CC BY
19Journal of Siberian Federal University. Chemistry 1 (2012 5) 24-36
УДК 553.96:661.183
Conversion of Alexandriya Brown Coal Into Microporous Carbons Under Alkali Activation
Yuliya V. Tamarkina*, Tatyana G. Shendrik, Vladimir A. Kucherenko and Tatyana V. Khabarova
L.M. Litvinenko Institute of Physical-Organic and Coal Chemistry,
National Academy of Sciences of Ukraine, 70 R. Luxemburg st., Donetsk, 83114 Ukraine 1
Received 2.03.2012, received in revised form 9.03.2012, accepted 16.03.2012
We studied the pore structure ofactivated carbons (ACs) formed during the thermolysis ofAleksandriya brown coal (Ukraine), which was preliminary impregnated with alkali metal hydroxides, МОН with M being Li, Na, or K. We explored how pore structure parameters such as surface area, a total pore volume, meso- and micropore volume are modified with changing processing parameters such as temperature, an alkali/coal ratio (up to 2 g/g) and the nature of alkali cation. We also determined pore size distribution for meso- and micropores and compared all the three hydroxides as to their effect on porosity development. The formation of microporous structure is evidenced on heating a KOH-impregnated coal (1 g/g) from 400 to 800 0С: a surface area rises from 13 to 1005 m2/g, while a micropore volume fraction increases from 1 to 68 % of the total pore volume. Microporous carbon is formed at 800 0C over 1 h in a yield of 30 % with SBET=1005 m2/g, a total pore volume of 0.55 cc/g and a micropore volume of 0.38 cc/g. The total pore volume grows monotonously in the range LiОН<NaОН<KОН. Potassium hydroxide is found to promote microporosity development, whereas both LiOH and NaOH (at 18 mmol/g) act as inhibitors for micropore growth.
Keywords: brown coal, alkali activation, microporosity.
Introduction
Various raw materials, especially coals of different rank, are able to be transformed into activated carbons (ACs) by chemical activation with alkali hydroxides, MOH (M=Li, Na, K) [1]. At the present time, the studies of alkali activation focus on high rank coals, especially anthracites [2-9]. Anthracites may be converted into АСs with highly developed surface area (Sbet=2000-3000 m2/g), but this requires a great amount of alkali to be used (up to 7g per gram of coal [9]).
* Corresponding author E-mail address: [email protected]
1 © Siberian Federal University. All rights reserved
Earlier, extensive studies were conducted for brown coals, which display a higher reactivity toward alkalis [10-13]. Alkali activation of brown coal results in AC with a surface area about 1000 m2/g under a relatively moderate alkali/coal ratio (~1 g/g). Brown coals contain a great amount of oxygenated functionalities reacting with MOH at room temperature for both traditional techniques of MOH-coal mixture preparation, i.e. impregnation and simple mixing. Such reactions involve interactions between alkalis and OH-acidic groups (phenolic and carboxylic) [14], heterolysis of C-O bonds in esters with phenolates and carboxylates as products [15], and the generation of semiquinone anion radicals [16]. The reactions proceed along with the diffusion of M+, OH- and MOH species into coal, helping the fixation of M+ and OH- anions in the neighboring electron-donor and electron-acceptor sites within the carbon framework, and also favoring the formation of alkali humates. The products of such interactions (particularly humates) enable stronger binding action [10-12], allowing starting materials to be used as pelletized, extruded or other molds for chemical activation with no additional binder. A similar molding ability can be developed for higher-rank coals (e.g., bituminous ones [17]), but this requires preliminary coal oxidation to be conducted for building up oxygenated functionalities, which provides better coal reactivity toward MOH.
