Conversion of ethanol to 1,3-butadiene over high-performance Mg–ZrOx/MFI nanosheet catalysts via the two-step method

Abstract

Mg–Zr/MFI nanosheet (NS) catalysts were prepared by a wet impregnation method for ethanol conversion to 1,3-butadiene (1,3-BD) via the two-step method in a dual fixed bed reaction system. Compared with Zr catalysts loaded on MFI(micro) or commercial SiO₂, 16%Zr/MFI(NS) gave the better performance, with 42.3% 1,3-BD selectivity and 60.5% total conversion of ethanol and acetaldehyde. Introducing 1.2 wt% Mg to 16%Zr/MFI(NS) improved the 1,3-BD selectivity to 54.7% at the expense of a 6% drop in the catalytic activity. Reaction conditions imposed remarkable influence on the reaction results. When the reaction was conducted at 350 °C, a WHSV of 1.44 h⁻¹ and a 2 : 1 ratio of ethanol to acetaldehyde, the 1,3-BD selectivity reached 74.6% with 41.5% total conversion. Such high performance over 1.2%Mg–16%Zr/MFI(NS) was maintained well in a 7 day (168 h) run without deactivation. The catalysts were characterized by XRD, N₂ adsorption, UV-Vis, Raman, and infra-red spectroscopy, NH₃-TPD, TEM and TG. The results showed that the Zr species on MFI(NS) are well distributed with the highest dispersion as compared with the microporous MFI and SiO₂ supported Zr catalysts. The Zr species preferentially occupied the silanol nests of MFI(NS) and eliminated the Brønsted acid sites at 4 wt% Zr loading, and afforded abundant Lewis acid sites in the form of Zr(OH)(OSi)₃ when the Zr loading was increased to 16 wt%. As a base site, Mg is inactive for MPVO reduction but slightly active for the aldol condensation of acetaldehyde, both of which are much inferior to that of the Lewis acid sites. The 1.2%Mg–16%Zr/MFI(NS) catalyst with hierarchical structures of meso- and micro-pores, abundant weak Lewis acid sites but nearly no Brønsted acid sites is competent for the two-step ethanol to 1,3-BD conversion process with high activity, selectivity and stability.