Environ. Sci. Technol. 2002, 36, 1620-1624
FTIR Investigation of Adsorption and Chemical Decomposition of CCl4 by High Surface-Area Aluminum Oxide ABBAS KHALEEL† AND BARRY DELLINGER* Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803
Chlorinated hydrocarbons are among the most recalcitrant pollutants for control by sorption or catalytic destruction. High surface-area alumina holds promise as a catalytic media as well as a component of other binary catalyst systems. We have prepared an alumina catalyst using the aerogel technique that has a very high surface area of 550 m2/g. This catalyst destroys carbon tetrachloride with an efficiency >99% at 400 °C. Its reactivity toward carbon tetrachloride is remarkably higher than that of commercial alumina, which has a surface area of 155 m2/g. Carbon dioxide is the major product. Minor products include hydrogen chloride and tetrachloroethylene along with traces of phosgene. Some of the carbon tetrachloride reacts with the alumina to form aluminum chloride, which vaporizes to reveal a fresh catalytic surface. A mechanism for adsorption and destruction has been developed that involves chemisorption followed by surface to adsorbate oxygen transfer and adsorbate to surface chlorine transfer.
Introduction Adsorption on solid surfaces and heterogeneous catalysis have been among the promising methods for removing hazardous and environmentally undesirable chemicals such as chlorinated hydrocarbons (CHCs) (1-8). Although adsorption can occur on a variety of surfaces, only few materials are known to possess adsorptive efficiencies or reactive surfaces sufficiently favorable for adsorbing chlorinated organic compounds. These include activated carbon, zeolites, and some metal oxide systems (9-18). Currently, the most widely used material as an adsorbent for environmental cleaning is high surface-area activated carbon (7, 8). Activated carbon presents some problems including the fact that the adsorbate molecules are very often not destroyed or decomposed (through irreversible dissociative chemisorption) but instead are only weakly held at the surface. There is a growing interest in high surface-area materials (nanoscale materials), especially metal oxides, and in their unique applications including chemical catalysis (19-22). The uniqueness of these materials stems largely from two properties: (i) large exposed surface areas for interactions and (ii) enhanced surface reactivities due to their richness in reactive, coordinatively unsaturated sites, usually edges, corners, and kinks. Herein, we report the results of our recent study on the adsorption and chemical degradation of carbon tetrachloride, * Corresponding author telephone: (225)578-6759; fax: (225)5783458; e-mail:
[email protected]. † Present address: P.O. Box 17551, Department of Chemistry, UAEU, Al-Ain, UAE. 1620
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as a representative of other chlorinated hydrocarbons, by high surface-area alumina. The presence of many different types of hydroxyl groups on its surface in many ways emulates the well-documented role of hydroxyl sites on several, complex metal oxide systems (23-27). Although an effective catalyst in its on right, the study of alumina serves as a basis for the development of binary oxide catalytic systems.
Experimental Section Reagents. Carbon tetrachloride (99.9+ pure) and commercial high surface-area alumina (C-Al2O3) were obtained from Aldrich and used as received. Sol-gel-prepared alumina (SGP-Al2O3) was prepared through the sol-gel method using aluminum tri-sec-butoxide. All chemicals used in the preparation were purchased from Aldrich and used as received. In a typical experiment, 9.0 mL of aluminum tri-sec-butoxide was dissolved in 300 mL of sec-butanol. While being stirred, the alkoxide solution was hydrolyzed by dropwise addition of 1.9 mL of water. The gel formed upon hydrolysis, and complete gelation was achieved by stirring for 12 h. Supercritical drying of the gel in an autoclave at 270 °C resulted in a high surface-area white powder of aluminum hydroxide. The aerogel hydroxide powder was heated at 500 °C under vacuum, leading to an ultrafine alumina powder. The heat treatment was performed slowly where the sample was heated first to 350 °C (where the dehydration was observed to be most extensive) at a rate of 6 deg/min and held at that temperature for 1 h. It was then heated at the same rate to 500 °C and held at that temperature for an additional 4 h to ensure a full conversion to oxide. The C-Al2O3 was subjected to identical heating to 500 °C prior to use. This procedure resulted in an oxide with a surface area of 550 m2/g for the SGP-Al2O3 versus 155 m2/g for the high surface-area commercial alumina, C-Al2O3. Characterization of Materials. Materials were characterized by FTIR spectroscopy, X-ray diffraction, and BET surface area analysis. FTIR spectra were recorded on a Perkin-Elmer FTIR spectrometer (1760X). Solid samples were prepared as KBr pellets (5% by mass of sample). X-ray diffraction analyses were obtained using a