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UV-Vis and ESR spectroscopic studies of the adsorption of arenes on the heteropoly acid H3PW12O40
Journal of Molecular Catalysis, 79 (1993) 21-28 Elsevier Science Publishers B.V., Amsterdam
21
MOO4
UV-Vis and ESR spectroscopic studies of the adsorption of arenes on the heteropoly acid H,PW,,O,, M.N. Timofeeva, A.V. Demidov, A.A. Davydov and I.V. Kozhevnikov* Institute of Catalysis, Novosibirsk 630090 (Russia); tel. (+ 7-383)2355750, fax. (+ 7-383)2355756 (Received May 7,1992; accepted June 18,1992)
Abstract Styrene, naphthalene, 2_methylnaphthalene, and anthracene are adsorbed on H,PW,,O, and H,PW,,0,,,/Si02 at 25-150°C to form carbonium ions and cation radicals. The carbonium ions are likely to be generated in the adsorption of o-xylene, m-xylene, pseudocumene, and durene on H,PW,,O,,. Key words: arenes; spectroscopy
ESR spectroscopy;
heteropoly
acids; 12-tungstophosphoric
acid; UV-Vis
Introduction Heteropoly acids (HPA) of Keggin structure are strong Brensted acids, exceeding in their strength such conventional solid acids as Si02-A1203, HX and HY zeolites, etc. [ 1,2]. Due to their high acidity, HPAs have been widely used as acid catalysts [ 1,3]. It has long been recognized that heterogeneous as well as homogeneous acid catalysis operates via carbonium ion (CI) intermediates’ or transition states. The validity of this concept has been recently discussed [ 4,5]. Many spectroscopic investigations have been done to detect the CIs on the surface of such solid acids as Si02-A1203 and zeolites [ 5131. Up to now, no convincing spectroscopic data supporting the formation of the adsorbed alkylcarbonium ions from the C, to C, alkenes have been obtained [4,14]. There is more or less substantiated evidence on the formation of more stable CIs from dienes and arenes such as arylolefins, alkylbenzenes, and polycyclic arenes [6-13,151. In some of these systems, aromatic cation radicals (CR) are also observed [ 8-10,161. It is still unclear which surface sites produce the above species, since Si02-A1203 and zeolites both have Brarnsted and Lewis acid sites of differing acid strengths in addition to the oxidative centers [ 161. *Author to whom correspondence should be addressed. ‘The term “carbonium ion” hereafter refers to any organic cations regardless of their structure.
0304-5102/93/$06.00
0 1993 - Elsevier Science Publishers
B.V. All rights reserved.
22
M.N. Timofeeva
et al.jJ. Mol. Catal. 79 (1993) 21-28
Unlike silica-aluminas, HPAs possess purely Bronsted acidity [ 1,171. Bulk HPAs of the Keggin structure have nearly uniform distribution of the proton sites [ 2,181. In this work we have attempted to detect the formation of CIs and CRs upon adsorption of arenes on the surface of the heteropoly acid H,PW,,O,, (PW), both bulk and supported on Si02, by means of UV-Vis diffuse reflectance spectroscopy and ESR. This HPA is the strongest acid in the Keggin HPA series [ 11. Experimental
Materials Chemical grade H,PW,,O,,*llH,O was recrystallized from water. The supported PW, 25% H,PW,,O,,JSiO, (PW/SiO,), was prepared by impregnation of SiOa (IKT-04-06, surface area 360 m2 g-l, pore volume 1.2 cm3 gg’, mean pore radius 170 A, particle size 0.1-0.2 mm) with an aqueous PW solution and dried overnight in a desiccator. Na,_,H,PW,,O,, (X = O-2) and CS~PW,~O,, salts were prepared by interaction of PW with Na,CO, or Cs,CO, [ 191. The BET surface areas of the samples used were (m” g-l): PW and Na,_,H,PW 1-7, Cs,PW 120, PW/Si02 344. Chemical grade arenes were dried and stored over NaA zeolite and outgassed prior to use. Adsorption The HPA samples were pre-heated in a quartz cell at 200’ C in vacua ( 10e2 Torr) for 3 h and exposed to arene vapor at room temperature for 0.5 h. After evacuating for 0.5 h, UV-Vis and ESR spectra were recorded. After re-heating the samples at lOO-200°C in vacua for 0.5 h, the spectra were again recorded. Spectra UV-Vis diffuse reflectance spectra were recorded with a Shimadzu UV300 instrument using a standard diffuse reflectance accessory and MgO as a reference. ESR spectra were recorded with a Bruker ER-200 D spectrometer; an admixture of Cr3+ in periclase crystal (MgO) was used as reference. Results and discussion
