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Redox Molecular Sieves: Recylable Catalysts for Liquid Phase Oxidations
Redox Molecular Sieves: Recyclable Catalysts for Liquid Phase Oxidations R.A. Sheldon, J.D. Chen, J. Dakka and E. Neeleman Delft University of Technology, Laboratory for Organic Chemistry and Catalysis, Julianalaan 136, 2628 BL Delft, The Netherlands
ABSTRACT Redox molecular sieves have been synthesized by isomorphous substitution in the framework of silicalite-1 and ALPO-5. CrAPO-5 and CrS-1 were shown to be effective catalysts for the decomposition of secondary alkyl hydroperoxides to the corresponding ketones. In the decomposition of cyclohexyl hydroperoxide the highest selectivity to cyclohexanone (86%) was observed with CrAPO-5. CrAPO-5 was also shown to be an effective catalyst for the oxidation of secondary alcohols to the corresponding ketones, alkylbenzenes to acetophenones and cyclohexane to cyclohexanone using tert-butyl hydroperoxide (TBHP) or 0, as the terminal oxidant. Evidence is presented in support of the reaction taking place inside the cavity of the molecular sieve.
INTRODUCTION Catalytic oxidation is widely used for the conversion of petroleum-derived hydrocarbons to commodity chemicals [I]. Moreover, in fine chemicals manufacture there
is increasing pressure to replace traditional stoichiometric oxidations with inorganic reagents such as dichromate and permanganate with cleaner, catalytic alternatives which do not generate excessive amounts of inorganic salts as byproducts. Catalytic oxidations in the liquid phase generally employ soluble metal salts or complexes as the catalyst. However, solid catalysts offer several potential advantages over their homogeneous counterparts, such as ease of recovery and recycling and enhanced stability. Moreover, site-isolation of discreet redox metal centers in inorganic matrices can lead to oxidation catalysts with unique activities and selectivities. One approach to designing stable solid catalysts with unique activities is to incorporate redox metal ions, by isomorphous substitution, into the lattice framework of molecular sieves, such as silicalites, zeolites, aluminophosphates (ALF'Os) and 407
408
R. A. Sheldon, J. D. Chen, J. Dakka and E. Neeleman
silicoaluminophosphates (SAPOs). Such redox molecular sieves [2-41 can be regarded as 'mineral enzymes'. Unlike conventional amorphous materials, such as silica and alumina, molecular sieves possess a regular microenvironment with homogeneous internal structures consisting of uniform, well-defined cavities and channels. Hence, redox molecular sieves can be tailor-made by fine-tuning of the size and hydrophobicity of the redox cavity to provide unique oxidation catalysts. Furthermore, incorporation of the redox metal ion into the stable lattice of a molecular sieve may provide enhanced stability towards leaching, a problem often encountered with conventional supported metal catalysts. A landmark in the development of redox molecular sieves was the titanium(1V)silicalite (TS-1) catalyst developed by Enichem workers [5]. TS-1 catalyzes a variety of industrially useful oxidations with 30% aqueous hydrogen peroxide, e.g. olefin epoxidation, phenol hydroxylation and cyclohexanone ammoximation. As part of an ongoing research program on redox molecular sieves we have synthesized and characterized a range of redox ALPOs, zeolites and silicalites. Chromium molecular sieves were of particular interest based on the widespread use of chromium(V1) compounds as stoichiometric oxidants in organic synthesis [6] and, more recently, the use of soluble chromium catalysts in combination with TBHP [7]. EXPERIMENTAL m l y s t synthesis CrAPO-5 was hydrothermally synthesized by essentially following a reported procedure [8], using the molar ratio: 0.05 Cr203:0.9 A1203:P205:Pr,N:S0 H,O. Crystallization was performed at 175 "C for 24 h. The template (Pr3N) was removed by subsequent calcination of the as-synthesized material at 500 "C for 10 h. Chromium silicalite (CrS-I) was hydrothermally synthesized by essentially following a reported procedure [9]. In order to obtain CrS-1 free of quartz it was necessary to rotate the autoclave, e.g. at 320 rpm, during the crystallization step. Other metal ALPOs and silicalites were prepared by similar procedures. m l v s t characterization CrAPO-5, CrS-1 and other metal ALPOs and silicalites were characterized by elemental analysis, X-ray diffraction (XRD), diffuse reflectance atomic absorption spectroscopy (DREAS) and scanning electron microscopy (SEM). XRD powder patterns
Redox Molecular Sieves
409
were recorded on a Philips PW 1877 automated powder diffractometer using CuKa radiation.
