J. Phys. Chem. 1983, 8 7 , 3735-3740
pentane12 and 35.9 f 0.2 kcal/mol for CH3,15then moo for reaction 1 is 81.4 f 1.3 kcal. Both the RRKM and adiabatic channel theories16 predict Arrhenius activation energies for reactions like reaction 1that are slightly higher so that our Arrhenius E of 84.8 kcal is not than moo, unreasonably high. Moreover, our least-squares calculation
3735
showed that the 95% confidence level for our Arrhenius
E is as large as 6 kcal, so that within experimental error (both ours and theirs) we are in agreement with the earlier workers in terms of Arrhenius parameters as well as the absolute values of the rate constants.
Acknowledgment. This research was supported by the
U.S. Departments of Energy under Contract DE-AC-02(15) K. M. Pamidimukkala, D. Rogers, and G. B. Skinner, J.Phys. Chem. Ref. Data, 11, 83 (1982). (16) M. Quack and J. Troe, Ber. Bunsenges. Phys. Chem., 78, 240 (1974).
76-ERO2944. We also recognize the assistance of Mr. John Dryden in preparing the figures. Registry No. 2,S-Dimethylpropane,463-82-1.
Reactions of Titanium Tetrachloride and Trimethylaluminum at Silica Surfaces Studied by Using Infrared Photoacoustic Spectroscopy John B. Klnneyt and Ralph H. Staley*t Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139 (Received: September 7, 1982; In Final Form: May 25, 1983)
The reactions of titanium tetrachloride and trimethylaluminum with high-surface-area silica are studied by using Fourier transform infrared photoacoustic spectroscopy. A peak at 980 cm-*in the spectrum of dehydrated silica is observed for the first time and assigned to the strained, surface siloxane bridge. Other previously unreported peaks in the low-frequencyregion are observed and assigned to various MO and MC stretch vibrations of the products of the surface reactions. The spectrum of an active Ziegler-Natta catalyst material is examined.
Introduction Reactions of surfaces with hydrogen-sequesteringagents are important in many areas of chemistry including heterogeneous catalysis' and surface modificationa2 The reactions of two such agents, titanium tetrachloride and trimethylaluminum, are of particular interest in studies of olefm-polymerization catalysts as a model Ziegler-Natta catalyst ~ y s t e m . Reaction ~~~ between surface TiCl, and Me3AIgas is known to give an active olefin-polymerization ~ a t a l y s t .However, ~ there is little understanding of the nature of the active site of this catalyst. Several groups have carried out infrared studies of the reactions of dehydrated silica with Me3A14-9and TiC14.4f8 The spectral information in these studies is primarily limited to the high-frequency CH and OH stretching regions of the IR, 4000-2600 cm-'. These studies have identified two types of reactive sites on the silica surface: silanols, I, and strained siloxane bridges, 11. These groups
I I1 I11 initially react with Me3A1 or TiC1, by reactions 1 and 2
0
Si
/ \si
-1
,%-1
0
+
ML,
Si
L
+ I
(2)
Si
'Current address: Central Research and Development Department, Experimental Station, E. I. du P o n t de Nemours and Co., Wilmington, DE 19898.
