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Zeolite titanium beta: A selective catalyst in the meerwein-ponndorf-verley-oppenauer reactions
3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 1997 Elsevier Science B.V.
1015
Zeolite T i t a n i u m Beta" A selective catalyst in the M e e r w e i n - P o n n d o r f V e r l e y - O p p e n a u e r reactions. J.C. van der Waal a, P.J. Kunkeler a, K. Tan b and H. van Bekkum a a Laboratory of Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands b Tianjin Research Institute of Chemical Industry, Dingzigu, Hongqiao, 300131 Tianjin, PR China Zeolite titanium beta has been tested in the liquid- and gas-phase Meerwein-PonndorfVerley reduction of cyclohexanones and the Oppenauer oxidation of cyclohexanols. A high selectivity towards the thermodynamically unfavourable cis-alcohol was observed, which has been ascribed to transition-state selectivity in the pores of the zeolite. Under gas-phase conditions the dehydration of alcohols to cycloalkenes is observed as a side reaction. The catalyst was found to be active even in the presence of water and ammonia.
1. INTRODUCTION The Meerwein-Ponndorf-Verley reduction of carbonyl compounds and the Oppenauer oxidation of alcohols, together denoted as MPVO reactions, are considered to be highly selective reactions. For instance, C =C double bonds are not attacked. In MPV reductions a secondary alcohol is the reductant whereas in Oppenauer oxidations a ketone is the oxidant. It is generally accepted that MPVO reactions proceed via a complex in which both the carbonyl and the alcohol are coordinated to a Lewis acid metal ion after which a hydride transfer from the alcohol to the carbonyl group occurs (Fig. 1) [1]. Usually, metal sec-alkoxides are used as homogeneous catalysts in reductions and metal t-butoxides in oxidations [1]. RI-_
"')==o+ ;XR, Fig. 1
u
R3
-
.,
The Meerwein-Ponndorf-Verley-Oppenauer reaction [1].
Zeolites are crystalline metal oxides which have potential as regenerable heterogeneous catalysts in various organic reactions [2]. Because of their unique microporous structure, zeolites are especially promising in the field of shape-selective
1016 catalysis. As far as we know, only very few examples of the use of zeolites in MPVO reactions have been reported [3-7,9]. The reactions were carried out in the gas-phase over zeolites A, X and Y, exchanged or impregnated with alkali or alkaline-earth cations [3-5]. Shape-selectivity was only observed by Shabtai et aL[4] in the conversion of citronellal over zeolite X. It was shown that selectivity could be tuned by the size of the exchanged metal ion. In NaX there was enough space for the substrate to perform an intramolecular ring closure to isopulegol, whereas over CsX reduction to the linear citronellol was observed. Similar steric effects were also found for various other substrates. Recently Creyghton et aL [6,7] reported the use of zeolite beta in the MPVO reduction of 4-t-butylcyclohexanone. The high selectivity towards the thermodynamically less favoured cis-alcohol is explained by a restricted transition-state around a Lewis-acidic aluminium in the zeolite pores. When using an aluminium-free zeolite, titanium beta, in the epoxidation of olefins, we have shown that Ti-beta has acidic properties when alcoholic solvents were employed [8]. This was ascribed to the Lewis-acidic character of titanium in the zeolite framework. As we reported very recently [9], Ti-beta is found to be an excellent catalyst in MPVO reactions with a tolerance for water. Here, results are presented on the high selectivity, stability and low by-product formation of the catalyst, Ti-beta, in both the liquid-phase and gas-phase MPVO reactions.
2. EXPERIMENTAL
Zeolite titanium beta (Ti-beta) was synthesized according to van der Waal et al. [8,9] using di(cyclohexylmethyl)dimethylammonium hydroxide (DCDMA.OH) as the template. For a typical synthesis, 0.25 g titanium(IV) ethoxide (TEOT) was added to 30.0 g of a 19.5 % w/w DCDMA.OH solution and the mixture was stirred until all TEOT was dissolved. To facilitate the dissolution of TEOT, 1 ml of H20 2 (30 % w/w aqueous) was added. To the resulting clear solution 3.0 g Aerosil 200 (Degussa), 0.15 g seeds (all-silica beta [10] and 11.8 g water were added and the gel was aged for at least 24 h. After crystallisation (14 days at 140 ~ the zeolite (1.4 g) was filtrated, washed, dried and calcined at 540 ~ in air. Elemental analysis, performed using a LINK EDX system, showed a Si:Ti molar ratio of 76 and confirmed the absence of oligomeric titanium dioxide phases. Zeolite aluminium beta (Al-beta, Si:AI = 10) was