Preparation and characterisation of carbon-coated monoliths for catalyst supports

Carbon coated monoliths have been prepared by dipcoating cordierite monoliths in a polymer mixture and subsequent carbonisation and activation. Preparation parameters that were varied were viscosity of the dipcoating mixture, carbon precursor and carbonisation temperature. Two different polymers have been used as carbon precursors, Novolac and Furan resins. Also monoliths have been coated with slurry of these resins and a commercial activated carbon, CP-97. The features of the final carbon that have been optimised are carbon loading, carbon layer thickness, coverage and mesoporosity. Coverage has been tested by leaching tests in acid media and SEM analysis. Both coverage and mesoporosity are considerably enhanced when the dipcoating mixture was a slurry of Furan resin and activated carbon. Leaching of the cordierite was considerably reduced but not completely eliminated. The morphology of the activated carbon could be transferred to the coating layer.     

Vanadium supported on carbon-coated monoliths for the SCR of NO at low temperature: effect of pore structure

Carbon-coated monoliths with different degrees of mesoporosity have been prepared. This has been accomplished by coating cordierite monoliths with a blend of two polymers, viz. Furan resin and polyethylene glycol (PEG), in different proportions. Upon carbonization at 973 K the former yields a carbon coating while the latter pyrolizes generating mesoporosity. Additionally the carbon-coated monoliths were activated with CO₂ to generate microporosity. Vanadium was impregnated in these carbon-coated monoliths by equilibrium adsorption using ammonium metavanadate as precursor and they were tested in the SCR of NO at low temperature. By increasing the amount of PEG, the mesopore volume increases in the range of narrow mesoporosity (2–5 nm). It was found that the more mesopore volume, the more oxygenated surface groups are formed. This turned out to be crucial for the deposition of vanadium in a dispersed fashion and also for the activity in the SCR of NO. On the contrary, the narrow microporosity (<0.7 nm) does not contribute to enhance the dispersion of the catalyst. The kinetic rate constants of the monolithic catalyst prepared are in the range of the most active catalyst reported in the literature for the SCR of NO at low temperature.     

Diffusion of gases in metal containing carbon aerogels

Carbon aerogels containing Fe, Ni, Cu or no metal were prepared by carbonisation of polymer aerogels synthesised from 2,4-dihydroxybenzoic acid and formaldehyde and modified by CVD of benzene. Uptakes and diffusion coefficients of CO₂, CH₄, N₂ and O₂ were measured and the results compared with those obtained using a commercial carbon molecular sieve. The results indicated that the diffusion of light gas molecules in carbon aerogels cannot be interpreted solely on the basis of micropore diffusion, but that the very high mesopore volumes of the aerogel monoliths exert a strong influence on the kinetics of diffusion in these materials. The mesoporosity is decreased when the % solids used during synthesis of the polymer precursor increases and this resulted in kinetic behaviour which was more similar to that predicted by Fickian or LDF models. Increasing % solids was also accompanied by generally slower diffusion rates and generally lower uptakes. The single gas uptakes and diffusion coefficients could be altered by varying the % solids used during synthesis of the polymer precursor, by introducing different metals into the polymer hydrogel by ion exchange, or by CVD of benzene on the carbon aerogel.     

Pore structure control of mesoporous carbon monoliths derived from mixtures of phenolic resin and ethylene glycol

Mesoporous carbon monoliths derived from phenolic resin mixtures have been prepared in the process based on polymerization-induced phase separation. The effect of the composition of resin mixtures and the resin curing temperature on the pore structure of carbon monoliths has been systematically investigated, with emphasis on controlling the apparent porosity and pore size distribution. Fractal dimensions have been introduced to evaluate the morphologies of the carbon monoliths. The results showed that mesoporous carbon monoliths with narrow pore size distribution were obtained. The pore structure of carbon monoliths could be controlled by changing the resin curing temperature and the content of ethylene glycol, curing catalyst and water in the resin mixtures. The apparent porosity of carbon monoliths varied from 54.27% to 26.83%. Carbon monoliths had narrower pore size distribution when more ethylene glycol and higher resin curing temperature were employed. The pore structure of carbon monoliths would be changed radically when the initial resin samples were prepared with excess water (9.8 wt%), i.e. porous carbon foams with sponge structure were obtained. Carbon monoliths inherit their porosity from precursing cured resins where it was formed as a result of phase separation of resin-rich and glycol-rich phases. Volume contraction had certain effect on the pore structure of carbon monoliths.     