There are large deposits of brown coal in Ukraine (6-8409 tons according to the Ukrianian ministry of coal industry). The most important deposit is found in Aleksandryia (Kyrovograd region), with its capacity being evaluated to be 0.5409 tons, making it an important natural resource for producing activated carbons. Aleksandriya brown coal possesses all the above properties of brown coals. Besides, this coal is known to have a great content of montan wax (up to ~8 %). The last contains long-chain (C24-C30) carboxylic acid esters (~60 wt %) and free long-chain organic acids (~13 wt %) [18]. When impregnated or mixed with alkali, waxes are able to form fatty acid salts and high molecular weight alcohols distributed within the coal framework. Thus, such transformations may affect the properties of activated carbons.
This study is generally devoted to the investigation of Alexandriya brown coal alkali activation and the properties of derived microporous carbons. We believe this to be a new point since chemical activation of this coal has not been studied and the quality of resulting activated carbons is not clear.
At the present point in time, our work focuses on exploring the effect of activation parameters (temperature, the type and content of alkali) on the properties of carbons from alkali activated brown coal, with particular attention paid to their pore structure.
1. Experimental
1.1. Brown coal
A brown coal of 500 kg selected from the Aleksandriya deposit (Kyrovograd region, Ukraine) was averaged, ground, sieved, and air-dried. All the experiments were conducted using particles of 0.5-1.0 mm in size. Coal is characterized as follows (%): Wa 12,4; Ad 11,7; Vdaf 57,6; Cdaf 70,4; Hdaf 6,0; Sdaf 3,8; Ndaf 2,0; Odafdif17,8.
1.2. Coal impregnation
Brown coal was preliminarily dried at 105 0C for 2 h. A 100 g coal sample was mixed with a 30% aqueous alkali solution. The mixture was held for 24 h at room temperature and dried at 120 0C until
constant weight. The alkali solution was taken in an amount providing the required MOH/coal ratio (RMOH), expressed as grams or moles of MOH per g of dry coal. In a blank experiment, a coal sample was soaked with the same quantity of water as that of alkali solution, then held for 24 h and finally dried under the same conditions as the KOH-treated samples.
1.3. Heat treatment
The solid products (SP) of thermolysis were produced by heating the original or alkali impregnated coal samples in a vertical furnace of 0.2 dm3.
The samples were heat-treated under a 2 dm3/h argon flow as follows: 1) temperature-programmed heating with a rate of 4°C/min up to a temperature selected between 400 °C and 800°C; 2) holding the sample at a given temperature for 1 hour; 3) cooling down to room temperature. The heat-treated samples were washed off with distilled water, then with 0.1 M HCl, and once again with water until the negative response of Cl- ions against AgNO3. The washed samples were dried at 105±5°C.
The solid products derived from brown coal thermolysis are referred to as SP. The products produced using LiOH, NaOH, KOH are denoted as AC-Li, AC-Na and AC-K, respectively.
1.4. Characterization of pore structure
Pore structure was explored by the analysis of N2 adsorption/desorption isotherms for the samples outgassed at 200 °C for 20 h. The isotherms were recorded at 77 K using a Quantachrome Autosorb 6B instrument. The specific surface area SBET (m2/g) was determined from the BET analysis of the isotherm portion corresponding to a relative N2 pressure p/po below 0.3 [19]. The total pore volume VE (cm3/g) was determined using the amount of nitrogen adsorbed at p/po~1.
Mesopore size distribution and the volume of mesopores - Vme (cm3/g) were evaluated by the BJH method [20] using the desorption branch of N2 gas isotherms. The mesopore volume Vme was determined from the cumulative volume curves by accounting only for the volume of pores 2-50 nm in diameter.
The micropore volume Vmi (cm3/g) was determined using the Dubinin-Radushkevich method [21]. Micropore size distribution was evaluated using the DFT method [22]. The surface area SDFT and the volume of subnanopores V1nm (pores less than 1 nm) were assessed from the DFT cumulative curves for the specific surface and volume, correspondingly.
