Mélodie Bourger-Bouveret

Esther Atlan

Minna Puustinen

The inclusion and education of pupils with PIMD: What contributions from special institutions?

. In France, the drive for inclusion has led to new coalitions between institutions administered by the separate Ministries of Education and Social Affairs. A paradigm shift is occurring, with staff from specialised institutions being deployed within mainstream schools to support the education of pupils with PIMD in special classrooms. The implementation of these arrangements requires close coordination between professionals from distinct communities of practice. To better understand this phenomenon, a focus group study was conducted in a specialised institution. Nine professionals working with children with PIMD (i.e., educators, carers, social and educational workers, and a service manager) participated in a semi-structured group interview. Data were collected on the representations of work (e.g., type of activities, duties, norms or values) and on educational practices (e.g., strategies, methods, attitudes or gestures). This interview was audio and video recorded for verbatim accuracy, and a thematic analysis was conducted on the transcripts. Results highlight a representation of the activity as being based on care and education for autonomy and necessarily reflexive, whereas the educational practices are anchored on both high-quality relationships and a deep understanding of the children's needs. Through an overview of their practices, this preliminary study illustrates some of the possible contributions of professionals from specialised institutions to inclusive education.

Introduction

The inclusion and education of students with profound intellectual and multiple disabilities (PIMD) remains a major challenge for most public policies but also for the practices of both schools and disability services [START_REF] Lyons | UNESCO Inclusion policy and the education of school students with profound intellectual and multiple disabilities: Where to now[END_REF]. Children with PIMD face many barriers in learning due to the severity of their cognitive, motor and/or sensory impairments and significant health problems [START_REF] Bellamy | A study to define: Profound and multiple learning disabilities (PMLD)[END_REF]. Nevertheless, considerable research efforts have recently led to the specification of a framework and key indicators for quality education [START_REF] Maes | Inclusion and Participation of Students with Profound Intellectual and Multiple Disabilities[END_REF], and previous work has shown that inclusive environments lead to better social participation outcomes [START_REF] Foreman | Evaluating the educational experiences of students with profound and multiple disabilities in inclusive and segregated classroom settings: An Australian perspective[END_REF][START_REF] Nijs | Social Peer Interactions in Persons with Profound Intellectual and Multiple Disabilities: A Literature Review[END_REF]. In several countries, meanwhile, children with PIMD remain excluded from school education as they fall under separate institutional systems [START_REF] Slee | Beyond special and regular schooling? An inclusive education reform agenda[END_REF]. In compliance with UNESCO's guidelines for inclusion in education ( 2009), many public policies now address this issue. However, their implementation can be challenging, as they may conflict with prevailing measures based on the segregation of general and special education. Unaccustomed to complex disabilities, mainstream schools may struggle to meet high support needs, while specialised services may be reluctant to engage with inclusion if they previously worked in silos [START_REF] Sailor | Access to the General Curriculum: Systems Change or Tinker Some More? Research and Practice for Persons with Severe Disabilities[END_REF]. In France, children with PIMD were until recently largely deprived of formal education [START_REF] Bryon | Polyhandicap: État des lieux de la scolarisation[END_REF], leading the government to make this issue one of the main priorities of the last five-year strategy for the improvement of disability services (Secretary of State for Persons with Disabilities, 2017). This resulted in new measures for pupils with PIMD, such as the provision of dedicated classrooms in mainstream schools. These arrangements involve new partnerships, as professionals from specialised institutions are mobilised alongside teachers to provide special care and educational assistance.

Previous research has produced a detailed review of French teachers' practices towards pupils with PIMD [START_REF] Toubert-Duffort | Conditions d'accès aux apprentissages des jeunes polyhandicapés en établissements médicosociauxde l'évaluation des potentiels cognitifs à la mise en oeuvre de leur scolarisation[END_REF], but no data were available regarding the practices of staff working in specialised institutions. Further research is needed to support the implementation of these programmes, particularly with regard to coordination between the professionals involved.

Methodology

A focus group study [START_REF] Kitzinger | Qualitative Research: Introducing focus groups[END_REF] was conducted in a specialised institution for children with PIMD, with the participation of nine of its professionals (i.e., four educators, three carers, two social and educational workers, and a service manager). The 105-minute interview was audio and video recorded and structured around a list of open-ended questions covering the professionals' representations of work (e.g., type of activities, tasks, norms or values) and educational practices (e.g., strategies, methods, attitudes or gestures). The data were transcribed using the Amberscript software and corrected from the recordings which were then destroyed. Finally, a thematic analysis was conducted to extract salient themes and sub-themes from the verbatim data. Results are presented along two dimensions, namely the characteristics and determinants of staff activity and professional practices related to the learning of children in educational activity.

Results

The staff activity dimension is divided into three themes, namely main orientations and values, reflexivity on practices and organisational determinants. Results suggests that professionals conceive their practices as an interrelated combination of proximal care and education for autonomy. The severity and complexity of children's disabilities often expose them to uncertainty about the outcome of their actions. This difficulty is addressed through a reflective approach to practice, in which three components are identified: active observation in various contexts, ongoing self-examination through action and reliance on cross-checking. These are related to ethical values and prevention of emotional distress or burnout, and seem to depend on organisational context. The educational practices dimension is divided into four themes, namely individual educational planning, environmental settings, adaptations and supports for learning activities, and identification of optimal learning moments. Results suggest that educational planning is both highly individualised and on-going, with a primary focus on autonomy. Professionals perceive affective continuity as an essential feature of learning environments, along with the reduction of external stimuli or the accessibility of educational contents. Support for exploration and engagement in learning is shaped by warm and proximate gestures and attitudes and linked to a deep understanding of individual needs and preferences. This specific knowledge also enables staff to recognise the optimal moments for learning, perceived as short and fleeting, and related to their students' states of attention and alertness.

Discussions

Various practices described in this study have an apparent similarity to teaching practices for students with PIMD, but are based on distinct concerns [START_REF] Sailor | Access to the General Curriculum: Systems Change or Tinker Some More? Research and Practice for Persons with Severe Disabilities[END_REF]. The care and assistance provided in specialised institutions takes place in close and continuous proximity to children with PIMD, so the professionals of those institutions have an in-depth knowledge of their specificities and needs. The proximal relationship and warm attitude nurtured by these professionals can provide a solid foundation for the quality of the classroom environment, given its impact on learning outcomes [START_REF] Arthur-Kelly | Socio-communicative perspectives on research and evidence-based practice in the education of students with profound and multiple disabilities[END_REF] and the need for children with PIMD to experience high quality and supportive interactions to engage in learning. [START_REF] De Bortoli | Mainstream teachers' experiences of communicating with students with multiple and severe disabilities[END_REF][START_REF] Imray | Care as an educational concept[END_REF]. As the identification of optimal learning moments is essential for quality teaching [START_REF] Petitpierre | Integration of Education and Care Given to Children With Profound Multiple Disabilities in Switzerland[END_REF], these professionals can help teachers by offering useful guidance to encourage their students to be active and alert in their learning [START_REF] Munde | Initiation of activities and alertness in individuals with profound intellectual and multiple disabilities: Initiation and alertness[END_REF]. Finally, their reflexive approach to activity could significantly support the refinement of teaching practices, given the complexity of PIMD situations which require joint analyses based on fine and repeated observations [START_REF] Atlan | L'observation et l'évaluation d'élèves en situation de polyhandicap: Une nécessaire réflexion méthodologique[END_REF], and the considerable benefits of peer-to-peer sharing for professional learning in the teaching of pupils with PIMD [START_REF] Jones | Trying, failing, succeeding, and trying again and again': Perspectives of teachers of pupils with severe profound multiple learning difficulties[END_REF]. In France, as in other countries, inclusive education now involves the shared commitment of professionals from both mainstream and special contexts. This study suggests that their respective practices and knowledge could complement each other in meeting the challenge of shaping education to meet the most complex needs. Given their different backgrounds, this convergence could be strengthened by opportunities to engage in dialogue on practices, which is a prerequisite for their joint development [START_REF] Engestrom | Activity theory as a framework for analyzing and redesigning work[END_REF].


