J . Phys. Chem. 1990, 94,6743-6748
6743
Characterization of the Adsorbed State of Isoprene on Alumina and Copper-Alumina Catalysts by 13C NMR and in Situ Diffuse Reflectance Infrared Spectroscopy M. Rigole,+ C. Depecker,t G. Wrobel,? P. Legrand,$ M. Guelton,*qt and J. P. Bonnellet Laboratoire de Catalyse HPtCrogCne et HomogCne.t Laboratoire de Spectrochimie Infrarouge et Raman,$ Unioersite des Sciences et Techniques de Liile Flandres- Artois, 59655 Villeneuoe d'Ascq CPdex. France (Received: February 21, 1989; In Final Form: April 18, 1990)
In situ 13CNMR spectroscopy and diffuse reflectance infrared spectroscopy (DRIFTS) have been used to characterize the adsorbed states of isoprene on both y-Alz03 and Cu/y-Alz03 catalysts. Both techniques have been shown to be quite complementary and provide very fine information. Isoprene molecules are physisorbed only at room temperature on alumina. On reduced copper-alumina, isoprene is mainly physisorbed, but a few chemisorbed species are evidenced. Taking into account the "C NMR line-width variations and the IR results, an adsorption model is proposed for Cu/y-A1203: reduced copper induces a selective orientation for the physisorbed isoprene molecules. The chemisorbed species detected by DRIFTS is attributed to a *-complex of isoprene with Cu' sites on the reduced copper-alumina spinel surface. The physisorbed species is the precursor of this *-complex, and this view agrees with the selectivity of isoprene hydrogenation on copper catalysts. Introduction
Copper-based catalysts are well-known and largely used in selective hydrogenation of polyenes into monoenes, particularly in the industrial treatment of fatty o i P or in fragrance chemi~try.~ They are also very active in the synthesis of methanol from carbon monoxide and hydrogen4 A large study of Cu-X-O systems (X = Cr, AI, Zn) has been undertaken in our l a b o r a t ~ r y . A ~ ~model ~ of the active site and a reaction mechanism have been proposed for the selective hydrogenation of isoprene and other pentadienes; in the pentadiene series, the isoprene molecule is the most sensitive to steric effect^.^ In the correctly reduced state, unsaturated Cu+ ions in an octahedral environment, bound to a hydride ion, are found to be responsible for the catalyst's activity. This work concerns the characterization of the adsorbed state of isoprene on a reduced y-alumina-supported copper catalyst, compared to pure y-alumina. To this purpose, two techniques have been used: I3C N M R spectroscopy and in situ diffuse reflectance infrared spectroscopy (DRIFTS), which have been proved quite complementary. I3C NMR is a powerful technique for the observation of weakly adsorbed small hydrocarbon molecules,' because it allows an easy characterization of each carbon atom; to our knowledge, such N M R studies had never been previously performed with isoprene. DRIFTS is a new technique in infrared spectroscopy that eliminates many inconveniences in the sampling p r o c e d ~ r e . *The ~ ~ IR technique, which associates the Fourier transform and a diffuse reflectance attachment, provides a high-performance toollo particularly in catalysis, since it can be applied even to very absorbing materials such as copper chromium oxides." In the case of the alumina and copper/ alumina catalysts, it provides very fine information on the adsorption mechanism of isoprene. Experimental Section
Catalysts Preparation. y-A1203(Merck) was calcined in air at 923 K for 6 h and finely ground. Surface area, as measured by BET isotherms at 77 K with Ar gas, was found to be 70 mz.g-l. This alumina contains less than 50 ppm N a 2 0 , 100 ppm CaO, 50 ppm MgO, 300 ppm Si02, 100 ppm FezOs, 300 ppm Ti02. Cu/y-A1203 was prepared by impregnation of the previous alumina with an aqueous copper(I1) nitrate solution (metal loading corresponding to 3 wt 7% CuO). The sample was dried and subsequently calcined in air for 20 h at 773 K. The BET surface area was 60 mZ.g-l. N M R Spectroscopy. For NMR measurements, 1 g of sample was placed in a 8-mm-0.d. NMR tube, degassed in vacuo at 623 To whom correspondence should be addressed 'URA CNRS No. 402. * LASIR, UPR CNRS A 263 I L.
