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Cyclohexene oxidation catalyzed by titanium modified hexagonal Y type zeolites
3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.
999
CYCLOHEXENE OXIDATION CATALYZED BY TITANIUM MODIFIED HEXAGONAL Y TYPE ZEOLITES Kenneth J. Balkus, Jr.*, Alia K. Khanmamedova and Jimin Shi
Department of Chemistry, University of Texas at Dallas, Richardson, TX 75083-0688, United States
SUMMARY Hexagonal NaY (Si/Al=5.1) and the dealuminated hexagonal NaY (Si/Al=63) type zeolites were modified with titanium by reacting sin-face silanol groups with titanocene chloride. After calcination to remove organics the resulting titanium containing hexagonal Y zeolites were found to be effective catalysts for the hydrogen peroxide based oxidation of cyclohexene. Preliminary results for XRD, UV-Vis and FT-IR characterization of the zeolite catalysts are presented. 1. INTRODUCTION The success of the titanium silicate molecular sieve TS-1 [1] as a commercial oxidation catalyst [2-4] has prompted a flurry of activity in the area of titanium zeolites. Titanosilicates and other transitional metal modified zeolites continue to show promise in many areas of oxidation catalysis [5]. Much effort has been expended on the characterization of TS-1 in order to relate the catalytic properties with the presence of isolated titanium ions incorporated into the zeolite framework [6,7]. This has certainly encouraged the synthesis of other titanosilicate molecular sieves such as the large pore Ti-UTD-1 [8,9], Ti-beta [10] and Ti-MCM-41 [11] for example. Although, titanium may be incorporated into the oxide framework of many zeolites by direct synthesis, a simpler approach involves the post synthesis reaction of surface silanols with reactive titanium organometallics. This strategy is exemplified by the work of Maschmeyer et al [12] which involved the modification of mesoporous MCM-41 with titanocene dichloride followed by thermal decomposition to yield an active oxidation catalyst. Related studies of molecular sieve surface modification with reactive metal species also appear promising [ 13-17]. We have employed this method in the modification of dealuminated hexagonal Y zeolites with titanium The structure of hexagonal Y has the EMT topology and is characterized by a three dimensional large pore (12 membered ring, 7.4 x 6.5A.) system which would have a size advantage over the medium pore TS-1 catalyst. Upon dealumination by acid treatment the structure of hexagonal Y is retained but the presence of defect sites is evident. We have reacted titanocene dichloride with the template free hexagonal NaY as well as the dealuminated zeolite. After template removal a si~ificant amount of titanium is retained. Subsequent, calcination heals the defect sites and
1000 produces a titanium aluminosilicate that is an effective epoxidation catalyst. Preliminary results for the characterization of titanium modified hexagonal NaY as well as results for the oxidation of cyclohexene using H202 are presented below. 2. EXPERIMENTAL Hexagonal NaY (Si/Al=5.1) was prepared and characterized as previously described [18]. Samples 1-4 were calcined 650~ to remove the crown ether template but not dealuminated while sample 5 was stirred at pH 1 (HC1) for 4 hours and then calcined at 650~ Titanocene dichloride, CpETiC12 (Aldrich) was used as the titanium source for modifying the surface of hexagonal NaY following a procedure similar to that previously described [ 12]. In a typical reaction 0.5 g of calcined hexagonal NaY was stirred in 20 mL of chloroform at room temperature followed by the addition of 0.2 g of titanocene dichloride dissolved in an additional 20 ml of chloroform The mixture was stirred for 0.5 hours and then 5 g of triethylamine were added with stirring for two more hours to scavenge HC1. During that period the color of the suspension changed from red to yellow. The resulting zeolite was suction filtered, washed with chloroform and dried at 80~ The organometallic modified hexagonal NaY was then calcined at 650~ in flowing dry oxygen for 4 hours. FT-IR spectra were recorded as KBr pellets using a Mattson 2025 FT-IR spectrophotometer. XRD patterns were collected using a Scintag XDS 2000 diffractometer. CaF2 was used as an internal standard. UV-Vis spectra were collected from samples prepared as nujol mulls between quartz plates using an Hitachi U-2000 IYV-Vis spectrophotometer. Elemental analyses were performed by Galbraith Laboratories, Knoxville, TN. The catalytic oxidations of cyclohexene were carded out as batch reactions in sealed glass vials (15 mL) under nitrogen at -~55~ In a typical experiment the reactor was loaded with 0.1 gram of catalyst, 8 mmol of cyclohexene, 2.5 mmol of H202 (30 wt.