Incorporation of molybdenum into mesoporous MCM-41

H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials

2067

Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.

INCORPORATION

OF MOLYBDENUM MCM-41

INTO MESOPOROUS

Samitha D Djajanti and Russell F Howe Department of Physical Chemistry University of New South Wales, Sydney 2052, Australia Molybdenum loading of mesoporous MCM-41 was attempted through different methods : incorporation during synthesis, MOCVD from the hexacarbonyl, gaffing with the pentachloride, and conventional impregnation or solid state mixing. The materials produced have been studied by FTIR, Raman, EPR, XPS and gravimetry. The merits and drawbacks of the different preparation methods will be discussed.

1. INTRODUCTION The catalytic chemistry of molybdenum has a long history. Molybdenum dispersed on high surface area supports has many applications in industrial catalysis, and the synthesis and characterisation of such catalysts has attracted wide attention. Methods used for loading molybdenum onto supports include impregnation from salt solution, solid state reaction, grafting or vapour phase adsorption of organometallic precursors [ 1-6]. The recent discovery of mesoporous MCM-41 materials by the Mobil group[7] has raised the possibility of using these silicate or aluminosilicate molecular sieves as supports for transition metal or transition metal oxide catalysts. The synthesis and properties of MCM-41 have been studied further since the original report[8-11] and the incorporation of different elements such as titanium [ 12,13] copper[ 14], manganese[ 15] and tin-molybdenum [ 16] has been described. In this paper we have investigated several different methods for incorporating molybdenum into the pore structure of MCM-41 and examined the materials produced spectroscopically. Our objective has been to find the optimum method for obtaining high dispersions of catalytically active molybdenum species within the MCM-41 pores.

2. EXPERIMENTAL Synthesis of MCM-41 followed the method described by Ryoo [11 ] using Ludox HS40 (DuPont) as silica source and sodium aluminate as aluminium source. Cetyltrimethylammonium chloride (CTACI) solution (25% in water, Aldrich) was used as template with the final gel composition CTACI : 6 SiO2 : 0.15(NH4)20 : 1.5 Na20 : 250 H20. The aluminosilieate MCM-41 was synthesised by adding dropwise the desired amount of sodium aluminate solution into the gel of silica MCM-41 and stirring for another 30 minutes. MoMCM-41 was prepared by adding either sodium molybdate or ammonium heptamolybdate solution to the silica MCM-41 gel to give gel compositions of Si:Mo

2068 between 15-30. Samples were calcined up to 823K (at l degree/min rate) and held at this temperature for 10 hours in air. The products were checked by XRD and chemical analysis. The sodium exchanged MCM-41 was obtained by ion-exchanging the calcined aluminosilicateMCM-41 with sodium chloride solution 0.02 M. Mo(CO)6 (98%, Aldrich) was purified by vacuum sublimation before being used in the MOCVD method. The adsorption and decomposition of the carbonyl was followed gravimetrically using a vacuum microbalance and by FTIR using a vacuum ceU.(Bomem MB100 spectrometer at 4cm"1 resolution). Samples of MCM-41 (in pellet form for gravimetric measurements and self supported wafer for FTIR) were activated up to 723 K and held under oxygen for 1 hour and then evacuated at this temperature. Upon cooling to room temperature, samples were exposed to molybdenum carbonyl vapour and monitored. The molybdenum grafted samples were prepared using molybdenum pentachloride (Aldrich, 95%) vapour in a vacuum cell with a breakable seal [5a]. The samples were activated as above and then after breaking the seal between sample and molybdenum pentachloride, the cell was put in oven at 473 K for 20 hours. The product was then washed with ammonia solution (1 M) for 2 minutes and heated under vacuum at 573K for 1 hour. EPR spectroscopy (Bruker ESP 300) was used to monitor the changes. Impregnated samples were prepared by the incipient wetness method using aqueous ammonia heptamolybdate solution and calcined at 723 K for 8 hours. Similar calcination treatment was used for solid state mixtures of MoO 3 and MCM-41. These samples were characterised by Rarnan spectroscopy ( Renishaw 2000 Raman microprobe, Ar ion laser). XRD measurements were obtained with a Cu source (~ = 1.54 A) using a Siemens instrument. Chemical analysis data were obtained by ICP Emission Spectroscopy and XPS measurements were done on Kratos XSAM800 using Mg Ka X-ray source.

