Vanadium containing crystalline mesoporous molecular sieves Leaching of vanadium in liquid phase reactions

~ ELSEVIER

APPLIED CATALYSIS A:GENERAL

Applied Catalysis A: General 148 11996) 7-21

Vanadium containing crystalline mesoporous molecular sieves Leaching of vanadium in liquid phase reactions Jale S u d h a k a r R e d d y , P i n g Liu, A b d e l h a m i d Sayari

*

Department of Chemical Engineering, Universit~ Lat,al, Ste-Foy, Que., Canada G1K 7P4 Received 20 November 1995; revised 12 June 1996: accepted 13 June 1996

Abstract Vanadium (V) containing mesoporous molecular sieves, V-HMS have been synthesized at room temperature in the presence of dodecylamine as template and vanadyl(IV) sulfate hydrate or vanadium(V) triisopropoxide as vanadium source. The presence of V in the silicate matrix was investigated by UV-visible spectroscopy, 5IV MAS NMR and catalytic measurements. Samples were also characterized using chemical analysis, XRD, SEM, FFIR and adsorption properties. It was tbund that the vanadium centers in V-HMS molecular sieves consist of three-legged (SiO)3V=O species, similar to those in amorphous V / S i O 2. V-HMS materials catalyze the oxidation of organic substrates such as phenol, 2,6-di-tert-butyl phenol, naphthalene and cyclododecanol in the presence of either dilute hydrogen peroxide or tert-butyl hydroperoxide as the oxidant. Leaching of active vanadium species from the silicate matrix during reaction was investigated. The stability of vanadium during liquid phase reactions was found to depend on the nature of the substrate, the solvent and the oxidant. Kevwords: Mesoporous molecular sieves: V-HMS; Oxidation: H202: TBHP; Solvent effect; Leaching

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

After achieving a tremendous success in the selective oxidation of organic substrates over titanium silicalites [1], researchers succeeded in incorporating various transition metal cations, particularly Ti and V, in the framework of a large number of zeolites and molecular sieves. For example, V has been introduced in the framework of the following zeolitic structures: ZSM-5 [2], Corresponding author. Tel.: (+ 1-418) 656 3563; fax: (+ 1-418) 656 5993: e-mail: [email protected]. 11926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0926- 860X(96)00222-0

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J. Sudhakar Reddy et al./Applied Catalysis A: General 148 (1996) 7-2 l

ZSM-11 [3], ZSM-12 [4,5], ZSM-48 [6], SSZ-24 [7], /3 [8], NCL-1 [91, A1PO-5 [10], A1PO-11 [11] and A1PO-21 [12]. Like their titanium counterparts, vanadium containing molecular sieves were found to be active in a number of liquid phase oxidation reactions in the presence of dilute hydrogen peroxide. Such reactions include the hydroxylation of aromatic compounds [13], the oxidation of alkanes [9,14], the sulfoxidation of thioethers [15], the oxidation of amines [16], etc. However, all these microporous materials have a common drawback which is the limited accessibility of active sites to large substrate molecules. Since their discovery [17], crystalline mesoporous silicates lent themselves to modification by transition metal cations. Both MCM-41 and HMS silicates have been modified by Ti [18-22] and V [23-25]. Progress in the area of mesoporous materials has been reviewed recently [26,27]. It should be noted that MCM-41 designates a family of crystalline mesoporous materials with hexagonal structure prepared mostly in the presence of ionic surfactants [17]. HMS designates similar materials prepared in the presence of amine surfactants [18,28]. V and Ti containing mesoporous materials allow the oxidation of bulky substrates such as naphthol, cyclododecanol, norbornene, 2,6-di-tert-butylphenol (2,6-DTBP) with improved efficiency compared to V and Ti modified microporous molecular sieves. The use of modified molecular sieves in liquid phase oxidation of organic compounds is part of a strong tendency to use heterogeneous catalysts for the synthesis of fine chemicals, an area traditionally based on extensive use of homogeneous catalysts. The advantages of such an approach have been addressed in a number of reviews [29,30]. However, this strategy may have a potentially fatal drawback in situations where the catalytically active ingredients leach into the liquid phase. Only few recent papers have been devoted in part to this important issue [25,31,32]. The objectives of the present investigation were to prepare and characterize V-HMS mesoporous materials and to address the liquid phase leaching of vanadium species from such materials during oxidation tests in the presence of dilute hydrogen peroxide or tert-butylhydroperoxide (TBHP).

