Selective isomerization of n-butenes to isobutene on high Si/Al ratio ferrierite in the absence of coke deposits: implications on the reaction mechanism

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Abstract

High Si/Al ratio ferrierite (Si/Al=59) synthesized using pyridine as structure-directing agent was found to show a high selectivity from very short times on stream (5 min TOS) during the skeletal isomerization of n-butenes. Such a high initial selectivity is accompanied by an unusually high stability with TOS even under reaction conditions that favor the oligomerization of the olefins (low temperatures and high n-butenes partial pressure). In contrast to what has been previously established for ferrierite with lower Si/Al ratios, the isomerization reaction occurs with a high selectivity without appreciable formation of carbonaceous deposits, as determined by elemental and thermogravimetric analysis of the zeolite after reaction. Thus, at 350°C and 0.1 atm partial pressure only about 0.3 wt% carbon was formed after 3.5 h on stream on the high-silica ferrierite (FER59 sample), whereas it amounted to ca. 5 wt% for a commercial low Si/Al ratio ferrierite (Si/Al=8.5, FERTOS sample) under the same reaction conditions. As a result, the acidity and porosity of the FER59 ferrierite remained unaltered after 3.5 h on stream under the above conditions, while both parameters significantly decreased for the FERTOS catalyst. These results, together with the analysis of the initial selectivity with temperature and partial pressure, indicate that isobutene is selectively formed in the ferrierite structure through a monomolecular reaction mechanism without the need of invoking the presence of coke deposits. It appears that reducing the density of acid sites in ferrierite by increasing the framework Si/Al ratio to the required levels is the key parameter to control the stability and selectivity of the zeolite during n-butenes isomerization.

Introduction

Skeletal isomerization of n-butenes has been considered as an attractive alternative for increasing the production of the refinery-deficient isobutene, a raw material used in various important chemical processes. Besides its known use in the production of methyl-tert-butyl ether (MTBE), a recently questioned additive for reformulated gasolines due to health concerns [1], isobutene is also used in the production of polymers and several chemicals, such as methacrolein, alkylated phenols and cresols, etc. [2]. In addition to its practical relevance, there has also been a growing interest in skeletal isomerization of n-butenes from a more fundamental point of view 3, 4.

Shape-selective medium pore zeolites have shown relatively high isobutene selectivities owing to a limited oligomerization activity inside the narrow zeolite pores [3]. Among them, ferrierite has shown unique catalytic properties giving high selectivity to isobutene under mild reaction conditions [5]. The advantages displayed by ferrierite with respect to other 10MR zeolites have been ascribed to its particular topology, rather than to differences in the zeolite acidity 6, 7, 8, 9. In spite of this, it is known that ferrierite does not show a high selectivity at the beginning of the reaction (fresh catalyst) producing, besides isobutene, large amounts of by-products. In fact, it has been repeatedly reported that a partial deactivation by carbonaceous deposits is required to achieve a selective and relatively stable behavior on ferrierite catalysts 10, 11, 12. There are, however, different opinions regarding the mechanism by which coke deposits might promote the selective formation of isobutene. Poisoning of non-selective acid sites (those of higher strength or located on the external surface), and a reduction of the free space around the acid sites limiting the competitive dimerization reaction have been proposed to explain the effect of coke 13, 14, 15, 16. Moreover, Guisnet et al. 10, 17, 18proposed that the change from a non-selective to a selective behavior is due to the development of a new isomerization mechanism (pseudo-monomolecular) involving, as active sites, aromatic carbocations formed from coke molecules located in the zeolite pores. More recently, these authors have proposed an autocatalytic reaction involving the alkylation of n-butene with a tert-butyl cation (formed from isobutene diffusing slowly from the zeolite pores) and cracking of the resulting trimethylpentyl cation in order to explain the increase of isobutene yield observed in ferrierite at short times on stream [19]. This has originated an interesting debate about the nature of the active sites responsible for the skeletal isomerization and the prevailing mechanism of isobutene formation 20, 21. Further studies are, then, necessary to answer the above and other important questions that might be relevant for the design of new active, selective, and stable isomerization catalysts.

