J.M. Lopes

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.

Structure-activity relationship in zeolites

Abstract In this review we will try to give an integrated view of the relation between the structure and the catalytic behaviour of zeolitic materials, covering both the compositional and the structural aspects. Due to the amount of work that has been done in this wide area of zeolite catalysis this will, necessarily, be an incomplete although, hopefully, unbiased work. The paper will refer, not only to the relation that can be observed with natural and as-synthesised zeolites, but also to the wide range of techniques that are currently available for the ‘tuning’ of the catalytic properties of zeolitic materials. A special emphasis will be placed in all the aspects concerning shape selective catalysis, which is certainly the most striking form of a structure-activity relationship in heterogeneous catalysis.

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.

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.

Activity–acidity relationship in zeolite Y: Part 3. Application of Brönsted type equations

The relation between activity and acidity in a variety of Y zeolite catalysts for the transformation of small olefins: ethene, propene, 1-butene and iso-butene, is presented. The acid site distribution of these samples was estimated by temperature-programmed desorption of ammonia, by using a numerical deconvolution technique. This information was then correlated with catalytic activity measurements using Bronsted type equations. Despite the complexity of calculations it is clear that this gas phase solid acid catalysed reactions follow simple rules, similar to the one observed in homogeneous processes.

Mixing effect of USHY+HZSM-5 for different catalyst ratios on the n-heptane transformation

Abstract The effect of mixing of an ultrastabilised acid form of Y zeolite (USHY, Si/Al=4.5) with two different HZSM-5 zeolites, synthesised by different methods, with template (CVN, Si/Al=45) and without template (CVM, Si/Al=26), and presenting quite different acidic properties, is studied using as model reaction the transformation of n-heptane at 350°C. The results obtained point to the existence of some interactions between the processes occurring in the two zeolites, and are helpful in the understanding of the known ZSM-5 addition effect in the cracking catalysts. There is a clear enhancement in the formation of ramified C4 products for the USHY:ZSM-5 mixtures (75:25, wt%) and (50:50), relative to the additive linear predictions, this effect being significantly more extensive for the mixtures with the ZSM-5 sample presenting the highest acid strength. In the mixtures (90:10) there is also an increment in the ramified/linear products ratio. The initial olefin/paraffin products ratio registered a slight increase, relative to the expected values, in all the mixtures with CVN. In the case of CVM there was also an increase of this parameter, but only for the mixture with 10% of ZSM-5, a significant reduction being observed in all the other cases.

Influence of cerium on the catalytic properties of ZSM-20 zeolite in the cracking of n-heptane: Comparison with rare earth Y zeolites

Abstract The catalytic properties of two series (sodium and hydrogen forms) of CeZSM-20 zeolites were studied in the cracking of n-heptane. The hydrogen series shows a maximum initial cracking activity for a cerium exchange level of about 30%; this maximum has a different location than that which would be expected based on the results obtained with rare earth Y zeolites. This difference is probably related to the particular crystalline structure of ZSM-20. The decrease in the initial activity at higher exchange levels is possibly due to the formation of hydroxyl bonds from the partial hydrolysis of two adjacent Ce 3+ cations. In the sodium series the initial activity increases steadily with increasing exchange level. The product distribution for n-heptane cracking is in accordance with the classical carbenium ion mechanism in which the limiting step should be the formation of the n-heptyl carbenium ions by hydride transfer.

Activity–acidity relationship in zeolite Y: Part 1. Transformation of light olefins

Reactions of ethene, propene, 1-butene and iso-butene over two series of catalysts, HNaY (prepared from NaY zeolite) and HNaUSY (prepared from NH4USY zeolite), were studied. The catalytic activity for the transformation of these olefins follows the expected order according to the ease of formation of the respective carbocations: ethene<propene<1-butene<iso-butene, for all the samples that were studied. The catalytic results obtained over the HNaUSY series of catalysts are compatible with what was expected for light olefin transformation over acid catalysts. The catalytic activity of HNaY series of catalysts, however, does not follow a regular trend with the protonic content, showing a decrease when the protonic content is high.