J . Phys. Chem. 1984,88, 4210-4213
4210
ARTICLES Photocatalytic Alcohol Dehydrogenation Using Ammonium Heptamolybdate Michael D. Ward,* James F. Brazdil, and Robert K. Grasselli Sohio Research Center, Cleveland, Ohio 44128 (Received: May 18, 1983; In Final Form: March 19, 1984)
Illumination of aqueous solutions of (NH4)6M07024*4H20 (1) in the presence of 2-propanol results in rapid formation of acetone with concomitant formation of a spectroscopically characterized reduced molybdenum species. A reaction sequence is proposed in which the excited state of 1 abstracts H- from the a-carbon atom of the alcohol. The relevance of this reaction to photochemical and thermal C-H activation on heterogeneous metal oxide catalysts is discussed.
Introduction
Experimental Section
Carbon-hydrogen bond activation has been a subject of extensive investigations as greater understanding of this process can eventually lead to desirable and selective transformations of hydrocarbons. Recently, there have been numerous reports pertaining to photochemical C-H bond activation at heterogeneous metal oxide surfaces. For example, illuminated Ti02suspensions have been reported to effect paraffin oxidation' and alcohol d e h y d r ~ g e n a t i o n . ~ The - ~ ~ primary chemical process can be envisioned as H. abstraction (from H 2 0 or C-H bonds) by photoproduced electron-deficient oxygen species which result from band gap excitation that is tantamount to a ligand-to-metal chargetransfer t r a n ~ i t i o n . ~ -Furthermore, ~ the observation of methane oxidation by similar, although chemically generated, electrondeficient oxygen species on MOO: suggests that these entities may be important for C-H bond activation on metal oxide catalysts. The similarity between these catalysts and the [Mo702$ ion (1) is evidenced from XRD and ESR ana lyse^^**^ of photoreduced [Mo7OUl6-ion which suggests the presence of an electron-deficient oxygen center and a reduced molybdenum ion in the excited state. This suggests that the photoprocess is essentially a ligand-to-metal charge transfer, consistent with reports for similar We report herein a detailed study of the photocatalytic dehydrogenation of alcohols by 1. Given the similarity between the ground-state chromophore of 1 and species considered to be photoactive on heterogeneous molybdenum oxides (Le., Mo6+==02entities),l33l4significant studies can be conducted to gain insights into photoinduced C-H activation on heterogeneous photocatalysts.
Aqueous solutions of (NH4)6M070244H20 (1, MCB) were prepared by using triply distilled water. Analytical-grade 2propanol (Fisher) was used as obtained. Deuterium-labeled 2propanol, (CH,),CDOH, was obtained from Merck Sharp and Dohme Isotopes, Inc. Reaction mixtures were deaerated by bubbling N 2 through the solution for at least 2 h. Ultravioletvisible spectra were recorded on a Beckman ACTA CIII UV-VIS spectrophotometer. Gas-chromatographic analysis was performed on a 6-ft 5% Carbowax on 80/100 Carbopack B column with a Hewlett-Packard 5710 gas chromatograph. Photochemical experiments were performed in a Rayonet photochemical reactor equipped with 3000-A mercury lamps so that reactions could be compared in parallel. Spectrophotometric determinations of the reaction progress as a function of [2propanol] were also performed in parallel by using the Rayonet apparatus and polystyrene disposable cuvettes. Concurrent determination of [acetone] and A720 was