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Supporting mechanism of non-toxic chromium (III) acetate on silica for preparation of Phillips ethylene polymerization catalysts.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2009; 4: 660–665
Published online 26 June 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.314
Special Theme Research Article
Supporting mechanism of non-toxic chromium (III) acetate
on silica for preparation of Phillips ethylene polymerization
catalysts
Pengyuan Qiu,1 Xiaofang Li,1 Shiliang Zhang,1 Ruihua Cheng,1 Qi Dong,1 Boping Liu*,1 , Liuzhong Li,2 Yongling Yu,2
Yan Tang,2 JianLing Xie,2 and Wenqing Wang2
1
2
State Key Lab of Chemical Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, China
Research Institute of Qilu Petrochemical Corp., SINOPEC Zibo 255400, Shandong, China
Received 2 November 2008; Revised 27 February 2009; Accepted 1 March 2009
ABSTRACT: Phillips catalyst is an important kind of industrial polyethylene catalyst. As early as in the late 1970s, CrO3
was substituted by chromium (III) acetate for the preparation of Phillips catalyst on the industrial scale owing to health
and environmental considerations. There is still considerable research focusing on the relations between the preparation
process and catalyst properties in academics. In this work, the supporting mechanism of chromium (III) acetate on
silica has been studied by Thermogravimetry–Differential Thermal Analysis (TG-DTA), and Electron Spin Resonance
(ESR), in comparison with that of supporting CrO3 on SiO2 . The basic chromium (III) acetate supported on high surface
area silica gel decomposed differently from that for bulk basic chromium acetate when decomposition temperature was
decreased by 15 ◦ C. The decomposition temperature was 299 ◦ C for Cr3 (OH)2 (Ac)7 /SiO2 catalyst precursor, which
would be firstly transferred into CrO3 followed by supporting on silica surface as chromate species. The further weight
loss came from thermal inductive reduction of chromate species into Cr2 O3 , which was also supported by the results of
colors of catalysts. Moreover, with the increase of chromium loading of Cr3 (OH)2 (Ac)7 /SiO2 , such thermal inductive
reduction became more severe. ESR spectra of Cr3 (OH)2 (Ac)7 /SiO2 and CrO3 /SiO2 catalyst precursors showed that
a small amount of supported Cr5+ can exist stably on silica gel surface at temperatures higher than 200 ◦ C.  2009
Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: basic chromium (III) acetate; CrO3 ; Phillips CrOx /SiO2 catalyst; Ethylene polymerization; ESR; TGDTA
INTRODUCTION
As an important industrial polyolefin catalyst, Phillips
CrOx /SiO2 catalyst is used for the production of several million tons of high density polyethylene (HDPE)
each year. Traditionally, this catalyst was prepared
from toxic CrO3 using silica gel as support by wet
impregnation method. From about 1978, trivalent Cr
compound, mostly basic chromium (III) acetate with
much weaker toxicity, has been used as raw material
instead of chromium (VI) trioxide for the preparation
of Phillips catalyst owing to environmental and health
considerations.[1 – 8] In the case of using basic chromium
(III) acetate as raw material, it would be firstly transferred into chromium (VI) trioxide, then transformed
*Correspondence to: Boping Liu, State Key Lab of Chemical
Engineering, East China University of Science and Technology,
Shanghai, PR China.
