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沸石催化剂中铝物种的分布:骨架、部分配位骨架和非骨架部分的铝核磁共振研究

Distribution of Aluminum Species in Zeolite Catalysts: Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties.

作者信息

Chen Kuizhi, Gan Zhehong, Horstmeier Sarah, White Jeffery L

机构信息

National High Magnetic Field Laboratory, Tallahassee, Florida 32310, United States.

School of Chemical Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, United States.

出版信息

J Am Chem Soc. 2021 May 5;143(17):6669-6680. doi: 10.1021/jacs.1c02361. Epub 2021 Apr 21.

Abstract

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 NMR data collected at field strengths ranging from 7.05 to 35.2 T, i.e., H 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 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 multiple-quantum magic-angle spinning (MQMAS) along with H-Al and Si-Al 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 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.

摘要

沸石H-ZSM-5催化剂内部及其中含铝部分的结构是催化剂元素组成、合成条件、暴露于湿气的情况以及热历史的复杂函数。在7.05至35.2 T的场强下收集的铝核磁共振(Al NMR)数据,即氢(H)拉莫尔频率在300至1500 MHz范围内的数据,表明在市售的脱水H-ZSM-5催化剂中,硅铝比(Si/Al)在11.5至40之间时,铝主要以骨架或部分配位的骨架物种形式存在。在本研究中使用的宽磁场强度范围内应用的定量直接激发和灵敏度增强的Al NMR技术表明,在显著的水热暴露之前,不存在可检测量的非骨架铝物种。二维铝多量子魔角旋转(MQMAS)以及氢-铝和硅-铝偶极相关(D-HMQC)NMR实验证实了这一结论,并表明在不同严重程度的水热暴露后产生的非骨架物种与部分配位的骨架位点能够清晰区分。讨论了水合作用对常用于评估骨架铝和非骨架铝的传统直接激发Al光谱的外观和解释的影响。脱水催化剂中的铝位点代表典型操作条件,其特征是具有大的四极相互作用,最好通过在多个场强下获取数据来进行归属。基于此处的结果,在任何水热处理之前对高铝含量MFI催化剂中的铝位点进行准确的初始评估,可用于指导反应条件、预测潜在的水影响,并识别除了与骨架桥连酸位点相关的羟基之外其他羟基的贡献。

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