accomplished by recording the visible spectrum of solutions of 1 irradiated in quartz cuvettes and sampling the reaction mixture with a 10-pL syringe through a rubber serum cap at periodic time intervals. The apparent second-order rate constant for 2-propanol oxidation was determined from the rate dependence of acetone formation upon either [ l ] or [2-propanol], assuming rate = k[ 11[2-propanol]. Quantum yields for acetone formation were determined by comparison to 4-pentenal production from a neat solution of cyclopentanone irradiated in parallel.lS
(1) I. Izumi, W. W. Dunn, K. 0. Wilbourn, F.-R. F. Fan, and A. J. Bard, J . Phys. Chem. 84, 3207 (1980). (2) S. Teratani, J. Nakamichi, K. Taya, and K. Tanaka, Bull. Chem. SOC. Jpn., 55, 1688 (1982). (3) P. Pichat, J. M. Herrmann. J. Disdier, H. Courbon, and M.-N. Mozzanega, Nouu. J . Chim., 5, 627 (1981). (4) (a) M. A. Malati and N. J. Seager, J . Oil Colloid Chem. Assoc., 64, 231 (1981); (b) J. Cunningham, B. K. Hodnett, M. Ilyas, E. M. Leahy, and J. P. Tobin, J . Chem. Soc., Faraday Trans. I , 78, 3297 (1982). ( 5 ) W. J. Lo, Y. W. Chung, and G . A. Somorjai, Surf. Sci., 71, 199 (1978). (6) C. D. Jaeger and A. J. Bard, J . Phys. Chem., 83, 3246 (1979). (7) R. I. Bickley and F. S. Stone, J . Catal., 31, 389 (1973). (8) R.3. Liu, M. Iwamoto, and J. H. Lunsford, Chem. Commun. 78 (1982). (9) T. Yamase, R. Sasaki, and T. Ikawa, J . Chem. Soc., Dalton Trans., 628 (1981). (10) T. Yamase, J . Chem. Soc., Dalton Trans., 1987 (1982). (11) G . M. Varga, Jr., E. Papaconstantinou, and M. T. Pope, Inorg. Chem., 9, 662 (1970). (12) H. So and M. T. Pope, Inorg. Chem., 11, 1441 (1972). (13) M. Anpo, I. Tanahashi, and Y. Kubokawa, J . Phys. Chem., 86, 1 (1982). (14) A. Mahay, S. Kaliaguine, and P. C. Roberge, Can. J . Chem. 60,2719 (1982).
0022-3654/84/2088-4210$01.50/0
Results and Discussion Illumination of a deaerated 8.3 X lo4 M aqueous solution (240 mL, pH 6.0) of (NH4)6M07024*4H20 (1) containing 2.0 M 2propanol resulted in the formation of acetone at an initial rate of ca. 9.0 turnovers min-' accompanied by formation of a yellow-green color attributed to a reduced molybdenum species (vide infra). The rate eventually decreased until the reaction slowed to an insignificant rate, a t which time the solution was deeply colored. When O2 was vigorously bubbled through a solution under identical conditions, the rate of acetone formation also decreased with time until a constant rate of ca. 1.0 turnover m i d was realized. The rate of acetone formation remained unchanged for at least 300 turnovers, thus indicating the photocatalyst was stable under the reaction condition. Also, the initial rate of acetone formation remained unchanged after (after exhaustive reoxidation of the reactive mixture with 0,) several hundred turnovers. The observation of a constant rate for 2-propanol oxidation that is lower than the initial rate suggests that the effective concentration of 1 is diminished under catalytic reaction condition, accompanied (15) P. Dunion and C. N. Trainbore, J. Am. Chem. Soc., 87,421 1 (1965).
0 1984 American Chemical Society
Photocatalytic Alcohol Dehydrogenation
The Journal of Physical Chemistry, Vol, 88, No. 19, 1984 4211
I
50.01
1.8 40.0
-
1.6
.H
1.4
c
1
30.0:
1.2 1.o
0.8 0.6 10.0
/ 0.5
1.0
1.5
2.0
2.5
0.4
0.2
c
1
3.0
[2-PROPANOL] (mol W1)
Figure 1. Dependence of the initial rate of acetone formation on [2propanol]. [Mol = 4.0 X M; reaction volume = 10 mL.
