Crystal structure of MCM-70: A microporous material with high framework density

Area of Science:

• Materials Science
• Crystallography
• Chemistry

Background:

• Zeolites and related porous materials are crucial in catalysis and separations.
• Understanding the precise atomic arrangement in novel frameworks like MCM-70 is essential for optimizing their performance.
• Borosilicate materials offer unique properties but require detailed structural characterization.

Purpose of the Study:

• To determine the crystal structure of the borosilicate MCM-70 material.
• To elucidate the channel system and identify extraframework species.
• To characterize the framework density and composition.

Main Methods:

• Synchrotron powder diffraction data collection and analysis.
• Rietveld refinement for structural model fitting.
• Electron energy dispersive spectroscopy (EEDS) and quantitative elemental analysis.
• Magic-angle spinning Nuclear Magnetic Resonance (MAS NMR) spectroscopy (11B, 29Si, 1H-29Si CP).

Main Results:

• MCM-70 crystallizes in space group Pmn2(1) with 1D ellipsoidal 10-ring channels (5.0 x 3.1 Å).
• Potassium (K) sites were identified, with evidence suggesting occlusion of potassium hydroxide (KOH) within the channels.
• High framework density (2.10 g/cm³) and a Si/B ratio of approximately 40:1 were determined.
• MAS NMR indicated sample heterogeneity, with varying degrees of interaction with hydrogen.

Conclusions:

• The detailed crystal structure of borosilicate MCM-70 was successfully elucidated.
• The presence of occluded KOH likely influences the material's properties and potential applications.
• The high framework density positions MCM-70 among dense silicate structures, offering unique structural characteristics.