Looking to the past, heading to the future. In this contribution we explain the reasons why the Cr/Al₂O₃ Phillips catalysts exhibit a faster kinetics profile in ethylene polymerization reaction with respect to Cr/SiO₂. Diffuse reflectance UV–Vis and FT-IR spectroscopies unequivocally demonstrate that, albeit several types of reduced Cr sites are stabilized by the Al₂O₃ support, only the 4-fold coordinated Cr²⁺ sites are active precursors in ethylene polymerization, as for Cr²⁺/SiO₂. Nevertheless, kinetic experiments indicate that ethylene polymerization is 15 times faster on CO-reduced Cr/Al₂O₃ than on CO-reduced Cr/SiO₂. The difference is even more striking (two order of magnitude) when the reaction rates per active Cr sites are compared. Our experimental results suggest two reasons behind the faster polymerization kinetic of Cr/Al₂O₃: (1) the higher ionic character of the CrOAl bond with respect to the CrOSi one; (2) the nature of the ancillary ligands in the coordination sphere of the Cr active sites (which are mainly carbonates for CO-reduced Cr/Al₂O₃ and siloxane bridges for CO-reduced Cr/SiO₂).

回顾过去，走向未来。在这一贡献中，我们解释了为什么Cr / Al ₂ O ₃ Phillips催化剂在乙烯聚合反应中相对于Cr / SiO ₂表现出更快的动力学曲线的原因。漫反射UV-Vis和FT-IR光谱法明确地表明，尽管几种类型的还原Cr位通过Al ₂ O ₃载体得以稳定，但只有4倍配位的Cr ²⁺位是乙烯聚合中的活性前体。 Cr ²⁺ / SiO ₂。尽管如此，动力学实验表明，在CO还原的Cr / Al ₂ O上乙烯聚合速度要快15倍₃比在CO还原的Cr / SiO _2上要高。当比较每个活性Cr位点的反应速率时，差异甚至更大（两个数量级）。我们的实验结果表明了Cr / Al ₂ O ₃更快的聚合动力学的两个原因：（1）Cr O Al键相对于Cr O Si离子具有更高的离子特性；（2）Cr活性位点配位域中辅助配体的性质（主要是CO还原的Cr / Al ₂ O _3的碳酸盐和CO还原的Cr / SiO _2的硅氧烷桥）。