Zeolitic imidazolate framework ZIF-7 based molecular sieve membrane for hydrogen separation

Abstract

Hydrogen-based energy system could address issues related to global climate change, energy security, and local air pollution. Thermally and hydrothermally stable microporous membranes with intrinsic high H 2/CO 2 selectivity are highly demanded. A novel zeolitic imidazolate framework (ZIF-7) membrane was tested for its gas separation performance. ZIFs are microporous materials and belong to the new class of metal–organic frameworks (MOFs). ZIF-7 is formed by bridging benzimidazolate anions and zinc cations resulting in a sodalite (SOD) topology with a pore size of about 0.3 nm. The ZIF-7 membrane exhibited promising H 2 separation abilities. At 220 °C, the H 2 permeance is ∼4.5 × 10 −8 mol m −2 s −1 Pa −1 and the mixture separation factors for H 2/CO 2, H 2/N 2, and H 2/CH 4 are 13.6, 18.0, and 14.0, respectively. As a result of molecular sieving mechanism, the ideal selectivities and mixture separation factors are identical. The permeation of H 2 through the ZIF-7 membrane is highly activated with an apparent activation energy of 11.9 kJ mol −1. Due to the ultra-hydrophobic properties of ZIF materials, the ZIF-7 membrane also showed excellent hydrothermal stability in the presence of steam. Our results clearly demonstrate that ZIF-7 membranes have an intrinsic high H 2/CO 2 selectivity and a promising application in hydrogen separation, which is based on its very narrow and well-defined crystal pore structure.