A transition-metal-free hydrogenation catalyst: Pore-confined sodium alanate for the hydrogenation of alkynes and alkenes

- A NaAlH4/C nanocomposite is active in alkene and alkyne hydrogenation at 150 °C, 100 bar H2.
- The catalyst is active in the hydrogenation of a number of substrates with different functionalities.
- A toluene solvent yields a lower activity than cyclohexane due to competitive adsorption.
- Isomerization from cis- to trans-stilbene is catalyzed by the catalyst.
- Pre-treatment under an argon atmosphere gives a higher selectivity towards cis-stilbene.

Hydrogenation catalysis is dominated by transition metals, but interest in alternative catalysts has been growing over recent years. Herein, a transition-metal-free catalyst is discussed consisting of carbon supported NaAlH4 as a selective catalyst for hydrogenation. This is illustrated using a range of substrates, and in more detail for the case of diphenylacetylene. Catalytic activity depends on the solvent utilized; in cyclohexane the activity is 2.3 mol (DPA) mol−1 (NaAlH4) h−1 at 100 bar H2, 150 °C with a slight preference for the formation of trans-stilbene. The catalyst selectivity is influenced by the loading, yielding a high selectivity toward the thermodynamically less stable cis-stilbene at low catalyst loadings. This proof of principle shows promise for using metal hydrides based on earth-abundant elements as effective hydrogenation catalysts.

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