DREAS
spectra
were
measured
with
a
Hitachi
150-20 UV-VIS
spectrophotometer equipped with a diffuse reflectance unit. SEM spectra were obtained using a JEOL JSM-35 scanning microscope. The samples were coated with an Au evaporated film. Elemental analyses were obtained using inductively coupled plasmaatomic emission spectroscopy (ICP-AES) on a Perkin-Elmer Plasma I1 instrument. TvDical reaction Drocedu re5 Typically, oxidation reactions were carried out by stirring a suspension of the catalyst (ca. 1 mol %) with a solution of the substrate and TBHP in the solvent (e.g. chlorobenzene) at 85-110 "C for 5 hours. Reactions employing molecular oxygen were caried out at atmospheric pressure by bubbling oxygen through the reaction mixture or 5 bar 0, in an autoclave. Hydroperoxides were analyzed by iodometric titration and other substrates and products by gas liquid chromatography. RESULTS AND DISCUSSION A e l1 hydroperoxide decomposition
In the manufacture of cyclohexanone via cyclohexane autoxidation initially formed cyclohexyl hydroperoxide (CHHP) is decomposed, often in a separate step, to give a mixture of cyclohexanol and cyclohexanone. From the viewpoint of practical utility it is desirable to achieve a high ratio of cyclohexanone to cyclohexanol. The ideal situation corresponds to decomposition of CHHP according to the stoichiometry:
OOH
0 catalyst
Moreover, a stable recyclable catalyst for reaction 1 would be particularly attractive. Consequently, we have tested a variety of redox molecular sieves as catalysts for reaction 1 (see Table 1). Both CrAPO-5 and CrS-1 were active catalysts for CHHP decomposition.
The highest selectivity to cyclohexanone (86%) was observed with CrAPO-5. CrS-1 was even more active but gave a lower cyclohexanone/cyclohexanol ratio. Other metal APOs and silicalites gave both lower activities and selectivities.
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R. A. Sheldon, J. D. Chen, J. Dakka and E. Neeleman
In one experiment with CrAPO-5 the catalyst was filtered, washed with cyclohexane, recalcined and reused with a fresh solution of CHHP. This was repeated five times without any noticeable loss of activity or selectivity. Recalcination is probably necessary in order to remove the water, formed in the reaction, from the pores of the catalyst. In practice this may be possible using other means, e.g. azeotropic distillation during reaction. Table 1. Catalytic decomposition of cyclohexyl hydroperoxide (CHHP) at 70 "C. Catalyst
Selectivity (%)
CHHP conversion (%)
CrAPO-5 Cr-silicalite VAPO- 11 CO-ZSM-5 VAPO-5 COAPO-5 MWO-5 V-silicalite TS-1 None
87 98
76 24 17 2 2 0
0 0
Cyclohexanone
Cyclohexanol
86 64 50 43 51 50 50 0 0
13 36 50 50 43 50 50
0
0 0 0
Conditions: CHHP (2.9 mmol) dissolved in cyclohexane (10 ml) stirred with the catalyst (0.029 mmol metal) at 70 "C for 5 hours. Similarly, other secondary hydroperoxides, e.g. ethylbenzene hydroperoxide and tetralin hydroperoxide, afforded high yields of the corresponding ketone with CrAPO-5. Tert-alkyl hydroperoxides were decomposed to the corresponding alcohol and dioxygen, together with small amounts of the ketone formed by 8-scission of intermediate alkoxy radicals (Table 2).
Redox Molecular Sieves
41 I
Table 2. CrAPO-5 catalyzed decomposition of alkyl hydroperoxidesa. ~~
R02H Cyclohexyl tert-Butylb Cumene Triphenylmethyl
Solvent C6H12 C,H,CI C,H,CI 1,2-C2H4CI,
~
~~~
~
Conversion (%) 87 49 24 1
Selectivity (%) Ketone
Alcohol
86 5 2
13 93 86
Conditions: R02H (2.9 mmol) in solvent (10 ml) stirred with CrAPO-5 (0.1 g containing 1.5% Cr = 0.029 mmol Cr) for 5 h at 70 "C. 50 "C.
