Andrea Katović

Vapor phase Beckmann rearrangement using high silica zeolite catalyst

Vapor phase Beckmann rearrangement of cyclohexanone oxime to e-caprolactam has been studied using high silica zeolite catalysts. Catalysts with different crystal sizes and gel-ageing times have been activated by ionic exchange in different conditions by means of a highly basic solution and a nearly neutral solution both containing ammonium salts. Samples have been calcined at different temperatures in order modify the number of defective sites. We observed that samples exchanged by means of a highly basic solution (pH > 10) and calcined at a relatively lower temperature (450 °C) show the most interesting catalytic results. X-ray powder diffraction patterns of these samples show retention of the unit cell symmetry (orthorhombic cell) if compared to the dried sample. NH3-TPD confirms the low acidity of high silica zeolites, however a higher amount of desorbed ammonia is observed for the samples exchanged at higher pH and calcined at 450 °C. Due to silanol nests the IR spectra of the same samples show the formation of Si–NH2 bonds which are absent in the same material exchanged by other methods. Such sites seem to promote the high stability of the high silica zeolite catalysts also to the regeneration which is needed to remove the heavy carbonaceous compounds from the catalyst surface.

Benzene Selective Oxidation with N2O on Fe/MFI Catalysts: Role of Zeolite and Iron Sites on the Deactivation Mechanism

The catalytic performances of Fe-zeolites having MFI structures and in which the Fe introduced either by ion exchange or during the hydrothermal synthesis has undergone partial framework to extra-framework migration induced by controlled heat treatment are reported. In particular, the catalytic behavior as function of time-on-stream and the formation of carbonaceous species were studied. The results suggest that only a small fraction of the iron is active in the selective oxidation of benzene to phenol in the presence of N2O. It is suggested that the active fraction is formed by isolated iron ions in a pseudo-octahedral configuration with the sites positioned in hydroxyl nests (defects) of the zeolite and is selective in phenol formation as a result of in situ reduction during the catalytic tests. Two possible pathways of carbonaceous species were identified, the first through the intermediate further hydroxylation of phenol and the second through the coupling of phenol with benzene or another phenol molecule. This second pathway is the dominant mechanism of formation of carbonaceous species, although the relative rate of the two pathways depends on the zeolite characteristics and iron loading. It is also suggested that the second pathway depends on the strong chemisorption of phenol, probably on Lewis acid sites, which hinders the fast back-desorption of phenol out from the zeolite channels and thus favors the formation of carbonaceous species. Catalysts prepared by hydrothermal treatment show a lower rate of deactivation than those prepared by ion exchange, although the latter show a comparable productivity to phenol for amounts of iron in extra-framework positions around 20 to 30 times lower. The results also indicate that the presence of Al in the zeolite framework is beneficial for reducing the rate of deactivation as compared to that of Fe-silicalite samples.

Isomorphous substitution in zeolites

Abstract This chapter deals with the isomorphous substitution in the zeolitic framework. Some specific cases among the research works of the authors have been selected. B-MFI, Fe-MFI, Fe-BEA and Fe-MOR zeolites have been examined optimizing the synthesis parameters. B-MFI zeolite has been synthesized in fluoride media, while the other zeolites have been prepared in alkaline media. In all cases isomorphous substitution in the zeolitic structure, even if not complete, is obtained. It should be pointed out that in fluoride media and in presence of K+ and Cs+ ions more than four boron atoms per unit cell can be incorporated in the MFI structure. To cite this article: R. Aiello et al., C. R. Chimie 8 (2005).

Active and spectator iron species in Fe/MFI catalysts for benzene selective hydroxylation with N2O

Abstract The presence and nature of active and spectator iron species in Fe/MFI catalysts is analyzed by comparing reactivity and physico-chemical properties (by ESR, Mossbauer and 29Si-NMR) of a series of samples pretreated in different conditions and in which part of the iron is selectively removed by extraction. A large part of the extra-framework (EFW) iron present inside zeolite channels could be removed without affecting significantly the productivity or selectivity to phenol. This shows that the most abundant species which may be identified in these samples are not those responsible for activity. During the catalytic reaction further modification occurs with the migration to EFW positions of the largest part of framework (FW) iron due to Fe3+ to Fe2+ reduction forming first disordered clustered iron oxide/hydroxide species (ferrihydrite) which then nucleate forming magnetite nanodomains. This initial dislodgement of FeFW determines an initial increase in phenol productivity. After about 3h the productivity to phenol reaches a nearly constant value, but this residual activity does not correlates with the amount of FeEFW further evidencing that the largest part of EFW iron species are only spectator in benzene selective hydroxylation, while the active species are very limited and possibly associated to sites interacting with defect sites suggested to be (-Si-O)2Fe2+ species which reacts with N2O to give rise to an (-Si-O)2Fe4+=O oxo-type species.