%) and 4 mL of acetonitrile solvent. All reactions were sampled by syringe through a rubber septum~ Products were analyzed by gas chromatography using an HP 5840 A capillary GC equipped with a 15 m AT-1 capillary column and a flame ionization detector. Products were verified by known standards. The data in Table 2 represent an average of multiple analyses in order to verfy the significance of the changes. 3. RESULTS AND DISCUSSION 3.1. Modification with Titanium
The organometallic Cp2TiC12 is a reactive 16 electron complex that is readily hydrolyzed. Reaction with silanols is expected to result in Ti-O linkages with concomitant production of HC1. This type of complexation has already been shown to occur with the silanols ofMCM-41 resulting in immobilized titanocene species [ 12]. One might also envision the partial exchange of sodium ions for Cp2TiCI~2n§ ions but this is probably not a si~ificant process in chloroform. In the present study Cp2TiC12 was reacted with calcined hexagonal Y and a dealuminated sample. Table 1 shows the results for elemental analysis of calcined hexagonal NaY treated with different amounts of titanocene dichloride (Samples 1-4) and the dealuminated titanium zeolite (sample 5). For the calcined only samples the amount of
1001 titanium incorporated appears to track with the amount of organometallic used. One might expect that dealumination would generate defect sites or silanol nests that would increase the Table 1. Results for elemental analysis of hexagonal NaY samples after calcination. Samples
Cp2TiC12
Titanium
Si/Al
(grams)
(weight %)
1
0.020
0.68
5.1
2
0.033
1.33
5.1
3
0.044
1.97
5.1
4
0.200
7.22
5.1
5
0.200
11.44
63.3
amount of organometallic that would react with the surface. Additionally, the more siliceous zeolite should become more hydrophobic which is a desirable feature titanosilicate catalysts. Table 1 shows that when hexagonal NaY is dealuminated more titanium is incorporated. For sample 5 the same concentration of Cp2TiC12 as in sample 4 was reacted but there was a -~60% increase in the resulting titanium loading. This is consistent with the greater number of silanols in sample 5. Additionally, if the titanium modification in samples 1-4 was predominately simple impregnation or deposition of titania in the channels, then one would not expect such a difference between samples 4 and 5. Interestingly the amount of titanium incorporated in sample 5 is almost an equimolar amount in relation to the quantity of aluminum removed. It should also be noted that HC1 treatment of hexagonal NaY did not result in any si~ificant loss in crystaninity as evidenced by XRD results and scanning electron microscopy. The high titanium loadings achieved with samples 4 and 5 seems to validate this approach to modifying zeolite uaTaces with reactive metal species. However, from a catalysis perspective one would prefer to have isolated titanium centers which will be difficult at such high loadings. One might expect formation of Ti-O-Ti oligomers or bulk TiO2 at such loadings. UV-VIS spectral analysis reveals that samples 4 and 5 have absorption edges above 325 nm indicating the possible presence of anatase [ 19]. Therefore, the nature of the surface titanium in these samples will have to be considered when evaluating catalytic reactivity. Infrared spectroscopy has proven to be a valuable tool in characterization of titanosilicates. In the case of TS-1 and other Ti-substituted molecular sieves the infamous band at -~960 cm-1 was held to be evidence of titanium incorporation into the framework [20,21]. However, the same band was found with titanium free silicalite [22] and not observed when silicalite was modified from aqueous fluorotitanate solutions [23]. Most would agree now that this band is an Si-O stretch derived from defect sites in the silicate yet there may be some overlap and contribution from Si-O-Ti stretches. It seems in some cases
1002 the intensity of the band at 960 cm1 correlates with the amount of titanium but only to the point where titanium oligomers begin to form [ 17].
