FREE-RADICAL CHAIN REACTIONS IN RADIATION-SENSITIZED PYROLYSIS OF ETHANOL VAPOR

The study of pyrolysis reactions can be assisted by use of the radiation sensitization technique. In this way the over-all kinetics are simplified and information can be obtained about the chain-propagation reactions. At temperatures above 300° in the radiolysis of ethanol vapor, freeradical chain reactions account for most of the decomposition of ethanol. The mechanisms were tested by the addition of propylene (freeradical scavenger), sulfur hexafluoride (electron scavenger), and ammonia (proton scavenger). Between 300 and 375°, at pressures in the range 102-103 torr, three modes of decomposition occur to similar extents. The stoichiometric equations are: I, C2H5OH → H2 + CH3CHO; II, C2H5OH → CH4 + CH2O; III, C2H5OH → C2H4 + H2O. The activation energies, in kilocalories/mole, and orders of the respective rate-limiting chain propagation reactions at 350° and 300-500 torr are: CH3CHOH → CH3CHO + H, ≥30,1.2; CH3CH2O→ CH3 + CH2O, 20, 1.9; CH2CH2OH → C2H4 + OH, 27, 1.5. These unimolecular reactions are in their pressure-dependent regions under the present experimental conditions. As the temperature of the system is increased, the relative extents of occurrence of the three mechanisms becomes I > III > II at about 420°, and the difference between their contributions increases with further increase in temperature. This conclusion also applies to the normal pyrolysis system. Based upon k(e- + (C2H5OH)mH+) = 1 × 10−6 cm3/(ion sec), the value k(e− + SF6) = 3 × 10−12 cm3/(ion sec) was obtained. © 1968, American Chemical Society. All rights reserved.