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Comparison of Physicochemical Properties of Nitrogen-enriched Activated Carbons Prepared by Physical and Chemical Activation of Brown Coal Piotr Nowicki, Robert Pietrzak,* and Helena Wachowska Laboratory of Coal Chemistry and Technology, Faculty of Chemistry, Adam Mickiewicz UniVersity, Grunwaldzka 6, 60-780 Poznan´, Poland ReceiVed April 11, 2008. ReVised Manuscript ReceiVed September 8, 2008
Nitrogen-enriched active carbon has been obtained from Polish brown coal from the “Konin” colliery. The process of ammoxidation by a mixture of ammonia and air at the ratio of 1:3 has been performed at two temperatures (300 and 350 °C) at different stages of the production, that is, at that of precursor, char, and active carbon. It has been shown that the stage at which the process of ammoxidation is conducted has profound effect on the amount of nitrogen introduced into the carbon structure. The carbonization and activation (by steam or KOH) of nitrogen-enriched samples leads to significant reduction of the nitrogen content. The final products were microporous active carbons of well-developed surface area varying from 604 to 3181 m2/g and having nitrogen content from 0.4 to 6.5 wt%, showing different acid-base character of the surface.
1. Introduction Because of low cost of production and the availability of relevant resources, coal has been extensively used as a precursor of active carbons needed in many industrial technologies. In view of increasing requirements as to the protection of the natural environment, the demand for active carbon continuously increases.1 In the developed countries, active carbons are widely applied in practically all branches of industry, for example, as adsorbents, catalysts, catalyst supports, electrode materials, and ion-exchangers.2-4 The wide spectrum of their use is a consequence of their unique physicochemical properties determined by the very well developed surface area and the presence of heteroatoms built in their structure, mainly oxygen, nitrogen, and sulfur. To meet particular needs it is possible to modify the acid-base, catalytic, or adsorption properties of the active carbons by a number of processes.5,6 Recently, much attention has been devoted to nitrogenenriched active carbons because of a wide gamut of their applications: for adsorption of compounds of acidic character such as SO2, H2S, NOx, and CO2 from the gas phase;7-10 and for adsorption of metal ions, for example, copper(II) from the * To whom correspondence should be addressed. Phone: +48-618291476; fax: +48-618291505; e-mail:
[email protected]. (1) Roskill Metals and Minerals Report, http://www.roskill.com/reports/ activated; Roskill Consulting Group Ltd. (2) Pokonova, Y. V. Carbon 1996, 34, 411–415. (3) Rodriguez-Reinoso, F. Carbon 1998, 36, 159–175. (4) Valente, A.; Palma, C.; Fonseca, I. M.; Ramos, A. M.; Vital, J. Carbon 2003, 41, 2793–2803. (5) Figueiredo, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Orfao, J. J. M. Carbon 1999, 37, 1379–1389. (6) Pradhan, B. K.; Sandle, N. K. Carbon 1999, 37, 1323–1332. (7) Boudou, J. P.; Chehimi, M.; Broniek, E.; Siemieniewska, T.; Bimer, J. Carbon 2003, 41, 1999–2007. (8) Bagreev, A.; Menendez, J. A.; Dukhno, I.; Tarasenko, Y.; Bandosz, T. J. Carbon 2004, 42, 469–476. (9) Huang, M.-C.; Teng, H. Carbon 2003, 41, 951–957. (10) Maroto-Valer, M. M.; Tang, Z.; Zhang, Y. Fuel Process. Technol. 2005, 86, 1487–1502.
