Potassium hydroxide influence on the structure and surface area development of brown coal under alkali activation

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 L_c, the average diameter of polyarene (graphene) layer L_a, the volume V_cr and the average number of layers N_cr. 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 (ω_t) for the thermolysis temperature t = 300, 700, 800 °C.

The relationships of BCI properties (d₀₀₂, L_c, L_a, V_кр N_кр, [e], ω₃₀₀, ω₇₀₀, ω₈₀₀,) as well as yield (Y) and specific surface area (S_BET) of асtivated carbons versus the alkali/coal ratio R_KOH≤15 mmol/g were obtained. They identified two regions: I (R_KOH≤6 mmol/g) and II (R_KOH>6 mmol/g).

In region I, the R_KOH increase enhances the degree of substitution of acid groups protons by K⁺-ions (with full replacement at R_KOH=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 R_KOH~2 mmol/g (43 % → 49 % → 39 %), the S_BET changes on a curve with a minimum (210 → 120 → 730 m²/g).

In region II, the R_KOH 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 S_BET surface grows linearly (from 730 to about 970 m²/g).

The KOH capability to develop the surface was proposed to evaluate by the coefficient K_EF=∆S_BET/∆R_KOH defining the S_BET increment with increasing alkali quantities. The K_EF values were determined to be significant (200–300 m²/mmol) within R_KOH=2.5–4.0 mmol/g. Catalysis of KOH-initiated pore-forming reactions in this R_KOH 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.