Surface Reactions of Oxygen Ions
The Journal of Physical Chemistry, Vol. 83, No. 6, 1979
(18) For a discussion of this point see ref 13a, Chapter 7. (19) P. Mukerjee in "Micellization, Solubilization and Microemulsions", Vol. 1, K. L. Mittal, Ed., Plenum Press, New York, 1977, p 171. (20) C. Gitler and A. Ochoa-Solano, J. Am. Chem. SOC.,90, 5004 (1968). (21) R. L. Venable and R. V. Nauman, J. Phys. Chem., 68, 3498 (1964). (22) F. M. Menger, J. M. Jerkunica, and J. C . Johnston, J . Am. Chem. Soc., 100, 4676 (1978); F. M. Menger, Acc. Chem. Res., in press. (23) D. Stigter, J . Phys. Chem., 68, 3603 (1964); 70, 1323 (1966).
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(24) P. Mukerjee, J. R. Cardinal, and N. R. Desai in "Micellization, Solubilization,and Microemulsions", Vol. 1, K. L. Mittal, Ed., Plenum Press, New York, 1977, p 241. (25) J. T. Davies and E. K. Rideal, "Interfacial Phenomena", Academic Press, New York, 1961, p 159. (26) M. S. Fernandez and P. Fromherz, J. Phys. Chem., 81, 1755 (1977). (27) C. A. Bunton in ref 3, p 73; C. A. Bunton and L. Robinson, J . Phys. Chem., 73, 4237 (1969); 74, 1062 (1970).
Surface Reactions of Oxygen Ions. 3. Oxidation of Alkanes by Of on MgO Yusaku Taklta and Jack H. Lunsford" Department of Chemistry, Texas A&M University, Coiiege Station, Texas 77843 (Received August 14, 1978) Publication costs assisted by the National Science Foundation
Ozonide ions on MgO reacted with C1 to C4 alkanes at 25 "C with half-lives between 1.7 and 5.2 min. A stoichiometry of one alkane molecule reacted per one 03-ion was determined. Following the reactions, only ethane gave an appreciable amount of gas phase product (ethylene) at 25 "C; however, upon heating the samples to elevated temperatures, other hydrocarbons and C 0 2 were obtained. At intermediate temperatures the corresponding C2 to C4 alkenes were the principal products, although the yields were less than observed in the corresponding reactions with 0-. The presence of gas phase oxygen during the initial reaction suppressed the formation of these alkenes. Infrared spectra of the surface complexes indicate that the reaction of 03-with ethane at 25 "C resulted in the formation of carbonate and ethoxide ions. At 150 "C the ethoxide ions decreased in concentration and acetate ions were formed. The acetate ions, as well as the carbonate ions, decomposed at elevated temperatures. The initial step in the reaction of 03- with alkanes is believed to be hydrogen atom abstraction, with the alkyl radical either forming an alkoxide ion or an alkyl peroxy radical. The alkoxide ion is largely responsible for the formation of the corresponding alkene, and the peroxy radical, as well as other oxidation products, result in the formation of C02 and CHI.
Introduction The role of' oxygen ions in promoting oxygen addition and oxidative dehydrogenation reactions is of interest both in homogeneous systems and in heterogeneous catalysis. Surface reactions of 0- ions with simple alkanes112and alkenes3 are facile, and the initial step probably involves hydrogen atom abstraction from the organic molcule. In this paper we describe the reactions of 03-on magnesium oxide with alkanes. The identification of 03-on metal oxides by electron paramagnetic resonance (EPR) spectroscopy was closely associated with the discovery of 0-, since the reaction 0- + O2 P 03-
(1)
occurs when small amounts of molecular oxygen are p r e ~ e n t . ~The ? ~ ozonide ion has now been identified on Mg0,495Ti02/Si02,6and V206/Si02.7 From oxygen-17 hyperfine data it has been shown that the ion on MgO is bonded to the surface by a terminal oxygen atom.5 Gas phase studies carried out by Parkes* demonstrate that the rate of reaction of 03-with hydrocarbons has an upper limit which is more than two orders of magnitude lower than that of 0-. Similarly, Naccache and Cheg have reported that 03-on MgO did not react a t -196 O C with molecules such as carbon monoxide, ethylene, or propylene. The 0; ion, however, has been proposed as an intermediate in the homomolecular oxygen isotope exchange reaction over Vz05/Si02.7 It has been suggested that catalytic oxidative dehydrogenation reactions on metal oxides may involve 0- ions, but since the oxidant is usually 02,one might expect that Os-would also be present on the surface. The extent of 0022-3654/79/2083-0683$0 1.OO/O
03-formation, its relative reactivity, and the products formed are thus important in understanding selective oxidation reactions. The MgO surface is well suited for such a study since eq 1 is shifted far in favor of the 03ion, which is not the case, for example, with V205/Si02. Nevertheless, one should recognize that the results reported here represent stoichiometric and not catalytic reactions.