prepared according to Wadlinger and Kerr [11] and all-silica beta (Si-beta, Si:A1 > 5000) was prepared according to van der Waal et aL [10]. Liquid-phase MPVO reactions were performed in 25 ml isopropanol (reductions) or 25 ml 2-butanone (oxidations) at 85 ~ using 2.5 mmol of the appropriate substrate: 4-t-butylcyclohexanone (4-Bu-ONE), 4-methylcyclohexanone (4-Me-ONE) or 4-t-butylcyclohexanol (4-Bu-OL, cis/trans mixture); 0.5 g zeolite or 0.25 mmol aluminium isopropoxide as the catalyst and 1,3,5-tri-t-butylbenzene as the internal standard. Samples were taken at regular intervals and analyzed by GC on a Carbowax CP-52 column and GC/MS. Gas-phase MPVO reactions were performed at 85 to 400 ~ in a fixed bed continuous down-flow reactor operated at atmospheric pressure under plug flow conditions. The catalyst, Ti-beta or N-beta (0.30 g), was diluted with 1.20 g a-quartz powder and processed to pellets then crushed to particles with a diameter of 0.7 - 1.0 mm. Reactant mixtures were pumped into a stream of preheated carrier gas (usually
1017 nitrogen) by means of a motor-driven syringe pump. The gas flow contained 10 vol.% isopropanol (reductions) or acetone (oxidations) and 1 vol.% of the appropriate substrate: 4-methylcyclohexanone (4-Me-ONE) or 4-methylcyclohexanol (4-Me-OL). The total gas flow was 50 ml/min and the molar gas flow of 4-Me-ONE or 4-Me-OL was 2.04 10-5 mol/min (WHSV = 2.9 gtotal goat-1 h'l) 9 Samples of the reactor effluent were taken regularly and analysed on a CP-Sil-19 column. 3. RESULTS
3.1 Liquid-phase MPVO In the liquid-phase, the MPVO reactions catalysed by Ti-beta are highly selective and have low by-product formation. In the reduction of 4-t-butylcyclohexanone and 4-methylcyclohexanone a high selectivity (99 % and 98%, respectively) towards the thermodynamically unfavourable cis-alcohol is observed, similar to that observed with A1beta (Table 1). The high selectivity towards the cis-alcohol over beta-type zeolites was explained by Creyghton et aL [6] on the basis of transition-state selectivity. It can be seen in Fig. 2, that the transition-state leading to the cis- and trans-alcohol differ substantially in spatial requirements. The transition-states leading to the cis-isomer is aligned with the zeolite channel while that leading to the trans-alcohol occupies a much more axial position. Table 1 MPVO reduction of cyclohexanones over beta-type catalysts in refluxing isopropanol. Substrate
Catalyst
TOF a
Conversion b (%)
Selectivityb cis : trans
4-Me-ONE
Ti-beta
1.04
33.7
99:1
4-Me-ONE
M-beta
> 12
100.0
98:2
4-Me-ONE
AI(OPr)3
16.7
99.8
27:73
4-Bu-ONE
Ti-beta
2.26
64.9
98:2
4-Bu-ONE
Al-beta
> 12
100.0
95:5
4-Bu-ONE
Si-beta
4-Bu-ONE
AI(OPr)3
0.0 8.72
100.0
36:64
a Initial turn-over-frequency in mol ketone per mol of titanium or aluminium per hour. b Conversion and selectivity after 6 h reaction based on the initial amount of ketone added.
1018
H-~Me Me
....
O~ /0
/ Fig. 2
/ Proposed transition-states for the formation of cis-4-t-butylcyclohexanol (left) and trans-4-t-butylcyclohexanol (right).
In the Oppenauer oxidation of an equimolar mixture of cis- and trans-4-tbutylcyclohexanol, the cis-alcohol is converted selectively over both Ti-beta and Al-beta (Table 2), which is probably due to the same spatial restriction on the transition-states as for the reduction depicted in Fig. 2. In this way, the MPVO reduction and oxidation are in harmony as to the high transition-state selectivity. Table 2 Liquid-phase Oppenauer oxidation of 4-t-butylcyclohexanol (1:1 cis/trans mixture)with butanone as the oxidant. Catalyst
Conversion a of cis-alcohol (%)
Conversion a of trans-alcohol (%)
By-products a'b (mg)
Ti-beta
52.1
4.6
22.4
Al-beta
98.6 49.4 d
47.3 5.7c
317.3 63.8c
Ti(OiPr)4
6.1
12.9
22.7
Al(OiPr)3
5.3
7.4
56.5
a After 6 h reaction time. b Not MPVO related, predominantly from 2-butanone via aldol condensation, c After 15 min reaction time. d After 15 min. The most important side reaction in heterogeneously catalysed MPVO reactions is the acid-catalysed aldol condensation. Aldol products are usually observed during the Oppenauer oxidation of alcohols, when a surplus of ketone or aldehyde is used as the oxidizing agent and the solvent. The low amount of by-products formed when Tibeta was used as the catalyst, demonstrates the advantage of the titanium system over A1beta. This is probably caused by the much weaker BrCnsted acidity of the solvated titanium site [8] compared with the strong H+-acidity of the aluminium site in Al-beta. As we have shown earlier Ti-beta has a high tolerance towards water, which further shows the catalytic potential of Ti-beta in MPVO reactions [9].
