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Room-temperature oxidation of hydrocarbons over FeZSM-5 zeolite
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
875
R o o m - t e m p e r a t u r e o x i d a t i o n of h y d r o c a r b o n s o v e r FeZSM-5 zeolite Mikhail A. Rodkin *~, Vladimir I. Sobolev, b Konstantin A. Dubkov, b Noel H. Watkins ~, and Gennady I. Panov b Solutia Inc., P.O. Box 97, Gonzalez, FL 32560-0097, USA b Boreskov Institute of Catalysis, 630090 Novosibirsk, Russia
SUMMARY Interaction of a variety of organic molecules with a-oxygen formed by N20 decomposition over FeZSM-5 zeolites leads to products of selective hydroxylation. O-insertion occurs rapidly at room temperatures and affects both aromatic and aliphatic C-H bonds.
1. INTRODUCTION N20 decomposition over FeZSM-5 zeolites is accompanied by the formation of a specific oxygen form called a-oxygen [1,2]. This oxygen exhibits a very high reactivity similar to that of monooxygenases and mimics their unmatched ability for selective oxidation of hydrocarbons at room temperature [3] The unique chemistry of a-oxygen has found practical application in the catalytic selective oxidation of benzene to phenol by nitrous oxide [4]. The process is being developed by Solutia, Inc. in cooperation with the Boreskov Institute of Catalysis and was successfully pilot-tested at Solutia's facilities in Pensacola [5]. To expand the application scope, room-temperature oxidations of various organic compounds by (z-oxygen using single-turnover experiments have been investigated and are reported in the present work.
2. EXPERIMENTAL The FeZSM-5 zeolite (SiOJA1203 = 72; 0.56 wt.% Fe) used in this work was prepared by hydrothermal synthesis with the introduction of iron in the form of FeC13 to the starting gel. The zeolite was converted into H-form by exchange with ammonia buffer and subsequent air calcination at 550~ To increase the concentration of a-sites, on which a-oxygen is formed, sample activation in vacuum at 900~ was used [6]. The experiments included three steps that were performed in the following sequence: 1. a-oxygen loading,
876 2. interaction of a-oxygen with organic substrate, 3. extraction of the product and its analysis. The first and the second steps were performed in a static setup equipped with an on-line mass-spectrometer. A catalyst sample (~1 g) was placed in a microreactor, which could be isolated from the rest of the setup. Before each experiment the sample was pretreated consecutively in vacuum and in oxygen at 550~ a-Oxygen was loaded by N20 decomposition at 250~ as described m reference [2]. N20 decomposition at this temperature is accompanied by stoichiometric binding of oxygen atoms to the active sites and the evolution of N,, into the gas phase (eq. (1)): N20+(
)a --> ( 0 ) a + N2
(1)
The n u m b e r of loaded oxygen atoms was determined from the amount of N2 evolved, as well as from the isotopic exchange with 1802, and found to be 2.0 2.2• m oxygen atoms per gram of catalyst in all cases. After a-oxygen loading the microreactor was isolated from the rest of the reaction volume and cooled to room temperature. Gas in the reaction volume was replaced with organic vapor which contacted with the catalyst sample for 10-15 min. The excess organic starting material was removed by evacuation, the sample was then taken out of the reactor and extracted with 2 ml of aqueous solution of acetonitrile (CH3CN : H20 = 1 : 1). The composition of the extract was determined using GC and GC/MS methods. The overall reaction is selective hydroxylation described by eq. (2). R-H + N20
-~ R-OH +N2
(2)
Additional information on the experimental details can be found in references [7, 8].