Activated carbon monoliths for methane storage: influence of binder

In the present work, the agglomeration of a high adsorption capacity powdered activated carbon suitable for methane storage has been studied. Activated carbon monoliths have been prepared using the starting activated carbon and six different binders. Porous texture characterization of all the monoliths has been carried out by physical adsorption and helium density. Experimental methane adsorption capacity and delivery values have been obtained for all the samples. The results show that the adsorption capacities of the activated carbon monoliths are reduced with respect to the starting activated carbon. In addition to the adsorption capacity and delivery, the monolith density is also a crucial parameter for methane storage applications. This parameter has been obtained for all the samples. Moreover, the evaluation of the mechanical properties of the monoliths has been carried out with compression tests. According to our results, among all the binders studied, the one which produces monoliths with the best equilibrium between adsorption capacity and piece density has a methane delivery of 126 V/V. The important effect of the percentage of this binder in piece density and mechanical properties has been shown. Finally, a preliminary kinetic study of methane adsorption up to 4 MPa for the monoliths has shown that activated carbon monoliths do not present diffusional problems for adsorption of methane.     

Carbon-based monolithic supports for palladium catalysts: The role of the porosity in the gas-phase total combustion of m-xylene

Three different carbon-based monoliths have been studied in their performance as Pd catalyst supports in the total gas-phase combustion of m-xylene at low temperatures. The first monolithic support (HPM) was a classical square channel cordierite modified with -Al₂O₃, blocking the macroporosity of the cordierite and rounding the channel cross-section, on which a carbon layer was applied by carbonization of a polyfurfuryl alcohol coating obtained by dipcoating. The other two monolithic supports were composite carbon/ceramic monoliths (MeadWestvaco Corporation, USA), microporous (WA) and a mesoporous (WB) sample.

All the catalysts have a comparable total Pd loading and very similar Pd particle size (around 5–6 nm). In sample Pd/WA the Pd is situated only in the macropores, while in the case of Pd/WB the Pd is distributed throughout the mesoporous texture. In the case of Pd/HPM, Pd particles are clearly situated at the external surface of the carbon layer.

The catalytic activities of the samples were very different, decreasing in the order: Pd/WB > Pd/WA > Pd/HPM. These results show that the carbon external surface area, the macropores and mainly mesopores, play an important role in this kind of gas-phase reactions, improving the contact between the Pd particles and the m-xylene molecules. The catalytic activity of the Pd supported on carbon-based monoliths correlates with the surface area developed in macro- and mesopores of the monolithic support.     

Analysis of the microporosity shrinkage upon thermal post-treatment of H3PO4 activated carbons

Starting from a commercial pelletized phosphoric acid based activated carbon, with a typical opened and developed micro and mesoporosity, a post-heat-treatment in KOH, at different KOH/activated carbon ratios, has been studied. In all the cases, a pore size shrinkage has been observed. To find an explanation for the reason of this micropore size distribution shrinkage different factors have been studied, among them: (a) effect of the presence of impurities coming from the activation process with phosphoric acid; (b) effect of the KOH post-treatment temperature; (c) heat-treatment temperature of the precursor (without chemical agent); (d) effect of the reagent nature (NaOH, NaCl and KCl vs. KOH). The variable that produces the most intense shrinkage effect, and the disappearance of the mesoporosity, is the heat-treatment in presence of hydroxide, which affects even using a low hydroxide/activated carbon ratio. Such a low hydroxide/activated carbon ratio does not produce activation, nor porosity development of the starting activated carbon during the treatment. This shrinkage phenomenon, which seems to be independent of the method of preparation used to prepare the activated carbon, can be understood considering our previous studies about the reactions involved during chemical activation by hydroxides.     

