Lithium-promoted hydrogenation of carbon dioxide to formates by heterobimetallic hydridozinc alkoxide clusters

Article abstract of DOI:10.1039/b714806b

The remarkably distinct reactivity of hydridozinc heterobimetallic cubanes [(HZnO^tBu)_4-n(thf·LiO^tBu)_n] 1a-1d towards CO₂ is reported - the hydride transfer from Zn-H to CO₂ is drastically a

Full text of DOI:10.1039/b714806b

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Lithium-promoted hydrogenation of carbon dioxide to formates by
heterobimetallic hydridozinc alkoxide clusters{
Klaus Merz,a Mariluna Moreno,a Elke Lo¨ffler,a Lamy Khodeir,a Andre Rittermeier,a Karin Fink,b
Konstantinos Kotsis,a Martin Muhlera and Matthias Driess*c
Received (in Cambridge, UK) 26th September 2007, Accepted 2nd November 2007
First published as an Advance Article on the web 12th November 2007
DOI: 10.1039/b714806b
increasing BET surface area but requires the presence of polar
ZnO facets5 and oxygen-vacancies as particularly important active
sites.6 Recently, it has also been shown that the catalytic activity of
pure ZnO supports for using feed gas mixtures containing CO and
H2 correlate with the amount of oxygen-vacancies, whereas CO2
has a poisoning effect presumably because it quenches oxygen
defect sites.7 Apparently, the exothermic conversion of CO2 with
H2 (water gas) to methanol on ZnO supports, according to eqn (1),
requires a different mechanism.
The remarkably distinct reactivity of hydridozinc heterobime-
tallic cubanes [(HZnOtBu)42n(thf?LiOtBu)n] 1a–1d towards CO2
is reported—the hydride transfer from Zn–H to CO2 is
drastically accelerated in the presence of Li ions in 1b–1d which
led to the respective metal formate hydrates; the systems are
inspiring models for the selective conversion of water gas into
formates on lithium-promoted ZnO supports.
Carbon dioxide (CO2) is an abundant yet low-value carbon source
of enormous impact in Nature. However, the extraordinarily high
stability of CO2 has hampered its utilization as efficient C1-
building block for large-scale industrial syntheses of useful organic
compounds. Thus, current efforts in developing efficient catalytic
processes that exploit CO2 as a source for valuable organic
products belong to one of the most ambiguous challenges in
industrial chemistry.1 Of particular interest is the reductive
transformation of CO2 in the presence of H2 into renewable
sources such as methanol (MeOH).2 The world demand for
MeOH is currently enormously increasing because of its role as a
precursor for many useful organic chemicals (e.g., formaldehyde,
acetic acid) and as a substitute for fuels.3 This can only be
managed by an efficient large-scale industrial process. In fact,
MeOH synthesis is very efficiently achieved by the heterogeneously
catalyzed conversion of syngas (CO, H2) and water gas (CO2, H2),
respectively, on heterometal-promoted ZnO carriers. Accordingly,
the most commonly commercialized unit for the production of
MeOH is the low-temperature ICI process, which converts a high-
pressure gas mixture of CO, CO2 and H2 into MeOH at 250–
300 uC, using Cu-promoted ZnO which is dispersed on alumina.4
As expected, the mechanisms of the heterogeneously catalyzed
reduction of carbon oxides depends sensitively on the nature of the
support material (i.e., composition, particle size, structure, defects)
and the presence of promoters (e.g., Cu). Notably, also Cu-free
pure ZnO model systems show considerable catalytic activities in
methanol synthesis under particular circumstances. The catalytic
activity of pure ZnO supports does not increase linearly with
CO2 + 3H2 A H3COH + H2O DRH = 240.9 kJ mol21 (1)
In line with that, five reactions on ZnO surfaces (eqn (2)–(6))
have been proposed in the literature, which play a crucial role for
the conversion of water gas to methanol.8
H2(g) A 2 H (ad)
CO2(g) A CO2 (ad)
(2)
(3)
(4)
(5)
CO2(ad) + H(ad) A HCO2(ad)
HCO2(ad) + 4H(ad) A H3CO(ad) + H2O
H3CO(ad) + H(ad) A H3COH(ad) A H3COH(g) (6)
The initial step of the catalytic process is the hydrogenation of
ZnO, leading to surface-terminated ZnH and OH sites9 which
indicate the heterolytic fission of dihydrogen. It has also been
shown for a few cases that alkali metals can promote the catalytic
performance of ZnO.10 On the molecular level, however, little is
known about the consecutive mechanism of chemisorption and
reduction steps. This lack of knowledge could be partially
overcome by using hydridozinc alkoxides as molecular models
which resemble some electronic features of hydrogenated ZnO.
