Heterogeneous Hydrocarbon Oxidation - American Chemical Society

Chapter 28

Alkane Oxidation on Vanadium Silicalite Compared to Titanium Silicalite

Downloaded by STANFORD UNIV GREEN LIBR on October 8, 2012 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0638.ch028

T. Tatsumi, Y. Hirasawa, and J . Tsuchiya Engineering Research Institute, Faculty of Engineering, University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113, Japan

Silicates containing Ti and V, TS-2 an VS-2, have been synthesized using tetrabutylammonium cation as the template. Contrasted with TS-2, VS-2 gives appreciable terminal oxidation products in the oxidation of hexane and allylic oxidation products in the oxidation of hexenes. The relative rate of 2-hexene/1-hexene epoxidation is much lower on VS-2 than on TS-2. These findings indicate the radical character of the active oxygen species on VS-2. Spin trapping experiments have revealed that the primary hexyl radical is generated in the VS-2-H O -hexane system and that no alkyl radical is observed with the TS-2 system. For both TS-2 and VS-2, hydroperoxy radicals are trapped by nitrones. It is proposed that in the oxidation of alkanes the oxidation of internal carbons and that of terminal carbons proceed by different mechanisms. 2

2

The Ti analogs of ZSM-5 (TS-1) (1) and ZSM-11 (TS-2) (2) catalyze the oxidation of simple alkanes at mild temperatures (3-5). Similar materials containing V have also been prepared (6-8) and exhibit interesting oxidation properties: the oxyfunctionalization with hydrogen peroxide of the primary carbon atom leading to the formation of primary alcohols and aldehydes was observed only with the vanadium silicalite VS-2 (8) and not with TS-1 or TS-2 (3-5). A comparison has been made of the oxidation efficiency of Ti and V silicalites and it has been postulated that a predominantly heterolytic pathway is operative in the case of TS-2 and that the principal pathway is homolytic in the case of VS-2 (9). The purpose of this study is to synthesize and characterize silicalites containing Ti and V and to investigate the similarities and differences in the metal environments and reaction pathway in the oxidation of hydrocarbons. Attention has been also focused on identifying the reactive radical intermediates. Although direct observation should generally provide the most reliable information of the radical, many radicals cannot be observed directly by ESR. The technique of spin trapping has been developed to detect and identify radicals too short lived or too scarce for direct ESR observation. In this approach, a transient radical reacts with a spin trap to produce a persistent radical, a spin adduct. The ESR spectrum of the adduct is characteristic of the trapped radical and can help in identifying the transient species.

0097-6156/96/0638-0374$15.00/0 © 1996 American Chemical Society In Heterogeneous Hydrocarbon Oxidation; Warren, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

28.

Alkane Oxidation on Vanadium Silicalite

TATSUMI ET AL.

375


Experimental Catalysts. Titanium silicalite (TS-2) was prepared by the method reported by Reddy and Kumar (2). Vanadium silicalite (VS-2) was prepared as follows: 54 g of tetrabutylammonium hydroxide (TBAOH) (40% aqueous solution, TCI) was added slowly under nitrogen atmosphere at 273 K to 136.5 g of tetraethyl orthosilicate (TEOS) (TCI) dissolved in 2-propanol (45 g). To the resultant mixture, a solution of 2.1 g of vanadium trichloride in 30 g of deionized water was added slowly together with 177 g of T B A O H (6.1 wt%) under vigorous stirring. Then 60 g of T B A O H (10 %) was added and the mixture was stirred for 90 min. Then the temperature was raised to 323-333 K and the solution was stirred at that temperature for 2 h. The clear green liquid was then transferred into a teflon flask, placed into an autoclave and heated at 433 K under static conditions for 4 days. After crystallization, the white solid materials were recovered by filtration, washed with water and dried overnight at 383 K. The template was removed by calcining the solid at 763 K in air for 6 h. Characterization. X-ray diffraction (XRD) powder patterns were collected on a Rigaku RINT 2400 X-ray diffractometer. Fourier-transform infrared spectra were recorded on a Perkin Elmer 1600 spectrometer. Raman spectra were obtained on a Nihon Bunko NR-1800 spectrometer. Electron spin resonance spectra were recorded at room temperature on a JEOL JM-EFIX spectrometer at 9.3 GHz (X-band). U V visible diffuse reflectance spectra were recorded on a Hitachi 340 spectrometer. Reactions. The oxidation of hexane and the epoxidation of 1 -hexene and 2-hexene were performed in a flask with 30% aqueous H 0 . The products were analyzed on a Shimadzu G C 14 gas chromatograph equipped with a Nukol capillary column for alkane oxidation and an OV-17 capillary column for alkene epoxidation. 2

