Unusual Electron Transfer Within Zeolites And Their Applications In Photo-Driven Methane Conversion

Effective activation of the methane C-H bond and conversion of methane (natural gas)to a mixture of more valuable hydrocarbon derivatives under mild conditions is always oneof the key scientific and practical challenges in the fields of catalysis and chemistry. As themost important industrial catalysts, zeolites, which contain uniform microporous structures,have been considered as a promising material for the usage of methane under mildconditons. It is a consensus that electrons can be transferred between zeolites and occludedreactants during catalytic reactions. The determination of the mechanism of electrontransfer of zeolites during the catalytic reactions is crucial for the improvement of catalyticperformance, and enables us to design better performing systems for methane conversion.Based on these considerations, in this dissertation, a systematic research has beencarried out centering on the electron donor/acceptor properties of zeolites framework. Withinvestigation of thermal-and photo-driven electron transfer processes in variouszeolite-based catalytic reactions, we aim to establish the relationship betweenmicrostructure and catalytic property. Based on the controllable electron transfer processwithin zeolites, we have prepared a series of novel zeolitic materials which exhibitssuperior performances for photoinduced methane conversion at room temperature,provided insights into the photoactivation of methane C-H bond from the view of electrontransfer, and revealed the essence of methane conversion at a molecular level. First, we demonstrate that aluminosilicate zeolites in the protonated form lose theirframework electrons when alkyl bromides are introduced into the pores of the zeolites (theliquid-solid reaction). The loss of electrons results in paramagnetic electron-deficient Ocenters in the framework of zeolites which function as single-electron redox sites, whilereduction of alkyl halide to its corresponding alkane has been observed after the reaction.To pinpoint the process that accounts for the formation of the paramagnetic centers, agas-solid reaction has been conducted. The control experiment indicates that the liquidsolution plays a role of "carrier" during the electron transfer, and the generation ofparamagnetic centers is closely related to the formation of alkane in the reaction system.The quantum chemical calculation gives rise to the optimized structure of the intermediatesformed in zeolites and we have proposed a mechanism concerning halogen switch andradical disproportionation processes. By using a series of acid zeolites and various alkylhalides as the reference catalysts and reactants, respectively, we have also systematicallyinvestigated the impacts of the composition and topology of zeolite framework and the typeof reactants on the electron transfer process. The paramagnetic centers formed on theframework of zeolites are chemically accessible to electron donor molecules, such as NOand CH₃OH. On the other hand, O₂, N₂O and CCl₃F, which are typical electron acceptors,do not interact with the paramagnetic centers in the zeolite sample. This research gives thedirect evidence for the existence of redox centers in zeolite framework and determines themechanism of electron transfer of zeolite during acid-catalyzed reaction.Second, we have synthesized a novel zinc-modified ZSM-5catalyst with an extraelectron delocalized on the zeolite framework through a solid-vapor reaction between adehydrated HZSM-5zeolite and metallic zinc vapor. By using in situ electronparamagnetic resonance (EPR) spectroscopy and a97%-enriched67Zn isotope, we havedemonstrated that the zinc atoms undergo two different oxidations. One is that each zincatom reduces two closely-positioned protons to form a Zn²⁺cation; the other is that eachzinc atom reduces one isolated proton to form a Zn²⁺cation with an extra electrondelocalized on the zeolite framework. Upon UV irradiation, these delocalized electrons canbe photoexcited from the zeolite framework to the4s orbital of the Zn²⁺cation, formingstable Zn⁺species. At above220℃, the4s electrons of Zn⁺will fall back to the zeolite framework reversibly. The transition of these extra electrons between the4s orbitals of theZn²⁺cations and zeolite framework make the zinc-modified ZSM-5become a superiorphotoactive material.Based on these, a (Zn⁺,Zn²⁺)-ZSM-5^-catalyst which exhibits the highestphotocatalytic activity for selective alkane C-H activation and methane conversion at roomtemperature has been developed. An optimized catalyst afforded9.8mol h-1g-1methaneconversion rate with>99%selectivity for ethane and hydrogen products. Mechanisticstudies suggest a two-stage photoexcitation process, with light of wavelengths shorter than390nm required to transfer electrons from the zeolite framework to Zn²⁺centers, and lightof wavelengths shorter than700nm required to photoexcited the4s-electron of Zn⁺to anempty C-H σ^*-antibonding orbital of methane to active the C-H bond. The two-stagephotoexcitation process makes the energy threshold (3.2eV) needed to power ourphotocatalytic system much lower than that (4.6eV) required by previously reportedsystems. For the first time, the activation of C-H bond and conversion of methane to ethanehave been successfully achieved under outdoor sunlight irradiation at room temperature.Third, through a facile ion exchange, we have introduced a series of metal cations intomicroporous ETS-10titanosilicate with a framework containing one-dimensionalO-Ti-O-Ti-O semiconducting nanowires. By employing various in situ techiques, we havedetermined the mechanism of the photoinduced cleavage of H₃C-H bond occurring on thesurfaces of these photoactive substrates. It is seen that a Ga³⁺-modified ETS-10materialexhibits the highest photoactivity for the cleavage of methane C-H bond atroom-temperature among all the samples tested. Further experiments demonstrate that boththe extra-framework metal ions and the photoactive TiO₂units in zeolite frameworks arecrucial for the photo-driven methane conversion reaction. The highly enhanced activity ofGa³⁺-modified ETS-10material is attributed to the synergistic effect of gallium-inducedC-H bond polarization and titania-based photoredox process (photogeneratedoxygen-centered radicals). The synergy between the metal species and the semiconductorphase is general and this result has broad implications for the design of superiorphoto-driven methane conversion systems. Finally, we have synthesized novel dye-zeolite hybrid materials through theincorporation of methyl viologen dye molecules (MV^2-) into two kinds of titanosilicatezeolites (TS-1and ETS-10). Upon UV irradiation, the photogenerated electrons from theframework of titanosilicate zeolites migrate rapidly to the surface (either internal orexternal) of the zeolite materials and are further photoexcited to MV²⁺to form stable MV+cation radicals. At the same time, the dye-zeolite hybrid materials turn light blue in colorand a single line ESR spectrum characteristic of the methyl viologen radical cation isobserved; whereas the reference MV^2-modified aluminosilicate ZSM-5zeolites do nothave this function, although the topology of ZSM-5is identical to that of TS-1. The controlexperiments shows that the liquid solvent plays a role of "carrier" during the electrontransfer, and the generation of stable MV^·+cation radicals and photochromism is closelyrelated to the presence of hydrogen atom in solutions. The hydrogen atom of solvent can becaptured by the photogenerated holes on the framework of zeolite, which reduces therecombination of the photogenerated charge carries (electrons and holes). Based on thisunique feature, we have chosen saturated alkanes (cyclohexane) or benzene as solvent tocarry out the photoinduced electron transfer and realized the formation of C-C bondthrough the direct cleavage of C-H bond of saturated alkanes and benzene.