(350e) Highly Efficient Bromine Capture and Storage Using N-Containing Porous Organic Cages

The diverse reactivity of bromine (Br₂), such as charge compensating anions in cross-coupling reactions and SN₂ reactions, is widely used in the industry to produce chemical intermediates and pharmaceuticals. However, the utility of bromine in the chemical industry is largely limited by its severe toxicity, corrosiveness, and volatility, which poses a significant safety concern for the chemical process operation. The high energy density provided by the bromine redox process in Zn-Br batteries (ZBBs) has been proposed for their low-cost energy storage system (ESS) operation. However, the large-scale deployment of the ZBB face significant hurdles due to the build-up of battery stack pressure which might induce leakage of Br₂ into the atmosphere. Toward this end, a small and efficient adsorption-based separation system capable of capturing bromine molecules could accelerate the deployment of ZBB system. Adsorption-based separation systems provide advantages such as a small device footprint, easy operation, and low energy usage. Until now, only a few adsorbent materials, such as cobalt- or zirconium-based metal-organic frameworks and vacancy-ordered perovskites, have been investigated for the adsorptive capture of Br₂. New porous adsorbents with high capacity, potential scalability, and chemical stability to Br₂ are highly desirable for capturing Br₂ under ambient conditions which can contribute to the safe handling of the hazardous chemicals.

Halogen compounds, frequently used as electron acceptors, have a good affinity for electron-donating groups. Extensive research into the capture of iodine (I₂) has discovered that electron-rich adsorbents would successfully adsorb electron-deficient iodine molecules by forming charge-transfer complexes. According to a recent study, ionic functionalized covalent organic frameworks (iCOFs), incorporating abundant binding sites with high electron densities, developed for I₂ capture performed exceptionally well in adsorption capacity and kinetics. The binding sites of iCOFs, including imine, triazine moieties, and cationic sites, would be attributed to their high adsorption capacity by charge-transfer and coulombic interactions with I₂. Br₂ could also preferentially interact with electron-rich species such as imine, triazine, etc. For example, a computational study shows that Br₂ interacts strongly with the electron-donating nitrogen atom from amine functional groups via charge-transfer interaction. Based on these studies, porous adsorbent materials with rich N atoms per volume could be effective for Br₂ capture.

Herein, we use two distinct N-containing POCs (CC3-R and FT-RCC3) to effectively capture bromine vapor under ambient conditions. CC3-R can be constructed from 1,3,5-triformylbenzene (TFB) and (1R,2R)-cyclohexane-1,2-diamine (CHDA), where the [4 + 6] cyclo-condensation reaction of the primary amines and aldehydes results in the formation of the discrete molecular cage through imine linkages (Scheme 1). FT-RCC3 can be readily prepared by reducing the imine group of CC3-R followed by the “tying” method with formaldehyde. Both cages are isostructural as tetrahedral space groups, and their crystal structures formed by window-to-window packing have a three-dimensional diamondoid pore network (Scheme 1b and c). We illustrate the excellent bromine adsorption capacity and kinetics of the CC3-R and FT-RCC3 (11.41 mmol/g and 10.08 mmol/g, respectively) under static adsorption conditions at room temperature. The drive behind the superior bromine adsorption onto the micropores of the POCs was identified to be the formation of strong charge-transfer complexes either with imine or tertiary amine sites. DFT calculations elucidated the evolution of polybromide species (Br₃^-, Br₅^-) when bromine vapors are adsorbed and stabilized

