No abstract

The catalytic cracking of n-hexane over ZSM-5 zeolite (MFI-type zeolite, Si/Al = 150 and 240) catalysts was examined at reaction temperatures ranging from 823 to 923 K under atmospheric pressure. The reaction rate constants and activation energies of n-hexane cracking over ZSM-5 zeolites with various crystal sizes and Si/Al ratios were evaluated. The catalytic cracking of n-hexane was first order with respect to the n-hexane concentration, and the activation energies of n-hexane cracking over ZSM-5 zeolites were found to be approximately 123-128 kJmol -1 . Compared with the macro-sized zeolite, the nano-sized zeolites exhibited high n-hexane conversion with stable activity for 50 h. This is because the cracking reaction with nano-sized zeolite proceeded under reaction-limiting conditions, whereas the reaction with macro-sized zeolite proceeded under transition conditions between reaction-and diffusion-limiting conditions. As a result, the application of nano-zeolite to the catalytic cracking of n-hexane was effective and gave light olefins with high yield and excellent stable activity.

Production of light olefins such as ethylene, propylene and isobutylene from acetone was examined over ZSM-5 zeolites. These light olefins are produced from acetone over the acid sites of the zeolite via a series of consecutive reactions where olefins such as ethylene and propylene are obtained by cracking of isobutylene produced from aldol condensation products of acetone. Macro-and nano-sized ZSM-5 zeolites were prepared by conventional hydrothermal and emulsion methods, respectively, and the ZSM-5 zeolites with nearly the same acidity and BET surface area were obtained regardless of the crystal sizes. From SEM observations, the crystal sizes of the zeolites were approximately 2000 nm and 30-40 nm.These zeolites with different crystal sizes were applied to light olefins synthesis from acetone, and the effect of crystal size on catalytic activity and stability was investigated. As compared with the macro-seized zeolite, the nano-sized zeolite exhibited a high activity over a long lifetime. However, because the nano-sized zeolite possesses a large external surface area, undesirable reactions to form aromatics from the produced light olefins occurred on the acid sites located near the external surface. To inhibit aromatics formation, selective deactivation of the acid sites located near the outer surface of the zeolite was achieved via the catalytic cracking of silane (CCS) method using diphenyl silane (DP-silane). The CCS method was effective in deactivating the acid sites located near the external surface of the ZSM-5 zeolite.Moreover, the nano-size zeolite after the CCS treatment using DP-silane exhibited high olefins and low aromatics yields under high acetone conversion conditions.

To reduce the consumption of hydrogen when converting heavy oil to light oil, the catalytic cracking of a heavy oil (residue of atmospheric distillation) with steam was examined. Two iron oxide-based catalystsshematite (R-Fe 2 O 3 ) and goethite (FeOOH, denoted herein as FeO X catalyst)swere used. It was found that the heavy oil was converted to a mixture of useful light hydrocarbons (i.e., gasoline, kerosene, and gas-oil) over iron oxide-based catalysts. Moreover, because the FeO X catalyst possessed mesopores with diameters of 6-10 nm, it exhibited higher activity than the R-Fe 2 O 3 catalyst without the production of carbonaceous residue. The catalytic activity could be enhanced by loading ZrO 2 on the FeO X catalyst. From the X-ray diffraction analysis and Mössbauer measurement, it was considered that the active oxygen species generated from H 2 O over ZrO 2 particles spilled over the FeO X surface, where the oxidized decomposition of heavy oil occurred.

The catalytic conversion of glycerol was performed with iron oxide-based catalysts for production of allyl alcohol using a fixed-bed flow reactor at 623 K under atmospheric pressure.The glycerol dehydration proceeds on acid sites of catalysts while the allyl alcohol production is assumed to be catalyzed by non-acidic sites of catalysts through a hydrogen transfer mechanism.Different alkali metals, including Na, K, Rb, and Cs were supported on ZrO 2 -FeO X and all of them gave impressively higher allyl alcohol yield and suppressed glycerol dehydration due to the reduced catalyst acidic property. K-supported ZrO 2 -FeO X (K/ZrO 2 -FeO X ) was chosen for further studies, and allyl alcohol yield remarkably increased up to 27 mol%-C at the K content of 3-5 mol%. Since no external hydrogen gas is supplied to the system, the hydrogen transfer mechanism should take place between the reaction of glycerol and either hydrogen atoms derived from formic acid forming during the reaction, or active hydrogen species produced from the decomposition of H 2 O by ZrO 2 .Addition of Al 2 O 3 to K/ZrO 2 -FeO X (K/Al 2 O 3 -ZrO 2 -FeO X ) was examined in order to improve structure stability during the glycerol conversion. Al 2 O 3 addition to the catalyst was effective to achieve higher structure stability, leading to high glycerol conversion with stable allyl alcohol yield of above 25 mol%-C. Moreover, K/Al 2 O 3 -ZrO 2 -FeO X can be applicable to the conversion of crude glycerol which is the waste solution obtained from biodiesel production.3