where ML, = Me3Al or TiC14.7 The surface-bound products of reactions 1and 2 can react further with neighboring surface sites to form bridged surface species. It has proven more difficult to obtain useful spectral information in the low-frequencyspectral region, 1000-600 cm-', where MO, MC1, and MC (M = Al, Ti, Si) stretching modes are located. The ability to obtain IR absorption spectra of surface species in this region has been recently demonstrated in a paper by Howe et a1.I0 They studied a silica-supported molybdena catalyst and were able to identify surface Mo-0 vibration frequencies. A recent study of the reactions of Me3A1 and TiC1, with silica using Raman spectroscopy has also given some useful information in this region." This technique is particularly sensitive to MCl vibrations. The symmetric MOM' vibrations are observed only weakly and the asymmetric MOM' vibrations are not observed at all. Raman studies of these systems are difficult due to the strong visible absorptions and fluorescence of the samples. In the present study, Fourier transform infrared (FTIR) photoacoustic spectroscopy is used to (1)See, for example: Pinnavaia, T. J.; Lee, J. G.-S.; Abedini, M. In 'Silylated Surfaces"; Leyden, D. E., Collins, W. T., Eds.; Gordon and Breach: New York. 1980 D 333. See also ref 3. (2) Leyden, D. E., Collih, W. T., Eds. 'Silylated Surfaces"; Gordon and Breach: New York, 1980. (3) Boor, J., Jr. 'Ziegler-Natta Catalysts and Polymerization"; Academic Press: New Yo&, 1979. (4) Murray, J.; Sharp, M. J.; Hockey, J. A. J. Catal. 1970, 18, 52. (5) Yaks, D. J. C.; Deinbinski, G. W.; Kroll, W. R.; Elliot, J. J. J.Phys. Chem. 1969, 73,911. (6) Peglar, R. J.; Murray, J.; Hambleton, F. H.; Sharp, M. J.; Parker, A. J.; Hockey, J. A. J. Chem. SOC.A 1970, 2170. (7) Peglar, R. J.; Hambleton, F. H.; Hockey, J. A. J. Catal. 1971, 20, 309. (8) Kunawicz, J.; Jones, P.; Hockey, J. A. Trans. Faraday SOC.1971, 67, 848. (9) Low, M. J. D.; Severdia, A. G.; Chan, J. J. Catal. 1981, 69, 384. (10) Seyedmonir, S.R.;Abdo, S.; Howe, R. F. J . Phys. Chem. 1982, 86, 1233. (11) Morrow, B. A.; Hardin, A.H. J. Phys. Chem. 1979, 83, 3135.
0022-3654/83/2087-3735$01.50/00 1983 American Chemical Society
Kinney and Staley
3736 The Journal of Physical Chemistry, Vol. 87, No. 19, 1983
observe spectral changes due to reactions of TiCl, and Me3A1 with silica. Information is obtained in both the high-frequency OH and CH stretch regions and the lowfrequency MO and MC stretch regions of the spectrum without difficult sample preparation and with no interference by the visible absorptions of the sample. The results reveal the infrared spectral changes occurring at the various stages of preparation of an active Ziegler-Natta catalyst more completely than has been previously observed. Several new bands are observed, assigned, and interpreted in terms of proposed mechanisms for the reactions.
Experimental Section The silica (99.8%) -400 m2/g, no. 89376)) titanium tetrachloride, and trimethylaluminum were purchased from Alfa Products. The ethylene was purchased from Matheson Gas Products. The photoacoustic cell and data collection procedures have been described elsewhere.12 Spectra were collected by using a Nicolet 7199 FTIR spectrometer at 8-cm-' resolution. Averages of 512 scans were used in recording the spectra. The silica was dried before use by baking at 450 "C under vacuum. The dried silica was then transferred to the cell in a water-free, controlled-atmosphere glovebox. Sample preparation was carried out in the cell by attaching the sample chamber to a glass vacuum line. The silica was exposed to the reactive vapors at the vapor pressure of the liquids, about 10 torr for both TiC1, and Me3A1. It was necessary to fill the cell with the TiC1, and Me3A1 vapor several times during these reactions since the sample chamber volume of 0.25 cm3 does not contain enough gas to give complete reaction at these pressures. The reacting gas was exposed to the sample for 1 min and then evacuated. Progress of the reaction was monitored visually. The color of the silica changed radically upon reaction with T i c 4or Me& After no further color change was observed, the silica was exposed to the gases 2 more times in order to ensure complete reaction. A total of 4-6 exposures was usually required. The polymerization of ethylene was carried out by pressurizing the cell containing the active catalyst with 30 psig of ethylene. The locations of the peaks in the spectra in this paper were determined by comparing the spectra obtained from several duplicate experiments. The spectra shown in the figures are representative of the results obtained. Results Spectra of silica before (wet) and after (dehydrated) drying under vacuum at 450 OC were obtained. There are notable differences in two regions of the infrared. The wet silica has a broad OH absorption between 3300 and 3700 cm-', while the dehydrated silica has a single sharp OH absorption at 3750 cm-l. In the low-frequencyregion, the spectrum of the dehydrated silica has a distinct shoulder at 980 cm-' that is not seen in the spectrum of the wet silica. Spectra of dehydrated silica before and after reaction with TiC1, are shown in Figure 1. The most apparent change in the spectrum is the loss of the peak at 3750 cm-'. The broad, weak absorption at about 3650 cm-' is unchanged. There are several changes evident in the region from 1100 to 600 cm-l (Figure 2). The shoulder at 980 cm-I is gone and new peaks are found at 990,920,790, and 730 cm-' with a shoulder at 1040 cm-'. The shape and the intensity of the peak at 990 cm-' are distorted by the loss of the peak at 980 cm-'. Spectra were taken between each (12) Kinney, J. B.; Staley, R.
