Add time:07/29/2019         Source:sciencedirect.com

This work studied the kinetics of copper-catalyzed decomposition of tetraphenylborate, triphenylborane, diphenylborinic acid and phenylboronic acid (NaTPB, 3PB, 2PB and 1PB, respectively) in aqueous alkaline solution over the temperature range of 25 to 70°C. The statistically designed test matrices added copper sulfate to maximum concentrations of 10 mg/l. The relative rates of decomposition increase in the order NaTPB < 1PB ∼ 3PB < 2PB. Dependence of decomposition on the amount of added copper increases in the order 3PB∼2PB<1PB∼NaTPB. Activation energies ranged from 82 to 143 kJ mol−1 over the temperature range studied. Final decomposition products involved benzene and phenol predominately. All 3PB, 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 temperature (23–25°C) with no added copper. Measurable (>10−7 M h−1) thermal hydrolysis of the phenylborate species occurs at 55 to 70°C in alkaline (0.6–2.3 M OH−, 2–4.7 M Na+) solution with no added copper. The experiments suggest an important role for oxygen in copper-catalyzed phenylborate decomposition. NaTPB decomposes promptly under anoxic conditions while 3PB, 2PB and 1PB decompose faster in aerobic solutions. Benzene and phenol form as the predominant end-products from alkaline copper catalysis in static systems sealed under air. Both 2PB and 1PB decompose with near equal rates and quantitatively produce phenol under flowing air-purge conditions at 25–60°C. Mechanisms for copper-catalyzed phenylborate decomposition likely involve a redox process giving loss of a phenyl group from the phenylborate with reduction of cupric ion, or dephenylation by reduced cuprous ion involving a phenylated copper intermediate.

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