Angela Carati

Reactions of titanium silicalite with protic molecules and hydrogen peroxide

The properties of titanium silicalite (TS-1) in reactions with water, hydrogen peroxide, and alcohol molecules have been studied by means of IR and NMR spectroscopy, and acid activity tests. An IR absorption is shown by TS-1 in the region 900–975 cm−1. Its exact position depends on the amount of adsorbed water, on exchange reactions with 17O- and 18O-labeled water, and on the average size of the crystallites. In 17O MAS NMR spectra a peak is shown at σ 360 ppm from H2 17O (σ = 0). IR and NMR spectra are discussed and interpreted in terms of Ti sites as constituted by tetrahedral [TiO4] units. TiOSi bonds react with water at near room temperature, producing TiOH and SiOH species. Reactions with hydrogen peroxide produce acidic derivatives, which catalyze the hydrolysis of cis and trans 2,3-epoxybutane in methanol, ethanol, and aqueous media. A five-membered ring hydrogen bonded structure, in which a titanium hydroperoxo moiety, TiOOH, and a protic molecule, ROH (R = H, CH3, C2H5), are involved, is proposed for these species. Their possible role in selective oxidations of hydrocarbons is discussed.

Production of titanium containing molecular sieves and their application in catalysis

Abstract This paper reviews several important issues related with industrial application of catalysts based on titanium silicalite-1 (TS-1). The catalyst preparation has been discussed, especially considering the most critical parameters for industrial application. Two processes already demonstrated on industrial scale (phenol hydroxylation and cyclohexanone ammoximation), and a process, under development by several industrial groups (propylene epoxidation), have been reported and discussed. The possibility to insert titanium in large pore zeolites, in order to widen the application field of this kind of materials, has been considered and the behaviour of several new materials has been reported and compared with the catalytic properties of TS-1.

Mesoporous silica-aluminas as catalysts for the alkylation of aromatic hydrocarbons with olefins

Abstract Two amorphous mesoporous silica-aluminas (MSA and MCM-41) are compared with the amorphous microporous silica-alumina (ERS-8) and the zeolite beta in the liquid-phase alkylation of toluene with propylene. The catalytic activity, the selectivity to o-, m- and p-cymenes and the formation of polyalkylates are discussed as a function of acidity and porosity. The catalytic behaviour of MSA, MCM-41, ERS-8 and zeolite beta is also compared with data reported in the literature for traditional alkylation catalysts (AlCl3–HCl and supported phosphoric acid, SPA) and for other acidic zeolites. MSA and MCM-41 show alkylation activities comparable with zeolite beta, while their isomerization and transalkylation activities are lower than those of zeolite beta and AlCl3–HCl, but similar to SPA. Such behaviour is in agreement with the Bronsted acidity of the materials studied.

New trends in the synthesis of crystalline microporous materials

Zeolites and related microporous materials form the most important class of microporous solids due to their practical importance in different technological areas. In spite of the extensive research carried out in previous decades, it is surprising to realize that there is still space for innovation in this area, making microporous crystalline solids special among all classes of materials known today. We illustrate here the most recent advances in the field, focusing on the research topics that, in our opinion, are most likely to provide results of practical interest. Three main topics are discussed: (1) the synthesis of new framework topologies, with particular attention to those having extra-large and/or multidimensional pore systems; (2) the modification of the morphological and textural properties of known zeolites, including the discussion on two-dimensional structures and on the synthesis of nanocrystals and of the hierarchical porous structures; (3) the still poorly explored field of silica-based hybrid organic–inorganic porous crystalline materials (hybrid zeolites). For each of these topics, a selection of the most relevant results reported in the literature is provided together with some considerations on the potentialities and future perspectives.

Synthesis and comparative characterization of Al, B, Ga, and Fe containing Nu-1-type zeolitic framework

We synthesized four different Nu-1-type zeolites containing, respectively, Al, B, Ga, and Fe as the trivalent element. With the exception of B-Nu-1, the synthesis is difficult because of the competitive formation of other crystalline phases as sodalite, ZSM-39-type, or glassy phases. The trivalent elements isomorphous substitution was demonstrated by X-ray powder diffraction, FTi.r., e.s.r., u.v.-Vis., and 27Al, 11B, and 71Ga-MAS-n.m.r. By the same techniques and t.g.a. measurements, we evaluated the thermal stability of the different Nu-1-type zeolites.