H.Anal. Chem. 1983, 55, 343.
I
I
I
I
I
I
I
1500
1000
1
t
L---
B 4000 3500
I
1
3000 2500 2000
500
Frequency (cm-') Flgure 1. Infrared PAS spectra of silica powder: (A) dried at 400 (9) sample A after reaction with TiCI, vapor.
I
(
I
I
I
OC;
I
I A
I
h
I \
I W
1200
1100
1000
I
I
I
900
800
700
600
Frequency (cm-') Figure 2. Expansion of low-frequency region of spectra in Figure 1: (A) dried silica; (9) after reaction with TiCI,; (C) difference.
exposure to Tielk These show the progressive reaction of the surface hydroxyls and formation of the various TiC1, surface species. These spectra indicate that the peak at 920 cm-l appears and the peak at 980 cm-' disappears after the first exposure. The peak at 990 cm-l only grows in after repeated exposures. When this sample is reacted with Me& to produce an active Ziegler-Natta catalyst, several changes in the spectrum are observed. In the CH stretch region, peaks similar to those found after the reaction of silica with Me& are observed (Figure 3). Only two broad peaks are found in the low-frequency region at 940 and 705 cm-' (Figure 4). The exposure of this sample to ethylene led to rapid polymerization as observed by the increase in the volume of the sample and the appearance of IR peaks characteristic of polyethylene (Figure 5). Spectra of dried silica which has been reacted with Me,A1 are shown in Figures 3 and 6. Peaks are found in the CH stretching region of the spectrum (Figure 3) at 2960,2940,2900,2860, and 2830 cm-' and a broad, weak shoulder between 3000 and 3050 cm-'. The low-frequency region (Figure 6) shows the loss of the shoulder at 980 cm-' and the growth of three new peaks, a broad peak at -700 cm-' and two weak peaks at 870 and 815 cm-'. The reaction of this sample with TiC1, leads to several changes in the IR spectrum (Figures 3 and 7). Peaks in the CH stretch region show substantial loss in intensity (Figure
Reactions of TICi, and AiMe, at Silica Surfaces I
I
I
The Journal of Physical Chemistry, Vol. 87, No. 19, 1983 3737
I
I
I
I
I
"\I 3200 3100
3000 2900 2800
2700
2600
1200
1100
Frequency (cm-')
1000
1
I 900
800
700
600
Frequency (cm-')
Figure 3. CH stretch absorptions of surface reaction products: (A) sillca reacted wlth TICI, and then Me,Ai vapors to give an active Ziegler-Natta catalyst; (B) silica reacted wlth Me,Ai vapor only; (C) sample B reacted with T U 4 vapor.
Figure 8. Silica reacted wlth Me,AI: (A) dried silica; (B)silica after reaction with Me,AI; (C) difference (X2). I
1200
1100
1
I
I
I
1000
900
800
700
600
Frequency (cm-') Figure 7. Sillca reacted wlth Me3AIand then TICI,: (A) dried silica; (B) silica reacted wlth Me,Ai and then TICI,; (C) difference (X2).