2. Results and discussion
2.1. Pore structure as function of temperature
It was established earlier that the optimum parameters for developing the pore structure of activated carbons includes isothermal exposure for 1 h at a KOH/coal ratio, RK0H=1 g/g [23]. With these parameters maintained, we studied the changes in AC properties as a function of processing temperature [24].
Yield. The yield of solid products decreases as temperature rises (Fig. 1), amounting to 44.4% for SP and 31% for AC-K at 800 0C. The lower yield in AC-K in comparison to SP is connected with a greater amount of volatile gaseous products produced as a result of the alkali destruction of C-C and C-0 bonds in the carbon framework, including those bonds related with cyclic esters and heterocycles [15].
120
100 80
s»
TJ 60 O
>" 40
20 0
0 200 400 600 800 1000 Temperature (oC)
Fig. 1. Changes in the SP and AC-K yield
• SP O AC-K
o*«a
r^ w |
°o0(
Changes in specific surface area for SP and AC-K products with increasing temperature are given in Table 1. At temperatures below 400oC, the specific surface area of parent coal equals SBET = 8-9 m2/g (as measured after drying at 120oC), whereas the SP products are formed with a lower specific surface area (5 m2/g). On the contrary, the AC-K products (KOH thermolysis) display an insignificant increase in Sbet, up to 13 m2/g. Between 400 oC and 500 oC, Sbet goes up abruptly and then increases continuously up to SBET=1005 m2/g at 800oC if KOH is present. By contrast, for the SP samples (prepared without potassium hydroxide), heating up to 600oC does not significantly alter the specific surface area (SBET=15 m2/g); further increasing temperature leads to a maximum of SBET = 272 m2/g at 700oC, with a little shift in SBET down to 235 m2/g at 800oC.
Total pore volume (V) does not change greatly up to 400oC for samples AC-K, but rises sharply if temperature is further increased up to 800 oC (Table 1). For temperatures between 400 and 800 oC, VE depends almost linearly on t and can be approximated as VE=0.0011t-0.3271 (R2=0.951). For temperatures between 400 oC and 500oC, no abrupt change is observed for the VE values, as distinct from the SBET values of AC-K.
KOH-free coal thermolysis does not induce a significant change in the pore volume up to temperatures of about 600oC, with VE=0.05-0.08 cm3/g. Further increasing temperature involves a larger pore volume of solid products, with their maximum values, VE=0.17-0.20 cm3/g for 750-800oC.
Potassium hydroxide promotes porosity development, with VE values increasing for samples AC-K over the whole temperature range. Between 400 and 600°C, alkali generates pores extensively, whereas this is not the case if no KOH is used. Strong porosity development is observable between 600-800°C for either SP or AC-K. However, in the presence of alkali, pore formation proceeds with more intensity, resulting in the products whose total pore volume is 3 times larger.
Mesoporosity. Mesoporosity is developed much better if KOH is present (Table 1). For AC-K products, a mesopore volume is progressively developed with maximum values being reached at 700oC (Vme=0,19±0,01 cm3/g). Higher temperature involves a Vme decay, although the total pore volume in fact increases (Table 1). A mesopore fraction, Vme/VE (Table 1), goes down from 0.58 to 0.22.
Table 1. Porosity characterization for samples SP and AC-K, produced at different temperatures
t, °C Sample SBETJ m2/g Vs, cm3/g Vmi cm3/g Vmi/ Vs Vme, cm3/g Vme/ Vs
120 SP AC-K1 8 0.048 0.003 0.06 0.028 0.58
400 SP AC-K 5 13 0.082 0.095 0.001 0.001 0.01 0.01 0.022 0.052 0.27 0.55
500 SP AC-K 8 273 0.070 0.281 0.003 0.079 0.05 0.28 0.018 0.123 0.26 0.44
550 SP AC-K 11 370 0.052 0.262 0.004 0.139 0.07 0.53 0.021 0.096 0.40 0.37
600 SP AC-K 15 450 0.077 0.384 0.005 0.167 0.06 0.43 0.019 0.162 0.25 0.42
650 SP AC-K 163 555 0.100 0.392 0.020 0.210 0.20 0.54 0.022 0.144 0.22 0.37
700 SP AC-K 272 695 0.152 0.496 0.068 0.265 0.44 0.53 0.030 0.186 0.20 0.38
750 SP AC-K 253 773 0.199 0.476 0.089 0.294 0.45 0.62 0.054 0.152 0.27 0.32
800 SP AC-K 235 1005 0.172 0.551 0.111 0.377 0.65 0.68 0.038 0.127 0.22 0.23
N.B. AC-K prepared at 120oC is not characterized as we were unable to remove KOH due to an intensive formation of potassium humates.
