CaO-Loaded Activated Carbon for Carbon Dioxide Capture

Article pubs.acs.org/IECR

MgO/CaO-Loaded Activated Carbon for Carbon Dioxide Capture: Practical Aspects of Use Jacek Przepiórski,*,† Adam Czyzė wski,† Robert Pietrzak,‡ and Antoni W. Morawski† †

Institute of Chemical and Environmental Engineering, West Pomeranian University of Technology in Szczecin, ul. Pulaskiego 10, 70-322 Szczecin, Poland ‡ Faculty of Chemistry, Adam Mickiewicz University of Poznan, ul. Umultowska 89b, 61-614 Poznan, Poland S Supporting Information *

ABSTRACT: Porous carbons loaded with CaO and MgO were prepared through a one-step process from mixtures of poly(ethylene terephthalate) and dolomite mineral. Obtained hybrid materials were examined as sorbents for CO2 gas contained in air. The influence of pore structure, loadings of the oxides, moisture, and temperature on the efficiency of gas removal is presented and discussed. Performance of the materials during CO2 capture was confirmed to be influenced by all mentioned factors. As a rule, efficiency of CO2 removal increased along with loading of the oxides. CO2 uptake increased also with moisture content and temperature of the bed. On the basis of attained results, optimal parameters for operational use are suggested and probable mechanisms of the CO2 removal from dry and humid air are proposed. Cyclic sorption−desorption experiments confirmed capability of the material for several-times use. Superior performance of the hybrid materials over a mixture of CaO and MgO is confirmed and discussed.

1. INTRODUCTION Purification of gaseous or liquid media can be realized according to various methods employing both physical and chemical processes.1,2 One of the popular methods to eliminate undesired components from fluids is adsorption on activated carbons.3,4 Typically, filtering beds using activated carbons are especially effective in removal of numerous organic contaminants contained in purified media at low concentrations. Much attention has been paid to application of activated carbon for deep purification of air. Such a task is often necessary to realize production of ultraclean air for specific applications. Highly pure air is required for certain processes carried out in clean rooms.2,5 Routinely, primary concern in clean rooms air is particulates and volatile organics. However, in recent years even traces of inorganic gaseous contaminants or organic vapors are considered as adversely affecting manufacturing of advanced microelectronic devices. For example, besides other inorganic species, carbon dioxide has been recognized as a contaminant in the dry etching process.7 Here CO2 is considered as an oxidant and as a source of carbon. The latter acts as a reducing agent, forming inhibitor films on surface layers, and thus decreases etch rate. In effect, even trace CO2 levels can lead to process variability that entails manufacturing yield losses and decreased tool uptimes. Hence, in order to continue manufacturing process reproducibly, control of gaseous contaminants is highly recommended. For that reason, specialty purifiers are mounted at the point of use, i.e., directly prior to vacuum chambers where the etching process is carried out. There are many variations of activated carbons offered on the market. These are sold in the form of a powder, granules, or even in a fibrous form known as an activated carbon fiber.2 Besides the physical form, carbon adsorbents differ in pore structure, which is one of the key factors affecting adsorption and thus effectiveness in a specific application.6 An important © 2013 American Chemical Society

role is attributed to the surface chemistry of activated carbon,8−10 especially to the presence of various oxygencontaining functional groups. The presence of some oxygen functionalities on the surface of porous carbon may even strongly enhance uptake of suitable adsorbates including polar or polarizable molecules like phenol11 or carbon dioxide.12 Porous materials including activated carbon are often intentionally loaded with chemicals. Usually such combined materials are used for specific applications, mainly as highly effective and selective sorbent materials toward some gases contained in gaseous mixtures. Conventionally, loading of chemicals to porous materials is realized by the impregnation method.13,14 Extensive works on highly efficient sorbent materials are ongoing, and numerous important reports concerning specific materials dedicated for removal of carbon dioxide from air have been reported. High CO2 uptakes, ca. 1− 2.5 mmol/g depending on experimental conditions, were observed for triamine-grafted mesoporous silica15 and as high as ca. 4 mmol/g for hyperbranched aminosilicas.16 The latter have been reported as highly selective toward CO2 and easy to regenerate by mild heating.17 Comparable or higher uptakes were reported18,19 for metal−organic frameworks proposed as regenerable sorbent materials for CO2 from ambient air or flue gases. An interesting optional routine to obtain porous carbon materials containing additives is thermal treatment of carbon precursors like pitches20 or polymeric materials,21,22 mixed with suitable chemicals. Because of the porous character and some content of reactive chemicals, such hybrid materials often Received: Revised: Accepted: Published: 6669

May 25, 2012 April 24, 2013 April 24, 2013 April 24, 2013 dx.doi.org/10.1021/ie302848r | Ind. Eng. Chem. Res. 2013, 52, 6669−6677

