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We performed n -heptane cracking over Cu-ZSM-5, Ag-ZSM-5, and P/H-ZSM-5 before and after steaming under various conditions. The reduced Cu-ZSM-5 and Ag-ZSM-5 exhibited similar n -heptane cracking activities to H-ZSM-5 due to generation of Brønsted acid sites by reduction of metal cations. The order of steaming stabilities in the presence of O 2 was Cu-ZSM-5 > Ag-ZSM-5 ≫ P/H-ZSM-5, which was the reverse order of cumene cracking activities in the presence of O 2 . Cumene cracking activity is a function of Brønsted acidity. Therefore, these findings indicate that the steaming stability of a catalyst is strongly related to its Brønsted acidity. Cu 2+ and Cu + cations in the 194%Cu-ZSM-5 were not reduced to Cu 0 at a high temperature of 1023 K in the presence of O 2 and, therefore, 194%Cu-ZSM-5 exhibited an ultra-high steaming stability in the presence of O 2 . The Brønsted acidity of 95%Ag-ZSM-5 was extremely low, almost equal to 194%Cu-ZSM-5. However, the thermal reduction of Ag + took place during steaming even in the presence of O 2 at a high temperature of 1023 K and, therefore, the steaming stability of 95%Ag-ZSM-5 in the presence of O 2 was much lower than that of 194%Cu-ZSM-5. [ABSTRACT FROM AUTHOR]

Highlights: [•] The steaming of H-ZSM-5(51) at 873K for 0.5h raised the heptane cracking activity. [•] Stronger Brønsted acid sites were generated on H-ZSM-5(51) by the steaming for 0.5h. [•] The activity of H-ZSM-5(51) decreased remarkably by the steaming for 10h. [•] The activity of H-ZSM-5(200) steamed for 10h was a similar to that of the parent. [•] P/H-ZSM-5(51) exhibited a much higher steaming stability than H-ZSM-5(51). [ABSTRACT FROM AUTHOR]

Abstract: H-ZSM-5 is an excellent catalyst to produce light olefin in naphtha cracking. But it is difficult to avoid the deactivation by coke formed, so the frequent regeneration of H-ZSM-5 by calcination is necessary. A little steam is generated during the calcination, resulting in the permanent deactivation by dealumination. We found that Cu-ZSM-5 exhibited an ultra-high steaming stability in the presence of oxygen and a comparable activity for heptane cracking in the reductive atmosphere, compared with H-ZSM-5. These results are attributed to a low Brønsted acidity in the presence of oxygen and a high Brønsted acidity in the reductive atmosphere. [Copyright &y& Elsevier]

Abstract: The catalytic cracking of n-heptane has been performed over HZSM-5 catalysts with various Si/Al ratios at 723–923K to form light olefins selectively. The HZSM-5 zeolites with various acid site densities exhibited almost the same selectivity at the same conversion. The ethylene+propylene selectivity increased, while the propylene/ethylene ratio decreased with an increase in reaction temperatures. It is found that a high temperature is required to obtain a high ethylene+propylene yield. The highest ethylene+propylene yield obtained in this study was 59.7 C-% with a propylene/ethylene ratio of ca. 0.72 at 99.6% conversion over HZSM-5 (Si/Al=31) at 923K. Moreover, it is concluded from the selectivities and activation energies that the monomolecular cracking is predominant at a high temperature as 923K. [Copyright &y& Elsevier]