3. RESULTS AND DISCUSSION In our previous w o r k [7, 8] a similar technique has been used for detailed studies of the reaction of benzene and methane with a-oxygen at room temperature. This interaction has been shown to yield selectively phenol and methanol, respectively. The amounts of detected products within experimental error matched the amounts of reacted starting materials. Accurate quantitative t r e a t m e n t of the results of such studies is quite challenging. Zeolites are known to strongly adsorb both the reaction products and the starting materials. The main goal of this work was to expand the scope of compounds that can react with a-oxygen, which inevitably led us to focus on the qualitative rather than quantitative information. Thus, we did not optimize the extraction procedure for every case. The extracted products accounted for 515 ~mole/g of catalyst, which enabled us to reliably identify them and determine
877 their relative ratios. However, taking into account the possibility of incomplete extraction for different products, the ratios of the products reported below should be treated as preliminary results, that will be refined in our more detailed studies. 3.1 Oxidation of alkanes and alkenes
In view of our earlier results with methane [7], it was no surprise that oxidation of ethane yielded ethanol as the only product. When more complex alkanes are subjected to interaction with a-oxygen, secondary alcohol formation is predominant over the hydroxylation of the primary C-H bond. Thus, when propane is reacted with a-oxygen, 1-propanol and 2-propanol are formed m 1:2 ratio. When the reaction was performed with n-hexane only secondary positions of the molecule have been affected, giving an approximately equimolar mixture of 2-hexanol and 3-hexanol (eq. (3)). In most cases, the secondary alcohols can be oxidized further and the corresponding ketones were also observed, usually in small quantifies. OH
a-oxygen
:
1
OH
(3)
In the case of branched 2-methylhexane, the hydroxylation exhibits high sensitivity to steric hindrances: hydroxylation affects neither the tertiary C-H bond, nor the secondary C-H bond next to the branching. Hydroxylation attack on position 7 to branching is much easier than that on the ~-position (eq. (4)). OH or-oxygen
1
:
4
(4)
Similar observations were made for cydoalkanes. Cyclohexane gives cydohexahol exclusively. In the case of methylcydohexane, having a methyl substituent on the cyclohexane ring led to almost complete blocking of positions 2 and 3 of the ring leaving position 4 as the only available for attack on the ring (2and 3-methylcydohexanols are formed in trace quantities). Cydohexanemethanol is another major product, the formation of which is probably also determined by specific steric environment of methylcyclohexane in the zeolite micropore space (eq. (5)).
878
?CH3
ct-oxygcn HOX
?CH3
?CH2OH
3 : 1 (5) The reaction of a-oxygen with olefins was studied using cyclohexene as the starting material. Allylic oxidation was found to be the predominant process with 2-cyclohexene-l-ol as the major product (along with minor amounts of the corresponding a,~-unsaturated ketone- 2-cyclohexene-l-one, the formation of which can be explained by enhanced reactivity of allylic alcohol towards oxidation). 3-Cyclohexene-l-ol was also detected; the ratio of the products of allylic and homoallylic attack being ca 4:1 (eq. (6)). OH
O
0
OH
(~-oxygen
4
:
1
(6)
3.2 Oxidation of aromatics
As has been shown previously [8], the interaction of benzene with ~oxygen resulted in selective hydroxy]ation of the aromatic nucleus. The competition between the hydroxy]ation of aIiphatic and aromatic carbons as we]] as the balance of electronic and steric factors influencing these transformations was studied using alkylaromatic compounds as starting materials. In all cases, hydroxylation of the aromatic nucleus was observed to be much more facile than hydroxylation of the side chain. Interaction of toluene with a-oxygen leads to both the products of benzylic and aromatic hydroxylation: benzyl alcohol and cresols (o:m:p = 1:1:2.5) (eq. (7)). CH2OH a-oxygen + 1
:
H 2.5
(7)
The oxidation of ethylbenzene (eq. (8)) and isopropylbenzene (eq. (9)) shows that an increase in the bulk of the substituent strongly suppresses hydroxylation of both ortho- and meta-positions. For ethylbenzene, ortho- and
879 meta-ethylphenols are found in trace quantities, whereas for isopropylbenzene, para-isopropylphenol is the only ring-hydroxylated product observed.
