Mixed-matrix carbon molecular sieve membranes using hierarchical zeolite: A simple approach towards high CO2 permeability enhancements

Membrane process is a robust technology for CO2/CH4 separation. To achieve the energy-efficiency necessary for a cost-effective membrane-based CO2/CH4 separation, high-performance membranes are often desirable. There are several approaches that can be undertaken to realize such membranes. In this study, we amalgamated three strategies: 1) filler design optimization by synthesizing hierarchical zeolite 5A comprising micro and mesoporous domains to strengthen the second strategy; 2) mixed-matrix approach by incorporating zeolite fillers to facilitate CO2 diffusion and reduce transport resistance through a Matrimid (R) matrix; and 3) membrane carbonization to induce thermal rearrangement, and create free volumes and pore apertures for a faster CO2 transport. These efforts afforded mixed-matrix carbon molecular sieve membranes with results confirming that our membrane at 30 wt% loading of hierarchical zeolite 5A was able to surpass the 2008 Robeson upper bound limit given a performance of 2450 barrers CO2 permeability and 19.3 CO2/CH4 selectivity. Notably, at this loading, the negative effect brought by the interfacial nanogaps between the filler and carbon matrix diminished, unleashing the potential of the mesopores in facilitating CO2 diffusion, which led to a two orders of magnitude enhancement in the CO2 permeability relative to the unfilled carbon molecular sieve membrane. Despite a decrease in the CO2/CH4 selectivity, the hierarchical zeolite 5A is effective in alleviating the intensity of the permeability-selectivity trade-off. Also, benchmarking of the filler effectiveness is first demonstrated using a new filler enhancement index, F-index. The largest F-index value in this study was 1.97. This attaches a label of "competent" to our hierarchical zeolite 5A filler at 30 wt% loading, demonstrating the success of our strategies.