2. Experimental The raw materials used in the synthesis of V-HMS and Si-HMS samples were tetraethyl orthosilicate (TEOS, 98%), vanadyl sulfate hydrate V O S O 4 . 5 H 2 0 , vanadium triisopropoxide VO(i-OC 3H 7)3, dodecylamine (DDA), hydrochloric acid (HC1, 1 N), ethanol (EtOH), isopropylalcohol (IPA) and water. V-HMS samples were prepared using three different methods. In method (1), vanadyl sulfate was used as the vanadium source and the procedure was similar to that of Ti-HMS [18,33]. In methods (2) and (3), the vanadium source was vanadium

J. Sudhakar Reddy et aL /Applied Catalysis A: General 148 (1996) 7-21

9

triisopropoxide. Typical synthesis procedures according to these methods are described below. Method (1): A solution (A) was prepared by adding 21 g of ethanol and 4.2 g of IPA to 15 g of TEOS, followed by addition of the required amount of vanadyl sulfate. The resulting solution was stirred for 30 min. Another solution (B) was prepared by adding 46 g of water and 1.4 ml of 1 N HC1 to 3.5 g of DDA and stirring for about 5 min. Solution (A) and (B) were mixed together and stirred moderately at room temperature for 18 h. The molar gel composition of the final reaction mixture was: SiOe:x VOe:0.27 DDA:0.02 HC1:6.5 EtOH:I.0 IPA:37 HeO, where x ranged from 0.0025 to 0.05. Pure silica HMS was prepared using the same procedure except that no vanadyl sulfate and no isopropyl alcohol were added. Method (2): This procedure was similar to Method (1) except that solution (A) was prepared by adding 0.17 g of vanadium triisopropoxide to a solution of 15 g of TEOS and 21 g of ethanol. The molar gel composition was: SIO2:0.01 VOe:0.27 DDA:0.02 HC1:6.5 EtOH:37 H20. Method (3): This procedure was also similar to Method (1) except that solution (A) was prepared using a sol-gel method [34]. 0.88 ml of 1 N HCI solution was added to a mixture of 15 g of TEOS and 21 g of ethanol. This solution was heated to 323 K and stirred for 30 min. After cooling it to room temperature, 0.17 g of vanadium triisopropoxide was added and stirred for an additional 30 min period before adding it to solution (B). The molar gel composition of the final reaction mixture was: SiO2:0.01 VO~:0.27 DDA:0.012 HC1:6.5 EtOH:37 H20. The final gels were stirred at room temperature for 18 h. Subsequently, the products were filtered, washed thoroughly with deionized water, dried at 353 K and calcined at 773 K for 6 h in dry air. It should be noted that as stated by Tanev and Pinnavaia [28], the template could also be removed by refluxing the as synthesized materials in ethanol. Three samples referred to as VeOs-SiO~ with different vanadium contents were prepared using Method (1) except that no surfactant was added. V2Os-SiO~ ~ xerogel was prepared using a sol-gel procedure as reported in the literature [34]. After calcination at 773 K for 6 h in dry air, these V205-SIO e mixed oxides were amorphous. Vanadium impregnated Si-HMS was prepared by dispersing 1 g Si-HMS in a solution of 0.04 g vanadyl sulfate in 25 ml water. The mixture was stirred and evaporated at room temperature until dry. Characterization techniques included X-ray diffraction (Philips PW 1010 using a nickel filtered Cu K a radiation), atomic absorption and flame emission spectroscopy (Jarrel-Ash 975 instrument), N 2 adsorption (Omnisorp 100 from Coulter), FTIR (Nicolet 550 spectrometer using the KBr pellet technique), scanning electron microscopy (JEOL 840A), magic angle spinning nuclear magnetic resonance (Bruker AMX-300 spectrometer) and UV-visible spec-