Adsorption of arenes on the surface of the strong solid acids such as HZSM-5, H-mordenite, SiO,-Al,O, proceeds to form CIs and CRs, which can be detected by UV-Vis and ESR spectroscopy [ 6-131. The carbonium ions generated from styrene and methylbenzenes exhibit characteristic bands in the UV-Vis at 320-360 and 400-460 nm [g-11,13]. The 400-460 nm bands are assigned to CIs derived from naphthalene and anthracene [6-8,121. Cation radicals show bands in the range 600-800 nm [ 6-8,121. The structure of these species and the mechanism of their formation are not yet clear. The following reaction paths have been suggested to explain the formation of the primary species using Brsnsted (eqn. ( 1) ) , Lewis (eqn. (2 ) ) , and oxidative (eqn. (3 ) ) sites [6-131:
M.N. Timofeeva
et al/J. Mol. Catal. 79 (1993)
23
21-28
ArH+H++ArH,+
(H+ addition)
(1)
ArH+L
(H- subtraction)
(2)
(electron transfer)
(3)
+Ar++HL-
ArH + Ox -+ ArH+ ’ + Ox-’ Ar’OfxAr+
The structure of the primary species depends on the structure of the parent ArH. Thus, styrene is certainly protonated at its alkene double bond to form the 1-phenylethyl cation. Methylbenzenes and polycyclic arenes are likely to form arenonium ions ArH,+ with a partial breaking of their n-aromatic system; the formation of benzylic cations also seems to be possible as a result of the H- subtraction (eqn. (2) ) [ 201. The primary species tend to undergo subsequent reactions, such as isomerization, alkylation and oligomerization, producing secondary cations and cation radicals [7,9-l 11. In the case of HPA, in contrast to the SiO,-Al,O, system, due to lack of the Lewis sites, the proton sites are expected to play a major role in generating the carbonium ions (eqn. (1) ). In addition to the proton sites, participation of the oxidative centers is also possible (eqn. (3 ) ) . PW itself has an intense absorption at II> 350 nm in its UV-Vis spectrum due to an oxygen-to-metal charge transfer (Fig. 1). Pre-heating of PW at 200°C caused a slight increase in the background absorption, without the appearance of any discrete maxima, probably due to a reduction of PW. Treatment of PW
I
1
300 Fig. 1. UV-Vis
500 spectra:
(1) PW/Si02
7oo pre-evacuated
A,nm at 200°C for 3 h; (2) PW/SiO*
with Hz at 150°C for 1.5 h; (3) styrene adsorbed on PW/Si02 at 25°C; 150°C for 0.5 h; (5) sample 4 after exposure to water at 25°C for 0.5 h.
pre-treated
(4) sample 3 heated at
24
M.N. Timofeeoa
et al/J. Mol. Catal. 79 (1993) 21-28
with H, at 150°C influenced the spectra in the same way (Fig. 1), i.e. the initially white colour of PW turns blue, which suggests the reduction of W (VI ) . S tyrene When styrene is adsorbed on PW or PW/Si02 at 25”C, an intense band at 425 nm appears (spectrum 3 in Fig. 1) . The same band has been observed on zeolites and Si02-A1203, indicating the formation of the l-phenylethyl carbonium ion [7]. This ion also has a band at 320 nm, which is observed on zeolites [7], but is not detected on PW owing to HPA absorption at A< 350 nm. The 425 nm band is also observed when styrene is adsorbed on Na,_.H,PW. Its intensity decreases with the acidity of the system: NaH,PW > Na,HPW; the neutral salts Na,PW and Cs,PW, and neat SiO, caused no absorption at 425 nm. These results support the assignment of the 425 nm band observed in the styrene-PW system to the carbonium ion. When styrene pre-adsorbed on PW is heated (lOO’C, lop2 Torr, 0.5 h) new bands at 460 and 630 nm (spectrum 4 in Fig. 1) appear. The 630 nm band may be assigned to the cation radical [ 7,8]. This sample showed an ESR spectrum (Fig. 2)) which coincides with that of the cation radical obtained in the
Fig. 2. ESR spectra: (1) PW/Si02 SiOp at 25°C;
pre-evacuated
at 200°C for 3 h; (2) styrene adsorbed on PW/
(3) sample 2 heated at 150°C for 0.5 h.
