Stability of the Square-Planar Cu2+ Sites in ZSM-5: Effect of Preparation, Heat Treatment, and Modification
Journal of Catalysis
, E-ISSN: 1090-2694
Scientific Research Laboratory, Ford Motor Company, MD 3179/SRL, P.O. Box 2053 Dearborn, Michigan 48121
Institute of Chemistry of Natural Organic Materials, Krasnoyarsk, Russia
Zelinsky Institute of Organic Chemistry, RAS, Moscow, Russia
HZSM-5 with SiO2/Al2O3=50 into which 0.5 wt% Cu (Cu/ Al∼0.25) was introduced after the removal of extralattice Al species by basic or acidic treatment shows no inhibition of the irreversible loss of active, tetrahedrally coordinated Cu2+ sites upon thermal treatment. Removal of the extralattice Al coupled with a complete exchange of all Brønsted groups for Cu2+ cations results in a noticeable stabilization of the reactive square-planar sites. FTIR data confirm the absence of free Brønsted groups in the exchanged samples. Quantitation of “ESR-visible Cu2+” in these exchanged samples, calcined at 500–800°C in air, gives values of 1.8 and 1.65wt% for base-treated and acid-treated samples, respectively. These values are near the limit of exchange assuming a Cu/Al=1 stoichiometry in this high-silica material. Formation of aggregated copper species is of minor importance, and the majority of Cu2+ remains well isolated and contributes to the ESR signal. It is evident from the comparison of partially exchanged and completely exchanged samples that one part of Cu2+ ions inhibits the irreversible loss of another portion of Cu2+ quite efficiently. Also, a strong stabilizing effect is noted for samples prepared on acid-pretreated zeolite containing 0.5 wt% Cu and additional Mg2+ cations. Here a large part of the copper ions (>50%) preserves the parent square-planar Cu2+ state even after calcination at 800°C. The effect of La, Ca, or Sr is also observable but less pronounced. Both the acidic pretreatment and the introduction of an appropriate amount of Mg2+ ions contribute to the better stabilization of the most reactive part of Cu2+ cations in CuMgZSM-5. Magnesium ions compete with Cu2+ for the cationic positions replacing first the less reactive, pyramidal Cu2+ cations. An increase in the Mg2+ loading results in a decrease in the number of the reactive cupric sites in samples calcined at 500–550°C but simultaneously improves further the thermal stability of the square-planar Cu2+ ions.