Tracing the acetalization of cyclohexanone in CO2-expanded alcohols by attenuated total reflection infrared spectroscopy.

The CO(2)-catalyzed acetalization is regarded as a promising alternative to the conventional acid-catalyzed method from a viewpoint of green chemistry (C. A. Eckert et al., Ind. Eng. Chem. Res. 43, 2605 (2004)). We have applied in situ attenuated total reflection infrared (ATR-IR) spectroscopy for elucidating and monitoring the acetalization of cyclohexanone in CO(2)-expanded ethylene glycol and methanol at 50 degrees C and 3 MPa. The ATR-IR spectra of the reaction mixtures periodically recorded with a ZnSe crystal demonstrate that ATR-IR spectroscopy is a practical tool for tracing the kinetics of acetalizations in situ. In addition, the rate of CO(2) dissolution as well as CO(2) solubility into the cyclohexanone-alcohol mixtures could be evaluated from the CO(2)-nu(3)-antisymmetric stretching band. The ZnSe ATR crystal, however, was corroded during longer use under the acidic conditions realized by the dissolution of CO(2) in the alcohols. In contrast, the corrosion did not occur when a Ge crystal was used instead of a ZnSe crystal, and therefore the application of a Ge ATR crystal is recommended for continuous long-term experiments with these media.

In situ diffuse reflectance infrared Fourier transform spectroscopy of MCM-41 mesoporous silica: mechanistic consideration on the chemical fixation of CO2 with N,N'-dimethylethylenediamine to 1,3-dimethyl-2-imidazolidinone.

In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy has been applied to evaluate the thermally activated MCM-41 mesoporous silica as a catalyst for CO(2) chemical fixation using N,N'-dimethylethylenediamine (DMEDA), and to elucidate the adsorption states of DMEDA, CO(2) and H(2)O on the activated silica surface at room temperature or 300 degrees C and at a pressure close to the atmospheric one. The thermal activation at 500 degrees C under a flow of dry air led to a slight loss of the hydrogen-bonded vicinal and a part of geminal silanols as well as physically adsorbed water, whereas the free isolated and most of the geminal silanols were not influenced, possibly due to the absence of neighboring silanols for condensation to form water and the unfavorable dehydration of geminal silanols. Respective adsorptions of Lewis acidic CO(2) and basic DMEDA revealed that the free silanols are rather basic, whereas the hydrogen-bonded silanols act as Brønsted acid sites. Treatment of the activated silica with a CO(2) flow containing DMEDA at 300 degrees C revealed that the diamine does not form the corresponding DMEDA-CO(2) adducts such as carbamic acids and carbamic acid salts under sufficiently CO(2)-diluted conditions. In addition, DMEDA was virtually unable to interact with the silanols under these conditions, due to the severe constraints imposed on these silanols by the surrounding dense CO(2) molecules. Based on these results, two plausible mechanisms were proposed for the CO(2) chemical fixation with DMEDA to 1,3-dimethyl-2-imidazolidinone (DMI): one involves the reaction of free DMEDA with the adsorbed CO(2) interacting with the surface silanols, while the other involves the formation of a cyclic carbamic acid salt which subsequently undergoes dehydration over the silica to give DMI. Finally, the spectroscopy revealed that H(2)O, a byproduct of the DMI formation, is smoothly extracted from the surface at 300 degrees C under gaseous CO(2) flow, and does not influence the parent surface structure of the silica under the conditions applied.

Mesoporous silica-catalysed continuous chemical fixation of CO(2) with N,N'-dimethylethylenediamine in supercritical CO(2): the efficient synthesis of 1,3-dimethyl-2-imidazolidinone.

MCM-41- and HMS-type mesoporous silicas were found to be efficient catalysts for the continuous chemical fixation of CO(2) into 1,3-dimethyl-2-imidazolidinone with N,N'-dimethylethylenediamine in supercritical CO(2).

In situ attenuated total reflection infrared spectroscopy of imidazolium-based room-temperature ionic liquids under "supercritical" CO(2).

