Ring opening, dimerisation and oligomerisation reactions of methyloxirane on solid acid and base catalysts

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Abstract

The reactions of methyloxirane were studied in a pulse microreactor over Nafion-H, K10 montmorillonite, Mg–Al L(ayered) D(ouble) H(ydroxide) and NaN3-loaded SiMCM-41, to cover a wide acid–base range. The first two solid substances that are acidic, produced appreciable conversion at 363 K, while the latter two that are basic, were hardly active at this temperature. At higher temperature (393 K), however, they displayed fair activity. The nearly superacidic Nafion-H was the most active and transformed the reactant in various ways: single ring opening producing exclusively propionaldehyde, cyclic dimerisation giving five- and six-membered ring compounds, di- and trimerisation producing linear oligomers all occurred. Over the less acidic K10 montmorillonite neither cyclic nor linear oligomer was observed. The NaN3-loaded SiMCM-41 preferred cyclic dimerisation to single ring opening, while the behaviour of the Mg–Al LDH was the opposite.

Methyloxirane undergoes a variety of reactions over catalysts of widely differing acid–base characteristics. Single ring opening, cyclic dimerisation and linear oligomerisation all occur on the nearly superacidic Nafion-H, while single ring opening is predominant over the basic Mg–Al hydrotalcite. The behaviour of K10 montmorillonite and NaN3-loaded SiMCM-41 are in-between of the two extremes.

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Introduction

Epoxides are extremely useful synthons in the synthesis of various complex organic molecules and macromolecules [1], [2]. There versatility lies in the fact that the ring may be opened by acids [3] or bases [4] in catalytic reactions as well as by electrophiles [5] or nucleophiles [6] in stoichiometric reactions. The epoxide ring can be opened on solids and for this purpose various oxides [7], zeolites [8], [9], [10], [11], [12] and even supported transition [13] or early transition [14] metals may be used. Over the solid materials ring opening may occur by the scission of either CO bond. This single ring opening may give open-chain isomers, cyclic dimers and in principle linear oligomers too. Occasionally, double CO scission, oxygen loss that is, also takes place. The actual product distribution depends very much on the nature of the solid substances and experimental conditions, particularly on the reaction temperature. Generally, higher temperature favors deoxygenation, and the sterically less hindered CO bond tends to be ruptured on solids of higher acidity. On porous materials dimerisation is the preferred transformation pathway [8], [9], [10], [11], [12] and for this reaction there exists an optimal pore size. In this contribution experimental findings concerning the transformations of methyloxirane over a series of solid substances with wide-ranging acid–base character are communicated.

Section snippets

Materials

The reactions of methyloxirane were studied in a pulse reactor over Nafion-H, K10 montmorillonite, Mg–Al L(ayered) D(ouble) H(ydroxide) or NaN3-loaded SiMCM-41. The first two substances are acidic, while the last two materials are of basic character.

Nafion is the registered trademark of the DuPont company and commercially available. The active H-form was generated from the potassium salt (Nafion 501) by treatment with 25% nitric acid then washing it with water and drying at 378 K for 24 h.

Results

The activities of transforming methyloxirane varied from catalyst to catalyst. Generally, the acidic materials were more active than the basic substances and in particular the nearly superacidic Nafion-H was more active than K10 montmorillonite. Reaction pathways as well as their proportion were also different (for a general picture of the identified products, please see Scheme 1).

In spite of its low surface area, Nafion-H proved to be the most active catalyst. It did produce almost all the

Discussion

It is known from any fundamental organic chemistry course that a substituted epoxide, like, e.g. propylene oxide (methyloxirane), gives a mixture of products even on simple ring opening (single CO scission and no further reaction of the ring-opened products), when the reaction is started with the attack of an acid but a single product when the attacking agent is a base [22]. The former is a mixture of SN1 and SN2 reactions, while the latter is a typical SN2 transformation. The SN1 reaction

Conclusions

Studying the transformations of methyloxirane on structurally diverse catalysts of different acid–base character, it was found that the isomerisation, the dimerisation and the linear oligomerisation pathway of ring opening can occur simultaneously only on porous catalysts with strong acid sites. Having porous system is a necessity for dimerisation to occur, appreciable linear oligomerisation requires strong acid sites, while the isomerisation type of ring opening requires sites of moderate

Acknowledgements

Research leading to this contribution was financed through a grant from the national Science Fund of Hungary (T034184). The support is highly appreciated.

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