Francesca Bonino

Electronic and vibrational properties of a MOF-5 metal–organic framework: ZnO quantum dot behaviour

UV-Vis DRS and photoluminescence (PL) spectroscopy, combined with excitation selective Raman spectroscopy, allow us to understand the main optical and vibrational properties of a metal-organic MOF-5 framework. A O(2-)Zn(2+)[rightward arrow] O(-)Zn(+) ligand to metal charge transfer transition (LMCT) at 350 nm, testifies that the Zn(4)O(13) cluster behaves as a ZnO quantum dot (QD). The organic part acts as a photon antenna able to efficiently transfer the energy to the inorganic ZnO-like QD part, where an intense emission at 525 nm occurs.

Oxidation of ethane to ethanol by N2O in a metal–organic framework with coordinatively unsaturated iron(II) sites

Enzymatic haem and non-haem high-valent iron-oxo species are known to activate strong C-H bonds, yet duplicating this reactivity in a synthetic system remains a formidable challenge. Although instability of the terminal iron-oxo moiety is perhaps the foremost obstacle, steric and electronic factors also limit the activity of previously reported mononuclear iron(IV)-oxo compounds. In particular, although natures non-haem iron(IV)-oxo compounds possess high-spin S = 2 ground states, this electronic configuration has proved difficult to achieve in a molecular species. These challenges may be mitigated within metal-organic frameworks that feature site-isolated iron centres in a constrained, weak-field ligand environment. Here, we show that the metal-organic framework Fe2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) and its magnesium-diluted analogue, Fe0.1Mg1.9(dobdc), are able to activate the C-H bonds of ethane and convert it into ethanol and acetaldehyde using nitrous oxide as the terminal oxidant. Electronic structure calculations indicate that the active oxidant is likely to be a high-spin S = 2 iron(IV)-oxo species.

Determination of the oxidation and coordination state of copper on different Cu-based catalysts by XANES spectroscopy in situ or in operando conditions

The use of XANES spectroscopy, both in classical and in dispersive geometries, is illustrated for the study of copper-based catalysts under in situ or in operando conditions. As case studies, copper-exchanged MFI zeolites and CuCl2/γ-Al2O3 systems are considered. In the former case, in situ XANES spectroscopy was used to characterise well defined complexes (Cu+N2, Cu+(CO)3, Cu+(NH3)(CO) and Cu+(NO)2) formed on copper ions inside the zeolite cavities under controlled conditions. From these results, useful information concerning the symmetry of the formed complexes can readily be gained. The latter case shows how the use of dispersive XANES spectroscopy allows to follow, in real time, the evolution of a system in working conditions. The simultaneous determination of the catalyst activity and of the average oxidation state of copper in the catalyst allows the evolution of a system in working conditions to be followed in real time. The criteria used for the quantification of the Cu(I) and Cu(II) fraction from XANES spectra are discussed in detail.

Probing zeolites by vibrational spectroscopies

This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Brønsted and Lewis acidity and surface basicity are treated in detail. The role of probe molecules and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely determine the IR absorption coefficients is given, making IR a quantitative technique. The thermodynamic parameters of the adsorption process that can be extracted from a variable-temperature IR study are described. Finally, cutting-edge space- and time-resolved experiments are reviewed. All aspects are discussed by reporting relevant examples. When available, the theoretical literature related to the reviewed experimental results is reported to support the interpretation of the vibrational spectra on an atomic level.

Furfuryl alcohol polymerization in H-Y confined spaces: reaction mechanism and structure of carbocationic intermediates.

The acid-catalyzed polymerization and resinification, in the 300-673 K interval, of furfuryl alcohol adsorbed in the framework of a protonic Y zeolite is studied by means of FTIR, Raman, and UV-vis spectroscopies. The idea is that restricted spaces can impose a constraint to the growth of the oligomeric chains, therefore moderating the formation of conjugated sequences responsible for the color of the products and allowing their observation by means of spectroscopic techniques. The detailed study of the evolution of UV-vis, FTIR, and Raman spectra upon dosed amount, contact time, and temperature has allowed the spectroscopic features of some of the single species, either neutral and positively charged (carbocationic intermediates), to be singled out and assigned to understand the mechanism of initiation. The vibrational assignments have been confirmed by computer simulations on model compounds and compared with the results of the mechanistic description of the reaction mechanism made in the past (Choura, et al. Macromolecules 1996, 29, 3839-3850). The spectroscopic methods have been applied in a large temperature range in order to follow also the formation of more complex products into the pores, associated with longer conjugated sequences, gradually filling the open spaces of the zeolite. For samples contacted with furfuryl alcohol at 673 K, this methodology gives information also on the incipient carbonization process, leading to the formation of a carbonaceous replica phase inside the internal porosity of the zeolite.