1019
3.2 Gas-phase MPVO From an industrial point of view, gas-phase reactions are often preferred due to their ease of operations. The reduction of 4-methylcyclohexanone and the oxidation of cis- and trans-4-methylcyclohexanol over Ti-beta and Al-beta in the gas phase were studied at 100~ As can be seen from Fig. 3, both Ti-beta and Al-beta are active, but Tibeta has a considerably lower rate of deactivation. The deactivation of Al-beta is probably caused by the higher acidic strength of the protonic aluminium site compared to the non-protic titanium site. Another difference between the two catalysts is the pronounced dehydration of the alcohols formed over Al-beta. Two important differences between the gas-phase and liquid-phase reaction were observed. The most striking is the selectivity towards the cis-alcohol. Under liquid phase conditions (Table 1.) a selectivity of 99 % towards the cis-alcohol was observed, while in the gas-phase over Ti-beta only 53 - 62 % cis-alcohol was obtained. Furthermore, dehydration of the alcohols formed to 4-methylcyclohexene (4Me-ENE) was observed in the gas phase, while no trace of dehydrated products could be detected in liquid phase MPV reductions. The somewhat higher temperature of 100 ~ compared with 85 ~ for the liquid-phase experiments, could not explain this behaviour, since lowering the temperature to 85 ~ still resulted in a 4-Me-ENE selectivity of approximately 10 % over Ti-beta. The remarkable differences in selectivity between the gas- and liquid-phase MPVO reaction induced us to study the evolution of products as a function of the temperature, in the reduction of 4-methylcyclohexanone. From Fig. 4 it can be seen that the selectivity to the cis-alcohol decreases at higher temperatures; initially the selectivity to the trans-alcohol increases but at higher temperatures dehydration to 4-methylcyclohexene becomes the major reaction. At still higher temperatures (> 200 ~ the 4-methylcyclohexene is subsequently isomerised to the more stable 1-methylcyclohexene. I00
80
'N
60
20
0
2
4 Time on stream [h]
6
8
a) Ti-beta Fig. 3a
The gas-phase MPV reduction of 4-methylcyclohexanone with isopropanol over Ti-beta at 100~ 9 = conversion; 9 = cis-4-Me-OL; x = trans-4-Me-OL; n = 4-Me-ENE.
1020
100
80
~.43
------'~
!o A
o
-
i
A
4 ' Time on stream [h]
i
b) Al-beta Fig. 3b
The gas-phase MPV reduction of 4-methylcyclohexanone with isopropanol over Al-beta at 100~ 9 = conversion; 9 = cis-4-Me-OL; x = trans-4-Me-OL; [] = 4-Me-ENE. 100
80 ,___,
c~
.~ 40
0
Fig. 4
50
100
150 200 250 Temperature [ * C]
300
350
Influence of the temperature on the selectivity in the reduction of 4-methylcyclohexanone with isopropanol. Temperature increment was 0.2 ~ 9 = conversion; 9 = cis-4-Me-OL; x = trans-4-Me-OL; [] = 4-Me-ENE.
Since the 4-methylcyclohexene can be formed via dehydration of either of the two alcohols formed, both alcohols were tested in the Oppenauer oxidation, using acetone as the oxidant at 100 ~ It was observed that the cis-alcohol (Fig. 5a) is easily oxidized to the corresponding ketone, while the trans-alcohol showed a much lower activity. This is in accordance with the transition-states assumed for liquid phase MPVO reactions (Fig. 2). It can be observed further that the dehydration of the trans-alcohol only occurred to very limited scale (Fig. 5b) whereas dehydration of the cis-alcohol was an important side-
1021 reaction. This can be understood by assuming an E2-mechanism, in which the axial OH group in the cis-isomer is in the ideal position for water elimination. Another important side reaction for both alcohols was isomerization. It is therefore proposed that the alkene is formed mainly from the cis-alcohol and that at low temperatures the trans-alcohol can only be dehydrated if it is first isomerised to the cisalcohol via an MPVO transition-state. Comparing the deactivation of the Ti-beta catalyst in the oxidation (Fig. 5) and reduction reactions (Fig. 3) shows that the deactivation is more pronounced during oxidative conditions. This is probably caused by the high amount of ketones present, which easily form aldol condensates which may plug the zeolite channels, thus inhibiting access to the micropore system. 100
80
r~
..~ 40
i
r,.)
0
2
4 Time on stream [h]
6
a) Oxidation of cis-4-Me-OL 100
80
"~
60
.~ 40
I 2o
r.,)
0
2
4 Time on stream [h]
6
8
b) Oxidation of trans-4-Me-OL Fig. 5
The gas-phase Oppenauer oxidation of the 4-methylcyclohexanols with acetone over Ti-beta at 100~ a) cis-4-Me-OL and b) trans-4-Me-OL. 9 = conversion; 9 = cis-4-Me-OL; x = trans4-Me-OL; 9 = 4-Me-ONE. [] = 4-Me-ENE.
1022 The commonly used MPVO catalysts consist of metal alkoxides, which are easily hydrolysed to inactive oxides in the presence of water. Since the proposed catalytic species for the MPVO reaction also consists of an alkoxide intermediate [6,9] the influence of water and strong Lewis bases on the catalytic activity and selectivity was investigated. As already reported for the liquid-phase reaction [9], Ti-beta has a high tolerance for water due to its hydrophobic interior. As can be seen from Fig. 6a the presence of water is not detrimental to the activity of Ti-beta in the MPV reduction of 4-methylcyclohexanone. The temperature of 110 ~ at which a ketone conversion of 50% is measured, is identical to the temperature required for 50% conversion in the absence of water (Fig. 3), i.e. water has no effect whatsoever on the overall MPVO activity of the titanium site. 100
50
100
150 200 250 Temperature [ * Q
300
350
a) Reduction in the presence of water 100
9
--
w--r-~
80
60
.~
40
i 0 100
150
200 250 Temperature * C]
300
350
400
b) Reduction in the presence of ammonia Fig. 6
Temperature-programmed gas-phase MPV reduction of 4-methylcyclohexanone over Ti-beta in the presence of a) 2.66 vol.% water or b) 5 vol% ammonia. 9 = conversion; 9 = cis-4-Me-OL; x = trans-4-Me-OL; [] = 4-MeENE.
