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Formation of isomeric terphenyls and triphenylene by pyrolysis of benzene
JoumaI of Ana@icat and Applied firofysi$ 5 (1983) 237-243 Ekevia Science Publishers B-V, Amsterdam - Printed in The Ncthcriands
237
FORMATION OF ISOMERIC TERPHENYLS AND TRIPHENYLENE BY PYROLYSIS OF BENZENE
C. JACOBELLI Isrituto di Tenoiogie Biomediche a21CNR, cia Morgagni 3a Rome fIta&)
G. PEREZ
l
Isrituto di Chimica Nuckare de1 CNR Stazione (Rome) (Ita&}
Ama delia Ricerca di Roma, c-p. JO. OtW16Monrerxwndo
C. POLCARO Isrihdo a? CromcItografiadef CNR. Ana della Ricerca di Roma c-p. Ia OtWZ6 Monrcrorondo Stazione (Rome) (Ita&) E POSSAGNO.
R BASSANELLI
and E LILLA
Issirurodi Chimica .VucIearedel CNR, Area della Ricewwxz di Romp, c-p. ia OtWI6 Momerottxndo Srazione (Rome) (Ira&)
(Receixd February 2&d. 1983: accepted March 30th. 1983)
SUM_MARY The pyrolysis of benzene has been inwstigatcd at 773. 923. and 1073 K and the mechanisms of formation of isomeric terphenyls and triphenylene have been studied. the results being compared with those obtained by pyrolysis of benzene soIutions containing diphenyl and u-terphenyl No differences in the mechanisms of the formation of ttrphenyls and triphenykne could be observed at the selected temperatures. an if ring ftion and a higher degree of polymerization occwred at 1073 K. The addition of diphenyl and c-terphenyl shomwl that the reactions leading to the formation of isomeric terphenyls and triphenylenc in the pyrol>Gs of pure benzene do not pass through stable intermediates.
INTRODUCTION
Although the pyrolysis of benzene is an i.nx~s~~~~t industrial process, As benzene is the there is still much uncertain ty about its mechanism. simplest aromatichydrocarbonand could provide the point of departurefor
a systematicunderstandingof aromatic thermaldecomposition,many stud0165-2370/83/SO3.00
.L11983 Else&r !kiencc Publishers B.V.
235
ies have been carried out using different techniques such as shock tubes [I-3] and flow systems [4.5]. It was observed that benzene undergoes ring fission at temperatures above 1020 K [6]. In the range 873- 1020 K carbon-hydrogen bonds are readily ruptured to give hydrogen atoms and phenyl radicaIs. which can react with further benzene molecules or with other phenyl radicals as reported by Badger in his detailed review 171. Equivalent amounts of hydrogen and diphenyl were detected among the products of benzene pyrolysis at 903 K by Pease and Morton [8]. It was reported that the reaction was strongly influenced by catalysts [9] and oxygen [IO] and partiaII_v inhibited by nitric oxide and propylene [ 1 I]. Although many studies on the py-rolysis of benzene had been carried out and the kinetics of benzene disappearance and diphenyl formation had been determined. Iittle attention had been devoted to inwstigatc the mechanism of formation of poIyphenyIs_ As trace amounts of isomeric terphenyls and triphcnylenc were detected among the pyrolysis products in the range 873- 1020 K [ 1 I]. a mechanism involving the phenylation of diphenyl leading to the formation of the three isomeric terphenyls. and the subsequent cycIodehydrogenation of o-terphenyl to triphenylene. was proposed. Such a mechanism seems to be supported by the observation that u-terphenyl undergoes ready cam-ersion to triphenylene at 763 K [ 121. To eIucidate the mechanism of formation of isomeric terphenyls and triphenylenc we compared the results of the pyrolysis of pure benzene with the yields of the products obtaind in the pyroI_vsis of benzene solutions of diphcnyl and o-terphenyl at very- low corn--ersions.
