ReviewCoordination chemistry and reactivity of copper in zeolites
Graphical abstract
Highlights
► The Cu coordination and location in zeolites is reviewed. ► Zeolites create a unique constrained microenvironment for Cu complexes (biomimics). ► Zeolites don’t merely act as support, but alter the coordination chemistry of ligated copper ions. ► The reactive species for C-H bond activation in Cu-ZM-5 is a mono(μ-oxo)dicopper core. ► This mono(μ-oxo)dicopper core is related to the exceptional deNOx catalysis of Cu-ZSM-5.
Introduction
Soon after their discovery the synthetic zeolites A, X and Y were ion-exchanged with transition metal ions (TMI) in order to modify the adsorption properties and to introduce new catalytic functionality [1], [2], [3], [4], [5], [6]. A whole research area was opened with subjects such as coordination of TMI with lattice oxygen, coordination with extra-lattice ligands in confined space, immobilization of homogeneous catalysts, redox chemistry, immobilization of clusters and nanoparticles of TMI and of transition metal oxides, substitution of Al3+ and Si4+ for TMI in the zeolitic lattice and redox catalysis.
Among the TMI, copper has always been the favorite of researchers in zeolite chemistry, mainly due to its relatively simple chemistry and ease of spectroscopic detection. Copper can occur in 3 oxidation states, viz. II, I and 0. Copper(II) is easily detectable by relatively simple and cheap techniques such as UV–vis–NIR spectroscopy and electron paramagnetic resonance (EPR), while copper(I) has characteristic luminescence spectra. Quantum-chemical calculations and, especially, density functional theory (DFT) have been of key importance in the development of models of the coordination chemistry of copper in zeolites that explain the spectroscopic data.
Cu–ZSM-5 is extraordinary in that it is active in deNOx catalysis [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] and in the selective hydroxylation of methane [19], [20], [21] and benzene [22], [23], [24] to methanol and phenol, respectively. Fe–ZSM-5 catalysts often closely compete for the same reactions [25], [26]. Cu–ZSM-5 and Fe–ZSM-5 differ in catalyst activation and in reaction temperature. Both catalysts are activated by heating at high temperatures in inert atmosphere. For Fe–ZSM-5 this treatment is followed by activation in N2O, while for Cu–ZSM-5 activation in O2 and N2O is possible. The selective oxidation of methane typically occurs at room temperature with Fe–ZSM-5 and in the range of 373–473 K for Cu–ZSM-5. Soluble methane mono-oxygenase (sMMO) with Fe in its active core and particulate methane mono-oxygenase (pMMO) with Cu in its active core are the enzymatic counterparts of Cu– and Fe–ZSM-5 [20], [26], [27]. Recently Hammond et al. reported on a bimetallic zeolite, viz. Fe–Cu–ZSM-5, that is active for the selective oxidation of methane to methanol with aqueous hydrogen peroxide. The addition of Cu to an Fe–ZSM-5 catalyst increased the methanol selectivity due to a synergetic effect between copper and iron, whereby copper merely overcomes the formation of hydroxyl radicals and the methanol over-oxidation coupled thereto [28].
In this review we mainly concentrate on the coordination chemistry of copper in zeolites with the following subjects: ion exchange, coordination to lattice oxygen, coordination of extra-framework ligands, deNOx catalysis and the selective oxidation of methane into methanol. Many reviews have been published over the years and our own reviews are referenced for the interested readers that want more detailed information [29], [30], [31], [32].
Section snippets
Ion exchange and coordination to lattice oxygen
The classical method of preparation of a Cu-zeolite is ion exchange of [Cu(H2O)6]2+ from aqueous solution into the pore system of zeolites. The pH in the pores must be below the pH of precipitation of Cu2+ in aqueous solution, which is 6 at [Cu2+] = 0.01 M [33]. As the pH in the pores is unknown, it is recommendable to keep the pH of the exchange solution below 6. As a consequence, exchange of a few protons accompanies Cu2+ exchange, giving rise to acidic bridging hydroxyl groups. If the pH in the
Complexation of copper in cavities and channels of zeolites
The pore system of zeolites is accessible through rings of oxygen atoms of which rings with 6, 8, 10 and 12 oxygen atoms (6–12 MR's) occur most frequently. They have pore openings of respectively 0.23–0.25 nm, 0.41–0.43 nm, 0.51–0.56 nm and 0.64–0.76 nm. Complexes of TMI can be introduced by ion exchange, if they are positively charged. They can also be prepared by adsorption of ligands in a zeolite, previously exchanged with Cu2+. One criterion for a successful introduction is that the cationic
Oxo complexes of Cu2+ in zeolites
When auto-reduction reactions (3), (4), (5) are reversed, the reaction of O2 with Cu+-zeolites formally should give Cu2+-zeolites. When reaction (3) is reversed, O2− must be incorporated in the zeolitic lattice. In the case reactions (4), (5) are reversed an intermediate oxo complex is formed, which needs water to return to the original [CuOH] species. Thus, in the absence of water a [CuOCu]2+ complex might form in the cages and channels of zeolites. The reverse of reaction (3) occurs in all
deNOx
Ever since the discovery of Iwamoto's group [99] that over-exchanged Cu–ZSM-5 has an exceptionally high activity in catalytic decomposition of nitrogen oxides, research on catalytic deNOx with Cu-zeolites has been booming [7], [8], [9], [10], [13], [14], [15], [16], [17], [18]. A variety of zeolite topologies and compositions have been investigated for a broad range of Cu loadings. Instead of using the total Cu content of the zeolite to correlate activity, we have used the amount of EPR-silent
Conclusions
The coordination chemistry of Cu2+ in zeolites is well established. Cu2+ preferentially coordinates in 6MR's with 1, 2 or 3 Al tetrahedra. It coordinates preferentially with the oxygen atoms of the Al tetrahedra, which are more basic than those of the Si-tetrahedra. Cu2+ takes a coordination number of 4 in these 6MR's. The presence of Al-tetrahedra in the 6MR's and the Cu2+ coordination lead to a strong local distortion and the symmetry of the 6MR's is lost. The observed d–d transitions and EPR
Acknowledgments
This work was performed within the framework of FWO (G.0596.11), IAP (Belspo), ERIC and Methusalem (long-term structural funding by the Flemish Government) projects.
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