Mechanism of MnSOD removing peroxy radical for inhibiting coal spontaneous combustion

Abstract

• The pathways of free radical small and large cyclic chain reactions were analyzed; • The disproportionation reaction of MnSOD and peroxy radical was analyzed; • The mechanism of MnSOD scavenging peroxy radical was revealed; • The effect of MnSOD inhibiting coal spontaneous combustion was verified. Superoxide dismutase (MnSOD) was proposed as an inhibitor to inhibit coal spontaneous combustion (CSC). The pathways of free radical cyclic chain reaction were analysed. The mechanism of MnSOD removing peroxy radical (–OO·) was calculated by quantum chemical method, and the effect of MnSOD inhibiting CSC was verified by electron paramagnetic resonance experiment. Results showed that –OO· is the key to triggering free radical small and large cyclic chain reactions. The Mn ion is the active site of the MnSOD. MnSOD disproportionates –OO· in four steps: firstly, –OO· undergoes an addition reaction with MnSOD, and a new coordination bond –OO attaching to the Mn ion is generated; secondly, Mn 3+ is reduced to Mn 2+ by –OO· with the –OO bond broken to release O 2 ; thirdly, Mn 2+ is oxidised to Mn 3+ by –OO·, and a new coordination bond –OOH is formed with the reaction of H + and –OO·, which is attached to the Mn ion; lastly, H 2 O 2 is formed when H + reacts with the broken –OOH bond. In the first step, only covalent bonds are formed with no bond breaking and energy barriers. The energy barriers for the second to fourth reactions are 52.51, 78.77 and 183.79 kJ/mol, and the reaction rates are 3.848 × 10 5 , 9.601 and 3.760 × 10 −18 s −1 M −1 , respectively. In this four-step reaction, the heat release values are 131.28, –78.77, 472.59 and –210 kJ/mol. The –OO· is consumed during the reduction and oxidation reactions of Mn ions to generate O 2 and H 2 O 2 , respectively. The g value of the MnSOD coal sample decreases, the line width narrows, and the free radical concentration decreases, indicating that MnSOD can effectively inhibit CSC.