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Silica polyethylene polymerisation catalyst by In-Situ infrared spectroscopy
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
3843
Investigation of a Cr/Silica Polyethylene Polymerisation Catalyst by In-Situ Infrared Spectroscopy Stewart F. Parker*, Christopher C. A. Riley and John E. Baker British Petroleum International, Group Research and Engineering, Chertsey Road, Sunburyon-Thames, Middlesex TW 16 7LN, UK. 1. I N T R O D U C T I O N Chromium based Phillips-type catalysts are employed for the production of high density polyethylene [1]. Despite the commercial importance of this process many aspects of the catalytic chemistry are incompletely understood. The aim of the work reported here is twofold. Firstly, to follow the activation of a commercial Phillips catalyst by in-situ infrared spectroscopy and secondly to study the growth of polyethylene on the catalyst immediately after exposure to ethylene. 2. EXPERIMENTAL The catalyst used throughout this work was EP30 (Cr(0.95wt%)/SiO2) supplied by Crosfield Catalysts. The catalyst (ca 50 mg) was loaded into the sample cup of a SpectraTech Catalytic Reaction Environmental Chamber in a SpectraTech Collector diffuse reflectance accessory. The cell was mounted in the sample compartment of a Digilab FTS-60A Fourier transform infrared spectrometer equipped with a narrowband high sensitivity mercury cadmium telluride detector, all spectra were recorded at 4 cm -~ resolution with triangular apodisation. The spectrometer was located inside a continuously extracted cabinet. Helium, dry air, carbon monoxide and ethylene were supplied on separate gas lines, each controlled by a Krohne mass flow controller. The total pressure was controlled by a back pressure regulator on the exhaust line. The system used commercial grade gases and the traces of oxygen and water present in these are sufficient to deactivate the catalyst. Clean-up traps (Phase Separation) filled with one-third Oxypure (in all lines, apart from the air feed, to remove O2) and two-thirds 5A molecular sieve (to remove water) were installed on the low pressure side of the gaslines. The catalyst was activated using the following conditions: i) Ramp to 500~ at 10 ~ per minute in air (50 ml/min) ii) Hold at 500~ for one hour in air iii) Cool to 350~ under helium (20 ml/min)
*Author to whom correspondence should be addressed. Present address: ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK Permission to publish has been given by BPAmoco plc
3844
iv) Hold at 350~ for 30 minutes under carbon monoxide (5 ml/min) v) Flush with helium at 350~ for 30 minutes (20 ml/min) vi) Cool to room temperature under helium. After activation, ethylene, either pure or diluted in helium, was flowed over the catalyst and spectra recorded as a function of time. 3. RESULTS AND DISCUSSION 3.1 The Activation Procedure A typical set of results activation procedure is shown in Fig. 1 a-f. For the untreated catalyst, Fig. 1 a, the spectrum is dominated by the broad absorption of the O-H stretch of hydrogen bonded adsorbed water centred at 3400 cm -1. The O-H bend is located at 1600 cm -1. The sharp band at 3750 cm -1 is assigned to isolated silanol (Si-O-H) groups (3). The remaining bands are assigned to fundamentals, overtones and combinations of the SiO2 support. Heating to 500~ in air, Fig. l b and l c, results in loss of the majority of the hydrogenbonded and adsorbed water, leaving silanol groups, both hydrogen bonded and free, on the surface. Introduction of CO at 350~ results in a gas phase CO spectrum as expected, Fig. l d, however, the envelope is distorted from the normal P and R branch structure, indicating the presence of an adsorbed CO species that is overlapped by the gas phase R branch. Flushing
9
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3000 2000 W a v e n u m b e r s / c m -1
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Fig.1. Activation of Cr(l%)/SiO2 (a) as received, at room temperature, (b) at 500~ under air, time = 0, (c) at 500~ under air, time - 1 hr, (d) at 350~ under CO, (e) at 350~ under He, (f) room temperature under He.
