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Epoxidation over niobium and titanium grafted MCM-41 and MCM-48 mesoporous molecular sieves.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
327
Epoxidation over niobium and titanium grafted MCM-41 and MCM-48 mesoporous molecular sieves. M. P. Kapoor' and Anuj Raj ^ ""Osaka National Research Institute, Department of Energy and Environment, Synthetic Chemistry Section, 1-8-31 Midorigaoka, Dceda, Osaka-563, Japan* ^National Institute of Materials and Chemical Research, Tailored Nanostructures Group, 1-1 Higashi, Tsukuba, Ibaraki-305-8565, Japan
Highly dispersed niobium or titanium containing molecular sieve catalysts were prepared by post synthesis modification of anchoring at surface silanol groups, using direct grafting of niobium and titanium compounds onto the totally accessible inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Different niobium and titanium compounds are used to accomplish the grafting. Catalytic epoxidation activity of niobium or titanium grafted mesoporous molecular sieves were studied using H2O2 as an oxidant. These niobium and titanium containing mesoporous molecular sieves exhibit higher epoxidation activity and better H2O2 selectivity.
1. INTRODUCTION Today's environmental concerns demand the streamlining of the catalytic processes for the production of fine chemicals. The utility of H2O2 as an oxidant is significant step in this direction due to the fact that its by-product is only water. Titanium incorporation in several silicalites such as silicalite-l(2), ZSM-12, ZSM-48 and zeolite-p are well documented and found to be effective for the selective oxidation of alkanes, the hydoxylation of phenols and the epoxidation of alkenes using H2O2 [1-6]. Later the synthesis and catalytic activity of titanium containing aluminophosphate molecular sieves viz. TAPO-5, TAPO-11, TAPO-31 and TAPO36 [7-8] were reported. The mesoporous silicas with large channel diameters (25-lOOA) could be used as selective oxidation catalyst to extend the capabilities to molecules with larger size [9-12]. The discovery of titanium containing siliceous mesoporous materials MCM-41 and MCM-48, where titanium is incorporated in the framework of mesoporous Si02, leads to remarkable catalytic performance utilising both diluted hydrogen peroxide and organic peroxides. These reactions are of prime importance to the fine chemical as well as pharmaceutical industries. Currently there is a great interest in titanium containing molecular sieve catalyst, in addition to these the incorporation of other metal ions (e.g., V, Cr, Co, Fe) by isomorphous substitution of Si has also been reported. Niobium is substitution in lattice position is less extensively studied but the use of niobium oxide as a support, promoter and as a solid acid has been studied in detail. Niobium containing materials in the ammoxidation of
328 propane and the oxidative dehydrogenation of propane is reported [13]. Recently, the synthesis of niobium silicalite [14] and niobium containing mesoporous molecular sieves [15] are reported with convincing evidence for isomorphous substitution of niobium ion. Another report describes the synthesis of a stable hexagonally packed mesoporous niobium oxide molecular sieve through a novel ligand-assisted templating mechanism [16]. Recently, the synthesis of niobium anchored in p structure was reported [17]. Also, the niobium containing materials has shown the significant activity in the large number of reactions [18-20]. Niobium and titanium incorporation in a molecular sieve can be achieved either by hydrothermal synthesis (direct synthesis) or by post-synthesis modification (secondary synthesis). The grafting method has shown promise for developing active oxidation catalyst in a simple and convenient way. Recently, the grafting of metallocene complexes onto mesoporous silica has been reported as alternate route to the synthesis of an active epoxidation catalyst [21]. Further the control of active sites, the specific removal of organic material (template or surfactant) occluded within mesoporous molecular sieves during synthesis can also be important and useful to develop an active epoxidation catalyst. Thermal method is quite often used to eliminate organic species from porous materials. However, several techniques such as supercritical fluid extraction ( S r e ) and plasma [22], ozone treatment [23], ion exchange [24-26] are also reported. In the present work the synthesis of highly dispersed niobium or titanium containing mesoporous molecular sieves catalyst by direct grafting of different niobium and titanium compounds is reported. Grafting is achieved by anchoring the desired compounds on the surface hydroxyl groups located on the inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Catalytic activity was evaluated in the liquid phase epoxidation of a-pinene with hydrogen peroxide as oxidant and the results are compared with widely studied titanium silicalites. The emphasis is directed mainly on catalytic applications of niobium or titanium anchored material to add a more detailed view on their structural physicochemical properties.
2.
