A Scalable Robust Microporous Al-MOF for Post-Combustion Carbon Capture

Abstract

Herein, a robust microporous aluminum tetracarboxylate framework, MIL-120(Al)-AP, (MIL, AP: Institute Lavoisier and Ambient Pressure synthesis, respectively) is reported, which exhibits high CO₂ uptake (1.9 mmol g^-1 at 0.1 bar, 298 K). In situ Synchrotron X-ray diffraction measurements together with Monte Carlo simulations reveal that this structure offers a favorable CO₂ capture configuration with the pores being decorated with a high density of µ₂-OH groups and accessible aromatic rings. Meanwhile, based on calculations and experimental evidence, moderate host-guest interactions Q_st (CO₂) value of MIL-120(Al)-AP (-40 kJ mol^-1) is deduced, suggesting a relatively low energy penalty for full regeneration. Moreover, an environmentally friendly ambient pressure green route, relying on inexpensive raw materials, is developed to prepare MIL-120(Al)-AP at the kilogram scale with a high yield while the Metal- Organic Framework (MOF) is further shaped with inorganic binders as millimeter-sized mechanically stable beads. First evidences of its efficient CO₂/N₂ separation ability are validated by breakthrough experiments while operando IR experiments indicate a kinetically favorable CO₂ adsorption over water. Finally, a techno-economic analysis gives an estimated production cost of ≈ 13 $ kg^-1, significantly lower than for other benchmark MOFs. These advancements make MIL-120(Al)-AP an excellent candidate as an adsorbent for industrial-scale CO₂ capture processes.

Keywords: CCS; CO2; MOFs; scale‐up; shaping; techno‐economic analysis.

Figure 1

Crystal structure of MIL‐120(Al). A) General view along [0 0 1] highlighting the MOF narrow channels (water molecules were omitted for clarity). B) Constitutive Al hydroxo‐chains built of trans‐cis edge sharing Al(OH)₄O₂ octahedra. C) Representation of one channel emphasizing its highly confined environment (represented by the yellow tube) due to dense network of µ ₂‐OH and stacked phenyl rings of BTeC. Color code: Al(OH)₄O₂, gray polyhedra; C, gray; O, light red; H, white. In C), one of the chains is highlighted using purple polyhedra.

Figure 2

CO₂ adsorption performances of MIL‐120(Al)‐HP and MIL‐120(Al)‐AP at 298 K. A) CO₂ (in red and blue, respectively) and N₂ (in gray) adsorption isotherms at 298 K. Enlargement on the step region is given in the inset. B) IAST selectivity at different compositions for MIL‐120(Al)‐AP using the Langmuir model. C) Isosteric enthalpy of adsorption versus CO₂ uptake for MIL‐120(Al)‐AP. D) Comparison of volumetric and gravimetric CO₂ uptakes at 0.1, 1 bar, at 298 K between MIL‐120(Al)‐AP and benchmark adsorbents including MOFs and zeolites. The volumetric uptake was calculated using the crystallographic density.

Figure 3

Variable temperature synchrotron SRPD data. Measurements performed under dynamic vacuum (activation step) for A) MIL‐120(Al)‐HP, showing no phase transition, while remaining monoclinic; and B) MIL‐120(Al)‐AP revealing phase transition, at ≈ 357 K, from monoclinic to triclinic phase. Heating rate is equal to 6 K min⁻¹, up to 373 K in (A) and 400 K in (B).

Figure 4

Crystal structure of MIL‐120(Al)‐HP cooled down to 200 K and loaded with 1 bar of CO₂. A) General view along [0 0 1]. B) Top view of a part of one channel, and C) a cut through a channel showing the arrays of alternated CO₂ molecules and their interactions with the frameworks throughout µ ₂‐OH (represented as green dashed lines) and phenyl groups (yellow dashed lines) of the BTeC, with the respective distances i): 3.6Å; ii): 3.3Å; iii): 3.6Å; iv): 3.7Å). Color code: Al(OH)₄O₂, gray polyhedra; C, gray; O, light red. H‐atoms were not localized.

Figure 5

Molecular simulations of MIL‐120(Al)‐AP structure and adsorption properties. A) Most stable DFT‐optimized structure of MIL‐120(Al). The unit cell parameter is not shown to highlight the different orientations of µ ₂‐OH group in the channel. The most stable Str1 and Str2 structures correspond to distinct orientations of µ₂‐OH groups [details in Supporting Information]. B) Calculated X‐ray diffraction patterns for the two most stable DFT‐optimized MIL‐120(Al)‐APs and the corresponding experimental data. C) GCMC‐simulated and experimental single‐component CO₂ adsorption isotherms at 298 K. D) Illustrations of the DFT‐optimized CO₂‐loaded MIL‐120(Al)‐AP‐Str1 and MIL‐120(Al)‐AP‐Str2 structures with different CO₂ loading (one [top] and two [bottom] CO₂ molecules per unit‐cell). The dashed line represents the interaction between the CO₂ molecule (O) and the µ ₂‐OH group (H) of the framework correspondingly. E) GCMC‐simulated 15CO₂:85N₂ binary mixture adsorption isotherm of MIL‐120(Al)‐AP‐Str1 at 298 K calculated by GCMC simulations, F) Corresponding snapshot (zoom in) showing the location of CO₂/N₂ in MIL‐120(Al)‐AP‐Str1 at 1 bar and 298 K from GCMC simulation (cf. full sharpshot in Figure S16, Supporting Information). Color code in A, D, and F: Al(OH)₄O₂, gray polyhedra; C, gray; O, light red; H, white (C‐H) and light yellow (O‐H); N, blue. Interactions between CO₂ molecules and µ ₂‐OH groups are represented as green dashed lines. All views are shown along [0 1 1].

Figure 6

CO₂ adsorption uptakes of MIL‐120(Al)‐AP. A) CO₂ adsorption isotherms at 298 K on different scale batch preparations. B) CO₂ adsorption isotherms comparison between pure and structured samples with 10% Bentonite or Silica. C) Breakthrough curves for MIL‐120(Al)‐AP beads with 10% Si. D) Breakthrough results of MIL‐120(Al)‐AP beads with 10% bentonite. The activation condition for both samples was heating at 50 °C for 12 h under vacuum, run 1 refers to the measurement in dry conditions, run 2 refers to the measurement in dry conditions after exposure to humid conditions, while in between the sample was reactivated at 50 °C under vacuum.

Figure 7

Operando IR spectra. IR spectra in the ν(OH) and ν(C═O) region for MIL‐120(Al)‐AP with time on stream during the first minutes of CO₂ adsorption in the presence of water (20% CO₂ and 1% H₂O in Ar flow at RT).

Figure 8

Sensitivity analysis of MIL‐120(Al)‐AP production costs. A) Effect of the ligand and energy costs; B) effect of the base equipment costs. The green dots over the lines represent the base economic scenario (2022 prices).