Siemens D5000 diffractometer with Cu KR radiation. Phase identification was performed by comparing the experimental spectra with the standard spectra using Powder Diffraction File database. BET surface areas were measured on a Quantochrome Autosorb-1 instrument using nitrogen gas adsorption. Studies of Destruction of CCl4. The adsorption and chemical degradation of carbon tetrachloride were performed with a fixed bed flow reactor system equipped with a vacuum line. In a typical experiment, a 1.06 × 10-3 mol (0.108 g of alumina) sample was placed in a U-shaped Pyrex tube reactor and heated under vacuum to the process temperature in about 30 min. This was achieved using a cylindrical heater placed around the reactor and monitored by a temperature controller. The evacuation was then stopped, and a carrier gas (He unless otherwise mentioned) was allowed to flow through the reactor at a rate of 5 mL/min. The desired amounts of CCl4 were injected through a septum inlet positioned at the beginning of the reactor so that the CCl4 in the flow stream evaporated immediately in the reactor. In all experiments, CCl4 was introduced in pulses of 2, 2, and 4 µL, followed by a succession of 8-µL pulses. The gaseous products, eluted by the carrier gas, were collected in a trap cooled by liquid nitrogen. The transfer line connecting the reactor and the trap was maintained at ∼150 °C using heating tape. The flow rate was adjusted to maintain a contact time between the carbon tetrachloride 10.1021/es010650i CCC: $22.00
2002 American Chemical Society Published on Web 03/01/2002
TABLE 1. Adsorption/Decomposition of CCl4 on SGP and Commercial Alumina sample
temp (°C)
carrier gas
wt beforea
wt aftera
wt AlCl3a
BTb (µL)
total removed (µL)c
% removedc
C-Al2O3 SGP-Al2O3
400 300 350 400
He He He He He + O2 He
0.108 0.108 0.108 0.108 0.108 0.108
0.104 0.105 0.090 0.052 0.060 0.030
0 0 0.050 0.109 0.08 0.125
2 2 8 56 56 4
60 43 46 77.5 79.2 78.7
70 54 57 97 99 98
500 a
The data reported in these three columns refer to the weight (in grams) of sample before adsorption and after adsorption and the weight of the aluminum chloride produced, respectively. These data are based on a total introduction of 120 µL of CCl4. b BT, break through, which refers to the amount of CCl4 completely removed before any undecomposed CCl4 was observed in the effluent. c The data in these two columns were calculated based on the first 80 µL of CCl4 introduced.
pulse and the solid of 8 s. After each injection, the products were collected for 15 min in the cold trap after which the flow was stopped, the reactor was closed, and the trap was evacuated, while at liquid nitrogen temperature, to a pressure of 2 × 10-2 Torr. The trap was then closed and warmed to room temperature where only vapor-phase products were observed. An 8-mL sample of the gaseous products was then collected, using a syringe equipped with a gastight lock, and injected into an IR cell, designed for gaseous samples, equipped with two KBr windows. FTIR spectrum was recorded for the products after each pulse of carbon tetrachloride. Quantitative measurements and comparisons were obtained by comparing the FTIR spectra in each experiment with those obtained from a background experiment that was performed under the same conditions without the presence of a solid sample.
Results Ultrafine powder of alumina having a surface of 550 m2/g was obtained using the sol-gel method as described above. The heat treatment of the aerogel powder hydroxide obtained from the autoclave drying, completely converted it into γ-Al2O3 as indicated from the X-ray diffraction analysis. The X-ray diffraction pattern showed very broad and weak peaks, indicating an amorphous structure of the SGP-Al2O3. In agreement with the literature, the FTIR spectrum showed the presence of a considerable concentration of surface hydroxyl groups, which are likely the a result of exposure of the sample to air during the analysis (23-27) and residual OH groups that stayed on the surface after heat treatment. Strong absorption peaks were also observed at 1445 cm-1, which are believed to be due to surface carbonate species as a result of CO2 adsorption from the atmosphere. The observed products from the decomposition of carbon tetrachloride over SGP-Al2O3 and C-Al2O3 included carbon dioxide, phosgene, hydrogen chloride, tetrachloroethylene, and aluminum chloride (identified by powder XRD). However, the two different alumina samples exhibited significant quantitative and qualitative differences in their behavior toward carbon tetrachloride. The temperature-dependent results for SGP-Al2O3 are compared to C-Al203 (at 400 °C) in Table 1 and in Figure 1a-e. We initially compare the result of SGP-Al2O3 and C-Al2O3 at 400 °C. In the case of SGP-Al2O3, carbon tetrachloride was completely removed in each injection until a total of 56 µL was injected. After that, a very small portion of each injection eluted undecomposed with the products. As shown in Figure 1, carbon dioxide was the major product for degradation over SGP-Al2O3 at all temperatures in the early injections. When the capacity of the sample started to decline and a portion of the carbon tetrachloride began to elute undestroyed, as additional carbon tetrachloride was injected, phosgene was observed with a concomitant reduction in carbon dioxide. Hydrogen chloride was also observed after about 24 µL of carbon tetrachloride was introduced and