1023
The selectivity towards alcohols increased from 8 1 % to 97% at 85~ This enhanced selectivity can be ascribed to either a kinetic suppression of an irreversible dehydration or to a change in the alcohol/olefin equilibrium due to the higher amount of water present. In the case of a shift in equilibrium, it should be possible to oxidise olefins with ketones in the presence of water via in situ formed alcohols. In an attempt to oxidise cyclohexene with acetone in the presence of a water, no conversion to cyclohexanone was observed between 85 and 400 ~ It is therefore concluded that under the experimental conditions used, 4-methylcyclohexene is formed irreversibly from the cisalcohol (Fig. 7) and the increased selectivity towards alcohols should therefore be ascribed to a deactivation of the dehydration sites in the presence of water. OH
cis-4-Me-OL
4-Me-ENE
1-Me-ENE
Ti 4-Me-ONE
~
OH
trans-4-Me-OL
Fig. 7
Proposed reaction scheme for the MPV reduction of 4-methylcyclohexanone.
For liquid-phase reactions at 85 ~ we reported that small amounts of a strong base, e.g. pyridine, completely poisoned the catalyst [2]. It can be seen from Fig. 6b that, in the presence of ammonia, higher temperatures are required to reduce 4-methylcyclohexanone. The higher temperatures are required for the desorption of ammonia from the catalytically active site. The relatively low temperature of 305 ~ at which 50 % conversion is observed, suggests that the ammonia is not bonded to a strong acidic site. Since Br0nsted-acidic aluminium sites desorb ammonia at about 480 ~ this confirms that the MPVO reactions proceed via the titanium sites and not via any residual aluminium sites (Si:AI > 5000). The latter can also be concluded from Table 1; the all-silica analogue of zeolite beta [10] was found to be completely inactive even though it has a Si:A1 ratio similar to Ti-beta. It was also observed that in the presence of ammonia, no isomerisation of 4-Me-ENE to 1-Me-ENE occurred even at 400 ~ suggesting that the isomerisation requires strong Brcnsted acid-sites, most probably the residual aluminium sites. As was already shown in Fig. 3 and 5a, Ti-beta exhibits a low deactivation rate in the reduction of 4-methylcyclohexanone and a significantly higher deactivation rate in the oxidation of cis-4-methylcyclohexanol. In both cases, frequent regeneration of the catalyst will be necessary. Catalyst stability was tested by regeneration at 480~ in air after each run. No significant loss in activity, selectivity or product distribution was observed, even after 35 consecutive runs. Similar results were also obtained for the Tibeta used in liquid-phase reductions; after regenerating the Ti-beta catalyst 5 times, the same initial catalytic activity per gram of catalyst was observed.
1024 4. CONCLUSION Ti-beta is found to be an excellent catalyst in MPVO reactions under both liquid- and gas-phase conditions. Under liquid-phase conditions, a very high selectivity in the reduction of 4-substituted cyclohexanones towards the thermodynamically unfavourable cis-alcohols was observed. By-products were observed only during the oxidation of alcohols using ketone solvents and consisted primarily of aldol condensation products. Remarkable differences exist between the liquid-phase and gas-phase reactions under otherwise similar conditions. The selectivity towards the cis-alcohol is still above the thermodynamically expected value but significantly lower than under liquid-phase conditions. In contrast to the liquid-phase reactions, dehydration of the alcohols to the corresponding alkene is an important side-reaction. The oxidation of both the cis- and the trans-alcohol clearly showed that the olefin is exclusively formed from the cis-alcohol. Dehydration of the trans-alcohol is assumed to proceed by isomerisation via a MPVO mechanism to the corresponding cis-alcohol. The catalytic potential of the titanium-based catalyst is shown by the low amount of by-products formed over Ti-beta compared with Al-beta, the high resistance to water and the excellent stability of the catalyst with respect to regeneration.
ACKNOWLEDGEMENT Dr. Eddy Creyghton is thanked for valuable discussion and the Dutch Institute for Scientific Research (NWO/SON) for financial support. REFERENCES
1
For a review, see : C.F. de Graauw, J.A. Peters, H. van Bekkum and J. Huskens, Synthesis, 10 (1994) 1007. 2 P.B. Venuto, Microporous Mater., 2, (1994) 297; W. H61derich and H. van Bekkum, Stud. Surf Sci. CataL, 68, (1991) 631. 3 J. Shabtai, R. Lazar and E. Biron, J. Mol. Catal., 27, (1984) 35. 4 M. Huang, P.A. Zielinski, J. Moulod and S. Kaliaguine, Appl. CataI. A, 118, (1994) 33. 5 M. Berkani, J.L. Lemberton, M. Marczewski and G. Perot, Catal. Lett., 31 (1995) 405. 6 E.J. Creyghton, S.D. Ganeshie, R.S. Downing and H. van Bekkum, J. Chem. Soc. Chem. Commun., (1995) 1859. 7 E.J. Creyghton, S.D. Ganeshie, R.S. Downing and H. van Bekkum, J. Mol. Catal., in press (1997). 8 J.C. van der Waal, P. Lin, M.S. Rigutto and H. van Bekkum, Stud. Surf Sci. Catal., 105, 1093 (1997). 9 J.C. van der Waal, K. Tan and H. van Bekkum, Catal. Lett., 41 (1996) 63. 10 J.C. van der Waal, M.S. Rigutto and H. van Bekkum, J. Chem. Soc., Chem. Commun., (1994) 1241. 11 R.L. Wadlinger and G.T. Kerr, US patent, Appl. 3.308.069 (1967).
