Synthetic zeolites and other microporous oxide molecular sieves

Use of synthetic zeolites and other microporous oxides since 1950 has improved insulated windows, automobile air-conditioning, refrigerators, air brakes on trucks, laundry detergents, etc. Their large internal pore volumes, molecular-size pores, regularity of crystal structures, and the diverse framework chemical compositions allow “tailoring” of structure and properties. Thus, highly active and selective catalysts as well as adsorbents and ion exchangers with high capacities and selectivities were developed. In the petroleum refining and petrochemical industries, zeolites have made possible cheaper and lead-free gasoline, higher performance and lower-cost synthetic fibers and plastics, and many improvements in process efficiency and quality and in performance. Zeolites also help protect the environment by improving energy efficiency, reducing automobile exhaust and other emissions, cleaning up hazardous wastes (including the Three Mile Island nuclear power plant and other radioactive wastes), and, as specially tailored desiccants, facilitating the substitution of new refrigerants for the ozone-depleting chlorofluorocarbons banned by the Montreal Protocol.

Isotropic petroleum pitch as a carbon precursor for the preparation of activated carbons by KOH activation

Results show that it is possible to activate a low softening point isotropic petroleum pitch, without intermediate pre-treatments, by chemical activation with KOH. The chemical activation is carried out by direct heat treatment of a mixture of the isotropic pitch and KOH. It produces activated carbons (ACs) with micropore volumes as high as 1.12 cm3/g, and BET surface areas around 3000 m2/g. The activating agent/precursor ratios studied (from 1/1 to 4/1; wt./wt.) show, as expected, that increasing the ratio enhances the adsorption characteristics of the resulting AC.

Activation of polymer blend carbon nanofibres by alkaline hydroxides and their hydrogen storage performances

In the present work we study the hydroxide activation (NaOH and KOH) of phenol-formaldehyde resin derived CNFs prepared by a polymer blend technique to prepare highly porous activated carbon nanofibres (ACNFs). Morphology and textural characteristics of these ACNFs were studied and their hydrogen storage capacities at 77 K (at 0.1 MPa and at high pressures up to 4 MPa) were assessed, and compared, with reported capacities of other porous carbon materials. Phenol-formaldehyde resin derived carbon fibres were successfully activated with these two alkaline hydroxides rendering highly microporous ACNFs with reasonable good activation process yields up to 47 wt.% compared to 7 wt.% yields from steam activation for similar surface areas of 1500 m2/g or higher. These nano-sized activated carbons present interesting H2 storage capacities at 77 K which are comparable, or even higher, to other high quality microporous carbon materials. This observation is due, in part, to their nano-sized diameters allowing to enhance their packing densities to 0.71 g/cm3 and hence their resulting hydrogen storage capacities.

Well-defined mesoporosity on lignocellulosic-derived activated carbons

Activated carbons with a highly developed mesoscale cavitation-linked structure have been prepared from natural products (e.g. peach stones) by combining chemical and physical activation processes. Characterization results show that these materials exhibit a large “apparent” surface area (∼1500 m2/g) together with a well-defined mesoporous structure, i.e. large cavities connected to the external surface through narrower mesoporous necks (cavitation effects).

Ammonia removal using activated carbons: effect of the surface chemistry in dry and moist conditions

The effect of surface chemistry (nature and amount of oxygen groups) in the removal of ammonia was studied using a modified resin-based activated carbon. NH3 breakthrough column experiments show that the modification of the original activated carbon with nitric acid, that is, the incorporation of oxygen surface groups, highly improves the adsorption behavior at room temperature. Apparently, there is a linear relationship between the total adsorption capacity and the amount of the more acidic and less stable oxygen surface groups. Similar experiments using moist air clearly show that the effect of humidity highly depends on the surface chemistry of the carbon used. Moisture highly improves the adsorption behavior for samples with a low concentration of oxygen functionalities, probably due to the preferential adsorption of ammonia via dissolution into water. On the contrary, moisture exhibits a small effect on samples with a rich surface chemistry due to the preferential adsorption pathway via Brønsted and Lewis acid centers from the carbon surface. FTIR analyses of the exhausted oxidized samples confirm both the formation of NH4+ species interacting with the Brønsted acid sites, together with the presence of NH3 species coordinated, through the lone pair electron, to Lewis acid sites on the graphene layers.

