Desulfurization of Digester Gas on Wood-Based Activated Carbons

850

Energy & Fuels 2008, 22, 850–859

Desulfurization of Digester Gas on Wood-Based Activated Carbons Modified with Nitrogen: Importance of Surface Chemistry Mykola Seredych and Teresa J. Bandosz* Department of Chemistry, The City College of New York, 160 ConVent AVenue, New York City, New York 10031 ReceiVed August 30, 2007. ReVised Manuscript ReceiVed NoVember 7, 2007

Activated carbon of wood origin with micro- and mesoporosity was modified with either melamine or urea by impregnation followed by heat treatment at 450 and 950 °C. The materials obtained were used as media for desulfurization of digester gas in the dynamic conditions. The initial and exhausted samples were characterized using elemental analysis, adsorption of nitrogen, Boehm titration, potentiometric titration, FTIR, and thermal analysis. The adsorbents revealed high capacities for H2S removal, and the presence of CO2 was not found detrimental for surface activity. Introduction of basic nitrogen functionalities, especially those thermally stable quaternary and pyridinic nitrogen, significantly increased the performance. It is proposed that these centers activate chemisorbed oxygen, which oxidizes H2S to elemental sulfur, in the first step. That sulfur can be further oxidized providing that enough oxygen is present in the system. Prehumidification of the carbon bed besides resulting in adsorption of water increases the performance by enhancing hydrogen sulfide dissociation in the basic environment, which increases the efficiency of its oxidation. In such conditions, besides elemental sulfur, SO2/sulfurous acid is deposited on the surface. Selectivity of oxidation to SO2 depends upon the availability of oxygen and pore sizes of the materials. When small pores are present, more SO2 is formed.

1. Introduction It is well-known that the performance of activated carbons as adsorbents is governed by their surface features.1–5 They are considered as excellent media for purification of gases and vapors, owing to their developed surface area and pore volume. Another factor governing application of activated carbons is their surface chemistry. Although it was neglected for many years when only its hydrophobicity advantage had been used, nowadays, when the applications of adsorbents require more and more sophisticated systems, leading to the low levels of pollutants removal, the surface chemistry research gains considerable attention. It was shown that by either impregnations, oxidations, reductions, complexation, or employing covalent bonding such heteroatoms as oxygen,1,3,4 nitrogen,6–14 sulfur,15–17 * To whom correspondence should be addressed. Telephone: 212-6506017. Fax: 212-650-6107. E-mail: [email protected]. (1) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. ActiVe Carbon; Marcel Dekker: New York, 1988. (2) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity; Academic Press: New York, 1982. (3) Leon y Leon, C. A.; Radovic, L. R. Interfacial chemistry and electrochemistry of carbon surfaces. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1992; Vol. 24, pp 213– 310. (4) Boehm, H. P. Carbon 1994, 32, 759–769. (5) Le Leuch, L. M.; Bandosz, T. J. Carbon 2007, 45, 568–578. (6) Matzner, S.; Boehm, H. P. Carbon 1998, 36, 1697–1703. (7) Adib, F.; Bagreev, A.; Bandosz, T. J. Langmuir 2000, 16, 1980– 1986. (8) Bagreev, A.; Menendez, J. A.; Dukhno, I.; Tarasenko, Y.; Bandosz, T. J. Carbon 2004, 42, 469–476. (9) Bashkova, S.; Bagreev, A.; Bandosz, T. J. Langmuir 2003, 19, 6115– 6121. (10) Bandosz, T. J.; Ania, C. O. Surface chemistry of activated carbon and its characterization. In ActiVated Carbon Surfaces in EnVironmental Remediation; Bandosz, T. J., Ed.; Elsevier: Oxford, U.K., 2006; pp 159– 230.

phosphorus,12,18 and halogens16,19 can be incorporated. Those species are usually bonded to the carbon matrix as functional groups analogous to those classified in organic chemistry. Moreover, the surface of carbon can be modified by deposition of metal oxides,20–22 salts,23 or polyoxycations.24 The modified adsorbents showed a good performance in the catalytic processes, such as removal of hydrogen sulfide,7,8 sulfur dioxide,21,25 nitrogen oxides,20–22 mercaptans,9 or ammonia5,23,24 from the gas phase. In all of those cases, although porosity was important to provide the space for reactions to occur, surface chemistry was crucial to make it happen. An example of the importance of surface chemistry for the technologically important process is desulfurization from the gas phase.7–9,26–34 It refers usually to the removal of reduced (11) Lahaye, J.; Nanse, G.; Bagreev, A.; Strelko, V. Carbon 1999, 37, 585–590. (12) Strelko, V. V.; Kuts, V. S.; Thrower, P. A. Carbon 2000, 38, 1499– 1504. (13) Stöhr, B.; Boehm, H. P.; Schlögl, R. Carbon 1991, 29, 707–720. (14) Elsayed, Y.; Bandosz, T. J. Langmuir 2002, 18, 3213–3218. (15) Boehm, H. P. Chemical identification of surface groups. In AdVances in Catalysis; Academic Press: New York, 1966; Vol. 1, pp 179– 274. (16) Puri, B. R. In Chemistry and Physics of Carbon; Walker, P. J., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6, p 191. (17) Chang, C. H. Carbon 1981, 19, 175–186. (18) Jagtoyen, M.; Derbyshire, F. Carbon 1998, 36, 1085–1097. (19) Tobias, H.; Soffer, A. Carbon 1985, 23, 281–289. (20) Pietrzak, R.; Bandosz, T. J. Carbon 2007, 45, 2537–2546. (21) Davini, P. Carbon 2001, 39, 2173–2179. (22) Lee, Y. W.; Park, J. W.; Jun, S. J.; Choi, D. E.; Yie, J. E. Carbon 2004, 42, 59–69. (23) Petit, C.; Karwacki, Ch.; Peterson, G.; Bandosz, T. J. J. Phys. Chem. C 2007, 111, 12705–12714. (24) Petit, C.; Bandosz, T. J. J. Phys. Chem. C 2007, 111, 16445–16452. (25) Bagreev, A.; Bashkova, S.; Bandosz, T. J. Langmuir 2002, 18, 1257–11264. (26) Bagreev, A.; Katikaneni, S.; Parab, S.; Bandosz, T. J. Catal. Today 2005, 99, 329–337.

