J. Am. Chem. SOC.1986,108,1502-1509
7502
Gas-Phase Studies of Alkane Oxidation by Transition-Metal Oxides. Selective Oxidation by CrO+ H. Kang and J. L. Beauchamp* Contribution No. 7319 from the Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91 125. Received February 24, 1986
Abstract: The gas-phase reactions of CrO+ with alkanes have been studied by using ion beam reactive scattering techniques. CrO+ undergoes facile reactions with alkanes larger than methane. CrO+ selectively oxidizes ethane to form ethanol. In addition to the possibility of alcohol formation, reactions with larger alkanes are more complex, yielding products in which dehydrogenation and loss of alkenes and alkanes occur. In reactions with cyclic alkanes, cyclopropaneand cyclobutane yield products characteristic of C-C bond cleavage. In contrast, reactions with cyclopentane and cyclohexane mainly involve dehydrogenation and elimination of H20. A series of hydrogen abstraction reactions are examined to determine the bond dissociation energy D'(Cr0'-H) = 89 5 kcal mol-'. This bond energy has implications for the reaction mechanisms of CrO+ with alkanes, leading to the suggestion of a multicenter reaction intermediate, in which alkyl C-H bonds add ucross the Cr+-O bond as an initial step. This is supported by an examination of the reactions of Cr+ with alcohols.
The development of new reagents and catalysts for the selective oxidation of saturated hydrocarbons remains an active area of interest among chemists. Desirable targets include such possibilities as the activation of methane and terminal oxidation of an alkane chain.'S2 However, there is an intrinsic need for better understanding of the mechanisms of oxidation processes with particular emphasis being placed on elucidation of elementary processes. Of particular interest is the question as to whether or not oxidations of organic substrates involve intermediates with metal-carbon bonds. In the absence of any M-C bond formation, radical intermediates formed by hydrogen atom abstraction are likely to be the mode of hydrocarbon oxidation,24 a route which is not well-designed for selectivity in general. On the other hand, reaction intermediates in which M-C bonds are formed offer the possibility of using the expanding body of knowledge in organometallic chemistry to design more selective alkane oxidation systems. The question of possible organometallic intermediates has been raised in a number of cases but seldom answered unequivocally in condensed phase studies. In this paper, we report gas-phase studies of simplified alkane oxidation systems which permit detailed examination of the reaction energetics as well as mechanisms. In a previous study from our laboratory: arguments based on energetic considerations were presented which suggested that CrO+ is an ideal reagent for studies of gas-phase oxidation processes. The conjecture that this species would be both reactive and selective was generally supported by studies of alkene oxidation. In our earlier study, the bond dissociation energy D'(Cr+-O) = 85.3 f 1.3 kcal mol-' was determined. With this value, the general oxidation process indicated in reaction 1 will typically be exoCrO+ + R H
-
Cr+
+ ROH
(1)
thermic by 10-15 kcal mol-' when R H is a saturated hydrocarbon (Table I). It would be naive to believe that the criterion of reaction exothermicity alone can be used to identify processes deserving of further study. However, Freiser et al. have shown that FeO+ reacts readily with saturated hydrocarbon^.^ These results, along with our study of alkene oxidation by CrO+, lend (1) Report of the International Workshop on "Activation of Dioxygen Species and Homogeneous Catalytic Oxidations'; Collins, T. J., Ed., Galzi'gnano, Italy, 1984. (2) Sheldon, R. A.; Kochi. J. A. Metal Catalvzed Oxidations of Orpanic Compounds; Academic Press: New York, 198i. (3) Martir, W.; Lunsford, J. H. J . Am. Chem. SOC.1981, 103, 3728. Schultz, J. C.; Beauchamp, J. L. J. Phys. Chem. 1983,87, 3587. (4) Kang, H.; Beauchamp, J. L . J . Am. Chem. SOC.,submitted for publication. (5) Jackson, T. C.; Jacobson, D. B.; Freiser, B. S.J. Am. Chem. Soc. 1984, 106, 1252. .