3. RESULTS AND DISCUSSION

Figure 1 shows XRD patterns of the silica MCM-41 and the materials obtained by incorporating aluminium and molybdenum into the synthesis, following calcination. In all cases well ordered MCM-41 is obtained with the d-spacing around 37-39A The incorporation of aluminium or molybdenum into the synthesis has no pronounced effect on the XRD patterns. The AI-MCM-41 with Si/Al=20 (in the gel) and its sodium exchanged form gave slightly broadened lines. Incorporation of molybdenum had no such effect, however chemical analysis of the MoMCM-41 materials revealed that the molybdenum contents achieved never exceeded 0.2%, regardless of the starting gel composition. XPS analysis gave Mo:Si ratios ca. 4X higher than that corresponding to the bulk composition, suggesting that the molybdenum present in these materials is largely adsorbed at the surface rather than incorporated into the silicate structure. The MOCVD method for loading microporous materials with transition metals involves adsorbing and decomposing precursors such as carbonyl complexes in the zeolite pores. In the case of MCM-41, the amount of molybdenum hexacarbonyl adsorbed on exposure to the vapour at room temperature depends on the composition of the MCM-41, as shown in figure 2.[17]. The silicate MCM-41 adsorbed around 20% of Mo(CO) 6 but almost all of this was lost upon evacuation. The adsorption of Mo(CO)6 increased with the incorporation of aluminium and sodium exchange, but evacuation at room temperature still removed most of the adsorbed carbonyl (unlike the situation in NaY, for example, where

2069

C.

b. A.

2 Theta Fig. 1 The XRD pattern of various calcined MCM-41; a. all silica ; b. Si/A1 = 20; c. Na-exchanged of Si/AI =20 ; d. Si/Mo = 20 (ammonium hepta-molybdate in synthesis); e. Si/Mo = 20 (sodium molybdate in synthesis)

adsorption of Mo(CO) 6 is completely irreversible [ 18]). Heating to 200~ decomposes the remaining adsorbed carbonyl, but the molybdenum loadings achieved are much lower than in the corresponding zeolite experiments [ 18] typically 1% in NaAIMCM-41. FTIR experiments showed clearly the origin of the differences between the different MCM-41 hosts. (Figure 3.) Adsorption of Mo(CO)6 in silica MCM-41 gives a single carbonyl band at 1990 cm-1 similar to that seen on silica gel[6], which was removed completely on evacuation at room temperature (not shown). In the AIMCM-41, which contains both H + and Na + aider calcination, room temperature adsorption of Mo(CO) 6 gave initially a carbonyl band at 2033 cm- 1; the 1990 cm- 1 band appeared and grew in intensity subsequently.

~o

"O

"d"

.Q O U) "O r-


.

_-_-. . . . .

