Photochemical and Thermal Behavior of Styrenes within Acidic and Nonacidic Zeolites. Radical Cation versus Carbocation Formation

Abstract

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.

Visualizing the Crystal Structure and Locating the Catalytic Activity of Micro‐ and Mesoporous ZSM‐5 Zeolite Crystals by Using In Situ Optical and Fluorescence Microscopy

“…More detailed information on the oligomerization reaction can be found elsewhere. [26,27,31,32] Comparison of the absorption spectra for micro-and mesoporous ZSM-5 crystals shows that, upon desilication, the band intensities associated with dimeric and trimeric carbocation species are changing. More specifically, the longer-wavelength absorption band, present in the microporous crystals, vanishes completely when the oligomerization is carried out over the mesoporous zeolite, showing that in the latter case the formation of trimeric or higher carbocations is limited.…”

Revealing Shape Selectivity and Catalytic Activity Trends Within the Pores of H‐ZSM‐5 Crystals by Time‐ and Space‐Resolved Optical and Fluorescence Microspectroscopy

“…The second promising approach, developed by our group, involves the use of insitu polarized-light optical microspectroscopy in combination with an in-situ cell, allowing a wide range of catalytic reactions under diverse experimental conditions, that is, both in the gas and liquid phase, with temperatures up to 1273 K. Making use of this experimental setup, it was possible to reveal the non-uniform catalytic behavior within H-ZSM-5 crystals. [15] More specifically, we have demonstrated for the oligomerization of different styrene molecules [16] at elevated temperatures that distinct pore geometries lead to different reaction products aligned within the straight pores of the zeolite material. Moreover, dimeric carbocation products are preferentially formed at the edges of the H-ZSM-5 crystals, whereas the main body of the zeolite crystal favors trimeric carbocation products.…”

“…[17] Secondly, the stability of the initial styrene carbocation (Scheme 1) is of great importance to the catalytic activity within the pore channels, being the rate-determining step. [16] In the case of 5, the position of the electron-withdrawing halogen atom on the aromatic ring stabilizes the carbocation by the electron density effect. In contrast, the chlorine atom, when placed at the meta-position, destabilizes the initial styrene carbocation, hence not showing reactivity in the case of 9.…”

“…In more detail, the band at 585 nm is due to the 1,3-bis-A C H T U N G T R E N N U N G (phenyl)buten-1-ylium cation (D in Scheme 1), as deduced form the comparison with the data for a nonsubstituted carbocation generated from 1,3-diphenylpropanediol upon interaction with H-ZSM-5. [16] As to the band at 630 nm, the shift to lower energies suggests an extended conjugated p system; therefore, it is reasonable to assume that the band originates from trimeric carbocations. The observation that this band arises later in time relative to that due to dimeric carbocations [15] supports this assumption.…”

Tracing Catalytic Conversion on Single Zeolite Crystals in 3D with Nonlinear Spectromicroscopy

“…The UV/vis response (not shown) is in very good agreement with monoenyl (300 nm), dienyl (360 nm), and trienyl (460 nm) carbenium ions reported in the literature for various alkene/ zeolite combinations. 22,23 Note that fluorescence cross sections are considerably (up to 10 orders of magnitude) larger than Raman scattering cross sections. Therefore, the absence of bands characteristic for such conjugated systems in the CARS spectra between −1505 and −1580 cm −1 22,24 does not exclude formation of small amounts of such species, below the CARS detection limit of a few millimoles.…”