Roberto Monaci

Activity and deactivation of Fe-MFI catalysts for benzene hydroxylation to phenol by N2O

Abstract Isomorphously substituted Fe-MFI zeolite catalysts with various Si/Al and/or Si/Fe ratios were synthesized and characterized by many different techniques, such as ICP, XRD, SEM, TPR, microcalorimetry, FTIR, and EPR. Under standard reaction conditions the best catalyst gave 20% benzene conversion and over 90% selectivity to phenol. For Fe-ZSM5 catalysts, addition of steam to the feed improved catalyst activity, selectivity, and durability. Phenol formed onto Fe-based sites only. Active sites could very likely be composed of oxygen-bridged, extraframework binuclear Fe redox species, charge-compensating the framework Fe 3+ or Al 3+ ions. Surface acidity was not responsible for activity in the main reaction, but it was heavily involved in catalyst deactivation by coking. Catalyst deactivation derived mainly from the decomposition-condensation of phenol onto acid sites; the stronger the latter, the quicker was the coking rate.

Oxidative dehydrogenation of propane over V2O5/TiO2/SiO2 catalysts obtained by grafting titanium and vanadium alkoxides on silica

The oxidative dehydrogenation (ODH) of propane have been studied on three different vanadium oxide catalysts, containing comparable amounts of vanadium. All the proven catalysts have been prepared by grafting but following different procedures. One has been prepared by grafting vanadyl tri-isopropoxide, dissolved in n-hexane on a support of silica coated with a multi-layer of TiO2. The support has been prepared by grafting in three different steps titanium alkoxide on silica. Another catalyst has been prepared by partially hydrolysing vanadyl tri-isopropoxide, dissolved in isopropanol, before grafting the obtained product on the same support. The third catalyst has been prepared by reacting partially hydrolysed vanadyl tri-isopropoxide with titanium alkoxide in isopropanol and anchoring then the reaction product, a vanadium–titanium bimetallic alkoxide, directly on silica. The first and second catalysts have similar activities and selectivities, while the third catalyst is less active but more selective than the other two ones. A kinetic approach has been made and a pseudo-first order kinetic law has been used to interpret the results. All the observed catalytic phenomena have been interpreted also with the aid of the several used characterisation techniques.

Thermodynamic properties of aqueous non-electrolyte mixtures. Alkanols + + water systems.

Abstract The molar excess enthalpies, H E , at 298.15 K, and the liquid-liquid equilibrium, LLE, of aqueous mixtures of alkanols have been measured. The results are interpreted qualitatively in terms of intercomponent molecular interactions and hydrophobic stabilization of the water structure by the alkanol molecules. The results obtained indicate that the geometry, the number of alkyl groups in the aliphatic chain and the relative position of the functional group -OH have an important action on these two effects.

4-Methylpentan-2-ol dehydration over zirconia catalysts prepared by sol-gel

Zirconia samples have been prepared from xerogels and aerogels obtained using zirconium n-propoxide as precursor. Structure and texture have been investigated by X-ray diffraction, thermal analysis, transmission electron microscopy, nitrogen adsorption/desorption. Surface acidity and basicity have been assessed by adsorption microcalorimetry, using ammonia and carbon dioxide as probe molecules. 4-Methylpentan-2-ol dehydration has been tested at atmospheric pressure in a fixed-bed flow microreactor. The xerogel gives tetragonal zirconia upon calcination, during which a mesoporous system is formed. The crystal phase depends on the presence of oxygen during the cooling step in the case of the aerogel, whose texture is partially retained upon calcination. Both kinds of catalysts have well-balanced concentrations of acid and base sites, but the acid sites are weaker in comparison with the basic ones. At 603 K the initial conversion of 4-methylpentan-2-ol over the calcined xerogel and aerogel is 45 and 63%, respectively; the selectivity to 4-methylpent-1-ene is 77% for both. The occurrence of an E2-like mechanism with the activated complex having a marked carbanionic character seems probable. The aerogel catalyst is quite stable during operation, whereas changes in activity and selectivity are observed for the xerogel catalyst.

CeO2–La2O3 catalytic system Part I. Preparation and characterisation of catalysts

The CeO2–La2O3 catalytic system was prepared by a sol–gel technique from inorganic precursors in the presence or absence of H2O2 . Samples were characterised by means of nitrogen physisorption, TG-DTA, XRD, IR, XPS and TEM. Preparation ia sol–gel in the presence of H2O2 produces certain interesting modifications in CeO2–La2O3, both structurally and morphologically. Higher surface areas have been found for the CeO2–La2O3 system prepared in the presence of H2O2. On the other hand, for La2O3 the structure obtained upon calcination at 873 K corresponds to La(OH)3, and for the system prepared with hydrogen peroxide, to a mixture of La(OH)3, and La2O3 which was detected by XRD. Interesting morphological differences were observed for both systems.

