Shape selective C-H activation using Molecular Palladium in Zeolites
An international collaborative study led by the Centre for Membrane separations, Adsorption, Catalysis and Spectroscopy for Sustainable Solutions (cMACS) at KU Leuven, in collaboration with the Center for Molecular Modeling (CMM) at Ghent University, the Smart Materials Research Institute at the Southern Federal University (Russia) and the National Institute of Chemistry (Slovenia) has shown that cationic palladium hosted in zeolites can catalyse C-H functionalization reactions with unprecedented shape-selectivity. The findings introduce a novel concept to the field of catalysis, in which exceptional control of selectivity can be achieved in transition metal (TM) catalysed arene C-H activation through the spatial confinement of TM sites in zeolite pores.
An ongoing challenge in the fine chemical industry is to design and identify catalytic systems that are both highly recyclable and easy to separate from products, and can exhibit high selectivity and activity towards the target transformation. In this context, TM-exchanged zeolites are promising candidates as they offer the possibility to combine the strengths of homogeneous and heterogeneous catalysis to afford a superior class of catalyst for use in industry. A particular reaction of high interest in industry is the catalytic coupling of aromatics to form biaryl motifs, which are extensively found in fine chemicals. Under traditional cross-coupling conditions the aromatic reagents must be pre-functionalized, therefore leading to extra steps and costs in the synthesis. For this reason, in recent years attention has shifted towards the direct C-H C-H coupling of aromatics. Homogeneous catalysts are now very well-developed for this purpose, but the development of analogous single-site heterogeneous catalysts can offer unique advantages. For example, the microscopic pores and channels present in the zeolite catalyst may direct regioselective activation of specific C-H bonds, while site isolation of the active metals can limit the catalyst deactivation caused by the aggregation of metallic nanoparticles.
Researchers at KU Leuven identified that cationic palladium can dock to zeolite H-Beta in a stable fashion, and effectively catalyse the oxidative homocoupling of toluene to produce bitolyl in high turnover numbers. Moreover, of the 6 possible isomers that could be formed in the reaction, p,p’-bitolyl is produced with an unprecedented 80 % selectivity, while the yield of products featuring at least one para-substituted aryl group reaches a staggering 97 %. Interested in both the mechanism of this Pd-Beta catalysed coupling reaction and the origin of this unprecedented shape-selectivity observed for the oxidative homocoupling of toluene, researchers at the CMM employed a combination of static and dynamic Density Functional Theory (DFT) calculations on the Pd-Beta system to investigate the reaction. The proposed catalytic cycle, supported by advanced experimental characterization techniques, shows how the confinement effects of the framework can induce the exceptional selectivity, thus leading the way to further development in the field of zeolite-supported single-site transition metal catalysts. The findings have been published in the journal Nature Catalysis. Read the article here.