EXPERIMENTAL
The mesoporous silica MCM-41 and Ti-MCM-41 samples were prepared using the procedure by Trong On et al. [27]. Similarly MCM-48 and Ti-MCM-48 were synthesized using the method reported by Tatsumi et. al. [28]. In the synthesis of Nb-MCM-41 and NbMCM-48 mesoporous molecular sieves, tetraethyl orthosilicate (TEOS) and niobium oxalate were used as sources of Si02 and NbjOs respectively. For Nb-MCM-41 the chemical composition of gels were SiO^: 0.02 N b P s : 0.13(CiJMA)2O: 0.13(TMA)2O: 0.13(NH4)2O : 55H2O. Oxalic acid was used to adjust the pH. For Nb-MCM-48, first a dispersed micellar solution of cetyltrimethylammonium bromide/hydroxide (Ci^TAMBr/OH) was prepared mixing a CigTAMBr solution (29 wt.% in water, Aldrich) with a hydroxide for halide exchange resin (IRA-400 (OH), Aldrich). The percentage exchange of bromide by hydroxide was 31.4 %. Another solution containing tetraethyl orthosilicate (TEOS) and niobium oxalate was prepared by stirring together for about 30 min. This solution was then added drop wise to the dispersed micellar solution under vigorous stirring at room temperature during a period of 1 h. The gel composition was Si02: 1.37 C^gTMA: 0.02 Nb.O,: 64 H2O. The gel was then transferred into a
329 Teflon-lined autoclave and healed to 373 K for 72 h for hydrothermal crystallisation. Solid products were recovered by filtration, washed thoroughly with distilled water and dried in air at 353 K. Finally the solids were calcined under continuous air tlow at 813 K for 6 h. 2.1. Template removal by solvent exchange process In a typical procedure, as made MCM-41 or MCM-48 (1.0 g) sample was added to 200-ml solution of 5% HCl in ethanol. The mixture was kept in closed container and stirred for 3 h at room temperature. The change in pH was constantly monitored. Sample was recovered by filtration and washed with distilled water and acetone. Finally, the samples were dried at 343 K overnight and calcined in air at 623 K for 2 h. These samples are designated as exchanged mesoporous MCM-41 and MCM-48 molecular sieves. 2.2. Liquid grafting procedures Titanium or niobium was grafted on both calcined as well as exchanged mesoporous molecular sieves MCM-41 and MCM-48. A desired amount of titanium butaoxide was first dissolved in methanol and the calcined or exchanged mesoporous MCM-41 and MCM-48 molecular sieves sample were add to the solution and kept for the 6 h at 323 K. Sample were recovered by filtration and initially dried at 373 K and finally calcined at 813 K for 6 h. In the other two steps procedure of titanium grafting, the weighed amount of titanocene dichloride was first diluted in chloroform and grafted on calcined or exchanged mesoporous molecular sieves sample. Solution was allowed to diffuse into pores of sample for at least 8 h at 333 K. The samples were then completely dried at 373 K for 5h. In the second step, the grafted samples (red color) were treated with triethylamine to activate the surface sites of the mesoporous sieves. The sufficient time (about 4 h) was allowed for the color of suspension to change from red to yellow. This confirmed the well establish substitution of the chloride with alkoxyl/siloxyl ligands had occurred. Samples were thoroughly washed with chloroform and remaining organic component were removed by calcination at 813 K for 6 h, leaving the white powdered mesopx^rous catalyst with 1.9 wt% grafted titanium. Niobium ethoxide or niobium oxalate was used to graft niobium on calcined as well as exchanged mesoporous MCM-41 and MCM-48 molecular sieves. Similar procedure was used as described for titanium grafting using titanium butaoxide. 2 3 . Characterization The X-ray diffraction (XRD) patterns of the sample were measured using Rigaku D-Max.II VC X-ray diftactometer using nickel filtered Cu Ka (X.= 1.5406 A) radiation. The specific BET surface area and average pore sizes were determined by Nj adsorption-desorption isotherms at 77 K using an Omnisorp-100. Diffuse reflectance UV-spectra were obtained using Perkin_Elmer Lambda 5 spectrophotometer using mesoporous silica MCM-41 or MCM-48 as a standard. The details are already reported earlier [27]. 2.4. Catalysis a-Pinene epoxidation reaction in a glass batch reactor under continuous stirring and reflux was performed using hydrogen peroxide as an oxidant over powdered titanium or niobium containing MCM-41 or MCM-48 catalyst prepared by direct hydrothermal synthesis and grafting route. Typical reaction procedure and related details are described elsewhere [27]
330 3. RESULTS AND DISCUSSION The powder X-ray diffraction patterns of the MCM-41 and MCM-48 samples are consistent with the XRD pattern of such material reported in literature and positively confirm the identity of the material [28-31]. The chemical composition and some important textural properties are given in Table 1 and 2. The pore radius was decreased on framework incorporation as well as grafting the niobium or titanium onto pure MCM-41 and MCM-48 but essentially were in similar range on grafting with different niobium or titanium compounds studied. The BET surface area of pure MCM-41 and MCM-48 calcined siliceous mesoporous were 1280 and 1310 m7g respectively and comparable to the ones previously reported for these materials. The surface area of the pure MCM-41 and MCM-48 sample where template was removed by solvent exchange method followed by calcination was lower than pure calcined samples but leads to larger pore size. Comparatively lower surface area for these samples is likely due to re-adjustment of the long-range order of the mesopores on removal of the template, into the silica framework. Further, both niobium and titanium grafted samples again showed lower surface area compare to the samples obtained by direct niobium or titanium incorporation by the hydrothermal synthesis. Table: 1 Descriptions, chemical composition and textural properties of MCM-41. Catalyst Pore radius, BET Surface area A m "g '^
cl.joo spacmg A
MCM-41' MCM-41 ^
18.0 19.0
1280 1158
36.5 37.1
2.0 wt% Nb-MCM-4r 2.0 wt% Ti-MCM-4r
14.2 16.5
1067 1230
31.8 32.7
1.9wt%Nb-MCM-^l='*' 1.9 wt%Nb-MCM-41 ^*^ 1.9 wt%Ti-MCM-41'*^ 1.9 wt%Ti-MCM-41 ^"^
13.1 13.5 15.5 16.0
924 908 1162 1146
30.6 30.5 31.3 314
1.9wt%Nb-MCM-4l'*' 1.9 wt%Nb-MCM-41'"^ 1.9wt%Ti-MCM-41'^' 1.9wt%Ti-MCM41'"s
13.2 13.5 15.6 16.0
902 891 1092 1073
30.9 30.9 31.5 31.7
* template removed by calcination. ^ template removed by solvent exchange and then calcined. ^' hydrothermal synthesis. ^' grafted with niobium ethoxide. " grafted with niobium oxalate. ^ grafted with titanium butaoxide. ^ grafted with titanocene dichloride. Diffuse reflectance UV-spectra showed the framework incorporation of titanium and niobium in the all sample studied. The detailed characterisation of the materials will be presented in the subsequent paper. The catalytic properties are studied by the epoxidation of apinene, using H2O2 as an oxidizing agent. The framework 2.0wt % Ti-MCM-41 and 2.0wt % Ti-MCM-48 mesoporous molecular sieves samples synthesized by direct hydrothermal route.