E-mail: boping@ecust.edu.cn
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
into mono-, di- or even polychromate species by esterification reaction with silanol groups during the thermal
activation process.[8] It is known that some researchers
have studied thermal activation of Phillips catalyst,
which is prepared by chromium (III) acetate as raw
material.[1,7,8,9,10] Ruddick et al . studied calcination
process of Phillips catalysts using basic chromium (III)
acetate and chromium (III) acetylacetonate by mass
spectroscopy (MS) and infrared spectroscopy (IR), the
authors concluded that chromium (III) acetate supported
on silica decomposed in a way similar to that observed
for bulk chromium acetate.[1] Liu utilized XPS to characterize the surface physico-chemical states of Phillips
catalyst. The results showed that chromium (III) acetate
could be oxidized and decomposed into bulk CrO3 even
at 120 ◦ C, then started to be transformed into supported
chromate species at 200 ◦ C and finished this esterification process at 400 ◦ C. Finally, a slight thermal
induced partial reduction of chromate species into Cr
(III) was observed at 600–800 ◦ C.[7] Also, McDaniel
et al . have investigated on the activation process of
Asia-Pacific Journal of Chemical Engineering
SUPPORTING MECHANISM OF NON-TOXIC CHROMIUM ACETATE
Phillips catalyst with several kinds of Cr compounds
as precursors and they observed only Cr6+ on the silica gel after calcination and each catalyst had a similar
polymerization activity.[9] However, Gaspar reported
that the distribution of Cr3+ /Cr6+ species in calcined
Cr/SiO2 catalysts depended on the precursor salt, and
the different distributions of Cr3+ /Cr6+ species caused
different polymerization activity.[8] This disagreement
about thermal activation of Phillips catalyst spurs us to
investigate this process, in order to get much deeper
understanding of the specific transformation process of
the valence states and its distribution of surface Cr
species from basic chromium (III) acetate into hexavalent chromate species during isothermal calcination
process.
The objective of this work is to get a much deeper
understanding of the structure and valence of Cr
species on Phillips CrOx /SiO2 catalyst prepared by
basic chromium (III) acetate as raw material. The catalyst samples prepared by different Cr compounds were
calcined at different temperatures, and were investigated by thermogravimetry–differential thermal analysis (TG–DTA), as well as electron spectroscopy resonance (ESR) methods. ESR is one of the most powerful tools to investigate the transition metal ion on
oxide surfaces.[11] However, as far as we know, it has
never been applied to study the structure and valence
nature of chromium (III) Phillips catalyst precursors
as well as its transformation during calcination process
for preparation of the final catalysts for ethylene polymerization. Three different Cr species can be detected
with ESR: isolated Cr5+ species, clustered Cr3+ and dispersed Cr3+ .[12] The combination of TG–DTA and ESR
is necessary in order to disclose the supporting mechanism of non-toxic basic chromium acetate on silica gel
support. At the same time, as a comparison, CrO3 was
also used as raw material to prepare Phillips CrOx /SiO2
catalyst.
EXPERIMENTAL SECTION
Preparation of the catalysts
The CrOx /SiO2 catalyst precursors were prepared by
wet impregnation of aqueous solutions using basic
chromium (III) acetate (Cr3 (OH)2 (Ac)7 ; Johnson
Matthey, 24% Cr content) on a high surface area silica
gel (Davison 955, surface area 299 m2 /g, pore volume
1.65 cm3 /g). After impregnation, the slurry was slowly
dried at 110 ◦ C for 12 h (the sample is named PCA110).
Thereafter, the solid was ground and finally calcined
in air for 6 h at 200, 300, 400 and 600 ◦ C to obtain
four Phillips catalysts named as PCA200, PCA300,
PCA400 and PCA600, respectively. Chromium loading
was changed from 0.5 wt% to 1 wt% and 2 wt%. For
each catalyst, 24 mg of catalyst precursor was placed
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
into a quartz tube. The tube was heated at a rate of
2 ◦ C min−1 to a certain temperature (200–600 ◦ C) and
then held at this temperature for 6 h prior to cooling down to room temperature. During the first stage
of the 6 h calcinations process, pure air was used as
gas media. It was switched to N2 (200 ml min−1 ) during the natural cooling process. As for Phillips catalyst precursor using CrO3 as raw material, the same
preparation process was undertaken, and the samples
calcined at 110, 200, 300, 400 and 600 ◦ C are named
PCT110, PCT200, PCT300, PCT400 and PCT600,
respectively. The chromium loading for PCT catalysts
is 1 wt%.
Characterizations
TG–DTA analyses were carried out in order to investigate how the supported catalyst precursors PCA110 and
PCT110 decompose over silica gel in a SDTQ 600 Thermal Analyzer under a flow of high purity air (100 ml
min−1 ) from 25 to 800 ◦ C at a temperature raising rate
of 10 ◦ C min−1 . And bulk basic chromium (III) acetate
and CrO3 were taken as reference for comparison.