Additionally, the promoting influence of heterometals could be
mimicked by using heterobimetallic hydridozinc clusters (e.g.,
mixed lithium hydridozinc alkoxides). Recently, we reported the
synthesis and structure of a series of well-defined homo- and
heterobimetallic hydridozinc tert-butoxide clusters of the formula
[(HZnOtBu)42n(LiOtBu)n] 1a–1d11 (Scheme 1) which could be
useful to study Zn–H assisted and heterometal promoted
hydrogenation of CO2.
aFaculty of Chemistry, Ruhr-University Bochum, Universita¨tsstrasse
150, D-44780 Bochum, Germany
bInstitute of Nanotechnology, Forschungszentrum Karlsruhe, PO Box
3640, D-76021 Karlsruhe, Germany
cTechnische Universita¨t Berlin, Institute of Chemistry: Metalorganics
and Inorganic Materials, Sekr. C2, Strasse des 17. Juni 135, D-10623
Berlin, Germany. E-mail: matthias.driess@tu-berlin.de;
Fax: +49(0)30-314-29732; Tel: +49(0)30-314-29731
{ Electronic supplementary information (ESI) available: Experimental
procedure, characterisation of [Zn(HCO2)2?2H2O] and DFT calculations
of IR vibrations. See DOI: 10.1039/b714806b
Here we report the remarkably different reactivity of the
lithium-free vs. lithium-containing hydridozinc heterocubane-like
clusters 1a vs. 1b–1d. The latter show the pivotal role of lithium
ions for an accelerated reduction of CO2 at Zn–H sites at ambient
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confirmed by quantum chemical calculations.14 From the literature
it is known that the formate ligand can adopt the g1- and
g2-coordination mode, respectively. We considered in our DFT
calculations three different coordination modes for the formate
ligand: the g1-, symmetric g2- and asymmetric g2-coordination
modes, respectively. As expected, the g2-coordination modes are
favoured with a slight preference for the asymmetric one. Our
calculations suggest that both coordination modes contribute to
the vibrational spectra of the formate species obtained by the
reaction of 1b and CO2. This is in accordance with the MAS 13C-
NMR spectrum of the product which shows two resonances for
the HCO2 moiety at d = 168 and 169 ppm. Furthermore, powder-
XRD studies confirm the presence of Zn(HCO2)2 and Li(HCO2)
hydrates as microcrystalline components of the reaction mixture.
Other hydrogenated products of CO2 such as formaldehyde and
methanol or CO could not be detected (IR, NMR, MS). The
experiments have been performed under rigorous anhydrous and
anaerobic conditions. Accordingly, using the deuterated isotopo-
mer of 1b, [(DZnOtBu)3(thf?LiOtBu)], leads to [Zn(DCO2)2?2D2O]
as proven by IR. Remarkably, contamination of CO2 with water
vapour (0.05–0.1%) prevents hydrogenation of CO2 and leads
merely to partial hydrolysis of Zn–H bonds in 1b and sorption of
CO2 to give carbonates exclusively. The water molecules of
[Zn(HCO2)2?2H2O] are presumably formed by secondary reduc-
tion of formate to give as yet unidentified reduction products.