10

J. Sudhakar Reddy et al./Applied Catalysis A: General 148 (1996) 7-21

troscopy (Perkin Elmer spectrometer using magnesium oxide for background correction). 51V solid state NMR measurements were carried out at a frequency of 78.9 MHz. Static 5~V NMR spectra were obtained using a 0-r-20 spin-echo sequence with a delay ~-= 40 /xs corresponding to a 7r/2 flip angle in the selective excitation limit. 51V MAS NMR spectra were acquired with one-pulse sequence and a'r/4 solid pulse, The speed of rotation was in the range of 8-12 kHz. NMR chemical shifts for V were determined using VOC13 as external reference. Catalytic reactions were performed batchwise in a round bottom flask. Product analysis was carried out on a gas chromatograph (Hewlett Packard 5890) equipped with a capillary column (HP-1; crosslinked methylsilicone gum; 50 m × 0.32 mm i.d.). Product identification was achieved by gas chromatography-mass spectrometry (GC-MS, HP-5972) and by comparison with authentic compounds. Leaching experiments were carried out using the oxidation of 2,6-DTBP in the presence of TBHP (70% in water). The reaction was run thrice on each catalyst. After each reaction, the catalyst was separated by filtration, washed thoroughly with acetone and calcined at 500°C for 6 h. The reaction was then run on both the catalyst and the filtrate. Since small amounts of active species may leach during reaction and play the role of a homogeneous catalyst, and readsorb completely on the solid phase upon cooling [35], the filtration was carried out while the mixture was hot.

3. Results and discussion

XRD dloo spacing, surface area and vanadium concentration before and after the crystallization are reported in Table 1. The XRD pattern of HMS samples

Table 1 Properties of V-HMS samples dloo spacing

SBET,m 2 / g

~ 405 232 124 60 -

40.1 40.9 40.2 40.8 44.6 41.2 35.3

1087 1080 1185 1097 1080 1108 1215

100

100

-

-

50 20 100

50 20 100

-

245

Sample no.

Sample a

S i / V ratio Gel

Product

1 2 3 4 5 6 7

Si-HMS(1) V-HMS(1) V-HMS(1) V-HMS(1) V-HMS(1) V-HMS(2) V-HMS(3)

~ 400 200 100 50 100 100

8

v2os-sio~

9 10 11

V205 -SiO~ V205-SiO~ V205 - S i O I ~

a The numbers in parentheses indicate the preparation method used.

,L Sudhakar Reddv et a l . / Applied Catalysis A: General 148 (1996) 7-21

11

Fig. 1. Scanning electron micrograph of V-HMS(1), sample no. 4.

prepared by all three methods matched well with literature data [18,33]. It consisted of only one low angle diffraction peak (100). The absence of other hk0 reflections for HMS materials was attributed to peak broadening due to small scattering domain size effects [18,26,33]. Davis et al. [36] reported however, that mesoporous materials having only one low angle XRD peak contain randomly ordered hexagonal channels. Likewise, using XRD and TEM, Bagshaw et al. [37] found that the so-called MSU materials prepared in the presence of polyethylene oxide surfactants exhibit only the (100) XRD peak and a highly disordered hexagonal-like array of channels. Our own TEM data of Ti-HMS samples showed that these materials are comprised of mostly spherical particles with poorly ordered porous structure [21]. SEM data (Fig. 1) indicate that V-HMS also consists of spherical particles with 0.1-0.5 /xm. The pore structure of HMS-based materials was found to be much less ordered than their MCM-41 counterparts [21,24,38]. The absence of other than the (100) XRD reflection may well be attributed to the lack of order of the porous structure. Chemical analysis data given in Table 1 indicate that most of the vanadium added during the gel preparation was retained after crystallization and calcination. As shown in Fig. 2, N 2 adsorption-desorption isotherms of Si-HMS and V-HMS (samples nos. 1 and 4) are essentially reversible, i.e., without hysteresis. In addition, these isotherms exhibited a sharp step characteristic of capillary condensation into mesopores with uniform sizes [39]. All samples displayed very high surface areas of more than 1000 m 2 / g (Table 1). Typical plots of pore size distributions for samples prepared by different methods are shown in Fig. 3. These distributions were calculated according to the Horvath-Kawazoe model [40].

