Abstract
Heterogeneous enantioselective hydrogenation is an ideal method for synthesizing important chiral compounds in pesticides and pharmaceuticals. Up to the present, supported noble-metal catalysts are most widely studied in heterogeneous enantioselective hydrogenations. However, it is found that the weak interactions existing on the surface of support may have negative effects on the enantioselectivity. Herein, a new category of TiO2 (Aeroxide® P25) supported Pd catalyst with ultrathin Pd shell was successfully prepared via a simple strategy based on the reduction of PdI carbonyl complex. Characterization results show that a well-dispersed ultrathin Pd shell with an average thickness of ~1.0 nm and a Pd loading of 36 wt.% was formed over the surface of P25 support. By excluding the negative weak interactions from the support, the P25@Pd core-shell catalyst with unique electronic properties of Pd exhibits higher activity and enantioselectivity than that of Pd/P25 catalyst prepared by the impregnation method and unsupported Pd black catalyst in the enantioselective hydrogenation of acetophenone.
Highlights
With the wide application of optically pure chiral compounds in pesticides, pharmaceuticals and fragrances, it is important to develop effective preparation methods for synthesizing single-enantiomer compounds [1,2,3]
A new category of TiO2 (Aeroxide® P25) supported Pd catalyst with unique core-shell structure (P25 core and Pd ultrathin shell, denoted as P25@Pd) was tactically prepared by a facile strategy under mild conditions based on the reduction of PdI carbonyl complex [19] over the surface of the P25 support
P25@Pd catalyst, a high angle annular dark field scanning transmission electron microscopy (TEM) (HAADF-scanning transmission electron microscopy (STEM)) image is presented in Figure 1c, which clearly exhibits the coverage of the Pd shells
Summary
With the wide application of optically pure chiral compounds in pesticides, pharmaceuticals and fragrances, it is important to develop effective preparation methods for synthesizing single-enantiomer compounds [1,2,3]. Among the many established methods, enantioselective hydrogenation over heterogeneous catalysts is one of the most ideal strategies owing to its inherent operational and economic advantages, e.g., atom economy, easy separation and recovery of catalysts. It has shown great potential in industrial research and caused extensive concern in academic circles [1,2,3,4]. Chirally modified supported noble-metal catalysts have been most widely used in heterogeneous enantioselective hydrogenations [2,3,4,5,6,7,9,10,11,12,13,14]. To make the metal nanoparticles disperse well for achieving high activity, the metal loading of the Catalysts 2019, 9, 513; doi:10.3390/catal9060513 www.mdpi.com/journal/catalysts
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