1200
1100
900
1000
700
800
600
Frequency (cm-') Figure 4. I R spectra of an actlve Zlegler-Natta catalyst: (A) dried silica; (B) active catalyst prepared by reaction of dried silica wlth TICI, and then Me,AI vapors; (C) difference. 1
I I 4000 3500
I
I
I
I
3000 2500
I
I
I
I
I
1
I
2000
1500
1000
500
Frequency (cm-') Flgure 5. IR spectrum of catalyst after ethylene polymerizations.
3) although the peaks at 2960,2900, and 2860 cm-' are less affected. In the low-frequency region, peaks are observed a t 970, 825, and 680 cm-'. No changes in the spectrum were observed when this sample was exposed to ethylene for a period of several hours.
Discussion The surface of silica consists primarily of three distinct species.13 The surface of fully hydrated silica is largely hydrogen-bonded silanol groups, 111. When the silica is heated under vacuum, the neighboring silanol groups react to give up free water (reaction 3) leaving strained siloxane
bridges, II.13 The dehydrated silica surface also has isolated silanol groups, I, that cannot react to form siloxane bridges. The spectrum of the dried silica used in this work (Figure 1A) shows a sharp peak at 3750 cm-' with a broad, weak absorption at lower energies. The sharp peak is the OH stretch of the isolated silanol groups. The lower frequency peak appears to be due to a class of hydrogenbonded silanols that are structurally inaccessible. These silanols are generally unreactive. They do not dehydroxylate at 450 "C and do not react with Me3A1or TiCl, at room temperature. The unreactive nature of these groups is probably due to steric factors. The high surface area of the silica makes it unlikely that these are true interstitial groups. Comparison of these spectra with data from the (13) Little, L. H. "Infrared Spectra of Adsorbed Species"; Academic Press: New York, 1966.
3738 The Journal of Physical Chemistry, Vol. 87, No. 19, 1983
Kinney and Staley
TABLE I: Surface MO and MC Stretch Vibration Frequencies
literature indicates that the silica surface is nearly completely dehydrated.I3 The shoulder at 980 cm-' that is seen after dehydration can be assigned to the Si0 stretch mode of the siloxane. Evidence for this assignment is given below. Silica Me,Al. The reactions of Me3Al with silanol and siloxane surface species have been studied by several group^.^-^ Reactions 4-7 have been proposed to account
v(Al0) = 870 or 815 cm-',' u(A1C) = 715 cm-'
,AIM%
1
si
+
Me
u(Al0) = 870 or 815 cm-',' v(A1C) = 715 cm-'
I
,AI,
'0
I
'0
I
+
AIMe3
SI
-I
+
i"
(4)
CH4
1
SI
SI
u(SiC) = 680 cm-'
SI
SI
u(Si0) = 990 cm-', u(Ti0) = 790 or 730 cm" a
TtC'3
,AlMe2
'
0
I
SI
u ( S i 0 ) = 920 cm-', u(Ti0) = 790 or 730 cm-'
C I\O/T'\O
Me
I
I
I
SI
SI
a Two peaks are listed for this vibration because the peaks cannot be assigned unambiguously.