Shenhui Li

Olivier Lafon

Weiyu Wang

Qiang Wang

Prof Xingxing X Y Wang

Yi Li

Jun Xu
email: xujun@wipm.ac.cn

Prof Feng S H Deng

Dr W Y Li

Prof Q Wang

Prof J Wang

Prof F Xu

Deng

Prof J Xu

Prof O Lafon

Prof Y Li

Recent advances of solid-state nmr spectroscopy for microporous materials

Keywords: porous materials, confined space, structure-property relationship, characterization, solid-state NMR

come

Introduction

Porous materials are of great interest from both fundmental and technological perspectives.

Amongst various types of porous materials, microporous materials (with pore diameter < 2 nm), such as zeolites, metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and porous aromatic frameworks (PAFs), combine high specific surface areas, large pore volumes and shape-selectivity effects, which makes them key materials for a wide range of applications, including catalysis, gas separation/purfication, ion exchange, gas storage, and sensing.

The diverse framework compositions and functionalities of microporous materials represent a huge challenge for their structural characterization as well as evaluation of their performances.

Analytic tools such as X-ray diffraction (XRD), electron microscopy and adsorptiondesorption isotherms are now routinely employed for characterization of microporous materials in order to establish the structure-property relationships, and hence, to facilitate the rational design of advanced materials with improved property. However, the structure determination of porous materials using single crystal XRD requires the high crystalline and long-range ordering of the framework. Solid-state NMR (SSNMR) has emerged as a powerful spectroscopic technique with atomic-level resolution, complementary to XRD, in the investigation of structures in materials science due to its sensitivity to geometries and orderings in short to medium range. [1] Information on the chemical composition, local environment, pore connectivity, and coordination network of porous materials can be retrieved from a variety of NMR parameters. Besides, SSNMR is able to provide detailed information on dynamic behavior of molecules at different time scales and probe host-guest interactions.

Tremendous progress has been made in the field of SSNMR study of microporous materials in the past decades. [1c, 2] Multinuclear and multidimensional SSNMR techniques have been extensively utilized to characterize the framework structure of porous materials and explore the dynamics of guest molecules. The self-diffusion coefficient and diffusion pathway of guest molecules along the channels of porous materials have be examined by pulsed field gradient (PFG) NMR spectroscopy. Hyperpolarized 129 Xe NMR can be employed to investigate the cage and channel structure and the communication in porous materials. [3] Variable temperature (VT) SSNMR is a well-established tool for elucidating the molecular flexibility and dynamic behavior of porous functional materials. [4] SSNMR exhibits great potential for investigating the host-guest interaction between small molecules and porous materials. [2c, 5] Furthermore, in situ SSNMR is becoming increasingly important in the study of catalytic reaction mechanism over microporous materials by observing reactive intermediates trapped in voids or channels. Instrumentation and techniques have been recently developed to improve the sensitivity of SSNMR of materials and detect the signal of nuclei of interests with low natural abundance, small gyromagnetic ratio (γ), or subject to large quadrupole interactions.

In this report, novel SSNMR methods and advanced instruments utilized to characterize microporous materials are briefly introduced. Recent advances in the application of SSNMR in porous materials are critically reviewed with focus on the progress on zeolites, MOFs, COFs, PAFs, and layered materials. We discuss how the scientific issues relevant to the structures, interactions and dynamics involved in these fascinating porous materials can be addressed by advanced SSNMR spectroscopy. The challenges and prospects of SSNMR techniques for the study of porous materials are also described.

Brief introduction of SSNMR methods and instruments

NMR parameters and relation with structural information for materials

SSNMR is an element-specific method, which provides a high content of information on solid materials. SSNMR spectra feature signal broadening as compared to the high resolution signals of solution-state NMR, owing to various anisotropic interactions in materials without sufficient molecular motion, including dipolar interaction, chemical shift anisotropy (CSA), and quadrupolar interaction for quadrupolar spin with I ≥ 1. Since these interactions are highly sensitive to the chemical and electronic environment, local geometry and dynamic behavior, they can be used to extract important information on the structure and chemical process. For example, adsorbed CO2 in MOFs exhibits restricted molecular motion due to the strong interaction between CO2 and metal center, which results in an evident CSA powder pattern in the 13 C NMR spectra. [6] The CSA can be determined through spectral fitting of onedimensional (1D) static spectral pattern or magic angle spinning (MAS) sidebands, as well as extracted from two-dimensional (2D) NMR experiments refocusing CSA in indirect dimension. For quadrupolar nuclei with half integer spin, such as the framework element 27 Al (I = 5/2) in zeolite and metal ion 25 Mg (I = 5/2) in MOFs, 2D multiple quantum (MQ) MAS NMR technique [7] allows achieving high-resolution isotropic spectra after eliminating secondorder quadrupolar interaction by which the framework site with different coordination numbers can be differentiated. Quadrupolar coupling constant (QCC) and asymmetry parameter η can be extracted from the MQMAS NMR spectra to provide refined information on the geometry of the structural unit with similar coordination state. An example is the identification of distorted tetrahedral Al in mordenite zeolite by its larger QCC from 27 Al triple-quantum (3Q) MAS NMR measurement. [8] The isotropic chemical shift (δiso) in high-resolution spectra provides a straightforward and simple way to get structural information for porous materials, such as 29 Si NMR for zeolites, 1 H and 13 C NMR for MOFs, COFs and PAFs. Dipolar decoupling is often required to obtain a high-resolution spectra of these nuclei by removing or reducing the broadening effect due to the dipolar couplings. On the other hand, the dipolar couplings, which depend on the internuclear distance, are very useful for extracting valuable information on connectivities and proximities between the same or distinct nuclei. Proximities between 29 Si nuclei in zeolites can be probed through 29 Si- 29 Si dipolar couplings (through-space), which can be reintroduced under MAS by application of recoupling schemes. This recoupling has been incorporated into 2D double-quantum-single-quantum (DQ-SQ) homonuclear correlation experiment, which gives information on the 29 Si-O- 29 Si proximities, as well as internuclear distances. [9] The proximities between protons in zeolites and MOFs have also been observed using 2D throughspace SQ-SQ or DQ-SQ homonuclear correlation experiments through dipolar recoupling. [10] DQ-SQ experiment allows the observation of the proximities between the sites with close δiso values. Additionally, 29 Si-O- 29 Si J-couplings (through-bond) have been used to identify the 29 Si sites linked by Si-O-Si bridge in zeolites. [11] This connectivity information has notably been obtained at natural abundance using 2D 29 Si refocused incredible natural abundance double-quantum transfer experiment (INADEQUATE). Information on the proximities between half-integer spin quadrupolar nuclei with I ≥ 3/2, such as 27 Al, can also been obtained using 2D through-space DQ-SQ homonuclear correlation experiments. [12] For example, detailed spatial correlations among various aluminum species in dealuminated HY zeolite were revealed by 27 Al DQ-MAS NMR spectroscopy. [12a] Spatial connectivities and proximities between two distinct nuclei can be probed using 2D heteronuclear correlation (HETCOR) experiment. The polarization transfer could be established through-bond correlation using heteronuclear multiple-quantum coherence (J-HMQC) or through-space correlation utilizing cross polarization (CP) or heteronuclear multiple-quantum coherence (D-HMQC) experiments. For example, the 11 B-O- 29 Si and 29 Si-O- 27 Al connectivities in zeolites have been probed using J-HMQC experiments. [13] Internuclear distances can be quantitatively determined by rotational-echo double-resonance (REDOR) [14] when the dephaser isotope has a spin I = 1/2, or its variants including transfer of population in double-resonance (TRAPDOR) [15] and the rotational-echo adiabatic-passage double-resonance (REAPDOR) [16] suitable for quadrupolar dephaser isotope. More recently, rotational-echo saturation-pulse double-resonance (RESPDOR) sequence has been introduced as an alternative to REAPDOR and TRAPDOR methods. [17] Compared to TRAPDOR and REAPDOR, RESPDOR experiment requiring moderate radio frequency (RF) field is compatible with high speed MAS, and for an isolated spin pair, its dipolar dephasing can be expressed by an analytical formula, which allows a rapid determination of internuclear distances. The host-guest interactions and Al-C proximities in dealuminated HY zeolite [17b] and in MIL-100(Al) framework [18] were explored using 27 Al-13 C S-RESPDOR NMR experiments.

