Photocatalytic Syntheis of L-Pipecolinic Acid from L-Lysine by Hollow Core-Shell Titania Particles

A possible approach for photocatalytic selective organic synthesis is utilization of photocatalysts of or in defined microstructures.  We have reported [1,2] fabrication of a novel core-shell photocatalyst which consisted of commercially available titania (TiO₂) particles incorporated in a hollow silica shell (SiO₂/void/TiO₂) showing size-selective properties in the photodecomposition of organic compounds.  Recently, we have attempted to use the SiO₂/void/TiO₂ photocatalyst for the synthesis of L-pipecolinic acid (PCA), a useful intermediate material for various fine chemicals, and found another function of silica shell to improve stereoselectivity, instead of molecular-size selectivity and report here.

Hollow core-shell silica (SiO₂)-TiO₂, "SiO₂/void/TiO₂", was prepared and platinized as reported (Fig. 1) [3,4].  For the photocatalytic reaction of redox-combined stereoselective synthesis of PCA from L-lysine (L-Lys), a platinum-loaded photocatalyst (0.05 g as TiO₂) was suspended in an aqueous solution (5.0 cm³) containing L-Lys (100 µmol) and photoirradiated by a mercury arc (400 W) under argon (Ar) at 298 K under vigorous magnetic stirring.  The yield of enantiomers of PCA, as well as the amount of unreacted L-Lys, was measured by HPLC.

Photoirradiation of the photocatalysts suspended in an aqueous L-Lys solution led to the formation of PCA.  Complete consumption of L-Lys was achieved using TiO₂ and also mec-SiO₂+TiO₂ (a mechanical mixture of silica and TiO₂).  These photocatalysts showed very similar results in terms of selectivity (S_PCA), optical purity (OP_PCA) and the rate of PCA formation (R_PCA), suggesting that the mechanical mixing of silica with TiO₂ does not give any effect on this reaction as only the TiO₂ part was responsible for PCA production.  As expected, dir-SiO₂/TiO₂, without void space between SiO₂ and TiO₂, showed poor photocatalytic activity to convert only 14% of L-Lys, thus proving that direct coverage of the TiO₂ surface with SiO₂ hinders the activity of the TiO₂ by prohibiting platinum deposition as well as L-Lys adsorption onto the bare TiO₂ surface. The SiO₂(0.5)/void/TiO₂ particles prepared with 0.5-h silylation period showed the performance almost the same as that of bare TiO₂.  Although the selectivity (43%) was slightly lower than that of bare TiO₂, SiO₂/void/TiO₂ exhibited the highest OP_PCA (70%), 13% more than that of platinized bare TiO₂, among all the samples.

In order to further prove the effectiveness of the structure, SiO₂/void/TiO₂ with thicker layer of silica shell were also prepared, by extending the silylation period (1.5 h and 3.0 h).  The thickness of the silica layer was increased to 14–32 nm and 28–45 nm, respectively, from 9–10 nm for 0.5-h silylated sample.  While SiO₂(1.5)/void/TiO₂ exhibited the best performance among the tested samples, it seemed that the photocatalytic performance (conversion, S_PCA, OP_PCA and R_PCA) was almost independent of the silica shell thickness. This suggests that the silica shell behaves as highly porous optically transparent penetration-free layer which surrounds the TiO₂ core and that this swollen sponge-like silica layer controls the stereoselectivity of the reaction through redox-combined process.

References

1. Ikeda, S.; Ikoma, Y.; Kobayashi, H.; Harada, T.; Torimoto, T.; Ohtani, B.; Matsumura, M. Chem. Commun. 2007, 3753.

2. Ikeda, S.; Kobayashi, H.; Ikoma, Y.; Harada, T.; Torimoto, T.; Ohtani, B.; Matsumura, M. Phys. Chem. Chem. Phys. 2007, 9 (2007) 6319.

3. Chandren, S.; Ohtani, B. J. Photochem. Photobiol. A: Chem. 2012, 246, 50.

4. Chandren, S.; Ohtani, B. Chem. Lett. 2012, 41, 677.