Ewa M. Serwicka

Oxidation of cyclohexene over Mn(TMPyP) porphyrin-exchanged Al, Si-mesoporous molecular sieves

The mesoporous silicas of the MMS (HMS)-type, containing different amounts of Al, have been prepared using dodecylamine as a structure directing agent. Encapsulation of the +4 charge cationic chloro[meso-tetra(4-N-methylopyridinio)-porphirynato] manganese(III) tetrachloride (Mn(TMPyP)) metalloporphyrin has been carried out by means of cation exchange. In all Al-substituted materials, the porphyrin species are firmly bound to the solid matrix, in contrast to the purely siliceous support. The preparation procedure provides a simple means of controlling the amount of the supported porphyrin. The solids are active in oxidation of cyclohexene with iodosylbenzene yielding only allylic oxidation products and no epoxide. It is argued that the regioselective effect is due to the narrowness of the structural pores, which prohibits the formation of the transient complex required for the formation of the epoxide. In contrast to previous studies in which increased loading of porphyrin has been found to result in a decrease in activity, in the present case an increase in the catalytic activity with increased porphyrin loading is observed.

ESR study on the interaction of water vapour with polycrystalline TiO2 under illumination

Abstract Electron spin resonance has been used to study the interaction of water vapour with polycrystalline TiO2 (anatase) under illumination. At 77 K, the illumination of TiO2 containing adsorbed water leads to the appearance of a strong ESR spectrum characterized g1 = 2.036 and g2 = 2.003, which is tentatively ascribed to the HO2, radical. Room-ternperature illumination of the TiO2—water vapour system results in the formation of a spectrum with g1 = 2.022, g2 = 2.009 and g3 = 2.003, which is ascribed to the O−2 radical. The influence of the degree of reduction of TiO2 on the observed photoeffects was examined. The use of light of wavelengths ⩽ 380 nm was necessary in order to observe radical formation. The results are interpreted in terms of the photocatalytic decomposition of water vapour at the TiO2 surface.

Mechanism of surface spreading in vanadia-titania system

Spreading of vanadia over TiO2 (anatase) supports of different specific surfaces has been investigated by XRD, XPS and ESR. It has been found that vanadia migration is favoured over well-developed anatase crystal planes. At 723 K spontaneous spreading takes place, controlled by diffusion of vanadium species through the vanadia monolayer. In reducing atmosphere an induction period is observed, possibly initiated by formation of a structurally modified titania surface layer. Amorphous vanadia is postulated as a transient form between crystalline V2O3 and the vanadia monolayer.

Vanadium-doped titania-pillared montmorillonite clay as a catalyst for selective catalytic reduction of NO by ammonia

Abstract A series of V-doped titania-pillared clay catalysts, characterized by ICP-AES chemical analysis, X-ray diffraction, BET surface area measurement, and ESR spectroscopy, have been tested in the selective catalytic reduction of NO by NH3. An ESR analysis shows that V dopant is anchored to the titania pillars. Vanadyl species with differing degrees of in-plane V-O π-covalent bonding are produced depending on the method of sample preparation. Polymeric V species appear as the V content is increased. Catalytic performance of these systems depends on the method of preparation and on the V content. The best catalyst, converting 90-100% NO in the temperature range 523-623 K, is obtained by exchange of pillared montmorillonite with vanadyl ions, at an extent of exchange below the level where significant amounts of polymeric V species appear. The co-pillared catalyst, containing vanadyl centres characterized by a higher degree of in-plane π-covalent bonding (according to ESR), is less selective than the exchanged samples.

Oxidation of aromatic hydrocarbons with hydrogen peroxide over Zn,Cu,Al-layered double hydroxides

Abstract Layered double hydroxides (LDHs) with general formula [(Zn1−yCuy)1−xAlx(OH)2](NO3)x·nH2O (x≅0.33, 0.6 ≤y≤1.0, n≅0.5) have been synthesized and characterized by XRD, chemical analysis, and ESR. According to ESR, copper ions exist both as isolated species and as clusters, their relative contribution depending on the overall Cu content. The tendency to form clusters is observed even at a very low absolute Cu concentration. The materials are active in the low temperature liquid phase oxidation of toluene, o-, m-, and p-xylene with hydrogen peroxide. The catalytic reaction involves both the side chain oxidation and the hydroxylation/oxidation of the aromatic ring. The catalytically active centres are formed by isolated copper species. The clustered copper ions are inactive in the oxidation reaction but accelerate the non-productive decomposition of hydrogen peroxide. The catalytic performance is discussed in terms of competition between the process of H2O2 decomposition and its consumption in the oxidation reaction.