liquid phase.11 Nitrogen-enriched active carbons are also very effective in removal of many organic compounds, such as aromatic and aliphatic amines and phenol and its derivatives.12-14 Of particular importance is the application of these carbons for production of electrodes in electrochemical capacitors in order to increase their capacity.15-18 The physicochemical properties of the nitrogen-enriched active carbons also depend on the amount and type of nitrogen species introduced into the carbon. Therefore, in order to obtain materials of different contents of nitrogen, the process of nitrogen-enrichment is performed with different agents, including ammonia, urea, or amines, and using different variants of thermal and pressure treatment.9,10,19-22 A very effective method of nitrogen-enrichment is ammoxidation.15,19,20 It is the process of simultaneous oxidation and nitrogenation of the coal leading to significant changes in its chemical structure, which can be enhanced or reduced depending on the further thermal treatment. The surface oxygen and nitrogen groups formed in the process of ammoxidation, upon further pyrolysis, can evolve to the species stronger bound to the parent coal structures or can be removed, which significantly changes the chemical character of the material modified. (11) Biniak, S.; Pakuła, M.; Szyman´ski, G. S.; S´wia˛tkowski, A. Langmuir 1999, 15, 6117–6122. (12) Koh, M.; Nakijima, T. Carbon 2000, 38, 1947–1954. (13) El-Sayed, Y.; Bandosz, T. J. Langmuir 2005, 21, 1282–1289. (14) Przepio´rski, J. J. Hazardous Mater. 2006, B 135, 453–456. (15) Jurewicz, K.; Babeł, K.; Ziółkowski, A.; Wachowska, A. Electrochim. Acta 2003, 48, 1491–1498. (16) Frac˛kowiak, E.; Lota, G.; Machnikowski, J.; Vix-Guterl, C.; Beguin, F. Electrochim. Acta 2006, 51, 2209–2214. (17) Lota, G.; Lota, K.; Frac˛kowiak, E. Electrochem. Commun. 2007, 9, 1828–1832. (18) Kodama, M.; Yamashita, J.; Soneda, Y.; Hatori, H.; Kamegawa, K. Carbon 2007, 45, 1105–1107. (19) Jansen, R. J. J.; Bekkum, B. Carbon 1994, 32, 1507–1516. (20) Pietrzak, R.; Wachowska, H.; Nowicki, P.; Babeł, K. Fuel Process. Technol. 2007, 88, 409–415. (21) Burg, P.; Fydrych, P.; Cagniant, D.; Nanse, G.; Bimer, J.; Jankowska, A. Carbon 2002, 40, 1521–1531. (22) Pietrzak, R.; Wachowska, H.; Nowicki, P. Energy Fuels 2006, 20, 1275–1280.
10.1021/ef800251w CCC: $40.75 2008 American Chemical Society Published on Web 10/15/2008
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Table 1. Characteristics of Raw and Demineralised Coal (wt %) coal
moisture
ashd
VMdaf
Cdaf
Hdaf
Ndaf
Sdaf
Odafa
raw demineralized
12.5 0.0
28.9 0.7
53.6 50.2
63.4 65.1
4.9 4.5
0.7 0.7
2.0 1.9
29.0 27.8
a
By difference.
In the hitherto studies, nitrogen-enriched active carbon was obtained by ammoxidation of coal and activated by the physical (thermal) method by oxygen, steam, or carbon dioxide. Much less attention has been paid to the activation by the chemical method with the use of KOH, ZnCl2, or H3PO4. The main aim of this study was to obtain nitrogen-enriched active carbons by ammoxidation and chemical activation of brown coal by KOH and to compare the properties of the active carbons obtained by the physical and chemical methods. 2. Experimental Section Samples. The starting raw sample was prepared from a Polish brown coal (the Konin mine). It was milled and sieved to the grain size of 0.2 mm. All measurements were carried out on samples demineralized (D) by concentrated hydrochloric and hydrofluoric acids according to the Radmacher and Mohrhauer method23 (Table 1). Carbonization. The carbonization (C) was carried out in a horizontal furnace under a stream of argon with a flow rate of 170 mL/min to about 40% burnoff. The samples were heated (5 °C/ min) from room temperature to the final carbonization temperature of 700 °C. In the final temperature, samples were kept 1 h and then cooled in inert atmosphere. Carbonization was applied to the precursor (DC) and samples obtained by ammoxidation of the precursor at 300 °C (DN1C) and 350 °C (DN2C). Activation. Carbonization products were subjected to activation by steam (AS) or by KOH (AK). Activation by steam was performed at 850 °C in a laboratory furnace for 60 min to about 50% burnoff. Water was fed by two microfeeding pumps, the steam leaving the reactor was directed to the cooler in which it was liquefied and the gases formed in the reaction after passing the cooler were combusted in a gas burner. Activation by KOH was preformed at 700 °C with an alkali/carbon weight ratio of 4/1 for 45 min in argon atmosphere (flow rate 330 mL/min) to about 50% burnoff for samples ammoxidized after carbonization and to about 70% burnoff for samples ammoxidized before carbonization. The obtained active carbons were washed first with 5% solution of HCl and later with distilled water until free of chloride ions. The washed active carbons were dried at 110 °C for 24 h. Incorporation of Nitrogen. Ammoxidation (N) was applied to