SI
SI
SI
SI
Te
0/A'Me3
-1
O/*'\O
+
I
SI
/o\s, SI
I
+
SI
SI
Me
1
(7)
SI
for the observed CH and OH ab~orptions.~ The initial reaction with isolated silanol groups (reaction 4) leads to the formation of surfaceSiUAlMe2groups that can then react further with neighboring surface sites (reactions 6 and 7). The reaction with siloxanes (reaction 5) results in the cleavage of one of the strained Si-0-Si bonds forming surface-Si-0-A1Me2 and surface-Si-Me species. The surface-Si-0-A1Me2 species can react further with neighboring silanols and siloxanes to give bridged aluminum alkyl surface groups. In the present work peaks are observed in the CH region for both the AlMe, (2940,2900, and 2830 cm-') and SiMe (2960 and 2900 cm-l) species in agreement with previous report^.^-^ Low et al. have recently suggested that reactions 8 and H
I I
1 + SI
-I
AIMe2
0
AIMe3
+
SI
MeOH
O/Me
AIMe2
0
SI
/ \
SI
+
AIMe3
-1
SI
(8)
+
I
(9)
SI
9 may also be important in this ~ y s t e m .These ~ reactions give product SiAlMe, surface species along with methanol (reaction 8) or surface methoxy (reaction 9). As in their work, the spectrum in Figure 3 shows significant absorption at 2860 cm-' which cannot be attributed to either the Si-Me or A1-Me species. The position of this peak corresponds to one of three CH stretch peaks in the spectrum of methoxy groups, along with peaks at 3000 and 2960 cm-'. These peaks may be due to either a true surface methoxy5 or chemisorbed-intact methanol. The changes observed in the low-frequency region of the spectrum after reaction with Me3Al can be accounted for by reactions 4-9. The peak a t 980 cm-' disappears, supporting its assignment to the S i 0 mode of a reactive surface species. The new peaks in the low-energy region can be assigned to the various MO and MC stretch vibrations of the SiOAlMe,, SiOAl(Me)OSi, and SiMe products of reactions 4-7 (Table I). The broad peak at 700 cm-' has been assigned from Raman datal1 to one of the AI-C vi-
brations of the surface species. Our results suggest two unresolved peaks at 680 and 715 cm-l.14 These peaks correspond to the Si-C and A1-C stretch vibrations, respectively. This interpretation is indicated by the observation that reaction with TiC14removes the component at 715 cm-' leaving only the peak at 680 cm-l as discussed below. The M-0 stretch vibrations can be assigned by comparison with spectra of MOAlMe2 complexes. The spectrum of Me3SiOAlMe2shows v(SiO), i.e., v,(SiOAl), at 1063 cm1.15 Spectra of (R2A1)20,R = Me, Et, i-C4Hg, show v(AlOA1) a t 780-815 cm-l.16 From these model compounds, we expect the u(Si0) peaks should be masked by the support absorption while the v(Al0) peaks should be observed a t around 800 cm-'. Our spectra show two peaks at 870 and 815 cm-'. We assign these peaks to the A10 stretch vibrations of the bridged and terminally bound Me,A1 species. The Raman work is consistent with our results; a broad Raman peak was found a t 820 cm-' but was not assigned.l' In summary, these results provide new direct evidence in support of both the singly bonded SiOAlMe2species and the bridged SiOAl(Me)OSispecies formed by reactions 4-7. No direct evidence is seen for a SiAlMe2species as would be formed in reactions 8 or 9. The reaction of the aluminum alkyl functionalized surface with TiC14 results in several changes in the IR absorption spectrum. The three peaks found in the CH region are due to the methoxy and surface-SiMe groups. All of the peaks due to the surface-AlMe, groups are gone. These changes are similar to the changes observed in the reaction of aluminum alkyl functionalized silica with water.5 The aluminum alkyls are highly reactive to both water and TiC1, while the silicon alkyls and methoxy groups do not readily react. In the low-frequency region, several changes are observed. A strong broad absorption appears a t -970 cm-' and a weak absorption is found at 825 cm-'. The peak at 700 cm-' has shifted to -680 cm-'. These changes are consistent with the changes in the CH region. All of the aluminum alkyls are gone, so the MC peak at 680 cm-' is now due to just the silicon alkyls. Only one peak is observed in the A10 region, 800-900 cm-', at 825 cm-'. The changes from two A10 peaks to one broad A10 peak indicate that in the reaction of the TiC14with (14)Peaks were resolved by comparing spectra before and after reaction with TiCI,. (15) Schmidbaur, H. J. Organomet. Chem. 1963,1, 28. (16) Veyama, N.; Araki, T.; Tani, H. Inorg. Chem. 1973, 12, 2218.