12

J. Sudhakar Reddy et al. /Applied Catalysis A: General 148 (1996) 7-21

v

g "l

> 0

0.2

0.4

0.6

0.8

1.0

Relative pressure, p/p0 Fig. 2. N 2 adsorption-desorption isotherms of (a) Si-HMS, (b) V-HMS(1) (sample no. 4) and (c) V-HMS(1) (sample no. 5). Figures were shifted vertically for clarity.

Infrared spectroscopy has been used extensively for the characterization of transition metal cations modified zeolites. It was often observed that the incorporation of such cations was accompanied by the development of an IR band at ca. 960 cm-1 which was assigned to the stretching mode of S i O 4 units bonded to a transition metal cation [41]. Alternatively, this band was assigned to the S i - O stretching vibration of Si-O...H groups [42]. In the present case, a band was observed at ca. 960 cm-1 for all V-HMS samples as well as for pure Si-HMS silicate. Moreover, the relative intensity of this band was almost the same for all samples. This indicates that in this case, no reliable information regarding vanadium incorporation can be drawn from IR data.

n," "10 '1o

I l l

1.0

I

3.0

i

i

i

]

i

5.0

Effective pore diameter, nm Fig. 3. Horwath-Kawazoe pore size distributions of (a) Si-HMS, (b) V-HMS(1) (sample no. 4), (c) V-HMS(2) and (d) V-HMS(3).

J. Sudhakar Reddy et al./ Applied Catalysis A: General 148 (1996) 7-21

13

a

i

200

i

I

350

i

5OO

Wavelength, nm Fig. 4. Diffuse reflectance UV-visible spectra of V-HMS(1) with different vanadium contents. Curves a, b, c and d correspond to S i / V ratios of 60, 124, 232 and 405, respectively.

Diffuse reflectance UV-visible spectra of V-HMS samples prepared using vanadyl sulfate are reported in Fig. 4. Other samples exhibited similar spectra. These spectra are dominated by two main absorption bands in the range of 373-385 nm and 252-272 nm, very close to those observed for V-silicalite-1 at 384 and 265 nm [2]. Centi et al. [2] found that vanadium oxide (454, 323 and 238 nm) and silicalite after impregnation with ammonium vanadate and calcination (263, 233 and a shoulder in the 385-313 nm region) exhibit different spectra. Based on their assignment, we concur that the charge transfer (CT) band at ca. 380 nm is attributable to V 5+ with a short V = O double bond and three longer V - O bonds. The intensity of this band increased somewhat upon exposure to moist air. Consequently, it may represent the state of vanadium centers in interaction with water molecules. The CT band at ca. 260 c m - i is consistent with V 5+ species in tetrahedral environment [2]. 51V N M R spectra of sample No. 5, a V-HMS(1) sample ( S i / V = 6 0 ) prepared by Method (1) are shown in Fig. 5. The isotropic chemical shifts were - 7 0 8 and - 5 8 0 ppm and the anisotropy was - 4 7 5 and - 6 4 0 ppm for the dehydrated and hydrated samples, respectively. Additional NMR data for V-HMS samples and other related materials are collected in Table 2. As seen, adsorption of water on V-HMS samples leads to a large increase in anisotropy. This behavior as well as the N M R parameters are very close to those previously reported for hydrated and dehydrated V / S i O 2 [44,43]. Based on this, we conclude that the structure of vanadium sites in V-HMS and in V / S i O 2 must be similar. By comparison to model compounds, Eckert and co-workers [44,43] showed that vanadium in dehydrated V / S i O e is present on the silica surface as

14

J. Sudhakar Reddy et al./ Applied Catalysis A: General 148 (1996) 7-21

V-HMS-60

V-MCM-41-60

b

C

-500

-1000

0

-500

-1000

-1500

Chemical shift, ppm Fig. 5. 5 I V N M R

spectra of V-HMS

( s a m p l e no. 5) and V - M C M - 4 1

(Si/V=60).

