In Aqueous Medium In The Ullmann Reaction And Efficient Nano-mesoporous Structure Of The Catalyst Developed And The Catalytic Mechanism

The recent discovery of novel silica-based mesoporous materials designated asM41S have developed a new class of porous materials exhibiting ordered porousstructure with uniform diameter between 1.5 nm and 10 nm. Such uniquecharacteristics together with their extremely high surface area exhibit potentialapplications in adsorption, separation, biochemistry, and nano-assemly etc. Ascatalysts, their activities could even match the homogeneous catalysts, which showsgood potential in future catalysis owing to the easy separation between the catalystsand reaction products.Today, green chemistry represents general trends owing to the requirements forenvironmental protection. To realize green chemistry, it is necessary to design newcatalysts with high activity and excellent selectivity toward desired products. Besides,the chemical reactions should be performed in clean media since various organicsolvents cause environmental pollution. Of course, the most clean and safety solventis water. However, nearly all the organic synthetic reactions need organic solventsrather than water due to the solubility limitation. Up to now, only the homogeneouscatalysts have been attempted in the organic reactions in aqueous solution andsometimes a phase-transferring catalyst should be used. As mentioned above, thehomogeneous catalysts could not be used repetitively since the separation betweencatalysts and reaction products is usually difficult. Although heterogeneous catalystscould overcome the separation difficulty, their poor activity in comparison withhomogeneous catalysts seems a problem. In this paper, we designed three kinds ofcatalysts, i.e., Pd/MCM-41, Pd/Al-MCM-41 and Pd/Ph-MCM-41 by impregnationfollowing the H2 reduction. Owing to the moseporous structure and high surface area,these catalysts exhibit good activity and selectivity toward biphenyl during couplingreaction between iodobenzene molecules (known as Ullmann coupling reactions) evenin aqueous solution, showing good potential in replacing the homogeneous catalystsfor the clean organic synthesis. Based on various characterizations and kinetic studies,the catalytic mechanism and the relationship between the catalytic properties and thestructure of the as-prepared catalysts are well-discussed. The follows are the details:(1) MCM-41 and Al-MCM-41 with different aluminum contents in MCM-41 skeletonwere synthesized by hydrothermal method. Meanwhile, the Ph-MCM-41 withdifferent Ph-contents was also synthesized through hydrolysis and co-polymerizationby using mixed inorganic silicon and organic silicon connecting with Ph-group assilicon sources. The Pd(II) ions were deposited on these supports by regularimpregnation. The Pd/Si-MCM-41, Pd/Al-MCM-41 and Pd/Ph-MCM-41 catalystswere obtained by reducing their precursors in H2 atmosphere at high temperature.(2) In Chapter 3, the effects of different supports and different Pd loading on thecatalytic activity were examined by using the Ullmann coupling reaction in organicsolvent. The Pd/MCM-41 exhibited much better catalytic properties than the Pd/SiO2.Over 12 wt% Pd/MCM-41, the PhI conversion could reach 90.7% and the biphenylyield may reach 50% after reaction for 10 h, nearly the same as those obtained withthe corresponding homogeneous catalyst.(3) During the Ullmann coupling reaction in aqueous solution over thePd/Al-MCM-41, the conversion of iodobenzene was increased with the increasing ofPd content and Si/Al ratio. The optimum Pd loading and Si/Al ratio were determinedas 12% and 60, respectively, at which the selectivity and yield to biphenyl couldreached 51.0% and 41.6%; The addition and the increase of reductant(Zn powder)dosage resulted in the increase iodobenzene conversion, but the decrease in theselectivity toward biphenyl; The acid and base and their contents also displayedimportant roles on the catalytic activity and selectivity to biphenyl. The KOH wasproved to be the best one for improving the catalytic efficiency and the its optimumcontent in the reaction system was also determined.(4) The Pd/Ph-MCM-41 catalyst was prepared by using Ph-MCM-41 as a support.During the Ullmann coupling reaction in aqueous solution over the Pd/Ph-MCM-41,the iodobenzene conversion and yield to biphenyl was increased with the increasingof phenyl content inlaid in the MCM-41 channel, possibly owing to the modificationof the surface chemistry of the Pd/Ph-MCM-41 catalyst, especially the hydrophilic...