~ APPLIED CATALYSIS A: GENERAL

ELSEVIER Applied Catalysis A: General 122 (1995) 85 97

Vanadium silicate xerogels in hydrogen peroxide catalyzed oxidations

Ronny Neumann *, Michal Levin-Elad Casali Institute of Applied Chemisto', Graduate School of Applied Science, The Hebrew University c?["

Jerusalem, 91904 Jerusalem, Israel

Received 1 March 1994: revised 22 September 1994: accepted 23 September 1994

Abstract

Vanadium silicate xerogels (V205-SiO2) were prepared by the sol-gel method by hydrolysis of vanadium and silicon alkoxides. The use of these xerogels as catalysts for oxidation of alkenes, alcohols and phenol was studied using 30% aqueous hydrogen peroxide as oxidant. It was found that the manner of xerogel preparation strongly influenced the catalytic activity of VzOs-SiO 2. Alcohols were the preferred solvents for the reaction and did not leach vanadium oxide into solution. For alkenes, epoxidation was the dominant oxidation reaction the yield and selectivity depending on the nucleophilicity and oxidizability of the substrate. For alkenes of intermediate nucleophilicity allylic oxidation was significant. Secondary alcohols were oxidized in low to fair yields whereas primary alcohols were inert. Phenol was oxidized selectively to a 2:1 mixture of hydroquinone/catechol. UV- vis and electron spin resonance spectra of V205-SIO2 treated with hydrogen peroxide conclusively showed formation of vanadium-peroxo species with polarization of the O-O bond as seen by formation of vanadium(IV) electron spin resonance spectra leading to oxidation by both heterolytic and hom- olytic cleavage.

Keywords: Alkene epoxidation: Catalyst preparation ( sol-gel ); Phenol oxidation: Vanadium/silicate

1. I n t r o d u c t i o n

The d e v e l o p m e n t for e c o l o g i c a l l y f r i end ly t e c hno log i e s is ce r t a in ly one o f the

m a j o r p resen t goa l s o f r e sea rch in chemis t ry . This is e spe c i a l l y t rue in the f ield o f

o x i d a t i o n o f o rgan ic c o m p o u n d s w h e r e there is an urgent need to r ep l ace h igh ly

e f fec t ive but was te fu l and toxic s t o i ch iome t r i c ox idan t s wi th a more p r e f e r r ed

t e c h n o l o g y b a s e d on ca t a ly t i c o x y g e n t ransfe r us ing ' c l e a n ' o x y g e n donors such as

h y d r o g e n pe rox ide . One r ecen t ly d e v e l o p e d ca ta ly t i c o x y g e n t ransfe r p roces s is

* Corresponding author. Fax. (+972-2) 663878.

0926-860X/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDIO926-860X (94)00206- I

86 R. Neumann, M. Levin-Elad / Applied Catalysis A: General 122 (1995) 85-97

based on the use of titanium substituted silicalite zeolites [ 1,2 ] as catalysts for the selective oxidation of organic substrates such as phenol [3,4], alkenes [5-7], and alkanes [8-10] with 30% hydrogen peroxide. Originally, the research centered on the titanium substituted ZSM-5 zeolite, TS-1, where it has been shown quite con- clusively that a titanium atom is incorporated in a silicon site within the MFI structure. This framework substitution and the high dispersion of the titanium within the zeolite are believed to be the major reasons for the unique activity of the TS- 1 catalyst which include fairly high activity, minimal non-productive hydrogen per- oxide decomposition and high catalyst stability [ 11,12 ]. An important very recent development is the standardization of the TS-1 catalyst so that differently prepared catalysts can now be compared [ 13 ]. Other research in this field includes synthesis and application of different titanium substituted zeolites based on ZSM-11 (TS-2) [ 14,15], ZSM-48 [ 16] and/3-silicalite [ 17] structures and use of analogous vana- dium substituted silicalites [18-22], VS-1 and VS-2 in similar oxygen transfer reactions.