Ads i

Si-MCM-41

e

Ilttlltlr"" e

,

Evac

200 C Illm Si/AI = 7

! Evac

Reads

200 C

m Si/AI=7; Na-exchanged

Fig.2 Gravimetric results of Mo(CO)6 adsorption into MCM-41

2070

In the Na + exchanged A1MCM-41 the 2033 cm-1 band was more dominant and was accompanied by an additional weak band at 2137 cm-1. We attribute these two bands to Mo(CO)6 perturbed through interaction with Na + cations in the MCM-41 pores. The spectrum obtained alter room temperature o 782 adsorption in Na exchanged MCM41 (Figure 3c) also shows an 213~ additional band of medium intensity at 1782 cm-1 and two weaker bands at 2060-2080 cm-l.(These bands are also evident, although less o intense, in Figure 3 b.). These additional bands are due to partially 2~ 15'oo decomposed molybdenum carbonyl wwenumber, cm-1 species not yet identified, since they persisted more strongly alter Fig.3 FTIR spectra of carbonyl vibration of Mo(CO)6 evacuation or mild heating. The adsorption into MCM-41 after 30 s. a. Si-MCM-41; infrared spectra indicate that b. Si-AI- MCM-41 with Si/AI= 20; c. Sodium-exchaMo(CO)6 has a specific interaction nged ofb. with ion exchanged Na + cations in MCM-41, which accounts for the higher uptake of Mo(CO)6 and greater retention of molybdenum on subsequent heating. In comparison with smaller pore materials such as NaY however the anchoring role of the cations is not sufficiently strong to achieve high loadings of molybdenum from a single adsorption and decomposition cycle. As figure 2 indicates, it is possible to incrementally increase the molybdenum loading of MCM-41 by carrying out repeated adsorption-decomposition cycles, but a stronger anchoring interaction will have to be found if this method is to be used to fully load the available pore volume of MCM-41. The gralting reaction of MoCI 5 with silica gel supports has been studied in detail by Louis and Che [5]. The method utilises reaction of MoCl 5 with SiOH groups of the support, forming Si-O-Mo bonds and eliminating HCI. The starting material is paramagnetic, so that EPR spectroscopy can be used to monitor the chemistry occurring during grafting, following washing to remove excess MoCI5, and subsequent treatment. Figure 4 shows EPR spectra measured following various stages of the grafting of MoCI 5 onto silica MCM-41. Exposure of the dehydrated MCM-41 to MoCI 5 vapour gave an orange-brown material with the EPR spectrum shown in figure 4a (the sample was transferred to the EPR sample tube under nitrogen). The signal observed is similar to that obtained by Louis and Che on reaction of MoCI 5 with silica gel, and also closely similar to that obtained by Seyedmonir and Howe on reaction of a molybdena-silica catalyst with HCI [19]. The g-tensor components are inverted relative to the Mo 5+ signal obtained on reduction of supported molybdenum oxide catalysts (gll = 1.964, g_L=l.94), which is due to the presence of chloride ligands in the coordination sphere of the Mo5+(as in species such as MOOC152- or MoOC14"). On exposure to air the o

2071

P IO

Fig.4 ESR spectra of grafted sample, a. after grafting under N2; b. exposed to air (blue eolour); e. aider washing by aq. ammonia 1M for 2 rains.; d. aider heating under vacuum at 573K for 1 hour. The sharp signal at g=2.00 is an artefact. sample turned blue. The resulting EPR spectrum still contains the original signal (with a slightly more anisotropic g-tensor), but shows in addition a broad new underlying signal whose parameters cannot be accurately determined. The blue r is characteristic of hydrated Mo 5+ species; exposure to air dearly causes partial hydrolysis of the grained MoCI5 species. Louis and Che found that washing the MoCI5 grafted silica catalysts with ammonia solution removed all traces of chloride from the surface, but also removed the hydrated molybdenum blue species, reducing the molybdenum content by an amount depending on the washing time. Similar results were found here for the MoCI 5 grained MCM-41. The EPR spectrum after washing (figure 4r shows a greatly reduced (ca. 100X) Mo 5+ signal intensity, and an almost isotropir g-tensor (gav = 1.92). The sample after washing was light brown in eolour. Subsequent heating in vaeuo gave the spectrum shown in figure 4d. The signal intensity increased 10-fold, and the g-tensor components gll = 1.90 and g_L = 1.95 are similar to those assigned by Louis, Che and Anpo [Sb] to an oetahedrally coordinated Mo 5+ species in an oxide environment ( a second gll component due to a 5-coordinate species is probably also present). XPS analysis of the MoC15 grained MCM-41 materials aider washing indicated that the Mo content was in the same range reported by Louis and Che, 1-3 %. These experiments indicate that the MCM-41 behaves in a closely similar way to conventional silica gel as far as the grafting method is concerned for producing molybdenum loaded catalysts. Several groups have reported that molybdenum loaded zeolites can be prepared by heating mixtures of zeolite and MoO3 to high temperatures in air [3,4]. It is supposed that under these conditions molybdenum can migrate into the zeolite pores in what is effectively a solid state ion exchange. We have applied this method also to MCM-41. Figure 5 shows XRD patterns obtained from such samples. Figure 5b is the diffraction pattern of a physical mixture (5%Mo) of MoO 3 and MCM-41 before heating. This consists as expected of a superposition of the diffraction patterns of MCM-41 and MoO 3. After heating to 723K in air for 8 hours