1015
Zeolite T i t a n i u m Beta" A selective catalyst in the M e e r w e i n - P o n n d o r f V e r l e y - O p p e n a u e r reactions. J.C. van der Waal a, P.J. Kunkeler a, K. Tan b and H. van Bekkum a a Laboratory of Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands b Tianjin Research Institute of Chemical Industry, Dingzigu, Hongqiao, 300131 Tianjin, PR China Zeolite titanium beta has been tested in the liquid- and gas-phase Meerwein-PonndorfVerley reduction of cyclohexanones and the Oppenauer oxidation of cyclohexanols. A high selectivity towards the thermodynamically unfavourable cis-alcohol was observed, which has been ascribed to transition-state selectivity in the pores of the zeolite. Under gas-phase conditions the dehydration of alcohols to cycloalkenes is observed as a side reaction. The catalyst was found to be active even in the presence of water and ammonia.
1. INTRODUCTION The Meerwein-Ponndorf-Verley reduction of carbonyl compounds and the Oppenauer oxidation of alcohols, together denoted as MPVO reactions, are considered to be highly selective reactions. For instance, C =C double bonds are not attacked. In MPV reductions a secondary alcohol is the reductant whereas in Oppenauer oxidations a ketone is the oxidant. It is generally accepted that MPVO reactions proceed via a complex in which both the carbonyl and the alcohol are coordinated to a Lewis acid metal ion after which a hydride transfer from the alcohol to the carbonyl group occurs (Fig. 1) [1]. Usually, metal sec-alkoxides are used as homogeneous catalysts in reductions and metal t-butoxides in oxidations [1]. RI-_
"')==o+ ;XR, Fig. 1
u
R3
-
.,
The Meerwein-Ponndorf-Verley-Oppenauer reaction [1].
Zeolites are crystalline metal oxides which have potential as regenerable heterogeneous catalysts in various organic reactions [2]. Because of their unique microporous structure, zeolites are especially promising in the field of shape-selective
1016 catalysis. As far as we know, only very few examples of the use of zeolites in MPVO reactions have been reported [3-7,9]. The reactions were carried out in the gas-phase over zeolites A, X and Y, exchanged or impregnated with alkali or alkaline-earth cations [3-5]. Shape-selectivity was only observed by Shabtai et aL[4] in the conversion of citronellal over zeolite X. It was shown that selectivity could be tuned by the size of the exchanged metal ion. In NaX there was enough space for the substrate to perform an intramolecular ring closure to isopulegol, whereas over CsX reduction to the linear citronellol was observed. Similar steric effects were also found for various other substrates. Recently Creyghton et aL [6,7] reported the use of zeolite beta in the MPVO reduction of 4-t-butylcyclohexanone. The high selectivity towards the thermodynamically less favoured cis-alcohol is explained by a restricted transition-state around a Lewis-acidic aluminium in the zeolite pores. When using an aluminium-free zeolite, titanium beta, in the epoxidation of olefins, we have shown that Ti-beta has acidic properties when alcoholic solvents were employed [8]. This was ascribed to the Lewis-acidic character of titanium in the zeolite framework. As we reported very recently [9], Ti-beta is found to be an excellent catalyst in MPVO reactions with a tolerance for water. Here, results are presented on the high selectivity, stability and low by-product formation of the catalyst, Ti-beta, in both the liquid-phase and gas-phase MPVO reactions.
2. EXPERIMENTAL
Zeolite titanium beta (Ti-beta) was synthesized according to van der Waal et al. [8,9] using di(cyclohexylmethyl)dimethylammonium hydroxide (DCDMA.OH) as the template. For a typical synthesis, 0.25 g titanium(IV) ethoxide (TEOT) was added to 30.0 g of a 19.5 % w/w DCDMA.OH solution and the mixture was stirred until all TEOT was dissolved. To facilitate the dissolution of TEOT, 1 ml of H20 2 (30 % w/w aqueous) was added. To the resulting clear solution 3.0 g Aerosil 200 (Degussa), 0.15 g seeds (all-silica beta [10] and 11.8 g water were added and the gel was aged for at least 24 h. After crystallisation (14 days at 140 ~ the zeolite (1.4 g) was filtrated, washed, dried and calcined at 540 ~ in air. Elemental analysis, performed using a LINK EDX system, showed a Si:Ti molar ratio of 76 and confirmed the absence of oligomeric titanium dioxide phases. Zeolite aluminium beta (Al-beta, Si:AI = 10) was prepared according to Wadlinger and Kerr [11] and all-silica beta (Si-beta, Si:A1 > 5000) was prepared according to van