The SP materials show poorly developed mesoporosity. The mesopore volume of SP has low values, comprised within Vme = 0.02-0.03 cm3/g, and only does the SP material prepared at 750-800oC show a sensible increase in the mesopore volume with Vme = 0.04-0.05 cm3/g. A mesopore fraction also decreases for the SP samples (Table 1), equaling that of the KOH-made AC-K material at 800oC.
There are virtually no micropores for the SP-K products produced in the presence of KOH at t<400°C, and the SP samples do not show microporosity development up to 600oC. These products have a micropore volume less than 0.005 cm3/g. In other words, their porosity is almost exclusively made up of mesopores and macropores.
Microporosity undergoes strong formation in the presence of alkali between 400°C and 800°C (Table 1). Particularly, there is a specific temperature range (400°C to 600°C) associated with micropore formation in the KOH treatment case only. The SP products (no KOH involved) do not possess micropores at the same temperatures, suggesting that micropores are developed solely owing to interactions between KOH and the organic coal matter.
At higher temperatures, between 600 and 800°C, the micropores are developed without KOH, most likely due to a deeper thermal destruction and some secondary reactions of gaseous coal products with a carbon-coated solid residue. A maximum micropore volume is seen for the products prepared at
0,35 0,3 0,25 0,2 0,15 0,1 0,05 0
400 500 600 700 Temperature (oC)
0,6 0,5 0,4
w
^ 0,3
c
* 0,2 0,1 0
800
B AC-K
• . • /
• f SP
—O-o— 'o -,-
400 500 600 700 Temperature (oC)
800
Fig. 2. Temperature dependence for the volume (A) and the fraction (B) of subnanopores for samples SP and AC-K
800oC: Vmi = 0.338 cm3/gfor AC-K and 0.11 cm3/g for SP. In thepresence of alkali, the micropore-volume is 4 fimes larger, whereas the micropore fractions are close in values.
Subnanopores. We additionally determined the volume (Vinm) of pores less than 1 nm and its fraction (defined as V1nm/VE) as a function of temperature. We draw particular attention to subnanopores since they are of utmost importance to both high hydrogen adsorption capacity [25] and high electrochemical capacitance [26].
The subnanopore volume V1nm of activated carbons is greatly dependent on the activation conditions (Fig. 2, A). For temperature below 400oC, materials SP and AC-K do not contain subnanopores (V1nm = 0). When alkali is used, V1nm rises from 240"4 cm3/g at 400oC to a maximum value of 0.28 cm3/g at 800oC, which makes up 51% of the total pore volume or 74 % of the micropore volume (Fig. 2, B). For the SP materials produced without KOH, such pores are formed if activation is conducted at temperature above 600oC with V1nm = 0.07 cm3/g at 800oC.
The products formed at 800 0C have the most developed porosity. Therefore, a temperature of 800 0C was selected to probe nanoporosity development as a function of both the KOH/coal ratio and the nature of cation.
2.2. Effect of KOH/coal ratio
The effect of KOH/coal ratio is studied for the samples prepared at varied Rkoh (up to 2 g/g), all other parameters being held constant: a temperature of 800 0C, a heating rate of 4 0C/min, isothermal exposure at 800 0C for 1 h.
