István Marsi

Isomorphously substituted Fe-ZSM-5 zeolites as catalysts: Causes of catalyst ageing as revealed by X-band EPR, Mössbauer and 29Si MAS NMR spectra

Abstract An unconstrained curve fitting/parameter estimation program adapted to PC was applied for the deconvolution of X-band EPR spectra of Fe(III) in ZSM-5 (MFI) zeolites. The sub-spectra of framework (FW) and extra-framework (EFW) Fe(III) ions sited in environments of different (ligand) symmetry could be identified. The EPR transition probability, r , of Fe(III) in the little oxide clusters (diads, triads) was 13.75 times larger than that incorporated into the lattice. Increase of the cluster size and ordering of the random structure led to reduction of r . When the co-ordination of Fe(III) ions in the oxide clusters approached the cubic symmetry of Fe(III) in the lattice, r converged to 1.0. The state: r =1.0 is equivalent to the development of a crystalline phase. The experimental determination of r revealed important details of catalyst action. The initial activity in the direct oxidation of benzene to phenol (oxidant: N 2 O) is due to oxide clusters of random structure (known as “ferrihydrite”). Partial and eventually (almost) complete loss of activity occurs when the clusters get ordered to hematite (or in reducing atmosphere to magnetite, as well). Contrary to fully inactive hematite, magnetite retains about 50% of the original activity because it exposes [Fe(III)] Th –O–[FeIII)] Oh linkages supposed to be responsible for oxidation activity.

Attempts to produce uniform Fe(III) siting in various Fe-content ZSM-5 zeolites: Determination of framework/extra-framework ratio of Fe(III) in zeolites by EPR and Mössbauer spectroscopy

Abstract Fe(III)-content ZSM-5 catalysts play important role in selective redox reactions, like biomimetic oxidations, or selective catalytic reactions (SCR) of pollutants, dangerous for the environment. There is a dispute in the literature whether the catalytic activity of these materials is due to isomorphously substituting framework (FW) iron, or extra-framework (EFW) oxide/hydroxide material of high dispersity, not incorporated into the lattice during the synthesis, or became ejected from the framework following the post-synthesis treatments (calcination, heat-treatment). In either case it is a must to know the FW/EFW ratio in order to correlate the catalytic activity with the number of active sites, or active surface area of the catalyst ingredient. In the case of EFW iron only (introduced, e.g. by ion-exchange) the chemical identity, particle size, location and thereby the probability of attainment from the fluid phase(s) are uncertain. Isomorphic substitution is more complicated, because in addition to these even the FW/EFW ratio is questionable. From both practical and academic interest it would be advantageous (e.g. from the point of view of above correlation) to have uniform FW, or EFW Fe(III) siting, or when it is mixed, to know the FW/EFW ratio. The authors approach this problem from several directions: they elaborate various synthesis methods to minimise EFW iron; extract unwanted EFW iron by reducing/complexing agents (SO 2 , hydroxylamine), or, if necessary, reintroduce EFW iron by (liquid and solid) ion-exchange and monitor the changes thus produced in the FW and EFW iron contents by X-band EPR and Mossbauer spectroscopy. Using mathematical methods and sophisticated analysis of the sub-spectra (intensity dependence on the iron content and temperature), the EPR spectrum-deconvolution made possible to reinterpret the X-band EPR Fe(III) spectra in weak ligand-field media, like zeolites. If the total iron contents of samples are known (e.g. from XRF spectra) the FW/EFW ratio can be computed on the basis of the deconvoluted EPR spectra alone. These values are in very good agreement with similar estimates obtained from the Mossbauer spectra, proving that the Mossbauer “loudness” for the various iron components is nearly equal.

Synthesis, Characterization and Catalytic Activity of V-Zsm-5 Zeolites

Abstract A V-ZSM-5 sample with a Si/V = 42 was synthetized outgoing from VO(COO)2 and Q-brand sodium silicate using TPA-Br as template. ESR spectroscopy proved that vanadium(IV) ions in the zeolitic framework exhibit a distorted square planar symmetry. Upon heat treatment a part of the framework vanadium ions migrate to extra-framework positions. After dehydration no Bronsted acidity was found. Treatment in oxygen and hydrogen above 570 K revealed the redox character of the V-ZSM-5 sample. In oxidation of n-butane (as catalytic test reaction) the V-ZSM-5 zeolite exhibits selective dehydrogenation and aromatization activity.

