Influence of the Brönsted and Lewis acid sites on the catalytic activity and selectivity of Fe/MCM-41 system

The system Fe2O3/MCM-41, with high iron dispersion, was synthesized in order to introduce Lewis acid sites into the MCM-41 structure. Two calcination atmospheres (inert and oxidant) were used to produce iron nanoclusters with different structural properties. Besides, a silylation treatment was realized on both solids with the aim of neutralizing the Brönsted acidity associated with the MCM-41 silanol groups. The samples were characterized by atomic absorption spectroscopy, X-ray diffraction at low angles, N2 adsorption, temperature-programmed reduction, Fourier transform infrared spectroscopy, nuclear magnetic resonance of 29Si, and Mössbauer spectroscopy. The influence of the structural changes of the nanoclusters and the effect of the simultaneous presence of Lewis and Brönsted acid sites were evaluated studying the product distribution from the α-pinene oxide isomerization reaction. It was determined that the Brönsted acidity of the Fe/MCM-41 system has not the sufficient acid strength to modify the product distribution which is mainly governed by the Lewis sites.

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► Lewis acid sites were produced using iron oxide in Fe/MCM-41 system.
► Brönsted acid sites were neutralized by silylation treatment.
► Brönsted acidity is not enough to modify the selectivity in POX isomerization.
► Different calcination atmospheres lead to different types of acid Lewis sites of Fe3+.