331 surface areas 1230 m'/g and 1191 mVg respectively, exhibit relatively fair conversion and the poor H2O2 efficiency with a epoxide as a sole reaction product. (Table 3). The presence of framework niobium showed an increase in activity and H2O2 efficiency compared to framework titanium. Over niobium containing mesoporous molecular sieves 1,2 pinane diol was also observed as the result of epoxy ring cleavage. Table 2 Descriptions, chemical composition and textural properties of MCM-48. Pore radius, BET Surface area d.^oo spacing Catalyst A m^g"^ A MCM-48' 35.3 16.3 1310 MCM-48 ^ 36.8 17.5 1124 2.0 wt% Nb-MCM-48'^ 2.0 wt% Ti-MCM-48'^
12.2 14.7
998 1191
30.1 30.9
1.9 wt%Nb-MCM-48 ^"^'^ 1.9wt%Nb-MCM^8^"'= 1.9wt%Ti-MCM^8^"' 1.9 wt%Ti-MCM-48 ^"s
10.6 11.1 12.4 13.5
972 847 1026 1008
29.2 29.4 30.1 30.0
1.9wt%Nb-MCM-48'*' 1.9 wt%Nb-MCM-48^"' 1.9wt%Ti-MCM-48^^' 1.9 wt%Ti-MCM-48'"2 Key as illustrated in Table 1.
10.5 10.8 12.5 13.4
892 801 966 913
29.6 29.6 30.0 29.8
Table 3 a-Pinene epoxidation over calcined niobium or titanium containing mesoporous molecular sieves prepared by direct hydrothermal synthesis a-Pinene Products (mol.%) H202(mol.%) Catalyst
ccpinene oxide 92.6 100
1,2 pinane diol 7.4
conversion
efficiency
2.0wt % Nb-MCM-41 2.0wt % Ti-MCM-41
conversion ("^^^•^^) 8.5 6.1
46.8 44.2
16.4 11.6
2.0wt % Nb-MCM-48 2.0wt % Ti-MCM-48
9.1 7.8
90.7 100
9.3
43.3 50.0
19.3 13.1
Reaction conditions: catalyst, O.lg; a-pinene, 0.037 mol; H2O2 (30% aqueous solution), 0.044 mol; reaction temperature, 328 K; reaction duration, 5 h. Acidity of niobium containing MCM-41 mesoporous materials, as reported by Ziolek et. al [15], are sufficient to provide the relatively mild acidic sites necessary to cleave the epoxy ring leading to diols. The a-pinene conversions and H2O2 selectivities were always higher with MCM-48 when compared with MCM-41 molecular sieves. This is probably due to the
332 topology, MCM-41 is comprised of unidimensional array of hexagonally arranged pore system (hexagonal), which consist of straight tube like channels while MCM-48 contains two independent three dimensional pore systems, which are interwoven and situated in a mirrorplane position to each other (cubic). Table 4 a-Pinene epoxidation on niobium or titanium grafted mesoporous molecular sieves where the template was removed by calcination. Products (moL%) a-Pinene H A ( mol.%) Conversion " Catalyst conversion efficiency 1,2 pinane a-pinene (mol.%) diol oxide 43.4 1.9wt % Nb-MCM-41' 18.8 9.2 8.9 90.8 1.9wt % Nb-MCM-41' 42.9 11.8 20.7 88.8 9.4 1.9wt % Nb-MCM-48' 22.5 49.7 13.7 86.3 11.7 1.9wt % Nb-MCM-48 ^ 16.9 51.3 25.3 83.1 13.2 1.9wt%Ti-MCM-41'= 1.9wt%Ti-MCM-41' 1.9wt%Ti-MCM-48'= 1.9wt % Ti-MCM-48'
6.9 7.7 8.6 9.4
100 97.9 100 97.3
— 2.1 — 2.7
42.3 45.6 45.7 44.8
13.7 14.5 15.8 18.1
Reaction conditions as described in Table 3. "" grafted with niobium ethoxide. ^ grafted with niobium oxalate. *" grafted with titanium butaoxide. ** grafted with titanocene dichloride. Catalytic data shown in Table 4 indicates that on systematic incorporation of niobium or titanium (~1.9wt % by ICP) onto calcined MCM-41 and MCM-48 molecular sieves by grafting results in an increase in the a-pinene conversion as well as improves the HjOj efficiency. However, the activity and the product selectivity differ with the type of niobium or titanium compound used for grafting. When niobium oxalate was used as grafting agent, the higher diols formation and maximum a-pinene conversions and H2O2 efficiency were observed. Very little diols formation was also seen when titanocene dichloride was used for grafting. This is likely due to the presence of traces of chloride ion, which could provide mild acidic sites and which are responsible for the ring opening of the epoxide. While the samples grafted with niobium ethoxide or titanium butaoxide also showed a reasonable increase in catalytic activity and H2O2 efficiency. Again the activity and selectivity of the niobium-grafted samples was always higher than that of titanium grafted samples. Table 5 lists the results of a-pinene conversion obtained over niobium or titanium grafted MCM-41 and MCM-48 samples where the template was first removed by solvent exchange method followed by calcination. The catalytic activities are comparatively higher than ones obtained over niobium or titanium grafted MCM-41 and MCM-48 samples where template was removed using a conventional thermal method (direct calcination). A similar trend for the H2O2 efficiency was noticed.