precursor, carbonization product, or active carbons. This process was performed with the mixture of ammonia and air at a ratio of 1:3 at 300 °C (N1) or 350 °C (N2) for 5 h. Weight loss upon ammoxidation process was about 40 wt % for precursor, about 20 wt % for char, and about 10 wt % for activated carbon. The active carbon samples were prepared by the above procedures, differing in the order of the technological processes: (a) ammoxidation of the precursor followed by carbonization and activation (DNCAS and DNCAK), (b) ammoxidation after carbonization (pyrolysis) (DCNAS and DCNAK), and (c) ammoxidation after activation (DCASN and DCAKN). The nonammoxidized samples (DCAS and DCAK) served as references. Analytical Procedures. The elemental analysis (C, H, N, S) of the products obtained at each stage of the processing were performed on an elemental analyzer CHNS Vario EL III (Elementar Analysensysteme GmbH, Germany). The surface oxide functional groups were determined by the Boehm method.24,25 (23) Radmacher, W.; Mohrhauer, O. Brennstoff-Chemie 1956, 37, 353– 358. (24) Boehm, H. P.; Diehl, E.; Heck, W.; Sappok, R. Angew. Chem. Int. Ed. Engl. 1964, 3, 669–677.
Characterization of the pore structure of activated carbons was performed on the ground of low-temperature nitrogen adsorptiondesorption isotherms measured on a sorptometer ASAP 2010, manufactured by Micrometrics Instrument Corp. (USA). Before the isotherm measurements samples were outgassed at 300 °C for 10 h, surface area and pore size distribution were calculated by BET and BJH methods, respectively. Total pore volume and average pore diameter were determined as well. Micropore volume and micropore surface area were calculated using t-plot method.
3. Results and Discussion Ammoxidation of the Precursor. Demineralized coal was subjected to ammoxidation at 300 °C (N1) or 350 °C (N2), carbonization of the nitrogen-enriched samples at 700 °C (C), followed by their activation by steam at 850 °C (AS) or by KOH at 700 °C (AK). Results of elemental analysis of the samples obtained are presented in Table 2. When brown coal is the precursor, the difference in temperatures of 50 °C is significant for the process. Brown coal shows a low thermal stability (much lower than bituminous coals), and a temperature close to 350 °C is also near its decomposition temperature. Therefore, an increase in the reaction temperature from 300 to 350 °C causes a significant increase in the precursor susceptibility to modifications, which is reflected by the results presented in our work. Analysis of data presented in Table 2 shows that the process of ammoxidation leads to very important changes in the elemental composition of demineralized coal and to the introduction of a great amount of nitrogen functional groups into the coal structure, accompanied by a considerable decrease in the content of hydrogen, oxygen, and carbon. The amount of the nitrogen introduced has been also found to depend on the temperature of the process. The content of nitrogen is much higher for the sample ammoxidized at 350 °C (DN2) than in that treated at 300 °C (DN1). At temperatures close to 350 °C, ammonia can be oxidized to a considerable degree, but only in the presence of catalysts, for example, metallic ones. When a mixture of air-ammonia is used to introduce nitrogen into carbon, this process is unlikely. At the temperature applied, ammonia reacts with oxygen-containing functional groups formed as a result of coal and/or active carbon oxidation. A significant decrease in the content of oxygen can be a result of nitrogen-building through the numerous oxygen functional groups present on the surface of the demineralized coal and partial degassing of brown coal taking part during the ammoxidation, especially at 350 °C. As a result of carbonization of coal subjected to the ammoxidation at the stage of precursor (DN1C and DN2C), the content of carbon significantly increases at the expense of decreasing content of the other elements, in particular nitrogen. Changes in the elemental composition of coal are mainly induced by high temperature (700 °C). Upon heating, the least-stable fragments of the coal structure (methylene, oxygen, and sulfur bridges) break, leading to formation of side products of carbonization rich in hydrogen, such as water, hydrogen sulfide, and hydrocarbons. The considerable decrease in the nitrogen content is probably a consequence of low thermal stability of nitrogen groups built into the coal structure upon ammoxidation. A large number of these groups probably decompose or undergo transformation to more thermally stable nitrogen species, for example, pyrrolic, pyridonic, or quaternary nitrogen.26 Analysis of the data collected in Table 2 has shown that the temperature of the ammoxidation of the demineralized coal also (25) Boehm, H. P. Carbon 1994, 32, 759–769. (26) Cagniant, D.; Gruber, R.; Boudou, J. P.; Bilem, C.; Bimer, J.; Sałbut, P. D. Energy Fuels 1998, 12, 672–681.
