J. Phys. Chem. 1081, 85,1251-1254
quire a longer time. The colloidal particles would have a larger active surface and consequently be redissolved more efficiently. 4.5. Final Conclusions. It has been shown here for the first time, that an electronegative metal can act as a catalyst in the colloidal state for the H2 formation from radicals in aqueous solution, a domain which has generally been thought to be occupied only by the noble metals. The methods developed in the earlier work for the investigation of the mechanism of catalysis at colloidal microelectrodes were again successfully applied. The results reveal that the mechanism of catalysis is more complex in the case of cadmium than for the noble metals. A base metal in the colloidal state may store reduction equivalents either in the form of highly reactive metal
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atoms resulting from the reduction of residual ions or in the form of excess electrons. Large electron-storage capacities close to 1F / L can be obtained as in the case of certain noble metals. It has previously been pointed out that reactions on colloidal metals can be considered as electrochemical reactions although there is no outer potential source that drives the reactions at the microelect r o d e ~ .The ~ electrochemical description of the processes is completed in the present paper by the presentation of an analogous electrical circuit. Acknowledgment. We thank Mrs. H. Pohl for her excellent assistance in the laboratory work. (9)Henglein, A. J.Phys. Chem. 1980,84, 3461-7.
Adsorption of Olefins on Zinc Oxide. Observations by Carbon-13 Nuclear Magnetic Resonance Ishmall T. All and Ian D. Gay' Department of Chemistw, Simon Fraser Unlversify, Burnaby, British Columbia V5A 1S6, Canada (Received: August 28, 1980)
A series of cyclic and acyclic olefins adsorbed on zinc oxide has been studied by 13C NMR. Comparison has been made with the corresponding molecules physically adsorbed on silica. In the case of olefins where the double bond is easily accessible,the chemical shifts show the presence of weak chemisorption in a a-complex bonding state. Multiple substitution of the olefinic carbons prevents the close approach of the olefin to the surface, and inhibits this mode of bonding. Introduction Zinc oxide is an effective catalyst for the hydrogenationl-ll and i s o m e r i z a t i ~ of n ~olefins. ~ ~ ~ ~Understanding ~~ of the catalytic mechanism requires an understanding of the mode of bonding of the olefin to the surface. The infrared studies of Dent and K ~ k e s ~ J show ~ J ~that y ~small olefins typically have two binding modes, which have been identified as a complexes and as a-allyl complexes. Formation of a complexes with ethylene and with propylene at high coverage has been confirmed16J6by proton NMR, where the downfield shift of olefinic protons upon adsorption is readily interpreted as due to electron donation from the olefinic a bond to a surface center. In the case of more complex molecules, such as cycloo l e f i n ~the ~ ~infrared studies are less clear, due to the greater complexity of the spectra, and proton NMR be(1)J. F. Woodman and H. S. Taylor, J. Am. Chem. SOC., 62, 1393 (1940). (2)E.H.Taylor and J. A. Wethington, , J. Am. Chem. Soc., 76,971 (1954). (3)D. L.Harrison, D. Nichols, and H. Steiner, J. Catal. 7,359(1967). (4)A. L. Dent and R. J. Kokes, J. Phys. Chem., 73, 3772 (1969). (5)A. L. Dent and R. J. Kokes, . J. Phys. Chen., 73,3781 (1969). (6)A. L. Dent and R. J. Kokes, J. Phys. Chem., 74 3653 (1970). (7)J. Aigueperse and S. J. Teichner, Ann. Chim., 7, 13 (1962). (8)J. Aigueperse and S. J. Teichner, J. Catal., 2, 359 (1963). (9)F.Bozon-Verduraz, B.Argiropoulos, and S.J. Teichner, Bull. SOC. Chim., 2854 (1967). (10)F. Bozon-Verduraz and S. J. Teichner, J. Catal., 11, 7 (1963). (11)F. Bozon-Verduraz and S. J. Teichner, Proc. 4th. Int. Congr. Catal., 85 (1968). (12)A. L. Dent and R. J. Kokes, J . Phys. Chem., 75, 487 (1971). (13)S. Oyekan and A. L. Dent, J. Catal., 52, 32 (1978). 92,1092(1970). (14)A. L.Dent and R. J. Kokes, J. Am. Chem. SOC., (15)A. G. Whitney and I. D. Gay, J. Catal., 25, 176 (1972). (16)A. G. Whitney, Thesis, Simon Fraser University, 1975.
