Roberto Millini

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.

Layered structure of ERB-1 microporous borosilicate precursor and its intercalation properties towards polar molecules

Abstract As-synthesized ERB-1 borosilicate has a bi-dimensional layered structure, and the three-dimensional (3D) microporous structure is formed after calcination at 270°C. X-Ray powder diffraction (XRD), temperature-programmed desorption (TPD) and 29 Si magic-angle spinning nuclear magnetic resonance (MAS NMR) analyses demonstrated that the formation of the 3D ordered structure occurs through the condensation of silanol groups present on the surfaces of the layers. As-synthesized ERB-1 displays intercalation properties towards polar molecules such as ethylene glycol, isopropanol and [2,2,2]-azabicyclooctane.

Experimental and computational study of beta, ZSM-12, Y, mordenite and ERB-1 in cumene synthesis

The catalytic performance of zeolite β in the liquid phase alkylation of benzene with propylene is compared with that of other zeolites: MOR, ERB-1, USY, MTW. The comparison is carried out both by catalytic tests and by computational evaluation of the energy barriers for diffusion of the product molecules and of the binding energies (BEs) of the same molecules, inside the zeolite pores. Among the zeolites studied, zeolite β appears to be one of the most efficient catalysts, producing the smallest amount of propylene oligomers and of n-propylbenzene. The energy diffusion barriers and the BEs, both calculated for cumene and DIPB isomers account for the outstanding performance of zeolite β.

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.

Stability of Ti in MFI and Beta structures : a comparative study

Abstract The structural stabilities of TS-1 (Ti-MFI) and Ti-Beta zeolites after exposure to aqueous H 2 O 2 (30% w/w) have been examined by X-ray diffraction and spectroscopic methods (UV–vis, IR and 29 Si MAS NMR). The catalytic properties of these molecular sieves are dependent on the stability of Ti(IV) in the framework. TS-1 was confirmed to be stable towards H 2 O 2 solution, in agreement with its well-known catalytic applications (e.g. hydroxylation, epoxidation, ammoximation). Ti-Beta is not stable. Physical studies confirm the partial hydrolysis of titanium during H 2 O 2 treatments with formation of highly defective situations and Ti movement to extraframework positions during the following calcination. Hence, dramatically different catalytic behaviors and lifetimes are expected for these two catalysts.

A priori selection of shape-selective zeolite catalysts for the synthesis of 2,6-dimethylnaphthalene

Modeling tools based on molecular mechanics and molecular dynamics were used for selecting shape-selective zeolite catalysts for the synthesis of 2,6-dimethylnapthalene (2,6-DMN) through the alkylation of naphthalene (NAPH) or via isomerization of other DMN isomers. A number of medium- (MFI and EUO) and large-pore zeolites (∗BEA, MOR, MAZ, FAU, LTL, OFF, and MTW) were considered and for each of them the minimum energy pathways for the diffusion of naphthalene, 1- and 2-methylnapthalene (MNs), and 1,5-, 1,6-, 2,6-, and 2,7-dimethylnapthalene (DMNs) were computed. The results of the simulations indicated that the diffusion of MNs and DMNs isomers in the medium-pore zeolites is impeded by high-energy barriers, leading to the conclusion that this kind of structure can be used neither in the isomerization nor in the alkylation reaction. In contrast, large-pore zeolites are more promising though their behavior strongly depends on the effective size of the pore openings. Among them, MTW was predicted to be the most promising candidate for the selective alkylation of NAPH to 2,6-DMN. In fact, the simulations indicated high-energy diffusion barriers not only for molecules bearing a CH3 group in the α-position but also for the undesired 2,7-DMN molecule. Catalytic tests, performed in the presence of 1,2,4-trimethylbenzene as a solvent, confirmed the prediction since MTW gave the highest 2,6-DMN yields with a 2,6-/2,7-DMN ratio in the range 2.0–2.6, well above the thermodynamic value of ≈1 obtained with the other zeolites. The good catalytic performances of MTW were explained by the fact that, unique among the large-pore zeolites considered, this zeolite showed a better stabilization of the 1,1-diarylmethane intermediate molecules leading to 2-MN, 2,6-DMN, and 2,7-DMN. Their formation can be considered more probable than for those deriving from the electrophilic attack of the benzyl carbocation in the α-position of the naphthalene ring.

