Langmuir 1998, 14, 5813-5820
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Low-Temperature IR Study of Ozone Interaction with Ethylene Adsorbed on Silica O. V. Manoilova,† J. C. Lavalley,‡ N. M. Tsyganenko,† and A. A. Tsyganenko*,† Institute of Physics, St. Petersburg University, St. Petersburg 198904, Russia, and UMR 6506, Laboratiore catalyze et spectrochimie ISMRA, Universite´ de Caen, 14050 Caen Cedex, France Received February 25, 1998. In Final Form: June 29, 1998 The advantages of the adsorbed state for spectral studies of the ozonolysis mechanism are illustrated by ozone reaction with ethylene on silica. The sample with preadsorbed ethylene or C2D4 was brought into contact with 16O3, 18O3, or mixed 16O3/18O3 from the solution in liquid oxygen at 60-77 K or from the gas phase at 77 K. The method enables us to detect the first products of ozonolysis, including primary and secondary ozonides or formaldehyde, arising already at 77 K or below and to follow their transformations in a wide temperature interval, not available for solid matrices or solutions. Formaldehyde, which should arise at the intermediate step of the reaction, is formed when an excess of ozone is admitted from the gas phase. It is supposed that O3 reaction with dioxymethyl (Criegee intermediate) prevents its recombination with formaldehyde, providing the stabilization of the latter.
Introduction The studies of the mechanism of ozone interaction with organic molecules are very important for the development of efficient and ecologically clean industrial processes, for water and wastewater purification. The destructive effect of ozone toward polymeric materials and its high toxicity awake an interest in searching for means to prevent this harmful action. For this purpose, detailed information about the structure of intermediate ozonolysis products and the conditions of their formation and destruction is needed. Such data could be of interest also for the understanding of the nature of atmospheric ozone interaction with organic components of soil or of troposphere aerosols as well as for the development of efficient and ecological industrial processes, such as paper bleaching or water purification. Most of the knowledge about the mechanism and the intermediate products of ozonolysis (see monographs in refs 1-3 and references therein) is based on the results of chemical analysis, reaction kinetics measurements, and spectroscopic studies carried out at ambient temperature in aqueous solutions or at low temperatures in nonaqueous solvents. In the latter case the temperature is limited by the solvent freezing point or by the solubility of the studied species and almost never is below 150 K. First, IR studies of ozone interaction with olefins at lower temperatures were performed by direct condensation of reactants upon cold windows.4 In this way spectra of primary ozonides were obtained. Later, using isolation in noble gas matrices, it was possible to study weak molecular ozone complexes with different molecules * To whom correspondence should be addressed. E-mail:
[email protected]. † St. Petersburg University. ‡ Universite ´ de Caen. (1) Bailey, P. S. Ozonation in Organic Chemistry. Olefinic Compounds; Academic Press: London, 1978; Vol. 1. (2) Bailey, P. S. Ozonation in Organic Chemistry. Nonolefinic Compounds; Academic Press: London, 1982; Vol. 2. (3) Razumovskii, S. D.; Zajkov, G. E. Ozone and its reactions with organic compounds; Studies in Organic Chemistry Series; Elsevier: Amsterdam, The Netherlands, 1984; Vol. 15. (4) Hull, L. A.; Hisatsune, I. C.; Heicklen, J. J. Am. Chem. Soc. 1972, 94, 4856.
including ethylene and formaldehyde5 or photostimulated processes with ozone as a source of atomic oxygen (see, e.g., ref 6). Initiation of chemical reactions normally requires temperatures too high for matrix isolation in noble gases. Nevertheless, Kohlmiller and Andrews7 succeeded in obtaining spectral evidence for different isotopomers of primary ozonides (POZ) of ethylene in a xenon matrix annealed at 80 K. The data obtained were in agreement with the study of ethylene and ozone codeposition in CO2 and CCl4 matrices that could be warmed to 150-160 K. In such a case the secondary ozonide (SOZ) was formed as a main product.8 Ozonolysis of adsorbed molecules seems to be a promising method for studying the reaction intermediates and their transformations by means of IR spectroscopy in a much wider temperature interval. At 77 K ozone has a vapor pressure high enough to be adsorbed on solid surfaces, and if needed, the temperature may be lowered to 60 K by using ozone adsorption from its solution in liquid oxygen. Then, changes in the spectrum on heating the cell could be followed until the decomposition or desorption of the last ozonolysis products. Such works, as far as we know, have not been reported yet. In the present paper we have tried the method using ethylene as a starting compound. C2H4 and C2D4 interactions with 16O3, 18O3, and 16O3/18O3 isotopic mixtures were studied on the surface of SiO2. This adsorbent is a comparatively inert support capable of fixing at low temperatures ethylene by a weak hydrogen bond between surface silanol groups and π-electrons of ethylene.9 It does not have other active surface sites that could decompose ozone or otherwise influence the ozonolysis process. However, silica has a very strong absorption between 1300 and 1000 cm-1, impeding observation in this region, where some of the bands of ozonolysis products should occur. To study spectral changes in this region and to find out the role of different active sites of solid (5) Nord, L. J. Mol. Struct. 1982, 96, 37. (6) Lugez, C.; Schriver, A.; Levant, R.; Schriver-Mazzuoli, L. Chem. Phys. 1994, 181, 129. (7) Kohlmiller, C. K.; Andrews, L. J. Am. Chem. Soc. 1981, 103, 2578. (8) Nelander, B.; Nord, L. Tetrahedron Lett. 1977, 32, 2821. (9) Busca, G.; Lorenzelli, V.; Ramis, G.; Saussey, J.; Lavalley, J.-C. J. Mol. Struct. 1992, 267, 315.