An AC-K yield exhibits an initial increase with increasing RKOH, which is obvious at low KOH/ coal ratios (RKOH<0.15 g/g) (Fig. 3). After reaching a maximum, a linear decrease in the AC-K yield is observed with growing RKOH.
SBET is decreasing on increasing RKOH up to RKOH=0.05-0.20 g/g, with an SBET minimum being reached at RKOH=0.05-0.20 g/g. An inverse relationship is observed between RKOH and SBET for the higher RKOH values (up to 0.8-1.0 g/g) (Fig. 3). Further increasing the alkali/coal ratio (RKOH=1-2 g/g) does not change the SBET value, which equals 980-1030 m2/g within experimental error.
N
E
U1
m W
1200 1000 800 600 400 200 0
0 0.5 1 1.5 2
-1>
KOH/coal ratio (g g-')
Fig. 3. The yield and specific surface area as a function of the KOH/coal ratio
A similar effect was observed earlier on the chemical activation of the same coal with sodium and lithium hydooxides [23]. At low RMOH values, Sbet goes down on incceasing an ¡alkali/coal oatio, but to a varied extent for different alkalis. Porosity development is enhanced in the range LiOH<NaOH<KOH. A yield of solid products decreases following the same alkali range.
Ar low RKOH, the increacing °ield and the decreasing specific surface area both provide evidence for the predominance of condensation processes, which are likely to be promoted by K+ ions or potassium metal, the latter being created due to a thermally initiated reduction of K+ ions by the electrons of the carbon framework [3, 7-9, 27]. Such condensation reactions were established on the heat-treatment of coal with NaOH [28] and polyarenes with KOH (e.g., the condensation of benzanthrone into violanthrone above 200oC [29]), giving rise to the formation of additional crosslinks and, finally, a more rigid spatial framework. This resulted in a yield increase of the solid product (Fig. 1), which is low-porosity carbon material, according to the values of SBET, VE, Vmi, Vmi/ VE and Smi/SBET-
Both further increase in specific surface area and decrease in yield are associated with the preponderance of alkali destruction reactions. These involve the heterolysis of C-O- and polarized C-C- bonds, the cleavage of O- and S-containing heterocycles [15], the dehydration of arene and alkane fragments of the carbon framework [3, 27]. All these interactions bring about low molecular weight products (such as H2, CH4, etc.). These reactions also decrease the yield of solid products, but they enable porosity growth, as evidenced by a substantial rise in the pore volume and specific surface area.
No change in the specific surface area and the fall in the AC-K yield for RKOH=1.0-2.0 g/g suggest that the preparation of activated carbons from the studied brown coal cannot be recommended at
Rkoh>1.0 g/g.
Fig. 4 shows the pore size distribution determined using the BJH method [20] for SP and AC-K. Potassium hydroxide favors an intensive growth of pores about 4 nm in diameter, with their differential volume (denoted as dV4nm/dD) exhibiting a complicated dependence on the KOH/coal ratio (Table 2).
0,3 0,25 0,2 0,15
g. 0,1
S 0,05
I
5 0
SP
AC- K(0.2) AC- K(0.3) -AC- K(1.0)
1 10
Pore diameter (nm)
Fig. 4. Mesopore size distribution (the BJH method)
100
Table 2. Porosity characterization for samples SP and AC-K, produced at different coal/KOH ratios
Rkoh, g/g SbetJ m2/g Vs, cm3/g Vmi cm3/g Vmi/ Vz Vme, zm31g Vme/ Vs dV4„m/dD, cm3/g
0 235 0.172 0.111 0.65 0.038 0.22 0.010
0.05 101 0.061 0 .037 0.61 0.006 0.1 <0.001
0.1 190 0./04 0.061 0.59 0.011 0.11 7.002
0.2 626 0.322 0.210 0.71 0.058 0.1 8 0.034
0.3 707 0.376 0.261 0.69 0.104 0.28 0.116
0.6 928 0.515 0.345 0.67 0.1741 0.334 0.206
1.0 1005 0.551 0.377 0.68 0.127 0.23 0.251
At rKOh^0.1 g/g, the value of dV4nm/dD goes first down and then up. The value of V4nm/VE is low for samples with VE<0.1 cm3/g; this value increases as porosity is more developed.