der Waal et aL [10]. Liquid-phase MPVO reactions were performed in 25 ml isopropanol (reductions) or 25 ml 2-butanone (oxidations) at 85 ~ using 2.5 mmol of the appropriate substrate: 4-t-butylcyclohexanone (4-Bu-ONE), 4-methylcyclohexanone (4-Me-ONE) or 4-t-butylcyclohexanol (4-Bu-OL, cis/trans mixture); 0.5 g zeolite or 0.25 mmol aluminium isopropoxide as the catalyst and 1,3,5-tri-t-butylbenzene as the internal standard. Samples were taken at regular intervals and analyzed by GC on a Carbowax CP-52 column and GC/MS. Gas-phase MPVO reactions were performed at 85 to 400 ~ in a fixed bed continuous down-flow reactor operated at atmospheric pressure under plug flow conditions. The catalyst, Ti-beta or N-beta (0.30 g), was diluted with 1.20 g a-quartz powder and processed to pellets then crushed to particles with a diameter of 0.7 - 1.0 mm. Reactant mixtures were pumped into a stream of preheated carrier gas (usually
1017 nitrogen) by means of a motor-driven syringe pump. The gas flow contained 10 vol.% isopropanol (reductions) or acetone (oxidations) and 1 vol.% of the appropriate substrate: 4-methylcyclohexanone (4-Me-ONE) or 4-methylcyclohexanol (4-Me-OL). The total gas flow was 50 ml/min and the molar gas flow of 4-Me-ONE or 4-Me-OL was 2.04 10-5 mol/min (WHSV = 2.9 gtotal goat-1 h'l) 9 Samples of the reactor effluent were taken regularly and analysed on a CP-Sil-19 column. 3. RESULTS
3.1 Liquid-phase MPVO In the liquid-phase, the MPVO reactions catalysed by Ti-beta are highly selective and have low by-product formation. In the reduction of 4-t-butylcyclohexanone and 4-methylcyclohexanone a high selectivity (99 % and 98%, respectively) towards the thermodynamically unfavourable cis-alcohol is observed, similar to that observed with A1beta (Table 1). The high selectivity towards the cis-alcohol over beta-type zeolites was explained by Creyghton et aL [6] on the basis of transition-state selectivity. It can be seen in Fig. 2, that the transition-state leading to the cis- and trans-alcohol differ substantially in spatial requirements. The transition-states leading to the cis-isomer is aligned with the zeolite channel while that leading to the trans-alcohol occupies a much more axial position. Table 1 MPVO reduction of cyclohexanones over beta-type catalysts in refluxing isopropanol. Substrate
Catalyst
TOF a
Conversion b (%)
Selectivityb cis : trans
4-Me-ONE
Ti-beta
1.04
33.7
99:1
4-Me-ONE
M-beta
> 12
100.0
98:2
4-Me-ONE
AI(OPr)3
16.7
99.8
27:73
4-Bu-ONE
Ti-beta
2.26
64.9
98:2
4-Bu-ONE
Al-beta
> 12
100.0
95:5
4-Bu-ONE
Si-beta
4-Bu-ONE
AI(OPr)3
0.0 8.72
100.0
36:64
a Initial turn-over-frequency in mol ketone per mol of titanium or aluminium per hour. b Conversion and selectivity after 6 h reaction based on the initial amount of ketone added.
1018
H-~Me Me
....
O~ /0
/ Fig. 2
/ Proposed transition-states for the formation of cis-4-t-butylcyclohexanol (left) and trans-4-t-butylcyclohexanol (right).
In the Oppenauer oxidation of an equimolar mixture of cis- and trans-4-tbutylcyclohexanol, the cis-alcohol is converted selectively over both Ti-beta and Al-beta (Table 2), which is probably due to the same spatial restriction on the transition-states as for the reduction depicted in Fig. 2. In this way, the MPVO reduction and oxidation are in harmony as to the high transition-state selectivity. Table 2 Liquid-phase Oppenauer oxidation of 4-t-butylcyclohexanol (1:1 cis/trans mixture)with butanone as the oxidant. Catalyst
Conversion a of cis-alcohol (%)
Conversion a of trans-alcohol (%)
By-products a'b (mg)
Ti-beta
52.1
4.6
22.4
Al-beta
98.6 49.4 d
47.3 5.7c
317.3 63.8c
Ti(OiPr)4
6.1
12.9
22.7
Al(OiPr)3
5.3
7.4
56.5
a After 6 h reaction time. b Not MPVO related, predominantly from 2-butanone via aldol condensation, c After 15 min reaction time. d After 15 min. The most important side reaction in heterogeneously catalysed MPVO reactions is the acid-catalysed aldol condensation. Aldol products are usually observed during the Oppenauer oxidation of alcohols, when a surplus of ketone or aldehyde is used as the oxidizing agent and the solvent. The low amount of by-products formed when Tibeta was used as the catalyst, demonstrates the advantage of the titanium system over A1beta. This is probably caused by the much weaker BrCnsted acidity of the solvated titanium site [8] compared with the strong H+-acidity of the aluminium site in Al-beta. As we have shown earlier Ti-beta has a high tolerance towards water, which further shows the catalytic potential of Ti-beta in MPVO reactions [9].
