The importance of modifier pH in the generation of enantioselective nickel catalysts
The importance of modifier pH in the generation of enantioselective nickel catalysts
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调节剂 pH 在对映选择性镍催化剂生成中的重要性
DOI：
10.1007/bf00771755
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发表时间：
1993
影响因子：
2.8
通讯作者：
M. Keane
中科院分区：
文献类型：
--
作者：
M. Keane
用（R）-（+）-酒石酸（TA）水溶液对Ni/SiO_2催化剂进行改性，制备了对映选择性Ni/SiO_2催化剂，并将其用于前手性β-酮酯（乙酰乙酸甲酯）不对称加氢合成β-羟基酯（（R）-（-）-甲基-3-羟基丁酸酯）。同时TA的吸附和表面镍金属的腐蚀性浸出的图形说明和TA在催化剂表面上的建设与改性的持续时间的进展。改性剂的pH值的变化被发现强烈影响的改性过程中，影响最终的加氢速率和对映选择性。TA的表面覆盖率与不对称活性相关，并确定了0.2的最佳覆盖率;在较高的覆盖率下，用碱性TA溶液改性产生了上级对映体选择性。虽然TA处理在碱性介质中的腐蚀性较小，但在氢化步骤期间，一定比例的表面对映选择性镍位点被浸出到溶液中。镍前体，通过浸渍和均匀沉淀/沉积，TA处理制备的响应的差异进行了比较和讨论的金属/支持相互作用。
Enantioselective Ni/SiO2 catalysts have been prepared by modification with aqueous solutions of (R)-(+)-tartaric acid (TA) and used in the asymmetric hydrogenation of a prochiral β-keto ester (methylacetoacetate) to a β-hydroxy ester ((R)-(-)-methyl-3-hydroxybutyrate). The simultaneous adsorption of TA and corrosive leaching of surface nickel metal are graphically illustrated and the progress of TA buildup on the catalyst surface with the duration of modification is presented. Variations of modifier pH were found to strongly affect the modification process and influence the ultimate hydrogenation rate and enantioselectivity. The surface coverage by TA is correlated to the asymmetric activity and an optimum fractional coverage of 0.2 is identified; at higher coverages modification with basic TA solutions yielded superior enantioselectivities. While TA treatment in basic media was less corrosive, a proportion of the surface enantioselective nickel sites was leached into solution during the hydrogenation step. The difference in the response of nickel precursors, prepared by impregnation and homogeneous precipitation/deposition, to TA treatment is compared and discussed in terms of metal/support interaction.