(f) (e) (d)
o t..)
(c) (b)
(a) 2000 1500 1000 Wavenumbers, cm-1
500
Figure 1. FT-IR spectra of (a) the as synthesized hexagonal NaY, (b) calcined hexagonal NaY, (c) hexagonal NaY after HC1 treatment, (d) hexagonal NaY after HC1 treatment and calcination, (e) dealuminated hexagonal NaY after reaction with Cp2TiC12 and (f) dealuminated hexagonal NaY after reaction with Cp2TiC12followed by calcination. The FT-IR spectra in figure 1 reveal that the as-synthesized hexagonal NaY has a broad shoulder between 950 and 850 cm-1. However, after calcination and removal of the crown ether template this region of the spectrum is clear and the bands associated with the asymmetric stretches become better defined. After HC1 treatment there is clear evidence of dealumination as shown in figure l c. The band assigned to the asymmetric Si-O stretching mode associated with internal tetrahedra shifts from 1032 to 1092 c m "1 w h i c h is consistent with an order of magnitude increase in Si/A1. The weaker bands associated with internal symmetric stretches and double ring vibrations also decrease in intensity, however, the zeolite structure is preserved according to XRD results. Dealumination also results in formation of a band at 954 cm1 which is relatively sharp compared to partially amorphous zeolites. Interestingly, this band at 954 cm1 disappears upon calcination with very little change in the rest of the spectrum (figure l d). In most cases, the zeolites that show such bands do not exhibit this type of behavior upon calcination but rather the band intensifies. The loss of the
1003 954cm-1 band suggests some healing process associated with the defect sites that does not involve reinsertion of T atoms since none of the other bands change. One might expect these defect sites to be reactive towards Cp2TiC12 which might affect this band. Figure l e indicates that modification with the organometallic reduces the intensity and slightly shifts the band to --~947cm"1. Any IR bands that might be associated with the Cp rings are not evident, possibly because they are masked by the zeolite bands or dissociated [12]. Nevertheless, we know from the results in Table 1 that a si~ificant amount of titanittm has been included at this stage. Calcination of this sample (figure If) results in loss of the 947cm-1 band and no shifts in the other bands. The reaction of Cp2TiC12 with the calcined hexagonal NaY in figure lb does not generate a 947cm 1 band nor is there any other si~ificant change in the resulting spectra (not shown). One would have to conclude that in all these cases the titanium species are being grafted to the surface of the zeolite pores. There is no evidence that would suggest that titanium is substituted for T atoms in the framework. 3.2 Oxidation of Cyclohexene The oxidation of cyclohexene using hydrogen peroxide was chosen as a test reaction for the catalytic evaluation of the titanium modified hexagonal NaY samples. Scheme I illustrates some of the typical products of cyclohexene oxidation. The epoxide and the diol which is a hydrolysis product of the epoxide, generally reflect a concerted process. In contrast the allylic alcohol and ketone are often ascribed to an autoxidation or radical process. We anticipated that some homolytic decomposition of the peroxide may be observed with these acidic zeolites. In fact, there was-~74% conversion of H202 over calcined hexagonal NaY after heating at 55~ for 24 hours. This resulted in only a 1% conversion of
[•0
+ H20 2 Cyclohexene
~Cyclohexene oxide
2-Cyclohexene-l-ol
,.