Sensitivity enhancement technique

A major limitation of solid-state NMR spectroscopy is its relatively low sensitivity owing to the small nuclear magnetic moments. The low sensitivity impairs the observation of diluted species or insensitive isotopes with low γ, such as 67 Zn and 91 Zr, or low natural abundance, such as 15 N and 17 O. Besides the isotope enrichments strategy, advanced SSNMR instruments and methods can effectively enhance NMR detection sensitivity. One approach to increase the sensitivity is to use high static magnetic fields, B0. Higher fields bring gains of sensitivity roughly proportional to B0 7/4 for spin-1/2 isotope and B0 11/4 for half-integer quadrupolar nuclei.

Higher fields also improve the resolution, which is proportional to B0 and B0 2 for spectral dimensions corresponding to spin-1/2 and half-integer quadrupolar nuclei, respectively.

Thanks to the availability of high-field (> 18.8 T) NMR spectrometers, the NMR spectra with significantly enhanced spectral resolution were recently reported for half-integer quadrupolar nuclei in microporous materials such as 27 Al in zeolite, [19] 25 Mg and 67 Zn in MOFs. [20] Further gain in sensitivity can be obtained using Carr-Purcell Meiboom-Gill (CPMG) detection or its QCPMG variants for quadrupolar nuclei. [21] In recent years, dynamic nuclear polarization (DNP) has also been demonstrated a promising technique to enhance the NMR signal of microporous materials. [22] This technique utilizes the microwave-driven transfer of polarization from unpaired electrons to the nuclear spins and yields sensitivity gain by one to three orders of magnitude. The DNP-NMR system generally includes a high-power microwave source, such as a gyrotron, and a cryogenic MAS probe, which operates at temperature of 100 K. Furthermore, MAS DNP experiments at B0 ≥ 5 T generally requires the introduction of nitroxide biradicals into the investigated materials, which act as the source of polarization. The sensitivity gain provided by DNP has enabled the detection of insensitive isotopes, such as 15 N,119 Sn and 195 Pt, [22][23] as well as diluted species. [24]

SSNMR characterization of zeolites

Zeolites are inorganic crystallites containing pores of molecular dimensions and cavities with well-defined structures and are widely applied in diverse areas such as catalysis, ion exchange, and separations in chemical and petrochemical industry. The three-dimensional fourconnected framework of classical aluminosilicate zeolite, made up of corner-sharing TO4 (T = Si and Al) tetrahedra, can be comprehensively characterized by the well-established NMR experiments. Advanced SSNMR techniques afford detailed information on the framework or extra-framework species, host-guest interactions and catalytic reactions occurring in the confined space of zeolites.

Characterization of zeolite framework

Multidimensional and multinuclear ( 29 Si, 27 Al, 17 O) SSNMR have been extensively applied in the structural characterization of zeolites. 29 Si NMR chemical shifts are very sensitive to the local chemical environments of framework. [25] The topology structure of purely siliceous zeolite could be determined by a combination of 29 Si DQ MAS NMR spectroscopy and powder XRD, which relies on the framework's crystallinity and spectral resolution of 29 Si NMR. The 29 Si- 29 Si distances of different Si pairs within framework can be elucidated from the 29 Si DQ build-up curves as a function of homo-nuclear recoupling times. [9a, b] Moreover, the 29 Si-O- 29 Si scalar couplings [11] and CSA [26] of pure silicon zeolite extracted from 2D refocused-INADEQUATE 29 Si{ 29 Si} NMR and 2D CSA recoupling NMR experiments could serve as structural constrains for solving the 3D framework structure. The 29 Si-O- 29 Si connectivities within the as-synthesized zeolites ITW and MTT were established through 2D 29 Si{ 29 Si} DQ MAS NMR experiments, which shed light on complicated order and disorder within their frameworks. [27] Smeets and coworkers applied DNP-enhanced 2D 29 Si homo-and hetero-nuclear correlation NMR to establish the through-bond connectivities of distinct Si sites and investigated the local structure of calcined Si-SSZ-70 at natural 29 Si isotopic abundance (4.7%). [28] Figure 1a shows 1D 29 Si MAS NMR spectrum of calcined Si-SSZ-70, which consisted of Q 4 sites (-105~-121 ppm) and Q 3 sites (-95~-104 ppm). In the DNP-enhanced 2D 29 Si{ 1 H} HETCOR spectrum (Figure 1b), 29 Si signal at -99 ppm from interlayer Q 3 silanol species was spatially correlated with the 1 H signal at 3.0 ppm from isolated silanol groups. Additionally, the 29 Si signal at -99 ppm was correlated with the 1 H signal at 8.8 ppm arising from the strongly H-bonded -SiOH moieties. The covalent 29 Si-O- 29 Si connectivities of nearest neighbor Q 3 and Q 4 sites were clearly manifested from the DNP-enhanced 2D 29 Si{ 29 Si} Jmediated correlation spectrum (Figure 1c). As indicated in blue lines, the Q 4 Si2b signals corresponding to the proposed model 2 (Figure 1d). Further DFT calculations would be helpful to support the assignments of various Si sites. The high sensitivity of the 2D DNPenhanced 29 Si NMR spectra provided direct evidence for the presence of two distinct types of Q 3 silanol species and their detailed bonding environments at local atomic-level connectivity in the partially disordered system (Figure 1e). Structural illustration of the interlayer region of calcined Si-SSZ-70. Reproduced with permission. [28] Copyright 2017, American Chemical Society.

The framework of doubly 29 Si-and 17 O-isotopically enriched Ge-UTL derived from assembly, disassembly, organization, reassembly (ADOR) method was fully examined by 29 Si and 17 O MAS NMR to monitor the hydrolysis and rearrangement process. [29] The 29 Si and 17 O NMR experiments reavelaed that the hydrolysis and rearrangement process proceeded over a much longer time scale than observed by XRD. To understand how the neutral liquid water affects the stability of the zeolite framework under mild conditions, Christopher and coworkers found that the covalent bonds in zeolite chabazite (CHA) underwent partial hydrolysis in contact with neutral liquid water at mild condition by using 27 Al, 29 Si and 17 O MAS NMR. [30] This process was fully reversible without framework degradation. 17 O MAS NMR was also used to obtian detailed structural information on the surface oxygen sites [31] and framework linkages [32] in zeolites and related materials. A quantitative analysis of the distribution and sitting of aluminum for different T-sites in H-BEA zeolite was achieved by using 27 Al MAS NMR in combination of extended X-ray absorption fine structure (EXAFS) spectroscopy. [33] Different Al distributions were observed in zeolite with the same framework topology. Tri-coordinated framework Al acting as Lewis acid sites was identified by using advanced NMR methods including 27 Al{ 1 H} REDOR MQMAS NMR, 31 P{ 27 Al} through-space HMQC and 31 P{ 27 Al} S-RESPDOR MAS NMR experiments. [34] Undertanding of synthesis mechainsm is essential for guding a rational design of robust materials. 27 Al, 29 Si, and 31 P SSNMR have been used as probes to monitor the crystallization of framework and examine intermediate phases in the synthesis of Beta and ZSM-5, [35] silicalite-1 [36] and microporous aluminophosphate molecular sieves such as SAPO-34 [37] and AlPO4-5 [38] . The evolution and rearrangment of the primary structure units composed of Si, Al NMR spectroscopy. [39] The concentration of these open site was determined to be to ca. 17 % of the total Sn atoms in framework. Furthermore, 2D 1 H{ 29 Si} D-HMQC NMR experiment revealed a reversible transformation between the open and closed Sn sites.

