A survey of the mechanism in catalytic isomerization of alkanes
Introduction
Isomerization of n-alkanes into the corresponding branched alkanes is an important way to enhance the octane number of gasoline. The octane numbers (RON) of pentane and hexane are 61.7 and 24.8, respectively, while those of isopentane and 2,3-dimethylbutane are 92.3 and 103.6, respectively. The isomerization of C4–C6 hydrocarbons is performed in industrial scale. Isobutane obtained by the isomerization of butane is also used for alkylation process and for MTBE synthesis after dehydrogenation to isobutene. Isomerization of heptane has been extensively studied. Skeletal rearrangement of hydrocarbons is also important in hydrocracking of higher hydrocarbons to obtain the products with higher octane numbers.
The mechanism of alkane isomerization has been discussed for years and is seemingly established. The details of the mechanism are, however, still unresolved. The main problems to be solved include the way of formation of carbenium ions, acidity requirement of the catalysts, and the roles of a metallic component and hydrogen during isomerization. This short review surveys the recent progress on the catalysts and the mechanisms in alkane isomerization.
Section snippets
Catalysts for alkane isomerization
The main catalysts of importance in alkane isomerization are briefly discussed. The mechanistic aspects of the catalytic systems are discussed later.
Mechanism of alkane isomerization
As described above, there are two types of catalysts for isomerization of alkanes; acid catalysts (homogeneous or heterogeneous) and solid acids loaded with a transition metal (mostly Pt), except a unique metallocycle mechanism proposed for molybdenum oxycarbonate. In both cases, carbenium ions are considered to be responsible for the skeletal rearrangements. Therefore, the most important steps in the isomerization are the formation of carbenium ions and its rearrangement on the catalyst
Bifunctional mechanism
As mentioned above, solid acids loaded with transition metal (Pt) are used in the industrial isomerization processes. The catalysts are often called bifunctional. One function is a function as an acid catalyst, the other being a function as a hydrogenation–dehydrogenation catalyst.
The original idea of “bifunctional catalysis” stems from the use of alumina loaded with a transition metal used for reforming processes. The idea was transferred to isomerizaton. Thus, it is supposed that alkane is
Activation of alkanes on S-ZrO2
As described above, Hino and Arata [2], [3] claimed that S-ZrO2 is an superacid from the color change of Hammett indicators. Lin and Hsu [6] claimed the superacidity by TPD of benzene for Fe, Mn-containing S-ZrO2. Another “strong evidence” was that S-ZrO2 could activate even butane molecules. Though many papers designate this material as superacid in their titles, these three pieces of “evidence” are recently questioned. Do we really need superacid to activate butane molecules?
The application
Quantum chemical modeling of alkane rearrangement
Most of our knowledge on the mechanism of alkane isomerization is based on the theory developed for accounting the findings in the isomerization in liquid phase. The applicability of the idea deduced from the events in homogeneous phase to those on solid surfaces has been a recent subject of quantum chemical calculations. The calculation has been made mainly on the transformations on zeolite surface.
Summary
The isomerization of alkanes has been industrially performed for years. The catalysts used in commercial processes have some disadvantages such as need for chlorinated compounds, high working temperature and high hydrogen pressure. The novel catalysts, which overcome these disadvantages of the present industrial catalysts are highly desired. The catalysts, which have a high selectivity in the isomerization of heptane and higher alkanes are also sought. As for the mechanism of the isomerization,
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