Adsorption of carbon monoxide, dinitrogen, and carbon dioxide on the porous metal−organic framework Mg-MOF-74 was investigated by means of a combined methodology comprising variable-temperature infrared spectroscopy and ab initio periodic DFT-D calculations using the CRYSTAL code. Both CO and N2 were found to form nearly linear (Mg2+···CO and Mg2+···NN) adsorption complexes, in contrast with CO2, which forms an angular Mg2+···OCO complex. From IR spectra recorded at a variable-temperature, the standard adsorption enthalpy (ΔH 0) was found to be −29, −21, and −47 kJ mol−1 for CO, N2, and CO2, respectively. Calculated values of ΔH 0, including an empirical correction for dispersion forces, resulted to be in a reasonably good agreement with those experimentally obtained. Calculations also showed the very significant role played by dispersion forces, which account for about one-half of the adsorption enthalpy for each of the three adsorbates, CO, N2, and CO2. The results are discussed in the broader context of the adsorption of the same gases on other MOFs and also on zeolites.
Siting of copper ions in Cu I -Y zeolite, prepared by gas-phase exchange of NH 4 -Y with CuCl, has been investigated employing XRPD, XAS and IR spectroscopies. An X-ray powder diffraction study of the zeolite in a vacuum shows that 23.4(2) cuprous ions are located at site I*, 6.1(3) at site II, and 11.5(3) at site II* (sites I* and II* are at the center of the plane of the six-membered ring connecting the hexagonal prism with the sodalite and the sodalite with the supercage, respectively). Addition of CO induces a relevant migration of copper ions from sites II and II* to a more exposed type II. EXAFS analysis shows that Cu I ions in the outgassed zeolite are surrounded by 2.8(3) oxygen atoms of the zeolite framework, the average Cu I -O distance being 1.99(2) Å. Both X-ray measurements and FTIR spectroscopy show that CO is adsorbed on the zeolite at room temperature with formation of carbonyl adducts. At liquid-nitrogen temperature and low CO pressure, two types of monocarbonyl species are observed, corresponding to CO adsorbed on copper ions located at sites II and II*. On increase of the CO pressure and subsequent formation of polycarbonylic species, cations at site II* move to the more exposed position II, and a single kind of tricarbonyl adducts is observed. IR spectroscopy also provides evidence for the interaction of NO with copper ions located at sites II and II*, which are the first sites able to adsorb up to two molecules of NO, whereas cations at site II*, because of their lower coordinative unsaturation, can only form Cu I (NO) adducts. NO proves to be a sensitive probe not only for cuprous but also for cupric ions.
We present a multitechnique (EPR, XANES, EXAFS, and IR of adsorbed NO) study of the coordination and oxidation chemistry of copper species hosted in mordenite (MOR) zeolite under different conditions. Starting from a 100% Cu 2+ -MOR, the progressive thermal activation causes first the loss of water molecules from the Cu 2+ coordination sphere, accompanied by a partial aggregation in Cu 2+ -O-Cu 2+ complexes, and then the Cu 2+ f Cu + reduction with oxygen elimination. The presence of EPR inactive cupric pairs, witnessed by EXAFS, explains the systematic underestimation of the fraction of Cu 2+ species evaluated by EPR, with respect to that obtained from XANES. The data discussed here confirm the interpretation of the so-called "self-reduction" phenomenon of cupric ions emerging in a previous study performed on Cu-ZSM-5 [J. Phys. Chem. B 2000, 104, 4064]. The reoxidation of the so obtained Cu + -MOR by O 2 is dramatically favored by the presence of water. This fact explains the poisoning effect of water in the deNO x activity of Cu-exchanged zeolites. The coordination of NO molecules on the Cu + -MOR system was studied in situ at liquid nitrogen temperature. The deNO x chemistry was then switched on by allowing the system to reach room temperature in the NO atmosphere. In all stages of this study, comparison is made with a Cu + -ZSM-5 model system. The differences observed between these two systems are explained in terms of the different structural (cation concentration and environment) characteristics.
The structural and spectroscopic similarities between homogeneous and heterogeneous [CuI(CO) n ]+A- (n = 1−3; A- = [AsF6]- or zeolite anion Z-) carbonyls are evidenced and discussed. While [CuI(CO)2]+ and [CuI(CO)3]+ have linear and planar structures in [CuI(CO) n ]+[AsF6]- solid compounds, they are bent in the zeolite channels. The [AsF6]- anion has a base strength lower than that of the zeolite anion; consequently, the [CuI(CO) n ]+ moieties have a more positive character in the solid compounds than in the intrazeolitic ones. The CuI−framework distance is influenced by the formation of the [CuI(CO) n ]+ complexes: this is demonstrated by both EXAFS and IR results concerning the effect of CO complexation on the CuI−framework distance and on the CuI-sensitive skeletal modes, respectively. The role of basic ligands in increasing the π-character of the Cu−CO bond (with simultaneous decrease of the electrostatic and σ contributions) has been studied on [CuI(CO) n (NH3)]+Z- (n = 1, 2) and [CuI(CO)(H2O) n ]+Z- (n = 1, 2) complexes synthesized in situ in the zeolite channels.
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