Highly efficient and selective electrophilic and free radical catalytic bromination reactions of simple aromatic compounds in the presence of reusable zeolites

Reactions of mono-substituted aromatics of moderate activity with bromine in the presence of stoichiometric amounts of zeolite NaY proceed in high yield and with high selectivity to the corresponding para-bromo products. The zeolites can easily be regenerated by heating and reused. Similar para-selectivity can be achieved in the case of toluene by use of tert-butyl hypobromite as reagent, zeolite HX as catalyst, and a solvent comprising a mixture of tetrachloromethane and diethyl ether. Radical bromination of ethyl 4-methylbenzoate using bromine in the presence of light is catalysed by various zeolites and affords a high yield of ethyl 4-(bromomethyl)benzoate but with no great improvement in selectivity for monobromination

Insights into CO2 and CH4 Adsorption by Pristine and Aromatic Amine-Modified Periodic Mesoporous Phenylene-Silicas

The synthesis and characterization of pristine and amine-functionalized periodic mesoporous phenylene-silicas having different pore sizes are reported. We explore the potential of these materials for CO2/CH4 separation, by studying the adsorption of pure CO2 and pure CH4 gases. The aminated periodic mesoporous phenylene-silica with the smallest pore size is the best adsorbent to CO2, presenting a Henry's constant of 0.56 mol.kg(-1).bar(-1) at 35 degrees C. However, the corresponding Henry's constant for CH4 is extremely low (0.06 mol.kg(-1).bar(-1) at 35 degrees C). There is a direct correlation between the % of T-SMP(2) silanols species and the values of the Henry's CO, constants, which may be used to theoretically predict experimental CO2 Henry's constants of potential adsorbents

Structure–property studies on the antioxidant activity of flavonoids present in diet

The screening of natural flavonoids for their bioactivity as antioxidants is usually carried out by determinination of their profile as chain-breaking antioxidants, by the evaluation of their direct free radical-scavenging activity as hydrogen- or electron-donating compounds. Since this may not be the only mechanism underlying the antioxidant activity it is important to check the ability of these compounds to act as chelators of transition metal ions. Accordingly, in the present study the acidity constants of catechin and taxifolin, as well as the formation constants of the corresponding copper (II) complexes, were investigated by potentiometry and/or spectrophotometry. Moreover, a detailed quantitative examination of the coordination species formed is presented. In addition, the partition coefficients of both catechin and taxifolin in a biomimetic system (micelles) were determined, since these properties may also contribute to the antioxidant behavior of this type of compound. The log P values determined depend on the electrostatic interactions of the compounds with the differently charged micelles (the highest values were obtained for zwitterionic and cationic micelles). The prooxidant behavior of the compounds was assessed through the oxidation of 2'-deoxyguanosine, induced by a Fenton reaction, catalyzed by copper. The data obtained reveal that the flavonoids under study did not present prooxidant activity, in this particular system. The results obtained are evidence of a clear difference among the pKa, the complexation properties, and the lipophilicity of the flavonoids studied, which can partially explain their distinct antioxidant activity. The most stable geometries of the free compounds were determined by theoretical (ab initio) methods, in order to properly account for the electron correlation effects which occur in these systems, thus allowing a better interpretation of the experimental data

Interaction of CO2 and CH4 with Functionalized Periodic Mesoporous Phenylene-Silica: Periodic DFT Calculations and Gas Adsorption Measurements

Nonfunctionalized and functionalized periodic mesoporous phenylene-silicas (Ph-PMOs) with different kinds of amine groups were prepared and their capacity to uptake CO2 and CH4 molecules were experimentally evaluated considering biogas upgrading. It was found that aminopropyl groups grafted to the free silanols of the Ph-PMO displayed the highest selectivity for CO2 gas, adsorbing 26.1 times more CO2 than CH4 at 25 degrees C. The interaction effect of the surface of these materials with the CO2 or CH4 molecules was obtained through the calculation of the Henry constants, and the adsorption mechanisms involved were elucidated from density functional theory calculations. The good synergy between experimental gas adsorption and computational studies suggests that the latter can be used to guide the experimental synthesis of more effective materials. Thus, our computational studies were extended to PMOs with other functional groups having different polarity for predicting interaction energies with CO2 and thus identifying the most promising candidates for experimental synthesis

