Potassium hydroxide influence on the structure and surface area development of brown coal under alkali activation
Journal Title: Хімія, фізика та технологія поверхні - Year 2017, Vol 8, Issue 2
Abstract
This work was aimed on 1) evaluating alterations of spatial structure and paramagnetic properties of brown coal (BC) caused by KOH intercalated under alkali impregnation and 2) studying KOH influence on BC thermolysis and formation of porous carbons under alkali activation in argon (800 °C, 1 h). The spatial structure of BC impregnated (BCI) was characterized by X-ray diffraction as parameters of coal crystallites: the interlayer distance d002, height Lc, the average diameter of polyarene (graphene) layer La, the volume Vcr and the average number of layers Ncr. The unpaired electron concentration [е–] was measured by ESR. Thermolysis of BC and BCI was described by differential thermogravimetry (4 deg/min) which gave the rates of volatile products formation (ωt) for the thermolysis temperature t = 300, 700, 800 °C. The relationships of BCI properties (d002, Lc, La, Vкр Nкр, [e], ω300, ω700, ω800,) as well as yield (Y) and specific surface area (SBET) of асtivated carbons versus the alkali/coal ratio RKOH≤15 mmol/g were obtained. They identified two regions: I (RKOH≤6 mmol/g) and II (RKOH>6 mmol/g). In region I, the RKOH increase enhances the degree of substitution of acid groups protons by K+-ions (with full replacement at RKOH=6 mmol/g), slightly changes the spatial structure, increases the concentration of [e‾] (from 2.7∙1018 to 5.0∙1018 spin/g), linearly decreases the rate ω300 (from 0.12 to 0.03 mg/g∙s) due to alkali promotion of condensation reactions. They are revealed under alkaline activation (800 °С): the solids yield changes on a curve with a maximum at RKOH~2 mmol/g (43 % → 49 % → 39 %), the SBET changes on a curve with a minimum (210 → 120 → 730 m2/g). In region II, the RKOH increase enhances the coal crystallite volume (from 1.37 до 2.05 nm3) due to KOH intercalation and formation of additional crystallite layers (4 vs 3), linearly raises the ω300 rate (0.03 to 0.20 mg/g∙s) by increasing the contribution of C-O and C-C bonds heterolysis. The active carbons yield decreases (from 39 % to 33 %), the SBET surface grows linearly (from 730 to about 970 m2/g). The KOH capability to develop the surface was proposed to evaluate by the coefficient KEF=∆SBET/∆RKOH defining the SBET increment with increasing alkali quantities. The KEF values were determined to be significant (200–300 m2/mmol) within RKOH=2.5–4.0 mmol/g. Catalysis of KOH-initiated pore-forming reactions in this RKOH interval is presupposed to be a useful approach of porous structure development at low KOH/coal ratios that is technologically attractive due to decreasing volumes of reagents and alkaline waste water during activated carbon separation.
Authors and Affiliations
V. O. Kucherenko, Yu. V. Tamarkina, G. F. Rayenko
Keywords
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- EP ID EP247822
- DOI 10.15407/hftp08.02.133
- Views 97
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