Base Strength Distribution Research Articles

1,3-DIGLYCERIDE-RICH EDIBLE OILS FROM GLYCEROLYSIS OF VEGETABLE OILS. EFFECT OF THE CATALYST BASIC PROPERTIES

Vegetable edible oils enriched in diglycerides were obtained by glycerolysis under heterogeneous catalysis conditions using MgO as catalyst. These oils contain up to 32% of the 1,3-diglyceride isomer, which is believed to present unique nutritional properties to prevent obesity, compared to triglycerides or 1,2-diglycerides. The base site density and strength distribution of the different sites (weak, medium and strong) present on MgO as well as the initial oil conversion rate depend on the preparation procedure. The density of medium-strength sites increases upon increasing the severity of the preparation procedure in the range of 673-873K in parallel to the initial oil conversion rate. These results confirm that medium-strength base sites promote the kinetically relevant reaction steps involved in the glycerolysis of vegetable oils.

Synthesis of dimethyl carbonate by transesterification with various MgO–CeO2 mixed oxide catalysts

The transesterification of ethylene carbonate (EC) with methanol to co-generate dimethyl carbonate (DMC) and ethylene glycol over the MgO, CeO2 and mixed oxide thereof was studied using a fixed-bed isothermal tube reactor. In all mixed oxide, the presence of cerium led to a decrease of BET surface area and pore volume as compared to pure magnesia. X-ray diffraction (XRD) revealed that no mixed phases were observed, but MgO (periclase) and CeO2 (cerianite) phases. The catalytic activity intensely depends on the surface basicity and the base strength distribution, but not on the surface area and pore volume. It was found that a high EC conversion of 67% with DMC selectivity of 95% could be obtained by catalyst with cerium content around 25 mol%.

Assessment of acid–base strength distribution of ion-exchanged montmorillonites through NH 3 and CO 2-TPD measurements

Distribution of the acid–base strengths of various ion-exchanged montmorillonites was assessed through thermal programmed desorption (TPD) of NH 3 and CO 2. Accurate acid–base measurements can be achieved via deconvolution of perfectly symmetrical peaks, under optimal carrier gas throughput and heating rate, estimated though a factorial experiment design. No neutral clay samples without interactions with carbon dioxide or ammonia were found. All clay samples displayed both acid and base characters, with specific TPD patterns, and no pure acidic or pure basic clay materials were obtained. Increasing cation radius led to increasing acidity and decreasing basicity, in terms of both total amount of desorbed probe gas and strength of the adsorption sites. It was suggested that basicity and weak acidity are intrinsic features of the clay structure, while medium and strong acidities are induced by the exchangeable cation.

Comparison of Alkali Metal Promoted MgO Catalysts for Their Surface Acidity/Basicity and Catalytic Activity/Selectivity in the Oxidative Coupling of Methane

Alkali metal (viz. Li, Na, K, Rb and Cs) promoted MgO catalysts (with an alkali metal/Mg ratio of 0·1) calcined at 750°C have been compared for their surface properties (viz. surface area, morphology, acidity and acid strength distribution, basicity and base strength distribution, etc.) and catalytic activity/selectivity in the oxidative coupling of methane (OCM) to C2-hydrocarbons at different temperatures (700–750°C), CH4/O2 ratios (4·0 and 8·0) in feed, and space velocities (10320 cm3 g−1 h−1). The surface and catalytic properties of alkali metal promoted MgO catalysts are found to be strongly influenced by the alkali metal promoter and the calcination temperature of the catalysts. A close relationship between the surface density of strong basic sites and the rate of C2-hydrocarbons formation per unit surface area of the catalysts has been observed. Among the catalysts calcined at 750°C, the best performance in the OCM is shown by Li–MgO (at 750°C). © 1997 SCI.

