Superbase catalysts from thermally decomposed sodium azide supported on mesoporous γ-alumina

doi:10.1016/j.cattod.2010.01.002

Catalysis Today

Volume 152, Issues 1–4, 1 July 2010, Pages 99-103

https://doi.org/10.1016/j.cattod.2010.01.002 Get rights and content

Abstract

Mesoporous γ-alumina because of its homogeneous pore size distribution, represents a good support for alkali metals. Controlled thermal decomposition of impregnated sodium azide on such support yields a superbasic catalyst for the double bond migration of vinylcyclohexane to ethylidene cyclohexane in continuous liquid flow operation. After slurry impregnation of the azide in methanol, ²³Na MAS NMR shows the presence of resonance lines corresponding to Na metal particles and sodium oxide on the support. When dry mixing of the catalyst components is done, only supported sodium oxide is found, in association with decreased catalytic activity. It is concluded that both species are necessary components of the superbasic sites required for the isomerization reaction mentioned. In the transesterification of soybean oil with methanol in a batch reactor, the same differences in activity are encountered. The ²³Na MAS NMR spectrum of the former catalyst remained unchanged after the transesterification reaction.

Introduction

The first successful preparation of mesoporous silica, viz. MCM-41 [1], [2], was at the basis of a whole pleiade of catalyst supports with high surface area, uniform pore-size distribution in the mesoporous region and high thermostability. Given the potential of traditional γ-alumina as catalyst support in the chemical and petrochemical industries [3], it is not unusual to encounter recent efforts studying the preparation of mesoporous γ-alumina. Materials with specific surface areas up to 800 m²/g and pore sizes ranging from 2.0 to more than 10 nm have been reported to be characteristic for organized mesoporous aluminas prepared by neutral, anionic and cationic synthesis routes [4].

Although solid base catalysts in general have been received much attention especially due to their advantages for easy separation and recovery, reduced corrosion and environmental acceptance [5], there is still a need to prepare solid superbasic catalysts. Such materials are able to catalyze specific reactions such as double bond shift in olefins such as vinylcyclohexane (VCH), vinylnorbornene and 2,3-dimethyl-1-butene, alkene dimerization and side chain alkylation of alkylaromatics. In particular, strong basic sites with H_ > 34 are needed to catalyze such reactions [6]. The reactions are therefore often used to probe the presence of superbasic sites, as they allow in situ inspection of the working catalysts, which is often very difficult using physico-chemical techniques.

The double bond isomerization of VCH is considered to be initiated by abstraction of an allylic H by a strongly basic site of the catalyst [7], [8], [9], [10], [11]. The allylic H of VCH is attached to a tertiary carbon and is more difficult to abstract in the form of H⁺ than the H atoms attached to a secondary or primary carbon atom. Therefore, it is suggested that superbasic sites are required for VCH double bond isomerization (Scheme 1).

There have been attempts to prepare superbasic catalysts by addition of both alkali hydroxides and alkali metals to alumina supports [12]. Such catalysts show high activity and selectivity in the reactions mentioned above to probe superbasicity. Controlled decomposition of sodium azide impregnated in zeolite NaY has been successfully used first to poison the acid sites and then to create strong basic sites in zeolites [6], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Martens et al. [16], [17], [18] have investigated the generation of both metallic particles and ionic alkali clusters in faujasite-type zeolites during controlled thermal decomposition of impregnated sodium azide. The ionic alkali metal clusters were accommodated in the sodalite cages of the faujasite structure, while the neutral metal particles were located in the zeolite large cages. The latter were found to be associated with the development of superbasic sites, able to activate some of the specific substrates mentioned.

Recently, potassium functionalized mesoporous γ-alumina containing impregnated K₂O as active compound, was claimed to act as a solid superbase [24]. Unfortunately, the low activity reported for the isomerization of 1-hexene, does not allow establishing unequivocally the presence of superbasic sites. A solid mixture of dry mesoporous alumina and sodium azide calcined at 400 °C has been shown to have moderate activity in the double bond shift in 2,3-dimethyl-1-butene [25]. Unfortunately no characterization of the sodium metal was provided.

In this paper we report the preparation and catalytic use of thermally decomposed sodium azide-on-mesoporous γ-alumina as a superbasic catalyst in the room temperature double bond migration of VCH, a reaction requiring superbasic catalytic sites to occur [6]. The potential of such catalyst in the transesterification of soybean oil with methanol, a reaction also known as biodiesel synthesis (Scheme 2), was examined as well. It is not implied that this reaction requires the presence of superbasic sites.

Section snippets

Synthesis of mesoporous γ-alumina

Mesoporous γ-alumina, denoted as MSU-γ, was prepared according to the method of Zhang et al. [26]. An aqueous mixture of aluminum tributoxide, 2-butanol and the tri-block (EO)₁₉(PO)₃₉(EO)₁₉ surfactant, Pluronic P84, was first aged at 65 °C for 8 h, followed by hydrolysis with excess water. The precipitate was aged again at 80 °C for 6 h and then at 100 °C for 24 h. The powder air-dried at 60 °C was then calcined at 350 °C for 3 h and then at 550 °C for 4 h. The powder obtained is denoted as MSU-γ 84.

Characterization of support and catalysts

The nitrogen adsorption isotherm measured at 77.3 K for the calcined mesoporous γ-alumina support, MSU-γ 84, is shown in Fig. 2. The data point to successful synthesis of the alumina mesoporous support, with a BET surface area of 298 m²/g, a pore volume of 0.97 mL/g and an average pore size of 8.8 nm.

The sodium species present in the catalysts after thermal activation were determined with solid state ²³Na NMR. Several sodium species can be present in such catalysts, namely metallic sodium

Conclusion

A sodium azide methanol slurry supported on mesoporous alumina was decomposed in a controlled way, yielding NMR resonance lines that could be assigned to Na metal and Na oxide species. This solid showed superbasicity as evidenced by its capability to catalyze the double bond shift in vinylcyclohexane. The minor deactivation of this catalyst in time using a continuous liquid flow reactor could be attributed to feed impurities. The NMR lines are assumed to correspond to sodium metal particles

Acknowledgements

Part of the work was done in the frame of a STWW project sponsored by IWT. R.M. Bota acknowledges a fellowship from KU Leuven R&D and subsequently (from 2008 onwards) in the frame of a GOA concerted action on catalysis, sponsored by the Flemish Government.

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Superbase catalysts from thermally decomposed sodium azide supported on mesoporous γ-alumina

Abstract

Introduction

Section snippets

Synthesis of mesoporous γ-alumina

Characterization of support and catalysts

Conclusion

Acknowledgements

Appl. Catal. A

Mater. Chem. Phys.

Stud. Surf. Sci. Catal.

J. Catal.

Stud. Surf. Sci. Catal.

Zeolites

Stud. Surf. Sci. Catal.

Stud. Surf. Sci. Catal.

Stud. Surf. Sci. Catal.

Stud. Surf. Sci. Catal.

Micropor. Mesopor. Mater.

Fuel

Nature

J. Am. Chem. Soc.