Double Substitution in Silicalite by Direct Synthesis: A New Route to Crystalline Porous Bifunctional Catalysts

Abstract We have synthesized silicalites containing both Ti and a trivalent element (Al, Ga, Fe) in lattice position. Syntheses are similar to that of TS-1. As we add increasing amounts of a trivalent metal nitrate to the reaction mixture, we obtain silicalites containing both titanium and the other metal. Titanium content decreases with increasing content of the trivalent metal. Physico-chemical analyses (e.g., XRD, FT-IR, EPR, MAS-NMR, exchange capacity) and oxidizing and acidic catalytic activities confirmed isomorphous substitution of both elements.

Structural characterization of as-synthesized B- and Ti-containing MFI-type molecular sieves☆

The influence of tetrapropylammonium ( TPA) ions on the structural properties of as-synthesized Ti- (TS-1) and B-containing (BOR-C ) MFI zeolites crystallized from alkali-free gels is investigated. Thermogravimetric analysis shows that the negative charge of the BOR-C framework is compensated by the TPA ions, while only ‘soft’ interactions between TPA and Ti sites are observed. Rietveld analysis of the synchrotron X-ray powder diVraction patterns reveals that incorporation of Ti and B causes an expansion and a contraction of the MFI framework, respectively. Regression curves describing the unit cell parameters variation can be used to determine the real framework composition. The presence of TPA ions inside the pores strongly influences the apparent d TiMO and d BMO tetrahedral bond distances, which are shorter than those in the calcined forms. The mobility of the TPA cations in silicalite-1, TS-1 and BOR-C samples with the highest Ti and B content is studied by 13C NMR and molecular dynamics simulations. As expected, the TPA cation mobility increases as the pore size increases (BOR-C

Synthesis and characterization of borosilicates with the EUO framework topology

Abstract B-EUO, a borosilicate with the EUO framework topology was successfully synthesized by using N,N,N,N′,N′,N′-hexamethyl-1,6-hexanediammonium (hexamethonium) as a structure-directing agent. The effective incorporation of B in the EUO framework was confirmed by 11 B MAS NMR, TG and XRD analyses. Up to ∼8 B atoms/unit cell were incorporated in the EUO framework, which corresponds to the number of ammonium groups accommodated in the pores. EUO was obtained also in the ternary B–Al–Si system, with the Al quantitatively incorporated into the framework. A structural model was derived for the as-synthesized EUO sample, with the position of the hexamethonium ion determined with the Quench Dynamics protocol, a procedure that combines high-temperature molecular dynamics and energy minimization techniques. The model was successfully employed for refining the experimental XRD profiles by the Rietveld method.

Ethylation of ethylbenzene over some aluminosilicates with molecular sieve structure

Abstract A β-zeolite, a ZSM-5 and an XRD-amorphous alumino silicate were prepared by the usual hydrother- mal technique in the presence of templating agents. The solids so obtained were characterised by various techniques and tested as catalysts for the title reaction, under typical industrial reaction conditions (T=180 to 260°C, P=35 bar, ethylbenzene WHSV =5 h -1 ). For none of the three catalysts was the structure and the pore size of the solid shown to play a significant role in directing the selectivity towards a particular diethylbenzene isomer. A major role appeared to be played by the concentration of Brensted acid surface sites and by reaction temperature, the distribution of diethylbenzene isomers being gov- erned by either kinetic or thermodynamic factors, rather than by shape-selective restraints.

Ring Opening of Methylcyclohexane over Platinum‐Loaded Zeolites

The activity of different platinum-loaded zeolites (Mordenite, ZSM-12, ZSM-5, ZSM-23) was investigated in the hydroconversion of methylcyclohexane (MCH), in the context of upgrading highly aromatic distillates for fuel blending. In all cases, conversion of MCH proceeds according to a pathway where the primary products are a mixture of dimethylcyclopentanes and ethylcyclopentane formed by isomerization and ring contraction of MCH. The primary products undergo consecutive ring-opening reactions with formation of n- and isoheptanes. The latter further react to form lower-molecular-weight n- and isoalkanes. The selectivity and distribution of products deriving from ring-contraction and ring-opening reactions are strongly affected by the pore size and topology of the zeolites. ZSM-5 exhibits a strong reactant shape-selectivity effect on ring-opening products. The evaluated zeolites show the following order of activity in the conversion of methylcyclohexane: Mordenite>ZSM-12>ZSM-5>ZSM-23.