ISSN 10634576, Journal of Superhard Materials, 2015, Vol. 37, No. 1, pp. 39–43. © Allerton Press, Inc., 2015. Original Ukranian Text © K.I. Veselovs’ka, V.L. Veselovs’kyi, O.M. Zaderko, V.E. Diyuk, O.V. Ishchenko, 2015, published in Sverkhtverdye Materialy, 2015, Vol. 37, No. 1, pp. 51–56.
PRODUCTION, STRUCTURE, PROPERTIES
Effect of the Oxidation and Thermal Treatment on Bromination of Activated Carbon K. I. Veselovs’ka, V. L. Veselovs’kyi*, O. M. Zaderko, V. E. Diyuk, and O. V. Ishchenko Shevchenko Kiev National University, vul. Volodymyrs’ka 64, Kiev, 01601 Ukraine *email: Veselovs’
[email protected] Received March 20, 2014
Abstract—It has been shown that the same active surface sites of carbon matrix take part in the oxidation and bromination of activated carbon. The thermal treatment of oxidized samples of activated carbon has been defined to partially or completely restore the reactivity of the surface by removing oxygencontaining functional groups. It has been found that just the number and not the nature of the removed groups deter mines the increment of the surface reactivity of activated carbon. The thermal treatment of oxidized sam ples of activated carbons at 800°C virtually completely restores their reactivity to the addition of bromine. DOI: 10.3103/S1063457615010062 Keywords: activated carbon, gaseousphase bromination, liquidphase bromination, surface modification, oxidation, thermal treatment.
1. INTRODUCTION Activated carbon (AC) is a material, which is characterized by a large specific surface and developed struc ture. The use of activated carbon in the processes of adsorption and catalysis requires a purposeful change of its chemical properties, which essentially depend on the nature and concentration of surface functional groups [1–4]. These groups either form in the course of producing AC or they may be formed by chemical modifica tion of activated carbon. At present the most universal method to modify AC is the bromination (halogena tion) of its surface and subsequent substitution of various functional groups for bromine [5–7]. Such an approach is efficient in producing various classes of derivatives, namely: amines, mercaptans, oxygencon taining compounds, etc. The bromination as a method of the AC functionalization has the following disadvantages: low output of the graft bromine and essential oxidation of the AC surface. To establish the prerequisites to the introduction of the maximum quantity of bromine, in this work we studied the effect of the AC oxidation and thermal treat ment on the AC modification capability. 2. EXPERIMENTAL For experiments we used carbon of the SKN grade (spherical nitrogencontaining activated carbon), whose specific surface SBET = 1100 m2/g and total volume of pores VS = 0.41 cm3/g. The SKN samples were oxidized by nitric acid of the corresponding concentration (5 and 15 wt %) on heating for 2 h with a subsequent careful washing with a lot of distilled water to pH of flushing water 5.5 and drying at 120°C for 2 h. The oxidized samples are denoted as SKN(5) and SKN(15). The thermal treatment was conducted at 300, 500, and 800°C for 1 h in an argon flow, whose rate was 40 cm3/min. By the thermal treatment certain types of oxygencontaining functional surface groups were removed from the AC surface. To compare the efficiency of bromine joining, we performed the liquid–phase and gaseousphase bromi nations. Brominated were the initial, oxidated, oxidated and thermal treated SKN samples. The liquid–phase bromination was performed in a water solution that contained the Br2·KBr complex for 1 h at room temperature with a subsequent careful washing with a solution of sodium oxalate to remove phys ically sorbed bromine and distilled water to achieve a negative response to the existence of Br– ions in flushing water. The gaseousphase bromination was carried out by Br2 vapor in an argon flow at isothermal conditions at temperatures of 300 and 500°C with a gravimetric control of the reaction proceeding. The rate of the Ar flow 39
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(gas carrier) was 40 cm3/min, bromine concentration in the flow was 8.7 torr (10–3 mole/l), which was attained by the Ar saturation with bromine vapor at 0°C. The weight of AC was 50 mg. The bromine content of samples under study was defined by the chemical analysis, i.e., reverse titration by the Folgard method [8]. A sample was transformed into a soluble form by burning in a NaNO3–NaOH solu tion. Thermodesorption studies were carried out using the thermogravimetric analysis (TGA) and thermal programmed desorption with IR– spectrometric registration of products (TPDIR). A sample was heated in argon in the 30–800°C temperature range at a rate of 10°C/min. Simultaneously with the registration of a change in weight we defined the gaseous products of desorption (CO and CO2) using a spectrometer. The con centration of oxygencontaining groups in the solution was found by the Boehm method. 3. RESULTS AND DISCUSSION According to the TGA and TPDIR data on the initial and oxidated SKN (Fig. 1), the oxidation causes an essential weight loss in the studied temperature range. The liberation of CO2 and CO increases, which is indic ative of the formation of functional surface groups of different types in oxidation of SKN. The titration by the Boehm method showed that the oxidation of SKN regardless of the HNO3 concentra tion results in the formation of groups of all types (Table 1). With increasing the HNO3 concentration, the concentrations of all types of functional groups also increase and in this case on the carbon surface phenol groups are in the greatest amount, carboxyl groups are in a somewhat smaller amount, and anhydride and lac ton groups are in the smallest amount. It should be noted that in oxidation the concentration of carboxyl groups increases by a factor of 3, phenol groups by a factor of 2, and anhydride and lacton groups appear in amounts to 0.18 mmole/g. 4
m, g/g dm/dT × 10 , g/°C
r × 10 , mol/min
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0
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0
0.25 –0.02
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–1
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–0.04
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4
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–3 0
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400 (a)
r × 10 , mol/min
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T, °C
800 4
m, g/g dm/dT × 10 , g/°C 0
600
1
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–0.02 2
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0.30
–0.08
0.20
–0.10
3
–0.12 –2
4
–0.14
0.10 0
400 600 800 T, °C (b) Fig. 1. Temperature dependences of the weight changes in the integral (1) and differential (2) forms, and the rate of the removal of CO2 (3) and CO (4) in the samples of SKN (a), SKN(5) (b), SKN(15) (c). 0
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m, g/g dm/dT × 10 , g/°C
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–0.20 –4
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0 0
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400 (c)
600
800
T, °C
Fig. 1. (Contd.)
