Crystallographic Determination of the Positions of the Copper Cations in Zeolite MFI

Abstract

Copper-exchanged Cu−MFI (Cu−ZSM-5) materials have been prepared by reacting H−MFI samples with\n0.1 M Cu(II) acetate solutions. Interpretation (Rietveld method) of X-ray powder diffraction (XRPD) profiles\ncorresponding to two fully dehydrated Cu−MFI phases, reveals the presence of several independent extra\nframework Cu centers. In H<sub>1.04</sub>Cu(I)<sub>1.19</sub>Cu(II)<sub>0.51</sub>MFI and H<sub>1.99</sub>Cu(I)<sub>2.37</sub>Cu(II)<sub>0.00</sub>MFI, four and five Cu cations\nare located, respectively. In these two phases, the Cu1/Cu2/Cu2‘/Cu3/Cu3‘ populations are 0.0/1.07/0.02/0.17/0.45 and 0.18/1.12/0.44/0.20/0.50 per unit cell (uc), respectively. Cu2 is the most populated one. The\nshortest distances between copper sites and framework oxygen atoms involve the Cu1, Cu2, and Cu2‘ sites\n(Cu−O in the 1.99−2.32 Å range). The Cu1, Cu3, and Cu3‘ sites are trapped in secondary five and six ring\nchannel sections of the MFI framework. Cu2 and Cu2‘ are very close to the 10-membered rings of the straight\nchannel sections. One or both of these cations, which are immediately accessible to guest molecules, appear\nto be the prime candidates for the catalytically active sites in copper-exchanged MFI.