Heterogenizing Homogeneous Catalysts and Their Use in a Continuous Flow Reactor

3 Heterogenizing Homogeneous Catalysts and Their Use in a Continuous Flow Reactor 

Converting homogeneous catalytic reactions to heterogeneous versions will benefit the pharmaceutical and fine chemical industries because heterogeneous catalysts are physically separated from reactants and products and can therefore be easily recycled. Dendrimers have been employed as a recoverable catalyst platform using specially designed techniques, such as nanofiltration, precipitation, and two-phase catalysis [25]. 

Recently, Pt DENs were used as an electrophilic catalyst in an intramolecular addition of phenols to alkynes (intramolecular hydroalkoxylation), as shown in Fig. 4.13b [100], The Pt DENs were supported on a mesoporous silica material known as SBA-15 (Fig. 4.13a). Since the as-synthesized SBA-15 consists of micrometer-sized particles, the supported Pt DENs can be easily separated from the reaction solution by centrifugation. This reaction has only been catalyzed by homogeneous catalysts (e.g., PtCy before this report. The use of the supported Pt DENs to catalyze this reaction was the first demonstration that a heterogeneous catalyst could also catalyze this conversion. It was also found that adding an oxidation agent, PhICF, would dramatically increase the benzofuran yield from 10 to 98 %, as shown in Fig. 4.13b. The authors proposed that PhICl2 could render the surface of the Pt nanoparticles more electrophilic, which is required for the Pt DENs to be active for the hydroalkoxylation reaction. 

The authors also designed and performed extensive experiments, such as three-phase and catalyst filtration tests, to prove the heterogeneous nature of the SBA-15 supported Pt DENs in the intramolecular hydroalkoxylation reaction. No leaching of the active catalytic species was detected in these tests. PVP-capped Pt nanoparticles were also tested for use in this reaction, but they could not be recycled. 

Pd DENs also catalyzed the same intramolecular hydroalkoxylation reaction [101]. SBA-15 supported Pd DENs achieved 95 % yield of benzofuran in 15 h at 20 °C, while PdBr2 and PdCla only achieved 10 % yield under the same conditions. Even at 100 °C, PdCl2 only gave 25 % yield of benzofuran after 15 h. After proving that there is no leaching of Pd during the reaction, the SBA-15 supported Pd DENs were applied in a continuous flow reactor. It was found that the presence of 5 tnM 

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