Stability of the Square-Planar Cu2+ Sites in ZSM-5: Effect of Preparation, Heat Treatment, and Modification Full article
Journal |
Journal of Catalysis
ISSN: 0021-9517 , E-ISSN: 1090-2694 |
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Output data | Year: 1999, Volume: 186, Number: 2, Pages: 334-344 Pages count : 11 DOI: 10.1006/jcat.1999.2574 | ||||||
Tags | FLOWING GAS-MIXTURES; OXIDATION-STATES; Cu-ZSM-5; COPPER; ESR; 500-DEGREES-C; COORDINATION; CuH-ZSM-5O-2; ACETONITRILE | ||||||
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Abstract:
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.
Cite:
Kucherov A.V.
, Shigapov A.N.
, Ivanov A.A.
, Shelef M.
Stability of the Square-Planar Cu2+ Sites in ZSM-5: Effect of Preparation, Heat Treatment, and Modification
Journal of Catalysis. 1999. V.186. N2. P.334-344. DOI: 10.1006/jcat.1999.2574 WOS Scopus РИНЦ
Stability of the Square-Planar Cu2+ Sites in ZSM-5: Effect of Preparation, Heat Treatment, and Modification
Journal of Catalysis. 1999. V.186. N2. P.334-344. DOI: 10.1006/jcat.1999.2574 WOS Scopus РИНЦ
Dates:
Submitted: | Jan 27, 1999 |
Accepted: | May 24, 1999 |
Published print: | Sep 10, 1999 |
Published online: | May 25, 2002 |
Identifiers:
Web of science | WOS:000082789800010 |
Scopus | 2-s2.0-0033543778 |
Elibrary | 13312413 |
Chemical Abstracts | 1999:592256 |
Chemical Abstracts (print) | 131:263143 |
OpenAlex | W1968901264 |