(a) static N M R

of

d e h y d r a t e d s a m p l e s , (b) static N M R o f h y d r a t e d s a m p l e s , (c) M A S N M R o f h y d r a t e d s a m p l e s . T h e r o t a t i o n s p e e d w a s 3.3 k H z for V - M C M - 4 1 and 11.2 k H z for V - H M S . Side b a n d s are indicated b y an asterix.

a three-legged species (SiO)3V=O (Scheme 1), which can coordinate water to form hexacoordinated vanadium species. This assignment is also consistent with the UV-visible data, and also with the reversible changes of color from yellow to white upon hydration and dehydration. The easy change in coordination state upon exposure of V-HMS to water vapor suggests that (SiO)3V=O species are

Table 2 P a r a m e t e r s o f 51V N M R s p e c t r a o f V - H M S a n d related s a m p l e s Samples

V/HMS(1)-D V/HMS(1)-H V/HMS(I)-D V/HMS(1)-H V/MCM-41-D V/MCM-41-H

Si/V

60 60 124 124 60 60

A n i s o t r o p y A 8,

Parameter

3iSTAT,

8iMAS,

p p m + 10

asymmetry, ~

p p m + 10

ppm + 3

0.15 0.15 0.15 0.15 0.3 ~ 0

-

720 600 715 590 650 527

-

708 580 711 576 665 527

-

508

-

714 736 710 609

-

475 640 480 640 320 ~ - 50

of

R e f e r e n c e [43] [ ( C 6 H 11)7(Si7 O12)VO]2 O V ( O S i P h 3)3 V/SiO 2 V/SiO 2 - H

- 398 - 422 -487 - 620

~ ~ ~ ~

0.05 0.05 0.05 0.13

D: d e h y d r a t e d s a m p l e s , H: h y d r a t e d samples.1833 - 8il ~ 1822 - 8il, lSli - 8il, 8 i = 1 / 3 ( 8 1 1 + 822 A 8 m 833 -- 8i ' "O m [822 -- 8111/ 1 8 3 3 - 8i1.

+ 833),

J. Sudhakar Reddy et al. / Applied Catalysis A: General 148 (1996) 7-21 O

O

I..o .~$i

!

o~Si'90

0

0

~'0~ "~Io H20

15

0 °

Scheme 1.

easily accessible. Most likely the vanadium resides on the surface of the channels in a relatively exposed position. Notice that as shown in Table 2 and in our previous studies [23,24], the effect of water vapor on V-HMS is different from V-MCM-41 samples. Exposure of V-MCM-41 to water vapor leads to two almost isotropic lines. Possible mechanisms of such a spectral 'symmetrization' have been discussed elsewhere [24]. The hydroxylation of 2,6-DTBP in the presence of our catalysts leads mainly to the corresponding quinone and diphenoquinone (Eq. (1)).

(1)

Data concerning the catalytic activity of Si-HMS, V-HMS(1), V-HMS(2), V-HMS(3) and V205-SIO 2 samples are reported in Table 3. It is seen that

Table 3 Oxidation of 2,6-DTBP over V and Ti containing catalysts a Sample no.

Conversion (%)

H202 Efficiency (%) b

Quinone selectivity (%)

4 6 7 8

63 70 83 5

59 67 76 5

80 85 87 53

4

4

46 (50)

4 94 26 15

4 87 7~ 48

47 (50) 63 (30) 54 93

11 No catalyst 4~ Ti-HMS d Ti-HMS ~

(14) (10) (4) (43)

The numbers in parentheses denote the selectivity towards diphenoquinone. a Catalyst = 0.1 g, 2,6-di-tert-butyl phenol = 1.03 g, hydrogen peroxide (30 wt.-%)= 1.7 g, acetone = 7.8 g, temperature = 335 K, reaction time = 2 h. b H202 efficiency = (mol H20 a utilized for the formation of quinone/mol H20 z added)× 100. ' tert-Butylhydroperoxide was used as the oxidant. d Ref. [33] (Si/Ti = 100). Calculated assuming total consumption of H202. ' Ref. [21] (Si/Ti = 100).

