Langmuir 1991, 7, 2160-2165
2160
Influence of Chromium Concentration and Addition of Fluorine, Titanium, or Boron on the Chromium Species of the Phillips Catalyst: A Quantitative Evaluation B. Rebenstorf and T.-C. Shengt Research Group on Catalysis, Inorganic Chemistry 1, University of Lund, Box 124, S-221 00 Lund, Sweden Received January 9,1991. I n Final Form: April 29, 1991 A new chromium(I1) surface species on the reduced Phillips catalyst is observed at low temperatures with a characteristic CO infrared band at 2027 cm-l (together with two more bands at 2120 and 2100 cm-1). This speciesis named chromium(I1)D and astructure model is proposed consistingof a dinuclear chromium(11) surface complex in which one chromium ion has three oxygen ligands and the other one only two oxygen ligands. Three chromium(I1) surface species (including the D)are determined quantitatively by low-temperature infrared spectra of adsorbed CO at 2027 (D), 2035 (A),and 2047 cm-1 (C). Specific trends in the relative amounts of the different chromium surface species are observed by varying the chromium concentration and modifying the catalyst with fluorine, titanium, or boron. The results are comparedwith catalyst performance in the industrial polymerization of ethylene as published in the literature. Modifying the catalyst by fluorine decreases the relative amounts of the C and D species, resulting in a more uniform distribution of catalytically active chromium(I1) surface sites [mainly chromium(I1) A species] and a narrower molecular weight distribution for the produced polymer. Modifying the catalyst with titanium increases the relative amount of the C species (from 9 to 45 76) and increases the polymer melt index (lower molecular weight of the polymer).
Introduction In recent years the Phillips catalyst for the polymerization of ethylene [chromium(VI) on silica gel]' has been modified by titanium and fluorine in order to improve the catalyst performancee2 This in turn has led to new interest in scientific studies of this catalyst. ' Since the first description of the Phillips catalyst many researchers have been involved in identifying and characterizing the polymerization-active chromium It is now established that chromium(I1) and chromium(111) surface ions can be active polymerization centers1*l6 and that chromium(I1) can be either two- or threecoordinated by oxygen ligands from the s ~ p p o r t . ~The J~J~ third oxygen ligand may be from either a surface silanol or siloxane group18and the chromium surface compounds can be either mononuclear or d i n ~ c l e a r . ~The J ~ latter ones show characteristic infrared bands from bridging CO molecules at 2120, 2100, and 2035 cm-l. t On leave from the Department of Chemistry, the School of Chemistry,ShandongUniversity, Jinan,Shandong,250100,People's Republic of China. (1) Clark, A.; Hogan, J. P.; Banks, R. L.; Laming, W. C. Znd. Eng. Chem. 1956,48,1152. (2)Karol, F.J.; Wagner, B. E.; Levine,I. J.; Goeke, G. L.; Noehay, A. In Advances in Polyolefins; Proceedings of the ACS International Sympoeium on Recent Advances in Polyolefins;Seymour, R. B., Cheng, T., Eds.; Plenum: New York, 1987;p 337. (3)Kraw, H. L.; Stach, H. Inorg. Nucl. Chem. Lett. 1968,4,393. ~~
~
(4)Baker, L. M.;Carrick, W. L. J. Org. Chem. 1968,33,616. (5)Krause, H. L.; Rebenetorf, B.; Weetphal, U. Z. Anorg. A&. Chem. 1975,414, 97. (6) Zecchina, A,; Garrone, E.; Ghiotti, G.; Morterra, C.; Borello, E. J. Phys. Chem. 1975,79,966. (7)Zecchina, A,; Garrone, E.; Ghiotti, G.; Coluccia, S.J. Phys. Chem. 197K. - - - -7 ,9. 972. (8)Rebenstorf, B.; Larsson, R. Z. Anorg. Allg. Chem. 1981,478,119. (9)Rebenstorf,B.; Lareaon, R. J. Mol.Catal. 1981,11, 247. (10)Rebenstorf, B. Z. Anorg. Allg. Chem. 1984,513,103. (11)Myers, D. L.; Lunaford, J. H. J. Catal. 1985,92, 260. (12)Rebenstorf,B. J. Mol.Catal. 1986,38,355. (13)Merryfield, R.;McDaniel, M. P.;Parks, G. J. Catal. 1982,77,348. (14)Luneford, J. H.; Fu, S.-L.; Myers, D. L. J. Catal. 1988,11, 231. (15)Ghiotti, G.; Garrone, E.; Zecchina, A. J. Mol. Catal. 1988,46,61. (16)Rebenstorf,B. 2.Anorg. Allg. Chem. 1989,571,148. (17)Rebenstorf,B. J. Mol. Catal. 1989,56,170. (18)K r a w , H. L.;Naumann, D. 2. Anorg. Allg. Chem. 1978,446,23.
