The asymmetric reduction of β-keto esters to their corresponding hydroxy alcohols can be performed by employing homogeneous and heterogeneous chemo- and bio-catalysis. This review covers the scope and limitations of ...The asymmetric reduction of β-keto esters to their corresponding hydroxy alcohols can be performed by employing homogeneous and heterogeneous chemo- and bio-catalysis. This review covers the scope and limitations of different catalysts and methodologies that were employed for the reaction and compare between them on the basis of catalytic performance, product separation and catalyst recycling procedure. In general, heterogeneous catalytic systems are advantageous from industrial point of views as they can be easily separated by filtration and re-used. Nickel modified with tartaric acid and sodium bromide was found to be suitable heterogeneous catalyst for the enantioselective hydrogenation, yet its performance is lower than this of homogeneous chiral metal catalysts such as Ru-BINAP. Heterogenization of the chiral complex via immobilization or entrapment using organic and inorganic supports was thus tested. However, though the resulted heterogeneous analogues were highly enantioselective and could be re-used, the activity of the system is often very low compared to homogeneous system due to mass transfer limitations. Alternatively, performing liquid phase hydrogenation under homogeneous conditions, using Ru-BINAP soluble derivatives, yielded high activit5＇ and enantioselectivity. Product separation and catalysts recycling were facilitated by either extraction of the product with solvent that does not dissolve the complex or by selective filtration of the product. Alternatively, precipitation of the complex at the end of the reaction was also reported.展开更多
Ecological geographic regions, also called eco-regions, can be used to divide a remotely sensed image, which is helpful for reducing the complexity of land cover types within eco-regions and for improving the classifi...Ecological geographic regions, also called eco-regions, can be used to divide a remotely sensed image, which is helpful for reducing the complexity of land cover types within eco-regions and for improving the classification accuracy of land cover. In this case study in China, we improved a method of ecological geographic regionalization that is more suitable for remote sensing mapping of regional land cover, and we obtained new eco-regions. The canonical correspondence analysis(CCA) and k-means clustering were adopted in the ecological geographic regionalization using both seasonal remotely-sensed vegetation information and environmental data including climate, elevation and soil features. Our results show that the combination of seasonal vegetation information and the CCA performed well in the selection of the dominant environmental factor of the biogeographic pattern, and it can be used as regionalization indicators of eco-regions. We found that thermal factors are the most important driving forces of the biogeographic pattern in China, which followed by moisture factors. Two global land cover products(MODIS MCD12C1 and Glob Cover) were used to assess our eco-regions. The results show that our eco-regions performed better than that of a previous study regarding the complexity of land cover types, such as in the number of types and the proportional area of the major/secondary type. These results indicate that the method of ecological geographic regionalization, which is based on environmental factors associated with seasonal vegetation features, is effective for reducing the regional complexity of land cover.展开更多
文摘The asymmetric reduction of β-keto esters to their corresponding hydroxy alcohols can be performed by employing homogeneous and heterogeneous chemo- and bio-catalysis. This review covers the scope and limitations of different catalysts and methodologies that were employed for the reaction and compare between them on the basis of catalytic performance, product separation and catalyst recycling procedure. In general, heterogeneous catalytic systems are advantageous from industrial point of views as they can be easily separated by filtration and re-used. Nickel modified with tartaric acid and sodium bromide was found to be suitable heterogeneous catalyst for the enantioselective hydrogenation, yet its performance is lower than this of homogeneous chiral metal catalysts such as Ru-BINAP. Heterogenization of the chiral complex via immobilization or entrapment using organic and inorganic supports was thus tested. However, though the resulted heterogeneous analogues were highly enantioselective and could be re-used, the activity of the system is often very low compared to homogeneous system due to mass transfer limitations. Alternatively, performing liquid phase hydrogenation under homogeneous conditions, using Ru-BINAP soluble derivatives, yielded high activit5＇ and enantioselectivity. Product separation and catalysts recycling were facilitated by either extraction of the product with solvent that does not dissolve the complex or by selective filtration of the product. Alternatively, precipitation of the complex at the end of the reaction was also reported.
基金Financial support for the study was provided by the China Postdoctoral Science Foundation (Grant No. 2015M570431)the Jiangsu Provincial Natural Science Foundation of China (Grant No. BK20150579)the State High Technology Funds of China (Grant No. 2009AA122001)
文摘Ecological geographic regions, also called eco-regions, can be used to divide a remotely sensed image, which is helpful for reducing the complexity of land cover types within eco-regions and for improving the classification accuracy of land cover. In this case study in China, we improved a method of ecological geographic regionalization that is more suitable for remote sensing mapping of regional land cover, and we obtained new eco-regions. The canonical correspondence analysis(CCA) and k-means clustering were adopted in the ecological geographic regionalization using both seasonal remotely-sensed vegetation information and environmental data including climate, elevation and soil features. Our results show that the combination of seasonal vegetation information and the CCA performed well in the selection of the dominant environmental factor of the biogeographic pattern, and it can be used as regionalization indicators of eco-regions. We found that thermal factors are the most important driving forces of the biogeographic pattern in China, which followed by moisture factors. Two global land cover products(MODIS MCD12C1 and Glob Cover) were used to assess our eco-regions. The results show that our eco-regions performed better than that of a previous study regarding the complexity of land cover types, such as in the number of types and the proportional area of the major/secondary type. These results indicate that the method of ecological geographic regionalization, which is based on environmental factors associated with seasonal vegetation features, is effective for reducing the regional complexity of land cover.
