Structural Characterization of Micro- and Mesoporous Carbon Materials Using In Situ High Pressure 129Xe NMR Spectroscopy

In situ high pressure 129Xe NMR spectroscopy in combination with volumetric adsorption measurements were used for the textural characterization of different carbon materials with well-defined porosity including microporous carbide-derived carbons, ordered mesoporous carbide-derived carbon, and ordered mesoporous CMK-3. Adsorption/desorption isotherms were measured also by NMR up to relative pressures close to p/p0 = 1 at 237 K. The 129Xe NMR chemical shift of xenon adsorbed in porous carbons is found to be correlated with the pore size in analogy to other materials such as zeolites. In addition, these measurements were performed loading the samples with n-nonane. Nonane molecules preferentially block the micropores. However, 129Xe NMR spectroscopy proves that the nonane also influences the mesopores, thus providing information about the pore system in hierarchically structured materials.

Synthesis of Ordered Mesoporous Carbon Materials by Dry Etching

A novel synthesis method for ordered mesoporous carbons is presented. The inverse replication of a silica template was achieved using the carbonization of sucrose within mesoporous KIT-6. Instead of liquid acid etching, as in classical nanocasting, a novel dry chlorine etching procedure for template removal is presented for the first time. The resultant ordered mesostructured carbon material outperforms carbons obtained by conventional hard templating with respect to high specific micro- and mesopore volumes (0.6 and 1.6 cm3 g−1, respectively), due to the presence of a hierarchical pore system. A high specific surface area of 1671 m2 g−1 was achieved, rendering this synthesis route a highly convenient method to produce ordered mesoporous carbons.

Textural Characterization of Micro- and Mesoporous Carbons Using Combined Gas Adsorption and n-Nonane Preadsorption

Porous carbon and carbide materials with different structures were characterized using adsorption of nitrogen at 77.4 K before and after preadsorption of n-nonane. The selective blocking of the microporosity with n-nonane shows that ordered mesoporous silicon carbide material (OM-SiC) is almost exclusively mesoporous whereas the ordered mesoporous carbon CMK-3 contains a significant amount of micropores (25%). The insertion of micropores into OM-SiC using selective extraction of silicon by hot chlorine gas leads to the formation of ordered mesoporous carbide-derived carbon (OM-CDC) with a hierarchical pore structure and significantly higher micropore volume as compared to CMK-3, whereas a CDC material from a nonporous precursor is exclusively microporous. Volumes of narrow micropores, calculated by adsorption of carbon dioxide at 273 K, are in linear correlation with the volumes blocked by n-nonane. Argon adsorption measurements at 87.3 K allow for precise and reliable calculation of the pore size distribution of the materials using density functional theory (DFT) methods.

Methoxylation of α-pinene over mesoporous carbons and microporous carbons: A comparative study

A biomass derived carbon, a commercial microporous carbon and a xerogel mesoporous carbon catalysts were used in the study of α-pinene methoxilation reaction and the influence of textural and physical–chemical properties of the carbons was evaluated. Biomass carbon presented the higher activity, whereas the commercial one is the less active in the conditions studied. The main product of the reaction was α-terpinyl methyl ether and good values of selectivity were obtained over all the catalysts. A kinetic model was developed assuming that the α-pinene is consumed according to the parallel reaction network. The kinetic model presents high quality fittings to the experimental concentration profiles. These results show that it is possible to activate a waste residue using H3PO4 and convert it to high added value product such as acid catalyst.

Novel synthesis of a micro-mesoporous nitrogen-doped nanostructured carbon from polyaniline

Nanostructured carbons with relatively high nitrogen content (3–8%) and different micro and mesoporosity ratio were prepared by activation of polyaniline (PANI) with a ZnCl2–NaCl mixture in the proportion of the eutectic (melting point 270 °C). It was found that the activated carbons consisted of agglomerated nanoparticles. ZnCl2 plays a key role in the development of microporosity and promotes the binding between PANI nanoparticles during heat treatment, whereas NaCl acts as a template for the development of mesoporosity of larger size. Carbons with high micropore and mesopore volumes, above 0.6 and 0.8 cm3/g, respectively, have been obtained. Furthermore, these materials have been tested for CO2 capture and storage at pressures up to 4 MPa. The results indicate that the nitrogen groups present in the surface do not seem to affect to the amount of CO2 adsorbed, not detecting strong interactions between CO2 molecules and nitrogen functional groups of the carbon, which are mainly pyridinic and pyrrolic groups.