a
Evidence for the reaction taking place inside the cavity of CrAPO-5 was provided by the observation that the bulky triphenylmethyl hydroperoxide, which cannot be accommodated in the cavity, was not decomposed. In contrast, homogeneous chromium(II1) acetylacetonate and the supported Cr02C12/silica-alumina were effective catalysts for the decomposition of this hydroperoxide giving 75% and 72% decomposition in 2 h, respectively, with equivalent amounts of catalyst (1% m) at 70 "C in 1,Zdichloroethane. Walyst structure and catalytic mechanism
In the case of both CrAPO-5 and CrS-1 the as-synthesized catalysts are green and contain chromium in the trivalent state. After calcination at 500 "C the catalysts were yellow and DREAS showed that most of the chromium is present as Cr(VI). ICP-AES analysis showed that chromium contents of up to 1% (CrS-1) to 1.5% (CrAPO-5) could be achieved. We tentatively assume that in the as-synthesized catalysts chromium(II1) is isomorphously substituted, in tetrahedral positions, for silicon (CrS-1) or aluminium (CrAPO-5). Subsequent oxidation during calcination is assumed to afford dioxochromium(V1) which is still attached to the internal framework in either tetrahedral or octahedral coordination. Decomposition of CHHP according to the stoichiometry of reaction 1 is consistent with a heterolytic mechanism. This can be envisaged as proceeding via /3-hydrogen elimination in an alkylperoxochromium(VI) complex (Figure 1). Such a mechanism is not possible with tert-alkyl hydroperoxides and we assume that a homolytic mechanism, involving tert-alkoxy radicals as intermediates, operates in this case. Acid-catalyzed heterolysis can be ruled out because it would lead to the formation of different products,
412
R. A. Sheldon, J . D. Chen, J . Dakka and E. Neeleman
e.g. phenol and acetone from cumene hydroperoxide.
>-,
VI
Cr=O
+ R,C=O + H,O
Fig. 1. Mechanism of decomposition of secondary alkyl hydroperoxide. CrAPO-5 catalyzed oxidation of secondary alcohols Based on the known use of homogeneous chromium catalysts for the oxidation of secondary alcohols with TBHP [7] we envisaged that CrAPO-5 should be an effective solid catalyst for this reaction. The results of CrAPO-5 catalyzed oxidations of secondary alcohols with TBHP at 85 "C are shown in Tdhk 3. Good to excellent selectivities to the corresponding ketones were observed with respect to both substrate and TBHP in most cases. Carveol underwent chemoselective oxidation of its alcohol group to give carvone in
94% selectivity, without any attack at its double bonds. Moreover, one of the (cis/trans) isomers appeared to react much faster indicating that some shape selectivity is observed. l-Phenyl-1,2-ethanediol was selectively oxidized at the secondary alcohol group to give ahydroxy acetophenone (73% selectivity). In one experiment with a-methylbenzyl alcohol the CrAPO-5 catalyst was filtered, washed 3 times with chlorobenzene and recalcined before reuse. The catalyst was recycled 4 times without any noticeable loss of activity or selectivity. DREAS spectra showed that most of the chromium remained in the hexavalent state within the ALPO, framework after recycling. Hence, we conclude that CrAPO-5 is a stable, recyclable catalyst for the selective liquid phase oxidation of secondary alcohols, to the corresponding ketones, using TJ3HP as the terminal oxidant. Interestingly, when the oxidation of a-methylbenzyl alcohol with TBHP was carried out in air instead of N2 a yield of acetophenone on TBHP of 216% was observed, suggesting that 0, could also act as the terminal oxidant. This was confirmed in subsequent experiments (Table 4). The best results were obtained using a small amount
(10 mol %) of TBHP to initiate the reaction.
Redox Molecular Sieves
413
Table 3. CrAPO-5 catalyzed oxidations of secondary alcohols with TBHP at 85 "Ca. Substrate
a-Ethylbenzyl alcohol a-Methylbenzyl alcohol Cyclohexanol Carveol l-Phenyl-1,2ethanediol
Time (h)
Product
Conversion
(%Ib
Selectivity (%) Substrate
TBHP
7
propiophenone
77
100
91
16 12 16
acetophenone cyclohexanone carvone a-hydroxyacetophenone
77 72 62
96 85 94
89 73 66
54
73
40
16
a Conditions: substrate, 10 mrnol; TBHP, 5 mmol; CrAPO-5 (0.14 mmol), chlorobenzene (solvent), 10 rnl; stirred at 85 "C for 16 h under N,. Conversion of substrate based o n the amount of TBHP charged.
We tentatively propose that the oxidation of secondary alcohols with TBHP in the presence of CrAPO-5 proceeds via a heterolytic mechanism involving 8-hydrogen elimination from an oxochrornium(V1) alkoxide followed by reoxidation of the reduced chromium(1V) by TBHP (Figure 2). When 0, is the terminal oxidant the a-hydroxyhydroperoxide, formed by (chrorniurn-catalyzed) autoxidation of the alcohol, can reoxidize the chromium(1V). Table 4. CrAPO-5 catalyzed oxidations of secondary alcohols with OZa. Conversion (%)
Selectivity (%)
cyclohexanone
30
97
acetophenone
31
96
propiophenone a-tetralone l-indanone
38 26 78
90 73 72
Substrate
Product
Cyclohexanol a-Methylbenzyl alcohol a-Ethylbenzyl alcoho I a-Tetralolb l-Indanolb
Conditions: substrate, 250 rnmol; 0, pressure 5 atm or 20 atm air; TBHP 25 mrnol; CrAPO-5 (3.65 mmol Cr); chlorobenzene (solvent), 65 ml; 3 A molecular sieve (drying agent), h g; 110 "C, 5 h. Conditions: substrate, 50 mmol; O,, 15 ml/min; TBHP, 5 mmol; CrAPO-5, 0.73 mmol; chlorobenzene, 5 ml, 110 "C, stirring 1000 rprn, 19 h.
