- Email: [email protected]
Direct catalytic hydroxylation of benzene with hydrogen peroxide over titanium—silicate zeolites
Applied Catalysis, 57 (1990) Ll-L3 Elsevier Science Publishers B.V., Amsterdam -
Ll Printed in The Netherlands
Direct Catalytic Hydroxylation of Benzene with Hydrogen Peroxide over Titanium-Silicate Zeolites A. THANGARAJ, National
R. KUMAR and P. RATNASAMY*
Chemical Laboratory,
Pune-411
008 (India)
(Received 20 October 1989, revised manuscript received 8 November 1989)
INTRODUCTION
The hydroxylation of benzene by hydrogen peroxide to phenol in the presence of oxidizable metal ions (such as ferrous, cuprous, titanous, etc. ) [ 11, and super acids [2] is well known. A direct catalytic method using heterogeneous, solid catalysts which can efficiently hydroxylate benzene, will have significant advantages. We have found that crystalline, titanium-silicate zeolites with the pentasil structure catalyse the hydroxylation of benzene with hydrogen peroxide to form phenol and other, disubstituted, products in good yields and with high selectivities. In the present communication, we report our preliminary findings on the catalytic efficiency and selectivity of various zeolites including titanium-silicate (TS-1 ), ferri-titanium-silicate (Fe-TS-1 ), alumino-titanium-silicate (Al-TS-1 ), ferri-silicate (Fe-ZSM-5) and alumino-silicate (ZSM5) in this novel catalytic process. EXPERIMENTAL
All zeolites used here were synthesized according to published procedures zeolites such as TS-1, Al-TS-1 and Fe-TS-1 the published synthesis procedure [ 51 was modified: In a typicalpreparation of TS-1 zeolite, 20 g tetrapropyl ammonium hydroxide (TPAOH) (20% aq., Aldrich) was slowly added under stirring to a mixture comprising of 45.5 g tetraethyl orthosilicate, 20 g isopropyl alcohol and 20 g distilled water. To the resultant mixture a solution of 2.24 g titanium tetra-butoxide (Ti(O-BU),) in 10 g dry isopropyl alcohol (in order to avoid instantaneous hydrolysis of Ti (0-Bu), prior to its addition to the silicate solution) was added very slowly under vigorous stirring. This clear liquid mixture was stirred at 320-330 K for about one hour before adding to it a solution of 60 g TPAOH (20 wt.-% aq. ) in [ 3,4]. In the case of titanium-containing
L2
80 g distilled water. This final clear liquid reaction mixture was stirred at 353 K for 6 h before transferring it into a SS autoclave. The crystallization was carried out for 5-6 days at 443 K. Similar procedures were adopted for preparing the aluminium and iron analogs of TS-1 (Al-TS-1 and Fe-TS-1, respectively). The structure and morphology of the zeolites were characterized by methods published earlier [ 3,6]. The presence of Fe3+ in the lattice framework positions in Fe-ZSM-5 and Fe-TS-1 was confirmed by ESR, Miissbauer spectroscopy and magnetic susceptibility methods. All zeolites were freed of sodium ions by ion exchange prior to use. The catalytic runs were carried out in batch reactors (100 ml capacity) at 300 K. The benzene (Merck, 99.9% GC pure) to-hydrogen peroxide (27% aq. sol.) molar ratio was 10. Products were analysed using capillary GC (H.P. 8090 B ) and occasionally checked by HPLC (Waters Associates). Good agreement was found in all cases. RESULTS AND DISCUSSION
