Jun-ichiro Take

Base-strength distribution studies of solid-base surfaces

Abstract A method has been developed for the determination of the base-strength distribution of solid surfaces. The present method consists of the titration of solids suspended in cyclohexane with benzoic acid, using a series of H − indicators. The base-strength distribution of solid surfaces has been measured by this method for alkaline-earth oxides and some other solid bases. Alkaline-earth oxides increased remarkably in base strength upon heat treatment in vacuum. The base strength of these oxides proved to decrease in the order SrO( H − ⩾ 26.5) ≈ CaO( H − ⩾ 26.5) > MgO( H − = 18.4 to 26.5), in accordance with the observation by Krylov et al. and also with the order expected from the partial negative charge of combined oxygen anions in these oxide surfaces. A distinct distribution of base strength was observed on these oxide surfaces. No basic sites having an H − equal to or higher than 12.2 were observed on the surfaces of ZnO, ZrO 2 , ThO 2 , or Na 2 CO 3 . The catalytic activity and selectivity of alkaline-earth oxides in the elimination of hydrogen chloride from 1,1,2-trichloroethane were quantitatively interpreted from the observed base-strength distribution.

Ethanol dehydration on alumina catalysts: I. The thermal desorption of surface compounds

Abstract The surface ethoxide on alumina made by ethanol adsorption was desorbed by heat treatment, with the desorbed species analyzed by mass spectrometry. Below 135 °C the main components of the desorbed species by the thermal treatment were ethanol, diethyl ether, and ethylene, whereas only ethylene was obtained above 135 °C. There was no difference in the infrared spectra which correlates with the change in product distribution. As the concentration of the surface ethoxide was increased, the formation of diethyl ether was favored at the low-temperature region. On the contrary, the formation of ethylene was suppressed with the increased concentration of both the surface ethoxide and the surface hydroxyl group. At the high-temperature region, however, ethylene was the only product without regard to the surface concentration. A diethyl ether molecule is formed from two surface ethoxide groups. The ethylene formation may take place by hydrogen abstraction from the methyl group of the surface ethoxide by the exposed oxide ion adjacent to the surface ethoxide. The structure of the ethanol adsorption on silica gel was investigated for comparison. The dehydration of ethanol over silica-alumina is also discussed.

Linear free energy relationships in heterogeneous catalysis: VIII. Isomerization of alkylbenzenes over solid acid catalysts

Abstract An attempt has been made to survey the applicability of linear free energy relationships (LFER) in the heterogeneous isomerization of dialkylbenzenes at high temperatures. The microcatalytic gas chromatographic technique was used to measure reactivities of a set of dialkylbenzenes at 400 °C over silica aluminas. The rate of the shift reaction, i.e., the migration of alkyl groups in one direction on a monosubstituted ring, increases markedly in the order: Me tert -Bu. The logarithms of the shift rate constants hold a linear relationship with Δ H C + , the enthalpy change for the hydride abstraction from the corresponding paraffin. This relationship has been reported to hold also in dealkylation of monoalkylbenzenes. The dependency on Δ H C + in isomerization was found to be three-fifths of that in dealkylation at 400 °C on the same catalyst. Thus, selectivity between both reactions is shown to change according to alkyl groups, and to be nearly unity at an iso-Pr group. On the other hand, the effects of the second substituted groups on isomerization were found to be described by the Hammett law. An agreement between the above-mentioned facts and the proposed reaction scheme is briefly discussed. These successes in LFER will make it possible to estimate the rate constants of isomerization at 400 °C.

Liquid-phase desorption kinetics of benzeneazodiphenylamine physisorbed on silica gel

Abstract Ultraviolet spectroscopic determination of the desorption rate of benzeneazodiphenylamine (BADA) was undertaken in cyclohexane at temperatures of 40–70 °C, and quantitative evidence was obtained to support our previous conclusion that an adsorption equilibrium, which has been assumed in the n -butylamine titration method, may not be realized under the conventional conditions. This determination was achieved by use of a pair of wafers which comprised silica gel producing physically adsorbed species and silica-alumina functioning as a trap for the BADA molecules desorbed from the silica gel. When BADA was added into the system, it was adsorbed onto both wafers with nearly equal probability at room temperatures, and the system reached a stationary state after 3–4 days. This stationary state was revealed to be not of the true but of the false adsorption equilibrium. The rate of the desorption obeyed well a first-order kinetics. An activation energy as high as 16 kcal/mol for the desorption was concluded to be probably too high to warrant the above assumption underlying the n -butylamine titration method. Molar absorptivity was also determined for both physically and chemically adsorbed BADA.

Linear free energy relationships in heterogeneous catalysis: X. Temperature effects in dealkylation and isomerization of dialkylbenzenes over a solid acid catalyst

Temperature effects in dealkylation and isomerization of dialkylbenzenes at the temperature range of 200 to 500 °C have been discussed from the viewpoint of linear free energy relationships (LFER). The effects of the second substituents on both reactions are well described by the Hammett law. The alkyl shift reaction, which is defined as the migration of an alkyl group in one direction on a monosubstituted ring, has been compared with the dealkylation of monoalkylbenzenes. The preexponential terms in both reactions have been shown to be constant independent of alkyl groups, whereas the activation energies are 24.8, 19.5, and 15.6 kcal/mole for the dealkylation of Et, isoPr, and tert-Bu groups, respectively, and 17.8, 14.1, 10.6, and 8.96 kcal/mole for the alkyl shift reaction of Me, Et, isoPr, and tert-Bu groups, respectively. The LFER equations for isomerization and dealkylation have been further extended over the temperature range of 200 to 500 °C, as follows: ki(R1, R2,T)=∑Rw(r)ki(0, ∞)exp [−{γ′iΔHc+(R)+EA, i(0)}RT]+2.3ρ(R)σ(Rf)] where the Hammett ϱ values are assumed to be practically independent of temperature. The generalized equation given above for dealkylation and isomerization has made it possible to predict the selectivity between two reactions of any kind of dialkylbenzenes. The reaction mechanism of both reactions proposed in a previous paper has been refined on the basis of the LFER found in the present work.

Catalytic Combustion of Methane–Experimental and Simulation Studies of the Effects of Catalyst Properties

The catalytic combustion of methane has been studied experimentally with a small flow reactor and by computor simulation with a one-dimensional steady-state two-phase model, placing stress on the effects of chemical and physical properties of catalyst on the combustion.