R. A. Sheldon, J . D. Chen, J . Dakka and E. Neeleman
414
c"j-0 o H
@ lv>
CR,
)
TBHP. R,CHOH -H,O.
+ R,CO
&-OH
TBA
I
Fig. 2. Mechanism of alcohol oxidation. CrAPO-5 catalyzed oxidations of hydrocarbons By analogy with the chemistry of soluble chromium catalysts [7] we reasoned that CrAPO-5 should also be an effective catalyst for benzylic oxidations with TBHP. Indeed, CrAPO-5 (1 mol % Cr) catalyzed the selective oxidation of ethylbenzene (reaction 3) and tetralin (reaction 4) with TBHP. 0
PhC1/7O0C/1 6 h
8 5 % selectivity
0
9 0 % selectivity
As was observed in alcohol oxidations (see above) when the oxidation of tetralin was carried out in air the selectivity to a-tetralone based on TBHP was greater than 100%. Subsquent experiments confirmed that CrAPO-5 is an effective catalyst for the autoxidation of benzylic hydrocarbons to the corresponding ketones. A small amount (10 mol %) of TBHP was added to initiate the reaction. For example, tetralin was oxidized with 0, at atmospheric pressure and 100 "C to give a mixture of a-tetralone (64%), a-tetralol (7%) and a-tetralin hydroperoxide (THP; 20%). Presumably, in practice a small
Redox Molecular Sieves
415
amount of the THP-containing product stream could be recycled to the oxidation reactor, thus obviating the need for TBHP as initiator. Recycling experiments showed that the CrAPO-5 could be recycled 5 times without loss of activity (Table 5). Recalcination of the catalyst prior to reuse was not necessary, presumably because the water formed was removed azeotropically during reaction. Table 5. Recycling of CrAPO-5 in the autoxidation of tetralin at 100 'Ca. Cycle nr.
1 2 3 4
Selectivity (%)
Conversion
(%I 44 58
57 53 57
a-tetralone
a-tetra101
64 65 60 61 65
7 6 5 5 6
THP~ 20 24 30 32 24
a Conditions: tetralin, 50 mmol; O,, 15 ml/min; TBHP, 5 mmol; CrAPO-5, 0.73 mmol Cr; 100 "C, stirring, 1000 rpm, 10 h. THP = cr-tetralin hydroperoxide. The CrAPO-5 was regenerated by calcination at 500 "C for 5 h.
CrAPO-5 was also found to catalyze the autoxidation of cyclohexane at 115 "C. At 3% cyclohexane conversion the major product was cyclohexanone (64%) together with cyclohexanol (10%) and CHHP (9%) and dicarboxylic acids (13%). In practice part of the CHHP-containing product stream could be recycled to the oxidation reactor to act as an initiator. 0
64%
OH
10%
OOH
9%
3% cyclohexane conversion
416
R. A. Sheldon, 1. D . Chen, J. Dakka and E. Neeleman
CONCLUDING REMARKS CrAPO-5, containing chromium(V1) in the A1P04-5 framework, is an active, recyclable catalyst for alkyl hydroperoxide decomposition and the selective liquid phase oxidation of secondary alcohols, alkylaromatics and cycloalkanes with TBHP or 0,as the terminal oxidant. The scope and mechanism of these and related liquid phase oxidations mediated by redox molecular sieves are under further investigation. REFERENCES 1 R.A. Sheldon and J.K. Kochi, ‘Metal-Catalyzed Oxidations of Organic Compounds’, Academic Press, New York, 1981. 2 R.A. Sheldon, CHEMTECH, (1991) 566. 3 R.A. Sheldon, Topics Curr. Chem., 164 (1993) 21. 4 J.D. Chen, J. Dakka, E. Neeleman and R.A. Sheldon, J. Chem. SOC.,Chem. Commun., in press. 5 U. Romano, A. Esposito, F. Maspero, C. Neri and M. Clerici, Chim. Ind. (Milan), 72 (1990) 610. 6 G. Cainelli and G. Cardillo, ‘Chromium Oxidations in Organic Chemistry’, SpringerVerlag, Berlin, 1984. 7 J. Muzart, Chem. Rev., 92 (1992) 113. 8 E.M. Flanigen, B.M.T. Lok, R.L. Patton and S.T. Wilson, US Patent, 4,759,919 (1988) to Union Carbide Corp. 9 M. Kawai and T. Kyoura, Japanese Patents, JP 0358,954 and JP 0356,439 (1991) to Mitsui Toatsu Chemicals; CA 115 (1991) 48863d and 48864e.