Cr-MIL-101 encapsulated Keggin phosphotungstic acid as active nanomaterial for catalysing the alcoholysis of styrene oxide

Mesoporous chromium-based terephthalate metal–organic framework (MIL-101) encapsulated Keggin phosphotungstic acid (HPW) [MIL-101(HPW)] was demonstrated to be an active heterogeneous catalyst for selective catalysis of the ring opening reaction of styrene oxide with methanol, achieving 99% yield of 2-methoxy-2-phenylethanol in 20 minutes at 40 °C. Similar MIL-101 samples prepared using one-pot microwave synthesis in the absence of HPW or in the presence of hydrofluoric acid (HF) were less active. The impact of fluoride and HPW polyanion incorporation on the acidity of MIL-101 was investigated by the in situ infrared spectroscopy technique using CO as a probe molecule. Additional hydroxyl groups and Lewis acid sites are present in MIL-101(HPW) explaining the observed superior catalytic performance in styrene oxide methanolysis.

Resonance Raman effects in TS-1: the structure of Ti(IV) species and reactivity towards H2O, NH3 and H2O2: an in situ study

The isomorphous insertion of 1–2 wt% of Ti into the MFI framework leads to a Titanium silicalite-1 (TS-1) material, which is an active and highly selective catalyst in a remarkable number of low-temperature oxidation reactions with aqueous H2O2 as oxidant. Such Ti(IV) species exhibit a local Td-like symmetry, forming [TiO4] units, and induces to the hosting MFI matrix two Ti-specific vibrational modes at 960 and 1125 cm−1. We report a Raman study on the perturbation caused by interaction with H2O, NH3 and H2O/H2O2 on the vibrational modes of the [TiO4] unit embedded in the MFI framework. The selective use of different excitation laser sources in the near-IR (1064 nm; 9398 cm−1), visible (442 nm; 22 625 cm−1), near-UV (325 nm; 30 770 cm−1) and far-UV (244 nm; 40 985 cm−1) allowed us to progressively enter into the oxygen to titanium charge transfer transition and thus to switch on the resonance effects on the 1125 cm−1 mode, which is the only Ti-specific mode exhibiting the same symmetry of the charge transfer transition. Interaction with both water and ammonia causes the formation of [Ti(H2O)2O4] or [Ti(NH3)2O4] complexes which destroy the Td-like symmetry and thus the Raman enhancement of the 1125 cm−1 mode. Upon dosing a H2O/H2O2 to TS-1, the powders turn yellow as a consequence of the appearance a new charge transfer transition around 385 nm (26 000 cm−1). In order to single out the vibrational mode of the active peroxo complex formed on Ti, we have performed Raman experiments using a visible laser source (442 nm; 22 625 cm−1). In these conditions we have observed the strong enhancement of a mode at 618 cm−1, which has been attributed to the symmetric breathing mode of the Ti(O)2 ring.

Response of CPO-27-Ni towards CO, N2 and C2H4

Coordinatively unsaturated Ni(2+) atoms in CPO-27-Ni form linear adducts with molecular nitrogen. The framework responds to the adsorption-modifying vibrational properties and local structure around adsorbing sites. The present paper deals with a fundamental infrared (IR) study of the interaction of gases on a microporous adsorbent metallorganic framework CPO-27-Ni containing, after solvent removal, coordinatively unsaturated Ni(2+) atoms [Dietzel et al., Chem. Commun. 2006, 959]. CO, N(2) and C(2)H(4) have been chosen. Notwithstanding the relative medium (CO and C(2)H(4)) and weak (N(2)) adsorption enthalpies and the low equilibrium pressures adopted (100-10(-3) mbar) the CPO-27-Ni framework responds promptly and reversibly to the adsorption process, modifying significantly both vibrational properties and local structure around Ni(2+) adsorbing sites as determined by a parallel EXAFS investigation locating the N(2) molecule 2.27 +/- 0.03 A apart from Ni(2+). For both N(2) and C(2)H(4), IR spectra have been discussed and carefully compared with literature data. Isosteric heat of adsorption of the Ni(2+)...N(2) complex formation has been evaluated from temperature dependent IR study to be -DeltaH(ads) = 17 kJ mol(-1).

Investigation of Acid Centers in MIL‐53(Al, Ga) for Brønsted‐Type Catalysis: In Situ FTIR and Ab Initio Molecular Modeling