Characterization of extra-framework species in zeolites

Apart from zeolite framework, there is an increasing of interest in the study of extraframework metal species in zeolites. The introduction of metals (Zn, Ga, Mo, etc.) or metal oxides into zeolites results in bifunctional catalysts of combined acidity and redox property, considerably enhancing their catalytic performances compared to unmodified ones. The property and catalytic performance of metal-modified zeolites depends on the metal speciation, distribution and interactions on zeolites.

SSNMR investigation of metals of interest, such as 67 Zn and 95 Mo, is very challenging due to their very low γ and low natural abundance, and large quadrupolar moment as well as the low metal loading (few wt% of the sample) in zeolite like Zn/ZSM-5 and Mo/ZSM-5. In order to tackle these challenges, it is always necessary to combine isotopic enrichments during sample preparation, ultra-high magnetic field spectrometer, and advanced pulse sequences to enable the NMR detection. Ultra-high field 95 Mo NMR (21.1 T) was employed to investigate the introduced Mo species on 95 Mo isotopically enriched-ZSM-5, which showed that the ionexchanged Mo species was active center for methane dehydroaromatization reaction. [40] Qi and coworkers applied 67 Zn and 1 H{ 67 Zn} double-resonance MAS NMR to detect the surface Zn species and their spatial interaction with Brønsted acid sites on Zn-modified ZSM-5 zeolite. [21b] A sensitivity-enhanced hyperbolic secant (HS) and Quadrupolar

Carr-Purcell-Meiboom-Gill (QCPMG) NMR technique combined with at high magnetic field (18.8 T) produced a 16-fold 67 Zn NMR signal enhancement on natural abundance ZnO powder and allowed structural characterization of the 67 Zn isotope-enriched ZSM-5 samples (Zn/H-ZSM-5). As shown in Figure 2a, 67 Zn HS-QCPMG NMR spectra of Zn/H-ZSM-5 exhibited two 67 Zn signals with isotropic chemical shifts of 224 and 238 ppm, ascribed to Zn 2+ ions located on the cation exchange sites of ZSM-5 and highly dispersed ZnO particles respectively. The 1 H{ 67 Zn} S-RESPDOR NMR spectra offered direct experimental evidence on the spatial proximity between Brønsted acidic protons and zinc species as reflected from the significant signal dephasing for the SiOHAl (at 4.3 ppm) [41] (Figure 2b). The 1 H- 67 Zn internuclear (2.70~3.34 Å) distance between the Brønsted acidic proton and zinc atom was determined by the S-RESPDOR experiment (Figure 2c). The spatial interaction between Zn 2+ Reproduced with permission. [42] Copyright 2018, American Chemical Society.

The wideband uniform-rate smooth truncation and QCPMG (WURST-QCPMG) 71 Ga NMR and 1 H{ 71 Ga} S-RESPDOR NMR have been employed to investigate the chemical environments of extra-framework gallium species in Ga-modified ZSM-5 zeolites. [42] Two 71 Ga signals with isotropic chemical shifts at 58 and 190 ppm were resolved in the 71 Ga WURST-QCPMG NMR spectra of Ga/ZSM-5 zeolite (Figure 2d), which can be ascribed to highly dispersed Ga2O3 clusters and extra-framework Ga species in the form of GaO + , respectively. The 1 H{ 71 Ga} S-RESPDOR NMR and the resulting build-up curve (Figure 2e)

suggested the cationic Ga species was involved in the formation of proximate Brønsted acid site-Ga pairs with an internuclear distance being 5.05 Å, consistent with the average 1 H-1 H distance between neighboring Brønsted acid sites in H- ZSM-5 (ca. 4.50 Å). This provided detailed information on the location of the Brønsted acid site-Ga pair: sitting on the next-nextnearest-neighboring Al in the 6-membered ring of zeolite channel (Figure 2f).

Based on the S-RESPDOR experiment, a viable NMR method capable of quantitative determination of the synergic active sites was developed, which provides a tool for the analysis of intrinsic activity of the metal-modified zeolites. From the maximum dephasing fraction, residual acidic protons on zeolite, isotopic abundance of metal nuclei and the scaling factor of the 1 H dephasing fraction in the 1 H{M} (M: metal) S-RESPDOR experiment, it is possible to determine the concentration of the synergic active sites. This quantification approach is useful for establishing the correlation of the synergic active sites with their catalytic activity in methane activation and methanol-to-aromatics conversion on zeolites. [2f] Dislodging Al from zeolite framework results in extra-framework Al species, which significantly impacts the catalytic reactions. The formation and nature of extra-framework Al in zeolites was investigated by using multidimensional 27 Al MAS NMR spectroscopy. [12a, 43] 2D sensitivity-enhanced 27 Al DQ-SQ MAS NMR revealed the detailed evolution of extraframework Al species and the spatial proximities of various aluminum species in dealuminated HY zeolites. [12a] The strucutres of other extra-framework metal species such as silver and titanium in zeolites and related materials were examined by SSNMR as well.

Popovych and coworkers monitored the changes of silver in AgSi-BEA zeolite as a function of Ag content and thermal treatment by 109 Ag MAS NMR. [44] The full characterization of Ti species in zeolites is essential to understand the detailed property of the Ti active center in heterogeneous catalysis and photocatlysis. 47 Ti/ 49 Ti SSNMR has been applied to uncover the titanium environment of fresh and reused titanium-immobilized mesoporous silica nanoparticles. [45] Additionally, 47,49 Ti solid-state NMR has been employed to probe the location and chemical environment of titanium in Ti substituted USY and MFI-type TS-1 zeolites although the spectra feature low resolution because of the low gamma nature and low natural abundance of NMR active 47,49 Ti nucleus. [46] It was found that Ti in the TS-1 framework was essentially tetrahedrally coordinated and became octahedrally coordinated in the presence of water.

Host-guest interactions and active intermediates in catalytic reactions

The host-guest interaction between organic structure-directing agents and zeolites framework plays essential role in the crystallization and self-assembly during zeolite synthesis. 2D hetero-nuclear correlation SSNMR experiments were used to obtain detailed information on the ordering and arrangement of surfactants confined in FER-type zeolite, [47] ECNU-7P, [48] pure silica MFI zeolite, [49] HOU-4, [50] and mesostructured zeolites. [51] 1 H-13 C and 1 H- 29 Si HETCOR experiments confirmed the existence of strong intermolecular interactions between the quaternary ammonium head groups of cetyltriethylammnonium bromide (CTAB) and the ECNU-7P zeolite framework during the self-assembly and structure evolution process. [48] 1 H DQ MAS and 1 H triple-quantum (TQ) MAS NMR spectroscopy provided clear evidence for the interaction and connectivity between the internal defect SiOH and organic structuredirecting agent in ZSM-12, ZSM-5, and SSZ-74, suggesting that the six-rings are preferred in the positions near the organic structure-directing agent for the formation of connectivity defects. [52] The interaction between the guest molecules and the zeolite framework is also a subject of intensive studies regarding molecule diffusion, reactants adsorption, catalytic reactions, and products desorption. SSNMR is powerful tool to characterize the host-guest interactions in zeolites by selectively correlating the interacting nuclei. 13 C- 27 Al double-resonance NMR technique enables direct observation of interactions between the adsorbed or confined organic molecules guest (containing 13 C atoms) and zeolite framework host (containing 27 Al atoms), by detecting their spatial dipolar 13 C - 27 Al coupling.

