Adsorption, Diffusion and Reaction Studies of Hydrocarbons on Zeolite Catalysts

Zeolites are crystalline microporous materials that are widely applied as catalysts in industries like oil refining, basic petrochemistry and fine chemistry. The major benefit of the use of zeolites as catalysts lies in their unique microporous structures. However, in some cases the presence of micropores limits the catalytic performance of zeolites due to diffusion limitation. This is caused by the similarity in size of the involved hydrocarbons and the micropore diameter. One of the most effective ways to minimize diffusion limitation in zeolites is the generation of mesopores in the zeolite crystals. In this way the length of the zeolite micropores is effectively shortened and the molecular accessibility is largely enhanced. In chapter 2 of this thesis, different routes for the generation and characterization of mesopores in zeolite crystals are reviewed. The beneficial effect that the presence of mesopores brings about for cracking reactions and fine chemical synthesis over zeolite Y, and for cumene production and alkane hydroisomerization over mordenite will be discussed. In chapter 3 of this thesis the experimental background and application of a new microbalance (tapered element oscillating microbalance, TEOM) is introduced and illustrated with two examples. In chapter 4 the TEOM is applied to perform uptake measurements under full catalytic conditions. In this way the effect of acid leaching on the diffusion and hydroisomerization of n-hexane over Pt/H-mordenite has been determined. In chapter 5, a new approach using adsorption and diffusion of hydrocarbons is introduced to verify the extent of accessibility of the micropore volume for the one-dimensional zeolite mordenite. Hydrocarbon conversions over zeolite catalysts often suffer from the concurrent formation of carbonaceous deposits or so-called coke. The coke molecules trapped inside the zeolite micropores are not always inert and therefore may participate in the catalytic reaction. In most cases this provokes a decrease in the catalytic activity and selectivity. However, in the case of the skeletal isomerization of n-butene to isobutene over the zeolite ferrierite the deposition of carbonaceous species induces a beneficial effect with regard to the isobutene selectivity. As a consequence, the skeletal isomerization of butenes receives much interest during the last decade. Especially much discussion is on the exact role of the carbonaceous deposits and the nature of the active sites for isobutene formation. In chapter 6 of this thesis a comprehensive review on the beneficial and harmful effects of carbonaceous deposits in butene skeletal isomerization is presented. In chapter 7 of this thesis, in situ infrared (IR) spectroscopy is applied to establish the nature of the carbonaceous deposits and the number of Brønsted acid sites during butene skeletal isomerization over ferrierite. It was possible to distinguish the differently located Brønsted acid sites in the ferrierite structure during reaction, hence as a function of the amount of carbonaceous deposits. In chapter 8 of this thesis d3-acetonitrile is applied as a probe molecule to determine the number and nature of the active sites on aged and highly selective ferrierite using IR spectroscopy. In chapter 9 of this thesis Electron Energy-Loss Spectroscopy (EELS) measurements are performed in a Scanning Transmission Electron Microscope (STEM) on aged zeolite ferrierite crystals. In this way the influence of the pore structure on the location and nature of the carbonaceous deposits in zeolite crystals is unraveled. The results in chapters 7, 8 and 9 support the view that the selective catalytic action over aged ferrierite takes place in the pore mouths of the 10 membered ring (MR) main channels. Moreover, since on the aged and selective ferrierite some Brønsted acid sites were still accessible and at the same time no aromatic carbenium ions were detected, it is suggested that the selective conversion of n-butene into isobutene will run over Brønsted acid sites