460276 Design of Functionalized Metal Organic Frameworks for CO2 Hydrogenation:the Effects of MOF Topology and Functional Group

Friday, November 18, 2016: 8:30 AM
Imperial B (Hilton San Francisco Union Square)
Jingyun Ye1, Benjamin Yeh2, Ronald A Reynolds2 and J. Karl Johnson3, (1)Chemical Engineering & Petroleum, University of Pittsburgh, Pittsburgh, PA, (2)Chemical & Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA, (3)Dept.of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA

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.

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