2

Spin trapping. The spin traps 2-methyl-2-nitrosopropane (MNP), phenyl-N-tertbutyl nitrone (PBN) and a-4-pyridyl 1-oxide N-tert-butyl nitrone (POBN) were used as purchased. To detect radicals in the presence of hexane, 0.3 ml of 30% aqueous H 0 was added to 1.25 ml of hexane in 4.0 ml of acetonitrile. To the resultant mixture, 4 mg of MNP and 16 mg of catalyst was added and the mixture was shaked for 30 seconds. To detect radicals in the absence of hexane, 20 mg of catalyst was added to the mixture of 1.0 ml of 30% aqueous H 0 and 10 mg of PBN or POBN was added and the resultant mixture was shaked for 30 seconds. A n aliquot of the supernant was transferred to an ESR cell and the ESR spectrum recorded. Highly dispersed M n on MgO was used as a marker. 2

2

2

2

2+

Results and discussion Structure of TS-2 and VS-2. X R D data show that TS-2 and VS-2 samples are highly crystalline and their patterns closely matched with those reported for M E L structures (8). However, T E M observation suggests that these materials are agglomerates of very small crystallites of MFI structure, not the M E L structure expected from the template (10). The absence of (110) reflection in their X R D pattern is also indicative of the MFI structure. The apparently singlet peak at 2 6 = 45° would be due to the coalescence resulting from line broadening of the (0 10 0), (0 8 4), (10 0 0) and (8 0 4) reflections. In the ESR spectrum of the as-synthesized VS-2, the anisotropic hyperfine splitting (8-fold) caused by V nucleus is very well resolved without the presence of appreciable superimposed broad singlet, indicating the atomic dispersion and immobility of the V species. The g-values (g „ =1.935, g = 1.994) and hyperfine 5 1

4 +

A

In Heterogeneous Hydrocarbon Oxidation; Warren, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

376

HETEROGENEOUS HYDROCARBON OXIDATION

4+

coupling constants (A,, = 188 G, A = 72 G) are typical of V = 0 complexes with square pyramidal coordination (11). The high dispersion is preserved after calcination at 763 K and subsequent photoreduction with H at 77 K (12). The IR spectrum of calcined VS-2 exhibits a medium intensity band at 965 cm ~\ although it shows no band around this wavenumber as synthesized. The Raman spectrum of VS-2 also shows an intense peak at 965 cm" . Photoluminescene studies indicates the presence of tetrahedrally coordinated V-oxide species; a vibrational fine structure owing to the vanadyl band was clearly observed (12). The energy gap between the (0 -> 0) and (0 -> 1) transition bands of the V=0 vibration was found to be ca. 965 cm" in good agreement with the IR and Raman V=0 stretching bands. This energy gap is slightly different from that of the vanadium oxide species highly dispersed on Vycor glass or silica (1035 cm" ), suggesting the presence of some electronic perturbation owing to the neighboring O H group in the zeolite lattice. Although there is an IR band at 960-970 cm' in the TS-1 and TS-2 samples, no evidence for the Ti=0 structure is obtained from their phosphorescence spectra. This IR band is assigned to Si-O" defect or Si-OH associated with the incorporation of Ti into the zeolite framework (13-15). ±

2

1

1


1

1

Oxidation Reaction. The catalytic activities of these materials are shown in Table I. In the oxidation of hexane, VS-2 gives appreciable terminal oxidation products as well as products formed by the oxidation of internal secondary carbons, in contrast with TS-2, which gives only internal oxidation products. These results are in agreement with the previously reported ones (8,9). Since the relative rate of 2hexene/l-hexene epoxidation is much lower on VS-2 than on TS-2, it is shown that VS-2 generates active oxygen species of less electrophilic character. This finding and formation of allylic oxidation products on VS-2 suggest radical character of the active oxygen species.

Table I. Oxidation of Hexane and Hexenes with Aqueous H 0 Catalyst Reactant Turnovers Terminal Oxidation Allylic Oxidation Turnover Rate (mol/mol-Ti or V) (%) (%} (x 1 0 s ) _ TS-2 2.8 Hexane 20 0 TS-2 1.9 1-Hexene 14 0 TS-2 10.3 2-Hexene 74 0 2

2

3

VS-2 VS-2 VS-2

Hexane 1-Hexene 2-Hexene

4.4 15 20

32

0.61 2.1 2.8

_

-

1

51 31

TS-2 (Si/Ti = 85) 50 mg, H 0 (30% aq.) 2.5 ml, substrate 2.5 ml, 333 K, 2 h. VS-2 (Si/V = 58) 50 mg, H O,T(30% aq.) 1.0 ml, substrate 3.9 ml, acetonitrile 12.5 ml, 333 K, 2 h. 2

