(600k) Hexane Cracking Over H-ZSM-5 with Different Al Contents

Light olefins such as ethylene and propylene are mainly manufactured through a thermal cracking of naphtha.  The thermal cracking process needs a high reaction temperature. Besides, it is difficult to raise the propylene/ethylene ratio to more than 0.6.  Therefore much attention is now focused on the catalytic cracking over acidic zeolite catalysts, which gives low ethylene but high propylene yields at lower temperatures [1]. Activation energies of paraffin cracking over acidic zeolite catalysts depend on acid strength [2] and reaction mechanism.  Haag and Dessau have reported that paraffin cracking proceeds along the monomolecular cracking mechanism including the protonation of paraffin to form a penta-cordinated carbonium ion and its decomposition and/or along the bimolecular cracking mechanism including the hydride transfer from paraffin to a carbenium ion to form a new carbenium ion and its b-scission.  They have also reported that the activation energy for the monomolecular cracking of hexane is 126 kJ mol^-1 and that for the bimolecular cracking is 27 kJ mol^-1[3].  Therefore the activation energy observed should change with the ratio of two mechanisms, which varies with reaction temperatures, paraffin pressures, conversion, and acid density of zeolites [4].  Kubo and co-workers have examined the cracking mechanism ratio in heptane cracking over H-ZSM-5 from selectivities to hydrogen, methane and ethane, which form solely by the monomolecular cracking. They have concluded that the monomolecular cracking is predominant at a high temperature as 923 K [5]. In this study, H-ZSM-5 zeolites with different Al contents were used as catalysts for catalytic cracking of hexane.  We focus on the behavior of the formation of hydrogen, methane, and ethane to clarify the effect of the Al content on the reaction mechanism and the activation energy.

      NH₄-ZSM-5 zeolites with different Si/Al ratios were synthesized by hydrothermal synthesis from the preparation gel, whose molar composition was 1 TEOS: 0.04-0.001 Al(NO₃)₃ : 0.25 TPAOH : 0.25 NaOH : 100 H₂O, followed by the conventional cation exchange procedure with 1 M NH₄NO₃.  NH₄- ZSM-5 samples were converted in situ into H-form by heating at 923 K in Ar flow before the hexane cracking.  The hexane cracking was carried out under the following reaction conditions by using a fixed-bed reactor equipped with an on-line gas-chromatograph.  Reaction temperature: 723 - 923 K, initial pressures of hexane: 5 kPa, W/F: adjusted to attain <20% conversion.

      Si/Al ratios of NH₄-ZSM-5 determined by ICP were 20 - 660.  The BET surface area and micropore volume for each sample were almost the same (ca. 420 m² g^-1 and 0.18 cm³ g^-1, respectively).  Crystallite sizes of all samples were less than 1 mm.  The IR spectra of CO adsorbed on H-ZSM-5 at 153 K did not reveal a significant difference in the Brønsted acid strength of these samples. All ²⁷Al MAS NMR spectra consist of two peaks at ca. 53 and 56 ppm.  The intensity ratios of the two peaks varied with Al contents.  Han et al. have reported similar results and have concluded that the preference of Al atoms for particular T-sites in ZSM-5 depends strongly on the framework Al content [6]. The activation energy for hexane cracking over H-ZSM-5 increased with a decrease in Al content.  The activation energies for Si/Al=20, 125 and 295 samples were 97, 129 and 136 kJ mol^-1, respectively.  To clarify the effect of cracking mechanism on activation energy, the selectivities to hydrogen, methane, and ethane, which formed solely by the monomolecular cracking, were examined.  The total selectivity to hydrogen, methane, and ethane was almost the same at every reaction temperature, regardless of Al content of H-ZSM-5.  This fact suggests that the ratio of the monomolecular cracking to the bimolecular cracking may not change considerably with Al content at high temperatures.  Furthermore, it can be estimated that at least ca. 65 % (823 K), 75 % (873 K) or 80% (923 K) of hexane cracking takes place via the monomolecular cracking mechanism at each temperature.  Therefore, monomolecular cracking is dominant, regardless of Al content at these reaction temperatures. To clarify the effect of the Al content on the activation energy solely for the monomolecular cracking, activation energies for the formations of hydrogen, methane, and ethane were examined.  Every activation energy decreased with an increase in Al content.  Therefore, it is apparent that the activation energy for the monomolecular cracking of hexane decreases with an increase in Al content. This fact indicates that Al distribution of H-ZSM-5 affect the hexane cracking activity.

Acknowledgments

This work was supported by the green sustainable chemistry project of New Energy and Industrial Technology Development Organization (NEDO).

References

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