Hydroxylation of the side chain of alkylaromatic compounds provides another good example of steric influences in a reaction happening in a zeolite micropore space. Hydroxylation of the side chain of ethylbenzene leads predominantly to ~phenetol. With the increase of the steric bulk of the alkyl substituent in isopropylbenzene, a-hydroxylation is strongly suppressed and 2-phenyl-1propanol is the predominant product of side chain hydroxylation, even though the tertiary C-H bond in the alkyl substituent of cumene is generally the most reactive. Even with severe steric restrictions, its reactivity is sufficient enough to make formation of 2-phenyl-2-propanol the only example in this study of the hydroxylation of the tertiary sp3-carbon. HO a-oxygen
O~
k
1
OH
9
3
(8)
OH
a-oxygen +
+
~" 1
OH 9
4
(9)
Hydroxylation of halobenzenes was also studied in the reaction with ~oxygen at room temperature (eq. (10)). In the case of fluorobenzene (X= F), fluorophenols were the major products (o:m:p = 1:2.6:5.1). When chlorobenzene (X=C1) is hydroxylated by a-oxygen, ortho- and para-chlorophenols in a 1:5.1 ratio are the major products. X
X a-oxygen v
(io)
880
In a,a,a-trifluorotoluene (X=CF3), the CF3 group, being a substituent with a strong -I effect and large steric bulk, directs hydroxylation predominantly in the meta-position (m:p = 1.5:1).
4. CONCLUSIONS A screening study of alkanes, alkenes and aromatic compounds in room temperature reactions with a-oxygen formed by N20 decomposition on the surface of Fe-containing zeolites is reported. Though the results are of qualitative nature, they significantly expand our knowledge of the chemistry of (z-oxygen and provide leads to other catalytic oxidations by N20. N20/FeZSM-5 zeolite was shown to be a versatile system for selective hydroxylation of a variety of organic compounds, effecting O-insertion into both aromatic and aliphatic C-H bonds. The reactivity of aromatic nucleus was found to be higher than that of the aliphatic substituents in all the studied alkylaromatic compounds. The regioselectivity of hydroxylation is determined by steric and electronic factors and is strongly influenced by the constraints imposed by the micropore space of the zeolite matrix. The reported results demonstrate the possibilities of novel routes to alcohols and phenols, most of which currently are manufactured via multi-step processes. To make these transformations commercially viable catalytic versions of these reactions need to be developed, a formidable challenge. We hope that the reported results will inspire researches to take a closer look at this novel and exciting field of oxidative catalysis.
REFERENCES 1. G.I. Panov, A.K. Uriarte, M.A. Rodldn and V.I. Sobolev, Catalysis Today, No. 41 (1-2) (1998) 365. 2. G.I. Panov, V.I. Sobolev, and A.S. Kharitonov, J. Mol. Catal., No. 61, (1990) 85. 3. G.I. Panov, V.I. Sobolev, K.A. Dubkov and A.S. Kharitonov in Proc. 1 lth Intern. Congr. Catal. J.Hightower, W.N. Delgass, E. Iglesia and A.T. Bell (eds.), Stud. Surf. Sci. Catal., Baltimore, 1996, Elsevier Science B.V. No.101 (1996) 493. 4. G.I. Panov, A.S. Kharitonov and V.I. Sobolev, Appl. Catal. No. 98 (1993) 1. 5. A.K. Uriarte, M.A. Rodkin, M.J. Gross, A.S. Kharitonov and G.I. Panov m Proc. 3rd Intern. Congress on Oxidation Catalysis, R.K. Grasselly, S.T. Oyama, A.M. Gaffney and J.E. Lyons (eds.) Stud. Surf. Sci. Catal., Elsevier Science B.V., No. 110 (1997) 857. 6. V.I. Sobolev, K.A. Dubkov, Ye.A. Paukshtis, L.V. Pirutko, M.A. Rodkin, A.S. Kharitonov and G.I. Panov, Appl. Catal. No. 141 (1996) 185. 7. K.A. Dubkov, V.I. Sobolev and G.I. Panov, Kinet. Katal., No. 39 (1998) 79. 8. V.I. Sobolev, A.S. Kharitonov, Ye.A. Paukshtis and G.I. Panov, J. Mol. Catal., No. 84 (1993) 117.
