A study of the phenylacetylene oxidative carbonylation reaction in oscillatory and non-oscillatory modes

Abstract

The palladium-catalysed phenylacetylene oxidative carbonylation (PCPOC) reaction operates in both oscillatory and non-oscillatory modes. In oscillatory mode oscillations have been observed in pH, reaction heat (Qr), redox potential and gas uptake. This work documents the steps towards understanding the process and uncovering the reaction mechanism. An extensive experimental study was undertaken involving small-scale (25 mL) experiments on the catalytic system and large-scale (450 mL) experiments on the whole reaction system. The large-scale experiments on the whole system included studies at temperatures from 0-40 °C. The effect of increasing the concentration of water in the system by increasing the solvent volume ratio of water from 0% to 40% was also studied. Decreasing the reaction temperature is known to affect the period and amplitude of the pH oscillations but this work found that it also changes the selectivity of the reaction: the major product is dimethyl (2Z)-2-phenyl-2-butenedioate at 40 °C whereas at 0 °C the major product is 5,5-dimethoxy-3-phenyl-2(5H)-furanone. Increasing the concentration of water in the system affected product selectivity and the oscillatory pH behaviour. As the water concentration increased, pH behaviour changed: the regular shark fin waveform observed when no water was added to the system gradually became step-wise oscillatory behaviour. The small-scale experiments uncovered the link between the mono-carbonylation of phenylacetylene and the generation of H+. They also showed the autocatalytic nature of the reaction between the PdI2 catalyst and carbon monoxide as well as the involvement of water in H+ generation. Based on the experimental findings a tentative model was produced using BatchCAD which accounted for the key features of the observed pH behaviour. The modelling study showed the need for autocatalysis in the model producing the best fit when HI catalysed the reaction between PdI2, CO and H2O.