I
Table 1. Reaction Enthalpies Used in Text AH' rxn, kcal mo1-I a -1 16.8 -89.2
reaction 0 ('P) + Hz 0 + CH4
H2O CH3OH CHzO + H2 H20 + CHI
0 + CZH6
CH4 + CHzO CH3CHzOH CHIOCH, C2H4 + H20 CHJOH + CH2
-67.1 -5.2'
-82.7 -95.0
-82.8 -84.1
-6.9
0 + CH3CH2CH,
1-propanol 2-propanol CH'OCzHS CH'OH + CzH4 H2 + allyl alcohol H2 + propanol H2 + acetone cyclopropane + HzO propene + HzO C2H6 + CH2O CH4 + CH'CHO Reference 10. Reference 3 1.
-95.3 -99.3 -85.9 -69.8 -63.8
-79.8 -86.2 -79.3 -87.1
-80.4 -91.8
Table 11. Low-Lying States of CrO+ and Cr' vib freq
ion
low-lying states
CrO+
x4z-
energy, eV 0
yieldc % 91 3
cm-l 640 k 30
4n 0.65 X6S 0 >99.9 6D 1.52 0 4D 2.46 0 'Reference 16. Energies are results of ab initio SCF + CI calculations. bReference 32. Cion beam composition at 1300 K.
Cr+
support to the expectation that CrO+ will also oxidize saturated hydrocarbons. The present study demonstrates that this contention is indeed correct. Ion beam studies of the reactions of CrO+ with alkanes are reported in which the oxidation of alkanes to alcohols (reaction 1) is observed along with other reaction pathways. To better characterize the energetics of these reactions, the CrO+-H bond dissociation energy is determined by examining a series of hydrogen atom abstraction reactions. The value determined for this bond energy implicates an intermediate in which alkyl C-H bonds add across the Cr+-O bond as an initial oxidation step. Reaction mechanisms are investigated by using deuterium labeled alkanes. In addition, reactions of Cr+ with alcohols are investigated in an attempt to further elucidate the potential energy
0002-7863/86/ 1508-7502%01.50/0 0 1986 American Chemical Society
J. Am. Chem. Soc., Vol. 108, No. 24, 1986 1503
Selective Oxidation by CrO'
Table 111. Reactions Providing Limits for the Bond Dissociation Energies Do(CrO+-H) and Do(CrO+-HT' Langevin cross sectn,
tot. cross sectn, A2
R-H
A2
D O(
R-H)~
kcal mol-'
Do (R+-H-)'
prod,
kcal mol
CrOH'
W of tot. yield CrOH,'
CrOH
88
67
82.5 f 1.3
24 1
0
5
0
140
67
85.6 i 1.5
244
0
7
0
==L
52
67
250
34
0
0
eH
70
58
86.3 f 1.5
256
12
0
0
17
83 68
88.0 f 1 93.2 f 2
238 233
0 0
0
110
0
100 0
14
60
95.1 f 1
252
0
13
0
100
68
95
248
0
11
0
70
75
100 i 2
264
0
0
0
8
49
110 f 2
287
0
0
0
R
H
C6HSCHz-H
k
L
-PH H
=/
-86
-
no reactn 110.9 f 2 n A t 0.5 eV relative kinetic energy. bReference 7. 'Reference 8. C6Hc-H
surfaces for these reactions and provide support for proposed reaction mechanisms.
282
Scheme I ,$+
+ H,O
Experimental Section The experimental techniques employed in the present study are the same as those described in ref 4, except that the Cr+ beam is generated by using both electron impact ionization and surface ionization sources. The surface ionization source produces a Cr+ beam which is almost entirely in the ground electronic state (>99.9%, Table 11) by thermal decomposition of Cr02C12and ionization of Cr on the hot filament surface (- 1300 K). The electron impact ionization source is used to generate Cr+ from Cr(C0)6, a process which is known to yield a significant fraction of the Cr+ beam in excited electronic states.6 The CrO' beam, produced from CrOzC12 with the surface ionization source, is predominantly in the electronic ground state (>97%), with 51% in the vibrational ground state, 25% in o = 1 state, 12% in u = 2 state, and the rest in higher vibrational states (Table 11). These distributions are calculated assuming that the ion is in thermal equilibrium with the surface. The ions are collimated, mass and energy selected, and allowed to react with the target gas in a collision chamber. Product ions scattered in the forward direction are detected by using a quadrupole mass spectrometer. Neutral products are not detected in these experiments.
