Oxygen Isotope Exchange over Copper-Containing Mordenite: The Effect of Copper Loading and Si/Al Ratio
2021
Artsiusheuski, Mikalai A. | van Bokhoven, Jeroen A. | Sushkevich, Vitaly L.
Oxygen isotope exchange over copper-containing mordenite with different copper loadings and Si/Al ratios was studied in temperature-programmed and isothermal regimes. The kinetics of the exchange was described quantitatively using a model comprising “fast” and “slow” sources of exchangeable oxygen atoms. The amount of “fast” exchangeable oxygen in the materials is determined by the aluminum content, while the number of atoms belonging to the “slow” source is independent of the composition of the material and the experimental conditions. Copper-containing mordenites with a Si/Al ratio of 6 containing dimeric species demonstrated measurable isotope exchange at 793–893 K. The decrease in the copper loading, from 4.3 to 1.7 wt %, lowered the number of catalytic sites and, consequently, the kinetic constants of isotope exchange by a factor from 3 to 5, while preserving the mechanism of exchange and the apparent activation energies. In contrast, the increase in the Si/Al ratio from 6 to 46 led to a change in the copper speciation to preferably monomeric sites. This resulted in a significant decrease in the exchangeability of the oxygen atoms in copper-containing mordenite and required a 200 K higher temperature for any prominent exchange rate.
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