Comparison of Alkali Metal Promoted MgO Catalysts for Their Surface Acidity/Basicity and Catalytic Activity/Selectivity in the Oxidative Coupling of Methane

Alkali metal (viz. Li, Na, K, Rb and Cs) promoted MgO catalysts (with an alkali metal/Mg ratio of 0·1) calcined at 750°C have been compared for their surface properties (viz. surface area, morphology, acidity and acid strength distribution, basicity and base strength distribution, etc.) and catalytic activity/selectivity in the oxidative coupling of methane (OCM) to C2-hydrocarbons at different temperatures (700–750°C), CH4/O2 ratios (4·0 and 8·0) in feed, and space velocities (10320 cm3 g−1 h−1). The surface and catalytic properties of alkali metal promoted MgO catalysts are found to be strongly influenced by the alkali metal promoter and the calcination temperature of the catalysts. A close relationship between the surface density of strong basic sites and the rate of C2-hydrocarbons formation per unit surface area of the catalysts has been observed. Among the catalysts calcined at 750°C, the best performance in the OCM is shown by Li–MgO (at 750°C). © 1997 SCI.

Surface properties of magnesium oxide obtained from basic magnesium carbonate: Influence of preparation conditions of magnesium carbonate

AbstractSurface area, total basicity and base strength distribution (weak, strong and intermediate strength basic sites) of magnesium oxide obtained from basic magnesium carbonate (by its decomposition at 873 K) prepared by precipitation using different magnesium slats, precipitating agents and precipitating conditions (viz. concentration of magnesium salt. pH, temperature, mode of mixing and ageing period) have been thoroughly investigated. The total basicity and base strength distribution of the different magnesium oxide samples have been determined by step‐wise thermal desorption of carbon dioxide from 323 to 1253 K. The chemical composition of basic magnesium carbonate and the surface properties and carbon dioxide content of magnesium oxide are found to be strongly influenced by the aforementioned preparation conditions of basic magnesium carbonate.

Oxidative coupling of methane over La2O3. Influence of catalyst preparation on surface properties and steady and oscillating reaction behaviour

The catalyst precursor and calcination conditions used in the preparation of La2O3 have been found to have a marked influence on the surface properties, basicity and base strength distribution, and catalytic activity and selectivity in the oxidative coupling of methane (OCM) under different reaction conditions. Among the La2O3 catalysts, only that prepared from lanthanum acetate by decomposition in N2(but not in O2) showed symmetric temperature and concentration oscillations in OCM at or below 933 K over a narrow temperature range. The unsteady reaction behaviour is found to be very complex and strongly dependent upon the OCM process parameters and the catalyst parameters. Furthermore, the results on the La2O3 obtained from the acetate indicated a strong possibility of the formation of new catalytic sites active at lower temperatures (below 873 K) during the OCM reaction at higher temperatures.

Synergistic effect of bialkali metal chlorides promoted magnesia on oxidative coupling of methane

Upon promoting MgO (prepared via a sol-gel process) with any binary mixture of the alkali metal chlorides, catalytic systems are obtained which are more active, selective and much more stable with time-on-stream than the respective monoalkali promoted MgO in the oxidative coupling of methane (OCM) to C2 hydrocarbons. The best catalytic performance is obtained over (5 mol% NaCl+5 mol% CsCl)/MgO, which exhibits a C2 yield of 19.7% compared to 5.9 and 4.1% over 10 mol% NaCl/MgO and 10 mol% CsCl/MgO, respectively, at atmospheric pressure, a temperature of 750 °C, a space velocity of 15000 cm3 g−1 h−1,\(P_{CH_4 }\) = 608 Torr and CH4/O2 = 4. A series of different combinations among the five alkali chlorides were made and the afore-mentioned synergistic effect was always observed. The basicity and base strength distribution of the bialkali chloride systems (measured by the gaseous acid adsorption or benzoic acid titration methods) are significantly higher than those of the respective monoalkali halide systems. The relationship between the catalytic performance and basicity/base strength distribution is explored.