Table 1 compares the catalytic activity and selectivity of various zeolites. Benzene remained unreacted over silicalite-1 (Si/Al> 3000 ), TiOp (both amorphous and crystalline), and also in the absence of catalysts. The selectivity for the conversion of hydrogen peroxide to hydroxy benzenes decreased in the order TS-1 > Fe-TS-1 > Al-TS-1 > Fe-ZSM-5 > Al-ZSM-5. The selectivity to phenol (amongst the hydroxy benzenes) however, follows the reverse order (Table 1, column 6). The decomposition of hydrogen peroxide was, however, almost complete in all cases except TiO, and silicalite-1. Initial kinetic experTABLE 1 Hydroxylation of benzene with hydrogen peroxide over various zeolite catalysts Reaction conditions: temperature = 300 K, benzene-to-hydrogen peroxide mole ratio = 10, reaction time = 6 h, catalyst wt. = 1.5 g; TS- 1= titanium silicate having silicalite- 1 structure; Ph = phenol and PBQ = P-benzoquinone. Catalyst
TS-1 Fe-TS-1 AI-TS-1 Fe-ZSM-5 Al-ZSM-5 Silicalite-1 TiO, No catalyst
Zeolite composition, mole fraction Si/Ti
Si/Al
Si/Fe
23 26 24 -
86 84 > 3000 -
87 _ 83 -
H,O, Selectivity” (%x)
Product, mol-% Phenol
p-Benzoquinone
100 50 37 30 27
76 91 96 100 100
24 9 4 _
no reaction no reaction no reaction
“Selectivity to hydroxy benzenes. The balance was decomposed to HzO+ l/2 0,. 0.012 mol H,O, per gram of zeolite.
L3
iments on the influence of residence time and temperature on product distribution indicated that phenol’is the primary product, and para-benzoquinone was formed by the further hydroxylation of phenol. A free radical mechanism involving hydroxycyclohexadienyl radical intermediates is generally accepted [l] for metal ion catalysed hydroxylation of benzene with hydrogen peroxide. An ionic mechanism was suggested by Olah and Onishi [2] in the case of superacid catalysed hydroxylation of benzene with hydrogen peroxide. The hydroxylation of phenol to form catechol and hydroquinone over TS-1 has been postulated [6] to occur via formation of titanium-peroxo species which facilitate the direct insertion of oxygen into the aromatic ring. This last mechanism is likely in the present case as well, where phenol is formed by oxygen insertion into the benzene ring. An interesting feature of Table 1 is that while phenol is the only product over pure acid zeolites (Al-ZSM-5 and Fe-ZSM-5)) P-benzoquinone, the secondary product is formed in appreciable quantities over titanium-containing zeolites (TS-1, Fe-TS-1, Al-TS-1). The selectivity for this p-benzoquinone decreases in the order: TS-1 > Fe-TS-l> Al-TS-1. The acid strength of these zeolites follows the reverse trend i.e., Al-TS-1 > Fe-TS-l> TS-1 [ 71. Protonation of phenol over the acid zeolites probably suppresses the further electrophilic reaction [ 21 leading to dihydroxy benzenes. ACKNOWLEDGEMENTS
This work was partly funded by UNDP. A.T. thanks CSIR for a research fellowship.
REFERENCES G. Sosnovsky Interscience,
and D.J. Rawlinson,
in D. Swern
(Editor)
Organic
peroxides,
Vol. II, Wiley
New York, 1971, p. 277.
G.A. Olah and R. Onishi, J. Org. Chem., 43 (1978) 865. S.B. Kulkarni, V.P. Shiralkar, A.N. Kotasthane, R.B. Borade and P. Ratnasamy, (1982) 313. P. Ratnasamy,
R.B. Borade,
S. Shivashankar,
V.P. Shiralkar
Zeolites,
and S.G. Hedge, Proc. Intern.
Symp. Zeolite Catalysis, Siofok, Hungary, 1985, Acta Phys. Chem., Szeged, 1985, p. 137. M. Taramasso, G. Perego and B. Notari, U.K. Patent GB 2 071 071B (1983), assigned Snamprogetti, Italy. B. Notari, in P.J. Grobet,
W.J. Mortier,
vation in Zeolite Material Science, Amsterdam, 1989, p. 413. A. Thangaraj
E.F. Vansant
and G. Schulz-Ekloff
Studies in Surface Science and Catalysis,
and R. Kumar, unpublished
results.