0022-3654/90/2094-6143$02.50/0
K for 18 h, and then reduced with hydrogen at the same temperature for 4 h. Adsorption of isoprene gas was achieved at room temperature at a pressure below the saturated vapor value, after evacuation of hydrogen at 623 K. A conventional gas-volumetric apparatus used for isoprene adsorption allows the calculation of the isoprene surface coverage. All 13C NMR spectra were run at 20.150 MHz at room temperature by using a WP 80 Bruker Fourier transform spectrometer, equipped with a DzO external lock. Proton noise decoupling was used in all experiments. Spectra were obtained by applying 3 ps (approximatively 30') pulses at 0.8-s intervals. Normally, 6 X IO4 scans were taken for each spectrum. Usually, the lowest isoprene coverage was limited to 5 pmolm-2 in order to obtain a significant signal-to-noise ratio. The highest coverage corresponds to 15 p m ~ l - m - ~Chemical . shifts were calibrated relative to liquid benzene and related to T M S (6 = 0). The resonance shifts were referred to the liquid state. Assignments of the I3C NMR lines in liquid isoprene spectrum were achieved from selective decoupling measurements. In the I3C NMR spectra of adsorbed isoprene on the solids, the overlapping of the lines of the different carbon atoms required a calculation treatment: a convolution program was used on a SORD 100 computer to obtain the line width and the chemical shift of each carbon. Inaccuracies were less than 5 Hz on line widths. IR Spectroscopy. The diffuse reflectance infrared Fourier transform spectra were recorded on a Bruker IFS 113 V spectrometer equipped with a wide-range mercury cadmium telluride (MCT) photoconductive detector, cooled with liquid nitrogen. The Harrick diffuse reflectance attachment DRA-2 CI is used with the heatable evacuable chamber HVC-DRP 1 for the in situ monitoring of the catalyst evolution. This system allows the gas flow passing through the sample to improve the gas-solid interaction. ( I ) Mounts, T. L.; Dutton, H. J. J . Am. Oil Chem. SOC.1967, 44, 67. (2) Koritala, S.; Selke, E.; Dutton, H. J. J . Am. Oil Chem. SOC.1973, 50, 310. (3) Pak, A. M.; Sokol'skii, D. V.; Konuspaev, S . R. Kinef. Cafal.1980,21, 670. (4) Pinto, A,; Rogerson, P. L. Chem. Eng. Prog. 1977, July, 95. (5) Jalowiecki, L.; Wrobel, G.;Daage, M.; Bonnelle, J. P. J . Cafal. 1987, 107, 375. (6) Wrobel. G.; Jalowiecki, L.; Bonnelle, J. P.; Bali, F.; Bettahar, A. New J . Chem. 1987, 11, 715. (7) Pfeifer, H.; Meiler, W.; Deininger, D. Annual Reports on NMR; Webb, G . A. Ed.; Academic Press: London, 1983; Vol. 15, p 291. (8) Griffiths, P. R.; Fuller, M. P. Aduances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1982; Vol. 9, p 64. (9) Ferraro, J. R.; Basile, L. Fourier Transform Infrared Spectroscopy; Academc Press: New York, 1985; Vol. 4. (IO) Depecker, C.; Legrand, P.; Sombret, B. Analusis 1985, 13, 349. ( 1 1) Depecker, C.; Legrand, P.; Sene, A.; Wrobel, G.Mikrochim. Acra 1988, 2, 119.
0 1990 American Chemical Society
6744
Rigole et al.
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990
I
I
c 5
200
b----_
A
i
0
100 ppm
Figure 1. I3C N M R spectra of adsorbed isoprene on (a) y-AI203 (surface coverage = 9.5 pmol.m-*) and (b) Cu/y-A1203(surface coverage = 10.2
pmo\.m-2).
