Abstract
A two step gas phase catalytic cycle for the dehydration of acetic acid to ketene was detected by multistage mass spectrometry experiments. The catalysts are Group VI mononuclear [MO 3(OH)] − and binuclear [M 2O 6(OH)] − oxo-anions (M=Mo, W), formed via electrospray ionization. These catalytic cycles proceed via: (i) initial condensation of acetic acid to form [MO 3(OCOCH 3)] − or [M 2O 6(OCOCH 3)] −, respectively, with elimination of water; and (ii) collisional activation of [MO 3(OCOCH 3)] − or [M 2O 6(OCOCH 3)] − to eliminate ketene, CH 2CO, and reform [MO 3(OH)] − or [M 2O 6(OH)] −, respectively. In contrast to the molybdenum and tungsten congeners, the analogous chromium systems are unreactive towards acetic acid in the gas phase. The chromium acetate complexes (formed by adding trace amounts of acetic acid to the electrospray solution) undergo a variety of different fragmentation reactions, with a considerable reduction in selectivity towards ketene compared with the molybdenum and tungsten congeners. The reactivity of various oxo-anions was quantified by examining the kinetics for condensation reactions as well as the kinetic isotope effect and threshold activation energy for ketene elimination.
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