An unprecedented c-oriented DDR@MWW zeolite hybrid membrane: new insights into H2-permselectivities via six membered-ring pores

Abstract

Despite the success of several types of homogeneous zeolite membranes, the manufacturing based on only one type of zeolite largely limits the flexible, reliable synthetic route toward the formation of continuous zeolite membranes. Herein, we demonstrate heterogeneous epitaxial growth of a seed layer using two structurally compatible zeolites. Specifically, MCM-22 (MWW type zeolites) seed layers were epitaxially grown with a synthetic precursor that leads to the DDR type zeolite. The resulting heterogeneous films, referred to as DDR@MWW hybrid films, were continuously formed on porous supports. Furthermore, heteroepitaxial growth of the c-oriented MCM-22 seed layer yielded a preferential c-out-of-plane orientation; accordingly, six membered-ring (6-MR) pores (having a maximum size of ∼0.28 nm) along the c-axis of the MWW and DDR zeolites were, for the first time, aligned in the out-of-plane direction. Subsequently, this good model sample was adopted for determining the H₂-permselectivities of the 6-MR pores; surprisingly, the 6-MR pores were rather impermeable to both H₂ and CO₂ molecules up to ∼200 °C so that the c-oriented hybrid membrane exhibited modest CO₂-permselectivities over N₂ (0.364 nm) or CH₄ (0.38 nm) at the expense of reduced CO₂ molar flux. Lastly, this experimental observation was further complemented with rigorous molecular dynamics simulations on well-designed DDR zeolite membranes.