Porous Organic Frameworks Supported Noble Metal Catalysts Prepared By Plasma

Porous organic framework materials，with metal organic frameworks (MOFs),covalent organic frameworks (COFs) and covalent organic polymers (COPs) as therepresentatives，offer a wide range of challenges and opportunities for the researchersin various fields. These materials are usually with high specific surface area and porevolume, variable pore structure, as well as low density and high porosity. In additionto applications in gas separation, gas storage, photoelectric conversion, drug delivery,they also have a promising prospect of application in the heterogeneous catalyticdomain. In this paper, porous organic frameworks were used as catalyst carriers.Activity of the catalysts was tested with two model reactions, the selective oxidationof benzyl alcohol to benzaldehyde and the catalytic oxidation of CO.Application of COPs as catalyst carrier is studied for the first time. First of all,with distilled water and N,N-dimethyl-formamide(DMF) as solvent respectively, twoPd/COP-4specimens were prepared by immersion method and glow dischargetreatment. Because COP-4has regular pore structure, and glow discharge can processat room temperature, Pd particles were highly dispersed with uniform particle size.For the remarkable hydrophobicity of COP-4, solvents have important influence onthe property of the Pd/COP-4specimens. With water as the solvent, Pd particlesdistributed outside the COP-4channel, with mean size of2.4nm; and in the solvent ofDMF, Pd particles were successfully into the COP-4channel, with size of1.1nm.Obviously, the channel of COP-4limited the growth of Pd particles. Since COP-4contains N atoms which have fixed effect on Pd particles, then even if there was nochannel constraint, no agglomeration of Pd particles happened. Therefore, COP-4is agood carrier material for preparation of ultra fine and uniform nanoparticles.Then two Pd/COP-4catalysts were prepared by impregnation method andhydrogen reduction. Compared with the plasma treated samples, solvent effect wasfurther studied, and the interaction among DMF, COP-4and Pd particles was explored.The catalytic activity of the Pd/COP-4samples was investigated by the oxidation ofbenzyl alcohol and the CO oxidation reaction. The characterization results implied,the relative high temperature and H2flow would destroy the interaction between Pdparticles and COP-4, leading to different dispersion of Pd particles. The Pd particles reduced by H2were much larger. However, high activity in the oxidation of benzylalcohol with TOF value up to95411h-1, further confirmed that COP-4was anexcellent catalyst support.COP-2is an analogue of COP-4. Ag/COP-2and Pd/COP-2samples wereprepared by glow discharge treatment. Meanwhile, Ag/COP-4and Au/COP-4sampleswere prepared, too. Results indicated that, Pd particles had better dispersion in COP-4,while Ag particles in COP-2. It implied that, the properties of COPs could bemodulated by changing the monomer, and the properties of COPs would affect thedistribution of the particles. The COPs supported noble metal catalysts had highactivity in the oxidation of benzyl alcohol.MIL-101, a kind of mesoporous MOF, with Langmuir specific surface area up to5900m2/g and good thermal stability, is favored in catalysis. Pd/MIL-101andPt-Pd/MIL-101were prepared through glow discharge for the first time, and theiractivity was tested by the oxidation of benzyl alcohol and CO oxidation. Thecharacterization results indicated ultrafine metal particles could be obtained in thechannel of MIL-101, whose structure kept intact. The addition of Pt will cause theredistribution of the Pd particles, resulting in better dispersion of bimetallic particles.IR results showed that Pd/MIL-101absorbed carbon species strongly, whilePt-Pd/MIL-101greatly weakened. Perhaps so, Pd/MIL-101had relatively low activityfor CO oxidation, while much higher activity for the oxidation of benzyl alcohol.MOF-5is a kind of excellent MOF too, but its poor water resistance and longersynthesis time limit its application in catalysis. However, through rapid synthesismethod, improved MOF-5can be obtained with relatively low specific surface areabut better hydrothermal stability. Pd/MOF-5catalyst with highly dispersed particlescan be prepared with the help of glow discharge. However, the activity of Pd/MOF-5in benzyl alcohol oxidation is limited by its small pore size.Plasma has good performance for preparation of porous organic frameworksupported noble metal catalysts. Under the assistance of plasma treatment, applicationof porous organic framework materials in catalysis can be further strengthened.