Ind. Eng. Chem. Res. 2007, 46, 1167-1173
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MATERIALS AND INTERFACES Effect of Process Conditions on Physicochemical and Electrical Characteristics of Denim-Based Activated Carbon Cloths Marı´a E. Ramos, Javier D. Gonza´ lez, Pablo R. Bonelli, and Ana L. Cukierman* Programa de InVestigacio´ n y Desarrollo de Fuentes AlternatiVas de Materias Primas y Energı´a-PINMATE, Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, UniVersidad de Buenos Aires, Intendente Gu¨iraldes 2620, Ciudad UniVersitaria, (C1428BGA) Buenos Aires, Argentina, and Ca´ tedra de Farmacotecnia II-Tecnologı´a Especial, Departamento de Tecnologı´a Farmace´ utica, Facultad de Farmacia y Bioquı´mica, UniVersidad de Buenos Aires, Junı´n 956, (C1113AAD) Buenos Aires, Argentina
Activated carbon cloths (ACCs) from denim fabric were developed by phosphoric acid activation in an inert atmosphere. The effect of acid concentration (5-15 wt %) and final thermal treatment temperature (600950 °C) on yield, chemical characteristics (elemental composition, ash, total acidity), porosity development, morphology, and electrical properties of the resulting ACCs were examined. Noticeable changes in the thermal behavior of the precursor due to acid impregnation were detected from thermogravimetric measurements using comparative samples of raw and acid-treated denim. ACCs properties were dependent on a combined effect of the conditions employed, though the temperature exerted a major influence. The ACCs prepared with 10 wt % acid concentration at 950 °C showed maximum BET surface area (1055 m2/g) and total pore volume (0.53 cm3/g), and well-preserved fibers integrity. Almost all the denim-derived ACCs were electrically conductive, following ohmic behavior. Increasing the final temperature from 600 to 800 °C led to a substantial change in electrical conductivity. The ACCs were heated by Joule effect, attaining surface temperatures in the range 300-500 K. Electrical resistivity of the ACCs was found to depend on the extent of porosity development. 1. Introduction Activated carbon cloths (ACCs) have attracted great interest in recent years because they offer several technological advantages over the traditional powder or granular forms of this widely used porous material. The small diameter of the fibers in ACCs and pores’ direct connection to the environment minimize diffusion effects and lead to faster adsorption rates, higher efficiency, larger capacity for adsorption, and lower pressure drops in flow units. Besides, ACCs can be arranged in different stable configurations, and their contiguous nature is suited for electrical and electrochemical applications.1-4 Due to ACCs advantages, a growing number of studies for potential applications in various fields have been reported in the last years, though most of them used commercial samples. Either untreated or modified ACCs have been investigated as catalyst supports5-8 and electrode materials9-12 and for abatement of gaseous and liquid pollutants13-20 and for gas storage and separation, H2 recovery and purification, and sour gas sweetening.21 As for the traditional forms, methods for preparing ACCs can be grouped into physical and chemical activation. The latter leads to activated carbons with developed pore structure in higher yield and at relatively lower temperatures than physical activation.1,22-24 Moreover, the activating agent alters the course of pyrolysis, leading to a considerable increase in mechanical strength,1 that is, especially relevant to ACCs. * To whom correspondence should be addressed. Fax: 54-1145763366. E-mail:
[email protected].
Studies in the literature concerned with preparation of activated carbon cloths or fibers include different precursors, mainly viscous rayon and poly(acrylonitrile) (PAN) as starting materials, applying both physical and chemical activation processes.21,25-32 Though cellulosic textiles as ACCs precursors have also been employed,33 to the best of our knowledge, denim, which is very popular due to its strength and durability, has not been investigated earlier for this purpose. Moreover, only a few studies have been devoted to examine the effect of process variables on electrical properties of ACCs on a macroscopic scale.34 They are relevant to extend applications of activated carbons and to be used in industrial air treatment processes for in situ regeneration of the cloths by Joule effect heating. As known, adsorption of volatile organic compounds (VOCs) from industrial gas effluents has been traditionally performed with granular activated carbons. Recovery of VOCs from the adsorption bed and regeneration of the adsorbent are then accomplished with steam, inert gas, or vacuum.1 ACCs provide an advantage over granular activated carbon for this purpose due to their suitability for the energy efficient desorption process of electrothermal regeneration. Accordingly, ACCs regeneration and recovery of the adsorbed compounds may be achieved by direct application of current using the Joule effect.19,34 Nevertheless, since properties of ACCs are strongly dependent on the operating conditions of the manufacturing processes and on the intrinsic characteristics of the precursor used, the choice of a particular precursor has to be made with regard to the desired ACCs electrical behavior.
10.1021/ie060842s CCC: $37.00 © 2007 American Chemical Society Published on Web 01/25/2007
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Table 1. Elemental Composition (Dry, Ash-Free Basis) and Ash Content (Dry Basis) of Denim Precursor denim fabric N (wt %) C (wt %) H (wt %) a
0.3 43.2 6.2
denim fabric O (wt %)a ash (wt %)
50.2 0.2
Estimated by difference.