started to decrease slowly. AlCl3 (0.109 g, 8.2 × 10-4 mol) formed and condensed at the cooler end of the reactor and was characterized by X-ray diffraction. The sample after the experiment was slightly darker in color (gray), which may indicate the formation of some carbon from the decomposition process. The sample weighed 0.060 g, indicating a weight loss of 44.4% as a result of conversion to AlCl3. In the case of C-Al2O3, a portion of each carbon tetrachloride injection remained undecomposed at 400 °C. The undecomposed increased as more carbon tetrachloride was introduced. The concentrations of carbon dioxide, phosgene, and hydrogen chloride were reduced because less carbon tetrachloride was reacted. There was no observable conversion of alumina to aluminum chloride and no weight loss. Some carbon was still observed as indicated by the color change of the sample from white to gray. The destruction of CCl4 on SGP-Al2O3 was studied at different temperatures (300, 350, 400, and 500 °C) to investigate the effect of the temperature on the efficiency as well as on the nature of the interaction (cf. Figure 2). The overall ability of the sample to adsorb and decompose carbon tetrachloride increased as the temperature was raised from 300 to 500 °C. At lower temperature (300 °C), phosgene, AlCl3, and carbon did not form (the sample after adsorption showed no weight loss or color change). These effects and products were not observed until temperatures g350 °C. At 500 °C, more carbon dioxide and much less phosgene formed as compared to that at 400 °C. On the other hand, traces of undecomposed CCl4 were observed with the products even during the initial injections, suggesting that a portion of the CCl4 removed at lower temperatures e400 °C was only molecularly adsorbed and consequently desorbed at higher temperature (500 °C). Traces of tetrachloroethylene were also observed at 500 °C. The adsorption/decomposition of carbon tetrachloride by SGP-Al2O3 in the presence of oxygen in the carrier gas was also studied at 400 °C (cf. Figure 3). Oxygen gas at a flow rate of 1.5 mL/min was allowed to mix with He of a flow rate of 3.5 mL/min as the carrier gas. The objective of these experiments was to determine if the presence of oxygen would facilitate conversion of the metal chloride back to the oxide and allow complete conversion of phosgene to carbon dioxide. In the presence of oxygen, the conversion of alumina to aluminum chloride was still similar to that in the case of He, but the amount of carbon tetrachloride being removed was increased slightly (see Table 1 and Figure 3) along with the expected increase in carbon dioxide. These observations lead us to believe the following: (i) SGP-Al2O3 has shown remarkable potential to adsorb and decompose CCl4 as compared to C-Al2O3. This can be, as one can expect, a result of higher surface area which, in turn, results in higher concentration of surface reactive sites, especially coordinatively unsaturated ones. (ii) The fact that relatively small quantities of gaseous effluents were observed for C-Al2O3 (lower surface area) VOL. 36, NO. 7, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. FTIR spectra of the gaseous products of carbon tetrachloride decomposition over SGP-Al2O3 at 300 (a), 350 (b), 400 (c), and 500 °C (d) and over C-Al2O3 at 400 °C (e). 1622
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SCHEME 1. Illustration of the Main Pathway for Adsorption and Decomposition of Carbon Tetrachloride by High Surface-Area Alumina
FIGURE 2. Temperature dependence of carbon tetrachloride decomposition over SGP-Al2O3.
indicates that the observed removal of carbon tetrachloride was mainly due to surface adsorption of CCl4 or other decomposition products. Also, since no AlCl3 formation was observed, the oxygen source for oxygen-containing products such as carbon dioxide and phosgene was likely surface hydroxyl groups. (iii) The formation of AlCl3 and more oxygen-containing products in the case of SGP-Al2O3 indicates that surface oxygen ions in SGP-Al2O3 are more weakly bound than in C-Al2O3 and consequently more reactive toward adsorbed CCl4. Removal of these surface oxygen ions exposed new oxygen sites for further adsorption and reaction. (iv) The fact that elution of gaseous products was low at low carbon tetrachloride injection volumes and increased with increasing carbon tetrachloride injection indicates that the removal process initially involved adsorption of carbon tetrachloride as well as readsorption of products and intermediates where they underwent decomposition. When the surface sites were completely occupied, adsorbed products were displaced and eluted by further carbon tetrachloride injection.
