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Adsorption/Oxidation of CH3SH on Activated Carbons Containing Nitrogen Svetlana Bashkova,†,‡ Andrey Bagreev,† and Teresa J. Bandosz*,†,‡ Department of Chemistry, The City College of New York, New York, New York 10031, and The Graduate School of The City University of New York, New York, New York 10031 Received January 2, 2003. In Final Form: March 27, 2003 Activated carbons of different origins were impregnated with urea and heat-treated at 450 and 950 °C. Surface properties of adsorbents were evaluated using nitrogen adsorption, potentiometric titration, Boehm titration, elemental analysis, and thermal analysis. Then, the CH3SH breakthrough capacity tests were carried out and the adsorption capacities were evaluated. The results showed that the amount adsorbed depends strongly on the modification conditions of the carbon surface. It increased from 1.4 to 10 times after introduction of nitrogen species. Immobilization of methyl mercaptan on the surface is likely governed by the redox process, and basic nitrogen groups can act as catalysts for electron transfer from sulfur to oxygen. Dimethyl disulfide and methyl methanethiosulfonate are the main products of methyl mercaptan oxidation.
Introduction Mercaptans (thiols) are sulfur-containing volatile organic species well-known for their disagreeable odors. Methyl mercaptan (CH3SH) is one of the natural sources of sulfur emitted into the atmosphere. It is a colorless gas with a smell like that of rotten or cooked cabbage. Mercaptans are found in crude petroleum, and methyl mercaptan is produced as a decay product of animal and vegetable matter. Methyl mercaptan is commonly used to “odorize” natural gas so that the gas can be detected if a leak in a gas line develops. It is done due to its low odor threshold (one part in 5 × 1010 parts of air1). Moreover, in high concentration it has significant toxicity. Methyl mercaptan is used in various applications, ranging from agricultural supplements to polymerization modifiers. To remove methyl mercaptan (MM) from air, activated carbons have been applied.2-8 Activated carbons are wellknown as adsorbents of gases and vapors.9 Their specific application depends on the properties of the molecules to be removed/adsorbed. It was reported that on the surface of the activated carbon MM is oxidized to dimethyl disulfide (DMDS), which is much more strongly adsorbed at room temperature than methyl mercaptan due to the larger size of the DMDS molecule and higher boiling point * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: (212) 650-6017. Fax: (212) 650-6107. † Department of Chemistry, The City College of New York. ‡ The Graduate School of The City University of New York. (1) Karchmer, J. H. The Analytical Chemistry of Sulfur and its Compounds; Wiley: New York, 1970; Vol. 1, p 466. (2) Katoh, H.; Kuniyoshi, I.; Hirai, M.; Shoda, M. Appl. Catal., B 1995, 6, 255. (3) Dalai, A. K.; Tollefson, E. L.; Yang, A.; Sasaoka, E. Ind. Eng. Chem. Res. 1997, 36, 4726. (4) Turk, A.; Sakalis, E.; Lessuck, J.; Karamitsos, H.; Rago, O. Environ. Sci. Technol. 1989, 23, 1242. (5) Shin, C. S.; Kim, K. H.; Yu, S. H.; Ryu, S. K. Presented at the 7th International Conference on Fundamental of Adsorption, Nagasaki, Japan, May 20-25, 2001. (6) Tanada, S.; Boki, K.; Matsumoto, K. Chem. Pharm. Bull. 1978, 26, 1527. (7) Tanada, S.; Boki, K. Chem. Pharm. Bull. 1978, 26, 3738. (8) Bashkova, S.; Bagreev, A.; Bandosz, T. J. Environ. Sci. Technol. 2002, 36, 2777. (9) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker: New York, 1988.