The probe of a Pyroprobe 100 Solid Pyrolyzer (Chemical Data Systems. Oxford. PA. U.S.A.) equipped with a platinum wire was introduced into a 6-c& two-necked cylindrical flask (I.D. 1 cm) containing 5 cn? of liquid s?mpIe. The platinum w:re was then heated to the selected temperature for 5 s at a non-linear heating rate of at least 75OC/ms. After benzene and other non-gaseous products had been allowed to reflux. the operation was repezted until the selected pyrolysis time was reached. Deaeration of the samples was ensured by bubbiing argon into the bottom of the flask from a side capiilary tube. The pyroiysed samples were analysed by gas chromatography on a 3-m silicon oil column at 220°C with a nitrogen flow-rate of 6.4 mI/min. Using these conditions. benzene. diphenyl. isomeric terphenyls and triphenylene were resolved and their concentrations were determined from the areas of the eluted peaks using appropriate calibration factors. The presence of these compounds in the pyrolysed samples was confirmed
239
by high-performance liquid chromatographic (HPLC) anaIysis and from fIuorescence spectra obtained directly from chromatographic peaks on-line scanning_
RESL’LTS
The yields of diphenyl, isomeric terphenyls and triphenylene from the pyrolysis at 923 and 1073 K are reported in Table 1. The results are the means of five independent runs and their consistency. measured by the standard deviation. is about 10% . During the pyrolysis at IO73 K the solution becomes brown. as a consequence of carbon formation due to the ring fission. The difference in the spectra of the pyrolysis products is shown in Fig. 1 where the eIution curves obtained by HPLC with a fluorescence detector of pure benzene pyrolysed at 773 and 1073 K are compared. The addition of different amounts of diphenyl to the benzene before its pyro!ysis did not affect the yields of the other products. within experimental error. when the diphenyl concentration was 2.152 mmol/dm’ (and lower). At a diphenyl concentration of 6.70 mmol/dm3 it was observed that the yields of the isomeric terphenyis and triphenylene are about twice those obtained by pyrolysis of pure benzene. AIso. when o-terphenyl is added to benzene its effect can be observed only at a relatively high concentration (13.45 mmol/dm3).
DISCUSSIOS
The formation of terphenyk has been explained by either a mechanism involving the dissociation of diphenyl: C,vHIO + C,,H,
+ H-
(1)
and the subsequent reaction of the diphenyl radical with a benzene molecule: C,,H;
+ C,H,
-
C,sH,,
+ H-
(2)
or the reaction of a phenyl radical. formed in the pyrolysis of benzene. with a diphenyl molecule: C,H;
i C,,H,,
+ &HI4
or the polymerization
C,H;=%,,H;, The
C+H,
relative yield
+
+ H’
(3) .
reaction
C,,H;,
etc.
of diphenyl.
(4 which in the pyrolysis
of pure benzene
240
I'-.--
I" i
I_
_.
.,. •
.,
.,
+
.,. ~
~
~
"
o
~.a.. !'1
I --
~
I"I
~
",'/
~
"a:
',,,6
7
...r
"F.
"F.
l'ql
~,
I +
i
i+ I I
et~
P+
!
~!__. '
~i-~
+
~
~
~
.,;,.m
~
¢.~
I
I
m~ +' "
+
,~
m+
g~
l
,',,,
,.+,+
m
~
¢q
m+
,~
241
A
lime. mfn
Fig. 1. High-performance liquid chroma:ography of pyrolysed benzene at (A) 1073 K and (B) 773 K. diiuted I:2 with acetonitrile (CX,CN). detected by fluorescence. Wavelength regulation corresponds to excitation uxvekngth = 280 nm. emission wawkngth = 340 nm for 33 min. then. corresponding uith the change of eluent to tetrahydrofuran (THF). excitation u-avekngth = 350 nm. emision u-avekngth = 430 nm. Pea’&: J = diphenyl: 2 = o-terphcnyl: 3 = m-terphcnyl: 4 - p-terphenyl_ Triphenylene. which is not detectable at this wavelength. is eluted immediately after p_terphenyL
exceeds the yieIds of the terphenyls about IOOO-fold, does not seem to conflict with the mechanisms proposed in reactions Z3 and 4; however, the results obtained in the presence of diphenyl, added at a relatively high concentration with reqect to its formation in the pyrolysis of pure benzene, rule out reactions 2 and 3 as principal paths leading to the formation of
34’ _-
terphenyls. Therefore. it seems that only the polymerization reaction 4 plays an important role in the production of terphenyis. The cyclodehydrogenation of o-terphenyl was suggested to expIain the formation of criphenylene:
(5)
Such a hypothesis does not seem to be in agreement uith the results of the pyrolysis of pure benzene and diphenyl solutions. as the yield of u-terphenyl is as high as the yield of triphenylene. Further. the fact that the addition of different amounts of eterphenyl to benzene hardly influences the yield of triphcnykne rules out reaction 5 as the principal pathway Ieading to the formation of triphenylene. Therefore. as for the formation of terphenyls. the polymerization reaction 4 seems the most appropriate mechanism to explain the results reported in Tabie 1, On& at relatively high solute concentrations can reactions 2. 3 and 5 not be neglected. but they cannot account for the formation of the products of the pyrolysis of benzene. In the high-temperature pyrolysis run higher yields of diphenyl. terphenyls and triphenylene are obtained. but it does not seem that terphenyis and triphenylene are formed by a mechanism other than that proposed in the pyroly-sis carried out at 923 K. In fact. the ratios between the yields of the products identified in this work at 933 and 1023 K. are the same_ within cxperimen ta1 error.
