The Coordination of CuII in Zeolites − Structure and Spectroscopic Properties

Abstract

Keywords: Zeolites / Copper / Ab initio calculations / Density functional calculations / ESR spectroscopy / UV/Vis spectroscopy / Heterogeneous catalysis Zeolites loaded with transition metal ions are promising heterogeneous catalysts. Knowledge about the location and structure of the metal centers is of paramount importance for the understanding of the catalytic potential of these materials. In this work, the spectroscopic studies of the coordination of Cu II in zeolite A, ZK4, X, Y and mordenite are reviewed. Experimentally, diffuse reflectance spectroscopy [a] teaches general chemistry and computational chemistry. Her main research interest is the application of ab initio methods for the study of spectroscopic and magnetic properties of systems containing transition metals. Marijke H. Groothaert (left center) was born in Neerpelt, Belgium in 1975. In 1998 she received her M. S. degree from the . Since 1998 she is a PhD student at the Center for Surface Chemistry and Catalysis under the supervision of Professor Robert A. Schoonheydt. Her research is concentrated on the coordination of transition metal ions in zeolites and combines spectroscopic techniques with computational methods. Robert A. Schoonheydt (left bottom) received his M. S. degree in 1966 and . Hab. in Leuven, where he teaches quantum chemistry, computational chemistry, and inorganic chemistry. His main research interests are in the application of computational methods in different areas of chemistry.MICROREVIEWS: This feature introduces the readers to the authors' research through a concise overview of the selected topic. Reference to important work from others in the field is included. 515(DRS) and electron spin resonance (ESR) spectroscopy have been applied to study the coordination of Cu II in zeolites. Ab initio calculations on model clusters, representing the Cu II sites in zeolites, are used for the interpretation of the experimental data. The combination of experimental spectroscopic information with theoretical results leads to a new and profound insight into the Cu II −zeolite interaction.

Evolution of Surface Copper(II) Environments in Cu2–xSe Nanoparticles

“…These tensors are diagonalized in their principal axis systems resulting in three principal components: g xx , g yy , and g zz and A xx , A yy , and A zz . The measured spectra were consistent with axially symmetric g and hf tensors, in which g xx = g yy ≡ g ⊥ , g zz ≡ g ∥ , A xx = A yy ≡ A ⊥ , and A zz ≡ A ∥ . , The splittings due to A ∥ are more pronounced as shown by dashed lines in Figure . The presence of a second component indicates more than one electronic environment.…”

Evolution of Surface Copper(II) Environments in Cu2–xSe Nanoparticles

“…These tensors are diagonalized in their principal axis systems resulting in three principal components: g xx , g yy , and g zz and A xx , A yy , and A zz . The measured spectra were consistent with axially symmetric g and hf tensors, in which g xx = g yy ≡ g ⊥ , g zz ≡ g ∥ , A xx = A yy ≡ A ⊥ , and A zz ≡ A ∥ . , The splittings due to A ∥ are more pronounced as shown by dashed lines in Figure . The presence of a second component indicates more than one electronic environment.…”

Methane Oxidation over Cu2+/[CuOH]+ Pairs and Site‐Specific Kinetics in Copper Mordenite Revealed by Operando Electron Paramagnetic Resonance and UV/Visible Spectroscopy

“…Due to the energetically favorable fourfold coordination and strong interaction with the MOR lattice, site S1 is present in all prepared materials [6a,f] . This is shown by the fact that all EPR spectra of the activated materials exhibit features with a g || value of 2.32 (Table 1).…”

“…The spectra of all activated materials are characterized by an absorption band (local minimum in reflectance) at ~13000 cm −1 , which has been attributed to the d‐d transition of sites S1 and S2 (Table 1). [6a–d,f,7b] A rather broad signal at ~36800 cm −1 can be observed in the spectra of activated Cu 5.2 MOR and Cu 0.6 MOR, which has been assigned to a LMCT transition from framework O to Cu 2+ [6f,10b,11b] . In the spectrum of activated Cu 5.2 MOR, another feature at ~26100 cm −1 can be observed, which has been associated with the O bridge 2p→Cu 2+ 3d/4s LMCT transition of dimeric Cu 2+ species (denoted as site S4, Table 1) such as [Cu 2 (μ‐O)] 2+ or [Cu 2 ( trans ‐μ‐1,2‐O 2 )] 2+ centers [7b,11b,15] .…”

Observation of Single-Crystal-Type EPR Spectra from Monolayers of Copper-Exchanged Zeolite Na-A Crystals Assembled on Glass Plates

“…24 It is well-known that Cu 2+ has a strong preference for a square planar coordination, and ab initio calculations on a copper complex in the six-ring plane have suggested that Cu 2+ is coordinated to three O(3) and one O(2), forming a distorted square plane. 10,13 Still, as long as Cu 2+ lies in the six-ring plane, the g z direction should be parallel to [111], contrary to our finding that it is tilted only 9°a way from the c axis. A trigonal complex with Cu 2+ displaced from the six-ring plane into the cage is more compatible with our finding.…”

