A. Corma

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

Infrared spectroscopic investigation of titanium in zeolites. A new assignment of the 960 cm–1 band

The formation of Si–O–defect groups due to the incorporation of Ti in the framework of zeolites is evidenced by means of solid-state tetraethylammonium (TEA) cation exchange; a new assignment for the infrared (IR) band in both β-Ti and TS-1 is presented.

The role of different types of acid site in the cracking of alkanes on zeolite catalysts

Abstract The cracking of n -heptane on a rare-earth (RE)HY zeolite has been studied in the temperature range 400–470 °C. By measuring the initial selectivities with respect to the various reaction products, and carrying out molecular orbital calculations it has been found that cracking can initially take place on Bronsted acid sites via protolytic cracking, or on Lewis acid sites via a classical β-scission mechanism. The activation energy for protolytic cracking is lower than for β-scission both for n -heptane and for the iso-heptanes. From energetic considerations, a scheme of cracking can be suggested where the cracking proceeds by hydride transfer and β-scission mainly via branched heptanes.

Isomerization and disproportionation of m-xylene : Selectivities Induced by the Void Structure of the Zeolite Framework

Abstract The conversion of m-xylene was performed on acid zeolites with ten- or twelve-membered rings, such as ZSM-5, ZSM-12, ZSM-48, OFF, MOR, FAU, Ω, β and L. All of these zeolites have distinctly different crystallographic structures with different shapes and dimensions of the intracrystalline cavities. This diversity of pore structures allowed to determine to what extent the pore shape and dimensions influence the selectivity of the reaction of m-xylene. The p/o selectivity in the isomerization of m-xylene is always high for ten-membered ring zeolites with crystals bigger than 1 μm. The moderate selectivity for p-xylene formation in some zeolites with twelve-membered rings is due to transition-state shape selectivity. The selectivity for disproportionation versus isomerization is very low for ten-membered ring zeolites and allows them to be distinguished easily from the twelve-membered ring zeolites, which show much more disproportionation. In the group of twelve-membered ring zeolites the disproportionation/isomerization selectivity does not correlate at all with the effective size of the intracrystalline cavities. The disproportionation/ isomerization selectivity is determined by the strong isolated acid sites which are present in cavities that allow the formation of at least one of the possible bimolecular transition-state complexes for disproportionation. The distribution of the trimethylbenzene isomers reflects important features of the shape of the pores, the presence and the position of larger spaces in lobes, cages or intersections adjacent to the twelve-membered ring channels of large pore zeolites.

Influence of preparation conditions on the structure and catalytic properties of SO42−/ZrO2 superacid catalysts

Abstract SO42−/ZrO2 samples were prepared by impregnation of hydrous zirconium oxides with 0.5M H2SO4 solutions. It was found that the pH of precipitation of the precursor Zr(OH)4 has a strong influence on the structure of crystalline ZrO2 and on the catalytic properties of the SO42−/ZrO2 for the isomerization of n-butane. The nature and strength of acid sites in SO42−/ZrO2; samples were studied by infrared spectra of absorbed pyridine and by temperature-programmed desorption (TPD) of ammonia. Acidity and catalytic results have been compared with those obtained on an acid zeolite as H-Mordenite. It has been found that infrared spectra of pyridine can not be used for the determination of the superacid character of SO42−/ZrO2. However, the NH3-TPD technique shown that in the case of SO42−/ZrO2 samples there are superacid sites in which the NH3-desorption peak appears at about 815 K. A correlation between the number of those superacid sites and the activity for the isomerization of n-butane has been observed.

Preparation and properties of Ti-containing MCM-41

Publisher Summary The new policies on environment preservation are directing the chemical processes to not only avoid the use of unfriendly reactants and catalysts but also to find more selective catalysts that minimize product waste and/or expensive separations and recycling. Zeolites doped with V and Zn can selectively oxidize n-butane to maleic anhydride. Pd/Cu–Y zeolites can replace the homogeneous Pd and Cu salts for the oxidation of short-chain olefins to carbonyl compounds using molecular oxygen. Metal phthalocyanine complexes entrapped in zeolites have also shown great activity and selectivity for the oxidation of alkanes, cycloalkanes, olefins, and alcohols using iodosobenzene and air as oxidants. An important advance in oxidation catalyst was given when Ti and V containing zeolites were shown to be active and selective to carry out the oxidation of different hydrocarbons with H 2 O 2 . Thus, the isomorphous substitution of Si by Ti and/or V into the framework of MFI and MEL zeolite structures leads to materials that show good activity and selectivity for the oxidation of phenol to catechol and hydroquinone, olefins to epoxides, and alkanes to alcohol and ketones using H 2 O 2 at relatively low temperatures. However, in the field of fine chemicals, it is sometimes required to oxidize large molecules that cannot penetrate in the narrow pores of the MFI structure.

Synthesis of an ultralarge pore titanium silicate isomorphous to MCM-41 and its application as a catalyst for selective oxidation of hydrocarbons

An ultralarge pore titanium silicate with MCM-41 structure has been prepared by direct hydrothermal synthesis; this material gives rise to useful catalysts for the selective oxidation of small and large organic compounds.

Characterization of acid surfaces by adsorption of 2,6-dimethylpyridine

Abstract 2,6-Dimethylpyridine has been adsorbed on different samples of γ-Al 2 O 3 , fluorinated γ-Al 2 O 3 , and decationated Y zeolites. The ir spectra of the adsorbed base have been obtained after desorption at different temperatures. It has been shown that the 2,6-dimethylpyridine cannot differentiate among different types of Lewis sites (Na + , tricoordinated aluminum, AlO + , etc.). The 1618-cm −1 absorption band appearing in γ-Al 2 O 3 is related to weak Bronsted sites, and its frequency increases when the acid strength is increased by fluorination. This band has not been observed on NaHY zeolites for which the Bronsted acid strength is higher.

The nature of acid sites on fluorinated γ-Al2O3

Abstract A study has been made of the effect of fluorination on the surface of γ-Al 2 O 3 , with reference to the influence of the amount of fluorine on the type of acid sites, their strength, and their number. The acidity has been measured by adsorption-desorption of pyridine, 2,6-dimethylpyridine, and ammonia, and by butylamine titration. It has been found that fluorinated γ-Al 2 O 3 presents both Bronsted and Lewis sites. The total number of Lewis acid sites decreases when the fluorine content increases, whereas the number of sites with strong acidity exhibits a maximum for samples with 2–4% of fluorine content. Also the number of Bronsted acid sites presents a maximum for the same fluorine content, despite the fact that only a small fraction of the sites created by fluorination exhibit strong acidity.

Isobutane/2-butene alkylation on ultrastable Y zeolites: Influence of zeolite unit cell size

The alkylation reaction of isobutane with trans-2-butene has been carried out on a series of steam-dealuminated Y zeolites with unit cell sizes ranging from 2.450 to 2.426 nm. A fixed-bed reactor connected to an automatized multiloop sampling system allowed us to make differential product analysis from very short (1 min or less) to longer times on stream. A maximum in the initial 2-butene conversion was found on samples with unit cell sizes between 2.435 and 2.450 nm. However, the TMPDMH ratio, i.e., the alkylation-to-oligomerization ratio, continuously increased with zeolite unit cell size. The concentration of reactants in the pores, the strength distribution of Bronsted acid sites, and the extent of hydrogen transfer reactions, which in turn depend on the framework SiAl ratio of a given zeolite, were seen to affect activity and product distribution of the catalysts. Finally, the influence of these factors on the aging characteristics of the samples was also discussed.