Triphasic liquid systems: generation and segregation of catalytically active Pd nanoparticles in an ammonium-based catalyst-philic phase

Abstract

A triphasic liquid system fabricated from isooctane, aqueous base, and trioctylmethylammonium chloride/decanol promoted the formation of Pd-nanoparticles in the size range of 2-4 nm which remained immobilised in the onium phase, catalysed organic reactions, and could be recycled.Onium salts, that are phase transfer agents, can be used to produce and stabilize metal nanoparticles (NPs) in situ, 1,2 and to control their size, 3 starting from appropriate precursors. Reetz and coworkers 2 have amply demonstrated that nanostructured R 4 N + X 2 -stabilized metal clusters can be prepared by a variety of methods. Recently, peculiar liquid systems formed by three separate liquid phases have attracted our attention, in particular with a view towards the possible effects on catalysis. 4 The interest was stimulated by knowledge of the formation of triphasic liquid systems made by mixtures of an organic solvent such as isooctane, water, and an onium salt (ammonium or phosphonium), and their use in heterogeneously catalysed organic reactions. 5 Increases in rates and selectivities could be achieved for a number of reactions catalysed, for example, by supported metals such as Pd, Pt, and by Raney-Ni. The onium salt was preferably liquid at the reaction temperature (20-50 uC), had a strong affinity for the catalyst, and formed a separate phase. The interaction between the liquid onium salt, and the catalyst was shown to be the factor controlling the improved catalytic effect. 6 In addition, the strong affinity of the onium salt for the catalyst made it a catalyst-philic phase, that simplified the separation of products from catalysts and from byproducts once the reaction was complete. 4 We reasoned that it would be interesting to take advantage of triphasic liquid systems, to generate, immobilise, and stabilise catalytically active metal NPs, starting from precursor metal complexes. The outcome was envisaged as shown in Fig. 1.This system offers a number of potential advantages in view of process intensification: (i) the active catalytic NPs can be prepared directly in the reactor, (ii) it is easy to separate the organic products from the catalyst by decanting, (iii) the catalyst morphology can be tuned, (iv) the metal NPs are stabilized by the third phase, and (v) inorganic reagents (i.e. base) and by-products remain in the aqueous phase, not to mention that the amount of catalyst-philic phase can be as small as convenient. All these issues are the basis for cleaner, cheaper catalytic systems, 7 and characterize some of the principles of green chemistry.This communication describes how a ternary system of this kind can be constructed, and used for catalytic reactions, through five subsequent steps: the preparation of a suitable ternary liquid system, formation of the active catalytic species, screening on a model organic reaction, catalyst recovery and recycle tests, and finally the characterisation of the active catalytic species by TEM.At first, the conditions for the reproducible formation of a model triphasic system wer...

Phase behaviour of trihexyl(tetradecyl)phosphonium chloride, nonane and water

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Triphasic Liquid Systems for Improved Separations. Trioctylmethylammonium Chloride‐Immobilised Rhodium Trichloride: A Phosphine‐Free Hydroformylation Catalytic System

“…These were catalytically active for selective hydrodehalogenation and Heck coupling reactions, and allowed easy productcatalyst separation thanks to triphasic separation, as well as straightforward catalyst recycle. [12] A biphasic system for catalytic hydroformylation was also reported, based on the use of water-soluble protein-rhodium complexes. [13] In this instance, the catalytic system allowed facile recovery of the Rh-HSA (human serum albumin) complex with high chemo-and regioselectivity in the hydroformylation reaction.…”

Ionic Liquids Made with Dimethyl Carbonate: Solvents as well as Boosted Basic Catalysts for the Michael Reaction

“…Here we describe a route based on the use of DMC for the synthesis for a series of new ionic liquids, in view of applications in multiphase catalysis, [22] nanoparticle formation, [23] and IL-mediated organic synthesis. Some of these ionic liquids showed peculiar and very remarkable built-in basicity and were investigated as catalysts for the formation of carbonucleophiles.…”