10.1021/ef700523h CCC: $40.75  2008 American Chemical Society Published on Web 01/25/2008

Desulfurization of Digester Gas

Energy & Fuels, Vol. 22, No. 2, 2008 851

Figure 1. H2S breakthrough curves measured in dry conditions (ED) on BAX carbon modified with melamine (A) and urea (B) and with 2 h of prehumidification (EP) on BAX carbon modified with melamine (C) and urea (D).

sulfur species, such as hydrogen sulfide or mercaptans. As reported in the literature,27,31–34 for this process, the basicity of the surface is essential because it helps in dissociation of H2S to HS-. Those sulfide ions are oxidized either to elemental sulfur or sulfur dioxide.32–34 The selectivity here depends upon the pH and the porosity of the materials. When the pH is very basic, the dissociation is shifted to the right and more HS- ions are formed, leading to the formation of polysulfides and thus bulky sulfur polymers. For this process, small pores are not favorable. They accept only small polymers and sulfur radicals, and thus, further oxidation of sulfur occurs resulting in the presence of SO2. SO2 can be weakly adsorbed on the surface or converted (27) Przepiorski, J.; Oya, A. J. Mater. Sci. Lett. 1998, 17, 679–682. (28) Bashkova, S.; Baker, F. S.; Wu, X.; Armstrong, T. R.; Schwartz, V. Carbon 2007, 45, 1354–1363. (29) Feng, S.; Kwon, W.; Feng, X.; Borguet, E.; Vidic, R. EnViron. Sci. Technol. 2005, 39, 9744–9749. (30) Mikhalovsky, S. V.; Zaitsev, Y. P. Carbon 1997, 35, 367–1374. (31) Bagreev, A.; Bandosz, T. J. Ind. Chem. Eng. Res. 2005, 44, 530– 538. (32) (a) Bandosz, T. J. J. Colloid Interface Sci. 2002, 246 (1), 1–20. (b) Bandosz, T. J. Desulfurization on activated carbons. In ActiVated Carbon Surfaces in EnVironmental Remediation; Bandosz, T. J., Ed.; Elsevier: Oxford, U.K., 2006; pp 231–292. (33) Bagreev, A.; Adib, F.; Bandosz, T. J. Carbon 2001, 39, 1987– 1905. (34) Seredych, M.; Bandosz, T. J. Ind. Eng. Chem. Res. 2006, 45, 3658– 3665.

to sulfuric acid.32,35–37 Thus, secondary pollutants are formed. In the case of methyl mercaptan, in the presence of oxygen, dimethyldisulfide is formed from dissociated methyl mercaptan molecules.9 The basicity of carbon surface important for desulfurization from air7–9,27–32 or digester gas26,34 can have its origin in impregnation with caustics38 or carbonates,39,40 in supplying NH3 to the system,41,42 in addition of basic oxides of calcium or magnesium31,34 or in incorporation of nitrogen to the carbon matrix.7–9 The latter was shown to have a tremendous effect upon enhancing the removal capacity of both H2S and CH3SH (35) Mochida, I.; Miyamoto, S.; Kuroda, K.; Kawano, S. l.; Yatsunami, S.; Korai, Y.; Yatsutake, A.; Yashikawa, M. Energy Fuels 1999, 13, 369– 373. (36) Adib, F.; Bagreev, A.; Bandosz, T. J. J. Colloid Interface Sci. 1999, 214, 407–415. (37) Lisovskii, A.; Shter, G. E.; Semiat, R.; Aharoni, C. Carbon 1997, 35, 1645–1648. (38) Bagreev, A.; Bandosz, T. J. Ind. Eng. Chem. Res. 2002, 41, 672– 679. (39) Przepiorski, J.; Abe, Y.; Yoshida, S.; Oya, A. J. Mater. Sci. Lett. 1997, 16, 1312–1314. (40) Graham, J. Activated carbon for odor control and method for making same. U.S. Patent 6,858,192, 2005. (41) Malhautier, L.; Gracian, C.; Roux, J.-C.; Fanlo, J.-L.; Le Cloirec, P. Chemosphere 2003, 50, 145–153. (42) Turk, A.; Sakalis, S.; Lessuck, J.; Karamitsos, H.; Rago, O. EnViron. Sci. Technol. 1989, 23, 1242.

