(768g) Design of Functionalized Metal Organic Frameworks for CO2 Hydrogenation:the Effects of MOF Topology and Functional Group

CO₂ capture coupled with chemical recycling of CO₂ to renewable fuels and valuable chemicals is a promising approach to reduce carbon emissions. An ideal process would combine both capture and chemical reduction of CO₂, taking advantage of process intensification. Metal organic frameworks (MOFs), a class of porous crystalline materials, have great potential for CO₂ capture. We have combined the adsorption selectivity of MOFs with functional groups having chemical activity toward CO₂ hydrogenation to produce a single material with the potential to both capture and convert CO₂. We have covalently bound catalytic functional groups based on Lewis pairs (LPs), which are organic molecules possessing both Lewis acid and base sites, to the linkers of a MOF. Our initial work utilized a highly stable and CO₂-selective Zr based MOF known as UiO-66 to create a heterogeneous catalyst, UiO-66-P-BF₂.^1,2 This new catalyst retains the catalytic activity of LPs and also avoids the limitations typical for homogeneous catalysts, such as difficulty with catalyst recycling and product separation. The energy favorable pathway for CO₂ hydrogenation to produce formic acid (FA) over UiO-66-P-BF₂ has two steps as revealed by density functional theory (DFT) calculation: (i) dissociation of H₂ to hydridic and protic hydrogens and (ii) CO₂ reacting with hydridic and protic hydrogens to produce FA.^1,2

However, one drawback of UiO-66-P-BF₂ is that CO₂ binds much stronger with Lewis acid-base sites than H₂, which could poison the catalytically active sites for H₂ dissociation. Additionally, a high density of LPs in the pore could lead to mutual quenching, which would deactivate the catalyst. In order to overcome this difficulty, we design a new catalyst we call UiO-67-NBF₂. Our DFT results suggest it has high selectivity for H₂ dissociation and activity for CO₂ hydrogenation to produce methanol. In addition, we have found that the framework of the MOF plays an important role in tuning the adsorption of H₂ and CO₂ at the functional groups. We have also examined a different MOF (MIL-140B) functionalized with NBF₂ in order to study the effect of MOF topology on CO₂ hydrogenation. Furthermore, we have screened hundreds of LPs searching for promising candidates for CO₂ hydrogenation based on their ability of selective dissociation of H₂, rather than adsorption of CO₂. 1. Ye, J.; Johnson, J. K. Design of Lewis Pair-Functionalized Metal Organic Frameworks for CO₂ Hydrogenation. ACS Catal. 2015, 2921-2928.

2. Ye, J.; Johnson, J. K. Screening Lewis Pair Moieties for Catalytic Hydrogenation of CO₂ in Functionalized UiO-66. ACS Catal. 2015, 5 (10), 6219-6229.