The intrinsic properties of metal-organic frameworks (MOFs) with regards to their isolated centers, regular cavities, dynamic flexibility, and framework polarity make them very attractive for catalysis. However, the general lack of short range characterizations and mechanistic investigations hinders the rational design of MOFs for catalytic applications. Indeed, very little is known on the nature and strength of potential catalytic centers. HKUST-1, for which the Cu paddle-wheel units were characterized as hard Lewis centers, is a rare exception. F rey and co-workers have reported the catalytic activity of two different MIL-100(Fe, Cr) for Friedel–Crafts benzylation. Despite their similar or closely related M3O(H2O)2F(btc)2 structures (M = Fe or Cr, btc = benzene-1,3,5-tricarboxylate), the Fe catalyst shows much higher catalytic activity than its Cr analogue and even surpasses H-BEA and H-Y zeolites. Because the structure possesses different potential catalytic centers, namely redox (Fe O), Brønsted and Lewis acids, a simple relationship between the structure and the activity cannot easily be established. 5] We have recently shown that Zn3(OH)2(bdc) (also called MOF69C), which exhibits m3-OH species, [7] is 100 % shape selective for the tertbutylation of large aromatics. We had anticipated that bridging OH groups, which are present in a number of rod-like 1D inorganic networks, would generate a Brønstedtype acid catalyst. Within this class, the MIL-53 structure, M(OH)(bdc) (M = Al, Cr, Fe, Ga or In, bdc = terephthalate) is probably the most studied (Figure 1). In this case, the inorganic framework is built from infinite chains of corner-sharing MO4(m2-OH)2 octahedra making 1D lozenge-shaped channels able to adsorb guest molecules. The objective of this study is to assess the acid properties of MIL-53(Al) and IM-19 (also called MIL-53(Ga)) for acid catalysis. It deals with the identification of the nature of the intermediate in model Friedel–Crafts alkylations and the characterization of acid centers by combined infrared (IR) and density functional theory (DFT) studies. IM-19 (Vmicro = 0.44 cm 3 g , particle size = 1–10 mm) and MIL53(Al) (Vmicro = 0.57 cm 3 g , particle size = 1–10 mm) were prepared according to optimized procedures from Ref. [12] , [13], and [14], respectively. For both solids, a solvothermal activation in DMF before calcination in air was carried to remove unreacted and occluded H2bdc molecules. The intermediate was soaked in EtOH before calcination in air. Brønsted and Lewis acidities of MIL-53(Al) and IM-19 were evaluated by IR spectroscopy by following the modification of spectroscopic features of CO (Figure 2). Before the measurements, samples were heated at 280 8C in air for 24 h and then at 250 8C under primary vacuum. The IR spectra collected were recorded at 100 K upon decreasing the partial pressures of CO probe molecule. IR peak positions and shifts have been simulated by DFT in order to propose assignments. Theoretical calculations are in very good agreement with experimental data for n(OH) bands: 3709 (th.) and 3704 cm 1 (exp.) for MIL-53(Al) ; and 3663 (th.) and 3669 cm 1 (exp.) for IM-19. Upon CO adsorption (Figure 2), small red shifts appeared in the OH vibration regions. A larger and broader shift is observed for IM-19 (Dn= 50/ 100 cm ) with respect to MIL-53(Al) (Dn(OH) = 30/ 50 cm ). This indicates a higher acidity for IM-19, although mild. Surprisingly, a parallel perturbation in the CO stretching region is not present. In fact, for both MIL-53 (Al) and IM-19, the main absorptions are detected Figure 1. Unit cells optimized by DFT of MIL-53(Al) and IM-19. a) View of the Al-(OH)-Al chains, b) perpendicular view of MIL-53(Al), c) and of IM-19, with representations of the dipolar moments (m) of OH groups (vectors).

Model oxide supported MoS2 HDS catalysts: structure and surface properties

Supported hydrodesulfidation (HDS) MoS2/SiO2, MoS2/γ-Al2O3 and MoS2/MgO catalysts having a model character have been synthesized by using CS2 as the sulfiding agent and deeply investigated by means of several techniques. XRPD, HRTEM, Raman and UV-Vis methods have been applied to obtain information on the morphology and the structure of the catalysts as well as on the vibrational and spectroscopic properties. It is shown that, when compared with HRTEM results, XRPD, Raman and UV-Vis data give realistic information on the stacking degree, on the particle size distribution and on the heterogeneity of supported MoS2 particles on the various supports. (S K-, Mo L3- and K- edges) EXAFS and XANES spectroscopies have been also used to set up the best sulfidation procedure. UV-vis analysis under controlled atmosphere has been performed to understand the presence of sulfur vacancies and the valence state of Mo ions associated with them. To explore the structure of coordinatively unsaturated Mo sites after reducing or sulfiding treatments (with CS2 or, occasionally, with H2S), in situFTIR of adsorbed CO has been performed. It is demonstrated that CO is a sensitive probe for coordinatively unsaturated sites and that the formation of sulfur vacancies on the MoS2 surface upon reduction in pure H2 at 673 K is accompanied by an increase of the coordinative unsaturation and a decrease of the valence state of a fraction of surface Mo cations, mainly located on corner and edge sites. Furthermore, it is demonstrated that this process can be reversed upon interaction with the sulfiding agent and that this reversible behavior is really mimicking some of the elementary acts occurring in the HDS process. The complexity of the IR results suggests that the adopted reduction procedure in pure H2 at 673 K induces the formation of several types of sulfur vacancies, presumably located in different crystallographic positions. It is also concluded that the sulfiding steps are strongly involving the surface of the support and that reductive treatments at high T in H2 are causing sulfur depletion not only from supported MoS2 particles, but also from the supporting phase. The involvement of the support is particularly relevant for Al2O3 and MgO.