A novel method for the separation of heterogeneous catalysts from liquid-phase reactions in High Throughput Experimentation (HTE) libraries was developed based on a magnetic recuperation procedure. Separation of heterogeneous catalysts from their reaction mixture is an essential step both for catalyst regeneration and for sampling in time from a batch reactor with the aim of determining the kinetic parameters of the chosen reaction. In a conventional approach, separation is achieved through centrifugation and/or filtration steps, which are difficult to automate and to apply with HTE catalyst libraries. Magnetic separation represents an attractive alternative for HTE applications. In this work, a library of ferromagnetic zeolite catalysts was prepared by impregnation of a set of H-zeolite structures with different amounts of iron particles (Fe3+) followed by calcination and reduction under H2. These magnetic zeolites can be efficiently stirred in the catalytic reaction mixture using conventional magnetic stirring bars (magnetic field of 15-25mT) and they automatically separate from the reaction mixture by depositing on the magnetic bar when the stirring is stopped. In order to reach 100% recovery using conventional magnetic stirring bars a minimum iron content of 10 wt.% is required. TEM analysis shows that the iron (Fe0) particles have an average size of ~10 nm, implying that they are too large to fit inside the zeolite micropores and, therefore, are located on the exterior surface of the zeolite crystals, where the interference with the catalytic active sites is limited.
For the efficient application of magnetic separation in HTE studies of libraries of catalysts, the activity of the heterogeneous catalysts should not be affected significantly by the presence of the iron particles. To investigate this, the library of 10%Fe magnetic zeolites was tested in the acid-catalysed benzylation of toluene with benzyl alcohol and the results were compared to those obtained with the corresponding untreated zeolites. Reaction conditions were: 5 ml toluene and 1 mmol benzyl alcohol with 100 mg of catalyst, at 80ºC. The catalytic test of the library of magnetic zeolites was performed on a HTE automated liquid-handling workstation. Separation of the catalyst from the reaction medium when stopping the magnetic stirring, followed by sampling via septum piercing and transfer to a 98 position rack, took around 17 s for each reactor. The conversions and products yields were determined by Ultra-fast GC analysis (3 min per sample). All the magnetic zeolites showed comparable but slightly lower activity than the unmodified samples. Such decrease is ascribed to the covering of some of the active Brönsted acid sites located at the external surface and/or partial blockage of the pore entrances by the iron particles. The selectivity among the mono-alkylated isomers obtained with the magnetic zeolites is analogous to that found with the unmodified materials. The highest catalytic activity was observed with an Ultrastable zeolite Y (USY), presenting large pores and an optimum combination of Brönsted site concentration and strength. Smaller pore zeolites, ZSM 5 and Mordenite, display higher selectivity towards the less bulky para-benzyltoluene, while with the USY zeolites the ortho- and the para-isomers are obtained with similar yields.
In summary, a novel and efficient method for the separation of heterogeneous catalysts from liquid-phase reactions in HTE libraries was achieved through the introduction of a ferromagnetic function into the catalysts, without significantly affecting their catalytic performance.