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J. Sudhakar Reddy et aL /Applied Catalysis A: General 148 (1996) 7-21

Table 4 Influence of vanadium content of V-HMS(1) and V205-SIO2(1) in the oxidation of 2,6-DTBP Sample no.

Conversion (%)

H202 efficiency (%) a

Quinone selectivity (%)

1 2 3 4 5 8b 9b 10 b 11 b VO(SO4)

4 71 80 63 88 69 88 83 52 77

4 67 70 59 81 66 76 72 50 73

52 (45) 75 (19) 83 (5) 8O (14) 89 (3) 68 (27) 81 (5) 88 (3) 64 (32) 90 (5)

The number in the parentheses denote selectivity towards diphenoquinone. Reaction conditions: catalyst = 0,1 g, 2,6-di-tert-butyl phenol = 1.03 g, hydrogen peroxide (30 wt.-%) = 1.7 g, acetone = 7.8 g, temperature = 335 K. reaction time = 2 h. a (mol H202 utilized for the formation of quinone/mol H202 added)× 100. b The catalyst was used in the as-synthesized form.

Si-HMS and both calcined V2Os-SiO 2 samples are almost inactive in this reaction. Likewise, negligible conversions were obtained in the absence of catalyst. However, V-HMS samples exhibited exceptional catalytic activity and hydrogen peroxide efficiency with very high selectivities towards 2,6-di-tertbutyl-l,4-benzoquinone and 3,3',5,5'-tetra-tert-butyl-4,4'-diphenoquinone (Eq. (1)). For comparison, relevant literature data regarding the oxidation of 2,6-DTBP over Ti-HMS with similar titanium content were included in Table 3. It is seen that V-HMS is by far a superior catalyst for this oxidation reaction as compared to its titanium analog. V-HMS exhibited better 2,6-DTBP conversion and H202 efficiency. Table 4 reports the catalytic performance of V-containing samples in the oxidation of 2,6-DTBP. It is seen that V-HMS(1) samples, prepared by Method (1) with different vanadium contents have comparable catalytic activities. Moreover, the as-synthesized form of V-HMS(1), i.e., without calcination, displayed very low activity indicating the importance of the accessibility of the pore walls to reactants. The as-synthesized V205-8iO2 samples gave considerable conversion of 2,6-DTBP. However, samples calcined at 823 K, exhibited almost no catalytic activity (Table 3, samples nos. 8 and 11), most probably because of V~O5 sintering. Similar to other vanadium containing molecular sieves, V-HMS exhibited very good activity in the hydroxylation of phenol to catechol and hydroquinone in the presence of water as solvent (Table 5). However, when acetone or acetonitrile was used as solvent, negligible conversions were obtained. The catalytic activity of V-HMS samples prepared by all three methods were also used in the oxidation of other substrates like naphthalene, 2-methyl naphthalene and cyclododecanol (Table 5). All catalysts exhibited very good activities and selectivities.

J. Sudhakar Reddy et al. /Applied Catalysis A: General 148 ~1996) 7-21

17

Table 5 Oxidation of different substrates over V-HMS catalysts Substrate

Sample no.

Solvent

Oxidant

Cony.

Product selectivity (%)

DTBP

4

Acetone

H 2 0 ~-

63

80% (14%)

Phenol ~ Naphthalene ~ 2-Methyl naphthalene b

4'