852 Energy & Fuels, Vol. 22, No. 2, 2008

Seredych and Bandosz

Table 1. H2S Breakthrough Capacity and the pH Values for the Initial and Exhausted Carbons in Dry Conditions pH

H2S breakthrough capacity sample

(mg/g)

(mg/cm3)

BAX CBAX-A CBAX-AM CBAX-AMO CBAX-B CBAX-BM CBAX-BMO CBAX-AU CBAX-AUO CBAX-BU CBAX-BUO

3.1 0.6 1.3 2.1 2.0 18.2 14.6 1.1 4.1 21.6 19.8

0.9 0.2 0.6 0.9 0.7 7.5 6.1 0.4 1.8 9.4 7.9

initial

exhausted

6.50 5.59 7.48 6.09 7.68 8.32 8.05 6.85 7.87 8.46 8.17

5.70 5.43 7.30 5.60 7.45 5.60 4.70 6.59 7.65 5.77 4.87

Table 2. H2S Breakthrough Capacity, Amount of Water Preadsorbed and the pH Values for the Initial and Exhausted Carbons in Wet Conditions H2S breakthrough capacity sample

(mg/g)

(mg/cm3)

BAX CBAX-A CBAX-AM CBAX-AMO CBAX-B CBAX-BM CBAX-BMO CBAX-AU CBAX-AUO CBAX-BU CBAX-BUO

5.7 2.6 2.5 8.6 6.6 64.1 52.7 6.7 13.4 34.1 51.6

1.7 0.7 1.1 3.6 2.7 26.0 23.2 2.8 5.2 14.4 23.8

water pH adsorbed (mg/g) initial exhausted 96 64 75 120 113 298 262 88 122 239 276

6.50 5.84 7.48 6.09 7.68 8.32 8.05 6.85 7.87 8.46 8.17

5.81 5.16 7.35 5.02 7.40 3.98 5.02 6.34 6.33 5.80 3.10

from air.7–9 This enhancement effect is governed by the extent of modification, which depends upon the pretreatment of carbon, type of nitrogen compounds used, and heat-treatment temperature.7–9,14,43,44 Preoxidation of carbons followed by heat treatment at high temperature (950 °C) usually results in incorporation of more active nitrogen than on the untreated carbons. Nitrogen in the carbon matrix was found to reduce the energy gap,12 and thus, it increased catalytic activity of the surface. Moreover, it was proposed that nitrogen in the carbon matrix is able to activate oxygen via the formation of superoxygen ions, which facilitate the oxidation reactions.13,44 Despite the fact that basicity is important for desulfurization, its presence in a particular form can be detrimental for the technological process. An example is desulfurization of digester gas on the activated carbon modified with alkaline earth oxides.31,34 In such systems, those oxides get deactivated by CO2, especially when the moisture is present, owing to the competition between hydrogen sulfide and carbon dioxide or carbonic acid for basic centers. Thus, for desulfurization of digester gas, the materials with less reactive basicity as sludgederived adsorbents were recommended as very efficient media.45 The objective of this paper is to evaluate the mechanism of hydrogen sulfide removal from dry digester gas on activated carbons modified with urea and melamine. Although CO2 can be considered as the competetive species for H2S adsorption on basic sites, we expect that those sites consisting of nitrogen should behave differently than inorganic basic species. Their impact on desulfurization is evaluated taking into account the surface features of carbons, such as porosity (size and volume) (43) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Carbon 1995, 33, 1641–1653. (44) Biniak, S.; Szymanski, G.; Siedlewski, J.; Swiatkowski, A. Carbon 1997, 35, 1799–1810. (45) Seredych, M.; Bandosz, T. J. Energy Fuels 2007, 21, 858–866.

Figure 2. Dependence of the H2S removal capacity on the amount of water adsorbed during the prehumidification.