The detailed host-guest interaction including the proximity and strength between dealuminated HY and acetone was investigated by 13 C- 27 Al double-resonance NMR. [17b] As shown in the 13 C{ 27 Al} S-RESPDOR NMR spectra of 2-13 C-acetone adsorbed on dealuminated HY (Figure 3a), the 13 C NMR signals in range of 228-234 ppm exhibited a strong 13 C- 27 Al dipolar dephasing. The distances between the carbonyl carbon of acetone and the neighboring framework aluminum were determined to be 3.4 Å by 13 C{ 27 Al} S-RESPDOR NMR build-up curve (Figure 3b). The 13 C signals at 228 and 234 ppm due to acetone adsorbed on Brønsted acid site also showed interaction with extra-framework aluminum (as Lewis acid site), as seen in the 2D 27 Al{ 13 C} D-HMQC spectrum (Figure 3c and 3d).

The host-guest interactions introduced by organic molecules in zeolite leads to the formation of supramolecular system. [53] The application of 13 C{ 27 Al} S-RESPDOR experiment coupled with 13 C NMR allowed for the characterization of the supramolecular reaction center (SRC) and its reactivity in the methanol-to-olefins (MTO) reaction, [54] which was supposed to be formed by the interaction between Brønsted acid site and trapped hydrocarbon pool (HP) species such as cyclic carbocations and multimethylbenzenes in zeolite. The 13 C{ 27 Al} S-RESPDOR spectra of 13 C-methanol reacted over H-ZSM-5 showed that the HP species (0~60 ppm) exhibited different degrees of 13 C- 27 Al dipolar dephasing (Figure 3e), suggesting the spatial interaction between the trapped HP species and Brønsted acid site and the formation of SRC. Further analysis of the 13 C- 27 Al dipole interaction indicated methylbenzenes (MBs)

interacted with Brønsted acid site by forming a π-complex, while cyclic carbocations interacted with Brønsted base site by forming an ion-pair complex (Figure 3f). The internuclear spatial interaction between the 13 C nuclei from the HP species and the 27 Al nuclei from Brønsted acid site dictated the reactivity of the HP species. The function of SRC in the MTO reaction was identified: closer HP species to zeolite framework Al resulted in higher reactivity of the HP species in MTO reaction. [54] Additionally, different interactions associated with the SRC in the MTO reaction were found in ZSM-5, H-SSZ-13, and H-MOR zeolites, revealing the influence of the shape selectivity of zeolites on the host-guest interactions. [55] Reproduced with permission. [54] Copyright 2016, Wiley-VCH.

The host-guest interactions on zeolite involving extra-framework Al species has been explored as well. [56] For the MTO reaction on dealuminated H-ZSM-5, a new surface methoxy species bound with extra-framework Al was experimentally identified using 13 C{ 27 Al} S-RESPDOR NMR. This species was demonstrated to initiate the direct formation of the first C-C bond product ethene in the MTO reaction. Recently, 13 C- 27 Al and 13 C- 29 Si SSNMR experiments were utilized to probe the interactions between the confined carbenium ions and the zeolite framework during methanol conversion on H-ZSM-5. [57] Considering the importance of the aforementioned metal species on zeolite, it can be envisioned that the characterizations of host-guest interactions by correlating metal nuclei (e.g., Zn, Ga and Sn)

and 13 C in zeolites would provide deep insights into their properties and catalytic role in reactions.

The observation and identification of active intermediates in zeolite catalyzed reactions is critical for elucidating catalytic reaction mechanism. It is desirable to monitor the species evolution from reactants, to intermediates, and to the final products. To this end, SSNMR particularly in situ NMR show its advantage in the exploration of active species during catalytic process. [START_REF] Li Received His Ph | degree from physical chemistry from Wuhan Institute of Physics and Mathematics (WIPM)[END_REF] Numerous reaction intermediates have been successfully discerned by in situ MAS NMR. [58c, 59] Due to the limited NMR detection sensitivity and time scale, the catalytic reactions are often quickly quenched and the frozen intermediates trapped on catalysts are characterized at batch mode. Nevertheless, NMR detection of active intermediates in catalytic reactions at operando condition has attracted increasing attention. [58a, 60] Figure 4a shows the in situ continuous-flow 13 C MAS NMR spectra of 13 CH3OH reacted at H-SSZ-13 and SAPO-34, in which two carbenium ions pentaMCP + and heptaMB + as important reaction intermediates were clearly identified (Figure 4b). [59c] Very recently, Zhou and co-workers reported the mechanistic study of ethanol dehydration to ethene over H-ZSM-5 by using in situ continuous-flow 13 C MAS NMR spectroscopy. for the first time observed during the reaction, which was confirmed by 2D 13 C-13 C INADEQUATE MAS NMR experiment (Figure 4c). The reactivity of this species was revealed by the in situ 13 C NMR experiment at elevated temperatures (Figure 4d). The facile transformation of triethyloxonium ion to ethene product pointed to its intermediate role in the ethanol dehydration process (Figure 4e).

In situ 13 C MAS NMR was also used to reveal the first carbon-carbon bond formation mechanism in the methanol-to-hydrocarbons reaction over H-ZSM-5. [60a] The development of novel in situ NMR probe and rotors allowed achieving the study of catalytic reaction with different phase and at high temperature and high pressure condition. [58c] For example, in the work by Jaegers and co-workers, [60c] cyclohexene carbenium ion was discerned as the intermediate in the phenol alkylation with cyclohexanol over zeolite H-Beta in liquid system.

SSNMR characterization of MOFs

MOFs are three-dimensionally connected networks consisting of metal centers or metal clusters linked by organic ligands. The vast variety of MOFs have found enormous applications in many areas, including gas storage, chemical separation, molecular catalysis, sensing, and drug delivery. [61] The wide application of MOFs is mainly ascribed to their intrinsic properties such as large surface area, framework flexibility, diverse pore structure and adjustable functional groups. Structural characterization of MOFs is crucial for understanding their structural-property relationships and the rational design of new types of functional materials.

Local structure of metal centers

SSNMR spectroscopy offers versatile approaches to characterize MOF lattice. The structures of metal centers can be examined by NMR experiments of quadrupolar nuclei including 91 Zr, 25 Mg, 67 Zn, 27 Al, 69/71 Ga, 47/49 Ti, etc. However, the low γ elements, such as 25 Mg and 67 Zn, are difficult for conventional NMR detection. The availability high magnetic field (>18.8 T) coupled with advanced pulse-sequence benefits NMR analysis of these challenging nuclei.

21

For example, Xu and coworkers applied 25 Mg 2D MQMAS NMR at a magnetic field of 21.1

T and natural abundance to detect the multiple non-equivalent Mg sites in α-Mg3-(HCOO)6

(Figure 5a). [20a] Different correlation peaks corresponding to four distinct Mg sites were clearly resolved in the 25 Mg MQMAS spectrum of the activated α-Mg3-(HCOO)6 (Figure 5b). Reproduced with permission. [20a] Copyright 2013, Wiley-VCH. 67 Zn MAS NMR spectra of crystalline ZIF-4 (c) and ZIF-4 glass (d) acquired at 19.5 and 35.2 T. (e) Schematic illustration of the structural alternation from crystalline ZIF to its glassy state during meltquenching. Reproduced with permission. [62] Copyright 2020, American Association for the Advancement of Science.