by an increase in the concentration of the yellow-green species. Therefore, there may be two causes for the observed lower rate: (a) a lower concentration of 1 due to inefficient reoxidation of the reduced species by O2 and (b) competitive light absorption by the yellow-green species. The yellow-green color and a pronounced change in the absorption spectrum of an irradiated solution of 1 and 2-propanol suggest the presence of a reduced molybdenum species. While 1 exhibited only a strong absorption maximum at 220 nm, an irradiated solution of 1 exhibited a strongly absorbing shoulder beginning at 460 nm and a weak band at 720 nm. Similar absorption spectra have been reported for one-electron reduced hetero- and isopolymolybdates. The absorption edge a t 460 nm is most likely due to the presence of a new charge-transfer transition(s) associated with the reduced heptamolybdate ion16 although others17 have assigned absorptions in this region for similar species (e.g., [Mo~O,,]~-)to d-d transitions. The absorption at 720 nm also cannot be unambiguously assigned. The molar extinction coefficient of the 720-nm band determined from an exhaustively reduced solution of 1 was estimated to be ca. 400 L mol-' cm-'. The position" and weak intensity of the absorption suggest that it arises from a d-d transition. However, the absorption may also be attributed to an intervalence charge-transfer transition from a highly localized ground state.18 Further evidence that the yellow-green color is associated with a reduced heptamolybdate species is the disappearance of the color and the absorptions at 460 and 720 mm when O2 is introduced to the irradiated sample. After exhaustive O2addition, the original UV spectrum of 1 is regained in unchanged form. The rate of acetone formation was observed to be linearly dependent on [2-propanol] over a wide concentration range (Figure 1). The rate of acetone formation was found to increase sharply as [ l ] was increased at low concentrations (Figure 2). Above [ 11 = 1.0 mM, however, the increase of rate with increasing [ 11 became substantially less pronounced. The molar absorption coefficient for 1 at 300 nm is sufficiently large (€300 = 8.6 X lo2 L mol-' cm-I) so that approximately 85% of the incident photons M. Therefore, further increases are absorbed at [ l ] = 1.0 X in [ 13 exerted a smaller influence on the reaction rate. Overall quantum yields for the process were estimated to be 0.17 f 0.02 mol einstein-I at 300 nm ( [ l ] = 9.5 X M; [2-propanol] = 2.2 M). The apparent second-order rate constant was determined as 3.76 0.04 L mol-' min-'. The results are consistent with the photoinduced formation of an excited state of 1 which is long-lived to the extent that de-
*
(16) C. D. Garner, L. H. Hill, F. E. Mabbs, D. L. McFadden, and A. T. McPhail, J . Chem. Soc., Dalton Trans., 853 (1977). (17) C. Sanchez, J. Livage, J. P. Launay, M. Fournier, and Y. Jeannin, J . Am. Chem. Soc., 104, 3194 (1982). (18) N. S. Hush, Prog. Inorg. Chem., 8, 391 (1967).
0'
I
I
1
1.o
2.0
3.0
4.0
[M070& (mmol lit-') Figure 2. Dependence of the initial rate of acetone formation on [l]. [2-propanol] = 1.32 M; reaction volume = 10 mL.
Scheme I
--
,e-
I*
1
hydrogenation of 2-propanol can be accomplished. The [Mo70,1b anion possesses two terminal Mo=(O), moieties located along the edge of edge-sharing octahedra.'2 Irradiation of 1 with ultraviolet light is thought to induce a ligand-to-metal chargetransfer transition on one of these two sites that can formally be envisioned as reduction of the molybdenum center with concomitant formation of an electron-deficient oxygen species (Scheme I). A similar mechanism was invoked to explain the formation of a MoV05(OH)species in a M o o 6 octahedral site upon irradiation of a [NH3Pri]6[Mo7024].3H20 (2) single c r y ~ t a l . ~The ~'~ resemblance of the absorption spectra of irradiated solutions of 1 (containing 2-propanol) and 2 suggests the presence of similar reduced specie^.'^ These observations are consistent with the formation of an excited state of 1 by illumination in its absorption region followed by H. abstraction from 2-propanol by an electron-deficient oxygen center to generate a reduced molybdenum site denoted as Mo5+-OH (Scheme 11; the other oxygen ligands of 1 have been omitted for greater clarity). Scheme I1
Mo5+FO1
+ (CH3)ZCHOH
MoS+-OH 3
+ (CH3)ZCOH
(CH3)2C=O
+ Mo5+-OH
+
(2) (CH,),COH
+ Mo6+=02-1 +
-
3
(3)
+
2M05+-OH '/202 2M06+=02- HzO (4) 3 1 overall: (CH3),CHOH O2 (CH3)$=0 2H20
+
-
+
The absorption at 720 nm, due to the reduced species, was found to increase linearly with [2-propanol] (Figure 3) as was acetone (19) The identity of the reduced species is strongly dependent upon the initial concentration of [ M O , ~ ~as~it] has ~ , been reported that blue solutions are formed upon irradiation of 2 when [2] 2 2.7 X M, due to condensed molybdates. (See T. Yamase, T. Ikawa, Y. Okaski, and Y. Sasada, Chem. Commun.,697 (1979).) We also have observed similar behavior for irradiated 2-propanol solution of 1 in these concentration regimes. The experiments described herein were performed with [l] < 2.0 X lo-) M, thus avoiding formation of these condensed structures.