ESR spectra of all samples were recorded on a
Bruker EMX-8/2.7 system at X band frequency at room
temperature (25 ◦ C) in quartz tubes. In general, the
spectra were measured at a microwave frequency of
about 9.842 GHz, and 100 kHz field modulation.
RESULTS AND DISCUSSION
TG–DTA characterization
TG–DTA characterization of PCA110 and PCT110 catalyst precursors was carried out in order to investigate
the decomposition of the basic chromium (III) acetate
and chromium trioxide dispersed on silica gel surface,
and corresponding bulk compounds were also taken as
reference for comparison.
Figure 1 shows the TG–DTA curves of bulk Cr3
(OH)2 (Ac)7 in dry air atmosphere at a temperature
increasing rate of 10 ◦ C min−1 . The main weight loss
process happened between 314 and 361 ◦ C, and the
total weight loss was 43%, which was very close to
the calculated weight loss of Cr3 (OH)2 (Ac)7 → CrO3 ,
i.e. 50.3%. The oxidative decomposition reaction of
basic chromium (III) acetate to bulk CrO3 happened
mainly between 314 and 361 ◦ C. An exothermic peak
appeared at 360 ◦ C was owing to the severest oxidative
decomposition reaction. The results were supported by
the work reported by Ruddick[1] and Gaspar.[5] The TG
curve showed another main weight loss process between
430 and 460 ◦ C, which indicated that CrO3 was unstable
Asia-Pac. J. Chem. Eng. 2009; 4: 660–665
DOI: 10.1002/apj
661
662
P. QIU ET AL.
Figure 1. TG–DTA analyses of bulk Cr3 (OH)2 (Ac)7 . This
figure is available in colour online at www.apjChemEng.
com.
at high temperature even in air and was converted into
Cr2 O3 at a temperature well below 500 ◦ C.[9,13]
Figure 2 presents the results from a TG–DTA run on
PCA110 precursor in dry air atmosphere. It was shown
that the decomposition temperature of catalyst precursor
PCA110 was 299 ◦ C, and there was an exothermic peak
at 347 ◦ C (Fig. 2). Comparison of Fig. 2 with Fig. 1
showed that the decomposition temperature decreased
about 13 ◦ C after the bulk Cr3 (OH)2 (Ac)7 was dispersed
on silica gel surface. After supporting, bulk chromium
(III) acetate was highly dispersed onto silica gel surface, which may induce the oxidative decomposition
reaction to occuring at lower temperature. The basic
chromium (III) acetate supported on high surface area
silica gel decomposed in a way different from that
observed for bulk basic chromium (III) acetate under
air atmosphere. These results were inconsistent with the
report by Ruddick,[1] who reported that chromium (III)
acetate supported on high surface area silica decomposed at 299 ◦ C in a way similar to that observed for
bulk chromium acetate under oxygen atmosphere. The
difference might be because of different carrier gas and
temperature raising rates.
Figure 3 presents the results from a TG–DTA run
on bulk CrO3 in dry air atmosphere. It can be seen
that there were two endotherms and one exotherm
peaks. Two endotherms peak at 207 and 492 ◦ C are
corresponding to the melting of CrO3 and the reduction
of CrO3 into Cr2 O3 , respectively, and a strong exotherm
at 347 ◦ C is corresponding to a partial reduction of
CrO3 into Cr2 [(Cr2 O7 )3 ]. These results are supported by
Guyot.[14] Figure 4 presents the results from a TG–DTA
run on PCT110 precursor in dry air atmosphere. It was
shown that there was an exotherm peak at 285 ◦ C, which
was ascribed to the esterification reaction between
bulk CrO3 and silanol groups on silica gel surface
and subsequently transformed into supported chromate
species.[15,16]
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
Figure 2.
TG–DTA analyses of Cr3 (OH)2 (Ac)7 /SiO2
catalyst precursor (PCA110).
Figure 3. TG–DTA analyses of bulk CrO3 .
