C L E A N E N E R G Y T R A N S I T I O N A N D S U S T A I N A B L E T E C H N O L O G I E S

S C I E N T I F I C H I G H L I G H T S

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X-ray spectroscopy shows ethylene- dimerising Pd-zeolites drastically improve HCN transfer reactions with propionitrile

Homogeneous transition metal complexes catalyse the transfer of hydrogen cyanide and aldehyde functionalities between two molecules, but are limited by thermodynamics. In-situ X-ray absorption spectroscopy revealed that Pd-zeolites overrule these thermodynamic limits by shape-selectively consuming one reaction product, greatly increasing the yields and synthetic utility of this reaction.

In the chemical industry, shuttle reactions entail an upcoming family of homogeneously catalysed reactions that transfer a functional group (e.g., a nitrile or aldehyde) from a donor molecule to an acceptor molecule, typically an olefin [1]. This strategy offers the great promise that highly toxic and hazardous reactants, like HCN or CO, can be avoided in labs or industries [1,2]. One of the biggest hurdles inherent to this type of catalysis is that, given transfer reactions are generally isodesmic, an equilibrium will be formed during the course of the reaction. For instance, Pd-catalysed removal of the cyano group from propionitrile to form ethylene is thermodynamically demanding and the equilibrium of the transfer reaction is largely at the left [2]. Thus, the scope of these reactions is restricted to specific donor and acceptor combinations that lead to a thermodynamically favourable outcome.

This work proposes to shift the equilibria of these transfer reactions by adding a zeolite catalyst. A well-selected zeolite irreversibly converts the olefin co-product of the transfer reaction, resulting in dramatic increases of the yield of the desired product(s). Critically, neither the olefin reactant nor the homogeneous transfer catalyst should be able to access the catalytic sites in the zeolites, while the small olefin co-product, e.g., ethylene, should smoothly enter the zeolite pores. This calls for strict control of pore sizes as a function of the co-product (Figure 90).

Extensive zeolite screening revealed that acidic, medium- pore zeolites such as H-ZSM-5 (MFI) and H-ZSM-48 (*MRE) with high Si/Al ratios (>100) drastically increase the yield of transfer hydrocyanation reactions with simple nitrile donors like propionitrile. For example, in the synthesis of a precursor of Lercanidipine, a heart disease drug, only traces of the nitrile product were observed in the absence of zeolite. By adding one of the aforementioned zeolites, the yield was improved from <2% up to 56%. In another example, the presence of these zeolites was shown to diminish the necessary reaction temperatures of nitrile transfer reactions, from up to 140°C as reported in literature with butyronitrile [3] down to just 80°C with propionitrile. Subsequent studies showed that the zeolite s remarkable thermodynamic effect is grounded

Fig. 90: Conceptual blueprint of a zeolite working as a shape-selective, equilibrium-shifting agent, exemplified by the transfer hydrocyanation of 1,1-diphenylethylene.