| Silica-supported Phillips catalyst is one of the most widely used polyolefin catalyst systems. In this work, Al-modified and N-modified chromium based polyolefin catalysts were designed to investigate the relationship between the composition and structure of catalyst and properties of polymer product, aiming to improve the distribution of short chain branch along with the co-polymer chain.The effects of Al-modification over silica support of Phillips catalysts on the nature of surface chromate species were investigated by high-resolution X-ray photoelectron spectroscopy (XPS) and Density Functional Theory (DFT) method. The decreased Cr 2p binding energies (BE) showed an increase in electron density of chromium by Al-modification. The photo stability of surface chromate species during XPS measurement was enhanced by Al-modification. Modeling results indicated that the initiation step undergoing a metallacycle reaction pathway was found to proceed through a spin crossover from the quintet energy surface via a minimum energy crossing point to the triplet energy surface. And the promotion effect on polymerization activity by Al-modification mainly originated from the chain initiation stage with lowered energy barrier for Al-modified active sites. The energy barrier difference between chain transfer and chain propagation decreased after Al-modification, demonstrating that an increased low molecular weight (MW) fraction of polymer was produced which resulted in an expansion of molecular weight distribution (MWD) to the low-MW side. Al-modification improved environmental stress-crack resistance (ESCR) property of polymer derived from the less short chain branches (SCBs) distribution in the low MW side.For N-modificated Cr-M catalyst, co-impregnation is better than separated impregnation method. In contrast to TEA, TiBA showed more effective to the activity as co-catalyst and also led to higher MW product. Both the activity and MW of polymer increased as the reaction temperature decreased. Although the activity was decreased, N-modification led to stronger hydrogen sensitivity and more SCBs in the high MW fraction of the product. |
