CO2 adsorption using amine-functionalized metal–organic framework (MOF), dmen-Mg2(dobpdc)

https://doi.org/10.1039/C5SC01191D

“From the N₂ uptake for 1-dmen at 77 K (Fig. S7†), it is observed that the BET surface area of 675 m² g⁻¹ is between those of en-Mg₂(dobpdc) (1253 m² g⁻¹) and mmen-Mg₂(dobpdc) (70 m² g⁻¹),^38,41 which is related to the bulkiness of the appended heterodiamines on the unsaturated metal sites. From the analysis of the pore size distributions, the average pore size of 1-dmen is smaller than those of both en-Mg₂(dobpdc) and mmen-Mg₂(dobpdc) (Fig. S8†). The CO₂ adsorption isotherms for 1-dmen were collected at several temperatures after the samples had been activated at 75 °C for 4 h prior to each measurement (Fig. 2a). The isotherm curves show steps at certain pressures, depending on the adsorption temperature. The shift of the step pressure may be associated with the greater thermal motion of the grafted dmen and CO₂ molecules at elevated temperatures. Similar behavior was also observed in Mg₂(dobpdc) functionalized with primary and secondary diamines.^38,41 At 1 bar, the adsorbed quantity is 4.34 mmol g⁻¹ at 40 °C, comparable to those at 50 and 60 °C. Remarkably, however, almost no CO₂ is adsorbed on 1-dmen at 75 °C and 1 bar, which is comparable with the adsorption behaviors of en-Mg₂(dobpdc) and mmen-Mg₂(dobpdc), which both show non-adsorption at temperatures above 120 °C.”

“Fig. 2 (a) Adsorption isotherms of CO₂ for 1-dmen at the indicated temperatures. The solid lines are eye-guides. (b) CO₂ and N₂ isotherms at 25 °C.”

“This notable feature indicates a weaker interaction between CO₂ and the amine groups in 1-dmen than that between CO₂ and the amines in en-Mg₂(dobpdc) and mmen-Mg₂(dobpdc).

The isosteric heat of adsorption (Q_st), which represents the mean binding energy of a gas molecule at a specific site of an adsorbent, was estimated by employing a dual-site Langmuir–Freundlich model to fit the CO₂ isotherms. The precise pressures corresponding to the adsorbed CO₂ amounts were introduced as input parameters to the Clausius–Clapeyron equation to calculate the adsorption enthalpy (Fig. S9†). Using this calculation, the estimated heat of adsorption (−Q_st) increases to 75 kJ mol⁻¹ at a loading of 0.25 mmol g⁻¹, and remains almost invariant in the range of 71–75 kJ mol⁻¹ below loadings of 2.6 mmol g⁻¹. This overall feature of the adsorption enthalpy for 1-dmen resembles that observed for the other Mg₂(dobpdc) moieties functionalized with diamines.^38,41 The initial low Q_st values at ultradilute CO₂ concentrations are unusual for chemisorbents. To investigate the adsorption behavior at the CO₂ pressures before and after the step in the isotherm, we measured the IR in situ at 40 °C by flowing simulated air (0.39 ppm CO₂ balanced with N₂) or pure CO₂ into the cell (Fig. S10†). There is no evidence of chemisorption at the CO₂ pressure before the step, while the peak at 3397 cm⁻¹ appears after the step pressure, indicating the formation of the chemisorbed species. To understand the CO₂ adsorption mechanism, we performed DFT calculations on the binding energy of each possible Mg–amine pair. From the results, the primary amine–Mg pair has a binding energy of −174.3 kJ mol⁻¹, which is thermodynamically more stable than that of the tertiary amine–Mg pair (−135.6 kJ mol⁻¹) (Fig. 1 and S11†). This result suggests that the open metal site is primarily occupied by the primary amine end of 1-dmen, although some tertiary amine ends are probably grafted onto the exposed metal site as well. On the basis of the in situ IR data and DFT calculations, one possible CO₂ adsorption mechanism is as follows: the low-affinity adsorption at low pressures is highly uncommon for chemisorbents, and cannot be rationalized in the absence of sorbent restructuring under CO₂ dosing conditions. At a certain partial pressure, the amine is reorganized so that it is now available to bind CO₂via chemisorption causing a jump in the binding energy. Therefore, we speculate that under CO₂ conditions, deprotonation of the coordinate amine group takes place with the assistance of the basic tertiary amine group of the dangling dmen molecule. Concomitant nucleophilic attack of CO₂ forms an alkylammonium carbamate. The generated ion-pair interacts to weaken the Mg–N bond strength of the neighboring dmen and in turn accelerates cooperative CO₂ insertion, which can explain the abrupt rise in the CO₂ isotherm (Fig. 2a). ”