S0743-7463(98)00226-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/10/1998
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Figure 1. IR spectra of silica pretreated at 770 K and cooled up to 77 K (1), after adsorption of ethylene and submergence in liquid O2 (2), and after O3 addition at 60 K and an increase of the temperature up to about 70 (3) and 77 K (4).
surfaces in the process of ozonolysis, experiments with other solid supports are in progress. Experimental Section The stainless steel cell described elsewhere10 for studying the IR spectra of adsorbed species at liquid nitrogen temperature was equipped with crystal ZnSe inner windows that, in combination with the outer KBr windows, enabled us to expand the available spectral region down to 450 cm-1. Two Barocel pressure gauges, one (Datametrix 600) attached directly to the sample containing an inner volume of the cell and the other connected with the preliminarily calibrated volume of the vacuum system, were used to measure the doses of gases introduced into the cell and to observe the pressure changes inside the closed cell during the experiments. Samples of silica (Aerosil Degussa, S ) 200 m2/g), typically 7-10 mg, were pressed into 2 cm2 pellets and heated first in oxygen (20-50 Torr) and then in a vacuum at 770-850 K for 1-2 h. Ethylene or C2D4 was introduced into the cell cooled up to 77 K, and then to accelerate the adsorption, the coolant was removed and the temperature raised to about 100 K. After that the cell was cooled again before running the spectrum of adsorbed ethylene. The amount of gas was chosen in order to saturate all of the OH groups with a small excess and normally was about 5 µmol (5-7 Torr in the dosing volume of 12 cm3). The same procedure was used to obtain the spectrum of adsorbed formaldehyde that is known to polymerize when adsorbed at 300 K. Ozone was brought into contact with adsorbed ethylene either from the gas phase or from the solution in liquid oxygen. In the first case, an excess portion of O3 (about 50 µmol or more) was admitted into the cell, preliminarily cooled to 77 K, and filled with about 0.5 Torr of He for thermostabilization. In the second procedure, oxygen was condensed in the sample-containing volume of the cell at 77 K. Then, after the background spectrum of the sample immersed in liquid oxygen was recorded, ozone, trapped beforehand in a U-trap, was added, carried in by a flux of oxygen. To achieve the temperature of solid nitrogen (60 K), the nitrogen used as a coolant was pumped off up to a pressure of about 50 Torr, and then ozone was introduced as before. Ozone was prepared from gaseous 16O2, 18O2 (“EURISO-TOP” CEA Group, 97.2% isotopic purity), or their mixtures in electric discharge and manipulated as before.11,12 To eliminate CO2 contamination, always present in thus-prepared ozonesthe latter before introduction into the cell was purified by distillations only the first fraction was used. Commercial 16O2 was used to prepare ozone and liquid oxygen. The presence of impurities was monitored spectroscopically. Usually, a small amount of methane (bands at 3027 and 1305 (10) Babaeva, M. A.; Bystrov, D. S.; Kovalgin, A. Yu.; Tsyganenko, A. A. J. Catal. 1990, 123, 396. (11) Bulanin, K. M.; Alexeev, A. V.; Bystrov, D. S.; Lavelley, J.-C.; Tsyganenko, A. A. J. Phys. Chem. 1994, 98, 5100. (12) Bulanin, K. M.; Bulanin, M. O.; Tsyganenko, A. A. Chem. Phys. 1996, 203, 127.
cm-1) could be detected, which was neither adsorbed nor oxidized in the conditions of the experiment. Commercial ethylene and C2D4 (98.2% isotopic purity) were used. Formaldehyde was obtained by evaporation in vacuo of solid paraformaldehyde (Carlo Erba, Milano, Italy). Spectra were recorded with a Nicolet FT-IR 710 spectrometer with 4 cm-1 spectral resolution. A germanium filter was installed in the beam before the cell to eliminate sample heating by IR radiation. For better thermal contact of the sample with the cooled environment, about 0.5-1 Torr of helium was introduced into the sample compartment before recording spectra at the liquid nitrogen temperature, unless liquid oxygen was in the cell. To eliminate the background absorption of silica below 2000 cm-1 as well as the induced absorption of liquid oxygen, the initial spectrum of the cooled sample either in helium or in liquid O2 was subtracted from the new one. To be sure that the observed bands of ethylene or ozone are due to adsorbed molecules but not to those dissolved in O2, spectra of the solution were registered as well, and when needed, the bands of the solution were subtracted.