Industrial & Engineering Chemistry Research

Article

2.2. Methods. 2.2.1. Pore Structure Parameters. Nitrogen adsorption/desorption (at 77 K) isotherms were measured using the Quadrasorb SI apparatus (Quantachrome Instruments, USA). Prior to the measurements, samples were degassed. For that purpose these were subjected to 24 h long heating at 290 °C under a high vacuum. Achieved nitrogen adsorption/desorption isotherms were used to determine textural parameters. Specific surface area values were calculated by applying the Brunauer−Emmett−Teller (BET) model. Using the αs analysis micropore surface areas (Smicro), external surface areas (Sext), and total surface areas (Stotal) could be calculated. Total pore volumes (Vtotal 0.95) were determined directly from the N2 adsorption isotherm considering amounts of the gas adsorbed at relative pressure p/p0 = 0.95. Micropore volumes (Vmicro DR) were estimated according to the Dubinin− Radushkevich equation using adsorption branches of the measured isotherms. Accordingly, mesopore volumes (Vmeso) were calculated by subtracting the micropore volume from the total pore volume. 2.2.2. X-ray Diffraction (XRD) Measurements and Mean Sizes of MgO and CaO Crystallites. All X-ray diffraction (XRD) patterns necessary to examine phase compositions and determine the mean sizes of MgO and CaO crystallites were recorded with the use of a Philips X’Pert PRO diffractometer operating with Cu Kα radiation (λ = 1.54056 Å). Mean sizes of MgO and CaO crystallites were determined according to Scherrer’s formula

perform very effectively in selective removal of particular impurities from various gaseous streams including air. Activated carbons containing chemicals find applications in manufacturing of various filter cartridges for respiratory protection23 or purification of air in closed ventilation systems.24 Application as sorbent beds for production of clean air for clean rooms25 may act as another example of practical utilization of the combined materials. Purifying media offered for clean room applications can be composed of an inorganic support holding chemicals reactive toward a gaseous impurity that is removed by irreversible mechanism. Such hybrid materials are capable to remove the contaminant to subparts per million level contents.7 Hence, a system composed of activated carbon containing basic metal oxides can be considered as potentially suitable for removal of CO2 from air. Recently, we reported on preparation of porous carbon materials containing both MgO and CaO.26 Thermogravimetric studies carried out in CO2 and N2 atmospheres revealed the capability of such hybrid materials to capture carbon dioxide. Adsorbent materials were confirmed to be easy to regenerate after saturation with the CO2 gas, and some loss of carbon material accompanying the regeneration process was observed. Because of the potential to fix CO2, we decided to determine if these hybrid materials are capable of eliminating CO2 contained in air at parts per million levels. Hence, this research shows how the MgO/CaO-loaded carbons prepared through pyrolysis of a mixture of poly(ethylene terephthalate) and natural dolomite can be practically used and how operational conditions may influence uptake of CO2 by the materials. Much attention has also been paid to the effect of the porous structure and loadings of the oxides on CO2 uptake. In addition, mechanisms of CO2 removal from dry and humid air are proposed.

D=

λ β cos θ

where D is the mean crystallite size (nm), λ is the wavelength of Cu Kα radiation (nm), θ is Bragg’s angle (deg), and β is the calibrated width of a diffraction peak at half-maximum intensity (rad). 2.2.3. MgO and CaO Contents. Contents of the oxides in the researched hybrid materials were determined thermogravimetrically using a precise thermobalance (STA 449 Netzsch system, Germany). For that purpose, a sample (ca. 10 mg) of a PET/DOL-based sorbent prepared according to the procedure described in section 2.1 was heated in an air atmosphere (30 N cm3/min) from room temperature to 950 °C at a heating rate of 10 °C/min. The residual mass registered at the end of each measurement was considered as an approximate summary content of both CaO and MgO. It should be said that obtained values are burdened with a certain content of impurities naturally included in the dolomite used for preparations. 2.2.4. Removal of CO2 from Air. Porous carbons loaded with CaO and MgO were tested as sorbents for CO2 contained in air. For that purpose high-purity synthetic air (N2, 79.5 vol %; O2, 20.5 vol %) contaminated with CO2 to 2000 ppm content was directed (50, 100, or 200 N cm3/min) onto a bed (300 mg) of a sorbent material at constant temperature, 20, 45, or 70 °C, and atmospheric pressure. In order to minimize the channeling effect and equalize CO2 concentration in the air contacting the sorbent, the bed was divided in three equal parts that were positioned in series in a testing glass tube. Details of the instrumental setup used along with superficial gas velocities and residence times determined for different flow rates used were described in our previous work.27 A concentration of 2000 ppm CO2 was chosen for experiments because the studied materials were thought to be suitable for purification of indoor air, and levels of CO2 normally measured in buildings are from 400 to 2000 ppm. The concentration of carbon dioxide in the

