Low temperature alkane activation over zeolites


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Pieterse, Johannis Alouisius Zacharias (2000) Low temperature alkane activation over zeolites. thesis.

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Abstract:The use of a catalyst, a material that selectively accelerates a chemical reaction by providing a new reaction path way (without being consumed itself), is of major importance in chemical industry. In a heterogeneously catalyzed reaction, at least one of the reactants must be adsorbed on the solid surface. Due to the interaction of the reactants and the surface of the catalyst, specific bonds in the molecules can be more easily formed or broken. For a monomolecular reaction, the action of the catalyst is to decrease the energy barrier in the rate determining step (i.e. energy of activation) or to increase the transition entropy (i.e. the entropy difference between the ground state prior to the rate determining step and the activated complex). For bimolecular reactions, the higher concentration of the reactants on the surface of the solid in comparison to the gas phase may also enhance the rate of reaction. The consequence of the use of a catalyst is that the chemical reaction can proceed under milder conditions so that less restraints have to be put on the design of reactors and less energy is consumed. The catalysts used in this thesis are zeolites which are widely employed in industrial catalysts. Their lattice consists of tetrahedrally, to oxygen coordinated Si and Al atoms. The latter introduce a negative charge to the crystalline structure, which is compensated by metal cations or protons. Compensation of this charge by protons generates Brønsted acid sites. Their location and their acid strength, and therefore their impact on catalytic reactions largely depends on the structure of the various zeolites. Because the pore dimensions of the channel structure in zeolites fall in the range of the kinetic radii of many hydrocarbons, these materials were also denoted as molecular sieves. The idea of molecular sieving lead to the concept of shape selectivity, which is one of the striking advantages zeolites offer. The work presented in this thesis describes the catalytic conversion of light alkanes over a variety of zeolites.
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