ABSTRACT Redox molecular sieves have been synthesized by isomorphous substitution in the framework of silicalite-1 and ALPO-5. CrAPO-5 and CrS-1 were shown to be effective catalysts for the decomposition of secondary alkyl hydroperoxides to the corresponding ketones. In the decomposition of cyclohexyl hydroperoxide the highest selectivity to cyclohexanone (86%) was observed with CrAPO-5. CrAPO-5 was also shown to be an effective catalyst for the oxidation of secondary alcohols to the corresponding ketones, alkylbenzenes to acetophenones and cyclohexane to cyclohexanone using tert-butyl hydroperoxide (TBHP) or 0, as the terminal oxidant. Evidence is presented in support of the reaction taking place inside the cavity of the molecular sieve.
INTRODUCTION Catalytic oxidation is widely used for the conversion of petroleum-derived hydrocarbons to commodity chemicals [I]. Moreover, in fine chemicals manufacture there
is increasing pressure to replace traditional stoichiometric oxidations with inorganic reagents such as dichromate and permanganate with cleaner, catalytic alternatives which do not generate excessive amounts of inorganic salts as byproducts. Catalytic oxidations in the liquid phase generally employ soluble metal salts or complexes as the catalyst. However, solid catalysts offer several potential advantages over their homogeneous counterparts, such as ease of recovery and recycling and enhanced stability. Moreover, site-isolation of discreet redox metal centers in inorganic matrices can lead to oxidation catalysts with unique activities and selectivities. One approach to designing stable solid catalysts with unique activities is to incorporate redox metal ions, by isomorphous substitution, into the lattice framework of molecular sieves, such as silicalites, zeolites, aluminophosphates (ALF'Os) and 407
408
R. A. Sheldon, J. D. Chen, J. Dakka and E. Neeleman
silicoaluminophosphates (SAPOs). Such redox molecular sieves [2-41 can be regarded as 'mineral enzymes'. Unlike conventional amorphous materials, such as silica and alumina, molecular sieves possess a regular microenvironment with homogeneous internal structures consisting of uniform, well-defined cavities and channels. Hence, redox molecular sieves can be tailor-made by fine-tuning of the size and hydrophobicity of the redox cavity to provide unique oxidation catalysts. Furthermore, incorporation of the redox metal ion into the stable lattice of a molecular sieve may provide enhanced stability towards leaching, a problem often encountered with conventional supported metal catalysts. A landmark in the development of redox molecular sieves was the titanium(1V)silicalite (TS-1) catalyst developed by Enichem workers [5]. TS-1 catalyzes a variety of industrially useful oxidations with 30% aqueous hydrogen peroxide, e.g. olefin epoxidation, phenol hydroxylation and cyclohexanone ammoximation. As part of an ongoing research program on redox molecular sieves we have synthesized and characterized a range of redox ALPOs, zeolites and silicalites. Chromium molecular sieves were of particular interest based on the widespread use of chromium(V1) compounds as stoichiometric oxidants in organic synthesis [6] and, more recently, the use of soluble chromium catalysts in combination with TBHP [7]. EXPERIMENTAL m l y s t synthesis CrAPO-5 was hydrothermally synthesized by essentially following a reported procedure [8], using the molar ratio: 0.05 Cr203:0.9 A1203:P205:Pr,N:S0 H,O. Crystallization was performed at 175 "C for 24 h. The template (Pr3N) was removed by subsequent calcination of the as-synthesized material at 500 "C for 10 h. Chromium silicalite (CrS-I) was hydrothermally synthesized by essentially following a reported procedure [9]. In order to obtain CrS-1 free of quartz it was necessary to rotate the autoclave, e.g. at 320 rpm, during the crystallization step. Other metal ALPOs and silicalites were prepared by similar procedures. m l v s t characterization CrAPO-5, CrS-1 and other metal ALPOs and silicalites were characterized by elemental analysis, X-ray diffraction (XRD), diffuse reflectance atomic absorption spectroscopy (DREAS) and scanning electron microscopy (SEM). XRD powder patterns
Redox Molecular Sieves
409
were recorded on a Philips PW 1877 automated powder diffractometer using CuKa radiation.