Adsorbent density impact on gas storage capacities

In the literature, different approaches, terminologies, concepts and equations are used for calculating gas storage capacities. Very often, these approaches are not well defined, used and/or determined, giving rise to significant misconceptions. Even more, some of these approaches, very much associated with the type of adsorbent material used (e.g., porous carbons or new materials such as COFs and MOFs), impede a suitable comparison of their performances for gas storage applications. We review and present the set of equations used to assess the total storage capacity for which, contrarily to the absolute adsorption assessment, all its experimental variables can be determined experimentally without assumptions, ensuring the comparison of different porous storage materials for practical application. These material-based total storage capacities are calculated by taking into account the excess adsorption, the bulk density (ρbulk) and the true density (ρtrue) of the adsorbent. The impact of the material densities on the results are investigated for an exemplary hydrogen isotherm obtained at room temperature and up to 20 MPa. It turns out that the total storage capacity on a volumetric basis, which increases with both, ρbulk and ρtrue, is the most appropriate tool for comparing the performance of storage materials. However, the use of the total storage capacities on a gravimetric basis cannot be recommended, because low material bulk densities could lead to unrealistically high gravimetric values.

NaOH and KOH for preparing activated carbons used in energy and environmental applications

Alkaline hydroxides, especially sodium and potassium hydroxides, are multi-million-ton per annum commodities and strong chemical bases that have large scale applications. Some of them are related with their consequent ability to degrade most materials, depending on the temperature used. As an example, these chemicals are involved in the manufacture of pulp and paper, textiles, biodiesels, soaps and detergents, acid gases removal (e.g., SO2) and others, as well as in many organic synthesis processes. Sodium and potassium hydroxides are strong and corrosive bases, but they are also very stable chemicals that can melt without decomposition, NaOH at 318ºC, and KOH at 360ºC. Hence, they can react with most materials, even with relatively inert ones such as carbon materials. Thus, at temperatures higher than 360ºC these melted hydroxides easily react with most types of carbon-containing raw materials (coals, lignocellulosic materials, pitches, etc.), as well as with most pure carbon materials (carbon fibers, carbon nanofibers and carbon nanotubes). This reaction occurs via a solid-liquid redox reaction in which both hydroxides (NaOH or KOH) are converted to the following main products: hydrogen, alkaline metals and alkaline carbonates, as a result of the carbon precursor oxidation. By controlling this reaction, and after a suitable washing process, good quality activated carbons (ACs), a classical type of porous materials, can be prepared. Such carbon activation by hydroxides, known since long time ago, continues to be under research due to the unique properties of the resulting activated carbons. They have promising high porosity developments and interesting pore size distributions. These two properties are important for new applications such as gas storage (e.g., natural gas or hydrogen), capture, storage and transport of carbon dioxide, electricity storage demands (EDLC-supercapacitors-) or pollution control. Because these applications require new and superior quality activated carbons, there is no doubt that among the different existing activating processes, the one based on the chemical reaction between the carbon precursor and the alkaline hydroxide (NaOH or KOH) gives the best activation results. The present article covers different aspects of the activation by hydroxides, including the characteristics of the resulting activated carbons and their performance in some environment-related applications. The following topics are discussed: i) variables of the preparation method, such as the nature of the hydroxide, the type of carbon precursor, the hydroxide/carbon precursor ratio, the mixing procedure of carbon precursor and hydroxide (impregnation of the precursor with a hydroxide solution or mixing both, hydroxide and carbon precursor, as solids), or the temperature and time of the reaction are discussed, analyzing their effect on the resulting porosity; ii) analysis of the main reactions occurring during the activation process, iii) comparative analysis of the porosity development obtained from different activation processes (e.g., CO2, steam, phosphoric acid and hydroxides activation); and iv) performance of the prepared activated carbon materials on a few applications, such as VOC removal, electricity and gas storages.