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comes unusable because of the great overlap of the broad lines which arise from adsorbed molecules of restricted mobility. It has been observed17that 13C spectra of adsorbed molecules yield much more information than proton spectra, both because the lines are narrower, and because the range of chemical shifts is much greater. 13C spectroscopy thus suggests itself as a good means of characterizing more complex olefins adsorbed on ZnO. Natural abundance 13C NMR has to date only been carried out for adsorbed species on surfaces of 200 m2/g or more, because of the unfavorable signal-to-noise ratio. The majority of the infrared and proton NMR studies have been done on Kadox-25 oxide of only 10 m2/g specific area. This difference in area creates a formidable signal-to-noise problem. We have showd8 that ZnO can be prepared from the oxalate with a higher surface area than the Kadox oxide, and with similar olefin adsorption properties. By using such an oxide, together with a wide-bore probe,lgwe have been able to obtain natural abundance 13C spectra for a wide range of adsorbed olefins.
Experimental Section All of the NMR spectra were measured at 25.2 MHz on a Varian XL-100 spectrometer, with TTI fourier-transform modification. Proton noise decoupling was used for all spectra. In order to study the relatively low area ZnO samples, we constructed a 22-mm probe, following the design of Zens and Grant.l9 This probe produced a signal-to-noise improvement of a factor of 5 over the Varian (17)I. D. Gay, J. Phys. Chem., 78,38 (1974). (18)I. T. Ali and I. D. Gay, J. Catal., 62, 341 (1980). (19)A. P. Zens and D. M. Grant, J. Magn. Reson., 30, 85 (1978).
0 1981 Amerlcan Chemical Society
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The Journal of Physical Chemistry, Vol. 85, No. 9, 1981
All and Gay
TABLE I adsorbate ethylene propene
6 5 -0.7
25
c-1
0.5 5.6 0.2
6 5 -1.3 70 -0.1 80 -1.1
20 20 40
c-1 c-2
0.2 7.6 0.5
25 -1.2 45 4.1 25 -1.5
15 20 20
c-1
0.4 4.2
25 -1.5 25 1.3
20 20
-0.2 1.6
25 -1.4 25 1.4
20 20
- 1.3
4.1 1.6 - 0.4
-0.8 -1.1 30 3.0 35 0.2 -0.9
25 30 60
c-1
0.4 1.7
110 -1.4 25 1.4
50 20
c-1
3.4 1.0 0.7
45 0.7 60 -0.8 50 -0.5
15 45 55
c-1
3.3 1.0 0.9
65 1.0 70 -0.6 40 0.5
25 55 55
c-1
3.6 1.3 0.5 0.5
75
35
c-3 cis-2-butene 150
I
I
I
I
I
50
100 ppm
Figure 1. Carbon-13 spectrum of cyclopentene adsorbed on ZnO.