333 Table 5 Epoxidation over niobium or titanium grafted mesoporous molecular sieves where template was removed by solvent exchange method prior to calcination Products (mol.%) a-Pinene H A ( mol.%) conversion a-pinene Catalyst 1,2 pinane conversion efficiency (mol.%) diol oxide 1.9wt % Nb-MCM-41 ^ 19.6 43.4 10.1 89.9 9.2 1.9wt % Nb-MCM-41 ^ 22.1 42.4 12.6 9.9 87.4 1.9wt % Nb-MCM-48 ^ 24.5 53.6 14.9 13.6 85.1 1.9wt % Nb-MCM-48' 27.2 55.2 18.3 15.1 81.7 1.9wt 1.9wt 1.9wt 1.9wt
% Ti-MCM-41 ^ % Ti-MCM-41"" % Ti-MCM-48"" % Ti-MCM-48""
7.3 8.0 9.1 9.6
100 97.1 100 95.8
2.9 4.2
40.4 38.9 38.4 40.8
15.2 17.8 19.9 20.6
Key as illustrated in Tables 3 & 4. Significantly higher catalytic activity of niobium or titanium grafted molecular sieves compared to sample prepared by direct hydrothermal route where niobium or titanium in framework, is likely due to the presence of surface sites which could be isolated and tetrahedral in nature to provide better performance. However, the presence of dimers or oligomers could be responsible for decomposition of the peroxide. The octahedrally co-ordinated titanium, which is usually inactive for the epoxidation of alkenes as it lacks free co-ordination sites [21,32]. Also, the high concentration of the silanol groups as well as hydroxy 1 groups present on the wall surface is responsible for the decomposition of H2O2. Therefore, in case of grafted mesoporous molecular sieves, probably the average structure of the catalytic site is mainly tetrahedral and/or the site might be composed of a mixture of different tetrahedral species. Further, the template removal through solvent exchange procedure is much more effective as evident, for complete removal of organic template as compared to standard calcination method. From general point of view the removal of organic species from mesoporous molecular sieves through calcination usually results water, decomposed hydrocarbons along with some forms of nitrogen and bromine compounds. The possibility of irreversible adsorption of decomposed material onto the inner wall of mesopores may cause the reduced pore size. However, in the case of template removed by solvent exchange procedure almost every organic species washed out from the mesopore and rendered the comparatively bigger pore size that eventually may enhance the catalytic activity.
4. CONCLUSION In agreement with catalytic results it is clear that upon direct grafting, a very high dispersion of isolated tetrahedral centres may be generated on the walls of mesoporous MCM-41 and MCM-48. This in turn allows for the possible tuning to improve the catalytic activity while preserving the mesoporous framework intact. Epoxidation with samples where template was removed by solvent extraction proceeds at better rate than with other mesoporous samples.
334 Further studies to clarify the exact nature of possible active sites are worth pursuing to design novel catalysts.
REFERENCES 1. M. Taramasso, G. Perego and B. Notari, US Patent 4,410, 501. 2. J. S. Reddy and R. Kumar, Zeolites , 12 (1992) 95. 3. T. Blasco, M.A.Comblor, A. Corma, J. Perez-Pariente, J.Am.Chem.Soc.,115(1993) 11806. 4. M. A. Comblor, A. Corma, A. Martine, J. Perez-Pari ente, Chem. Commun., (1992) 589. 5. A. Tuel, Zeolites, 15 (1995) 236. 6. D. P. Serrano, H.X. Li and M.E.Davis, Chem. Commun., (1992) 745. 7. M.H.Z. Niaki, M. P. Kapoor and S. Kaliaguine, ProcH"' Int'l zeolite conf. (1999) 1221. 8. M.H.Z. Niaki, M. P. Kapoor and S. Kaliaguine, J. Catal, 177 (1998) 231. 9. C. T. Kresge, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 10. J. S. Beck, J. C. Vartuli, W. J. Roth, J. Am. Chem. Soc, 114 (1992) 10834. 11. P. T. Tanev, M. Chibwe and T. J. Pinnavaia, Nature, 368 (1994) 321. 12. P. T. Tanev and T. J. Pinnavaia, Science, 267 (1995) 865. 13. O. Desponds, R.L.Keiski and G.A.Somarjai, Catal. Lett., 19 (1997) 17. 14. A. M. Prakash and Larry Kevan, J. Am. Chem. Soc., 120 (1998) 13148. 15. M. Ziolek and L Nowak, Catal. Lett., 45 (1997) 259. 16. D.M.Antonelli and J.Y.Ying, Angew. Chem. Int. Ed. Engl., 35(1995) 426. 17. M. P. Kapoor, J. Matr. Chem., (1999) communicated. 18. Y. Wada and A. Morikawa, Catal. Today, 8(1990) 13. 19. K. Tanabe, Catal. Today, 8 (1990) 1. 20. J. M. Jehng and I.E. Wachs, Catal. Today, 8 (1990) 37. 21. T. Maschmeyer, F. Ray, G. Sankar and J. M. Thomas, Nature, 378, (1995), 159 22. S. Kawi and M.W. Lai, Chem. Commun., (1998),1407. 23. M.T.J Keene, R. Denoyel and P. L. Llewellyn, Chem. Cummun., (1998), 2203. 24. S. Hitz and R. Prins, J. Catal., 168,(1997),194. 25. C.Y.Chen, H.X.Li and M.E. Davis, Microporous Mater., 2, (1993), 17. 26. P. T. Tanev and T.J. Pinnavaia, Chem. Matr., 8, (1996), 2068. 27. D. Trong On, M. P. Kapoor, L. Bonneviot and S. Kaliaguine, Catal. Lett., 44 (1997) 157. 28. K. A.Koyano and T.Tatsumi, Chem.Commun.(1996) 145. 29. A. Corma, Topics in Catalysis, 4 (1997) 249. 30. L.Y. Chen, G.K. Chuah and S. Jaenicke, Catalysis Letter, 50 (1998) 107. 31. J.C Vartuli, K. D. Schmidt et. al., Chem. Mater., 6 (1994) 2317. 32. B.Notari, Adv. Catal., 41 (1996) 252.