3845 with helium at 350~ removes both the gas phase and the adsorbed CO, see Fig. le. Cooling to room temperature in flowing helium, Fig. l f, results in a small CO band at 2188 cm l , presumably due to pick-up from the traces of CO present in the helium. From the work on CO chemisorption described later, it is estimated that the CO coverage was --20% of the saturation coverage. (The helium and CO lines, both feed into a common line which is then fed to the cell, thus there will be adsorbed CO on the stainless steel tube, slow desorption of this and subsequent adsorption by the catalyst is probably sufficient to account for the CO). The assignment of the adsorbed CO bands will be discussed further later. Spectroscopically, the effect of the activation procedure is to decrease the amount of water and silanols present. Visually, the initially orange catalyst changes to a pale green after activation, consistent with reduction of Cr(VI) to a lower oxidation state.
3.2 Ethylene Polymerisation The results of the introduction of ethylene at room temperature are shown in Fig. 2. The growth of the asymmetric and symmetric C-H stretch modes of the methylene groups of polyethylene are clearly seen at 2940 and 2850 cm l respectively, in addition to the gas phase ethylene bands. In agreement with previous work [2], there is no evidence for the presence of methyl groups (bands at 2967 and 2884 cm-l).
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Fig. 2: Growth of polyethylene Cr(l%)/SiOz.from He/CzH4(4%). Spectra are shown sequentially at 1 minute intervals. After 5 minutes, C2D4 was switched for the He/CzH4(4%). (Trace marked D onwards). The feature marked with ? may represent an intermediate species.
3846 After 5 minutes reaction, the gas feed was switched to 100 % C2D4 (trace marked D in Fig. 2). This is seen in the spectra by the immediate appearance of bands assigned to perdeuteropolyethylene at 2195 and 2085 cm 1. However, the C-H bands continue to grow as the C-D bands appear. In part this may be due to some residual C2H4 in the gas lines, but the quantity seems too large for this and suggests that there is a substantial reservoir on the surface. This is supported by the observation of a weak band at 2987 cm -~ that is present at constant intensity throughout the reaction, which suggests that it is an intermediate species (band marked by ? in Fig. 2). This band has been previously assigned [3,4] to ethylene ~bonded to Cr. The persistence of the band in the presence of C2D4 indicates either slow exchange between the gas phase and the adsorbed state or that the species is a stable byproduct rather than an intermediate. There is no overlap of the gas phase ethylene bands with the 2850 cm -1 band of polyethylene. The area of the band can be used directly, therefore, without having to subtract a gas phase contribution, to give a measure of the extent of polymerisation. Fig. 3 shows the area of the 2850 cm -1 band as a function of time (for an experiment using 100% C2H4 and no isotopic switch). There is an initial rapid, almost linear, growth; measurements after several hours show a plateau and a slow fall-off. This is probably due to blocking of the catalyst pores by polymer. It is noteworthy that most of the adsorbed CO is immediately displaced by ethylene at room temperature. From the spectra (not shown), at the first instance of gas phase ethylene being present, most of the adsorbed CO has gone.
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Time/secs Fig. 3: Area of 2850 cm -~ band of polyethylene as a function of time.
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3847
3.3 CO Adsorption The activation procedure in this instance was somewhat different in that the sample was heated to 500~ in flowing helium and then switched to air. The sample was cooled to 350~ as usual and then exposed to CO. At the end of the CO reduction period instead of cooling in helium the sample was cooled in CO to room temperature and then flushed with helium. Fig. 4 shows the region 2 2 5 0 - 2050 cm 1 at different stages in this process. Subtraction of the gas phase CO contribution allows the adsorbed CO to be clearly seen. The subtraction reveals a number of interesting features. The position of the peak at 2183 cm -1 does not change between 350~ Fig. 4a, and room temperature, Fig. 4b, although its intensity does. This suggests that the same type of site is populated at both temperatures, although more sites are populated at the lower temperature. Interestingly, an additional site is populated at room temperature, as shown by the band at 2097 cm -I in Fig. 4b. Flushing the gas phase away shows that the linear site is still populated, however, the bridged site is no longer visible, Fig. 4c. The CO is desorbed in helium as shown by Fig. 4d (100~ and 4e (150~ At 200~ there was no evidence for adsorbed CO.