Insights into CO₂ and CH₄ Adsorption by Pristine and Aromatic Amine-Modified Periodic Mesoporous Phenylene-Silicas

The synthesis and characterization of pristine and amine-functionalized periodic mesoporous phenylene-silicas having different pore sizes are reported. We explore the potential of these materials for CO₂/CH₄ separation, by studying the adsorption of pure CO₂ and pure CH₄ gases. The aminated periodic mesoporous phenylene-silica with the smallest pore size is the best adsorbent to CO₂, presenting a Henry’s constant of 0.56 mol·kg^–1·bar^–1 at 35 °C. However, the corresponding Henry’s constant for CH₄ is extremely low (0.06 mol·kg^–1·bar^–1 at 35 °C). There is a direct correlation between the % of T²_SMP silanols species and the values of the Henry’s constants, which may be used to theoretically predict experimental CO₂ Henry’s constants of potential adsorbents

Accurate Model for Predicting Adsorption of Olefins and Paraffins on MOFs with Open Metal Sites

Metal–organic frameworks (MOFs) have shown tremendous potential for challenging gas separation applications, an example of which is the separation of olefins from paraffins. Some of the most promising MOFs show enhanced selectivity for the olefins due to the presence of coordinatively unsaturated metal sites, but accurate predictive models for such systems are still lacking. In this paper, we present results of a combined experimental and theoretical study on adsorption of propane, propylene, ethane, and ethylene in CuBTC, a MOF with open metal sites. We first propose a simple procedure to correct for impurities present in real materials, which in most cases makes experimental data from different sources consistent with each other and with molecular simulation results. By applying a novel molecular modeling approach based on a combination of quantum mechanical density functional theory and classical grand canonical Monte Carlo simulations, we are able to achieve excellent predictions of olefin adsorption, in much better agreement with experiment than traditional, mostly empirical, molecular models. Such an improvement in predictive ability relies on a correct representation of the attractive energy of the unsaturated metal for the carbon–carbon double bond present in alkenes. This approach has the potential to be generally applicable to other gas separations that involve specific coordination-type bonds between adsorbates and adsorbents

Interaction of CO₂ and CH₄ with Functionalized Periodic Mesoporous Phenylene–Silica: Periodic DFT Calculations and Gas Adsorption Measurements

Nonfunctionalized and functionalized periodic mesoporous phenylene–silicas (Ph–PMOs) with different kinds of amine groups were prepared and their capacity to uptake CO₂ and CH₄ molecules were experimentally evaluated considering biogas upgrading. It was found that aminopropyl groups grafted to the free silanols of the Ph–PMO displayed the highest selectivity for CO₂ gas, adsorbing 26.1 times more CO₂ than CH₄ at 25 °C. The interaction effect of the surface of these materials with the CO₂ or CH₄ molecules was obtained through the calculation of the Henry constants, and the adsorption mechanisms involved were elucidated from density functional theory calculations. The good synergy between experimental gas adsorption and computational studies suggests that the latter can be used to guide the experimental synthesis of more effective materials. Thus, our computational studies were extended to PMOs with other functional groups having different polarity for predicting interaction energies with CO₂ and thus identifying the most promising candidates for experimental synthesis

Accurate model for predicting adsorption of olefins and paraffins on MOFs with open metal sites

Metal–organic frameworks (MOFs) have shown tremendous potential for challenging gas separation applications, an example of which is the separation of olefins from paraffins. Some of the most promising MOFs show enhanced selectivity for the olefins due to the presence of coordinatively unsaturated metal sites, but accurate predictive models for such systems are still lacking. In this paper, we present results of a combined experimental and theoretical study on adsorption of propane, propylene, ethane, and ethylene in CuBTC, a MOF with open metal sites. We first propose a simple procedure to correct for impurities present in real materials, which in most cases makes experimental data from different sources consistent with each other and with molecular simulation results. By applying a novel molecular modeling approach based on a combination of quantum mechanical density functional theory and classical grand canonical Monte Carlo simulations, we are able to achieve excellent predictions of olefin adsorption, in much better agreement with experiment than traditional, mostly empirical, molecular models. Such an improvement in predictive ability relies on a correct representation of the attractive energy of the unsaturated metal for the carbon–carbon double bond present in alkenes. This approach has the potential to be generally applicable to other gas separations that involve specific coordination-type bonds between adsorbates and adsorbents