Selective Synthesis of Primary Anilines from NH₃ and Cyclohexanones by Utilizing Preferential Adsorption of Styrene on the Pd Nanoparticle Surface

Abstract

Dehydrogenative aromatization is one of the attractive alternative methods for directly synthesizing primary anilines from NH₃ and cyclohexanones. However, the selective synthesis of primary anilines is quite difficult because the desired primary aniline products and the cyclohexanone substrates readily undergo condensation affording the corresponding imines (i.e., N-cyclohexylidene-anilines), followed by hydrogenation to produce N-cyclohexylanilines as the major products. In this study, primary anilines were selectively synthesized in the presence of supported Pd nanoparticle catalysts (e.g., Pd/HAP, HAP=hydroxyapatite, Ca₁₀ (PO₄ )₆ (OH)₂ ) by utilizing competitive adsorption unique to heterogeneous catalysis; in other words, when styrene was used as a hydrogen acceptor, which preferentially adsorbs on the Pd nanoparticle surface in the presence of N-cyclohexylidene-anilines, various structurally diverse primary anilines were selectively synthesized from readily accessible NH₃ and cyclohexanones. The Pd/HAP catalyst was reused several times though its catalytic performance gradually declined.