Mechanistic understanding of methanol carbonylation: Interfacing homogeneous and heterogeneous catalysis via carbon supported Ir-La
Alyssa J.R. Hensley a, Jianghao Zhang a, Ilka Vinçon b, Xavier Pereira Hernandez a, Diana Tranca b, Gotthard Seifert b,*, Jean-Sabin McEwen a,c,d,e,*, Yong Wang a,e,*
a The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164, United States
b Theoretical Chemistry, Technische Universität Dresden, Dresden 01062, Germany
c Department of Physics and Astronomy, Washington State University, Pullman, WA 99164, United States
d Department of Chemistry, Washington State University, Pullman, WA 99164, United States
e Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352, United States
*Corresponding authors at: The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164, United States (J.-S. McEwen and Y. Wang).
E-mail addresses: gotthard.seifert@chemie.tu-dresden.de (G. Seifert), js.mcewen@wsu.edu (J.-S. McEwen), yong.wang@pnnl.gov (Y. Wang)
1. Introduction
The carbonylation of methanol is one of the largest processes that is still being carried out under homogeneous catalysis. It allows to produce more than 6 million tonnes of acetic each year [1]. The energy requirements of the separation of catalyst and products, and the contamination associated to them, are considerably high for a process of this magnitude, therefore, finding and understanding the fundamentals of a heterogeneous catalysis homologous process is necessary and of high importance.
In this work, we report the synthesis, activity and characterization of a carbon-supported Ir-La catalyst which exhibited high activity and selectivity towards acetyls for methanol carbonylation, compared to the unpromoted carbon-supported Ir catalyst, as well as the homogenous Ir-Ru catalyst and a heterogeneous analog carbon-supported Ir-Ru catalyst [2]. The work focuses and answers two questions [3]: 1) what is the reaction mechanism and 2) what is the role La plays on the catalyst.
To provide information about these questions, X-ray photoelectron spectroscopy (XPS), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), in situ attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy, density functional theory (DFT) and ab initio molecular dynamics (AIMD) were used.
2. Results and Discussion
The activity measurements for the Ir-based catalysts mentioned above can be observed in Figure 1. The Ir-La/C catalyst is the only one that exhibits activity higher than the homogenous catalyst (close to 40% higher) and it exhibits around 3 times the activity of the unpromoted Ir/C catalyst. Selectivity to acetyls was > 99%. Even though Ir and Ru are used during the homogeneous process, a heterogeneous catalyst involving the same metals has about half the activity of the Ir-La/C catalyst. The reaction conditions are 240°C and 17 bar, a MeOH/MeI ratio of 70/30wt.% and a syngas molar composition of CO/H2 = 4/1. Previous to reaction, the catalyst is activated by exposing it to a pretreatment with syngas of the same composition at the same reaction temperature for 1 h.
Figure 1. Activity results of Ir and M-promoted Ir catalysts supported on carbon compared to the homogeneous Ir-Ru catalyst. Reaction conditions: 17 bar, 240°C, MeOH/MeI: 70/30wt.%, syngas composition of CO/H2 = 4/1.
To understand why the Ir-La/C catalyst exhibits good activity for the carbonylation of methanol, microscopic and spectroscopic characterization of the catalyst was performed. Figure 2 (left) shows that the catalyst has a very high dispersion and many of the particles contain 1-3 atoms, in which dimers are the predominating structure (50-60%). Dimers also show the pairing of atoms with high and low intensity, suggesting that Ir-La closely interact with each other. Intensity profiles indicate that the interatomic distances range between 2-3 Å. Furthermore, figure 2 (right) shows that exposing the catalyst to a syngas treatment changes the oxidation state of Ir from Ir3+ (that results from an inert treatment with He) to Ir+. In all cases, La is always present as La3+. CO-TPD experiments (not shown) indicated that the CO/Ir ratio is 1.9. This information suggests the heterogeneous Ir-La/C catalyst has a structure very similar to the homogeneous one, in which 2 CO molecules are bonded to an Ir+ metal center.
