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...

Upgrade of Biomass-Derived Levulinic Acid via Ru/C-Catalyzed Hydrogenation to γ-Valerolactone in Aqueous–Organic–Ionic Liquids Multiphase Systems

“…Notwithstanding that the reaction was not quantitative, a remarkable change of the reacting mixture was observed: although the starting mixture of isooctane, water, and [N 8,8,8,1 ][Cl] with RuCl 3 appeared biphasic initially, it became triphasic, however, once subjected for 16 h to the reaction conditions, as shown in Figure 7. On the basis of our previous experience with homogeneous Pd catalyst precursors that over time formed catalytically active nanoparticles in multiphase systems, 34 we therefore conducted a number of recycling experiments on the triphasic mixture. These were carried out by the same method described previously (Figure 5): after a first reaction catalyzed by RuCl 3 , the lower aqueous phase containing the product GVL was removed and a fresh aqueous layer containing LA was added.…”

“…An analogous behavior was observed by us on previous investigation of Ru-and Pdmultiphase systems. 33,34 Further studies of this aspect appeared beyond the scope of the present paper.…”

“…Experiments demonstrated that such metal nanoparticles behaved as the active species and they could be easily recovered and recycled. In this case the best operative conditions were so identified: 150 °C, [Cl] as the IL phase was based on our previous experience: 34 this ionic liquid was chosen notwithstanding that it was slightly partitioned in the aqueous phase (Table 3), mainly for its affinity with RuCl 3 . The performance of such a "homogeneous-multiphase" system was evaluated as a function of the IL/Cat ratio.…”

“…On the basis of our previous experience with homogeneous Pd catalyst precursors that over time formed catalytically active nanoparticles in multiphase systems, 34 we therefore conducted a number of recycling experiments on the triphasic mixture. These were carried out by the same method described previously ( Figure 5): after a first reaction catalyzed by RuCl 3 , the lower aqueous phase containing the product GVL was removed and a fresh aqueous layer containing LA was added.…”

“…For example, trioctylmethylammonium chloride [N 8,8,8,1 ][Cl] was used previously for the formation of catalytic Pd nanoparticles and allowed easy product−catalyst separation as well as straightforward catalyst recycle, thanks to built-in separation. 34 Another triphasic organic/aqueous/IL (again with [N 8,8,8,1 ][Cl]) system was used by us for the hydroformylation reaction. 33 In this case also, simple recovery and recycle of the catalytic system for repeated runs was possible, without detectable loss of activity, and without any leaching of metal to the organic phase.…”