OH trans..1,2-Cyclohexanediol
7-Oxabicyclo[4.1.0]heptan-2-oi
__. d>O 2-Cyclohexene-l-one
7-Oxabicyclo[4.1.0]heptan-2-one
Scheme 1
1004 cyclohexene during this period to a 3:1 mixture of 2-cyclohexen-1-one and 2-cyclohexen-1ol. No epoxide was formed under these conditions. Table 2 shows the results for the oxidation of cyclohexene catalyzed by a series of titanium modified hexagonal NaY zeolites. The epoxide is produced in all cases but under the present conditions is transformed to the diol as the major product. Additionally, there are smaller amounts of autoxidation products (K + A). This product distribution was to be expected given the acidity of these zeolites and has certainly been noted before with other titanium modified zeolites [24,25]. After aluminum removal from the zeolites, the side reactions and selectivity improves [26]. Most of the catalyst activity occurs in the first few hours since after one day there is only a small increase in the conversion of substrate or peroxide. There is an apparent improvement in selectivity for the epoxide after a day but the rate of reaction is quite low. This would suggest clogging of the zeolite pores which retards the reaction chemistry. The spent catalyst is typically pale yellow in color while the starting zeolite is white. The MCM-41 modified with titanium in a similar fashion to our study also turns yellow in a peroxide based cyclohexene oxidation but deactivates after only 90 minutes [ 12]. The used catalyst may be calcined at 500~ in flowing oxygen to remove the color and restore the original catalytic activity. Controlling tile catalyst acidity as well as evaluating the influence of solvent will be important factors to consider in improving catalyst lifetime. Table 2. Results for cyclohexene oxidation catalyzed by titanium modified hexagonal NaY.
% Conversion Sample
1 2 3 4 5
% Selectivitya'b
Hours
CY
H202
CYO
diol
K
A
% Efficiencyc
5 24 5 24 5 24 5 24 5 24
5.7 6.4 8.2 10.5 10.8 12.8 7.2 9.5 4.5 6.4
50 54 69 75 56 65 36 50 30 47
5.0 4.8 6.1 5.5 5.7 4.3 4.3 4.0 6.3 5.7
58.1 60.0 57.9 50.2 69.3 53.1 59.1 68.1 52.5 51.8
24.2 22.9 31.1 31.0 15.4 25.8 27.5 19.9 32.0 31.9
12.6 12.2 16.8 13.2 9.3 1.6.7 8.9 7.8 9.2 10.6
36 37 38 45 59 61 64 62 54 53
CY = cyclohexene; CYO = cyclohexene oxide; diol = 1,2-cyclohexanediol; K - 2-cyclohexen- 1-one; A = 2-cyclohexen- 1-ol. b Selectivity = (mmol product / mmol total products) x 100. c Efficiency = (retool of converted cyclohexene /mmol of converted H202) x 100 a
1005 Table 2 indicates that the conversion of cyclohexene as well as the peroxide efficiency increases as the titanium content increases for the first three samples. Recall samples 4 and 5 showed evidence of bulk TiO2 occlusion which must have adverse effects on the reaction including pore blockage. Sample 5 is the dealuminated zeolite which should have fewer but stronger acid sites. In spite of having the highest titanium loading this sample exhibits the lowest conversion of both substrate and peroxide. The peroxide efficiency is comparable to the better catalysts which may reflect the lower acid site density. A slightly lower level of epoxide hydrolysis may also represent a more hydrophobic environment at the active site. These result certainly warrant further investigation of the high silica hexagonal Y type zeolites as support materials for isolated titanium species. 4. CONCLUSIONS We have grafted titanocene derived species onto the surface of hexagonal NaY before and after dealumination resulting in effective epoxidation catalysts. The organometallic was intended to reduce the possibility of forming oligomers and bulk titania but it is clear that at high enough loadings this is unavoidable. However, this method of zeolite surface modification has resulted in epoxidation activity one would associate with isolated titanium centers which then represents a viable alternative to framework titanosilicate catalysts. Clearly more work is needed to better define the optimum zeolite composition and reaction conditions that will stabilize the system and maximize selectivity. ACKNOWLEDGMENTS The support of the Robert A. Welch Foundation and the National Science Foundation are gratefully acknowledged. REFERENCES
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4. 5. .
7. 8.
