Characterization of Specific N₂-Adsorption Site Existing on CuZSM-5 Type Zeolite:  Effect of Ion-Exchange Level on Adsorption Properties

Abstract

The specific adsorption property of copper-ion-exchanged ZSM-type zeolite (CuZSM-5) for dinitrogen molecules (N 2 ) has been elucidated by methods such as infrared (IR) and emission (ES) spectroscopy and by the measurements of heat of adsorption and adsorption isotherms of N 2 and CO. In the IR spectra an intense band appears at 2295 cm -1 , which is attributed to the adsorbed N 2 species. The amount of adsorbed CO, as well as the adsorbed N 2 , increases with increasing copper-ion-exchange level of the ZSM-5 sample. By use of CO as a probe molecule, it was found that on the 873 K treated CuZSM-5 sample there are at least three types of adsorption sites available for CO adsorption; these are responsible for giving the IR bands due to the adsorbed CO species at 2159, 2151, and 2135 cm -1 . The adsorption behavior of N 2 molecules on the samples, which have various copper-ion-exchange levels and preadsorbed CO species, has been investigated, and it was found that the 2151 cm -1 band in the IR spectra reflects an N 2 adsorption site. Emission spectra were also obtained at each step of N 2 adsorption; the emission band due to the exchanged copper-ion species decreases in intensity with increasing pressure of N 2 . These results can be interpreted as follows. The monovalent copper ion (Cu + ) formed during the evacuation of the sample at 873 K acts as an effective site for N 2 adsorption. Moreover, the site responsible for giving the IR band at 2151 cm -1 and the ES band at 18500 cm -1 plays an important role in the N 2 adsorption. By reference to the results of X-ray absorption fine structure (XAFS) measurements reported previously, it is thought that the active site, i.e., Cu + species, for N 2 adsorption interacts with the lattice oxygen and with CO or N 2 molecules to produce a pseudotetrahedral four-coordination structure. The relation between the heat values and the frequency of the IR band due to the adsorbed CO species gives a linear regression, which indicates that σ-bonding is predominantly operative in the bonding of Cu + with CO or N 2 molecule.

Unusual Forms of Molecular Hydrogen Adsorption by Cu+1 Ions in the Copper-Modified ZSM-5 Zeolite

“…Therefore, there is no doubt that after reduction in CO, the predominant part of copper was transformed into Cu þ ions. This conclusion also well agrees with the previously reported literature data on self-reduction of bivalent copper after vacuum treatment at high temperature or reduction in CO [9][10][11][12][13].…”

“…It was also reported in Refs. [9][10][11][12][13] that reduced Cu þ ions very strongly adsorb CO and nitrogen. In Ref.…”

Synthesis and ionic exchanges of zeolites for gas adsorption

“…They demonstrated that the presence of lithium cations strongly affected the zeolite framework. The comparison between bond lengths and valence angles in Li A and Na A zeolites revealed a distortion of the skeleton, especially around the oxygen linking the AlO 4 and SiO 4 tetrahedra due to the formation of a molecule of Li 2 O in the pore entry after a total ionic exchange by Li C . Also, Knops-Gerrits et al 25 investigated the zeolites using Raman spectroscopy.…”

“…The mid-IR region of the spectrum is useful in this regard, because it contains fundamental vibrations of the framework Al, Si-O 4 or (TO 4 ) tetrahedral. 19 The IR spectrum was determined by KBr wafer technique and obtained with a Perkin-Elmer FTIR 1600 spectrophotometer.…”

Adsorption of Nitrogen Monoxide and Carbon Monoxide on Copper-Exchanged ZSM-5: A Cluster and Embedded Cluster Study

“…However, recent IR and adsorption experiments suggested that CO molecules bind to Cu-ZSM-5 zeolite stronger than NO molecules. 24,43 Adsorption of CO and NO have also been the subject of numerous theoretical studies. 18,[27][28][29][30]34,35,[44][45][46][47][48][49][50] The calculated results for adsorption energy, frequency shift, geometry, and the coordination number are still scattered due to differences in the topological structure of the model, cluster size, constraints, basis sets, and the levels of theory employed.…”