Lewis Acids: From Conventional Homogeneous to Green Homogeneous and Heterogeneous Catalysis

Hot compressed water (HCW, here water above 200 °C) possesses very interesting properties This series of articles gives an overview of the state of the art as regards the understanding of reactions in HCW In the first part the macroscopic and microscopic properties of HCW will be described, followed by a summary of synthesis reactions published The impact of the unique properties shall be discussed in part II, with the thermal degradation of tert-butylbenzene and the oxidation of methanol being used as examples The studies reported will show that the microscopic properties are of more importance to understanding reactions in HCW than assumed in the past

Hot compressed water as reaction medium and reactant properties and synthesis reactions

The effects of TiO2 (anatase TiO2 or rutile TiO2) and ZrO2 (monoclinic/tetragonal mixture ZrO2) on glucose and fructose reactions were examined in hot-compressed water at 473 K with a batch type reactor. Rutile TiO2 (r-TiO2) is inactive during glucose reactions. A monoclinic/tetragonal mixture of ZrO2 (m/c-ZrO2) was the basic catalyst for the reaction at this temperature. Anatase TiO2 (a-TiO2) showed both acidity and basicity during the reactions. In order to understand the catalytic activity of a-TiO2, r-TiO2, and m/c-ZrO2, we measured the acidity and basicity by means of NH3- and CO2-TPD, respectively. The TPD analysis showed us that the amount of acid (670 μmol/g) and base (550 μmol/g) sites on m/c-ZrO2 were the highest among these three catalysts, while the density of acid site (17 μmol/m2) and that of basic site (9 μmol/m2) of the a-TiO2 were the highest. Such the acidity and basicity analyses suggested that the amount of basicity was the key factor for the isomerization while the density of acidity and basicity was important for the HMF formation from glucose.

Catalytic glucose and fructose conversions with TiO2 and ZrO2 in water at 473 K: Relationship between reactivity and acid–base property determined by TPD measurement

Hot compressed water (HCW, here water above 200 °C) owns interesting properties The impact of the unique properties is discussed exemplary for the thermal degradation of tert-butylbenzene and the oxidation of methanol Both reactions have been conducted not only in HCW but also in other high-pressure media and from the comparison the impact of the special properties of HCW can be settled In addition the degradation of glycerol, a model substance for carbohydrates and biomass in HCW was studied This reaction shows a strong dependence on the properties of HCW The examples picture an increased specific impact of HCW with rising polarity of the reactants and intermediates The studies also points to higher importance of microscopic properties for understanding reactions in HCW than assumed in the past

Hot compressed water as reaction medium and reactant. 2. Degradation reactions

Chemical reactions of organic compounds in supercritical water gasification and oxidation.

We carried out a catalytic hydration of propylene with MoO3/Al2O3 in sub- and supercritical water. MoO3/Al2O3 catalyst promoted hydration of propylene to form 2-propanol as a sole product and exhibited stronger activity than Al2O3, which is known to function as an acid catalyst. It was also experimentally found that the reaction temperature near the critical temperature of water gave a maximum conversion of propylene. The rate of this catalytic hydration could be expressed as a function of both the reaction temperature and the ion product of water, implying that the catalytic activity of the acid sites in MoO3/Al2O3 is strongly affected by the concentration of H+ in the bulk phase.

Catalytic hydration of propylene with MoO3/Al2O3 in supercritical water

A kinetic analysis was made for the phenol disappearance rate in catalytic oxidation of phenol over MnO{sub 2} in supercritical water at a fixed temperature of 425 C and pressures between 22.7 and 27.2 MPa. The nonsupported MnO{sub 2} catalyst possessed a strong activity for promoting phenol oxidation, though the overall reaction rate was appreciably influenced by internal mass-transfer resistance. From the kinetic analysis on the reaction rate of the phenol disappearance, the global rate expression of the surface reaction was obtained, where the reaction orders with respect to phenol, oxygen, and water were almost unity, 0.7, and {minus}2.0, respectively. A Langmuir-type mechanism, in which phenol and oxygen adsorbed on the catalytic sites and water adsorbed on the same site to inhibit the phenol and oxygen adsorption, was proposed to explain the reaction orders for phenol, oxygen, and water.

Kinetics of the catalytic oxidation of phenol over manganese oxide in supercritical water

We have examined the stability of catalytic activity and the effect of supercritical water on the phase and structural properties of MnO2 through a catalytic supercritical water oxidation of phenol. From the experimental result, the conversion of phenol slightly declined during the first 6 h. It is suggested by X-ray diffraction analysis that the change of the activity of the MnO2 catalyst is caused by the transformation of the crystal structure to the Mn2O3 catalyst in supercritical water. In addition, it is found that the lattice oxygen in the catalyst could be involved in the oxidation and play a role as an oxidizer. The oxygen supplied to the oxidation system functions to compensate for the used lattice oxygen as well as the reactant as the adsorbed oxygen on the catalyst.

Stability of manganese oxide in catalytic supercritical water oxidation of phenol

Acid catalyzed reactions of 1-octene on TiO2 in sub- and supercritical water were investigated (T = 250–450 °C, P = 11–33 MPa). The main products were 2-octene and 2-octanol. Additionally, other liner C8 alkenes and liner secondary C8 alcohols were produced as by-products. Through kinetic analysis, acid catalyzed reactions can divide into the reaction catalyzed by Lewis acidic sites on TiO2 and the reaction catalyzed by protons produced by the dissociation of water molecules. Each type of the reaction is affected by water density or ionic product of water, respectively, therefore, reaction mechanism changes with temperature and pressure. From the contribution of each reaction type, the temperature dependence of cis/trans ratio of produced 2-octene could also be explained.

Kinetics of solid acid catalyzed 1-octene reactions with TiO2 in sub- and supercritical water

To mimic in vivo condition for exposure to gaseous compounds, we employed the Air-Liquid Interface Culture (ALIC) of a human bronchial epithelial cell line, Calu-3, and developed a simple batch-type closed system for direct gas exposure. This system enabled relatively simple cytoxicity evaluation of various gaseous chemicals in an in-vivo mimicking manner. As a preliminary evaluation of the system developed, we tested the toxicity expression of Calu-3 to benzene, tetrachloroethylene and acetone gases in terms of lactate dehydrogenase (LDH) release during 48 hours of the loading. The toxicity in ALIC exposure was higher than that in conventional exposure in the liquid phase. The reason was largely explained by numerical estimation that chemical concentrations exactly on the cell surface in the liquid culture is lower in such acute phase exposure than that in ALIC culture, in the cause of the diffusion process of molecules in the surface liquid layer. These results indicate that basic concept of the combination of ALIC of lung cells and a simple batch-type closed system is promising as a cytotoxicity test of wide ranges of gaseous compounds or samples.

Development of a toxicity evaluation system for gaseous compounds using air-liquid interface culture of a human bronchial epithelial cell line, Calu-3

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