Hydrogenation of aqueous mixtures of calcium carbonate and carbon dioxide using a water-soluble rhodium(I)-tertiary phosphine complex catalyst

Article abstract of DOI:10.1016/j.molcata.2004.08.045

Aqueous suspensions of calcium carbonate were hydrogenated to yield calcium formate under a gas phase containing both H₂ and CO₂. A rhodium(I)-complex of monosulfonated triphenylphosphine (mtppms), [RhCl(mtppms)₃] was used as catalyst. Reaction temperatures were in the range of 20-70°C, total pressure 10-100 bar with an optimum p(CO ₂):p(H₂) ratio of 1:4. Due to the mobile bicarbonate-formate equilibrium, the highest available formate concentration is decreased with increasing temperature but increased with increasing pressure. Interestingly, at 50°C and 100 bar total pressure the yield of formate was 143% (based on CaCO₃) which implies the formation of free formic acid in the reaction of CO₂ from the gas phase. The dependence of the hydrogenation rate on the catalyst and ligand concentrations, as well as that of the decomposition of formate as a function of temperature were also studied. A mild method has been developed for the hydrogenation of calcium carbonate to calcium formate in aqueous suspensions under CO₂ + H₂ pressure catalyzed by [RhCl(mtppms)₃] (mtppms = monosulfonated triphenylphosphine) at 20-70°C and 10-100 bar total pressure. In certain cases the yield of formate exceeded 100% (based on CaCO₃) which implies the concomitant hydrogenation of carbon dioxide, too.

Full text of DOI:10.1016/j.molcata.2004.08.045

Journal of Molecular Catalysis A: Chemical 224 (2004) 87–91
Hydrogenation of aqueous mixtures of calcium carbonate
and carbon dioxide using a water-soluble rhodium(I)–tertiary
phosphine complex catalyst
a
a,b,∗
´
´
´
Istvan Joszai , Ferenc Joo
a
Institute of Physical Chemistry, University of Debrecen, P.O.B. 7, Debrecen H-4010, Hungary
b
Research Group of Homogeneous Catalysis, Hungarian Academy of Sciences, P.O.B. 7, Debrecen H-4010, Hungary
Received 1 March 2004; received in revised form 18 June 2004; accepted 21 August 2004
Dedicated to Professor Jo´zef J. Zio´łkowski on the occasion of his 70th birthday in recognition of his outstanding contributions to organometallic
chemistry and catalysis.
Abstract
Aqueous suspensions of calcium carbonate were hydrogenated to yield calcium formate under a gas phase containing both H2 and CO2. A
rhodium(I)-complex of monosulfonated triphenylphosphine (mtppms), [RhCl(mtppms)3] was used as catalyst. Reaction temperatures were
in the range of 20–70 ◦C, total pressure 10–100 bar with an optimum p(CO2):p(H2) ratio of 1:4. Due to the mobile bicarbonate-formate
equilibrium, the highest available formate concentration is decreased with increasing temperature but increased with increasing pressure.
Interestingly, at 50 ◦C and 100 bar total pressure the yield of formate was 143% (based on CaCO3) which implies the formation of free formic
acid in the reaction of CO2 from the gas phase. The dependence of the hydrogenation rate on the catalyst and ligand concentrations, as well
as that of the decomposition of formate as a function of temperature were also studied.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Calcium carbonate; Carbon dioxide; Hydrogenation; Rhodium; Water-soluble
1. Introduction
Perhaps the largest volume process on Earth is the fix-
ation of CO2 as carbohydrates during the green plant pho-
tosynthesis – these reactions take place in a bulk aqueous
environment. It is also noteworthy that the CO2 content of
industrial gases is removed by washing with aqueous solu-
tions of bases particularly of amines or amino-alcohols. This
has generated active research into the use of aqueous solu-
tions for the hydrogenation of carbon dioxide with water-
soluble catalysts [7,13]. In the first successful hydrogenation
of CO2 in a fully aqueous system the K[RuCl(edta-H)] cat-
alyst (edta-H = monoprotonated ethylenediaminetetraacetate
trianion) converted CO2 with a turnover frequency (TOF)
of 375 h−1 under mild conditions [14] (TOF = mol prod-
uct/mol catalyst × time). Importantly, this reaction did not
require the addition of bases. Later it was found that sev-
eral water-soluble Rh(I)- and Ru(II)-complexes, in many
cases with hydrophilic tertiary phosphine ligands, were
Homogeneous catalytic conversion of carbon dioxide into
useful products has attracted much recent interest [1–8]. Car-
bon dioxide is cheap and easily available not only highly di-
luted in the atmosphere but in high concentrations, too, at
natural or industrial sources (fossil-carbon fuelled generator
plants, beer factories, etc.). In the last few years, important
advances have been made in the homogeneous catalytic hy-
drogenation of carbon dioxide under supercritical or near-
critical conditions [9–11], in coupling of CO2 and epoxides
yielding polycarbonates [3] and in the practical synthesis of
2-ethylheptanoic acid from carbon dioxide, butadiene and
hydrogen [12].