The pyrolysis of benzene solutions of diphenyl and u-terphenyl. which have been supposed to be intermediates of terphenyls and triphenylene. respectively. shows that in the pyrolysis of benzene such polymeric compounds are produced by chain reactions involving the formerly formed phenyl radical. while the reactions of the phenyl radicals with diphenyl molecules and cyclodehydrogenation of eterphenyl play a significant role onIy in the presence of large excesses of diphenyl and u-terphenyl. respectively. At temperatures above IO25 K. it seems that the proposed mechanism for the formation of terphenyls and triphenylene is the same. even if ring fission and formation of compounds uith a higher degree of polymerization are observed_
243
REFERENCES 1 2 3 4 5 6 7 8 9 10 1I
C-F. Aten and S.H. Bauer. J. Chem. Phys., 39 (1963) 1253. IL Mafpsin and ZA. Nubuto\sky. Kinetics and Catalysis. 10 (1969) 983. W.B. Ftichardson. S_H. Lin and DL Evans. J. Chem. Sot. Faraday Trans. 2 78 (1982) 1. R.L. Slj-sh and CR Kinney, J. Phys Chem, 65 (1961) 1044. KC_ Hou and W.B. Palmer. J. Phys. Chcm, 69 (1965) 863. K. BoIton. J.E. Cullingworth. B-P. Gosh and J-W. Cobb. J. Chem. Sw.. ( 1942) 252 G-M. Badger. Progr. Phys. Or& Chem.. 3 (1965) 1. R.N. Pease and J-M. Morton. J. Amer. Chem. Sot, 55 (1933) 3190. F.C. Mead and RE Burk. Ind. Eng. Chem, 27 (1935) 229. DA. Leathard and J.H. Pumell. Ann. Rev_ Php. Chem. ( 1970) 197. CT_ Brook SJ. Peacock and B-G. Reuben J. Chem. Sot. Faraday Trans_ 1. 75 ( 1979) 652. 12 P-G. Copeland. RE Dean and D. &McNeil.J. Chem. Socv (1960) 266.
237
FORMATION OF ISOMERIC TERPHENYLS AND TRIPHENYLENE BY PYROLYSIS OF BENZENE
C. JACOBELLI Isrituto di Tenoiogie Biomediche a21CNR, cia Morgagni 3a Rome fIta&)
G. PEREZ
l
Isrituto di Chimica Nuckare de1 CNR Stazione (Rome) (Ita&}
Ama delia Ricerca di Roma, c-p. JO. OtW16Monrerxwndo
C. POLCARO Isrihdo a? CromcItografiadef CNR. Ana della Ricerca di Roma c-p. Ia OtWZ6 Monrcrorondo Stazione (Rome) (Ita&) E POSSAGNO.
R BASSANELLI
and E LILLA
Issirurodi Chimica .VucIearedel CNR, Area della Ricewwxz di Romp, c-p. ia OtWI6 Momerottxndo Srazione (Rome) (Ira&)
(Receixd February 2&d. 1983: accepted March 30th. 1983)
SUM_MARY The pyrolysis of benzene has been inwstigatcd at 773. 923. and 1073 K and the mechanisms of formation of isomeric terphenyls and triphenylene have been studied. the results being compared with those obtained by pyrolysis of benzene soIutions containing diphenyl and u-terphenyl No differences in the mechanisms of the formation of ttrphenyls and triphenykne could be observed at the selected temperatures. an if ring ftion and a higher degree of polymerization occwred at 1073 K. The addition of diphenyl and c-terphenyl shomwl that the reactions leading to the formation of isomeric terphenyls and triphenylenc in the pyrol>Gs of pure benzene do not pass through stable intermediates.
INTRODUCTION
Although the pyrolysis of benzene is an i.nx~s~~~~t industrial process, As benzene is the there is still much uncertain ty about its mechanism. simplest aromatichydrocarbonand could provide the point of departurefor
a systematicunderstandingof aromatic thermaldecomposition,many stud0165-2370/83/SO3.00
.L11983 Else&r !kiencc Publishers B.V.