2072

10000

i 5000

i

10

"

i

20 2theto

i

50

4o

Fig. 5 XRD patterns of a. MCM-41; b. MoO3/MCM-41 (5%Mo) mixture without heating; c. MoO3/MCM-41 (5%Mo) heated at 723K for 8 h ; d. MoO3/MCM-41(I%Mo ) heated at 723K for 8h; e. (NH4)2Mo7024 impregnated MCM-41 heated at 723K for 8h (figure 5c) the diffraction pattern of MoO 3 is almost completely attenuated, while that of MCM-41 is unaffected. For a sample containing l%Mo the MoO 3 pattern was completely removed after heating (Figure 5d).The disappearance of the MoO 3 diffraction pattern on heating the physical mixtures implies a decrease in the average crystallite size or increase in dispersion. Raman spectroscopy can follow changes in the molybdenum speciation at the molecular level [2]. Figure 6 shows Raman spectra of molybdenum MCM-41 catalysts. The Raman spectrum of the 5%Mo physical mixture of MoO 3 and MCM-41 before heating (figure 6a) shows only the characteristic Raman bands of MoO 3 (e.g. major bands at 998,822 and 669cm-1). After heating (figure 6b), these bands are still present, although some additional weak features can be observed between the 822 and 988cm-1 bands. The Raman spectrum of the l%Mo mixture before heating (not shown) likewise contained only bands due to MoO 3. After heating, the spectrum of the l%Mo sample (figure 6c) shows a complex set of new bands at for example 956, 840,609 and 500 cm-1, as well as residual MoO 3 bands. There is an extensive literature on the Raman spectra of supported molybdenum catalysts. Detailed discussion of the Raman data will be presented elsewhere; we note however that some of the new bands match those reported previously for hydrated molybdenum on silica catalysts and assigned to adsorbed polymolybdate clusters or other surface oxide phases[2]. Clearly some migration of molybdenum and spreading over the internal surface of the MCM-41 occurs on heating of the physical mixture s in air to 450C. At low molybdenum loadings (1%) this appears to occur quite efficiently, but the MCM-41 lacks the capacity to accommodate high loadings of surface bound molybdenum oxide (this same feature distinguishes silica gel from other more reactive oxide supports such as alumina or titania [2]).

2073

3000

998

I 1822

a.

2000

1000

1000

75"0 Raman shift, cm-1

560

25"0

Fig.6 Raman spectra of: a. physical mixture of 5% MoO3/MCM-41 without heating; b. a) heating at 723K for 8 h in air; c. 1% MoO3/MCM-41 calcined like b; d. 5% (NH4) 2 Mo7024 impregnated MCM-41 calcined like b. The % refers to Mo/MCM-41. As an alternative to dry mixing, MCM-41 was also impregnated with ammonium heptamolybdate solution using the incipient wetness technique (the standard method for preparing conventional supported molybdenum catalysts). Figure 5d shows the XRD pattern of such an impregnated material (5%Mo) after calcining in air at 450C. Impregnation with ammonium heptamolybdate greatly reduced the intensity of the diffraction lines of MCM-41 (the higher order peaks can hardly be detected in figure 5d), but after calcining no XRD pattern of MoO 3 could be detected for the impregnated catalysts. Likewise in the Raman spectrum of the same material (Figure 6d) the peaks due to MoO 3 are totally absent. The Raman spectrum shows some features similar to those seen in the physical mixtures after heating, but we have insufficient data at this point to make detailed assignments. An impregnated MCM-41 containing 10%Mo, on the other hand did show the MoO 3 Raman bands seen for the physical mixtures.