Nitrogen-enriched ActiVated Carbons from Brown Coal
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Table 2. Elemental Analysis of the Coal Samples Subjected to Ammoxidation Followed by Carbonization and Activation (wt %) Cdaf
Hdaf
Ndaf
Sdaf
Odaf a
sample
before
after
before
after
before
after
before
after
before
after
DN1 DN2 DN1C DN2C DN1CAS DN2CAS DN1CAK DN2CAK
65.1 65.1 59.7 60.0 76.7 75.1 76.7 75.1
59.7 60.0 76.7 75.1 87.8 87.3 87.3 85.7
4.5 4.5 2.4 2.2 1.3 1.3 1.3 1.3
2.4 2.2 1.3 1.3 1.1 1.1 0.9 0.9
0.7 0.7 20.8 23.4 14.7 16.7 14.7 16.7
20.8 23.4 14.7 16.7 2.8 3.7 2.6 3.8
1.9 1.9 1.9 1.9 0.9 0.8 0.9 0.8
1.9 1.9 0.9 0.8 0.4 0.4 0.2 0.2
27.8 27.8 15.2 12.5 6.4 6.1 6.4 6.1
15.2 12.5 6.4 6.1 7.9 7.5 9.0 9.4
a
By difference.
Table 3. Elemental Analysis of Coal Samples Subjected to Carbonization Followed by Ammoxidation and Activation (wt %) Cdaf
Hdaf
Ndaf
Sdaf
Odaf a
sample
before
after
before
after
before
after
before
after
before
after
DC DCN1 DCN2 DCN1AS DCN2AS DCN1AK DCN2AK
65.1 93.3 93.3 86.9 83.2 86.9 83.2
93.3 86.9 83.2 94.5 87.2 90.5 89.1
4.5 2.2 2.2 1.6 1.6 1.6 1.6
2.2 1.6 1.6 1.0 1.5 0.6 0.5
0.7 1.0 1.0 4.3 7.5 4.3 7.5
1.0 4.3 7.5 1.8 1.5 0.8 2.1
1.9 1.5 1.5 1.5 1.5 1.5 1.5
1.5 1.5 1.5 0.2 0.2 0.0 0.0
27.8 2.0 2.0 5.7 6.2 5.7 6.2
2.0 5.7 6.2 2.5 9.6 8.1 8.3
a
By difference.
