J. Phys. Chem. 1995,99,16519-16522
16519
Oxidation of Methyl to Methoxy on Oxidized Cu/SiOz M. D. Driessen and V. H. Grassian" Department of Chemistry, University of Iowa, Iowa City, Iowa 52242 Received: July 12, 199.5;In Final Form: September 11, 199.5@
The room-temperature adsorption of methyl iodide on an oxidized copper silica catalyst has been investigated using FT-IR spectroscopy. After reaction of methyl iodide with an oxidized Cu/SiO2 catalyst, oxygenated hydrocarbons including methoxy and bidentate formate are present on the copper surface. The data suggest that the carbon-iodine bond in adsorbed methyl iodide dissociates on the oxidized copper particles, methyl groups then react with surface oxygen to form adsorbed methoxide and formate. The reactions of adsorbed methoxide and formate were monitored in the temperature range extending from approximately 300 to 600 K.
Introduction
consists of a 2-3/4 in. stainless steel cube fitted with two differentially pumped barium fluoride windows and a sample Alkyl fragments and their oxygenated counterparts are known holder through which thermocouple and power feedthroughs to be important intermediates in several industrial processes. are connected to a tungsten sample grid. The sample holder Therefore, studies of the oxidation of alkyl fragments may design is such that the sample may be cooled to near liquid provide useful information on the mechanism of hydrocarbon nitrogen temperatures and heated resistively up to 1300 K. The oxidation reactions. Recently, studies of the oxidation of alkyl cell is attached to an all stainless steel vacuum system through fragments adsorbed on single crystal surfaces have been a 2 ft stainless steel bellows hose. The vacuum system is reported.'-4 These investigations have, at least in part, been pumped by an 80 W s ion pump and rough pumped using a concemed with discerning mechanistic information about the turbomolecular pump. selective oxidation of hydrocarbons. Zhou et al. have examined Samples are made by spraying a slurry of copper nitrate the partial oxidation of methylene to produce formaldehyde. (Strem Chemicals, 99.999%)/silica (Cabosil, M-5,200 m2 g-l) The oxidation of methylene was studied by adsorbing chlorsuspended in acetone and water onto a photoetched tungsten oiodomethane on an oxygen-covered Pt(111) surface. Both grid (Buckbee-Mears). A template is used to mask half of the carbon-halogen bonds dissociate below room temperature to tungsten grid, allowing one side to be coated with Cu/Si02 and form methylene. Methylene then reacts with surface oxygen the other with Si02. Metal loadings on the order of 15% are to produce formaldehyde. Solymosi and Klivenyi have also typically used in these experiments. investigated the oxidation of methylene groups (formed from After the sample is prepared, it is mounted inside the IR cell, the dissociation of diiodomethane) to formaldehyde over an wrapped in heating tape and baked out for several hours at 473 oxygen covered Rh( 111) surfacee2 K. The sample is resistively heated during bakeout to 573 K The oxidation of methyl, ethyl, and 2-propyl has been studied for 2 h to decompose the nitrate salt and then reduced in 400 by Bo1 and Friend on an oxygen covered Rh( 111) s ~ r f a c eIt. ~ ~ ~ Torr of hydrogen for 15 min at 573 K followed by evacuation. was determined from high-resolution electron energy loss This procedure is repeated for 15, 30, 30, and 60 min. The spectroscopy that gas-phase methyl radicals can react with sample is then oxidized in 5 Torr of 0 2 at 523 K for 2 h, reduced adsorbed oxygen to form methoxy on Rh( 11l).3 For reactions for 2.5 h, and oxidized again. The first oxidation treatment is of ethyl and 2-propyl on Rh( 11l), acetaldehyde and acetone done to remove hydrocarbon impurities from the sample and were produced from ethyl and propyl oxidation, respectively, the second oxidation treatment is done to oxidize the surface with a high degree of selectivity at half monolayer coverage of of the copper particles. oxygen? After sample processing, the IR cell is placed on a linear Although a previous attempt by Rasko et al. to form methoxy translator inside the spectrometer sample compartment. Either from methyl on an oxidized PdSi02 catalyst was unsuc~essful,~ the Si02 or the Cu/SiO2 half of the sample can be translated Tong and Lunsford have produced methoxy on the surface of into the IR beam for data acquisition. This enables the CeO2 using a gas-phase methyl radical source.6 Here we present chemistry of both Cu/SiO2 and Si02 to be examined during the evidence for the oxidation of methyl, formed from the dissociacourse of a particular experiment. tive adsorption of methyl iodide, to