327
Epoxidation over niobium and titanium grafted MCM-41 and MCM-48 mesoporous molecular sieves. M. P. Kapoor' and Anuj Raj ^ ""Osaka National Research Institute, Department of Energy and Environment, Synthetic Chemistry Section, 1-8-31 Midorigaoka, Dceda, Osaka-563, Japan* ^National Institute of Materials and Chemical Research, Tailored Nanostructures Group, 1-1 Higashi, Tsukuba, Ibaraki-305-8565, Japan
Highly dispersed niobium or titanium containing molecular sieve catalysts were prepared by post synthesis modification of anchoring at surface silanol groups, using direct grafting of niobium and titanium compounds onto the totally accessible inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Different niobium and titanium compounds are used to accomplish the grafting. Catalytic epoxidation activity of niobium or titanium grafted mesoporous molecular sieves were studied using H2O2 as an oxidant. These niobium and titanium containing mesoporous molecular sieves exhibit higher epoxidation activity and better H2O2 selectivity.
1. INTRODUCTION Today's environmental concerns demand the streamlining of the catalytic processes for the production of fine chemicals. The utility of H2O2 as an oxidant is significant step in this direction due to the fact that its by-product is only water. Titanium incorporation in several silicalites such as silicalite-l(2), ZSM-12, ZSM-48 and zeolite-p are well documented and found to be effective for the selective oxidation of alkanes, the hydoxylation of phenols and the epoxidation of alkenes using H2O2 [1-6]. Later the synthesis and catalytic activity of titanium containing aluminophosphate molecular sieves viz. TAPO-5, TAPO-11, TAPO-31 and TAPO36 [7-8] were reported. The mesoporous silicas with large channel diameters (25-lOOA) could be used as selective oxidation catalyst to extend the capabilities to molecules with larger size [9-12]. The discovery of titanium containing siliceous mesoporous materials MCM-41 and MCM-48, where titanium is incorporated in the framework of mesoporous Si02, leads to remarkable catalytic performance utilising both diluted hydrogen peroxide and organic peroxides. These reactions are of prime importance to the fine chemical as well as pharmaceutical industries. Currently there is a great interest in titanium containing molecular sieve catalyst, in addition to these the incorporation of other metal ions (e.g., V, Cr, Co, Fe) by isomorphous substitution of Si has also been reported. Niobium is substitution in lattice position is less extensively studied but the use of niobium oxide as a support, promoter and as a solid acid has been studied in detail. Niobium containing materials in the ammoxidation of
328 propane and the oxidative dehydrogenation of propane is reported [13]. Recently, the synthesis of niobium silicalite [14] and niobium containing mesoporous molecular sieves [15] are reported with convincing evidence for isomorphous substitution of niobium ion. Another report describes the synthesis of a stable hexagonally packed mesoporous niobium oxide molecular sieve through a novel ligand-assisted templating mechanism [16]. Recently, the synthesis of niobium anchored in p structure was reported [17]. Also, the niobium containing materials has shown the significant activity in the large number of reactions [18-20]. Niobium and titanium incorporation in a molecular sieve can be achieved either by hydrothermal synthesis (direct synthesis) or by post-synthesis modification (secondary synthesis). The grafting method has shown promise for developing active oxidation catalyst in a simple and convenient way. Recently, the grafting of metallocene complexes onto mesoporous silica has been reported as alternate route to the synthesis of an active epoxidation catalyst [21]. Further the control of active sites, the specific removal of organic material (template or surfactant) occluded within mesoporous molecular sieves during synthesis can also be important and useful to develop an active epoxidation catalyst. Thermal method is quite often used to eliminate organic species from porous materials. However, several techniques such as supercritical fluid extraction ( S r e ) and plasma [22], ozone treatment [23], ion exchange [24-26] are also reported. In the present work the synthesis of highly dispersed niobium or titanium containing mesoporous molecular sieves catalyst by direct grafting of different niobium and titanium compounds is reported. Grafting is achieved by anchoring the desired compounds on the surface hydroxyl groups located on the inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Catalytic activity was evaluated in the liquid phase epoxidation of a-pinene with hydrogen peroxide as oxidant and the results are compared with widely studied titanium silicalites. The emphasis is directed mainly on catalytic applications of niobium or titanium anchored material to add a more detailed view on their structural physicochemical properties.
2.