12-mm probe, and a line width of about 5 Hz for static liquid samples. Chemical shifts were obtained by substitution, using an external lock. They are referenced to the neat liquid, or to the gas in the case of ethylene and propene. Susceptibility corrections (0.7 ppm) were applied, based on observation of the proton resonance of (CHJ4Si (Me&) physically adsorbed on the same adsorbent, and the assumption16that the physically adsorbed species have the same chemical shift as the pure liquid. Samples of ZnO were degassed in a 22-mm tube, and a measured quantity of adsorbate introduced via the vapor phase. For liquids, a coverage of 0.8 monolayer was used, based on the molecular area estimated from liquid density. For gases, the sample was equilibrated with the gas at a pressure of 40 torr. Samples with Si02as adsorbate were prepared by the same method, in 12-mm tubes. All samples were frozen at 77 K and sealed off following adsorption. All spectra were measured at 28 f 1 OC. Materials. The ZnO was prepared by pyrolysis of zinc oxalate dihydrateSz0After heating in vacuo at 400 "C for 3 h, 170 torr of O2was admitted at the same temperature. After 1 h the O2 was evacuated for 1 h, 150 torr of O2 admitted, the sample cooled to room temperature, and then degassed for 6 h before adsorption. The surface area of the ZnO was 53 m2/g, determined by the BET method, using N2 at 77 K. The silica gel used was from Baker Chemical Co., lot no. 1-3405. Samples were degassed for 4 h at 400 "C before use. The BET surface area was 253 m2/g. 1-Methylcyclohexeneand 1,2-dimethylcyclohexenewere obtained from Chemical Sample Co., and had purities of 99.5 and 99.9% respectively. Cycloheptene, 2,3-dimethylbutene-2, and 2-methylbutene-2 were from Aldrich Chemical Co. Cyclohexene was obtained from the Baker Chemical Co. Ethylene, propene, and butenes were Matheson CP grade gases, and were vacuum distilled before use.
Results Figure 1shows a spectrum typical of those obtained in this study. As can be seen, lines from all nonequivalent carbons are observed. This is generally the case, except as noted below. The results of all of our measurements are collected in Table I, which gives chemical shift changes on adsorption, and peak widths at half-maximum for all molecules, adsorbed on both ZnO and SiOP Positive shifts are in the direction of lower field. As noted above, shifts (20) R. Sh. Mikhail, N. M. Guindy, and I. T. Ali, J. Appl. Chem. Biotech., 24,583 (1974).
silica shift width
6.4
c-2 c-3 2-methylpropene
I
zinc oxide shifta widthb
c-2 trans-2-butene
c-1
c-2 2-methvlbutene-2 cis-c-i trans-C-1 c-2 c-3 c-4 2.3-dimethylbutene-2 c-2 cyclopentene c-3 c-4 cyclohexene c-3 c-4 cycloheptene
c-3 c-4 c-5 1-methylcyclohexene c-1 c-2 c-3 c-4,5
C-6 CH3
- 0.9
2.8 0.6 0.0 unresolved -0.2 unresolved
0.5 0.8 - 1.0 -0.9
95 1.5 40 -0.9 40 -1.8 unresolved 40 -1.8 unresolved
30 25 40
25 2.4 35 -1.0 25 -0.8 65 -1.2
25 45 35 60
60
1,2-dimethylcyclohexene
c-1 c-3 c-4 CH3
2.6 1.6 1.6 1.2
In ppm with respect to liquid, except ethylene and propylene with respect to gas. Positive shifts to lower field. k0.2 ppm. In Hz, corrected for apodization. 55 Hz. Blank entries unmeasurable due t o insufficient resolution.
are referred to the liquid, except for ethylene and propene, which are referred to the gas. It should be noted that the shifts of the other molecules would appear 1-2 ppm to lower field, had they been referred to the gas. This is because liquid hydrocarbons typically have their 13Cresonances a t slightly lower field than the corresponding gases.21p22
Discussion From an inspection of Table I it can be seen that the main chemical shift effect is a downfield shift of olefinic carbons upon adsorption on zinc oxide. In some cases this (21)I. D.Gay and J. F. Kriz, J. Phys. Chem., 82, 319 (1978). (22)B.Tiffon and J.-P. Doucet, Can. J. Chem., 54, 2045 (1976).