Figure 4.
TG–DTA analyses of CrO3 /SiO2 catalyst
precursor (PCT110).
Colors of catalyst precursors
Figures 5 and 6 show the effect of calcination temperature on the colors of these two kinds (PCA and
PCT) of catalyst precursors. Though with different raw
Asia-Pac. J. Chem. Eng. 2009; 4: 660–665
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
SUPPORTING MECHANISM OF NON-TOXIC CHROMIUM ACETATE
Colors produced from the activation of
Cr3 (OH)2 (Ac)7 /SiO2 (1.0 wt%) at various temperatures
(from left to right: PCA110, PCA200, PCA300, PCA400
and PCA600, respectively). This figure is available in colour
online at www.apjChemEng.com.
Figure 5.
from the color of the samples. Both kinds of catalyst
precursors undergo oxidation and/or esterification reaction between 200 and 400 ◦ C, and a small part of CrO3
happens to transferred into more stable Cr2 O3 by thermal inductive reduction between 400 and 600 ◦ C. The
results are also supported by TG–DTA of the catalyst
precursors PCA110 and PCT110 as mentioned earlier
and agreed with the report by Liu.[7]
The influence of calcination temperature on the
colors of Cr3 (OH)2 (Ac)7 /SiO2 catalyst precursors with
different chromium loading has also been investigated
(Figs 5,7,8). The color changed from blue to orange,
then to green by calcination process, except PCA600
with 0.5 wt% Cr loading (Fig. 7), which was still orange
instead of green, the same color as PCA400.
From Figs 5, 7 and 8, it can be seen that at a
loading of 0.5–2 wt% Cr on silica gel, the CrO3 is
well stabilized as Cr (VI) (chromate species) at 400 ◦ C.
However, with the Cr loading increasing, more and
more chromate species are converted into Cr2 O3 during
further increasing calcination temperature from 400 to
600 ◦ C. It can be rationalized that water moisture from
dehydroxylation at high temperature might react with
chromate species to produce CrO3 , followed by further
decomposition into Cr2 O3 through thermal inductive
reduction as shown in Eqn. 1. The TG–DTA results
also support this point, which is in agreement with the
report by Hogan[13] .
ESR characterization
The ESR spectra of Cr3 (OH)2 (Ac)7 /SiO2 samples after
calcination at various temperatures are shown in Fig. 9.
Both the PCA110 and PCA200 samples had an isotropic
Colors produced from the activation of
CrO3 /SiO2 (1.0 wt%) at various temperatures (from left
to right: PCT110, PCT200, PCT300, PCT400 and PCT600,
respectively). This figure is available in colour online at
www.apjChemEng.com.
Figure 6.
materials, the colors of the two kinds of catalyst precursors were the same at temperature 400 ◦ C or above
(orange at 400 ◦ C, green at 600 ◦ C). However, the colors of these two kinds of catalyst precursors were
absolutely different initially when calcination temperature was 200 ◦ C or below. According to Hogan,[13]
for Phillips CrOx /SiO2 catalyst precursor prepared from
CrO3 using silica gel as support by wet impregnation
method, chromate species after esterification reaction
were orange, and the transformation of partial chromate
species into Cr2 O3 with more stable valence may happen at higher calcination temperature indicative from
the green color. A simple conclusion might be drawn
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Colors produced from the activation of
Cr3 (OH)2 (Ac)7 /SiO2 (0.5 wt%) at various temperatures
(from left to right: PCA110, PCA200, PCA300, PCA400
and PCA600, respectively). This figure is available in colour
online at www.apjChemEng.com.
Figure 7.
Asia-Pac. J. Chem. Eng. 2009; 4: 660–665
DOI: 10.1002/apj
663
664
P. QIU ET AL.
Asia-Pacific Journal of Chemical Engineering
O
O
Cr
O
O
+ H2O (gas)
OH
OH
Si
Si
Si
Si
O
O
O
+
O
Air
Cr
O
O
O
Cr
O
Cr
Chromate species
Colors produced from the activation of
Cr3 (OH)2 (Ac)7 /SiO2 (2.0 wt%) at various temperatures
(from left to right: PCA110, PCA200, PCA300, PCA400
and PCA600, respectively). This figure is available in colour
online at www.apjChemEng.com.