M. Taramasso, G. Perego and B. Notari, Preparation of porous crystalline synthetic material comprised of silicon and titanium oxides, U. S. Patent No. 4,410,501 (1983). M. Taramasso, G. Manara, V. Fattore and B. Notari, Silica based synthetic material containing titanium in the crystal lattice and process for its preparation, U.S. Patent No. 4,666,692 (1987). B. Notari, Catal. Today, 18 (1993) 163. B. Notari, Stud. Surf. Sci. Catal., 37 (1987)413. D.E. De Vos, P.L. Buskens. D.L. Vanoppen, P.P. Knops-Gerrits and P.A. Jacobs in Comprehensive Supramolecular Chemistry 7 (1996) 647 and references there in. 1L Millini and G. Perego, Gaz. Chim Ital., 126 (1996) 133. B. Notari, Adv. Catal., 41 (1996) 253. K~J. Balkus, Jr., A.G. Gabrielov and S.I. Zones, Stud. Surs Sci. Catal., 97 (1995) 519. I~J. Balkus, Jr., A.A. Khanmamedova, A. Gabrielov and S.I. Zones, Stud. Surf Sci. Catal., 101 (1996) 1341.
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999
CYCLOHEXENE OXIDATION CATALYZED BY TITANIUM MODIFIED HEXAGONAL Y TYPE ZEOLITES Kenneth J. Balkus, Jr.*, Alia K. Khanmamedova and Jimin Shi
Department of Chemistry, University of Texas at Dallas, Richardson, TX 75083-0688, United States
SUMMARY Hexagonal NaY (Si/Al=5.1) and the dealuminated hexagonal NaY (Si/Al=63) type zeolites were modified with titanium by reacting sin-face silanol groups with titanocene chloride. After calcination to remove organics the resulting titanium containing hexagonal Y zeolites were found to be effective catalysts for the hydrogen peroxide based oxidation of cyclohexene. Preliminary results for XRD, UV-Vis and FT-IR characterization of the zeolite catalysts are presented. 1. INTRODUCTION The success of the titanium silicate molecular sieve TS-1 [1] as a commercial oxidation catalyst [2-4] has prompted a flurry of activity in the area of titanium zeolites. Titanosilicates and other transitional metal modified zeolites continue to show promise in many areas of oxidation catalysis [5]. Much effort has been expended on the characterization of TS-1 in order to relate the catalytic properties with the presence of isolated titanium ions incorporated into the zeolite framework [6,7]. This has certainly encouraged the synthesis of other titanosilicate molecular sieves such as the large pore Ti-UTD-1 [8,9], Ti-beta [10] and Ti-MCM-41 [11] for example. Although, titanium may be incorporated into the oxide framework of many zeolites by direct synthesis, a simpler approach involves the post synthesis reaction of surface silanols with reactive titanium organometallics. This strategy is exemplified by the work of Maschmeyer et al [12] which involved the modification of mesoporous MCM-41 with titanocene dichloride followed by thermal decomposition to yield an active oxidation catalyst. Related studies of molecular sieve surface modification with reactive metal species also appear promising [ 13-17]. We have employed this method in the modification of dealuminated hexagonal Y zeolites with titanium The structure of hexagonal Y has the EMT topology and is characterized by a three dimensional large pore (12 membered ring, 7.4 x 6.5A.) system which would have a size advantage over the medium pore TS-1 catalyst. Upon dealumination by acid treatment the structure of hexagonal Y is retained but the presence of defect sites is evident. We have reacted titanocene dichloride with the template free hexagonal NaY as well as the dealuminated zeolite. After template removal a si~ificant amount of titanium is retained. Subsequent, calcination heals the defect sites and
1000 produces a titanium aluminosilicate that is an effective epoxidation catalyst. Preliminary results for the characterization of titanium modified hexagonal NaY as well as results for the oxidation of cyclohexene using H202 are presented below. 2. EXPERIMENTAL Hexagonal NaY (Si/Al=5.1) was prepared and characterized as previously described [18]. Samples 1-4 were calcined 650~ to remove the crown ether template but not dealuminated while sample 5 was stirred at pH 1 (HC1) for 4 hours and then calcined at 650~ Titanocene dichloride, CpETiC12 (Aldrich) was used as the titanium source for modifying the surface of hexagonal NaY following a procedure similar to that previously described [ 12]. In a typical reaction 0.5 g of calcined hexagonal NaY was stirred in 20 mL of chloroform at room temperature followed by the addition of 0.2 g of titanocene dichloride dissolved in an additional 20 ml of chloroform The mixture was stirred for 0.5 hours and then 5 g of triethylamine were added with stirring for two more hours to scavenge HC1. During that period the color of the suspension changed from red to yellow. The resulting zeolite was suction filtered, washed with chloroform and dried at 80~ The organometallic modified hexagonal