Yves Trambouze

Catalytic dehydration of ethanol over silica-alumina

Abstract On silica-alumina the dehydrations of ethanol into olefin and ether are parallel surface reactions. The kinetic law for ether production is v = K ap 1 2 (1 + ap 1 2 ) ; ethylene formation is zero order with respect to ethanol. The role of active sites has been investigated by poisoning the catalyst with pyridine, Na + ions, perylene, and tetracyanoethylene; both acids and bases inhibit dehydration. A mechanism is proposed assuming the intervention of basic and acidic sites for the dissociative adsorption of ethanol.

Dehydration of methanol and tert-butyl alcohol on silica-alumina

The catalytic dehydration of tert-butyl alcohol and of methanol has been studied over silica-alumina. Both reactions obey the rate law, v=kap½a/(1 +ap½a+bpw). Pyridine, Na+ and tetracyanoethylene poison the dehydration reactions. These results show that the reaction requires both acidic and basic sites. However, the dehydration of tert-butyl alcohol is much more sensitive to the presence of strongly acidic sites then the methanol dehydration. Alumina, which has a basic character, is more active than silica-alumina for methanol dehydration. These experimental results and those previously published in the literature accord with a conventional carbonium ion mechanism for the dehydration of tert-butyl alcohol. A new mechanism which involves a nucleophilic attack by the alcohol molecule is proposed for the dehydration of methanol.

The state of platinum on reforming catalysts

Platinum has been deposited on a series of silica-alumina supports with different alumina contents; the dispersion of platinum is found to be constant. The catalytic activity for cyclohexane dehydrogenation increases when the alumina content decreases. The activity for tert-butanol dehydration and for perylene cation formation decreases when platinum is deposited on the support. In contrast, the number of sites producing tetracyanoethylene anion increases. The existence of a charge-transfer complex between the metal and the oxidizing sites of the support could explain the variations of platinum activity and redox properties.

Kinetics of the catalytic dehydration of 2-propanol

Abstract The dehydration of 2-propanol is chosen as an activity test of oxide catalysts. The thermal kinetics of the decomposition are studied. Catalytic kinetics are examined on silica, alumina, thoria, and modified silica-aluminas. In the range T = 100 − 160 °C, p (2-propanol) = 8 − 23 mm Hg, the reaction is zero order with respect to alcohol, inhibited by water and acetone, but not by propene. Silica-alumina catalysts have a higher activity than alumina, silica or sodium silica-alumina.

The isomerization of cis-2-Butene over silica-alumina catalysts: III. Dependence on alumina content

Abstract cis-2-Butene isomerization catalyzed by silica-alumina is shown to be affected by dealumination. The initial rate can be broken down into two parts. The first, vL, corresponding to sites isomerizing in the first minutes of a run, parallels Lewis acidity and the oxidizing and reducing properties of the materials. The second initial rate, Vb, corresponding to sites which undergo only little poisoning, is related to Bronsted acidity and to the reducing properties of the four catalysts. Deactivation occurs and is explained in terms of the Lewis/Bronsted sites ratio. Particular changes in vl near 2.4% alumina content show the existence of different kinds of aluminum sites in silica-aluminas, as found previously for other properties.

The isomerization of cis-2-butene over silica-alumina catalysts: I. Study of the deactivation of active surface sites

Abstract Two independent exponential laws are involved simultaneously to describe the deactivation which occurs in the isomerization of butenes on silica-alumina. This permits the calculation of two complementary initial rates for the reaction on a fresh catalyst. The action of poisoning reagents on such rates demonstrates which types of surface sites (Lewis or Bronsted acid-base sites, oxidizing or reducing centers) are acting as primary catalytic centers. Furthermore the experimental conditions (pressure and temperature) for which one of them is more active are determined.

The isomerization of cis-2-Butene over silica—Alumina catalysts: II. Kinetic study

Abstract cis -2-Butene isomerization has been studied on four silica-aluminas of different aluminum contents. The reaction leads to double-bond migration and cis-trans conversion. The deactivation of all catalysts obeys two exponential laws which allow two initial rates to be calculated. One of these is related to Lewis-oxidizing sites, the other one to Bronsted acidic sites. Kinetic studies show that the rate is first order in reactant pressure for one of the catalysts whereas it obeys the Langmuir equation for the other ones. For the latter, activation energies and chemisorption enthalpies have been calculated which allow a mechanism for the isomerization to be proposed. The isomerization proceeds through the same mechanism on both types of active site. A common activated complex is involved in both double-bond and cis-trans . isomerizations.