The Journal of Physical Chemistry, Vol, 88, No. 19, 1984
4212
Ward et al.
- 0.80 0.468
. - $_ _ _ _ _ Q--o.80
i
Q
- 0.70 - 0.60 - 0.50 $ - 0.40
- 0.30 - 0.20 - 0.10
a
0
2
4
6
8
10
12
14
16
18
20
22
24
t(m1n.)
Figure 4. Variation of A720and [acetone] with time for irradiated solutions of 1.6 X lo-) M 1 and 26.3 X lo-) M 2-propanol.
I
1.o
0
I
2.0
I
Scheme I11
3.0
Figure 3. Linear dependence of A720on [2-propanol]after illumination of 1.6 X M 1 in the presence of 2-propanol for 15 min.
formation (Figure 1). These results clearly demonstrate that alcohol oxidation occurs with concurrent formation of Mo5+-OH, suggesting that this species is indeed an intermediate in the catalytic cycle. The initial step (H. abstraction) is believed to occur at the a-carbon atom as this C-H bond is the weakest one in 2-propanol (ca. 91 kcal/mol). The radical nature of this reaction is supported by the occasional observation of minute amounts of acetaldehyde in the reaction mixture after illumination. Similar observations of competing C-C bond cleavage have been reported for alcohol oxidation by Co(II1) in aqueous acid solutions which is also reported to proceed by a-(C-H) activation.20 The remainder of the steps outlined in Scheme I are reasonable on the basis of the known reduction potential of Mo5+-OH ( E (Mo6+*Mo5+) = -0.30 V vs. SCE at pH 6.0),21which is substantially more negative than Eredax for O2 reduction (Eredox = +0.875 V at pH 6.0). A similar mechanism has recently been ~ ~ ~ ~ to proposed elsewhere for h e t e r o p o l y t u n g ~ t a t e s . ~According Scheme 11, the 2-hydroxy-2-propyl radical is oxidized by the heptamolybdate anion to yield acetone, similar to current doubling effects observed for photooxidation reaction of organic compounds by semicond~ctors.~~ Alternatively, acetone formation may result via disproportionation of two radicals, thus yielding 1 equiv of acetone (eq 5 ) . Either pathway requires 2 equiv of 1 to yield 2(CH3)&0H (CH3)2CHOH + (CH&2=0 (5) 1 equiv of acetone in the absence of oxygen. The presence of this condition is indeed supported by experiments in which the intensity of the 720-nm absorption was monitored concurrently with the acetone concentration as a function of irradiation time (Figure 2). When a deaerated 2.6 X M 2-propanol solution containing 1.6 X M 1 was irradiated, the absorption at 720 nm and the acetone concentration increased until the reaction did not progress at a significant rate (a plateau was observed; see Figure 4). Assuming complete depletion of 1, this plateau corresponds to [acetone]/[l](initial) = 0.68. This result suggests that the stoichiometry defined in Scheme I1 is correct; the slightly higher number may arise from either adventitious oxygen (introduced
-
(20) J. K. Kochi, "Organometallic Mechanisms and Catalysis", Academic Press, New York, 1978, p 106. (21) The estimated Ersdor for the process MoS+-OH Mo6+=O2-+ H+ + e- was reported as -0.25 V at pH 5.2. As this value is pH dependent, Ercdox (pH 6.0) is estimated at -0.30 V. See T. Yamase and T. Ikawa, Inorg. Chem. Acta, 37, L529 ( 1 919). (22) J. R.Darwent, J . Chem. Soc., Chem. Cornmun., 198 (1982). (23) E. Papaconstantinou, J . Chem. Soc., Chem. Commun. 12 (1982). (24) (a) S:R. Morrison, "Electrochemistry at Semiconductors and Oxidized Metal Electrodes", Plenum Press, New York, 1980, and references therein; (b) W. P. Gomer, T. Freund, and S. R. Morrison, J . Electrochem. Soc., 115, 818 (1968).
