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

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, N-2 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.