Effect of the stabilisation time of pitch fibres on the molecular sieve properties of carbon fibres

The stabilisation of pitch fibres (PFs) is the most important step for their subsequent use in the preparation of carbon fibres (CFs) and their resulting characteristics. The present work studies the influence that the stabilisation time has on the porosity of the CFs, and on the subsequent properties as carbon molecular sieve (CMS). The increase of the stabilisation time carried out at 573 K, from 2 to 8 h favours their CMS properties producing a decrease in the microposity accessible to N2, which gets completely blocked after 6 and 8 h, while the narrow microporosity (V-DR CO2) remains accessible. Adsorption kinetic studies with CH4 and CO2 were performed to assess the possibility of using these CFs as CMS by comparing them with Takeda 3A CMS. The results suggest that there is an optimal stabilisation time which allows the preparation of CFs from an abundant raw precursor with properties similar to Takeda 3A CMS.

Effect of the porous structure in carbon materials for CO2 capture at atmospheric and high-pressure

Activated carbons prepared from petroleum pitch and using KOH as activating agent exhibit an excellent behavior in CO2 capture both at atmospheric (∼168 mg CO2/g at 298 K) and high pressure (∼1500 mg CO2/g at 298 K and 4.5 MPa). However, an exhaustive evaluation of the adsorption process shows that the optimum carbon structure, in terms of adsorption capacity, depends on the final application. Whereas narrow micropores (pores below 0.6 nm) govern the sorption behavior at 0.1 MPa, large micropores/small mesopores (pores below 2.0–3.0 nm) govern the sorption behavior at high pressure (4.5 MPa). Consequently, an optimum sorbent exhibiting a high working capacity for high pressure applications, e.g., pressure-swing adsorption units, will require a poorly-developed narrow microporous structure together with a highly-developed wide microporous and small mesoporous network. The appropriate design of the preparation conditions gives rise to carbon materials with an extremely high delivery capacity ∼1388 mg CO2/g between 4.5 MPa and 0.1 MPa. Consequently, this study provides guidelines for the design of carbon materials with an improved ability to remove carbon dioxide from the environment at atmospheric and high pressure.

Micro/Mesoporous Activated Carbons Derived from Polyaniline: Promising Candidates for CO2 Adsorption

A series of activated carbons were prepared by carbonization of polyaniline at different temperatures, using KOH or K2CO3 as activating agent. Pure microporous or micro/mesoporous activated carbons were obtained depending on the preparation conditions. Carbonization temperature has been proven to be a key parameter to define the textural properties of the carbon when using KOH. Low carbonization temperatures (400–650 °C) yield materials with a highly developed micro- and mesoporous structure, whereas high temperatures (800 °C) yield microporous carbons. Some of the materials prepared using KOH exhibit a BET surface area superior to 4000 m2/g, with total pore volume exceeding 2.5 cm3/g, which are among the largest found for activated carbons. On the other hand, microporous materials are obtained when using K2CO3, independently of carbonization temperature. Some of the materials were tested for CO2 capture due to their high microporosity and N content. The adsorption capacity for CO2 at atmospheric pressure and 0 °C achieves a value of ∼7.6 mmol CO2/g, which is among the largest reported in the literature. This study provides guidelines for the design of activated carbons with a proper N/C ratio for CO2 capture at atmospheric pressure.

Enhancement of the hydrogenation activity of a Pd-tridecilamine (TDA) complex by confinement in carbon nanotubes

An active hydrogenation Pd complex has been immobilised by impregnation on CNTs submitted to several treatments that lead to important differences in their surface chemistry and in the proportion of tubes with both ends open. Most of the hybrid catalysts are more active than the complex in homogeneous phase, but the support properties have an important impact in the catalytic activity. In general, the more developed the surface chemistry, the lower the activity. However, when CNTs are open at both ends, the Pd complex can enter the tubular cavity and an important enhancement of the catalytic activity due to a confinement effect is observed.

Ir supported over carbon materials for the selective hydrogenation of chloronitrobenzenes

In the present work we have studied the effect of carbon supports with different graphitic character (carbon nanotubes, mesoporous graphite and activated carbon) on the catalytic performance of iridium nanoparticles on the liquid phase chemoselective hydrogenation of para-chloronitrobenzene at room temperature. The effect of the oxygen groups was also evaluated by oxidizing a portion of the carbon nanotubes. The Raman and XRD spectra showed that the mesoporous graphite displayed the strongest graphitic character. The characterization of the catalysts by HR-TEM, XPS and TPR-H2, showed that the catalysts had similar particle size and that the catalysts prepared over the previously oxidized support, Ir/CNTox, was not fully reduced. The activity and selectivity achieved with the catalyst Ir/CNT was the best among the samples and the presence of irdium oxide on Ir/CNTox diminished the yield to p-chloroaniline, being the worse catalyst. The reactivity of different isomers was also studied over Ir/CNT and it followed the order m > o > p.