catalysts, and molecular modeling. Much deeper mechanistic understanding,
together with successive catalyst innovations through modifications of the Phillips
catalyst, has been achieved. The advances in the field of Phillips catalysts during the
past half-century have been reviewed in depth by McDaniel in 1985 [2], Zecchina
and coworkers in 2005 [11], and McDaniel in 2008 and 2010 [3, 4]. This present
contribution aims at an overview of the achievements of the last decade, unraveling
the mechanistic aspects of the activation, the nature of the active chromium species,
and the polymerization mechanisms through both experimental and computational
approaches, as well as catalyst innovation through modification of the Phillips
catalyst with particular emphasis on the studies undertaken in the authors’
laboratory.
2 Approaches Using Spectroscopic Methods
The state of surface Cr species, which is closely related to the molecular structure,
texture, and orientation of the chromium oxide on the catalyst surface, is crucial for
a deeper understanding of the Phillips catalyst. Many modern analytical methods
[8, 11, 31], such as oxygen chemisorptions [32], magnetic susceptibility measurement [33], XRD [5, 34–36], EPR [33, 37–41], SIMS [42], Raman [35, 43–50],
UV-vis DRS [33, 39, 43, 50], XAS (EXAFS-XANES) [48, 51–55], PIXE [56], TPR
[36, 39], SEM/EDS [57], FTIR [11, 22, 50, 54, 55, 58–63], XPS [6, 8, 56, 64–71],
NMR [41, 71, 72], AFM [73], EPMA [8], TPD-MS [67], TG-DTA [10], RBS [74],
LA-MS, and LDI-MS [75], have been used separately or jointly to characterize the
physico-chemical state of Cr species on Phillips catalysts [11, 31]. These
approaches attempt to provide direct or indirect evidence for the anchoring of
chromate species at the surface during activation and the ability of the catalyst to
polymerize ethylene, especially at the early stage of polymer chain formation. For
example, based on the combination of the FTIR, Raman, and UV-vis spectroscopic
results, monochromate species were identified anchored on the surface of the
Cr/silica catalyst at low chromium loadings [47]. The monochromate structure on
a highly diluted Cr/SiO 2 /Si(100) system was also confirmed by EXAFS results
[53]. In the polymerization mechanism study, the in-situ FTIR spectroscopy
suggested that the initiation mechanism followed a metallacycle route [23]. Very
recently, Groppo and coworkers [76, 77] reviewed the spectroscopic investigations
into the Phillips catalyst. However, the lower concentration of the Cr species,
diversity of the amorphous silica gel surface, and high sensitivity to moisture and
air are major obstacles for exploring the structure of Cr species in relation to
polymerization activities. At the same time, these obstacles have led to difficulty
in combining different experimental findings from different groups into one
unifying picture. Although a definite explanation of the nature of the active site
relating to the polymerization mechanism has not yet been achieved, it will be
demonstrated in the following sections that valuable understanding has been
Phillips Cr/Silica Catalyst for Ethylene Polymerization
141
together with successive catalyst innovations through modifications of the Phillips
catalyst, has been achieved. The advances in the field of Phillips catalysts during the
past half-century have been reviewed in depth by McDaniel in 1985 [2], Zecchina
and coworkers in 2005 [11], and McDaniel in 2008 and 2010 [3, 4]. This present
contribution aims at an overview of the achievements of the last decade, unraveling
the mechanistic aspects of the activation, the nature of the active chromium species,
and the polymerization mechanisms through both experimental and computational
approaches, as well as catalyst innovation through modification of the Phillips
catalyst with particular emphasis on the studies undertaken in the authors’
laboratory.
2 Approaches Using Spectroscopic Methods
The state of surface Cr species, which is closely related to the molecular structure,
texture, and orientation of the chromium oxide on the catalyst surface, is crucial for
a deeper understanding of the Phillips catalyst. Many modern analytical methods
[8, 11, 31], such as oxygen chemisorptions [32], magnetic susceptibility measurement [33], XRD [5, 34–36], EPR [33, 37–41], SIMS [42], Raman [35, 43–50],
UV-vis DRS [33, 39, 43, 50], XAS (EXAFS-XANES) [48, 51–55], PIXE [56], TPR
[36, 39], SEM/EDS [57], FTIR [11, 22, 50, 54, 55, 58–63], XPS [6, 8, 56, 64–71],
NMR [41, 71, 72], AFM [73], EPMA [8], TPD-MS [67], TG-DTA [10], RBS [74],
LA-MS, and LDI-MS [75], have been used separately or jointly to characterize the
physico-chemical state of Cr species on Phillips catalysts [11, 31]. These
approaches attempt to provide direct or indirect evidence for the anchoring of
chromate species at the surface during activation and the ability of the catalyst to
polymerize ethylene, especially at the early stage of polymer chain formation. For
example, based on the combination of the FTIR, Raman, and UV-vis spectroscopic
results, monochromate species were identified anchored on the surface of the
Cr/silica catalyst at low chromium loadings [47]. The monochromate structure on
a highly diluted Cr/SiO 2 /Si(100) system was also confirmed by EXAFS results
[53]. In the polymerization mechanism study, the in-situ FTIR spectroscopy
suggested that the initiation mechanism followed a metallacycle route [23]. Very
recently, Groppo and coworkers [76, 77] reviewed the spectroscopic investigations
into the Phillips catalyst. However, the lower concentration of the Cr species,
diversity of the amorphous silica gel surface, and high sensitivity to moisture and
air are major obstacles for exploring the structure of Cr species in relation to
polymerization activities. At the same time, these obstacles have led to difficulty
in combining different experimental findings from different groups into one
unifying picture. Although a definite explanation of the nature of the active site
relating to the polymerization mechanism has not yet been achieved, it will be
demonstrated in the following sections that valuable understanding has been
Phillips Cr/Silica Catalyst for Ethylene Polymerization
141