NaY was then calcined at 650~ in flowing dry oxygen for 4 hours. FT-IR spectra were recorded as KBr pellets using a Mattson 2025 FT-IR spectrophotometer. XRD patterns were collected using a Scintag XDS 2000 diffractometer. CaF2 was used as an internal standard. UV-Vis spectra were collected from samples prepared as nujol mulls between quartz plates using an Hitachi U-2000 IYV-Vis spectrophotometer. Elemental analyses were performed by Galbraith Laboratories, Knoxville, TN. The catalytic oxidations of cyclohexene were carded out as batch reactions in sealed glass vials (15 mL) under nitrogen at -~55~ In a typical experiment the reactor was loaded with 0.1 gram of catalyst, 8 mmol of cyclohexene, 2.5 mmol of H202 (30 wt.%) and 4 mL of acetonitrile solvent. All reactions were sampled by syringe through a rubber septum~ Products were analyzed by gas chromatography using an HP 5840 A capillary GC equipped with a 15 m AT-1 capillary column and a flame ionization detector. Products were verified by known standards. The data in Table 2 represent an average of multiple analyses in order to verfy the significance of the changes. 3. RESULTS AND DISCUSSION 3.1. Modification with Titanium
The organometallic Cp2TiC12 is a reactive 16 electron complex that is readily hydrolyzed. Reaction with silanols is expected to result in Ti-O linkages with concomitant production of HC1. This type of complexation has already been shown to occur with the silanols ofMCM-41 resulting in immobilized titanocene species [ 12]. One might also envision the partial exchange of sodium ions for Cp2TiCI~2n§ ions but this is probably not a si~ificant process in chloroform. In the present study Cp2TiC12 was reacted with calcined hexagonal Y and a dealuminated sample. Table 1 shows the results for elemental analysis of calcined hexagonal NaY treated with different amounts of titanocene dichloride (Samples 1-4) and the dealuminated titanium zeolite (sample 5). For the calcined only samples the amount of
1001 titanium incorporated appears to track with the amount of organometallic used. One might expect that dealumination would generate defect sites or silanol nests that would increase the Table 1. Results for elemental analysis of hexagonal NaY samples after calcination. Samples
Cp2TiC12
Titanium
Si/Al
(grams)
(weight %)
1
0.020
0.68
5.1
2
0.033
1.33
5.1
3
0.044
1.97
5.1
4
0.200
7.22
5.1
5
0.200
11.44
63.3
amount of organometallic that would react with the surface. Additionally, the more siliceous zeolite should become more hydrophobic which is a desirable feature titanosilicate catalysts. Table 1 shows that when hexagonal NaY is dealuminated more titanium is incorporated. For sample 5 the same concentration of Cp2TiC12 as in sample 4 was reacted but there was a -~60% increase in the resulting titanium loading. This is consistent with the greater number of silanols in sample 5. Additionally, if the titanium modification in samples 1-4 was predominately simple impregnation or deposition of titania in the channels, then one would not expect such a difference between samples 4 and 5. Interestingly the amount of titanium incorporated in sample 5 is almost an equimolar amount in relation to the quantity of aluminum removed. It should also be noted that HC1 treatment of hexagonal NaY did not result in any si~ificant loss in crystaninity as evidenced by XRD results and scanning electron microscopy. The high titanium loadings achieved with samples 4 and 5 seems to validate this approach to modifying zeolite uaTaces with reactive metal species. However, from a catalysis perspective one would prefer to have isolated titanium centers which will be difficult at such high loadings. One might expect formation of Ti-O-Ti oligomers or bulk TiO2 at such loadings. UV-VIS spectral analysis reveals that samples 4 and 5 have absorption edges above 325 nm indicating the possible presence of anatase [ 19]. Therefore, the nature of the surface titanium in these samples will have to be considered when evaluating catalytic reactivity. Infrared spectroscopy has proven to be a valuable tool in characterization of titanosilicates. In the case of TS-1 and other Ti-substituted molecular sieves the infamous band at -~960 cm-1 was held to be evidence of titanium incorporation into the framework [20,21]. However, the same band was found with titanium free silicalite [22] and not observed when silicalite was modified from aqueous fluorotitanate solutions [23]. Most would agree now that this band is an Si-O stretch derived from defect sites in the silicate yet there may be some overlap and contribution from Si-O-Ti stretches. It seems in some cases
1002 the intensity of the band at 960 cm1 correlates with the amount of titanium but only to the point where titanium oligomers begin to form [ 17].