DREAS
spectra
were
measured
with
a
Hitachi
150-20 UV-VIS
spectrophotometer equipped with a diffuse reflectance unit. SEM spectra were obtained using a JEOL JSM-35 scanning microscope. The samples were coated with an Au evaporated film. Elemental analyses were obtained using inductively coupled plasmaatomic emission spectroscopy (ICP-AES) on a Perkin-Elmer Plasma I1 instrument. TvDical reaction Drocedu re5 Typically, oxidation reactions were carried out by stirring a suspension of the catalyst (ca. 1 mol %) with a solution of the substrate and TBHP in the solvent (e.g. chlorobenzene) at 85-110 "C for 5 hours. Reactions employing molecular oxygen were caried out at atmospheric pressure by bubbling oxygen through the reaction mixture or 5 bar 0, in an autoclave. Hydroperoxides were analyzed by iodometric titration and other substrates and products by gas liquid chromatography. RESULTS AND DISCUSSION A e l1 hydroperoxide decomposition
In the manufacture of cyclohexanone via cyclohexane autoxidation initially formed cyclohexyl hydroperoxide (CHHP) is decomposed, often in a separate step, to give a mixture of cyclohexanol and cyclohexanone. From the viewpoint of practical utility it is desirable to achieve a high ratio of cyclohexanone to cyclohexanol. The ideal situation corresponds to decomposition of CHHP according to the stoichiometry:
OOH
0 catalyst
Moreover, a stable recyclable catalyst for reaction 1 would be particularly attractive. Consequently, we have tested a variety of redox molecular sieves as catalysts for reaction 1 (see Table 1). Both CrAPO-5 and CrS-1 were active catalysts for CHHP decomposition.
The highest selectivity to cyclohexanone (86%) was observed with CrAPO-5. CrS-1 was even more active but gave a lower cyclohexanone/cyclohexanol ratio. Other metal APOs and silicalites gave both lower activities and selectivities.
410
R. A. Sheldon, J. D. Chen, J. Dakka and E. Neeleman
In one experiment with CrAPO-5 the catalyst was filtered, washed with cyclohexane, recalcined and reused with a fresh solution of CHHP. This was repeated five times without any noticeable loss of activity or selectivity. Recalcination is probably necessary in order to remove the water, formed in the reaction, from the pores of the catalyst. In practice this may be possible using other means, e.g. azeotropic distillation during reaction. Table 1. Catalytic decomposition of cyclohexyl hydroperoxide (CHHP) at 70 "C. Catalyst
Selectivity (%)
CHHP conversion (%)
CrAPO-5 Cr-silicalite VAPO- 11 CO-ZSM-5 VAPO-5 COAPO-5 MWO-5 V-silicalite TS-1 None
87 98
76 24 17 2 2 0
0 0
Cyclohexanone
Cyclohexanol
86 64 50 43 51 50 50 0 0
13 36 50 50 43 50 50
0
0 0 0
Conditions: CHHP (2.9 mmol) dissolved in cyclohexane (10 ml) stirred with the catalyst (0.029 mmol metal) at 70 "C for 5 hours. Similarly, other secondary hydroperoxides, e.g. ethylbenzene hydroperoxide and tetralin hydroperoxide, afforded high yields of the corresponding ketone with CrAPO-5. Tert-alkyl hydroperoxides were decomposed to the corresponding alcohol and dioxygen, together with small amounts of the ketone formed by 8-scission of intermediate alkoxy radicals (Table 2).
Redox Molecular Sieves
41 I
Table 2. CrAPO-5 catalyzed decomposition of alkyl hydroperoxidesa. ~~
R02H Cyclohexyl tert-Butylb Cumene Triphenylmethyl
Solvent C6H12 C,H,CI C,H,CI 1,2-C2H4CI,
~
~~~
~
Conversion (%) 87 49 24 1
Selectivity (%) Ketone
Alcohol
86 5 2
13 93 86
Conditions: R02H (2.9 mmol) in solvent (10 ml) stirred with CrAPO-5 (0.1 g containing 1.5% Cr = 0.029 mmol Cr) for 5 h at 70 "C. 50 "C.
a
Evidence for the reaction taking place inside the cavity of CrAPO-5 was provided by the observation that the bulky triphenylmethyl hydroperoxide, which cannot be accommodated in the cavity, was not decomposed. In contrast, homogeneous chromium(II1) acetylacetonate and the supported Cr02C12/silica-alumina were effective catalysts for the decomposition of this hydroperoxide giving 75% and 72% decomposition in 2 h, respectively, with equivalent amounts of catalyst (1% m) at 70 "C in 1,Zdichloroethane. Walyst structure and catalytic mechanism
In the case of both CrAPO-5 and CrS-1 the as-synthesized catalysts are green and contain chromium in the trivalent state. After calcination at 500 "C the catalysts were yellow and DREAS showed that most of the chromium is present as Cr(VI). ICP-AES analysis showed that chromium contents of up to 1% (CrS-1) to 1.5% (CrAPO-5) could be achieved. We tentatively assume that in the as-synthesized catalysts chromium(II1) is isomorphously substituted, in tetrahedral positions, for silicon (CrS-1) or aluminium (CrAPO-5). Subsequent oxidation during calcination is assumed to afford dioxochromium(V1) which is still attached to the internal framework in either tetrahedral or octahedral coordination. Decomposition of CHHP according to the stoichiometry of reaction 1 is consistent with a heterolytic mechanism. This can be envisaged as proceeding via /3-hydrogen elimination in an alkylperoxochromium(VI) complex (Figure 1). Such a mechanism is not possible with tert-alkyl hydroperoxides and we assume that a homolytic mechanism, involving tert-alkoxy radicals as intermediates, operates in this case. Acid-catalyzed heterolysis can be ruled out because it would lead to the formation of different products,
412
R. A. Sheldon, J . D. Chen, J . Dakka and E. Neeleman
e.g. phenol and acetone from cumene hydroperoxide.
