The detailed kinetics and mechanism of ethyl ethanoate synthesis over a Cu/Cr2O3 catalyst

Ethyl ethanoate is produced industrially in a process that came on-stream in March 2003 using liquid ethanol as the feed. The catalyst was Cu/Cr2O3, and the reactor temperature was 473 K. The selectivity to ethyl ethanoate was 95%. Detailed kinetic and mechanistic studies of this process were conducted in a multipurpose microreactor system, using an on-line mass spectrometer controlled and interrogated by computer. Isothermal frontal gas adsorption chromatography experiments using ethanol as the adsorbate in the temperature range 300-473 K found that a H2 pulse was detected immediately after the ethanol contacts the catalyst. The fact that H2 evolved from the Cu/Cr2O3 catalyst at 300 K indicates that the dehydrogenative adsorption of ethanol to form an adsorbed ethoxy species [(CH3CH2O)(a)] occurred on the Cu component of the catalyst. A plot of the logarithm of the amount of H2 evolved on adsorption against the reciprocal of the adsorption temperature gives an activation energy to adsorption of 31 kJ mol−1. Temperature-programmed studies on the adsorbed ethoxy species showed that some of these ethoxy species dehydrogenated at 425 K with a surface activation energy of 94 kJ mol−1 to form the adsorbed acetyl species (CH3CO)(a). The adsorbed ethoxy and acetyl species combined on the surface to form adsorbed ethyl ethanoate. The ethyl ethanoate desorbed at a peak maximum temperature of 680 K. Ammonia temperature-programmed desorption experiments produced a desorption peak for NH3 at 680 K, revealing the existence of a strong Brönsted acid site on the surface of the Cr2O3. Thus the ethyl ethanoate desorbing at 680 K was bonded to the Cu/Cr2O3. Temperature-programmed reaction (TPR) studies, in which ethanol was passed continuously over the Cu/Cr2O3 catalyst while raising the temperature linearly with time and monitoring the products of reaction continuously on the mass spectrometer, gave an overall activation energy of 92 kJ mol−1 for ethyl ethanoate formation, the same as that for the dehydrogenation of the adsorbed ethoxy species to form an adsorbed acetyl, suggesting that this might be the rate-determining step for the reaction. TPR studies of ethanol over unsupported polycrystalline Cu gave identical results to those of the Cu/Cr2O3 catalyst, confirming that Cu is the active component of the catalyst.