Deducing CO₂motion, adsorption locations and binding strengths in a flexible metal–organic framework without open metal sites

Abstract

Microporous metal-organic frameworks (MOFs) have high surface areas and porosities, and are well-suited for CO2 capture. MIL-53 features corner-sharing MO4(OH)2 (M = Al, Ga, Cr, etc.) octahedra interconnected by benzenedicarboxylate linkers that form one-dimensional rhombic tunnels, and exhibits an excellent adsorption ability for guest molecules such as CO2. Studying the behavior of adsorbed CO2 in MIL-53 via solid-state NMR (SSNMR) provides rich information on the dynamic motion of guest molecules as well as their binding strengths to the MOF host, and sheds light on the specific guest adsorption mechanisms. Variable-temperature (13)C SSNMR spectra of (13)CO2 adsorbed within various forms of MIL-53 are acquired and analyzed. CO2 undergoes a combination of two motions within MIL-53; we report the types of motion present, their rates, and rotational angles. (1)H-(13)C CP SSNMR experiments are used to examine the proximity of (1)H atoms in the MOF to (13)C atoms in CO2 guests. By replacing (1)H with (2)H in MIL-53, the location of the CO2 adsorption site in MIL-53 is experimentally confirmed by (1)H-(13)C CP SSNMR. The binding strength of CO2 within these MIL-53 MOFs follows the order MIL-53-NH2 (Al) > MIL-53-NH2 (Ga) > MIL-53 (Al) > MIL-53 (Ga).

Solid‐State NMR Spectroscopy: A Powerful Technique to Directly Study Small Gas Molecules Adsorbed in Metal–Organic Frameworks

“…CO 2 dynamics in MOFs without OMSs, such as MIL‐53(Al, Ga)‐ np and MIL‐53‐NH 2 (Al, Ga)‐ np , have also been studied . In these two cases, static VT 13 C experiments revealed that CO 2 undergoes a localized wobbling about the hydroxyl groups on the linker and a two‐fold hopping between adjacent hydroxyl sites (Figure ).…”

“…CO 2 dynamics in MOFs without OMSs, such as MIL-53(Al, Ga)-np and MIL-53-NH 2 (Al, Ga)-np,h ave also been studied. [16] In these two cases, static VT 13 Ce xperiments revealed that CO 2 undergoes al ocalized wobbling aboutt he hydroxyl groups on the linker and at wo-fold hopping between adjacent hydroxyl sites ( Figure 2). As compared to MIL-53,s maller wobbling angles, a,w ere observed for MIL-53-NH 2 .T his indicates ad ecrease in dynamics and thus an increase in CO 2 -MOF interaction, consistent with previouso bservations that amine functionalization can enhanceC O 2 capturea bility.…”

“…The lack of OMSs in some MOFs can make the CO 2 binding sites less than trivial to determine. Nevertheless, the CO 2 binding sites in these MOFs can be identified via cross polarization (CP) NMR experiments . Under favorable conditions, CP will transfer magnetization from abundant spins (e.g., 1 H) to nearby dilute spins (e.g., 13 C), enhancing the signal of the dilute spins .…”

“…MOFs are promising CO 2 adsorbents, [13] and SSNMR spectroscopy has been employed to studyC O 2 chemisorptiona nd physisorption in severalM OFs. [12] This section focuses on physisorptiona nd demonstrates how guest dynamics, [14][15][16][17][18][19][20][21][22] MOF-CO 2 interaction strengths [16] and binding sites [16,17] can be investigated.…”

“…Long-and short-range CO 2 dynamics can both be studied using SSNMR spectroscopy.E ven thoughl ong-ranges tudies can provide important information on diffusivity, [19,22] local CO 2 dynamics has been the main focus for many of the recent SSNMR investigations.Hence, we restrict our currentd iscussion to local CO 2 dynamics studies. Various SSNMR experiments (e.g.,e xchange spectroscopy, [20] T 1 relaxation studies, [21] variable-temperature (VT) experiments [14][15][16]18] )h ave been used to probe CO 2 motions, and static VT 13 Ce xperiments are one of the most commonly employed techniques. Since the appearance of powder patterns is influenced by molecular orientations, and therefore changes in orientations (i.e.,d ynamics), motional modelsc an be directly extracted by simulating the VT spectra.…”