Experimental Section The magnesium oxide was prepared according to the method reported previously.1° Samples were evacuated a t 530 "C overnight before each experiment in order to eliminate hydrocarbons, carbon dioxide, and water. 'The ozonide ion was produced on the MgO surface by UV irradiation (254 nm) of the sample in the presence of l V 2 0 at 25 "C for 10 min-3 h. The concentration of 0, on MgO was controlled by varying the N 2 0 pressure and the duration of UV irradiation. The EPR spectra of 0; were measured using the Varian E-6S, X-band spectrometer with the sample either a t - 196 or 25 "C. The g values were evaluated relative to a phosphorus-doped silicon standard having a g value of 1.9987. The concentration of 03-was determined by comparing the second integral of the EPR spectrum with that of the standard. Infrared spectra of self-supporling MgO wafers with adsorbed molecules were obtained in a manner similar to that described elsewhere,'l using an Beckman IR-9 spectrophotometer. The alkanes were purified in the same manner as reported previous1y.l Hydrogen (Matheson 99.999% purity) and helium (Airco 99.999% purity) were used without further purification. 0 1979 American
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The Journal of Physical Chemistry, Vol. 83,
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Y. Takita and J. H. Lunsford 1
TABLE I: Paramagnetic Oxygen Species and Desorbed Oxygen Following the Thermal Treatment of MgO
temp, "C
0
25 100 150 200 250 300
concn of o speciesa (nmol/g MgO) 3 02- 0 -
701 90
36 39 24
161 88
1116 477 452
8
4
782
858 546 154
350
TIME
( h )
Flgure 1. Relative intensity of the Os-spectrum as a function of time with the sample (a) in a static system and (b) under dynamic vacuum.
Reactants and products were analyzed using a Carle AGC 311 gas chromatograph with both a thermal conductivity detector (TCD) and a flame ionization detector (FID). The chromatograph was connected directly to the reaction system. The column packings were bis(ethoxyethyladipate) for the separation of propane, propylene, n-butane, n-butenes, and butadiene; porapack Q for the separation of methane, ethane, ethylene, carbon dioxide, and nitrous oxide; 5A molecular sieve for the separation of hydrogen, oxygen, and nitrogen, and carbon monoxide; and Carbowax 600 for the separation of oxygen-containing hydrocarbons. After the UV irradiation, the gas phase was removed from the reactor and the spectrum of the 03-ion on MgO in the quartz side arm of the reactor was recorded. The reactor was then attached to the same closed circulation system described in the previous paper.l About 900 nmol of alkane (0.3 torr) diluted in helium was introduced and circulated at 25 "C for 2-3 h. After the reaction the condensable gas phase products were collected for 1 h in the cold trap (-196 "C) located in the circulation system. The uncondensable gas phase remaining in the system was analyzed separately. Following a brief evacuation of the reactor about 30 torr of pure He was introduced in the system, and the reactor was heated to a prescribed temperature for 1h. Substances desorbed during the thermal treatment were collected at -196 "C and subsequently analyzed.
