recently reported silica-supported inorganic Cr-V bimetallic catalysts [30, 170] and
silica-supported organic Cr-V bimetallic catalysts [171] have been successfully
synthesized. Much improved SCB distribution of the PE products with better
properties and performance made from such bimetallic catalysts within single
polymerization reactor could be expected commercially in the near future.
8 Conclusions and Outlook
Since the discovery of the Phillips Cr/silica catalyst by Hogan and Banks in the
early 1950s, it has achieved great success as one of the most important industrial
catalysts for production of more than ten million tons of HDPE per year. However,
academic progress regarding basic understanding of the nature of active sites and
polymerization mechanisms is lagging far behind due to the complexity of this
heterogeneous catalyst system and the limitation of current technologies. The
complexity of Phillips-type catalysts mainly originates from the low percentage
of active Cr species in the total Cr loading, the multiple valence states of Cr
including +1, +2, +3, +4, +5, and +6, the high surface heterogeneity of the
amorphous silica support, the concealment of over 99% of the active sites on the
inner surface within the micro- and mesopores of the silica support, the instant
encapsulation of active sites by produced polymer and the very short lifetime of
the growing polymer chains due to the ultrafast polymerization rate, as well as the
coexistence of many side reactions in the polymerization system during the whole
process, such as catalyst deactivation and various chain transfer reactions.
During the last decade, increasing research efforts have focused on Phillips-type
catalysts through various approaches, including spectroscopic methods, polymerization kinetics, heterogeneous model catalysts, homogeneous model catalysts, and
molecular modeling, accompanied by successive catalyst innovations through
modification of the traditional Phillips catalyst. Much deeper and better understanding of the nature of active sites and polymerization mechanisms has been achieved
by various explorations concerning the activation by high temperature calcination,
CO, or Al-alkyl cocatalysts during catalyst preparation; activation by ethylene
monomer and Al-alkyl cocatalysts during polymerization; promotional effects of
modification of the catalyst by Ti; spin-crossover phenomenon and its effects on
the reactivity; and analysis of the microstructures of the produced PE chains,
etc. Combined experimental and computational methodologies have been used.
Investigations of polymerization kinetics over Phillips-type catalysts combined
with different Al-alkyl cocatalysts have provided deeper understanding on formation and transformation of plausible active sites as well as strategies of cocatalyst
introduction for design and optimization of commercial polymerization processes.
It was also made clear that coordination of divalent active site precursor with
siloxane ligands on the silica surface in terms of catalyst calcination temperature
was crucial for determination of the precise microstructure and coordination environment of the active Cr species and thus for the performance of the catalyst.
Phillips Cr/Silica Catalyst for Ethylene Polymerization
197
silica-supported organic Cr-V bimetallic catalysts [171] have been successfully
synthesized. Much improved SCB distribution of the PE products with better
properties and performance made from such bimetallic catalysts within single
polymerization reactor could be expected commercially in the near future.
8 Conclusions and Outlook
Since the discovery of the Phillips Cr/silica catalyst by Hogan and Banks in the
early 1950s, it has achieved great success as one of the most important industrial
catalysts for production of more than ten million tons of HDPE per year. However,
academic progress regarding basic understanding of the nature of active sites and
polymerization mechanisms is lagging far behind due to the complexity of this
heterogeneous catalyst system and the limitation of current technologies. The
complexity of Phillips-type catalysts mainly originates from the low percentage
of active Cr species in the total Cr loading, the multiple valence states of Cr
including +1, +2, +3, +4, +5, and +6, the high surface heterogeneity of the
amorphous silica support, the concealment of over 99% of the active sites on the
inner surface within the micro- and mesopores of the silica support, the instant
encapsulation of active sites by produced polymer and the very short lifetime of
the growing polymer chains due to the ultrafast polymerization rate, as well as the
coexistence of many side reactions in the polymerization system during the whole
process, such as catalyst deactivation and various chain transfer reactions.
During the last decade, increasing research efforts have focused on Phillips-type
catalysts through various approaches, including spectroscopic methods, polymerization kinetics, heterogeneous model catalysts, homogeneous model catalysts, and
molecular modeling, accompanied by successive catalyst innovations through
modification of the traditional Phillips catalyst. Much deeper and better understanding of the nature of active sites and polymerization mechanisms has been achieved
by various explorations concerning the activation by high temperature calcination,
CO, or Al-alkyl cocatalysts during catalyst preparation; activation by ethylene
monomer and Al-alkyl cocatalysts during polymerization; promotional effects of
modification of the catalyst by Ti; spin-crossover phenomenon and its effects on
the reactivity; and analysis of the microstructures of the produced PE chains,
etc. Combined experimental and computational methodologies have been used.
Investigations of polymerization kinetics over Phillips-type catalysts combined
with different Al-alkyl cocatalysts have provided deeper understanding on formation and transformation of plausible active sites as well as strategies of cocatalyst
introduction for design and optimization of commercial polymerization processes.
It was also made clear that coordination of divalent active site precursor with
siloxane ligands on the silica surface in terms of catalyst calcination temperature
was crucial for determination of the precise microstructure and coordination environment of the active Cr species and thus for the performance of the catalyst.
Phillips Cr/Silica Catalyst for Ethylene Polymerization
197