affects the changes in the content of nitrogen on carbonization. The char DN2C obtained from the coal ammoxidized at 350 °C shows a greater content of Ndaf than the samples ammoxidized at 300 °C DN1C. This difference can follows from the fact that upon ammoxidation at a higher temperature, nitrogen is built into deeper layers of the coal structure, and because of this it is more resistant to the effect of temperature during carbonization. The activation of the ammoxidized and then carbonized samples (DNCAS and DNCAK) either by steam or by KOH brings further increase in the content of carbon in these samples, accompanied by a decrease in the content of nitrogen and hydrogen. The content of oxygen also increases, which suggests that these samples are oxidized in the process of activation, in particular upon activation with KOH. Such a drastic decrease in the content of nitrogen upon activation confirms the earlier observation that the nitrogen species built into the demineralized coal are thermally unstable. Ammoxidation of the Product of Carbonization. The second part of the study included the carbonization at 700 °C of the coal demineralized, followed by ammoxidation of the char DC at 300 or 350 °C, and by activation of the ammoxidized chars by KOH or steam. The elemental composition of the samples obtained is given in Table 3. The great increase in the content of Cdaf is probably caused by a considerable loss of the volatile matter in the process of carbonization. The small increase in the content of nitrogen, observed upon carbonization of brown coal, can be explained by decomposition of the less thermally stable fragments of the coal structure exposed to high temperature, leading to an increase in the percent contribution of nitrogen in the char. This fact indicates that the initial coal (D) contains only the thermally stable nitrogen species. Further analysis of the results given in Table 3 brings a conclusion that the effectiveness of the ammoxidation applied at the stage of char is much lower than that of the same process at the stage of the precursor modification. The content of nitrogen in the DCN1 sample is almost five times lower than in the DN1 sample (20.8 wt %), ammoxidized at the stage of demineralised coal. Such a drastic decrease in the efficiency of ammoxidation at the stage of char is most probably due to changes in the coal structure taking place upon carbonization at a high temperature. The processes of coal pyrolysis are
accompanied by polymerization, polycondensation, and aromatization of the coal substance, leading to increased ordering of the coal structure and a significant decrease in the reactivity of the carbon materials. As follows from a comparison of the content of nitrogen in samples DCN1 and DCN2, ammoxidised at 300 and 350 °C, respectively, the amount of nitrogen built into the char structure significantly depends on the temperature of ammoxidation. Almost twice as much nitrogen species were built into the char structure upon its ammoxidation at 350 °C (DCN2) than at 300 °C (DCN1). The process of ammoxidation applied at the stage of the char also significantly changes the contents of carbon, hydrogen, and oxygen. The considerable decrease in the content of carbon is a result of the incorporation of nitrogen species into the coal structure and the oxidation of the char by the air-ammonia mixture, as indicated by a significant increase in the content of oxygen in DCN1 and DCN2 samples relative to that in DC sample. The ammoxidized chars were subsequently activated by steam at 850 °C (DCNAS) and by KOH at 700 °C (DCNAK). As follows from the elemental analysis of the activated carbon samples obtained (Table 3), upon activation the majority of nitrogen groups built into the char structure undergo decomposition, irrespective of the activating agent. This fact proves a very low thermal stability of the nitrogen species generated in the process of ammoxidation and their low resistance to alkalies. The activation of the ammoxidized chars also leads to an increase in the content of carbon in all samples and to a decrease in the content of hydrogen, in particular in the samples activated with KOH. The majority of the active carbon samples (except DCN1AS) also show a slightly higher content of oxygen than the corresponding chars. Ammoxidation of the Active Carbon. In the third stage of the study, the demineralized coal was at first carbonized and then activated, and the active carbon samples obtainedsDCAS and DCAKswere subjected to ammoxidation at 300 or 350 °C. The elemental compositions of the samples are given in Table 4. According to the results, the activation of the char DC with steam gives the active carbon samples of much different elemental composition than that of the samples activated with KOH. The activation with steam results in an increase in the
4136 Energy & Fuels, Vol. 22, No. 6, 2008
Nowicki et al.
Table 4. Elemental Analysis of Coal Subjected to Carbonization Followed by Activation and Ammoxidation (wt %) Cdaf
Hdaf
Ndaf
Sdaf
Odaf a
sample
before
after
before
after
before
after
before
after
before
after
DCAS DCAK DCASN1 DCASN2 DCAKN1 DCAKN2
93.3 93.3 97.6 97.6 91.3 91.3
97.6 91.3 84.6 86.3 87.3 85.5
2.2 2.2 0.7 0.7 0.3 0.3
0.7 0.3 1.0 1.0 0.6 0.7
1.0 1.0 0.4 0.4 0.5 0.5
0.4 0.5 4.3 5.7 5.1 6.5
1.5 1.5 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0
2.0 2.0 1.3 1.3 7.9 7.9
1.3 7.9 10.1 7.0 7.0 7.3
a
By difference.