Electrooxidation methods to produce pseudocapacitance-containing porous carbons

Surface oxygen groups play a key role on the performance of porous carbon electrodes for electrochemical capacitors in aqueous media. The electrooxidation method in NaCl electrolyte using a filter press cell and dimensionally stable anodes is proposed as a viable process for the generation of oxygen groups on porous carbon materials. The experimental set-up is so flexible that allows the easy modification of carbon materials with different configurations, i.e. cloths and granular, obtaining different degrees of oxidation for both conformations without the requirement of binders and conductivity promoters. After the electrooxidation method, the attained porosity is maintained between 90 and 75% of the initial values. The surface oxygen groups generated can increase the capacitance up to a 30% when compared to the pristine material. However, a severe oxidation is detrimental since it may decrease the conductivity and increase the resistance for ion mobility.

Relevance of porosity and surface chemistry of superactivated carbons in capacitors

We show, through some examples, that chemical activation by alkaline hydroxides permits the preparation of activated carbons with tailored pore volume, pore size distribution, pore structure and surface chemistry, which are useful for their application as electrodes in supercapacitors. Examples are presented discussing the importance of each of these properties on the double layer capacitance, on the kinetics of the electric double-layer charge-discharge process and on the pseudo-capacitative contribution from the surface functional groups or the addition of a conducting polymer.

Electrocatalytic activity of Ni-doped nanoporous carbons in the electrooxidation of propargyl alcohol

Herein, we explore the immobilization of nickel on various carbon supports and their application as electrocatalysts for the oxidation of propargyl alcohol in alkaline medium. In comparison with massive and nanoparticulated nickel electrode systems, Ni-doped nanoporous carbons provided similar propargyl alcohol conversions for very low metallic contents. Nanoparticulated Ni on various carbon supports gave rise to the highest electrocatalytic activity in terms of product selectivity, with a clear dependence on Ni content. The results point to the importance of controlling the dispersion of the Ni phase within the carbon matrix for a full exploitation of the electroactive area of the metal. Additionally, a change in the mechanism of the propargyl alcohol electrooxidation was noted, which seems to be related to the physicochemical properties of the carbon support as well. Thus, the stereoselectivity of the electrooxidative reaction can be controlled by the active nickel content immobilized on the anode, with a preferential oxidation to (Z)-3-(2-propynoxy)-2-propenoic acid with high Ni-loading, and to propiolic acid with low loading of active Ni sites. Moreover, the formation of (E)-3-(2-propynoxy)-2-propenoic acid was discriminatory irrespective of the experimental conditions and Ni loadings on the carbon matrixes.

Assessment of CO2 adsorption capacity on activated carbons by a combination of batch and dynamic tests

In this work, batch and dynamic adsorption tests are coupled for an accurate evaluation of CO2 adsorption performance for three different activated carbons obtained from olives stones by chemical activation followed by physical activation with CO2 at varying times, i.e. 20, 40 and 60 h. Kinetic and thermodynamic CO2 adsorption tests from simulated flue-gas at different temperature and CO2 pressure are carried out both in batch (a manometric equipment operating with pure CO2) and dynamic (a lab-scale fixed-bed column operating with CO2/N2 mixture) conditions. The textural characterization of the activated carbon samples shows a direct dependence of both micropore and ultramicropore volume on the activation time, hence AC60 has the higher contribution. The adsorption tests conducted at 273 and 293 K showed that, when CO2 pressure is lower than 0.3 bar, the lower the activation time the higher CO2 adsorption capacity and a ranking ωeq(AC20)>ωeq(AC40)>ωeq(AC60) can be exactly defined when T= 293 K. This result can be likely ascribed to a narrower pore size distribution of the AC20 sample, whose smaller pores are more effective for CO2 capture at higher temperature and lower CO2 pressure, the latter representing operating conditions of major interest for decarbonation of a flue-gas effluent. Moreover, the experimental results obtained from dynamic tests confirm the results derived from the batch tests in terms of CO2 adsorption capacity. It is important to highlight that the adsorption of N2 on the synthesized AC samples can be considered negligible. Finally, the importance of a proper analysis of characterization data and adsorption experimental results is highlighted for a correct assessment of CO2 removal performances of activated carbons at different CO2 pressure and operating temperature.