The effect of KOH on the micropore size distribution of solid products can be followed from the comparison between curves 4 (AC-K) and 1 (SP) at the figure below where the DFT pore size distribution is discussed.
2.3. Effect of the nature of cation
The effect of the nature of cation was assessed for the samples prepared under the same processing conditions (4 0C/min, 800 0C, 1 h) at an equal molar MOH/coal ratio. The equal molar alkali/coal ratios were adopted for the following reason. We suppose that the development of the pore structure is caused by the same reactions with different alkali metal hydroxides. In case different alkalis result in different porosities, this is caused by the nature of the cation. For KOH, an optimum ratio was 1 g/g, i.e. 18 mmol
Table 3. Porosity parameters for the samples prepared using different alkalis (800 0C, 4 0C/min, 1h, Rmoh =18 mmol/g)
SbetJ Ve, V v mi Vmi/ V v me; V / me V1nm, Vmm/ dV4nm/
m2/g m31g cm3/g Ve cm3/g Ve cm3/g Ve dD
SP 235 0.172 0.111 0.65 0.038 0.22 0.069 0.40 0.009
AC-Li 273 0.270 0.096 0.36 0.132 0.49 0.052 0.19 0.112
AC-Na 173 0.350 0.062 0.18 0.199 0.57 0.020 0.06 0.142
AC-K 1005 0.551 0.377 0.68 0.127 0.23 0.280 0.45 0.251
Cation diameter (nm)
Fig. 5. Pore structure parameters of SP, AC-Li, AC-Na, AC-K vs alkali cation diameter
of КОН per 1 gram of coal. The same molar ratio was adopted when LiOH and NaOH were used. Porosity parameters for the samples of activated carbons are summarized in Table 3.
In comparison to the SP material (i.e. prepared without alkali), the surface of alkali activated carbons rises slowly with LiOH as an activating agent and much more significantly if KOH is used. Sodium hydroxide taken in an amount of 18 mmol/g suppresses surface area development (Table 3).
A rise in the total pore volume (VE) is observed as the alkali cation diameter increases (DM) (Table. 3). The relation between VE and DM can satisfactorily be approximated as VE=5,6 DM 2 - 0,13 DM +0,174 (R2=0,996). The micropore volume (pores at D<2 nm) experiences a 2-times decrease in the range SP - AC-Li - AC-Na, whereas it rises greatly on going to AC-K, VE = 0.38 cm3/g (Table 3). A similar trend is seen for the micropore fraction (Vmi/VE): the adsorbent derived from parent coal has the greatest Vmi/VE value, close to that for AC-К. For AC-Na, the Vmi/VE value is about 4 times lower.
The microporosity of SP from parent coal is formed due to the interaction between the carbon framework and the thermolysis gases only. KOH is an agent most efficiently contributing to micropore formation (Fig. 5). Microporosity development is suppressed when LiOH and NaOH are used (at least with an MOH/coal ratio of 18 mmol/g). This is surprising since NaOH is known to be an efficient
- 32 -
Pore diameter (nm)
Fig. 6. DFT pore size distribution for AC-Na (1), AC-Li(2), SP (3), andAC-K (4))
activating agent [4, 6]. NaOH activation results in ACs with developed surface area, but at a large NaOH/coal ratio, usually 2-4 g/g. In our work, this ratio is essentially lower (RNaOH=0.72 g/g), and condensation processes may come about as those taking place under KOH activation at RKOH<0.1 r/r (Fig. 3).
The micropore decrease in the range SP - AC-Li - AC-Na is compensated by mesopore increase in the same range (Fig. 5). As a result, monotonous increase of total pore volume is observed.