Despite the obvious attraction of the titanium silicalite zeolites as catalysts for hydrogen peroxide activation their somewhat complicated synthesis and small pore size will perhaps limit their use as a general tool for oxidation in organic chemistry. We have shown in a preliminary communication [23] that amorphous metallosil- icate xerogels prepared by the simple sol-gel method may be an interesting alter- native for catalytic liquid-phase heterogeneous oxygen transfer with hydrogen peroxide as oxygen donor. In the sol-gel method well-dispersed and 'molecularly mixed' metallosilicates, a generally required requisite for effective catalytic activity [ 11,12 ], may be prepared by using metal alkoxides as precursors [ 24 ]. Evaporation of the solvent yields a xerogel with dispersed metal oxides in a silica framework. In this paper we present our results on the use of such a V 2 O s - S i O 2 xerogel as catalyst in the oxidation of various organic substrates with 30% aqueous hydrogen peroxide as oxygen donor.

2. Experimental

2.1. Catalyst preparation

The catalyst preparation [ 25 ] is based on a two-step procedure where in the first stage silicon tetraethoxide is partially hydrolyzed and/or condensed with two equiv- alents of water under acidic or basic conditions. In the second stage, the vanadyl triisopropoxide (5 mol-%) is added and hydrolysis and condensation completed to form the required gel with another two equivalents of water. Finally, the solvent is slowly evaporated yielding the desired xerogel. In this manner, five different V205-SIO2 xerogels were prepared according to the following specific procedures. V2Os-A was prepared by mixing 12.5 g (60 mmol) silicon tetraethoxide (TEOS; Fluka >98%) with 2.15 ml (120 mmol) of aqueous 0.15 M HC1 in 20 ml of

R. Neumann, M. Levin-Elad /Applied Catalysis A: General 122 (1995) 85-97 87

ethanol at 60°C for 1.5 h. The solution was then cooled to room temperature and 0.74 g (3 mmol) of vanadyl triisopropoxide (VTIP; Strem) was added followed by addition of 2.15 ml (120 mmol) distilled water. The orange-red solution was then left in an open 50 ml beaker for two days during which the ethanol evaporated yielding a green xerogel which was then ground and dried at 100°C for 12 h. The V2Os-B and the VzOs-P xerogels were prepared in an analogous manner to the V2Os-A xerogel. The only difference was that in the first stage 2.15 ml of 0.15 M NH4OH and 2.15 ml 30% H202 (Merck) were added instead of 0.15 M HC1 for V2Os-B and V2Os-P, respectively. The V2Os-AP and VzOs-BP xerogels were pre- pared by adding 2.15 ml of 0.15 M HCI and 0.15 M NH4OH, respectively, in the first stage and 2.15 ml 30% H,O, instead of distilled water in the second part of the synthesis. A sixth catalyst, V205-I was prepared by attaching vanadium pentaoxide to course silica gel. Thus, 1.1 g vanadyl trichloride (Strem) was dissolved in 25 ml of dry benzene and 6.6 g of silica gel (BDH) was added. The mixture was stirred for 1 h at 60°C and the benzene evaporated under vacuum. The compound was at first heterogeneous in color (orange to green), but after further drying under vacuum at room temperature overnight turned dark green. The impregnated silica was then further dried at 100°C for 12 h.

2.2. Catalyst characterization

Infrared spectra of the V2Os-A xerogel were taken as KBr pellets on a Nicolet 510M Fourier transform IR (FT-IR) spectrometer. UV-vis spectra were taken in the reflectance mode of a Hitachi U-2000 spectrophotometer equipped with an integrating sphere. Electron spin resonance (ESR) spectra were taken on a Varian E-12 instrument at both room temperature and 150 K. Spectra were taken at a frequency of 9.082 GHz, microwave power 10 roW, modulation frequency 100 kHz, modulation amplitude 0.5 G and time constant 0.1 s. Spectra of hydrogen peroxide treated samples were taken after 0.5 g V2Os-A xerogel was mixed for 5 min with 10 m130% H202, the sample filtered and dried at room temperature under vacuum for 30 rain.

2.3. Catalytic reactions

Catalytic reactions were run in 5 ml sealed vials and stirred with a magnetic stirrer. Typically, 5 mg of catalyst, 1 mmol substrate and 0.5 ml solvent were stirred and the reaction temperature equilibrated in an oil bath. The 30% hydrogen peroxide was then added to initiate the reaction. Samples were taken and analyzed using gas chromatography. Analysis was on a 15 m long, 0.25 mm ID capillary silica column bonded with 0.25 kcm coating of methyl silicone, RTX-100 (Restex). Where standards were available and all peaks identified, analyses were preformed on a HP 5980 (Hewlett Packard) instrument equipped with a flame ionization detector. In other cases, the identity of products was deduced from mass spectra obtained on a

88 R. Neumann, M. Levm-Elad / Applied Catalvsi.r A." General 122 (1995) 85-97

HP 5790A chromatograph (Hewlett Packard) equipped with a mass selective detector.

