Molecular-level fabrication of highly selective composite ZIF-8-CNT-PDMS membranes for effective CO2/N2, CO2/H2 and olefin/paraffin separations

Abstract

The molecular-scale fabrication method of unique CNT-ZIF-8-PDMS composite membranes with enhanced gas separation performance mainly focusing on CO2 capturing (CO2/N2, CO2/H2). Also olefin/paraffin separation is reported. For the first time, a successful synthesis of ZIF-8 gutter-layer on the free-standing chemically modified carbon nanotube (CNT) platforms was conducted via the ZnO linkages enabling the proper inter-grown and strong anchoring ZIF-8 crystals onto the CNT. The formation of aligned ZIF-8 arrays onto the CNT surface acted as an effective buffering layer that boosted the molecular gas transport through the membrane. In contrast, the conventional gutter layers which usually reduced the gas permeability. To diminish the interstitial pores at the surface/bulk of the CNT/ZIF-8 formations, thin polydimethylsiloxane (PDMS) layer was deposited via the spray-coating technique on the CNT/ZIF-8. The resulting non-defect morphology was confirmed by the SEM and the 3D optical profilometer observations. Furthermore, the PDMS deposition improved mechanical stability (stress vs. strain) of the composite with a twelvefold increased elongation and thermal stability led to a 46% mass reduction. Moreover, composite membranes exhibited outstanding gas separation performance with high CO2 and C3H6 permeabilities of 8705 and 4965 Barrer, respectively and selectivities overcoming the appropriate 2008 Robeson upper bounds for various gas pairs CO2/N2 (ideal selectivity α = 45.6) and CO2/H2 (α = 23.9) gas pairs. Similarly, achieved results of the other tested gas pairs, e.g. CO2/SF6 (α = 17.2), C3H6/N2 (α = 26.0) and C3H6/C3H8 (α = 4.9) were further remarkable in comparison with the literature. Hence, the CNT-ZIF-8-PDMS composite membrane concept has the excellent potential for the fabrication of efficient membranes suitable for targeted gas separation with high commercial and environmental relevance, such as the greenhouse gasses, hydrocarbons recovery for energy harvesting, CO2 capturing and olefin/paraffin separation.