318
J. Handzlik
3 CrO x /SiO 2 System
Chromia–silica system is well known as the Phillips catalyst for ethene polymerization, discovered in 1950s [17–20]. Approximately 40–50% of the worldwide production of high-density polyethylene is related to this catalyst [17]. A unique property of
the Phillips catalyst is the fact that no cocatalyst is necessary to achieve the catalytic
activity, in contrast to other transition metal systems used for ethene polymerization,
such as Ziegler–Natta and metallocene catalysts. Silica-supported chromium oxide
systems are also effective in other catalytic reactions, like dehydrogenation of alkanes [21], oxidative dehydrogenation of hydrocarbons in presence of oxygen [22, 23]
or carbon dioxide [24, 25], and various selective oxidation reactions [20, 26–28].
3.1 Structure of Surface Chromium Oxide
Species—Experimental Data
Many techniques, including UV-vis, Raman, XANES (X-ray absorption near-edge
structure), EXAFS (extended X-ray absorption fine structure), EPR (electron paramagnetic resonance), XPS (X-ray photoelectron spectroscopy) and IR (infrared) spectroscopy, have been used to characterize chromium oxide species on silica. After hightemperature calcination of chromia–silica systems, well-dispersed surface Cr(VI)
oxide species are mainly present [17–26, 28–36], although small amounts of Cr(V)
and Cr(III) species were also detected, as well as Cr 2 O 3 clusters at higher Cr loadings
[17, 19–24, 26, 32, 34]. The nature of the Cr(VI) species on silica has been discussed
for many years. Monomeric, dimeric and polymeric Cr(VI) species were often postulated, mainly based on the UV-vis DRS (diffuse reflectance spectroscopy) data
[19, 20, 24]. On the other hand, Raman spectroscopy, UV-vis, XANES and EXAFS
studies conclusively demonstrated that the monomeric form is the major or even the
only surface Cr(VI) oxide species [19, 23, 26, 29–36]. It is described as a tetrahedral
dioxo species with two Cr–O–Si linkages (Fig. 2a). Minor five-coordinate monooxo
Cr(VI) species, bonded to the silica surface by four Cr–O–Si linkages (Fig. 2b), was
also suggested [32–34]; however, this proposal has been recently questioned [35,
36].
Even more complex and problematic is nature of reduced chromium species,
which seems to be strongly dependent on the reduction conditions. If CO is used
as the reducing agent, Cr(VI) species are almost selectively converted to Cr(II) [5,
17–20, 30, 37–39], whereas reduction with H 2 results in Cr(III) [34, 37] or both
Cr(III) and Cr(II) [5, 24] oxidation states. The presence of water during reduction
favours formation of Cr(III) species [5]. When the calcinated Phillips catalyst is
contacted with ethene, Cr(VI) is reduced to Cr(II) and/or Cr(III) [17–19, 34, 40, 41].
According to the UV-vis DRS studies, there are three distinct reduced chromium
oxide species on silica: pseudo-tetrahedral Cr(II), pseudo-octahedral Cr(II) and
pseudo-octahedral Cr(III) [20, 24, 42]. In addition to the surface reduced chromium
J. Handzlik
3 CrO x /SiO 2 System
Chromia–silica system is well known as the Phillips catalyst for ethene polymerization, discovered in 1950s [17–20]. Approximately 40–50% of the worldwide production of high-density polyethylene is related to this catalyst [17]. A unique property of
the Phillips catalyst is the fact that no cocatalyst is necessary to achieve the catalytic
activity, in contrast to other transition metal systems used for ethene polymerization,
such as Ziegler–Natta and metallocene catalysts. Silica-supported chromium oxide
systems are also effective in other catalytic reactions, like dehydrogenation of alkanes [21], oxidative dehydrogenation of hydrocarbons in presence of oxygen [22, 23]
or carbon dioxide [24, 25], and various selective oxidation reactions [20, 26–28].
3.1 Structure of Surface Chromium Oxide
Species—Experimental Data
Many techniques, including UV-vis, Raman, XANES (X-ray absorption near-edge
structure), EXAFS (extended X-ray absorption fine structure), EPR (electron paramagnetic resonance), XPS (X-ray photoelectron spectroscopy) and IR (infrared) spectroscopy, have been used to characterize chromium oxide species on silica. After hightemperature calcination of chromia–silica systems, well-dispersed surface Cr(VI)
oxide species are mainly present [17–26, 28–36], although small amounts of Cr(V)
and Cr(III) species were also detected, as well as Cr 2 O 3 clusters at higher Cr loadings
[17, 19–24, 26, 32, 34]. The nature of the Cr(VI) species on silica has been discussed
for many years. Monomeric, dimeric and polymeric Cr(VI) species were often postulated, mainly based on the UV-vis DRS (diffuse reflectance spectroscopy) data
[19, 20, 24]. On the other hand, Raman spectroscopy, UV-vis, XANES and EXAFS
studies conclusively demonstrated that the monomeric form is the major or even the
only surface Cr(VI) oxide species [19, 23, 26, 29–36]. It is described as a tetrahedral
dioxo species with two Cr–O–Si linkages (Fig. 2a). Minor five-coordinate monooxo
Cr(VI) species, bonded to the silica surface by four Cr–O–Si linkages (Fig. 2b), was
also suggested [32–34]; however, this proposal has been recently questioned [35,
36].
Even more complex and problematic is nature of reduced chromium species,
which seems to be strongly dependent on the reduction conditions. If CO is used
as the reducing agent, Cr(VI) species are almost selectively converted to Cr(II) [5,
17–20, 30, 37–39], whereas reduction with H 2 results in Cr(III) [34, 37] or both
Cr(III) and Cr(II) [5, 24] oxidation states. The presence of water during reduction
favours formation of Cr(III) species [5]. When the calcinated Phillips catalyst is
contacted with ethene, Cr(VI) is reduced to Cr(II) and/or Cr(III) [17–19, 34, 40, 41].
According to the UV-vis DRS studies, there are three distinct reduced chromium
oxide species on silica: pseudo-tetrahedral Cr(II), pseudo-octahedral Cr(II) and
pseudo-octahedral Cr(III) [20, 24, 42]. In addition to the surface reduced chromium