2.2 Activation of Phillips Catalysts by CO or Al-alkyl
Cocatalysts
The reduction of Cr(VI) to lower oxidation state is the first step in the induction
period in the ethylene polymerization process. Active site precursors for polymerization can be formed after reduction of the chromate species by CO (usually at
350
C) or Al-alkyl cocatalyst (e.g., TEA) in a separated pre-activation step, or by
ethylene monomer itself during the initial stage of polymerization [2]. Activation
by ethylene monomer is most frequently used in the commercial processes. The use
of CO or Al-alkyl cocatalyst as reduction agent may shorten or remove the
induction period, which is frequently used at the laboratory scale. The effect of
activation using different reducing agents for the Phillips catalyst was systematically investigated by XPS characterization.
Phillips catalyst reduced by CO at 350
C can exhibit instantaneous polymerization activity upon contact with ethylene [2] and is generally considered as an ideal
catalyst system in spectroscopic investigations of the early stages of ethylene
polymerization [11]. Comparison of the oxidation states of surface Cr species on
PC600 (calcined) and PC600/CO (PC600 pre-reduced by CO at 350
C for 1 h)
catalysts measured by XPS method suggested that about 63% of surface Cr species
were reduced into surface-stabilized Cr(II) species by CO, leaving a certain amount
of residual chromate species on PC600/CO [80]. The DRS results [83] showed that
the Cr active sites could be fully available for reduction at higher reduction
temperature (600
C).
The activation of the Phillips catalyst by Al-alkyl cocatalyst was also systematically studied by XPS and solid state NMR [69, 84]. XPS quantified the existence of
four oxidation states, including +2, +3, +5, and +6, of surface Cr species on
TEA-modified catalysts. It was found that the relative concentration of active
sites was around 14.4–24.9 mol% Cr for the TEA-modified Phillips catalysts
depending on the calcination temperature and Al/Cr molar ratio. The correlation
of polymerization activities as well as the oxidation states of surface chromium
species with the molar ratio of Al/Cr is shown in Fig. 4. It seemed that only the
surface chromium species in oxidation states of +2 and +6 were possibly related
with the activity of the ethylene polymerization catalysts. The correlation suggested
that the active precursor of the chromium cluster can be named as a Cr
2+ ·2Cr
6+
cluster composed of one Cr(II)O x,surf species and two Cr(VI)O x,surf species, in
which the Cr(II)O x,surf species act as the real center of active chromium precursor
and the residual Cr(VI)O x,surf species are also necessary components acting as the
neighboring ligand environment with electronic and steric effects. Three plausible
chemical structural models of the Cr
2+ ·2Cr
6+ cluster are proposed in Scheme 5,
based on the correlation between XPS and polymerization results and our previous
understanding of the surface chemical nature of calcined Phillips Cr(VI)O x /SiO 2
catalysts and pre-reduced Cr(II)O x /SiO 2 catalysts.
The
1 H and
27 Al MAS solid state NMR spectra clearly demonstrated that the
existing states of surface Al species in the TEA-modified Phillips catalysts strongly
Phillips Cr/Silica Catalyst for Ethylene Polymerization
147
Cocatalysts
The reduction of Cr(VI) to lower oxidation state is the first step in the induction
period in the ethylene polymerization process. Active site precursors for polymerization can be formed after reduction of the chromate species by CO (usually at
350
C) or Al-alkyl cocatalyst (e.g., TEA) in a separated pre-activation step, or by
ethylene monomer itself during the initial stage of polymerization [2]. Activation
by ethylene monomer is most frequently used in the commercial processes. The use
of CO or Al-alkyl cocatalyst as reduction agent may shorten or remove the
induction period, which is frequently used at the laboratory scale. The effect of
activation using different reducing agents for the Phillips catalyst was systematically investigated by XPS characterization.
Phillips catalyst reduced by CO at 350
C can exhibit instantaneous polymerization activity upon contact with ethylene [2] and is generally considered as an ideal
catalyst system in spectroscopic investigations of the early stages of ethylene
polymerization [11]. Comparison of the oxidation states of surface Cr species on
PC600 (calcined) and PC600/CO (PC600 pre-reduced by CO at 350
C for 1 h)
catalysts measured by XPS method suggested that about 63% of surface Cr species
were reduced into surface-stabilized Cr(II) species by CO, leaving a certain amount
of residual chromate species on PC600/CO [80]. The DRS results [83] showed that
the Cr active sites could be fully available for reduction at higher reduction
temperature (600
C).
The activation of the Phillips catalyst by Al-alkyl cocatalyst was also systematically studied by XPS and solid state NMR [69, 84]. XPS quantified the existence of
four oxidation states, including +2, +3, +5, and +6, of surface Cr species on
TEA-modified catalysts. It was found that the relative concentration of active
sites was around 14.4–24.9 mol% Cr for the TEA-modified Phillips catalysts
depending on the calcination temperature and Al/Cr molar ratio. The correlation
of polymerization activities as well as the oxidation states of surface chromium
species with the molar ratio of Al/Cr is shown in Fig. 4. It seemed that only the
surface chromium species in oxidation states of +2 and +6 were possibly related
with the activity of the ethylene polymerization catalysts. The correlation suggested
that the active precursor of the chromium cluster can be named as a Cr
2+ ·2Cr
6+
cluster composed of one Cr(II)O x,surf species and two Cr(VI)O x,surf species, in
which the Cr(II)O x,surf species act as the real center of active chromium precursor
and the residual Cr(VI)O x,surf species are also necessary components acting as the
neighboring ligand environment with electronic and steric effects. Three plausible
chemical structural models of the Cr
2+ ·2Cr
6+ cluster are proposed in Scheme 5,
based on the correlation between XPS and polymerization results and our previous
understanding of the surface chemical nature of calcined Phillips Cr(VI)O x /SiO 2
catalysts and pre-reduced Cr(II)O x /SiO 2 catalysts.
The
1 H and
27 Al MAS solid state NMR spectra clearly demonstrated that the
existing states of surface Al species in the TEA-modified Phillips catalysts strongly
Phillips Cr/Silica Catalyst for Ethylene Polymerization
147