2. EXPERIMENTAL SECTION 2.1. Materials and Preparations. Hybrid sorbent materials examined in this work were prepared from dolomite mineral (hereinafter DOL) and poly(ethylene terephthalate) (PET). The first served as a source of MgO and CaO, and the latter was a carbon precursor. The total share of main natural impurities contained in the dolomite with reference to the oxides, SiO2, Fe2O3, Al2O3, was ca. 2 wt %. Both raw materials were mixed in three PET/DOL weight ratios (83:17, 50:50, and 30:70) and pyrolyzed under Ar gas flow at either 850 or 900 °C or 1000 °C. Thus, carbon materials loaded with MgO and CaO could be easily obtained. Details of the preparation procedure and some properties of the raw materials have been reported in our earlier work,26 concerning thermogravimetric studies on effects accompanying regeneration of the hybrid sorbent saturated with carbon dioxide. In addition, porous carbon free from CaO and MgO was obtained from an exemplary product. For that purpose a small part of the hybrid material was washed with an excessive amount of 3.2 M aqueous HCl to convert both oxides to watersoluble chlorides. After overnight agitation the remaining solids were rinsed with distilled water to constant pH, filtered out, and finally dried for 24 h at 120 °C in air. In order to verify whether the oxides could be removed from the hybrid material or not, the solid obtained was subjected to thermogravimetric (TG) analysis in air up to 950 °C. The negligible residual mass detected at the end of the TG measurement confirmed a high degree of separation of the oxides from the carbonaceous material subjected to the acid washing described above. 6670

dx.doi.org/10.1021/ie302848r | Ind. Eng. Chem. Res. 2013, 52, 6669−6677


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− Mean sizes of MgO and CaO crystallites determined for DOL-based material are considerably larger than those found for PET/DOL-based samples. − Washing out inorganics from the hybrid materials results in obtaining highly porous carbon material rich in micropores and mesopores. 3.2. Uptake of CO2 by the Prepared Materials: Effect of MgO and CaO Contents. Breakthrough curves measured for the materials prepared from different PET/DOL mixtures and for reference materials, together with calculated adsorption capacities toward CO2 gas, are shown in Figure 1.

outlet air leaving the testing tube loaded with the sorbent bed was continuously monitored with a mass spectrometer (QGA Gas Analysis system, Hiden Analytical Ltd., England), serving as CO2 detector (m/z = 44 signal). Capability of the materials to remove CO2 from the air was estimated from obtained breakthrough curves, i.e., from c/c0 plotted vs time. Here, c0 is the initial concentration of the gas and c is the CO2 concentration measured in the effluent gas leaving the testing tube. The breakthrough point was assumed to be at c/c0 = 0.05. Consequently, the bed was considered as saturated as c/c0 reached 0.95. Amounts of CO2 captured by the tested beds were calculated through integration of the areas above the breakthrough curves, starting from the beginning of a measurement (c/c0 = 0) until the saturation point. The response time of the system used for breakthrough measurements, 25 s, was taken into account during the calculations. The influence of moisture on CO2 uptake was examined by carrying out extra measurements with the use of prehumidified beds of a sorbent. In order to obtain the necessary humid samples, 300 mg portions of fresh sorbent materials were contacted with flowing nitrogen saturated with water. This operation continued from 15 to 90 min. The outcome of this humidification procedure was a regular increase in the mass of a sorbent, ranging from ca. 16 to ca. 30 mg. 2.2.5. Cyclic Sorption/Regeneration Studies. An exemplary sorbent material prepared was subjected to cyclic CO 2 adsorption/regeneration measurements using a precise thermobalance. TG measurements were carried out with ca. 10 mg samples. The adsorption step was carried out at 30 °C in a flowing equimolar CO2/He mixture (60 N cm3/min). After mass indication became stable, CO2 gas in the TG system was exchanged with nitrogen (30 N cm3/min) and the TG furnace was heated (10 °C/min) to 850 °C. After 60 min exposure to the final temperature, the system was cooled to 30 °C and nitrogen flow was changed back to CO2. The above adsorption/desorption procedure was repeated. For comparison, analogous cyclic experiments were carried out using a mixture of CaO and MgO prepared through thermal decomposition of dolomite at 850 °C.

Figure 1. Removal of CO2 (2000 ppm) from dry air (20 °C, 100 N cm3/min) by hybrid materials prepared from PET/DOL mixtures and reference samples.

Results shown in Figure 1 reliably confirm that the performance of all hybrid materials exceeds the effects revealed by the reference materials, including MgO/CaO mixture prepared from the dolomite, the char produced from the PET, acid-washed PET/DOL 50:50, and the commercially available activated carbon impregnated with KI and KOH. Taking into account the textural parameters (Table S1, Supporting Information) determined for the char and CaO/ MgO mixture, one may conclude that the rather low CO2 uptakes revealed by them are due to generally very low specific surface areas and pore volumes. However, in spite of relatively high pore structure parameters, low CO2 uptakes reveal also the commercial reference activated carbon and in particular the PET/DOL 50:50 after acid washing. Moreover, despite the extremely low micropore area and volume, uptake of CO2 by the DOL-based material is almost two times of that determined for the more microporous char produced from PET. In view of the fact that adsorption of CO2 on activated carbon is known to be dependent predominantly on microporosity,28 the superior performance of the reference MgO/CaO mixture must be due to some other effects. Because the metal oxides are of basic character and CO2 is an acid anhydride, chemical reaction between the latter and the metal oxides seems to proceed readily. Though in spite of many trials and even prolonged exposure of the CaO/MgO mixture to CO2, no newly formed phase could be detected in the material using XRD measurements. Such results were considered to be caused by either a slow rate of the carbonation reaction or chemisorption of CO2 on the studied materials without formation of new phases. Sorption of CO2 on the surface of MgO and CaO was studied