0

o

Spin trapping is a valuable tool for the study of free radical processes. Two kind of spin traps have been developed, nitrone and nitroso compounds. Nitroso compounds, such as MNP, can provide considerably more information than nitrones as the radical to be trapped adds directly to the nitroso nitrogen. When MNP is added to the hexane oxidation system catalyzed by VS-2, we have obtained an ESR spectrum consisting of a triplet (1:2:1) of triplets (Figure 1). This signal is identified as originating from the MNP-CH (CH ) CH . This clearly shows that the C H ( C H ) C H • radical is generated' in the VS-2-H 0 -hexane 2

3

2

4

2

2

4

3

2

2


28. TATSUMI ET AL.


system. If secondary hexyl radical such as CH (CH ) CH(CH )» had been trapped instead of CH^CH^CFL,*, the ESR signal would have been doublet of triplets. In spite of the predominant oxidation of secondary carbon atoms of hexane in the case of VS-2, no MNP-sec-hexyl radical adduct is observed. There are two explanations for this finding. One explanation is that, owing to the transition state shape selectivity in the restricted space in the zeolite channel, the interaction of sec-hexyl radical with MNP may be hindered. The other explanation is that only the terminal oxidation proceeds by a homolytic mechanisM.; a distinct homolytic mechanism involving free radicals operates for this terminal oxidation whereas a kind of heterolytic mechanism seems to operate in the oxidation of secondary carbon atoms. In line with this hypothesis, as shown in Figure 2, the ESR signal assignable to MNP-radical adduct was hardly observed with the TS-2-H 0 -hexane system which gives only the products resulting from the oxidation of secondary carbons. 3


377

2

3

3

2

2

Postulated Reaction Mechanism. While oxygen-centered radical adducts of MNP are quite unstable, nitrones can be used for the study of oxygen-centered radicals. To detect the reactive species generated directly from H 0 in the presence of VS-2, PBN and POBN were employed. The ESR spectrum of the PBN adduct is shown in Figure 3. Its coupling constants, A = 14.4 G and A = 2.63 G, suggest that the trapped radical is «OOH (16). For the PBN-OH radical adduct in aqueous solution, A larger than 15.2 G is expected (16). This means that the VS-2-H 0 system is clearly different from the Fenton system, in which the reactive species is revealed to be the hydroxy radical by using PBN (17) and POBN (18). The coupling constants of the POBN adduct, A = 13.8 G and A = 1.66 G, also support this conclusion. The POBN- OH radical adduct should have A = ca. 15 G (16). Similar spectra are obtained with the reaction of H 0 and TS-2 in the presence of the spin trapping agents (Figure 4). 2

N

2

H

N

2

N

2

H

N

2

2

N

O

HOO

0

^

•

Thus, it has been revealed that the «OOH radical is generated and is the major product both in the VS-2-H 0 system and in the TS-2-H 0 system. Since -OOH radical is relatively stable, it is unlikely that the »OOH radical abstracts the H» radical from an alkane. Therefore it is not unreasonable that no alkyl radical is detected in the TS-2-H 0 -hexane system. Considering that VS-2 has vanadyl groups, the following reaction scheme is proposed. On addition of aqueous H p ^ 2

2

2

2

2

2


378



Figure 2. ESR spectrum of spin adduct of MNP in the hexane oxidation by aqueous H 0 catalyzed by TS-2. 2

2



TATSUMI ET AL.


10 G

379

A = 1 4 . 4 G A =2.63 G N

H

Figure 3. ESR spectrum of spin adduct of PBN in the reaction of VS-2 with aqueous H 0 . 2

2

f\

ft

A

n

rh

Figure 4. ESR spectrum of spin adduct of PBN in the reaction of TS-2 with aqueous H 0 . 2

2




380

Figure 5. ESR spectrum of VS-2 contacted with aqueous H 0 2

2


28. TATSUMI ET AL.

381

Alkane Oxidation on Vanadium Silicalite 1

the IR band at 965 cm" assignable to the vanadyl group disappears, while it is virtually unchanged on addition of water. Although the calcined VS-2 is ESRsilent, the typical spectrum of V reappears when VS-2 is contacted with H 0 . The ESR spectra of the VS-2 which has been calcined, treated with H 0 and evacuated at increasing temperatures are shown in Figure 5. The spectra are only partly anisotropic at room temperature. Evacuation of the H 0 -adsorbing sample at temperatures higher than 343 K reduces the ESR signal intensity, indicating reoxidation of V to V . 4+

2

2

2


4+

2

5+

O H

II .-0 -Si

2

2

/

sr

HOO

HoO:

2^2

OH

N

/

/

- Si /

\

-Si

/

Si' N

•OOH

OH

-Si -Si

/ \

4+

As shown below, we propose that the formation of V - 0 0 • concomitant with the release of water might be responsible for the oxidation of primary carbons of alkanes. However, we could not detect such species, probably because they are too short-lived.

HOO.

