Pore selectivity and electron transfers in HZSM-5 single crystals: a Raman microspectroscopy mapping and confocal fluorescence imaging combined study

Electron transfers at the single particle level in HZSM-5 zeolite are followed by combining Raman microspectroscopy mapping and confocal fluorescence imaging. The effects of pore accessibility and guest diffusion on reactivity are investigated.

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

From experimental results on H-ZSM-5 and H-*BEA zeolites, it is shown that the stability of radical cations and of charge transfer complexes (CTC) is highly dependent on the distance between Brønsted sites and strong Lewis sites or Brønsted Strong Lewis Pairs (BSLPs).

Influence of hierarchization on electron transfers in structured MFI-type zeolites

H-ZSM-5 zeolite (Si/Al = 19.3) was hydrothermally synthesized. Alkaline and/or acid post-synthesis treatments were carried out to give rise to an interconnected mesoporous volume. The desilication treatment parameters have been tuned (temperature, organic base addition) to obtain a series of samples with increasing mesoporous volume and a constant number of acid sites. The physico-chemical properties of the resulting materials were fully characterized by many techniques (NMR, BET, PXRD, and pyridine thermal desorption followed by infrared spectroscopy). To assess the effect of post-treatments on sample reactivity, the charge separation processes between the zeolite framework and adsorbed trans-stilbene (t-St) molecule were investigated by UV-visible diffuse reflectance. The spectra obtained after t-St adsorption show clear differences depending on the applied post-treatments. It appears that the desilication treatments performed without acidic washing highly stabilize the radical cation resulting from the t-St spontaneous ionization. In contrast, by applying acidic washing after desilication, the ionization process becomes significantly weaker. The results show that the proportion of strong Lewis acid sites in the vicinity of Brønsted sites named Brønsted Strong Lewis Pairs (BSLP), are responsible for the amount of radical cations observed in the different samples. More precisely, it exists an optimal proportion of BSLP to achieve a high ionization rate. On the basis of the experimental results a mechanism for the formation of the t-St radical cation and the charge transfer complex (CTC) is proposed.

A theoretical study of the confinement effects on the energetics and vibrational properties of 4,4'-bipyridine adsorption on H-ZSM-5 zeolite

The confinement effects of a zeolite framework on the adsorption of a bidentate 4,4'-bipyridine (44BPY) ligand on the straight channel of H-ZSM-5 have been investigated by density functional theory calculations using B3LYP, B3LYP-D3, M06-2X, M06-2X-D3 functionals, and the MP2 method with two basis sets 6-31+G* and 6-31++G(2d,2p). The straight channel is simulated by a realistic cluster model of 32 tetrahedra (T), having two aluminum atoms located in the straight and intersection regions, respectively. The potential energy surface of the double proton transfer reaction from the Brønsted acid sites of H-ZSM-5 to 44BPY is characterized by three minima corresponding to two monodentate ion pair complexes 44BPYH⁺/32T^- and one bidentate ion pair complex 44BPYH₂²⁺/32T^2- formed consecutively via two distinct pathways. No energy minimum is found for a neutral hydrogen bonding structure. The relative stabilities of these 44BPY adsorption complexes and the transition states connecting them do not exceed 4.9 and 4.2 kcal mol^-1, respectively. Consequently equilibrium between the mono and bidentate complexes could be established. Our results clearly show that the adsorption energy for all complexes is substantially governed by the confinement effects executed through steric constraints and dispersive van der Waals interactions. The calculated vibrational frequencies and frequency shifts of 44BPY adsorbed as mono or diprotonated species are in good agreement with our Raman spectra of 44BPY occluded in H-ZSM-5 of different Si/Al ratios.

Sorption of 3-hydroxyflavone within channel type zeolites: the effect of confinement on copper(ii) complexation

