We have more deeply studied SiO₂-, Al₂O₃-, MgO-, SBA-15-, TiO₂- and ZrO₂-supported Pd^II systems derived by calcination of supported Pd(acac)₂ in air and their respective supported Pd⁰ systems prepared by reduction of supported Pd(acac)₂ in H₂ for the Heck coupling of PhBr and styrene. The supported Pd^II systems turn out to exhibit incomparable catalytic performance. The advantage of the supported Pd^II systems is elucidated in terms of the dependence of catalytic activity on the concentration of soluble active Pd species measured by selective scavenging of soluble molecular Pd species over a supported Pd system. Under Heck reaction conditions used, a supported Pd^II system is able to produce a larger amount of fine Pd particles by in situ reduction of the supported Pd^II, which results in a higher fraction of molecular Pd species in solution owing to an Ostwald ripening effect. The correlation that the smaller the size of supported Pd particles, the higher the catalytic activity is further suggested for homogeneous catalysis in nature. The catalyst recyclability of the supported Pd^II systems under equivalent reaction conditions is dependent on the size of the supported Pd particles and the interaction between the Pd particles and support. The supported Pd^II systems having a Pd loading of 1% or above can give rise to a strong catalyst recyclability. At the end of the Heck reaction, the contamination level of residual Pd to products can be controlled to <5 ppm with the supported Pd^II systems having a loading of 1% Pd, but increases at a higher Pd loading. Properly selecting the Pd loading allows a compromise between strong catalyst recyclability and low Pd contamination to products for a supported Pd^II system.

中文翻译：

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精细负载的Pd颗粒催化Heck反应的新见解

我们已经对由空气中负载的Pd（acac）₂和它们各自负载的煅烧得到的SiO _2-，Al ₂ O _3-，MgO-，SBA-15-，TiO _2-和ZrO ₂负载的Pd ^II系统进行了更深入的研究。通过在H _2中还原负载的Pd（acac）₂制备的Pd ⁰体系，用于PhBr和苯乙烯的Heck偶联。支持的Pd ^II系统显示出无与伦比的催化性能。受支持的Pd ^II的优势通过催化活性对可溶性活性Pd种类浓度的依赖性来阐明该体系，该活性是通过在支持的Pd系统上选择性清除可溶性分子Pd种类而测得的。在所用的Heck反应条件下，负载的Pd ^II系统能够通过原位还原负载的Pd ^II而产生大量的Pd细颗粒，由于奥斯特瓦尔德（Ostwald）熟化作用，溶液中的分子Pd种类比例更高。对于自然界中的均相催化，进一步暗示了负载的Pd颗粒的尺寸越小，催化活性越高的相关性。负载型钯^II的催化剂可回收性在等效反应条件下的体系取决于负载的Pd颗粒的大小以及Pd颗粒与载体之间的相互作用。具有1％或更高的Pd负载量的负载的Pd ^II系统可产生强的催化剂可循环性。在Heck反应结束时，可以使用负载为1％Pd的负载型Pd ^II系统将残留Pd对产品的污染水平控制为<5 ppm ，但在Pd负载较高时会增加。正确选择Pd负载量可以在强大的催化剂可回收性和对支持的Pd ^II系统的产品造成的低Pd污染之间做出折衷。