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0743-7463 f 91f 2407-2160$02.50/0
Experimental Section A. Preparation of the Chromium Samples. The silica gel 952 (Grace, Davison; specific surface area 280 m2/g)was used as the support without further purification. For the unmodified chromiumsamples the appropriate amount of Cr03 in water was added and the mixture dried at 120 OC in air. Samplesmodified by fluorinewere prepared in the sameway with the only difference that 2.5 w t % (NH4)tSiFswas added to the water solution.1eThe sample modified by titanium was the same as used previously [silica gel 60 (Merck 7733), 0.5% Cr, 0.2 mmol Ti].m Samples modified by boron were prepared from silica gel 952 by adding the necessary amount of Ha03 in water, drying this mixture at 120 "C in air, and calcining the resulting material at 700 "C in air. Chromium (0.5% Cr) was added as chromium acetylacetonate in ethanol and the product was dried at 80 "C in air. B. Spectra. Samples (65 mg) were pressed into a selfsupportingdisk and placed in an infrared cell described before.ll The infrared cell was heated to 500 O C under vacuum and 0 2 added for 10min. Thereafterthe infrared cell was further heated under vacuum to 800 OC, 0 2 added for 30 min, and the sample cooled to 350 O C . The 02 waa removed and the sample reduced withCOfor 30min. TheCOwasremovedat 300°C byevacuation, the infrared cell cooled to room temperature, and the first FTIR spectrum recorded. Now CO (10 Wa) was added to the infrared cell, the sample cooled to -120 "C, and the second spectrum recorded. Thereafter the infrared cell was evacuated at room temperature and for a short time (1min) at 400 OC and two more spectra were measured at each step. The first FTIR spectrum was subtractedfrom all other spectra. This preparation procedure was used for the samples with varied chromium concentration, and the samples were modified by fluorine and titanium. The sample noted as Cr2 and those modified by boron were oxidized with 02 at 800 O C and reduced with CO at 350 O C for only 10 or 15 min, respectively. The FTIR spectra were recorded on a Nicolet 2OSXC spectrometer with a resolution of 2 cm-' and 1024 spectra were averaged. If necessary, spectra were smoothed before the second derivative was calculated from infrared spectra by using inkrpolation. The curve-fittingprogram from SpectraCalcwas used (19)Rebenstorf, B. J. Mol.Catal. 1991,66,59. (20)Rebenstorf,B.;Andereeon, S. L. T. J.Chem. Soc., Faraday Trons. 1990,86,2783. (21)Rebenstorf, B.; Lareson, R. 2.Anorg. Allg. Chem. 1979,453,127.
0 1991 American Chemical Society
Cr Species of the Phillips Catalyst
2200
2150 2100 2050 WAVENUMBER (cm-1)
Langmuir, Vol. 7, No. 10,1991 2161
2000
Figure 1. Low-temperature (-120 "C) infrared spectra of CO adsorbed on chromium ions of the unmodified reduced Phillips catalyst (0.5% Cr). The sample of spectrum 1 was oxidized at 800 O C for 30 min and that of spectrum 2 at 800 "C for 10 min. Spectrum 3 is the difference spectrum of spectrum 1 minus spectrum 2. For spectrum 3 the absorbance scale has been enlarged by a factor of 10.
for separatingCO infrared bands and calculatingtheir integrated area. None of the parameters (position, width at half-height, height, Gauss/Lorenz curve form) were fixed. XPS spectra were recorded with a Kratos XSAM 800 instrument and the atomic concentrations of B, 0,and Si were calculated from the B la (198.0eV), 0 1s (537.1 eV), and the Si 2p (107.5eV) peaks.