Table 1. Concentrations of oxygencontaining groups on the surface of SKN samples defined by the Boehm method Sample SKN SKN(5) SKN(15)
carboxyl 0.12 0.34 0.38
Concentration of oxygencontaining groups, mmole/g anhydride and lacton phenol total concentration of groups ~0 0.49 0.61 0.16 0.87 1.37 0.18 1.04 1.60
The liquid–phase bromination of SKN, SKN(5), and SKN(15) showed that the joining of bromine to oxi dized surface is slight (Table 2). The treatment even with a diluted HNO3 solution (5 wt %) results in a virtually complete loss of the SKN surface ability for bromine joining, which is caused by the oxidation of active cen ters. Table 2. Bromine content (mmole/g) of samples, produced as a result of the SKN thermal treatment and a subsequent liquid or gaseousphase bromination Sample
25
SKN SKN(5) SKN(15)
0.41 0.07 0
Bromine content, mmole/g at the thermal treatment temperature,°C 300 500 800 300 500 Liquid–phase bromination Gaseousphase bromination 0.42 0.42 0.45 2.33 2.26 0.22 0.24 0.38 1.55 1.75 0.25 0.29 0.43 1.30 1.49
The SKN(5) and SKN(15) samples were thermally treated according to the temperature intervals of the disintegration of different types of oxygencontaining functional groups. Carboxyl groups, which are the least stable on the AC surface, decompose to liberate CO2 in the 150–400°C temperature range (see Fig. 1) with the maximum at Tmax = 250–300°C. Anhydride and lacton groups are more stable and decompose to liberate a mixture of CO2 and CO in the range of 400–700°C with a maximum at Tmax = 500–550. Phenol groups, which are most stable AC surface groups of the acid nature, decompose to liberate CO at temperatures above 600°C. Taking into account the above data, to analyze the effect of the thermal treatment, we chose three tem peratures, at which the desorption of corresponding types of functional groups takes place: 300, 500, and 800°C. The results of the bromination of the SKN oxidized samples after thermal treatment at different tem peratures are given in Table 2. It is evident that as the temperature increases, the reactivity of the AC surface to join bromine increases as well. The thermal treatment at 300°C and removing carboxyl groups bring about the regeneration of 0.22–0.25 mmole/g active centers, which make up more than half of centers of the initial SKN. The highest temperature of the thermal treatment (800°C) and removal of carboxyl, anhydride, lacton, and phenol groups promotes virtually complete restoration of the reaction ability of the SKN surface to bro JOURNAL OF SUPERHARD MATERIALS
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mination. These regularities are more evident for SKN(15), which points to the possibility of the activity res toration in the case of a heavy oxidation of the AC surface. It should be noted that such regularity verifies the assumption about the regeneration of SKN active centers as a result of the thermodesorption of surface func tional groups. Thus, a higher effect of the increment of the joint bromine amount in samples of SKN(15) as compared with SKN(5) is caused by a greater amount of oxygencontaining groups (see Table 1) that are des orbed at the thermal treatment. According to the data gathered by the Boehm method (see Table 1), we evaluated the influence of the func tional groups nature on the regeneration of AC active centers. Figure 2 shows the dependences between the amounts of oxygencontaining functional groups removed by the thermal treatment and an increase of the grafted bromine concentration for SKN(5) and SKN(15). These dependences are linear, which not only sup port the regeneration of active centres during the desorption of functional groups, but prove that the nature of groups does not affect the amount of active centers. Thus, the thermal treatment allows us to regenerate active centers on the surface of SKN samples, which were previously oxidized. The temperature of the thermal treat ment should be chosen only according to the concentrations of oxygencontaining groups, which will be des orbed under the chosen conditions.
