Functional Metal-Organic Frameworks. Ali Morsali

dioxide interaction change into strong CO₂ chemisorption by alkylamine (N)∙(C)carbon dioxide interaction which leads to high CO₂ adsorption enthalpy and selectivity. Homodiamine N,N’-dimethylethylenediamine (mmen) ligand applied in the structure of CuBTTri and Mg(dobpdc)₂ to develop post-synthetically modified materials with high affinity to carbon dioxide molecules

, −96, −47 and −71 for Cu-BTTri, mmen-Cu-BTTri, Mg(dobpdc)₂ and mmen-Mg(dobpdc)₂ respectively) [6, 34].

Originally, Mg-MOF-74 could adsorb exceptional amount of carbon dioxide (20.6 wt%) under relevant post-combustion flue gas conditions while it display only about 16% recovery of its initial amount of CO₂ in breakthrough experiment with 70% humidity [35]. To remove this barrier and improvement of CO₂ capacities at lower CO₂ partial pressures and presence of humidity, Mg(dobpdc)₂ post-synthetically modified with homodiamine ligands like ethylenediamine and dimethylethylenediamine [31]. Anyway, the working capacity of these alkylamine grafted FMOFs is not high at low temperatures. This is another issue that must be eliminated for efficient release of CO₂ and increase in carbon dioxide working capacity.

Chang Seop Hong and coworkers synthesized a diamine-grafted FMOF (with the name of dmen-Mg₂(dobpdc) where dmen = N,N’ dimethylethylenediamine and H₄dobpdc = 4,4’-dihydroxy-(1,10-biphenyl)-3,3’dicarboxylic acid) and applied it for carbon dioxide capture and evaluation of working capacity at post-combustion conditions (Figure 2.1) [8]. dmen is a heterodiamine with both primary and tertiary amines and incorporated into framework through post-synthesis modification. The results reveal that dmen-Mg₂(dobpdc) represent high CO₂/N₂ selectivity (S = 554 at 25 °C and p(CO₂) = 0.15 bar) which is higher that mmen-Mg₂(dobpdc) (S = 200) and en-Mg₂(dobpdc) (S = 230). At 25 °C and 0.15 bar, activated dmen-Mg₂(dobpdc) adsorb 3.77 mmol·g⁻¹ of CO₂ which is higher than en-Mg (dobpdc) (3.62 mmol·g⁻¹) and mmen-Mg (dobpdc) (3.13 mmol·g⁻¹). Efficiency of an adsorbent in post-combustion process is assessed by measuring the working capacity which is defined as difference between the adsorbed quantities at P_ads = 0.15 bar CO₂/T_ads = 40 °C and P_des = 1 bar CO₂/T_des. The higher working capacity at lower desorption temperatures, the lower energy consumption. dmen-Mg₂(dobpdc) could adsorb 18.8 wt% for pure CO₂, and 14.1 wt% for N₂/CO₂(85/15) which is close to that at the corresponding CO₂ partial pressure in the isotherm at 40 °C, while no obvious adsorption was observed for pure N₂ at 40 °C. Regeneration of the material is evaluated via vacuum-swing adsorption (VSA) and temperature-swing adsorption (TSA) methods. In TSA method, CO₂ was adsorbed at 40 °C for 1 h and desorbed at 75 °C for 1 h under Ar. After 24 cycles, no capacity loss was observed, revealing that the dmen-Mg₂(dobpdc) is thermally stable under these experimental conditions as well as maintenance in its adsorption capacity. In VSA method, at 25 °C, material was saturated with CO₂ at 1.2 bar and then placed under high vacuum. The removal of adsorbed CO₂ from the solid was performed repeatedly by applying a vacuum to the adsorbent. Based on observed results, such a large amount of adsorbed CO₂ (4.5 mmol·g⁻¹) at 1.2 bar can be completely desorbed only under vacuum, without heating. The working capacities of dmen-Mg₂(dobpdc) at 130 to 90 °C desorption temperature is in the range of 11.7–13.5 wt%. Notably, experimental results reveal that at T_des = 75 °C working capacity is 11.6 wt% which is higher that top performing MOFs such as Mg-MOF-74 (3.7 wt%), Mg₂(dobpdc) (4.5 wt%), en-Mg₂(dobpdc) (2.9 wt%), mmen-Mg₂(dobpdc) (2.1 wt%), and tmen-Mg₂(dobpdc) (3.9 wt%) (tmen = N,N,N’,N’-tetramethylethylenediamine). However, the working capacity of dmen-Mg₂(dobpdc) sharply reduced to almost zero at 70 °C. To evaluate the reusability of dmen-Mg₂(dobpdc) in the presence of water vapor, the material exposed to water vapor (100% RH for 10 min). Since the solid sample can be fully saturated with CO₂ within the exposure time (10 min), it exposed to water vapors for 10 min. Then, dmen-Mg₂(dobpdc) reactivated under a pure CO₂ flow at 130 °C for 4 h, followed by CO₂ adsorption at 40 °C. After 5 cycles, a capacity loss of 5% is observed which is markedly higher than working capacities of the other MOFs during the first cycle (lower than 4.5 wt%). Clausius–Clapeyron equation applied for estimation of heat of adsorption (−Q_st ) so that 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. Based on DFT calculations, the open metals site are mostly occupied by the primary amine end of dmen-Mg₂(dobpdc), although some tertiary amine ends are probably grafted onto the exposed metal site as well. Possible mechanism based on DFT calculations and in-situ FT-IR analysis possible CO₂ adsorption mechanism is illustrated in Figure 2.1f.