Water

H202

13

58%

4

Acetonitrile Acetonitrile

H202

10

72%

4

H202

9

94%

DTBP

7

Acetone

Naphthalene Naphthalene Cyclododecanol ~ Cyclododecanol Cyclododecanol

7 7

Acetonitrile Acetonitrile

TBHP H,O 2

94 4

71% 60%

TBHP

9

64%

7

Acetone

TBHP

12

77%

7 7

Acetonitrile Acetonitrile

TBHP

24

92%

DTBP

6

Acetone

H202 TBHP

25 96

94% 74%,

Cyclododecanol

6

Acetonitrile

TBHP

26

89%

(%)

quinone cathechol, 42% hydroquinone 1,4-naphthoquinone 1,4-naphthoquinone (20%) quinone 1,4-naphthoquinone 1,4-naphthoquinone cyclododecanone cyclododecanone cyclododecanone (16%) quinone cyclododecanone

The number in the parentheses denote the selectivity towards diphenoquinone. J

Catalyst = 0.2 g, phenol = 2.0 g, p h e n o l / p e r o x i d e ( m o l a r ) = 3.0, solvent = 15 g. t e m p e r a t u r e = 353 K,

reaction time = 6 h. b Catalyst = 0.1 g, naphthalene = 1.0 g, n a p h t h a l e n e / p e r o x i d e (molar) = 0.33, solvent = 10 g, temperature = 333 K, reaction time = 2 h. Catalyst = 0.1 g, cyclododecanol = 1.4 g, ture = 333 K, reaction time = 3 h.

naphthalene/peroxide (molar) = 0.3 g, solvent = 10 g, tempera-

Data obtained in liquid phase in the presence of flesh solid catalysts may be misleading. Indeed, it is crucial to ascertain that no leaching of active ingredients occurs and no homogeneous catalysis is taking place. To address this problem we first used a method employed by Neumann and Levin-Alad [32]. A V-HMS(1) catalyst (sample no. 4) was suspended in 30 wt.-% H202 at 353 K for 2 h in the presence of one of the four solvents, water, ethanol, acetone or acetonitfile without any substrate. After separation of the catalyst, The filtrate was used in the oxidation of 2,6-DTBP. The organic substrate and H 2 0 2 were added to each filtrate and no catalytic activity was detected in any of the solvents. Notice that under the same conditions, vanadyl sulfate dissolved in water led to an excellent conversion. At this stage, it is still premature to conclude that no vanadium leaching takes place. Indeed, after actual hydroxylation of 2,6-DTBP in the presence of 30 wt.-% H202 and filtration of the spent catalyst (V-HMS(I)), addition of reactants to the filtrate gave more products, while the solid catalyst after drying or calcination had no catalytic activity. This indicates that leaching is not necessarily due to the solvent, it may be induced by the substrate. We carried out additional experiments using TBHP as the oxidant instead of hydrogen peroxide in the presence of acetone as solvent. These experiments were performed over three V-HMS samples prepared by different methods, and over vanadium impregnated Si-HMS as well as over V2Os/SiO ~ in both as-prepared and calcined forms. In the presence of V-HMS(2) and V-HMS(3)

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J. Sudhakar Reddy et al. /Applied Catalysis A: General 148 (1996) 7-21

Table 6 Data on leaching of vanadium from different V-HMS samples during the oxidation of 2,6-DTBP a Sample

Number

4

6

7

A

B

C

D

Fresh catalyst

Conversion Selectivity

94 63(30)

96 74(16)

94 71(20)

94 50(48)

23 18(62)

84 63(28)

4 25(60)

Filtrate

Conversion Selectivity

7 30(70)

27 10(68)

27 44(56)

68 51(42)

7 25(50)

19 32(58)

-

Spent catalyst

Conversion Selectivity

100 24(76)

96 63(23)

91 44(56)

7 b 1(64)

_ -

83 28(70)

-

Filtrate

Conversion Selectivity

14 0(96)

14 31(65)

6 30(55)

-

-

16 4(96)

-

Spent catalyst

Conversion Selectivity

10 33(56)

98 51(48)

87 19(79)

-

-

70 10(89)

-

Selectivity (%): quinone (diphenoquinone), A: VOSO 4 impregnated Si-HMS; B: V 2 0 5 / S i O I I ; C: calcined sample A; D: calcined sample B. a Catalyst = 0.2 g, 2,6-di-tert-butyl phenol = 2.06 g, tert-butylhydroperoxide (70 wt.-%) = 3.9 g, acetone = 16.0 g, temperature = 335 K, reaction time = 2 h. b Sample was filtrated and washed with acetone but not calcined.