M.N.Timofeeva et al./J. Mol. Catal. 79 (1993) 21-28
25
adsorption of styrene on zeolites and Si02-A&O3 [ 7,8]. On exposure to water vapor the bands at 460 and 630 nm disappear completely within 0.5 h, and a new band at 500 nm develops, the colour of the sample changing from blue to white. Re-heating the sample in vacua restores the colour and the bands at 460 and 630 nm. The appearance of the bands at 460 and 500 nm is probably a result of subsequent transformations of the primary cations to secondary structures such as the dimeric cations [ 7,9-111. Thus, the interaction of styrene with the HPA proton sites at 25’ C most likely occurs via the protonation of the double bond: PhCH=CH,+H+-tPhCHCH,
(4)
The oxidative centers do not play a significant role in this interaction because at 25’ C neither the cation radicals are detected, nor does the reduction of PW occur. At lOO”C, however, styrene does react with oxidative centers to yield the cation radicals. Methylbenzenes The UV-Vis study of the adsorption of o- and m-xylene, pseudocumene, and durene on PW and PW/SiO, at 25°C reveals the spectral bands at 410450 nm (spectra 2 and 5 in Fig. 3). No spectral changes are observed in the adsorption of benzene and toluene. When methylbenzenes are adsorbed on zeolites or Si02--A1203, bands at 420-430 nm are found, which have been assigned to arene carbonium ions [g-11]. On the basis of these data, we may assign the bands at 410-450 nm detected in the PW system at 25°C to the carbonium ions.
I
300
500
700 h,nm
Fig. 3. UV-Vis spectra: (1) PW pre-evacuated at 200’ C for 3 h; (2 ) durene adsorbed on PW at 25°C; (3) sample 2 heated at 100°C for 0.5 h; (4) sample 3 heated at 200°C for 0.5 h; (5) o-xylene adsorbed on PW at 25°C; (6) sample 5 heated at 150°C for 0.5 h.
26
M.N. Timofeeua et al/J. Mol. Catal. 79 (1993) 21-28
After heating (200’ C, lo-’ Torr, 0.5 h), the PW-durene system shows a peak at 460 nm (spectrum 4 in Fig. 3). Similarly, o-xylene heated in the same manner at 150’ C gives a peak at 470 nm (spectrum 6 in Fig. 3 ) . These peaks may be assigned to the dimeric cations formed via an alkylation of ArH by the primary carbonium ions [ 7,9-111.
Polycyclic arenes When naphthalene, 2-methylnaphthalene, or anthracene are adsorbed on PW and PW/Si02 at 25-150°C two bands are observed: 420-460 and 570-800 nm (spectra 2-5 in Fig. 4). The first band is assigned to CIs, the second to CRs [ 6-8,121. The ESR spectra (Fig. 5) confirm that cation radicals are formed in this system. Our ESR spectra are in good agreement with those reported previously [8]. It should be noted that when anthracene is adsorbed, in contrast to styrene, the cation radical is generated at 25°C while the carbonium ion appears at higher temperatures (Fig. 4). When naphthalene is adsorbed the two species appear simultaneously at 25’ C. Thus, we can conclude that HPAs, similarly to other strong solid acids, are capable of generating the carbonium ions and cation radicals from arenes. These species may take part as intermediates in HPA catalyzed reactions of arenes. It is noteworthy that the formation of such species takes place under mild conditions at 25-lOO”C, which is in agreement with the low-temperature catalytic performance of HPAs.