Discussion High surface-area oxide powders possess surface electronic structures that are difficult to relate to theoretically modeled surfaces or to bulk structures (22, 23, 28-30). High surfacearea powders possess vacant and other coordinatively unsaturated sites that can be oxygen or metal ions (29, 30). These complex and poorly understood surfaces usually lead to a variety of chemical interaction pathways. Since there was no oxygen in the reaction atmosphere of the system, it is evident that surface oxygens are the source of oxygen-containing products. The formation of AlCl3 with concurrent formation of oxygenated products, i.e., phosgene and carbon dioxide, also supports the reaction of carbon tetrachloride with surface oxides resulting in oxygen-chlorine exchange. Consequently, the surface area determined the degree of destruction. This interaction had to occur on the surface, exchanging the oxide ions on the surface with chloride ions. Sublimation of the resulting aluminum chloride regenerated the reactive surface by exposing a deeper layer of reactive sites. The fact that this did not take place in the case of the lower surface-area alumina indicates the presence of highly reactive oxygen ions on SGP-Al2O3 that did not exist on C-Al2O3. These sites are likely to be reactive, surface defects (i.e., coordinatively unsaturated sites such as those on corners and edges).
FIGURE 3. Effect of oxygen on the decomposition of carbon tetrachloride over SGP-Al2O3 at 400 °C. The interaction of CCl4 with the alumina surface leads to a dissociative adsorption of carbon tetrachloride molecules (see Scheme 1). This dissociation is likely heterolytic since oxide surfaces, due to their ionic nature, generally stabilize ionic or polar species and hence favor heterolytic dissociation of adsorbate molecules. A molecularly adsorbed species (I in Scheme 1) could be a reasonable intermediate where the relatively positive carbon center interacts with a negative oxygen site and the relatively negative chlorine atoms interact with positive metal ion sites. As shown in Scheme 1, species II can lead to the formation of phosgene as a result of oxygen abstraction from the surface (path a), which was found to occur at temperatures g350 °C. One might suggest that species II can possibly undergo further C-Cl dissociation to form more Al-Cl bonds and carbon monoxide (path b), which may readsorb and be converted to carbon dioxide after abstraction of another oxygen from the surface. Pathway b is not adopted as a likely mechanism as explained below. Phosgene formed in path a may readsorb and be oxidized to carbon dioxide as a result of abstraction of another oxygen ion from the surface. Extraction of oxygen ions from the surface is facilitated by two factors. First, they are coordinatively less saturated than bulk ions and hence are expected to be bonded to the lattice less strongly. Second, the chlorination of the metal centers weakens the Al-O bonds. Although we do not have direct experimental evidence to prove that phosgene was an intermediate in the formation of carbon dioxide, we lean toward this proposal for three reasons. First, the fact that its elution increased as the efficiency of the samples declined indicates that it was forming in considerable quantities but was adsorbed and decomposed. Second, phosgene has been reported in the literature to adsorb and decompose easily on metal oxide surfaces, forming carbon dioxide and the VOL. 36, NO. 7, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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corresponding metal chloride (15). Third, the orientation of species II does not favor the direct abstraction of an additional oxygen to form the linear carbon dioxide molecule. After abstraction of the first oxygen, the intermediate has to approach the surface from the other side of the carbon center to abstract the second oxygen. We do not believe that this intermediate is carbon monoxide because it was never observed as a product in significant yields, and carbon monoxide is known to adsorb only molecularly on metal oxides (1). This allows us to exclude pathway b shown in Scheme 1. The formation of hydrogen chloride during the process could be a result of a more minor involvement of surface hydroxyl groups, where the carbon center of carbon tetrachloride interacts with hydroxyl oxygens resulting in weaker O-H bonds. The fact that, in many cases, hydrogen chloride did not form early in the process (when the surfaces were fresh) may indicate that the chlorine in carbon tetrachloride initially reacts with a surface aluminum species. Addition of more carbon tetrachloride to the surface allows the release of relatively acidic hydrogen atoms to combine with chloride ions to form hydrogen chloride. As more HCl forms, the hydroxyl content declines and so does the formation of HCl. The formation of traces of tetrachloroethylene appears to be a result of combination of two intermediate dCCl2 species. This was not a favored pathway since tetrachloroethylene was observed only in small quantities at 500 °C. In the case of C-Al2O3, the considerable removal of carbon tetrachloride that took place without the formation of AlCl3 may indicate that only dissociative adsorption occurred involving oxygen or surface hydroxyl groups and surface oxygen ions that are bound relatively strongly to the lattice. The oxygen in the small amounts of phosgene and carbon dioxide that were observed could be from hydroxyl groups. In conclusion, it appears that it is possible to prepare an ultrafine powder of alumina with a remarkably reactive surface through sol-gel synthesis. This alumina adsorbed and decomposed large quantities of carbon tetrachloride through surface interactions that drove further interactions with the bulk of the particles as the AlCl3 forming on the surface sublimed. The probable mechanism was through dissociative adsorption and heterolytic decomposition of the carbon-chlorine bonds.
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Literature Cited
Received for review February 16, 2001. Revised manuscript received December 4, 2001. Accepted December 6, 2001.
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ES010650I