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Fig. 4: Adsorption of CO on Cr(1%)/SIO2. (a) at 350~ under 1 bar CO (gas phase CO subtracted), (b) at room temperature under 1 bar CO (gas phase CO subtracted), (c) at room temperature after flushing with He, (d) at 100~ under 1 bar He, (e) at 150~ under 1 bar He.
3848 The literature [2] assigns the band at 2183 cm l to linear CO and the band at 2097 cm ~ to two-fold bridging CO. (The latter has been used as evidence for binuclear Cr sites being the active centre). The linear CO band is usually described as a triplet, however, the band contour is pressure dependent [2] and as most of the literature has employed a maximum pressure of 40 torr rather than atmospheric pressure as used here, some differences are not unexpected. 4. CONCLUSIONS It has been shown that it is possible to follow the activation of a commercial Cr/silica catalyst and the polymerisation of ethylene by in-situ infrared spectroscopy. The growth of alkyl chains on the surface of the active catalyst has been followed. The presence of a surface species, assigned to ethylene n-bonded to Cr has been observed. CO adsorption experiments show that both mononuclear and dinuclear Cr sites exist on the surface, the latter is only observable in the presence of CO. Adsorbed CO is displaced very rapidly by ethylene. REFERENCES 1. C. E. Marsden, Plastics, Rubber and Composites Processing and Applications, 21 (1994) 193. 2. G. Ghiotti, G. Garrone and A. Zecchina, J. Mol. Cat. 46 (1988) 61. 3. B. Rebenstorf, J. Mol. Cat., 45 (1988) 263. 4. O. M. Bade, R. Blom, I. M. Dahl and A. Karlsson, J. Cat. 173 (1998) 460.
3843
Investigation of a Cr/Silica Polyethylene Polymerisation Catalyst by In-Situ Infrared Spectroscopy Stewart F. Parker*, Christopher C. A. Riley and John E. Baker British Petroleum International, Group Research and Engineering, Chertsey Road, Sunburyon-Thames, Middlesex TW 16 7LN, UK. 1. I N T R O D U C T I O N Chromium based Phillips-type catalysts are employed for the production of high density polyethylene [1]. Despite the commercial importance of this process many aspects of the catalytic chemistry are incompletely understood. The aim of the work reported here is twofold. Firstly, to follow the activation of a commercial Phillips catalyst by in-situ infrared spectroscopy and secondly to study the growth of polyethylene on the catalyst immediately after exposure to ethylene. 2. EXPERIMENTAL The catalyst used throughout this work was EP30 (Cr(0.95wt%)/SiO2) supplied by Crosfield Catalysts. The catalyst (ca 50 mg) was loaded into the sample cup of a SpectraTech Catalytic Reaction Environmental Chamber in a SpectraTech Collector diffuse reflectance accessory. The cell was mounted in the sample compartment of a Digilab FTS-60A Fourier transform infrared spectrometer equipped with a narrowband high sensitivity mercury cadmium telluride detector, all spectra were recorded at 4 cm -~ resolution with triangular apodisation. The spectrometer was located inside a continuously extracted cabinet. Helium, dry air, carbon monoxide and ethylene were supplied on separate gas lines, each controlled by a Krohne mass flow controller. The total pressure was controlled by a back pressure regulator on the exhaust line. The system used commercial grade gases and the traces of oxygen and water present in these are sufficient to deactivate the catalyst. Clean-up traps (Phase Separation) filled with one-third Oxypure (in all lines, apart from the air feed, to remove O2) and two-thirds 5A molecular sieve (to remove water) were installed on the low pressure side of the gaslines. The catalyst was activated using the following conditions: i) Ramp to 500~ at 10 ~ per minute in air (50 ml/min) ii) Hold at 500~ for one hour in air iii) Cool to 350~ under helium (20 ml/min)