and surface chemistry. The diversity on those features is provided using different degrees of modification, which include the source of carbon, nitrogen precursor, surface pretreatment, and the temperature of nitrogen incorporation. 2. Experimental Section 2.1. Materials. Commercial-activated carbon of wood origin BAX-1500 (MeadWestvaco) was used in this study. Before modification with nitrogen-containing species, a subsample of carbon was oxidized with 50% HNO3 for 4 h and then washed out with water to remove excess acid and water-soluble products of oxidation. To introduce nitrogen groups, the initial carbon and oxidized carbon (30 g) were treated with urea or melamine suspension (20 g of urea or melamine in 100 mL of ethanol) and stirred at room temperature for 5 h. Then, the mixture was boiled to evaporate alcohol, and the carbon sample was dried at 120 °C. The samples impregnated with urea or melamine were heated in nitrogen at 10 °C/min to 450 or 950 °C and maintained at these temperatures for 0.5 h. After modifications, the samples were washed with boiling water to remove any excess of urea or melamine decomposition products. The carbons after treatment are referred to as CBAX. The urea-modified carbons have either letter U or M added to their names, representing urea or melamine, respectively. The preoxidized samples are referred to with letter O. The temperature of heat treatment is denoted either as A or B, which refer to 450 or 950 °C, respectively. Thus, CBAX-AU is BAX-1500-modified with urea and heated at 450 °C, whereas CBAX-BMO is BAX-1500-preoxidized-treated with melamine and heated at 950 °C. To distinguish between the effects of thermal treatment and incorporation of nitrogen, untreated wood-based carbon samples were heated under the same conditions as those used for samples impregnated with nitrogen-containing species. Those samples are referred to as, for example, CBAX-A and CBAX-B, where A represents 450 °C and B represents 950 °C. 2.2. Methods. Boehm Titration. A total of 0.5 g of carbon sample was placed in 25 mL of the following 0.05 N solutions of sodium hydroxide and hydrochloric acid. The vials were sealed and shaken for 24 h, then 5 mL of each filtrate was pipetted, and the excess of 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.15 The number of surface basic sites was calculated from the amount of hydrochloric acid that reacted with the carbon. Because nitrogen-containing groups can have pKa similar to those containing oxygen, the bases of different strength to distinguish lactonic, carboxylic, or phenolic groups were not used.



Figure 3. Dependence of the decrease in the pH between the initial and exhausted samples on the amount of water adsorbed during prehumidification. Table 3. Carbon, Hydrogen, and Nitrogen Contents (%) in the Samples Studied sample

C

H

N

N/C

BAX CBAX-AU CBAX-AUO CBAX-AM CBAX-AMO CBAX-BU CBAX-BUO CBAX-BM CBAX-BMO

73.8 73.5 72.7 67.3 65.1 82.2 81.4 83.3 79.6

3.6 1.8 1.7 1.9 1.5 0.7 0.8 0.6 0.9

0.2 15.6 14.3 21.9 22.8 5.1 5.9 5.9 8.0

0.003 0.21 0.20 0.33 0.35 0.06 0.07 0.07 0.10

Table 4. Results of the Boehm Titration (Numbers of Surface Groups) and Surface pH Values sample

pH

basic

acidic (mmol/g)

all (mmol/g)

BAX CBAX-A CBAX-AU CBAX-AUO CBAX-AM CBAX-AMO CBAX-B CBAX-BU CBAX-BUO CBAX-BM CBAX-BMO

6.50 5.59 6.85 7.87 7.48 6.09 7.68 8.46 8.17 8.32 8.05

0.352 0.153 0.756 0.845 0.512 0.656 0.228 0.881 0.815 0.988 0.688

0.764 0.241 0.752 0.857 0.637 0.645 0.0 0.543 0.536 0.614 0.572

1.116 0.394 1.508 1.702 1.149 1.301 0.228 1.424 1.351 1.602 1.260

pH of the Carbon Surface. Carbon powder (0.4 g) was placed in 20 mL of distilled water and equilibrated during the night. Then, the pH of the suspension was measured. Potentiometric Titration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm). The instrument was set at the mode when the equilibrium pH was collected. Subsamples of the materials studied of about 0.100 g in 50 mL of 0.01 M NaNO3 were placed in a container thermostatted at 298 K and equilibrated overnight with the electrolyte solution. To eliminate the influence of atmospheric CO2, the suspension was continuously saturated with N2. The suspension was stirred throughout the measurements. Volumetric standard NaOH (0.1 M) was used as the titrant. The experiments were performed in the pH range of 3–10. In the evaluation of the data,46 it is assumed that the population of sites can be described by a continuous pKa distribution, f(pKa). (46) Jagiello, J.; Bandosz, T. J.; Schwarz, J. A. Carbon 1994, 32, 1026– 1028.

Figure 4. pKa distributions for the BAX series of samples.

The experimental data can be transformed into a proton-binding isotherm, Q, representing the total amount of protonated sites, which is related to the pKa distribution by the following integral equation: ∞

Q(pH) )

∫ q(pH, pK )f(pK )dpK a

a

a

(1)

-∞

The solution of this equation is obtained using the numerical procedure,46,47 which applies regularization combined with nonnegativity constraints. On the basis of the spectrum of acidity constants and the history of the samples, the detailed surface chemistry was evaluated. (47) Jagiello, J. Langmuir 1994, 10, 2778–2785.