Proton exchange and isomerisation reactions of photochromic and reverse photochromic spiro-pyrans and their merocyanine forms

Photochromic 1′,3′,3′-trimethyl-6,8-dinitro-spiro- [2H-1-benzopyran-2,2-indoline], (6,8-dinitro BIPS) has been studied in different solvents using 1H-NMR techniques, UV/visible spectroscopy, X-ray diffraction and electrospray ionisation mass spectroscopy (ESI-MS). Molar decadic absorption coefficients for 6,8-dinitro BIPS were found to range between 35000–45000 d mol-1 cm-1. 1H–1H NOESY and 1H-NMR experiments have established that in solution the merocyanine form of 6,8-dinitro BIPS is in rapid exchange between the trans-trans-cis (TTC) and trans-trans-trans (TTT) isomers. The 3B proton on the central bridging double bond of spiro-pyran merocyanines is labile in organic solvents. The closed form of 6,8-dinitro BIPS enantiomerises about the spiro C(2) carbon at a rate of 18 s-1 at 300 K. The activation energy for this process was estimated to be 46 kJ mol-1 using dynamic NMR.

Metal Substitution in Keggin-Type Tridecameric Aluminum-Oxo-Hydroxy Clusters

The species resulting from a typical preparation for metal-substituted hybrids of the Keggin tridecamer, Al13 or [AlO4Al12(OH)24(OH2)12]7+, were examined by performing 27Al NMR on the solutions during aging and by studying the precipitated sulfate salts via solid state 27Al NMR and powder X-ray diffraction (XRD). Aqueous mixtures (0.25 mol L-1) of AlCl3 and another metal ion (M), in a 12:1 mole ratio (Al:M), where M = Fe3+, Zn2+, Ga3+, In3+, Sn2+, La3+, and Bi3+, were subjected to forced hydrolysis by addition of NaOH (1.0 mol L-1) until OH/(Al + M) = 2.25, and the kinetics of Al13 formation and disappearance with aging at 80 °C was monitored by 27Al NMR spectroscopy. Al13 units polymerize on aging with an apparent rate constant (k) of 4.8(8) × 10-2 h-1 to form a species referred to as AlP2. Only the solutions containing Ga3+ and Sn2+ exhibited faster Al13 conversion rates. GaAl12 forms quickly at 80 °C (k = 0.54 h-1) and is more stable than AlP2. Sn2+ apparently promotes AlP2 formation (k = 0.38 h-1). XRD and solid state NMR reveal that only the Ga hybrid can be prepared by this method. No hybrid formation was evidenced using M = Mg2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, In3+, La3+, or Ce3+ at 25 °C or M = Co2+ or La3+ under reflux conditions. Isostructural (cubic symmetry) single crystals were obtained for the sulfate salts of Al13 and GaAl12. Single-crystal XRD analysis of these two polyoxocations provides the first rigorous comparison between them and shows they have very similar structures. The main crystallographic data for Al13 and GaAl12 are as follows:  Na[AlO4Al12(OH)24(H2O)12](SO4)4·10H2O, cubic, F4̄3m, a = 17.856(2) Å, Z = 4; Na[GaO4Al12(OH)24(H2O)12](SO4)4·10H2O, cubic, F4̄3m, a = 17.869(3) Å, Z = 4. Thus, the greater thermal stability of GaAl12 cannot be rationalized in terms of the overall geometric considerations, as suggested by others. Solid state NMR also shows the coordination symmetries of the outer 12 Al nuclei in both clusters to be similar.

Hierarchical hybrid organic-inorganic materials with tunable textural properties obtained using zeolitic-layered precursor.

Novel layered zeolitic organic-inorganic materials have been synthesized using a two-dimensional zeolite precursor IPC-1P prepared by a top-down approach from zeolite UTL. The formation of porous materials containing organic linkers or polyhedral oligomeric siloxane covalently bonded to zeolite layers in the interlayer space was confirmed by a variety of characterization techniques (N2/Ar sorption analysis, XRD, (29)Si and (13)C NMR, TEM). The organic-inorganic porous hybrids obtained by intercalation with silsesquioxane posessed layered morphology and contained large crystalline domains. The hybrids exhibited mesoporous or hierarchical micro-/mesoporous systems, stable up to 350 °C. Textural properties of the formed zeolitic organic-inorganic materials can be controlled by varying the linker or synthetic conditions over a broad range. Surface areas and pore volumes of synthesized hybrids significantly exceed those for parent zeolite UTL and corresponding swollen material; the amount of micropores increased with increasing rigidity and size of the organic linker in the order biphenyl > phenylene > ethanediyl.

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.