differentiate non-equivalent Zn sites in ZIF-8, ZIF-14 and ZIF-4. [20b] Recently, the structural alternation from crystalline ZIFs to their glassy state during melt-quenching treatment has been monitored by ultra-highfield 67 Zn SSNMR spectroscopy. [62] In the 67 Zn MAS NMR spectra of crystalline ZIF-4 and ZIF-4 glass acquired at 19.5 and 35.2 T (Figure 5c), there were two crystallographically distinct Zn sites at a 1:1 ratio in the parent ZIF-4 sample. Upon melt-quenching treatment, the transformation of two distinct Zn sites in the parent ZIF-4 crystal into a single disordered tetrahedral site in the ZIF-4 glass was reflected in the 67 Zn NMR spectra (Figure 5d). A glass formation mechanism was proposed according to the structural change from crystalline ZIFs to their glassy state (Figure 5e). [62] With respect to the variety of metal elements in framework, other metal nuclei such as 91 Zr, [21a] 47/49 Ti, [21a] 43 Ca, [63] 69/71 Ga, [64] , 27 Al [65] have been detected by high-field SSNMR spectroscopy to explore the environment of the metal centers in various MOFs. Furthermore, 17 O MAS NMR has been utilized to differentiate the crystallographically nonequivalent oxygen sites in MOFs lattice. In 17 O-enriched UiO-66, three chemically unique oxygens including carboxylate (COO -) alongside μ3-O 2-anions and μ3-OH groups were clearly distinguished. [66] Similarly, the carboxylate groups linking with metals and bridging μ2-OH were well resolved in the 17 O MAS NMR spectrum of 17 O-enriched MIL-53. [66][67]

Local structure of organic linkers

The structures and their functionalities of organic linkers in MOFs can be routinely characterized by 1 H, 13 C and 15 N SSNMR. [10c, 68] The combination of high magnetic field (21.1 T), ultrafast MAS (62.5 kHz) and 1 H isotopic dilution protocol enables the unambiguous assignment of complicated 1 H and 13 C chemical shifts and the structural determination of guest molecules confined in α-Mg-formate. [68a] The influence of chemical linker modification on their ring rotational rates in the functionalized UiO-66 was unraveled by 1 H-13 C separated-local-field (SLF) SSNMR. [69] Specifically, variable-temperatures (VT) 2 H NMR provides a versatile approach to measure the rotational frequencies of the linkers and monitor the flexibility and molecular dynamics of various MOFs. [70] Multivariate MOFs (MTV-MOFs) were fabricated through mixing variable linkers and variable metals ions. [71] In order to establish the structure-property relationships of MTV-MOFs, it is desirable to obtain the spatial arrangements information of various linkers or metal ions. Kong and coworkers determined the spatial apportionment of functional groups in a series of MTV-MOF-5 using SSNMR quantitative distances measurements in combination with molecular simulations. [72] Figure 6a shows a representative 13 C-15 N spin pair between two distinct linkers in 15 N isotopically enriched MTV-MOF-5-BF, where the average 13 C-15 N distances could be extracted from the 13 C{ 15 N} REDOR NMR experiments (Figure 6b). By fitting the 13 C{ 15 N} REDOR decay curves and calculating the effective interaction energies using Monte Carlo simulations, the spatial apportionment of mixing linkers in MTV-MOF-5-BF was determined to be small cluster (Figure 6c). Theoretical prediction of the CO2 adsorption isotherms compared with the corresponding experimental data further confirmed the spatial apportionment of mixing linkers in MTV-MOFs. This work provided a viable strategy to resolve apportionment in disordered MTV-MOFs through inter-linker heteronuclear distance measurements. and calculated 1 H spin-diffusion buildup curves of ML-DUT-5. Reproduced with permission. [73] Copyright 2015, Wiley-VCH.

The spatial distribution of the mixed linkers in MTV-MOFs could also be elucidated from 1 H spin-diffusion SSNMR measurements whereas the 15 N isotope enrichment of MTV-MOFs is unnecessary. [73] Figure 6d shows the labeling of carbon and hydrogen atoms of ML-DUT-5, which contained biphenyl dicarboxylic acid (BPDC) and bipyridyl dicarboxylic acid (BPyDC) linkers. In the 2D 1 H spin-diffusion MAS NMR spectrum of ML-DUT-5 (Figure 6e), the cross correlation peaks between the H1 site of BPDC and H2' site of BPyDC were clearly observable. The existence of the cross peaks suggested that BPyDC and BPDC linkers partially shared the same crystallites. Three models with different linkers apportionments were proposed to fit the 1 H-1 H spin diffusion buildup curves (Figure 6f). It is clear that only the model in which BPyDC and BPDC linkers are homogeneously distributed within the framework agreed well with the experimental results. [73] The work shows that high resolution 1 H spin-diffusion MAS NMR experiment is simple but robust to explore the spatial distribution of mixed linkers in MOFs.

Advanced SSNMR methods were developed to improve the 1 H spectral resolution and explore the spatial apportionments of MTV-MOFs. Carbon-detected version of the proton spin-diffusion NMR experiment was recently used to effectively improve NMR detection resolution and thus yield structural information of mixed-linker UiO-66-type metal-organic material. [74] Moreover, high resolution 1 H CRAMPS spin diffusion SSNMR in combination with computational modeling was employed to elucidate structural arrangements and determine the short-range order (SRO) parameter of mixed-linkers in ZIF-8 and ZIF-90. [75]

Host-guest interactions between confined gas molecule and MOFs

Hydrogen and light alkanes in MOFs

MOFs exhibit promising capability of adsorption and storage of the clean energy gas including hydrogen and methane. In order to fabricate new types of MOFs to enhance storage capacity, detailed knowledge of the host-guest interactions and the mobility of gas molecules confined inside MOFs is required. SSNMR is particularly suitable for characterization of the interaction between host framework and adsorbed molecules as well as the dynamics of the latter. 2 H SSNMR is in particular a powerful probe for this purpose.

Huang and coworkers investigated the hydrogen adsorption in UiO-66 and M-MOF-74 (M = Zn, Mg, Ni) using VT 2 H SSNMR. [76] The static VT 2 H NMR spectrum of D2 in Mg-MOF-74 as shown in Figure 7a exhibited a narrow peak in range of 253~293 K, suggesting highly mobile D2 exhibiting rapid isotropic reorientation in Mg-MOF-74 at the temperature range.

The 2 H NMR signal became broad as the temperature decreased to 173 K, indicative of onset of D2 adsorption on Mg-MOF-74. At 153 K, the adsorbed D2 (broad power pattern) and free D2 (narrow central peak) coexisted inside the Mg-MOF-74 pore (Figure 7a). The fitting of the wide 2 H lineshapes at 133 K revealed that D2 exhibits a local wobbling rotation with respect to the open metal site (OMS) as well as a non-localized six-site hopping along the pore edge (Figure 7b). The VT 2 H NMR spectra of D2 adsorbed on UiO-66 and Zn-MOF-74 exhibited

narrow 2 H resonances, indicating rapid reorientation of the D2 molecules and weaker interaction between D2 and frameworks. [76] Therefore, the VT 2 H NMR of D2 adsorption on MOFs provided detailed information on H2 dynamics and its host-guest interactions with MOFs. Reproduced with permission. [76] Copyright 2016, Royal Society of Chemistry. (c) Experimental (blue) and simulated (red) VT 2 H NMR spectra of CH3D adsorbed in α-Mg3(HCO2)6 and (d) simplified illustration of the Nucleus-Independent Chemical Shift (NICS) effect on methane adsorbed in MOFs. Reproduced with permission. [77] Copyright 2018, Wiley-VCH.

