23 Hydrogen Activation by Soluble Metal Oxide Complexes
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R. J. Klingler, T. R. Krause, and J. W. Rathke Chemical Technology Division, Argonne National Laboratory, Argonne, IL 60439
To better define the chemistry associated with nucleophilic oxide centers, the catalysis of the water-gas shift reaction was investigated in triethylene glycol solution (150-250 °C and 1-300 atm) by employing alkali metal hydroxide catalysts. Under reaction conditions most of the sodium salt is in the form of the formate complex, which is produced through the carbonylation of hydroxide ion. The resultant water-gas shift reaction is first order in sodium formate over a concentration range from 0.01 to 1.0 M. Both formic and acetic acids have a beneficial effect on the rate of dihydrogen evolution within the sodium hydroxide-formate water-gas shift system. Deuterium, D , is produced in 93% isotopic purity when the reaction is conducted in D O-triethylene glycol-d . It exhibits a substantial inverse kinetic isotope effect (kH/kD = 0.5) compared to the analogous water-gas shift reaction conducted in H O-triethylene glycol. The kinetic results indicate that the sodium hydroxide-formate water-gas shift system proceeds through an intermediate (or transition state) with substantial H-H bond order. This activity contrasts with earlier proposals for nucleophilic dihydrogen activation, which focused on a free hydride ion intermediate. 2
2
2
2
D I H Y D R O G E N IS H E T E R O L Y T I C A L L Y C L E A V E D (I) on most metal oxides, with the hydride and the proton residues going to the metal and oxygen centers, respectively. For example, dihydrogen is reversibly
chemisorbed
onto zinc oxide at -45 °C with the formation of zinc hydride and surface hydroxyl groups (eq 1) (2, 3), both of which are observed by IR spectroscopy. {-Zn-0-Zn-}
s
+ H
2
{-ZnH + HOZn-}
0065-2393/92/0230-0337$06.00/0 © 1992 American Chemical Society
Moser and Slocum; Homogeneous Transition Metal Catalyzed Reactions Advances in Chemistry; American Chemical Society: Washington, DC, 1992.
(1)
338
H O M O G E N E O U S TRANSITION
METAL CATALYZED
REACTIONS
Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch023
The heterolytic cleavage of dihydrogen by metal oxides is usually described as a four-centered transition state in which an empty Lewis acid site on the metal center and a Lewis base site on the oxygen center simultaneously interact with opposite ends of the dihydrogen molecule. Furthermore, although the metal-centered Lewis acid and the oxygen-centered Lewis base act in concert in the four-centered transition state, they are not necessarily equal partners in the process. Indeed, the metal is usually considered to be the primary point of interaction for the dihydrogen molecule (4). However, the role of the nucleophilic oxygen centers may be underestimated in many of these metal oxide systems because the hydrogénation of an organic substrate can be catalyzed with a hydroxide ion catalyst in the complete absence of a metal center (5). Deuterium exchange has been known to be catalyzed by bases since the 1936 work of Wirtz and Bonhoeffer (6) (eqs 2 and 3). OH-
H 0
+ D
2
NH
3
> HOD + HD
2
+ D - A 2
(2)
N H D 4- H D
(3)
2
Base-catalyzed deuterium exchange can be quite rapid. The half-life in the liquid ammonia-sodium amide system (7) is less than 1 min at -53 ° C . The kinetics of the base-catalyzed deuterium-exchange reaction has been investigated by a number of groups (6-12). The early mechanistic work is described in terms of hydroxide attack on the dihydrogen molecule to displace a free hydride ion intermediate, which subsequently reacts rapidly with the proton source (eqs 4 and 5). D
2
+ OH"
>D" + DOH
(4)
> HD + OH"
(5)
fast
D" + H 0 2
However, more recent gas-phase work (13) is able to rule out a free hydride ion intermediate, at least in the gas phase. The heat of formation of gaseous hydride ion is known, and the observed barrier for deuterium exchange is too small, by a factor of 5, to generate a high-energy hydride ion intermediate. Instead, Grabowski et al. (13) propose an addition adduct between the hydroxide ion and the dihydrogen molecule. The proposed intermediate, H 0 ~ ion, is a long-lived species in ion cyclotron resonance experiments (14-16). The nature of the bonding within this species has been investigated by extended basis set molecular orbital calculations (17, 18). The molecular orbital calculations on the H 0 ~ ion 3
3
Moser and Slocum; Homogeneous Transition Metal Catalyzed Reactions Advances in Chemistry; American Chemical Society: Washington, DC, 1992.
23.
KLINGLER ET AL.
Hydrogen Activation by Metal Oxide Complexes
339
Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch023
system indicate that hydride ion is stabilized by 26 kcal through coordination to the Lewis acidic end of a water molecule, [ H O H · H]~. Stabilization of the hydride ion increases the lifetime of this species and could facilitate hydride transfer to organic substrates. Indeed, it is possible to hydrogenate organic substrates with base catalysts, as demonstrated by Walling and Bollyky (19, 20). These workers observed the hydrogénation of benzophenone and nitrobenzene employing a potassium f-butoxide catalyst. Walling (20) proposed a mechanism similar to that suggested for the early solution work on deuterium exchange. Thus, Walling's mechanism for base-catalyzed hydrogénation contains a free hydride ion intermediate. Alternatively, the hydride ion might exist as a tight complex with the Lewis acidic end of the alcohol solvent. No kinetic data are available for this system. We attempted to probe the potential application of nucleophilic dihydrogen activation processes for synthesis gas transformations with metal oxide catalysts. Our recent efforts in this direction have been focused on determining the kinetics for the reduction of carbon dioxide catalyzed by alkali metal hydroxide complexes in triethylene glycol solution (eq 6). OH
H
2
+ C0
2
> H 0 + CO 2
(6)
This reaction is commonly referred to as the water-gas shift equilibrium, and standard commercial catalysts are available. The experimentally reversible reaction is usually conducted in the thermodynamically favorable dihydrogen evolution direction. The homogeneous alkali metal system is of fundamental mechanistic interest. Measurements (5) have demonstrated that the activation barrier observed for the catalysis of the water-gas shift reaction by solvated hydroxide ion is comparable to the activation barrier exhibited by an industrially used iron oxide catalyst. In this industrial reaction the metal and oxide centers are free to exert synergistic effects.
Results The reactions were conducted in gold-plated stainless steel autoclaves, as described elsewhere (5). Identical results were obtained in a Teflon [poly(tetrafluoroethylene)] block reactor in which no part of the solution or gas phase contacts the stainless steel support walls. These precautions were necessary because formic acid produced through the carbonylation of water (eq 7) rapidly builds up to nearly equilibrium concentration levels under steady-state water-gas shift reaction conditions at 180 ° C .
H 0 + CO 2
OH
> H CO