3. Results and discussion

The first part of the research was to evaluate the activity of the vanadium silicate xerogels as catalysts in reactions using hydrogen peroxide as oxygen donor. The epoxidation of cyclooctene was chosen as the probe reaction. Various different preparations of 2.5 mol-% V2Os in a silicate xerogel matrix were tested, Fig. 1. When the first stage of the xerogel preparation is performed under acidic conditions in the presence of 0.15 M HC1 (V2Os-A) or 30% H202 (V2Os-P) followed by addition of water in the second step one obtains active catalysts with similar activity. On the other hand, xerogels prepared under basic conditions in the first stage in the presence of 0.15 M NHgOH (VzO5-B) and/or by the addition of 30% H202 in the second step yielded inactive catalysts (VzOs-AP, V2Os-BP). The V2Os-SiO 2 com- pound prepared by impregnating or depositing V205 on an silica support (V205- I) also appears to yield an active catalyst. However, in this case the vanadium oxide is immediately leached (see below) from the silica support and the reaction is in fact catalyzed by a soluble vanadium species. The difference between acid and base catalyzed gel formation is probably the reason for the different activity of the xerogels. In acid catalyzed preparations, hydrolysis of the alkoxide precursors is faster than condensation and gels with a 'good molecular mix' or highly dispersed V205 in SiO2 is obtained [24]. In base catalyzed gel preparations, condensation is faster than hydrolysis and thus segments of V205 are formed within SiO2. It would seem apparent, therefore that singular V205 sites dispersed in SiO2 are more effec-

o--t - - 30- O

E E o 20- 2 > r ' -

o 10- o

[ ] Conversion (7 hr) 33.9

26.9 [ ] Co~ , , e~o~ (24 h~ ~ . tiiiiiiiii!iiiiii ................. ' zb.7 iii!iii!iiiii!i~iiiiiiii

0 - ' . . . . . . ' . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . " . . . . . .

V205-A V205-B V205-P V205-AP V205-BP V205-1

C a t a l y s t

Fig. 1. Catalytic activity of the V2Os-SiO 2 metallosilicate xerogel as a function of the method of preparation. Reaction conditions: ! mmol cyclooctene, 2 mmol of 30% H202, 5 mg of the V205-SIO2 xerogel in 0.5 ml t- butanol at 60°C. Conversions were measured by GLC analysis, cyclooctene oxide was the only product. The preparation and nomenclature of the V205-SIO2 metallosilicate xerogel is given in the experimental section. In the case of V2Os-I the V205 was leached into the solvent and the reaction took place in the organic liquid phase.

R. Neumann, M. Levin-Elad / Applied Catalysis A: General 122 (1995) 85-97 89

o~ 0 E t- O

g 0

60-

40-

20-

O"

59

21 1 22.7

CH3OH C2HSOH i C3H7OH I-C4H9OH CH3CN

14.2

02r ~ 0,2r HCON(CH3)2 CH3CQCH3 CICH2CH2CI

S o l v e n t

Fig. 2. Catalytic activity of the V2Os-A xerogel as a function of the solvent. Reaction conditions: l mmol cyclooctene, 2 mmol of 30% H202, 5 mg of V2Os-A in 0.5 ml solvent at 60°C. Conversions were measured after 24 h by GLC analysis, cyclooctene oxide was the only product.

tive than polymeric V205 (commercial V205 is an ineffective catalyst). The det- rimental effect of hydrogen peroxide in the second stage of gel formation is as yet without a firm explanation. However, one may surmise that formation of vanadium peroxo intermediates are detrimental to formation of catalytically active xerogels. The V2Os-A xerogel was used exclusively in further studies.

