(229d) The Dilemma of Formic Acid Stabilization: Key Influences of Stabilizers on the Esterification Reaction

The production of methyl formate
(MF) by hydrogenation of CO₂ to formic acid (FA) and subsequent
esterification is a promising approach for the utilization of CO₂ as
a new resource for the chemical industry.¹ Due to the positive Gibbs
free energy difference of the homogeneous catalyzed hydrogenation, the addition
of a stabilizer e.g. in form of an amine has a strongly positive impact on the
conversion of CO₂.² The stabilized FA is removed from the
catalytic phase into a so-called product phase. This in-situ extraction shifts
the reaction equilibrium towards FA. When using methanol as a solvent for this extraction
step, the product phase contains all educts needed for the esterification
without further addition of chemicals. However, the influence of the choice of amine
and operating conditions of the hydrogenation on the economic assessment of both
reactions cannot be neglected.³ Thus, selection of a suitable amine
impacts the overall process performance and has to be investigated in detail
with regard to its thermodynamic and chemical impact.

In this work, we consider the influence
of different amines on a novel process for MF synthesis from CO₂
with special remark on the esterification reaction where the stabilizer of FA
is present as side component. A conceptual process design was carried out in
order to identify promising amines, thermodynamic properties of amines are
taken into account in order to prevent relevant azeotropes and thus avoiding
energy intensive separation steps. Batch experiments for the esterification
reaction are performed in a 100 mL reactor and in situ spectroscopy is used to
detect the change of component concentrations. Thus, the influence of the amine
on the esterification reaction concerning reaction velocity and yield is
investigated. However, experiments show that the influence of stabilizers on
the reaction kinetics cannot be neglected. A theoretical kinetic model from
literature⁴ is expanded with regard to the impact of stabilization
on the equilibrium constant and reaction rate. The model is validated based on the
experimental results in order to define operating parameters for high
time-space yields.

The evaluation of the experiments
and model with relevant amines shows that all selected stabilizers have a
negative effect on the yield of MF in the esterification, thus shifting the
equilibrium of the esterification to the substrate side. Furthermore, the
influence of temperature and feed ratio on the reaction yield are taken into account.
In order to increase the product yield in the presence of amines, destabilization
of the FA-amine complex has to take place. Destabilization can be achieved by
exploiting the temperature dependence of the exothermal stabilization reaction.
Since the esterification is an endothermal reaction, equilibrium constant of
this reaction is decreasing with increasing temperature, thus, a trade-off has
to be found. Another promising approach to increase the time-space yield of MF
is the in situ removal of the product. Therefore, we built a reactive
distillation in mini plant scale with 10 bubble trays and operated it for a
given amine at moderate temperature ranges. Results show a drastically improved
reaction time, improved conversion of FA as well as a product separation of MF
with a purity of over 90 mol% at the top of the reactive distillation column.

Acknowledgments

We gratefully acknowledge funding
for the Carbon2Chem® project by the German Federal Ministry of Education and
Research (BMBF). Many thanks to Covestro Deutschland AG for providing
supporting information.

References

[1]       Klankermayer, J.,
Wesselbaum, S., Beydoun, K., Leitner, W. (2016). Angewandte Chemie Int. Ed., 55,
7296–7343. DOI: 10.1002/anie.201507458.

[2]       Scott, M.,
Blas, Molinos, B., Westhues, C., Franciò, G., Leitner, W. (2017). ChemSusChem,
10, 1085 – 1093. DOI: 10.1002/cssc.201601814.

[3]       Kaiser, T., Rathgeb, A.,
Gertig, C., Bardow, A., Leonhard, K., Jupke, A. (2018). Chemie Ingenieur
Technik, 90, 1497-1503. DOI: 10.1002/cite.201800029.

[4]       Tischmeyer,
M. (2004). Ph.D. Thesis, Technical University Berlin, Germany.