Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Metal Active Sites and Their Catalytic Functions in Zeolites: Insights from Solid-State NMR Spectroscopy Jun Xu,* Qiang Wang, and Feng Deng*
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State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, National Center for Magnetic Resonance in Wuhan, Key Laboratory of Magnetic Resonance in Biological Systems, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China CONSPECTUS: Zeolites are important heterogeneous catalysts widely used in the modern chemical and petrochemical industries. Metal-containing zeolites show distinct performance in the catalytic processes such as fluid catalytic cracking, activation and conversion of light alkanes, methanol-to-aromatics conversion, biomass transformation, and so on. The metal speciation, distribution, and interactions on zeolites have enormous impact on their property and catalytic performance. Significant efforts have been devoted to the synthesis of more active and selective zeolites by engineering the metal active sites. However, the nature of metal species and their role in the reactions are still poorly understood, which makes it difficult to establish the structure−activity relationship toward the rational design and application of zeolites. For example, synergic active sites are often present on the metal-containing zeolites, but their structure, property and quantification still remain to be resolved. Solid-state NMR is a powerful tool for the characterization of heterogeneous catalysts and catalytic reactions by providing information about both molecular structure and dynamics. The heterogeneity and low concentration of the metal sites on zeolites usually leads to a great challenge for their characterization. In this Account, we will describe our effort to study the metal active sites, host−guest interactions, and reaction intermediates by using solid-state NMR spectroscopy, with the aim to highlight recent advances in solid-state NMR techniques for probing the structure and property of metal-containing zeolites as well as the relevant reaction mechanisms. Using sensitivity-enhanced NMR methods such as 67Zn, 71Ga, and 119Sn, NMR enables the identification of metal speciation on zeolites. The synergic active sites constituted by metal species (as Lewis acid sites) and acidic protons (as Brønsted acid sites) on zeolites that amount to only a small fraction of the whole system can be directly probed and quantified with advanced 1 H−67Zn or 1H−71Ga double-resonance solid-state NMR. We developed NMR methods to study the host−guest interactions in zeolites by observing the spatial interaction/proximity between aluminum sites (associated with Brønsted or Lewis acid sites) in zeolite host and carbon atoms in organic molecule guest formed during catalytic reaction, which leads to the formation of supramolecular reaction centers in the methanol-to-olefins reaction. The mechanisms underlying the catalytic reactions on metal-modified zeolite are revealed by the identification of key reaction intermediates with in situ 13C MAS NMR spectroscopy. Our discussion based on the representative examples shows how the metal species serving as active sites significantly affect the property and activity of zeolites and related reaction pathways. The structural information obtained by the state-of-the-art solidstate NMR techniques provides new insights into the structure−activity relationship of zeolites in heterogeneous catalysis, which should be beneficial for rational design of highly efficient zeolite catalysts.
1. INTRODUCTION
and higher catalytic performance. On the other hand, the introduction of metal elements such as Zn, Ga, Mo, Ti, and Sn onto zeolites often gives rise to Lewis acid sites along with the intrinsic BAS. These metal-containing zeolites are intensively used for multifunctionally catalyzed reactions for the production of chemicals and fuel not only in the petrochemical field but also in the biomass-transformation processes.3 Despite their unparalleled catalytic activity, the structure and property of metal active sites on zeolites are not well understood. There is increasing evidence that the metal elements can be present in different states, such as isolated
As one kind of the most important heterogeneous catalysts, zeolites have found a wide range of catalytic applications in various petroleum refining and petrochemical processes. The bridging hydroxyl groups (Si−OH−Al) associated with neighboring SiO4− and AlO4− tetrahedra in zeolite framework act as the Brønsted acid sites (BAS) which are responsible for the ability of zeolites to catalyze traditional refinery reactions. Partial release of the Al species from zeolite framework by dealumination treatment typically results in the formation of Lewis acid sites (LAS) due to the formation of extraframework Al species. New character can be generated on the dealuminated zeolites such as Brønsted/Lewis acid synergy,1,2 which leads to enhanced Brønsted acid strength © XXXX American Chemical Society
