No abstract

The kinetics of formation of tetrahydrofuran from dibutyl succinate were studied. The mechanism of catalytic hydrogenation of dialkyl succinates was found to involve consecutive formation of γ-butyrolactone, butane-1,4-diol, and tetrahydrofuran. Parameters of kinetic equations that properly describe the system of concurrent and consecutive reactions were determined.Tetrahydrofuran is widely used as solvent in organic synthesis, membrane technologies [1], processing of natural and synthetic resins, and liquefaction of bituminous coal [2]. Polymerization of tetrahydrofuran or its copolymerization with alkylene oxides and diols gives oligomeric polyalkylene ether glycols which are then converted into polyurethanes and thermoplastic polyesters [3][4][5].Several procedures for the preparation of tetrahydrofuran have been reported: from furfural through furan [6], from maleic anhydride obtained from benzene or by oxidation of butane [7], from butadiene via oxidation [8] or acetoxylation [9], and from butane-1,4-diol via dehydration [10]. In the past decade, the synthesis of tetrahydrofuran by hydrogenation of maleic acid esters has attracted increased attention in many countries [11][12][13][14][15]. However, the mechanism of this process was not studied. Kanetaka et al. [16] and Loktev et al. [7] described the mechanism of catalytic hydrogenation of maleic anhydride, which involved intermediate formation of γ-butyrolactone. It may be presumed that tetrahydrofuran is formed in a similar way in the hydrogenation of maleic acid esters (Scheme 1), though the reactivity of a linear dialkyl maleate could differ considerably from the reactivity of cyclic maleic anhydride.The first step is hydrogenation of the double bond activated due to conjugation with electron-withdrawing ester carbonyl groups. This step is fast, and it does not affect the kinetics of the overall process [16]. Therefore, the goal of the present work was to examine in detail the kinetics of the formation of tetrahydrofuran from dibutyl succinate (I) and determine the mechanism of this process. Dibutyl succinate was selected as substrate, taking into account that butyl alcohol liberated during the hydrogenation process can readily be separated from tetrahydrofuran and dehydrated for further recycling.Catalytic hydrogenation of dibutyl succinate (I) was carried out in a rocking high-pressure reactor in the temperature range from 180 to 260°C under a hydrogen pressure of 8-14 MPa over nickel-cobalt catalyst which showed a high efficiency in analogous processes [18]. We found that the reaction rate does not depend on the grain size of the catalyst in the range from 0.4 to 2 mm and on the rate of stirring within 180-480 rpm, i.e., the reaction occurs in the kinetic region under these conditions.Initially we examined the kinetics of hydrogenation of γ-butyrolactone (II). Figure 1 shows that lactone II is rapidly consumed in the initial period of time and that the reaction slows down when 50% conversion is

No abstract

SHORT COMMUNICATIONSN-Benzyl-tert-butylamine is used as starting compound in the synthesis of drugs and catalytic systems for polymerization of olefins, blocking agent for biuret groups in polyisocyanate compositions, and adsorbent of carbon dioxide from gas systems [1]. N-alkylbenzylamines I can be synthesized in two steps (Scheme 1) including condensation of benzaldehyde (II) with alkylamines III in lower aliphatic alcohols (C 1 -C 4 ) without separation of water and subsequent hydrogenation of Schiff base IV in the presence of transition metal catalysts [2]. containing catalysts was accompanied by side hydrolytic processes, leading to the formation of up to 8% of benzyl alcohol (V) and up to 9% of toluene (VI) (Scheme 2).Apart from inefficient expenditure of the initial compounds, these side processes impair the purity of the target product (compound Ia). Though toluene (VI) can readily be removed by fractional distillation, benzyl alcohol (V) and N-benzyl-tert-butylamine (Ia) have similar boiling points, 99.5°C [3] and 100°C [4] at 17 mm, respectively. Owing to the presence of benzyl alcohol among the products, the concentration of the main substance in the target amine cannot exceed 90% without using special expensive separation methods. Benzyl alcohol is formed with participation of water liberated in the condensation leading to N-benzylidene-tert-butylamine (IVa), whereas dehydration of an alcoholic solution containing compound IVa is a laborious and energy-consuming process.We have found a simple and effective solution of the above problem. It implies synthesis of Schiff base IVa under solvent-free conditions with separation of most liberated water (no less than 90% of the theoretical amount) immediately after the condensation of benzaldehyde with tert-butylamine. The hydrogenation of Schiff base IVa is also carried out in the absence of Scheme 1.