*Author to whom correspondence should be addressed. Present address: ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK Permission to publish has been given by BPAmoco plc
3844
iv) Hold at 350~ for 30 minutes under carbon monoxide (5 ml/min) v) Flush with helium at 350~ for 30 minutes (20 ml/min) vi) Cool to room temperature under helium. After activation, ethylene, either pure or diluted in helium, was flowed over the catalyst and spectra recorded as a function of time. 3. RESULTS AND DISCUSSION 3.1 The Activation Procedure A typical set of results activation procedure is shown in Fig. 1 a-f. For the untreated catalyst, Fig. 1 a, the spectrum is dominated by the broad absorption of the O-H stretch of hydrogen bonded adsorbed water centred at 3400 cm -1. The O-H bend is located at 1600 cm -1. The sharp band at 3750 cm -1 is assigned to isolated silanol (Si-O-H) groups (3). The remaining bands are assigned to fundamentals, overtones and combinations of the SiO2 support. Heating to 500~ in air, Fig. l b and l c, results in loss of the majority of the hydrogenbonded and adsorbed water, leaving silanol groups, both hydrogen bonded and free, on the surface. Introduction of CO at 350~ results in a gas phase CO spectrum as expected, Fig. l d, however, the envelope is distorted from the normal P and R branch structure, indicating the presence of an adsorbed CO species that is overlapped by the gas phase R branch. Flushing
9
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3000 2000 W a v e n u m b e r s / c m -1
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Fig.1. Activation of Cr(l%)/SiO2 (a) as received, at room temperature, (b) at 500~ under air, time = 0, (c) at 500~ under air, time - 1 hr, (d) at 350~ under CO, (e) at 350~ under He, (f) room temperature under He.
3845 with helium at 350~ removes both the gas phase and the adsorbed CO, see Fig. le. Cooling to room temperature in flowing helium, Fig. l f, results in a small CO band at 2188 cm l , presumably due to pick-up from the traces of CO present in the helium. From the work on CO chemisorption described later, it is estimated that the CO coverage was --20% of the saturation coverage. (The helium and CO lines, both feed into a common line which is then fed to the cell, thus there will be adsorbed CO on the stainless steel tube, slow desorption of this and subsequent adsorption by the catalyst is probably sufficient to account for the CO). The assignment of the adsorbed CO bands will be discussed further later. Spectroscopically, the effect of the activation procedure is to decrease the amount of water and silanols present. Visually, the initially orange catalyst changes to a pale green after activation, consistent with reduction of Cr(VI) to a lower oxidation state.
3.2 Ethylene Polymerisation The results of the introduction of ethylene at room temperature are shown in Fig. 2. The growth of the asymmetric and symmetric C-H stretch modes of the methylene groups of polyethylene are clearly seen at 2940 and 2850 cm l respectively, in addition to the gas phase ethylene bands. In agreement with previous work [2], there is no evidence for the presence of methyl groups (bands at 2967 and 2884 cm-l).
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Fig. 2: Growth of polyethylene Cr(l%)/SiOz.from He/CzH4(4%). Spectra are shown sequentially at 1 minute intervals. After 5 minutes, C2D4 was switched for the He/CzH4(4%). (Trace marked D onwards). The feature marked with ? may represent an intermediate species.