Heteropolysalt-supported heteropolyacids as a new class of acid-base and redox catalysts

Heteropolyacid catalysts of composition H3+nPVnMo12−nO40 (n = 0,1,2,3) when supported on K3PMo12O40 desplay new types of acid-base and redox properties and have enhanced thermal stability.

Acid—base properties of H3+nPVnMo12-nO40 heteropolyacids, pure and supported on K3PMo12O40

Abstract The acidity of unsupported H3+nPVnMo12-nO40·xH2O heteropolyacids is primarily affected by the degree of hydration, and to a lesser extent by the sample composition. Thermal pretreatment reduces the number of hydrated, strongly acidic protons leaving behind protons which are devoid of the hydration shell, representing weak acid sites, only partially detectable by pyridine, their amount increasing withn. After treatment at 573 K a shift towards weaker acid sites is observed for increasing vanadium substitution due to the growing susceptibility to dehydration. Supporting on K3PMo12O40 enhances the acidic properties by a factor of 3–4. Acid sites present in surface acid layers show a similar strength irrespective of acid composition. The results are consistent with the previously postulated formation of an epitaxial acid coat of increased resistivity against dehydration, isostructural with the support, where the most hydrated, hence most acidic, protons exist as diaquahydrogen ions.

Acid–base and oxidation catalysis on heteropolysalts with surface acid layers

Physicochemical and catalytic properties of heteropolyacids of the series H3 +nPVnMo12–nO40(n= 0–3), both pure and supported on the potassium salt K3PMo12O40, have been investigated. Thin acid coats formed on such a support display modified properties and enhanced thermal stability. In particular, it is postulated that the change in the acidic properties of the supported acids is a consequence of their modified hydration ability resulting from the epitaxial relationship with the support. Results of catalytic experiments for the oxidation of acrolein, methanol and alkanes are presented and compared for both series of the catalysts. Possible mechanisms of all these processes are proposed on the basis of experimental data and quantum-chemical calculations.

Layered double hydroxide-derived vanadium catalysts for oxidative dehydrogenation of propane: Influence of interlayer-doping versus layer-doping

Abstract Mixed-oxide vanadium catalysts for oxidative dehydrogenation (ODH) of propane have been prepared by thermal decomposition of Mg, Al-layered double hydroxides (LDHs) containing vanadium either in the brucite layer or in the interlayer. The materials have been characterised by XRD, ICP-AES chemical analysis, XPS, BET and ESR. The catalytic performance of the samples depended on the manner of incorporation of the vanadium into the LDH structure. The sample obtained from interlayer-doped precursor was more active and more selective than mixed oxides obtained from layer-doped LDHs. The difference in the catalytic properties was attributed to the different magnesium vanadates nucleating in the calcined samples, the pyrovanadate formed from the interlayer-doped LDH giving better performance than ortho-vanadate crystallising from the layer-doped precursor. It has been suggested that one of the factors contributing to the difference in the behaviour of both types of catalysts might be the difference in the covalency of VO in-plane bonds around the reduced V centres.

ESR evidence for structural rearrangements occurring upon MoO3 reduction

Reduction of polycrystalline MoO3 in the course of heat treatmentin vacuo has been studied by means of ESR. The formation of two different Mo(V) centers has been identified: center A withgx = 1.946, gy = 1.960, gz = 1.868; and center B characterized bygx = 1.943, gy = 1.954, gz = 1.878. Center B exhibited a well-resolved hyperfine structure resulting from naturally abundant odd isotopes of Mo:Ax = 3.2mT, Ay = 3.0mT, Az = 7.0mT. Species A formed at an early stage of reduction has been identified as Mo(V) in rhombically distorted square pyramidal surrounding, whereas center B appearing in strongly reduced samples shows a distorted octahedral coordination. The results have been interpreted in terms of the crystallographic shear mechanism.