Characterization of Active Sites on Copper Ion-Exchanged Mordenite for Dinitrogen Adsorption by Using CO as a Probe Molecule

Abstract

The peculiar feature of copper ion-exchanged mordenite in dinitrogen (N 2 ) adsorption has been investigated through the measurements of infrared (IR) and emission spectra (ES) and X-ray absorption fine structure (XAFS). IR spectra provided the definitive evidence for the existence of at least two kinds of adsorption sites on the 873 K-treated copper ion-exchanged mordenite; the absorption bands centered at 2160 and 2150 cm -1 appeared when CO molecules were adsorbed. The adsorbed species assigned to the former band was desorbed at 573 K in Vacuo and that to the latter one at 473 K. It was found that the site responsible for the latter band acts as effective sites for N 2 adsorption. XAFS spectra also showed new bands due to the CO adsorption on copper ion at 1.48 Å, 2.52 Å, and 8.981 keV, together with bands at 1.85 Å and 8.993 keV. The former set of bands was concomitant with the appearance of an IR band at 2150 cm -1 due to the adsorbed CO species. The two bands (at 8.981 and 8.993 keV) observed in the XAFS spectra for the CO-adsorbed sample are explained in terms of the formation of a planar three-or four-coordinate site through the reaction of copper ions with CO molecules. CO adsorption also caused a significant shift of the emission band from 470 to 430 nm. This band reverted to the original position by evacuating the sample at 473 K, accompanying a liberation of CO. This makes possible the interpretation that the adsorption site responsible for respective bands at 2150 cm -1 in IR, at 430 nm in ES, and at 8.981 keV in XAFS spectra through the interaction with CO molecules is effective for N 2 adsorption at ambient temperature.

Cu(I)/O2 Chemistry Using a β-Diketiminate Supporting Ligand Derived from N,N-Dimethylhydrazine: A [Cu3O2]3+ Complex with Novel Reactivity

“…Rationale (a) seems unlikely because while several [Cu(CO) 2 ] + species have been reported (cf. entry 2, Table 1), 16,27–31 they display ν(CO) values higher in energy than free CO (2143 cm −1 ) due to competition between the multiple CO ligands for dp electron density at the metal center. 27 In support of rationale (b), equilibria between Cu(I) species with bridging or terminal CO ligands (Figure 3) have been described for complexes supported by bidentate peralkylated diamines, which exhibit a pattern of ν(CO) values similar to that which we observe (entries 4–6, Table 1).…”

Relating Structure and Chemical Composition with Lewis Acidity in Zeolites: A Spectroscopic Study with Probe Molecules

“…This sample was a gift from Exxon Chemical Europe. NH 4 + -exchange of the Na + -zeolites was performed in NH 4 Cl (pro analyse, Merck) solutions with a 10-fold excess of NH 4 + with respect to the cation exchange capacity (CEC) at a solid: liquid ratio of 1 g/dm 3 . The samples were exchanged 3-times overnight under continuous stirring and washed with bidistilled water until Cl --free.…”

“…While cations in zeolites are well characterized, [2][3][4][5][6] the structure of true Lewis sites is still controversial. Some authors associate LAS with trigonal Al atoms formed as a result of zeolite dehydroxylation by thermal treatment.…”

“…Some authors associate LAS with trigonal Al atoms formed as a result of zeolite dehydroxylation by thermal treatment. 7 Other research groups have proposed Al 3+ -species, or various other nonframework species (AlO + , Al(OH) 2+ , Al(OH) 2 + , Al(OH) 3 , AlO(OH), Al 2 O 3 , etc. ), leached from the zeolite framework during chemical or thermal treatment.…”

Nature of Copper Active Sites in CuZSM-5: Theory and Experiment

“…Another interesting property of Cu + in ZSM-5 is the possibility of bonding N 2 at room temperature [2].…”