Chemistry, Physics and Technology of Surface

ISSN 2518-1238 (Online), ISSN 2079-1704 (Print)

The aim of the work is to establish the effect of temperature on the porous structure characteristics of thermolysis solids (TS) prepared from brown coal (BC) during a novel process - alkaline activation with thermal shock. The BC sample is the Alexandria deposit coal, demineralized to ash content 0.5±0.1 % by treatment with HCl and HF acids. The elemental composition of the organic coal substance is as follows (%): C 70.6, H 5.9, S 3.6, N 1.9, O 18.0 (by difference). The treatment of BC with an alkaline activator (KOH) was performed by impregnation; the mass ratio of KOH/coal is 1.0. The preparation of TS was carried out in argon in three successive stages: 1) rapid introduction of the sample into the reactor, preheated to the temperature of thermal shock t_TS, varied in the interval t_TS = 400–800 °С; 2) isothermal holding at t_TS (1 h); 3) cooling, washing from alkali compounds and drying. Based on low-temperature (77 K) nitrogen adsorption-desorption isotherms (Micromeritics ASAP 2020), there were determined pore size distributions, total volume (V_t, cm³/g) and surface (S, m²/g) of adsorbing pores, volumes of macro- (V_ma) , meso- (V_mе) and micropores (V_mi), as well as micropores with a diameter of D≤1 nm (V_1nm). The temperature dependences of these characteristics are obtained. An increase in the t_TS temperature was found to result in the forming TS with increasing specific surface areas from 14.7 (400 °C) to 1947 m²/g (800 °C): half S is formed in a narrow interval t_TS=700–800 °C. The yield of TS is reduced from 67 to 25 %. The V_t value increases by a factor of 7.2 times (from 0.124 to 0.892 cm³/g), the volumes of mesopores and macropores increase equally - 2.9 times. The main growth of V_t volume is due to micropores: their volume V_mi increases from 0 to 0.547 cm³/g, the contribution of micropores with D≤1 nm becomes dominant (84–98 %) at t_TS=600–800 °C. Pores with D≤5 nm were found to develop most dynamically under combined effect of KOH and thermal shock. Pore size distribution is characterized by three maxima: dV₁ for micropores with D≤1 nm, dV₂ for micropores with D=1–2 nm, dV₃ for mesopores with D=3–5 nm. The dependence of dV₁ on temperature t_TS is strictly exponential (R²=0.988), that allows us to calculate the parameter E(V1), which characterizes the effect of temperature on the increase in the volume of micropores with D≤1 nm. It has the dimension of the "classical" activation energy and is 56.1 kJ/mol. As t_TS values increase, the dV₂ maximum value decreases by a factor of 22, and the pore diameter shifts from 1.85 to 1.39 nm. Values of dV₃ are an order of magnitude lower than dV₁ and approximately replicate the dependence of dV₁-t_TS in the interval of 400–750 °C. An increase in the thermal-shock temperature is concluded to promote the micropores formation (especially pores with D≤1 nm), which is limited by the diffusion of the activator (KOH or K atoms as the products of K⁺ ion reduction) within the forming three-dimensional framework of carbonaceous solids.

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