Results IR Spectra of Adsorbed Ethylene and Deuterioethylene. After ethylene is admitted at 77 K into the cell with a SiO2 sample pretreated at 773 K, the band of surface silanol groups at 3750 cm-1 gradually diminishes, while a wide band of perturbed hydroxyls arises at 3598 cm-1 simultaneously with the bands of adsorbed molecules at 3098, 3071, 3011, 2979, and 1441 cm-1, in accordance with the earlier data.9 A couple more bands at 966 and 955 cm-1 and a broad band with a maximum at 888 cm-1 were detected after the background spectrum subtraction. Ethylene adsorption at 77-100 K on the deuterated silica surface results in a similar displacement of the OD band from 2765 to 2645 cm-1, while the 888 cm-1 band does not appear. After condensation of liquid oxygen in the cell containing the sample with preadsorbed ethylene, the CH bands of the latter are shifted by about 4 cm-1 to higher wavenumbers, while the band of perturbed OH groups becomes narrower and moves to lower frequencies by about 16 cm-1, from 3598 to 3582 cm-1. Bands at 1441, 966, 955, and 888 cm-1 remain practically at the same positions (curve 2 in Figure 1 compared with curve 2 from Figure 4). C2D4 adsorption at about 100 K on silica from the gas phase, besides the OH group perturbation with the maximum of H-bonded hydroxyls at 3570 cm-1, results in the appearance of strong bands at 2336 and 2194 cm-1 together with weaker maxima at 2312, 2253, and 2215 cm-1, a broad band at 891 cm-1, and a couple of peaks at 735 and 723 cm-1. The intensity ratio of the two latter bands is not constant: that at 735 cm-1 reaches its
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Table 1. Position of the Bands (cm-1) of Ozonolysis Products Observed after 16O3 and 18O3 Reaction with Ethylene and Deuterioethylene Adsorbed on a Silica Sample Immersed in Liquid Oxygen compound 16O
18O
3
3
+ C2H4
+ C2H4
primary products on silica
primary ozonides in a matrix7
3013, 1464, 1328, 927, 724, 646
1214, 983, 927, 846, 727, 647, 409
1464, 1328, 912, 703
16O /18O 3 3
16O
18O
3
3
+ C2H4
+ C2D4
+ C2D4
1464, 1328, 927, 919, 912, 725, 714, 703
2226, 2188, 2156, 2127, 2119, 892, 637
1208, (955), 909, 802, 705, 615, 390 1210, 927, 917, 909, 727, 719, 705, 647, 641, 636, 630, 623, 616, 400
secondary products on silica
secondary ozonides in a matrix13
2979, 2906, 2725, 1392, 1352,
2989, 2973, 2967, 2895, 2716, 1387, 1346, 1201, 1196, 1129, 1078, 1029, 952, 927, 808, 736, 698
948 (sh), 935 2979, 2906, 2720, 1392, 1348, 946, 932, 768 1392, 1350, 945, 934, 764
2269, 2200, 2178, 2094, 1089, 892, 636, 387
2223, 2198, 2150, 2124, 2114, 874, 664
maximum when the band of free silanol groups at 3750 cm-1 disappears completely, and another one continues to grow with the increasing amount of adsorbed ethylene. Ethylene Oxidation by Ozone from the Solution in Liquid Oxygen at 60-77 K. Changes in the spectrum of adsorbed ethylene observed on O3 addition through the solution in liquid oxygen are illustrated in Figure 1. Ozone addition into liquid oxygen at 60 K results in the appearance of the bands of dissolved O3, similar to those observed in ref 12, but no changes occur in the spectrum of adsorbed ethylene (curve 2). However, on an increase of the temperature up to 77 K (curves 3 and 4) or after ozone introduction at liquid nitrogen temperature, the bands of adsorbed ethylene disappear in several minutes, and a new group of bands arise at 3013 (sh), 2979, 2906, 2725 (weak), 1464, 1392, 1352, 1328, 952 (broad), 948, 935, 927, 724, and 646 cm-1 simultaneously with a new band of perturbed OH groups at 3446 cm-1, which demonstrates even stronger perturbation of silanol groups by reaction products. Among the bands of ozonolysis products one can distinguish several groups of maxima that always have constant relative intensities, disappear simultaneously, and evidently could be attributed to the same surface species. Pumping off the liquid oxygen does not cause any notable changes in the spectrum of adsorbed species. One could only notice the shift of bands due to ozone ν1 + ν3 combination and ν2 fundamental from the values typical of dissolved molecules at 2102 and 700 cm-1, respectively, to the positions observed for H-bonded O3 on silica (2106 and 703 cm-1). On an increase of the sample temperature, the intensities of the adsorbed ozone bands first increase with increasing vapor pressure and then start to diminish up to complete desorption at about 150 K. At about 120150 K the disappearance of the 927 cm-1 band together with those at 1464, 1328, 724, and 646 cm-1 takes place as well. These latter bands evidently belong to a less stable product of ozonolysis, referred to later as “primary product”. Concurrently, a decrease in the intensity of the OH band at 3446 cm-1 could be observed.