4. CONCLUSIONS The major conclusion for this study is that the silica MCM-41 behaves in every respect like silica gel as a support for molybdenum. Thus, the MoCI 5 grafting technique, impregnation with molybdate, or heating of physical mixtures with MoO 3 at low loading levels all appear to produce high dispersions of molybdenum in the MCM-41 pores; the silanol groups at the internal surface of MCM-41 have similar low reactivity to those on silica gal. To achieve high loadings of molybdenum in MCM-41 via graiting or MOCVD it will be necessary to have

2074 stronger interaction of the precursors with the MCM-41 surface. We note that this may be done by choosing a more reactive organometallic precursor, such as the metallocenes recently described by Maschmeyer et al.[ 13], or by modifying the surface reactivity.

5. ACKNOWLEDGMENTS

We acknowledge Prof. Ryong Ryoo's help for MCM-41 synthesis and Dr. Vittofio Luca for taking care of ESR measurements. The financial support from the Australian Research Council and AusAID(to SDD) is also gratefully acknowledged.

6. REFERENCES

1. R.F.Howe in Tailored Metal Catalysts, ed. Y. Iwasawa, Reidel, Dordrecht, 1986 2. H.Hu and I.E.Wachs, J.Phys.Chem., 99 (1995),10897 3. A.V.Kucherov and A.A.Slinkin, J.Mol. Catal., 90 (1994), 323 4. J.Thoret, C.Marchal, C.Doremieux-Morin, P.P.Man, M.Gruia and J.Fraissard, Zeolites, 13 (1993), 269 5. a.) C.Louis and M.Che, J.Catal, 135(1992), 156 and b.) C.Louis, M.Che and M.Anpo, J.Catal, 141(1993), 453 6. R.F.Howe, Inorg. Chem., 15 (1976), 486 7. a.) C.T.Kresge, M.E.Leonowicz, W.J.Roth, J.C.Varttfli, and J.S.Beck, Nature, 359 (1992), 710 and b.) 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 8. C.Y.Chen, H.X. Li and M. E. Davis, Microporous Mater., 2 (1993), 17 9. P.J.Branton, P.G.Hall, K.S.W.Sing, H.Reichert, F.Schuth and K.K.Unger, J.Chem.Soc.Faraday Trans., 90 (1994), 2965 10. Z.Luan, H.He, W.Zhou, C.Cheng and J.Klinowski, J.Chem.Soc.Faraday Tans., 91 (1995), 2955 11. a.) R.Ryoo and J.M.Kim, J.Chem. Soc., Chem.Commun., (1995), 711 and b.) J.M.Kim, J.H.Kwak, S.Jun and R.Ryoo, J.Phys. Chem., 99(1995), 16742 12. T.Blasco, A.Corma, M.T.Navarro and J.P.Pariente, J.Catal., 156 (1995), 65 13. T.Maschmeyer, F.Rey, G.Sankar and J.M.Thomas, Nature, 378 (1995), 159 14. A.Poppl, P.Baglioni and L.Kevan, J.Phys. Chem, 99 (1995), 14156 15. D.Zhao and D.Goldfarb, J.ChemSoc.,Chem.Commun., (1995), 875 16. C.Huber, K.MoHer and T.Bein, J.Chem.Soc., Chem.Commun., (1994), 2619 17. S.D.Djajanti and R.F.Howe in Zeolites : A Refined Tool for Designing Catalytic Sites; Studies in Surface Science and Catalysis, vol.97, L.Bonneviot and S.Kaliaguine,eds., (1995), 197 18. Yong, Y-S and R.F.Howe, J.Chem. Soc., Faraday Trans.l, 82 (1986), 2887 19. S.R.Seyedmonir and R.F.Howe, J.Chem.Soc.Faraday Trans.l, 80(1984), 87