CO2 uptake capacity under humid conditions using amine-functionalized metal–organic framework (MOF), dmen-Mg2(dobpdc)

https://doi.org/10.1039/C5SC01191D

“The reusability of a solid adsorbent in the presence of water vapor is also essential to be relevant for CO₂ capture applications.⁵⁴ We assessed the material’s performance by repeated exposure to humid environments. The solid sample can be fully saturated with CO₂ within the exposure time (10 min), because its initial rate of adsorption is estimated to be roughly 1.7 wt% min⁻¹ (Fig. S18†). In this experiment, 1-dmen was exposed to 100% RH for 10 min. The humidified sample was then reactivated under a pure CO₂ flow at 130 °C for 4 h, followed by CO₂ adsorption at 40 °C. After 5 cycles run in this sequence (adsorption at 15% CO₂ and 40 °C, humidification at 100% RH, activation at 130 °C under pure CO₂), a capacity loss of 5% is observed (Fig. 4c). To compare the CO₂ working capacity of 1-dmen upon exposure to humidity with those of other adsorbents, we applied the same experimental protocols to the amine-grafted MOFs, en-Mg₂(dobpdc) and mmen-Mg₂(dobpdc), and the MOFs with open metal sites, Mg–MOF-74 and Mg₂(dobpdc) (Fig. 4d). The capacity of 1-dmen retains significantly high values in subsequent cycles, while in comparison, the working capacities of the other MOFs during the first cycle are below 4.5 wt% and are reduced even further in the second cycle. From these results, the CO₂ working capacity of 1-dmen outperforms those of the other tested materials that have primary and secondary diamines or open metal sites.

“Fig. 4 (a) Adsorption–desorption cycling of CO₂ for 1-dmen. The adsorption of simulated flue gas (0.15 bar CO₂ balanced with N₂) was recorded at 40 °C and desorption was performed at 130 °C under flowing pure CO₂. The symbol C represents the influx of pure CO₂ to regenerate the sample. The working capacity is determined as the difference between A and B. (b) The working capacities of 1-dmen and the other porous solids obtained under the same conditions. (c) CO₂ adsorption of 1-dmen in flue gas using the sequence (adsorption at 40 °C – desorption at 130 °C under pure CO₂ – 10 min exposure to 100% RH). (d) Working capacities of the CO₂ uptake in the first and second cycles for 1-dmen, mmen-Mg₂(dobpdc), en-Mg₂(dobpdc), Mg₂(dobpdc), and Mg–MOF-74 using TG experiments. The first cycle was measured with fully activated samples in flue gas, and the second cycle was measured by following the sequence.”

To examine the CO₂ adsorption capability under humid conditions, we performed time-dependent in situ IR spectroscopy at 40 °C upon simultaneous exposure to 15% CO₂ and 100% RH (Fig. S19†). The N–H stretching band (3397 cm⁻¹) of the chemisorbed species appears under humid CO₂ flowing conditions. The characteristic peak still survives after purging the cell with pure N₂ for 1 min, while the free CO₂ peaks disappear. This observation definitely supports the conclusion that CO₂ is preferably adsorbed onto the grafted amine groups even under humid conditions.

Moreover, to probe the true effect of water vapor on CO₂ uptake, we performed humid adsorption–desorption cycles, exposing 1-dmen to 15% CO₂ and 3.75% H₂O balanced with He. The sample was placed in an automated chemisorption analyzer (Autochem II 2920) to test its adsorption of CO₂ at 40 °C for 30 min, followed by CO₂ desorption at 130 °C for 1 h. The reversible adsorption–desorption events occurred while a CO₂ uptake of around 14.6 wt% was maintained (Fig. 5a), similar to the CO₂ adsorption amount under dry conditions. The framework structure of 1-dmen remains intact after the cycling experiment in humid conditions, as shown by the PXRD data (Fig. S20†). To explore the dynamic separation capability, we ran breakthrough tests for a gas mixture containing 15% CO₂ in an N₂ gas stream. The sample bed was preheated at 130 °C for 4 h in a He atmosphere. The gas mixture was introduced to the solid packed in a column, and the effluent was detected by a gas chromatograph (Agilent 7890A). Under dry conditions, the solid held CO₂ for up to 14.7 min g⁻¹ relative to the N₂ breakthrough time (Fig. 5b). To identify dynamic CO₂ separation in the presence of moisture, we carried out humid breakthrough experiments. While 5% water vapor was flowed into the adsorbent bed prior to the breakthrough runs, a gas mixture of 15% CO₂ and 80% N₂ was infused into the fixed bed. The breakthrough time for CO₂ was equivalent to 21.1 min g⁻¹, which is greater than that of the dry CO₂ capacity. ”

“Fig. 5 (a) CO₂ adsorption–desorption cycling curve for 1-dmen in humid conditions. (b) Breakthrough curves for 1-dmen under dry (N₂, red circle; CO₂, orange circle) and humid conditions (N₂, green triangle; CO₂, blue triangle).”