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Research was conducted to examine skeletal T-O-T variations as a tool for characterization of divalent cation complexation in ferrierite. The effect of several divalent cations such as Mn2+, Mg2+, Ni2+ and Co2+ exchanged in Na-ferrierite and ammonia adsorption/desorption was investigated using FTIR spectroscopy in the range 4000-400/cm. Results demonstrate that FTIR spectroscopy has a potential for highly sensitive identification of equilibrium states between the metal ion-ligand complexes in zeolites and bare metal ions.

Simplified cluster models representing cation bindings sites in a zeolite of ferrierite structure is optimized with a series of divalent and monovalent metal cations. It is observed that the coordination of metal cations significantly perturbs the char ge distribution over the cation binding sites.

Highlights: [•] Ratio of Al pairs and single Al atoms in zeolites can be tuned in a wide range. [•] Corresponding local negative charges balance bare divalent cations or metal-oxo species. [•] These cation/metal-oxo species control the activity in N2O and NOx related reactions. [ABSTRACT FROM AUTHOR]

Abstract: Two parent ferrierites with similar Si/Al ratio (8.5 and 10.5), a commercial and a laboratory specimen with different distribution of Al atoms in the framework, were used for the preparation of model Fe-ferrierite catalysts for N2O decomposition. As proven by UV–Vis analysis of the fully exchanged Co(II)-ferrierites, the two ferrierites differ in the distribution of framework Al between Al–O–(Si–O)2–Al sequences in one ring (Al pairs) and single Al atoms in different rings (Al–O–(Si–O) n >2–Al sequences). The part of the framework Al atoms arranged in Al pairs mostly in β six-member rings was 66% and 30% of Al for the two ferrierites. Two series of Fe-ferrierites with low concentration between 0.4 and 2.1 wt.% Fe were prepared, containing Fe ions in cationic sites, and valence and siting analyzed by Mössbauer spectroscopy of 57Fe isotope enriched (96%) samples. The higher extent of Fe(III) ions reduced to Fe(II) and predominantly located in β sites was found in the ferrierite with the higher content of Al pairs, while that with prevailing single Al atoms possessed higher content of Fe(III) ions and Fe(II) in α sites compared to the former ferrierite. It has been shown that the activity in N2O decomposition is in the whole Fe concentration range higher for the Fe-ferrierite with higher proportion of Al pairs. The Fe(II) ions in β sites were indicated as the active sites, but not all β-type Fe(II) ions exhibited equal activity. The highest unique activity in N2O decomposition is attributed to two cooperating Fe(II)-β⋯Fe(II)-β ions each balanced by Al pair in six-member ring in an opposite wall of ferrierite cavity. For both series of Fe-ferrierites the vacuum heat-treatment at 700°C increased reduction of Fe(III) ions and occupation of the β sites by Fe(II) ions, dehydroxylation of OH groups with formation of Lewis sites and boosted the N2O decomposition, but not proportionally to the change of the β-type Fe(II) ions. For the activity increase the effect of cooperation of the Fe active sites with Al-Lewis acid sites was suggested, operating through stabilization of the surface NO x species on Lewis sites. The optimum distance (7.5Å) of the most active Fe⋯Fe ion in β sites of Fe-ferrierite is in agreement with the lower activity of Fe-β and Fe-ZSM-5 zeolites. [Copyright &y& Elsevier]

Abstract: Four series of tungstated zirconia of different surface area ranging from 100 to 20m2 g−1 and of micro- and nano-meter crystal size were investigated with respect to nuclearity and reducibility of WO x species, concentration of strong and weak acid sites, and n-heptane isomerization over Pt/WO3–ZrO2. The nuclearity of WO x species was analyzed by diffuse reflectance UV–Vis spectroscopy and their reducibility by TPR-H2. TPD of ammonia distinguished acid sites of different strengths. n-Heptane isomerization proceeds over Pt/WO3–ZrO2 in hydrogen/nitrogen mixture at 250°C. At least four types of WO x species differing in nuclearity were identified in tungstated zirconia by a maxima of absorption at 39000, 35000, 30000 and 27000cm−1, intensity of which depended on the surface area, crystal size and WO3 loading. From the changes in the relative intensities of the characteristic individual bands of WO x species with tungsten loading, at different surface area and crystal size, it follows that at the lowest loading mononuclear W–O–Zr species (39000cm−1) and two-dimensional poly-tungstate like WO x species (35000cm−1) are formed, which are at loading above a nominal monolayer started to transform into a larger defined WO x , likely tri-dimensional clusters (30000cm−1). The absorption maximum at 27000cm−1 corresponds to bulk-like WO3. The nuclearity of WO x species was reflected in their reducibility, which increased with increased nuclearity, and it was higher for the nano-size tungstated zirconia. The isomerization activity followed the relative population of poly-tungstate WO x species (35000cm−1), but was not in strict agreement with the concentration of strong acid sites. This might be connected with the rate determining step of the isomerization process or formation of radical cations depending on reaction conditions. The nano-size tungstated zirconia exhibited high isomerization activity advantageously at low W loading (6–12wt.%) compared to the micro-meter size ones. This finding corresponds to formation of poly-tungstate WO x species already at low loadings, likely at the defective sites of the nano-size zirconia. [Copyright &y& Elsevier]