Screening Lewis Pairs for CO2 Hydrogenation
Ronald Reynolds (firstname.lastname@example.org), Jingyun Ye (email@example.com), J. Karl Johnson (firstname.lastname@example.org)
Over the past decade, increasing awareness of the damaging effects of greenhouse gases on our atmosphere has exposed a need to seek new ways to manage and reduce the amount of greenhouse gas emissions we produce on a daily basis. Of these greenhouse gases, carbon dioxide (CO2) is most prevalent. In addition to being produced in abundance it is very chemically stable, making it hard to convert or even capture to prevent it from reaching the atmosphere. A sustainable process is needed to reduce CO2 emissions that can be reproduced on an industrial scale.
Recent research on Lewis pair moieties (entities having both a Lewis acid and Lewis base site) have shown that these molecules possess promising CO2 hydrogenation abilities. They provide a metal free, catalytic solution to controlling CO2 emissions by using hydrogen gas (H2) as a reducing agent to convert CO2 into formic acid (HCOOH). We use computational chemistry as a method of screening different Lewis pairs for their ability to hydrogenate CO2. In this work we attempt to tune the activity of the Lewis pair by modifying the Lewis base site. We use the CO2 and H2 binding energies (adsorption energies) as surrogates for the activity of the Lewis pair toward hydrogenation of CO2. This approximation is based on a tested Brønsted-Evans-Polanyi relationship for similar Lewis pair functional groups.
We have screened a total of 28 different functional groups. These groups were generated by modifying the substituents on the Lewis base site and by using different parent structures for the functional group. These 28 functional groups gave binding energies for H2 adsorption spanning the range of -1.98 to 1.29 eV and CO2 binding energies from -2.08 to 0.9 eV. Our target binding energies are -0.3 to -0.6 eV for H2 while having CO2 binding energies less favorable than H2 binding (preferably greater than 0 eV). None of the functional groups tested fell into the desired combined range for H2 and CO2 binding energies. We have concluded that it is not possible to design an optimum functional group by modulating the strength of the Lewis base site alone.