Influence of Controlled Structural Changes on the Catalytic Properties of Zeolites

Influence of Controlled Structural Changes on the Catalytic Properties of Zeolites

137 INFLUENCE OF CONTROLLED STRUCTURAL CHANGES ON THE CATALYTIC PROPERTIES OF ZEOLITES P. JIRU J. Heyrovsky Institute of Physical Chemistry and El...

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INFLUENCE OF CONTROLLED STRUCTURAL CHANGES ON THE CATALYTIC PROPERTIES OF ZEOLITES

P.

JIRU

J. Heyrovsky Institute of Physical Chemistry and Electrochemistry,

Czechoslovak Academy of Sciences, 121 38 Prague 2 (Czechoslovakia)

ABSTRACT The paper is aimed at a detailed discussion of the results of an experimental investigation of the effect of changes in surface and/or crystallographic structure of zeolites of the types Y and ZSM on their reactivity. The test interactions were: i) the activation of hydrogen by the zeolites Fe(II)Y; ii)the interaction of the dealuminated zeolites HY and that of the zeolites ZSM with methanol. INTRODUCTION A series of new catalytic processes using zeolitic catalysts with a higher content of Si has been realized recently in the world. In order to explain from the theoretical point of view the remarkable activity and selectivity of these zeolitic catalysts the laboratories taking part in this research have published a great amount of more generally valid data about their synthesis and modification. The achieved results give suggestions for controlled changes both in the crystallographic and surface structure of the zeolites (the former concerning e.g. the dimensions of the cavities and their electrostatic field, the latter namely the type and the amount of OH groups, the presence and the location of protons, the composition of the surface layers of the zeolite crystallites and the Si :Al ratio) as well as for interpretations of their influence on the catalytic behaviour of cations in zeolites. In the preparation of metal-loaded zeolite catalysts (e.g. by reduction) these cations may act as the precursors of the corresponding metal particles. Such changes may then affect the structural modification in question and optimize it from the point of view of a preferential course of a certain reaction. The aim of our 4 contributions on the occasion of this workshop is to present the results of investigations in this field,

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both experimental and theoretical. The following investigations were performed: i) a quantum chemical study on the molecular level of the properties of model clusters of faujasites with a varying Si :Al ratio and various forms of Cr, Fe, Co and Ni; ii) an experimental study of the influence of the pretreatment of the zeolites on the valence state, the localization and possibly the coordination of the cations of Fe, Cr and Ni as well as on the structural stability and catalytic activity of the respective zeolite; iii) the characterization of the structure of HY zeolites with a high degree of dealumination and the comparison of their surface structure and reactivity with the zeolites H-ZSM. A more detailed discussion of the results of i) and ii) is presented in three separate paper~ in this workshop - see Beran et al., Wichterlovi et al., Patzelovi et al .. My own paper will be dedicated to a survey of some recent results concerning the point iii) and the catalytic activity of the zeolites Fe(II)HY and that of the high-silica zeolites. In order to obtain an information as full as possible a series of experimental techniques, such as IR spectroscopy, ESCA, ESR, X-ray diffraction analysis and TPD, have been used. CATALYTIC ACTIVITY OF THE Fe(II)HY ZEOLITES IN THE ACTIVATION OF HYDROGEN The Fe(III) ions, introduced by ionic exchange up to 23 , occupied preferentially the cationic sites and the stability of zeolitic structure increased. Cationic Fe(III) exhibited considerable tendency to self-reduction: after thermal treatment in vacuo of Fe(II). Thermal treatment (670 K) the zeolites contained 75 up to 1070 K in vacuo and/or in hydrogen was accompanied by the dehydroxylation of structural OH groups and by the migration of Fe(II) cations into the large cavities. Iron-rich Fe(III)NH 4Y zeolite (63 ~ of ion exchange) contained an appreciable amount of the hydroxo-oxidic form of Fe(III) in addition to Fe(III) in cationic sites. This hydroxo-oxidic form was more resistant against self-reduction than the cationic one. In contrast to zeolites with low content of iron no movement of Fe(II) species was found at high pretreatment temperatures; the structural stability was lower and nonspecific Si-OH groups predominated over the structural ones. In all zeolites a very small amount of metallic iron (less than 10 %) was found in the surface layers after hydrogen reduction at

