Selective and Direct Hydrogenation of CO₂ for the Synthesis of Formic Acid over a Rhodium Hydrotalcite (Rh‐HT) Catalyst

Abstract

Rhodium hydrotalcite (Rh‐HT), a heterogeneous catalyst was synthesized, characterized and investigated for CO2 hydrogenation. The catalyst was effectual for the CO2 hydrogenation and selective formation of formic acid at a moderate temperature and was efficiently recycled up to 5 times without any significant loss in activity. A TON of 15840 for formic acid was selectively obtained at 50 bar pressure (1:1, p/p, CO2 and H2) and 60 °C temperature in a mixture of methanol:water (5:1 v/v, 60 mL) as a solvent in 24 h with no additional base. The effect of various reaction parameters was investigated for the reaction rate and CO2 conversion. The formic acid formation rate follows the 1st order kinetic trend regarding the catalyst amount and partial pressure. The determined activation parameters from the temperature dependence rate of formic acid were Ea=33.5 ± 2.5 kJmol−1, ΔH#=30.9 ± 2.5 kJmol−1, ΔS#=‐275 ± 5 Jdeg−1mol−1. The presence of water as a solvent together with methanol have effectively enhanced the performance of the catalyst. The mechanistic routes for CO2 hydrogenation to formic acid are proposed and discussed by the kinetic and experimental observations.

Liquid phase hydrogenation of CO2 to formate using palladium and ruthenium nanoparticles supported on molybdenum carbide

“…This result is comparable to Mori et al's Pd/TiO 2 and PdAg/TiO 2 catalysts, achieving TOFs of 12 and 31 h À1 respectively, 51 also their Ru/LDH catalyst, with TOF of 29. However, this catalyst indeed has a lot of competition and achieves TOFs less than Song et al's Pd/Chitin (TOF = 257 h À1 ) 42 and Maru et al's Pd/ g-C 3 N 4 (TOF = 660 h À1 ) 81 both achieving results at higher pressure but lower temperatures. A comparison of recent CO 2 hydrogenation catalysts, their TOFs and reaction conditions can be found in the ESI † (Table S1).…”

Zinc Oxide Morphology‐Dependent Pd/ZnO Catalysis in Base‐Free CO2 Hydrogenation into Formic Acid

“…However, the final product is formate instead of FA in the presence of base, which weakens the importance of this reaction. Currently, there are few reports of heterogeneous CO 2 hydrogenation into FA without the addition of base and the production rate of FA is also inferior [49–52] . Such as, Mondelli et al .…”

Enhanced Base‐Free Formic Acid Production from CO2 on Pd/g‐C3N4 by Tuning of the Carrier Defects

“…Still, it has been highlighted that the establishment of a large‐scale CO 2 ‐based manufacture of FA also requires an economic advantage and that reducing catalyst and energy costs is most important to achieve this goal. The availability of an industrially viable renewable process is regarded as crucial considering that the global market of FA is expected to increase by approximately 4 % by 2021 owing to its broadening application in different industrial sectors and, especially, to its emerging role as an efficient and safe hydrogen carrier in fuel cell technologies and, potentially, in biomass upgrading . It is worth mentioning that the use of FA as a hydrogen vector supports the concept of circular economy, as CO 2 is used for its production and is recovered upon hydrogen release …”

“…Targeting a higher sustainability, heterogeneous systems have been preferably tested in water or by employing amine or amidine base, or CO 2 as the medium without reaching better results . More recently, it has been realized that only a base‐free process would truly fulfill ecologic requirements, minimizing the implications associated not only with the use of this stoichiometric additive but also of the acid needed to convert the formate salt produced into FA in free form . First attempts in this direction include Pd supported on graphitic carbon nitride (g‐C 3 N 4 ) applied in water and, lately, a Rh‐hydrotalcite operated in water‐containing methanol, which exhibited TOF values of 4.5 and 660 h −1 , respectively.…”