Adsorption of Olefins on Zinc Oxide
shift is found for adsorption on silica, but in all cases it is of greater magnitude on zinc oxide, indicating an electronic interaction with this surface which is not present with silica. When the olefinic carbons are unsymmetrically substituted, the more substituted carbon experiences a greater shift on both surfaces. In the case of cis- and trans-butenes, it is seen that a much larger effect occurs for the cis isomer, which is not observed on SiOz. Triple or quadruple substitution of the double bond tends to reduce the shift with respect to that found for single or double substitution, probably indicating steric hindrance of the access of the double bond to the surface. Some care must be exercised in comparing line widths between the two adsorbents, since differing levels of paramagnetic impurities might make such comparison meaningless. It can be seen that lines are generally broader on ZnO, and if it is assumed that differential impurity levels do not dominate, this would indicate generally lower mobility for the molecules adsorbed on ZnO. It can be seen that nonprotonated carbons tend to have narrower lines than protonated ones. This indicates some contribution of C-H dipolar coupling to the line width, and at least indicates that widths are not completely dominated by impurities. It is interesting to note that for cyclohexene, C-3 and C-4 have distinctly different line widths on ZnO, but not on SiOD This indicates that the rigid C-1,2,3,6 unit is preferentially held to the surface on ZnO, with C-4,5 having a narrower line due to their extra degrees of freedom, as compared with a more isotropic motion of the whole molecule on Si02. It is interesting, in view of the present data, to reconsider the mode of binding of olefins to ZnO. Dent and Kokes6,12*23 identified two types of species by infrared spectroscopy. Those which have a smaller shift of olefinic C-C stretch, with respect to the free olefin, were identified as ‘‘T complexes”, and the more strongly bound species, with larger shifts, as “a-allyls”. Studies with isotopically labeled propenesZ4showed conclusively in this case that the strongly bound species is a symmetrical C3H5entity, formed by dissociation of a hydrogen from the methyl group. Later Raman studies of the C-H stretching region, by Nguyen and Sheppard,26show that the C3H5species is more similar to that in allyl magnesium chloride, than to the classical palladium a-allyl complexes, and these authors suggest that the species on ZnO is best described as an allyl anion. If one examines the NMR data for olefin P complexes of the d’O species Pt(0),27v26 Ag(I),29-31 and Hg(II),32one observes a distinct trend in behavior. For Pt olefin complexes, the lH spectra show the olefin resonance upfield of the free olefin, and a large (50-100 ppm) upfield shift in the 13Cspectrum. Ag complexes show the proton resonance downfield, with much smaller (10-20 ppm) upfield shifts in the carbon spectrum. Finally, the Hg complexes (23) A. L. Dent and R. J. Kokes, J . Am. Chem. SOC.,92,6709 (1970). (24) A. L. Dent and R. J. Kokes, J . Am. Chem. SOC.,92,6718 (1970). (25) T. T. Nguyen and N. Sheppard, J.Chem. SOC.,Chem. Commun., 868 (1978). (26) C. C. Chang, W. C. Conner, and R. J. Kokes, J. Phys. Chem., 77, 1957 (1973). (27) C. E. Holloway, G. Hulley, B. F. G. Johnson, and J. Lewis, J . Chem, SOC.A, 1653 (1970). (28) M. H. Chisholm, H. C. Clark, L. E. Manzer. and J. B. Stothers. J. Am. Chem. SOC.,94, 5087 (1972). (29) H. W. Quinn, J. S. McIntyre, and D. J. Peterson, Can. J . Chem., 43, 2896 (1965). (30) K. R. Ark, V. Aris, and J. M. Brown, J. Organometal. Chem., 42, C67 (1972). (31) D.’Michel, W. Meiler, and E. Angele, Z. Phys. Chem. (Leipzig), 255. 389 (1974). (32) G. A. Olah and P. R. Clifford, J . Am. Chem. SOC.,95,6067 (1973).