Results Thermal Stability of 03-on MgO. The thermal stability of 0; on MgO depends strongly on the partial pressure of oxygen in the system. In a static system at 25 "C where the partial pressure of O2 became 0.1 torr in 25 h, the 03concentration decreased by only 20%; whereas, under a dynamic vacuum of torr a half-life of 22 min as observed as shown in Figure 1. As the 03-spectrum decreased, no new spectrum was observed and the spectrum of 0-,present originally, disappeared after 4 h. In view of previous result^^,^ it is noteworthy that 02-,which was present a t a low concentration in some samples, was not formed as the 03-dissociated. As the temperature was increased with the sample in a static system, the concentration of the 03-ion decreased as indicated in Table I. At temperatures greater than 150 "C no 0,remained on the surface. The 0- concentration, which amounted to 9% of the total paramagnetic oxygen spins, increased at 100 "C and then disappeared at 200 "C; whereas, the small amount of 0,- (4.4%) remained constant until 100 "C and then decreased to nearly zero at 200 "C.
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amt of 0 desorbedb (nmol/ gMgO)
400 a Calculated from EPR spectra. chromatograph.
0
Calculated from gas
TABLE 11: Half-Life for O ; , Under Vacuum and During Reaction with Alkanes 0 3 -
ambient atmosphere
half-life, min
under dynamic vacuum CH,
22.4 5.2 2.4 1.7 3.1
c* C3H8 n-c,H, H6
0
Under similar conditions the gas phase O2 was monitored and the results are included in Table I. As the 03ion decayed comparable amounts of O2 appeared in the gas, although a considerable amount of oxygen remained on the surface until 400 "C. A second desorption maximum was observed between 250 and 300 "C. EPR Evidence for the Reaction of 0; with Alkanes. In the presence of C1 to C4 alkanes the 03-signal decayed at room temperature with half-lives which ranged from 5 min for methane to 1.7 min for propane as listed in Table 11. A small fraction of the ozonide ions did not react, and in the case of CH, 10% of the ions were observed even after 16 h. The concentration of 0- and 02-remained unchanged, which is surprising since 0- is known to react very rapidly with alkanes. This result suggests that the 0formed in these experiments was not on the surface, but was present as a bulk V center. As the 03-reacted, a weak spectrum was observed having g, = 2.013 and g, = 2.003. This signal which was obscured because of overlap with the remaining 03spectrum disappeared after several minutes. The origin of the spectrum is not certain, although the g values are consistent with those reported for an alkoxy radical.12 The stoichiometry for the surface reaction was determined by measuring the amount of ethane reacted as a function of the 03-concentration. The amount of ethane reacted was determined by allowing the gas to react with the solid, and then condensing the gas phase and physically adsorbed ethane at -196 "C. The difference between the initial amount of ethane and the amount recovered was taken as the amount of gas reacted. Since only small amounts of ethane were desorbed at elevated temperatures, one may conclude that relatively little chemisorption occurred. It is apparent from Figure 2 that the amount of C2H, which reacted increased linearly with respect to the concentration of 03-.A slope of unity confirms that one ethane molecule reacted with one 03-ion on the MgO surface, Furthermore, within experimental error, the total amount of ethane consumed was the same as the amount of 03-ions detected on the surface. Product Determination. Following the reaction of CH4 with 03-no gas phase products were observed at tem-
The Journal of Physical Chemistry, Vol. 83, No. 6, 1979
Surface Reactions of Oxygen Ions
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rr
n 40W I-
O
a W
cc W
2
3O-
0 E
n
z 0
s 0
CONCENTRATION OF 0; ON MgO(nmol/S-MgO) Figure 2. Amount of ethane reacted with 03at 25 of the concentration of Os-on MgO.
OC
as a function
E
v
10
n
.J
w> 0 0
100
200
500
400
500 2h
500
(
TEMPERATURE
O C
)
Figure 4. Yield of products as a function of the thermal desorption program following the reaction of propane with Os-at 25 O C .
0
0
100
200
100
TEMPERATURE
-
400
500
P S"
500 3h
("C )
Figure 3. Yield of products as a function of the thermal desorption program following the reaction of ethane with Os-at 25 OC.
peratures between 25 and 300 OC. A t 400 "C COz began to appear in the gas phase, and after heating the sample to 500 "C for 3 h C 0 2 amounting to 95 mol 9'0 of the reacted CH4 was collected. At 400 " C and above trace amounts (