875
R o o m - t e m p e r a t u r e o x i d a t i o n of h y d r o c a r b o n s o v e r FeZSM-5 zeolite Mikhail A. Rodkin *~, Vladimir I. Sobolev, b Konstantin A. Dubkov, b Noel H. Watkins ~, and Gennady I. Panov b Solutia Inc., P.O. Box 97, Gonzalez, FL 32560-0097, USA b Boreskov Institute of Catalysis, 630090 Novosibirsk, Russia
SUMMARY Interaction of a variety of organic molecules with a-oxygen formed by N20 decomposition over FeZSM-5 zeolites leads to products of selective hydroxylation. O-insertion occurs rapidly at room temperatures and affects both aromatic and aliphatic C-H bonds.
1. INTRODUCTION N20 decomposition over FeZSM-5 zeolites is accompanied by the formation of a specific oxygen form called a-oxygen [1,2]. This oxygen exhibits a very high reactivity similar to that of monooxygenases and mimics their unmatched ability for selective oxidation of hydrocarbons at room temperature [3] The unique chemistry of a-oxygen has found practical application in the catalytic selective oxidation of benzene to phenol by nitrous oxide [4]. The process is being developed by Solutia, Inc. in cooperation with the Boreskov Institute of Catalysis and was successfully pilot-tested at Solutia's facilities in Pensacola [5]. To expand the application scope, room-temperature oxidations of various organic compounds by (z-oxygen using single-turnover experiments have been investigated and are reported in the present work.
2. EXPERIMENTAL The FeZSM-5 zeolite (SiOJA1203 = 72; 0.56 wt.% Fe) used in this work was prepared by hydrothermal synthesis with the introduction of iron in the form of FeC13 to the starting gel. The zeolite was converted into H-form by exchange with ammonia buffer and subsequent air calcination at 550~ To increase the concentration of a-sites, on which a-oxygen is formed, sample activation in vacuum at 900~ was used [6]. The experiments included three steps that were performed in the following sequence: 1. a-oxygen loading,
876 2. interaction of a-oxygen with organic substrate, 3. extraction of the product and its analysis. The first and the second steps were performed in a static setup equipped with an on-line mass-spectrometer. A catalyst sample (~1 g) was placed in a microreactor, which could be isolated from the rest of the setup. Before each experiment the sample was pretreated consecutively in vacuum and in oxygen at 550~ a-Oxygen was loaded by N20 decomposition at 250~ as described m reference [2]. N20 decomposition at this temperature is accompanied by stoichiometric binding of oxygen atoms to the active sites and the evolution of N,, into the gas phase (eq. (1)): N20+(
)a --> ( 0 ) a + N2
(1)
The n u m b e r of loaded oxygen atoms was determined from the amount of N2 evolved, as well as from the isotopic exchange with 1802, and found to be 2.0 2.2• m oxygen atoms per gram of catalyst in all cases. After a-oxygen loading the microreactor was isolated from the rest of the reaction volume and cooled to room temperature. Gas in the reaction volume was replaced with organic vapor which contacted with the catalyst sample for 10-15 min. The excess organic starting material was removed by evacuation, the sample was then taken out of the reactor and extracted with 2 ml of aqueous solution of acetonitrile (CH3CN : H20 = 1 : 1). The composition of the extract was determined using GC and GC/MS methods. The overall reaction is selective hydroxylation described by eq. (2). R-H + N20
-~ R-OH +N2
(2)
Additional information on the experimental details can be found in references [7, 8].