∗
Corresponding author. Tel.: +36 52 512900; fax: +36 52 512915.
E-mail address: jooferenc@tigris.klte.hu (F. Joo´).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.08.045

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I. Jo´szai, F. Joo´ / Journal of Molecular Catalysis A: Chemical 224 (2004) 87–91
able to catalyze the hydrogenation of CO2 in aqueous sys-
tems in the presence of organic amines or amino-alcohols
yielding the corresponding ammonium formates or for-
mamides with TOFs up to several thousand [15]. For ex-
ample, in aqueous HNMe2 solutions with H2:CO2 = 1:1,
p = 40 bar and T = 81 ◦C the water-soluble [RhCl(mtppts)3]
(mtppts = 3,3ꢀ,3ꢀꢀ-phosphinetriylbenzenesulfonic acid, m-
trisulfonated triphenylphosphine, usually as sodium salt) cat-
alyst afforded dimethylammonium formate with an initial
TOF of 7260 h−1 [15]. However, in the absence of HNMe2 or
other nitrogen bases this catalyst was completely inactive for
the hydrogenation of CO2. Several other rhodium complexes
are also known to catalyze the hydrogenation of carbon diox-
ide in partially or fully aqueous solutions [16–19]. In all cases
the addition of either an amine or an alcohol was found essen-
tial to achieve reasonable rates and total conversions. It was
also established, that the nitrogen bases not only facilitated
the reaction through their reaction with the product formic
acid but played an active role also in the formation of the cat-
alytically active monohydridorhodium(I) species such as e.g.
[RhH(S)(mtppts)3] (S = solvent or amine) [15]. An important
feature of the reactions run in the presence of amines is that
the final concentration of the product formic acid (formate,
formamide) never exceeded that of the amine. This is in con-
trast to the case of dipolar aprotic solvents (e.g. dmso) when
with certain amines HCOOH/amine > 1 could be observed
(for example with NEt3 this ratio was 1.6) [20].
The last few years saw extensive studies on the homo-
geneous hydrogenation of bicarbonates (Eq. (1)) or carbo-
nates in aqueous solutions. The catalysts used inclu-
ded [RuCl2(pta)4] (pta = 1,3,5-triaza-7-phosphaadamantane)
[21], [{RuCl2(mtppms)2}2] (mtppms = (3-diphenylph-
osphino)benzensulfonic acid, m-monosulfonated triph-
enylphosphine, sodium salt) [22], [RuCl3(NO)(PR3)2]
(PR3 = mtppms or mtppts) [23], and the [(␩6-
arene)RuH(PR3)2]+ complexes formed under hydrogen
from [{(␩6-arene)RuCl2}2] and PR3 (arene = benzene or
p-cymene, PR3 = mtppms or mtppts or pta) [24]. To a lesser
extent, [RhCl(mtppms)3] has also been studied as catalyst
for reaction (1) [25].
sures in the absence of added acids or bases the composition
of the solution can be regarded as that of a solution of the
hypothetical Ca(HCO3)2, however, at lower pH (higher CO2
pressures) the molar ratio of the Ca(HCO3)+ ion pair becomes
significant [26] leaving less free bicarbonate available for hy-
drogenation. Since the dissolution requires aqueous solvents,
water-soluble catalysts [13] are uniquely suited for this reac-
tion. Calcium formate is a valuable product used in leather
tanning and in the production of silage in animal nutrition
[27]. In addition, cheap CaCO3 is available in huge quanti-
ties and its use may contribute to the utilization of CO2 by
affordable means.