Seredych and Bandosz

Table 5. Peak Positions (pKa) and Numbers of Species in mmol/g (in Parentheses) for the BAX Series of Samples sample BAX CBAX-A CBAX-AU CBAX-AUO CBAX-AM CBAX-AMO CBAX-B CBAX-BU CBAX-BUO

pHa

pKa 4–5

pKa 5–6

pKa 6–7

pKa 7–8

pKa 8–9

pKa 9–10

pKa 10–11

7.02

4.53 (0.097) 4.26 (0.099) 4.24 (0.113) 4.26 (0.139) 4.61 (0.050) 4.51 (0.068) 4.55 (0.063) 4.24 (0.117) 4.55 (0.091) 4.21 (0.115)

5.77 (0.060) 5.61 (0.052) 5.38 (0.053) 5.48 (0.074)

6.87 (0.114) 6.55 (0.059) 5.92 (0.047) 6.48 (0.050) 6.05 (0.038)

11.10 (0.319) 10.20 (0.190) 10.24 (0.124)

6.15 (0.047)

8.43 (0.059) 8.87 (0.126) 8.15 (0.068) 8.75 (0.075) 8.74 (0.053) 8.66 (0.042) 8.92 (0.037) 8.83 (0.088) 8.39 (0.052) 8.80 (0.095)

9.61 (0.137)

5.85 (0.037) 5.85 (0.043) 5.53 (0.080) 5.87 (0.054) 5.38 (0.066)

9.60 (0.138) 9.90 (0.154)

4.49 (0.102)

5.83 (0.074)

7.08 (0.079)

7.55 (0.052) 7.29 (0.074) 7.04 (0.109) 7.42 (0.065) 7.25 (0.041) 7.09 (0.043) 7.87 (0.041) 7.62 (0.064) 7.05 (0.085) 7.13 (0.076) 7.84 (0.025) 7.79 (0.029)

8.81 (0.064)

9.78 (0.112)

6.12 7.18 7.29 7.68 7.21 7.87 8.09 8.11 8.03

CBAX-BM CBAX-BMO a

8.03

6.92 (0.068) 6.76 (0.097)

9.33 (0.132) 9.72 (0.177) 9.80 (0.121) 9.53 (0.108) 9.70 (0.064)

all (mmol/g) 0.838 0.600 0.646 0.580 0.303 0.298

10.31 (0.049) 10.01 (0.146)

0.365 0.592 0.420 0.578 0.460

pH of the suspension used for PT experiments.

Thermal Analysis. TG curves were obtained using a TA instrument thermal analyzer. About 30 mg of the sample were submitted to a regular increase of temperature with a heating rate of 10 °C/min, while the nitrogen flow rate was 100 mL/min. Adsorption of Nitrogen. On the materials obtained, sorption of nitrogen at its boiling point was carried out using ASAP 2010 (Micromeritics). Before the experiments, the samples were outgassed at 120 °C to constant vacuum (10-4 kPa). From the isotherms, the surface areas (BET method), total pore volumes, Vt (from the last point of isotherm at a relative pressure equal to 0.99), volumes of micropores, Vmic, mesopore volume, Vmes, along with pore size distributions were calculated. The last four quantities were calculated using the density functional theory (DFT).48,49 CHN. The content of carbon, hydrogen, and nitrogen was evaluated in the commercial Schwarzkopf laboratory, New York. Fourier Transform Infrared (FTIR). FTIR spectroscopy was carried out using a Nicolet Magna-IR 830 spectrometer using the attenuated total reflectance method (ATR). The spectrum was collected 16 times and corrected for the background noise. The experiments were performed on the powdered samples, without KBr addition. EValuation of H2S Sorption Capacity. A custom-designed dynamic test was used to evaluate the performance of adsorbents for H2S adsorption from gas streams as described in the technical literature.36 Adsorbent samples were ground (1–2 mm particle size) and packed into a glass column (length of 370 mm, internal diameter of 9 mm, and bed volume of 3 cm3). Digester gas mixture (60% CH4 and 40% CO2) containing 0.1% (1000 ppm) of H2S was passed through the column of adsorbent at 0.150 L/min. The flow rate was controlled using Cole Palmer flow meters. The samples run without or with 2 h of prehumidification with moist air (70%). They are referred to as ED and EP, respectively. The breakthrough of H2S was monitored using electrochemical sensors. As a breakthrough concentration, 100 ppm was arbitrary chosen. The adsorption capacities of each sorbent in terms of milligrams of sulfur containing gases per gram of adsorbent were calculated by integration of the area above the breakthrough curves and from (48) Olivier, J. P. J. Porous Mater. 1995, 2, 9–17. (49) Lastoskie, Ch. M.; Gubbins, K. E.; Quirke, N. J. Phys. Chem. 1993, 97, 4786–4796.

the H2S concentration in the inlet gas, flow rate, breakthrough time, and mass of the sorbent. The experiments were run on as-received samples.

3. Results and Discussion The performance of the carbons studied in desulfurization of digesters gas is presented in Figure 1. As seen on the basis of the breakthrough time, the significant differences exist, and generally speaking, modifications with nitrogen-containing species improve the performance of materials when the treatment is performed at 950 °C. Prehumidification also noticeably improved the performance; however, the effects of urea and melamine treatment do not follow the same trends. All curves rise steeply to 100 ppm, indicating fast deactivation of the adsorption centers. The calculated capacities are summarized in Tables 1 and 2. A comparison of the values indicates about 100% improvement in the capacity when water is present in the system. Modifications with urea and melamine at 950 °C increase the capacity of carbons between 5- and 10-fold. The values obtained are comparable to those on catalytic carbons.26,34 On the other hand, the treatment at 450 °C visibly decreases the performance. The negative changes are also noticed for carbons exposed only to heat treatment without the impregnation with nitrogen-containing species. In comparison to this treatment, the effect of the nitrogen incorporation at the corresponding temperatures is even more visible. Preoxidation with nitric acid has a consistently positive effect on the capacity, especially for the urea-modified samples run in the presence of water on the carbon surface, where a big improvement in the performance is found after treatment at 950 °C. Because water has a positive effect on the H2S removal capacity from the digester gas, which is consistent with the results found previously for sludge derived adsorbents45 and some catalytic carbons,34 the relationship between these two values was analyzed. Figure 2 shows the linear trend for both dry ad wet conditions, with R2 equal to 0.91 and 0.96, respectively. It is interesting that both curves cross the x axis at the same point at about 70 mg/g of water adsorbed, which



