Do Cu(II) ions need Al atoms in their environment to make CuSiBEA active in the SCR of NO by ethanol or propane? A spectroscopy and catalysis study

The two-step postsynthesis method (creation of vacant T-sites and associated SiOH groups by dealumination of BEA zeolite with nitric acid followed by incorporation of copper in the resulting SiBEA by impregnation with an aqueous solution of copper nitrate) allows to obtain a CuSiBEA zeolite which contains 0.8 Cu wt%. The incorporation of Cu(II) into the lattice of SiBEA is evidenced by XRD while the concomitant consumption of SiOH groups is monitored by FTIR. The presence of mainly isolated mononuclear Cu(II) in D_2d-distorted tetrahedral symmetry is evidenced by diffuse reflectance UV–vis-NIR, EXAFS and XANES. The CuSiBEA zeolite is active in the selective catalytic reduction (SCR) of NO with ethanol or propane with maximum NO conversion of 40 and 20% and selectivity toward N₂ close to 80–90 and 90–100%, respectively. These results suggest that the SCR process occurs on isolated mononuclear Cu(II) in D_2d-distorted tetrahedral symmetry after Al atoms have been removed from the zeolite structure. Thus, Cu(II) ions do not need Al atoms in their environment to be catalytically active. The lack of correlation between the SCR activity in presence of ethanol and the oxidation of NO to NO₂ suggests that the two reactions are more competitive than sequential. The higher activity of CuSiBEA with ethanol than with propane may be due to different activation energies and/or reaction mechanisms.