L. Forni

Activity and deactivation of Fe-MFI catalysts for benzene hydroxylation to phenol by N2O

Abstract Isomorphously substituted Fe-MFI zeolite catalysts with various Si/Al and/or Si/Fe ratios were synthesized and characterized by many different techniques, such as ICP, XRD, SEM, TPR, microcalorimetry, FTIR, and EPR. Under standard reaction conditions the best catalyst gave 20% benzene conversion and over 90% selectivity to phenol. For Fe-ZSM5 catalysts, addition of steam to the feed improved catalyst activity, selectivity, and durability. Phenol formed onto Fe-based sites only. Active sites could very likely be composed of oxygen-bridged, extraframework binuclear Fe redox species, charge-compensating the framework Fe 3+ or Al 3+ ions. Surface acidity was not responsible for activity in the main reaction, but it was heavily involved in catalyst deactivation by coking. Catalyst deactivation derived mainly from the decomposition-condensation of phenol onto acid sites; the stronger the latter, the quicker was the coking rate.

Liquid-phase alkylation of naphthalene by isopropanol over zeolites. Part II : Beta zeolites

The title reaction has been studied on various HY-based catalysts. Surface acidity has been evaluated by adsorption microcalorimetry and FTIR analysis, using pyridine as probe molecule. Catalytic tests were carried out at 623 K and 40 bar in liquid phase, with decalin as solvent, in flow reactor. Isopropylation is accompanied by oligomerisation/cracking of the reactant alcohol, cracking of the solvent, isomerization of the products of the main reaction. Pore filling by coke is fast for non-dealuminated HY, where naphthalene conversion remains high due to the occurrence of transalkylation between polyisopropylnaphthalenes trapped in the pores and naphthalene; this does not occur on dealuminated HY, owing to the low density of the acid sites and the presence of a secondary mesoporous system which allows easy diffusion of the reaction products in the zeolite pores.

LaCeCo perovskites as catalysts for exhaust gas depollution

Abstract A series of La 1 − x Ce x CoO 3 + δ perovskite-type catalysts, with x ranging from 0 to 0.20, showed to be quite active for reduction of NO by CO (reaction 1) and for oxidation of CO by air oxygen (reaction 2) at temperatures ranging from 373 to 723 K. Analysis by X-ray diffraction, electron probe microanalysis, temperature-programmed desorption-temperature-programmed reaction and electron paramagnetic resonance, coupled with catalytic activity data, showed that the active sites on these catalysts are very likely localised onto Co ions, which coordinate O 2 − ions as intermediates for reaction 2, the latter taking place essentially through a suprafacial mechanism. For reaction 1, taking place through both a suprafacial and an intrafacial mechanism, the Co-based sites play also an additional role, favouring the electron transfer from site to site and so enhancing the transfer of oxygen species from surface to bulk and vice versa. Ce ions seem to act only as a stabiliser of O 2 − ions, helping in keeping them at the catalyst surface.

Electron paramagnetic resonance spectra of CeO2 catalyst for CO oxidation

The EPR spectra of freshly prepared CeO2 samples as well as of samples used as a catalyst in the oxidation of CO to CO2 are reported. Attribution of these patterns to oxygen species O2−, O2, O−, or to Ce3+ ions is discussed and dynamic phenomena involving these species are described. Bottlenecked systems between conduction electrons and Ce3+ ions are envoked to explain the observation of the Ce3+ EPR spectrum up to temperatures of about 370 K. Oxygen-deficient systems are obtained when the sample is used as a catalyst for CO oxidation in the presence of pyrex or quartz powder as a diluent, showing that the latter can play some role in stabilizing such species.

TPD-TPR-MS mechanistic study of the ammoxidation of 2-methylpyrazine over Sb-V-Mn-O catalyst

The setting up of a TPD-TPR-MS apparatus particularly suited to the study of the mechanisms of heterogeneous catalytic reactions is described. The system has been employed for the study of the ammoxidation of 2-methylpyrazine. The information collected showed that all the reactants and products except oxygen adsorb competitively on at least two types of site on the catalyst surface. From sites of the first type (the lower-energy sites) they desorb unaltered, while sites of the second type are higher-energy sites on which the reactants are activated both for the desired reaction to 2-cyanopyrazine and for the decomposition to by-products. Ammonia adsorbs also on a third type of site (very high-energy sites), which are able to activate the molecule for the oxidation to N2 and NOx Oxygen does not compete in adsorption with any of the other species. Very likely it dissolves in the solid, rapidly diffusing through the bulk and is involved in the reaction through a Mars-van Krevelen mechanism. A Rideal-Eley type of mechanism, with either oxygen or ammonia coming from the gas phase, seems not to be supported by the present results.

