Electron Transfers Induced by t-Stilbene Sorption in Acidic Aluminum, Gallium, and Boron Beta (BEA) Zeolites

Abstract

The adsorption of trans-stilbene (t-St) on the acidic aluminum-, gallium-, and boron-containing zeolite beta (H-AlBEA, H-GaBEA, and H-BBEA, respectively) is investigated using UV/vis and timeresolved UV/vis spectroscopy, electron paramagnetic resonance spectroscopy, and Raman spectroscopy. On H-AlBEA, the results show a fast and spontaneous one-electron oxidation of t-St, quickly followed by the recapture of an electron from the zeolite framework by the t-St •+ radical cation and the formation of a long-lived charge-transfer complex (t-St H-AlBEA •−•+ ). This charge-transfer complex (I) evolves over a period of months into a spectroscopically distinct charge-transfer complex (II). Evidence for the (undetected) intermediate t-St •+ radical cation is obtained using time-resolved UV/vis spectroscopy. Similar electron-transfer processes have been observed in the acidic ferrierite, ZSM-5, and mordenite aluminosilicate zeolites. The key difference is that the rate of electron recapture by the t-St •+ radical cation is much faster than that in all of the other zeolites. This is in agreement with the trend observed in the ferrierite, ZSM-5, and mordenite zeolites: looser fit leads to a lower stability of the radical cation intermediate and leads to a rapid hole transfer to the zeolite framework to form a charge-transfer complex. In the case of boron-containing beta, the rate of formation of the radical cation is slow, and the yield is small. Gallium-containing zeolite beta shows intermediate behavior when compared with the Al and B forms of zeolite beta.

Electron transfers in a TiO2-containing MOR zeolite: synthesis of the nanoassemblies and application using a probe chromophore molecule

“…The constant spectrum thus observed after one day is assigned to the electron-hole pair as already reported after t-St ionization in TiO 2 -free mordenite as well as in ZSM-5 as in BEA zeolites. [96][97][98] Again, identical behaviors were observed for the three Ti-containing MOR samples but with weaker signal intensities and lower signal to noise ratios for t-St@TiO 2 -HMOR ox and t-St@TiO 2 -KMOR ox .…”

Comparison between Spontaneous and Photoinduced Ionization Mechanisms for p-Quaterphenyl in M-ZSM-5 (M = H+, Na+) Zeolites

“…[10][11][12][13][14][15][16][17] In case of high internal electrostatic field and if the confinement is high enough, ionization can occur spontaneously. [18][19][20][21][22][23][24][25] This feature was often observed with sorption of polyaromatics in the channels of medium pore acid zeolites [20][21][22][23][24] or in frameworks containing cations like Li + small enough to generate highly polarizing environment. 26 Thus, the ionizing capacity of the host was shown to depend significantly on the zeolite structure and on the Si/Al ratio but also on the charge balancing cation nature.…”

“…To our knowledge, this spectrum does not correspond to any previously reported spectrum for p-QPh related compounds. Based on our own experience in such systems and by analogy with previous works related to electron donor ability of ZSM-5 zeolite framework towards electron deficient radical cations of biphenyl, p-terphenyl, naphthalene, tetracene, and t-stilbene, 21,22,23,24,28,29,39,40,41 this spectrum is tentatively assigned to a charge transfer complex (AlO 4 •+ -p-QPh) associated with the formation of an electronhole pair due to subsequent electron transfer occurring after the radical formation according to equation 3.…”

“…Only the observation of a broad signal in the near infrared region around 1400-1600 nm could be associated with such trapped electron as already reported for analogous zeolite systems after spontaneous ionization. 24,27 The evolutions of the spectral concentrations C k (t) as function of time of both the extracted species involved in the p-QPh sorption in H-ZSM-5 zeolite were also calculated by the MCR method using the spectra obtained for one year and are presented on figure 2B. Curve a shows that the spectral concentration of the radical cation remained very low during the first week following the mixing.…”

Electron Transfers in Donor–Acceptor Supramolecular Systems: Highlighting the Dual Donor and Acceptor Role of ZSM-5 Zeolite

“…By analogy with the spectral signature reported in the near-infrared region after t-stilbene in aluminated BEA and GaZSM-5 zeolites, the broad band observed between 1000 and 1800 nm might correspond to the spectral signature of electron trapped within the framework. 46,49 Note that no absorption characteristic of the radical anion DCB •− could be detected in the UV and visible domains. Moreover, the spectral region around 300 nm includes intense absorptions of nonionized PTP and DCB molecules which overlap the possible contributions of radical species with weaker intensities appearing in the same region.…”