Figure 6. DTG curves for the BAX series of samples.

Figure 5. FTIR spectra for the BAX series of samples.

suggests that there is the minimum affinity of the surface toward water at which the H2S can be retained on the surface at the dynamic conditions of our experiments. This behavior can be linked either to the influence of water on the surface reactions or on the specific nature of surface sites, which have affinity toward both water and hydrogen sulfide. In fact, good correlation in dry conditions, where water was actually not supplied to the system and the correlation should be considered as indirect one, supports that water likely plays a role in the H2S retention on the surface, and/or the sites that are active in the retention of water are also active in the retention of hydrogen sulfide. The changes in the pH of the carbon surface after exposure to digester gas also seem to be related to the conditions of the experiments. As seen in Figure 3, linear trends are found for both experimental conditions and more affinity of the carbon surface toward water, which is related to surface polarity,32 results in more pronounced decrease in the pH after contact with H2S and CO2. The decrease in pH can be related either to formation of amphiprotic salts, which affect the pH of the system, such as sulfates or carbonates, or the formation of sulfur oxyacid or carbonic acid. Because the carbon was modified only with nitrogen and its ash content is less than 2%,14 it is

reasonable to exclude the formation of inorganic salts and the deposition of oxyacids, if oxidation can take place, seems to be a more plausible scenario. As shown above, the behavior of our carbons in desulfurization from digester is certainly affected by surface chemistry. Treatment of carbons with nitrogen-containing compounds at 950 °C introduced about 5-8% of nitrogen, whereas treatment at 450 °C increased nitrogen up to 22% when melamine was used as a nitrogen source (Table 3). It is interesting that preoxidation of carbon before low-temperature treatment does not affect the amount of nitrogen left in the structure of those carbons. Thus, the differences in the performance must be related to its specific chemical environment. The high content of nitrogen in the low-temperature-treated samples is expected, owing to not complete decomposition of the nitrogen source molecules.8,13,50 Also, wood-based carbons obtained using chemical activation, as BAX, are known to receive appreciable amounts of nitrogen, even at high temperature, owing to their low degree of aromatization. On the basis of the previously published results, it is expected that at low-temperature nitrogen is present as NH or NH2 species, whereas after heating at 950 °C, they are converted to quaternary, pyrrolic, and pyridinic nitrogen.43,44 For high-temperature-treated samples, preoxidation results in more nitrogen incorporated to the matrix, likely as a result of chemical interaction between the oxygen-containing acidic groups and NH2 groups of nitrogen precursors. (50) Schaber, P. M.; Colson, J.; Higgins, S.; Thielen, D.; Anspach, B.; Brauer, J. Thermochim. Acta 2004, 424, 131–142.


Seredych and Bandosz Table 7. Structural Parameters Calculated from Nitrogen Adsorption Isotherms

Figure 7. DTG for the samples exposed to digester gas in dry conditions (ED) and after prehumidification (EP). Table 6. Weight Loss from TA Analysis for the Exhausted Samples Modified with Nitrogen Urea and Melamine at 950 °C mass loss (∆W) (%) sample

200–300 °C

300–400 °C

400–500 °C

SH2Sa (wt %)

STAb (wt %)

CBAX-B-ED CBAX-B-EP CBAX-BM-ED CBAX-BM-EP CBAX-BMO-ED CBAX-BMO-EP CBAX-BU-ED CBAX-BU-EP CBAX-BUO-ED CBAX-BUO-EP

0.29 0.31 1.32 3.51 1.16 2.72 1.23 2.40 1.41 3.82

0.32 0.49 0.45 1.51 1.14 1.05 0.75 1.48 0.42 1.54

0.16 0.08 0.17 1.31 0.25 0.56 0.18 0.27 0.16 0.57

0.19 0.62 1.71 6.03 1.37 4.96 2.03 3.21 1.86 4.86

0.62 0.72 1.28 4.58 1.97 2.97 1.55 2.95 1.29 4.02

a Calculated from the breakthrough test as the content of sulfur in the H2S molecule. b Calculated assuming that the first peak represents SO2.