Apparently, the formation of water necessitates the presence of
lithium ions, since the Li-free cluster 1a leads exclusively to
anhydrous zinc formate. The formation of water during the
hydrogenation of CO2 on ZnO supports has previously been
discussed by Bailey et al..15 In the latter case, however, formate
species are also formed as initial products at elevated temperature
via hydride transfer apart from other hydrogenation products (e.g.,
methanolate) and water. Interestingly, the formation of
Zn(HCO2)2 and Li(HCO2) hydrates occurs also in solution.
Thus, clear solutions of 1b in thf react with CO2 leading
immediately to precipitation of [Zn(HCO2)2?2H2O] which has
been characterized by IR, 1H- and 13C-NMR spectroscopy.
Crystals of the latter have been characterized by a single-crystal
X-ray diffraction analysis (Fig. 2). The crystal structure16 consists
of a coordination polymer with two octahedrally coordinated Zn
ions linked by a formate anion as a bridging bidentate ligand.17
The two Zn ions are in different environments and lie on
independent inversion centres: while Zn1 is coordinated to six
formate ligands, Zn2 is surrounded by two formate ligands and
four water molecules. As proven by 1H-NMR spectroscopy of the
filtrate, compound 1b has been completely consumed and neither
another Zn–H compound nor formate species remain in solution.
Instead, resonance signals of as yet unknown metal tert-butoxide
Scheme 1 The homometallic cubane 1a vs. heterobimetallic lithium zinc
tert-butoxide cubanes 1a–1d.
temperature and atmospheric pressure to give selectively zinc
formate.
The reaction progress for the consumption of CO2 has been
monitored by in situ IR measurements. In contrast to ZnH2 which
remains unchanged even after one day in a pure CO2 atmosphere,
the powdered hydridozinc heterocubane cluster 1a reacts slowly
with CO2 to give additional vibrational bands in the region of
1330, 1600–1650, 1798–1813 and 2700–2800 cm21 in the IR
spectrum which can be assigned to the formate ion in different
coordination modes.12 The consumption of 1a is complete after ca.
three days. The formation of formate is similar to the process of
insertion of CO2 into the Zn–H bond of a hydridozinc
tris(pyrazole)borate described by Parkin et al. which enabled the
isolation of an g1-formate zinc complex.13 Interestingly, the
insertion of CO2 into the Zn–H bond is drastically accelerated in
the presence of Li ions in the heterocubane framework: thus,
powders of 1b react immediately with CO2 as shown by IR
measurements (Fig. 1).
The characteristic Zn–H vibrational mode of 1b at 1769 cm21
decreases gradually during the reaction progress. Concomitantly,
an additional band at 1805 cm21 is growing in and additional new
bands appear at 1336, 1630–1674, and 2700–2800 cm21
.
Furthermore, the appearance of a broad OH stretching vibration
in the region of 3200–3500 cm21 suggests the formation of water
which is coordinated to zinc and/or lithium formate. In fact, the
vibrational modes can be unequivocally assigned to zinc formate–
hydrate species based on characteristic reference data13 and
Fig. 1 In situ IR spectra of the gradual conversion of 1b with CO2. (a)
Vibrational bands between 1900 and 1500 cm21. (b) Vibrational modes
Fig. 2 Structural unit of polymeric [Zn(HCO2)2?2H2O] in the crystal.
between 1450 and 1250 cm21
.
The Zn2-atom coordinates four water molecules.
74 | Chem. Commun., 2008, 73–75
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conversion of water gas to formic acid derivatives are currently
underway.
Notes and references
1 Review: H. Arakawa, M. Aresta, J. N. Armor, M. A. Barteau,
E. J. Beckman, A. T. Bell, J. E. Bercaw, C. Creutz, E. Dinjus,
D. A. Dixon, K. Domen, D. L. DuBois, J. Eckert, E. Fujita,
D. H. Gibson, W. A. Goddard, D. W. Goodman, J. Keller,
G. J. Kubas, H. H. Kung, J. E. Lyons, L. E. Manzer, T. J. Marks,
K. Morokuma, K. M. Nicholas, R. Periana, L. Que, J. Rostrup-
Nielson, W. M. H. Sachtler, L. D. Schmidt, A. Sen, G. A. Somorjai,
P. C. Stair, B. R. Stults and W. Tumas, Chem. Rev., 2001, 101, 953–996
and references therein.