catalysts, though the filtrate gave some conversion, the catalysts retained their original activity after three successive runs (Table 6). This indicates that there is some partial leaching of vanadium during the reaction, but most of the vanadium in V-HMS(2) and V-HMS(3) remained in the silicate matrix. When V-HMS(1) was used under the same reaction conditions using TBHP as the oxidant, the catalyst activity dropped to ca.10% of its initial activity after two runs. This indicates that the source of vanadium used in the catalyst preparation may have an effect on the stability of vanadium species in the silicate matrix and thereby their resistance to leaching. In addition, it is important to recall that during the oxidation of 2,6-DTBP in the presence of H202 as the oxidant, most of the V was leached from all V-HMS catalysts. The as-synthesized V impregnated Si-HMS (Sample A, Table 6) lost its activity after one reaction run while the filtrate showed very high conversion, indicating that V leached during the reaction. On the other hand, its calcined form (Sample C, Table 6) gave similar conversion during the three runs. This suggests that in this case, V may have been incorporated in the silicate framework during calcination. The as-prepared V2Os-SiO II gave quite low conversion and its calcined form was not active. No further test were undertaken to examine their stability. In summary, The V-HMS samples prepared with vanadium (V) triisopropoxide (Samples 6 and 7) were more stable as compared to that prepared using vanadyl sulfate (Sample 4). The VOSO 4 impregnated Si-HMS was also stable after calcination. Using TBHP as the oxidant, we have also carried out the oxidation of cyclododecanol in the presence of acetone and acetonitrile, then used the filtrates and the spent catalysts separately for further reactions. The filtrates of

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all three catalysts exhibited significant catalytic activity. Surprisingly, even V-HMS(2) and V-HMS(3) spent catalysts showed negligible activity. Earlier we have reported that during the hydroxylation of phenol in water over V-HMS, no leaching takes place [25]. All these findings demonstrate that the leaching of active ingredients from solid catalysts during liquid phase reactions is a rather complex issue. The few examples documented here show that the nature of vanadium sources used in the synthesis of catalysts, the solvent, the substrate and even the oxidant have a dramatic influence on the stability of active vanadium centers against leaching during liquid phase reactions. In particular, dilute H202 which is supposed to be more economical than TBHP induces extensive leaching of vanadium. Recently, Dutoit et al. [45] showed that none of the vanadium containing aerogels or xerogels they tested was resistant towards dilute H202.

4. Conclusions V-HMS materials were prepared at room temperature using different sources of vanadium. The presence of vanadium in the silicate matrix was evidenced by UV-vis, NMR and catalytic test reactions. V-HMS catalyzed the oxidation of various substrates like naphthol, 2,6-di-tert-butylphenol, naphthalene, 2-methylnaphthalene and cyclododecanol. The vanadium in V-HMS sample prepared using vanadium (V) triisopropoxide is more stable in the presence of TBHP as the oxidant during the liquid-phase oxidation reactions as compared to that prepared using vanadyl sulfate. However, all V-HMS samples are not resistant towards dilute H20 2. During the evaluation of this manuscript, two papers on vanadium modified mesoporous materials appeared. Based on the results of UV-vis, EPR and NMR, Gonfier and Tuel [46] concluded that V v species in calcined V-HMS were tetracoordinated and rapidly changed their coordination in the presence of water. Morey et al. [47] characterized V/MCM-48 samples prepared by adsorption of vanadium isopropoxide on the MCM-48 silicate. It was inferred from 51V NMR and UV-vis that pseudotetrahedral O3/2V=O coordinated by three S i - O - V bridges were grafted to the mesoporous walls and introducing water led to a change of the V environment to higher coordination number. These results are consistent with our assignment of the V coordination in V-HMS reported here and elsewhere [24,25].

Acknowledgements This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). JSR thanks NSERC for Canada International Fellowship. We are very grateful to I.L. Moudrakovski for NMR data.

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