Received: March 6, 2019
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DOI: 10.1021/acs.accounts.9b00125 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research Table 1. Topics and Targeted Nuclei Studied by Suitable SSNMR Experiments topic metal active site
nucleus 67
Zn, 71Ga,
95
SSNMR experiment Mo
119
Sn
Brønsted/Lewis acid synergy host−guest interaction surface active intermediates
1
H−1H, 27Al−27Al H−67Zn, 1H−71Ga 13 C−27Al 13 C 1
ref
HS-QCMPG (hyperbolic secant + quadrupolar Carr−Purcell−Meiboom−Gill) WURST-QCPMG (wideband uniform-rate smooth truncation + quadrupolar Carr−Purcell− Meiboom−Gill) D-HMQC (dipolar-mediated heteronuclear multiple quantum correlation) CPMG (Carr−Purcell−Meiboom−Gill) DNP (dynamic nuclear polarization)
7, 10, 14
DQ (double quantum)−SQ (single quantum) homonuclear correlation S-RESPDOR (symmetry-based resonance-echo saturation-pulse double-resonance) S-RESPDOR, double resonance for nuclei with close Lamor frequency in situ NMR
1, 2, 13 10, 14 26, 31−33 37−39, 41−44
22 21 20
Figure 1. (a) 67Zn HS-QCPMG NMR spectra recorded at 18.8 T of 67Zn-enriched ZnO/ZSM-5 (prepared by mixing ZnO with ZSM-5) and 67Znenriched Zn/ZSM-5 (prepared by incipient wetness impregnation), (b) 1H−67Zn S-RESPDOR NMR spectra and dephasing curves, and (c) schematic of the proximate acidic proton and Zn2+ ion in Zn/ZSM-5. Reproduced with permission from ref 10. Copyright 2016 Wiley-VCH.
tional BAS, the ununiformity of metal species with respect to speciation and distribution on zeolites brings about great difficulty in their structural characterization by SSNMR. It is not an easy task to distinguish between different metal species which often exist with heterogeneous distribution. Moreover, the ill-defined metal active sites make it highly challenging to elucidate the catalytic reaction mechanism. In this Account, we focus on the utilization of state-of-theart SSNMR techniques for the study of structure and property of metal active sites and their catalytic functions in zeolites. Table 1 lists the discussed topics and the targeted nuclei studied by suitable SSNMR experiments. We highlight our recent progress in the characterization of the metal speciation, spatial proximities of different active sites that lead to synergic effect, and reaction intermediates formed in the catalytic reactions. We also show how the advanced NMR methods provide new insights into the host−guest interactions and their catalytic consequences in zeolite catalysis.
species, clusters, and bulk metal oxides, constituting a variety of active sites as well as spectator species. Moreover, the position of the metal species affects their property and activity in the catalytic reactions of interest. To rationally design metal-containing zeolites with high activity and selectivity, a detailed knowledge of the precise nature and behavior of the metal active sites at atomic level is required. Among the spectroscopic techniques, solid-state NMR (SSNMR) spectroscopy is a powerful technique for the characterization of heterogeneous catalysts and porous materials.4−6 In particular, SSNMR is unique to detect different active sites as well as their connectivities and correlations. The successful application of SSNMR in zeolites and related catalytic reactions has significantly increased our understanding of the composition, structure and property of the active sites on zeolites as well as their catalytic functions. For example, the nature of BAS on acidic zeolite has been well characterized with SSNMR spectroscopy.4 Unlike the convenB
DOI: 10.1021/acs.accounts.9b00125 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 2. (a) 71Ga WURST-QCPMG NMR spectrum of Ga/ZSM-5 (prepared by incipient wetness impregnation) and 1H−71Ga S-RESPDOR dephasing curve, (b) 2D 1H−1H DQ-SQ MAS NMR spectra and the models for BAS pair and BAS-Ga pair (internuclear distance is indicated), and (c) schematic of the proximate BAS-Ga site in the channel of Ga/ZSM-5. Reproduced with permission from ref 14. Copyright 2018 American Chemical Society.
2. METAL ACTIVE SITES AND THEIR SYNERGIC EFFECT IN ZEOLITES Both qualitative and quantitative analysis of metal active sites is essential for assessing their catalyst performance. The metals of interest in zeolites such as Zn, Mo, and Ti often suffer from formidable challenge for NMR characterization because of the low detection sensitivity arising from low gyromagnetic ratio (γ) and large quadrupolar moment. The difficulty is exacerbated by the low metal loading (