973, 928, 904, 756, 669
2990, 2967, 2895, 2710, 1388, 1340, 1197, 1189, 1124, 1052, 1013, 938, 906, 764, 703, 668 1389, 1341, 1128, 1125, 1078, 1076, 1071, 1058, 1055, 1052, 952, 940, (927), 915, 907, 807, 787, 765, 736, 729, 722, 717, 711, 703, 698, 693, 684, 679, 673, 668 2257, 2249, 2204, 2181, 2131, 2117, 2091, 1057, 1020, 981, 972, 929, 911, 904, 848, 830, 758, 706, 672
2267, 2184, 2088, 939, 919, 903, 892, 850, 844, 733
It should be noted that complete removal of oxygen after ozonolysis of adsorbed ethylene from the solution can lead to explosive decomposition of the products with destruction of the pellet; that is why the quantity of admitted ethylene was always limited, and after removal of the liquid oxygen, 10-50 Torr of O2 was kept in the cell and further heating of the sample was carried out in the presence of this gas. The spectrum in the CH stretching region does not change substantially. However, heating to 120 K clearly reveals the disappearance of a band at 3013 cm-1 and simultaneous increases of absorption at 2930 and 1365 cm-1. Two other features arise at 2998 and 2898 cm-1, as shoulders of the bands at 2979 and 2906 cm-1. Bands at 2979, 2906-2898, 2725, 1392, 1352, 948 (sh), and 935 cm-1, referred to later as due to the secondary product, diminish in intensity after heating to 190 K and all disappear completely at about 230 K. Meanwhile, the absorption of perturbed OH groups almost disappears and the intensity of isolated silanols becomes close to that in the initial spectrum before ethylene addition. Bands at 2998, 2930, and 1365 cm-1 remain in the spectrum up to 300 K. Effect of Isotopic Substitution. Experiments on ozonolysis of ethylene from the solution in liquid oxygen were performed as well with ethylene-d4. Both C2H4 and C2D4 combined with 16O3 and 18O3 were studied. Band positions are summarized in Table 1, where the bands of products that all disappear on heating to about 120-150 K and those that remain or increase in intensity at the same temperature but could all be removed on further heating to about 250 K are placed in two separate columns and referred to as primary and secondary products, respectively. The evolution of IR spectra of ethylene ozonolysis products with temperature is illustrated by the case of C2D4 interaction with 18O3 in Figure 2. Bands of primary products, such as those at 2223 and 664 cm-1, are weakened in curve 3 and absent in curve 4, while those of secondary products, e.g., at 733 cm-1, display their maximum intensities in curve 4 and decrease greatly in curve 5.
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Figure 2. Evolution of the IR spectra of low-temperature ethylene ozonolysis products with temperature. SiO2 pretreated at 770 K after C2D4 adsorption, submergence in liquid O2, 18O3 addition, and pumping off the liquid phase at 77 K (1) and after a subsequent increase of the temperature up to ca. 100 (2), 130 (3), 200 (4), and 230 K after removing the gas phase (5).
Figure 3. Effect of isotopic substitution on the spectra of ozonolysis products obtained by O3 addition to the silica sample exposed to ethylene and submerged into liquid oxygen at 77 K: (1) C2H4 + 16O3, (2) C2H4 + 18O3, (3) C2H4 + mixed 16O3/18O3, (4) C2D4 + 16O , (5) C D + 18O on deuterated silica. 3 2 4 3
Figure 3 illustrates the changes caused by isotopic substitution in the spectra of ozonolysis products obtained by O3 addition through liquid oxygen at 77 K. As seen from curves 1-3 of Figure 3 and from Table 1, 18O substitution practically does not affect the spectrum of the C-H stretchings, and in the 1500-1300 cm-1 region, only the bands at 2725 and 1352 cm-1 exhibit measurable shifts to lower wavenumbers by about 5 cm-1. Bands due to primary products at 927 and 724 cm-1 display more prominent shifts to 912 and 703 cm-1, respectively, while after reaction with isotopically mixed 16O3/18O3, a third component at the intermediate position shows up for both of the bands. For deuterated ethylene, as one can see from the left part of Figure 2, instead of CH stretchings we have a set of CD vibrations, much better resolved as compared with corresponding CH spectra presented in Figure 1. Up to five bands in this region with slightly different positions for 16O3 or 18O3 should be attributed here to the primary ozonolysis products. Deuterium substitution in ethylene results in the disappearance of all of the bands in the 1500-1300 cm-1 region of both primary and secondary products (Figure 3, curves 4 and 5), which are, evidently, due to the bending CH2 modes and cause also the appearance of a number of new peaks below 1000 cm-1. After the removal of secondary products by an increase of the temperature up to about 250 K, a pair of CD bands remain at 2258 and about 2105 cm-1, accompanied by those at 880 cm-1 for 16O3 and at 768 cm-1 in the case of ozonolysis by 18O3. Deuterium exchange of surface silanol groups does not affect the spectra of adsorbed ethylene or ozonolysis