2
(Editors)
to
Inno-
Vol. 37 Elsevier,
Ll Printed in The Netherlands
Direct Catalytic Hydroxylation of Benzene with Hydrogen Peroxide over Titanium-Silicate Zeolites A. THANGARAJ, National
R. KUMAR and P. RATNASAMY*
Chemical Laboratory,
Pune-411
008 (India)
(Received 20 October 1989, revised manuscript received 8 November 1989)
INTRODUCTION
The hydroxylation of benzene by hydrogen peroxide to phenol in the presence of oxidizable metal ions (such as ferrous, cuprous, titanous, etc. ) [ 11, and super acids [2] is well known. A direct catalytic method using heterogeneous, solid catalysts which can efficiently hydroxylate benzene, will have significant advantages. We have found that crystalline, titanium-silicate zeolites with the pentasil structure catalyse the hydroxylation of benzene with hydrogen peroxide to form phenol and other, disubstituted, products in good yields and with high selectivities. In the present communication, we report our preliminary findings on the catalytic efficiency and selectivity of various zeolites including titanium-silicate (TS-1 ), ferri-titanium-silicate (Fe-TS-1 ), alumino-titanium-silicate (Al-TS-1 ), ferri-silicate (Fe-ZSM-5) and alumino-silicate (ZSM5) in this novel catalytic process. EXPERIMENTAL
All zeolites used here were synthesized according to published procedures zeolites such as TS-1, Al-TS-1 and Fe-TS-1 the published synthesis procedure [ 51 was modified: In a typicalpreparation of TS-1 zeolite, 20 g tetrapropyl ammonium hydroxide (TPAOH) (20% aq., Aldrich) was slowly added under stirring to a mixture comprising of 45.5 g tetraethyl orthosilicate, 20 g isopropyl alcohol and 20 g distilled water. To the resultant mixture a solution of 2.24 g titanium tetra-butoxide (Ti(O-BU),) in 10 g dry isopropyl alcohol (in order to avoid instantaneous hydrolysis of Ti (0-Bu), prior to its addition to the silicate solution) was added very slowly under vigorous stirring. This clear liquid mixture was stirred at 320-330 K for about one hour before adding to it a solution of 60 g TPAOH (20 wt.-% aq. ) in [ 3,4]. In the case of titanium-containing
L2
80 g distilled water. This final clear liquid reaction mixture was stirred at 353 K for 6 h before transferring it into a SS autoclave. The crystallization was carried out for 5-6 days at 443 K. Similar procedures were adopted for preparing the aluminium and iron analogs of TS-1 (Al-TS-1 and Fe-TS-1, respectively). The structure and morphology of the zeolites were characterized by methods published earlier [ 3,6]. The presence of Fe3+ in the lattice framework positions in Fe-ZSM-5 and Fe-TS-1 was confirmed by ESR, Miissbauer spectroscopy and magnetic susceptibility methods. All zeolites were freed of sodium ions by ion exchange prior to use. The catalytic runs were carried out in batch reactors (100 ml capacity) at 300 K. The benzene (Merck, 99.9% GC pure) to-hydrogen peroxide (27% aq. sol.) molar ratio was 10. Products were analysed using capillary GC (H.P. 8090 B ) and occasionally checked by HPLC (Waters Associates). Good agreement was found in all cases. RESULTS AND DISCUSSION
Table 1 compares the catalytic activity and selectivity of various zeolites. Benzene remained unreacted over silicalite-1 (Si/Al> 3000 ), TiOp (both amorphous and crystalline), and also in the absence of catalysts. The selectivity for the conversion of hydrogen peroxide to hydroxy benzenes decreased in the order TS-1 > Fe-TS-1 > Al-TS-1 > Fe-ZSM-5 > Al-ZSM-5. The selectivity to phenol (amongst the hydroxy benzenes) however, follows the reverse order (Table 1, column 6). The decomposition of hydrogen peroxide was, however, almost complete in all cases except TiO, and silicalite-1. Initial kinetic experTABLE 1 Hydroxylation of benzene with hydrogen peroxide over various zeolite catalysts Reaction conditions: temperature = 300 K, benzene-to-hydrogen peroxide mole ratio = 10, reaction time = 6 h, catalyst wt. = 1.5 g; TS- 1= titanium silicate having silicalite- 1 structure; Ph = phenol and PBQ = P-benzoquinone. Catalyst
TS-1 Fe-TS-1 AI-TS-1 Fe-ZSM-5 Al-ZSM-5 Silicalite-1 TiO, No catalyst
Zeolite composition, mole fraction Si/Ti
Si/Al
Si/Fe
23 26 24 -
86 84 > 3000 -
87 _ 83 -
H,O, Selectivity” (%x)
Product, mol-% Phenol
p-Benzoquinone
100 50 37 30 27
76 91 96 100 100
24 9 4 _
no reaction no reaction no reaction
“Selectivity to hydroxy benzenes. The balance was decomposed to HzO+ l/2 0,. 0.012 mol H,O, per gram of zeolite.