The confinement effect on the complexation process of Cu(ii) by 3-hydroxyflavone (3HF) was investigated by studying 3HF incorporation in channel-type copper-containing ZSM-5 and mordenite (MOR) zeolites characterized by different pore diameters. Complementary electronic and vibrational spectroscopy techniques point out two distinct behaviors upon 3HF sorption and subsequent complexation depending on the channel diameter in CuZSM-5 and CuMOR. To determine the influence of the internal environment on the interaction between the copper cation and the guest molecule, and to predict the structure of the complexes formed within the narrow-pore ZSM-5 and in the larger pore mordenite, the vibrational spectra of the complexes were calculated using quantum chemical calculations at the DFT level. From the calculations, it is derived that the Cu(3HF)⁺ chelate is formed in CuMOR indicating a weak interaction with the pore walls. In contrast, due to high confinement in CuZSM-5, interactions between copper cations and the narrower pore walls are assumed to take place in addition to 3HF metal complexation. To emphasize the fact that zeolites act as a solid solvent, 3HF complexation was also investigated in methanol solution. In such liquid media, a stable complex Cu(3HF)₂ of 1 : 2 stoichiometry resulting in a double chelation with the metal cation was found to coexist with a minor species [Cu(3HF)(MeOH)₂]⁺ of 1 : 1 stoichiometry. These two complexes show striking analogy with those observed in CuZSM-5 and CuMOR, respectively. Thus, it appears clearly that zeolites can constitute an ideal tool to control and orientate molecular reactivity for the guest in the isolated state.

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

New assemblies constituted by a microporous matrix of mordenite (MOR) zeolite on which TiO2 nanoclusters are deposited were synthesized using ionic oxalate complexes and TiCl3 titanium precursors. The samples were used to investigate the transfer of electrons produced by spontaneous or photo-induced ionization of a guest molecule (t-stilbene, t-St) occluded in the porous volume towards the conduction band of a conductive material placed nearby, in the pores or at least close to their entrance. The reaction mechanisms were compared in these Ti-rich solids and in a Ti-free mordenite sample. The characterization by XRD, N2 physisorption, TEM, XPS and DRIFT spectroscopy of the supramolecular TiO2/MOR systems before t-St adsorption showed the preservation of the crystalline structure after Ti addition and thermal activation treatments. They also revealed that titanium is mainly located at the external surface of the zeolite grains, in the form of highly dispersed and/or aggregated anatase. After incorporation of the guest molecule in the new assemblies, diffuse reflectance UV-visible and EPR spectroscopies indicate that the electron transfer processes are similar with and without TiO2 but strongly stabilized t-St˙(+) radicals are detected in the TiO2-MOR samples whereas such species were never detected earlier in TiO2-free mordenite using these techniques. The stabilization process is found to be more efficient in the sample prepared with TiCl3 as the precursor than with titanium oxalates. It is proposed that the proximity of TiO2 with the formed t-St˙(+) radicals provokes the stabilization of the radical through capture of the ejected electron by the semi-conductor and that confinement effects can also play a role.

Spectroscopic evidence of 3-hydroxyflavone sorption within MFI type zeolites: ESIPT and metal complexation

Due to the possibility of modulating the molecule surrounding, the internal volume of zeolite provides a specific environment to study the photochemical behaviour of 3-hydroxyflavone (3HF) molecules incorporated in the pores.

Conformation and protonation of 2,2'-bipyridine and 4,4'-bipyridine in acidic aqueous media and acidic ZSM-5 zeolites: a Raman scattering study

In situ FT-Raman scattering spectroscopy was used to monitor the sorption kinetics of 2,2'- and 4,4'-bipyridine in acidic ZSM-5 zeolites. The data processing of all the Raman spectra was applied to extract the characteristic Raman spectra of occluded species and respective Raman contribution generated from many spectral data which resolves spectrum of mixture into pure component spectra without any prior information. The assignment of the extracted spectra was performed according to careful comparison with corresponding spectra extracted from a set of Raman spectra recorded during the protonation of 2,2'- or 4,4'-bipyridine (bpy) in hydrochloric acid aqueous solutions. The data processing of the Raman spectra recorded during the slow sorption of 4,4'-bpy in acidic H(n)ZSM-5 (n = 3, 6) zeolites provides specific Raman spectrum of N,N'-diprotonated dication 4,4'-bpyH(2)(2+) as unique species generated in the void space of acidic ZSM-5 zeolites. No evidence of Lewis acid sites was found during the sorption of 4,4'-bpy by Raman scattering spectroscopy. The data processing of the Raman spectra recorded during the slow sorption of 2,2'-bpy in acidic H(n)ZSM-5 (n = 3, 6) zeolites provides specific Raman spectrum of trans-N-monoprotonated cation 2,2'-bpyH+ as major species generated in the void space of acidic ZSM-5 zeolites at loading corresponding to 1 mol per unit cell. The trans/cis interconversion occurs at higher loading even after the complete uptake of the sorbate and indicates some rearrangement in the void space over a long time. The cations were found to be located in straight channels in the vicinity of the intersection with the zigzag channel of the porous materials with the expected conformations deduced from ab initio calculations. However, the motions of occluded species within the channel of ZSM-5 are hindered but remain in the range of the isotropic limit of a liquid at room temperature.