We next ascertained the effect of the reaction solvent on the activity of the catalyst, Fig. 2. Alcohols (except methanol) are similarly effective solvents, with acetone slightly less effective. Almost no activity was found in dimethylformamide or 1,2-dichloroethane. Acetonitrile appears at first glance to be the best solvent. However, when testing the stability of the xerogel or more specifically the propen- sity for formation of soluble vanadium oxides by complexation to the solvent, we found that in acetonitrile significant leaching could be observed. To test for leaching, for each solvent a reaction mixture without cyclooctene was stirred and heated at 60°C for 2 h and then filtered. Cyclooctene was added to the filtrate and the mixture heated overnight. Only in the case of acetonitrile did the liquid phase contain active catalytic species and a yield of 45% cyclooctene oxide was obtained. It is apparent, therefore, that the acetonitrile is sufficiently coordinating and the vandium-oxygen- silicon bonds are sufficiently labile for formation of soluble vanadium oxides in acetonitrile. Alcohols are relatively non-coordinating solvents, thus no leaching occurs when using them as solvents.

Concentrating on the results presented so far for the V2Os-A xerogel one notes that conversions are appreciable but still far from quantitative although reactions were preformed using a two-fold excess of hydrogen peroxide. In fact yields given in all cases at 24 h are maximum yields, with all the remaining hydrogen peroxide being decomposed. It was believed that addition of more 30% hydrogen peroxide, Fig. 3, or alternatively more catalyst, Fig. 4, would bring about higher conversions. For t-butanol as solvent, a two- to four-fold excess of hydrogen peroxide gave the highest conversion of cyclooctene to cyclooctene oxide. Greater excess reduced the conversion by about 50%. With methanol as solvent this trend is more obvious

90 R. Neumann, M. Levin-Elad/Applied Catalysis' A: General 122 (1995) 85 97

O E

E o

0 >

0 0

40 -4 38.5

23.2

20-

O-

34.6 31.2

22.6

[ ] t-butanol

[ ] methanol

24.4

1 2 4 6 10

Amount of 30% H202, mmol

Fig. 3. Catalytic activity of the V2Os-A xerogel as a function of the amount of hydrogen peroxide. Reaction conditions: 1 mmol cyclooctene, 1 I 0 mmol of 30% H202, 5 mg of VzOs-A in 0.5 ml t-butanol at 70°C. Conversions were measured after 24 h by GLC analysis, cyclooctene oxide was the only product.

where a large excess of 30% hydrogen peroxide reduced yields very significantly. These results can be explained by the negative effect that the addition of water added together with hydrogen peroxide has on the reaction. The water and oxidant compete for complexation to the vandium center, therefore excess water relative to hydrogen peroxide inhibits the reaction. Inhibition could alternatively be because of the excessively hydrophilic nature of the xerogel pores preventing the diffusion of the hydrophobic substrate to the reaction site. This effect was verified by addition of water (10% H202) to a reaction with two equivalents hydrogen peroxide per equivalent of cyclooctene. The effect is more appreciable in methanol as solvent for the addition of a larger excess of 30% hydrogen peroxide causes in addition phase separation (methanol/water/hydrogen peroxide and substrate). The best method to obtain high conversions would be to use more concentrated hydrogen peroxide, not commonly available in the laboratory because of safety problems

50

40 - 36.6 o

E 30 l[i[iiiiiiiiiiiii i 28.2 0 : : : : : : : : : : :5 '~ ii!ii!:!:!i!i!i!i 22.6 ~, 20 ilililiiiiiiiiiii .... ~ g i~iiiiiiii!iiiii . . . . . . . .

tO 10 - ii!i!iii!iiiii!il , . , . ...,. . , , , y , . . . , .

: : : : % : : : : : : : : • . . . . . . . , . . . . . , . . . . , . . . . . . . . . . .

O . . . . i . . . . i

5 10 20 50

Amount of Catalyst, mg

Fig. 4. Catalytic activity of the V2Os-A xerogel as a function of the amount of V2Os-A xerogel. Reaction conditions: 1 mmol cyclooctene, 2 mmol of 30% H 2 O 2 , 5-50 mg of V2Os-A in 0.5 ml t-butanol at 60°C. Conversions were measured after 24 h by GLC analysis, cyclooctene oxide was the only product.

