Synthesis and characterization of transition metal containing mesoporous silicas

Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.

157

Synthesis and Characterization of Transition Metal Containing Mesoporous Silicas S. Gontier and A. Tuel Institut de Recherches sur la Catalyse. C.N.R.S. 2, av. A. Einstein 69626 Villeurbanne Cedex France Titanium and Vanadium mesoporous silicas (MS) have been synthesized at ambient temperature using a neutral templating route with primary alkyl amines as surfactant. Spectroscopic characterization of the samples showed that transition metal cations were highly dispersed in the silica framework. The metal content, the surfactant chain length and the amine/SiO 2 ratio greatly influenced the properties of the final product. These materials were found to be active as catalysts in oxidation reactions with alkyl peroxides at mild temperature. 1. I N T R O D U C T I O N There has been an extensive interest during the last years for the synthesis of transition metal containing molecular sieves, due to their remarkable properties as catalysts in oxidation reactions with organic peroxides [1,2]. A very beautiful example is TS-1, the titanium substituted silicalite-1, that catalyzes many oxidation reactions in the liquid phase with aqueous hydrogen peroxide. However, as far as zeolitic supports are concerned, reactions are limited to small substrates with a kinetic diameter smaller than 7~. Recently, the discovery of a novel family of silica-based mesoporous molecular sieves M41S by Mobil researchers [3] and a group from Waseda University [4] opens new perspectives in the field. Corma et al. [5]

were the first to synthesize a Ti

mesoporous silica analog to MCM-41 and to show that the material was active as catalyst in the oxidation of bulky substrates. In a similar manner, Franke et al. [6] also prepared Ti-MCM-41, but they did not report any catalytic data over these solids. Reddy et al. [7] also reported the possibility of preparing vanadium-containing MCM-41, active in oxidation reactions.

158 Very recently, Tanev et al. [8] have shown that mesoporous silicas (MS) could be prepared using a neutral templating route with primary alkyl amines as surfactant. A great advantage of the recipe was that the template could be removed from the mesopores by a solvent extraction, which is of great interest for environmental protection. We have followed a similar procedure to prepare Ti and V-containing mesoporous silicas. The influence of synthesis parameters, e.g. the metal content, the amine chain length or the synthesis time have been examined. These materials have shown very interesting properties as catalysts in oxidation reactions with organic peroxides as compared to zeolitic materials. 2. E X P E R I M E N T A L Ti or V-MS were prepared at ambient temperature by mixing a first solution containing tetraethyl orthosilicate (1 mole), ethanol (6.5 moles), isopropyl alcohol (1 mole) and the metal precursor (tetrabutyl orthotitanate or vanadyl acetylacetonate) to a second solution containing the alkylamine (0.3 mole) in water (36 moles). The resulting solution was homogeneized, stirred for about 30 min and aged for different periods at room temperature under static conditions. Solids were then filtered, washed several times with distilled water and air dried. Calcination of the samples was performed at 650 ~C in air for 6 h. When the organics were removed by a solvent extraction, 1 g of dried solid was dispersed in 100 ml of ethanol and the mixture refluxed for about 1 h. The solid was recovered by filtration, washed with cold ethanol and the procedure repeated once. A Ti-Beta sample was synthesized following the recipe of Camblor et al. [9] and contained about 1.5 wt % Ti and 0.4 wt % AI. A TS-1 sample was also synthesized following the patent literature [10]. Samples have been characterized using X-Ray diffraction (Philips PW 1710, CuKa radiation), IR spectroscopy (Perkin Elmer 580), U.V-Vis spectroscopy (Perkin Elmer Lambda 9) and EPR (Varian, E9). N 2 adsorption/desorption isotherms were carried out on a Catasorb apparatus. 3. R E S U L T S AND DISCUSSION

3.1. Synthesis and Characterization A series of samples have been synthesized with dodecylamine (n = 12 carbon atoms) and varying the amount of metal precursor in the gel. Gels were aged for 12 h

159 before recovering as-synthesized solids by filtration. As shown in Table i, the amount of metal incorporated in the silica matrix is very close to that introduced during the preparation. Table i Characterization of the different samples Sample Gel .

.

M~

oc

Si/Ti Product .

S(m2/g) .

.

r

V(cm3/g)

.