We measured the vapor Br₂ adsorption capacities and uptake curves of N-containing POCs (CC3-R and FT-RCC3) under ambient, static conditions (25℃ and 0.3 bar of Br₂ vapor pressure). The bromine uptake capacity was measured using two kinds of methods; the weight increment of the POCs and iodometric titration method. The experimental setup is shown in Fig. 3. Based on the gravimetric measurements, CC3-R and FT-RCC3 exhibited high Br₂ adsorption capacities of 9.49 and 9.71 mmol/g in the first 24 hours of adsorption, respectively; 11.02 mmol/g and 11.64 mmol/g in the first 48 hours of adsorption, respectively (Fig. 1a and 1b). They outperformed the Br₂ adsorption capacity (4 – 70 times) of previously reported adsorbents (Table 1). The high capture performance would be attributed to their large number of N atoms per discrete molecular cage (14.3% of unit cell atomic composition). Notably, FT-RCC3 showed faster adsorption kinetics than CC3-R. These findings suggest that tertiary amine groups interact with bromine molecules more favorably than imine groups; additionally, the equilibrium sorption capacities were affected by the total pore volume and BET surface area of these materials (Table 1). Interestingly, the variations in uptake capacities evaluated by two different methods (gravimetric versus titration) were greater in FT-RCC3 than in CC3-R, indicating stronger irreversible chemisorption occurring in FT-RCC3 (Fig. 1c). The titration was performed using the collected Br₂ after the thermal desorption of the bromine from the POCs, and the adsorption capacity calculated from the gravimetric method was higher than that of the titration method due to the existence of a small portion of the condensed Br₂ phase. CC3-R with abundant imine sites showed partially-reversible sorption of Br₂ with the recovery of Br₂ by thermal desorption (> 100 – 200℃) was observed up to 70%. While FT-RCC3 with sterically hindered tertiary amine sites showed almost irreversible chemisorption—Br₂ recovery was only up to 10%. With further analyses such as the evolved gas analyzer with a mass spectrometer (EGA-MS), X-ray Photoelectron Spectroscopy (XPS) and time-resolved Fourier transform infrared spectrometer (FT-IR), we confirmed that varying amounts of the Br₂ remained within the interior cavity of two distinct POCs, and would not be desorbed by heat treatment due to a strong interaction with host cages, indicating that bromine molecules formed charge-transfer complexes with POCs.

To better understand of the mechanism of charge-transfer complex formation and physicochemical states of bromine species, DFT calculations were carried out to compute the binding and formation energies as a function of bromine species and the number of unreacted bromine molecules within the two cages. The calculated adsorption energies between CC3-R and Br₃^- are 33.9 kcal/mol and binding energies between FT-RCC3 and Br₃^- are 74.0 kcal/mol, respectively (Fig. 2 and Table 2). The differences in the adsorption energies of both POCs indicate the difficulty of desorbing the adsorbed bromine species in CC3-R and FT-RCC3, which is likely the culprit of the irreversible desorption of bromine species from FT-RCC3. We demonstrated that several charge transfer processes lead to a longer NBr interatomic distance and a shorter HBr interatomic distance resulting in that FT-RCC3 can form significantly more stable polybromide species, through more favorable deprotonation of the “tying” methylene group. We hypothesized that the tied-methylene groups in FT-RCC3 decrease the mobility of trapped bromine species in the host cages, resulting in more stable storage of bromine in polybromide species forms that are not easily desorbed. For CC3-R, bromine species can move more freely in the cage structure compared to FT-RCC3. DFT calculation results provide additional support for this hypothesis. We found that the calculated formation energy of carbocation species (Br₃^- and Br₅^-) is significantly more exergonic for FT-RCC3 than it is for CC3-R. Based on these calculations, we conclude that HBr and Br₅^- species are the most energetically stable species compared with other bromine species. This is consistent with experimental Raman spectra analyses that bromine molecules would mainly exist as polybromide states, and EGA-MS analysis which showed that the hydrogen bromide was detected at both POCs after the thermal treatment process.

In this work, we present the new application of porous organic cages as potential candidates for highly toxic and volatile Br₂ vapor capture. For efficient bromine capture, it should be considered several factors including textural properties, affinity with bromine molecules and densities of binding sites. Two different N-containing POCs (CC3-R and FT-RCC3), which have high porosity and many favorable binding sites for Br₂, show high Br₂ vapor capture performance under static conditions. FT-RCC3, which has a lower total pore volume, exhibits a faster bromine adsorption kinetics than CC3-R with similar adsorption capacities, indicating that affinity between host materials and guest molecules is the critical factor for enhancing capture performance. Interestingly, both POCs showed different desorption abilities owing to their structural difference. DFT calculation results show that the formation of carbocationic species (Br₃^- and Br₅^-) and HBr is energetically more favorable within the cage. The energy decomposition analyses confirmed that both POCs showed highly stabilizing halogen bonding interaction with bromine. This study demonstrates the essential role of strong host-guest interaction in the development of highly efficient capture and storage performance adsorbents. We believe that these N-containing POCs, coupled with their good processability and synthetic scalability, can be utilized for toxic and volatile halogen vapor capture.