>-,
VI
Cr=O
+ R,C=O + H,O
Fig. 1. Mechanism of decomposition of secondary alkyl hydroperoxide. CrAPO-5 catalyzed oxidation of secondary alcohols Based on the known use of homogeneous chromium catalysts for the oxidation of secondary alcohols with TBHP [7] we envisaged that CrAPO-5 should be an effective solid catalyst for this reaction. The results of CrAPO-5 catalyzed oxidations of secondary alcohols with TBHP at 85 "C are shown in Tdhk 3. Good to excellent selectivities to the corresponding ketones were observed with respect to both substrate and TBHP in most cases. Carveol underwent chemoselective oxidation of its alcohol group to give carvone in
94% selectivity, without any attack at its double bonds. Moreover, one of the (cis/trans) isomers appeared to react much faster indicating that some shape selectivity is observed. l-Phenyl-1,2-ethanediol was selectively oxidized at the secondary alcohol group to give ahydroxy acetophenone (73% selectivity). In one experiment with a-methylbenzyl alcohol the CrAPO-5 catalyst was filtered, washed 3 times with chlorobenzene and recalcined before reuse. The catalyst was recycled 4 times without any noticeable loss of activity or selectivity. DREAS spectra showed that most of the chromium remained in the hexavalent state within the ALPO, framework after recycling. Hence, we conclude that CrAPO-5 is a stable, recyclable catalyst for the selective liquid phase oxidation of secondary alcohols, to the corresponding ketones, using TJ3HP as the terminal oxidant. Interestingly, when the oxidation of a-methylbenzyl alcohol with TBHP was carried out in air instead of N2 a yield of acetophenone on TBHP of 216% was observed, suggesting that 0, could also act as the terminal oxidant. This was confirmed in subsequent experiments (Table 4). The best results were obtained using a small amount
(10 mol %) of TBHP to initiate the reaction.
Redox Molecular Sieves
413
Table 3. CrAPO-5 catalyzed oxidations of secondary alcohols with TBHP at 85 "Ca. Substrate
a-Ethylbenzyl alcohol a-Methylbenzyl alcohol Cyclohexanol Carveol l-Phenyl-1,2ethanediol
Time (h)
Product
Conversion
(%Ib
Selectivity (%) Substrate
TBHP
7
propiophenone
77
100
91
16 12 16
acetophenone cyclohexanone carvone a-hydroxyacetophenone
77 72 62
96 85 94
89 73 66
54
73
40
16
a Conditions: substrate, 10 mrnol; TBHP, 5 mmol; CrAPO-5 (0.14 mmol), chlorobenzene (solvent), 10 rnl; stirred at 85 "C for 16 h under N,. Conversion of substrate based o n the amount of TBHP charged.
We tentatively propose that the oxidation of secondary alcohols with TBHP in the presence of CrAPO-5 proceeds via a heterolytic mechanism involving 8-hydrogen elimination from an oxochrornium(V1) alkoxide followed by reoxidation of the reduced chromium(1V) by TBHP (Figure 2). When 0, is the terminal oxidant the a-hydroxyhydroperoxide, formed by (chrorniurn-catalyzed) autoxidation of the alcohol, can reoxidize the chromium(1V). Table 4. CrAPO-5 catalyzed oxidations of secondary alcohols with OZa. Conversion (%)
Selectivity (%)
cyclohexanone
30
97
acetophenone
31
96
propiophenone a-tetralone l-indanone
38 26 78
90 73 72
Substrate
Product
Cyclohexanol a-Methylbenzyl alcohol a-Ethylbenzyl alcoho I a-Tetralolb l-Indanolb
Conditions: substrate, 250 rnmol; 0, pressure 5 atm or 20 atm air; TBHP 25 mrnol; CrAPO-5 (3.65 mmol Cr); chlorobenzene (solvent), 65 ml; 3 A molecular sieve (drying agent), h g; 110 "C, 5 h. Conditions: substrate, 50 mmol; O,, 15 ml/min; TBHP, 5 mmol; CrAPO-5, 0.73 mmol; chlorobenzene, 5 ml, 110 "C, stirring 1000 rprn, 19 h.