Table 5. Porous Structure of the Samples Obtained by Physical Activation sample
total surface area (BET) [m2/g]
micropore area [m2/g]
total pore volume [cm3/g]
micropore volume [cm3/g]
Vm/ VT
average pore diameter [nm]
DN1CAS DN2CAS DCN1AS DCN2AS DCAS DCASN1 DCASN2
839 1005 826 909 664 604 660
802 937 762 844 613 554 595
0.43 0.54 0.47 0.50 0.35 0.32 0.36
0.37 0.43 0.35 0.39 0.28 0.26 0.28
0.86 0.80 0.74 0.78 0.80 0.81 0.78
2.0 2.1 2.3 2.2 2.1 2.1 2.2
Table 6. Porous Structure of the Samples Obtained by Chemical Activation sample
total surface area (BET) [m2/g]
micropore area[m2/g]
total pore volume [cm3/g]
micropore volume [cm3/g]
Vm / VT
average pore diameter [nm]
DN1CAK DN2CAK DCN1AK DCN2AK DCAK DCAKN1 DCAKN2
3181 2922 2292 2616 2156 2018 1877
3090 2852 2252 2569 2117 1982 1846
1.92 1.72 1.11 1.29 1.05 0.98 0.91
1.76 1.60 1.05 1.21 0.99 0.92 0.86
0.92 0.93 0.95 0.94 0.94 0.94 0.95
2.42 2.36 1.94 1.97 1.95 1.94 1.94
carbon content relative to the initial char, accompanied by decrease in the contents of the other elements. The activation with KOH gives samples with much lower contents of Cdaf, Hdaf, and Ndaf than in the DC char, but with much higher content of oxygen. This result implies that upon activation with KOH the char undergoes significant oxidation. According to literature data,27 oxygen content in the active carbon significantly increases upon activation with KOH. This increase is a result of KOH decomposition and the reaction of K2O with the carbon, which is consistent with the mechanism of coal gasification upon the activation with alkalies. Therefore, KOH can cause a greater extent of oxidation of carbon than steam. As follows from Table 4, the process of ammoxidation applied at the stage of active carbon (DCAS and DCAK) also leads to considerable changes in the elemental composition of the samples obtained. First of all, it causes a decrease in the content of carbon, which is more profound in the sample activated with steam (DCAS); an increase in the content of nitrogen built into the bulk or on the surface of the active carbon; and an increase in the content of hydrogen. The effectiveness of the process of ammoxidation applied to active carbon samples at 300 °C (DCAKN1 and DCASN1) is similar to that applied to char DCN1, whereas at 350 °C (DCAKN2 and DCASN2) the amounts of nitrogen introduced into the active carbon are by 1-2 wt % lower than in the DCN2 sample, subjected to ammoxidation at the stage of char (Table 3). Similarly as upon the ammoxidation of the demineralized coal (D) (Table 2) and the char (DC) (Table 3), the amount of nitrogen groups introduced into active carbon significantly depends on the temperature of the process. The content of nitrogen is much higher in the samples ammoxidized at 350 °C (DCAKN2, DCASN2) than in those ammoxidized at 300 °C (DCAKN1, DCASN1). The efficiency of the ammoxidation applied at the stage of active carbon also depends on the (27) Otowa, T.; Nojima, Y.; Miyazaki, T. Carbon 1997, 35, 1315–1319.
type of the activation. The amount of nitrogen introduced into the active carbon sample activated with KOH (DCAKN) is by about 1 wt % higher than in the analogous sample (DCASN) activated with steam. Also of interest is the different character of changes in the content of oxygen upon the ammoxidation of the active carbons DCAKN and DCASN. The sample obtained by ammoxidation of the carbon activated with KOH (DCAKN) shows lower content of oxygen than the active carbon not subjected to ammoxidation. This observation can be explained by the supposition that upon the ammoxidation some amount of nitrogen is built into the active carbon structure through the oxygen functional groups on its surface. A different tendency was observed for the sample obtained by ammoxidation of the carbon activated with steam (DCASN); its content of oxygen is much higher than in the active carbon not subjected to ammoxidation. These results suggest that the active carbon DCAS obtained by activation with steam is not only enriched in nitrogen but is also, to a significant degree, oxidized. Textural Studies of Active Carbons. The textural parameters of the active carbon samples were determined from the measurements of the low-temperature nitrogen adsorption, performed on a sorptometer ASAP 2010. The results presented in Tables 5 and 6 indicate that the textural parameters of the active carbon samples depend on the type of the activating agent and on the conditions of thermal treatment and modifications of coal. The samples activated with KOH show much more developed surface areas (1877-3181 m2/g) and much greater pore volumes (0.91-1.92 cm3/g) (Table 6) than those activated with steam, which have surface areas of 604-1005 m2/g and total pore volume varying from 0.32 to 0.54 cm3/g (Table 5). This result proves that KOH is a much more effective activating reagent than steam. Analysis of the data presented in Tables 5 and 6 also shows the two possible effects of the process of
