F. Ramôa Ribeiro

Conversion of light alkanes to aromatic hydrocarbons: II. Role of gallium species in propane transformation on GaZSM5 catalysts

Abstract The conversion of propane into benzenic compounds was studied at 530°C on a series of GaHZSM5 catalysts prepared by impregnation of HZSM5 with a solution of gallium nitrate. Gallium species increase the rate of propane conversion and the selectivity for aromatics. These species have dehydrogenating activity, increasing both propane activation and naphthene aromatization. The conversion of propane into benzenic compounds on GaHZSM5 catalysts can be considered, therefore, as a bifunctional process in which dehydrogenation reactions are catalyzed by the gallium species and oligomerization and oligomer cyclization by the acid sites. The active gallium species seem to be gallium oxide dispersed in the zeolite rather than gallium cations in an exchange position.

Infrared spectroscopic study of the acid properties of dealuminated BEA zeolites

Abstract A series of HBEA samples were prepared by dealumination through three different methods (steaming, treatments with hydrochloric acid and ammonium hexafluorosilicate (HFS) solutions) of a parent sample with total and framework Si:Al ratios of 12.5 and 33, respectively, constituted of 20 nm crystallites. The samples were characterized by XRD, nitrogen adsorption and pyridine adsorption–desorption followed by IR spectroscopy. Whereas the three dealumination treatments have practically no effect on crystallinity and nitrogen adsorption properties, they cause large changes in the ranges of the O–H bond stretching modes (3300–3800 cm−1) and of the most intense IR absorption of pyridine (1400–1700 cm−1). Treatments with HCl or HFS cause the most significant changes: disappearance of the 3662 and 3782 cm−1 bands corresponding to extraframework Al species and to tricoordinated Al species partially connected to the framework, respectively, appearance of additional bands at 1603 and 1446 cm−1 ascribed to a new type of pyridine species coordinated to Lewis sites (PyL2) and for desorption temperatures above 350 °C, of a band at 1462 cm−1 generally ascribed to iminium ions, apparently at the expense of the PyL2 band. In agreement with this formation of iminium ions, desorption at high temperatures causes the complete disappearance of pyridinium ions without complete restoration of the acidic bridging hydroxyl band. PyL2 species are proposed to be pyridine molecules coordinated to Lewis sites and interacting through hydrogen bond with protonic sites.

The Acidity of Zeolites: Concepts, Measurements and Relation to Catalysis: A Review on Experimental and Theoretical Methods for the Study of Zeolite Acidity

In this article, we considered all aspects of acidity (nature of acid sites, strength, density, etc.) in solid catalysts and in zeolites in particular. After reminding the definition of acidity in liquid and solid acids, we emphasized acidity characterization by the most used physical techniques, such as Hammetts indicator titration, microcalorimetry of adsorbed probe molecules (ammonia, pyridine or other amines for acidity characterization and CO2 or SO2 for basicity characterization), ammonia or any amine thermodesorption, IR spectroscopy of hydroxyl groups and of several probe molecules adsorbed (ammonia, pyridine, piperidine, amines, CO, H2, etc.), MAS-NMR of 27Al, 29Si, 1H elements and of 1H, 13C, 31P, etc. of adsorbed probe molecules, and model catalytic reactions. Modeling the way the acid features of zeolites influence the catalytic activity of these catalysts toward acid-catalyzed reactions (relation between ammonia desorption activation energy values and catalytic activities, reaction mechanism, and kinetics) completes the general analysis of acidity and zeolite chemistry.

Zeolites as supports for enzymatic hydrolysis reactions. Comparative study of several zeolites

Abstract Fusarium solani pisi recombinant cutinase was immobilised by adsorption on different zeolites and its activity towards the hydrolysis reaction of tricaprylin was measured. It was observed that the enzyme immobilised over most of the zeolites used (NaA, NaX, NaY, NaUSY and modified forms) was active for this reaction, although the specific activity depended strongly on the structure and composition of the zeolite support. This preliminary study of the use of zeolites as supports for enzymes was also extended by the analysis of the influence of the water content and of the thermal stability of one of the preparations. It can be concluded that the prospective use of zeolites as supports for enzymes in reactions involving organic media is very promising.

ZEOLITES AS SUPPORTS FOR AN ENZYMATIC ALCOHOLYSIS REACTION

A recombinant cutinase from Fusarium solani pisi was immobilized by adsorption on several zeolites and its activity towards the alcoholysis reaction of butyl acetate with hexanol, in organic media (isooctane), was measured as a function of the water content and water activity. The effects of the zeolite framework composition (including cation nature) and acidity were studied. The results were compared with other commonly used supports: polyamide Accurel-PA6, silica and alumina. Both the nature of the cation and the silica:alumina (Si:Al) ratio of the framework revealed to be important parameters. The most promising results were obtained for supports with little acidity and with lower Si:Al ratio. This last observation is in accordance with the results obtained with silica and alumina.