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.

Spherical carbons: Synthesis, characterization and activation processes

Spherical carbons have been prepared through hydrothermal treatment of three carbohydrates (glucose, saccharose and cellulose). Preparation variables such as treatment time, treatment temperature and concentration of carbohydrate have been analyzed to obtain spherical carbons. These spherical carbons can be prepared with particle sizes larger than 10 μm, especially from saccharose, and have subsequently been activated using different activation processes (H3PO4, NaOH, KOH or physical activation with CO2) to develop their textural properties. All these spherical carbons maintained their spherical morphology after the activation process, except when KOH/carbon ratios higher than 4/1 were used, which caused partial destruction of the spheres. The spherical activated carbons develop interesting textural properties with the four activating agents employed, reaching surface areas up to 3100 m2/g. Comparison of spherical activated carbons obtained with the different activating agents, taking into account the yields obtained after the activation process, shows that phosphoric acid activation produces spherical activated carbons with higher developed surface areas. Also, the spherical activated carbons present different oxygen groups’ content depending on the activating agent employed (higher surface oxygen groups content for chemical activation than for physical activation).

Factors governing the adsorption of ethanol on spherical activated carbons

Ethanol adsorption on different activated carbons (mostly spherical ones) was investigated covering the relative pressure range from 0.001 to 1. Oxygen surface contents of the ACs were modified by oxidation (in HNO3 solution or air) and/or by thermal treatment in N2. To differentiate the concomitant effects of porosity and oxygen surface chemistry on ethanol adsorption, different sets of samples were used to analyze different relative pressure ranges (below 1000 ppmv concentration and close to unity). To see the effect of oxygen surface chemistry, selected samples having similar porosity but different oxygen contents were studied in the low relative pressure range. At low ethanol concentration (225 ppmv) adsorption is favored in oxidized samples, remarking the effect of the oxidizing treatment used (HNO3 is more effective than air) and the type of oxygen functionalities created (carboxyl and anhydride groups are more effective than phenolic, carbonyl and derivatives). To analyze the high relative pressure range, spherical and additional ACs were used. As the relative pressure of ethanol increases, the effect of oxygen-containing surface groups decreases and microporosity becomes the most important variable affecting the adsorption of ethanol.

New insights on electrochemical hydrogen storage in nanoporous carbons by in situ Raman spectroscopy

In situ Raman spectroscopy was exploited to analyze the interaction between carbon and hydrogen during electrochemical hydrogen storage at cathodic conditions. Two different activated carbons were used and characterized by different electrochemical techniques in two electrolytes (6 M KOH and 0.5 M Na2SO4). The in situ Raman spectra collected showed that, in addition to the D and G bands associated to the graphitic carbons, two bands appear simultaneously at about 1110 and 1500 cm−1 under cathodic conditions, and then they disappear when the potential increases to more positive values. This indicates that carbon–hydrogen bonds are formed reversibly in both electrolytes during cathodic conditions. Comparing the two activated carbons, it was confirmed that, in both electrolytes, the hydrogenation of carbon atoms is produced more easily for the sample with lower amount of surface oxygen groups. In KOH medium, for the two samples, the formation of carbon–hydrogen bonds proceeds at more positive potential with respect to the thermodynamic potential value for hydrogen evolution. Furthermore, changes in the shape of the D band (due to an intensity increase of the D1 band) during the formation of carbon–hydrogen bonds suggest that hydrogenation of the carbon atoms increases the number of edge planes.