Physico-chemical and catalytic properties of Zr- and Cu–Zr ion-exchanged H-MCM-41

Abstract

H-MCM-41 catalyst ion-exchanged by Zr and/or Cu-cations were synthesized and characterized by XPS, ²⁷Al- and ²⁹Si-MAS-NMR, FTIR of pyridine adsorption, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and direct current plasma atomic emission spectrometry. The catalysts were evaluated towards 1-butene skeletal isomerization after pre-treatment with hydrogen or synthetic air. The dispersion of metals after ion-exchange was reasonably good, as revealed by LA-ICP-MS. Binding energies observed by XPS indicated that after reduction or oxidation pre-treatments copper is mainly present as Cu²⁺, except in case of reduced Cu-H-MCM-41. Pre-treatment had no influence on the oxidation state of Zr⁴⁺ species. Zr ion-exchange decreased the concentrations of Brønsted and Lewis acid sites. As a result, the initial conversion of 1-butene was decreased, but the selectivity and stability of the catalyst were increased. As reported before, the copper introduction decreased the Brønsted acid site density, but increased the conversion. The enhancement in the catalytic activity has been connected to the presence of copper with oxidation state +1, which is formed under reaction conditions due to the hydrogen formation. This led to a peculiar effect over oxidized bimetallic Cu–Zr-H-MCM-41 at a low temperature and high weight hourly space velocity (523 K and 15 h⁻¹, respectively): The initial conversion of 1-butene was almost zero but increased during the time-on-stream reaching the value of about 13 mol%. These observations are related to the reduction of Cu²⁺ species to Cu⁺ during the reaction. The catalyst evaluation and characterization results led to a conclusion that the presence of Zr stabilizes copper’s oxidation state of Cu⁺² under reductive conditions in the bimetallic Cu–Zr-H-MCM-41 catalyst.