Copper-containing zeolites, such as mordenite (MOR), have recently gained increased attention as a consequence of their catalytic potential. While the preferred copper loadings in these catalytic studies are generally high, the literature lacks appropriate spectroscopic and structural information on such Cu-rich zeolite samples. Higher copper loadings increase the complexity of the copper identity and their location in the zeolite host, but they also provide the opportunity to create novel Cu sites, which are perhaps energetically less favorable, but possibly more reactive and more suitable for catalysis. In order to address the different role of each Cu site in catalysis, we here report a combined electron paramagnetic resonance (EPR), UV/Vis-NIR and temperature-programmed reduction (TPR) study on highly copper-loaded MOR. Highly resolved diffuse reflectance (DR) spectra of the CuMOR samples were obtained due to the increased copper loading, allowing the differentiation of two isolated mononuclear Cu(2+) sites and the unambiguous correlation with extensively reported features in the EPR spectrum. Ligand field theory is applied together with earlier suggested theoretical calculations to determine their coordination chemistry and location within the zeolite matrix, and the theoretical analysis further allowed us to define factors governing their redox behavior. In addition to monomeric species, an EPR-silent, possibly dimeric, copper site is present in accordance with its charge transfer absorption feature at 22200 cm(-1) , and quantified with TPR. Its full description and true location in MOR is currently being investigated.
[Show abstract][Hide abstract] ABSTRACT: Cu-CHA combine high activity for the SCR reaction with better hydrothermal stability and selectivity compared to other copper substituted zeolites. At the same time Cu-CHA offers an opportunity for unraveling the coordination environment of the copper centers since the zeolite framework is very simple with only one crystallographically independent tetrahedral site (T-site). In this study the results of an X-band EPR investigation of ion exchanged Cu-CHA zeolite with a Si/Al ratio of 14±1 is presented. Different dehydration treatments and rehydration experiments are performed in-situ while monitoring with EPR. The results are compared with recent literature evidence from temperature programmed reduction, X-ray methods, IR spectroscopic methods and UV-visible spectroscopy. Based on these findings quantitative information is obtained for the different copper positions in dehydrated Cu-CHA. The well-defined copper sites in the 6-membered ring of the CHA structure are found to be EPR active, to give two distinct sets of signals in an approximate 1:1 ratio and add up to 19±2 % of the total copper in the material. The long-standing question of the EPR silent monomeric Cu2+ in copper substituted zeolites is suggested to be copper species with an approximate trigonal coordination sphere appearing during the dehydration. After complete dehydration at 250 °C the majority of the EPR silent Cu2+ are suggested to exist as Cu2+-OH− coordinated to two framework oxygen atoms located in the microenvironment of an isolated Al T-site.
The Journal of Physical Chemistry C 09/2014; DOI:10.1021/jp5065616 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Copper-doped nanosized zeolite A crystals were synthesized by an in situ templating approach using [Cu(EDTA)](2-)-complex. The structural properties of the copper containing zeolite crystals were characterized by a suite of different techniques including SEM-EDX, ESR, mid-IR and Far-IR, Raman, in situ XRD and non-ambient neutron powder diffraction. The SEM investigations on the morphology show spheroidal zeolite A crystals with average size similar to 200 nm. The asymmetric ESR spectrum shows that the Cu2+ ion is in a tetragonal-distorted octahedral crystal field. FT-IR and Raman spectroscopies provide information on coordination environment of the copper ion. The band due to stretching vibration of C-N bond, where N is coordinated to the copper ion (C-N-Cu), was observed at 1109 cm(-1) in the mid-infrared region. The Raman band due to the Cu-O bond is present at 630 cm(-1) indicating the coordination of the Cu2+-cation to COO--group of the EDTA-ion. The XRD data shows an enlarged d-spacing between the adjacent zeolite lattice planes due to the presence of the [Cu(EDTA)](2-)-complex in comparison to template-free LTA zeolite structure. LeBail fitting approach on temperature-dependent in situ X-ray and neutron diffraction profiles have demonstrated the expansion of the zeolite cell during the thermal treatment followed by subsequent contraction with the decomposition of the organic template. Crown Copyright
[Show abstract][Hide abstract] ABSTRACT: Copper-exchanged zeolites with mordenite structure mimic the nuclearity and reactivity of active sites in particulate methane monooxygenase, which are enzymes able to selectively oxidize methane to methanol. Here we show that the mordenite micropores provide a perfect confined environment for the highly selective stabilization of trinuclear copper-oxo clusters that exhibit a high reactivity towards activation of carbon-hydrogen bonds in methane and its subsequent transformation to methanol. The similarity with the enzymatic systems is also implied from the similarity of the reversible rearrangements of the trinuclear clusters occurring during the selective transformations of methane along the reaction path towards methanol, in both the enzyme system and copper-exchanged mordenite.
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