R. Neumann, M. Levin-Elad / Applied Catalysis A." General 122 (1995) 85 97

50

4O- 36 .6 0 : . : : : . : . : , :+ : , :

. . , . . . . . . . .

o 2 2 7 i:i i: ::: ::i

~. 20- iiii::;:: :Z . . . . ' - " 5 : : : : : : : : : . : : o :i:?i: :?! i 0 1 O- iiiiiiii !iii[:

0 lii!iii=:;: i, ii 0 2~5 40 50 80

91

T e m p e r a t u r e , ° C

Fig. 5. Catalytic activity of the V2Os-A xerogel as a function of the temperature. Reaction conditions: 1 mmol cyclooctene, 2 mmol of 30% H20> 5 mg of V2Os-A in 0.5 ml t-butanol at various temperatures. Conversions were measured after 24 h by GLC analysis, cyclooctene oxide was the only product.

associated with use of concentrated, > 20%, hydrogen peroxide in organic phases. Addition of different amounts of catalyst to the reaction mixture had an irregular effect. Apparently, the rates of non-productive hydrogen peroxide decomposition and epoxidation are similar. Therefore, there is no advantage in addition of addi- tional catalyst as concerns the conversion to cyclooctene oxide. Another way to improve yields is to use higher reaction temperatures, Fig. 5. The fact that the reaction efficiency (cyclooctene oxide formed/hydrogen peroxide consumed) increases with increased temperatures suggests that the activation energy for hydro- gen peroxide decomposition is lower than the activation energy for the cyclooctene epoxidation.

After gaining a basic understanding of the parameters affecting the epoxidation of cyclooctene, the scope of the V2Os-A xerogel as catalyst for the oxidation of other alkenes, Fig. 6, and the oxidation of alcohols and phenol, Fig. 7, was inves- tigated. In reactions with hydrogen peroxide, basically two modes of oxygen trans- fer can be identified [26,27]. Heterolytic cleavage is common for electron-poor d o complexes of Ti, V, Mo, W and Re. In the intermediate peroxo complex, the O-O bond is polarized leading to a transfer of an electrophilic oxygen to a nucleophilic substrate as in the epoxidation of alkenes. Heterolytic cleavage may also be observed with electron-rich transition complexes of metals such as Rh, Pd or Pt, where inverse polarization leads to transfer of a nucleophilic oxygen. Homolytic cleavage is often more dominant and of importance with all metals, especially those having high oxidation potentials. Cleavage of the O-O bond in the peroxo inter- mediate leads to formation of hydroperoxy and hydroxy radicals and in the case of alkenes predominantly to allylic oxidation. The final result for the oxidation of alkenes is dependent on the Lewis acidity and oxidation potential of the catalyst and the nucleophilicity and susceptibility to autoxidation (oxidizability) of the alkene [ 26,27 ]. From Fig. 6, one can see that for substrates with low nucleophilicity and oxidizability such as 1-octene, cyclododecene, and 2-methyl-1-heptene there is low conversion to the epoxide only. For substrates with high nucleophilicity such

92 R. Neumann, M. Levin Elad/Applied Catalysis A: General 122 (1995) 85-97

Substrate

©

@

• ] cyclohexene oxide

cyclohexene-2 ol

"ii!iiiii!ii!iiiiiiiiiiiiiiiiiiiiiiiii~ cyclohexene-2-one

~.iiiii!!!i!i!iiiii!i!i!i!i!i!iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii~ benzaldehyde

::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 2,3-dimethyl-2-butene oxide

;i!!!iii!:!iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii!i!:.!:!iii!i!i!iiiii!i!:i !ii:.!i!iii!i!iii!iiiiiiiiiiiiiiiii~ pinaoo~

J 2-met hyl-2-heptene-4-one

iiiiiiii!i!i!!!i!!i!!!!!!!!ii~ 2-methyl-2,3-heptanediol

2-methyl-2-heptene oxide

2-methyl-1 -heptene oxide

2,3 oclanediol

2-octene-4-one

2-oc ene oxide

1 -octene oxide"

cyclododecene oxide

ii:.iiii:.i:.:-!:.!::!::!:-!!i::!!!!i!::!i!i!!iii!i!iiiii!iii!iii!i!i!i!i!i!i~ cyclooctene oxide i