than those for MM.1 The removal of the adsorption products from the carbon depends on the features of the carbon surface and the chemical nature of the oxidation products and their adsorption energies.10 The surface chemistry of activated carbon is governed by the presence of heteroatoms such as oxygen, hydrogen, nitrogen, and phosphorus. Their origin is in the chemical nature of the organic precursor and the method of carbon preparation/ activation.9 They exist in the form of acidic, basic, or neutral organic functional groups.11-13 Sometimes the “natural” chemistry of the activated carbon surface is not efficient enough to enhance the specific adsorbate-adsorbent interactions or catalytic activity. In such cases, the chemical modification based mainly on impregnation processes is used. It was demonstrated that activated carbons impregnated with such chemicals as potassium iodide, potassium iodate, potassium carbonate, or ammonia work well as methyl mercaptan adsorbents.5 The sorption properties of activated carbons are strongly influenced by the presence of surface oxygen groups. They may, depending upon conditions, enhance the sorption of methyl mercaptan.14 This study focuses on the effect of nitrogen modification of the carbon surface on the adsorption of methyl mercaptan. It was shown before that activated carbon fibers and activated carbons modified with pyridine at elevated temperatures and having basic nitrogen functionalities show a significant increase in SO2 and H2S adsorption capacity compared to unmodified samples and they convert these gases to sulfuric acid.15,16 Activated carbons used in this study are modified with urea. The conditions of modification were chosen to impose changes in the chemistry of nitrogen within the carbon matrix.17 This (10) Lisovskii, A.; Semiat, R.; Aharoni, C. Carbon 1997, 35, 1639. (11) Puri, B. R. In Chemistry and Physics of Carbon; Walker, P. J., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6, p 191. (12) Boehm, H. P. Carbon 1994, 32, 759. (13) Boehm, H. P. In Advances in Catalysis; Academic Press: New York, 1966; Vol. 1, pp 179-274. (14) Bashkova, S.; Bagreev, A.; Bandosz, T. J. Ind. Eng. Chem. Res. 2002, 41, 4346. (15) Adib, F.; Bagreev, A.; Bandosz, T. J. Langmuir 2000, 16, 1980. (16) Bagreev, A.; Bashkova, S.; Bandosz, T. J. Langmuir 2002, 18, 1257.
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nitrogen incorporated into the carbon increases its basicity and plays a catalytic role in the adsorption-oxidation processes.15-20 The results are discussed in terms of adsorption capacity, oxidation products, and a specific role of the carbon surface in the enhancement of the amount of MM adsorbed. Experimental Section Materials. Four commercial activated carbons of various origins were used in this study. They are as follows: BAX (wood based, Westvaco), S208 (coconut shell, Waterlink Barnabey and Sutcliffe), BPL (bituminous coal, Calgon), and PCB (coconut shell, Norit). Before testing, the carbons were washed with water in a Soxhlet apparatus to a constant pH of a leachate. To introduce nitrogen groups, the carbons were treated with a saturated aqueous solution of urea (10 g of carbon was treated with 60 mL of saturated urea solution and left on the stirrer for 48 h) and then they were heated in nitrogen at the rate of 10 °C/min to 450 and 950 °C and maintained at these temperatures for 1 h. After modification, the samples were water-washed to remove any excess of urea decomposition products. The modified carbons are referred to as, for instance, BAXU-450 and BAXU-950 where U refers to urea modifications and 450 and 950 represent heat treatment temperatures. The exhausted samples after methyl mercaptan adsorption and purging with 80% humidified air are designated with an additional letter E. For the sake of general trend finding, the polymeric-based carbons SCN-1, SCN-3, SCN-4,16,19 and Centaur (urea modified, manufactured by Calgon)8,15 were used. SCN-1 was obtained by carbonization of vinylpyridine resin at 950 °C in an argon atmosphere. SCN-3 and SCN-4 are derived from SCN-1 using steam activation to the 20 and 50% burn off, respectively. The nitrogen content in these materials was estimated to be 4.3, 3.6, and 2.4% for SCN-1, SCN-3, and SCN-4. Correspondingly, the surface areas are 340, 917, and 973 m2/g.19 Methods. CH3SH Breakthrough Capacity. Dynamic tests were carried out at room temperature to evaluate the capacity of the sorbents for CH3SH removal under wet conditions. Adsorbent samples were ground (1-2 mm particle size), packed into a glass column (length, 370 mm; internal diameter, 9 mm; bed volume, 6 cm3), and prehumidified with moist air (relative humidity