EXPERIMENTAL
The mesoporous silica MCM-41 and Ti-MCM-41 samples were prepared using the procedure by Trong On et al. [27]. Similarly MCM-48 and Ti-MCM-48 were synthesized using the method reported by Tatsumi et. al. [28]. In the synthesis of Nb-MCM-41 and NbMCM-48 mesoporous molecular sieves, tetraethyl orthosilicate (TEOS) and niobium oxalate were used as sources of Si02 and NbjOs respectively. For Nb-MCM-41 the chemical composition of gels were SiO^: 0.02 N b P s : 0.13(CiJMA)2O: 0.13(TMA)2O: 0.13(NH4)2O : 55H2O. Oxalic acid was used to adjust the pH. For Nb-MCM-48, first a dispersed micellar solution of cetyltrimethylammonium bromide/hydroxide (Ci^TAMBr/OH) was prepared mixing a CigTAMBr solution (29 wt.% in water, Aldrich) with a hydroxide for halide exchange resin (IRA-400 (OH), Aldrich). The percentage exchange of bromide by hydroxide was 31.4 %. Another solution containing tetraethyl orthosilicate (TEOS) and niobium oxalate was prepared by stirring together for about 30 min. This solution was then added drop wise to the dispersed micellar solution under vigorous stirring at room temperature during a period of 1 h. The gel composition was Si02: 1.37 C^gTMA: 0.02 Nb.O,: 64 H2O. The gel was then transferred into a
329 Teflon-lined autoclave and healed to 373 K for 72 h for hydrothermal crystallisation. Solid products were recovered by filtration, washed thoroughly with distilled water and dried in air at 353 K. Finally the solids were calcined under continuous air tlow at 813 K for 6 h. 2.1. Template removal by solvent exchange process In a typical procedure, as made MCM-41 or MCM-48 (1.0 g) sample was added to 200-ml solution of 5% HCl in ethanol. The mixture was kept in closed container and stirred for 3 h at room temperature. The change in pH was constantly monitored. Sample was recovered by filtration and washed with distilled water and acetone. Finally, the samples were dried at 343 K overnight and calcined in air at 623 K for 2 h. These samples are designated as exchanged mesoporous MCM-41 and MCM-48 molecular sieves. 2.2. Liquid grafting procedures Titanium or niobium was grafted on both calcined as well as exchanged mesoporous molecular sieves MCM-41 and MCM-48. A desired amount of titanium butaoxide was first dissolved in methanol and the calcined or exchanged mesoporous MCM-41 and MCM-48 molecular sieves sample were add to the solution and kept for the 6 h at 323 K. Sample were recovered by filtration and initially dried at 373 K and finally calcined at 813 K for 6 h. In the other two steps procedure of titanium grafting, the weighed amount of titanocene dichloride was first diluted in chloroform and grafted on calcined or exchanged mesoporous molecular sieves sample. Solution was allowed to diffuse into pores of sample for at least 8 h at 333 K. The samples were then completely dried at 373 K for 5h. In the second step, the grafted samples (red color) were treated with triethylamine to activate the surface sites of the mesoporous sieves. The sufficient time (about 4 h) was allowed for the color of suspension to change from red to yellow. This confirmed the well establish substitution of the chloride with alkoxyl/siloxyl ligands had occurred. Samples were thoroughly washed with chloroform and remaining organic component were removed by calcination at 813 K for 6 h, leaving the white powdered mesopx^rous catalyst with 1.9 wt% grafted titanium. Niobium ethoxide or niobium oxalate was used to graft niobium on calcined as well as exchanged mesoporous MCM-41 and MCM-48 molecular sieves. Similar procedure was used as described for titanium grafting using titanium butaoxide. 2 3 . Characterization The X-ray diffraction (XRD) patterns of the sample were measured using Rigaku D-Max.II VC X-ray diftactometer using nickel filtered Cu Ka (X.= 1.5406 A) radiation. The specific BET surface area and average pore sizes were determined by Nj adsorption-desorption isotherms at 77 K using an Omnisorp-100. Diffuse reflectance UV-spectra were obtained using Perkin_Elmer Lambda 5 spectrophotometer using mesoporous silica MCM-41 or MCM-48 as a standard. The details are already reported earlier [27]. 2.4. Catalysis a-Pinene epoxidation reaction in a glass batch reactor under continuous stirring and reflux was performed using hydrogen peroxide as an oxidant over powdered titanium or niobium containing MCM-41 or MCM-48 catalyst prepared by direct hydrothermal synthesis and grafting route. Typical reaction procedure and related details are described elsewhere [27]
330 3. RESULTS AND DISCUSSION The powder X-ray diffraction patterns of the MCM-41 and MCM-48 samples are consistent with the XRD pattern of such material reported in literature and positively confirm the identity of the material [28-31]. The chemical composition and some important textural properties are given in Table 1 and 2. The pore radius was decreased on framework incorporation as well as grafting the niobium or titanium onto pure MCM-41 and MCM-48 but essentially were in similar range on grafting with different niobium or titanium compounds studied. The BET surface area of pure MCM-41 and MCM-48 calcined siliceous mesoporous were 1280 and 1310 m7g respectively and comparable to the ones previously reported for these materials. The surface area of the pure MCM-41 and MCM-48 sample where template was removed by solvent exchange method followed by calcination was lower than pure calcined samples but leads to larger pore size. Comparatively lower surface area for these samples is likely due to re-adjustment of the long-range order of the mesopores on removal of the template, into the silica framework. Further, both niobium and titanium grafted samples again showed lower surface area compare to the samples obtained by direct niobium or titanium incorporation by the hydrothermal synthesis. Table: 1 Descriptions, chemical composition and textural properties of MCM-41. Catalyst Pore radius, BET Surface area A m "g '^
cl.joo spacmg A
MCM-41' MCM-41 ^
18.0 19.0
1280 1158
36.5 37.1
2.0 wt% Nb-MCM-4r 2.0 wt% Ti-MCM-4r
14.2 16.5
1067 1230
31.8 32.7
1.9wt%Nb-MCM-^l='*' 1.9 wt%Nb-MCM-41 ^*^ 1.9 wt%Ti-MCM-41'*^ 1.9 wt%Ti-MCM-41 ^"^
13.1 13.5 15.5 16.0
924 908 1162 1146
30.6 30.5 31.3 314
1.9wt%Nb-MCM-4l'*' 1.9 wt%Nb-MCM-41'"^ 1.9wt%Ti-MCM-41'^' 1.9wt%Ti-MCM41'"s
13.2 13.5 15.6 16.0
902 891 1092 1073
30.9 30.9 31.5 31.7
* template removed by calcination. ^ template removed by solvent exchange and then calcined. ^' hydrothermal synthesis. ^' grafted with niobium ethoxide. " grafted with niobium oxalate. ^ grafted with titanium butaoxide. ^ grafted with titanocene dichloride. Diffuse reflectance UV-spectra showed the framework incorporation of titanium and niobium in the all sample studied. The detailed characterisation of the materials will be presented in the subsequent paper. The catalytic properties are studied by the epoxidation of apinene, using H2O2 as an oxidizing agent. The framework 2.0wt % Ti-MCM-41 and 2.0wt % Ti-MCM-48 mesoporous molecular sieves samples synthesized by direct hydrothermal route.
