Elisabetta Rombi

Liquid-phase alkylation of naphthalene by isopropanol over zeolites. Part II : Beta zeolites

The title reaction has been studied on various HY-based catalysts. Surface acidity has been evaluated by adsorption microcalorimetry and FTIR analysis, using pyridine as probe molecule. Catalytic tests were carried out at 623 K and 40 bar in liquid phase, with decalin as solvent, in flow reactor. Isopropylation is accompanied by oligomerisation/cracking of the reactant alcohol, cracking of the solvent, isomerization of the products of the main reaction. Pore filling by coke is fast for non-dealuminated HY, where naphthalene conversion remains high due to the occurrence of transalkylation between polyisopropylnaphthalenes trapped in the pores and naphthalene; this does not occur on dealuminated HY, owing to the low density of the acid sites and the presence of a secondary mesoporous system which allows easy diffusion of the reaction products in the zeolite pores.

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.

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.

MeOx/SBA-15 (Me = Zn, Fe): highly efficient nanosorbents for mid-temperature H2S removal

Zinc oxide/ and iron oxide/SBA-15 composites were synthesized using the innovative Two-Solvents procedure and tested as sorbents for the mid-temperature (300 °C) removal of hydrogen sulphide, and then compared with a commercial unsupported ZnO sorbent. The sulphur retention capacity results showed the superior performance of the iron oxide/SBA-15 composite (401 mg S g−1 Fe2O3) in comparison with the zinc oxide/SBA-15 composite (53 mg S g−1 ZnO), both these sorbents being much more efficient than the commercial sorbent (6 mg S g−1 ZnO). The different sorption behaviour was discussed in terms of the nature of the nanocomposites where: (i) the mesostructure of the support was retained with a high surface area and pore volume; (ii) the zinc oxide phase was incorporated inside the SBA-15 channels as a thin amorphous homogeneous layer while the iron oxide was dispersed in form of small maghemite crystallites; and (iii) significant interactions occurred between the silica matrix and the zinc oxide phase. Remarkable differences in the regeneration behaviour of the exhaust sorbents were revealed by temperature-programmed experiments under an oxidizing atmosphere. After regeneration, the sorption properties of the zinc oxide/SBA-15 composite appeared to be enhanced compared to the commercial sorbent. Incomplete recovery of the sorption activity was observed for the regenerated iron oxide/SBA-15 sorbent, whose performance remained far better than that of the ZnO-based one, either fresh or regenerated. In view of its higher sulphur retention capacity and appropriate regeneration temperature (T ≤ 350 °C), the iron oxide/SBA-15 composite is a promising material for the design of advanced sorbents for a thermally efficient H2S removal process from hot gas streams.

Adsorption of Pentafluorophenol onto Powdered, Granular, and Cloth Activated Carbons

In the present study, the adsorption of pentafluorophenol from aqueous solution onto granular activated carbon, powdered activated carbon, and activated carbon cloth has been investigated from the equilibrium and kinetic points of view. To the best of our knowledge, the removal of pentafluorophenol from aqueous solution onto activated carbon have not been reported in the literature. The experimental equilibrium data, suitably fitted by the Toth, Langmuir-Freundlich, and Redlich-Peterson isotherms, have shown that the cloth and powdered carbons exhibit the highest adsorption capacity. For all the investigated samples, the experimental kinetic data have been satisfactorily interpreted by a pseudo-second-order model. From the fitting results it has been observed that the highest rate constant and initial rate of adsorption is shown by powdered activated carbon.

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.

Evidence of transalkylation during liquid-phase isopropylation of naphthalene

The alkylation of naphthalene with isopropanol was investigated over a HY (Si/Al=4) zeolite under the following conditions: flow reactor, liquid phase with decalin as solvent, 623 K, 40 bar. Despite complete filling of the zeolite pores by coke during the first hour of reaction, the rate of formation of isopropylnaphthalene remained very high. This is due to the development of a new reaction process involving transalkylation between the polyisopropyl naphthalenes entrapped in the pores and naphthalene followed by the realkylation of the former compounds with isopropanol.

FTIR and EPR characterisation of copper-exchanged mordenites and beta zeolites

Sodium mordenites and beta zeolites, partially exchanged with Cu 2+ , were characterised by means of Fourier transform infrared (FTIR) and both continuous-wave and pulse electron paramagnetic resonance (EPR) spectroscopies, in order to shed light on the catalytic role of copper ions in the ammoxidation of 1-methylnaphthalene. FTIR analysis showed that ammonia interacts with both Bronsted and Lewis acid sites, while 1-methylnaphthalene is mainly coordinated through π-complexes. The acidity of the catalysts was also determined through FTIR analysis of the adsorbed pyridine. The main changes in the coordination sphere of Cu 2+ ions were observed by EPR analysis after ammonia adsorption and after catalyst use, while adsorption of 1-methylnaphthalene had a very minor effect. Primary electron spin echo envelope modulation spectra after catalytic use showed the presence of organic radicals, having a relaxation time of longer than the Cu 2+ ions. The latter are confirmed as the active reaction centres and should be located in the side pockets of the mordenites, and be unaccessible to the aromatic system of 1-methylnaphthalene. This explains the higher selectivity observed with mordenites. In the more acidic beta zeolites the presence of copper ions on the walls of large channels initially favours side reactions, but also makes adsorbed ammonia more available for ammoxidation.