2070
2050 2030 WAVENUMBER (cm-11
2010
Figure 2. Low-temperatureinfrared spectra between 2080 and 2000 cm-l of CO adsorbed on chromium(I1) surface species. Spectrum 1 is the same as spectrum 1 in Figure 1. Spectrum 2 shows the second derivative.
n
al
z g
fl
I
I I
9
I I I
Results During the work on the present paper we found that in order to get reliable and reproducible results from curve fitting three different and independent pieces have to be consistent: (1) difference spectra demonstrate the trends in the number of each surface species from one sample to another, (2) the second derivative gives the peak positions of shoulders or even hidden bands, and (3) the curve fitting provides the integrated absorbance area and the width at half-height. A. The New Chromium(I1) D Species. In the first three figures our procedure is exemplified by the observation of a new chromium(I1) surface species on silica gel. Figure 1, spectra 1 (0.5% Cr, 800 OC, 02,30 min) and 2 (0.5% Cr, 800 OC, 02, 10 min), shows typical low-temperature spectra of CO adsorbed on the reduced Phillips catalyst. The two CO spectra (1 and 2 in Figure 1) are from chromium silica gel samples with a slight difference in the oxidation time. The difference spectrum 3 in Figure 1, calculated by subtracting spectrum 2 from spectrum 1 and multiplied by 10, shows now two trends that were hardly noticable before: a shoulder near 2030 cm-1 is converted into the band at 2035 cm-l and the CO band at 2198 cm-l is converted into the band at 2185 cm-l. This means that comparing the two samples one can say that a (new) dinuclear chromium(I1) species is converted into normal chromium(I1) A and that some chromium(II1) is converted into mononuclear chromium(I1) on going from sample 1 to sample 2. The second derivative of the broad CO band at 2035 cm-l from spectrum 1 in Figure 1 is shown in Figure 2, spectrum 2. It becomes clear now that this CO band has three components a t 2046,2035, and 2027 cm-l. The much sharper (negative) band at 2046 cm-' in spectrum 2 indicates that the CO infrared band at this position will be narrower than the other two.
I
2200
2150
2100 2050 WAVENUMBER fcm-11
2000
Figure 3. Curve fitting of spectrum 1 in Figures 1 and 2. The broad CO band around 2035 cm-' is resolved into three components, as indicated by the second derivative in Figure 2. A shoulder near 2090 cm-l is also resolved. In Figure 3 the results from curve fitting are shown for the same spectrum as in Figure 2. It becomes obvious that indeed three CO bands can be fitted to the broad band a t 2035 cm-l and that the band a t 2047 cm-l is indeed narrow. The new chromium(I1) surface species with CO IR bands a t 2120,2100, and 2027 cm-l is named chromium(11)D, because the other first letters of the alphabet are already used for chromium(I1) surface species. It was also observed in difference spectra (not shown) that this species has an additional CO band a t 2156 cm-l in low-temperature infrared spectra.
B. Changes for theChromium(I1)Surface Species on Modifications. From curve fitting of low-temperature CO infrared spectra one can calculate the percentage of each chromium(I1) surface species, as shown in Figure 4. This procedure is based on the assumption that the extinction coefficients of the three CO bands around 2035 cm-' are the same. This assumption will be tested below. From Figure 4 (left part) and Table I it becomes clear that with lower chromium concentration the chromium(I1) D species decreases from some 35 to 15%,while the chromium(I1) A species increases from 60 to 80%, when the chromium concentration is lowered from 0.5 to 0.05 % Cr. The third species (C) is only little affected. The shorter oxidation time at 800OC (10 min, sample Cr2) extrapolates these trends, which is in agreement with Figure 1.