methoxy on oxidized Cu/ A Mattson RS-1 FT-IR spectrometer equipped with a Si02. As discussed in detail below, transmission FT-IR narrowband MCT detector was used in these experiments. spectroscopy is used to show the presence of methoxide on the Spectra were recorded by averaging 1000 scans at an instrument surface of the oxidized copper particles, along with adsorbed resolution of 4 cm-' . Absorbance spectra reported represent formate. single beam spectra referenced to single beam spectra recorded for the freshly prepared sample (either Si02 or Cu/SiOz prior Experimental Section to reaction). For the temperature-dependent infrared spectra, the sample was warmed to the desired temperature and held The infrared cell used in these experiments is a modified there for 30 s before it was cooled down to room temperature version of a cell designed by Yates and c o - ~ o r k e r s . ~The . ~ cell after which a spectrum was recorded. The oxidation state of the Cu catalyst was determined from * To whom correspondence should be addressed. CO adsorption. For adsorbed CO, the frequency of the CO Abstract published in Advance ACS Abstracts, October 15, 1995 @
0022-3654/95/2099-16519$09.00/0
0 1995 American Chemical Society
Letters
16520 J. Phys. Chem., Vol. 99, No. 45, 1995
146.9
TABLE 1: Vibrational Assignment of Adsorbed Methoxide on Copper Surfaces mode description" C ~ / S i 0 2 ~Cu/Si02' C ~ / S i 0 2 ~ Cu(100)' Cu( 1l0y 2930 2901 2940 2920 v,,(CH,) 2927 2824 2821 2787 2840 v,(CH~) 2814 v,,(CH~) 2895 2880 2861 1443 1460 &CHd 1468 a Mode description taken from refs 20 and 21. This work. Reference 17. Reference 18. e Reference 20. f Reference 21.
'T
c
L 3100 2900 2700
1600
1500
1400
13
Wavenumbers Figure 1. Infrared spectra of (a) oxidized Cu/SiO;?and (b) Si02 after reaction with 15.0 Torr methyl iodide for 80 min at room temperature. The spectra were recorded after evacuation of the IR cell.
stretch was observed at 2116 cm-' which correlates predominately with (21,102,although some CuO may be present as we11.9~10
Results and Discussion IR Spectroscopy of CH31 Adsorbed on an Oxidized Cu/ Si02 Catalyst at 298 K. It is well established that the carboniodine bond in adsorbed methyl iodide readily dissociates on single-crystal metal surfaces to yield methyl groups and iodine atoms.' '-I5 Recent studies on silica-supported metal catalysts5-I6 also suggest that methyl iodide dissociates on the metal particles to form adsorbed methyl groups and iodine atoms. In this study, we focus on the reaction of methyl iodide on an oxidized Cu/ Si02 catalyst. Figure l a shows the infrared spectrum of an oxidized Cul Si02 sample after room temperature adsorption of 15.0 Torr of CH31. After evacuation of gas-phase CH31, there are several bands present in the CH stretching region (2700 to 3200 cm-') and several more observed in the spectral region extending from 1350 to 1600 cm-'. The spectral region below 1300 cm-' is not shown because it is opaque due to strong absorption by the silica support. There are no new absorption bands observed in the pure Si02 spectrum (Figure lb) after reaction with CH31. Although the spectrum shown in Figure l a is much too complex to be attributed to adsorbed methyl groups, it has been observed that alkyl iodides dissociate on oxygen covered single crystal rhodium and platinum surfaces to form adsorbed alkoxy intermediate~l-~Several of the bands observed in the CdSi02 spectrum shown in Figure l a correlate well with bands observed for methoxide (OCH3) on copper surface^.'^-^' In particular, the bands near 2927, 2895, 2814, and 1468 cm-' are assigned to methoxide adsorbed on the copper particles (see Table 1). It is well-known that methoxide can decompose to form surface formate."-I9 A number of the bands that do not correlate with methoxide in the spectrum shown in Figure l a do match those of adsorbed formate species. By comparison with literature spectra, the bands near 2962, 2861, 1570, 1388, and 1357 cm-' can be assigned to bidentate formate adsorbed on the copper particle^.'^,*^-^^ The shoulder at 1584 cm-' in the spectrum shown in Figure l a is near the 1583 cm-' band observed for unidentate formate adsorbed on an oxidized Cu/ Si02 catalyst.26 Although distinct bands in the CH stretching
b
52%-
) h
1564
S
I
0
r b
a n
i"
+ =I
2867
-P 2
1460
30 2900 2600 1700 1600 1500 1400 1300
Wavenumber s Figure 2. Infrared spectra recorded as a function of temperature under static conditions (IR cell isolated from pumping system) after reaction for 80 min, and subsequent evacuation, of 15 Torr of methyl iodide with an oxidized CdSiO2 catalyst at 298 K. (Note that the 298 K spectrum shown above is similar to the spectrum shown in Figure la).