Reactions of TiCI, and AIMe, at Silica Surfaces
The Journal of Physical Chemistry, Vol. 87,No. 19, 1983
the aluminum alkyls one of the legs of the bridged species is broken, leaving an aluminum species that is bound to the surface with one A1-0-Si bond. There is not enough data at the present time to determine the other ligands on the aluminum atom. In summary TiC14 reacts rapidly with surface-AlMe, groups. One of the SiOAl legs of the bridged SiOA1(Me)OSi species is cleaved and all of the methyl groups attached to A1 atoms are displaced. Silica Tic/,. The reactions of TiC14 with silica are analogous to the reactions of Me3A1 with silica. These reactions are outlined in reactions 10-13. The loss of the
+
ONH
1
+
Tic14
Si
Si
/"\si
-1
,TIC13
0
+
Si
+
Tic14
-I
Si
Si
(10)
HCI
Si
Si
Si
Si
Si
TiCI,
I
0 Si
Si
I
Si
(13)
OH peak at 3750 cm-' (Figure 1)indicates that all of the free hydroxyl groups have reacted with the Tic&. The only new peaks that should be observed above 500 cm-' are due to the S i 0 and T i 0 bonds of the new species. There are no reported assignments of SiOTi vibrations in the literature. However, assignmentsof similar complexes can help in the assignment. A survey of the literature shows that the Si0 vibration in SiOM complexes (M = Al, Au, Bi, Ge, Hg, Pb, Sn, T1) ranges from 900 to 1070 cm"." The T i 0 vibration in TiOTi complexes is found in the 720-795-cm-' region.18 Thus, we assign the peaks observed in this work at 990 and 920 cm-' to the S i 0 stretch vibrations and the peaks at 790 and 730 cm-' to the T i 0 stretch vibrations. Kunawicz et al. observed that significant amounts of some hydrogen sequestering agents react with dehydroxylated silica before any change in the OH stretch band intensity is observed.8 This was attributed to the reaction between siloxane bridges and the reactant. Similar results are observed here. When only a small amount of TiCl, is reacted with the silica, essentially no change in the OH band intensity is observed. However, the shoulder at 980 cm-l completely disappears and a new peak at 920 cm-' is seen. Only after further reaction is the OH peak lost and does the peak at 990 cm-l appear. These changes are caused by two processes. First, the siloxane bridges are reacting more quickly than the hydroxyl groups. The shoulder at 980 cm-l is the asymmetric stretch of the surface siloxane species, so this peak disappears at a more rapid rate than the OH stretch peak. Second, when the TiC14first starts to react with the surface, there are many sites where the surface-attached TiC13 species can react (17)(a) Schmidbaur, H.; Bergfeld, M.; Schindler, F. Z. Anorg. Allg. Chem. 1968,363,73.(b) Schmidbaur, H.; Bergfeld, M.Ibid. 1968,363, 84. (c) Schmidbaur, H.; Hussek, H. J. Organomet. Chem. 1964,1,257. (d) Schmidbaur, H., bergfeld, M. Inorg. Chem. 1966, 5, 2069. (e) Schmidbaur, H.; Hussek, H. J. Organomet. Chem. 1964, 1, 235. (fJ Schmidbaur, H.; Hussek, H. Ibid. 1964,1, 244. (18)Reid, A. F.;Shannon, J. S.; Swan, J. M.; Wailes, P. C. Aust. J. Chem. 1965,18,173.