According to the differential distribution curves calculated using the BJH method, the adsorbent from parent coal exhibits approximately uniform pore size distribution (Fig. 4). Mesopore size distribution for AC-Li h AC-Na corresponds to that for AC-K. In the alkali-made samples, the fraction of pores with D~4 nm rises sharply. The effect is seen Sor all the studied alkalis, and is progressirely stronger tn tee range AC-Li < AC-Na < AC-K (Table 3).
Micropore size distribution (pores with D<2 nm) is presented in Fig. 6. Lithium and sodium hydroxides do not induce the development of subnanoporne,s, which is also noticeable from the dependence between V1nm and cytion diameter (Fig. 5). The differential pore volume within D=1-2 nm has close values for SP, AC-Li h AC-Na. KOH develops micropaees of all sizes ane is the most eifiaient pore-forming agent. The values of total micropore volume calculated with the DFT method are (in cm3/g): 0.100 foa SP; 0.092 (AC-Li); 0.05S (AC-Na); 0.365 (AC-K). Thfse values are close to those determined using the DubininiRaduskemiah method (Table 3).
3. Conclusions
1. Aleksandriya brown coal can be transformed into microporous carbon using heat-treatment with potassium hydroxide at 800 0C for 1 h. Derived microporous carbon has a specific surface area of ~1000 m2/g, a total pore volume of ~0.55 cm3/g and a micropore volume of ~ 0.38 cm3/g.
2. The pore structure of KOH activated carbon is created at temperature 400-800оС. Increasing temperature induces an increase in SBET, from 13 to 1005 m2/g, whereas VE is increased from 0.095 cm3/g to 0.551 cm3/g, and Vmi from 0.001 cm3/g to 0.377 cm3/g.
3. Lithium and sodium hydroxides (at 18 mmol/g ratio) suppress the development of microporosity, which is compensated by the additional development of mesoporosity. As a result, the total pore volume grows monotonously in the range LiOH<NaOH<KOH.
4. Potassium hydroxide is the most efficient reagent for microporosity develoment in activated carbons made from Alexandriya brown coal.
Acknowledgments
This work was supported by the National Academy of Sciences of Ukraine (under contract 0107U010881). We thank N.N. Tsyba from the Institute for Sorption and Problems of Endoecology, NAS of Ukraine, for cooperation in the N2 adsorption study.
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Конверсия александрийского бурого угля в микропористый углеродный материал в условиях щелочной активации
Ю.В. Тамаркина, Т.Г.Шендрик, В.А. Кучеренко, Т.В. Хабарова
Институт физико-органической химии и углехимии им. Л.М. Литвиненко НАН Украины, Украина 83114, Донецк, ул. Р. Люксембург, 70
Была изучена пористая структура активированного угля (АУ), полученного термолизом бурого угля Александрийского месторождения (Украина), предварительно импрегнированного гидроксидом щелочного металла, МОН, где М = Li, Ш, К. Было изучено, как меняются удельная поверхность, суммарный объем пор, объемы мезо- и микропор с изменением параметров процесса, таких как температура, соотношение щелочь/уголь (до 2 г/г) и природа катиона. Также было установлено распределение мезо- и микропор по размерам и влияние разных щелочей на развитие пористости. Очевидно, что микропористая система развивается при нагревании (от 400 оС до 800 оС) угля, импрегнированного КОН: удельная поверхность возрастает от 13 до 1005 м2/г, тогда как доля микропор увеличивается с 1 до 68 % общего объема пор. Микропористый материал формируется при 800 оС за 1 ч с выходом 30 %, удельной поверхностью 1005 м2/г, общим объемом пор 0,55 см3/г и объемом микропор 0,38 см3/г. Суммарный объем пор растет монотонно в ряду LiОН<NaОН<KОН.
Ключевые слова: бурый уголь, щелочная активация, микропористость.