3. RESULTS AND DISCUSSION In our previous w o r k [7, 8] a similar technique has been used for detailed studies of the reaction of benzene and methane with a-oxygen at room temperature. This interaction has been shown to yield selectively phenol and methanol, respectively. The amounts of detected products within experimental error matched the amounts of reacted starting materials. Accurate quantitative t r e a t m e n t of the results of such studies is quite challenging. Zeolites are known to strongly adsorb both the reaction products and the starting materials. The main goal of this work was to expand the scope of compounds that can react with a-oxygen, which inevitably led us to focus on the qualitative rather than quantitative information. Thus, we did not optimize the extraction procedure for every case. The extracted products accounted for 515 ~mole/g of catalyst, which enabled us to reliably identify them and determine
877 their relative ratios. However, taking into account the possibility of incomplete extraction for different products, the ratios of the products reported below should be treated as preliminary results, that will be refined in our more detailed studies. 3.1 Oxidation of alkanes and alkenes
In view of our earlier results with methane [7], it was no surprise that oxidation of ethane yielded ethanol as the only product. When more complex alkanes are subjected to interaction with a-oxygen, secondary alcohol formation is predominant over the hydroxylation of the primary C-H bond. Thus, when propane is reacted with a-oxygen, 1-propanol and 2-propanol are formed m 1:2 ratio. When the reaction was performed with n-hexane only secondary positions of the molecule have been affected, giving an approximately equimolar mixture of 2-hexanol and 3-hexanol (eq. (3)). In most cases, the secondary alcohols can be oxidized further and the corresponding ketones were also observed, usually in small quantifies. OH
a-oxygen
:
1
OH
(3)
In the case of branched 2-methylhexane, the hydroxylation exhibits high sensitivity to steric hindrances: hydroxylation affects neither the tertiary C-H bond, nor the secondary C-H bond next to the branching. Hydroxylation attack on position 7 to branching is much easier than that on the ~-position (eq. (4)). OH or-oxygen
1
:
4
(4)
Similar observations were made for cydoalkanes. Cyclohexane gives cydohexahol exclusively. In the case of methylcydohexane, having a methyl substituent on the cyclohexane ring led to almost complete blocking of positions 2 and 3 of the ring leaving position 4 as the only available for attack on the ring (2and 3-methylcydohexanols are formed in trace quantities). Cydohexanemethanol is another major product, the formation of which is probably also determined by specific steric environment of methylcyclohexane in the zeolite micropore space (eq. (5)).
878
?CH3
ct-oxygcn HOX
?CH3
?CH2OH
3 : 1 (5) The reaction of a-oxygen with olefins was studied using cyclohexene as the starting material. Allylic oxidation was found to be the predominant process with 2-cyclohexene-l-ol as the major product (along with minor amounts of the corresponding a,~-unsaturated ketone- 2-cyclohexene-l-one, the formation of which can be explained by enhanced reactivity of allylic alcohol towards oxidation). 3-Cyclohexene-l-ol was also detected; the ratio of the products of allylic and homoallylic attack being ca 4:1 (eq. (6)). OH
O
0
OH
(~-oxygen
4
:
1
(6)
3.2 Oxidation of aromatics
As has been shown previously [8], the interaction of benzene with ~oxygen resulted in selective hydroxy]ation of the aromatic nucleus. The competition between the hydroxy]ation of aIiphatic and aromatic carbons as we]] as the balance of electronic and steric factors influencing these transformations was studied using alkylaromatic compounds as starting materials. In all cases, hydroxylation of the aromatic nucleus was observed to be much more facile than hydroxylation of the side chain. Interaction of toluene with a-oxygen leads to both the products of benzylic and aromatic hydroxylation: benzyl alcohol and cresols (o:m:p = 1:1:2.5) (eq. (7)). CH2OH a-oxygen + 1
:
H 2.5
(7)
The oxidation of ethylbenzene (eq. (8)) and isopropylbenzene (eq. (9)) shows that an increase in the bulk of the substituent strongly suppresses hydroxylation of both ortho- and meta-positions. For ethylbenzene, ortho- and
879 meta-ethylphenols are found in trace quantities, whereas for isopropylbenzene, para-isopropylphenol is the only ring-hydroxylated product observed.