.

m

973

26.5

0.52

Ti-MS

i0()

85

i086

28

0.62

Ti-MS

50

45

i 066

29

0.65

I1-M~

30

3i

894

28

0.54

Ti-MS

20

i 9.5

667

25

0.36

V-Mb

i00

108

998

28

0.55

V-MS

50

62

i0 i5

28

0.64

V-MS

30

36

794

28.5

0.48

9 p()~) is the mean pore diameter V(cm3/g) is the porous volume measured at P/P0 = 0.5 in the N 2 isotherm. This suggests that all the Si and Me precursors were in the solid phase as the yield in Ti or V-MS was always very high (> 95 %). For relatively low Me contents (< 2 wt %), U.V-Vis as well as EPR spectroscopies showed that the cations were highly dispersed in the solid. Ti-MS materials exhibited a U.V. absorption band around 240 nm. The absorption edge decreased by about 10 nm to 230 nm upon calcination in air. For V-MS, EPR parameters were characteristic of vanadyl ions in an axially symmetric crystal field (A ![ = 190 G, g [] = 1.94, A__k_ = 72 G and gA_ = 2.00). The totality of the EPR signal disappeared upon calcination of V-MS samples in air. Dried calcined samples were white, but their color rapidly turned to bright yellow upon exposure to air, suggesting a change in the cation coordination. This was clearly evidenced in U.V-Vis and 51V NMR spectra. In dry samples, V 5 + cations are more likely in a tetrahedral environment but water molecules can easily enter their coordination sphere to give hexacoordinated cations. The process was perfectly reversible as original U.V-Vis and NMR spectra were restored after outgasing the samples at 200~

for 3 h. This clearly showed the relatively

160 high hydrophilic character of mesoporous silicas as compared to substituted zeolites like TS-1 or VS-1. All calcined samples exhibited relatively high surface areas, typically 1000-1200 m2/g and a mean pore diameter close to 28/~. Both slightly increased with the metal incorporation up to about 2 wt % of metal in the solid. Beyond that limit, for higher Me contents, the surface area as well as the pore diameter decreased. In the same time, U.VVis and NMR spectra revealed the presence of extrawall dispersed oxide species in the samples. However, the decrease in pore diameter could also arise from a loss of thermal stability of the structure for high Me incorporations. Similar observations have been made by Franke et al. [6] for Ti-MCM-41, and the authors interpreted the decrease in pore dimensions to the presence of extrawall species. A sample has been prepared with Si/Ti = 100 in the precursor gel and aged at room temperature for different periods ranging from a few minutes to 18 h. Indeed, Chen et al. [11] had reported that even though an X-Ray powder pattern typical of MCM-41 could be obtained after heating a silica-alumina gel at 70~

for 3 h, the

material was not thermally stable and collapsed upon calcination in air at 540 ~C. Table 2 clearly shows that using the present synthesis route, Ti-MS was obtained after a few minutes after mixing of all the reagents. There was no significant evidence for modifications occuring during the aging period ; the Ti content as well as the BET surface area and the pore dimension remained unchanged. The same observations could be made with vanadium-containing samples. Table 2 Characterization of Ti-MS samples recovered at different time intervals t(h)

Si/Ti

S(m2/g)

0

93

1081

28

0.58

1

85

1212

28

0.62

2

96

1091

28

0.60

3

94

1112

28

0.63

6

92

1174

28

0.62

18

85

1085

28

0.62

For the definition of ~p and V, see Table 1

~p()~)

V(cm3/g)

161 it has been widely reported that the pore dimension of MCM-4i synthesized with aikyitrimethyiammonium cations depended on the number of carbon atoms of the alkyi chain. For pure silica materials, data from Beck et ai. [3] showed an almost linear increase in the pore diameter from i8 ~ with octyiamine to about 37 /~ with hexadecyiamine. Very recently, Tanev et ai. [8] also reported that the pore dimensions of mesoporous silicas prepared using a neutral templating route increased with the amine chain length. For both Ti and V-substituted materials, we also observed that the dimension of the mesopores increased from about 25 ~ for n = i0 carbon atoms to 35 for n = i6. The corresponding isotherms are shown in Fig. i.

(c) o') A

I...03

1I~176

/

(b)

E "6 ..Q L_

o

Ca)

"13 <

E

0'

'

' 0.2

'

' 0.4

'

' 0.6

'

0'8.

'

I 1.0

plpo Figure i. N 2 adsorption/desorption of Ti-MS synthesized with CnH2n + i N H 2. n = i0 (a), n - i2 (b) and n - 16 (c)

