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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.
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.
[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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Full-text available
Eschewing the common trend towards use of catalysts comprising of Cu, it is reported that PdZn alloys are active for CO2 hydrogenation to oxygenates. It is shown that enhanced CO2 conversion is achievable through the introduction of Brønsted acid sites, which promote dehydration of methanol to dimethyl ether. We report that deposition of PdZn alloy nanoparticles onto the solid acid ZSM-5, via chemical vapour impregnation affords catalysts for the direct hydrogenation of CO2 to DME. This catalyst shows dual functionality; catalysing both CO2 hydrogenation to methanol and its dehydration to dimethyl in a single catalyst bed, at temperatures of > 270 oC. A physically mixed bed comprising 5% Pd 15% Zn/ TiO2 and H-ZSM-5 shows a comparably high performance, affording a dimethyl ether synthesis rate of 546 mmol kgcat⁻¹ h⁻¹ at a reaction temperature of 270 oC.
Full-text available
Tröger's base-derived microporous organic polymers (TB-MOPs) were designed, which could adsorb CO2 and coordinate with Ru(III) complex. The resultant TB-MOP-Ru showed good performances for adsorbing CO2 and H2, and high efficiency for catalysing CO2 hydrogenation into HCOOH with a turnover number up to 2254 at 40 oC.
Hydrogen gas is a storable form of chemical energy that could complement intermittent renewable energy conversion. One of the main disadvantages of hydrogen gas arises from its low density, and therefore, efficient handling and storage methods are key factors that need to be addressed to realize a hydrogen-based economy. Storage systems based on liquids, in particular, formic acid and alcohols, are highly attractive hydrogen carriers as they can be made from CO2 or other renewable materials, they can be used in stationary power storage units such as hydrogen filling stations, and they can be used directly as transportation fuels. However, to bring about a paradigm change in our energy infrastructure, efficient catalytic processes that release the hydrogen from these molecules, as well as catalysts that regenerate these molecules from CO2 and hydrogen, are required. In this review, we describe the considerable progress that has been made in homogeneous catalysis for these critical reactions, namely, the hydrogenation of CO2 to formic acid and methanol and the reverse dehydrogenation reactions. The dehydrogenation of higher alcohols available from renewable feedstocks is also described. Key structural features of the catalysts are analyzed, as is the role of additives, which are required in many systems. Particular attention is paid to advances in sustainable catalytic processes, especially to additive-free processes and catalysts based on Earth-abundant metal ions. Mechanistic information is also presented, and it is hoped that this review not only provides an account of the state of the art in the field but also offers insights into how superior catalytic systems can be obtained in the future.
The recent advances in the development of heterogeneous catalysts and processes for the direct hydrogenation of CO2 to formate/formic acid, methanol, and dimethyl ether are thoroughly reviewed, with special emphasis on thermodynamics and catalyst design considerations. After introducing the main motivation for the development of such processes, we first summarize the most important aspects of CO2 capture and green routes to produce H2. Once the scene in terms of feedstocks is introduced, we carefully summarize the state of the art in the development of heterogeneous catalysts for these important hydrogenation reactions. Finally, in an attempt to give an order of magnitude regarding CO2 valorization, we critically assess economical aspects of the production of methanol and DME and outline future research and development directions.
The integration of COSMO-RS methodology to Aspen Tech’s process simulators is used in this work to elaborate new criteria for the design/selection of ionic liquids to specific applications. These criteria are related to the operating conditions and energetic consumptions being the procedure capabilities demonstrated through the example of the IL regeneration in separation processes. Previously, the predictive capacity of the COSMO-RS method respect to VLE of mixtures (organic solvent + IL) is assessed and the correct transferability of the COSMO-RS results to Aspen Plus/Aspen HYSYS process simulators demonstrated.
The use of carbon dioxide as a raw material for chemical syntheses is an ecologically and economically valuable extension to the carbon sources used at the present time. In order to convert the thermodynamically stable and comparatively unreactive CO2 molecule into the desired product in an efficient manner, suitable reaction conditions and activation mechanisms must be found. The catalytic reduction of CO2 to formic acid and its derivatives has been intensively studied in recent years. A number of new approaches to the synthesis of formic acid from CO2 have reached such a state of knowledge that continuing development may well lead to industrial-scale operation in the near future. This can to a large extent be attributed to the fruitful interaction between investigative work into reaction mechanisms and the development of new catalytic systems.