(448d) Catalytic Hydrogenation of Carbon Monoxide to Formaldehyde in Functionalized Metal Organic Frameworks: An Investigation of Pathway and Uncertainty

Formaldehyde is a highly desirable platform chemical with an annual demand of over 30 million metric tons. The current most common method of generating formaldehyde industrially involves conversion of methanol to formaldehyde and suffers from low exergy efficiency.[1] The methanol used is typically produced by the oxidation of methane to syn-gas, followed by the reduction of CO to methanol via Fischer-Tropsh synthesis. In this work, we will present a direct pathway for converting CO to formaldehyde by direct hydrogenation in Metal Organic frameworks (MOFs) functionalized with Lewis pairs (LPs). MOFs are a class of porous materials having potential for both gas adsorption and catalysis. Functionalization of MOFs with catalytic moieties results in heterogeneous catalysts with active sites similar to those of molecular catalysts

Based on our previous studies, the one of the biggest advantage of using LPs as hydrogenation catalyst is the fact that LPs are good at heterolytic hydrogen dissociation. [2-4] In those pathways, the concerted transfer of a hydride and proton to the C and O atoms (respectively) of CO₂ had a much lower activation barrier than sequential addition of hydrogens to produce formic acid. In this work we test whether similar pathways can be used to directly reduce CO to formaldehyde with low barriers. The density functional theory (DFT) results presented here employed the GPAW code, with the BEEF-vdW functional [5] to describe the exchange correlation energy. We computed reaction pathways for H₂ dissociation, CO chemisorption, CO hydrogenation to formaldehyde and methanol. Using the BEEF ensemble, we also quantified the uncertainly associated with each DFT calculation due to the choice of exchange-correlation functional.

We have investigated the proposed pathway in gas phase, inside functionalized UiO-67, and on a freestanding fragment containing the LP (N-BF₂). We found that the poisoning of the LP by CO is reduced when the LPs were incorporated into the MOF. The rate-limiting step is the hydrogenation of CO, with a barrier of 1.30 ± 0.15 eV. We believe that conversion of CO to formaldehyde could be further enhanced in MOFs due to the increase in effective pressure inside confined pores. Additionally, this confinement effect also reduces translational degrees of freedom, which makes the free energy of the overall reaction more favorable.

1. Bahmanpour, A., Hoadley, A., Tanksale, A. Critical Review and Exergy Analysis of Formaldehyde Production Processes. Reviews in Chemical Engineering, 30(6), pp. 583-604. (2014).
2. Ye, J.; Johnson, J. K. ACS Catal. 5, 6219-6229 (2015)
3. Ye, J.; Johnson, J. K. Sci. Technol. 6, 8392-8405 (2016)
4. Ye, J.; Johnson, J. K. ACS Catal. 5, 2921-2928 (2015)
5. Wellendorff, J. et al. Phys. Rev. B. 85, 235149 (2012)