Study on the mechanism of free radical scavenger TEMPO blocking in coal oxidation chain reaction

The inhibitory effect of free radical scavenger 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) on the spontaneous combustion process of coal has been experimentally confirmed. However, the mechanism of free radical trapping in coal by TEMPO is not clear. This paper revealed the mechanism of free radical capture in coal by TEMPO from three perspectives by quantum chemistry method, including capture radicals directly, regenerative cycle chain and hydrogen transfer. It was regarded as a beginning that the formation of free radicals (R-CH2∙, CH3∙) by breaking alkane groups in the side chain of the coal molecule. The oxidation process of these free radicals was analyzed and the capturing models of radicals were described. The thermodynamic data for reactions (R2 ∼ R5) are calculated to reveal the initial reaction process of coal. The adsorption energy and thermodynamic data for reactions (Process1 ∼ 4, R6 ∼ R10, R12 ∼ R13) are computed to illustrate spontaneity and difficulty of traps free radicals by TEMPO. The results indicate that the initial reaction (R2, R4) of free radicals with oxygen in coal is exothermic and some other reactions (R3, R5) need to overcome large reaction energy barriers. TEMPO can capture free radicals in coal directly, reducing the concentration of free radicals, and is most effective in capturing ∙OH and alkyl radicals. At the same time, TEMPO can capture peroxide radicals and alkyl radicals in coal through regenerative cycle chains and keep reborn to maintain the efficiency of its capture. For coals under acidic conditions, TEMPO can bind to H+ first, then transfers hydrogen with oxygen-containing radicals in the coal. This process is prone to occur due to the presence of hydrogen bonds. In future applications, the pH value of TEMPO can be appropriately reduced to enhance the effect of capturing free radicals in coal.