region for a unidentate formate species (2978 and 2904 cm-') are not observed, these bands could be easily obscured by the intense absorptions for adsorbed methoxy and bidentate formate. A possible assignment for the shoulders observed at 1446 and 1428 cm-' is that of the d(CH3) of methoxy groups adsorbed on different surface sites. This band at 1530 cm-' does not correlate with any of the above species. The band may correlate with some type of a carbonyl species but for now it is left unassigned. Temperature-Dependent Infrared Spectra of CH31 Adsorbed on an Oxidized Cu/SiOz Catalyst. To confirm the assignments of the methoxy, bidentate formate, and unidentate formate species, we have monitored the CH3I-oxidized CulSi02 spectrum as a function of temperature. The temperaturedependent infrared spectra (Figure 2 ) show the formation and decomposition of several species which have also been observed in the decomposition reaction of methanol on supported copper catalyst^.'^-'^ Between room-temperature and 423 K, there are three hydrocarbon species present on the surface which can be distinguished by differences in their thermal stability. If we have correctly assigned the bands for methoxy groups adsorbed on copper particles, we would expect to see these bands decrease
Letters
J. Phys. Chem., Vol. 99, No. 45,1995 16521
TABLE 2: Vibrational Assignment of Adsorbed Bidendate Formate on Copper Surfaces mode sodium format$ description" Cu/Si026 Cu/SiOlc CuISi02d Cu( 1 combinationband 2958 2936 2935 2960 2953 [~as(COO)+
WW1
Vs(CH)
combination band [ W O O )+
W-UI
V,dCOO) WH) Vs(CO0)
2867 27 188
2856
2851
2920
2841 2720
1564 1369 1360
1579
1553
1560
1358
1351
1360
1567 1377 1366
"Mode description taken from ref 24. 6This work. Note the frequencies of bidendate formate are coverage and temperature dependent. See text for further details. Reference 23. Reference 17. e Reference 21. f Reference 25. g Observed only at high coverages.
for bidentate formate as a function of temperature on Cu( Invoking the metal surface selection rule, one can ascribe this effect to the orientation of the formate species. At low temperatures and coverage, the formate layer has near CZ, symmetry, as shown by the weak va,(COO) band at 1570 cm-' seen in the 298 K spectrum shown in Figure la. At higher temperatures and coverages, the formate species must be tilted toward the surface allowing the va,(COO) band to have a component of its dynamic dipole parallel to the surface normal as shown by the very intense band at 1564 cm-' in the 473 K spectrum shown in Figure 2. Surface Reactions. Clarke et al. have investigated the decomposition of methoxide (from adsorption of methanol) on an oxidized CdSi02 catalyst and observe both unidentate and bidentate formate, methylenebis(oxy), and f~rmaldehyde.'~ Rochester et al. have reported fairly similar results; however, they did not find any evidence for CH2(0)2.I8 The room temperature CH3I-oxidized Cu/SiOz spectrum (Figure la) shows evidence for the presence of several adsorbed species. Methoxy, bidentate formate and unidentate formate are all present after the initial adsorption of methyl iodide. The data suggest that methyl iodide dissociates on the copper surface to form adsorbed methyl groups and iodine atoms (eq 1).