In situ high-pressure attenuated total reflection infrared (ATR-IR) spectroscopy has been applied to elucidate the molecular interactions between dissolved CO(2) and three different imidazolium-based room-temperature ionic liquids, 1-n-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF(4)], 1-n-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF(6)], and 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [bmim][Tf(2)N], and also for 1-n-butylpyridinium tetrafluoroborate [bpy][BF(4)] swollen under "supercritical" CO(2) conditions (50 degrees C, 12.0 MPa). The results show that cation species as well as anion species of the ionic liquids affect the molecular state of the dissolved CO(2), because the Lewis acid-base interaction between the ionic-liquid anion species and CO(2) generates new anion species [X-CO(2)](-) (X = anion species of ionic liquid) which are more basic than X(-). Good correlation was found between the extent of imidazolium-ring C-H stretching band shifts and that of the anion species B-F, P-F, C-F, and SO stretching band shifts after the dissolution of CO(2). Largest shift of the ring C-H bands as well as the B-F band was observed for [bmim][BF(4)], indicating that the newly formed anion species [BF(4)-CO(2)](-) is most basic, which seems to be related to the higher activity of [bmim][BF(4)] compared to [bmim][PF(6)] for the catalytic cycloaddition of CO(2) to propylene oxide. Thus, the spectroscopy was applied also for the [bmim][BF(4)]-propylene oxide-CO(2) system (3.0 MPa, room temperature and 80 degrees C) to evaluate the reactivity of [BF(4)-CO(2)](-) and to acquire mechanistic information on the cycloaddition. Based on the spectroscopic results, a new plausible catalytic cycle was proposed, in which [BF(4)-CO(2)](-) species first attacks the electrophilic epoxide carbon, followed by cyclization of the corresponding intermediate to give propylene carbonate product as well as [bmim][BF(4)]. On the other hand, the shift of B-F stretching band of [bpy][BF(4)] under identical supercritical-CO(2) conditions was less drastic compared to [bmim][BF(4)], indicating that the Lewis acid-base interaction between BF(4)(-) and the dissolved CO(2) can be tuned by the choice of the cation species of the ionic liquid. Finally, the studies revealed that the strong Lewis acid-base interaction between the ionic liquids and the dissolved CO(2) has no promotional effect on the solubility of CO(2), because [bmim][Tf(2)N] which showed very small shifts of CF(3) and SO(2) bands and no imidazolium ring C-H band shifts after the dissolution of CO(2) exhibited much higher solubilizing power for CO(2) than [bmim][BF(4)]. The solubility of CO(2) in the ionic liquids seems to be determined by other factors such as the affinity of fluorine atoms for CO(2) and free volumes.

Continuous catalytic "one-pot" multi-step synthesis of 2-ethylhexanal from crotonaldehyde.

2-ethylhexanal is synthesized directly from crotonaldehyde over a bifunctional acidic resin-supported palladium catalyst via hydrogenation and aldol condensation in supercritical CO2.

Sulfated mesoporous alumina: a highly effective solid strong base catalyst for the Tishchenko reaction in supercritical carbon dioxide.

Heterogeneous strong base catalysis for the intramolecular Tishchenko reaction of aromatic 1,2-dicarbaldehydes to the corresponding phthalides in supercritical CO2CscCO2 has been realized with mesoporous alumina containing SO4(2-) ions in the alumina framework (mesoAl2O3/SO4(2-)). Infrared spectroscopy of pyrrole adsorbed on the alumina and strong poisoning by a weak Brönsted acid of methanol revealed that the SO4(2-) ions in the framework slightly suppressed the average strength of base sites (O2-) on mesoAl2O3/SO4(2-), but there exists a small number of strong base sites that promote the Tishchenko reaction in scCO2. Although the intramolecular Tishchenko reaction of phthalaldehyde to phthalide in scCO2 was somewhat slower than those in organic solvents such as tetrahydrofuran (THF) and benzene, the addition of a small amount of THF as a cosolvent remarkably increased the reaction rate; the reaction in the scCO2-THF system proceeded 1.5-fold faster than those in pure benzene and THF solvents.