Juan Alcañiz-Monge

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.

Formation of mesopores in phenolic resin-derived carbon fiber by catalytic activation using cobalt

Abstract Preparation of an activated carbon fiber containing mesopores was attempted by catalytic activation using cobalt. Phenolic resin and cobalt-acetylacetonate were mixed intimately, spun, cured, carbonized at 900 C in nitrogen and finally activated at 750 900 C in steam. The carbon fibers with the cobalt contents of 38 ppm and 100 ppm were prepared together with a fiber without cobalt as a reference. The cobalt deposited a characteristic turbostratic carbon by catalytic graphitization at carbonization process. Simultaneously the cobalt accelerated activation of the fiber in steam catalytically to form mesopores preferentially. The maximum surface area. 170 m2g 1. for mesopores of several 10s of nm in radius was obtained in the fiber containing 38 ppm cobalt after 38% burn-off. This fiber also contained a relatively large amount of micropores. No damage on the liber surface after activation was observed by scanning electron microscopy, but the fiber became fragile. The formation mechanism of mesopores is discussed briefly.

Methane storage in activated carbon fibres

Abstract Methane storage in two series of activated carbon fibres (ACF) has been studied. The two series have been prepared by activation with CO2 and steam of a petroleum-pitch based carbon fibre. Methane isotherms have been carried out in a gravimetric system up to 4 MPa. ACF are very promising materials for this purpose because they are essentially microporous activated carbons. The paper analyzes the different correlations found in the literature between methane storage capacity and parameters related with the porous texture. Our best correlation is found with the total micropore volume, noting that this micropore volume has to include both the narrow microporosity (size lower than 0.7 nm) and supermicroporosity (size between 0.7–2.0 nm). This statement is specially relevant for samples in which N2 adsorption has diffusional limitations. Steam activation produces lower micropore volumes than CO2 and, hence, lower storage capacities. Methane storage capacities of 163 v/v are reached with the ACF prepared by CO2 activation. The delivery of methane obtained for the best samples is about 143 v/v. This value is very close to that aimed for a commercial application of space-filling shapes of activated carbons.

Effect of the activating gas on tensile strength and pore structure of pitch-based carbon fibres

Abstract Petroleum pitch-based activated carbon fibres have been prepared by CO2 and steam activation. The effects of the activating gas on porosity and mechanical properties of the activated carbon fibres have been analyzed. The original carbon fibre contains a narrow porosity that is well developed upon activation in both CO2 and steam (an apparent surface area of about 1700 m 2 g is obtained after a 50% burn-off). The activated carbon fibres obtained exhibit different porous texture evolution with burn-off depending on the activating gas used. CO2 essentially develops microporosity and causes a steady decrease in the tensile strength with burn-off, while the fibre diameter does not change significantly. Contrarily, steam produces a wider porous texture and, after the initial stages of the activation process, the tensile strength remains nearly constant and the fibre diameter decreases. The results have been interpreted considering the different behaviour of the two molecules (CO2 and steam) involved in the reaction in the narrow microporosity. In this sense, CO2 seems to react to a larger extent than steam inside the narrow microporosity due to its larger coefficient diffusion.

Theoretical and experimental studies of methane adsorption on microporous carbons

Abstract GCEMC molecular simulations of methane adsorption in model slit-shaped carbon pores show that variations in the density of carbon atoms in the pore wall have a significant influence on adsorbed methane density, although variations in inter-layer spacing and the number of layer planes in the pore wall, n, (n > 2) have little effect. A model is proposed for calculating the influence of pore wall thickness on the stored volumetric methane capacities of a void-free microporous carbon monolith and model monoliths formed from close-packed powders and fibers. Volumetric methane capacities for the models were in reasonable agreement with measured values for carbon monoliths made from KOH-activated meso-carbon microbeads and for compacted activated carbon fibers.