Rebenstorf and Sheng
2162 Langmuir, Vol. 7, No. 10, 1991
I
I L
\
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0
0.25 K Cr
1
0.5
d 2
Cr3
1
,
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and a much larger one for the catalysts modified with fluorine. The latter effect has been observed previously.1Q It should also be noted that the positions of the CO bands of all three chromium(I1) species are not shifted for the catalyst modified with titanium. C. Samples Modified with Boron. It was now interesting to see if the above procedures also can be used to study other modified chromium catalysts. For this purpose boron was chosen as the modifier and different amounts from 0.1 to 3.2 mmol of B per gram of Si02 were used. The chromium content was 0.5% Cr, which is equivalent to 0.1 mmol of Cr. As shown in Figure 7 and Table I, a decrease in the concentration of the chromium(I1)D species was observed on addition of relatively small amounts of boron (0.1-0.4 mmol of B)with a corresponding increase in the chromium(11)A species. The C species was relatively little affected. Figure 8 shows that the decrease for chromium(I1)D is associated with a narrowing of the CO infrared band at 2027 cm-1: an effect that was already observed above for samples with low chromium concentration (compare with Figures 4 and 5). The positions of the three CO infrared bands around 2035 cm-I did not show any specialdeviation from those of the unmodified catalyst and are therefore not shown here. In order to be sure that the boron did not sublime from the samples during the preparation at high temperatures, XPS spectra of samples calcined in air at 800 "C were measured. The samples with 0.2,0.4, and 1.6 mmol of B per gram showed an atomic concentration of 0.35,0.59, and 3.27% (*O.lO% 1. This is larger by a factor of nearly 2 than what would be expected for a complete mixture of boron and Si02. This means boron is concentrated at the silica gel surface. The amount of boron used for the impregnation is in accordance with that found by the XPS measurements. It is concluded that no removal of B by sublimation takes place. Figure 9 shows the integrated absorbance for all three CO infrared bands around 2035 cm-l. For all samples, the height of the infrared band at 1870 cm-l from the silica gel was used as an internal standard. In order to compare the samples with low chromium concentration with those of 0.5% Cr, the former ones are normalized to the latter value (multiplied by the appropriate factor). The samples modified by boron are also included. It is observed that most values (70%) cluster around 33 f 5 % ,but a number of samples are better fitted by 31 f 10% (90%1. Although two samples with relatively low boron content (low amount of D species) show low values of integrated absorbance, this trend is not consistent with the low chromium content samples (which also show a low amount of the D species). The CrFl sample, which has a very low C amount, does not deviate from the dashed line. We therefore conclude that the extinction coefficients of the CO infrared bands from the different chromium(I1) surface species investigated here must be similar within f 5 !?& , The Cr3 sample (heated to 700 OC after reduction) shows a chromium(I1) concentration, which has decreased by a factor of 2. The sample CrF2 modified by fluorine and with low chromium concentration (only 0.05% Cr) shows an even lower value of 10. These effects are due to either oxidation of chromium(I1) to chromium(II1) or perhaps the formation of chromium fluorides. The first effect is also observed in low-temperature CO spectra (see Figure 1) and can be evaluated quantitatively, if necessary. Figure 10shows the CO infrared spectra and the second derivatives from the chromium samples Cr2 (spectrum 1) and Cr3 (spectrum 2) in Figure 4 at room temperature
Cr Species of the Phillips Catalyst
Langmuir, Vol. 7, No. 10,1991 2163
Table I. Relative Amounts of Chromium(I1) Surface Species in Different Samples of the Reduced Phillips Catalyst samples conditions of treatment 5% of Cr(I1) species Cr,wt % Tom "C tox,min Td,O C trd,min A C D 0.05 0.125 0.25
800 800 800
0.5
0.5 (Cr2) 0.5 (Cr3) 0.5/F (CrF1) 0.05/F (CrF2) 0.5/Ti 0.5/0.0 mmol B 0.5/0.1 mmol B 0.5/0.2 mmol B 0.5/0.4 mmol B 0.5/0.8 mmol B 0.5/1.6 mmol B 0.5/3.2 mmol B
I
I
30 30 30 30 10 30 30 30 30 10 10 10 10 10 10 10
800
800 800 800 800 800
800 800 800 800 800 800
800
350 350 350 350 350 700 350 350 350 350 350 350 350 350 350 350
30 30 30 30 15 30 30 30 30 15 15 15 15 15 15 15
78 75 68 58 53 81 70 97 24 53 66 70 73 63 60 58
7 9 8
15 16 24 34 38 4 27 2 31 38 22 20 18 26 30 30
8 9
15 3 1
45 9 12 10 9 11 10 12
Cr-A
cr-C
0.5
%cr
Ci2
Ck
CrFl C;F2