Temperature-programmed desorption study of ammonia desorption-diffusion in molecular sieves: II. Application to partially decationated Y-zeolites

Abstract An analysis of TPD data relating to the desorption of ammonia from two different samples of partially decationated Y-zeolite is reported, assuming various chemical or physical steps to be rate-determining. The process is shown to be controlled by intracrystalline surface diffusion, with values of the apparent activation energy E a = 30.8 and 27.1 kcal/mol, respectively, for the two zeolite samples. These values include the retardation effect associated with the hindering interaction of the diffusing ammonia molecules with the acid sites of the zeolite. Values of the apparent effective diffusion coefficient ranging from ca. 3 10 −18 to ca. 4 10 −14 cm 2 /s have been calculated for the 423–573 K temperature range.

Acid sites investigation of simple and mixed oxides by TPD and microcalorimetric techniques

The present work deals with the acid properties of silica, alumina and two mixed oxides obtained by grafting silica on alumina (SA sample) or alumina on silica (AS sample). The surface acidity of the samples, of both Lewis and Bronsted type, was determined by means of microcalorimetry and temperature programmed desorption (TPD), using pyridine and 2,6-dimethylpyridine as probe molecules. Both techniques point out that the grafted mixed oxides SA and AS have acidic properties different from those of the pure alumina and silica starting supports. Some differences exist, in terms of acid sites strength distribution, between TPD and calorimetric results, which can be reasonably attributed to the different activating conditions of the samples.

Dehydration of 4-methylpentan-2-ol over lanthanum and cerium oxides

Lanthanum and cerium oxides have been tested for the title reaction at 623 K and atmospheric pressure in a flow reactor. Lanthanum oxide (prepared from the corresponding nitrate) gives mainly 4-methylpent-1-ene (80% of the products). Similar results are observed with cerium oxide obtained from the corresponding hydroxide, whereas cerium oxide prepared from nitrate is less selective towards alk-1-enes. In addition to dehydration, dehydrogenation to 4-methylpentan-2-one is also observed to a limited extent for all the catalysts. Information on the acid–base properties of the samples was obtained by adsorption microcalorimetry of ammonia and carbon dioxide and correlated to reaction selectivities. Possible changes in the oxidation state of cerium ions due to the reaction atmosphere are considered. The present results are compared with former data for zirconia catalysts. Modification of cerium oxide via immersion in NaOH solution does not appear to be useful for improving alk-1-ene selectivity.

Synthesis and characterization of MCM-22 zeolites for the N2O oxidation of benzene to phenol

MCM-22 zeolites with various Si/Al and/or Si/Fe ratios were synthesized and characterized by XRD, microcalorimetry and other techniques. The catalytic activity and selectivity of MCM-22 zeolites were investigated in the gas-phase oxidation of benzene with N2O at 673 K. The oxidation of benzene produces phenol as main product. The influence of the nature of catalysts (i.e., the content of iron, the nature of active sites) on catalytic activity of MCM-22 was investigated. In addition, some results about the nature of coke, which causes the Fe-MCM-22 deactivation, are exposed.

CeO2–La2O3 catalytic system. Part II. Acid–base properties and catalytic activity for 4-methylpentan-2-ol dehydration

CeO2–La2O3 mixed oxides, formerly prepared ia a sol–gel procedure and characterised by several techniques, have been further investigated as to their acid–base properties and catalytic activity. Surface acidity and basicity have been assessed by adsorption microcalorimetry, using ammonia and carbon dioxide as probe molecules. Catalytic activity for 4-methylpentan-2-ol dehydration has been tested at atmospheric pressure in a fixed-bed flow microreactor. The acid and base features of the catalysts markedly depend on the relative amounts of the two component oxides. For pure ceria, cerium and oxygen ions are the source for acidity and basicity, respectively. Some contribution of OH groups to the acid–base character is probable for the mixed oxides. Most of the acid sites of each catalyst are weak, whereas most of its base sites are strong. The concentration of the base sites tends to prevail over that of the acid sites as the lanthanum content increases. 4-Methylpent-1-ene is by far the most abundant product of 4-methylpentan-2-ol conversion for all the catalysts, which are quite stable, even after repeated operation cycles. Subtle differences in the relative extents of dehydration and dehydrogenation activity seem to be due to a shift in the reaction mechanism, originated by changes in the relative amounts of acid and base sites.