In summary, investigation of the polymerization kinetics over Phillips-type
catalysts could provide deep mechanistic understanding and valuable information
to guide the design and optimization of the polymerization processes. It has been
demonstrated that the polymerization kinetics could be drastically affected by the
types of cocatalysts, the timing of introduction of the cocatalyst, and the types of
polymerization. From the industrial point of view, the direct activation of Phillipstype catalysts by AlR 3 -type cocatalysts during the polymerization process within
the polymerization reactor should be avoided in ethylene slurry polymerization. As
for gas phase ethylene polymerization, activation of Phillips-type catalysts by AlR 3
during catalyst preparation before polymerization should also be forbidden. We
could expect that more efforts should be devoted to the investigation of gas phase
polymerization kinetics, combination of experiments with kinetic modeling and
microkinetic modeling based on first principle calculations in the near future.
4 Approaches Using Heterogeneous Model Catalysts
The surface complexity of the traditional Phillips Cr/silica catalyst derives from the
following aspects: the coexistence of mono-, di-, and polychromate species, the
inevitable formation of Cr 2 O 3 microcrystals [8, 11], the formation of surface
chromium species in lower oxidation states due to thermally induced reduction of
surface chromate species at high temperature, the low fraction of active Cr species
in the total Cr loading [4, 11], the ambiguous and complicated reactions for the
formation of the first chromium–carbon bond between ethylene monomer and
surface chromate species. These factors greatly contribute to the surface complexity
of the industrial Phillips catalyst and thus hinder academic progress in basic
understanding of the nature of active sites and polymerization mechanisms for
this important commercial polyolefin catalyst. During the last decades, various
heterogeneous models with more uniform and well-defined structure of surface
chromium species have been designed to facilitate the fundamental investigations
in this field. Typical reported heterogeneous models for Phillips catalysts are listed
in Scheme 9. These models can be generally divided into two groups: surface
hexavalent chromate species (models 1c, 2c, 6c, 9c, and 10c) and surface chromium
species with lower oxidation states (models 3c, 4c, 5c, 7c, and 8c).
S-2 catalyst prepared by wet impregnation of BC into thermally pretreated silica
gel could be considered as a commercial heterogeneous model (1c) for Phillips
catalysts [26, 112]. The S-2 catalyst shows an increased activity after supporting on
silica gel compared with BC and produces HDPEs with even broader molecular
weight distribution than that produced by the Phillips catalyst. Model 2c was firstly
prepared by McDaniel [113] via mild grafting (at 200
C) of CrO 2 Cl 2 onto thermally
pretreated silica. The CrO 2 Cl 2 grafted onto silica pretreated at 400
C showed
similar surface chromate structure and polymerization activity to the Phillips
catalyst. Recently, Scott and colleagues [54, 114] prepared similar catalysts via
ambient anhydrous grafting of CrO 2 Cl 2 onto silica pretreated at 200
C, 450
C and
Phillips Cr/Silica Catalyst for Ethylene Polymerization
163
catalysts could provide deep mechanistic understanding and valuable information
to guide the design and optimization of the polymerization processes. It has been
demonstrated that the polymerization kinetics could be drastically affected by the
types of cocatalysts, the timing of introduction of the cocatalyst, and the types of
polymerization. From the industrial point of view, the direct activation of Phillipstype catalysts by AlR 3 -type cocatalysts during the polymerization process within
the polymerization reactor should be avoided in ethylene slurry polymerization. As
for gas phase ethylene polymerization, activation of Phillips-type catalysts by AlR 3
during catalyst preparation before polymerization should also be forbidden. We
could expect that more efforts should be devoted to the investigation of gas phase
polymerization kinetics, combination of experiments with kinetic modeling and
microkinetic modeling based on first principle calculations in the near future.
4 Approaches Using Heterogeneous Model Catalysts
The surface complexity of the traditional Phillips Cr/silica catalyst derives from the
following aspects: the coexistence of mono-, di-, and polychromate species, the
inevitable formation of Cr 2 O 3 microcrystals [8, 11], the formation of surface
chromium species in lower oxidation states due to thermally induced reduction of
surface chromate species at high temperature, the low fraction of active Cr species
in the total Cr loading [4, 11], the ambiguous and complicated reactions for the
formation of the first chromium–carbon bond between ethylene monomer and
surface chromate species. These factors greatly contribute to the surface complexity
of the industrial Phillips catalyst and thus hinder academic progress in basic
understanding of the nature of active sites and polymerization mechanisms for
this important commercial polyolefin catalyst. During the last decades, various
heterogeneous models with more uniform and well-defined structure of surface
chromium species have been designed to facilitate the fundamental investigations
in this field. Typical reported heterogeneous models for Phillips catalysts are listed
in Scheme 9. These models can be generally divided into two groups: surface
hexavalent chromate species (models 1c, 2c, 6c, 9c, and 10c) and surface chromium
species with lower oxidation states (models 3c, 4c, 5c, 7c, and 8c).
S-2 catalyst prepared by wet impregnation of BC into thermally pretreated silica
gel could be considered as a commercial heterogeneous model (1c) for Phillips
catalysts [26, 112]. The S-2 catalyst shows an increased activity after supporting on
silica gel compared with BC and produces HDPEs with even broader molecular
weight distribution than that produced by the Phillips catalyst. Model 2c was firstly
prepared by McDaniel [113] via mild grafting (at 200
C) of CrO 2 Cl 2 onto thermally
pretreated silica. The CrO 2 Cl 2 grafted onto silica pretreated at 400
C showed
similar surface chromate structure and polymerization activity to the Phillips
catalyst. Recently, Scott and colleagues [54, 114] prepared similar catalysts via
ambient anhydrous grafting of CrO 2 Cl 2 onto silica pretreated at 200
C, 450
C and
Phillips Cr/Silica Catalyst for Ethylene Polymerization
163