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 0.1 M Cu(II) acetate solutions. Interpretation (Rietveld method) of X-ray powder diffraction (XRPD) profiles corresponding to two fully dehydrated Cu-MFI phases, reveals the presence of several independent extra framework Cu centers. In H 1.04 Cu(I) 1.19 Cu(II) 0.51 MFI and H 1.99 Cu(I) 2.37 Cu(II) 0.00 MFI, four and five Cu cations are 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 shortest distances between copper sites and framework oxygen atoms involve the Cu1, Cu2, and Cu2′ sites (Cu-O in the 1.99-2.32 Å range). The Cu1, Cu3, and Cu3′ sites are trapped in secondary five and six ring channel sections of the MFI framework. Cu2 and Cu2′ are very close to the 10-membered rings of the straight channel sections. One or both of these cations, which are immediately accessible to guest molecules, appear to be the prime candidates for the catalytically active sites in copper-exchanged MFI.

A [Cu 2 O] 2+ core in Cu-ZSM-5, the active site in the oxidation of methane to methanol

“…4B). This placement is also consistent with the crystal structure of Cu I -ZSM-5, which resolves two Cu I atoms in the 10-membered ring separated by two intermediate T-sites (28). Thus, a larger model was formed with each Al T-site ligand extended with covalently bonded Si T-sites (Fig.…”

“…This can result in an active core containing either two Cu III s or two Cu II s with an accompanying 2-electron transfer from nearby Cu I ions to reduce the additional O atom. ‡ In ZSM-5, the 10-membered rings of the lattice are the only sites with pore-sizes appropriate for access of O 2 , N 2 O, and CH 4, and within these rings, a Cu 2 O core could only bind two lattice Os to each Cu (28). We therefore initially modeled the site with a tetrahedral Al(OH) 4 (Fig.…”

Changing active sites in Cu–CHA catalysts: deNOx selectivity as a function of the preparation method

“…A recent study by Fickel et al using Rietveld refinement of synchrotron-based powder X-ray diffraction (XRD) data revealed an exclusive occupation of Cu 2+ cations in the plane of the six-ring that comprise each end of the double six membered ring (d6r) subunits of SSZ-13 (CHA structure) [6,7]; in comparison Cu-ZSM-5 is known to possess 5 cationic positions for Cu 2+ within its framework [8]. EXAFS data further revealed that the Cu 2+ ions do not sit in the center of the plane of the six-ring but rather tend to be displaced towards the edge of the ring in a 3-4 coordinate planar arrangement [5].…”

ZSM-5 and ferrierite single crystals with lower Si/Al ratios: Synthesis and single-crystal synchrotron X-ray diffraction studies

“…Therefore, catalytic active metal sites have been extensively studied by various methods such as synchrotron powder X-ray diffraction (XRD), X-ray absorption fine structure, theoretical modeling, and others [1][2][3][4][5][6][7]. Especially, some ZSM-5 and ferrierite structures including non-framework cation positions have been determined by powder XRD [5,6,8,9]. Although theoretical studies for NO in Fe-exchanged ferrierite provided some useful but indirect information about NO and Fe interactions [3], the detailed structures and environments of transition metals and adsorbed molecules including reactants and intermediates in metal exchanged ZSM-5 and ferrierite have been rarely confirmed due to some limitations on the analytical methods and materials.…”