Copper-catalyzed sodium tetraphenylborate, triphenylborane, diphenylborinic acid and phenylboronic acid decomposition kinetic studies in aqueous alkaline solutions

This work studied the kinetics of copper-catalyzed decomposition of tetraph enylborate, triphenylborane, diphenylborinic acid and phenylboronic acid (N aTPB, 3PB, 2PB and 1PB, respectively) in aqueous alkaline solution over the temperature range of 25 to 70 degrees C. The statistically designed test m atrices added copper sulfate to maximum concentrations of 10 mg/l. The rela tive rates of decomposition increase in the order NaTPB < 1PB similar to 3P B < 2PB. Dependence of decomposition on the amount of added copper increase s in the order 3PB similar to 2PB < 1PB similar to NaTPB. Activation energi es ranged from 82 to 143 kJ mol(-1) over the temperature range studied. Fin al decomposition products involved benzene and phenol predominately. All 3P B, 2PB and 1PB intermediate phenylborate species proved relatively stable ( < 8% decomposition over ca. 500 h) towards thermal hydrolysis in 1.5 M NaOH when contained in carbon-steel vessels sealed under air at ambient tempera ture (23-25 degrees C) with no added copper. Measurable (> 10(-7) M h(-1)) thermal hydrolysis of the phenylborate species occurs at 55 to 70 degrees C in alkaline (0.6-2.3 M OH-, 2-4.7 M Na+) solution with no added copper. Th e experiments suggest an important role for oxygen in copper-catalyzed phen ylborate decomposition. NaTPB decomposes promptly under anoxic conditions w hile 3PB. 2PB and 1PB decompose faster in aerobic solutions. Benzene and ph enol form as the predominant end-products from alkaline copper catalysis in static systems sealed under air. Both 2PB and 1PB decompose with near equa l rates and quantitatively produce phenol under flowing air-purge condition s at 25-60 degrees C. Mechanisms for copper-catalyzed phenylborate decompos ition likely involve a redox process giving loss of a phenyl group from the phenylborate with reduction of cupric ion, or dephenylation by reduced cup rous ion involving a phenylated copper intermediate. (C) 1999 Westinghouse Savannah River Company. Published by Elsevier Science S.A. All rights reser ved.