Hiroki Konno

Kinetics of the catalytic cracking of naphtha over ZSM-5 zeolite: effect of reduced crystal size on the reaction of naphthenes

The catalytic cracking of model naphthenes (cyclohexane and methylcyclohexane) over ZSM-5 zeolites of different crystal sizes (macro- and nano-ZSM-5) was examined at reaction temperatures ranging from 748 to 923 K under atmospheric pressure, focusing on the associated reaction rate constants and activation energies. The catalytic cracking was found to follow first-order kinetics with respect to the naphthene concentrations and the activation energies for cyclohexane and methylcyclohexane cracking over nano-ZSM-5 were determined to be 119 and 116 kJ mol−1, respectively. In order to elucidate the rate-limiting step in the cracking process, the Thiele modulus and the effectiveness factor obtained from cracking over the two ZSM-5 zeolites were evaluated. Cracking with nano-ZSM-5 proceeded under reaction-limiting conditions, whereas the reaction over macro-ZSM-5 at 923 K took place under transition conditions between reaction- and diffusion-limiting. Nano-ZSM-5 was applied to the catalytic cracking of a model naphtha and the results demonstrated that this catalyst was both effective and stable and generated a high yield of light olefins.

n-ヘキサン接触分解による低級オレフィン選択合成におけるMFI型ゼオライトの結晶サイズとSi/Al比の影響

Light olefins are important basic raw materials for the petrochemical industry, and demand for light olefins such as ethylene and propylene has been increasing every year1),2). Light olefins have been mainly produced by thermal cracking of naphtha, which gives yields of ethylene and propylene of approximately 25 % and 13 %, respectively3)~5). However, the naphtha cracking process consumes more than 30 % of the total amount of energy required in petrochemical production, so more efficient processes for the production of light olefins are highly desirable. Moreover, the relative demand for propylene has increased due to the large-scale production of ethylene in the Middle East and China. Catalytic cracking of naphtha over solid-acid catalysts can provide a high propylene/ethylene ratio at low reaction temperatures compared with thermal cracking6), so use of this process could provide energy savings together with the selective production of propylene. Accordingly, the catalytic cracking of naphtha is expected to be an effective alternative to the thermal cracking process. Promising catalysts for n-hexane cracking include the zeolites, which are crystalline aluminosilicate materials with various properties, such as strong acidity and high surface area, and catalytic cracking of alkane over zeolite catalysts has been investigated7)~9). Zeolites incorporate intracrystalline micropores and nanospaces close to the molecular diameters of light hydrocarbons, so have remarkable molecular-sieving effects for light hydrocarbons and have been widely used as shapeselective catalysts in various hydrocarbon processes, such as the alkylation of aromatics10),11) and synthesis of olefins from alcohol and acetone12),13). However, the crystal sizes of zeolites are usually much larger than the sizes of the micropores, so the rate-limiting step of the reaction tends to be diffusion of the reactant/product molecules within the micropores. Moreover, carbon solid (coke) readily forms near the external surface of the crystal under diffusion-controlled conditions, resulting in rapid blocking of the pores, leading to a short catalyst lifetime. Nano-sized zeolites are effective to achieve low diffusion resistance, because the diffusion length for reactant/product hydrocarbons, which depends on the zeolite crystal size, is reduced. We have successfully prepared MFI-type and MORtype zeolite nanocrystals via hydrothermal synthesis in a water/surfactant/organic solvent (emulsion method)14)~18). The nano-sized zeolites are expected to be effective catalysts with low diffusion resistance as well as large external surface area, which will improve the catalytic activity and lifetime. In the present study, catalytic cracking of n-hexane, as a model reaction for the catalytic cracking of naphtha, was examined over MFI-type zeolites, and the effects of the Si/Al ratio and 267 Journal of the Japan Petroleum Institute, 55, (4), 267-274 (2012)