Attempts to produce uniform Fe(III) siting in Fe content SOD and LTA zeolites: An EPR and Mössbauer study

The authors’ studies dealing with Fe(III) content sodalites (SOD) and NaA (LTA) zeolites [Appl. Catal. A 223 (2002) 147] revealed a lesser degree of framework (FW) aluminium substitution by Fe(III) than anticipated on the basis of previous publications [Solid State Ionics 53–56 (1992) 1282]. The fairly high stability of SOD structure “withstood” the attack by diluted mineral acids to some degree in that amounts in about 30–40% excess compared to the ion-exchange capacity of the zeolite did not result in complete dissolution, but in amorphisation only, while the solid habit was retained. Fe content LTA samples exhibited lower tolerance against mineral acids. The acid treatment of SOD’s (or industrial by-products with high SOD content) combined with addition of reducing and/or complexing agents (like sulphur dioxide, hydroxylamine, EDTA, “phen”, “dipy”, etc.) permits extraction of extra-framework (EFW), or EFW and FW iron, if necessary. The recrystallisation of fully deferrated, amorphised material makes the synthesis of various zeolites possible with acceptable white colour. X-band EPR and Mossbauer spectra of SOD’s and LTA samples and those enriched in 57Fe tracer, revealed uniform Fe(III) siting in the (partially) deferrated and recrystallised samples, although at low Fe(III) levels. It is an interesting observation that this uniform siting is dominantly FW in SOD’s and EFW in LTA’s with unexpected low symmetry, unfilled co-ordinations. Besides that, these investigations may contribute to the understanding of Fe(III) ion substitution in three-dimensional tectosilicate frameworks, the LTA preparations can be attractive as selective oxidation catalysts for reactants not excluded from the structure by sieving effects.

Kinetics and mechanism of oxidation of n-butenes over SnO2Sb2O4 catalysts

Abstract Over Sn:Sb=3:l catalyst at 423–673 K the kinetics of oxidation and isomerization of 1-butene and cis- and trans-2-butene have been studied in a recirculatory batch reactor. The oxygen uptake of the catalysts was determined with a microbalance and kinetic parameters of oxidation were calculated. Comparison of the oxidation of the n-butenes demonstrates that both the conversion and the selectivity for butadiene are much lower in the reaction of both 2-butenes than in the oxidation of 1-butene. Since the double-bond isomerization (taking place over the Bronsted-type acidic sites of the catalyst) is a much slower reaction even at higher reaction temperatures, it was supposed that the only reaction step yielding butadiene is 1-butene→butadiene on an oxidation site.

Determination of the Arrhenius parameters of the decomposition of azoisopropane: investigation of possible systematic errors via computer simulation

Abstract The different methods for the determination of Arrhenius parameters of the initiation step of azoisopropane (AIP) thermal decomposition in the presence of isobutene were investigated via computer simulation. The systematic errors in the methods were estimated.

Estimation of kinetic parameters in oxidative dehydrogenation of n-butenes over SnO2-Sb2O4 catalysts

Kinetic curves measured in the oxidation of n-butenes over a Sn∶Sb=3∶1 mixed oxide catalyst were fitted by the kinetic model put forward in a previous paper. The goodness of fitting shows that the kinetic behavior of this complex reaction system can be described by a mechanism involving acidic and redox sites on the catalyst surface.AbstractПроведено приближение экспериментальных кинетических кривых окисления н-бутиленов на смешанном оксидном катализаторе Sn∶Sb=3∶1 с помощью кинетической модели, предложенной в предыдущей работе. Благодаря надежности этого приближения кинетические свойства этой сложной реакции могут быть описаны механизмом, включающим как кислотные, так и окислительно-восстановительные центры на поверхности катализатора.

Kinetic simulation of oxidative dehydrogenation of n-butenes over SnO2-Sb2O4 catalysts

A previously suggested reaction mechanism was utilized to evaluate a kinetic model for the oxidation of n-butenes over Sh∶Sb=3∶1 mixed oxide catalyst. With this model, kinetic curves measured at 673 K were simulated, illustrating the changes of amounts of undetectable surface components, too.AbstractДля оценки кинетической модели окисления н-бутиленов на смешанном оксидном катализаторе Sn∶Sb=3∶1 применен нредложенный ранее механизм реакции. С помощью этой модели симулировали кинетические кривые, полученные при 673 K, иллюстрируя также изменения количеств недетектируемых поверхностных соединений.

Facilitation of computer estimation of kinetic parameters of a heterogeneous catalytic reaction via the use of adsorption data

Abstract Over CoNa-A zeolite catalysts cyclopropane is transformed into propene. The reaction was carried out in a recirculatory batch reactor and its progress has been followed by detecting the concentrations of the components in the gas phase. The measured kinetic curves thus obtained can then be fitted to the functions obtained by numerical integration of the kinetic equations. The present paper shows how to apply results of adsorption measurements as independent information in order to decrease the uncertainty and correlation of estimated parameters involving the number of active sites besides kinetic parameters. Some problems of the statistical analysis are discussed.

Spectroscopic characterization of Fe-BEA zeolite

Abstract Fe-BEA zeolite samples are characterized by Mossbauer and Electron Paramagnetic Resonance (EPR) spectroscopies, 29 Si-, 27 A1- and 13 C-NMR spectroscopy and thermal analysis. Particular attention is devoted to the determination of framework and extra framework iron species.