OH -HoO

H 0 2

o

0

o

o

RH

2

o -ROH

o^l^o

•00

HOO + R-

o

/

x

o


382


This scheme is similar to the one proposed by Hari Prasada Rao et al. (19). Since only the primary hexyl radical is observed, only the oxidation of terminal carbons may be envisaged to occur via this pathway; the oxidation of internal carbons might proceed by a different mechanism, which may be common to both VS-2 and TS-2 systems. Since titanyl groups are absent on TS-2 and most Ti is considered to be surrounded by three SiO and one OH groups (14,15) we can depict the following scheme. Although the redox potential of the T i / T i couple suggests that T i is hardly reducible, it can be expected that the highly sensitive ESR measurements enabled us to detect the • OOH radical trapped by the nitrones. Downloaded by STANFORD UNIV GREEN LIBR on October 8, 2012 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0638.ch028

4+

HO V i

o

HOO

p 4

+

\ >

\ i - H

2

O

3+

-OOH O

O 4

o

'

4+

+

^

\>

- i f o

/

x

o

It is conceivable that oxidation of internal carbons of alkanes on both TS-2 and VS-2 proceeds via a mechanism involving a metal coordinated OOH group (5, 20) while the electrophilicity of this group should be influenced by the difference in the electron-withdrawing nature between T i and V . 4+

4 +

Conclusions A comparison of oxidation reactions on VS-2 and TS-2 shows that the active oxygen species on VS-2 have more radical character than those on TS-2. Spin trapping experiments have revealed that the primary hexyl radical is generated in the VS-2H 0 -hexane system, in contrast with the TS-2 system where no alkyl radical is observed. The formation of hydroperoxy radicals is observed both for TS-2 and VS-2. It is proposed that the oxidation of internal carbons of alkanes on VS-2 and TS-2 proceeds by a mechanism involving a metal coordinated OOH group and that V - 0 0 • is the active species for oxidation of terminal carbons of alkanes on VS-2. 2

2

4 +

Acknowledgment We thank Prof. M . Anpo for measuring photoluminescence spectra of TS-2 and VS-2. Literature Cited 1. Taramasso, M.; Perego, G.; Notari, B., U.S. Patent 4,410,501 (1983). 2. Reddy, J.S.; Kumar, R.; Ratnasamy, P., Appl.Catal.,1990, 58, L1. 3. Tatsumi, T.; Nakamura, M.; Negishi, S.; Tominaga, H., J. Chem. Soc. Chem. Commun., 1990, 476. 4. Huybrechts, D.R.C.; DeBryuker, L.; Jacobs, P.A., Nature, 1990, 345, 240. 5. Clerici, M.G., Appl.Catal.,1991, 68, 249. 6. Rigutto, M.S.; Van Bekkum, H., Appl.Catal.,1991, 58, L1. 7. Centi, G.; Perathoner, S., Trifilo, F., Aboukais, A.; Aissi, C.F.; Guelton, M., J. Phys. Chem., 1993, 96, 123. 8. Hari Prasada Rao, P.R.; Ramaswamy, A.V.; Ratnasamy, P., J. Catal., 1992, 137, 225. 9. Ramaswamy, A.V.; Sivasanker, S.; Ratnasamy, P., Microporous Mater., 1994, 2, 451.



28. TATSUMI ET AL. Alkane Oxidation on Vanadium Silicalite

383

10. Tatsumi, T., Koyano, K.A.,Terasaki, O., unpublished data. 11. Kucherov, A.B.; Slinkin, A.A. ; Zeolites, 1987, 7, 583. 12. Anpo, M.; Zang, S.; Yamashita, H.; Hirasawa, Y.; Tatsumi, T., Preprints of the 7th Japan-China-USA Symposium on Catalysis, 1992, p.55. 13. Camblor, M.A.; Corma, A.; Perez-Pariente, J., J. Chem. Soc. Chem. Commun., 1993, 557. 14. Deo, G.; Turek, A.M.; Wachs, I.E.; Huybrechts, D.R.C.; Jacobs, P.A., Zeolites, 1993, 13, 365. 15. Khouw, C.B.; Davis, M.E., J.Catal.,1995, 151, 77. 16. Buettner, G.R., Free Radical Biology & Medicine, 1987, 3, 259. 17. Schaich, K.M.; Borg, D.C., In Autoxidation in Food and Biological Systems, M.G. Simic, M. Karel, Ed., Plenum Press, New York, NY, 1980, pp 45-70. 18. Lain, C.S.; Piette, L.H., Biochem. Biophys. Res. Commun., 1977, 78, 51. 19. Hari Prasada Rao, P.R.; Ramaswamy, A.V.; Ratnasamy. P., J. Catal., 1993, 141, 604. 20. Khouw, C.B.; Dartt, C.B.; Labinger, J.A.; Davis, M.E., J. Catal., 1994, 149, 195.


Heterogeneous Hydrocarbon Oxidation - American Chemical Society

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