Advances in the study of methane storage in porous carbonaceous materials

This paper presents an overview of the results of our research group in methane storage, in which the behaviour of different carbon materials in methane storage has been studied. These materials include physically activated carbon fibres (ACFs), chemically activated carbons (ACs) and activated carbon monoliths (ACMs), all of them prepared in our laboratories. These results have been compared with those corresponding to commercial ACFs, commercial activated carbon cloths and felts, and a commercial activated carbon.An in depth analysis (different raw materials, activating agent and preparation variables) has been done in order to obtain the carbon material with the best methane adsorption capacity by unit volume of adsorbent. The important effect of the micropore volume, micropore size distribution (MPSD) and packing density of the carbon materials in the methane adsorption capacity and delivery has been analysed. After this study, activated carbons with volumetric methane uptake as high as 166 v/v and delivery of 145 v/v have been prepared. In addition, ACM with methane uptake of 140 v/v and a delivery of 126 v/v has also been obtained.Moreover, the results corresponding to preliminary in situ small angle neutron scattering (SANS) study of CD₄ adsorption under pressure in different porous carbons and a zeolite are also included. These experiments have established SANS as a viable technique to investigate high-pressure methane adsorption. CD₄ adsorption at supercritical conditions produces changes in the SANS curves. The changes observed are in agreement with theoretical speculations that the density of the adsorbed phase depends upon the pore size.     

Adsorption Characteristics of Microporous Carbons from Apricot Stones Activated by Phosphoric Acid

Crushed apricot stone shells were impregnated with varying H₃PO₄ acid concentrations (20–50 wt%), followed by carbonisation at 573–773 K. The products were characterised by nitrogen gas adsorption. Analysis of the nitrogen isotherms by the DR and α_s methods proved that most of the obtained carbons are highly microporous, with high surface areas (⩾1000 m² g^-1) and very low mesoporosity. Increasing acid concentration, at 573 and 673 K, increases surface area and pore volume, whereas at 733 K a small decrease in both parameters appears at higher H₃PO₄ concentrations. Whole apricot stones produce activated carbon of inferior porous characteristics. Development of the extensive pore structure was described in light of the effect of H₃PO₄ on the lignocellulosic mataerial during carbonisation.     

Dispersion and Distribution of Ruthenium on Carbon-Coated Ceramic Monolithic Catalysts Prepared by Impregnation

Carbon-coated monolithic catalysts were prepared by dipcoating a cordierite substrate in a Furan resin, after which the ruthenium was incorporated by impregnation with a [RuCl₅]^2- complex. Immobilization of the precursor proceeded particularly via weak physisorption. Since the carbon-coated monolithic supports have a broad pore-size distribution, redistribution of the ruthenium precursor occurred upon drying due to capillary forces, resulting in low dispersion. A significant amount of ruthenium is present on the surface of the carbon inclusions located deep in the cordierite walls, making the diffusion path of the reactants to the active ruthenium sites considerably larger than the average carbon-coating thickness. For successful application of the carbon-coated monolithic catalysts in gas–liquid reactions, the deposition of carbon in the walls of the monolith should be prevented and maldistribution of ruthenium upon drying should be solved.     

One-pot synthesis of carbonaceous monolith with surface sulfonic groups and its carbonization/activation

The carbonaceous monoliths rich in surface sulfonic acid groups were synthesized by one-pot hydrothermal carbonization of the mixture of p-toluenesulfonic acid/glucose/resorcinol at 180 °C. The Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy characterizations confirmed the presence of surface sulfonic groups on these monoliths. The catalytic performance of this kind of carbonaceous material as a solid-acid catalyst was studied in the reaction of acetalization of benzaldehyde and the results showed that it has high activity and reusability. Then, these monoliths were further carbonized and activated to form monolithic carbons with high surface area and large pore volume. The surface area and pore volume per mass increased with prolonging the activation time (0–6 h) and the best results on 6-h activated samples were 2337 m²/g and 2.12 cm³/g. Due to the decrease in bulk density the volumetric surface area increased initially until maximum and then slightly dropped down during the activation. These carbonized and activated samples showed better oxidation resistance than one commercial activated carbon under air. Moreover, the adsorption capacity for dye molecules with different size on these activated samples was significant higher than that on commercial activated carbons and a synthetic ordered mesoporous carbon.     