(f) (e) (d)
o t..)
(c) (b)
(a) 2000 1500 1000 Wavenumbers, cm-1
500
Figure 1. FT-IR spectra of (a) the as synthesized hexagonal NaY, (b) calcined hexagonal NaY, (c) hexagonal NaY after HC1 treatment, (d) hexagonal NaY after HC1 treatment and calcination, (e) dealuminated hexagonal NaY after reaction with Cp2TiC12 and (f) dealuminated hexagonal NaY after reaction with Cp2TiC12followed by calcination. The FT-IR spectra in figure 1 reveal that the as-synthesized hexagonal NaY has a broad shoulder between 950 and 850 cm-1. However, after calcination and removal of the crown ether template this region of the spectrum is clear and the bands associated with the asymmetric stretches become better defined. After HC1 treatment there is clear evidence of dealumination as shown in figure l c. The band assigned to the asymmetric Si-O stretching mode associated with internal tetrahedra shifts from 1032 to 1092 c m "1 w h i c h is consistent with an order of magnitude increase in Si/A1. The weaker bands associated with internal symmetric stretches and double ring vibrations also decrease in intensity, however, the zeolite structure is preserved according to XRD results. Dealumination also results in formation of a band at 954 cm1 which is relatively sharp compared to partially amorphous zeolites. Interestingly, this band at 954 cm1 disappears upon calcination with very little change in the rest of the spectrum (figure l d). In most cases, the zeolites that show such bands do not exhibit this type of behavior upon calcination but rather the band intensifies. The loss of the
1003 954cm-1 band suggests some healing process associated with the defect sites that does not involve reinsertion of T atoms since none of the other bands change. One might expect these defect sites to be reactive towards Cp2TiC12 which might affect this band. Figure l e indicates that modification with the organometallic reduces the intensity and slightly shifts the band to --~947cm"1. Any IR bands that might be associated with the Cp rings are not evident, possibly because they are masked by the zeolite bands or dissociated [12]. Nevertheless, we know from the results in Table 1 that a si~ificant amount of titanittm has been included at this stage. Calcination of this sample (figure If) results in loss of the 947cm-1 band and no shifts in the other bands. The reaction of Cp2TiC12 with the calcined hexagonal NaY in figure lb does not generate a 947cm 1 band nor is there any other si~ificant change in the resulting spectra (not shown). One would have to conclude that in all these cases the titanium species are being grafted to the surface of the zeolite pores. There is no evidence that would suggest that titanium is substituted for T atoms in the framework. 3.2 Oxidation of Cyclohexene The oxidation of cyclohexene using hydrogen peroxide was chosen as a test reaction for the catalytic evaluation of the titanium modified hexagonal NaY samples. Scheme I illustrates some of the typical products of cyclohexene oxidation. The epoxide and the diol which is a hydrolysis product of the epoxide, generally reflect a concerted process. In contrast the allylic alcohol and ketone are often ascribed to an autoxidation or radical process. We anticipated that some homolytic decomposition of the peroxide may be observed with these acidic zeolites. In fact, there was-~74% conversion of H202 over calcined hexagonal NaY after heating at 55~ for 24 hours. This resulted in only a 1% conversion of
[•0
+ H20 2 Cyclohexene
~Cyclohexene oxide
2-Cyclohexene-l-ol
,.
