The loss of Cu/ZnO/Al 2 O 3 activity for methanol synthesis has been studied over 50 h under a range of synthesis gas compositions. Experiments have been carried out at differential and finite conversions in order to compare deactivation under the pure feed to that in the presence of products. Under differential conversion, it was found that the degree of deactivation was related to the CO concentration, being particularly acute under CO/H 2 . Under CO/H 2 , there was a close correlation between activity loss and loss of Cu surface area, suggesting that an important cause of deactivation is sintering, which may be promoted by over-reduction of Cu. At finite conversion with CO/H 2 feed the deactivation was less severe. This lower rate of deactivation is most likely due to exposure of the catalyst to products that can stabilize oxidized Cu species. Interestingly, under CO 2 /H 2 , the deactivation was negligible at differential conversion. However some deactivation was apparent at finite conversion. The exposure of the catalyst to the products, CO and water, at finite conversion can explain these results.
Detailed kinetic experiments of higher alcohol synthesis over K-promoted CuO/ZnO/ AI2O3 catalysts have been performed using an internal recycle reactor operated at 4 MPa and between 548 and 578 K. It was found that a 0.5% K-promoted catalyst, a stoichiometric H2/CO ratio (approximately 2), and a low (2%) CO2 concentration were favorable for isobutanol yield. In contrast, for the synthesis of methanol, a higher K loading, H2/CO ratio, and CO2 concentration were found to be favorable. The results suggest that isobutanol synthesis requires a balanced level of oxidation (Cu°/Cu1+), as confirmed by in situ X-ray photoelectron spectroscopy, while methanol synthesis requires a greater degree of oxidation (more Cu1+). Contact time experiments showed the selectivity of aldehydes to pass through a maximum, suggesting the intermediacy of aldehydes in a classic "aldol" condensation mechanism. In extended time-on-stream experiments, initial deactivation was accounted for by a loss in BET surface area and a loss in Cu/Zn ratio. Greater deactivation at higher temperatures resulted in lower final activities; nevertheless steady-state isobutanol yields increased significantly at higher temperatures.
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