3846 After 5 minutes reaction, the gas feed was switched to 100 % C2D4 (trace marked D in Fig. 2). This is seen in the spectra by the immediate appearance of bands assigned to perdeuteropolyethylene at 2195 and 2085 cm 1. However, the C-H bands continue to grow as the C-D bands appear. In part this may be due to some residual C2H4 in the gas lines, but the quantity seems too large for this and suggests that there is a substantial reservoir on the surface. This is supported by the observation of a weak band at 2987 cm -~ that is present at constant intensity throughout the reaction, which suggests that it is an intermediate species (band marked by ? in Fig. 2). This band has been previously assigned [3,4] to ethylene ~bonded to Cr. The persistence of the band in the presence of C2D4 indicates either slow exchange between the gas phase and the adsorbed state or that the species is a stable byproduct rather than an intermediate. There is no overlap of the gas phase ethylene bands with the 2850 cm -1 band of polyethylene. The area of the band can be used directly, therefore, without having to subtract a gas phase contribution, to give a measure of the extent of polymerisation. Fig. 3 shows the area of the 2850 cm -1 band as a function of time (for an experiment using 100% C2H4 and no isotopic switch). There is an initial rapid, almost linear, growth; measurements after several hours show a plateau and a slow fall-off. This is probably due to blocking of the catalyst pores by polymer. It is noteworthy that most of the adsorbed CO is immediately displaced by ethylene at room temperature. From the spectra (not shown), at the first instance of gas phase ethylene being present, most of the adsorbed CO has gone.
25'
20 O0 O0
QO
0
15"
10'
.
O
v ~0
0
16o
260
360
a6o
Time/secs Fig. 3: Area of 2850 cm -~ band of polyethylene as a function of time.
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3847
3.3 CO Adsorption The activation procedure in this instance was somewhat different in that the sample was heated to 500~ in flowing helium and then switched to air. The sample was cooled to 350~ as usual and then exposed to CO. At the end of the CO reduction period instead of cooling in helium the sample was cooled in CO to room temperature and then flushed with helium. Fig. 4 shows the region 2 2 5 0 - 2050 cm 1 at different stages in this process. Subtraction of the gas phase CO contribution allows the adsorbed CO to be clearly seen. The subtraction reveals a number of interesting features. The position of the peak at 2183 cm -1 does not change between 350~ Fig. 4a, and room temperature, Fig. 4b, although its intensity does. This suggests that the same type of site is populated at both temperatures, although more sites are populated at the lower temperature. Interestingly, an additional site is populated at room temperature, as shown by the band at 2097 cm -I in Fig. 4b. Flushing the gas phase away shows that the linear site is still populated, however, the bridged site is no longer visible, Fig. 4c. The CO is desorbed in helium as shown by Fig. 4d (100~ and 4e (150~ At 200~ there was no evidence for adsorbed CO.
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Fig. 4: Adsorption of CO on Cr(1%)/SIO2. (a) at 350~ under 1 bar CO (gas phase CO subtracted), (b) at room temperature under 1 bar CO (gas phase CO subtracted), (c) at room temperature after flushing with He, (d) at 100~ under 1 bar He, (e) at 150~ under 1 bar He.
3848 The literature [2] assigns the band at 2183 cm l to linear CO and the band at 2097 cm ~ to two-fold bridging CO. (The latter has been used as evidence for binuclear Cr sites being the active centre). The linear CO band is usually described as a triplet, however, the band contour is pressure dependent [2] and as most of the literature has employed a maximum pressure of 40 torr rather than atmospheric pressure as used here, some differences are not unexpected. 4. CONCLUSIONS It has been shown that it is possible to follow the activation of a commercial Cr/silica catalyst and the polymerisation of ethylene by in-situ infrared spectroscopy. The growth of alkyl chains on the surface of the active catalyst has been followed. The presence of a surface species, assigned to ethylene n-bonded to Cr has been observed. CO adsorption experiments show that both mononuclear and dinuclear Cr sites exist on the surface, the latter is only observable in the presence of CO. Adsorbed CO is displaced very rapidly by ethylene. REFERENCES 1. C. E. Marsden, Plastics, Rubber and Composites Processing and Applications, 21 (1994) 193. 2. G. Ghiotti, G. Garrone and A. Zecchina, J. Mol. Cat. 46 (1988) 61. 3. B. Rebenstorf, J. Mol. Cat., 45 (1988) 263. 4. O. M. Bade, R. Blom, I. M. Dahl and A. Karlsson, J. Cat. 173 (1998) 460.