Analysis of Active Sites on Copper Ion-Exchanged ZSM-5 for CO Adsorption through IR and Adsorption-Heat Measurements

Abstract

The state of copper ion exchanged in ZSM-5-type zeolite has been investigated through the IR and adsorption-heat measurements at 301 K by using CO as a probe molecule. As a result, it was proved that there are at least three kinds of adsorption sites for a CO molecule on the copper ion-exchanged ZSM-5, which are responsible for the IR bands at 2159, 2149, and 2136 cm-1, as well as the differential adsorption-heats of 91, 82, and ∼70 kJ mol-1. Corresponding to these data, TPD also gives three desorption peaks at 363, 442, and 542 K. An excellent linear relationship has been established between the stretching vibrational frequencies due to the adsorbed CO species and the differential adsorption-heat values. When CO gas is introduced to the 723 K-treated sodium ion-exchanged ZSM-5, this sample provides the adsorption-heat of about 35 kJ mol-1 in the whole adsorption region studied and the weak IR bands at 2175 and 2112 cm-1. For the sodium ion-exchanged ZSM-5 and CO system, the above correlation does not hold in the same plot containing Cu−CO species. This fact is interpreted in terms of the difference in the nature of bonding between the electrostatic force for the sodium ion-exchanged ZSM-5 and CO system and the covalent nature for the copper ion-exchanged ZSM-5 and CO system. More detailed discussions are also made on the nature of the Cu−CO bond.

Investigation of the effect of carbon monoxide on the oxidative carbonylation of methanol to dimethyl carbonate over Cu+X and Cu+ZSM-5 zeolites

“…The heat of adsorption for CO on Cu + ZSM-5 is found to be 80.5 ± 3.5 kJ/mol, and the equilibrium constant can be fit to K eq (T) = 5.5 × 10 −6 exp(9680/T) atm −1 . This heat of adsorption is in the range of previous microcalorimetry investigations for CO on Cu + ZSM-5, which determined heats of adsorption that vary from 91 to 70 kJ/mol [19], 120 to 100 kJ/mol [20], and 130 to 40 kJ/mol [21]. The pre-exponential factor is higher than the value estimated using Transition State Theory [18], Fig.…”

“…12), the Cu + ZSM-5 shows a much higher ratio of CO coverage/methoxide coverage than Cu + X, which agrees qualitatively with the surface coverage of CO predicted by the modeling of the kinetic experiments. It is difficult to obtain a quantitative measurement for these ratios, because the Cu + ZSM-5 shows a strong peak at 2137 cm −1 , which has been observed previously as CO adsorption on a weaker Cu + site [19].…”

New frontier in transmission IR spectroscopy of molecules adsorbed on high surface area solids: Experiments below liquid nitrogen temperature

“…The bands at 3130 and 3079 cm À1 are assigned to similar Cu + (H 2 ) adducts formed on the ions counterbalancing the ZSM-5 framework negative charge. The presence of a doublet can be accounted for by recalling that in the ZSM-5 structure Cu + can occupy slightly different locations [29,31,96] differing for the number of framework oxygen ligands entering the Cu + first coordination sphere. It is worth noticing that H 2 is able to distinguish between these two different types of sites, giving rise to two bands separated by 51 cm À1 .…”

Ethene and ethyne molecules interacting with Cu+ sites in zeolites—Quantitative infrared spectroscopic studies

“…In fact, the value of the heat of adsorption is higher for ethene (E ads. C 2 H 4 = 90-110 kJ/mol) [51][52][53] than for CO molecules (E ads. CO = 65-80 kJ/mol) [36,54,55].…”