A separated gas introduction system is connected to the cell and permits the admittance of H2 (N55from Air Liquide) and/or isoprene (purum grade, Fluka) diluted with He (N55)on the sample. The total flow was 2 L-h-l. The treatment conditions were chosen as near as possible those for the NMR studies. The solids (about 300 mg) were first dried in a He flow at 373 K for 1 h and then reduced for 16 h at 623 K in a H e H 2 mixture (50%). After a purge at 623 K and cooling in He, isoprene with/without H2 was admitted at various temperatures. Typically spectra were recorded with a 2-cm-' nominal resolution over the range 4000-500 cm-' with 48,96, or 512 scans; both the nature of the chemical treatment and the signal-to-noise ratio determined this number. Dried and finely ground KBr was employed as reference to calibrate the spectra. Subtraction was performed directly on spectra in Kubelka Munk units. Indeed, the Kubelka Munk function F(R,)12 gives a good quantitative representation of the diffuse reflectance spectra:I3 F(R,) = (1 - R,)2/2R,
= K/S
where K is the absorption coefficient and S the diffusion coefficient. In practice, the reflectance R , is the ratio between the sample reflectance and the reference reflectance. I n our case, as in the Beer-Lambert law commonly used in transmission measurements, F(R,) is directly proportional to the isoprene concentration, since S may be considered as a constant in the whole infrared spectral region, because the particle size is the same for all samples. Results
Isoprene Adsorption. N M R Results. Figure 1 shows typical spectra of isoprene adsorbed on y-A1203and Cu/y-A1203. On both adsorbents, the narrowness of the detected I3C NMR lines (25-80 Hz) can be explained by the rapid motion of the diene molecules on the solid surfaces, indicating that the main part of isoprene is physisorbed.' This view is also supported by the fact that chemical shifts corresponding to the different carbons in adsorbed isoprene are between 0 and 1 ppm to lower field referred to the liquid (methyl group excepted). Then, the order of magnitude of these shifts is the same as the necessary corrections for medium effects, and furthermore they do not depend on surface coverage. Therefore, they cannot be discussed here. I n Figure 2 are reported the I3C NMR line-width variations for the different carbon atoms versus isoprene coverage on both (12) Kubelka, P.; Munk, F. Z.Tech. Phys. 1931, 12, 593. (13) Hecht. H. G.Appl. Spectrosc. 1983, 37, 348.
5
10 coverage ( p m o 1 . m - 2 )
Figure 2. I3C NMR line-width variations versus (a) y-A120, and (b) Cu/y-AI20,.
15
isoprene coverage on
SCHEME I
.c '6 -
Qd
H-
" H '
/ Iq, H
I
H
y-A1203and Cu/y-A1203catalysts. Whereas all the carbon line widths follow the same trends on y-A1203(except at low coverage), the behavior of the adsorbed isoprene on Cu/y-AI2O3 is quite different. On this catalyst, the lines are broader and rather different from one carbon to another. It is interesting to remark that at the lowest coverage (5 gmol.m-2), respectively on Cu/y-A1203 and y-A1203, different lines appear much broader in the adsorbed isoprene spectra: the C, line on Cu/y-A1203(61 Hz); the C, and C2 lines on y-A1203 (respectively 49 and 5 5 Hz). On alumina, the marked increase of the line widths when the molecular coverage varies from 5 to 8 pmol.m-2 has been already evidenced.', It should be noted that at the highest isoprene coverages, all the carbon NMR lines exhibit the same line width (about 30 Hz) on alumina but not on Cu/y-AI2O3. Indeed, on Cu/y-AI2O3 we observe an important increase for the line widths corresponding to carbons C2 and C5 at high coverage (1 5 Kmol.m-2). At this great coverage, the different behavior of the carbon atoms arises from strong intermolecular interactions, which seem to be related to the surface modification due to copper. Isoprene Adsorption. IR Results. In any case, the IR spectra of the adsorbed isoprene are weak. On the spectra presented here (Figure 3), the alumina dehydration is sufficient to observe the 1200-800-cm-' region, in which isoprene bands can occur. (14) Rigole, M.; Choain, C.; Pietrzyk, S.; Guelton, M.; Bonnelle, J. P. C. R . Acad. Sci. Paris 1986, 303, Ser. I I ( 1 4 ) , 1289.