products. The only significant feature, besides the stretching OH displacement, is the absence of the broad absorption at about 950 cm-1 (Figure 3, curve 5), which is present in all of the other curves where silanol groups were not deuterated. Ethylene Interaction with 16O3 from the Gas Phase. Changes in the regions of OH and CH stretching vibrations of the IR spectrum of ethylene adsorbed on silica, observed on ozonolysis from the gas phase at 77 K, are illustrated in Figure 4. After ozone addition the bands of adsorbed ethylene disappear in several minutes, the band of perturbed OH groups shifts to 3477 cm-1, and a set of new bands of ozonolysis products arise. The band of residual free silanols decreases in intensity, while a new maximum at 3664 cm-1 arises, which according to ref 11 is due to OH groups perturbed by adsorbed ozone. Most of the bands of ozonolysis products are the same as those in experiments with ozonolysis from the solution in liquid oxygen. One could distinguish bands of primary and secondary products that disappear in the same conditions as described previously, leaving the species characterized by bands at about 2990 and 2930 cm-1, which first increase in intensity up to about 200 K and remain in the spectrum at 300 K. The main dissimilarity between the spectra of products of ozonolysis from liquid and gas phases is the presence in the latter case of a new group of bands at 2833, 1721, and 1500 cm-1, which arise among the others at 77 K and all disappear on heating to 150-200 K when the bands of primary products are already gone. It will be shown
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Figure 4. IR spectra of silica pretreated at 773 K and cooled up to 77 K in the presence of He (1); after addition of ethylene (2); after O3 admittance (3); and after 6 min (4). Spectra below 2000 cm-1 are obtained by subtraction of curve 1 from curves 2-4.
Figure 5. Effect of isotopic substitution on the spectra of low-temperature adsorbed ethylene ozonolysis products: (1) C2H4 + 16O3 (same as curve 4 with the subtracted bulk absorption in Figure 4); (2) C2H4 + 18O3; (3) C2H4 + mixed 16O3/18O3; (4) C2D4 + 16O3; (5) C2D4 + 18O3.
that this group of bands, not observed after ozonolysis from the liquid phase, is due to adsorbed formaldehyde molecules. Decomposition of primary products on an increase of the sample temperature is accompanied by a remarkable increase in the CO2 absorption at 2340 cm-1 but does not cause an intensity increase of formaldehyde bands. An intensity increase was detected for the bands of secondary products in some experiments when the absence or low intensity of the bands of adsorbed ozone shows that there is no excess of it in the cell. The disappearance of formaldehyde bands is accompanied by an intensity diminution of the perturbed OH group absorption and a remarkable increase of bands at 2990 and 2930 cm-1. After 18O3 is admitted from the gas phase to the sample with preadsorbed ethylene at 77 K, a new maximum becomes visible at 2873 cm-1, which disappears together with those of formaldehyde. The bands of the latter are shifted now to 1692, 1489, and 911 cm-1 (Figure 5, curves 2 and 3). C2D4 interaction with 16O3 gives rise to the only band due to formaldehyde at 1674 cm-1 (curve 4), which shifts further to 1637 cm-1 if 18O3 has been used (curve 5). We have not succeeded in finding out the accompanying bands of CD stretching vibrations. Adsorption of Formaldehyde on Silica and Its Reactivity toward Ozone. The bands at 1721 and 1500