3. RESULTS AND DISCUSSION 3.1. Properties of the Obtained Materials. Textural properties, chemical composition, and mean sizes of MgO and CaO crystallites determined for the hybrid materials are listed in Table S1, Supporting Information. For reference, appropriate data were collected also for the char formed from PET, the solid product remaining after pyrolysis of the dolomite, acidwashed PET/DOL 50:50 850 °C material, and commercially available activated carbon impregnated with KOH and KI. While studying the data concerning materials prepared in this study, Table S1, Supporting Information, the following regularities could be found: − Micropore areas and volumes determined for materials prepared from PET/DOL mixtures tend to lower along with MgO and CaO loadings. − Pore parameters revealed by PET- and DOL-based materials are substantially lower compared to samples prepared from PET/DOL mixtures. − Specific surface area values tend to decrease along with preparation temperature; however, the influence of this factor is not as substantial as of the PET/DOL ratio used for preparations. 6671



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process is directly connected with the concentration of the adsorbate on the adsorbent’s surface, contact between the adsorbed CO2 and the metal oxides being dispersed over the carbon matrix must be facilitated. Thus, chemical interactions between the reagents, i.e., CO2 and the metal oxides, are more probable. Interestingly, the performance of the PET/DOL 83:17 material is comparable to that revealed by PET/DOL 50:50. The reason for this must originate from both relative textural properties and contents of CaO and MgO in these materials. In view of data listed in Table S1, Supporting Information, less microporous PET/DOL 50:50 comprises more MgO and CaO. On the other hand, more microporous PET/DOL 83:17 contains lower amounts of these oxides. As a result, the summary effect on amounts of CO2 retained by these materials due to chemisorption and physisorption may be comparable. The mean sizes of MgO and CaO crystallites contained in the solid remaining after thermal decomposition of the dolomite are much larger compared to those determined in PET/DOL-based samples, Table S1, Supporting Information. Since the larger species reveal lower specific surface area, this factor is considered as unfavorable for area of contact between the CO2 and the oxides. This must be another reason why the mixture of MgO and CaO does not capture the gas as efficiently as the oxides contained in the hybrid materials. 3.3. Effect of Preparation Temperature on CO 2 Removal. As reported elsewhere,26 thermal decomposition of the dolomite contained in a mixture with PET material is complete at ca. 800 °C. However, there is no obstacle to carry out preparations applying temperatures above the safe value, guaranteeing completeness of the decomposition process. Nevertheless, an increase in the preparation process may entail changes in the structure of a product and thus may affect the capability of the hybrid sorbent to remove CO2 from air. In order to examine whether the preparation temperature influences uptake of the gas or not, two additional preparations were carried at 900 and 1000 °C, both using the PET/DOL (50:50) mixture as starting material. For comparative reasons, obtained hybrid products were also subjected to CO2 capture tests, and results attained during breakthrough experiments are presented in Figure 2.

in detail by other teams, but no unequivocal conclusions were reported. IR studies on CO2 sorption on the oxides at room temperature confirmed29,30 chemisorption of the gas with formation of mono- and bidentate carbonates on MgO and monodentate carbonate on CaO. Possible chemisorption of CO2 to a form of a carbonate was confirmed by Krischok et al.31 According to others,32 exposure of CaO to CO2 at room temperature results in very fast carbonate production. However, after carbonate monolayer has been formed, the carbonation reaction stops. In other studies33 formation of carbonate layers next to monodendate and bidendate carbonates was reported as possible. In any case, no XRD pattern reliably confirming formation of either MgCO3 or CaCO3 in dry conditions was reported. The occurrence of chemical interaction between CO2 and the oxides in our materials seems to be supported by the characteristic shapes of the breakthrough curves, diverging from the classical S-type shape. The shape of the curve depends on a number of factors, and the occurrence of chemisorption may be a reason for the sharp rise in outlet concentration of an adsorbate immediately after breakthrough,34 followed by less radical changes. Because chemisorption has been confirmed as playing an important role during capturing CO2 by the MgO/ CaO-loaded carbons, it may be stated that before the breakthrough point certain portions of unreacted oxides are available for interaction with CO2. Hence, the presence of the unreacted MgO and CaO is favorable for capture of the gas, and therefore, complete CO2 removal can be achieved only during the first moments of the tests. After the mass transfer zone reaches the last portions of the bed no more fresh CaO and/or MgO is available for chemical interaction with the adsorbate. As a consequence, portions of unmaintained CO2 are detected at the outlet. The interaction of CO2 with the oxides illustrates the so-called shrinking core model.35,36 Here products of the reaction are assumed to form a shell on the surface of CaO and MgO crystallites, and thus, contact between the reagents becomes more and more constrained. As an effect, the breakthrough curve becomes gentler and gentler as the total amount of CO2 captured by the bed increases. A comparable sharp rise followed by less drastic changes can be also considered as resulting from physisorption. Such effects were reported for certain adsorbent−organic adsorbate systems of low mass transfer resistance and high heat of adsorption.37,38 However, in view of (a) numerous reports on chemical interaction between CO2 and CaO and MgO and (b) the clear effect of metal oxides loadings on CO2 uptake, chemisorption was assumed to play a key role in our breakthrough experiments. CO2 retention by the hybrid samples increases along with loadings of CaO and MgO, Figure 1. Such a tendency is observed in spite of the reverse order in the textural parameters, especially in micropore areas and volumes. This finding supports the statement that the basic oxides contained in the carbon material play a crucial role during CO2 capture. Nevertheless, because the performance of MgO/CaO mixture prepared through thermal decomposition of the sole dolomite is, compared to the hybrid sorbents, relatively poor, there must be other phenomena taking place during the removal process, boosting the resultant effect to the superior level. Such an advantageous outcome must be caused by a synergism resulting from physisorption of CO2 in micropores and chemical interaction between the chemicals (MgO and CaO) contained in the sorbent material and the gas. Since the adsorption