Here we present in detail our findings on the hydrogena-
tion of aqueous suspensions of CaCO3 under CO2 pressure
using [RhCl(mtppms)3] as catalyst. The effects of the varia-
tion of experimental conditions are also reported and allow
some insight into the kinetic details of the reaction. An earlier
communication of us has already disclosed the suitability of
this Rh-complex as catalyst for this purpose [28].
2. Experimental
2.1. Materials and analytical techniques
mtppms and [RhCl(mtppms)3] were prepared according
to published procedures [29] and checked by 1H, 31P NMR
and IR spectroscopy. H2, CO2 and Ar were obtained from
Linde. Analytically pure CaCO3 was supplied by Reanal,
Hungary. Other materials were highest purity commercial
products of Aldrich and Merck and were used as received.
Doubly distilled water was used throughout.
1H and 31P NMR spectra were recorded on a Bruker
DRX400 equipment in D2O. Infrared spectra were recorded
on a PE Paragon 1000 PC FT-IR spectrometer in KBr discs.
The reaction mixtures were analyzed for formate (as formic
acid) by HPLC (Waters 501 pump, Supelcogel® 610H col-
umn thermostated to 30 ◦C, Waters 490E multiwavelength
UV–vis detector at λ = 210, 205 and 254 nm, sample volume
10 ␮L, eluent 0.1% H3PO4 in water, flow rate 1 mL min−1).
HCO3− + H2 ꢀ HCO2− + H2O
(1)
2.2. Hydrogenation experiments
These reactions did not require the addition of an or-
ganic base, however, with the above catalysts hardly any
hydrogenation of aqueous CO2 solutions (carbonic acid)
could be observed. In contrast, NaHCO3 was hydrogenated
to NaHCO2 at 20–80 ◦C and p(H2) = 1–100 bar with TOFs
up to 9600 h−1 [22] with nearly quantitative yields.
Based on our experience in the homogeneous hydrogena-
tion of bicarbonates, it seemed reasonable to us to explore the
possibility of the hydrogenation of alkaline earth metal car-
bonates, specifically CaCO3 in aqueous suspensions under a
CO2 atmosphere. On the action of CO2 and H2O (carbonic
acid) CaCO3 dissolves and takes part in several equilibria in
solution, depending mostly on the pH [26]. At low CO2 pres-
These experiments were run either in home-made
heavy-walled glass reactors (total volume 150 mL) up to
≤10 bar total pressure or in glass-lined stainless steel re-
actors (up to ≤100 bar) both equipped with a gas inlet,
pressure gauge and an inlet/sampling port. In a typical
run 100 mg (1 mmol) CaCO3, 13.4 mg (1 × 10−5 mol Rh)
[RhCl(mtppms)3], 6.0 mg (1.5 × 10−5 mol) mtppms and a
Teflon covered stir bar were placed into the reactor. After
closing the reactor it was deoxygenated with several evac-
uation/refill (Ar) cycles. Then 10 mL of water was injected
through the inlet port. The reactor was evacuated and filled
first with CO2 followed by H2 of the desired pressure at room
temperature, then placed into an oil bath of the desired tem-

I. Jo´szai, F. Joo´ / Journal of Molecular Catalysis A: Chemical 224 (2004) 87–91
89
perature which was controlled by a Lauda K4R circulator
(glass reactor) or wrapped around with a heating mantle con-
nected to a toroid transformer (stainless steel reactors). The
reaction mixture was vigorously stirred using a magnetic stir-
rer. At the end of the desired reaction time the reactor was
cooled in ice water, the pressure vented carefully and the
formate content of the product mixture was determined by
HPLC (vide supra).
3. Results and discussion
Fig. 2. Effect of the amount of CaCO3 on the rate of the hydro-
genation of CaCO3/CO2 catalyzed by [RhCl(mtppms)3] in aqueous
suspension/solution. [Rh] = 1 mM, [mtppms]total = 4.5 mM, p(CO2) = 2 bar,
p(H2) = 8 bar, T = 60 ◦C, V = 10 mL, t = 15 h.
When a fine powder of calcium carbonate was stirred in
water in the presence of [RhCl(mtppms)3] under an atmo-
sphere containing both CO2 and H2 a clean hydrogenation
occurred yielding formate as the sole product (Eq. (2), Fig. 1).