Isoprene on Alumina and Copper-Alumina Catalysts
The Journal of Physical Chemistry, Vol. 94, No. 17. 1990 6745
L
3100 + .-
300q cm
2900
1800
1600 0.1
1400
Figure 5. IR spectrum of adsorbed isoprene on Cu/y-Al2O3,obtained = 7 Torr). by a subtraction of spectra (Pisoprsns
I
i
M
3500
2500
500
1500
cm 1
Figure 3. IR spectra during adsorption and desorption of isoprene on y-A1203 ( I ) after alumina dehydration at 623 K, (2) after isoprene
adsorption in an isoprene-helium flow (PuoPm= 7 Torr) at 308 K during 20 min, (3) as in (2) after 30 min, (4) desorption phase in pure helium at 308 K after 8 min, ( 5 ) as in (4) at 323 K after 11 min, (6) as in (4) at 343 K after 25 min, and (7) as in (4) at 343 K after 30 min.
200
0
100 ppm
Figure 6. I3C NMR spectra of adsorbed isoprene on y-A1203 (a) and Cu/y-Al2O3 (b) after heating at 373 K for 5 min.
3100
3000 cm.1
2900
G
t
- .
. .
1800
'
1600 c m.1
1400
Figure 4. Subtraction between the IR spectrum of isoprene adsorbed on y-A1203and the spectrum of pure y-AI2O3.
In Table I are summarized the results of a normal-mode analysis done by Sverdlov and co-workers15 (the notations are mentioned in Scheme I ) . The corresponding frequencies have not been reported here since they are in total agreement with the experimental results obtained with gaseous isoprene. Comparison between adsorbed isoprene on y-Al,O, (Figure 4) and isoprene gas (Table I ) shows that the diene is physisorbed on the alumina surface, since the different shifts do not exceed 7 cm-I. Furthermore, they are smaller than the shifts corresponding to liquid isoprene, and this is consistent with the amount of adsorbed isoprene on the surface (smaller than a monolayer) as determined by thermogravimetric measurements. These results agree with Busca's, who reported the butadiene adsorption on hydroxylated y-alumina and silica,16and they can be correlated with the NMR data. However, it must be noted that (i) the relative intensities of the C-H stretching bands corresponding to isoprene vary, (ii) the absorption attributed to the overtone of the out-of-plane deformation at 1800 cm-I is weaker, and (iii) the bands at 1070 and 990 cm-' disappear (Figure 3), whereas they are strong for the pure gas. Moreover, the hydroxyl stretching region is also strongly affected: the 3730-cm-I band is less visible, whereas the 3590-cm-I band is enhanced: a good reversibility is observed after isoprene desorption (343 K, 1 h). This fact has already been mentioned by numerous authors for hydrocarbons adsorbed on oxides.l6-I9 In this study, the isoprene molecules (15) Tarasova, N. V.; Sverdlov, L. M. Opt. Mol. Spectrosc. 1967,3, 140. (16) Busca, G . J . Mol. Struct. 1984, 117, 103. (17) Knozinger, H.; Ratnasamy, P. Catal. Reo. Sci. Eng. 1978, 17, 31. (18) Gordymova, T. A.; Davydov, A. A. Kinet. Catal. 1979, 20, 721.
f
1
3500
2500 em-1
1500
5 00
Figure 7. IR spectra of adsorbed isoprene on y-Al2O3at 363 K after a treatment of 10 (2), 19 (3), and 40 min (4), compared with pure y-A1203 (1); (Piroprcns = 8 Torr).
may perturb the distribution of the hydroxyl groups on alumina surface. On Cu/y-A1203, physisorption also occurs (Figure 5 ) , but differences with pure alumina concern the 1800-cm-' band and mainly a new band that appears at 15 10-1 5 15 cm-I; this important result will be discussed below. Llnfortunately, the reduced Cu/y-A120, catalyst contains many hydroxyl groups, and no significant information can be drawn from the corresponding spectral region. Isoprene Polymerization. In our experimental conditions, I3C NMR spectra of adsorbed isoprene at room temperature did not show any polymerization (Figure 1). But, after the sample was heated at 373 K for only 5 min, all the lines are broadened (Figure (19) Busca, G.; Ramis, G . ; Lorenzelli, V.; Janin, A,; Lavalley, J. C. Spectrochim. Acta 1987, 43A, 489.
