HIGH-TEMPERATURE THERMAL PHOTOLYTIC OXIDATION OF MONOCHLOROBENZENE

It has recently been demonstrated that the rate of many photolytic reactions can be increased by more than an order of magnitude by raising the temperature of the reaction system. Conversely, it has also been shown that the rate of many high-temperature, gas-phase oxidation and pyrolysis reactions may be increased by initiating thermally assisted photochemical reactions with concentrated solar radiation, intense ultraviolet arc lamps, or high-powered laser systems. This general phenomenon of 'high-temperature photochemistry' has many potential applications including hazardous waste destruction, chemical synthesis, and materials processing. Initial laboratory studies using simulated, broad-band solar radiation (filtered xenon arc emission) in conjunction with a thermoelectrically heated flow reactor have clearly shown that the photochemical destruction rates of many toxic organic compounds can be significantly increased and the production of stable reaction intermediates reduced by increasing the system temperature. More detailed studies of the thermal and thermal-photolytic oxidation of monochlorobenzene have been performed using a Nd:YAG pumped, pulsed-dye system, lasing at 280 nm with an intensity of 44.2 mJ cm-2 per pulse as the photoexcitation source. The pseudo-first-order rate constant for the gas-phase photodecomposition of chlorobenzene increased from 0.0737 s-1 to 0.837 s-1 when the temperature was raised from 300-degrees-C to 700-degrees-C. This results in a thermal-photolytic destruction efficiency which is 4300 times higher than the thermal efficiency over the same temperature range. Although some products were common to both the thermal and thermal-photolytic exposures, a startling change in mechanism was observed as the organic product distribution was shifted from substituted benzenes and naphthalenes for the thermal reaction to cyclic esters and anhydrides for the thermal-photolytic reaction. This change in mechanism and rate of reaction is attributed to a combination of a thermally induced increase in the rate of photon absorption, an increase in the quantum yield of the primary photoreaction, and initiation of photo-induced radical-chain reactions.