期刊
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
卷 143, 期 17, 页码 6669-6680出版社
AMER CHEMICAL SOC
DOI: 10.1021/jacs.1c02361
关键词
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资金
- National Science Foundation [CHE-1764116]
- NSF [DMR-1039938, DMR-0603042, DMR-1157490]
- NIH [P41 GM122698]
- State of Florida
The structure of aluminum-containing moieties in and within zeolite H-ZSM-5 catalysts is influenced by various factors and can be analyzed using Al-27 NMR data. Nonframework aluminum species can be detected post hydrothermal exposure and distinguished from partially coordinated framework sites. Hydration impacts the appearance and interpretation of conventional Al-27 spectra, and initial assessment of aluminum sites in high-Al content MFI catalysts can guide reaction conditions.
The structure of aluminum-containing moieties in and within zeolite H-ZSM-5 catalysts is a complex function of the elemental composition of the catalyst, synthesis conditions, exposure to moisture, and thermal history. Al-27 NMR data collected at field strengths ranging from 7.05 to 35.2 T, i.e., H-1 Larmor frequencies from 300 to 1500 MHz, reveal that Al primarily exists as framework or partially coordinated framework species in commercially available dehydrated H-ZSM-5 catalysts with Si/Al ranging from 11.5 to 40. Quantitative direct-excitation and sensitivity-enhanced Al-27 NMR techniques applied over the wide range of magnetic field strengths used in this study show that prior to significant hydrothermal exposure, detectable amounts of nonframework Al species do not exist. Two-dimensional Al-27 multiple-quantum magic-angle spinning (MQMAS) along with H-1-Al-27 and Si-29-Al-27 dipolar correlation (D-HMQC) NMR experiments confirm this conclusion and show that generation of nonframework species following varying severities of hydrothermal exposure are clearly resolved from partially coordinated framework sites. The impact of hydration on the appearance and interpretation of conventional direct-excitation Al-27 spectra, commonly used to assess framework and nonframework Al, is discussed. Aluminum sites in dehydrated catalysts, which are representative of typical operating conditions, are characterized by large quadrupole interactions and are best assigned by obtaining data at multiple field strengths. On the basis of the results here, an accurate initial assessment of Al sites in high-Al content MFI catalysts prior to any hydrothermal treatment can be used to guide reaction conditions, anticipate potential water impacts, and identify contributions from hydroxyl groups other than those associated with the framework bridging acid site.
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