This successful attempt to hydrogenate CaCO3 in a homo-
geneously catalyzed reaction under mild conditions is made
possible by the combination of acid–base chemistry in water
and aqueous organometallic catalysis.
tion obtained in 1 h reactions is used for the characterization
of the initial reaction rate.
The activity of the catalyst is influenced by the concen-
tration of the phosphine ligand. An optimum value is ob-
tainedbytheadditionof1.5 molmtppmsto[RhCl(mtppms)3]
([mtppms]total/[Rh] = 4.5). Increasing the concentration of
the catalyst increases the rate almost linearly up to
[Rh] ≤ 2 mM, however, at higher concentrations the rate of
the reaction tends to level off. This latter behaviour is the con-
sequence of the bicarbonate/formate equilibrium (vide infra).
In most of the experiments discussed here we used 10 bar to-
tal pressure; a maximum in the reaction rate as a function of
the gas phase composition was found at p(CO2):p(H2) = 1:4,
i.e. at p(CO2) = 2 bar and p(H2) = 8 bar. (The effects of the
above parameters are shown as supplementary information,
Supplementary Figures 1–3.) The amount of formate ob-
tained in the first hour of the hydrogenations initially shows
a rapid increase as a function of the amount of solid CaCO3
in the suspension (Fig. 2), however it soon reaches a limiting
value and a further increase of the amount of CaCO3 leads
only to negligible improvement of the formate yield.
CO32− + CO2 + 2H2 ꢀ 2HCO2− + H2O
The reaction is rather slow and under the conditions of
Fig. 1 the concentration of HCO2 levels off in time much
(2)
−
below the stoichiometric value (200 mM) based on Eq. (2).
Earlier attempts of CaCO3 hydrogenation are sporadic and
the heterogeneous catalytic processes described in the liter-
ature [30] require harsh conditions. In general CaCO3 has to
be decomposed to yield CO2 which becomes subsequently
hydrogenated. There are also observations on that when Pd or
Ir metal is deposited on calcium carbonate the surface layer
of the support is able to react with spilt-over hydrogen to
yield CH4 already at 300 ◦C, well below the decomposition
temperature of CaCO3. Nevertheless, this temperature is still
much higher than those applied in this study.
It is also seen in Fig. 1 that at the beginning of the reaction
the concentration of [HCO2−] increases linearly in time and
therefore in the following discussion the formate concentra-
The effect of the temperature on the reaction is two-
fold. As can be seen in Fig. 3, the initial rate increases ex-
ponentially with increasing temperature and the Arrhenius
plot of the data leads to an apparent activation energy of
36 kJ mol−1. This value is close to those obtained in the hy-
Fig. 1. Time course of the hydrogenation of calcium carbonate in aque-
ous suspension under CO2 pressure catalyzed by [RhCl(mtppms)3] in the
presence of added mtppms. m(CaCO3) = 100 mg (1 mmol), [Rh] = 1 mM,
[mtppms]total = 4.5 mM, p(CO2) = 2 bar, p(H2) = 8 bar, T = 60 ◦C, V = 10 mL.
Fig. 3. Effect of the temperature on the initial rate of the hy-
drogenation of CaCO3/CO2 catalyzed by [RhCl(mtppms)3] in aque-
ous suspension/solution. m(CaCO3) = 100 mg (1 mmol), [Rh] = 1 mM,
[mtppms]total = 4.5 mM, p(CO2) = 2 bar, p(H2) = 8 bar, V = 10 mL, t = 1 h.

90
I. Jo´szai, F. Joo´ / Journal of Molecular Catalysis A: Chemical 224 (2004) 87–91
Fig. 4. Effect of the temperature on the maximum formate concentration ob-
tained in the hydrogenation of CaCO3/CO2 catalyzed by [RhCl(mtppms)3]
in aqueous suspension/solution. m(CaCO3) = 100 mg (1 mmol),
[Rh] = 1 mM, [mtppms]total = 4.5 mM, p(CO2) = 2 bar, p(H2) = 8 bar,
V = 10 mL. T = 50 ◦C (ꢁ), 60 ◦C (ꢂ), 70 ◦C (᭹).
Fig. 5. Decomposition of Ca(HCO2)2 in aqueous solution cat-
alyzed by [RhCl(mtppms)3] as function of the temperature.