The incorporated nitrogen should change the apparent acid– base properties of our carbons. These properties should be also changed by either oxidation or heat treatment at elevated temperatures. The first indications of these changes were the pH values listed in Table 1. On the basis of them, the most pronounced effect of an increase in basicity is observed after treatment at 950 °C, even though less nitrogen is present than in the corresponding samples obtained at 450 °C. Boehm titration results collected in Table 4 confirm those changes showing up to a 3-fold increase in the number of basic groups after the treatment applied, especially at 950 °C. For those samples, the number of acidic groups decreases. On the other

sample

SBET (m2/g)

Vmic (cm3/g)

Vmeso (m2/g)

Vt (cm3/g)

Vmic/Vt

BAX BAX-ED BAX-EP CBAX-A CBAX-A-EP CBAX-AM CBAX-AMO CBAX-AMO-EP CBAX-B CBAX-B-ED CBAX-B-EP CBAX-BM CBAX-BM-ED CBAX-BM-EP CBAX-BMO CBAX-BMO-ED CBAX-BMO-EP CBAX-AU CBAX-AU-EP CBAX-AUO CBAX-AUO-EP CBAX-BU CBAX-BU-ED CBAX-BU-EP CBAX-BUO CBAX-BUO-ED CBAX-BUO-EP

2176 2073 2092 2116 2092 881 393 299 1477 1497 1498 1435 1366 1259 721 667 629 1165 1115 888 836 1364 1364 1238 963 849 765

0.818 0.774 0.811 0.786 0.805 0.313 0.153 0.116 0.577 0.580 0.582 0.560 0.531 0.481 0.308 0.286 0.245 0.430 0.414 0.385 0.368 0.535 0.545 0.486 0.411 0.371 0.327

0.701 0.662 0.649 0.687 0.653 0.359 0.133 0.109 0.383 0.422 0.409 0.394 0.386 0.367 0.143 0.138 0.180 0.366 0.357 0.150 0.127 0.354 0.339 0.326 0.163 0.126 0.121

1.519 1.436 1.460 1.473 1.458 0.672 0.286 0.225 0.960 1.002 0.991 0.954 0.917 0.848 0.451 0.424 0.425 0.796 0.771 0.535 0.495 0.889 0.884 0.812 0.574 0.497 0.448

0.54 0.54 0.56 0.53 0.55 0.47 0.54 0.52 0.60 0.58 0.59 0.59 0.58 0.57 0.68 0.68 0.58 0.54 0.54 0.72 0.74 0.60 0.62 0.60 0.72 0.75 0.73

hand, for the samples obtained at 950 °C, the effective number of acidic groups is not significantly changed. This can be explained by the protonation of the NH2 groups in water.7,14 The surface functionality also changed as a result of the treatment at 950 °C, as demonstrated by an increase in the pH for CBAX-B, resulting in the decomposition of oxygen containing acidic and basic groups.3,10,51–53 That change in their number must affect the capacity because a noticeable decrease in the performance is noticed after thermal treatments at both 450 and 950 °C. Potentiometric titration (PT) combined with the solution of adsorption integral equation using splines (SAIEUS) procedure46,47 provides information about the acidity constant distribution of species with 3 < pKa < 10 present on the carbon surface.54,55 In the case when the only heteroatom is oxygen, groups with pKa < 8 are classified as carboxylic acids and those with pKa > 8 are classified as phenols and quinines.15 As mentioned previously, NH3+, NH+, and NH4+ ions can also be found in this range.56 The pKa distribution curves are collected in Figure 4. The numbers of groups and the positions of peaks are summarized in Table 5. The surface of BAX series of samples is very heterogeneous with a variety of compounds representing nitrogen- and oxygen-containing functionalities (in the case of carbon heat-treated at 450 °C). Treatment with nitrogencontaining compounds increase the number of species, especially those with pKa about 4.5 and about 10 in comparison to the only heat-treated samples. Comparing the pKa distributions for two BAX series, treated with melamine and urea, suggests that (51) Papirer, E.; Dentzer, J.; Li, S.; Donnet, J. B. Carbon 1991, 29, 69–72. (52) Otake, Y.; Jenkins, R. B. Carbon 1993, 31, 109–121. (53) Krishnankutty, N.; Vannice, A. Chem. Mater. 1995, 7, 754–763. (54) Bandosz, T. J.; Jagiello, J.; Contescu, C.; Schwarz, J. A. Carbon 1993, 31, 1193–1202. (55) Bandosz, T. J.; Putyera, K.; Jagiello, J.; Schwarz, J. A. Carbon 1994, 32, 659–664. (56) Kortum, G.; Vogel, W.; Andrusso, K. Dissociation Constants of Organic Acids in Aqueous Solutions; Butterworth: London, U.K., 1961.



