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SYSTEMATIC SCREENING OF IONIC LIQUIDS FOR HYDROGENATION OF CO2 TO FORMIC ACID

Authors:
SYSTEMATIC SCREENING OF IONIC LIQUIDS FOR HYDROGENATION OF CO2 TO
FORMIC ACID
T.O. BELLO1, P. A. PESSOA FILHO1, A.E. BRESCIANI1, R.M.B. ALVES1, C.A.O. NASCIMENTO1
1 Polytechnic School, University of Sao Paulo, Sao Paulo, Brazil.
1. Keywords
Ionic liquids, Formic acid, CO2 hydrogenation, COSMO Screening
2. Highlights
Screening on phase equilibria, physical properties, toxicity, and separation
Estimation by COSMO-RS
Analysis on gas capacity/solubility
3. Purpose
The utilization of carbon dioxide (CO2) as a C1 feedstock to produce valuable chemicals is of great
significance for green chemistry and sustainable development. A variety of chemicals such as formic
acid, [1] methanol, [2] and hydrocarbons, [3] can be manufactured through hydrogenation of CO2.The
hydrogenation of carbon dioxide to formic acid is thermodynamically disfavoured starting from
gaseous reactant with a standard Gibbs energy (ΔG°298) of +32.9 kJmol−1 [5], but is somewhat exergonic
in aqueous solution [2,4]. In order to convert the thermodynamically stable and relatively unreactive
CO2 molecule into the desired product efficiently, suitable reaction conditions and activation
mechanisms must be found. Ionic liquids (ILs) may be viewed as a new, remarkable class of solvents
and as an alternative to make this reaction feasible. One significant benefit of ILs as solvent in
hydrogenation reactions is the ability to fine-tune the properties of the solvent by altering the
structure, catalyst immobilization and activating the CO2, consequently leading to reduction in the
Gibbs energy of reaction of formic acid.
4. Materials and methods
A systematic strategy for the selection of ILs as solvent for the hydrogenation of CO2 combining Liquid-
Liquid equilibrium calculation (LLE), gas capacity(solubility), physical property prediction, and
separation performance is presented. The first step consists of two independent parallel screening
(LLE & gas capacity) of the reactants (CO2 & H2), ILs and formic acid. The gas solubility & LLE of the
reactants in different ILs are predicted with the conductor-like screening model (COSMO-RS).
Consequently, the ILs with greater gas capacity, distribution coefficient, selectivity and lower solvent
loss than a benchmark solvent (conventional organic solvent) are selected for the next stage. Further,
physical property (melting point & viscosity) are estimated by COSMO-RS. ILs with low melting point
and viscosity proceed to the next stage. Third step is qualitative analysis of the ILs that satisfy both gas
capacity and LLE route. Afterwards, quantitative estimation of the ILs impact on the environment
based on octanol /water partition coefficient is performed. Finally, the separation performance of the
most promising ILs candidates with formic acid mixture in a continuous process using a simple flash
separation is analyzed in Aspen Plus to finally identify process-based optimal solvents.
5. Results and discussion
The recovery of ILs from formic acid is feasible by evaporating the volatile component (formic acid)
under vacuum conditions at ~0.22-0.66bar and temperature range of 140-150OC. Approximately
99.99% of the ILs were regenerated at T= 150oC and 0.22 < 0.66bar with molar fraction of formic acid;
0.970, 0.982, 0.996 and 0.999 respectively in the vapour product of each separator.
The energy consumption of formic acid vaporisation from its feed mixture of ILs is in the range 998-
1134 kJ /h regeneration. These values are lower compared with the benchmark solvent regeneration
duty of 3971KJ/h. At this value of energy consumption, there is small fraction of formic acid lost with
the benchmark solvent at 0.11bar and 150oC. The calculated heat of vaporization (Qvap) of ILs in the
mixture are in the range of 310-605 kJ/kg and formic acid; 503kJ/kg unlike Qvap of formic acid;
433.5KJ/Kg in its pure state. This differences in Qvap of formic acid can be seen as a measure of the
strength of the interactions between the ILs and formic acid in the liquid phase [6]. From the results,
it can be shown that the separator duty decreases with ILs with more branched chain alkyl group. ILs
with more branch alkyl group proved to be optimal for this application compared to others with just
branch group because of its less energy demand for its regeneration.
6. Conclusions and perspectives
Regeneration of 99% of ILs from organic solvent/solute mixtures is feasible using a simple flash
separation process to evaporate the volatile component under vacuum condition because of their
relative stability under high temperature conditions. The four most promising ILs (1-ethyl-3-
imidazolium nitrite, 1-ethyl-2,3-dimethyl-imidazolium nitrite, 1-methy-3-limidazolium nitrite and 1-
pentyl-3-imidazolium nitrite) for this process are consequently selected based on their operation
performance. The screening method can be easily extended to select practically attractive IL solvents
for other multi-objective applications.
Acknowledgments
The authors gratefully acknowledge the support of the RCGI Research Centre for Gas Innovation,
hosted by the University of São Paulo (USP) and sponsored by FAPESP São Paulo Research
Foundation (2014/50279-4) and Shell Brasil. This study was financed in part by the Personnel
Coordination of Improvement of Higher Level - Brazil (CAPES) - Finance Code 001.
7. References
[1] Leitner, W.: Carbon dioxide as a raw material — the Synthesis of Formic Acid and its Derivatives
from CO2. Angew. Chem., Int. Ed. Engl. 34, 2207−2221 (1995)
[2] Alvarez, A., Bansode, A., Urakawa, A., Bavykina, A. V., Wezendonk, T. A., Makkee, M., Gascon, J.,
Kapteijn, F.: Challenges in the Greener Production of Formates/Formic Acid, Methanol, and DME
by Heterogeneously Catalyzed CO2 Hydrogenation Processes. Chem. Rev. 117, 9804−9838 (2017)
[3] Bahruji, H., Armstrong, R. D., Esquius, J. R., Jones, W., Bowker, M., Hutchings, G. J. :Hydrogenation
of CO2 to Dimethyl Ether over Bronsted Acidic PdZn Catalysts. Ind. Eng. Chem. Res. 57, 6821− 6829.
(2018)
[4] Sordakis, K.,Tang, C., Vogt, L.K., Junge, H., Dyson, P. J., Beller, M., Laurenczy, G.: Homogeneous
catalysis for sustainable hydrogen storage in formic acid and alcohols. Chemical Reviews. 118(2),
372-433 (2018)
[5] Yang, Z. Z., Zhang, H., Yu, B., Zhao, Y., Ji, G., Liu, Z.: A Troger’s Base-derived Microporous Organic
PolymerDesign and Applications in CO2/H2 Capture and Hydrogenation of CO2 to Formic Acid.
Chem. Commun. 51, 1271−1274 (2015)
[6] Ferro, V.R., Ruiz, E., De Riva, J., Palomar, J.: Introducing process simulation in ionic liquids
design/selection for separation processes based on operational and economic criteria through the
example of their regeneration. Sep. Purif. Technol. 97, 195204 (2012))
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