Figure 2. Left: HAADF-STEM image of the Ir-La/C catalyst. Right: XPS of the Ir-La/C catalyst after a) He pretreatment at 300°C and b) syngas pretreatment (CO/H2 = 4/1) at 240°C after the He pretreatment at 300°C.
To provide more insights into the likely structure present in the catalyst, different configurations were studied by DFT. Three different configurations involving one trans and two cis were optimized and are presented in figure 3. The cis configurations were found to be ~ 65 kJ/mol more stable than the trans and of the two cis configurations, the open structure was found to be more stable by ~ 5 kJ/mol. ATR-FTIR spectra of the catalyst during activation conditions and the DFT calculated IR spectra of the CO stretching modes in the trans and open cis configurations of the Ir-La complex are shown in figure 4. These experimental and theoretical spectra suggest the cis structure is present when the catalyst is activated and the complexes are formed, which agrees well with its higher stability.
Figure 3. Optimized configurations for the trans, closed cis and open cis configurations of the Ir-La complex with iodine ligands.
Figure 4. a) ATR-FTIR spectra of the Ir-La/C catalyst obtained during syngas pretreatment (CO/H2 = 4/1) at 240°C and 3.75 bar, b) Spectra calculated for cis and trans configurations by density functional perturbation theory (DFPT).
Figure 5. Reaction scheme for methanol carbonylation on the Ir-La heterogeneous catalyst complex with the La promoter acting as an iodine acceptor
Even though HAADF-STEM results would suggest the presence of the closed cis configuration due to the shorter bond between Ir and La, AIMD showed that at reaction conditions, this structure relaxes into the open cis structure. Therefore, the open cis configuration was used as a starting point in the reaction mechanism for the carbonylation of methanol obtained by DFT (figure 5). It was found that the catalyst follows a similar reaction mechanism to the homogenous analog. It starts with the oxidative addition of MeI to the Ir center to form a 6-fold coordinated structure, with a change in the oxidation state of Ir from Ir+ to Ir3+. After the addition of MeI, La accepts an iodine from the Ir center which enables a third CO molecule to bind with the Ir center. The migratory insertion of CO into the Ir-Me bond is the next step, leading to the formation of the acetyl (Ac) group. Finally, a reductive elimination of AcI closes the cycle and restores the catalyst, in which the oxidation state of Ir changes from Ir3+ to Ir+. It was also found that a possible variation exists in which first the third CO molecule binds to the Ir+ center before the oxidative addition of MeI, while the next steps remain the same. In both cases, La acts as an iodine acceptor.
Conclusions
We presented a carbon-supported Ir-La catalyst that exhibits high activity for the carbonylation of methanol. The catalyst exhibited around 40% higher activity than the homogeneous existing process and around 3 times the activity of the unpromoted carbon-supported Ir catalyst. Characterization of the catalyst showed the structure, reaction mechanism and promoter effect in the heterogeneous catalyst resembles that of the homogeneous catalyst with a very high dispersion of the metal. The present work has great implications on the possibility of using an industrially relevant heterogeneous catalyst that would not only lead to the reduction of energy requirements characteristic of homogeneous processes but also to the use of cheaper metals.
References
[1] A. Haynes, Chapter 1 – Catalytic Methanol Carbonylation, 1st ed., vol. 53, no. 10. Elsevier Inc., 2010.
[2] J. H. Kwak, R. Dagle, G. C. Tustin, J. R. Zoeller, L. F. Allard, and Y. Wang, “Molecular active sites in heterogeneous Ir-La/C-catalyzed carbonylation of methanol to acetates,” J. Phys. Chem. Lett., vol. 5, no. 3, pp. 566–572, 2014.
[3] A. J. R. Hensley, J. Zhang, I. Vinçon, X. I. Pereira Hernandez, D. Tranca, G. Seifert, J. Mcewen, and Y. Wang, “Mechanistic understanding of methanol carbonylation: Interfacing homogeneous and heterogeneous catalysis via carbon supported Ir-La,” J. Catal., vol. 361, pp. 414–422, 2018.