Effect of the ligand functionalization on the acid-base properties of flexible MOFs

By Béatrice Moulin, Fabrice Salles, Sandrine Bourrelly, Philip L. Llewellyn, Thomas Devic, Patricia Horcajada, Christian Serre, Guillaume Clet, Jean-Claude Lavalley, Marco Daturi, Guillaume Maurin and Alexandre Vimont

DOI: 10.1016/j.micromeso.2014.04.025

Highlights

1. IR and modelling approaches for specify the dual interaction of propyne in MIL-53.
2. Halogen atom acts as basic site toward propyne in functionalized MOFs.
3. Free hydroxyl interact with propyne as hydrogen donor but also as basic site.

Abstract

The natureof the sites contributing to the acid-base properties of the functionalized highly flexible MIL-53(Fe)-X MOFs, X= CH3, Cl, Br was assessed by Infra-red measurements usingpropyne as a probe molecule since it could act as a proton acceptor via its C≡C triple bond and as a proton donor through the ≡C-H group. These experiments performed in a wide range of propyne concentrationwere coupled with Density Functional Theory (DFT) calculations performed for all the propyne/MIL-53(Fe) systems based on structures simulated from the in situ X-ray Powder diffraction data. This joint experimental-modelling approach evidenced that the bridging hydroxyl groupspresent inthe MOF structurefree of any intra-framework interactions play a preponderant role in the interaction with propyne as hydrogen donor but also as basic sites through the O atom for all functionalized MIL-53(Fe) solids. For X= Cl and Br, an additional interaction is evidenced at higher propyne concentration, involving the halogen atom, which plays the role of basic site toward propyne. These findings are further discussed in light of complementary experimental data issued from gravimetry and microcalorimetry measurements.

Keywords

Metal Organic Framework; Propyne; Basicity; Infrared; DFT calculations;