Carbon adsorbents from waste ion-exchange resins

A series of activated carbons was prepared from different waste commercial ion-exchange resins and studied by means of adsorption, SEM and IR methods. Samples were additionally washed or washed/frozen. This resulted in increases in micro- and mesoporosity in comparison with initial activated carbons. For some samples, the latter treatment gives enhancement of mesoporosity but reduction of microporosity and vice versa comparing with only washed carbons due to different localization of water droplets in mesopores or micropores. Changes in the morphology of chars and activated samples depended on resin composition and history. Relatively high values of porosity (V_p ≈ 0.4 cm³/g) and specific surface area (S_BET ≈ 600 m²/g) show that activated carbons prepared from waste ion-exchange resins can be utilized for different purposes, especially after additional treatment (such as washing, impregnation by certain compounds and subsequent thermal activation).     

Active carbons from almond shells as adsorbents in gas and liquid phases

Almond shells have been used as raw material to prepare activated carbons. The preparation conditions, by direct activation of the raw material with CO₂ or air, or by activation after carbonisation under nitrogen, have been compared in respect to the percentage burn-off and the adsorptive properties of the resulting carbons. It is shown that direct activation with CO₂ in the temperature range 1023–1173 K (750–900°C) gives activated carbons with similar or larger surface areas than those obtained by the conventional method (carbonisation followed by activation). On the other hand, the results show the role played by the particle size of the carbonised products on the adsorptive properties of the activated carbons prepared from them. Activation with air (in the temperature range from 573 to 673 K) does not produce carbons with large surface areas, but the yield is large and this could be of practical interest. The adsorption of methylene blue and phenol, both in aqueous solution, has allowed further comparison of the adsorptive properties of the activated carbons. In all, the direct activation with CO₂ seems to be a very good way to prepare activated carbons from almond shells; they can be favourably compared with commercial activated carbons, two of which have been used in this study.     

Highly efficient adsorbents based on hierarchically macro/mesoporous carbon monoliths with strong hydrophobicity

Hierarchically porous carbon monoliths (HPCMs) with both ordered hexagonal mesoporosity and three-dimensionally connected macroporosity were synthesized via a simple and time-saving hydrothermal process followed by a nanocasting pathway. These monoliths have high macropore volumes and specific surface areas, strong hydrophobicity, low densities and regular shapes. Importantly, these HPCMs showed excellent performance in cleaning/recycling spilled oils or organic solvents with high adsorption capacity, rate, stability, and reusability, and in adsorbing bilirubin with high adsorption capacity, blood compatibility and durability in plasma as well. The adsorption of oil was based on the dispersion interaction between gasoline molecules and the carbon basal planes of HPCMs, while the isotherm of bilirubin adsorption on the optimized HPCMs and the corresponding kinetic data were found better fitted by Langmuir adsorption model and pseudo-second-order kinetic model, respectively.     

Fabrication of activated carbons with well-defined macropores derived from sulfonated poly(divinylbenzene) networks

Poly(divinylbenzene) (PDVB) monoliths with well-defined macropores that have been sulfonated and carbonized to obtain macroporous carbon monoliths. The original macroporous PDVB networks have been synthesized by living radical polymerization accompanied by spinodal decomposition. Sulfonation prevents polymer networks from large shrinkage and weight loss during carbonization by heat-treatment in an inert atmosphere. In the case of PDVB gels sulfonated at 120 °C using conc. H₂SO₄, mesopores in the original skeletons as well as macropores are retained after carbonization. The obtained carbon monoliths are subsequently activated by CO₂, which resulted in activated carbons. The specific surface area of the obtained activated carbons reaches up to 2360 m² g⁻¹.     

