The role of lithium tetrahydroaluminate in the formation of nanodimensional nickel hydrogenation catalysts

The principal stages of formation and the nature of nanoparticles of the reduced nickel formed in the system Ni(acac)2–LiAlH4 and catalytically active in hydrogenation catalysis are established. The reduction of Ni(II) to Ni(0) under the action of LiAlH4 is shown; the Ni(0) atoms form clusters whose growth results in the formation of nanoparticles of the average size of 2–3 nm. Being adsorbed on the surface, the tetrahydroaluminate anions act as stabilizers of the nickel nanoparticles. Besides, the tetrahydroaluminate anions and the products of catalytic decomposition of LiAlH4 (LinAln+2H2n+2), by occupying free positions on the surface, can be active as catalytic poisons. Highly efficient hydrogenation catalysts with well reproduced properties are obtained by the reaction of the products formed in the system Ni(acac)2–LiAlH4 with proton-donating compounds (water, alcohols, weak acids). Alcoholysis (hydrolysis) of the surface compounds results in their dissolution, release of the centers of the catalytically active nickel nanoparticles and their stabilization.

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► Formation of nickel hydrogenation catalysts under the action of LiAlH4.
► Nanoparticles formed in the system Ni(acac)2–LiAlH4 are inactive in the hydrogenation.
► LiAlH4 is reducing agent, stabilizer of nanoparticles and catalytic poison.
► The promoting effect of proton containing compounds.
► Strategy for synthesis of the highly effective hydrogenation nickel catalysts.