162 These values are somewhat higher than those of Beck et al. [3] but also than those of Tanev et al. [8] who prepared their samples following a very similar recipe. As for MCM-41, the use of an amine with 18 carbon atoms led to materials with the same pore dimensions as those prepared with hexadecylamine. Another interesting parameter in the synthesis of mesoporous silicas is the surfactant/silica (Surf/SiO2) ratio. Beck et al. [3] have reported that hexagonal phases could only be prepared for Surf/SiO 2 < 1.1. For higher ratios, a cubic phase MCM-48 or unstable lamellar materials were obtained. We have synthesized a series of Ti-MS with Si/Ti = 100 in the gel and varying the Surf/SiO 2 ratio from 0.13 to 1.1. Whilst the surface area was high for relatively low ratios (< 0.5), it decreased to 350 m2/g for Surf/SiO 2 = 1.1 (Table 3). X-Ray diffraction showed that the cubic phase similar to MCM-48 was never formed, at least at a significant level. In contrast to the surface area and porous volume, the pore diameter increased continuously with the amount of amine introduced in the gel. These experiments clearly demonstrated that the best materials were obtained for Surf/SiO 2 ratios close to 0.3. Table 3 Characterization of Ti-MS synthesized with hexadecylamine and various Surf/SiO 2 ratios Surf/SiO 2

Si/Ti

S(m2/g)

~p(*.)

V(cm3/g)

I).13

81

968

34

0.59

I).27

85

1045

34

0.70

I).54

76

887

35

0.39

[).81

90

526

36.5

0.21

1.1

41

354

37.5

0.14

For the definition of ~p and V, see Table 1

3.2. Catalytic experiments Because of the high dispersion of Ti and V cations in mesoporous silicas, these materials are potentially interesting catalysts for the oxidation of large substrates with alkyl peroxides in the liquid phase. Ti-MS have been used as catalysts in the epoxidation of cyclohexene with both aqueous H202 and tert-butyl hydroperoxide (TBHP). The performances are compared with those of TS-1, that is known to be inactive in these

163 reactions because of restriction limitations, and Ti-Beta prepared following the recipe given by Camblor et al. [9]. Results are summarized in Table 3. With hydrogen peroxide, the activity of Ti-MS is very high as compared to those obtained over TS-1 and Ti-Beta. With TBHP, activities over Ti-MS and Ti-Beta are similar but the nature of the products formed differ considerably" because of the presence of aluminium in Ti-Beta, the diol derivatives is the major product formed whilst a selectivity in epoxide close to 9(1% is reached over Ti-HMS. This makes these materials particularly interesting for the selective epoxidation reactions with organic peroxides in the liquid phase. Table 3 Oxidation of cyclohexene over various Ti-containing catalysts Catalyst

Oxidant

COx(%)

SEp(%)

SD(%)

TS-1

H20 2

16

97

3

Ti-Beta

H20 2

77

2

98

Ti-MS

H20 2

95

79

21

Ti-Beta

TBHP

92

90

10

Ti-MS

TBHP

97

88

12

COx is the conversion in oxidant, SEp the selectivity in epoxide and SD that in diol. Reaction conditions "0.1 mole cyclohexene, 20 ml ethanol, H202/cyclohexene = (}.2, T = 7{1~C. Data obtained after l h30 of reaction. The same catalysts have been used for the oxidation of substituted anilines. Over all Ti-containing catalysts, aniline was converted selectively into azoxybenzene with a selectivity close to 95 %. The activity over Ti-MS was comparable to that over Ti-Beta, and both were very high as compared to that obtained over TS-1. Over V-MS materials, the only product formed was nitrobenzene, but the catalysts were only active with TBHP and conversions relatively low. Interesting differences between Ti-MS and Ti-Beta were observed during the oxidation of 3-chloroaniline.

CI

,.,,,vmw2r'2~

CI

0

CI

164 The selectivity in 3-3' dichloroazoxybenzene was only 6() % over Ti-Beta after 2 h 30 of reaction whereas it was already 95 % over Ti-MS after 15 min. In both cases, all the H 2 0 2 introduced at the beginning of the reaction had been consummed. This example clearly evidenced the advantage of mesoporous systems with respect to zeolites in the liquid phase oxidation of organic molecules. 4. R E F E R E N C E S

1

G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo and A. Esposito, Stud. Surf. Sci. Catal., 28 (1986) 129.

2 3

B. Notari, Catal. Today, 18 (1993) 163. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834.

4

S. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. Soc., Chem. Commun. (1993) 680.

5

A. Corma, M.T. Navarro and J. Perez-Parient6, J. Chem. Soc., Chem. Commun. (1994) 147.

6

O. Franke, J. Rathousky, G. Schulz-Ekloff, J. Starek and A. Zukal, Stud. Surf. Sci. Catal., 84 (1994) 77.

7

K.M. Reddy, I. Moudrakovski and A. Sayari, J. Chem. Soc., Chem. Commun.

8 9

(1994) 1059. P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865. M.A. Camblor, A. Corma and J. Perez-Parient6, Zeolites, 13 (1993) 82.

10

M. Taramasso, G. Perego and B. Notari, US Pat, 4 410 5[)1 (1983).

11

C-Y. Chen, H-X. Li arid M.E. Davis, Micorporous Mater., 2 (1993) 17.