particles were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and nitrogen sorption techniques.     

Functionalization of carbon nanobeads and their use as metal ion adsorbents

Ethylenediamine, dithiocarbamate and thioureido ligand functionalized nanoparticles, with average diameters ranging from 4 to 8 nm and containing 1.06 to 1.26 mmol of ligand/g, are readily obtained when carbon nanobeads are modified with a silane coupling agent,and then functionalized with carbon disulfide and phenylisothiocyanate. Their sorption characteristics for transition metal cations have been studied. The nanometer metal ion adsorbents exhibit very high selectivities, capacities and rates of complexing. This study demonstrates that these nanoparticles have potential applications as catalyst supports.     

Template synthesis of novel porous carbons using various types of zeolites

As previously reported, high surface area microporous carbons with long-range order can be synthesized by using zeolite Y as a template. In this work, an attempt is made to synthesize porous carbon using several other types of zeolites (zeolite β, ZSM-5, mordenite and zeolite L). Special attention is paid to whether the structural regularity of each zeolite can be transferred to the carbon structure as well as with the case of zeolite Y. The carbon filling method was then examined to see which gives the highest regularity to the carbon structure. It has been found that the optimum carbon filling method for zeolite Y is not an optimum one for the other zeolites and the degree of the regularity of long-range ordering in the carbons strongly depends on zeolite type. The order of the regularity in the resultant carbons is zeolite β>>zeolite L>mordenite>ZSM-5. The effect of zeolite type on the regularity is discussed in relation to the size and the shape of the zeolite channels.     

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.     

Adsorption of monosaccharides, disaccharides, and maltooligosaccharides on activated carbon for separation of maltopentaose

To study separation of maltopentaose from maltooligosaccharide mixtures, we investigated the adsorption equilibrium and kinetics of monosaccharides (fructose, galactose, and glucose), disaccharides (sucrose, maltose, and lactose), and maltooligosaccharides (maltotriose, maltotetraose, and maltopentaose) on activated carbon. The adsorption amount of maltose consisting of two glucosidic units was the highest among the nine adsorbates studied. It was also found that the different chemical structure of disaccharides with the same molecular weight highly affected the adsorption amount compared to that of monosaccharides. A simple but practical dynamic model was proposed, based on both the ideal adsorbed solution theory (IAST) for multicomponent equilibria and the surface diffusion model for intraparticle mass transport. In spite of its simplicity, the proposed dynamic model successfully simulated the adsorption breakthrough curves up to ternary mixtures.