Photochemical and Thermal Behavior of Styrenes within Acidic and Nonacidic Zeolites. Radical Cation <i>versus</i> Carbocation Formation
作者:Frances L. Cozens、Roumiana Bogdanova、Michèle Régimbald、Hermenegildo García、Vinente Martí、J. C. Scaiano
DOI:10.1021/jp9708963
日期:1997.8.1
Laser flash photolysis of a series of substituted styrenes embedded within the cavities of the large pore zeolite NaY leads to the formation of the corresponding styrene radical cation. The reactivity and spectra of these radical cations embedded within NaY are examined and compared to the reactivity of the same radical cations in solution. It is found that for the highly reactive parent styrene radical cation the zeolite framework provides a strong stabilizing effect. For the 4-methoxy-substituted styrene radical cation the zeolite framework plays less of a role in stabilizing the radical cation as compared to the reactivity of the same radical cation in acetonitrile solution. Rigorous analysis of the thermal stability of 4-methoxystyrene, 4-methylstyrene, and anethole in the zeolite micropores was carried out using two sources of NaY zeolite (Aldrich and The PQ Corporation). It was found that the thermal stability was surprisingly dependent on the source of the NaY zeolite. 4-Methoxystyrene, 4-methylstyrene, and anethole were thermally stable in NaY (Aldrich) but rapidly dimerized in NaY (PQ) upon incorporation with dichloromethane. We observed the formation of the same type of dimers not only for 4-methoxystyrene but also for 4-methylstyrene and anethole. In addition, 4-methoxystyrene was incorporated into a series of different acid zeolites (HZSM-5, HMordenite, HBeta, and HY) varying in the shape and size of their micropores where rapid thermal protonation occurs. Dimerization of the thermally formed 4-methoxyphenethyl cation with a neutral molecule of 4-methoxystyrene took place within all the acid zeolites examined. The generation of this secondary 1,3-bis(4-methoxyphenyl)-1-butylium ion was clearly observed in the medium pore ZSM-5. This carbocation was found to be thermally unstable in the acidic environment provided by the four acidic zeolites and underwent a proton and hydride transfer to form the more stable allylic 1,3-bis(4-methoxyphenyl)buten-1-ylium cation. In the large round cavities of HY a competing cyclization reaction took place which led to the formation of the 3-methyl-5-methoxy-1,4-methoxyphenylindanyl cation.