On the other hand, when an intrinsically less selective zeolite structure is used as isomerization catalyst, it has been shown that it is possible to improve the isobutene selectivity by modifying the acidic properties of the given zeolite. Thus, lowering the acid strength of the bridging OH groups by replacing framework Al3+ for another trivalent element, such as B, Ga, and Fe, significantly increases the isobutene selectivity as it has been reported in the case of ZSM-5 22, 23, ZSM-11 [24], ZSM-22 [25], and ZSM-23 [26]zeolites. Moreover, reducing the density of acid sites, for instance by increasing the Si/Al ratio, also results in a higher selectivity to isobutene 25, 27, 28. This has been explained by a lower extent of undesired consecutive reactions, such as olefin dimerization and cracking, that lead to by-product formation. When the Si/Al ratio of ferrierite has been increased, either by hydrothermal synthesis 9, 13, 29or post-synthesis treatments 12, 30, better initial selectivities and lower deactivation rates have been reported. However, in all these cases it was concluded that formation of coke deposits is a necessary requisite to achieve the high selective and stable behavior characteristic of ferrierite.

In this work we report the results obtained with a high Si/Al ratio ferrierite (Si/Al=59) synthesized using pyridine as structure-directing agent for the skeletal isomerization of n-butenes under a wide range of reaction conditions. The results are compared with those obtained with a commercial ferrierite sample with lower Si/Al ratio (Si/Al=8.5, from TOSOH). It will be shown that, in contrast to what has been previously reported, the high Si/Al ratio ferrierite displays an exceptional selectivity from the beginning of the reaction and that the skeletal isomerization proceeds with a high selectivity and stability without appreciable formation of coke deposits. The implications of these results on the nature of active sites and the reaction mechanism will be also discussed.

Section snippets

Preparation and characterization of catalysts

High Si/Al ratio Ferrierite (FER59) was synthesized following a slightly modified procedure reported in the patent literature and using pyridine as template [31]. For the preparation of the gel a solution of NaOH (99%, Scharlau) and SiO2 (Aerosil 200, Degussa) in water was prepared and agitated, to which a second solution of Na2SO4 (99%, Merck) and Al2SO4 18H2O (Merck) in water was added. The gel formed was agitated for 10 min and then a third solution of pyridine (99.5%, SDS) in water was

Characterization of catalysts

The high Si/Al ratio ferrierite synthesized in this work (FER59) presented the XRD pattern characteristic of the FER structure with no other phases detected. The SEM pictures showed for both the FERTOS and FER59 samples the presence of platelet shaped crystals with an irregular morphology, being the size of most of the crystals between 0.5–1 μm for FERTOS and between 1 and 3 μm for FER59 (not shown).

The 27Al MAS NMR spectra of the two calcined ferrierite samples showed the presence of one peak at

Conclusions

High Si/Al ratio ferrierite (FER59, Si/Al=59) synthesized using pyridine as structure-directing agent shows a very high selectivity to isobutene and stability with time on stream (TOS) during the skeletal isomerization of n-butenes. This behavior is observed from very short TOS (5 min) even under reaction conditions which are more favorable for olefin dimerization (low reaction temperatures and high partial pressures). This is ascribed to the low density of acid sites in FER59, which prevents

Acknowledgements

Financial support by the Comisión Interministerial de Ciencia y Tecnologı́a (CICYT) of Spain is gratefully acknowledged (project MAT97-1010). The authors are grateful to Dr. J. Cejka and Prof. V. Fornés for their contribution in the synthesis and characterization of the zeolites. M.A. Asensi thanks the Ministerio de Educación y Ciencia of Spain for a personal grant.

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