1
300 Fig. 4. UV-Vis
500 spectra:
(1) PW/Si02
700 pre-evacuated
h.nm at 200°C for 3 h; (2) anthracene
adsorbed
on PW/SiO, at 25°C; (3) sample 2 heated at 150°C for 5 min; (4) naphthalene adsorbed on PW at 25°C; (5) naphthalene adsorbed on PW/SiO, at 25°C and then heated at 150°C for 5 min.
M.N. Timofeeva
et al./J. Mol. Catal. 79 (1993) 21-28
g- 2.0032 all-lit
g - 2.0033 Ati -1OG
H 1OOG
Fig. 5. ESR spectra (Cr3+ with g= 1.9796 as a reference; starred signals are from Mn2+ in the reference): (1) anthracene adsorbed on PW at 25°C and then heated at 150°C for 10 min; (2) naphthalene treated similarly.
Acknowledgements The authors thank Prof. E.A. Paukshtis for valuable discussions. References I.V. Kozhevnikov, Russ. Chem. Rev., 56 (1987) 811. G.N. Kapustin, T.R. Brueva, A.L. Klyachko, S.M. Kulikov, I.V. Kozhevnikov and M.N. Timofeeva, Kinet. Katal., 31 (1990) 1017. M. Misono and N. Nojiri, Appl. Catal., 64 (1990) 1. V.B. Kazansky, Izv. Akad. Nauk SSSR, Ser. Khim., (1990) 2261. A.G. Pelmenshchikov, N.U. Zhanpeisov, E.A. Paukshtis, L.V. Malysheva, G.M. Zhidomirov and K.I. Zamaraev, Dokl. Acad. Nauk SSSR, 293 (1987) 915. V.A. Barachevsky, V.E. Kholmogorov, G.M. Belotserkovsky, A.N. Terenin, Kinet. Katal., 6 (1965) 258. H.P. Leftin and M.C. Hobson in D.D. Eley, H. Pines and P.B. Weisz (eds)., Advances in Catalysis, Vol. 14, Academic Press, New York, NY, 1963, p. 115. L.A. Kupcha, V.I. Lygin and L.V. Mineeva, Kinet. Katal., 4 (1968) 840. I.L. Yatovt, E.I. Kotov and N.R. Bursian, Zhur. Prikl. Khim., 58 (1985) 312.
28 10 11 12 13 14 15 16 17 18 19 20
M.N. Timofeeva et al./J. Mol. Catal. 79 (1993) 21-28 I.L. Yatovt, E.I. Kotov and N.R. Bursian, Zhur. Prikl. Khim., 59 (1986) 457. W.R. Hall, J. Catal., 1 (1962) 53. L.M. Kustov and V.L. Zholobenko, Kinet. Katal., 29 (1988) 928. A.V. Demidov, A.A. Davydov and L.N. Kurina, Zzv. Akad. Nauk SSSR, Ser. Khim., (1989) 1486. E.A. Paukshtis, DSc. Thesis, Institute of Catalysis, Novosibirsk, 1991. A.V. Demidov, A.A. Davydov and L.N. Kurina, Zzv. Akad. Nauk SSSR, Ser. Khin., (1989) 1229. M.I. Loktev and A.A. Slinkin, Usp. Khim., 45 (1976) 1594. M. Misono, N. Mizuno. K. Katamura, A. Kasai, Y. Konishi, K. Sakata, T. Okuhara and Y. Yoneda, Bull. Chem. Sot. Jpn., 55 (1982) 400. V.M. Mastikhin, S.M. Kulikov, A.V. Nosov, I.V. Kozhevnikov, I.L. Mudrakovsky and M.N. Timofeeva, J. Mol. Catal., 60 (1990) 65. G.A. Tsigdinos, Znd. Eng. Chem. Res. Dev., 13 (1974) 261. D. Bethel1 and V. Gold, Carbonium Zons, Academic Press, London and New York, 1976.