3739
further to give a bridged TiClz species as in reactions 12 and 13. As the reaction progresses, an increasing number of TiC13 species do not have neighboring hydroxyl and siloxane groups for further reaction. This gives rise to an increase in the proportion of TiCl, species on the surface. Thus, the first peak to appear, at 920 cm-l, is due to the Si0 stretch of the bridged species and the second peak at 990 cm-l is due to the Si0 stretch of the terminally bound species. Reactions 11and 13 produce surface Si-C1 groups. No infrared evidence is seen for Si-C1 or TiCl groups. The infrared absorption for these groups should be in the 400-'700-~m-~region.lg Our PA spectra are effectively cut off below 600 cm-'. The signal-to-noise ratio of our spectra between 600 and 700 cm-l permits the identification of only the strongest peaks, such as the M-C vibration. It is thus not surprising that Si-Cl and Ti-C1 features are not observed in our work either because they fall below 600 cm-' or because they fall between 600 and 700 cm-' but are too weak to be distinguished above the noise. Morrow et al. have reported the occurrence of peaks at 888 and 908 cm-' due to a highly reactive surface species.M These peaks appear only after degassing at >600 "C. These peaks were assigned to a strained surface siloxane species formed by the dehydroxylation of isolated surface hydroxyl groups. No mechanism for this reaction was proposed. Morrow's studies were carried out only at dehydration temperatures of 800 "C and above and were blind to the 980-cm-l area, since he is limited to the region between 850 and 950 cm-l where absorption by the lattice is relatively weak. Morrow's species are formed only at much higher temperatures than we used and are present only at a fraction of the concentration of the species we observe. The concentration after dehydration at 600 "C of the reactive siloxane species observed in the present work can be estimated to be 0.4/100 A2.8,21 Morrow reports the species in question are only 0.15/100 A2 after degassing at 1200 "C and perhaps half this concentration after degassing at 800 "C. After degassing at only 450 "C, the concentration is presumably lower still. The reactive surface species observed by Morrow is thus different from the siloxane species observed in the prsent work which is formed by reaction 3. As expected, no evidence of peaks at 888 and 908 cm-' is seen at dehydration temperatures up to 450 OC studied in the present work. In summary, TiC14 reacts first with siloxane bridge groups and then with siloxol groups. The results from this reaction system clearly show that the peak at 980 cm-' is due to the reactive siloxane bridge groups. New direct evidence is observed for both the singly bound and bridged titanium chloride surface species, and the reaction between Tic&and surface siloxane groups (reactions 11 and 13). The reactions of Me3A1 with titanium chloride functionalized silica are similar to the reactions of Me3A1with the silica surface. The CH stretch absorptions in both reactions have the same basic features. Peaks that can be assigned to silicon methyls (2840, 2900, 2940 cm-') and aluminum methyls (2900,2960 cm-') are observed. In the low-frequencyregion, only two broad peaks are observed. The peak at 940 cm-l is due to the Si0 stretch of the titanium surface species. The peak at 705 cm-' is undoubtedly a multiple peak consisting of at least the two MC (M = Si, Al) stretch peaks. It is also likely that the T i 0 stretch is part of the peak. As in the systems pre(19)Nakamoto, K. "Infrared and Raman Spectra of Inorganic and Coordination Compounds";Wiley: New York, 1978. (20)(a) Morrow, B.A.; Devi, A. J. Chem. Soc., Faraday Trans. I 1972, 68, 403. (b) Morrow, B. A.; Cody, I. A. J. Phys. Chem. 1976,80, 1995. (21)Hair, M. L.; Hertl, W. J.Phys. Chem. 1969,73,2372.
J. Phys. Chem. 1903, 87. 3740-3747
3740
Only one S i 0 peak is observed, giving further support for the cleavage of the bridged species. While no direct evidence for a Ti-0-A1 species is observed, the stretch absorptions for this species may well be part of one of the broad peaks observed. In summary, the spectrum of the active catalyst indicates that one of the TiO-Si bonds of the bridged SiOTiOSi species reacts with the Me3A1.
(1O00400 cm-’) in these systems for the first time. A peak at 980 cm-’ is observed in dehydrated unreacted silica and is assigned to reactive siloxane groups (surface Si-0-Si) formed by high-temperature dehydration of the silica. Direct evidence for the various Si-0-M- species and SiMe is also observed in the low-frequency region of the spectrum. Me3A1 reacts quickly with surface siloxane groups and then also hydroxyl groups. In addition, bridged surface species are formed by reaction between the Me3A1 and more than one reactive surface site. The present work shows that the reactions of T i c 4with silica follow the same general reaction scheme as for Me3A1. A mixture of both terminally bound and bridged TiC1, species is observed. No direct evidence is observed for the Si-Ti or Si-A1 species that would be formed in reactions 8 and 9, however. The reactions of TiC14 with silica pretreated with Me3Al and Me3A1with silica pretreated with TiC14are similar to the reactions with the clean silica surface. The bridged species in both systems are cleaved by the reactive gas.