Figure 2. Removal of CO2 (2000 ppm) from dry air (20 °C, 100 N cm3/min) by hybrid materials prepared from the PET/DOL (50:50) mixture at different temperatures. 6672



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and observed for numerous systems.39,40 Water plays the role of a solvent for both CO2 gas and loaded chemicals, and thus, some of the adsorbate is maintained on the wet surface. Hence, a concentration of CO2 on the surface of the sorbent material must occur. In consequence, the possibility of reaction between the absorbed CO2 and the reactive species, just like CaO or MgO contained in the researched materials, must increase. In this way the intensified uptake of CO2 in the presence of water may be explained easily. Interestingly, the PET/DOL-based sorbent exposed to humid air containing CO2 revealed the presence of newly formed CaCO3, Figure 4 c. Nevertheless, the

Even though temperatures used for preparations were not much different from those normally used by us 850 °C, some weak trends in the properties and performance of the prepared materials could be noticed. As shown in Table S1, Supporting Information, the increase in preparation temperature entails noticeable lowering in SBET and Stotal areas. Because preparation temperature did not affect mesopore areas, lowered BET and total areas were assumed to be predominantly due to the decrease in the micropore areas. Shifting the preparation temperature to higher values entailed also minor lowering in the micropore volume. Breakthrough curves presented in Figure 2 are of similar shape, and this confirms the occurrence of analogous mechanisms during capturing CO2 by the hybrid materials prepared at various temperatures. However, CO2 uptakes revealed by the sample prepared at the lowest temperature, 850 °C, is ca. 11% higher compared to the capacity determined for materials prepared at 900 and 1000 °C. In view of the data listed in the Table S1, Supporting Information, indicating no straight relation between the preparation temperature and the mean sizes of CaO and MgO crystallites, such effect seems to be due to the less favorable for CO2 sorption textural parameters revealed by the samples prepared at high temperatures. As noticed above, specific surface areas determined for PET/DOL (50:50) materials tend to decrease along with preparation temperature. Hence, the area of contact between CO2 being captured from air and the sorbent materials and in consequence the possibility of chemical interactions between the gas and the active species included in the hybrid sorbents must follow the same trend. For these reasons, the amount of CO2 captured by the PET/DOL 50:50 prepared at 850 °C is higher compared to uptakes determined for materials prepared at higher temperatures. In view of above explanations it may be stated that there is no merit to carry out preparations at temperatures exceeding 850 °C. 3.4. Uptake of CO2 by Prepared Materials: Effect of Humidity. Performance of the PET/DOL-based materials strongly depends on the humidity, Figure 3. Thus, while in dry conditions uptake of gas is below 10 mg/ g, the amount of CO2 captured increases progressively along with the amount of water deposited in the material prior to breakthrough tests. Such effect is usually explained by formation of a water film on the surface of the sorbent material

Figure 4. XRD patterns of the MgO/CaO-loaded carbon material prepared from PET/DOL (50:50) mixture, measured for (a) virgin material, (b) dry material exposed to air containing CO2 gas, (c) prehumidified material exposed to air containing CO2.

carbonate was confirmed to coexist with the original oxides, MgO and CaO. In contrast, no formation of carbonate phases was confirmed by XRD analyses measured for the sorbent exposed to CO2 in the absence of humidity, Figure 4b. Hence, the presence of water makes the mechanism of CO2 capture different compared to that occurring in dry conditions. Transformation of CaO to CaCO3 in the presence of humidity was reported by others,41,42 and enhanced uptake of CO2 by CaO in humid conditions was explained as an outcome of facilitated reaction between CO32− and Ca2+ ions, both present in the water adsorbed on the surface of a sorbent material. Formation of hydroxides, Mg(OH)2 and Ca(OH)2, confirmed by appropriate peaks on the diffraction pattern shown in Figure 4, must be due to hydration of the hydrophilic metal anhydrides, MgO and CaO. Because no carbonate was detected in the reference material exposed to the CO2 in dry conditions, the CaCO3 newly formed in humid conditions may also be a product of Ca(OH)2 carbonation. Interestingly, no new phases authorizing chemical fixation of CO2 by MgO or Mg(OH)2 were detected. The lack of corresponding magnesium carbonate in the material exposed to CO2 captured in humid conditions must be due to the relatively poor, compared to CaO or Ca(OH)2, capacity of MgO or Mg(OH)2 to fix CO2.43 Nevertheless, because CO2 uptake by carbon materials loaded with MgO only also considerably increases along with humidity content (data not shown), the contribution of magnesium oxide or hydroxide contained in the hybrid material to the total amount of CO2 captured should not be omitted. 3.5. Effect of Bed Temperature on CO2 Uptake. Apparently, amounts of CO2 removed from air by the PET/

Figure 3. Removal of CO2 (2000 ppm) from air (20 °C, 100 N cm3/ min) by prehumidified hybrid materials prepared from an exemplary PET/DOL (30:70) mixture. 6673



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DOL-based sorbent materials increase along with the temperature of the adsorbent bed, Figure 5.

