Enhanced hydrogen separation with ZIF-8/α-Al2O3 membranes synthesized via PDA/PEI-chelated zinc ions

Abstract

In this study, the coupling agent polydopamine/polyethyleneimine (PDA/PEI) was utilized to modify the surface of an α-Al₂O₃ support. Owing to the cross-linking characteristics of the –NH₂ and –OH functional groups with Zn²⁺ ions on the support by the coupling agent, a continuous and dense 2D-zeolitic imidazolate framework-8 (ZIF-8) membrane was induced and regulated. The hydrogen (H₂) separation performance, mechanism and preparation optimization of the ZIF-8 membrane grown on the modified layer of PDA/PEI were investigated. The results indicated that a ZIF-8 membrane with an approximate thickness of 12 μm formed on a Zn²⁺@PDA/PEI-modified α-Al₂O₃ support. The H₂ permeance of the ZIF-8 membrane was 3.02 × 10^–7 mol·Pa⁻¹·m⁻²·s⁻¹ at 25 °C and 0.15 MPa, with ideal selectivities of 12, 51, and 59 for H₂/CO₂, H₂/N₂, and H₂/CH₄, respectively. Notably, these values significantly exceeded the Robeson upper limits. The primary mechanism of zinc ion complexation in the PDA/PEI codeposit layer involves the formation of complexes between the O and N atoms on the layer and Zn²⁺, and a significant number of imines (NH=) can be employed to chelate Zn²⁺ and facilitate the formation of Zn =N compounds. This facilitated the heterogeneous nucleation of ZIF-8 on the support surface, resulting in the synthesis of continuous ZIF-8 membranes with ultrahigh H₂/N₂ and H₂/CH₄ selectivity and excellent intergrowth.