Recreation of Brønsted acid sites in phosphorus-modified HZSM-5(Ga) by modification with various metal cations

• Recreation of Brønsted acid sites in the M/P/HZSM-5 zeolite was confirmed.
• La and Zr (higher valences) are more effective than Mg and Ca (di-valence).
• Propylene yield increased with increasing La(Zr)/Ga ratio and gave a maximum value.
• Catalytic activity and hydrothermal stability of HZSM-5(Ga) were improved by co-modification of M and P.

To investigate the recreation of Brønsted acid sites in phosphorus-modified HZSM-5(Ga) zeolites by modification with various metal cations, we prepared several HZSM-5(Ga) zeolites co-modified with Mg, Ca, La or Zr and P (M/P/HZSM-5(Ga)). From FT-IR, 31P MAS NMR, and NH3-TPD measurements, it was found that the introduced metal cations (M) of Mg, Ca, La, and Zr react with the pre-introduced phosphorus to generate phosphorus species such as Mx(PO4)y, resulting in the recreation of some native Brønsted acid sites (Si(OH)Ga). The degree of recreation of the Brønsted acid sites was strongly dependent on the M/Ga ratio and doped metal valence. Modification with La and Zr that have higher valences more effectively recreated the Brønsted acid sites compared to Mg and Ca, which have di-valence. The conversion of ethanol to propylene was carried out over the M/P/HZSM-5(Ga). The propylene yield increased with increasing M/Ga ratio and reached maximum values of ca. 26 C-% for Zr/P/HZSM-5(Ga) with a Zr/Ga ratio of 0.25 and ca. 29 C-% for La/P/HZSM-5(Ga) with a La/Ga ratio of 0.4. The propylene yield hardly changed for the co-modified HZSM-5(Ga) zeolites during the 24 h of reaction time, indicating the suppression of both carbonaceous deposition and the release of gallium from the zeolite framework.

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