Figure 6. Effect of flow rate on removal of CO2 (2000 ppm) from dry air (20 °C) by the CaO/MgO-loaded sorbent material.

sorbent material, only ca. 19% of the total amount of CO2 is captured until the breakthrough point. Hence, a great majority of the gas is removed between the breakthrough and the saturation points. In contrast, outcomes of using the low flow mode, 50 N cm3/min, are equal removals before and after the breakthrough point. Taking into account the above observations it may be stated that low flow rates are recommended for complete purification of larger volumes of air. Nevertheless, because the total CO2 uptake does not considerably depend on the flow rate, comparable utilization of the bed can be achieved using the different flow rates. In view of considerations related to the shrinking core model (see section 3.2), lengthy contacting of CO2 with MgO and CaO must increase the probability of passing the shell by the gas. In effect larger amounts of CO2 are maintained by the bed. Following this explanation it is easy to conclude that the sorbent materials studied in this work should reveal potential to remove CO2 from air in static conditions. 3.7. Cyclic CO2 Sorption/Regeneration Studies. The susceptibility of the hybrid sorbent material for repeated CO2 removal/regeneration was examined by adequate cyclic sorption/desorption measurements using a thermobalance. Obtained results are illustrated by Figure 7. Considering the results obtained for both the hybrid sorbent and the CaO/MgO mixture, one can easily notice that the two materials perform very differently. CO2 uptake revealed by the hybrid sample significantly exceeds the effect demonstrated by the mixture of CaO and MgO and decreases by ca. 12% after each regeneration. In contrast, the corresponding decrease in the amount of CO2 retained by the CaO/MgO mixture is considerable. Besides, while the mass of the CaO/MgO mixture registered after each regeneration remains at practically an unchanged level, the mass of the hybrid sorbent available for next use for CO2 sorption obviously decreases. The significant gradual decrease in performance of the CaO/MgO mixture must be due to the well-known46 sintering effect that may entail a significant decrease in the surface area of the sorbent material. Such a consequence is generally considered as unfavorable for CO2 capture. On the other hand, a much less extensive drop in CO2 uptake as revealed by the hybrid material after each cycle may indicate a hindered sintering effect. This must be due to the effective separation of the CaO and MgO grains dispersed over the char being a structural component of the hybrid material. As seen in the data listed in Table S3, Supporting Information, the mean sizes of CaO and MgO crystallites

Figure 5. Removal of CO2 (2000 ppm) from dry air (100 N cm3/min) by hybrid material prepared from PET/DOL (30:70) mixture: influence of bed temperature.

This effect is opposite to that generally observed for CO2 adsorption on activated carbons free from additives.44 However, one has to remember that materials studied in this work employ metal oxides that are reactive toward the adsorbate gas, and the rate of reaction between CO2 and both oxides depends on temperature. It is generally accepted that CaO and MgO conversion degree increases along with carbonation temperature until the opposing reaction, i.e., decomposition processes, starts to proceed efficiently.41,45 Hence, even though hybrid materials carrying MgO and CaO were examined, the results collected at different working temperatures reflect the tendencies known for pure oxides. The noticeable rise in CO2 uptake along with temperature (Figure 5) confirms that the increase in reaction rate is the factor that dominates over the opposite effect, i.e., decreasing amounts of CO2 captured by the sorbent material owing to physisorption. 3.6. Influence of Flow Rate on CO2 Removal from Air. Results of breakthrough experiments carried out for porous carbon loaded with CaO and MgO, exposed to air containing 2000 ppm CO2 at different flow rates, are presented in Figure 6. Complementary data determined on the basis of the attained curves are shown in Table S2, Supporting Information. Even the breakthrough curves determined for the higher flow rates are steeper compared to those attained using lower flow rates, total amounts of CO2 captured from air passed through the bed are comparable and range from 9.5 to nearly 13 mg/g. Obviously, owing to the longest time of contact between the contaminated air and the sorbent material, the breakthrough time determined for the lowest flow rate is considerably longer than these observed for higher flows. The above observations and the data listed in Table S2, Supporting Information, are important from a practical point of view. Hence, for the used experimental setup, high flow rates are not recommended to be used for prolonged complete purification of larger volumes of air from the gas. In contrast, the low flow rate of 50 N cm3/min is suitable to completely eliminate CO2 from the air for ca. 10 min. This corresponds to ca. 500 N cm3 of purified air, which is ca. 3.0−3.5 times larger volume compared to the volumes achieved for higher flows. When either 100 or 200 N cm3/min of contaminated air is directed onto the bed of the hybrid 6674