Hydroxylation of the side chain of alkylaromatic compounds provides another good example of steric influences in a reaction happening in a zeolite micropore space. Hydroxylation of the side chain of ethylbenzene leads predominantly to ~phenetol. With the increase of the steric bulk of the alkyl substituent in isopropylbenzene, a-hydroxylation is strongly suppressed and 2-phenyl-1propanol is the predominant product of side chain hydroxylation, even though the tertiary C-H bond in the alkyl substituent of cumene is generally the most reactive. Even with severe steric restrictions, its reactivity is sufficient enough to make formation of 2-phenyl-2-propanol the only example in this study of the hydroxylation of the tertiary sp3-carbon. HO a-oxygen
O~
k
1
OH
9
3
(8)
OH
a-oxygen +
+
~" 1
OH 9
4
(9)
Hydroxylation of halobenzenes was also studied in the reaction with ~oxygen at room temperature (eq. (10)). In the case of fluorobenzene (X= F), fluorophenols were the major products (o:m:p = 1:2.6:5.1). When chlorobenzene (X=C1) is hydroxylated by a-oxygen, ortho- and para-chlorophenols in a 1:5.1 ratio are the major products. X
X a-oxygen v
(io)
880
In a,a,a-trifluorotoluene (X=CF3), the CF3 group, being a substituent with a strong -I effect and large steric bulk, directs hydroxylation predominantly in the meta-position (m:p = 1.5:1).
4. CONCLUSIONS A screening study of alkanes, alkenes and aromatic compounds in room temperature reactions with a-oxygen formed by N20 decomposition on the surface of Fe-containing zeolites is reported. Though the results are of qualitative nature, they significantly expand our knowledge of the chemistry of (z-oxygen and provide leads to other catalytic oxidations by N20. N20/FeZSM-5 zeolite was shown to be a versatile system for selective hydroxylation of a variety of organic compounds, effecting O-insertion into both aromatic and aliphatic C-H bonds. The reactivity of aromatic nucleus was found to be higher than that of the aliphatic substituents in all the studied alkylaromatic compounds. The regioselectivity of hydroxylation is determined by steric and electronic factors and is strongly influenced by the constraints imposed by the micropore space of the zeolite matrix. The reported results demonstrate the possibilities of novel routes to alcohols and phenols, most of which currently are manufactured via multi-step processes. To make these transformations commercially viable catalytic versions of these reactions need to be developed, a formidable challenge. We hope that the reported results will inspire researches to take a closer look at this novel and exciting field of oxidative catalysis.
REFERENCES 1. G.I. Panov, A.K. Uriarte, M.A. Rodldn and V.I. Sobolev, Catalysis Today, No. 41 (1-2) (1998) 365. 2. G.I. Panov, V.I. Sobolev, and A.S. Kharitonov, J. Mol. Catal., No. 61, (1990) 85. 3. G.I. Panov, V.I. Sobolev, K.A. Dubkov and A.S. Kharitonov in Proc. 1 lth Intern. Congr. Catal. J.Hightower, W.N. Delgass, E. Iglesia and A.T. Bell (eds.), Stud. Surf. Sci. Catal., Baltimore, 1996, Elsevier Science B.V. No.101 (1996) 493. 4. G.I. Panov, A.S. Kharitonov and V.I. Sobolev, Appl. Catal. No. 98 (1993) 1. 5. A.K. Uriarte, M.A. Rodkin, M.J. Gross, A.S. Kharitonov and G.I. Panov m Proc. 3rd Intern. Congress on Oxidation Catalysis, R.K. Grasselly, S.T. Oyama, A.M. Gaffney and J.E. Lyons (eds.) Stud. Surf. Sci. Catal., Elsevier Science B.V., No. 110 (1997) 857. 6. V.I. Sobolev, K.A. Dubkov, Ye.A. Paukshtis, L.V. Pirutko, M.A. Rodkin, A.S. Kharitonov and G.I. Panov, Appl. Catal. No. 141 (1996) 185. 7. K.A. Dubkov, V.I. Sobolev and G.I. Panov, Kinet. Katal., No. 39 (1998) 79. 8. V.I. Sobolev, A.S. Kharitonov, Ye.A. Paukshtis and G.I. Panov, J. Mol. Catal., No. 84 (1993) 117.