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.

No abstract

The catalytic cracking of representative hydrocarbons of naphtha (n-hexane, cyclohexane, and methyl-cyclohexane) over ZSM-5 zeolite catalysts was examined at reaction temperatures ranging from 823 K to 923 K under atmospheric pressure. It was found that the Si/Al ratio of the zeolite affected the product selectivity and conversion. 5In order to investigate the effects of the crystal size of the ZSM-5 zeolites on catalyst lifetime, macro-and nano-scale ZSM-5 (Si/Al = 150) with crystal sizes of 2300 nm and 90 nm, respectively, were used for the cracking of representative naphtha hydrocarbons.In the cracking of naphthenes (cyclohexane and methyl-cyclohexane), coke was readily formed from the beginning of the reaction leading to significant deactivation of the 10 catalyst for the macro-scale ZSM-5. In contrast, the nano-scale ZSM-5 exhibited a high conversion and high light olefins yield with stable activity, regardless of the type of reactant. As a result, the application of nano-scale ZSM-5 zeolites to the catalytic cracking of naphtha was effective and gave light olefins with high yield and excellent stable activity.

For the application of zeolites as heterogeneous catalysts, low diffusion resistance for hydrocarbons within the micropore is essential for improving product selectivity and catalyst lifetime. This problem has been overcome by reducing the crystal size. This review introduces size-controlled preparation of nano-sized zeolites via hydrothermal synthesis in water/surfactant/organic solvent (emulsion method) and their application to heterogeneous catalysts. The ionicity of the hydrophilic group in surfactant molecules and the concentration of the Si source affected the crystallinity and morphology of zeolites prepared using the emulsion method. When using a non-ionic surfactant, mono-dispersed silicalite-1 nanocrystals approximately 60 nm in diameter were successfully prepared. Nano-and macro-ZSM-5 zeolites with crystal sizes of approximately 150-200 nm and 1.5 µm, respectively, were prepared and applied to n-hexane cracking and acetone-to-olefin reactions to investigate the effect of zeolite crystal size on catalytic stability and light olefin yield. Application of nano-zeolite to light olefin production was effective in achieving faster mass transfer of hydrocarbon molecules within the micropore, which led to improvements in olefin yields and catalyst lifetime.