F.X. Llabrés i Xamena

Engineering Metal Organic Frameworks for Heterogeneous Catalysis

2.2. MOFs with Metal Active Sites 4614 2.2.1. Early Studies 4614 2.2.2. Hydrogenation Reactions 4618 2.2.3. Oxidation of Organic Substrates 4620 2.2.4. CO Oxidation to CO2 4626 2.2.5. Phototocatalysis by MOFs 4627 2.2.6. Carbonyl Cyanosilylation 4630 2.2.7. Hydrodesulfurization 4631 2.2.8. Other Reactions 4632 2.3. MOFs with Reactive Functional Groups 4634 2.4. MOFs as Host Matrices or Nanometric Reaction Cavities 4636

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.

MOFs as multifunctional catalysts: One-pot synthesis of menthol from citronellal over a bifunctional MIL-101 catalyst

A bifunctional MOF catalyst containing coordinatively unsaturated Cr(3+) sites and palladium nanoparticles (Pd@MIL-101) has been used for the cyclization of citronellal to isopulegol and for the one-pot tandem isomerization/hydrogenation of citronellal to menthol. The MOF was found to be stable under the reaction conditions used, and the results obtained indicate that the performance of this bifunctional solid catalyst is comparable with other state-of-the-art materials for the tandem reaction: Full citronellal conversion was attained over Pd@MIL-101 in 18 h, with 86% selectivity to menthols and a diastereoselectivity of 81% to the desired (-)-menthol, while up to 30 h were necessary for attaining similar values over Ir/H-beta under analogous reaction conditions.

Sol-gel method for preparing high surface area CoAl2O4 and Al2O3-CoAl2O4 spinels

Abstract Cobalt aluminate, CoAl 2 O 4 , and Al 2 O 3 –CoAl 2 O 4 solid solutions (at Co:Al=1:4, and Co:Al=1:8) were prepared by hydrolysis of mixed metal alkoxides, followed by mild calcination of the resulting gels. Powder X-ray diffraction showed that all samples prepared were single phase materials having the spinel-type structure. The cubic lattice parameter, a 0 , was found to change gradually with aluminium content of the mixed metal oxides. Nitrogen adsorption–desorption isotherms, at 77 K, were used to determine surface area and pore texture. The BET surface area was found to be 235 m 2 g −1 for CoAl 2 O 4 , and higher values (up to 365 m 2 g −1 ) were found for the Al 2 O 3 –CoAl 2 O 4 solid solutions. Samples having the same composition were also prepared by the classical ceramic method (starting from mechanical mixtures of the parent metal oxides); for these materials the corresponding surface areas were found to be in the range 1.7–3.3 m 2 g −1 . The sol–gel method, starting from mixed metal alkoxides, was thus shown to be a convenient route to prepare cobalt aluminate spinels in a very high surface area form.

An in situ temperature dependent IR, EPR and high resolution XANES study on the NO/Cu+–ZSM-5 interaction

We report an in situ IR, EPR, and high resolution XANES study on the Cu+(NO) and Cu+(NO)2 complexes formed at 80 K in Cu+–ZSM-5. Bare Cu2+ ions (<1%) exhibit an axial EPR spectrum, while bare Cu+ ions exhibit well resolved 1s→4pxy and 1s→4pz transitions (pxy/pz splitting of 3.1 eV), reflecting the same local symmetry. Cu+(NO) complex exhibits an anisotropic EPR signal, while XANES spectrum of dinitrosyl adducts presents three resolved components ascribed to the 1s→4px, 1s→4py, and 1s→4pz transitions (px/py and py/pz splitting of 3.8 and 2.6 eV, respectively) testifying the rupture of the axial symmetry of Cu+ bare cations upon NO adsorption. Upon increasing the temperature to 300 K, NO oxidizes a fraction of 75–85% of the cuprous ions.