2 (a) G. C. Chinchen, K. C. Waugh and D. A. Whan, Appl. Catal., 1986,
25, 101; (b) M. Kurtz, H. Wilmer, T. Genger, O. Hinrichsen and
M. Muhler, Catal. Lett., 2003, 86, 77.
3 (a) M. S. Spencer, Catal. Lett., 1989, 50, 37–40; (b) G. A. Olah, Angew.
Chem., 2005, 117, 2692, (Angew. Chem., Int. Ed., 2005, 44, 2636).
4 Review: L. Lloyd, D. E. Ridler and M. V. Twigg, in Catalyst Handbook,
ed. M. V. Twigg, Wolfe, London, 1989.
5 H. Wilmer, M. Kurtz, K. V. Klementiev, O. P. Tkatschenko,
W. Gru¨nert, O. Hinrichsen, A. Birkner, S. Rabe, K. Merz, M. Driess,
C. Wo¨ll and M. Muhler, Phys. Chem. Chem. Phys., 2003, 5, 4736.
6 (a) S. A. French, A. A. Sokol, S. T. Bromley, C. R. A. Catlow,
S. C. Rogers, F. King and P. Sherwood, Angew. Chem., Int. Ed., 2001,
40, 4437; (b) S. A. French, A. A. Sokol, S. T. Bromley, C. R. A. Catlow
and P. Sherwood, Top. Catal., 2003, 24, 161.
7 S. Polarz, J. Strunk, V. Ischenko, M. W. E. Van den Berg,
O. Hinrichsen, M. Muhler and M. Driess, Angew. Chem., Int. Ed.,
2006, 45, 2965–2969.
8 I. A. Fisher and A. T. Bell, J. Catal., 1997, 172, 222.
9 (a) G. Ghiotti, A. Chioino and F. Boccuzzi, Surf. Sci., 1993, 297,
228–234; (b) J. C. Lavalley, S. Saussey and T. Rais, J. Mol. Catal., 1982,
17, 289–298.
10 (a) D. M. Minahan, W. S. Epling and G. B. Hoflundy, J. Catal., 1998,
179, 241–257; (b) J. G. Nunan, C. E. Bogdan, K. Klier, K. J. Smith,
C. W. Young and R. G. Herman, J. Catal., 1989, 116(1), 195–221; (c)
P. Winiarek and J. Kijenski, J. Chem. Soc., Faraday Trans., 1998, 94(1),
167–172; (d) A. Szabo, Prog. Catal., 2000, 9(1–2), 65–72.
11 W. Marchiniak, K. Merz, M. Moreno and M. Driess, Organometallics,
2006, 25, 4931–4933.
12 D. G. Rethwisch and J. A. Dumesic, Langmuir, 1986, 2, 73–79.
13 R. Han, I. B. Gorell, A. G. Looney and G. Parkin, J. Chem. Soc.,
Chem. Commun., 1991, 717.
14 R. Ahlrichs, M. Ba¨r, M. Ha¨ser, H. Horn and C. Ko¨lmel, Chem. Phys.
Lett., 1989, 162, 165–169.
15 S. Bailey, G. F. Froment, J. W. Snoeck and K. C. Waugh, Catal. Lett.,
1994, 30, 99.
16 Crystal data for [Zn(HCO2)2?2H2O] see ESI.{ Crystal data for
C2H6O6Zn, Mr = 191.44, monoclinic, space group P21/c (No. 14), a =
Fig. 3 In situ IR spectra of the gradual conversion of 1c with CO2. (a)
Vibrational bands between 1250 and 1450 cm21. (b) Vibrational modes
between 1500 and 1900 cm21
.
aggregates and uncharacterized organic side-products can be
observed.