Figure 8. Pore size distributions for the initial and exhausted samples.

the latter results in more heterogeneous chemistry and the larger number of surface species. This is consistent only with the performance in desulfurization in dry conditions, which showed slightly better capacity for the samples treated with urea (Table 1). The materials obtained at 450 °C seem to be more heterogeneous than their high-temperature-treated counterparts, owing to the abundance of nitrogen functionalities. Effects of chemical and thermal modifications on BAX carbon are seen on the FTIR spectra (Figure 5). The 1700 cm-1 band in the FTIR spectra of the initials carbons, without nitrogen modifications, is characteristic of stretching vibrations of carbonyl functional groups band CdO in carboxylic acids. It is accompanied by band at 1150 cm-1 representing -C-O stretching and O-H bending modes (ether, lactone, carboxyl, and phenolic structures).44,57,58 The 1570 cm-1 band is probably a composite band that has some contributions from the aromatic ring mode, conjugated carbonyl, and carboxylate groups.57 The reactions of carboxylic groups with urea or melamine followed by thermal treatment at 450 °C resulted in an decrease in the intensity of the 1700 cm-1 band, especially for the not oxidized samples, which is accompanied by an increase in the 1570 cm-1 band associated in this case with amides.59 This indicates that some COO- groups are involved in the retention of amines.57 (57) Zawadzki, J. IR spectroscopy in carbon surface chemistry. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; Vol. 21, pp 180–200. (58) Silverstein, R. M.; Webster, F. X. Spectroscopic Identification of Organic Compounds; Wiley: New York, 1998. (59) Ko, Y. G.; Sung, B. H.; Choi, U. S. Colloids Surf., A 2007, 305, 120–125.

After treatment of carbon with urea or melamine, three additional bands are observed at 1245, 1314, and 1407 cm-1 in the FTIR spectrum. The absorption peak at 1245 cm-1 seen on the spectra for the low-temperature-treated samples can be assigned to the presence of tertiary nitrogen species (C-N stretching vibrations) incorporated into the carbon structure.44,57,60 The bands 1314 and 1407 cm-1 are possibly due to C-Nvibrations in secondary aromatic amines and from NdN arrangements, respectively.61,62 Although weak, owing to only few percentages of nitrogen, these bands can be distinguished for the high-temperature-treated samples. The absorption below 900 cm-1 can be assigned to out-of-plane deformation vibrations of C-H groups located at the edges of aromatic planes.57,58 The complex bands about 1570 cm-1 for nitrogen-treated samples heated at high temperature represent pyridinic nitrogen.57 The presence of N-H or O-H bonds can be detected in the spectra between 2500 and 3500 cm-1. The band of O-H stretching vibrations because of the existence of surface hydroxyl groups is observed after heat treatment of carbon BAX at 450 °C. Because the -NH2, -NH, and NH3 species vibrate in the range of 3200–3500 cm-1, the most pronounced band, which appears in the spectra of the carbons with urea or melamine after heat treatment at 450 °C, can be attributed to their presence. In the case of these samples, the content of nitrogen is the (60) Quillard, S.; Louarn, G.; Lefrant, S.; MacDiarmid, A. G. Phys. ReV. B: Condens. Matter Mater. Phys. 1994, 50, 12496. (61) Stejskal, J.; Trchova, M.; Prokes, J.; Sapurina, I. Chem. Mater. 2001, 13, 4083–4086. (62) Devallencourt, C.; Saiter, J. M.; Fafet, A.; Ubrich, E. Thermochim. Acta 1995, 259, 143–151.


highest (Table 3) and we expect it to be in the form of -NH, -NH2, or NH4+.43,44 The high-temperature treatment at 950 °C removed some surface oxygen-containing groups and resulted in the incorporation of nitrogen to the carbon structure. This is seen as a slight indication of the bands in the region between 1000 and 1750 cm-1. Nevertheless, some oxygen is still present, which can be detected from the balance of elements listed in Table 3, knowing the small ash content in those carbons. One has to remember that those carbons are expected to contain some phosphorus because they were obtained using phosphoric acid activation. Examination of the FTIR spectra for the exhausted samples does not reveal any significant differences in the surface chemistry pattern, which suggests that elemental sulfur, which is not expected to leave its fingerprints on FTIR spectra, should be an important product of surface reactions. The presence of nitrogen-containing species in the carbon structure and on the surface is consistent with the weight change that occurs during thermogravimetric analysis for the samples after heat treatment with urea or melamine (Figure 6). For all carbons, the peaks at temperature higher than 400 °C represent decomposition of melamine and urea-derived species deposited on the surface.50,62 They are released mainly in the form of ammonia. In the case of preoxidized samples, also oxygencontaining groups and those engaged in the compounds with melamine and urea-derived nitrogen decompose mainly between 500 and 600 °C. On the other hand, the samples heated at 950 °C are very stable. DTG curves for the samples exposed to digester gas are presented in Figure 7. We show only curves for the samples modified with urea and melamine obtained at 950 °C, on which the high capacity for H2S removal was measured. For all of them, the distinguishable differences are at the temperature range between 200 and 500 °C where one complex peak, which in some cases can be considered as consisting of three peaks, is revealed. All samples have the main peak centered at 250 °C with a shoulder at 350 °C of varying intensity. For moist conditions, a new peak appears at about 425 °C. On the basis of the pervious studies, the first peak represents the removal of strongly adsorbed SO225 and the next two peaks can be assigned to the removal of elemental sulfur from larger and smaller pores, respectively. SO2 can be responsible for the decrease in the pH after the exposure to the digester gas. We do not expect a noticeable amount of H2S being physically adsorbed at the conditions of our experiments. If adsorbed, it should be removed at very low temperatures (

Desulfurization of Digester Gas on Wood-Based Activated Carbons

Recommend Documents