in intensity and disappear near 500 K.I7 This is indeed what is observed. As the sample is heated between 298 and 473 K, the bands near 2927, 2895, 2814, 1468, 1446, and 1428 cm-' decrease in intensity concomitant with the growth of bands near 3538, 2958, 2867, 2718, 1564, 1369, and 1360 cm-I. Upon further warming to 523 K, these intense bands decrease sharply as new bands near 3608, 1664, 1615, and 1428 cm-' develop. CH,I(g) Cu- CH,(ads) I(ads) (1) The hydrocarbon bands which grow in intensity between 323 and 473 K can now be definitively attributed to bidentate formate (CH(0)z) on the copper surface due to their frequencies CH3(ads) O(ads) CH,O(ads) (2) and the fact that they reach maximum intensity and then decrease sharply near temperatures where bidentate formate has CH,O(ads) 3O(ads) CH(O),(ads) 20H(ads) (3) been seen to decompose (> 500 K) on oxidized C u / S i 0 ~ and '~ C U ( ~ O O )Table . ~ ~ 2 shows the assignment of these bands using literature comparisons. It should be noted that the frequencies OH(ads) 4-CH(O),(ads)and relative intensities of the bidendate formate bands are H,O(ads) CO,(ads) C02(g) (4) coverage and temperature dependent. The frequencies noted in Table 2 are for the high-coverage spectrum of bidendate Methyl groups subsequently react with adsorbed oxygen to form formate, i.e., for the 473 K spectrum shown in Figure 2. The adsorbed methoxy groups (eq 2). Although it should be noted band which develops (along with the bidentate formate bands) that there is little evidence for adsorbed CH3 in these experiat 3538 cm-' is assigned to the v(0H) of Cu-OH as has been ments. Therefore, the C-I bond breaking and C-0 bond previously done.28 The significance of the Cu-OH band is making steps (eqs 1 and 2) may be occurring either in a discussed in the next section. The bands which have been concerted fashion or very rapidly in a two-step process. Due attributed to unidentate formate either decrease or become to the high coverage of oxygen, methoxide can react further obscured by the intense bidentate formate bands above 373 K. with adsorbed oxygen and subsequently dehydrogenate to form Heating to 673 K evolved yet another band (not shown) near surface formate species, both bidentate and unidentate (eq 3). 1893 cm-I. Also observed but not shown is the development This would suggest that a methylenebis(oxy)(CH2(0)2) species and constant growth in intensity of a band centered at 2342 may form as an intermediate, although we do not observe it cm-' (gas-phase COP) after heating to 423 K and above. The here. The dehydrogenation of methoxy groups to form CHbands formed at higher temperatures can be attributed to a ( 0 )results ~ in the formation of Cu-OH groups28which appear combination of surface COXspecies which are formed from the as the 3538 cm-' band that develops at the same time the decomposition of the surface formate species. Frequency ranges bidentate formate coverage is increasing (and methoxy is for adsorbed COXspecies are as follows: bidentate carbonate 1590-1630 and 1260- 1270 cm-I, carboxyl ( C O Z ) ; ~ ~ ~decreasing). At higher temperatures (523 K), bidentate formate and Cu-OH bands begin to decrease and adsorbed water begins 1550-1630 and 1350-1420 cm-I, organic-likecarbonate (COto appear on the silica support as bands located near 3608 and (0)2);29a 1870-1750 and 1250-1280 cm-I, bridged CO;30b 1664 cm-I, which correlate to the stretching and bending modes 1700-1900 cm-I. It is most likely that it is a COS species of adsorbed water.29b This would suggest that additional which is responsible for the bands at 1625 and 1429 cm-' in hydrogen from the dehydrogenation of surface species is reacting the spectrum recorded after heating to 623 K. The 1893 cm-' with the Cu-OH to form H20 which is desorbing from the band can not be firmly assigned to a specific COXspecies since copper surface and readsorbing on the nearby silica support. any other associated bands may be masked by silica absorptions The static conditions under which this experiment was per(below 1300 cm-I). formed make the desorption of water from the copper particle It is interesting to note that at low temperataures and and readsorption on the silica support feasible. The formate seemingly low coverages of bidentate formate, the v,(COO) species decompose upon warming to 523 K to form gas-phase band is more intense than the v,,(COO) band. But as the COz and adsorbed COXspecies (eq 4, not balanced). By 523 K temperature and formate coverage increase (as the methoxy concentration decreases), the relative band intensities become all hydrocarbon species have been completely oxidized leaving reversed. The relative intensity change has also been reported only COXspecies on the surface. Lower oxygen coverages on
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the catalyst should result in a decrease in the amount of gasphase C02 and adsorbed COXformed.