Study of Fe-silicalite catalyst for the N2O oxidation of benzene to phenol

Abstract A set of Fe-silicalite samples of MFI structure have been prepared by the hydrothermal technique, followed by steaming and by further chemical treating of the solid. After characterisation by nitrogen adsorption/desorption, X-ray diffraction (XRD), scanning electron microscopy-electron probe micro analysis (SEM-EPMA), the samples have been tested as catalysts for the oxidation of benzene to phenol by N 2 O. The best performing catalyst has been studied also by temperature programmed desorption-mass spectrometry and temperature programmed reaction-mass spectrometry (TPD-TPR-MS), after pre-adsorption of both reactants and products. It was found that phenol forms when N 2 O is adsorbed first, followed by benzene. Almost no phenol formation was observed when adsorbing benzene before N 2 O. Furthermore, on this catalyst N 2 O decomposed since 50°C or less, forming gaseous N 2 and adsorbed oxygen, which started to become available for the oxidation of benzene since 100–200°C. However, the so formed phenol remained adsorbed onto the catalyst. It desorbed within the 225–425°C temperature range, with a maximum around 300°C.

TPD-TPR-MS mechanistic study of the synthesis of 2-methylpyrazine over palladized Zn-Cr oxide

Abstract A mechanistic study of the cyclization of ethylenediamine (ED) and propyleneglycol (PG) to 2-methylpyrazine is reported, carried out by means of the TPD-TPR-MS technique. The reacting system proved to be complex, due to the presence of numerous intermediates and by-products, deriving either from one of the two reactants, or from both. The optimal reaction temperature, permitting the best selectivity to the desired product, was about 660 K. At least two different types of surface sites are present on the catalyst, and only the higher-energy ones activated the reactants. The reaction probably involves a rate-determining step of the Rideal-Eley type, between adsorbed PG and ED coming from the gaseous phase. This step leads to a fully hydrogenated intermediate, 2-methylpiperazine, which quickly dehydrogenates and aromatizes to 2-methylpyrazine.

Propylene oligomerisation over TiAlsilicalite Effect of catalyst pelleting pressure

Abstract The nature of diffusion phenomena in TiAlsilicalite and ZSM-5 catalysts has been investigated by means of the GC pulse method. Bulk, Knudsen, and intracrystalline contributions to the overall diffusion resistance have been evaluated by employing benzene as a probe molecule. The analysis of these results, together with those coming from a more general physicochemical characterisation of the solid, provide insight into the effect of pelleting pressure on the structure of the catalyst. There are shown to be considerable changes in catalytic activity and aging behaviour in propylene oligomerisation as a function of pelleting pressure.

Kinetics and mechanism in catalytic dehydrogenation of n-butane over chromia-alumina

Abstract The dehydrogenation reaction of n -butane over chromia-alumina has been kinetically investigated employing the differential reactor technique at temperatures ranging from 510 ° to 550 °C. The runs were performed on n -butane and on mixtures of n -butane + 1-butene or n -butane + hydrogen, in the presence of an inert gas (nitrogen), with complete analysis of reaction products. The reaction rate data fit satisfactorily the dual-site mechanism. A set of runs have been made on 10% chromia catalyst, containing different amounts of Li 2 O or Na 2 O. The progressive addition of Li 2 O gave a continuous deactivation of the catalyst, while the progressive addition of Na 2 O gave a deactivation followed by an activation. An interpretation of the reaction mechanism on the basis of electronic theory of catalysis is offered.

Kinetic study of 1-butene isomerization on alumina at 510–550 °C

Abstract A kinetic study of 1-butene isomerization on nonactivated alumina at 510 °, 530 °, and 550 °C has been performed in a flow system. The kinetic runs were made by feeding mixtures of 1-butene and nitrogen or 1-butene, nitrogen, and ammonia at various partial pressures. The results, interpreted in the Langmuir-Hinshelwood scheme, confirmed the existence of a thermal activation of the catalyst. The presence of ammonia decreases the reaction rate consistently with a competitive adsorption of this gas on the isomerization active centers.