Evolution of Surface Copper(II) Environments in Cu2–xSe Nanoparticles

“…These tensors are diagonalized in their principal axis systems resulting in three principal components: g xx , g yy , and g zz and A xx , A yy , and A zz . The measured spectra were consistent with axially symmetric g and hf tensors, in which g xx = g yy ≡ g ⊥ , g zz ≡ g ∥ , A xx = A yy ≡ A ⊥ , and A zz ≡ A ∥ . , The splittings due to A ∥ are more pronounced as shown by dashed lines in Figure . The presence of a second component indicates more than one electronic environment.…”

Methane Oxidation over Cu2+/[CuOH]+ Pairs and Site‐Specific Kinetics in Copper Mordenite Revealed by Operando Electron Paramagnetic Resonance and UV/Visible Spectroscopy

“…Due to the energetically favorable fourfold coordination and strong interaction with the MOR lattice, site S1 is present in all prepared materials [6a,f] . This is shown by the fact that all EPR spectra of the activated materials exhibit features with a g || value of 2.32 (Table 1).…”

“…The spectra of all activated materials are characterized by an absorption band (local minimum in reflectance) at ~13000 cm −1 , which has been attributed to the d‐d transition of sites S1 and S2 (Table 1). [6a–d,f,7b] A rather broad signal at ~36800 cm −1 can be observed in the spectra of activated Cu 5.2 MOR and Cu 0.6 MOR, which has been assigned to a LMCT transition from framework O to Cu 2+ [6f,10b,11b] . In the spectrum of activated Cu 5.2 MOR, another feature at ~26100 cm −1 can be observed, which has been associated with the O bridge 2p→Cu 2+ 3d/4s LMCT transition of dimeric Cu 2+ species (denoted as site S4, Table 1) such as [Cu 2 (μ‐O)] 2+ or [Cu 2 ( trans ‐μ‐1,2‐O 2 )] 2+ centers [7b,11b,15] .…”

Observation of Single-Crystal-Type EPR Spectra from Monolayers of Copper-Exchanged Zeolite Na-A Crystals Assembled on Glass Plates

“…24 It is well-known that Cu 2+ has a strong preference for a square planar coordination, and ab initio calculations on a copper complex in the six-ring plane have suggested that Cu 2+ is coordinated to three O(3) and one O(2), forming a distorted square plane. 10,13 Still, as long as Cu 2+ lies in the six-ring plane, the g z direction should be parallel to [111], contrary to our finding that it is tilted only 9°a way from the c axis. A trigonal complex with Cu 2+ displaced from the six-ring plane into the cage is more compatible with our finding.…”

Evolution of Surface Copper(II) Environments in Cu2–xSe Nanoparticles

“…These tensors are diagonalized in their principal axis systems resulting in three principal components: g xx , g yy , and g zz and A xx , A yy , and A zz . The measured spectra were consistent with axially symmetric g and hf tensors, in which g xx = g yy ≡ g ⊥ , g zz ≡ g ∥ , A xx = A yy ≡ A ⊥ , and A zz ≡ A ∥ . , The splittings due to A ∥ are more pronounced as shown by dashed lines in Figure . The presence of a second component indicates more than one electronic environment.…”

Methane Oxidation over Cu2+/[CuOH]+ Pairs and Site‐Specific Kinetics in Copper Mordenite Revealed by Operando Electron Paramagnetic Resonance and UV/Visible Spectroscopy

“…Due to the energetically favorable fourfold coordination and strong interaction with the MOR lattice, site S1 is present in all prepared materials [6a,f] . This is shown by the fact that all EPR spectra of the activated materials exhibit features with a g || value of 2.32 (Table 1).…”

“…The spectra of all activated materials are characterized by an absorption band (local minimum in reflectance) at ~13000 cm −1 , which has been attributed to the d‐d transition of sites S1 and S2 (Table 1). [6a–d,f,7b] A rather broad signal at ~36800 cm −1 can be observed in the spectra of activated Cu 5.2 MOR and Cu 0.6 MOR, which has been assigned to a LMCT transition from framework O to Cu 2+ [6f,10b,11b] . In the spectrum of activated Cu 5.2 MOR, another feature at ~26100 cm −1 can be observed, which has been associated with the O bridge 2p→Cu 2+ 3d/4s LMCT transition of dimeric Cu 2+ species (denoted as site S4, Table 1) such as [Cu 2 (μ‐O)] 2+ or [Cu 2 ( trans ‐μ‐1,2‐O 2 )] 2+ centers [7b,11b,15] .…”

Observation of Single-Crystal-Type EPR Spectra from Monolayers of Copper-Exchanged Zeolite Na-A Crystals Assembled on Glass Plates

“…24 It is well-known that Cu 2+ has a strong preference for a square planar coordination, and ab initio calculations on a copper complex in the six-ring plane have suggested that Cu 2+ is coordinated to three O(3) and one O(2), forming a distorted square plane. 10,13 Still, as long as Cu 2+ lies in the six-ring plane, the g z direction should be parallel to [111], contrary to our finding that it is tilted only 9°a way from the c axis. A trigonal complex with Cu 2+ displaced from the six-ring plane into the cage is more compatible with our finding.…”