Figure 8.
and broad signal (g = 1.98), which was assigned to
Cr3+ ion. It was noted that the same ESR signal
was observed for bulk basic chromium acetate (not
shown). After the calcination temperature increased
up to 300 ◦ C, this ESR signal disappeared and was
substituted by another ESR signal, which was a sharp
axially symmetric signal around g = 1.97 of the γ
signal of Cr5+ according to the literature.[17] Owing
to the absence of ESR signal of Cr3+ at 300 ◦ C, it
was concluded that the oxidation reaction of basic
chromium (III) acetate to CrO3 was completed before
300 ◦ C. It was very interesting to note that a weak
γ signal can still be measured even at 600 ◦ C. It is
known that the bulk Cr5+ is rather unstable and can be
easily disproportionate to Cr3+ and Cr6+ .[12] It is most
probably that small amount of Cr5+ can exist as a stable
state by anchoring onto silica gel in a certain form.
As to PCT110, no ESR signal was observed (Fig. 10),
and it was similar to bulk CrO3 (not shown). When the
calcination temperature reached 200 ◦ C, a weak γ signal
could be measured and maintained stable up to 600 ◦ C.
This phenomenon indicated that a small amount of Cr
(VI) was reduced to Cr (V) during calcination process,
which can exist as a stable state probably by anchoring
onto silica gel in a certain form. The ESR spectra of
Cr3 (OH)2 (Ac)7 /SiO2 sample with different chromium
loading after calcination at various temperatures were
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 9. ESR spectra of Cr3 (OH)2 (Ac)7 /SiO2 sample after
calcination at various temperatures in air (from top to
bottom: PCA110, PCA200, PCA300, PCA400 and PCA600,
respectively). This figure is available in colour online at
www.apjChemEng.com.
also studied, but there was no difference from those of
the samples with 1 wt% Cr loading.
CONCLUSION
Phillips CrOx /SiO2 catalysts have been prepared at different Cr loading and calcination temperatures using
non-toxic basic chromium (III) acetate and toxic CrO3
as the raw material, respectively. And the supporting
mechanism of basic chromium (III) acetate over silica gel has been investigated by comparison with that
of CrO3 on silica gel. The decrease of the decomposition temperature was about 15 ◦ C under air atmosphere after Cr3 (OH)2 (Ac)7 was being supported on
silica gel. It was suggested that high dispersion of
bulk basic chromium acetate onto silica support would
induce the oxidative decomposition reaction to occur
at lower temperature. Thus, the basic chromium (III)
acetate supported on silica decomposed into CrO3 in a
Asia-Pac. J. Chem. Eng. 2009; 4: 660–665
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
SUPPORTING MECHANISM OF NON-TOXIC CHROMIUM ACETATE
665
color of samples. The ESR results indicated that a small
amount of Cr5+ supported on silica gel can be produced
at temperature higher than 200 ◦ C and can be very stable
up to 600 ◦ C.
Acknowledgement
This work is financially supported by the National Natural Science Foundation of China (No. 20774025) and
Research Institute of Qilu Petrochemical CorporationSINOPEC.
REFERENCES
ESR spectra of CrO3 /SiO2 sample after
calcination at various temperatures in air (from top to
bottom: PCT110, PCT200, PCT300, PCT400 and PCT600,
respectively). This figure is available in colour online at
www.apjChemEng.com.
Figure 10.
way different from that of bulk basic chromium acetate.
Thereafter, esterification reaction of CrO3 with silanol
groups on silica gel surface gave rise to the supported
chromate species. Both reactions were completed below
400 ◦ C. Further weight loss observed at higher temperature was ascribed to the transformation of chromate
species into Cr2 O3 owing to thermal induced reduction.
With chromium loading increase, such thermal induced
reduction became more severe. This was supported by
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
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Asia-Pac. J. Chem. Eng. 2009; 4: 660–665
DOI: 10.1002/apj
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