cm-1, arising on ethylene ozonolysis by gaseous O3, are close to those known for the spectrum of formaldehyde, which could be regarded as one of the possible products of the reaction. Taking into account the lack of such data in the literature, we have studied formaldehyde adsorption on silica at 300 K and low temperature as well as the influence of ozone addition to the products of formaldehyde adsorption. If formaldehyde vapor is admitted into the cell with the silica sample pretreated at 850 K and preliminarily cooled by liquid nitrogen, no changes occur until the temperature is raised to about 110 K, when the vapor pressure becomes sufficient for adsorption. If then the sample is cooled again to 77 K, the two bands at 1726 and 1500 cm-1, with an intensity ratio very close to that observed in the spectrum of ozonolysis products, arise together with a wide band at about 945 cm-1. The intensity of the band due to free silanol groups decreases, and a wide absorption of the perturbed OH groups appears at 3430 cm-1. In the CH stretching region a group of bands at 2994, 2891, and 2830 cm-1 arises, accompanied by a less intense one at 2729 and a shoulder at about 2914 cm-1. The latter pair of bands, shifted to 2983 and 2923 cm-1, and a broad absorption centered at about 940 cm-1 are the most intense if formaldehyde is adsorbed at 300 K and then the sample is cooled to 77 K. Other weak bands arise also in this case at 2874, 2859, 2847, 1470, 1432,
5818 Langmuir, Vol. 14, No. 20, 1998
1383, and 635 cm-1. Weak bands at about 1720 and 1500 cm-1, absent at 300 K, show up on cooling of the sample. Ozone introduction at 77 K does not affect the bands of adsorbed formaldehyde species and only causes residual OH group perturbation and shifts the maximum of OH groups bound to formaldehyde species from 3430 to 3373 cm-1. No new bands appear on heating of the sample in the presence of ozone; only the increased intensity of the 1720 and 1500 cm-1 bands could be noticed. Discussion Ethylene and Formaldehyde Adsorption on Silica. The presented spectra of adsorbed ethylene are in agreement with the earlier results9 and show that at least part of the adsorbed molecules form H bonds with surface silanol groups. The two bands at 966 and 955 cm-1, not reported so far, whose relative intensities vary in a different way as well as the corresponding maxima in the spectrum of deuterated ethylene at 735 and 723 cm-1 should be attributed to the ν7 bending vibration of two different forms of adsorbed ethylene. The intensity increase of the high-frequency peak at 966 cm-1 (735 cm-1 for C2D4) is saturated when all of the free hydroxyls are perturbed. This band is due, evidently, to the hydrogenbonded ethylene molecules. The maximum at 955 cm-1 (723 cm-1) should naturally be attributed to the physisorbed ethylene. As one might expect, H-bond formation leads to a slight increase of the ν7 frequency value, while physisorption accounts for bands at wavenumbers close to 949 and 720 cm-1, reported for free C2H4 and C2D4, respectively.14 Other vibrations are less sensitive to the adsorption and, apparently, coincide for H-bonded and physisorbed ethylene. In fact, four bands of CH stretching vibrations (ν9, ν5, ν1, and ν11, according to ref 14), at 3098, 3071, 3011, and 2980 cm-1, or the corresponding CD bands in the case of C2D4 at 2336 (s), 2312 (sh), 2215, and 2194 (s) cm-1 as well as the peak due to the bending CH2 mode ν12 at 1441 cm-1 are close enough to the corresponding values known for free molecules in the gas phase (3105.5, 3102.5, 3026.4, and 2988.7 and 2345, 2310, 2260, 2200, and 1443.5 cm-1, respectively15). The broad maximum at 888 cm-1, observed both for usual and deuterated ethylene but absent in the spectrum of both isotopic modifications adsorbed on deuterated silica, should be assigned to the bending νOH vibration of surface silanol groups, perturbed by H bond with ethylene. For free SiOH groups it has a frequency about 840 cm-1,16 which increases as a result of hydrogen bonding. The weak maximum at 2253 cm-1 could be attributed to the admixed C2HD3 molecules. Our data on formaldehyde adsorption on silica are in accordance with the earlier results by Busca et al.,17 who have first claimed the observation of H-bonded formaldehyde monomer bands at 2995, 2894, 2830, 2732, 1725, 1717, and 1501 cm-1 at 150 K and polymerized species at 300 K (bands at 2980, 2915, 2805, 1480, 1425, and 1385 cm-1), with the latter also studied earlier in ref 18. In our spectra, bands of adsorbed formaldehyde monomers at 77 (13) Hawkins, M.; Kohlmiller, C. K.; Andrews, L. J. Phys. Chem. 1982, 86, 3154. (14) Herzberg, G. Molecular spectra and molecular structure. Infrared and Raman spectra of polyatomic molecules; van Nostrand: New York, 1945; Vol. 2. (15) Sverdlov, L. M.; Kovner, M. A.; Krainov, E. P. Vibrational spectra of polyatomic molecules; Nauka: Moscow, 1970 (in Russian). (16) Tsyganenko, A. A. Russ. J. Phys. Chem. 1982, 56, 1428. (17) Busca, G.; Lamotte, J.; Lavalley, J.-C.; Lorenzelli, V. J. Am. Chem. Soc. 1987, 109, 5197. (18) Kubelkova, L.; Jiru, P.; Schurer, P. Collect. Czech. Chem. Commun. 1969, 34, 3842.
