Publisher Summary This chapter discusses the specific poisoning of a catalyst surface. A probe molecule other than the reactant is preadsorbed as a poison and its effect on the catalytic activity studied. From the nature of interaction of this probe molecule with the surface sites one may obtain information regarding the chemical nature of these sites and from the number of poisoning molecules necessary to bring the activity to zero one may estimate an upper limit of the active site density. Besides the obvious difficulty to discriminate between surface sites that purely absorb the probe molecule and those that are involved in the catalytic process as active sites, there are a number of other limitations making it difficult to obtain clear-cut information on the nature of active sites. The chapter describes the proposals that are indispensable for a proper application of this method and the precautions that have to be kept in mind while drawing conclusions from specific poisoning experiments. The chapter illustrates two examples—namely, the dehydration of alcohols and the isomerization of olefins on alumina.

Specific Poisoning and Characterization of Catalytically Active Oxide Surfaces

The evolution of the acid/base properties of a series of oxide supports (alumina, magnesia and silica) modified by increasing loadings of additive ions (Li+, Ni2+, and SO4
 2-) from 1 to 50% of the support surface coverage is reported using the catalytic test of isopropanol decomposition, studied as a function of the reaction temperature. The calculated kinetic parameters Ea, A, and ΔS≠ permit interpretation of the reaction mechanism with relation to the acidity/basicity of the modified surfaces. The series of alumina oxides, due to the amphoteric properties of the surfaces, decomposed isopropanol through an E2 mechanism leading to propene and di-isopropyl ether formation. The selectivity to the two products was dependent on the strength of the basicity (addition of lithium and nickel) or on the acidity (addition of sulfate) of the surfaces. Magnesia series oxides dehydrated isopropanol through an E1b mechanism due to the presence of very strong basic surfaces possessing some weak acid sites. The very weak amphoteric character of silica was strengthened by the loading of the three additives; the modified silica surfaces displayed enhanced decomposition activity with respect to pure silica.

Study of the acidic character of modified metal oxide surfaces using the test of isopropanol decomposition

Steady state, isotopic, and chemical transient studies of ethanol dehydration on γ-alumina show unimolecular and bimolecular dehydration reactions of ethanol are reversibly inhibited by the formation of ethanol–water dimers at 488 K. Measured rates of ethylene synthesis are independent of ethanol pressure (1.9–7.0 kPa) but decrease with increasing water pressure (0.4–2.2 kPa), reflecting the competitive adsorption of ethanol–water dimers with ethanol monomers; while diethyl ether formation rates have a positive, less than first order dependence on ethanol pressure (0.9–4.7 kPa) and also decrease with water pressure (0.6–2.2 kPa), signifying a competition for active sites between ethanol–water dimers and ethanol dimers. Pyridine inhibits the rate of ethylene and diethyl ether formation to different extents verifying the existence of acidic and nonequivalent active sites for the dehydration reactions. A primary kinetic isotope effect does not occur for diethyl ether synthesis from deuterated ethanol and onl...

Kinetics and Mechanism of Ethanol Dehydration on γ-Al2O3: The Critical Role of Dimer Inhibition

Solid-solid wetting and formation of monolayers in supported oxide systems

Catalysts for the methanol/dimethyl ether reaction are reviewed with particular emphasis on the acid/base properties of the catalysts. The focus is on temperatues less than around 300°C and atmospheric pressure conditions, which correspond to potential automotive applications. Mechanisms including both dual site (acid/base) Langmiur-Hinshelwood and single site Rideal-Eley mechanisms have been postulated, with both Bronsted and Lewis acid sites involved in the dehydration reaction. Temperatures higher than about 300°C, as well as a strong surface acidity, tend to promote the further dehydration of DME to form C2-Q5 olefins and eventually hydrocarbon products in a series type reaction sequence. Methods exist, particularly in zeolites, to control the strength and type of surface acidity, and therefore the selectivity to DME.

Review: dehydration catalysts for the methanol/dimethyl ether reaction

The dehydration of alcohols over alumina: VII. The dependence of reaction direction on the substrate structure

Es wird die Kinetik der Ätherbildung aus Äthanol über 1203 im Temperaturbereich zwischen 150 und 195°C bei Drucken zwischen 30 und 300 Torr Äthanol beschrieben. Im Bereich hoher Drucke vorläuft die Reaktion nach nullter Ordnung mit einer „wahren" Aktivierungsenergie von 25,9 kcal/Mol. Wasser hemmt dio Reaktion stark. Die Druckabhängigkeit der Reaktionsgeschwindigkeit wird durch die Gleichung

Einhard Ress

Papers

The dehydration of alcohols over alumina: VII. The dependence of reaction direction on the substrate structure

Die Dehydratisierung von Alkoholen an Aluminiumoxid

Wasserstoffbrücken bei der Adsorption von Alkoholen an Al2O3