20 4~0 6'0

Conversion, m o l %

Fig. 6. Catalytic activity of the V2Os-A xerogel in the oxidation of alkenes. Reaction conditions: l mmol substrate, 2 mmol of 30% H202, 5 mg of V2Os-A in 0.5 ml t-butanol at 80°C. The conversion to the different products is given a mol product/mol substrate.

as 2,3-dimethyl-2-butene the epoxidation is predominant with high yield including formation of diol by hydrolysis. Carbon-carbon bond cleavage only is observed for styrene with benzaldehyde as sole product. Substrates of intermediate nucleo- philicity, 2-methyl-2-heptene and trans-2-octene yield a mixture of products with epoxidation more favored over oxidation at the allylic position. Cyclohexene, known to be very sensitive to allylic oxidation, yields almost no epoxide. In the oxidation of alcohols the reactivity is as would be commonly expected with the following order: secondary benzylic = secondary cyclic > primary benzylic > secondary acyclic > primary acyclic. Surprisingly, an allylic alcohol such as 1-

R. Neumann, M. Levin-Elad /Applied Catalysis A: General 122 (1995) 85-97 93

Substrate

Ho..

HOA O

OH

OH

benzoquinone

iTililiTililililililililililililili i~ calechol i!i!i!i!i!i!i!!!!!!!i!!!ii!ii!iiiiiiiiii ! !iii!iii!i!i!i!i!i!i!i!i!i!iii ~ hydroquinone

i~!~!i!i!i!!!i!!!!!!!!!i!i!i!i!iiiiii!!iiiiiiiiiiiiiiiiiiiiiiiii!i!iiiiii!i!!!!! ~ benzaldehyde

2-heptanone

1 heptanoic acid

!i!i!i!i!i!i!i!i!i!i!i!!!i!i!!!i!i!!!!!i!!!i!!i!i!i!!!!!!!i!ii!iiiiiii!!i!!i!i!i!i!i!i!i!i!~ cyclohexanone

i!i!i!i!i!i!i ii iiiiiiiiiiiii:i:i:i:i~ benzaldehyde

ii~i~i~i~i~ii~i~ii~i~i~i~i~i~i~i~i~iiiii!i~i:iii~iii!i!iiiiiii!i!i~ acetophenone

1 ~0 2'0 3 O

Conversion, mo[%

Fig. 7. Catalytic activity of the V2Os-A xerogel in the oxidation of alcohols and phenol. Reaction conditions: 1 mmol substrate, 2 mmol of 30% H202, 5 mg of V20~-A in 0.5 ml t-butanol at 80°C. The conversion to the different products is given a tool product/mol substrate.

octene-3-ol was completely inert. An interesting result was found in the oxidation of 2-phenylethanol where benzaldehyde was the only observed product. This result is especially curious since neither ethylbenzene nor phenyl-l,2-ethanediol were oxidized in this system, ruling out hydroxylation at the benzylic position and formation of the diol as the reactive intermediate in carbon-carbon bond cleavage. An interesting result was also observed for the hydroxylation of phenol; hydroqui- none and catechol were formed in a 2:1 ratio with practically no formation of benzoquinone or tars. This selectivity is rather desirable although the yield is relatively low compared to the published for TS- 1.

In order to gain more information on the properties of the vanadium silicate xerogels, some physicochemical measurements were made. The UV-vis diffuse reflectance spectra of the V2Os-A xerogel and the V2Os-A xerogel after treatment with 30% hydrogen peroxide are illustrated in Fig. 8. The VzOs-A xerogel is green, indicated by the very broad absorption at 650-1000 nm. This color and absorbance is due to the presence of reduced vanadium(IV) species, most probably VO z+,

94 R. Neumann, M. Levin-Elad/Applied Catalysis A: General 122(1995)&5 97

o x ~ <

\

L

400 600 B00 1000

Wavelength, nm

Fig. 8. UV-vis diffuse reflectance spectra of the V2Os-A xerogel. (a) the V2Of-A xerogel (green) ; (b) the VzOs- A xerogel after treatment with hydrogen peroxide (orange).

present in the xerogel in an unknown amount (see also EPR spectra below). Upon addition of hydrogen peroxide the xerogel immediately turns orange-red, typical for the formation of vanadium-peroxo species, with significant increase in absorp- tion in the 300-400 nm region. The IR spectra of the V2Os-A xerogel with and without treatment by hydrogen peroxide are shown in Fig. 9. The peak at 941 cm- attributable to the V-O-Si bond vibration [ 11,12] is unaffected by the presence of

) C 03

I--"

" I " , • I I

1200.0 1000.0 800.0 600.0

Wavenumber, c m -1

Fig. 9. IR spectra of the V2Os-A xerogel. ( a ) The V2Os-A xerogel ( green ) ; (b) the V 2Os-A xerogel after treatment with hydrogen peroxide (orange).