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K were detected at 2891 (ν4), 2824 (ν1), 1726 (ν2), and 1500 cm-1 (ν3), while polymeric forms account for the strong bands at 2980, 2923, and about 940 cm-1 and, evidently, also for weaker maxima at 2874, 2859, 2847, 1470, 1432, 1383, and 635 cm-1. Ozonolysis Products. Analysis of results obtained reveals at least four products of adsorbed ethylene ozonolysis. One, which appears as a result of ozone admittance from the gas phase and has characteristic bands at about 2833-2825, 1721, and 1500 cm-1, is evidently a formaldehyde monomer, as follows from the comparison with the spectra of adsorbed formaldehyde discussed above. Band positions of formaldehyde arising among other products of ozonolysis are slightly different because of perturbation by other molecules. So, the band first arising at 2833 cm-1 shifts to 2825 cm-1 after the removal of primary products. The formaldehyde band at 2891 cm-1 in the spectrum of products is overlapped with stronger bands due to other species, but the band at 2873 cm-1 well-resolved after reaction with 18O3 is, evidently, due to the same vibration of the isotopically substituted molecule. The isotopic shift of the νCO (ν2) band at 1721 cm-1 by 47 cm-1 and the disappearance of the νCH2 (ν3) band at 1500 cm-1 when using ethylene-d4 are in agreement with the assignment proposed, since, according to ref 15, for the ν2 vibration of formaldehyde the shift value on deuteration is 46 cm-1, while the ν3 band should displace to 1106 cm-1, just in the region of strongest bulk absorption of silica. The bands that remain at about 2980 and 2930 cm-1 after decomposition of other species on raising the temperature up to 300 K are very close to those observed after formaldehyde polymerization. In fact, the increase of the 2930 cm-1 band intensity was observed when the bands due to formaldehyde monomer disappeared, which can be explained by the formation of polymers when the mobility of adsorbed formaldehyde molecules becomes high enough. However, these bands grow not only when the bands of formaldehyde monomer diminish but also after ozonolysis from the liquid phase, when formaldehyde was not formed at all. Thus, we could deduce that these final products of ozonolysis are poly(oxomethylene) species resulting from formaldehyde polymerization. At 77 K, the ozonolysis either from the liquid or gas phase leads to the appearance of at least two more species, denoted above as the primary and secondary products. The former disappears after raising the temperature up to about 120-150 K. The latter is much more stable; it appears together with the first one but remains on the surface almost up to 190-230 K. To identify these products, we have compared the spectra obtained with those of different isotopic modifications of primary and secondary ozonides observed earlier in inert gas matrices by Andrews and co-workers.7,13 The reported positions of band maxima for these compounds are presented in separate columns of Table 1. One could see that peaks of primary and secondary products of the adsorbed ethylene reaction with ozone almost coincide with the bands reported for POZ in a xenon matrix7 or for SOZ in an argon matrix13 for all the studied isotope modifications, if only there are data for the same spectral region. Nice correspondence could be seen as well in the character of isotopic splitting and in the isotopic shift values. In fact, as seen from Table 1, if mixed isotopic ozone is used, the bands of primary products, observed in our experiments with 16O3 at 927 and 724 cm-1, are split into three components, almost at the same positions as in the spectrum of POZ in the matrix. Bands of secondary
Ozone Interaction with Ethylene Adsorbed on Silica
Langmuir, Vol. 14, No. 20, 1998 5819
products at 2725 and 1352 cm-1, although their positions differ from those of the corresponding bands in the matrix by up to 9 cm-1, undergo the same frequency shifts by 4-6 cm-1, coinciding within the accuracy of measurements with the shifts of analogous bands of SOZ. Reaction Mechanism. Data obtained enable us to clear up the detailed mechanism of adsorbed ethylene ozonolysis. The general olefin ozonolysis mechanism proposed by Criegee infers the sequential formation of POZ and SOZ in one chain of sequential transformations (Scheme 1). Our results, nevertheless, show that the bands of SOZ, most intense after ozonolysis from the solution in liquid O2, appear simultaneously with those of POZ and practically do not grow after decomposition of the latter on the increase of the temperature. The same bands of POZ and SOZ, although less intense, could be distinguished along with those of adsorbed formaldehyde in the spectra of initial products of ozonolysis from the gas phase. We can again state that all three products appear from the very beginning and should thus arise as a result of three parallel processes. The probability of POZ, SOZ, or formaldehyde formation depends, evidently, on the conditions of the experiment or the form of ethylene adsorption. It is possible that the way of ozonolysis is not the same for H-bonded and physisorbed ethylene and that in the presence of liquid oxygen formaldehyde formation is almost completely inhibited. It is noteworthy that formaldehyde cannot be formed alone as a result of interaction between ethylene and ozone. According to Criegee’s mechanism, it should arise as a product of POZ decomposition together with the unstable “Criegee intermediate” H2COO, known as dioxymethyl (DOM), the simplest carbonyl oxide. This very unstable compound, as far as we know, has not been observed yet by IR spectroscopy. It is isoelectronic to ozone, and quantum chemical calculations19 support a singlet biradical structure (1; Chart 1) rather than a zwitterion one (2). Then, it should be expected that the frequencies of coupled CO and OO stretching and vibrations of this compound are close to those of the ν1 and ν3 modes of ozone and fall in the 1300-1000 cm-1 region of bulk silica absorption. In fact, in the spectrum of cyclopentadienone O-oxide, a carbonyl oxide derived from cyclopentadienylidene,20 the band detected at 1114 cm-1 splits into four components when prepared using a 16O/18O isotope