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Figure 7. Results of cyclic CO2 sorption/regeneration for (a) PET/DOL 50:50 850 °C and (b) mixture of MgO and CaO prepared from dolomite.

the gas are used. Even so, as demonstrated, other materials of higher performance can be easily found. Systematic studies allowed us to explain some phenomena accompanying CO2 uptake and point to several conclusions to be taken into account in practical use. First, it is important to ensure optimal humidity of the air subjected to cleaning from CO2 gas. For that reason, if necessary, additional humidification carried out prior to directing a gaseous stream onto the sorbent’s bed is recommended. It is noteworthy that CO2 removal from air in humid conditions follows a different scheme compared to the process carried out in dry conditions. Removal of carbon dioxide with the use of the hybrid sorbent is a temperature-dependent process. Even though the examined performance of the bed is within a rather narrow temperature range (20−70 °C), the observed tendency allows to state that further increases in bed temperature may be followed by even higher performance. However, this needs further experimental work. Considering the impact of preparation conditions on the effect revealed by the sorbent material, it appears that using preparation temperatures above 850 °C is not recommended. This is due to unfavorable changes in the textural parameters of the materials reflected by lowered total CO2 uptake. Because the performance of the hybrid materials increases markedly along with MgO and CaO contents and dolomite is a readily available mineral, high loadings of these oxides are recommended to be used to achieve a high effect. It should be mentioned that compared to the mixture of CaO and MgO, the hybrid sorbent studied in this work reveals some advantages. In addition to much higher CO2 uptakes, sorption capacity toward the gas can be kept at a reasonable level over at least three sorption/desorption cycles. In contrast, the uptake of CO2 by a mixture of CaO and MgO obtained from dolomite is far lower and radically decreases with each regeneration. It is noteworthy that all components of the hybrid sorbent material researched in this work are nonvolatile. This can be considered as an advantage in specific applications where even traces of volatile contaminations are undesirable in purified gaseous media. Even though the results presented and discussed in this paper concern a particular case, which is removal of CO2 from air, the spectrum of potential applications of the studied materials seems to be wider. In view of literature reports and our other

incorporated in the hybrid material do not change significantly during cyclic use. However, already the as-prepared MgO/CaO mixture obtained from dolomite has a content of much larger crystallites. Moreover, both oxides tend to form larger species with the number of sorption/regeneration cycles. The above statement particularly concerns calcium oxide. The effect of the carbonaceous component of the hybrid sorbent on the mean size of CaO and MgO crystallites was also clearly demonstrated in our previous work.26 Similar findings were reported for other systems containing carbon.47 Apart from playing a role as a specific barrier diminishing the sintering effect, the presence of carbonaceous component considerably influences textural parameters of the hybrid sorbent. Thus, in spite of lower loadings of the basic oxides, uptake of CO2 by PET/DOL-based materials is superior compared to MgO and CaO obtained through thermal decomposition of dolomite. As reported in our previous work,26 this effect results from a synergism arising from some content of metal oxides and sorption on porous surface, both favorable for CO2 uptake. In the same work we explained that regeneration of the sorbent material after saturation with CO2 proceeds in two stages. Below 200 °C weakly bound CO2 is freed from the sorbent material, and chemically bound CO2 needs much more drastic temperature conditions, 850 °C, to be removed. The ca. 2 wt % decrease in the mass of the hybrid sorbent accompanying the regeneration step, Figure 7a, can be considered as owing to partial gasification of char by CO2 being released during treatment at high temperature. Such an effect was studied in detail in our previous works dealing with sorption of CO2 on porous carbon materials loaded with metal oxide.26,48

4. CONCLUSIONS The results presented in this study clearly confirmed the capability of porous carbon materials loaded with MgO and CaO to capture CO2 gas from air already at room temperature. Uptake of CO2 by these materials can be considered as comparable to that of zeolite 13X49 used at analogous conditions, i.e., at low CO2 concentrations and similar temperatures. Thermogravimetric experiments proved that uptake of CO2 could be even higher if high concentrations of 6675



