Wen-Qing Xu

Isomerization reactions of n-butenes over isomorphously substituted B/A1-ZSM-11 zeolites

Abstract Low levels of boron have been isomorphously substituted into pentasil zeolites and are active in butene isomerization reactions. The acidity of B/A1-ZSM-11 zeolites, measured by NH3 adsorption, is significantly lower than the acidity of B3+-free Al/ZSM-11 zeolites. These materials exhibit both Bronsted and Lewis acidities as determined by pyridine adsorption studies. Adsorption/desorption experiments with 1-C4H8 and i-C4H8 were studied to determine reactant and product shape selectivities. Complete isomorphous substitution of framework Al3+ by B3+ in ZSM-11 zeolites results in inactivity. Our data show that the reaction is first order in 1-C4H8 with slow deactivation via coke formation. The effect of flow rates on conversion, yields, selectivities, and product distributions has been studied on B/A1-ZSM-11 materials. Isobutene yields of 22% and selectivities of 47% have been obtained under optimal operating conditions using B/Al-ZSM-11 catalysts. Lower yields of i-C4H8 were obtained using Pt/B/Al-ZSM-11 zeolites with n-C4H10 as a feed gas. A temperature of 523°C was used in isomerization reactions while a temperature of 580°C was required in dehydrogenation/isomerization reactions. Propene is the major byproduct of this reaction, believed to form as a result of cracking of dimerized (and polymerized) olefins. This side reaction is the primary cause for deactivation and coke formation on the catalyst.

Skeletal Isomerization of n-Butenes on Zeolite Catalysts: Effects of Acidity

Abstract ZSM-5 and ZSM-11 zeolites with different contents of aluminum and boron have been synthesized to control their acidity. Results of catalytic studies suggest that there are synergistic effects between framework boron and aluminum to enhance skeletal isomerization activity. In the absence of boron, the competitive reactions like oligomerization, cracking, and H-transfer will dominate, which produce undesired products. In the absence of aluminum, the isomerization activity is low. Incorporation of boron into the framework of aluminum zeolites can reduce the acidity. Consequently, the isomerization activity increases, and the activity of oligomerization, cracking, and H-transfer reaction decrease to some extent. Alumina binder also has effects on boron zeolites because of the migration of aluminum into the framework.

Temperature Program Desorption and Reduction Studies of Octahedral Molecular Sieves

Publisher Summary The reactivity of oxygen species in single or mixed transition metal oxides plays an important role in determining the catalytic performance of an oxide as a consequence of the Mars–van Krevelen redox mechanism. Manganese dioxides, such as octahedral molecular sieves (OMSs), are a family of promising compounds for studying relationships between solid and catalytic properties. The incorporation of additional transition metal(s) into OMS materials lends great variability for adjusting oxidative and reductive stoichiometry and, consequently, the valency states and Mn–O binding strengths. Three or more oxygen species can be discriminated in the tunnel structure of both doped OMS 1 and OMS 2 in view of the emerging temperature of peaks during temperature-programmed desorption (TPD) in He. The doping cations do not markedly affect the evolution of TPD peaks. The predominance of oxygen species related to the high-temperature (HT) region peaks in TPD suggests that they are mainly under the influence of Mn ions in the framework because of their relative interatomic distances from Mn and from doping cations.