1019
3.2 Gas-phase MPVO From an industrial point of view, gas-phase reactions are often preferred due to their ease of operations. The reduction of 4-methylcyclohexanone and the oxidation of cis- and trans-4-methylcyclohexanol over Ti-beta and Al-beta in the gas phase were studied at 100~ As can be seen from Fig. 3, both Ti-beta and Al-beta are active, but Tibeta has a considerably lower rate of deactivation. The deactivation of Al-beta is probably caused by the higher acidic strength of the protonic aluminium site compared to the non-protic titanium site. Another difference between the two catalysts is the pronounced dehydration of the alcohols formed over Al-beta. Two important differences between the gas-phase and liquid-phase reaction were observed. The most striking is the selectivity towards the cis-alcohol. Under liquid phase conditions (Table 1.) a selectivity of 99 % towards the cis-alcohol was observed, while in the gas-phase over Ti-beta only 53 - 62 % cis-alcohol was obtained. Furthermore, dehydration of the alcohols formed to 4-methylcyclohexene (4Me-ENE) was observed in the gas phase, while no trace of dehydrated products could be detected in liquid phase MPV reductions. The somewhat higher temperature of 100 ~ compared with 85 ~ for the liquid-phase experiments, could not explain this behaviour, since lowering the temperature to 85 ~ still resulted in a 4-Me-ENE selectivity of approximately 10 % over Ti-beta. The remarkable differences in selectivity between the gas- and liquid-phase MPVO reaction induced us to study the evolution of products as a function of the temperature, in the reduction of 4-methylcyclohexanone. From Fig. 4 it can be seen that the selectivity to the cis-alcohol decreases at higher temperatures; initially the selectivity to the trans-alcohol increases but at higher temperatures dehydration to 4-methylcyclohexene becomes the major reaction. At still higher temperatures (> 200 ~ the 4-methylcyclohexene is subsequently isomerised to the more stable 1-methylcyclohexene. I00
80
'N
60
20
0
2
4 Time on stream [h]
6
8
a) Ti-beta Fig. 3a
The gas-phase MPV reduction of 4-methylcyclohexanone with isopropanol over Ti-beta at 100~ 9 = conversion; 9 = cis-4-Me-OL; x = trans-4-Me-OL; n = 4-Me-ENE.
1020
100
80
~.43
------'~
!o A
o
-
i
A
4 ' Time on stream [h]
i
b) Al-beta Fig. 3b
The gas-phase MPV reduction of 4-methylcyclohexanone with isopropanol over Al-beta at 100~ 9 = conversion; 9 = cis-4-Me-OL; x = trans-4-Me-OL; [] = 4-Me-ENE. 100
80 ,___,
c~
.~ 40
0
Fig. 4
50
100
150 200 250 Temperature [ * C]
300
350
Influence of the temperature on the selectivity in the reduction of 4-methylcyclohexanone with isopropanol. Temperature increment was 0.2 ~ 9 = conversion; 9 = cis-4-Me-OL; x = trans-4-Me-OL; [] = 4-Me-ENE.
Since the 4-methylcyclohexene can be formed via dehydration of either of the two alcohols formed, both alcohols were tested in the Oppenauer oxidation, using acetone as the oxidant at 100 ~ It was observed that the cis-alcohol (Fig. 5a) is easily oxidized to the corresponding ketone, while the trans-alcohol showed a much lower activity. This is in accordance with the transition-states assumed for liquid phase MPVO reactions (Fig. 2). It can be observed further that the dehydration of the trans-alcohol only occurred to very limited scale (Fig. 5b) whereas dehydration of the cis-alcohol was an important side-
1021 reaction. This can be understood by assuming an E2-mechanism, in which the axial OH group in the cis-isomer is in the ideal position for water elimination. Another important side reaction for both alcohols was isomerization. It is therefore proposed that the alkene is formed mainly from the cis-alcohol and that at low temperatures the trans-alcohol can only be dehydrated if it is first isomerised to the cisalcohol via an MPVO transition-state. Comparing the deactivation of the Ti-beta catalyst in the oxidation (Fig. 5) and reduction reactions (Fig. 3) shows that the deactivation is more pronounced during oxidative conditions. This is probably caused by the high amount of ketones present, which easily form aldol condensates which may plug the zeolite channels, thus inhibiting access to the micropore system. 100
80
r~
..~ 40
i
r,.)
0
2
4 Time on stream [h]
6
a) Oxidation of cis-4-Me-OL 100
80
"~
60
.~ 40
I 2o
r.,)
0
2
4 Time on stream [h]
6
8
b) Oxidation of trans-4-Me-OL Fig. 5
The gas-phase Oppenauer oxidation of the 4-methylcyclohexanols with acetone over Ti-beta at 100~ a) cis-4-Me-OL and b) trans-4-Me-OL. 9 = conversion; 9 = cis-4-Me-OL; x = trans4-Me-OL; 9 = 4-Me-ONE. [] = 4-Me-ENE.
1022 The commonly used MPVO catalysts consist of metal alkoxides, which are easily hydrolysed to inactive oxides in the presence of water. Since the proposed catalytic species for the MPVO reaction also consists of an alkoxide intermediate [6,9] the influence of water and strong Lewis bases on the catalytic activity and selectivity was investigated. As already reported for the liquid-phase reaction [9], Ti-beta has a high tolerance for water due to its hydrophobic interior. As can be seen from Fig. 6a the presence of water is not detrimental to the activity of Ti-beta in the MPV reduction of 4-methylcyclohexanone. The temperature of 110 ~ at which a ketone conversion of 50% is measured, is identical to the temperature required for 50% conversion in the absence of water (Fig. 3), i.e. water has no effect whatsoever on the overall MPVO activity of the titanium site. 100
50
100
150 200 250 Temperature [ * Q
300
350
a) Reduction in the presence of water 100
9
--
w--r-~
80
60
.~
40
i 0 100
150
200 250 Temperature * C]
300
350
400
b) Reduction in the presence of ammonia Fig. 6
Temperature-programmed gas-phase MPV reduction of 4-methylcyclohexanone over Ti-beta in the presence of a) 2.66 vol.% water or b) 5 vol% ammonia. 9 = conversion; 9 = cis-4-Me-OL; x = trans-4-Me-OL; [] = 4-MeENE.
