Hydrolysis of diphenylether in supercritical water: Effects of dissolved NaCl

Dilute solutions of NaCl in supercritical water (SCW), in concentrations up from 0 to 3.1 wt.% have unexpected influence on the rate of hydrolysis of diphenylether (DPE). The rate at 430°C and at a SCW density of 0.46 g/cc showed a sharp decrease with incremental addition of salt, down from the value measured at zero salt. But the rate increased again with continued addition of salt and reached a value almost twice that of the zero-salt rate when the salt concentration was 3.1 wt.%. The decrease of the rate is attributed to the formation of the ion pairs H+ Cl−; the excess of Cl− ions that prevail from ionic (partial) dissociation of NaCl capture the protons generated by self-dissociation of water, according calculations of the relevant ionic dissociation equilibria. The formation of protonated DPE, as a critical intermediate species in the chemistry of DPE hydrolysis according the postulated SN1 mechanism, competes with this ion pair for protons and is consequently restrained. At the higher salt content a new mechanism predominates. This mechanism is rationalised with Lewis acid/base behaviour of Na+ and Cl− ions in SCW solution. Charge transfer in the water cluster surrounding these ions supposedly generates H+ and OH− ions at the outer hydration shells of the clusters and form the active species for DPE hydrolysis. The rate of hydrolysis, according this Lewis acid/base theory is proportional to the square root of the salt concentration in the SCW reaction medium and this was indeed corroborated by experiments.