ΔCBr, mmole/g
0.5 0.4 2 0.3 1 0.2 0.1
0
0.25
0.50
0.75
1.00
1.25
1.50
1.75
–ΔCfg, mmole/g Fig. 2. Correlation dependences between the amount of removed functional oxygencontaining groups and increasing con centration of grafted bromine for SKN(5) (R = 0.998) (1) and SKN(15) (R = 0.988) (2).
Kinetic dependences of gaseousphase bromination of SKN, SKN(5), and SKN(15) at isothermal condi tions and temperatures 300 and 500°C are given in Fig. 3. The greatest amounts of grafted bromine at gaseous phase bromination of SKN are given in Table 2. It should be noted that the greatest amount of the joint bro mine is observed for the initial SKN: 18.4 and 18.1 wt % (2.33 and 2.26 mmole/g) at temperatures of 300 and 500°C, respectively. Gaseousphase bromination of SKN(5) and SKN(15) results in joining of the lower amount of bromine: 10–14 wt % (1.30–1.75 mmole/g). The amount of the grafted bromine increases with the Δm, g/g
Δm, g/g
0.20
0.20 1
1 0.16
0.16
2 2
0.12
3
0.12
3
0.08
0.08
0.04
0.04
0 20 40 60 80 100 t, min 60 80 100 120 t, min (b) (a) Fig. 3. Kinetic curves of gaseousphase bromination of the SKN initial (1) and oxidized samples SKN(5) (2), SKN(15) (3) at 300 (a) and 500 (b) °C. 0
20
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bromination temperature, which is caused by the regeneration of the greater amount of centers as the bromi nation temperature increases. Regardless of the bromination temperature the amount of grafted bromine is greater in the SKN(5) than in the SKN(15), which is due to the formation of the greater amount of phenol groups and deactivation of the greater amount of SKN centers in oxidation of 15 wt % with HNO3 solution. 4. CONCLUSIONS It has been found that the same active surface centers take part in oxidation and bromination of SKN and the thermal treatment of oxidized SKN may partially or completely restore its reaction ability due to the removal of oxygencontaining functional groups. Reaction ability of SKN to join bromine has been defined to virtually completely restore as a result of the thermal treatment of oxidized samples at 800°C. The reaction ability of the SKN is defined by the amount of removed oxygencontaining functional groups and not by their nature. REFERENCES 1. Saha, B., Tai, M.N., and Streat, M., Study of activated carbon after oxidation and subsequent treatment character ization, Process Saf. Environ Protection, 2001, vol. 79, no. 4, pp. 211–217. 2. Stavropoulos, G.G., Samaras, P., and Sakellaropoulos, G.P., Effect of activated carbons modification on porosity, surface structure and phenol adsorption, J. Hazardous Mater., 2008, vol. 151, no. 2–3, pp. 414–421. 3. Xianglan, Zh., Shengfu, D., Qiong, L., Yan, Zh., and Lei, Ch., Surface functional groups and redox property of mod ified activated carbons, Mining Sci. Technol. (China), 2011, vol. 21, no. 2, pp. 181–184. 4. Ledesma, B., Román, S., ÁlvarezMurillo, A., Sabio, E., and González, J.F., Cyclic adsorption/thermal regenera tion of activated carbons, J. Anal. Appl. Pyrolysis, 2014, vol. 106, pp. 112–117. 5. Klimenko, I.V., Zhuravleva, T.S., Geskin, V.M., and Jawhary, T., Study of the bromination of pitchbased carbon fibers, Mater. Chem. Phys., 1998, vol. 56, pp. 14–20. 6. Barpanda, P., Fanchini, G., and Amatucci, G.G., Structure, surface morphology and electrochemical properties of brominated activated carbons, Carbon, 2011, vol. 49, no. 7, pp. 2538–2548. 7. Hanelt, S., Friedrich, J.F., OrtsGil, G., and MeyerPlath, A., Study of Lewis acid catalyzed chemical bromination and bromoalkylation of multiwalled carbon nanotubes, Ibid., 2012, vol. 50, no. 3, pp. 1373–1385. 8. Alekseevskii, E.V., Gol’ts, R.K., and Musakin, A.P., Quantitative analysis, Leningrad: Goskhimizdat, 1955. 9. Goertzen, S.L., Thériault, K.D., Oickle, A.M., Tarasuk, A.C., and Andreas, H.A., Standardization of the Boehm titration. Part I. CO2 expulsion and endpoint determination, Carbon, 2010, vol. 48, no. 4, pp. 1252–1261. 10. Oickle, A.M., Goertzen, S.L., Hopper, K.R., Abdalla, Y.O., and Andreas, H.A., Standardization of the Boehm titra tion. Part II. Method of agitation, effect of filtering and dilute titrant, Ibid., 2010, vol. 48, no. 12, pp. 3313–3322. Translated by G. Kostenchuk
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