Influence of various synthesis parameters on the morphology and crystal size of zeolite Zn-MFI

The use of the zinc-phosphate complex in the hydrothermal synthesis has been proven to be a good way for obtaining zinc-zeolite materials, especially in the case of the zeolite Zn-MFI that can be prepared in a large range of Si/Zn ratios [1, 2]. Moreover, the pH values of the starting hydrogel play an important role on the nature of the crystalline phases obtained. For that reason the first step in the synthesis optimization is the definition of the pH range that is specific for each zoelite type. The crystal size and morphology can be varied by altering the chemical synthesis parameters (zinc, organic and water content as the main ones) as well as the source of silica. Moreover, the isopropanol decomposition is carried out with the purpose to valuate the acidity of the prepared Zn-MFI samples.

One-step benzene oxidation to phenol. Part I: Preparation and characterization of Fe-(Al)MFI type catalysts

The characteristics of iron-containing MFI type zeolites synthesized by different methods (direct synthesis, chemical vapor deposition, solid state reaction and ion exchange), in relation to their application for the one-step synthesis of benzene from phenol, were investigated by Mossbauer and NMR techniques. In the direct synthesis, the amount of iron incorporated in the zeolite framework depends on both its amount in the initial hydrogel and the TPABr template concentration. Part of these framework iron ions migrate to extra-framework position during catalytic tests and/or catalyst hydrothermal pretreatment forming active (Fe,Al) framework −O−(Fe,Al) extra-framework pair sites in which iron is in a distorted octahedral coordination. During long-term catalytic tests in benzene hydroxylation the iron ions migrate to more stable positions in the zeolite which is one of the cause of catalyst deactivation together with the formation of carbonaceous species and strongly chemisorbed phenol as detected by 13 C-NMR. Introduction of iron by post-synthesis methods leads to a lower dispersion of iron and less stable species during the catalytic reaction.

One step benzene oxidation to phenol. Part II: Catalytic behavior of Fe-(Al)MFI zeolites

The catalytic behavior in gas phase hydroxylation of benzene to phenol with nitrous oxide on Fe-(Al)MFI zeolites prepared by different methods is reported. The amount of iron, thermal treatment, presence of aluminium ions and method of iron addition to the zeolite are all important parameters which determine the catalyst life time as well as the selectivity and productivity to phenol. Control of these parameters make it possible to improve the catalyst life-time and productivity to phenol.

04-P-14-Preparation of zinc containing zeolite catalysts

Publisher Summary This chapter discusses the preparation of zinc (Zn)-containing zeolite catalysts by direct synthesis from hydrogels. The postsynthesis introduction of copper (Cu) is also presented. The samples are characterized by X-ray diffraction (XRD), atomic absorption spectroscopy (AAS), thermal analysis, scanning electron microscopy (SEM), and 29 silicon (Si)-nuclear magnetic resonance (NMR) spectroscopy. The catalytic results on the cumene conversion are described.

Spanish Broom (Spartium junceum L.)—History and Perspective

The Spanish Broom (Spartium junceum L.) plant is almost a forgotten textile raw material. In this paper a review of procedures practiced in the past and nowadays for obtaining fibers from this plant is presented. New discoveries about its use as a component of special composite fibers as well as its probable benefits for the households of poorer limestone areas are discussed. Some chemical and physical properties of the fibers extracted from local plants are also reported.

An NMR characterization of MTW zeolite synthesized inpresence of TEA+ and/or MTEA+ cations

Prompted by the recently obtained results on the number, type and strength of acid sites in MTW zeolite, 13C-, 17Al- and 29Si-NMR spectra of the final MTW products, as well as of the starting hydrogels, prepared from reaction mixtures containing one or both organic cations were recorded in order to better understand the zeolite properties. As the Si/Al ratio in the zeolite is one of the most important factors that determines its usefulness as a catalyst, the influence of these cations (MTEA+ and TEA+) on the aluminium incorporation in the zeolite framework has been investigated.