2 H NMR offers a straightforward method for the investigation of methane adsorption locations and dynamics within MOFs pores. [78] Methane adsorption behavior on several representative MOFs has been examined by VT 2 H NMR spectroscopy. [77] Figure 7c displayed the static VT 2 H static NMR spectra of singly deuterated methane (CH3D) adsorbed on α-Mg3(HCO2)6. At 293 K, a sharp resonance corresponds to rapidly and isotropically tumbling CH3D. 2 H powder patterns was clearly observable at 123~173 K, ascribed to adsorbed CH3D. The relatively narrow 2 H NMR linewidth at 123 K was resulted from the fast exchange between CH3D experiencing isotropically tumbling and adsorbed CH3D undergoing anisotropic motion. As CH3D molecules approach and adsorb on the MOF surface, the increase of Nucleus-Independent Chemical Shift (NICS) magnitude indeced pronounced 2 H CSA of CH3D (Figure 7d). According to the determined CQ values of CH3D adsorbed on MOFs at low temperature, Mg-MOF-74 shows the weakest methane binding strength compared to that of α-Mg3(HCO2)6, -Zn3(HCO2)6, and SIFSIX-3-Zn. [77] SSNMR enables direct investigation of adsorbed light alkanes in MOFs. The host-guest interaction and primary adsorption site of light alkanes (methane, ethane, and propane) confined inside UiO-67 and MTV-UiO-66 was revealed by 1 H-1 H spin diffusion SSNMR experiments. [79] Methane diffusion in M2(dobdc) as different methane loadings was investigated by PFG Stimulated Echo NMR and MD simulations. It was found that the selfdiffusion coefficient of methane in M2(dobdc) was inversely related to the binding energy at the unsaturated metal sites. [80]

CO2 in MOFs

MOFs exhibit great potential for CO2 capture to cut down greenhouse gas emissions owing to their large gas adsorption capacities and ability to selectively separate CO2 from gases mixtures. Understanding of dynamic behavior of adsorbed CO2 is important for the practical application of MOFs as adsorbents. The dynamics of CO2 in the Mg-MOF-74 was revealed by the analysis of 13 C static NMR line shapes and spin-lattice relaxation times. [6] It was found that CO2 preserved uniaxial rotation with a fixed rotation angle along the rotation axis of Mg-O(CO2) vector in Mg-MOF-74. The motion of CO2 and the interactions with framework in various MOFs were comprehensively studied by VT 13 C static NMR experiments. [81] Diamine-appended MOFs are highly efficient for CO2 adsorption from various gas mixtures. [82] Recently, multinuclear SSNMR spectroscopy has been utilized to characterize CO2 chemisorption in diamine-appended metal-organic frameworks. [83] with the H atoms from different NH groups on framework. (Figure 8c) The detailed NMR assignments with the assistance of DFT theoretical calculations in the 1 H-13 C HETCOR spectrum of CO2 adsorbed on dmpn-Mg2(dobpdc) were manifested in Figure 8d. It should be noted that the hydrogen of the carbamic acid -OH group is in spatial proximity to the carbon site of the ammonium carbamate. On the basis of the NMR observations and computational CO2 adsorption energies, a mixed chemisorption structure was proposed (Figure 8e). The formation of a 1:1 mixture of ammonium carbamate and carbamic acid resulted in highly efficient CO2 adsorption properties of dmpn-Mg2(dobpdc). [83] carbamic acid in a 1:1 ratio. Reproduced with permission. [83] Copyright 2018, American Chemical Society. (f) Two CO2 diffusion pathways in Zn2(dobpdc) as determined by 13 C PFG NMR. Reproduced with permission. [84] Copyright 2018, American Chemical Society.

SSNMR provided a deeper understanding of CO2 adsorption within diamine-M2(dobpdc) compounds, which benefits the design of improved materials for the CO2 capture. The detailed interactions between CO2 and dmpn-Mg2(dobpdc), 2-ampd-Mg2(dobpdc) were investigated by means of 1 H and 13 C MAS NMR spectroscopy as well. [85] It is interesting to note that water molecules enhance CO2 capture in oxidation-resistant 2-ampd-Mg2(dobpdc) through hydrogen-bonding interactions with the ammonium carbamate chains formed upon CO2 adsorption. [85a] Additionally, the host-guest interactions between alcoholamine-and alkoxyalkylamine-functionalized Mg2(dobpdc) and CO2 were explored using 13 C and 15 N CP/MAS as well as 1 H-13 C HETCOR NMR to clarify the cooperative chain-forming mechanisms towards energy-efficient CO2 separations. [86] Insight into the long-range diffusive motion of confined molecules within the nanosized pores of MOFs is critical for guiding the design of robust materials with improved gas transport properties. Pulsed field gradient (PFG) NMR spectroscopy that is a well-established technique for the measurement of intracrystalline self-diffusion in porous materials was used to study the diffusion of CO2 within the pores of Zn2(dobpdc). [84] Self-diffusion coefficients were determined by the analysis of the 13 C NMR line shape as a function of the applied field gradient. 13 C PFG NMR in combination with molecular dynamics (MD) simulations revealed unexpected CO2 diffusion: in addition to the diffusion through the channels parallel to the crystallographic c axis, CO2 diffused between the hexagonal channels due to the presence of framework defects in the crystallographic ab plane (Figure 8f). The work demonstrated an example for the measurement of the self-diffusion in different crystallographic directions in MOFs from residual chemical shift anisotropy for pore-confined CO2 using PFG NMR spectroscopy. The results also show the importance of defects for multidimensional CO2

transport particularly in MOFs one-dimensional porosity.

SSNMR investigation of COFs

COFs constructed by the covalent linkage of building units represent a new class of porous polymers with high chemical and thermal stability and well-defined structures. [87] COFs show great potential for a range of applications in gas storage, chemical separation, chemical sensor, energy conversion and molecular catalysis. The chemical composition of COFs has been comprehensively characterized by multinuclear SSNMR. 1 H→ 13 C CP/MAS NMR serves as a robust and straightforward tool to characterize the functional groups in various COFs. [88] 13 C NMR chemical shifts of the synthesized COFs and starting monomers can be directly compared to assess the introduction of functional groups to COFs. 13 C CP/MAS NMR spectroscopy verified the anticipated covalent bonding with atomic-level precision and high crystallinity in the structure of large single crystals of 3D imine-based COFs including COF-300, In addition to 13 C MAS NMR, 11 B, 15 N MAS NMR and 129 Xe NMR have also been utilized to elucidate the local framework structure and pore property of various COFs. [89] Three consecutive transformations in porous, crystalline cyclic carbamate and thiocarbamate-linked frameworks through postsynthetic modifications of imine-linked COF-170 were identified from 1 H→ 15 N CP/MAS NMR spectroscopy. [89b] The local structure of ionic covalent organic framework (ICOF) capable of transporting lithium ions was characterized by 13 C and 11 B MAS NMR. [90] The structural transformation and dynamic behavior of a new 3D COF LZU-301 was observed using 129 Xe NMR spectroscopy, in which a gradual switch of 129 Xe NMR signal from high field to low field indicated a state transition from contracted LZU-301 to expanded form. [89d] The detection sensitivity and spectral resolution is often problematic in the SSNMR characterization of COFs, since the framework elements, such as 15 N, have low natural abundance (0.37 %) and low γ, leading to low NMR detection sensitivity. DNP is a technique of choice for COFs due to its capability of signal enhancement by 2-3 orders of magnitude.

Cao and coworkers constructed a radical-embedded COFs which allows for DNP enhancement without introducing extraneous polarizing agents. [91] Reproduced with permission. [91] Copyright 2018, American Chemical Society. (c) Schematic structure of COF-LZU8 and (d) 13 C CP/MAS NMR spectra of COF-LZU8, Hg/COF-LZU8, Hg/COF-LZU8 after the treatment with Na2S, and Cd/COF-LZU8. Reproduced with permission. [92] Copyright 2016, American Chemical Society.

∼27 (Figure 9b). The resonances at 91, 104, and 315 ppm assigned to -N-H, terminal -NH2, and -N=C-sites, respectively, can be clearly distinguished in the DNP-enhanced 1 H→ 15 N CP/MAS NMR spectra, whereas these signals cannot be observed without DNP. The extraneous polarizing agent such as a stable organic radical is typically required in the DNP experiments. This work provided a strategy for direct construction of radical COFs from radical monomers suitable for direct DNP NMR characterization, which circumvents the problem caused by the chemical compatibility between the polarizing agent and the sample that has to be addressed in the conventional DNP approach.

