On the formation of the acid sites in lanthanum exchanged X zeolites used for isobutane/cis-2-butene alkylation

doi:10.1016/j.micromeso.2005.04.024

Microporous and Mesoporous Materials

Volume 83, Issues 1–3, 1 September 2005, Pages 309-318

https://doi.org/10.1016/j.micromeso.2005.04.024 Get rights and content

Abstract

The acid sites generated at different steps during the preparation of La-H-X zeolites were characterized by physicochemical methods. The resulting materials were tested in isobutane/cis-2-butene alkylation in a continuously operated stirred tank reactor, under industrially relevant conditions.

The concentration and strength of acid sites depend subtly on the ion exchange procedure. Especially, the rehydration of materials calcined for the first time after ion exchange changes the distribution of hydroxyl groups, the Brønsted acidic bridging hydroxyl groups (3640 cm⁻¹) being strongly affected. Rehydration leads to dealumination and, as consequence, the concentration of silanol groups (3740 cm⁻¹) and of Lewis acid sites increases. This in turn results in an enhanced stability towards the subsequent thermal treatments and rehydration processes of the rare earth zeolite in the next steps of catalyst preparation. The strength of the Brønsted acid sites was shown to be a function of the hydrolysis of hydrated lanthanum cations and removal of sodium cations.

The catalytic activity in isobutane/cis-2-butene alkylation and the fraction of strong Brønsted to total Brønsted acid sites are directly related. Catalysts with similar concentration of strong Brønsted acid sites and higher concentration of weak Brønsted sites showed shorter lifetime.

Introduction

Isobutane/butene alkylation is an important refining process in which butenes and isobutane are converted into a complex mixture of branched alkanes (alkylate), which is an excellent blending component in the gasoline pool [1]. The catalysts used in commercial processes are sulfuric and hydrofluoric acids [2]. The development of new alkylation technologies based on more environmentally friendly solid catalysts has seen high interest over the past decades. In this context, highly Brønsted acidic zeolites and especially large pore zeolites are a potential alternative able to overcome the problems related to liquid acids. However, the industrial application of zeolites is constrained by their rapid deactivation.

The overall cycle in the alkylation reaction comprises the addition of linear butene (1- or 2-butene) to a tert-butyl carbenium ion to form a secondary octyl carbenium ion, which can undergo isomerization to a tert-octyl carbenium ion. Finally, the octyl carbenium ion is removed from the acid site by hydride transfer from isobutane leading to a tert-butyl carbenium ion [3] (Scheme 1). Competition between hydride transfer and addition of butene to a carbenium ion determines the lifetime of the catalyst. A high ratio of hydride transfer vs. olefin addition leads to enhanced trimethylpentane formation and reduction of catalyst deactivation [4].

Among the large pore zeolites, rare earth exchanged faujasites have been shown to have a good ability to catalyze hydride transfer in alkylation reaction. This is related to two factors, i.e. (i) the high concentration of aluminum in the framework leads to an optimum strength of the bond between the zeolite oxygen and the secondary or tertiary carbon atom of the alkoxy groups being the ground state for the carbenium ions in the transition state of olefin addition or hydride transfer and (ii) the resulting high concentration of strong Brønsted acid sites allows to generate a (relatively) high concentration of alkoxy groups/carbenium ions increasing the probability of hydride transfer over olefin addition for the individual alkoxy group. Both factors help to compensate the higher energy barrier required for the hydride transfer in solid catalysts compared to liquid acids [5], [6], [7].

The nature of the acid sites in the rare earth exchanged FAU zeolites has been extensively studied. It has been proposed that the rare earth ions exchanged in X or Y zeolites are hydrolyzed upon calcination. The resulting protons generate Brønsted acid sites [8], [9], [10]: [La(H₂O)_n]³⁺ = [La(OH)(H₂O)_n − 1]²⁺ + H⁺.

Direct evidence for hydrolysis is deduced from IR spectra showing O–H stretching bands that are attributed to OH groups attached to the exchanged metal cation and to the aluminosilicate framework [11]. Similarly, inelastic neutron diffraction has provided direct evidence for metal cation hydrolysis [12].