CrTi 0
1
2 mmol B/gsio
3 2
Figure 7. Relative amounts of the different chromium(I1) species as resolved by curve fitting. The chromium samples are modified in this case by different amounts of boron from 0.1 to 3.2 mmol of B per gram of Si02 (0.5% Cr = 0.1 mmol of Cr). a x
/ *\
t
30
20
0
0.25 % Cr
Cr2 Cr3
CrFl CrF2 CrTi
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0.2s % Cr
as
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Cr3
CrFl CrF2
B
2164 Langmuir, Vol. 7,No. 10, 1991
Rebenstorf and Sheng Chart I
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Si
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might consider two different models: (1)the chromium(11)A has no, the chromium(I1) D has one, and the chro"(11) C species has two additional oxygen ligands (from surface silanol or siloxane groups) at each chromium ion, or (2)the chromium species are dinuclear and the chromium(I1) A has no, the chromium(I1) D has one, and the chromium(I1) C species has two additional oxygen ligands at each dinuclear chromium surface compound. The latter possibility is shown in the chart. How can one distinguish between the two models described above? In Figure 4 an inversion of the relative amounts for the chromium(I1) species D and C is observed with the sample heated to 700 O C after the reduction with CO. If the first model would be realistic, three CO infrared bands [one for each chromium(I1) surface species] should I I 1 I I I be observed from terminal CO ligands after evacuation at 2210 2200 2190 2180 2170 2160 high temperature. The second model needs only two CO WAVENUMBER (cm-1) infrared bands from terminally bonded CO ligands for Figure 10. Room-temperature infared spectra of CO adsorbed the chromium(I1) samples. In Figure 10 only two chroterminally (after short evacuation at 400 "C). The samples are mium surface species are indeed noticed by terminal CO Cr2 (spectrum 1, solid line) and Cr3 (spectrum 2, dashed line) as in Figure 4. Spectra 3 (points)and 4 (pointsand dashes) show ligands around 2180 (A species) and 2190cm-l (Cspecies). the second derivatives of spectra 1and 2, respectively. Notice The three chromium species detected at low temperathe slight shifts between spectrum 1(and 3) and spectrum 2 (and tures with CO infrared bands around 2035 cm-' can 4)and that the second derivative always resolves only two bands. therefore only be explained with the structures A, D, and C from the second model 88 shown in the chart. chromium(I1) species to &3% (Figures 4 and 71, and for We think that these arguments are additional evidence the width at half-height and the position of the CO infrared that the catalytically active sites of the Phillips catalyst bands to better than fl cm-l (Figures 5, 6,8, and 9). are indeed dinuclear chromium(I1) surface complexes. Discussion Hydrogen atoms of the surface silanol groups in the structures D and C in the chart should be more acidic Chromium Surface Species. The three large CO than normal single-surface silanol groups. Such acidic infrared bands at 2120,2100,and -2035 cm-l have been surface silanol groups should adsorb CO at low temperpreviously interpreted by one of the authors as due to ature, giving CO IR bands higher than 2156 cm-'. These three CO molecules bridging between two chromium(I1) surface ions of a dinuclear chromium surface c ~ m p o u n d . ~ ~ CO ~ bands should be noticable in FTIR spectra as shown in Figure 1. Although a CO band is observed around 2160 More specifically, the main band at 2035 cm-l (together cm-l in spectra of all samples, a correlation to the chrowith the ones at 2120 and 2100 cm-l) was ascribed to mium(I1) species C and D is not found. Especially the CO bridging CO on the chromium(I1) A species [only two spectra of samples with addition of fluorine showed no oxygen ligands at the chromium(II)] and the shoulder at decrease and the spectra of samples with addition of 2045 cm-l (again together with the bands at 2120 and 2100 titanium no increase in the intensity of this band. Acidic cm-l) to the chromium(II)-C species (three oxygen ligands surface silanol groups coordinating to chromium(I1) surface at the ~hromium(II)).~*~ The two small CO infrared bands ions will be rather unstable at (reduction) temperatures at 2160 and 2145 cm-l are also due to CO adsorbed on the chromium(I1) A and chromium(I1)C species, respectively. of 350 "Cor higher and could easily oxidize chromium(I1) to chromium(II1). CO terminally absorbed on chromium(I1) at 2198 cm-l and on mononuclear chromium(I1) surface compounds at While in our opinion the dinuclear character of the 2185 cm-l are also observed.8~9 chromium surface compoundsis essential for the initiation For the above three chromium(I1) surface species one of the polymerization (without alkylating agents), the f\
