Influence of Confinement Effect on Electron Transfers Induced by t-Stilbene Sorption in Medium Pore Acidic Zeolites

Abstract

The mere exposure of trans-stilbene (t-St) to three types of dehydrated medium pore acid zeolites that differ by their pore diameter induces t-St spontaneous ionization in high yield. In situ diffuse reflectance UVÀvisible, EPR, and Raman spectra recorded over several months highlight the exceptional stability of the charge separated states formed in ferrierite (H-FER), H-MFI, and mordenite (H-MOR). The increase in the pore diameter from H-FER to H-MOR induces different behaviors after radical cation formation. t-St •+ is stabilized for months in the narrow pores of H-FER, whereas in the larger pore H-MFI, relatively fast electron abstraction (hole transfer) takes place from the zeolite framework to create charge transfer complexes. Pulsed EPR experiments were performed using t-St and marked [D 12 ]t-St and [ 13 C 2 ]t-St molecules to reveal the structural environment of the unpaired electrons through the assignment of the couplings with 1 H, 2 H, 13 C, 27 Al, and 29 Si nuclei.

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.…”

Influence of framework Si/Al ratio and topology on electron transfers in zeolites

“…The spectral features of this long lived charge separated state and its formation mechanism related to the electron donor property of zeolite were detailed in previous papers. [16] The spectra recorded for ten hours, fourteen days and nine months after mixing t-stilbene with H-ZSM-5 zeolites of different Si/Al ratios are shown in Fig. 7.…”

“…The effects of high intrachannel electrostatic field due to aluminium, charge balancing cation and confinement of the guest molecule give rise to intermolecular non-covalent interactions that can induce many interesting reactions. [1,2] Particularly, these remarkable intrinsic properties can include spontaneous ionization, stabilization of otherwise unstable radicals, polymerization and hole-catalyzed reactions. [3][4][5] Indeed, recent studies have demonstrated the determinant role of electron transfer processes in the reaction mechanisms involved in zeolite catalysis.…”

“…[1] The stabilization of radical cations highlights the electron acceptor property of zeolite but the zeolite can also act as an electron donor through AlO 4 moieties. [14][15][16] Although some bibliographic studies suggest a correlation between the increase in aluminium content and the ionization rate of incorporated molecules and their stabilization, the interpretations proposed to explain this behaviour remain incomplete because zeolites are very often not sufficiently characterized. [1] For example, it has been proposed that increasing the aluminum content that reduces the average distance between two aluminum atoms can facilitate electron transfer and provide better stability to the system.…”