Preparation Of Palladium And Ruthenium Nanoparticle Catalysts In Halogen-free-anion Ionic Liquids And Study Of Their Catalytic Properties

Transition metal nanoparticles (NPs) are attracting much interest due to their enhancedcatalytic activity that arises from their high surface area and rotational degrees of freedomcaused by their nanoscale dimensions. Nevertheless, NPs show a tendency forself-agglomeration, resulting in a decrease in their catalytic activity during reaction processes.To circumvent this problem, the one of most attractive methods is currently the preparation ofNPs in the presence of stabilizers. Ionic liquids (ILs) have emerged as important stabilizingand recyclable agents for metal NPs, simultaneously avoiding the use of highly toxic solvents.Hence, transition metal NPs stabilized in ILs have been employed for hydrogenation,oxidation, C C coupling reactions, and so on. To date, halogen-anion ILs have beenpredominantly adopted as stabilizers for the preparation of metal NP catalysts for chemicalreactions. However, halides are the most common and persistent contaminants in theenvironment. Thus, there is an urgent need for the development of highly active nanocatalystsstabilized by ILs based on halogen-free anions. In this paper, a1-butyl-3-methylimidazolium-lactate IL-stabilized palladium NP (PdNPs@[Bmim]Lac) and a1-butyl-3-methylimidazolium-tartrate IL-stabilized ruthenium NP (RuNPs@[Bmim]Tar)catalyst were prepared and systematically characterized and their catalytic properties werealso studied.The PdNPs@[Bmim]Lac catalyst was prepared and systematically characterized. Theeffect of the molar ratio of [Bmim]Lac to Pd(OAc)2, temperature and the type of alcohol onthe size of the PdNPs was examined. TEM of the resulted PdNPs reveals that the size ofPdNPs increases with the molar ratios of [Bmim]Lac/Pd(OAc)2decreasing and temperatureincreasing. When ethanol was used as a reducing agent the size of the PdNPs is smallest. Thus,the appropriate prepared conditions are [Bmim]Lac/Pd(OAc)2(mol/mol,20:1), ethanol usedas a reducing agent and room temperature and the size of PdNPs about1.5nm is obtainedunder the conditions.The activity of PdNPs@[Bmim]Lac catalyst was tested by Suzuki–Miyaura andHeck-Mizoroki reactions. The effect of bases, solvents, reaction temperature and amount of catalyst on reactions were all discussed in detail. It is found that PdNPs@[Bmim]Lac catalystshows high efficiency and good functional group tolerance in the Suzuki–Miyaura reaction atroom temperature under the optimized conditions: aryl halide (1.0mmol), arylboronic acid(1.1mmol), Na3PO4·12H2O (3.0mmol), EtOH/H2O (2:1;6.0mL), and PdNPs@[Bmim]Lac(1.0mol%of Pd) in air. Aryl bromides and iodides bearing electron-donating (-CH3,-OCH3),electron-neutral (H), and electron-withdrawing (-NO2,-CHO,-CN) groups are all efficientlycoupled with phenylboronic acid to afford the desired biaryls in good to excellent yields. Inaddition, the coupling reaction can be efficiently accomplished with aryl boronic acidsbearing electron-donating (-CH3) or electron-withdrawing groups (-Cl). The catalyst can beused for eight runs and retained its catalytic activity in the Suzuki–Miyaura reaction. ThePdNPs@[Bmim]Lac catalyst can also catalyze efficiently the phenyl bromides and iodidessubstituted with electron-withdrawing (-NO2,-CN,-CHO), electron-neutral group (H) orelectron-donating groups (-OCH3,-CH3) coupling well with styrene and give thecorresponding products of high yields under the optimized conditions of the Heck-Mizoroki:1.0mmol aryl halide,1.5mmol olefin,2.0mol%catalyst PdNPs@[Bmim]Lac,3.0mmolEt3N and6.0mL DMF/H2O (4:1) at90oC. The iodobenzene with electron-donating groups(-OCH3,-CH3) is very active with olefins such as acrylic acid and methyl, butyl acrylate. Thecatalyst can be recycled up to six consecutive times in the Heck-Mizoroki reaction.RuNPs@[Bmim]Tar catalyst was prepared and systematically characterized. The effect ofthe type of alcohol and the molar ratio of [Bmim]Tar to RuCl3·3H2O on the size of the RuNPswas examined. It is found that the size of RuNPs increases with the molar ratios of[Bmim]Tar/RuCl3·3H2O decreasing and the size of RuNPs about3.0nm is smallest usingmethanol as a reducing agent at130℃. The RuNPs@[Bmim]Tar catalyst can catalyzeefficiently the hydrogenation and oligomerization reaction of phenylacetylene substituted with-Cl, H,-OCH3, or-CH3group under the optimized conditions: aryl acetylene (1.5mmol),Na3PO4·12H2O (1.5mmol) and EG/H2O=3:1(4.0mL) and RuNPs@[Bmim]Tar catalyst (2mol%of Ru) at90℃under air. The catalyst can be recycled up to six consecutive times.