Chemo- and regioselective Meerwein–Ponndorf–Verley and Oppenauer reactions catalyzed by Al-free Zr-zeolite beta
Introduction
The Meerwein–Ponndorf–Verley (MPV) reduction provides a highly selective reduction of the CO functional group in unsaturated carbonyl compounds using secondary alcohols as hydrogen donors [1]. The reverse Oppenauer oxidation of alcohols is carried out with oxidants such as furfural, benzophenone, and cyclohexanone. Both reactions (collectively denoted as MPVO) can be catalyzed using homogeneous catalysts such as metal alkoxides. The homogenously catalyzed MPVO reaction has many advantages, such as chemoselectivity, mild reaction conditions, and ready adaptation both in the laboratory and on a large scale. However, the need for almost stoichiometric amounts of catalyst, the moisture sensitivity, and problems with separation limit the practical applications. Hence, heterogeneous catalysts have been developed for the reaction. These include metal oxides, such as Al2O3 [2], hydrous ZrO2 [3], [4], magnesium oxide or phosphates [5], [6], and grafted alkoxides or alkyl complexes [7], [8], [9].
Al-zeolite beta has been reported to be a highly active and regioselective catalyst for the reduction of 4-tert-butylcyclohexanone to the thermodynamically less stable cis-4-tert-butylcyclohexanol [10]. The high selectivity toward the cis-alcohol was explained by a restricted transition state around a Lewis-acidic aluminum atom in the straight channels of the zeolite beta pore system. Whilst the presence of water was important for the activation of the catalyst prior to reaction, moisture during the reaction severely decreased the activity of the catalyst [11]. Corma et al. [12] recently reported that Sn-zeolite beta showed excellent activity and selectivity in the MPV reduction of several ketones. In addition, the Sn-zeolite beta was found to be more resistant to the presence of water in the reaction media than Ti- or Al-zeolite beta.
Zirconium is increasingly applied as a catalyst in many reactions due to its moderate acidity and oxidizing capabilities [13]. We have found that zirconium in the form of hydrous zirconia and zirconium 1-propoxide grafted on supports is a good catalyst for the MPV reaction [14], [15]. Besides good activity, both types of catalysts could be easily handled in the ambient environment without a need for moisture-free conditions. In the homogeneous form, zirconium 1-propoxide easily undergoes hydrolysis but after grafting on a support, the catalyst was not deactivated by the presence of water and showed good stability even after exposure to the ambient for 48 h [15]. In contrast, grafted aluminum 2-propoxide required stringent moisture-free conditions for activity and was deactivated when exposed to air. In the present paper, we report on the incorporation of zirconium into zeolite beta, combining the properties of zirconium with the shape selectivity and possibly higher acidity offered by the zeolite. In an earlier communication [16], we have reported on the successful synthesis of Al-free Zr-zeolite beta and showed that Zr-zeolite beta was an excellent catalyst for the MPV reduction of 4-tert-butylcyclohexanone. Here, we present the results of a thorough characterization of Al-free Zr-beta and its application to MPV reduction of a number of ketones and Oppenauer oxidation. The robustness of the catalyst is tested by poisoning experiments, ease of regeneration, and activity after successive cycles.
Section snippets
Preparation of zeolite beta seeds
Nanocrystalline zeolite beta seeds were synthesized following the procedure described in Ref. [17]. A quantity of 0.216 g of metallic Al (Goodfellow) was dissolved in 41.23 g of tetraethylammonium hydroxide (TEAOH) (40 wt% aqueous solution), and 29.26 g of deionized water and 12 g of fumed silica were added and stirred for 2 h. The molar composition of the final gel mixture was 1.0 SiO2:0.56 TEAOH:0.02 Al2O3:15 H2O. The mixture was placed in a Teflon-lined stainless steel autoclave and kept at
Catalyst characterization
The physical properties of the catalysts used are summarized in Table 1. All the samples possess large specific surface area, . The crystal size of the unseeded pure Si-beta was in the range of 10–15 μm, while the seeded samples were much smaller (1–2 μm). It was difficult to determine the crystal size for the Al-zeolite beta (Si/Al 12.5) synthesized according to Wadlinger et al. [20] due to aggregation of particles and the rather small crystal size.
The incorporation of Zr into the
Conclusion
Well-crystallized Al-free Zr-zeolite beta can be obtained in a fluoride medium using nanocrystalline zeolite seeds. Framework substitution of Zr in the zeolite beta structure is possible up to about 1.3% (Si/Zr 75) in a seeded synthesis in fluoride medium. The incorporation of Zr into zeolite beta resulted in an active catalyst for the MPV reduction of several ketones. The high catalytic ability of Zr-beta zeolite for the MPV reaction can be attributed to the presence of Lewis acid sites with
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