Supported Catalysts Leaching versus Recycling

The idea that Mizoroki-Heck reactions in the presence of immobilized precatalysts are catalysed by homogeneous palladium species formed through leaching was declared by Shmidt and Mametova as early as 1996 [172] and further corroborated by subsequent studies [32]. Currently, we believe that no conclusive evidence of the opposite exists  

The knowledge accumulated so far on supported catalysts in Mizoroki-Heck reactions leaves little room for doubt that these reactions involve leaching of palladium and are actually catalysed by homogeneous dissolved palladium complexes. Thus, supported systems provide another example of SRPCs, which, however, pursue a different goal than common nonsupported SRPCs. 

Palladium complexes, immobilized through appropriate linkers on various supports, might be regarded as a palladium complex precursor. In our opinion, from the viewpoint of reactivity and catalytic performance, there is no significant difference between these and other common precatalysts. All of them enter into the Mizoroki-Heck catalytic cycle via a preactivation procedure. 

Palladium salts or complexes directly deposited on an appropriate support without a linker, but rather through surface groups native to the support material. These pre-catalysts are cheaper than the aforementioned systems. Although these possess a less understandable and more disordered structure, their activity in Mizoroki-Heck reactions (type 1 and, very rarely, type 2 systems) is comparable to precatalyst of more sophisticated design and architecture. 

Palladium metal particles (nanoparticles) embedded into various supports. While such catalysts can be specially prepared, many investigations of suported precatalysts of other types show that sooner or later, during pretreatment or the reaction itself, palladium tends to be reduced and redeposited as clusters and nanoparticles [173]. Consequently, almost any supported catalyst, if not in the first run, turns into a nanoparticles-on-support material on recycling. Moreover, this kind of precatalyst is obtained directly in situ if the reaction is run first in the presence of a palladium salt and an appropriate polymer for example, poly-A-vinylpyrrolidone or poly-A-vinylimidazole-A-vinylcaprolactam. The resulting supported nanoparticle precatalysts are further used in recycling runs [168]. 

---

Harmer et al.196 used 1,1,2,2-tetrafluoroethanesulfonic acid in the alkylation of para-xylene with 1-dodecene. The silica-embedded catalyst prepared by the sol-gel method showed much higher activity than the neat acid (almost complete conversion in 15 min at 100°C over the sol-gel-derived material versus 10% conversion, using the same molar amounts of acid). Practically no leaching was detected and the catalyst could be recycled with a slight decrease in conversion. It is in sharp contrast with silica-supported triflic acid, which showed much lower activity due to the loss of volatile triflic acid. 

The platelike shape of the [Au]-SMAP-Rh (C) chip enables easy separation of the catalyst and reaction mixture. Indeed the catalyst can be recycled by physically transferring the used catalyst chip into another reaction vessel. The second use of [Au]-SMAP-Rh (C) resulted in a similar performance (TON of 61000 for 16h), demonstrating that catalytic activity was maintained. In the fourth run, a slight decrease in TON was observed (Table 7.1), but the superior recyclability of supported [Au]-SMAP-Rh (C) versus the homogeneous systems was evident. Leaching of the Rh complex was lesser than 0.5%. 

---