G. Leofanti

Surface area and pore texture of catalysts

Abstract The origin, the classification and the effect on catalytic performance of pore texture of heterogeneous catalysts are briefly examined. The techniques and the methods suitable for the determination of related properties (surface area, pore volume and pore size distribution) are reviewed. Particular attention is paid to the most widely used ones: vapour adsorption at low temperature, mercury porosimetry, incipient wetness impregnation and picnometry. Advantages, disadvantages and applicability of each method to different pore textures are pointed out. The best choice of a group of methods, allowing a complete characterization of catalysts, is proposed.

Spectroscopic Characterization of Silicalite and Titanium-Silicalite

Surface characterization of silicalite and titanium-silicalite is made by combined use of IR and UV-Vis reflectance spectroscopy. The nature of the IR modes and of the electronic transitions associated with framework titanium is discussed in detail. The perturbation caused by the adsorption of small polar molecules (H 2 O, NH 3 , CH 3 OH) on both the IR and UV-Vis spectra is discussed in terms of the formation of six-coordinated complexes by ligand insertion in the Ti(IV) coordination sphere. All the Ti(IV) is accessible to the interaction with the molecules.

DRS UV-VIS AND EPR SPECTROSCOPY OF HYDROPEROXO AND SUPEROXO COMPLEXES IN TITANIUM SILICALITE

The most important spectroscopic features in the UV-Vis of framework and extraframework Ti(IV) in titanium silicalite are briefly summarized. The spectroscopic manifestations of the complexes formed by framework Ti(IV) in presence of H2O2 are reported. The formation of EPR active species is also considered.

FOURIER-TRANSFORM INFRARED AND RAMAN SPECTRA OF PURE AND AL-, B-, TI- AND FE-SUBSTITUTED SILICALITES : STRETCHING-MODE REGION

A systematic investigation of the IR and Raman spectra of pure and Al-, B-, Ti- and Fe-substituted silicalites in the SiO stretching region (1500–700 cm–1) is presented. As well as the characteristic stretching modes of the skeleton, silicalites containing hydroxylated nests show also broad bands at ca. 960 cm–1(IR) and at ca. 976 cm–1(Raman), associated with O3Si—OH group modes, with prevailing Si—OH stretching character. The replacement of Si with heavier elements (like Ti or Fe) causes the appearance of new IR- and Raman-active modes (i) at 960 cm–1(IR and Raman) and at 1127 cm–1(Raman) in Ti silicalite; (ii) at 1015 cm–1(IR) and at 1020 cm–1(Raman) in Fe silicalite. Neither the Raman nor the IR spectra of the skeletal modes are substantially modified by the introduction of Al (ZSM5). The presence of boron induces the appearance in the IR spectra of a complex absorption at 1380 cm–1 and at 960–930 cm–1, which corresponds to the absorptions at 1417 and 976 cm–1 in the Raman spectra. The assignment of the absorptions associated with heteroatoms and hydroxylated nests is discussed in detail.

Structural characterization of Ti centres in Ti-silicalite and reaction mechanisms in cyclohexanone ammoximation

Abstract The main results obtained by means of many physical methods (IR, Raman, UV-Vis and XAFS spectroscopies) concerning the structure of the Ti centre in titanium silicalite and the reaction intermediates in the ammoximation of cyclohexanone are concisely reviewed. The Ti is in tetrahedral coordination in vacuo and expands its coordination sphere upon interaction with adsorbates. In the presence of H2O and H2O/H2O2 solutions one of the SiOTi bridges is hydrolyzed with formation of (ue5fcSiO)3L2TiOH (Lue5fcH2O) and (ue5fcSiO)3L2TiOOH species, respectively. When NH3 is dosed on (ue5fcSiO)3L2TiOOH structures (ue5fcSiO)3L2TiOO−NH4+ species are formed. These species are thought to play an important role in the ammoximation reaction. The geometries of the peroxidic species (open or bridged) are discussed also on the basis of ab initio calculations.

Framework and Extraframework Ti in Titanium-Silicalite: Investigation by Means of Physical Methods

Abstract The application of UV-Vis diffuse reflectance, IR transmission, Raman and ESR spectroscopies to the determination of extraframework and framework Ti in Titanium-silicalite is discussed in detail. The four methods give complementary information. In particular the utility of the UV-Vis and Raman spectroscopies to detect extraframework Ti is definitely established, while IR proves to be more useful for framework Ti. ESR spectroscopy can be also utilized on reduced samples to distinguish between the two types of Ti.

Deactivation Phenomena on Ti-Silicalite

Abstract The stability and deactivation behavior of Ti-silicalite employed as a catalyst in the ammoximation of cycloexanone to cycloexanonoxime have been investigated. The reaction was performed in liquid phase using H2O2 as an oxidizing agent. The solid was characterized by various techniques: chemical analysis, thermogravimetry, IR and UV-Vis spectroscopy, XRD, gas adsorption and a Standard Activity Test. Three main deactivation processes are identified: (i) slow dissolution of the framework with accumulation of Ti on the external surface of the remaining solid; (ii) direct removal of Ti from the framework and (iii) pore filling by by-products.

Toluene ethylation on ZSM zeolites

Abstract The activity and selectivity to para -ethyltoluene in toluene alkylation with ethanol have been correlated to the structural and morphological features of the ZSM-5 and ZSM-11 zeolites employed as catalysts. In particular, as the particle size increases and correspondingly the surface area outside the zeolite channels decreases, lower activities and higher para -isomer selectivities (up to 90%) are obtained. This behavior has been interpreted by supposing a two-step mechanism: in the first step, para -isomer forms by toluene alkylation with high selectivity inside the zeolitic channels (shape selectivity), while in the second step the isomerization toward the thermodynamic equilibrium of the three isomers proceeds just on the external surface,

Cyclohexanone Ammoximation: A Break Through In The 6-Caprolactam Production Process

As a part of a research devoted to selective oxidation of organic compounds, we have studied the synthesis of cyclohexanonoxime by direct ammonia oxidation in presence of cyclohexanone. By using titanium silicalite as catalyst the reaction took place in a smooth and selective way. The effect of the most important experimental conditions was investigated. The role of the catalyst and some possible reaction mechanisms were discussed. Our results provided a promising starting point for the development of a new technology for cyclohexanonoxime synthesis.

Dispersion measurement by the single introduction method coupled with the back-sorption procedure: A chemisorption and TPD study of the different chemisorbed hydrogen species: II. Pd on Alumina

Abstract In this work a comparison is made between values of the metal surface areas of several alumina-supported Ru, Rh, and Pt catalysts obtained by hydrogen adsorption isotherms, here defined as the “Multiple Introductions” method, and those calculated by a very simple technique defined as the “Single Introduction” method. With the systematic application of the procedure designated “Back-Sorption,” particularly good agreement is observed between the two techniques if dispersion is calculated on the basis of the irreversibly adsorbed hydrogen. “Back-Sorption” is a necessary practice because of the presence on the above-mentioned metals of more than one adsorbed species not to be considered for the calculation of dispersion, as shown in a more detailed manner for a Pt Al 2 O 3 catalyst using temperature-programmed desorption. Whenever the conditions for its applicability are satisfied, the Single Introduction method provides a faster and more reliable way to measure the dispersion of metal-supported catalysts than the Multiple Introductions technique.