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8Z0 K. The changes of Fe content and Fe localization as well as of the zeolite structure stability and of the nature and number of OH groups affect the deuterium exchange which was investigated with regard to the zeolite pretreatment on Fe(II)HY zeolites. Both types of deuterium exchange (OZ + HZ and 0z + OH) were found not to be influenced by added Fe up to 7 % of ion exchange. A higher amount of Fe(ll) accelerated the 0z + OH exchange in the pretreatment temperature range of 670 - 8Z0 K. This acceleration resulted probably from the presence of a sufficient number of Fe(II) in the vicinity of OH groups. In this way the dissociation of the OH bond was enhanced and the 02 exchange with OH groups predominated. With further increase of the pretreatment temperature the number of OH groups substantially decreased and thus the probability of the interaction of activated hydrogen species leading to 0z + HZ exchange increased. The presence of Fe(ll) in large cavities seems to catalyze this type of exchange more likely than the small amount of metallic iron on the zeolite surface. This can be seen from the comparison of zeolites with Z3 and 63 % of Fe ion exchange: the former zeolite with high 0z + HZ exchange rate exhibited a considerable number of Fe(ll) in large cavities and a sma1 ler amount of metallic iron in the surface layer, the latter one had a lower amount of Fe(II) in large cavities, a higher concentration of surface Fe o and a very low 0z + HZ exchange rate. This behaviour of Fe-rich zeolite can be explained by its structural collapse. COMPARISON OF STRUCTURE ANO REACTIVITY OF OEALUMINATEO ZEOLITES WITH THOSE OF THE H-ZSM ZEOLITES In this part of my paper I would like to discuss the independent influence of changes in the surface or crystallographic structure of the zeolites (i .e. in the absence of cations of transition metals and therefore also without their influence) on the course of the catalytic reaction. This independent effect must be taken into account also in the case of metal-loaded zeolite catalysts, as these represent in many cases typical bifunctional catalysts. The X-ray diffraction data together with the IR spectra of the skeletal vibrations have shown the good crystallinity of our zeolites of the type H-ZSM (Si :Al ratios ranging between 17 and 49)

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and dea1uminated type HY (Si :Al ratios ranging between 2.5 and 18). The dea1uminated catalysts were prepared by the method published by Beyer (ref.1). The subsequent cationic exchange with NH 4C1 was performed with all these zeolites in the usual way. The dealumination of HY just mentioned is interpreted (ref.l) by the reaction of the zeolite with SiC1 4 at elevated temperatures, where Al from the lattice is exchanged for Si under the formation of volatile A1C1 3 as well as of NaCl possibly of Na(A1C1 4 ) · These products are formed in the cavities of the zeolite and - according to (ref.1) - probably mostly removed from them in the course of the washing of the product with water. The substitution mechanism suggested is in good agreement with our experimental results comparing the dependence of the unit cell dimensions and that of the OH groups vibrations on the content of aluminium as (A1/Si+Al) in the series of the dealuminated zeolites. A part of aluminium released from the lattice, however, is probably maintained in the cavities as extra-lattice aluminium or even as cationic A1 3+. The dealuminated zeolites are highly decationated, which is probably the result of the hydrolysis of Cl complexes in the course of the preparation. The further exchange of Na+ for NH~ has a minimum effect. The content of strongly acidic structural groups in the dehydrated and deammonized Y zeolites decreases with the content of Al whereas the amount of nonspecific SiOH hydroxy1s (IR band at 3745 cm- 1) increases. In dealuminated HY zeolites hydroxyl groups corresponding to the IR band near 3600 cm- l were observed: this band indicates obviously the presence of extra-lattice aluminium, as it has been found by many authors in the case of hydrothermally stabilized Y zeolites, where the delocalization of Al from the lattice into the cavities is typical. The increase of the skeletal Si:Al ratio, the presence of extra-lattice aluminium and the decationization strongly influence the acido-basic properties of dealuminated Y zeolites, their stability and catalytic activity. In some cases also other factors may be important: the high enrichment of the surface layers with A1 and the presence of traces of Cl in these layers, as determined by X-ray photoelectron spectroscopy. In this respect, our results complete some conclusions about similar zeolitic systems presented in (ref.2). The products of the interaction of methanol with H-ZSM and highly dealuminated HY zeolites, which are released successively with

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increasing temperature, were investigated by the TPD technique with mass-spectrometric analysis of the desorbates. The TPD measurements were performed in the temperature range 300 - 800 K. The dependence of the relative ratios of saturated, olefinic and aromatic desorbates (products of interaction) on the atomic Si :Al ratios on dealuminated HY zeolites (with the same crystallographic but varying surface structure) will be discussed in deta i 1 . On the whole, a comparable amount of dimethyl ether is formed on all zeolite probes. The highest amount of olefins (namely of propylene) and of aromates appears on HY zeolites with the Si:Al ratio between 3 and 4; it is about twice as high as that observed on HY zeolites with the Si:Al ratio of about 17. No paraffins (including methane) were found after the interaction of these zeolites with methanol. Further attention to differences in the reactivities of both types of zeolites HZSM-5 and HY with the same ratio of Si:Al = 17 and different crystallographic structures is given. On the whole, again a comparable amount of dimethyl ether is formed on both types of zeolites. In contrast to the dealuminated HY zeolite, in the interaction of methanol with the H-ZSM-5 zeolite among the paraffins mainly methane is formed and no olefins were observed. The products of interaction contain comparable amounts of aromates. REFERENCES 1 H.K. Beyer and Imelik et al., 2 P.A. Jacobs, J. 1981, preprint

1. Belenykya, Catalysis by Zeolites, Ed. B. Elsevier, Amsterdam, 1980, 203 pp. . Weitkamp and H.K. Beyer, Faraday Discussion, 72/21.