+ H20
Mo5+-O-
[P-PROPANOL] (mol lW1)
HO.
-+
+ (CH3)ZCHOH
+
Scheme IV MoHo
Mo5+-OH 3
t He0
+ HO.
HzO + (CH3)ZCOH
-
(6)
(7)
/a
Mo-OH
\O
\OH
4
//O
MO-OH
4 O
L M&OH
\OH
(9)
\OH
4* 5+/.
MO -OH ' 0 H
4* HO' t (CH,),CHOH
0
5+/
MO
t HO'
(10)
O 'H
-
3 H ~ Ot
(CH,I,EOH
(7)
during sampling) or slow 2-propanol oxidation by the reduced species (3). The presence of hydroxyl radical, HO-, in irradiated aqueous solutions of [NH3Pri]6[Mo7024].3Hz0 has been invoked in light of EPR spin trapping studiesg Additionally, strong evidence exists for the intermediacy of HO- in the photooxidation of organic substrates with semiconductor powders. Therefore, one must consider the possibility that hydroxyl radicals are the species responsible for H. abstraction from 2-propanol (Scheme 111). However, this scheme can be distinguished from that described earlier (Scheme 11) by deuterium labeling studies. A significant isotope effect (kH/kD = 3.7) was observed when isopropyl-2-d alcohol was employed as the substrate. This result confirms that the a-C-H bond is indeed the site of H. abstraction and that this process is the rate-determining step. Additionally, irradiation of a D 2 0 solution of 1 and 2-propanol resulted in a rate of acetone formation identical with that observed in H20. This observation suggests that HO. formation neither is involved in the rate-determining step nor precedes it according to Scheme 111. It is interesting to note that while photoreduction of [NH3Pri],[Mo,02,] .3H20 occurs readily in aqueous solution, no evidence for photoreduction of 1 was observed in the absence of 2-propanol, in agreement with a previous report.25 Our observations do not exclude the formation of HO. radical by 1* in the absence of 2-propanol. The detection of this species, albeit indirectly,21 supports the contention that H-OH, abstraction by the photoexcited [Mo,02,16- ion is possible. Addition of products presumed (25) T. Yamase and T. Ikawa, Bull. Chem. Soc., Jpn., 50, 746 (1977).
The Journal of Physical Chemistry, Vol. 88, No. 19, 1984 4213
Photocatalytic Alcohol Dehydrogenation Scheme V H
I
-
t R2CHOH
MOHO
N O
//O Mo-OCR~ \OH
(1 1)
5
1 H
/o- I
5
5+/OH*
--c
MO --0CR2 ‘OH
(13)
6 6
-’
4+/OH MO ‘OH
,
+
R2C=O
7 0
overall:
~06~4t \O
RzCHOH
-
nation readily occurs in the gas phase on the surfaces of illuminated metal oxide powders in the absence of water,4bsuggesting direct interaction of the alcohol with the excited states (photogenerated holes) on the surface. The results are also consistent with an alternative mechanism which involves condensation of the alcohol with 1 prior to H. abstraction. Indeed, the proximity of the a-C-H bond to the Mo6=02- chromophore in the condensate may account for the significant activity observed for alcohol dehydrogenation (Scheme V; the other oxygen ligands of 1 have been omitted for the sake of clarity). This mechanism implies the formation of Mo4+species 7 contrary to the reported formation of a MoSf center when 1 is irradiated. However, rapid comproportionation of 7 and 1 to yield a Mo5+species may account for the observation of the latter (eq 15). Unfortunately, at the present time the mechanisms are not distinguishable.