The Journal of Physical Chemistry, Vol. 85, No. 9, 198 1
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show downfield shifts for both protons and carbons, with respect to the free olefins. This trend can readily be interpreted in terms of a simple Dewar-Chatt model of olefin bonding as being due to progressively smaller contribution of metal d orbitals to the metal-olefin bond, as one proceeds from Pt(0) to Hg(I1). Although there are no examples of stable Zn(I1) olefin complexes for comparison, one might expect from analogy with the above, that modest downfield shifts would be expected for olefii P-complex formation with Zn(I1). This is exactly what we observe in our spectra of adsorbed species, and strengthens the argument for ?r-complex formation. It is interesting to note that the chemical shifts observed above correlate well with the relative thermodynamic stabilities of different Ag(1)-olefin complexes. Generally, we find larger shifts for those olefins whose complexes are more ~table.3~ The correlation is even more striking if one plots the average shift of olefinic carbons on ZnO, minus that on SiOzvs. the equilibrium constant for formation of the Ag(1) complex. A good straight line results, with a correlation coefficient of 0.95. The most serious discrepancy from this correlation is cycloheptene, where the observed shift is too small to correlate with the unexpectedly large formation constant. The marked difference in shift behavior of cis- and trans-2-butene, in particular, is in accord with the much greater stability of the cis complex. Muhs and we is^^^ attribute this difference in stability to a relief of steric interactions in the cis isomer, upon complex formation. If this is so, one would expect a downfield shift of the methyl resonance upon complex formation, since the upfield position of this resonance, with respect to that of the trans isomer, is also attributed to steric interaction^.^^ This in fact is exactly what is observed, as can be seen in Table I. A similar effect may be operating in 1,2-dimethylcyclohexene,where the methyl resonance is at unexpectedly low field on ZnO, particularly when compared with the shift on Si02. In none of the spectra do we see any lines attributable to a P-allyl species. If one takes allyl magnesium bromide as a model, the 13Cresonances are at 113 and 57 ppm with respect to Any lines in the latter region would clearly have been noticed, and certainly with propene, the coverage of such species should b e sufficiently high to observe. At least in the case of propene, then, one is forced to fall back on the explanation that the line width is too great for a resonance to be observed in the present experiment. Indeed, careful study by proton NMR of a low coverage propene sample shows a resonance with a Tzvalue of about 100 ps. This clearly represents a line too wide to observe on a high-resolution spectrometer, although the proton spectra do not of course distinguish between an allyl species and surface hydroxyls. Such a line width indicates a very restricted motion of the adsorbed allyl species, and one may well ask why this is so. The proton relaxation data for ethylene (a complex) on ZnO can be interpreted16 as indicating rapid rotation of the molecule with respect to the surface normal, and less frequent flips about an axis parallel to the surface. The data in Table I suggest a bond involving only sp orbitals on Zn, with minimal participation of d orbitals. If an allyl species formed a similar bond with a single Zn, there is no obvious reason why similar freedom of motion (33) M. A. Muhs and F. T. Weiss, . J. Am. Chem.Soc., 84,4697 (1962). (34) J. B. Stothers, “Carbon-13 NMR Spectroscopy”, Academic Press, New York, 1972. (35) D. Leibfritz, B. 0. Wagner, and J. D. Roberts, Ann. Chem., 763, 173 (1972).
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J. Php. Chem. 1981, 85,1254-1261
should not be observed. A possible explanation of the restricted motion would be interaction of the allyl with an ensemble of two or more Zn ions. The existence of such ensembles on ZnO has been suggested by Boccuzzi et al.= As might be expected, 2-butene samples show spectra which change in time due to catalytic isomerization. On our oxide preparation, this takes place on a time scale of hours, at room temperature. It is interesting to note that, starting from a sample prepared with trans-2-butene, the equilibrium cis to trans ratio is 0.8, as judged from the relative intensities of the well-resolved methyl peaks. This is appreciably above the gas-phase equilibrium value of 0.3. Chang et a1.2ealso suggested that the adsorbed cis/ trans ratio in this system is higher than the gas-phase equilibrium value, but could not definitely confirm this because of possible differences in extinction coefficients. Greater stability of the adsorbed cis species would of course be expected, from the difference in shifts, as discussed above. On a short time scale, the methylcyclohexenes do not show any evidence of isomerization due to double bond migration. However, after 6 months at room temperature, the 1-methylcyclohexenesample shows new lines at 132.6 and 130.0 ppm with respect to Me4Si. These are most likely adsorption-shifted resonances of 4-methylcyclo(36)F. Boccuzzi, E. Garrone, A. Zecchina, A. Bossi, and M. Camia, J. Catal., 51, 160 (1978).