Preparation and properties of an antibacterial activated carbon fiber containing mesopores

Preparation of an antibacterial activated carbon fiber with mesopores was attempted. Phenolic resin containing cobalt as an activation catalyst and silver as an antibacterial agent was spun, stabilized, carbonized and activated in steam. The number of the metal particles increased with as activation proceeded and reached to 100–200 nm in diameter at the largest. The activated carbon fiber with 72 m2 g−1 of mesopore surface area was obtained after 51 wt% burn-off of the carbon fiber containing 0.22 wt% of Ag and 52 ppm of Co. The silver somewhat disturbed formations of both micro- and mesopores through catalytic activation by cobalt. A silver content of 0.22 wt% in the activated carbon fiber was rapidly decreased to 0.0006 wt% after immersion in flowing tap water for 20 days, but the resulting fiber exhibited antibacterial activity against Escherichia coli and Staphylococcus aureus. The above behaviors can be reasonably explained by the formation of an alloy of cobalt and silver.

Characterisation of coal tar pitches by thermal analysis, infrared spectroscopy and solvent fractionation

Four commercially available coal tar pitches have been characterised through the combination of conventional techniques used for pitch characterisation, like elemental analysis and solvent fractionation, and infrared spectroscopy (FTIR) and thermal analysis (thermogravimetry (TG)-differential thermal analysis (DTA)). The results obtained from this set of experiments provide suitable information to establish differences between the materials according to their industrial origin and preparation method. The differences found are a consequence of the distinct chemical composition of the pitches (determined by FTIR), as well as the dissimilar chemical interaction that exists between the molecules of different fractions of the parent pitch (evidenced from DTA). These observations are responsible of the clear differences found between a given pitch and its respective insolubles, and affect their applications, i.e. in the area of carbon fibre preparation.

Preparation of general purpose carbon fibers from coal tar pitches with low softening point

This paper describes the different procedures applied for the preparation of general purpose carbon fibers from four coal tar pitches. The four raw materials have a low softening point (of about 373 K) for this application and, hence, must be subjected to a treatment before spinning to increase this temperature. The treatments applied are the following: (i) heating in N2, (ii) heating in air, (iii) consecutive heating in N2 and air, and (iv) blending of the coal tar pitch with a petroleum one and further treatment in air. The changes in chemical composition, softening point and yield of the treatments used have been followed by different techniques. Only treatments (iii) and (iv) produce materials with an adequate viscosity for spinning and a sufficiently high softening point to be transformed in carbon fibers. The carbon fibers obtained have similar mechanical properties to those prepared in a previous work from a suitable petroleum pitch with the same experimental system. However, the mechanical properties of these fibers are inferior to those of a commercial carbon fiber, the differences being due to the lower diameter of the commercial fibers.

Molecular sieve properties of general-purpose carbon fibres

Abstract Molecular sieve properties of general-purpose carbon fibres (GPCFs) from different origins are studied. Selectivity of the materials for CO2 and CH4 separation and uptake of these gases are analysed and compared with a commercial pelleted carbon molecular sieve appropriate for this process. A set of four GPCFs, one of them commercially available, obtained from petroleum or coal tar pitches is used in this work. Kinetics of CO2 and CH4 uptake at different pressures and temperatures have been followed. The results obtained show that the GPCFs have a high selectivity to CO2, the kinetics of the process being very slow. The treatments, which comprise either the elimination of surface oxygen groups or controlled gasification, produce materials with a high selectivity and CO2 uptake and fast kinetics. In this sense, the general-purpose carbon fibres can have a performance as good as a commercial carbon molecular sieve.

Development of new carbon honeycomb structures from cellulose and pitch

Abstract In this paper the development of a new, low-cost method for the preparation of carbon honeycomb structures for gas adsorption applications is described. The method comprises the impregnation of a petroleum pitch into a cellulose-based corrugated paper. The resultant material has a high carbon content and retains the original structure of the paper, making it suitable for usage in gas flow applications. TEM and SAD studies on the carbonised material suggest the presence of two different types of carbon structures, a disorganised structure and a more organised one. The porosity of the samples was characterised by CO 2 and N 2 adsorption. The results indicated an appreciable narrow microporosity with a high structural stability to high temperatures (presence of the porosity at high temperatures). Finally, the molecular sieve properties of the materials were studied by CH 4 and CO 2 adsorption kinetics and compared favourably with those of a commercial carbon molecular sieve (CMS), indicating their promise for high temperature applications, such as catalyst supports or for gas separations.