This aspect has been almost unexplored, at least in the open literature. Within this context, the present work explores the feasibility of using denim in the form of woven cloth as a starting material for the production of activated carbon cloths by phosphoric acid activation. Thermogravimetric measurements for the raw and acid-treated precursor were carried out in order to examine changes in the precursor’s thermal behavior caused by the activation reagent. The influence of the acid concentration and final thermal treatment temperature on yield, physicochemical characteristics, and electrical behavior of the resulting ACCs was systematically investigated. Besides, to gain insight into the effect of the surface chemistry of the denim-based ACCs, some measurements of the capability of selected ACCs in removing Zn(II) ions from dilute solutions, under fixed preestablished conditions were additionally performed. 2. Experimental Section 2.1. Preparation of Activated Carbon Cloths. Denim fabric with a serge weave, a warp density of 25.6 yarns/cm, a weft density of 18.8 yarns/cm, and specific mass of 457 g/m2 was used. Elemental composition and ash content of the denim fabric used, following the methods detailed in section 2.2, are reported in Table 1. Cloth specimens, (80 mm × 80 mm) previously weighed, were soaked in H3PO4 acid (analytical grade) solutions of concentration in the range of 5-15 wt % at 100 °C for 30 min. The impregnated samples were thermally treated in a tubular stainless steel reactor of horizontal configuration under a N2 flow (100 mL/min). It was externally heated by an electric furnace with a programmable temperature controller, which provided control of the heating rate, temperature, and heating time. Preliminary measurements of the internal temperature enabled determination of the constant-temperature zone in the reactor. The samples were placed in the center of this zone and heated at a rate of 5 °C/min to different final temperatures (FT), varying between 600 and 950 °C. To improve resistance of the ACCs, an isothermal step at initial cellulose decomposition temperature was included in the thermal schedule.26 Once the desired final temperature was attained, it was held for 1 h. The N2 flow was kept throughout the thermal treatment and further cooling of the samples up to room temperature. Afterward, the resulting cloths were rinsed thoroughly with distilled hot water until the wash water had a neutral pH, in order to remove the acid. They were subsequently dried, periodically weighted until constant weight, and stored in sealed bags for further characterization. Yields were calculated from weight differences. For comparison, blank samples, namely, carbon cloths using the precursor soaked only in distilled water (acid-free samples, FT ) 800 °C), were prepared. Furthermore, to explore the effect of more drastic conditions on some characteristics of the ACCs, samples using the precursor soaked with a high acid concentration solution (50 wt % H3PO4, FT ) 800 °C) and carbon cloths thermally treated holding the final temperature for 3 h (H3PO4 10 wt %, FT ) 950 °C) were also obtained. Duplicates of all
the samples were prepared for the different experimental conditions examined. Further details of the equipment and procedure used may be found elsewhere.35 Moreover, measurements by non-isothermal thermogravimetric analysis (TGA) for pristine denim and 10 wt % acidtreated samples were carried out under a N2 flow (100 mL/ min) using a Netzsch STA 409 thermal analyzer equipped with a data acquisition system. The samples (∼10 mg) were placed in the thermobalance and heated from room temperature to 800 °C using a heating rate of 10 °C/min. 2.2. Characterization of the Activated Carbon Cloths. N2 adsorption experiments at (-196 °C) were conducted to determine the specific surface area and pore volume of the ACCs using a Micromeritics Gemini 2360 instrument. The isotherms were determined on the samples prior outgassed under helium flow at 120 °C up to constant weight. The Brunauer-EmmettTeller (BET) model was applied to fit the isotherms and to evaluate the specific surface area of the samples, following the conventional procedure. Total pore volumes were estimated from the amount of N2 adsorbed at the highest relative pressure, near unity. The total content of acidic surface oxygen functional groups of the ACCs was assessed by titration of the samples with a 0.1 N sodium hydroxide solution following a conventional procedure.36 Besides, on the basis of the reported effect of the surface chemistry on metal adsorption performance of conventional activated carbons,37-39 the influence of the total acidity of the denim-derived ACCs was further explored by carrying out some measurements of Zn(II) ion adsorption from dilute solutions, under fixed, preestablished equilibrium conditions. For this purpose, those ACCs samples showing maximum and minimum contents of acidic surface groups and the blank were used. The uptake of Zn(II) ions was determined by contacting a weighed amount of each sample (0.1 g) to 5 mL of a zinc nitrate (analytical grade) solution (20 ppm) in capped glass flasks. They were kept in a shaker. Adsorption assays were carried out at room temperature and pH ) 3. The contact time and other conditions were selected on the basis of preliminary experiments, which demonstrated that equilibrium was established in 48 h. After this period, the solution was filtered and concentrations of the Zn(II) ions remaining were determined by spectrophotometry measurements at λ ) 510 nm using dithizone as colorimetric reagent at pH 5.5.40 Elemental composition of the ACCs was determined using a Carlo Erba EA 1108 elemental analyzer. No evidence for the presence of sulfur was found. Ash content was assessed according to standard ASTM D5142-04. The surface morphology of the denim fabric and derived ACCs were examined by scanning electron microscopy (SEM) with a Philips XL-30 microscope. Before the measurements, all the samples were attached to mounting stubs, dried under vacuum, and sputtercoated with Au-Pd.41 Macroscopic measurements of the electrical resistance of the denim-based ACCs were carried out to determine electrical resistivity and thermal coefficient, following a procedure similar to the one earlier reported by Subrenat et al.34 for commercial ACCs samples. Briefly, rectangular ACCs samples were assembled between two copper electrodes. A standard DC method in a four probe configuration was used. Measurements were carried out in ambient air and in the warp direction. The temperature at the center of the samples’ surface was measured with a copper-constantan thermocouple. It should be mentioned that, from preliminary assays, no significant variations were determined by performing the measurements in the weft
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the acid-treated sample. Acid impregnation led to degradation of the precursor at lower temperatures compared to that of the pristine fabric. Thermal degradation of the latter began at around 250 °C, proceeding through a sharp weight loss between 250 and 350 °C, which remained nearly constant at higher temperatures. For the acid-treated precursor, degradation took place steadily almost throughout the whole range of temperatures, including the low-temperature region (