235
ies have been carried out using different techniques such as shock tubes [I-3] and flow systems [4.5]. It was observed that benzene undergoes ring fission at temperatures above 1020 K [6]. In the range 873- 1020 K carbon-hydrogen bonds are readily ruptured to give hydrogen atoms and phenyl radicaIs. which can react with further benzene molecules or with other phenyl radicals as reported by Badger in his detailed review 171. Equivalent amounts of hydrogen and diphenyl were detected among the products of benzene pyrolysis at 903 K by Pease and Morton [8]. It was reported that the reaction was strongly influenced by catalysts [9] and oxygen [IO] and partiaII_v inhibited by nitric oxide and propylene [ 1 I]. Although many studies on the py-rolysis of benzene had been carried out and the kinetics of benzene disappearance and diphenyl formation had been determined. Iittle attention had been devoted to inwstigatc the mechanism of formation of poIyphenyIs_ As trace amounts of isomeric terphenyls and triphcnylenc were detected among the pyrolysis products in the range 873- 1020 K [ 1 I]. a mechanism involving the phenylation of diphenyl leading to the formation of the three isomeric terphenyls. and the subsequent cycIodehydrogenation of o-terphenyl to triphenylene. was proposed. Such a mechanism seems to be supported by the observation that u-terphenyl undergoes ready cam-ersion to triphenylene at 763 K [ 121. To eIucidate the mechanism of formation of isomeric terphenyls and triphenylenc we compared the results of the pyrolysis of pure benzene with the yields of the products obtaind in the pyroI_vsis of benzene solutions of diphcnyl and o-terphenyl at very- low corn--ersions.
The probe of a Pyroprobe 100 Solid Pyrolyzer (Chemical Data Systems. Oxford. PA. U.S.A.) equipped with a platinum wire was introduced into a 6-c& two-necked cylindrical flask (I.D. 1 cm) containing 5 cn? of liquid s?mpIe. The platinum w:re was then heated to the selected temperature for 5 s at a non-linear heating rate of at least 75OC/ms. After benzene and other non-gaseous products had been allowed to reflux. the operation was repezted until the selected pyrolysis time was reached. Deaeration of the samples was ensured by bubbiing argon into the bottom of the flask from a side capiilary tube. The pyroiysed samples were analysed by gas chromatography on a 3-m silicon oil column at 220°C with a nitrogen flow-rate of 6.4 mI/min. Using these conditions. benzene. diphenyl. isomeric terphenyls and triphenylene were resolved and their concentrations were determined from the areas of the eluted peaks using appropriate calibration factors. The presence of these compounds in the pyrolysed samples was confirmed
239
by high-performance liquid chromatographic (HPLC) anaIysis and from fIuorescence spectra obtained directly from chromatographic peaks on-line scanning_
RESL’LTS
The yields of diphenyl, isomeric terphenyls and triphenylene from the pyrolysis at 923 and 1073 K are reported in Table 1. The results are the means of five independent runs and their consistency. measured by the standard deviation. is about 10% . During the pyrolysis at IO73 K the solution becomes brown. as a consequence of carbon formation due to the ring fission. The difference in the spectra of the pyrolysis products is shown in Fig. 1 where the eIution curves obtained by HPLC with a fluorescence detector of pure benzene pyrolysed at 773 and 1073 K are compared. The addition of different amounts of diphenyl to the benzene before its pyro!ysis did not affect the yields of the other products. within experimental error. when the diphenyl concentration was 2.152 mmol/dm’ (and lower). At a diphenyl concentration of 6.70 mmol/dm3 it was observed that the yields of the isomeric terphenyis and triphenylene are about twice those obtained by pyrolysis of pure benzene. AIso. when o-terphenyl is added to benzene its effect can be observed only at a relatively high concentration (13.45 mmol/dm3).
DISCUSSIOS
The formation of terphenyk has been explained by either a mechanism involving the dissociation of diphenyl: C,vHIO + C,,H,
+ H-
(1)
and the subsequent reaction of the diphenyl radical with a benzene molecule: C,,H;
+ C,H,
-
C,sH,,
+ H-
(2)
or the reaction of a phenyl radical. formed in the pyrolysis of benzene. with a diphenyl molecule: C,H;
i C,,H,,
+ &HI4
or the polymerization
C,H;=%,,H;, The
C+H,
relative yield
+
+ H’
(3) .
reaction
C,,H;,
etc.
of diphenyl.
(4 which in the pyrolysis
of pure benzene
240
I'-.--
I" i
I_
_.
.,. •
.,
.,
+
.,. ~
~
~
"
o
~.a.. !'1
I --
~
I"I
~
",'/
~
"a:
',,,6
7
...r
"F.