Acidity/basicity of rare-earth oxides and their catalytic activity in oxidative coupling of methane to C 2-hydrocarbons

Rare-earth metal oxides (viz. La 20 3, Ce0 2, Sm 20 3, Eu 20 3, and Yb 20 3) have been compared for their acid and base strength distribution (measured by stepwise thermal desorption of C02 from 323 to 1173 K and TPD of NH 3 from 323 to 1223 K, respectively) and their catalytic activity/ selectivity in oxidative coupling of methane to C2-hydrocarbons at 973–1123 K (space velocity = 108,000 cm 3 · g −1 h −1 and CH 4/O 2 = 4 and 8). The catalyst activity and selectivity showed dependence on both the surface acidity and basicity. However, the relationship of the catalytic activity/ selectivity with the surface acidity /basicity is not straightforward; it is quite complex. Stronger acid sites are found to be harmful for the selectivity. There is a possibility of an involvement of an acid-base pair (M LC n+i c 0 LC 2− , where subscript LC denotes low coordination) on the surface in the abstraction of the H-atom from adsorbed methane molecule by its polarization followed by heterolytic C-H bond rupture to form CH 3 − (which interacts with M LC n+ .c) and (OH) LC − and a transfer of electron from the carbanion to O 2, resulting methyl radical and O z. Regeneration of basic sites and the possibility of formation of 0 − species on the catalyst surface are discussed.

Surface properties of magnesium oxide obtained from magnesium hydroxide: Influence on preparation and calcination conditions of magnesium hydroxide

Surface area, basicity and base strength distribution (weak, strong and intermediate strength basic sites) of magnesium oxide obtained from magnesium hydroxide prepared using different magnesium salts, precipitating agents, precipitating conditions (viz. concentration, pH and temperature) and ageing periods, and calcined at different temperatures (873–1173 K) have been investigated. The basicity and base strength distribution on the different magnesium oxide samples have been determined by stepwise thermal desorption of carbon dioxide from 323 to 1173 K. The surface properties are found to be strongly influenced by the above preparation conditions of magnesium hydroxide and also by its calcination temperature.

Bestimmung von Oberflächenoxiden auf Rußen durch potentiometrische Direkttitration in nichtwäßrigen Lösungsmitteln / Determination of Surface Oxides on Carbon Blacks by Direct Potentiometric Titration in Non-Aqueous Solvents

An attempt is made to quantitatively determine acidic and basic surface oxides on carbon blacks and also to monitor their acid and base strength distribution by direct potentiometric titrations in non-aqueous media. In contrast to direct titration procedures proposed so far, our method allows to distinguish clearly between several types of acidic surface oxides of different acid strength. This is mainly achieved by improvements of the non-aqueous titration media. The total contents of weakly and strongly acidic and also of basic oxides determined by this very fast method is comparable to results obtained by slow and laborious indirect titrations.

A novel method for measuring base strength distribution on basic solid catalysts under operating conditions

Stepwise thermal desorption (STD) of CO2 with gravimetric analysis of the desorbed CO2 from 50–900 °C provides a simple and faster method for measuring total basicity and base strength distribution on basic solid catalysts under their operating conditions.

Acid-base properties of modified γ-alumina

Studies of the surface acidity of binary oxide catalysts were carried out. The major component of all the catalysts was γ-Al 2O 3, modified by means of coprecipitation with Fe 2O 3, Cr 2O 3, NiO, MoO 3 and MgO in the amounts up to 7 wt%. All the catalysts were characterized by determining their total acidities on the basis of their ability to chemisorb gaseous ammonia, whereas with white catalysts and those showing a slight colouration, titration measurements of acid and base strength distribution were carried out. Moreover, the activity of all catalysts for the skeletal isomerization of cyclohexene was examined. It was found that the modification of γ-Al 2O 3 with small amounts of the above oxides considerably changes its acid properties. The most substantial change consists in a reduction in the number of the strongest acid sites and the formation of centres of moderate and weak acid strength.

A new procedure for determination of the acid and base strength distribution of colored catalysts

A new procedure for determining the strength distribution of acid and base centers of colored catalysts by the indicator method has been proposed. The procedure basing on the use of a platinum gauze container for the colored catalyst enables us to eliminate the difficulties in the identification of the color of the indicator.

Base-strength distribution studies of solid-base surfaces

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 2CO 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.