Sustainable production of ethylene from bioethanol over hierarchical ZSM-5 nanosheets

Abstract

Hierarchical aluminosilicate nanosheets composed of an MFI structure with various Si/Al ratios have been successfully prepared via a one-pot hydrothermal process with the aid of tetrabutylphosphonium hydroxide (TBPOH) as the bifunctional structure-directing agent (SDA). The MFI zeolite nanosheets exhibit outstanding properties, such as an extremely high external surface area and appropriate acidic properties. To illustrate the beneficial effect of the hierarchical structure of the zeolite nanosheets towards green and sustainable catalytic conversion of renewable resources to high value-added chemicals, direct bioethanol dehydration to ethylene over Brønsted-acid MFI nanosheets has been studied from both experimental and theoretical points of view. Interestingly, the hierarchical structure strongly effects an increase in surface acid density at the external surfaces, eventually resulting in an improvement in ethylene selectivity. The results obtained from density functional theory (DFT) calculations also reveal that ethylene is possibly produced over the Brønsted acid sites located at the external surfaces, whereas diethyl ether (DEE) formation is the predominant pathway over the internal acid sites of MFI. From these findings, hierarchical ZSM-5 nanosheets consisting of a high fraction of active sites at the external silanol surfaces can significantly enhance the selectivity of ethylene from ethanol/bioethanol dehydration.