of 80% at 25 °C) for 1 h. The amount of water adsorbed was estimated from the increase in the sample weight. Moist air (relative humidity of 80% at 25 °C) containing 0.3% (3000 ppm) CH3SH was then passed through the column of adsorbent at 0.5 L/min. The breakthrough of CH3SH was monitored using a Micromax monitoring system (Lumidor) with an electrochemical sensor calibrated with MM. The test was stopped at the breakthrough concentration of 50 ppm. The adsorption capacities of each sorbent in terms of milligrams of CH3SH per gram of carbon were calculated by integration of the area above the breakthrough curves and from the CH3SH concentration in the inlet gas, flow rate, breakthrough time, and mass of sorbent. For each sample, the CH3SH test was repeated at least twice. The determined capacities agreed to within 4%. The amount of weakly adsorbed CH3SH was evaluated by purging the adsorbent column with air at 0.35 L/min immediately after the breakthrough experiment. The CH3SH concentration was monitored until it dropped to 1 ppm. The process took about up to 3 h depending on the type of adsorbent. pH of the Carbon Surface. Carbon powder (0.4 g) was placed in 20 mL of water and equilibrated during the night. Then the pH of the suspension was measured. For exhausted samples, an additional letter E is added (pHE). Boehm Titration. One gram of carbon sample was placed in 50 mL of the following 0.1 N solutions of sodium hydroxide and hydrochloric acid. The vials were sealed and shaken for 24 h, and then 5 mL of each filtrate was pipetted and the excess of (17) Strohr, B.; Boehm, H. P.; Schlogl, R. Carbon 1991, 26, 707. (18) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Carbon 1995, 33, 1641. (19) Bagreev, A.; Strelko, V.; Lahaye, J. International Conference on Carbon, Carbon 96, Newcastle, U.K., 1996; p 527. (20) Biniak, S.; Szymanski, G.; Siedlewski, J.; Swiatkowski, A. Carbon 1997, 35, 1799.
Bashkova et al. base or acid was titrated with HCl or NaOH. The numbers of all acidic sites (of various types) were calculated under the assumption that NaOH neutralizes carboxyl, phenolic, and lactonic groups.21 The number of surface basic sites was calculated from the amount of hydrochloric acid that reacted with the carbon. Sorption of Nitrogen. Nitrogen isotherms were measured using a ASAP 2010 (Micromeritics) at -196 °C. Before the experiment, the samples were heated at 120 °C and then outgassed overnight at this temperature under a vacuum of 10-5 Torr to a constant pressure. The isotherms were used to calculate the specific surface areas (S), micropore volumes (Vmic), volume of pores smaller than 50 Å, total pore volumes (Vt), average micropore sizes (Lmic), characteristic energy of adsorption, (E0),22 and pore size distributions (density functional theory (DFT)).23,24 Thermal Analysis. Thermal analysis was carried out using a TA Instruments Thermal Analyzer. The instrument settings were a heating rate of 10 °C/min in a nitrogen atmosphere with a 100 mL/min flow rate. Elemental Analysis. The content of carbon, hydrogen, and nitrogen was determined by Huffman Laboratories, Golden, CO. Potentiometric Titration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm).25-27 The instrument was set at the mode when the equilibrium pH was collected. Subsamples of the carbons of about 0.100 g in 50 mL of 0.01 M NaNO3 were placed in a container thermostated at 25 °C and equilibrated overnight with the electrolyte solution. To eliminate the influence of atmospheric CO2, the suspension was continuously saturated with N2. The carbon suspension was stirred throughout the measurements. Volumetric standard NaOH (0.1 M) was used as a titrant. The experiments were done in the pH range of 3-10. Each sample was titrated with base after acidifying the carbon suspension. Gas Chromatography-Mass Spectrometry (GC-MS). GC/MS study was done using a Shimadzu gas chromatograph/mass spectrometer model QP 5050 with split injection (split ratio, 15). The separation was done on a Shimadzu XTI-5 column (bonded 5% phenyl) 30 m long, with a 0.25 mm internal diameter and 0.025 µm df (liquid film thickness). The instrument settings were a column temperature of 30 °C with a 10 °C/min increase to 250 °C. The column was held for 10 min at 250 °C. The injector temperature was 60 °C. The results were analyzed using unrestricted library search. The samples for analysis were extracted by placing 1.2 mL of carbon in a vial where 2 mL of methanol was added. Then the suspensions were heated at 60 °C for 1 h. After separation of the liquid phase from carbon, 0.5 µL of extracted solution was injected to the GC column. The mass spectra were collected for M/Z from 12 to 500.