21
MOO4
UV-Vis and ESR spectroscopic studies of the adsorption of arenes on the heteropoly acid H,PW,,O,, M.N. Timofeeva, A.V. Demidov, A.A. Davydov and I.V. Kozhevnikov* Institute of Catalysis, Novosibirsk 630090 (Russia); tel. (+ 7-383)2355750, fax. (+ 7-383)2355756 (Received May 7,1992; accepted June 18,1992)
Abstract Styrene, naphthalene, 2_methylnaphthalene, and anthracene are adsorbed on H,PW,,O, and H,PW,,0,,,/Si02 at 25-150°C to form carbonium ions and cation radicals. The carbonium ions are likely to be generated in the adsorption of o-xylene, m-xylene, pseudocumene, and durene on H,PW,,O,,. Key words: arenes; spectroscopy
ESR spectroscopy;
heteropoly
acids; 12-tungstophosphoric
acid; UV-Vis
Introduction Heteropoly acids (HPA) of Keggin structure are strong Brensted acids, exceeding in their strength such conventional solid acids as Si02-A1203, HX and HY zeolites, etc. [ 1,2]. Due to their high acidity, HPAs have been widely used as acid catalysts [ 1,3]. It has long been recognized that heterogeneous as well as homogeneous acid catalysis operates via carbonium ion (CI) intermediates’ or transition states. The validity of this concept has been recently discussed [ 4,5]. Many spectroscopic investigations have been done to detect the CIs on the surface of such solid acids as Si02-A1203 and zeolites [ 5131. Up to now, no convincing spectroscopic data supporting the formation of the adsorbed alkylcarbonium ions from the C, to C, alkenes have been obtained [4,14]. There is more or less substantiated evidence on the formation of more stable CIs from dienes and arenes such as arylolefins, alkylbenzenes, and polycyclic arenes [6-13,151. In some of these systems, aromatic cation radicals (CR) are also observed [ 8-10,161. It is still unclear which surface sites produce the above species, since Si02-A1203 and zeolites both have Brarnsted and Lewis acid sites of differing acid strengths in addition to the oxidative centers [ 161. *Author to whom correspondence should be addressed. ‘The term “carbonium ion” hereafter refers to any organic cations regardless of their structure.
0304-5102/93/$06.00
0 1993 - Elsevier Science Publishers
B.V. All rights reserved.
22
M.N. Timofeeva
et al.jJ. Mol. Catal. 79 (1993) 21-28
Unlike silica-aluminas, HPAs possess purely Bronsted acidity [ 1,171. Bulk HPAs of the Keggin structure have nearly uniform distribution of the proton sites [ 2,181. In this work we have attempted to detect the formation of CIs and CRs upon adsorption of arenes on the surface of the heteropoly acid H,PW,,O,, (PW), both bulk and supported on Si02, by means of UV-Vis diffuse reflectance spectroscopy and ESR. This HPA is the strongest acid in the Keggin HPA series [ 11. Experimental
Materials Chemical grade H,PW,,O,,*llH,O was recrystallized from water. The supported PW, 25% H,PW,,O,,JSiO, (PW/SiO,), was prepared by impregnation of SiOa (IKT-04-06, surface area 360 m2 g-l, pore volume 1.2 cm3 gg’, mean pore radius 170 A, particle size 0.1-0.2 mm) with an aqueous PW solution and dried overnight in a desiccator. Na,_,H,PW,,O,, (X = O-2) and CS~PW,~O,, salts were prepared by interaction of PW with Na,CO, or Cs,CO, [ 191. The BET surface areas of the samples used were (m” g-l): PW and Na,_,H,PW 1-7, Cs,PW 120, PW/Si02 344. Chemical grade arenes were dried and stored over NaA zeolite and outgassed prior to use. Adsorption The HPA samples were pre-heated in a quartz cell at 200’ C in vacua ( 10e2 Torr) for 3 h and exposed to arene vapor at room temperature for 0.5 h. After evacuating for 0.5 h, UV-Vis and ESR spectra were recorded. After re-heating the samples at lOO-200°C in vacua for 0.5 h, the spectra were again recorded. Spectra UV-Vis diffuse reflectance spectra were recorded with a Shimadzu UV300 instrument using a standard diffuse reflectance accessory and MgO as a reference. ESR spectra were recorded with a Bruker ER-200 D spectrometer; an admixture of Cr3+ in periclase crystal (MgO) was used as reference. Results and discussion
