Crystal phase competition by addition of a second metal cation in solid solution metal-organic frameworks

Area of Science:

• Materials Science
• Inorganic Chemistry
• Crystallography

Background:

• Metal-organic frameworks (MOFs) are advanced porous materials with tunable properties.
• Solid-solution MOFs offer enhanced functionality through compositional variation.
• Controlling phase formation in MOF synthesis is crucial for targeted applications.

Purpose of the Study:

• To investigate the synthesis of solid-solution MOFs using a mixed-metal approach.
• To explore the impact of incorporating zinc and cobalt into MOF structures.
• To understand the competition between kinetic and thermodynamic factors in MOF crystallization.

Main Methods:

• Solvothermal synthesis of MOFs using zinc, cobalt, 1,2,4-triazole, and 4,4′-hexafluoroisopropylidenebisbenzoic acid (H2hfipbb).
• Isolation and characterization of four new MOF phases: TMPF-88, TMPF-90, TMPF-91, and TMPF-95.
• Analysis of the influence of metal composition and reaction conditions on phase formation.

Main Results:

• Four new transition metal polymeric frameworks (TMPFs) were successfully synthesized.
• The addition of a second metal (Co or Zn) significantly altered MOF phase formation, even at high metal ratios.
• Kinetically favored phases often crystallized rapidly, hindering the formation of thermodynamically stable solid-solution phases.

Conclusions:

• Mixed-metal MOF synthesis presents complex phase behavior.
• Kinetic control during synthesis can lead to the preferential formation of specific MOF phases.
• Further research is needed to achieve pure solid-solution MOFs by overcoming kinetic barriers.