Nitrogen-enriched ActiVated Carbons from Brown Coal
ammoxidation on the porous structure of carbon. The active carbons ammoxidized at the stage of precursor (DNCAK and DNCAS) or char (DCNAK and DNCAS) have a much more developed porous structure than those not subjected to the mixture of air-ammonia (DCAK and DCAS). The opposite effect, that is, deterioration of the textural parameters, was observed for samples DCAKN and DCASN, subjected to ammoxidation after activation. The changes are more pronounced for the coals subjected to ammoxidation at 350 °C, both in the samples with improved properties (DNCAS, DCNAK, and DCNAS) and in those with deteriorated properties (DCAKN). The exceptions are samples DNCAK and DCASN. The reason for such a strong development of the porous structure of the active carbons ammoxidized before activation is probably increased reactivity of the modified precursors and chars, following from the presence of great amounts of nitrogen and oxygen groups introduced upon ammoxidation, which can additionally activate coal samples. The beneficial effect of ammoxidation applied before activation is even stronger in the samples ammoxidized at the stage of precursor (DNCAK and DNCAS), whose surface areas are much greater (even by 1025 m2/g, as in, e.g., DN1CAK) than those of the active carbons not subjected to ammoxidation. The most probable reason is that the chars DN1C and DN2C, obtained from the nitrogenenriched precursors, are more reactive toward the activating agent (KOH or steam) than samples DCN1 and DCN2 ammoxidized after carbonization, as the former contain much more oxygen and nitrogen bound to the coal matrix. As mentioned above, a significant amount of nitrogen groups introduced upon ammoxidation undergo decomposition, whereas the others are transformed to more thermally stable species or are built into deeper layers of the coal structure. Upon activation, these species react with the activating agent and undergo decomposition (as indicated by a drastic decrease in the content of nitrogen), facilitating the activating agent penetration into the deeper layers of the char, which leads to a greater development of the porous structure. On the basis of the literature data,28 it is known that ammoxidation leads first of all to formation of nitrogen species of the amine, amide, and nitrile types. As a result of the exposure to high temperature, the majority of these groups decompose and are transformed into the groups of the types N-5, N-6, and N-Q, characterized by greater thermal resistance. In the samples ammoxidized after carbonization (DCN), the majority of nitrogen species are built into the surface layers of the char because of a more ordered and stable structure of the coal matrix. Upon activation they can hinder the access of the activating agent to the surface of coal, which would lead to gasification of the surface layers of the char only, and the development of the porous structure is less effective. The deterioration of the textural parameters of samples DCAKN and DCASN, relative to those not subjected to ammoxidation (DCAK and DCAS), can be a consequence of oxidation of the active carbon surface upon contact with the air-ammonia mixture, leading to destruction of walls between pores or to pore-blocking by the numerous nitrogen species introduced on ammoxidation. Analysis of the data from Table 4 (elemental composition of samples ammoxidized at the stage of active carbon) and Tables 5 and 6 suggests that the second of the above reasons is more probable because of a significant amount of nitrogen (4.3-6.5 wt %) introduced into active carbons on ammoxidation and no increase in the average pore diameter after the ammoxidation. The data from Tables 5 and 6 also imply (28) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Carbon 1995, 33, 1641–1653.