Conclusions This work supports the general reaction schemes presented by earlier workers for the reaction between Me3A1 and ~ilica.“~ The photoacoustic detection technique used in this work has permitted study of the absorption spectra in the low-frequency end of the mid-infrared range
Acknowledgment. This work was supported by the National Science Foundation through the Center for Materials Science and Engineering a t M.I.T. and by the Joint Services Electronics Program through the Research Laboratory for Electronics at M.I.T. Registry No. Tic&,7550-45-0;Me& 75-24-1;SOz, 7631-86-9.
viously discussed, the bridged Si-0-Ti-&Si surface group is cleaved by the reacting gas, Me3A1. The formation of Si-Me groups can only be obtained by a reaction such as reaction 14, resulting in the formation of a Ti-0-Al species. ,CI
,CI
O/Ti
I
Si
\o
I +
Si
AlMe3
-I
Si
+? Si
Surface Characterization of CuO-ZnO Methanol-Synthesis Catalysts by X-ray Photoelectron Spectroscopy. 1. Precursor and Calcined Catalysts Yasuakl Okamoto, Klyotaka Fuklno, Torhlnobu Imanaka, and Schllchlro Teranlshl’ Departmeflt of Ch”cal Englneerlng, Faculty of Engineering Science, Osaka Unlverslty, Toyonaka, Osaka 560, Japan (Received: September 15, 7982; I n Final Form: May 4, 1983)
A surface characterization of the precursor and calcined CuO-ZnO catalysts prepared both by coprecipitation and by impregnation methods was carried out by using X-ray photoelectron spectroscopy. Strong oxide-oxide interactionswere found to result in the formation of several surface copper species in the coprecipitated catalysts. With the catalysts containing 80-30 wt % CuO, it is proposed that the CuO structure is distorted to produce amorphous copper oxide phases in the surface layer. The copper species in the catalysts with >80 wt 9’0 CuO are composed of the Cu2+ions in crystalline CuO and the amorphous phases. As for the catalysts containing less than 30 wt % CuO, XPS results showed the presence of Cu2+ions, which dissolved substitutionally in the ZnO lattice and formed highly covalent bondings with oxygen anions. In contrast to the coprecipitated catalysts, the impregnated catalysts showed no appreciable interactions between copper and zinc oxides. The surface copper species in the precursor catalysts were attributed to copper hydroxycarbonate in the high copper content catalysts and to the copper in zinc hydroxycarbonate in the low copper content catalysts.
Introduction Effective synthesis of methanol from Hzand CO has become more and more important in industry, since methanol, which can be selectively converted to, for example, gasoline and acetic acid, is considered to be one of the materials that can overcome the forthcoming shortage of petroleum by being derived from coal. Methanol synthesis is carried out over Catalysts containing copper, such as CuO/ZnO/AlZO3and CuO/ZnO/Crz03 with various compositions, at low pressure and temperat~re.’-~These (1) Natta, G.‘Catalysis”; Emmett, P. H., Ed., Reinhold New York, 1955; Vol. 3, p 109. 0022-365418312087-3740$01.5010
catalysts are also highly active for the low-temperature water-gas shift reactions.“’ For both reactions, the active species of copper are still open to question. (2) Kotera, Y.; Oba, M.; Ogawa, K.; Shimomura, K.; Uchida, H. “Preparation of Catalysts”; Delmon, B., Jacobs, P. A,, Poncellet, G., Eds.; Elsevier: Amsterdam, 1976; p 589. (3) Shimomura, K.; Ogawa, K.; Oba, M.; Kotera, Y. J . Catal. 1978,52, 191. (4) Herman, R. G.; Klier, K.; Simmons, G. W.; Finn, B. P.; Bulko, J. B.;Kobilinski, T.P. J . Catal. 1979,56, 407 and references therein. (5) Newsome, D. S. Catal. Rev.-Sci. Eng. 1980,21, 275. (6) Campbell, J. S.;Craven, P.; Young, P. W. “Catalyst Handbook”; Springer-Verlag: New York, 1970; p 97. (7) van Hewijnen, T.;de Jong, W. A. J . Catal. 1980, 63, 83.
0 1983 American Chemical Society
