DOI: 10.1002/cssc.200800152 Recovery of Enlarged Olefin Metathesis Catalysts by Nanofiltration in an Eco-Friendly Solvent Adel Keraani, [a] Thierry Renouard, [a] CØdric Fischmeister,* [b] Christian Bruneau, [b] and Murielle Rabiller-Baudry* [a] Introduction The development of chemical reactions catalysed by homoge- neous organometallic complexes has allowed a significant breakthrough in numerous fields of synthetic chemistry. [1] The search for more efficient catalysts is one of the major goals of homogeneous catalysis, but, in addition, novel research axes have to integrate catalyst recycling strategies at the early stage of their conception. A dual objective has then to be reached: 1) to increase their productivity and simultaneously lower their environmental impact as a result of inadequate reuse of the catalyst, and 2) to decrease the residual metal content in the reaction products, as that will influence the purification steps and their resulting downstream treatment. To address these problems, a current solution consists in designing highly effi- cient catalysts that can be used in “homeopathic” amounts and finally left in the final product as trace amounts. However, this approach cannot be extended to the entire field of homo- geneous catalysis. Furthermore, very low catalyst loadings usu- ally require ultrapurification of reagents and solvents, which has environmental (energy, solvent, wastes) and economic im- plications (costs). A second strategy involves methods that allow the catalysts to be separated from the reaction products and then eventually reused. The solution mainly consists of im- mobilisation of the catalyst to allow simple extraction and re- cycling by phase separation. Numerous catalysts have been heterogenised by grafting onto soluble (polymers [2] or den- drimers [3] ) or insoluble (silica, alumina) [4] supports. Other possi- bilities include immobilisation of the catalyst in non-conven- tional solvents, [5] such as ionic liquids, [6] supercritical fluids, [7] perfluoroalkanes [8] or water. [9] Although these techniques have led to significant breakthroughs in the recycling of homogene- ous catalysts, their scale-up is difficult and the modified cata- lysts suffer from some depletion of activity as compared to their homogeneous homologues. Among homogeneous catalytic processes, olefin metathesis is of particular interest. [10] During the last 20 years, tremendous efforts have been focussed on this reaction which has become one of the major tools for organic [11] and polymer chemists. [12] Two types of complexes are well known to catalyse metathesis reactions. Those based on the early transition metals are very active, but their sensitivity towards polar functional groups is a problem for further recycling applications. Meanwhile, rutheni- um-based catalysts are more stable and resistant, thus poten- tially recyclable. For an overall eco-friendly approach, particular attention has to be paid to the solvent. Toluene and dichloro- methane are conventional solvents very often used for meta- thesis reactions, but they have a significant environmental impact owing to their toxicity. Recently, dimethyl carbonate (DMC) was shown to be an eco-friendly solvent that is compat- ible with metathesis transformations. [13] Membrane processes are well-known separation processes widely used at the industrial scale in aqueous media for water treatment as well as in the food industry for both concentra- tion and specific extractions of target molecules. The separa- tion driving force is the pressure difference, the so-called trans- This study was aimed at integrating a green separation process without phase change, namely nanofiltration, with olefin meta- thesis to recover the homogeneous catalyst. As the commercially available Hoveyda II catalyst was not sufficiently retained by the membrane, a set of homogeneous ruthenium-based catalysts were prepared to enhance the recovery of the catalyst by sol- vent-resistant commercial membranes made of polyimide (Star- mem 228). The molecular weights of the catalysts were gradually increased from 627 to 2195 g mol À1 , and recovery was found to increase from around 70 % to 90 % both in toluene and dimethyl carbonate. The most retained catalyst was then engaged in a series of model ring-closing metathesis reactions associated to a final nanofiltration step to recover and recycle the catalyst. Up to five cycles could be performed before a deterioration in the per- formance of the process was observed. [a] A. Keraani, Dr. T. Renouard, Prof. M. Rabiller-Baudry CNRS – UMR “Sciences Chimiques de Rennes” UniversitØ Rennes 1, Equipe Chimie et IngØnierie des ProcØdØs 263 avenue du gØnØral Leclerc, CS 74205, Bâtiment 10A, Case 1011 35042 Rennes cedex (France) Fax: (+ 33) 22-323-5765 E-mail : murielle.rabiller-baudry@univ-rennes1.fr [b] Dr. C. Fischmeister, Dr. C. Bruneau CNRS – UMR “Sciences Chimiques de Rennes” UniversitØ Rennes 1, Laboratoire Catalyse et OrganomØtalliques 263 avenue du gØnØral Leclerc, Bâtiment 10C 35042 Rennes cedex (France) Fax: (+ 33) 22-323-6939 E-mail : cedric.fischmeister@univ-rennes1.fr ChemSusChem 2008, 1, 927 – 933  2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 927