Shedding light on the atomic-scale structure of amorphous silica-alumina and its Brønsted acid sites.

Phys Chem Chem Phys

US DOE, Ames Laboratory, Ames, IA 50011, USA and Department of Chemistry, Iowa State University, Ames, IA 50011, USA.

Published: September 2019

AI Article Synopsis

  • Despite their widespread use in industrial processes, the detailed structures of amorphous silica-aluminas (ASAs) have remained poorly understood due to their lack of long-range order.
  • Researchers utilized a combination of conventional and DNP-enhanced solid-state NMR techniques to analyze ASAs generated through flame-spray-pyrolysis, overcoming previous limitations of sensitivity.
  • The study confirmed that ASAs follow Loewenstein's rule, showing aluminum avoidance, and developed new experiments to measure specific distances between atoms, indicating that most Brønsted acid sites come from pseudo-bridging silanol groups.

Article Abstract

In spite of the widespread applications of amorphous silica-aluminas (ASAs) in many important industrial chemical processes, their high-resolution structures have remained largely elusive. Specifically, the lack of long-range ordering in ASA precludes the use of diffraction methods while NMR spectroscopy has been limited by low sensitivity. Here, we use conventional as well as DNP-enhanced Si-Si, Al-Al, and Si-Al solid-state NMR experiments to shed light on the ordering of atoms in ASAs prepared by flame-spray-pyrolysis. These experiments, in conjunction with a novel Monte Carlo-based approach to simulating RESPDOR dephasing curves, revealed that ASA materials obey Loewenstein's rule of aluminum avoidance. 3D O{H} and 2D O{H,Al} experiments were developed to measure site-specific O-H and HO-Al distances, and show that the Brønsted acid sites originate predominantly from the pseudo-bridging silanol groups.

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Source
http://dx.doi.org/10.1039/c9cp04099dDOI Listing

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