Mechanistic Aspects of Acrylic Acid Formation from CO₂–Ethylene Coupling over Palladium‐ and Nickel‐based Catalysts

Abstract

The detailed mechanism of catalytic conversion of CO2 and ethylene to acrylic acid has been investigated through DFT and DLPNO‐CCSD(T) calculations. Four bidentate ligands, dmpe (1,2‐bis(dimethylphosphino)ethane), dcpe (1,2‐bis(dicyclohexylphosphino)ethane), dtbpe (1,2‐bis(di‐tert‐butylphosphino)ethane), and tmeda (tetramethylethylenediamine) were systematically studied to understand the different catalytic behavior of the Pd‐based catalyst and the well‐known Ni‐based catalyst. The energy barrier of Pd‐/Ni‐catalyzed C−C coupling appears to increase with the larger steric hindrance of the ligand, whereas the barrier of the corresponding β‐H elimination shows an opposite tendency. The barrier for C−C coupling is likely higher than the associated barrier for β‐H elimination in both catalytic systems provided that the coordinated ligand is bulky enough. The palladium catalytic center is more effective than the nickel center for the C−C coupling and β‐H elimination steps in the presence of bulky ligands, whereas the nickel catalyst in turn performs better with small ligands. The palladium catalyst tends to be superior to the nickel catalyst during the following hydrogen transfer to the O atom. In general, the diamine ligand (tmeda) is less efficient than the diphosphine ligands for both systems. In the absence of the auxiliaries, a novel chelating carbene ligand was proposed to be quite efficient for Pd‐catalyzed C−C coupling. Two ligands (dcpm, dtbpm) showed relatively better performance towards the palladium‐catalyzed acrylic acid formation reaction among all the investigated ligands.

Insights into the Base‐Assisted Acrylate Formation from CO2/C2H4 Coupling by Pd‐ and Ni‐catalyst: A DFT Mechanistic Study

“…However, the Pd(dcpe)/E system was apparently less hindered in routes 7→6 and 8→6 than the Ni(dcpe)/E system. In our previous study, we also found that the Pd center was more active than the Ni center during the β‐H elimination when bulky ligand was selected, while the Ni‐catalyst showed better performance with small ligands . Here during the base‐assisted β‐H elimination, the nickel center was also more efficient than the palladium center under less hindered environment, while the palladium center surpassed the nickel center in crowded circumstance.…”

“…So the purpose and primary function of the base is to eliminate the β‐H to produce acrylate, and it does not play an important role in the metallalactone formation according to former reports ,. In our last publication, the detailed C−C coupling mechanism has already been thoroughly studied . Based on the current aim to clarify the base‐assisted acrylate formation mechanism in both the Ni‐ and Pd‐catalyzed systems, hereby, we started the investigation with the metallalactone rather than the M−C 2 H 4 complex.…”

“…For the first time the β‐agostic intermediates 7 and 9 were located in the base‐assisted mechanism, since they were suggested to be able to lower the β‐H elimination barrier and explain the experimentally discovered isomerization between the five‐membered lactone and the four‐membered lactone ,,. The elongation of the M−O bond was supposed to generate the unstable β‐agostic intermediates though no experiments detected this yet ,,. By carrying out a rigid potential energy surface (R‐PES) scan for the cleavage of the nickelalactone 2A Ni to yield 7A Ni , a transition state TS‐2‐7A Ni was optimized with a free energy of 24.2 kcal/mol (see ESI , Figure S1 ).…”

“…In addition, the effects of solvents as well as diverse ligands owning different steric and electronic properties were examined through DFT simulations . In early 2018, we also reported a comprehensive study on the coupling of CO 2 /C 2 H 4 catalyzed by nickel and palladium catalysts without auxiliaries, by using the latest developed state‐of‐the‐art method DLPNO‐CCSD(T) to ensure the predictive power of the computations . In consideration of the blank of mechanistic studies about the palladium catalyst, a further investigation for the palladium center was performed to clarify the effects of ligands.…”

“…More detailed studies to elucidate the role of bases, the difference between the base‐involved Ni and Pd systems and other potential reaction mechanisms are highly demanded. Hence, on the basis of our former mechanistic understanding of the reaction, here we further focused on the influence of bases on both the Ni‐ and Pd‐catalyzed acrylate synthesis mechanism . Seven possible reaction routes were designed for both metal systems in the presence of diverse bases (A, lithium methoxide; B, sodium methoxide; C, potassium methoxide; D, sodium phenoxide; E, sodium tert‐butoxide), aiming to clarify the difference between the base‐assisted nickel and palladium systems, as well as to find out the most promising β‐H eliminating path.…”