Zinc oxide-modified mordenite as an effective catalyst for the dehydrogenation of (bio)ethanol to acetaldehyde

Abstract

The direct transformation of ethanol to acetaldehyde is an important step in the cascade conversion of bioethanol to higher value chemicals and for the development of sustainable fuels. Herein, zinc oxide supported on alkali cation-exchanged mordenite (ZnO/M–MOR) prepared by a simple wetness impregnation method, is shown to be a selective and stable catalyst for the direct dehydrogenation of ethanol to acetaldehyde at 400 °C under continuous flow conditions. Through variation of the ZnO loading and the zeolite counter-cation (Na, K, Rb, Cs), an optimum catalyst material was identified, ZnO/Rb–MOR loaded at 3.5 wt% Zn. Acetaldehyde productivity (normalised to Zn) could be increased by over 80% and ethylene selectivity reduced to 0.9% through simple variation of the extra-framework alkali cation. Very low ethylene production leads to low levels of carbonaceous deposits and therefore minimal deactivation at short time on stream (<5 h). Detailed analysis of the optimized system reveals excellent selectivity and stability beyond 120 h time on stream, resulting in an average acetaldehyde productivity of 16 mmol g_cat⁻¹ h⁻¹ and overall acetaldehyde selectivity of 90% whilst operating at an ethanol conversion level of 40%. Additionally, the use of a zeolite support is shown to greatly improving the usage efficiency of Zn atoms by virtue of an acetaldehyde productivity increase from 20 to 48 mmol mmol_Zn⁻¹ h⁻¹ for unsupported and supported ZnO, respectively. The new catalyst system shows that ZnO can be tuned to give very low ethylene selectivity and extended lifetimes in ethanol dehydrogenation to acetaldehyde which has not previously been reported.