[Ca(HCO2)2]0 = 100 mM, [Rh] = 1 mM, [mtppms]total = 4.5 mM, V = 10 mL,
t = 6 h, under argon (1 bar; ꢂ) and under carbon dioxide and hydrogen
(p(CO2) = 2 bar, p(H2) = 8 bar; ᭹).
a
drogenation of CO2 in aqueous solution with K[RuCl(edta-
H)] (31 kJ mol−1) [14], with [RhCl(mtppts)3] in the presence
of amines (25 kJ mol−1) [15] and in the hydrogenation of
still 65%. Conversely, in a CO2/H2 atmosphere the concomi-
bicarbonate with a heterogeneous Pd catalyst (39 kJ mol−1
)
−
tant hydrogenation of HCO3 formed in the decomposition
[31]. The familiar exponential increase of the reaction rate at
higher temperatures, however, involves the temperature de-
pendence of all the component processes (solubility of CO2,
its hydration equilibrium, the dissociation equilibrium of car-
bonic acid, the dissolution equilibrium of CaCO3 under CO2,
and the hydrogenation reaction, itself a reversible process).
Therefore the activation barrier obtained from these experi-
ments should be regarded as a composite value and cannot be
related directly to the hydrogenation reaction let alone one of
the steps of the catalytic cycle.
From a practical point of view, it is more important to note
that the maximum available formate concentration shows an
inverse dependence on the temperature (Fig. 4), so that the
20% yield observed at 50 ◦C drops to 11% at 70 ◦C. It is
reasonable to suppose that this is a consequence of the cat-
alytic decomposition of formate (Eq. (3)). Therefore we have
studied the decomposition of formate in aqueous solutions
catalyzed by [RhCl(mtppms)3].
reaction establishes a new equilibrium and only 46% of the
initial formate is decomposed at 70 ◦C. The data of Figs. 3–5
show that although both the hydrogenation of CaCO3 and the
decomposition of formate is accelerated by increased tem-
peratures, this effect is more pronounced in the case of the
decomposition. This explains that at higher temperatures the
hydrogenations start faster but reach a lower equilibrium for-
mate concentration than at lower temperatures (Fig. 4).
In the investigations of the effect of the total gas
pressure on the reaction rate the same gas composition,
p(CO2):p(H2) = 1:4, was used which led to optimum results
at ptotal = 10 bar. These studies brought an unexpected result.
As seen from Fig. 6, the yields of formate at ptotal ≥ 50 bar
are higher than 100% (200 mM, based on CaCO3); indeed at
such high pressures all the −solid CaCO3 goes into solution.
The highest yield of HCO2 was 143% obtained at 100 bar
pressure. The significance of this result is that the product
mixture now contains free formic acid, which – in principle
– can be recovered by well-established methods of the present
day industrial production of formic acid [27,32].
HCO2− + H2O ꢀ HCO3− + H2
(3)
Concerning the mechanism of the hydrogenation pro-
cess, the effect of the various reaction parameters (concen-
Since in most of our hydrogenation experiments we used
100 mg (1 mmol) CaCO3 in 10 mL suspension, the decom-
position studies were run using 0.1 M Ca(HCO2)2 solutions
which corresponds to the maximum formate concentration
obtainable from the above amount of CaCO3. The catalyst
concentration was the same both in the hydrogenation and in
the decomposition experiments. One set of experiments were
carried out under an inert atmosphere (Ar), while others were
run under a CO2/H2 gas phase of the same composition and
pressure used for the hydrogenations. The results are shown
in Fig. 5.
Substantial decomposition of formate was observed both
under Ar and under CO2/H2. A 6 h heating of the reaction
mixture at 70 ◦C under argon results in the complete de-
Fig. 6. Effect of the total pressure on the rate of the hydro-
genation of CaCO3/CO2 catalyzed by [RhCl(mtppms)3] in aque-
ous suspension/solution. m(CaCO3) = 100 mg (1 mmol), [Rh] = 1 mM,
[mtppms]total = 4.5 mM, p(H2):p(CO2) = 4:1, T = 50 ◦C, V = 10 mL, t = 24 h.
−
composition of HCO2 (accompanied by the precipitation
of solid CaCO3) and at 50 ◦C the extent of decomposition is

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91
tration of the catalyst, reactants, temperature) can be ra-
tionalized by assuming that it involves the free HCO3
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