"F.
l'ql
~,
I +
i
i+ I I
et~
P+
!
~!__. '
~i-~
+
~
~
~
.,;,.m
~
¢.~
I
I
m~ +' "
+
,~
m+
g~
l
,',,,
,.+,+
m
~
¢q
m+
,~
241
A
lime. mfn
Fig. 1. High-performance liquid chroma:ography of pyrolysed benzene at (A) 1073 K and (B) 773 K. diiuted I:2 with acetonitrile (CX,CN). detected by fluorescence. Wavelength regulation corresponds to excitation uxvekngth = 280 nm. emission wawkngth = 340 nm for 33 min. then. corresponding uith the change of eluent to tetrahydrofuran (THF). excitation u-avekngth = 350 nm. emision u-avekngth = 430 nm. Pea’&: J = diphenyl: 2 = o-terphcnyl: 3 = m-terphcnyl: 4 - p-terphenyl_ Triphenylene. which is not detectable at this wavelength. is eluted immediately after p_terphenyL
exceeds the yieIds of the terphenyls about IOOO-fold, does not seem to conflict with the mechanisms proposed in reactions Z3 and 4; however, the results obtained in the presence of diphenyl, added at a relatively high concentration with reqect to its formation in the pyrolysis of pure benzene, rule out reactions 2 and 3 as principal paths leading to the formation of
34’ _-
terphenyls. Therefore. it seems that only the polymerization reaction 4 plays an important role in the production of terphenyis. The cyclodehydrogenation of o-terphenyl was suggested to expIain the formation of criphenylene:
(5)
Such a hypothesis does not seem to be in agreement uith the results of the pyrolysis of pure benzene and diphenyl solutions. as the yield of u-terphenyl is as high as the yield of triphenylene. Further. the fact that the addition of different amounts of eterphenyl to benzene hardly influences the yield of triphcnykne rules out reaction 5 as the principal pathway Ieading to the formation of triphenylene. Therefore. as for the formation of terphenyls. the polymerization reaction 4 seems the most appropriate mechanism to explain the results reported in Tabie 1, On& at relatively high solute concentrations can reactions 2. 3 and 5 not be neglected. but they cannot account for the formation of the products of the pyrolysis of benzene. In the high-temperature pyrolysis run higher yields of diphenyl. terphenyls and triphenylene are obtained. but it does not seem that terphenyis and triphenylene are formed by a mechanism other than that proposed in the pyroly-sis carried out at 923 K. In fact. the ratios between the yields of the products identified in this work at 933 and 1023 K. are the same_ within cxperimen ta1 error.
The pyrolysis of benzene solutions of diphenyl and u-terphenyl. which have been supposed to be intermediates of terphenyls and triphenylene. respectively. shows that in the pyrolysis of benzene such polymeric compounds are produced by chain reactions involving the formerly formed phenyl radical. while the reactions of the phenyl radicals with diphenyl molecules and cyclodehydrogenation of eterphenyl play a significant role onIy in the presence of large excesses of diphenyl and u-terphenyl. respectively. At temperatures above IO25 K. it seems that the proposed mechanism for the formation of terphenyls and triphenylene is the same. even if ring fission and formation of compounds uith a higher degree of polymerization are observed_
243
REFERENCES 1 2 3 4 5 6 7 8 9 10 1I
C-F. Aten and S.H. Bauer. J. Chem. Phys., 39 (1963) 1253. IL Mafpsin and ZA. Nubuto\sky. Kinetics and Catalysis. 10 (1969) 983. W.B. Ftichardson. S_H. Lin and DL Evans. J. Chem. Sot. Faraday Trans. 2 78 (1982) 1. R.L. Slj-sh and CR Kinney, J. Phys Chem, 65 (1961) 1044. KC_ Hou and W.B. Palmer. J. Phys. Chcm, 69 (1965) 863. K. BoIton. J.E. Cullingworth. B-P. Gosh and J-W. Cobb. J. Chem. Sw.. ( 1942) 252 G-M. Badger. Progr. Phys. Or& Chem.. 3 (1965) 1. R.N. Pease and J-M. Morton. J. Amer. Chem. Sot, 55 (1933) 3190. F.C. Mead and RE Burk. Ind. Eng. Chem, 27 (1935) 229. DA. Leathard and J.H. Pumell. Ann. Rev_ Php. Chem. ( 1970) 197. CT_ Brook SJ. Peacock and B-G. Reuben J. Chem. Sot. Faraday Trans_ 1. 75 ( 1979) 652. 12 P-G. Copeland. RE Dean and D. &McNeil.J. Chem. Socv (1960) 266.