ConcIusions The room-temperature adsorption of methyl iodide on an oxidized silica-supported copper catalyst results in the formation of adsorbed oxygenated hydrocarbon fragments. Methoxide, unidentate formate and bidentate formate were identified on the copper surface. IR band frequencies, decomposition temperatures, and decomposition products (initially adsorbed formate and ultimately gas-phase C02 and adsorbed CO,) provide definitive evidence for the formation of methoxide on the surface of the oxidized copper catalyst from the dissociative adsorption of methyl iodide. This is the first report of the direct oxidation of methyl groups on an oxidized supported metal catalyst. IR studies of alkyl oxidation on supported metal catalysts can provide important mechanistic information for these reactions.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research and the National Science Foundation (Grant CHE-9300808). References and Notes (1) Zhou, X.-L.; Liu, Z.-M.; Kiss, J.; Sloan, D. W.; White, J. M. J. Am. Chem. Soc. 1995, 117, 3565. (2) Solymosi, F.; Klivenyi, G. J. Phys. Chem. 1995, 99, 8950. (3) Bol, C. W. J.; Friend, C. M. J. Am. Chem. Soc. 1995, 117, 8053. (4) Bo!, C. W. J.; Friend, C. M. J. Phys. Chem. 1995, 99, 11930. (5) Rasko, J.; Bontovics, J.; Solymosi, F. J. Catal. 1993, 143, 138. (6) Tong, Y.; Lunsford, J. H. J. Am. Chem. SOC. 1991, 113, 4741. (7) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr. Rev. Sci. lnstrum. 1988, 59, 1321.
Letters (8) Fan, J.; Yates, J. T., Jr. J. Phys. Chem. 1994, 98, 10621. (9) Kohler, M. A.; Cant, N. W.; Wainwright, M. S.; Trimm, D. L. J. Catal. 1989, 117, 188. (10) Millar, G. J.; Rochester, C. H.; Waugh, K. C. J. Chem. SOC., Faraday Trans. 1991, 87, 1467. (11) Zhou, X.-L.; Zhu, X.-Y.; White, J. M. Ace. Chem. Res. 1990, 23, 327. (12) Zaera, F. Acc. Chem. Res. 1992, 25, 260. (13) Lin, J.-L.; Bent, B. E. J. Vac. Sci. Technol. A 1992, 10, 2202. (14) Lin, J.-L.; Chiang, C.-M.; Jenks, C. J.; Yang, M. X.; Wentzlaff, T. H.; Bent, B. E. J. Catal. 1994, 146, 1. (15) Lin, J.-L.; Bent, B. E. Chem. Phys. Lett. 1992, 194, 208. (16) McGee, K. C.; Driessen, M. D.; Grassian, V. H. J. Catal., submitted. (17) Clarke. D. B.: Lee, D.-K.: Sandoval, M. J.: Bell. A. T. J. Catal. 1994, 350, 81. (18) Millar. G. J.: Rochester, C. H.: Waugh, K. C. J. Chem. Soc., Faraday Trans. 1991, 87, 2795. (19) Sakata, Y.; Domen, K.; Maruya, K.; Onishi, T. Appl. Surf: Sci. 1988/89, 35, 363. (20) Ryberg, R. Phys. Rev. B 1985, 31, 2545. (21) Sexton, B. A,; Hughes, A. E.; Avery, N. R. Surf: Sei. 1985, 155, 366. (22) Hayden, B. E.; Prince, K. C.; Woodruff, D. P.; Bradshaw, A. M. Surf: Sci. 1983, 133, 589. (23) Millar, G. J.; Newton, D.; Bowmaker, G. A,; Cooney, R. P. Appl. Spectrosc. 1994, 48, 827. (24) Hayden, B. E. In Vibrational Spectroscopy of Molecules on Sugaces; Yates, J. T., Jr., Madey, T. E., Eds.; Plenum: New York, 1987: p 303. (25) Ito, K.; Bemstein, H. J. Can. J. Chem. 1956, 34, 170. (26) Millar, G. J.; Rochester, C. H.; Waugh, K. C. J. Chem. SOC., Faraday Trans. 1991, 87, 1491. (27) Sexton, B. A. Surf: Sci. 1979, 88, 319. (28) Ellis, T. H.; Wang, H. Langmuir 1994, 10, 4083. (29) Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: London, 1966; (a) p 76, (b) p 261. (30) Davydov, A. A. Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides; Wiley and Sons: Chichester, 1990; (a) P 38, (b) P 118. I
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