Results and Discussion Methyl mercaptan breakthrough curves for the carbons studied are presented in Figure 1. Results showed that after impregnation with urea and heat treatment the adsorption capacities of the carbons studied increased from 2 to 10 times depending on the carbonization temperature. The experimental data are presented in Table 1. From these data, it is obvious that the higher the heat treatment temperature after impregnation with urea, the more methyl mercaptan is adsorbed on the carbon surface. High capacities were found for BPLU-950, BPLU-450, BAXU950, and S208U-950, while the lowest capacity was recorded for BAXU-450. The greatest effect of urea modification was observed for BAX carbon carbonized at 950 (21) Boehm, H. P. In Advances in Catalysis; Academic Press: New York, 1966; Vol. 1, p 179. (22) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: New York, 1982. (23) Lastoskie, C. M.; Gubbins, K. E.; Quirke, N. J. Phys. Chem. 1993, 97, 4786. (24) Olivier, J. P. J. Porous Mater. 1995, 2, 9. (25) Jagiello, J.; Bandosz, T. J.; Putyera, K.; Schwarz, J. A. J. Colloid Interface Sci. 1995, 172, 341. (26) Jagiello, J.; Bandosz, T. J.; Schwarz, J. A. Carbon 1994, 32, 1026. (27) Puziy, A. M.; Poddubnaja, O. I.; Ritter, J. A.; Ebner, A. D.; Holland, C. E. Carbon 2001, 39, 2313.
Adsorption/Oxidation of CH3SH on Activated Carbon
Langmuir, Vol. 19, No. 15, 2003 6117 Table 2. Structural Parameters and Characteristic Energy of Adsorption Calculated from Adsorption of Nitrogen sample
Figure 1. Methyl mercaptan breakthrough curves for the carbons studied. Table 1. pH of the Surface, Amount of Water Preadsorbed, Breakthrough Time, and Amount of CH3SH Adsorbed and Desorbed for the Materials Studied
sample
pH
pHE
BAX BAXU-450 BAXU-950 BPL BPLU-450 BPLU-950 S208 S208U-450 S208U-950 PCB PCBU-450 PCBU-950
7.20 6.18 7.43 7.41 7.84 8.46 7.47 8.49 9.41 7.57 8.81 9.07
6.78 5.45 4.88 3.82 5.98 3.67 5.94 6.56 6.77 5.34 7.26 7.35
amount CH3SH CH3SH of water breakthrough capacity desorbed [mg/g] [mg/g] [mg/g] time [min] 163.4 161.1 145.4 89.9 129.4 102.0 92.6 93.9 68.9 78.2 81.7 75.7
13 15 180 155 232 303 143 199 254 59 162 156
28.2 30.0 299.0 216.8 321.1 440.6 162.2 221.9 272.9 68.2 203.5 192.3
0.48 1.82 0.03 0.03 0.04 0.02 0.04 0.05 0.03 0.11 0.09 0.05
°C, whose adsorption capacity for methyl mercaptan increased 10-fold compared to that of the initial sample. On the other hand, for the same carbon but carbonized at 450 °C no effect of urea treatment was found and a very small amount of MM was immobilized on the surface. For the PCB carbon series, the adsorption capacities are not very sensitive to the carbonization temperature. The capacity of PCBU-450 is only slightly higher than that for PCBU-950. To analyze the strength of methyl mercaptan adsorption, the amount of MM desorbed from the carbon surface during air purging was estimated. The data collected in Table 1 suggest that higher MM adsorption capacity is related to the smaller amount desorbed. For all the samples except BAXU-450, for which the amount desorbed is relatively high (about 6%), only traces of methyl mercaptan are desorbed, suggesting strong adsorption of the CH3SH molecule and/or its oxidation products. After modifications with urea, an increase in the pH values of carbons is observed (Table 1). This can be explained by the appearance of additional basic sites, which are probably some nitrogen structures incorporated into the carbon matrix during urea treatment.20 After methyl mercaptan adsorption, the pH of all carbons decreased, which can be due to the formation of such products as DMDS and methyl methanethiosulfonate (MMTS) found as two major products of CH3SH adsorption/oxidation in our previous work.8,14 To explain the differences in the CH3SH adsorption capacity, both porosity and surface chemistry should be
BAX BAXU-450 BAXU-450E BAXU-950 BAXU-950E BPL BPLU-450 BPLU-450E BPLU-950 BPLU-950E PCB PCBU-450 PCBU-450E PCBU-950 PCBU-950E S208 S208U-450 S208U-450E S208U-950 S208U-950E
S Vmic Vt V