Selective aerobic oxidation of activated alkanes with MOFs and their use for epoxidation of olefins with oxygen in a tandem reaction

MOFs with Cu2+ centers linked to four nitrogen atoms from azaheterocyclic compounds, i.e., pyrimidine [Cu(2-pymo)2] and imidazole [Cu(im)2], are active catalysts for aerobic oxidation of activated alkanes, such as tetralin, cumene and ethylbenzene. Differences in activity among the two MOFs appear to be related to differences in their ability to decompose the hydroperoxide and to coordinate to the resulting radical ˙OH species. Copper ions in [Cu(im)2] can coordinate by expanding their coordination sphere from 4 to 5 in a reversible way, while in the case of [Cu(2-pymo)2] it results in a displacement of one of the pyrimidine ligands. The MOFs can be used in combination with a silylated Ti-MCM-41 to catalyze the epoxidation of olefins with oxygen by means of a tandem reaction in which the MOF produces cumene hydroperoxide, which is used by Ti-MCM-41 to epoxidize the olefin.

FTIR spectroscopy of carbon dioxide adsorbed on sodium- and magnesium-exchanged ETS-10 molecular sieves

The adsorption of carbon dioxide on sodium- and magnesium-exchanged ETS-10 molecular sieves was investigated by FTIR spectroscopy. CO2 was found to adsorb reversibly onto the charge-balancing cations with formation of linear end-on adducts of the type M⋯OCO (with M=Na+ or Mg2+). Such adducts are characterised by intense ν3 bands at 2353 and 2365 cm−1, for CO2 interacting with Na+ and Mg2+, respectively. More interestingly, carbon dioxide was also found to form different carbonate-like species upon adsorption on the Na-ETS-10 (and, to a minor extent, also on Mg-ETS-10), which are the origin of the activity of ETS-10 in heterogeneous base-catalysed processes.

The combined use of acetonitrile and adamantane–carbonitrile as IR spectroscopic probes to discriminate between external and internal surfaces of medium pore zeolites

Abstract The interaction of acetonitrile and adamantane–carbonitrile with hydroxy groups of zeolites causes changes in the OH stretching mode that render the zeolite surface IR-readable. Acetonitrile can penetrate inside the channels of medium-pore zeolites, while adamantane–carbonitrile (due to its larger molecular size) cannot. By using both IR probe molecules, discrimination between external and internal OH groups can be accomplished. The method is demonstrated by studying the interaction of the probe molecules with an H-ZSM-5 zeolite sample. Within the limits of experimental error, bridged Si(OH)Al groups (Bronsted acid sites) were all found to be located at the internal surface of the zeolite, while SiOH groups (silanols) were found to be mainly at external surfaces.

Vibrational spectroscopy of carbon monoxide and dinitrogen adsorbed on magnesium‐exchanged ETS‐10 molecular sieve

Carbon monoxide and dinitrogen adsorbed, at nominally liquid‐nitrogen temperature, on Mg2+‐exchanged ETS‐10 were found to form Mg2+...CO and Mg2+...N2 adducts involving cations located in the main channels (12‐membered rings) of the titanosilicate molecular sieve. These adducts gave main IR absorption bands at 2190 and 2203 cm-1 for CO, and at 2336 and 2339 cm-1 for N2, which were assigned to the fundamental stretching mode of the diatomic molecules polarized by the electric field created by Mg2+ ions. Corresponding adducts with Na+ and K+ ions, not exchanged with Mg2+, were also present. The results, which are relevant to the potential use of ETS‐10 as a catalyst, lend support to previous structural studies suggesting the existence of two different cation sites along the main channel of ETS‐10.

Free radical activity of natural and heat treated amphibole asbestos

The amphibole minerals amosite and crocidolite were subjected to calcination and to hydrothermal treatment in order to study the effect of these heat treatments on the ability of the minerals to trigger formation of free radicals, which is known to be a main factor causing asbestosis and other asbestos-induced diseases. Free radical activity of the natural and heat treated minerals was studied by using supercoiled DNA (pUC18 plasmid) as a target molecule, and also by means of EPR spectroscopy. It was shown that after calcination of the natural minerals at 1073 K their free radical activity was strongly decreased These results, which may have relevant consequences for asbestos technology, were correlated with concomitant alteration of the structure and surface chemistry of the minerals during calcination.