hexene, for which both olefinic carbons resonate at 127.1 ppm in the liquid, or of 3-methylcyclohexene, for which C-1 resonates at 126.6.34 The thermodynamics of the methylcyclohexene isomers is not clear, since the heats of formation of the 1and 4 isomers quoted by Stull et al.37 are in conflict with the temperature dependence of the equilibrium composition found by Herling et al.% It does appear, however, from either set of data, that measurable amounts of the 4 isomer, and probably also of the 3 isomer, should be present at equilibrium. Thus an isomerization would be expected on a suitable catalyst. A similar phenomenon occurs with 1,2-dimethylcyclohexene, where, after 6 months, new lines appear at 122.8 and 20.9 ppm. In the absence of both 13C NMR and thermodynamic data, one cannot be sure of the origin of these, but it seems most likely that they also indicate double bond migration within the ring. If one accepts the allyl" mechanismB for double bond isomerization of alkenes on ZnO, these results would imply the formation of intraring allyl species, which have not previously been observed on this catalyst. The slow rate of this reaction, compared with butene isomerization, however, probably implies that very few sites are capable of forming such species. ~
~~
~~~
(37)D. R. Stull, E. F. Westrum, Jr., and G. C. Sinke, “The Chemical Thermodynamics of Organic Compounds”, Wiley, New York, 1969. (38)J. Herling, J. Shabtai, and E. Gil-Av, J. Am. Chem. Soc., 87,4107 (1966).
Vapor Pressure Osmometry and Nuclear Magnetic Resonance Investigations of Some Bile Acid Methyl Esterst Julle Robeson, Bruce W. Foster, Steven N. Rosenthal, E. 1.Adams, Jr.,* and Eleanor J. Fendler Chemistry Depattment, Texas A & M Universlty, College Station, Texas 77843 (Received: September 3, 1980)
Vapor pressure osmometry (VPO) and nuclear magnetic resonance (NMR) studies have been carried out on solutions of methyl lithocholate, methyl deoxycholate, and methyl cholate in nonaqueous solvents (CC14,CHC13, CH2C12,and CS2). At 37 O C the VPO experiments indicated that the self-association of each bile-acid ester was greater in CC14than in CHC13solutions and that an increase in the number of hydroxyl groups promoted self-associationin both solvents. The concentration dependence of the proton chemical shift of the hydroxyl group(s) tended to parallel the VPO experiments. As the number of hydroxyl groups increases, the concentration dependence of the chemical shift becomes more pronounced. The type of self-associationpresent, the values of the equilibrium constant(s),and the values of the nonideal term were calculated from the VPO experiments. The NMR experiments could not provide this information, but they did give insight into the nature of the bonding involved in the self-association,the sites of interaction, and the internal mobility in the aggregates.
Introduction Bile salts are natural surfactants that aid in the solubilization of various dietary lipids, such as cholesterol and fat-soluble vitamins. Ideally one would like to study the interaction of bile salts with cholesterol and other lipids in aqueous solutions, but relatively little cholesterol is solubilized by the bile salts themselves. The solubility in aqueous solutions is markedly increased when lecithin is present.l From a physical-chemical viewpoint it is more difficult to study these systems, since three or more solute components are present-the bile salt, cholesterol, and ‘Presented in part at the 176th National Meeting of the American Chemical Society, Miami Beach, FL, Sept 11-14, 1978. 0022-3654/81/2085-1254$01.25/0
lecithin (which is heterogeneous in nature2) in addition to any supporting electrolytes and/or buffer components. An alternative is to study the interaction of the bile-acid steroidal nucleus with cholesterol in organic solvents; this is a three-component system which is much easier to study. Here one can see how changes in the number of hydroxyl groups and their stereochemistry can influence the interaction (the mixed association) between bile acid esters and cholesterol. Before the mixed association can be studied, (1)D. M. Small in “The Bile Acids: Chemistry, Physiology and Metabolism”, Vol. I, D. P. Nair and D. Kutchevsky, Eds., Plenum Press, New York, 1971,pp 249-356. (2) B. J. White, C. L. Tipton, and M. Dressel, J. Chem. Educ., 51, 533 (1974).
0 1981 American Chemical Society