R. A. Sheldon, J . D. Chen, J . Dakka and E. Neeleman
414
c"j-0 o H
@ lv>
CR,
)
TBHP. R,CHOH -H,O.
+ R,CO
&-OH
TBA
I
Fig. 2. Mechanism of alcohol oxidation. CrAPO-5 catalyzed oxidations of hydrocarbons By analogy with the chemistry of soluble chromium catalysts [7] we reasoned that CrAPO-5 should also be an effective catalyst for benzylic oxidations with TBHP. Indeed, CrAPO-5 (1 mol % Cr) catalyzed the selective oxidation of ethylbenzene (reaction 3) and tetralin (reaction 4) with TBHP. 0
PhC1/7O0C/1 6 h
8 5 % selectivity
0
9 0 % selectivity
As was observed in alcohol oxidations (see above) when the oxidation of tetralin was carried out in air the selectivity to a-tetralone based on TBHP was greater than 100%. Subsquent experiments confirmed that CrAPO-5 is an effective catalyst for the autoxidation of benzylic hydrocarbons to the corresponding ketones. A small amount (10 mol %) of TBHP was added to initiate the reaction. For example, tetralin was oxidized with 0, at atmospheric pressure and 100 "C to give a mixture of a-tetralone (64%), a-tetralol (7%) and a-tetralin hydroperoxide (THP; 20%). Presumably, in practice a small
Redox Molecular Sieves
415
amount of the THP-containing product stream could be recycled to the oxidation reactor, thus obviating the need for TBHP as initiator. Recycling experiments showed that the CrAPO-5 could be recycled 5 times without loss of activity (Table 5). Recalcination of the catalyst prior to reuse was not necessary, presumably because the water formed was removed azeotropically during reaction. Table 5. Recycling of CrAPO-5 in the autoxidation of tetralin at 100 'Ca. Cycle nr.
1 2 3 4
Selectivity (%)
Conversion
(%I 44 58
57 53 57
a-tetralone
a-tetra101
64 65 60 61 65
7 6 5 5 6
THP~ 20 24 30 32 24
a Conditions: tetralin, 50 mmol; O,, 15 ml/min; TBHP, 5 mmol; CrAPO-5, 0.73 mmol Cr; 100 "C, stirring, 1000 rpm, 10 h. THP = cr-tetralin hydroperoxide. The CrAPO-5 was regenerated by calcination at 500 "C for 5 h.
CrAPO-5 was also found to catalyze the autoxidation of cyclohexane at 115 "C. At 3% cyclohexane conversion the major product was cyclohexanone (64%) together with cyclohexanol (10%) and CHHP (9%) and dicarboxylic acids (13%). In practice part of the CHHP-containing product stream could be recycled to the oxidation reactor to act as an initiator. 0
64%
OH
10%
OOH
9%
3% cyclohexane conversion
416
R. A. Sheldon, 1. D . Chen, J. Dakka and E. Neeleman
CONCLUDING REMARKS CrAPO-5, containing chromium(V1) in the A1P04-5 framework, is an active, recyclable catalyst for alkyl hydroperoxide decomposition and the selective liquid phase oxidation of secondary alcohols, alkylaromatics and cycloalkanes with TBHP or 0,as the terminal oxidant. The scope and mechanism of these and related liquid phase oxidations mediated by redox molecular sieves are under further investigation. REFERENCES 1 R.A. Sheldon and J.K. Kochi, ‘Metal-Catalyzed Oxidations of Organic Compounds’, Academic Press, New York, 1981. 2 R.A. Sheldon, CHEMTECH, (1991) 566. 3 R.A. Sheldon, Topics Curr. Chem., 164 (1993) 21. 4 J.D. Chen, J. Dakka, E. Neeleman and R.A. Sheldon, J. Chem. SOC.,Chem. Commun., in press. 5 U. Romano, A. Esposito, F. Maspero, C. Neri and M. Clerici, Chim. Ind. (Milan), 72 (1990) 610. 6 G. Cainelli and G. Cardillo, ‘Chromium Oxidations in Organic Chemistry’, SpringerVerlag, Berlin, 1984. 7 J. Muzart, Chem. Rev., 92 (1992) 113. 8 E.M. Flanigen, B.M.T. Lok, R.L. Patton and S.T. Wilson, US Patent, 4,759,919 (1988) to Union Carbide Corp. 9 M. Kawai and T. Kyoura, Japanese Patents, JP 0358,954 and JP 0356,439 (1991) to Mitsui Toatsu Chemicals; CA 115 (1991) 48863d and 48864e.