Controlled Covalent Functionalization of ZIF-90 for Selective CO2 Capture & Separation

Abstract

Mixed Matrix Membranes (MMM) with enhanced selectivity and permeability are preferred for gas separations. The porous metal-organic frameworks (MOFs) materials incorporated in them play a crucial part in improving the performance of MMM. In this study, Zeolitic imidazolate frameworks (ZIF-90) are selected to fabricate Polyetherimide (PEI) MMMs owing to their lucrative structural and chemical properties. This work reports new controlled post-synthetic modifications of ZIF-90 (50-PSM-ZIF-90) with ethanolamine to control the diffusion and uptake of CO2. Physical and chemical properties of ZIF-90, such as stability and presence of aldehyde functionality in the imidazolate linker, allow for easy modulation of the ZIF-90 pores and window size to tune the gas transport properties across ZIF-90-based membranes. Effects of these materials were investigated on the performance of MMMs and compared with pure PEI membranes. Performance of the MMMs was evaluated in terms of permeability of different gases and selective separation of CO2 and H2 gas. Results presented that the permeability of all membranes was in the following order, i.e., P(H2) > P(CO2) > P(O2) > P(CH4) > P(C2H6) > P(C3H8) > P(N2), demonstrating that kinetic gas diffusion is the predominant gas transport mode in these membranes. Among all the membranes, permeability of pure PEI membrane was highest for all gases due to the uniform porous morphology. The pure PEI membrane showed highest permeability of H2, which is 486.5 Barrer, followed by 49 Barrer for O2, 29 Barrer for N2, 142 Barrer for CO2, 41 Barrer for CH4, 40 Barrer for C2H6 and 39.6 Barrer for C3H8. Results also confirm the superiority of controlled PSM-ZIF-90-PEI membrane over the pure PEI and ZIF-90-PEI membranes in CO2 and H2 separation performance. The 50-PSM-ZIF-90 PEI membrane exhibited a 20% increase in CO2 separation from methane and a 26% increase over nitrogen compared to the ZIF-90-PEI membrane. The 50-PSM-ZIF-90 PEI membrane showed 15% more H2/O2 separation and 9% more H2/CH4 separation than ZIF-90 PEI membrane. Overall, this study represents the role of controlled PSM in enhancing the property of new materials like ZIF and its application in MMMs fabrication to develop a promising approach for the CO2 capture and separation.

Keywords: CO2 capture; CO2 separation; ZIF-90; ethanolamine; functionalization; membranes.

Figure 1

Schematic presentation of 50-PSM-ZIF-90-PEI membranes in CO₂ separation.

Figure 2

Imine condensation reaction of ZIF-90 with ethanolamine for 50-PSM-ZIF-90.

Figure 3

PXRD patterns of the simulated ZIF-90, parent ZIF-90, and 50-PSM-ZIF-90.

Figure 4

FESEM images of ZIF-90 and 50-PSM-ZIF-90.

Figure 5

FT-IR spectra of parent ZIF-90 and 50-PSM-ZIF-90.

Figure 6

(A) TGA of the pure ZIF-90 and 50-PSM-ZIF-90 (B) PXRD analysis of TGA residue of ZIF-90, 50-PSM-ZIF-90 and simulated ZnO.

Figure 7

N₂ adsorption isotherms of ZIF-90 and 50-PSM-ZIF-90 at 77 K.

Figure 8

Cross-section FESEM images of Pure PEI membrane, ZIF-90 PEI and 50-PSM-ZIF-90 PEI membranes.

Figure 9

TGA of the pure PEI, ZIF-90 PEI and 50-PSM-ZIF-90 PEI membranes.

Figure 10

Gas isotherms for (A) ZIF-90 and (B) 50-PSM-ZIF-90 at 273, 298 and 313 K.

Figure 11

Adsorption isotherms of CO₂ isotherms for ZIF-90 and 50-PSM-ZIF-90 at 273K.

Figure 12

Qst value of (A) ZIF-90 and (B) 50-PSM-ZIF-90.

Figure 13

Pure gas permeability of Pure PEI membrane, ZIF-90 PEI and 50-PMS-ZIF-90-PEI membranes.

Figure 14

Single Gas permeation curves of (A) Pure PEI membrane, (B) ZIF-90 PEI, and (C) 50-PSM-ZIF-90 PEI membranes; Selectivity of Pure PEI membrane, ZIF-90 PEI and 50-PSM-ZIF-90 PEI membranes (D) O₂ and CO₂ separation (E) H₂ separation.

Figure 15

Comparison of CO₂ separation performance of Pure PEI membrane, ZIF-90 PEI and 50-PSM-ZIF-90 PEI membranes on Robeson’s Upper Bound.