Nilton Rosenbach

Complex Adsorption of Short Linear Alkanes in the Flexible Metal-Organic-Framework MIL-53(Fe)

This investigation is based on a combination of experimental tools completed by a computational approach to deeply characterize the unusual adsorption behavior of the flexible MIL-53(Fe) in the presence of short linear alkanes. In contrast to the aluminum or chromium analogues we previously reported, the iron MIL-53 solid, which initially exhibits a closed structure in the dry state, shows more complex adsorption isotherms with multisteps occurring at pressures that depend on the nature of the alkane. This behavior has been attributed to the existence of four discrete pore openings during the whole adsorption process. Molecular simulations coupled with in situ X-ray powder diffraction were able to uncover these various structural states.

Adsorption of light hydrocarbons in the flexible MIL-53(Cr) and rigid MIL-47(V) metal–organic frameworks: a combination of molecular simulations and microcalorimetry/gravimetry measurements

The adsorption of short linear alkanes has been explored in the highly flexible MIL-53(Cr) porous metal-organic framework by means of molecular simulations based on configurational bias grand canonical Monte Carlo. The unusual shape of the adsorption isotherms with the existence of steps has been successfully modelled by creating a (narrow pore, large pore) phase mixture domain, the composition of which varies with pressure. A further step consisted of combining our computational approach with several experimental tools including microcalorimetry, gravimetry and in situ X-ray diffraction, to fully characterize the adsorption behaviour of the isostructural MIL-47(V) rigid MOF, i.e. the preferential arrangement of each type of alkane inside the pores and the resulting interaction energy. Finally, relationships are established between the adsorption enthalpies and both alkyl chain length and polarisability of the alkanes that can be further utilised to predict the energetics of the adsorption process for longer alkane chains.

Rearrangement, Nucleophilic Substitution, and Halogen Switch Reactions of Alkyl Halides over NaY Zeolite : Formation of the Bicyclobutonium Cation Inside the Zeolite Cavity

Rearrangement and nucleophilic substitution of cyclopropylcarbinyl bromide over NaY and NaY impregnated with NaCl was observed at room temperature. The first-order kinetics are consistent with ionization to the bicyclobutonium cation, followed by internal return of the bromide anion or nucleophilic attack by impregnated NaCl to form cyclopropylcarbinyl, cyclobutyl, and allylcarbinyl chlorides. The product distribution analysis revealed that neither a purely kinetic distribution, similar to what is found in solution, nor the thermodynamic ratio, which favors the allylcarbinyl halide, was observed. Calculations showed that bicyclobutonium and cyclopropylcarbinyl carbocations are minimal over the zeolite structure, and stabilized by hydrogen bonding with the framework structure. A new process of nucleophilic substitution is reported, namely halogen switch, involving alkyl chlorides and bromides of different structures. The reaction occurs inside the zeolite pores, due to the confinement effects and is an additional proof of carbocation formation on zeolites. The results support the idea that zeolites act as solid solvents, permitting ionization and solvation of ionic species.

A DFT Study of the Conversion of CO2 in Dimethylcarbonate Catalyzed by Sn(IV) Alkoxides

Density functional theory (DFT) calculations of intermediates and transition states of the reaction between CO2 and methanol over different R2Sn(OCH3)2 catalysts (R = alkyl, phenyl and halogens) were carried out. The interaction of the CO2 molecule with the tin catalyst was controlled by the entropic term, being disfavored at room temperature and atmospheric pressure. On the other hand, the insertion of the CO2 molecule into the Sn–OCH3 bond is thermodynamic favorable for all the catalysts studied. The computed free-energy of activation varied with the nature of the substituent R. Phenyl groups exhibit the smallest barrier, whereas halogen atoms the highest. Alkyl groups present intermediate barriers. The results are in agreement with recent experimental results that indicated a higher turnover number (TON) for dimethylcarbonate (DMC) formation when Ph2SnO was used as catalyst. The whole mechanistic scheme was then computed for phenyl and methyl as substituents, considering a dimer tin species.