Figure 2.1 Application of dmen-Mg₂(dobpdc) in selective capture-release of carbon dioxide. (a) TSA process: Adsorption (40 °C)-desorption (75 °C) cycling of CO₂ under Ar. (b) Vacuum-swing adsorption at 25 °C. (c) The working capacities of 1-dmen and the other porous solids obtained under the same conditions. (d) 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). (e) Estimated working capacity from q_ads (P_ads = 0.15 bar CO₂, T_ads = 40 °C)-q_des (P_des = 1 bar CO₂, T_des = 75). (f) Framework structure of 1 with open metal sites, grafting modes of dmen onto the open metal sites, and subsequent CO₂ adsorption. The schematic diagram (bottom) indicates the arrangement of ammonium carbamates running along the c-axis [8].

      Although extensive studies conducted on application of amine decorated MOFs is carbon dioxide capture and release, but there is an urgent need to clarify the effective conditions of arylamine or alkylamine groups in practical CO₂ capture and release for real-life applications.

      Christian Serre and coworkers applied amine decorated MIL-125(Ti) (MIL-125 formula is (Ti₈O₈(OH)₄(BDC)₆) where BDC is benzenedicarboxylate), denoted as NH₂-MIL-125(Ti) for separation of CO₂ and H₂S [9]. They mentioned this material could improve separation of these acid gases from biogas or natural gas markedly. Based on in-situ FT-IR analysis, they mentioned that –NH₂ function interact weakly with CO₂ through lone pair of relatively negative N-atom of amine and relatively positively charged C-atom of CO₂. Also, hydrogen bonding is the main reason for improved H₂S separation in a way that H₂S acts as hydrogen bond donor and amine, through its N atom, acts as hydrogen bond acceptor.

      Amine function is an ideal gust-adsorptive site to interact with different type of guests. Since it contains positively charged H atoms, it can interact as hydrogen bond donor site. Also, it can interact as hydrogen acceptor or hydrogen bond acceptor site as well as Lewis basic site through its N atom. These multiple chemical features enable amine decorated MOFs to apply as a host for different type of analytes. Their Lewis basicity is potentially suitable to interact with metal ions and their ability to participate in hydrogen bonding is ideal to interact with small