Investigation of the impact of crystal sizes of Metal-Organic Frameworks on their heterogeneous catalytic activity for oxidation reactions

Abstract

Metal-organic frameworks (MOFs) are porous crystalline materials comprised of metal nodes spanned by organic linkers with desirable properties for catalysis, such as high metal content and surface areas. MOFs crystals grow through self-assembly of their key components. Coordination modulation offers an efficient way to control crystal growth and was utilized in this work for the synthesis of HKUST-1 (Hong Kong University of Science and Technology)- a framework that consists of copper ions and 1,3,5-benzene tricarboxylic acid (BTC)-to scale the micron to nanometer regimes. By varying equivalents of benzoic and dodecanoic acid, HKUST-1 crystals with sizes ranging from 220 nm to 1.5 μm were synthesized. The diffusion of reactants to the active surface area within porous catalyst particles is a crucial step in liquid-phase organic oxidation reactions. Using reaction-diffusion theory, as-synthesized crystal sizes were then used to model diffusion and reaction within HKUST-1 crystals. The transport of reactants to the active surface areas by diffusion limited impaired the conversion of cyclooctene when sizes were larger than 300 nm.