Influence of the cerium content of CeHY catalysts on their physicochemical and catalytic properties

Abstract The physicochemical and catalytic properties of a stabilized HY zeolite and of CeHY zeolites were compared. The Bro˝nsted acid sites of CeHY zeolites are definitely stronger than those of HY whereas their Lewis acid sites are weaker; the total number of acid sites decreases with the cerium content. The strength of protonic sites and the activity for n-heptane cracking at 350°C are maximal for an exchange rate of about 45%. For all the samples the cracking activity is clearly related to the strong protonic acidity. The rates and selectivities of the transformations of n-heptane and of its isomers (methylhexanes, 2,4-dimethylpentane) were compared. It can be shown that on all the catalysts, n-heptane cracking occurs through the classical carbenium ion chain mechanism in which the formation of nC 7 secondary carbenium ions is the limiting step; cracking products (essentially C 3 and iC 4 ) result mainly from the scission of 2,4- and 2,2-dimethyl carbenium ions,

Total oxidation of toluene over calcined trimetallic hydrotalcites type catalysts.

Two trimetallic ZnCuAl and MnCuAl hydrotalcites have been successfully synthesized by a co-precipitation method. The manganese based material was identified as a new hydrotalcite phase. Both lamellar precursors were calcined at 450 and 600 degrees C and the resulting catalysts were tested on reaction of total oxidation of toluene. The solids were characterized by X-ray diffraction, thermal analysis, atomic absorption spectroscopy, Fourier transformed infrared spectroscopy, N(2) adsorption and H(2) temperature-programmed reduction. It was found that ZnCuAl materials are composed of copper and zinc oxides supported on alumina; while MnCuAl ones comprise basically spinel phases, which were not completely identified. The catalytic behavior of the calcined samples showed that Mn hydrotalcite calcined at 450 degrees C exhibited the best catalytic performance that corresponds to 100% toluene conversion into CO(2) at about 300 degrees C.

Activity–acidity relationship in zeolite Y: Part 2. Determination of the acid strength distribution by temperature programmed desorption of ammonia

Abstract In this paper we will have a detailed look at a numerical procedure that allows the estimation of the acid site strength distribution using a single temperature programmed desorption (TPD) experiment. The possibility of estimating these parameters is of paramount importance in the determination of quantitative relationships between activity and acidity. The application of the procedure to a set of simulated thermograms gives a clear view of the applicability of this method. The procedure was tested both in single and multi energy distributions, as well as quasi-continuum distributions. Then the method has been applied on TPD thermograms obtained from catalysts based on two different forms of zeolite Y. In part 3 of this series, the acid site distributions obtained here will be used in an activity–acidity relationship, using a Bronsted type equation, similar to the one used in homogeneous acid catalysis.

Effect of the immobilization support on the hydrolytic activity of a cutinase from Fusarium solani pisi

Fusarium solani pisi recombinant cutinase was immobilized on two support materials (NaY zeolite and polyamide-Accurel PA6). These preparations were used to catalyze the hydrolysis of tricaprylin in a nonconventional medium. Water content is a very important factor in relation to enzyme activity in organic media, and the effect of the water in the reaction medium as well as the effect of the organic solvent at different water concentrations on the hydrolytic activity of the immobilized cutinase were investigated. Factors that are also important for the catalytic activity of the enzyme such as immobilization pH, ionic strength, and pH of the reaction medium and temperature were optimized. The selected conditions for further work were pH 9 (the immobilization pH) and 30°C. In order to evaluate cutinase selectivity, the kinetic parameters of the immobilized enzyme were determined with triglycerides containing fatty acids ranging from C4 up to C14. A selectivity to C8 was observed with both supports. The thermal stability of the enzyme was also investigated and a particularly good stability was observed with cutinase immobilized on the NaY zeolite with no loss of initial activity after 45 days.

Carbon dioxide in Y and ZSM-20 zeolites : adsorption and infrared studies

Abstract Adsorption of carbon dioxide was performed on sodic and protonic forms of Y and ZSM-20 zeolites. The adsorption isotherms of carbon dioxide were determined in the temperature range of 293—318 K by the volumetric method. Isosteric heats were obtained by the Clausius-Clapeyron equation. In order to obtain information on the adsorbed molecules at low coverages, the infrared technique was used. It is shown that, in spite of having closely related structures, the Y and ZSM-20 zeolites present structures with different polar sites. This difference in textural properties can influence the adsorption of molecules capable of having specific interactions with surface polar groups.