Adsorption of arenes on the surface of the strong solid acids such as HZSM-5, H-mordenite, SiO,-Al,O, proceeds to form CIs and CRs, which can be detected by UV-Vis and ESR spectroscopy [ 6-131. The carbonium ions generated from styrene and methylbenzenes exhibit characteristic bands in the UV-Vis at 320-360 and 400-460 nm [g-11,13]. The 400-460 nm bands are assigned to CIs derived from naphthalene and anthracene [6-8,121. Cation radicals show bands in the range 600-800 nm [ 6-8,121. The structure of these species and the mechanism of their formation are not yet clear. The following reaction paths have been suggested to explain the formation of the primary species using Brsnsted (eqn. ( 1) ) , Lewis (eqn. (2 ) ) , and oxidative (eqn. (3 ) ) sites [6-131:
M.N. Timofeeva
et al/J. Mol. Catal. 79 (1993)
23
21-28
ArH+H++ArH,+
(H+ addition)
(1)
ArH+L
(H- subtraction)
(2)
(electron transfer)
(3)
+Ar++HL-
ArH + Ox -+ ArH+ ’ + Ox-’ Ar’OfxAr+
The structure of the primary species depends on the structure of the parent ArH. Thus, styrene is certainly protonated at its alkene double bond to form the 1-phenylethyl cation. Methylbenzenes and polycyclic arenes are likely to form arenonium ions ArH,+ with a partial breaking of their n-aromatic system; the formation of benzylic cations also seems to be possible as a result of the H- subtraction (eqn. (2) ) [ 201. The primary species tend to undergo subsequent reactions, such as isomerization, alkylation and oligomerization, producing secondary cations and cation radicals [7,9-l 11. In the case of HPA, in contrast to the SiO,-Al,O, system, due to lack of the Lewis sites, the proton sites are expected to play a major role in generating the carbonium ions (eqn. (1) ). In addition to the proton sites, participation of the oxidative centers is also possible (eqn. (3 ) ) . PW itself has an intense absorption at II> 350 nm in its UV-Vis spectrum due to an oxygen-to-metal charge transfer (Fig. 1). Pre-heating of PW at 200°C caused a slight increase in the background absorption, without the appearance of any discrete maxima, probably due to a reduction of PW. Treatment of PW
I
1
300 Fig. 1. UV-Vis
500 spectra:
(1) PW/Si02
7oo pre-evacuated
A,nm at 200°C for 3 h; (2) PW/SiO*
with Hz at 150°C for 1.5 h; (3) styrene adsorbed on PW/Si02 at 25°C; 150°C for 0.5 h; (5) sample 4 after exposure to water at 25°C for 0.5 h.
pre-treated
(4) sample 3 heated at
24
M.N. Timofeeoa
et al/J. Mol. Catal. 79 (1993) 21-28
with H, at 150°C influenced the spectra in the same way (Fig. 1), i.e. the initially white colour of PW turns blue, which suggests the reduction of W (VI ) . S tyrene When styrene is adsorbed on PW or PW/Si02 at 25”C, an intense band at 425 nm appears (spectrum 3 in Fig. 1) . The same band has been observed on zeolites and Si02-A1203, indicating the formation of the l-phenylethyl carbonium ion [7]. This ion also has a band at 320 nm, which is observed on zeolites [7], but is not detected on PW owing to HPA absorption at A< 350 nm. The 425 nm band is also observed when styrene is adsorbed on Na,_.H,PW. Its intensity decreases with the acidity of the system: NaH,PW > Na,HPW; the neutral salts Na,PW and Cs,PW, and neat SiO, caused no absorption at 425 nm. These results support the assignment of the 425 nm band observed in the styrene-PW system to the carbonium ion. When styrene pre-adsorbed on PW is heated (lOO’C, lop2 Torr, 0.5 h) new bands at 460 and 630 nm (spectrum 4 in Fig. 1) appear. The 630 nm band may be assigned to the cation radical [ 7,8]. This sample showed an ESR spectrum (Fig. 2)) which coincides with that of the cation radical obtained in the
Fig. 2. ESR spectra: (1) PW/Si02 SiOp at 25°C;
pre-evacuated
at 200°C for 3 h; (2) styrene adsorbed on PW/
(3) sample 2 heated at 150°C for 0.5 h.
