Investigation of the spin-lattice relaxation of 13CO and 13CO2 adsorbed in the metal-organic frameworks Cu3(btc)2 and Cu3−xZnx(btc)2

Abstract

The (13)C nuclear spin-lattice relaxation time of (13)CO and (13)CO2 molecules adsorbed in the metal-organic frameworks (MOFs) Cu2.97Zn0.03(btc)2 and Cu3(btc)2 is investigated over a wide range of temperatures at resonance frequencies of 75.468 and 188.62 MHz. In all cases a mono-exponential relaxation is observed, and the (13)C spin-lattice relaxation times (T1) reveal minima within the temperature range of the measurements and both frequencies. This allows us to carry out a more detailed analysis of the (13)C spin relaxation data and to consider the influence due to the spectral functions of the thermal motion. In a model-free discussion of the temperature dependence of the ratios T1 (T)∕T1,min we observe a motional mechanism that can be described by a single correlation time. In relation to the discussion of the relaxation mechanisms this can be understood in terms of dominating translational motion with mean jump distance being larger than the minimum distances between neighboring adsorption sites in the MOFs. A more detailed discussion of the jump-like motion observed here might be carried out on the basis of self-diffusion coefficients. From the present spin relaxation measurements activation energies for the local motion of the adsorbed molecules in the MOFs can be estimated to be 3.3 kJ∕mol and 2.2 kJ∕mol, for CO and CO2 molecules, respectively. Finally, our findings are compared with our recent results derived from the (13)C line shape analysis.

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

“…CO is a poisonous pollutant produced from fossil fuel combustion and can potentially be removed using porous solids such as MOFs. Similar to the case of CO 2 , CO dynamics and binding in MOFs can be studied using 13 C SSNMR experiments . For instance, static and magic angle spinning VT 13 C measurements were utilized by Bertmer and co‐workers to study CO in Cu 3− x Zn x (btc) 2 .…”

“…The fast exchange between CO attached to Cu 2+ and adsorbed CO that are not directly attached to Cu 2+ resulted in temperature‐dependent δ iso ( 13 C), which were employed to calculate the activation energy ( E a ) for the motion of the adsorbed CO. The E a was found to be 6.1 kJ mol −1 and the motion was concluded to be influenced by the desorption of CO. 13 C T 1 relaxation analysis was also conducted to analyze the CO dynamics . CO molecules were found to undergo a jump‐like motion, where the jump length is greater than the smallest distance between adjacent binding sites, and an E a of 3.3 kJ mol −1 was obtained.…”

“…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.…”

“…Similart ot he case of CO 2 ,C Od ynamics and binding in MOFs can be studied using 13 CS SNMR experiments. [21,26,27] For instance, static and magic angle spinning VT 13 Cm easurements were utilized by Bertmer and co-workerst os tudy CO in Cu 3Àx Zn x (btc) 2 . [26] The fast exchange between CO attached to Cu 2 + anda dsorbed CO that are not directly attached to Cu 2 + resultedi nt emperature-dependent d iso ( 13 C), which were employed to calculate the activation energy (E a )f or the motion of the adsorbed CO.…”