Aqueous medium Ullmann reaction over a novel Pd/Ph–Al-MCM-41 as a new route of clean organic synthesis

Abstract

By using the Ph- and Al-functionalized MCM-41 (Ph–Al-MCM-41) as a support, a new and powerful Pd/Ph–Al-MCM-41 catalyst was designed and used in iodobenzene (Ar–I) Ullmann coupling reaction with the attempt to develop new routes of clean organic synthesis by using water instead of organic solvents as the reaction medium. The as-prepared Pd/Ph–Al-MCM-41 exhibited higher activity, better selectivity to biphenyl (Ar–Ar) and stronger durability than other supported Pd catalysts and the maximum Ar–Ar yield reached 79.8% at the optimum Ph/Al/Si molar ratio (15/1/60). Meanwhile, the Pd/Ph–Al-MCM-41 catalyst could be recycled 5 times, indicating the good stability, especially good hydrothermal stability. According to various characterizations, such as FTIR, XRD, TEM, ICP, NMR, N₂ sorption isotherms, the correlation of the catalytic performance to the structural characteristics of the Pd/Ph–Al-MCM-41 was discussed. Besides the high dispersion of Pd active sites on the surface and the easy diffusion in the pore channels of the Ph–Al-MCM-41 owing to the mesoporous structure, the higher activity of the Pd/Ph–Al-MCM-41 could be attributed to the increase of both the surface hydrophobicity resulting from Ph-modification and the surface Lewis acidity owing to the Al-modification, which were favorable for the adsorption of the Ar–I molecules, and thus enhanced the activity. The surface hydrophobicity could also account for the promotion on the selectivity to Ar–Ar by the Ph-modification, since less water could enter the pore channels, which was favorable for Ar–I dehalogenation to form the Ar–H byproduct. The promoting effect of the Al-modification on the selectivity was limited since the Lewis acidic site could adsorb Ar–I molecules in two modes, which were favorable for both the coupling reaction to form Ar–Ar and dehalogenation to form Ar–H. The hydrothermal stability test demonstrated that both the Ph-groups and Al-dopants could stabilize the mesoporous structure in aqueous solution, which could account for the strong durability of the Pd/Ph–Al-MCM-41 catalyst.