(368g) Design of Frustrated Lewis Pair Functionalized Metal Organic Frameworks for CO2 Hydrogenation

Design of Frustrated Lewis Pair Functionalized Metal Organic

Frameworks for CO2 Hydrogenation

Jingyun Ye1, Karl Johnson1,2

1Department of Chemical & Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, 15261, USA

2National Energy Technology Laboratory, 626 Cochrans Mill Road, Pittsburgh, PA 15236, USA

Metal organic frameworks (MOFs) are highly versatile nanoporous materials because of their structural and functional tailorability. We use density functional theory (DFT) methods to design functionalized MOFs that are capable of chemically binding and hydrogenating CO2. We employ UiO-66 [shown in Fig. 1(a)] as our starting base material because it is a MOF that has chemical and thermal stability, is highly selective toward CO2 and can be modified (functionalized) post- synthetically. We design functional moieties for CO2 hydrogenation based on frustrated Lewis pairs (FLPs) because these have been shown to be very active for chemisorption of CO2 and the heterolytic dissociation of H2. One candidate FLP is 1-[bis( pentafluorophenyl)boryl]-3,5-di-tert- butyl-1H-pyrazole1,2 [1 shown in Fig. 1(b)]. We have developed a family of functional groups based on a modified version of 1, which we denote 2, as shown in Fig. 1(c). Structure 2 has both Lewis acid and base sites embedded within the molecule, but has much less steric hindrance than the FLP 1 because internal steric hindrance is not needed once the Lewis pair moiety is chemically bound within a MOF. We have examined the properties of UiO-66 functionalized with derivatives of 2 [Fig.
1(d)]. We have replaced the BF2 group on 2 with the following moieties: -BH2, -B(CN)2, -B(CF3)2, - B(NO2)2 and -AlCl2. We computed the electronic structure, charge distribution, and CO2/H2 binding energies and found that chemisorption of CO2 and dissociation of H2 are both energetically favorable with many of the functional groups. We have also investigated UiO-66 having two functional groups per cell in order to provide both CO2 binding sites and H2 dissociation sites in close proximity. We have identified possible scenarios for CO2 hydrogenation through an enzyme-like pathway where one of the functional groups in the pore binds and activates CO2, while the second functional group heterolytically cleaves H2 and supplies the resulting hydride and proton to the activated CO2.

(a) (b) (c) (d)

Fig 1. (a) UiO-66 primitive cell. (b) Structure of 1-[bis( pentafluorophenyl)boryl]-3,5-di-tert- butyl-1H-pyrazole, 1, a frustrated Lewis pair. Color code: gray=C, white=H, blue=N, cyan=F, pink=B. (c) Structure of 1-(difluoroboranyl)-4-methyl-1H-pyrazole, 2, a modified version of 1 (d) UiO-66 functionalized with single instance of 2.
References

1. Theuergarten, E., et.al., Chem. Commun. 2010, 46 (45), 8561-8563.

2. Theuergarten, E., et.al., Dalton Trans. 2012, 41 (30), 9101-9110.