Symmetric and asymmetric zeolitic imidazolate frameworks (ZIFs)/polybenzimidazole (PBI) nano-composite membranes for hydrogen purification at high temperatures

Abstract

Hydrogen is one of the most strategically important sustainable fuel sources among many energy alternatives. In addition to be viable and versatile in widely applications such as fuel for transportation and feedstock for chemical industries, a hydrogen-based energy system has other advantages including low emissions and environmentally friendly, clean, and efficient use. Currently, the favored route of hydrogen production is from the steam reforming of methane followed by the water-gas shift reaction. The main by-product in the leaving stream from the operation is carbon dioxide. The removal of CO2 from H2 or its sequestration is a critical requirement for hydrogen to be a sustainable energy system as well as for the minimization of environmental impact. We have molecularly designed high performance zeolitic imidazolate frameworks (ZIFs)/ polybenzimidazole (PBI) nano-composites for hydrogen separation at high temperatures, and demonstrated it in a useful configuration as dual-layer hollow fibers for the first time. By incorporating as-synthesized nanoporous ZIF-8 nano-particles into the high temperature but extremely low permeability polybenzimidazole (PBI), the resultant mixed matrix membranes show an impressive enhancement in H2 permeability as high as hundred times without any significant deduction in H2/CO2 selectivity. The 30/70 ZIF-8/PBI dense membrane has a H2 permeability of 105.4 Barrer and a H2/CO2 selectivity of 12.2. This performance is far superior to ZIF-7/PBI membranes and is the best ever reported data for H2-selective polymeric materials in the literature. Meanwhile, defect-free ZIF-8-PBI/Matrimid dual-layer hollow fibers have been successfully fabricated without post annealing and coating by optimizing ZIF-8 nano-particle loadings, spinning conditions and solvent-exchange procedures. Two types of hollow fibers targeted at either high H2/CO2 selectivity or high H2 permeance have been developed; namely, (1) PZM10-I B fibers with a medium H2 permeance of 64.5 GPU at 180 °C and a high H2/CO2 selectivity of 12.3, and (2) PZM33-I B fibers with a high H2 permeance of 202 GPU at 180 °C and a medium H2/CO2 selectivity of 7.7. This work not only molecularly designs novel nano-composite materials for harsh industrial applications such as syngas and hydrogen production, but also, for the first time, synergistically combines the strengths of both ZIF-8 and PBI for energy-related applications.