331 surface areas 1230 m'/g and 1191 mVg respectively, exhibit relatively fair conversion and the poor H2O2 efficiency with a epoxide as a sole reaction product. (Table 3). The presence of framework niobium showed an increase in activity and H2O2 efficiency compared to framework titanium. Over niobium containing mesoporous molecular sieves 1,2 pinane diol was also observed as the result of epoxy ring cleavage. Table 2 Descriptions, chemical composition and textural properties of MCM-48. Pore radius, BET Surface area d.^oo spacing Catalyst A m^g"^ A MCM-48' 35.3 16.3 1310 MCM-48 ^ 36.8 17.5 1124 2.0 wt% Nb-MCM-48'^ 2.0 wt% Ti-MCM-48'^
12.2 14.7
998 1191
30.1 30.9
1.9 wt%Nb-MCM-48 ^"^'^ 1.9wt%Nb-MCM^8^"'= 1.9wt%Ti-MCM^8^"' 1.9 wt%Ti-MCM-48 ^"s
10.6 11.1 12.4 13.5
972 847 1026 1008
29.2 29.4 30.1 30.0
1.9wt%Nb-MCM-48'*' 1.9 wt%Nb-MCM-48^"' 1.9wt%Ti-MCM-48^^' 1.9 wt%Ti-MCM-48'"2 Key as illustrated in Table 1.
10.5 10.8 12.5 13.4
892 801 966 913
29.6 29.6 30.0 29.8
Table 3 a-Pinene epoxidation over calcined niobium or titanium containing mesoporous molecular sieves prepared by direct hydrothermal synthesis a-Pinene Products (mol.%) H202(mol.%) Catalyst
ccpinene oxide 92.6 100
1,2 pinane diol 7.4
conversion
efficiency
2.0wt % Nb-MCM-41 2.0wt % Ti-MCM-41
conversion ("^^^•^^) 8.5 6.1
46.8 44.2
16.4 11.6
2.0wt % Nb-MCM-48 2.0wt % Ti-MCM-48
9.1 7.8
90.7 100
9.3
43.3 50.0
19.3 13.1
Reaction conditions: catalyst, O.lg; a-pinene, 0.037 mol; H2O2 (30% aqueous solution), 0.044 mol; reaction temperature, 328 K; reaction duration, 5 h. Acidity of niobium containing MCM-41 mesoporous materials, as reported by Ziolek et. al [15], are sufficient to provide the relatively mild acidic sites necessary to cleave the epoxy ring leading to diols. The a-pinene conversions and H2O2 selectivities were always higher with MCM-48 when compared with MCM-41 molecular sieves. This is probably due to the
332 topology, MCM-41 is comprised of unidimensional array of hexagonally arranged pore system (hexagonal), which consist of straight tube like channels while MCM-48 contains two independent three dimensional pore systems, which are interwoven and situated in a mirrorplane position to each other (cubic). Table 4 a-Pinene epoxidation on niobium or titanium grafted mesoporous molecular sieves where the template was removed by calcination. Products (moL%) a-Pinene H A ( mol.%) Conversion " Catalyst conversion efficiency 1,2 pinane a-pinene (mol.%) diol oxide 43.4 1.9wt % Nb-MCM-41' 18.8 9.2 8.9 90.8 1.9wt % Nb-MCM-41' 42.9 11.8 20.7 88.8 9.4 1.9wt % Nb-MCM-48' 22.5 49.7 13.7 86.3 11.7 1.9wt % Nb-MCM-48 ^ 16.9 51.3 25.3 83.1 13.2 1.9wt%Ti-MCM-41'= 1.9wt%Ti-MCM-41' 1.9wt%Ti-MCM-48'= 1.9wt % Ti-MCM-48'
6.9 7.7 8.6 9.4
100 97.9 100 97.3
— 2.1 — 2.7
42.3 45.6 45.7 44.8
13.7 14.5 15.8 18.1
Reaction conditions as described in Table 3. "" grafted with niobium ethoxide. ^ grafted with niobium oxalate. *" grafted with titanium butaoxide. ** grafted with titanocene dichloride. Catalytic data shown in Table 4 indicates that on systematic incorporation of niobium or titanium (~1.9wt % by ICP) onto calcined MCM-41 and MCM-48 molecular sieves by grafting results in an increase in the a-pinene conversion as well as improves the HjOj efficiency. However, the activity and the product selectivity differ with the type of niobium or titanium compound used for grafting. When niobium oxalate was used as grafting agent, the higher diols formation and maximum a-pinene conversions and H2O2 efficiency were observed. Very little diols formation was also seen when titanocene dichloride was used for grafting. This is likely due to the presence of traces of chloride ion, which could provide mild acidic sites and which are responsible for the ring opening of the epoxide. While the samples grafted with niobium ethoxide or titanium butaoxide also showed a reasonable increase in catalytic activity and H2O2 efficiency. Again the activity and selectivity of the niobium-grafted samples was always higher than that of titanium grafted samples. Table 5 lists the results of a-pinene conversion obtained over niobium or titanium grafted MCM-41 and MCM-48 samples where the template was first removed by solvent exchange method followed by calcination. The catalytic activities are comparatively higher than ones obtained over niobium or titanium grafted MCM-41 and MCM-48 samples where template was removed using a conventional thermal method (direct calcination). A similar trend for the H2O2 efficiency was noticed.
