Silica-Promoted Diels−Alder Reactions in Carbon Dioxide from Gaseous to Supercritical Conditions

Abstract

Amorphous fumed silica (SiO2) was shown to increase yields and selectivities of several Diels−Alder reactions in gaseous and supercritical CO2. Pressure effects on the Diels−Alder reaction were explored using methyl vinyl ketone and penta-1,3-diene at 80 °C. The selectivity of the reaction was not affected by pressure/density. As pressure was increased, the yield decreased. At the reaction temperature, adsorption isotherms at various pressures were obtained for the reactants and the Diels−Alder adduct. As expected when pressure is increased, the ratio of the amount of reactants adsorbed to the amount of reactants in the fluid phase decreases, thus causing the yield to decrease. The Langmuir adsorption model fit the adsorption data. The Langmuir equilibrium partitioning constants all decreased with increasing pressure. The effect of temperature on adsorption was experimentally determined and traditional heats of adsorption were calculated. However, since supercritical CO2 is a highly compressible fluid, it is logical to examine the effect of temperature at constant density. In this case, entropies of adsorption were obtained. The thermodynamic properties that influence the real enthalpy and entropy of adsorption were derived. Methods of doping the silica and improving yields and selectivities were also explored.

A Study of the Diastereoselectivity of Diels-Alder Reactions on the Ce-SiO2as Support

“…Previous theories to account for the promotion of reactions on the surface of silica were developed for processes taking place in the absence of solvent. 8 However, the catalytic mechanism in this work is proposed by the adsorption and coordination of Lewis acid site of cerium ion with the carbonyl of methylacrylate and probably hydrogen bonding or protonation of the dienophile by acidic SiOH groups on the silica surface and then the DA reaction carries out with facility. The amount of cerium ion loaded on silica was determined by ICP method.…”

“…This effect decreases the energy level of the LUMO of dienophiles and allows more efficiently overlapping with HOMO of the diene. 8 Therefore, the reaction proceeds with great ease. According to the Pearson's HSAB classification, trivalent state of rare earths which located between Mg 2+ and Ti 4+ are hard acids.…”

Density‐Dependent Formation of the Pure trans‐Isomer of the Unsaturated Alcohol by Selective Hydrogenation of Citral in Supercritical Carbon Dioxide

“…Density-dependent adsorption of CO 2 with different solutes on the catalyst surface is a well known phenomenon. [17][18][19] We are aware that the density of the supercritical medium changes substantially either by changing the pressure or temperature. For example, a change in pressure from 4 to 14 MPa the density changes from 1.6181 to 10.406 mol/L (NIST Chem web book) and the solubility of the citral increases.…”

A multiphase reaction medium including pressurized carbon dioxide and water for selective hydrogenation of benzonitrile with a Pd/Al2O3 catalyst

“…The authors mentioned that the primary amines were protected by CO 2 and so their further hydrogenation to the secondary amines was effectively suppressed. Note, in addition, that the pressurization with CO 2 is also effective for promoting the rate of Heck coupling reactions and modifying the yield and/or the selectivity in Diels-Alder reactions in which no gaseous reactants are involved [11][12][13].…”