Article

(12) Shafeeyan, M. S.; Daud, W. M. A. W.; Houshmand, A.; Shamiri, A. A Review on Surface Modification of Activated Carbon for Carbon Dioxide Adsorption. J. Anal. Appl. Pyrol. 2010, 89, 143. (13) Henning, K. D.; Schäfer, S. Impregnated Activated Carbon for Environmental Protection. Gas Sep. Purif. 1993, 7, 235. (14) Fortier, H.; Westreich, P.; Selig, S.; Zelenietz, C.; Dahn, J. R. Ammonia, Cyclohexane, Nitrogen and Water Adsorption Capacities of an Activated Carbon Impregnated with Increasing Amounts of ZnCl2, and Designed to Chemisorb Gaseous NH3 from an Air Stream. J. Colloid Interface Sci. 2008, 320, 423. (15) Belmabkhout, Y.; Serna-Guerrero, R.; Sayari, A. Adsorption of CO2 - Containing Gas Mixtures over Amine-Bearing Pore-Expanded MCM-41 Silica: Application for Gas Purification. Ind. Eng. Chem. Res. 2010, 49, 359. (16) Choi, S.; Drese, J. H.; Eisenberger, P. M.; Jones, C. W. Application of amine-tethered solid sorbents for direct CO2 capture from the ambient air. Environ. Sci. Technol. 2011, 45, 2420. (17) Choi, S.; Gray, M. L.; Jones, C. W. Amine-Tethered Solid Adsorbents Coupling High Adsorption Capacity and Regenerability for CO2 Capture from Ambient Air. ChemSusChem. 2011, 4, 628. (18) Choi, S.; Watanabe, T.; Bae, T. H.; Sholl, D. S.; Jones, C. W. Modification of the Mg/DOBDC MOF with amines to enhaance CO2 adsorption from Ultradilute Gases. J. Phys. Chem. Lett. 2012, 3, 1136. (19) McDonald, T. M.; Lee, W. R.; Mason, J. A.; Wiers, B. M.; Hong, C. S.; Long, J. R. Capture of Carbon dioxide from air and flu gas in the Alkylamine-Appended Metal-Organic Framework mmen-Mg2(dobpdc). J. Am. Chem. Soc. 2012, 134, 7056. (20) Przepiórski, J.; Abe, Y.; Yoshida, S.; Oya, A. Preferential Supporting of Potassium Carbonate Around the Peripheral Region of Activated Carbon Fiber. J. Mater. Sci. Lett. 1997, 16, 1312. (21) Wu, Z.; Hao, N.; Xiao, G.; Liu, L.; Webley, P.; Zhao, D. OnePot Generation of Mesoporous Carbon Supported Nanocrystalline Calcium Oxides Capable of Efficient CO2 Capture Over a Wide Range of Temperatures. Phys. Chem. Chem. Phys. 2011, 13, 2495. (22) Inagaki, M.; Kojin, F.; Tryba, B.; Toyoda, M. Carbon-Coated Anatase: the Role of the Carbon Layer for Photocatalytic Performance. Carbon 2005, 43, 1652. (23) Smith, J. W. H.; Westreich, P.; Abdellatif, H.; Filbee-Dexter, P.; Smith, A. J.; Wood, T. E.; Croll, L. M.; Reynolds, J. H.; Dahn, J. R. The Investigation of Copper-Based Impregnated Activated Carbons Prepared from Water-Soluble Materials for Broad Spectrum Respirator Applications. J. Hazard. Mater. 2010, 180, 419. (24) Cheng, H. H.; Tan, C. S. Removal of CO2 from Indoor Air by Alkanolamine in a Rotating Packed Bed. Sep. Purif. Technol. 2011, 82, 156. (25) Muller, C. Comparison of Chemical Filters for the Control of Airborne Molecular Contamination. J. IEST 2007, 50, 52. (26) Przepiórski, J.; Czyżewski, A.; Pietrzak, R.; Tryba, B. MgO/ CaO-Loaded Porous Carbons for Carbon Dioxide Capture. Effects Accompanying Regeneration Process. J. Therm. Anal. Calorim. 2013, 111, 357. (27) Przepiórski, J.; Czyżewski, A.; Kapica, J.; Moszyński, D.; Grzmil, B.; Tryba, B.; Mozia, S.; Morawski, A. W. Low Temperature Removal of SO2 Traces from Air by MgO-Loaded Porous Carbons. Chem. Eng. J. 2012, 191, 147. (28) Hsiao, H. Y.; Huang, C. M.; Hsu, M. Y.; Chen, H. Preparation of High-Surface-Area PAN-Based Activated Carbon by Solution-Blowing Process for CO2 Adsorption. Sep. Purif. Technol. 2011, 82, 19. (29) Fukuda, Y.; Tanabe, K. Infrared Study of Carbon Dioxide Adsorbed on Magnesium and Calcium Oxides. Bull. Chem. Soc. Jpn. 1973, 46, 1616. (30) Yanagisawa, Y.; Takaoka, K.; Yamabe, S.; Ito, T. Interaction of CO2 with Magnesium Oxide Surfaces: a TPD, FTIR, and ClusterModel Calculation Study. J. Phys. Chem. 1995, 99, 3704. (31) Krischok, S.; Höfft, O.; Kempter, V. Adsorption on Metal Oxides: Studies with the Metastable Impact Electron Spectroscopy. Nucl. Instrum. Methods Phys. Res., Sect. B 2002, 193, 466.

studies, porous carbon materials loaded with CaO and MgO, obtained from mixtures of PET and dolomite, should be suitable for removal of other contaminants susceptible to chemical interaction with the oxides. One should remember that both MgO and CaO are basic oxides that can be easily washed out from the hybrid materials, and thus, new highly porous carbons rich in micropores and mesopores can be obtained. This valuable feature widens the spectrum of possible applications of the researched materials.

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ASSOCIATED CONTENT

* Supporting Information S

Structural and textural properties of the materials, results attained during breakthrough experiments with CO2 gas, and data illustrating effect of regeneration process on mean size of CaO and MgO crystallites. Tables S1, S2, and S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +48 914494326. Fax: +48 914494686. E-mail: jacek. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Polish Ministry of Science and Higher Education, Grant No. N R050004 10.

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REFERENCES

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CaO-Loaded Activated Carbon for Carbon Dioxide Capture

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