R. Neumann, M. Levin-Elad / Applied Catalysis A: General 122 (1995) 85 97 95

L,

I I 250 G

Fig. 10. ESR spectra of the V20~-A xerogel. (a) 298 K, (b) 150 K. g~ = 1.965; glq = 1.942; g • = 1.977; A~ = 113

G ; A I p = 2 1 1 G ; A ± = 6 4 G .

peroxo complexes. Unfortunately, the relatively small tool-% of vanadium in the xerogel and the strong silica absorption precludes observation of mononuclear

- - I peroxo species expected at approximately 800 cm The most information can be gained by ESR spectroscopy. In Fig. 10, the ESR

spectra of the V2Os-A xerogel at 298 K and 150 K are shown. These are clearly anisotropic spectra of a reduced vanadium(IV), VO 2+, species in a distorted octahedral or tetragonal environment. After treatment of the VzO5-A xerogel with hydrogen peroxide, the ESR spectrum at 298 K significantly changes, Fig. 11. The spectrum at room temperature may be viewed as a superposition of the anisotropic vanadium (IV) species and an oxygen centered radical species (g = 2.005 ) split to eight lines due to interaction with the vanadium-51 nucleus ( I = 7/2) . On the other hand, the when measuring the ESR spectrum of the s a m e compound at 150 K one again obtains a spectrum similar to that of a simple vanadium(IV) species in a tetragonal field. It is clearly apparent that at low temperature the electron density is entirely transferred to the vanadium nucleus. Vanadium(IV) is formed even

96 R. Neumann, M. Levin-Elad / Applied Catalysis A: General 122 (1995) 85-97

25O G

I

Fig. l 1. ESR spectra of the V2Os-A xeroge] after treatment with hydrogen peroxide at 298 K. g = 2.00; insert is

an enlarged picture at the center of the spectrum.

though the xerogel is definitely the well known orange peroxo compound and not the green VO 2 + species. Importantly, this phenomenon is reversible, i.e. reheating the xerogel to room temperature reverts the ESR spectrum to that found in Fig. 11, ruling out decomposition of vanadium peroxy intermediates. From the ESR and UV-vis spectra and reaction and hydrogen peroxide selectivity one may envisage the following vanadium silicate to hydrogen peroxide interactions. The interme- diates formed lead to alkene epoxidation, allylic (radical oxidation) or hydrogen peroxide decomposition (at reaction temperature).

H \ O

o o I II . . . . . . . . I / , , o

' ! 1

+ H20

/ 1k \ O ° O

o I o I o O .... II , , ,o .... II , , ,o ..... IV ~o

Vv'" ~ "'VtV "" ', "~O--H d " l '~O--H d V V'~o H t i i - -

i

3 2 4

Thus, an inorganic peracid type species, 1, or other possible peroxo intermediates such as 4 will react with alkenes to form epoxides by the usual heterolytic mecha- nism. On the other hand, the species 2 and 3 observable in the ESR spectrum (3 more predominant at high temperatures and 2 at lower temperatures) will yield dioxygen or allylic oxidation products via radical intermediates. In the latter case, the vanadium (IV) compound thus formed will be quickly reoxidized by the excess hydrogen peroxide present [ 26,27 ].

R. Neumann, M. Levin-Elad / Applied Catalysis A: General 122 (1995) 85-97 97

4. Conclusions

Our research results show that vanadium silicate xerogels are active catalysts for the activation of aqueous hydrogen peroxide for a variety of reaction including, epoxidation of alkenes, oxidation of secondary alcohols to ketones and the hydrox- ylation of phenol. Although in the past titanium and vanadium substituted zeolites have been used in similar reactions, we have now demonstrated that in the case of vanadium silicates the crystalline zeolite structure is not required for catalytic activity and certain simple amorphous 'molecularly mixed' xerogels have catalytic potential.

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