Active copper species in 1-butene skeletal isomerization: Comparison between copper-modified MCM-41 and beta catalysts

The role of copper was studied in the skeletal isomerization of 1-butene over copper-modified mesoporous MCM-41 molecular sieve and Beta zeolite. The Cu-H-MCM-41 and Cu-H-Beta catalysts were synthesized in our laboratory and characterized by XRD, nitrogen adsorption, X-ray fluorescence, FTIR of adsorbed pyridine and direct current plasma atomic emission spectrometry. The oxidation state of copper after oxidation and reduction in Cu-H-MCM-41 was evaluated by FTIR with probe molecules. Copper ion-exchanged and the proton forms of MCM-41 and Beta catalysts were tested towards 1-butene skeletal isomerization by varying the weight hourly space velocity and temperature. Quantum chemical calculations at the B3LYP/6-31 + G** level were performed in order to understand the role of copper at the molecular level. Copper in Cu-H-MCM-41 pretreated in synthetic air was mostly in the form of Cu²⁺ but reduced during the catalytic experiment to the metallic form Cu⁰ via Cu⁺. Even if the copper exchange decreased the amount of Brønsted acid sites, Cu-H-MCM-41 pretreated in synthetic air was more active than H-MCM-41 towards 1-butene skeletal isomerization. The enhanced catalytic activity is due to copper Cu⁺, which was formed during the reaction. Introduction of copper into H-Beta, however, did not have any effect at all on the performance of the catalyst. The probable reason for this is the high initial activity of copper-modified H-Beta causing a very fast reduction of copper to the inactive metallic form Cu⁰.