M.N.Timofeeva et al./J. Mol. Catal. 79 (1993) 21-28
25
adsorption of styrene on zeolites and Si02-A&O3 [ 7,8]. On exposure to water vapor the bands at 460 and 630 nm disappear completely within 0.5 h, and a new band at 500 nm develops, the colour of the sample changing from blue to white. Re-heating the sample in vacua restores the colour and the bands at 460 and 630 nm. The appearance of the bands at 460 and 500 nm is probably a result of subsequent transformations of the primary cations to secondary structures such as the dimeric cations [ 7,9-111. Thus, the interaction of styrene with the HPA proton sites at 25’ C most likely occurs via the protonation of the double bond: PhCH=CH,+H+-tPhCHCH,
(4)
The oxidative centers do not play a significant role in this interaction because at 25’ C neither the cation radicals are detected, nor does the reduction of PW occur. At lOO”C, however, styrene does react with oxidative centers to yield the cation radicals. Methylbenzenes The UV-Vis study of the adsorption of o- and m-xylene, pseudocumene, and durene on PW and PW/SiO, at 25°C reveals the spectral bands at 410450 nm (spectra 2 and 5 in Fig. 3). No spectral changes are observed in the adsorption of benzene and toluene. When methylbenzenes are adsorbed on zeolites or Si02--A1203, bands at 420-430 nm are found, which have been assigned to arene carbonium ions [g-11]. On the basis of these data, we may assign the bands at 410-450 nm detected in the PW system at 25°C to the carbonium ions.
I
300
500
700 h,nm
Fig. 3. UV-Vis spectra: (1) PW pre-evacuated at 200’ C for 3 h; (2 ) durene adsorbed on PW at 25°C; (3) sample 2 heated at 100°C for 0.5 h; (4) sample 3 heated at 200°C for 0.5 h; (5) o-xylene adsorbed on PW at 25°C; (6) sample 5 heated at 150°C for 0.5 h.
26
M.N. Timofeeua et al/J. Mol. Catal. 79 (1993) 21-28
After heating (200’ C, lo-’ Torr, 0.5 h), the PW-durene system shows a peak at 460 nm (spectrum 4 in Fig. 3). Similarly, o-xylene heated in the same manner at 150’ C gives a peak at 470 nm (spectrum 6 in Fig. 3 ) . These peaks may be assigned to the dimeric cations formed via an alkylation of ArH by the primary carbonium ions [ 7,9-111.
Polycyclic arenes When naphthalene, 2-methylnaphthalene, or anthracene are adsorbed on PW and PW/Si02 at 25-150°C two bands are observed: 420-460 and 570-800 nm (spectra 2-5 in Fig. 4). The first band is assigned to CIs, the second to CRs [ 6-8,121. The ESR spectra (Fig. 5) confirm that cation radicals are formed in this system. Our ESR spectra are in good agreement with those reported previously [8]. It should be noted that when anthracene is adsorbed, in contrast to styrene, the cation radical is generated at 25°C while the carbonium ion appears at higher temperatures (Fig. 4). When naphthalene is adsorbed the two species appear simultaneously at 25’ C. Thus, we can conclude that HPAs, similarly to other strong solid acids, are capable of generating the carbonium ions and cation radicals from arenes. These species may take part as intermediates in HPA catalyzed reactions of arenes. It is noteworthy that the formation of such species takes place under mild conditions at 25-lOO”C, which is in agreement with the low-temperature catalytic performance of HPAs.
1
300 Fig. 4. UV-Vis
500 spectra:
(1) PW/Si02
700 pre-evacuated
h.nm at 200°C for 3 h; (2) anthracene
adsorbed
on PW/SiO, at 25°C; (3) sample 2 heated at 150°C for 5 min; (4) naphthalene adsorbed on PW at 25°C; (5) naphthalene adsorbed on PW/SiO, at 25°C and then heated at 150°C for 5 min.
M.N. Timofeeva
et al./J. Mol. Catal. 79 (1993) 21-28
g- 2.0032 all-lit
g - 2.0033 Ati -1OG
H 1OOG
Fig. 5. ESR spectra (Cr3+ with g= 1.9796 as a reference; starred signals are from Mn2+ in the reference): (1) anthracene adsorbed on PW at 25°C and then heated at 150°C for 10 min; (2) naphthalene treated similarly.
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