1 Mo4’/0H O ‘H
t R2C=0
to result from formation of HO. radical (HzOzor 0,)to irradiated solutions of 1 caused oxidation of the reduced molybdenum species; such behavior may explain the lack of net photoreduction of 1 in the absence of 2-propanol. These observations suggest that net reduction of 2 may occur via H-abstraction from the alkyl group contained in the countercation of 2. Indeed, formation of products resulting from cleavage of the isopropyl group was reported for illuminated solutions of 2.19 Thus, the reactivity exhibited by 1 may be due to more facile reaction of 1* with 2-propanol compared to HzO, behavior consistent with the bond strength of the C-H and 0-H bonds (91 and 104 kcal/mol, respectively). An alternative mechanism involving a hydroxyl radical intermediate which is consistent with the above observation may also be considered. For example, it is possible that the photochemistry arises from a hydrated form of 1 (shown in Scheme IV as 4), and that the excited state, 4*,results via a LMCT originating from a hydroxyl group. Loss of hydroxyl radical from 4* would result in generation of 3 and subsequent H- abstraction from 2-propanol. Note that this mechanism is consistent with lack of an observable isotope effect when D 2 0 was employed as a solvent as only secondary effects would be expected. The position of the equilibrium defined by eq 8 may not differ significantly in D20. Although Scheme IV cannot be entirely discounted, given the preponderance of evidence that Mo=O species are responsible for photoactivity in analogous s y s t e m ~ ’ ~(vide J ~ , ~infra) ~ we believe Scheme I1 to be more likely. It is interesting to note that alcohol dehydroge(26) B. N. Shelimov, A. N. Pershin, and V. B. Kazansky, J . Catal., 64, 426 (1980).
7
3
Concluding Remarks The results described above illustrate that photocatalytic C-H bond activation can be observed in the presence of a simple molybdenum polyoxyanion. The observation of a first-order dependence on [Zpropanol] is consistent with reaction of the alcohol with the excited state of 1; high concentrations of 2-propanol are desirable as extensive decay of 1* to the ground state is obviated under these conditions. Similar results have been observed for photocatalytic methanol dehydrogenation in the presence of the SiW,,04$- anion.22 The activity is attributable to a well-characterized photoactive center, specifically octahedral MoV1=O centers as observed for the Mo,O,,~- and H2Mo8OzX6anions. Similar species on the surfaces of molybdenum oxide either or dispersed on have supported on porous Vycor been shown to exhibit UV absorptions characteristic of LMCT transitions similar to those proposed above for 1. The observation that CH, is activated by chemically generated electron-deficient oxygen speciesx similar to those present in the excited state of 1 suggests a common theme for oxidation catalysis on metal oxides performed under photochemical or thermal conditions. Indeed, the observation of photocatalytic oxidation of propylene on TiO?’ and the reaction of methane with photogenerated “0-” species on other metal oxideszx which contain M=O surface species suggests that species having metal-oxygen double-bond character may be a necessary prerequisite for C-H bond activation. Registry No. (NH4)6Mo,0,4, 12027-67-7; 2-propanol, 67-63-0 (27) P. Pichat, J.-M. Herrmann, J. Disdier, and M.-N. Mozzanega, J . Phys. Chem., 83, 3122 (1979). (28) S. L. Kaliaguine, B. N. Shelimov, and V. B. Kazansky, J . Catal., 55, 384 (1978).