Rigole et al.
The Journal of Physical Chemistry, Vol. 94, No. 17, I990
6746
TABLE I
mode
attribution
sym
N' 1
A'
3 6 23
A' A' A"
7
A'
IRgas
liq
3106 3083 3091 3086 3026 2975 2987 2957
2MlB
overtone of paCH2 A'
9
A'
~ - c H , ( H C H )BcH,. , ~
A'
24
A"
II
A'
12
A'
13
A'
C
H
(. T > 363 Kl
I
I
,
3087
3092
3090
m F F
2965
2985 2957
2980 2960
2982
2985
1800
1800
1800
1640
I640
1648
1597
1598
1598
1510
1515
1465
1460
1440
1445
1370
1380
1475 1460 1467 m I458 1436 1443 m 1427 1421 m 1417
~
AI2O3
CulALO,
3089
Q,(C=C). Q,(C=C), Qb(CC) antisym 1646 1650 f 161 I 1598 1604 F Q,(C=C). Q,(C=C), sym 1595
10
.,
isoprene
3092 m
2925TF
2947 2947 m 2886 m 1824 1793 1800 m 1783
q+CH,(CH)
I
isoprene + H2
F
2935TF
8
2M2B
isoprene AIZO?, Cu/Al,O,
1770 f 1642 F
I650 tf
1447 F
1441
1600
1450
1430 m 1426
1380m
1396 1378
1365 14
A' 1245
1247 1100 m
16
A'
FcH,(C=CH),
26 28
A" A"
XCH,CCH, P'CCH PaCH2
1078 1070 m 1066 1061 990 991 F 905 T F 890 893 T F
fib33 P C H 3
1065 f 975 880 T F
900
900
895
900
790 m
1
I
21 3 El
z Y I
I
L--.--. x
3200
3000
2000
/
\
,.
,'
1800
1600
1400
.., 1000
800
600
cml
Figure 8. IR spectrum of the adsorbed species on Cu/-y-AI2O3during isoprene hydrogenation (P,soprcne = 7 Torr; P H J P H=~ I ; T = 323 K).
6). On y-A1203,formation of light polymers can explain such spectral modifications. On Cu/y-A1203,adsorbed isoprene is still observed, as opposed to y-A1203,in which case polymerization appears less important. Some differences also appear on IR spectra in the dynamic conditions (Figure 7). The bands are weaker and broader. The 1800-cm-I absorption totally disappears. On the other hand, a large band near 975-950 cm-' appears at the end of the adsorption. With reference to Table I, these results could be explained by the formation of light polymers and agree with the NMR data. Isoprene Hydrogenation. The IR results obtained only on Cu/y-AI20, at 323 K under a flow mixture of helium, hydrogen, and isoprene are reported here, since isoprene is not hydrogenated in these conditions on pure alumina. The spectral subtractions corresponding to the reduced catalyst spectra (Figure 8) show slight differences between hydrogenation and adsorption. The main features are the decrease of the 3092-cm-' band and the appearance of new absorption bands at
Figure 9. I3C NMR spectrum of adsorbed species on Cu/y-A1203,after hydrogenation (PH = 300 Torr; T = 423 K; 5 min) (a); "C NMR reference spectra ot liquid 2-methyl-2-butene(b) and liquid 2-methylI-butene (c).
