Zeolite catalysed synthesis of 5-ethyl-2-methylpyridine under high pressure

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Abstract

Simple and selective single-step synthesis of 5-ethyl-2-methylpyridine (EMP) under high pressure (autogeneous pressure) conditions is achieved with high conversion and high selectivity over zeolites. The catalyst can be reused many times without significant loss of activity.

Graphical abstract

Simple and selective single-step synthesis of 5-ethyl-2-methylpyridine (EMP) under high pressure (autogeneous pressure) conditions is achieved with high conversion and high selectivity over zeolites. The catalyst can be reused many times without significant loss of activity.

Introduction

Pyridine bases are industrially important compounds. Pyridine, 2-picoline and 5-ethyl-2-methylpyridine (EMP) are in strong demand, whereas 3-picoline and 4-picoline have a limited market. All these bases can be produced from acetaldehyde. Industrially, large amounts of EMP are used as starting material for the production of nicotinic acid and its derivatives and 2-methyl-5-vinylpyridine (MVP), these chemical transformations involving the alkyl groups bound to the pyridine ring [1].

The condensation of acetaldehyde or paraldehyde with ammonia to form EMP has been described by a number of investigators [2], [3], [4] but low yields have been obtained due to the formation of pyridine derivatives having high boiling points and large amounts of tar. The tarry by-products are most likely the result of polymerization of acetaldehyde or its reaction with the substituted pyridines formed.

Later it is found that the excess of ammonia might increase the ratio of EMP to pyridine derivatives having high boiling points and tar. Various methods have been developed and reported for the synthesis of EMP such as paraldehyde and ammonia in the presence of ammonium salts as catalysts [5], [6], [7] and the continuous process using acetaldehyde and ammonia in the presence of ammonium acetate and ammonium salts of other organic acids, sodium acetate, sodium carbonate, sodium fluoride and other metal halides as catalysts [8], [9], [10]. Frank and Seven have widely investigated this reaction and were able to obtain high yields of single products in certain instances by the use of an excess of aqueous ammonia. However, these investigators were able to obtain an improved yield of EMP by their method only when a derivative of acetaldehyde such as acetal or paraldehyde was employed and their yield under the same conditions using acetaldehyde was much lower (34%) [11]. All the processes for the production of EMP are carried out in the liquid phase under pressure with paraldehyde and ammonia in the presence of an ammonium salt as catalyst. This synthesis is essentially based on the studies carried out by Farbwerke Hochst [12] and Frank et al. [4]. Although the reaction has been widely studied [13], [14], the yields were always low for instances if acetaldehyde is fed instead of paraldehyde [8], [9], [10] and if the reaction takes place in liquid ammonia [15] or in benzene [16]. The variations observed in the different plants are presumably due to the nature of the ammonium salt employed and to the technical solutions used in the different phases of the process. While in the patent literature the use of various salts as catalysts is reported, ammonium acetate and some fluorides particularly ammonium fluoride, difluorides are the most interesting catalysts [17], [18]. These latter two show a catalytic activity even higher than that of ammonium acetate. However, in actual operation, they can cause corrosion.

Shimizu et al. [19] reported the aminocyclization of acetaldehyde and ammonia over modified zeolite catalysts. The main products were pyridine and picolines. The yield of pyridine was 68% over silylated ZSM-5 and 60% over Pb-ZSM-5. Kulkarni and co-workers [20], [21], [22] reported 61.5% yield of pyridine and 16.8% 3-picoline over W-ZSM-5 in the reaction of acetaldehyde, formaldehyde and ammonia under atmospheric pressure. In this paper, we report the formation of EMP using acetaldehyde and ammonia as reactants and zeolite as a catalyst under high pressure (autogeneous pressure) conditions.

However, most of the above methods are suffering from one or more of the following disadvantages such as long reaction times, vigorous reaction conditions, occurrence of side reactions, homogeneous catalysts, separation of catalyst and selectivity of product. Nevertheless, there is still a great demand for solid acid catalysts and selective synthesis of EMP under mild conditions.

The use of heterogeneous catalysts in various areas of the organic synthesis has now reached significant levels not only for the possibility to perform environmentally benign synthesis but also for the good yields frequently, accompanied by excellent selectivities that can be achieved. Zeolites are uniform microporous crystalline materials [23], [24] and have been investigated extensively and used as solid catalysts in the field of petrochemistry. Zeolites are also known to catalyze various synthetic organic transformations much more effectively and selectively than Lewis acid catalysts. In view of this, we have developed a process for the synthesis of EMP with high selectivity and high conversion of acetaldehyde using zeolite catalysts under high pressure conditions.

Section snippets

Experimental

All the reactions are carried out in a Parr autoclave (600 mL capacity, 20 cm height and 6.3 cm inner diameter) in the temperature range of 150–225 °C under constant stirring (90–100 rpm) at the respective autogeneous pressure. The reactants acetaldehyde (0.1 mol), 25% aqueous ammonia (0.5 mol), 2 g of catalyst and 200 mL of solvent (methanol) are loaded into an autoclave. At room temperature the autoclave is sealed and then the temperature is increased (150–225 °C) under stirring. Stirring is maintained

Results and discussion

We have synthesized 5-ethyl-2-methylpyridine using acetaldehyde and ammonia (25% aqueous) using zeolites as catalysts under autogeneous pressure at 150–225 °C (Scheme 1).

We have investigated the use of various zeolites, K10-montmorillonite, SiO2 and SiO2-Al2O3 as catalysts for the cyclization of acetaldehyde and aqueous ammonia (Table 1). H-beta, HY, HZSM-5(40) and K10-montmorillonite show better results compared to other catalysts (S. No. 1, 5, 6 and 13). By considering the high conversion of

Conclusion

The present study demonstrates the development of the direct conversion of acetaldehyde and ammonia into 5-ethyl-2-methylpyridine under high pressure. Moreover it demonstrates the novelty of zeolite catalysts exercising a unique activity in the synthesis of EMP. The catalyst can be reused without any loss of activity. Inexpensive and environmentally friendly catalysts and commercially available reactants make our method valuable from a preparative point of view.

Acknowledgements

Dr. K.V.V. Krishna Mohan is thankful to Dr. B.M. Reddy for financial assistance in the form of fellowship and K.S.K. Reddy is thankful to CSIR, New Delhi for the award of SRF-GATE fellowship.

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    Present address: Instituto de Tecnologia Quimica e Biologica da Universidade Nova de Lisboa, Apt. 127, Quinta do Marques, EAN, 2781-901 Oeiras, Portugal.

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