Unambiguous Observation of Electron Transfer from a Zeolite Framework to Organic Molecules

Abstract

Zeolites are crystalline aluminosilicates with microporous structures. These materials have been widely used in the field of heterogeneous catalysis. [1] In most catalytic reactions involving a zeolite catalyst, the acidic sites of the zeolite play a pivotal role; hence, much attention has been paid to the nature of the acidic sites and the interactions of these sites with organic molecules.[2] The oxidation states of the framework atoms (Si, Al, and O) are thought to remain unchanged during acid-catalyzed reaction processes. However, electron loss from the zeolite framework may occur when a zeolite is subjected to a high temperature [3] or high-energy g-ray (or Xray) irradiation.[4] Electron transfer between zeolites and occluded molecules has also been proposed previously.[5] The electron-donating ability of the zeolite framework [6] and the ability of zeolites to generate organic radical cations [7] have been established by several research groups. Although it is recognized that electrons may be transferred between zeolites and occluded guest species, the nature of the electron donor/ acceptor in zeolites has not yet been elucidated clearly, and the whereabouts of the electrons/holes formed in the electron-transfer process has been under intense debate. [5,7] In particular, when zeolites act as electron donors, only occluded molecules that accept electrons from zeolites can be observed to change, whereas a difference in the zeolite framework can hardly be detected after the electron-transfer process.Herein, we demonstrate that aluminosilicate zeolites in the protonated form lose their framework electrons when alkyl bromides are introduced into the pores of the zeolites. The loss of the framework electrons results in paramagnetic centers, which function as single-electron redox sites and can be detected by electron paramagnetic resonance (EPR) spectroscopy. More importantly, this process of electron transfer accompanies the acid-catalyzed reaction between zeolites and alkyl bromides; thus, framework electron transfer of acidic zeolites should be considered to be partially responsible for the formation of some of the organic products during acid-catalyzed reactions involving zeolites.A mixture of the dehydrated zeolite HY (protonated zeolite Y with a framework Si/Al ratio of 2.46; see Figure S2 in the Supporting Information) and ethyl bromide was stirred and heated at reflux in an airtight vessel at 50 8C under argon for 24 h. We refer to this process as the "liquid-solid" (L-S) reaction. After the L-S reaction, the HY material (designated Y-R) was orange in liquid ethyl bromide and turned light pink upon the removal of the liquid alkyl bromide with a liquid-nitrogen trap (see Figure S3 in the Supporting Information). The liquid ethyl bromide remained colorless throughout the whole reaction process, both before and after trapping; in the gas phase of the product of the catalytic reaction, a small quantity of ethane was detected, besides ethene and hydrogen bromide. Figure 1 a shows the EPR spectrum of the HY zeo...

Influence of Gallium Isomorphous Substitution in the Acidic MFI Zeolite Framework on Hole Formation, Transfer, and Trapping upon Incorporation of Anthracene

“…14, 48 The formation of a hole on an AlO 4 •þ center of the H-AlZSM-5 has been studied previously by hybrid quantum mechanics. Generation of a hole produces a substantial geometry relaxation of the AlÀO distance to the oxygen atom with the unpaired electron, and the zeolite framework stabilizes the positive charge by longrange effects.…”

Encapsulating Palladium Nanoparticles Inside Mesoporous MFI Zeolite Nanocrystals for Shape‐Selective Catalysis

“…Efforts were also devoted towards evaluating the catalytic performance of Pd@mnc-S1 nanocrystals for organic synthesis as long as it is our long-term interest. [9] Hydrogenation of nitrobenzene,a ni mportant industrial reaction to produce high-value-added aniline and its derivatives,w as selected as am odel reaction to survey the catalytic performance of Pd@mnc-S1. As shown in Figure 3a,t he hydrogenation of nitrobenzene proceeded smoothly over Pd@mnc-S1.…”

“…[9] Hydrogenation of nitrobenzene, an important industrial reaction to produce high-value-added aniline and its derivatives, was selected as a model reaction to survey the catalytic performance of Pd@mnc-S1. As shown in Figure 3 a, the hydrogenation of nitrobenzene proceeded smoothly over Pd@mnc-S1.…”

Single-site photocatalysts with a porous structure

“…The energy needed to power the Zn + -modified ZSM-5 photocatalytic system is much lower than those systems reported previously (Choudhary et al 1997;Arakawa et al 2001;Zheng et al 2008). In a related research system, electron transfer from a zeolite framework to guest organic molecules has been unambiguously observed (Li, J. R. et al 2009;Li, L. et al 2009), demonstrating that electron transfer in zeolite structures is not unusual. These results suggest that the design and synthesis of highly photoactive SSPCs with a zeolitic feature is quite feasible.…”

“…MOFs exhibit many unique properties, such as large pore size, high surface area, good selectivity for small molecules and optical or magnetic responses to the inclusion of guests (Rowsell & Yaghi 2004). Therefore, MOFs have many potential applications in the fields of, for example, gas storage (Zhao et al 2004), compound separation (Li, J. R. et al 2009;Li, L. et al 2009), chemical sensing (Lu & Hupp 2010), biomedical imaging (Liu et al 2011) and heterogeneous catalysis (Song et al 2010). The MOF-based catalysts are mainly classified into three types: MOFs with metal-active sites, MOFs with reactive functional groups and MOFs as host matrices or nanometric reaction cavities (Corma et al 2010).…”