1648 and 1245 cm-I. These last bands can exclusively be attributed to 2-methyl-1-butene (2MlB, Table I). Isoprene hydrogenation can also be displayed by I3C NMR (Figure 9), but in these experimental conditions (heating the sample 5 min at 423 K in the stztic atmosphere hydrogen/ isoprene), the main reaction product is 2-methyl-2-butene (2M2B) but we also observe 2-methyl-1-butene (2MlB). In this experiment, no significant polymerization is observed. Discussion Adsorption of Isoprene on y A l 2 O 3 . The narrowness of the observed lines shows that the main part of the adsorbed molecules is in any case in a rapid motional state, but the interpretation of these line widths in terms of simple motional narrowing models indicates correlation times much longer than those observed in the liquid state. Furthermore, although the adsorbed molecules
Isoprene on Alumina and Copper-Alumina Catalysts are in rapid motion, they may have different mobilities (distribution of correlation times);20 the fast exchange displays an averaged NMR spectrum, which is correlated with the number and the strength of the different adsorption sites and also with the texture of alumina.21 So, for y-alumina, the important broadening of the isoprene 13C N M R spectrum observed when the surface coverage is increased up to 8 pmol.m-2 can be explained by the presence of mesopores (4-6-nm median diameter),21in which the motions of isoprene molecules are relatively hindered. This phenomenon is well discussed in literature, especially for adsorption of organic molecules in zeolites,20v22and it is often associated with a drastic decrease of the translational mobility of the molecules. The line widths exhibit a maximum value at a coverage of about 8 pmol-m-2, whereas the BET monolayer coverage is only 6 pmol.m-2. The small discrepancy between these results has already been described.14 At the highest isoprene coverage, all the carbons exhibit the same line width; this result can easily be interpreted by the formation of a liquidlike phase in which the molecules are reorienting isotropically. As a matter of fact, a t such a coverage the intermolecular interactions greatly exceed the physisorption effects. At the lowest surface coverage ( 5 pmol.m-2), liquidlike intermolecular interactions between neighboring adsorbed molecules are unlikely, and NMR spectra can yield information about adsorbent-adsorbate interactions. Particularly, the different carbon line widths of adsorbed isoprene can point out the most tightly bound atoms to the catalyst surface or the nearest ones. As observed on y-alumina (Figure 2), C2 and C1 lines are broader than the others. This result is surprising for C2 carbon because usually nonprotonated carbons tend to exhibit narrower lines than do protonated ones.23 Nevertheless, these results suggest that C2 and C1 carbons are close to the surface or involved in a specific interaction with the solid. On the contrary, the C5 carbon line is always the narrowest, and therefore the methyl group appears relatively mobile (its free rotation is never hindered by the surface or by specific intermolecular interactions). The IR spectroscopic dynamic method provides further information, since the surface coverage, as already mentioned, is smaller than a monolayer. Indeed, the ratio of the stretching vinylic C-H band intensities for the methylene and methyl groups is higher in the case of the adsorbed isoprene (0.40 in comparison with 0.18 for the gas). Since the CH3 group is little affected by the adsorption, as shown by the N M R results, it can be assumed from the IR technique that the C , and/or C4carbon(s) is(are) concerned. Such modifications of the relative signal intensities have been already theoretically interpreted in the case of adsorption experiments on single crystals;24however, few explanations concerning similar IR results obtained with adsorbed molecules on supported metals or pure oxides have been reported.25 Moreover, the interaction of the CI and/or C4 carbons with the surface is clearly evidenced by the intensity decrease of the overtone of the out-of-plane C-H deformation at 1800 cm-l. Furthermore, an important experimental feature is the total disappearance of the strong bands at 990 and 1070 cm-I. From Table I, this can be interpreted by a relative immobilization of the C3 carbon atom. Then, the comparison between N M R and IR data show that the CI and C3 carbons of the isoprene molecule are close to the alumina surface. This interaction probably occurs via weak hydrogen bonding between the hydrogen atoms of the C1 and C3 carbons and the oxygen anions of the solid lattice, as already proposed for olefin adsorption.26-28 (20) Michel, D.; Pfeifer, H.; Delmau, J. J . Magn. Reson. 1981, 45, 30. (21) Rigole, M.; Pietrzyk, S.;Guelton, M., to be published. (22) Borovkov, V. Y.;Hall, W. K.; Kazanski, V. B. J. Coral. 1978,52,437. (23) Ali, I. T.;Gay, 1. D. J . Phys. Chem. 1981, 85, 1251. (24) Greenler, R. G.; Snider, D. R.; Witt, D.; Sorbello, R. S.Surf. sci. 1982, 118, 415. (25) Campione, T. J.; Ekerdt, J. G. J . Catal. 1986, 102, 64. (26) Gay, I . D. J . Phys. Chem. 1974, 78, 38.
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6147 CHART I
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