The heterobimetallic cubanes 1c (ratio Li : Zn = 2 : 2) and 1d
(ratio Li : Zn = 3 : 1) react also with CO2 which has been
monitored by in situ IR spectroscopy. Fig. 3 shows the changes of
selected characteristic vibrational modes for the gradual conver-
sion of 1c which are practically identical with those of 1d.
However, reaction progress is significantly slower than that for the
monolithium cluster 1b. While the conversion of 1b with CO2 is
complete after ca. 5 min, it takes ca. 30 min to consume the same
molar quantity of 1c and 1d, respectively, affording
[Zn(HCO2)2?2H2O] and Li(HCO2) hydrates as major products.
The distinct reactivity of 1b vs. 1c and 1d suggests that hydride
transfer from the Zn–H bond to CO2 is significantly reduced by
increasing the molar ratio of Li : Zn. In line with that, the relatively
low reactivity of 1a indicates that the presence of at least one Li ion
as a Lewis-acidic centre in proximity to the Zn–H moiety fosters
the hydride transfer to CO2.
˚
8.6803(9), b = 7.1241(10), c = 9.3060(12) A, b = 97.664(3)u, V =
570.3(1) A , rcalcd = 2.229 g cm23, m = 4.265 mm21, Z = 4,
3
˚
On the other hand, increasing the Li : Zn ratio reduces the
basicity of the Zn–H moiety due to a stronger O A Li vs. O A Zn
coordination. In conclusion, our model systems demonstrate the
pivotal role of Li ions for an accelerated reduction of CO2 at Zn–H
sites. Although, the mechanism for the accelerated reduction of
CO2 through the presence of Li ions is still unknown, our
preliminary results on the model systems 1a–1d suggest that the
selective conversion of water gas (hydrogenation of CO2) into
formic acid derivatives (e.g., formic acid methylester) could be
strongly favoured by using lithium-promoted ZnO supports. In
line with our previous results on synthesizing nanoscaled zinc
oxide materials through the organometallic precursor approach,18
1b–1d are promising molecular single-source precursors for the
synthesis of Li-promoted, nanoscaled ZnO materials. Respective
investigations on the synthesis and catalytic performance of
Li-promoted ZnO nanoparticles for the selective catalytic
˚
l = 0.71073 A, T = 213 K, 209 reflections collected (¡h, ¡k, ¡l), [(H
range: 3.71 to 25.02], 926 independent (Rint=0.019) and 734 observed
reflections [I . 2s(I)], 110 refined parameters, R = 0.022, wR2 = 0.062.
CCDC 650772. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b714806b.
17 A. S. Lipton, M. D. Smith, R. D. Adams and P. D. Ellis, J. Am. Chem.
Soc., 2002, 124, 410–414.
18 (a) K. Merz, R. Schoenen and M. Driess, J. Phys. IV, 2001, 11, 467; (b)
M. Driess, K. Merz, R. Schoenen, R. Rabe, F. E. Kruis, A. Roy and
A. Birkner, C. R. Chim., 2003, 6(3), 273; (c) J. Hambrock, S. Rabe,
K. Merz, A. Wohlfarth, A. Birkner, R. A. Fischer and M. Driess,
J. Mater. Chem., 2003, 13, 1731; (d) M. Kurtz, N. Bauer, C. Buescher,
H. Wilmer, O. Hinrichsen, R. Becker, S. Rabe, K. Merz, M. Driess,
R. A. Fischer and M. Muhler, Catal. Lett., 2004, 92, 49; (e) S. Polarz,
A. Roy, M. Merz, S. Halm, D. Schro¨der, L. Schneider, G. Bacher,
F. E. Kruis and M. Driess, Small, 2005, 1, 2; (f) V. Ischenko, S. Polarz,
D. Grote, V. Stavarache, K. Fink and M. Driess, Adv. Funct. Mat.,
2005, 15, 1945; (g) D. Schro¨der, H. Schwarz, S. Polarz and M. Driess,
Phys. Chem. Chem. Phys., 2005, 7, 1049.
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Chem. Commun., 2008, 73–75 | 75