1023
The selectivity towards alcohols increased from 8 1 % to 97% at 85~ This enhanced selectivity can be ascribed to either a kinetic suppression of an irreversible dehydration or to a change in the alcohol/olefin equilibrium due to the higher amount of water present. In the case of a shift in equilibrium, it should be possible to oxidise olefins with ketones in the presence of water via in situ formed alcohols. In an attempt to oxidise cyclohexene with acetone in the presence of a water, no conversion to cyclohexanone was observed between 85 and 400 ~ It is therefore concluded that under the experimental conditions used, 4-methylcyclohexene is formed irreversibly from the cisalcohol (Fig. 7) and the increased selectivity towards alcohols should therefore be ascribed to a deactivation of the dehydration sites in the presence of water. OH
cis-4-Me-OL
4-Me-ENE
1-Me-ENE
Ti 4-Me-ONE
~
OH
trans-4-Me-OL
Fig. 7
Proposed reaction scheme for the MPV reduction of 4-methylcyclohexanone.
For liquid-phase reactions at 85 ~ we reported that small amounts of a strong base, e.g. pyridine, completely poisoned the catalyst [2]. It can be seen from Fig. 6b that, in the presence of ammonia, higher temperatures are required to reduce 4-methylcyclohexanone. The higher temperatures are required for the desorption of ammonia from the catalytically active site. The relatively low temperature of 305 ~ at which 50 % conversion is observed, suggests that the ammonia is not bonded to a strong acidic site. Since Br0nsted-acidic aluminium sites desorb ammonia at about 480 ~ this confirms that the MPVO reactions proceed via the titanium sites and not via any residual aluminium sites (Si:AI > 5000). The latter can also be concluded from Table 1; the all-silica analogue of zeolite beta [10] was found to be completely inactive even though it has a Si:A1 ratio similar to Ti-beta. It was also observed that in the presence of ammonia, no isomerisation of 4-Me-ENE to 1-Me-ENE occurred even at 400 ~ suggesting that the isomerisation requires strong Brcnsted acid-sites, most probably the residual aluminium sites. As was already shown in Fig. 3 and 5a, Ti-beta exhibits a low deactivation rate in the reduction of 4-methylcyclohexanone and a significantly higher deactivation rate in the oxidation of cis-4-methylcyclohexanol. In both cases, frequent regeneration of the catalyst will be necessary. Catalyst stability was tested by regeneration at 480~ in air after each run. No significant loss in activity, selectivity or product distribution was observed, even after 35 consecutive runs. Similar results were also obtained for the Tibeta used in liquid-phase reductions; after regenerating the Ti-beta catalyst 5 times, the same initial catalytic activity per gram of catalyst was observed.
1024 4. CONCLUSION Ti-beta is found to be an excellent catalyst in MPVO reactions under both liquid- and gas-phase conditions. Under liquid-phase conditions, a very high selectivity in the reduction of 4-substituted cyclohexanones towards the thermodynamically unfavourable cis-alcohols was observed. By-products were observed only during the oxidation of alcohols using ketone solvents and consisted primarily of aldol condensation products. Remarkable differences exist between the liquid-phase and gas-phase reactions under otherwise similar conditions. The selectivity towards the cis-alcohol is still above the thermodynamically expected value but significantly lower than under liquid-phase conditions. In contrast to the liquid-phase reactions, dehydration of the alcohols to the corresponding alkene is an important side-reaction. The oxidation of both the cis- and the trans-alcohol clearly showed that the olefin is exclusively formed from the cis-alcohol. Dehydration of the trans-alcohol is assumed to proceed by isomerisation via a MPVO mechanism to the corresponding cis-alcohol. The catalytic potential of the titanium-based catalyst is shown by the low amount of by-products formed over Ti-beta compared with Al-beta, the high resistance to water and the excellent stability of the catalyst with respect to regeneration.
ACKNOWLEDGEMENT Dr. Eddy Creyghton is thanked for valuable discussion and the Dutch Institute for Scientific Research (NWO/SON) for financial support. REFERENCES
1
For a review, see : C.F. de Graauw, J.A. Peters, H. van Bekkum and J. Huskens, Synthesis, 10 (1994) 1007. 2 P.B. Venuto, Microporous Mater., 2, (1994) 297; W. H61derich and H. van Bekkum, Stud. Surf Sci. CataL, 68, (1991) 631. 3 J. Shabtai, R. Lazar and E. Biron, J. Mol. Catal., 27, (1984) 35. 4 M. Huang, P.A. Zielinski, J. Moulod and S. Kaliaguine, Appl. CataI. A, 118, (1994) 33. 5 M. Berkani, J.L. Lemberton, M. Marczewski and G. Perot, Catal. Lett., 31 (1995) 405. 6 E.J. Creyghton, S.D. Ganeshie, R.S. Downing and H. van Bekkum, J. Chem. Soc. Chem. Commun., (1995) 1859. 7 E.J. Creyghton, S.D. Ganeshie, R.S. Downing and H. van Bekkum, J. Mol. Catal., in press (1997). 8 J.C. van der Waal, P. Lin, M.S. Rigutto and H. van Bekkum, Stud. Surf Sci. Catal., 105, 1093 (1997). 9 J.C. van der Waal, K. Tan and H. van Bekkum, Catal. Lett., 41 (1996) 63. 10 J.C. van der Waal, M.S. Rigutto and H. van Bekkum, J. Chem. Soc., Chem. Commun., (1994) 1241. 11 R.L. Wadlinger and G.T. Kerr, US patent, Appl. 3.308.069 (1967).