Kinetics of the Birch reduction

Abstract

Because of contradictions in the literature, we reinvestigated the kinetics of the Birch reduction, i.e. the hydrogenation of benzene and its derivatives in metal ammonia solutions (MAS: containing solvated electrons e− and metal cations M+) with alcohols to yield the corresponding cyclohexa‐1,4‐dien compounds (e.g. 2 Li+2CH3OH+C6H6⟹2CH3OLi+C6H8). The kinetics of this reaction are obscured since the hydrogen reaction proceeds parallel to it (2Li+2CH3OH⟹2CH3OLi+H2). The two reactions differ in their activation energies (6.5 and 22.5 kJ/Mol resp.); and in the series of the alkali metals Li, Na and K the rate of the Birch reduction decreases, whereas that of the hydrogen reaction increases. However, in the metal concentration range around 0.01 M. both reactions have within the experimental error the same reaction order with respect to the metal (≈︁0.8). Both are accelerated by addition of alkali cations common to the dissolved alkali metal, and both are decelerated by addition of alkali cation complexing cryptands. Thus we conclude that the cations are involved in the kinetics of both reactions, probably by forming intermediate ion pairs or shifting pre‐equilibria in which solvated electrons are involved. The experimental data of both reactions can be described very well with the rate laws v(B) = kBf2[e−][Li+)[CH3OH][C6H6]and v(H)=kHf2[e−][Li+](CH3OH]resp. (f activity coefficients after Debye‐Hückel) inserting the concentrations of e− and Li+ as calculated from the known thermodynamics of LiAS. The experimental rate constants kB and kH are the products of the rate constants of the rate determining steps and the equilibrium constants of the pre‐equilibria.

Hydrogen storage on graphene using Benkeser reaction

“…Pekker et al [17] and Tang et al [5] have also reported that the decomposition of covalent hydrogen functionalized to multiple-walled carbon nanotubes takes place in the range of 400-600°C. Pekker et al [18] detected the effluent hydrogen and methane via a mass spectrometer in this temperature range. At the temperature 400-600°C, the weight loss percent of hydrogenated graphene is much more than that of graphene.…”

“…Cheng et al [1] reported that at room temperature and 6 Mpa pressure, hydrogen absorption of graphene was less than 0.2 wt.%, whereas hydrogen uptake was 0.4 wt.% at 77 K temperature and 100 kPa. In order to chemically adsorb hydrogen, different methods have been implemented to hydrogenate the graphene including Birch reaction [17,18], high pressure hydrogenation [14,19], H-plasma procedure [20,21], and polyamine hydrogenation [5,9]. Wenhui yuan et al [14] showed that hydrogen absorption was about 2.7 wt.% at 298 K and 25 bars on glucose.…”

“…Wang et al [13] reported that hydrogen adsorption in a vacuum system at room temperature was about 0.1 to1 vol.%. Birch reaction is a low-temperature chemical hydrogenation process involving liquid ammonia, which is very difficult to handle [17,18]. Very recently, Xian Guo et al [15] reported that extent of physisorption of hydrogen by hierarchical graphene was 4.0 wt.%.…”

“…In order to chemically adsorb hydrogen, different methods have been implemented to hydrogenate the graphene including Birch reaction [17,18], high pressure hydrogenation [14,19], H-plasma procedure [20,21], and polyamine hydrogenation [5,9]. Birch reaction is a low-temperature chemical hydrogenation process involving liquid ammonia, which is very difficult to handle [17,18]. High-pressure hydrogenation process is carried out generally in a hydrogen atmosphere under a high pressure (60-150 bars) and a high temperature (200-500°C), which needs very spatial care of handling.…”

“…Here, in this paper, we report the chemisorption of hydrogen on graphene by using lithium in ethylenediamine under Benkeser reaction [22,23]. Benkeser reaction is often termed as modified Birch reaction [17,18] and earlier has been employed to store hydrogen on multiwall carbon nanotubes [5]. However, no reports are available on hydrogen storage on graphene employing Benkeser reduction reaction.…”