Two-Dimensional Covalent Organic Frameworks for Carbon Dioxide Capture through Channel-Wall Functionalization

Ordered open channels found in two-dimensional covalent organic frameworks (2D COFs) could enable them to adsorb carbon dioxide. However, the frameworks’ dense layer architecture results in low porosity that has thus far restricted their potential for carbon dioxide adsorption. Here we report a strategy for converting a conventional 2D COF into an outstanding platform for carbon dioxide capture through channel-wall functionalization. The dense layer structure enables the dense integration of functional groups on the channel walls, creating a new version of COFs with high capacity, reusability, selectivity, and separation productivity for flue gas. These results suggest that channel-wall functional engineering could be a facile and powerful strategy to develop 2D COFs for high-performance gas storage and separation.

[HO] X% -H 2 P-COFs (30 mg) was weighed into a 10-mL glass vial, to which succinic anhydride (6 mL, 1.0 M solution in anhydrous acetone) was added. The reaction mixture was heated at 60 °C for two days. The precipitate was collected by centrifugation, washed with anhydrous THF for five times. The crude product was rinsed with THF for 48 h using a Soxhlet extractor. The powder was dried at 100 °C under vacuum overnight to give the corresponding products of [HO 2 C] 25% -H 2 P-COF, [HO 2 C] 50% -H 2 P-COF, [HO 2 C] 75% -H 2 P-COF, and [HO 2 C] 100% -H 2 P-COF, quantitatively.
After filtration, 5 mL of aqueous HCl solution (6 M) was added slowly to the filtrate and the mixture was stirred for 1 h. The greenish porphyrin precipitate was removed by filtration. The filtrate was evaporated under vacuum and submitted to 1 H NMR spectroscopy in d 6 -DMSO. The content of carboxylic acid was calculated by using the proton integrates (Table S3, Figure S5).

Section C. Fitting of pure component isotherms
The salient properites of two different COFs ([HO] 100% -H 2 P-COF and [HO 2 C] 100% -H 2 P-COF) are specified in Table 1. The potential of these COFs are evaluated for the separation of CO 2 /N 2 mixtures that is relevant for CO 2 capture from flue gases. For our evaluations, we assume the CO 2 /N 2 mixtures to contain 15% CO 2 , and 85% N 2 , following the earlier work of Mason et al. S1 The experimentally measured excess loadings of CO 2 , and N 2 , obtained at different temperatures, were first converted to absolute loadings before data fitting. The procedure for converting to absolute loadings is the same as described in the Supporting Information accompanying the paper of Wu et al. S2 For the purpose of converting to absolute loadings, the pore volumes used are specified in Table S4. The isotherm data for CO 2 were fitted with the Langmuir-Freundlich model: The Langmuir-Freundlich parameters for adsorption of CO 2 are provided in Table S5. The simpler Langmuir model was adequate for fitting the isotherm data for N 2 ; Table S6 provides the T-dependent Langmuir parameters for N 2 in different materials.

Section D. Isosteric heat of adsorption
The isosteric heat of adsorption, Q st , defined as (3) was determined using the pure component isotherm fits using the Clausius-Clapeyron equation.

Section E. IAST calculations
The selectivity of preferential adsorption of component 1 over component 2 in a mixture containing 1 and 2, perhaps in the presence of other components too, can be formally defined as

Section F. Simulation methodology for transient breakthrough in fixed bed absorbers
The separation of CO 2 /N 2 mixtures is commonly carried out in fixed bed absorbers in which the separation performance is dictated by a combination of three separate factors: (a) adsorption selectivity, (b) uptake capacity, and (c) intra-crystalline diffusivities of guest molecules within the pores. Transient breakthrough simulations are required for a proper evaluation of MOFs; the simulation methodology used in our work is described in earlier publications. S10,S11 A brief summary of the simulation methodology is presented below.
Assuming plug flow of an n-component gas mixture through a fixed bed maintained under isothermal conditions (see schematic in Figure 4a), the partial pressures in the gas phase at any position and instant of time are obtained by solving the following set of partial differential equations for each of the species i in the gas mixture. S12 In equation (5), t is the time, z is the distance along the adsorber, r is the framework density, e is the bed voidage, v is the interstitial gas velocity, and ) , ( z t q i is the spatially averaged molar loading within the crystallites of radius r c , monitored at position z, and at time t.
At any time t, during the transient approach to thermodynamic equilibrium, the spatially averaged molar loading within the crystallite r c is obtained by integration of the radial loading For transient unary uptake within a crystal at any position and time with the fixed bed, the radial distribution of molar loadings, q i , within a spherical crystallite, of radius r c , is obtained from a solution of a set of differential equations describing the uptake The molar flux N i of component i is described by the simplified version of the Maxwell-Stefan equations in which both correlation effects and thermodynamic coupling effects are considered to be of negligible importance 12

S8
Summing equation (6) The interstitial gas velocity is related to the superficial gas velocity by In industrial practice, the most common operation uses a step-wise input of mixtures to be separated into an absorber bed that is initially free of adsorbents, i.e. we have the initial condition At time t = 0, the inlet to the absorber, z = 0, is subjected to a step input of the n-component gas mixture and this step input is maintained till the end of the adsorption cycle when steady-state conditions are reached.