Results and Discussion Determination of the CrO+-H Bond Dissociation Energy. The 00'-H bond dissociation energy is an important quantity in characterizing the reactivity of CrO+ toward alkanes. Hence, the determination of Do(CrO+-H) will be presented first in order to permit logical development of discussions in later sections. Do(CrO+-H) can be obtained by examining hydrogen atom abstraction reactions (reaction 2) with a series of hydrocarbons of CrO+ + R H
-
CrOH'
+R
(2)
varying bond energies7**(Table 111). The results for alkenes are taken from ref 4. Hydrogen abstraction is observed from propene and 2-methylpropene, which have allylic C-H bond dissociation energies -86 kcal mol-'.' I-Butene and cis-2-butene also have allylic C-H bonds with comparable bond dissociation energies, but they do not yield CrOH'. The study in ref 4 suggested a mechanism in which hydrogen atom abstraction from these molecules results in the formation of CrOH2+rather than CrOH' (6) Halle, L. F.; Armentrout, P. B.; Beauchamp, J. L. J. Am. Chem. SOC. 1981, 103, 962. (7) McMillen, D. F.; Golden, D. M. Ann. Reu. Phys. Chem. 1982,33,493. (8) Lossing, F. P.; Holmes, J. L. J . Am. Chem. Soc. 1984, 106, 6917.
(Scheme I). Briefly, both 1-butene and cis-2-butene undergo H abstraction to form an allyl-hydroxy ion complex 1 followed by a facile P-H transfer from the allyl substituent to the Cr center to form a water-butadiene ion complex 2, which may lose either butadiene or H 2 0 . Hence, H abstractions from 1-butene and cis-2-butene are considered to yield CrOH2+. The analogous P-H transfer pathway is not available in the reactions of propene and 2-methylpropene, rendering generation of CrOH' a major process. The H abstraction reactions from propene and 2-methylpropene exhibit the typical cross section behavior of an exothermic proce~s.~ The exothermic hydrogen abstraction from propene provides a lower limit for Do(CrO+-H) of 86.3 kcal mol-'.7 No hydrogen abstraction reaction is observed from the hydrocarbons with alkyl, vinyl, and phenyl C-H groups, which provides an upper limit of 93.2 kcal mol-' for the bond energy. Since failure to observe a reaction does not necessarily imply that a reaction is endothermic, the upper limit on the bond strength is less rigorous than the lower limit. Reaction with toluene gives rise to C7H7+and CrOH, representing a formal hydride abstraction from t o l ~ e n e . ~Hydride abstraction is not observed from the other hydrocarbons. The hydride abstraction from toluene provides a lower limit for the bond energy, DO(CrO+-H-) > Do(C6H5CH2+-H-) = 233 kcal mol-'.9 The reaction with 2-methylpropene, which yields CrOH' and a neutral product 2-methylpropenyl radical, suggests IP(CrOH) < IP[CH2=C(CH,)CH,] = 7.90 eV.' This value, combined with DO(CrO+-H), yields an upper limit for Do(CrO+-H-) of 253 kcal mol-' from the thermodynamic cycle depicted by eq 3. Do(CrO+-H-) = D(CrO+-H)
+ IP(Cr0H) - EA(H)
(3)
(9) The product C7H7+can be either benzyl or cycloheptatrienyl cation. Do(C6HSCHz+-H-)= AHa&C7H,+)+ AHa&H-) - AHoAC6HSCH3)is used to calculate the bond dissociation energy, which gives 233 kcal mol-' when the product is cyclohe tatrienyl cation. For the formation of benzyl cation, it gives 237 kcal mol- for the bond energy.
P
Kang and Beauchamp
7504 J. Am. Chem. SOC..Vol. 108, No. 24, 1986 Table IV. Summary of Thermochemical Data" D'(FeO+-H) D'(Cr0'-H) 89 f 5 D'(Fe+-OH) D'(Cr+-OH) 73 f 5 PA(Cr0) 221 f 5 D'(CoO+-H)
Table V. Product Distribution for the Exothermic Reactions of CrO+ with Linear and Branched Alkanes"
106 f 4e 73 f 3d 107 f 4e 71 f 3d
Do(Cr+-0) 85.3 f 1.3b D'(Co+-OH) D'(Cr0-H) >67, 231, 189, 6.9.