Functional mesoporous poly(ionic liquid)-based copolymer monoliths: From synthesis to catalysis and microporous carbon production

Ionic liquid-functionalized mesoporous polymeric networks with specific surface area up to 935 m²/g have been successfully synthesized one pot by solvothermal copolymerization of divinylbenzene and monomeric ionic liquids. The as-obtained polymers exhibit a monolithic structure featuring large pore volumes, an abundant mesoporosity and an adjustable content of ionic liquids. The effect of the reaction conditions on the pore structure has been studied in detail. These poly(ionic liquid)-based porous networks (PILPNs) have then been employed as precursors in two distinct applications, namely organocatalysis and production of microporous carbon monoliths. Selected organocatalyzed reactions, including carbonatation of propylene oxide by cycloaddition with carbon dioxide, benzoin condensation, and cyanosilylation of benzaldehyde have been readily triggered by PILPNs acting as crosslinked polymer-supported (pre)catalysts. The two latter reactions required the prior deprotonation of the imidazolium salt units with a strong base to successfully generate polymer-supported N-heterocyclic carbenes, referred to as poly(NHC)s. Facile recycling and reuse of polymer-supported (pre)catalysts was achieved by simple filtration owing to the heterogeneous reaction conditions. Furthermore, PILPNs could be easily converted into microporous carbon monoliths via CO₂ activation.     

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 m²/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.     

Novel carbon based catalysts for the reduction of NO: Influence of support precursors and active phase loading

Carbon-supported catalysts in the form of powder, briquettes and monolitos have been prepared. Powder and briquette samples have been obtained using a Spanish low-rank coal as raw material for carbon support through a pyrolysis process whereas monoliths were prepared by coating cordierite monoliths with a blend of two polymers. Vanadium was chosen as active element and impregnated by equilibrium adsorption from 1 to 8 wt% on the surface of as-prepared supports. All samples were tested in the selective catalytic reduction of NO with NH₃ as reducing agent at low temperature (150 °C), demonstrating a considerable efficiency which was enhanced mainly by oxidation treatments and an increase of vanadium loading up to vanadium agglomerates formation. The nature of carbon precursor determines the porosity development and surface chemistry of supports, what results in a different dispersion and fixation of active phase. An enhancement of NO efficiency is achieved by increasing microporosity and the amount of surface oxygen groups in pyrolysed coal whereas in polymer blend, mesoporosity and just a certain amount of surface oxygen groups should be promoted. An excess in vanadium loading decreases NO reduction efficiency because of a pore blockage and the formation of vanadium agglomeration what makes to expose a lower vanadium surface to the reactants.     

Activation de la tourbe de sphaigne par le dioxyde de carbone et agglomération du charbon actif produit

The physical activation of peat moss using CO₂ has been investigated with respect to the operational variables and the characteristics of the active carbon produced. Peat coke is produced during carbonisation with a 30% yield. The activated carbon accounts for 25% of the initial air dried peat moss. Analysis of the active carbon has revealed a surface area up to 600 m²/g and a chemical reactivity similar to commercial active carbon as measured by the iodine, phenazone and phenol indexes. The active carbon thus produced is a powdered material, with a weak abrasive resistance. Agglomeration of the carbon and subsequent pelletization has been done with ammonium ligno-sulfonate, water, and the application of pressure. Resistant pellets can be obtained using 15-20% lignosulfonate and 3000 psi.     

Activation de la tourbe de sphaigne par le dioxyde de carbone et agglomération du charbon actif produit

The physical activation of peat moss using CO₂ has been investigated with respect to the operational variables and the characteristics of the active carbon produced. Peat coke is produced during carbonisation with a 30% yield. The activated carbon accounts for 25% of the initial air dried peat moss. Analysis of the active carbon has revealed a surface area up to 600 m²/g and a chemical reactivity similar to commercial active carbon as measured by the iodine, phenazone and phenol indexes. The active carbon thus produced is a powdered material, with a weak abrasive resistance. Agglomeration of the carbon and subsequent pelletization has been done with ammonium ligno-sulfonate, water, and the application of pressure. Resistant pellets can be obtained using 15–20% lignosulfonate and 3000 psi.