Modeling and Experiment for Oxygen Isotope Exchange over Copper-Containing Mordenite

Abstract

Oxygen isotope exchange for sodium- and copper-containing mordenite was studied in the isothermal regime using a plug-flow reactor. No prominent exchange was observed over sodium mordenite at 873 K. In contrast, the presence of copper significantly increases the lability of oxygen atoms resulting in a substantial rate of isotope exchange in the temperature range between 793 and 873 K. A novel numerical model accounting for two kinetically distinct sources of exchangeable oxygen atoms was proposed and shown to provide high quality of fitting of experimental data. Two types of exchangeable atoms correspond to about 9 and 25% of the total amount of oxygen atoms, respectively, and these values do not change with the exchange temperature. The exchange mechanism involving two oxygen atoms from the surface dominates in the case of the first source of exchangeable atoms, while the second source undergoes exchange mainly via a single-atom exchange mechanism.

Oxygen Isotope Exchange over Copper-Containing Mordenite: The Effect of Copper Loading and Si/Al Ratio

“…As reported by Leglise et al, FeMOR has around 34% of the exchangeable oxygen atoms at 773 K. Similar values were obtained in our recent study on oxygen isotope exchange over copper-containing mordenite . Notably, the sodium and proton forms of zeolites do not undergo the isotope exchange under similar conditions at a prominent rate, , showing that the hosted metal species induce the lability of the framework oxygen atoms. Importantly, the mechanism of the exchange depends significantly on the nature of the hosted species.…”

“…Depending on the number of oxygen atoms from the solid phase participating in the exchange reaction, zero-atom (R 0 ), single-atom (R 1 ), and two-atom (R 2 ) kinetically distinct types of exchange were suggested. We previously showed that the isothermal oxygen isotope exchange over copper-containing mordenite can be adequately described using the kinetic model with two independent sources of exchangeable oxygen, which undergo both R 1 and R 2 types of exchange . The corresponding set of partial differential equations (PDE) is as follows with the following initial and boundary conditions where C­(O 2 ) gas represents the concentration of the oxygen molecules in the gas phase; α fast and α slow are the fraction of 18 O atoms in the “fast” and “slow” source, respectively; and C­(O) (mol·L –1 ), k 1 (L·mol –1 ·s –1 ) and k 2 (L 2 ·mol –2 ·s –1 ) are the concentration of exchangeable oxygen atoms and kinetic constants of single-atom and two-atom exchange, respectively.…”

“…Voskoboinikov et al showed that isolated Fe 3+ cations or small iron-oxo clusters in ZSM-5 zeolite induce the single-atom type of exchange, while the material with hosted larger iron-oxo clusters mainly exchanges oxygen atoms via the two-atom exchange mechanism . Moreover, the kinetics of the isotope exchange over iron- and copper-containing zeolites is complicated by the presence of at least two temporary resolved processes: initial fast exchange followed by a secondary slow process. ,,, The effect of the sample composition, such as Si/Al ratio and copper content, on the isotope exchange kinetics over copper-containing zeolites has not been studied systematically thus far.…”

“…Valyon et al reported that about 13 and 33% of the oxygen atoms in copper-containing MFI and FAU, respectively, can be exchanged with the gas phase at 773 K. 40 As reported by Leglise et al, 42 FeMOR has around 34% of the exchangeable oxygen atoms at 773 K. Similar values were obtained in our recent study on oxygen isotope exchange over coppercontaining mordenite. 67 Notably, the sodium and proton forms of zeolites do not undergo the isotope exchange under similar conditions at a prominent rate, 57,67 showing that the hosted metal species induce the lability of the framework oxygen atoms. Importantly, the mechanism of the exchange depends significantly on the nature of the hosted species.…”

“…39 Moreover, the kinetics of the isotope exchange over iron-and copper-containing zeolites is complicated by the presence of at least two temporary resolved processes: initial fast exchange followed by a secondary slow process. 39,41,42,67 The effect of the sample composition, such as Si/Al ratio and copper content, on the isotope exchange kinetics over copper-containing zeolites has not been studied systematically thus far.…”