L3
iments on the influence of residence time and temperature on product distribution indicated that phenol’is the primary product, and para-benzoquinone was formed by the further hydroxylation of phenol. A free radical mechanism involving hydroxycyclohexadienyl radical intermediates is generally accepted [l] for metal ion catalysed hydroxylation of benzene with hydrogen peroxide. An ionic mechanism was suggested by Olah and Onishi [2] in the case of superacid catalysed hydroxylation of benzene with hydrogen peroxide. The hydroxylation of phenol to form catechol and hydroquinone over TS-1 has been postulated [6] to occur via formation of titanium-peroxo species which facilitate the direct insertion of oxygen into the aromatic ring. This last mechanism is likely in the present case as well, where phenol is formed by oxygen insertion into the benzene ring. An interesting feature of Table 1 is that while phenol is the only product over pure acid zeolites (Al-ZSM-5 and Fe-ZSM-5)) P-benzoquinone, the secondary product is formed in appreciable quantities over titanium-containing zeolites (TS-1, Fe-TS-1, Al-TS-1). The selectivity for this p-benzoquinone decreases in the order: TS-1 > Fe-TS-l> Al-TS-1. The acid strength of these zeolites follows the reverse trend i.e., Al-TS-1 > Fe-TS-l> TS-1 [ 71. Protonation of phenol over the acid zeolites probably suppresses the further electrophilic reaction [ 21 leading to dihydroxy benzenes. ACKNOWLEDGEMENTS
This work was partly funded by UNDP. A.T. thanks CSIR for a research fellowship.
REFERENCES G. Sosnovsky Interscience,
and D.J. Rawlinson,
in D. Swern
(Editor)
Organic
peroxides,
Vol. II, Wiley
New York, 1971, p. 277.
G.A. Olah and R. Onishi, J. Org. Chem., 43 (1978) 865. S.B. Kulkarni, V.P. Shiralkar, A.N. Kotasthane, R.B. Borade and P. Ratnasamy, (1982) 313. P. Ratnasamy,
R.B. Borade,
S. Shivashankar,
V.P. Shiralkar
Zeolites,
and S.G. Hedge, Proc. Intern.
Symp. Zeolite Catalysis, Siofok, Hungary, 1985, Acta Phys. Chem., Szeged, 1985, p. 137. M. Taramasso, G. Perego and B. Notari, U.K. Patent GB 2 071 071B (1983), assigned Snamprogetti, Italy. B. Notari, in P.J. Grobet,
W.J. Mortier,
vation in Zeolite Material Science, Amsterdam, 1989, p. 413. A. Thangaraj
E.F. Vansant
and G. Schulz-Ekloff
Studies in Surface Science and Catalysis,
and R. Kumar, unpublished
results.
2
(Editors)
to
Inno-
Vol. 37 Elsevier,