Photocatalytic Syntheis of L-Pipecolinic Acid from L-Lysine by Hollow Core-Shell Titania Particles
Authors: Ohtani Bunsho , Chandren Sheela 
Published: February 2020

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 (SPCA), optical purity (OPPCA) and the rate of PCA formation (RPCA), 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 OPPCA (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, SPCA, OPPCA and RPCA) 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.

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