Stability of the square-planar Cu2+ sites in ZSM-5: Effect of preparation,heat treatment, and modification

HZSM-5 with SiO2/Al2O3 = 50 into which 0.5 wt% Cu (Cu/ Al similar to 0.25) was introduced after the removal of extralattice Al species by basic or aci dic treatment shows no inhibition of the irreversible loss of active, tetra hedrally coordinated Cu2+ sites upon thermal treatment. Removal of the extr alattice Al coupled with a complete exchange of all Bronsted groups for Cu2 + cations results in a noticeable stabilization of the reactive square-plan ar sites. FTIR data confirm the absence of free Bronsted groups in the exch anged samples. Quantitation of "ESR-visible Cu2+" in these exchanged sample s, calcined at 500-800 degrees C in air, gives values of 1.8 and 1.65 wt% f or base-treated and acid-treated samples, respectively. These values are ne ar the limit of exchange assuming a Cu/Al=1 stoichiometry in this high-sili ca 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 comple tely exchanged samples that one part of Cu2+ ions inhibits the irreversible loss of another portion of Cu2+ quite efficiently. Also, a strong stabiliz ing effect is noted for samples prepared on acid-pretreated zeolite contain ing 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 calc ination at 800 degrees C. The effect of La, Ca, or Sr is also observable bu t less pronounced, Both the acidic pretreatment and the introduction of an appropriate amount of Mg2+ ions contribute to the better stabilization of t he most reactive part of Cu2+ cations in CuMgZSM-5. Magnesium ions compete with Cu2+ for the cationic positions replacing first the less reactive, pyr amidal 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 d egrees C but simultaneously improves further the thermal stability of the s quare-planar Cu2+ ions. (C) 1999 Academic Press.