(715g) Accelerating CO2 Capture of Highly Permeable Polymer through Incorporating Highly Selective Hollow ZIF

The current large-scale CO₂ capture and storage (CCS) technology based on liquid phase adsorption tower is extremely energy intensive. Membrane separation is widely regarded as an alternative to conventional absorption, however the gas fluxes through commercially selective membranes are so low that hundreds of thousands of square meters of membrane area are required. The key to future membrane technology is to significantly decrease the total membrane area, which requires high flux membrane materials that retain high selectivity.

A facile strategy is via mixed matrix membranes (MMMs), which could combine the attractive properties of polymers with molecular-sieve materials. PIM-1 is an ideal candidate for large scale CO₂ separation MMMs because of its intrinsically high permeability. The tightly regulated pore structure of zeolite imidazolate frameworks (ZIFs) and the compatibility with the polymer matrix can be finely-tuned through rationally matching metal ions and organic ligands. In this study, we prepared MMMs that are both highly permeable and selective through the synergistic combination of PIM-1 with an imidazole-based ZIF-67 hollow nanoparticles (ZIF-HNPs). The molecular sieving shell of ZIF-HNPs can be efficiently utilized, improving both selectivity and permeability of the PIM-1 matrix.

High monodispersity of ZIF-HNPs with mean diameter of 330 nm were achieved. As a control, ZIF-67 solid nanoparticles (ZIF-SNPs) with mean diameter of 370 nm were prepared. A high magnification of ZIF-HNPs revealed that ZIF-HNPs with a cavity diameter of 220 nm consist of fused ZIF-67 nanoparticles. The measured powder XRD patterns of ZIF-HNPs matched well with the simulated XRD pattern of ZIF-67, indicating the intact crystalline structure of ZIF-HNPs. The N₂ sorption test for ZIF-HNPs revealed a type I isotherm with BET surface area of 1170 m²/g and micro-pore volume of 0.501 cm³/g, comparable to those of ZIF-67 nanoparticles. Also, ZIF-HNPs are well dispersed in PIM-1 matrix.

Single-component permeation measurements of CH₄ and CO₂ were performed on membranes with different loading of ZIF-HNPs. When less than 28 vol% ZIF-HNPs were embedded into PIM-1 matrix, the CO₂ permeability significantly increased with the loading, whereas the CH₄ permeability nearly remained unchanged, leading to the enhancement of selectivity by 31.4%. The improved selectivity strongly confirmed that the defect-free MMMs were successfully prepared. Nevertheless, as the loading further increased to 36 vol%, the selectivity for CO₂/CH₄ mixture dropped to 15.1, while the CO₂ permeability kept increasing, resulting from the non-selective interfacial defects occurred around the agglomeration of ZIF-HNPs. We also compared the separation performance between PIM/ZIF-HNPs and PIM/ZIF-SNPs. By replacing ZIF-SNPs with ZIF-HNPs at the same volume loading, the CO₂ permeability further increased by factor of 57.7% without sacrificing the selectivity enhancement.

The membranes loaded with ZIF-SNPs shows well matched permeability with Maxwell Model. However, the permeability of membranes loaded with ZIF-HNPs are much higher than the Maxwell model predicted values. This behavior indicates the non-ideal nature of the PIM/ZIF-HNPs originated from the inner cavity. Therefore, a modified Maxwell model was developed in this study to describe the gas permeability of membranes loaded with ZIF-HNPs. In our modified Maxwell model, the membrane was assumed to be a pseudo-two-phase with the first phase of the filler’s cavity and shell. The second phase consists of the first phase with the polymer matrix. The effect of cavity size and ZIFs loading on gas permeability were quantified by the modified Maxwell model. It should be noted that the cavity size of the hollow fillers should not be too large regarding the overall gas permeation performance. According to the modified Maxwell model, an optimal permeability ratio of continuous polymer matrix to the dispersed ZIF-HNPs (R_c/d) equaling to 0.2-0.3 and cavity size ranging from 200 to 240 nm are preferred.

In conclusion, ZIF-67 hollow nanoparticle was synthesized and embedded as fillers in PIM membranes to prepare mixed matrix membranes. Compared with pristine PIM-1 membrane, the optimal CO₂ permeability and CO₂/CH₄ selectivity of mixed matrix membranes increased by 57.7% and 31.4%, respectively. Compared with membranes incorporated with the same volume loading of ZIF-67 solid nanoparticles, the gas permeability of mixed matrix membranes increased by 37%. These increments can be explained by the enhanced gas diffusivity coefficient resulting from the hollow cavity, which creates extra fractional free volume for fast gas diffusion. Moreover, a modified Maxwell model was developed to analyze the effects of cavity size and filler loading on gas permeability. The predicted results well match with the experimental gas permeability of membrane loaded with ZIF-HNPs (with optimal permeability ratio R_c/d equals to 0.2-0.3), indicating the validity of this modified model. The optimized permeation results for CO₂/CH₄ gas pairs surpass their state-of-the-art upper bounds, manifesting that the strategy of combining PIMs and ZIF hollow nanoparticles is an attractive approach to prepare highly permeable membranes for a number of applications such as carbon capture and hydrogen purification.