Francesco Frigerio

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.

Solvent-free phenyl-C61-butyric acid methyl ester (PCBM) from clathrates: insights for organic photovoltaics from crystal structures and molecular dynamics

The first solvent-free crystal structure of PCBM, an organic semiconductor widely used in solvent-free nanocrystalline films in plastic solar cells, is reported and its relevance to structure-property relationships discussed. The PCBM structure, obtained from o-dichlorobenzene solvates by solvent abstraction, was solved using powder diffraction, demonstrating this possibility for functionalized fullerenes.

Molecular dynamics simulations of the solvent- and thermal history-dependent structure of the PCBM fullerene derivative

The fullerene derivative PCBM ([6,6]phenyl-C61-butyric acid methyl ester) is one of the best electron acceptors used so far in solution-processed organic photovoltaic devices. The reasons for this success depend partly on its favourable electronic properties, partly on its solubility in common organic solvents and plausibly also on the possibility to optimize its structure and morphology by post-deposition treatments (solvent or thermal annealing). The latter feature is still largely a matter of speculation, as experimentally validated structural models of PCBM molecular organization within the devices are still unavailable. This structural characterization is non-trivial, given that poorly ordered PCBM nanocrystals and amorphous domains appear to often coexist in bulk-heterojunction films based on this system. Here we address some of these issues using molecular dynamics (MD) simulations. Our starting points are the only two published PCBM crystal structures, which were obtained by crystallization from oDCB (ortho-dichlorobenzene) and MCB (monochlorobenzene). Both contain guest molecules of the specific solvent. We simulated their thermal behavior, from room temperature up to their apparent melting points. Additional MD simulations involved model crystals obtained by removing solvent molecules from these co-crystal structures. Models that can apply to the amorphous phase or to nanocrystalline samples have been obtained by cooling molten PCBM, after removing the solvent at different stages in the simulation. Their densities are close to the experimental values and they present a well interconnected network of fullerene moieties, where each of them has an average of seven close neighbours available for charge hopping. Pre- and post-melting structural features such as intermolecular pair distribution functions are discussed in the framework of organic solar cell production and host–guest system dynamics.

Dynamic behaviour of azonia-spiro-alkanes within the MOR and MTW zeolite pore structures

The molecular packing stability and the dynamic behaviour of three azonia-spiro-alkanes involved as structure determining agents in the zeolite synthesis has been examined in the frameworks of MOR and MTW. A detailed molecular model has been set up by computer graphics methods and used to perform Monte Carlo packing and molecular dynamics simulations. The energetic stabilisation and molecular mobility have been calculated and compared to previous experimental characterisation data. The results are found useful to discuss the packing preferences of this class of molecules in the zeolite pores under investigation, with the aim of rationalising their templating ability and selectivity.

Characterisation of the Surfactant Shell Stabilising Calcium Carbonate Dispersions in Overbased Detergent Additives: Molecular Modelling and Spin-Probe-ESR Studies

The surfactant shell stabilising the calcium cabonate core in overbased detergent additives of lubricant base oils was characterised by computational and experimental methods, comprising classical force-field based molecular simulations and spin-probe Electron Spin Resonance spectroscopy. An atomistic model is proposed for the detergents micelle structure. The dynamical behaviour observed during diffusion simulations of three nitroxide spin-probe molecules into micelle models could be correlated to their mobility as determined from ESR spectra analysis. The molecular mobility was found to be dependent on the chemical nature of the surfactants in the micelle external shell.