Characteristics of Silver Ions Exchanged in ZSM-5-Type Zeolite, Aluminosilicate, and SiO₂ Samples:  In Comparison with the Properties of Copper Ions Exchanged in These Materials

Abstract

The IR technique in combination with adsorption microcalorimetry was used to picture the bonding nature of silver ion exchanged or supported on solid materials, such as ZSM-5-type zeolite, aluminosilicate, and SiO2, by utilizing CO as a probe molecule. It has become apparent that there exists an adsorption site on which the CO molecule is adsorbed to give an IR absorption band: IR band at around 2193 cm-1 for silver-ion exchanged ZSM-5 (AgZSM-5) and aluminosilicate (Ag/SiO2·Al2O3), and at 2177 cm-1 for silver-ion supported SiO2 (Ag/SiO2). The CO adsorption took place accompanying a large heat evolution of 100−80 kJ mol-1 on the former two samples and relatively small heat of 70−40 kJ mol-1 on the latter sample; silver ions exchanged with protons acting as Brønsted acid sites are responsible for the strong adsorption sites for CO adsorption. Taking account of the relationship between the differential heat of adsorption (q diff) and the stretching vibrational frequency of adsorbed CO (ν CO), it was concluded that the electrostatic interaction is dominantly operative in these systems. The large adsorption heats in the initial stage of CO adsorption on the AgZSM-5 and Ag/SiO2·Al2O3 samples may be successfully explainable by considering a little contribution of σ-bonding in addition to the electrostatic interaction. The quantum chemical calculation was performed to justify the two types of ion-exchange model for silver ion coordinated to two or three lattice oxygen atoms in AgZSM-5, as well as to clarify the bonding nature between the exchanged silver ion and CO molecule. As the results, the two-coordinated silver ions in AgZSM-5 can adsorb CO molecules and give the values of about 100 kJ mol-1 and 2193 cm-1, and the three-coordinated silver ions weakly adsorb CO to give the values of about 80 kJ mol-1 and 2184 cm-1. These adsorption energies are much smaller and the stretching frequencies due to the adsorbed CO are higher, compared with the case of CO adsorption on copper-ion exchanged ZSM-5 (CuZSM-5). From these results it can be interpreted that the dominant force operating in the AgZSM-5−CO system is electrostatic attraction, as is different from the case of CuZSM-5−CO system in which the σ-donation is dominant in the bonding. This difference is explained by taking account of the differences in energy gap between 4d−5s for silver ion and 3d−4s for copper ion; in the former case the hybridization of orbital is limited to result in a large σ-repulsion. The present experiment clearly proved that the evaluation of bonding nature from the relationship between q diff and ν CO is useful in the characterization of exchanged ions. The specific bonding nature between copper ion (Cu+) exchanged in ZSM-5 and CO molecule was also clarified.

On the possibility of AgZSM-5 zeolite being a partial oxidation catalyst for methane

“…After evacuation at 300 K, the EXAFS spectrum of Ag(H)ZSM-5 gives a broad band at around 1.69 Å that is not converted for phase shift. It is assigned to the back scattering from the nearest-neighbor oxygen atoms composed of the lattice oxygen atoms and/or the adsorbed water molecules, by comparison with the spectrum of the reference sample, Ag 2 O [7]. After evacuation at 473 K, a similar spectral pattern was observed, indicating that the Ag + ions remained at the ion-exchanged site, releasing H 2 O molecules in the evacuation process.…”

“…This tendency did not change, even after the temperature was raised to 373 and 473 K in the presence of CH 4 and O 2 (spectra 3 and 4). However, new bands appeared at 2192 and 2330 cm −1 at temperatures above 573 K, which are attributed to the stretching vibrations of CO and CO 2 molecules adsorbed onto the Ag + ions in ZSM-5 [7], indicating the occurrence of partial oxidation of CH 4 . The band-area ratio of CO to (CO 2 + CO) increased after the heat treatment at 673 K. Raising the temperature to 773 K brought about a further increase in the production of CO. As can be seen in Fig.…”

“…The HZSM-5 sample thus prepared was then ion exchanged with Ag + ions by the conventional ion-exchange method of submerging it in an aqueous solution of silver nitrate at 363 K. This ion-exchange process was repeated several times to obtain a silver-ion-exchanged ZSM-5-type zeolite sample (Ag(H)ZSM-5) with the desired exchange capacity. The ionexchange capacity was determined to be 92% by the chelatometric method, where the sample was treated with a solution of HClO 4 and the concentration of silver ions liberated was measured [7]. It was assumed that one monovalent H + ion is exchanged with one monovalent silver ion.…”

“…This result corresponds with the results of activation observed by Baba and Sawada, indicating the same mechanism at work, i.e., Ag-CH 3 and Ag-H formation on Ag + exchanged in the ZSM-5 sample [12][13][14]. One of the reasons for the lower efficiency observed in the Ag/SiO 2 ·Al 2 O 3 sample may be the difference in the acid strength of the Ag(H)ZSM-5 sample, as is apparent from the observed positions of IR bands for CO molecules adsorbed onto the respective samples [7]. Another reason for the lower efficiency observed may be the difference in the states of metal particles among the systems Ag(H)ZSM-5 and Ag/SiO 2 ·Al 2 O 3 .…”

Determination of Acid/Base Properties by Temperature Programmed Desorption (TPD) and Adsorption Calorimetry

“…CO adsorption microcalorimetry has been used in combination with the IR technique to picture the bonding nature of silver ion-exchanged ZSM-5 type zeolites (Ag-ZSM-5) [196]. CO adsorption at 301 K gave rise to a large heat Fig.…”