Here the DFT approach has been employed to examine both the concerted and stepwise mechanisms of the carbon dioxide hydrogenation to formic acid over Cu-alkoxide functionalized metal organic framework. In the concerted mechanism, the CO2 was hydrogenated to generate formic acid in one step with a large activation barrier of 67.2 kcal/mol. In the stepwise mechanism, the reaction occurs via two steps of CO2 hydrogenation. In the first step, CO2 is hydrogenated to a formate intermediate. In the second step, the formate is further hydrogenated into formic acid. Lower activation barriers compared with the concerted mechanism were associated with this pathway (24.2 and 18.3 kcal/mol for the first and second steps, respectively); therefore, this stepwise mechanism seemed to be favored over the concerted one. For the gas-phase uncatalyzed reaction, the hydrogenation of CO2 occurred in a single step. The calculated reaction barrier was 73.0 kcal/mol, which was much higher than that of the Cu-MOF-5 system. Therefore, it can be suggested that the Cu alkoxide functional on MOF-5 might be a good candidate material for use as a catalyst for the carbon dioxide hydrogenation reaction.
In addition, we also found that the activity of this reaction was sensitive to the charge transfer between catalysts and the adsorbing molecules. This charge transfer played an important role for activating the CO2 sufficiently to promote its hydrogenation and also assisting the H–H bond breaking of the hydrogen molecule. Therefore, the appropriate strategy for enhancing the reactivity of MOF for this reaction was the improvement of the charge transfer that was the substitution of the electron-donating group on the organic linker. This assumption was confirmed in our work. The activation energy of the rate-determining step for the preferred pathway was decreased with the electron-donating group substitution.
Supporting Information
Optimized structure of the rate-determining step transition state of the stepwise mechanism (TS1_S) on the Cu-MOF-5 substituted with NH2 and NO2.This material is available free of charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Here the DFT approach has been employed to examine both the concerted and stepwise mechanisms of the carbon dioxide hydrogenation to formic acid over Cu-alkoxide functionalized metal organic framework. In the concerted mechanism, the CO2 was hydrogenated to generate formic acid in one step with a large activation barrier of 67.2 kcal/mol. In the stepwise mechanism, the reaction occurs via two steps of CO2 hydrogenation. In the first step, CO2 is hydrogenated to a formate intermediate. In the second step, the formate is further hydrogenated into formic acid. Lower activation barriers compared with the concerted mechanism were associated with this pathway (24.2 and 18.3 kcal/mol for the first and second steps, respectively); therefore, this stepwise mechanism seemed to be favored over the concerted one. For the gas-phase uncatalyzed reaction, the hydrogenation of CO2 occurred in a single step. The calculated reaction barrier was 73.0 kcal/mol, which was much higher than that of the Cu-MOF-5 system. Therefore, it can be suggested that the Cu alkoxide functional on MOF-5 might be a good candidate material for use as a catalyst for the carbon dioxide hydrogenation reaction.
In addition, we also found that the activity of this reaction was sensitive to the charge transfer between catalysts and the adsorbing molecules. This charge transfer played an important role for activating the CO2 sufficiently to promote its hydrogenation and also assisting the H–H bond breaking of the hydrogen molecule. Therefore, the appropriate strategy for enhancing the reactivity of MOF for this reaction was the improvement of the charge transfer that was the substitution of the electron-donating group on the organic linker. This assumption was confirmed in our work. The activation energy of the rate-determining step for the preferred pathway was decreased with the electron-donating group substitution.
Supporting Information
Optimized structure of the rate-determining step transition state of the stepwise mechanism (TS1_S) on the Cu-MOF-5 substituted with NH2 and NO2.This material is available free of charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
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