OH trans..1,2-Cyclohexanediol
7-Oxabicyclo[4.1.0]heptan-2-oi
__. d>O 2-Cyclohexene-l-one
7-Oxabicyclo[4.1.0]heptan-2-one
Scheme 1
1004 cyclohexene during this period to a 3:1 mixture of 2-cyclohexen-1-one and 2-cyclohexen-1ol. No epoxide was formed under these conditions. Table 2 shows the results for the oxidation of cyclohexene catalyzed by a series of titanium modified hexagonal NaY zeolites. The epoxide is produced in all cases but under the present conditions is transformed to the diol as the major product. Additionally, there are smaller amounts of autoxidation products (K + A). This product distribution was to be expected given the acidity of these zeolites and has certainly been noted before with other titanium modified zeolites [24,25]. After aluminum removal from the zeolites, the side reactions and selectivity improves [26]. Most of the catalyst activity occurs in the first few hours since after one day there is only a small increase in the conversion of substrate or peroxide. There is an apparent improvement in selectivity for the epoxide after a day but the rate of reaction is quite low. This would suggest clogging of the zeolite pores which retards the reaction chemistry. The spent catalyst is typically pale yellow in color while the starting zeolite is white. The MCM-41 modified with titanium in a similar fashion to our study also turns yellow in a peroxide based cyclohexene oxidation but deactivates after only 90 minutes [ 12]. The used catalyst may be calcined at 500~ in flowing oxygen to remove the color and restore the original catalytic activity. Controlling tile catalyst acidity as well as evaluating the influence of solvent will be important factors to consider in improving catalyst lifetime. Table 2. Results for cyclohexene oxidation catalyzed by titanium modified hexagonal NaY.
% Conversion Sample
1 2 3 4 5
% Selectivitya'b
Hours
CY
H202
CYO
diol
K
A
% Efficiencyc
5 24 5 24 5 24 5 24 5 24
5.7 6.4 8.2 10.5 10.8 12.8 7.2 9.5 4.5 6.4
50 54 69 75 56 65 36 50 30 47
5.0 4.8 6.1 5.5 5.7 4.3 4.3 4.0 6.3 5.7
58.1 60.0 57.9 50.2 69.3 53.1 59.1 68.1 52.5 51.8
24.2 22.9 31.1 31.0 15.4 25.8 27.5 19.9 32.0 31.9
12.6 12.2 16.8 13.2 9.3 1.6.7 8.9 7.8 9.2 10.6
36 37 38 45 59 61 64 62 54 53
CY = cyclohexene; CYO = cyclohexene oxide; diol = 1,2-cyclohexanediol; K - 2-cyclohexen- 1-one; A = 2-cyclohexen- 1-ol. b Selectivity = (mmol product / mmol total products) x 100. c Efficiency = (retool of converted cyclohexene /mmol of converted H202) x 100 a
1005 Table 2 indicates that the conversion of cyclohexene as well as the peroxide efficiency increases as the titanium content increases for the first three samples. Recall samples 4 and 5 showed evidence of bulk TiO2 occlusion which must have adverse effects on the reaction including pore blockage. Sample 5 is the dealuminated zeolite which should have fewer but stronger acid sites. In spite of having the highest titanium loading this sample exhibits the lowest conversion of both substrate and peroxide. The peroxide efficiency is comparable to the better catalysts which may reflect the lower acid site density. A slightly lower level of epoxide hydrolysis may also represent a more hydrophobic environment at the active site. These result certainly warrant further investigation of the high silica hexagonal Y type zeolites as support materials for isolated titanium species. 4. CONCLUSIONS We have grafted titanocene derived species onto the surface of hexagonal NaY before and after dealumination resulting in effective epoxidation catalysts. The organometallic was intended to reduce the possibility of forming oligomers and bulk titania but it is clear that at high enough loadings this is unavoidable. However, this method of zeolite surface modification has resulted in epoxidation activity one would associate with isolated titanium centers which then represents a viable alternative to framework titanosilicate catalysts. Clearly more work is needed to better define the optimum zeolite composition and reaction conditions that will stabilize the system and maximize selectivity. ACKNOWLEDGMENTS The support of the Robert A. Welch Foundation and the National Science Foundation are gratefully acknowledged. REFERENCES
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