Energy & Fuels, Vol. 22, No. 6, 2008 4137 Table 7. Acid-Base Properties of the Modified Carbons sample
total content of surface oxides [mmol/g]
acidic groups [mmol/g]
basic groups [mmol/g]
DN1CAS DN2CAS DCN1AS DCN2AS DCAS DCASN1 DCASN2
1.98 1.77 1.92 1.73 0.80 0.88 1.00
0.54 0.74 0.20 0.39 0.20 0.39 0.30
1.44 1.03 1.72 1.34 0.60 0.49 0.70
DN1CAK DN2CAK DCN1AK DCN2AK DCAK DCAKN1 DCAKN2
2.70 2.75 2.16 2.63 2.06 2.00 2.19
1.86 1.98 1.42 1.74 1.36 0.97 0.97
0.84 0.77 0.74 0.89 0.70 1.03 1.22
that the activation of already ammoxidized coal brings active carbon samples of microporous structure. Much more microporous character was seen in the samples obtained by activation with KOH, in which the contribution of micropores reaches from 92 to 95% of the total pore volume, whereas in the samples activated with steam the contribution of micropores reaches from 74 to 86%. Moreover, the latter samples have greater average diameter of the pores than the samples activated with KOH, except the samples DN1CAK and DN2CAK, showing the greatest pore diameters of all active carbon samples obtained. Acid-Base Properties of Modified Active Carbons. To get the information on the surface properties of the active carbon samples obtained, the content of the oxygen functional groups of acidic and basic character was determined by the Boehm method. As follows from Table 7, the active carbon samples obtained have a considerable amount of surface oxygen groups besides the nitrogen species. The important effect on the content and type of the surface oxides generated on the active carbon surfaces has the method of activation. The carbons activated with KOH have much more oxygen functional groups on the surface than those activated with steam. The differences are most probably consequences of the fact that upon activation with KOH the processes of the surface oxidation are more intense than upon activation with steam, as suggested by a greater content of oxygen in the former samples (Tables 2 and 3). The active carbon samples obtained by activation with KOH have a much greater content of the surface groups of acidic character and much lower content of the groups of basic character than the samples activated with steam. These differences are probably a consequence of the different chemical character of the activating agents and different thermal conditions of the activation. The activation with steam was performed at 850 °C, which is known to favor the formation of basic oxygen functional groups on the carbon surfaces. Another factor influencing the content of the oxygen functional groups is the sequence of the processes of ammoxidation, carbonization, and activation. The greatest amount of the oxygen species was found on the surface of the samples subjected to ammoxidation at the stage of precursor (DNCAK and DNCAS), whereas the lowest was on the samples modified after the process of activation (DCAKN and DCASN). The sequence of the three processes to which the demineralized coal is subjected also has a significant effect on the acid-base character of the active carbon samples obtained. The samples ammoxidized at the stage of precursor or char have great prevalence of acidic groups (when activated with KOH) or basic groups (when activated with steam), whereas the samples modified at the stage of active carbon have an
4138 Energy & Fuels, Vol. 22, No. 6, 2008
intermediate acidic-basic character of the surface, irrespective of the method of activation. Some influence on the type and content of oxygen groups has been found to have the temperature of ammoxidation, see Table 7. The samples subjected to ammoxidation at 350 °C at the stage of precursor or char contain a little more functional groups of acidic character and fewer groups of basic character than the samples ammoxidized at 300 °C. The opposite relation has been observed for the samples ammoxidized after the process of activation. For such samples the higher temperature of ammoxidation favors the formation of a greater number of functional groups of basic character and lower (DCASN) or the same (DCAKN) number of functional groups of acidic character. 4. Conclusions The above-discussed results permit drawing a few important conclusions on the preparation and physicochemical properties of active carbons enriched in nitrogen. Substantial effects on the amount of nitrogen built into the coal structure have two factors: the temperature of ammoxidation and the stage at which this process is applied. The processes of carbonization and activation of the ammoxidized coal, in particular that of
Nowicki et al.
activation with KOH, lead to active carbons of very well developed surface area and pore structure, with the dominant contribution of micropores. Unfortunately, these processes also significantly reduce the content of nitrogen in the samples. Our results have also shown that the enrichment of brown coal in nitrogen at different stages of production of active carbon together with different methods of activation leads to obtaining samples whose surfaces can have different chemical character. The steam activation of the samples ammoxidised at the stage of precursor or char gives the carbon samples with surfaces of basic character, while the analogous samples activated with KOH have surfaces of acidic character. The samples subjected to ammoxidation at the stage of the active carbon have surfaces of an intermediate acidic-basic character, irrespective of the activation agent used. Acknowledgment. This work was supported by The Polish Ministry of Science and Higher Education project No. N204 043 32/0906. The authors are grateful to Dr. hab. Krzysztof Babeł from Institute of Chemical Wood Technology, Poznan´ University of Life Sciences for assistance at preparation of some active carbon samples. EF800251W