The effect of the calcination temperature on the migration of lanthanum ions from the supercages (the initial location of La³⁺ after ion exchange) to the sodalite cages has been likewise investigated [13]. It has been reported that at temperatures higher than 333 K the hydrated La³⁺ cations begin hydrolyzing water and migrating to sodalite cages. However, during calcination, the hydrolysis of the rare earth cations produces not only Brønsted acid sites, but also Lewis acid sites [8]. Potential processes involving the framework during calcination have not been well characterized. Especially, it is unclear under which conditions water may lead to the partial or full hydrolysis of framework aluminum atoms and what role the rare earth metal cations play in this process.

Here, primarily IR spectroscopy is used to provide information about the formation of the acid sites in lanthanum exchanged X zeolites and to investigate the influence of water on the OH groups of the calcined materials. A series of lanthanum exchanged X zeolites was examined to establish, how the acidic properties (concentration and strength) of the modified materials vary with the lanthanum exchange degree. These findings have been correlated with the catalytic activity in alkylation of isobutane with cis-2-butene.

Section snippets

Material preparation

The samples in this study were prepared from a Na-X zeolite obtained from Chemische Werke Bad Köstritz (Si/Al = 1.2). The Na-X material was ion exchanged with 0.2 M lanthanum nitrate solution with pH = 3 (from La(NO₃)₃ · 6H₂O, Fluka, puriss. p.a., ⩾99.0%), using a liquid-to-solid ratio between 5 and 10 mL/g. The ion exchange was carried out at 353 K for 2 h. This step was performed one or two times. After washing and drying, the resulting materials were calcined in 100 mL/min flowing air for 1 h at 723 K

Hydroxyl group formation on La³⁺ exchanged Na-X zeolites

The acidity of La-X zeolites has been attributed to the protons that are generated, when water of the hydrated lanthanum cations is hydrolyzed at temperatures between 333 and 573 K [8], [9], [10]. The transformations during the activation procedure of a Na-X zeolite exchanged two times with lanthanum cations were monitored by mass spectrometry (MS) analysis of the outlet of the calcination oven and by in situ IR spectroscopy. Fig. 1 shows the MS signal for m/e 18 (water) recorded during

Hydroxyl groups formation on La³⁺ exchanged Na-X zeolites

The presence of bridging hydroxyl groups (IR bands at 3600 and 3640 cm⁻¹) and of lanthanum hydroxyl groups (IR band at 3520 cm⁻¹) on lanthanum exchanged Na-X zeolites after calcination at 393 K indicates that the hydrolysis of hydrated lanthanum cations starts already at such low temperatures. Upon further increase of the calcination temperature, physisorbed water is removed (decrease of the IR band at 3570 cm⁻¹) and the concentration of free hydroxyl groups, bridging and associated with lanthanum,

Conclusions

Hydrolysis of the hydrated lanthanum cations starts below 393 K as evidenced by the bands corresponding of La-OH (3520 cm⁻¹) and bridging hydroxyl groups (3600 and 3640 cm⁻¹). As the calcination temperature is further increased, dehydroxylation of the zeolite and hydrolysis of the hydrated lanthanum cations take place. While the La-OH (3520 cm⁻¹) and one type of bridging hydroxyl groups (3600 cm⁻¹) decrease in concentration, the other bridging hydroxyl groups (3640 cm⁻¹) increase.

Local changes of the

Acknowledgements

Financial support from Süd-Chemie AG is gratefully acknowledged.

The authors wish to thank Prof. Dr. Michael Hunger of University of Stuttgart for the NMR measurements of some of our samples and for his discussions on structural properties of zeolitic materials.

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¹: Present address: Universidad de Pamplona, Km 1 vía a Bucaramanga, Pamplona-Colombia.

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On the formation of the acid sites in lanthanum exchanged X zeolites used for isobutane/cis-2-butene alkylation

Abstract

Introduction

Section snippets

Material preparation

Hydroxyl group formation on La3+ exchanged Na-X zeolites

Hydroxyl groups formation on La3+ exchanged Na-X zeolites

Conclusions

Acknowledgements

Catal. Today

J. Catal.

J. Catal.

J. Catal.

J. Catal.

Chem. Phys. Lett.

J. Catal.

Zeolites

J. Catal.

Solid State Nucl. Magn. Reson.

J. Catal.

Hydroxyl group formation on La³⁺ exchanged Na-X zeolites

Hydroxyl groups formation on La³⁺ exchanged Na-X zeolites