Synthesis and Characterization of a Crystalline Imine-Based Covalent Organic Framework with Triazine Node and Biphenyl Linker and Its Fluorinated Derivate for CO2/CH4 Separation

Abstract

A catalyst-free Schiff base reaction was applied to synthesize two imine-linked covalent organic frameworks (COFs). The condensation reaction of 1,3,5-tris-(4-aminophenyl)triazine (TAPT) with 4,4'-biphenyldicarboxaldehyde led to the structure of HHU-COF-1 (HHU = Heinrich-Heine University). The fluorinated analog HHU-COF-2 was obtained with 2,2',3,3',5,5',6,6'-octafluoro-4,4'-biphenyldicarboxaldehyde. Solid-state NMR, infrared spectroscopy, X-ray photoelectron spectroscopy, and elemental analysis confirmed the successful formation of the two network structures. The crystalline materials are characterized by high Brunauer-Emmett-Teller surface areas of 2352 m²/g for HHU-COF-1 and 1356 m²/g for HHU-COF-2. The products of a larger-scale synthesis were applied to prepare mixed-matrix membranes (MMMs) with the polymer Matrimid. CO₂/CH₄ permeation tests revealed a moderate increase in CO₂ permeability at constant selectivity for HHU-COF-1 as a dispersed phase, whereas application of the fluorinated COF led to a CO₂/CH₄ selectivity increase from 42 for the pure Matrimid membrane to 51 for 8 wt% of HHU-COF-2 and a permeability increase from 6.8 to 13.0 Barrer for the 24 wt% MMM.

Keywords: CO2/CH4 separation; covalent organic framework (COF); fluorinated COF; imine-COF; mixed-matrix membrane (MMM).

Scheme 1

Schematic formation of HHU-COF-1 and HHU-COF-2 from TAPT and BPDCA or OF-BPDCA, respectively, indicating the idealized hexagonal ring and network in the HHU-COF products (HHU = Heinrich-Heine University). The given triazine-centroid triazine-centroid (tz-tz) distance along the edges and the edge-edge distances were determined from the most intense (100) reflexes in the powder X-ray diffractograms, assuming a hexagonal unit cell for the honeycomb layer (cf. Figure S16). The given tz-tz distance along the edge is half the tz-tz distance across the ring.

Figure 1

¹³C CP MAS NMR spectra of (a) HHU-COF-1 and (b) HHU-COF-2. Spectra were obtained at 150 MHz, cross-polarization with decoupling spinal and 35 kHz spinning.

Figure 2

¹⁹F CP MAS NMR spectra of HHU-COF-2.

Figure 3

IR-spectra of (a) HHU-COF-1 and (b) HHU-COF-2 in comparison with the spectra of the educts.

Figure 4

(a) SEM image of HHU-COF-2 and (b) associated fluorine elemental mapping.

Figure 5

(a) TGA curves acquired under nitrogen atmosphere with a heating rate of 5 K/min and (b) PXRD pattern of HHU-COF-1 and HHU-COF-2. The first derivative of the TGA curves is given in Figure S15.

Figure 6

(a) Nitrogen sorption isotherms at 77 K and (b) pore size distributions of HHU-COF-1 and HHU-COF-2.

Figure 7

Cross-section SEM images of HHU-COF-1/Matrimid with (a) 8 wt%, (b) 16 wt%, and (c) 24 wt% filler and HHU-COF-2/Matrimid MMMs with (d) 8 wt%, (e) 16 wt%, and (f) 24 wt% filler.

Figure 8

CO₂ and CH₄ permeabilities (P) and CO₂/CH₄ selectivity(α) for (a) HHU-COF-1/Matrimid and (b) HHU-COF-2/Matrimid MMMs.

Figure 9

Lg permeability versus 1/FFV for 0 to 24 wt% (a) HHU-COF-1 and (b) HHU-COF-2 as dispersed phases in Matrimid MMMs.

Figure 10

P_eff /P_c of the gas CO₂ versus ${\varphi }_d$ for 0 to 24 wt% HHU-COF-1 and HHU-COF-2 as dispersed phases in Matrimid matrices.