mixture, demonstrating that both oxygen atoms participate in this vibration. The bands of dioxymethyl that could be observable when adsorbed on silica are those due to CH stretching, CH2 bending, and COO bending, analogous to ν2 of ozone. Dioxymethyl isomerization could lead to cyclic dioxirane (structure 3) that was detected21 among the products of gas-phase ethylene ozonolysis or finally to formic acid (structure 4). However, no other bands except those discussed above and assigned to the mentioned products were detected in our study. To our notion, the following observations should be taken into account to explain the adsorbed ethylene ozonolysis mechanism: (i) The bands of POZ, SOZ, and formaldehyde appear simultaneously at 77 K or even below. All these products are stable at 77 K, and their bands remain unchanged until the temperature is raised. (ii) The spectra of the ozonolysis products were almost always recorded for samples exposed and further heated in excess of ozone, and then formaldehyde was always detected. (iii) In these experiments SOZ band intensity remained constant on heating until it decomposed. (iv) In a few experiments, where ozone was completely consumed, POZ decomposition was accompanied by an increase of the intensities of the SOZ band. In these latter experiments, formaldehyde formation was not detected, or the bands of the latter were much weaker. (v) In all of the experiments, dioxymethyl or its isomers were never observed. These facts together with the above-presented results suggest that the adsorbed POZ is stable enough at 77 K and decomposes only after important temperature increases. Simultaneous formation of POZ, formaldehyde, and SOZ, or POZ and SOZ only, when the liquid nitrogen temperature is stabilized by the presence of gaseous helium or of liquid oxygen; to our knowledge, this infers that the activation energy, necessary for further POZ transformation, could be taken from the initial excitation of POZ by the heat of the reaction. Either excited POZ could be deactivated and thus stabilized or it decomposes following Criegee’s mechanism. Dioxymethyl, which forms together with formaldehyde at the first stage of the reaction or during the thermal POZ decomposition, is an extremely reactive compound22-24 and could be supposed to react with the excess of ozone. Meanwhile, it rather undergoes complete oxidation with the formation of water and CO2, in fact detected when POZ decomposes. There is evidence neither for dioxymethyl isomerization to dioxirane or formic acid nor for its decomposition into simpler products such as CO and H2, as it occurs in the gas-phase low-temperature ethylene ozonolysis.24 Such products were not detected in our spectra, and these monomolecular processes should not depend on the presence of extra ozone. Cycloaddition reaction of dioxymethyl with O3 is not excluded and should lead to formaldehyde and oxygen; however, the increase of formaldehyde bands as a result of POZ decomposition could scarcely be observed because of the beginning of formaldehyde desorption or polymerization in the same temperature interval of 120-150 K. In any case, either the excess of ozone will promote the formation of formaldehyde from dioxymethyl or, at least, O3 could act as a scavenger, removing dioxymethyl and
(19) Wadt, W. R.; Goddard, W. A. III. J. Am. Chem. Soc. 1975, 97, 3004. (20) Bell, G. A.; Dunkin, R. I. J. Chem. Soc., Chem. Commun. 1983, 1213.
(21) Lovas, F. J.; Suenram, R. D. Chem. Phys. Lett. 1977, 51, 453. (22) Kuczkowski, R. L. J. Chem. Soc. Rev. 1992, 79. (23) Sander, W. Angew. Chem., Int. Ed. Engl. 1990, 29, 344. (24) Bunelle, W. H. Chem. Rev. 1991, 91, 335.
Scheme 1
Chart 1
5820 Langmuir, Vol. 14, No. 20, 1998 Scheme 2
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decomposition, the adsorbed O3 could prevent SOZ formation, but in these conditions formaldehyde, if it appears, is already able to desorb or polymerize and not be detected as a monomer. Conclusions
leaving formaldehyde that now has no partner for recombination. Formaldehyde is not reactive with respect to ozone and remains at the surface until it finally polymerizes with a further temperature increase. The overall scheme of the reaction that occurs at 77 K could then be presented as Scheme 2. The proposed scheme enables us to explain the difference between oxidation from the solution in liquid oxygen and from the gas phase. In the latter case the excess of ozone could not be easily avoided. If even the amount of O3 is comparatively small, it is first condensed at the exterior surface of the pellet, where it predominates over the adsorbed ethylene, and then diffuses inside. When the dose of O3 is dissolved in oxygen, the rate of the process is lower and there is never a large ozone concentration at the surface during ozonolysis until all the adsorbed ethylene is oxidized. Then, when liquid oxygen is removed, the excess of dissolved ozone could be condensed at the surface, but it is too late to affect the initial reaction with ethylene. When the sample is heated enough for POZ
Ozone interaction with ethylene and C2D4 preliminarily adsorbed on a silica surface has been studied by means of FT-IR spectroscopy from the solution in liquid oxygen at about 60 K or from the gas phase at 77 K. Using 16O3, 18 O3, or mixed 16O3/18O3, it was possible to identify the first products of ozonolysis, including POZ, SOZ, and formaldehyde that arise already at 77 K or below, and to follow their transformations in a wide temperature interval, not available for solid matrices or solutions. Simultaneous formation of POZ, formaldehyde, and SOZ at 77 K when POZ is stable enough is explained by supposing that the needed activation energy is taken from the initial excitation of POZ by the heat of the reaction. Either excited POZ could be deactivated and thus stabilized or it decomposes following Criegee’s mechanism. Formaldehyde, which should arise together with dioxymethyl (Criegee intermediate) in the intermediate step of the reaction, is formed when the excess of ozone is admitted from the gas phase. To account for this fact, we suppose that O3 acts as a scavenger, removing dioxymethyl and thus preventing its recombination with formaldehyde. The latter, as shown in special experiments, is not reactive with respect to ozone and remains at the surface until it polymerizes when the temperature increases. The spectral method of the ozonolysis mechanism studies developed here could be of interest for synthesis of different ozonides. Immobilization of ozonolysis intermediates by adsorption is believed to be promising to avoid undesired side reactions in the preparative procedures.24 LA9802264