333 Table 5 Epoxidation over niobium or titanium grafted mesoporous molecular sieves where template was removed by solvent exchange method prior to calcination Products (mol.%) a-Pinene H A ( mol.%) conversion a-pinene Catalyst 1,2 pinane conversion efficiency (mol.%) diol oxide 1.9wt % Nb-MCM-41 ^ 19.6 43.4 10.1 89.9 9.2 1.9wt % Nb-MCM-41 ^ 22.1 42.4 12.6 9.9 87.4 1.9wt % Nb-MCM-48 ^ 24.5 53.6 14.9 13.6 85.1 1.9wt % Nb-MCM-48' 27.2 55.2 18.3 15.1 81.7 1.9wt 1.9wt 1.9wt 1.9wt
% Ti-MCM-41 ^ % Ti-MCM-41"" % Ti-MCM-48"" % Ti-MCM-48""
7.3 8.0 9.1 9.6
100 97.1 100 95.8
2.9 4.2
40.4 38.9 38.4 40.8
15.2 17.8 19.9 20.6
Key as illustrated in Tables 3 & 4. Significantly higher catalytic activity of niobium or titanium grafted molecular sieves compared to sample prepared by direct hydrothermal route where niobium or titanium in framework, is likely due to the presence of surface sites which could be isolated and tetrahedral in nature to provide better performance. However, the presence of dimers or oligomers could be responsible for decomposition of the peroxide. The octahedrally co-ordinated titanium, which is usually inactive for the epoxidation of alkenes as it lacks free co-ordination sites [21,32]. Also, the high concentration of the silanol groups as well as hydroxy 1 groups present on the wall surface is responsible for the decomposition of H2O2. Therefore, in case of grafted mesoporous molecular sieves, probably the average structure of the catalytic site is mainly tetrahedral and/or the site might be composed of a mixture of different tetrahedral species. Further, the template removal through solvent exchange procedure is much more effective as evident, for complete removal of organic template as compared to standard calcination method. From general point of view the removal of organic species from mesoporous molecular sieves through calcination usually results water, decomposed hydrocarbons along with some forms of nitrogen and bromine compounds. The possibility of irreversible adsorption of decomposed material onto the inner wall of mesopores may cause the reduced pore size. However, in the case of template removed by solvent exchange procedure almost every organic species washed out from the mesopore and rendered the comparatively bigger pore size that eventually may enhance the catalytic activity.
4. CONCLUSION In agreement with catalytic results it is clear that upon direct grafting, a very high dispersion of isolated tetrahedral centres may be generated on the walls of mesoporous MCM-41 and MCM-48. This in turn allows for the possible tuning to improve the catalytic activity while preserving the mesoporous framework intact. Epoxidation with samples where template was removed by solvent extraction proceeds at better rate than with other mesoporous samples.
334 Further studies to clarify the exact nature of possible active sites are worth pursuing to design novel catalysts.
REFERENCES 1. M. Taramasso, G. Perego and B. Notari, US Patent 4,410, 501. 2. J. S. Reddy and R. Kumar, Zeolites , 12 (1992) 95. 3. T. Blasco, M.A.Comblor, A. Corma, J. Perez-Pariente, J.Am.Chem.Soc.,115(1993) 11806. 4. M. A. Comblor, A. Corma, A. Martine, J. Perez-Pari ente, Chem. Commun., (1992) 589. 5. A. Tuel, Zeolites, 15 (1995) 236. 6. D. P. Serrano, H.X. Li and M.E.Davis, Chem. Commun., (1992) 745. 7. M.H.Z. Niaki, M. P. Kapoor and S. Kaliaguine, ProcH"' Int'l zeolite conf. (1999) 1221. 8. M.H.Z. Niaki, M. P. Kapoor and S. Kaliaguine, J. Catal, 177 (1998) 231. 9. C. T. Kresge, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 10. J. S. Beck, J. C. Vartuli, W. J. Roth, J. Am. Chem. Soc, 114 (1992) 10834. 11. P. T. Tanev, M. Chibwe and T. J. Pinnavaia, Nature, 368 (1994) 321. 12. P. T. Tanev and T. J. Pinnavaia, Science, 267 (1995) 865. 13. O. Desponds, R.L.Keiski and G.A.Somarjai, Catal. Lett., 19 (1997) 17. 14. A. M. Prakash and Larry Kevan, J. Am. Chem. Soc., 120 (1998) 13148. 15. M. Ziolek and L Nowak, Catal. Lett., 45 (1997) 259. 16. D.M.Antonelli and J.Y.Ying, Angew. Chem. Int. Ed. Engl., 35(1995) 426. 17. M. P. Kapoor, J. Matr. Chem., (1999) communicated. 18. Y. Wada and A. Morikawa, Catal. Today, 8(1990) 13. 19. K. Tanabe, Catal. Today, 8 (1990) 1. 20. J. M. Jehng and I.E. Wachs, Catal. Today, 8 (1990) 37. 21. T. Maschmeyer, F. Ray, G. Sankar and J. M. Thomas, Nature, 378, (1995), 159 22. S. Kawi and M.W. Lai, Chem. Commun., (1998),1407. 23. M.T.J Keene, R. Denoyel and P. L. Llewellyn, Chem. Cummun., (1998), 2203. 24. S. Hitz and R. Prins, J. Catal., 168,(1997),194. 25. C.Y.Chen, H.X.Li and M.E. Davis, Microporous Mater., 2, (1993), 17. 26. P. T. Tanev and T.J. Pinnavaia, Chem. Matr., 8, (1996), 2068. 27. D. Trong On, M. P. Kapoor, L. Bonneviot and S. Kaliaguine, Catal. Lett., 44 (1997) 157. 28. K. A.Koyano and T.Tatsumi, Chem.Commun.(1996) 145. 29. A. Corma, Topics in Catalysis, 4 (1997) 249. 30. L.Y. Chen, G.K. Chuah and S. Jaenicke, Catalysis Letter, 50 (1998) 107. 31. J.C Vartuli, K. D. Schmidt et. al., Chem. Mater., 6 (1994) 2317. 32. B.Notari, Adv. Catal., 41 (1996) 252.