A
Cr Species of the Phillips Catalyst chain-propagation and chain-transfer reactions may only depend on the direct ligand environment at a single chromium ion. With this argument the effect of the two catalyst modifications (fluorine and titanium) on the produced polymer may be compared with the above results. The fluorine modification is known to form polyethylene with a narrower molecular weight distributioa2 Indeed the single chromium(I1)specieswith one additional oxygen ligand [chromium(II)C and halftheamount of chromium(11)D] decreases from 25 % for the normal catalyst to only 16% for the catalyst modified by fluorine with a chromium concentration of 0.5% Cr. The fluorinated sample with low chromium content (0.05% Cr) shows an even more uniform chromium(I1) species distribution of 98 % chromium(I1) A. The molecular weight distribution of the polyethylene should therefore be a function of the number and amount of the different chromium(I1)species, as already concluded p r e v i o u ~ l y .Above ~ ~ we have determined the quantitative value for one side of the coin. As far as we know, a quantitative value for the change in the molecular weight distribution of the normal Phillips catalyst and the one modified by fluorine has not been published. That would be the other side of the coin. The above value of 25 % for the normal reduced Phillips catalyst is in quite good agreement with measurements using an entirely different investigation method (25%).22 The titanium modification of the Phillips catalyst is known to yield polyethylene with a lower molecular weight.2 The amount of the chromium(I1) surface species with one additional oxygen ligand can be calculated for this case from the results above as 60%. This means that the chromium(I1) surface species dominating in this catalyst [the one with an extra oxygen ligand at the chromium(II)] has a smaller ratio of the chain-propagation to the chain-transfer reaction compared to the chromium(11) species without an extra oxygen ligand. Only a relatively small effect was observed for the samples modified by boron. This may be due to the migration of boron into the silica gel matrix at 700 or 800 "C and the formation of a borosilicate phase inside the gel particles. Titanium and fluorine are more resident at the (22) Hoepfl, R. Dhrtation,
Bayreuth, Germany, 1981.
Langmuir, Vol. 7, No. 10, 1991 2165 silica gel surface. Titanium may prevent the condensation of surface silanol groups at higher temperaturesm or stabilize siloxane groups that are somewhat lifted up over the support surface. Fluorine removes the silanol surface groups to a large extent.19 These conclusions are in agreement with the observation of CO terminally adsorbed on titanium(1V)m with a CO infrared band at 2184 cm-l and with the decrease of the intensity of the 0-Hinfrared band from silanol groups on addition of fluorine.'@ For the catalyst modified by boron, neither surface BO-H nor CO adsorbed on boron could be detected for samples with low boron content (0.1-0.8 mmol of B/g of Si02). It was noticed recently that the electron density (Lewis acidity) at the chromium as measured by the stretching frequency of one terminally adsorbed CO per chromium ion increases in line with the rates of the chain-propagation and chain-transfer reactions for the polymerization of ethylene by Phillips-type catalysts (chromium ions on supp o r t ~ ) This . ~ ~ observation together with the above results will be essential ingredients for further research in this area and may convert the improvement of the Phillips catalyst from an art to a matter of science.
Conclusion The investigation of chromium surface species on the reduced Phillips catalyst with and without modifications by low-temperatureinfrared spectroscopy of adsorbed CO makes an attempt of a quantitative determination of the catalytically active chromium surface species possible. A consistentpicture of the structureand electronic properties of chromium surface species and their catalytic properties is emerging from this research and should in the future provide a spectroscopic basis for a scientific search of modified Phillips catalysts with improved catalytic properties. Acknowledgment. This research has been supported by the Swedish Board for Technical Development (STU) and the National Energy Administration (SEV). We thank Anders Wideldv for the XPS measurements. Registry No. CrOa,1333-82-0;(NH&SiFa, 16919-19-0;Ti, 7440-32-6;HaBOa, 10043-35-3; chromiumacetylacetonate,2167931-2;polyethylene, 9002-88-4. (23)Rebenstarf, B. J. Polym. Sei., Polym. Chem. Ed., in press.