SSNMR exhibits potential for exploring the host-guest interaction mechanism in COFs. Ding and coworkers [92] fabricated a thioetherfunctionalized COF material, COF-LZU8, for selective detection and facile removal of highly toxic mercury(II). COF-LZU8 showed strong fluorescence upon excitation at 390 nm, which was quenched after introducing Hg 2+ to the material. The 1 H→ 13 C CP/MAS NMR spectrum of COF-LZU8 showed that the peak at 28 ppm assigned to the methylene carbons adjacent to the S atoms in COF-LZU8 was obviously shifted to 35 ppm upon the adsorption of Hg 2+ (Figure 9d), while the chemical shifts of the other signals remained unchanged. This provided evidence for the strong interaction between S and Hg 2+ in Hg/COF-LZU8. The treatment of Hg/COF-LZU8 with Na2S removed Hg 2+

allowing the recycling of COF-LZU8. This Hg 2+ elimination was reflected by the recovery of the 13 C CP/MAS NMR spectrum (Figure 9d). The introduction of other ions, such as Cd 2+ , into COF-LZU8 was examined using 13 C CP/MAS NMR spectrum as well (Figure 9d). As indicated in this work, 13 C CP/MAS NMR was deomstrated to be an effective tool for probing the interaction between metal ions and the functional groups in COFs, which is critical for understanding the adsorption and separation property of porous material as adsorbents.

More information about the interaction between COFs and guest molecules can be obtained from multinuclear and multidimensional SSNMR. Solid-state transverse relaxation T2 measurement and 2D 1 H-1 H DQ MAS NMR spectroscopy revealed the location and dynamic of inserted PEO chains in COFs. These chains were responsible for the fast ion conductivities and transportation as well as the COF structural stability. [93] The detailed interactions between small organic molecules and the COFs framework were investigated using multidimensional and multinuclear SSNMR as well. 2D 1 H-1 H DQ MAS NMR experiments showed the presence of strong host-guest interaction between residual mesitylene solvent and the framework in both the AB staggered and AA eclipsed polymorphs of COF-1, which plays an important role in the phase change. [94] The stabilization effects mechanism of pyridine on the structure of COF-5 and COF-10 were revealed by 1 H→ 15 N CP/MAS NMR and 2D 11 B MQMAS NMR spectroscopy, in which pyridine formed a Brønsted-type interaction with both COF-5 and COF-10. [95] MOFs and their derivatives, and COFs are emerging as important catalysts in thermocatalysis, electrocatalysis, photocatalysis due to their adjustable acid property. [96] SSNMR has exhibited great potential for exploring the acidic features of active sites and elucidating the catalyzed reaction mechanism. [97] The detailed structure and the superacidity characteristic of Brønsted acid site in sulfated MOF-808 as solid acid catalyst for dimerization of isobutene were comprehensively examined by SSNMR in conjunction with probe molecule techniques.

[ 97a, b] Using SSNMR and deprotonation energy calculations, the phosphorus chemical environment and the acidic property of phosphonate-modified UiO-66 as Brønsted acid catalyst for biomass conversion were determined. [97c] TADDOL-embedded chiral porous polymer (TADDOL-CPP) can act as a highly efficient and recyclable catalyst in the asymmetric addition of diethylzinc to aromatic aldehydes. The key intermediates formed upon the reaction of 13 C-labeled benzaldehyde reactant were identified and thus the detailed catalyzed reaction mechanism was revealed accordingly. [97d] 6. SSNMR characterization of other microporous materials

PAFs

PAFs built by C-C aromatic-based building units hold great promise for gas storage and capture because of their ultra-high surface area and high physicochemical stability. [98] Multinuclear 1 H, 13 C, and 29 Si SSNMR has been efficiently employed for structural characterization of various PAFs including PAF-1, [99] PAF-5, [100] JUC-Z2, [101] JUC-Z12, [102] mPAF-1, [103] sulfonated PAFs [104] and PAFPORP [105] . PFA-1 possesses high surface area of 7100 m 2 /g and exceptional thermal/hydrothermal stabilities. The local diamond-like tetrahedral bonding of tetraphenylene methane building unit in PAF-1 was resolved by 1 H MAS, 1 H→ 13 C CP/MAS, and 2D 1 H-13 C HETCOR NMR spectra. [99] 1 H→ 13 C CP/MAS NMR and 29 Si MAS NMR experiments was employed to characterize the structure of JUC-Z1 capable of selectively adsorbing benzene [106] as well as PAF-3 and PAF-4 with excellent adsorption ability to hydrogen, methane and carbon dioxide. [107] Recently, Li and coworkers [108] constructed PAF-100 and PAF-101 with high surface areas, large pore volumes, and high performance of methane uptakes. 1 H→ 13 C and 1 H→ 29 Si CP/MAS NMR experiments suggested the uniform distribution of carbon and silicon sites in the framework units. VT 2 H spin-echo SSNMR has been utilized to explore the dynamics of p-phenylene rotors of

[D4]PAF-3 framework, in which the rotational rates and the librational amplitude at high temperature were determined. [109] The constructive elements (p-phenylene) of the [D4]PAF-3 are robust struts and ultra-fast rotors. The rotational motion of p-phenylene rotors can be actively regulated by controlling the temperature and the presence of guest molecule. It is noteworthy that the adsorption and desorption of I2 can effectively regulate the rotor speed of

[D4]PAF-3. 2 H NMR spectroscopy revealed that the rotational rate of p-phenylene of

[D4]PAF-3 drastically slowed down as I2 diffused into the PAF-3 pores due to the presence of I2-phenylene interactions. [109] 6.2 Porous layered materials

Graphene derivatives

Porous layered materials including inorganic and organic open frameworks have been extensively developed and studied due to their great potential and tremendous applications in catalysts, sorbents and electronic devices, etc. The porous layered materials, such as montmorillonite [110] and sodium disilicate [111] , have been studied with SSNMR. The recent progress regarding the SSNMR characterization of 2D layered graphene derivatives, layered double hydroxides (LDHs) and layered silicates is briefly described in the following sections.

Graphene derivatives, such as graphene oxide (GO), is an important class of 2D layered materials with a wide range of application in energy storage and conversion devices, separation membranes due to their outstanding electrical, optical, thermal, and mechanical

properties. [112] The interlayer spacing of graphene derivatives can be well controlled by introducing diamine pillars to adjust their molecular selectivity, permeability and ionic conductivity. SSNMR has been utilized to probe the cross-linking interaction between pillared graphene and diamine. [113] Figures 10a shows the schematic representation of cross-linking interaction. Reproduced with permission. [113] Copyright 2019, American Chemical Society. Static 13 C NMR of 13 CO2 in (e) GO-EDA-0.2 under elevating temperatures and (f) GOs with different EDA loadings at 153 K. (g) graphic illustration of CO2 adsorbed in diamine-cross-linked GO layers. Reproduced with permission. [114] Copyright 2019, American Chemical Society.

C=O (δ13C ∼ 165-180 ppm) suggested that the diamines have reacted with the carboxyl functions on the GO sheets. The C-N pillar carbons (δ13C ∼ 40 ppm) are correlated with the N site in α-position to carbons (δ15N ∼ 83 ppm), indicating that diamine was grafted to the graphene surface through reactions with epoxides. The pillared graphene materials constructed by cross-linking the graphene sheets with a bifunctional pillar molecule (Figure 10d) facilitated ion transportation and storage capacity for supercapacitors. A sparse filling of pillars in the galleries improved the supercapacitors performances with higher capacitances and larger power capability.

The interlayer spacing of GO with diamine can be utilized for the capture of greenhouse gas CO2. The introduction of cross-linking molecules leads to complicated host-guest interactions between CO2 and GO matrix. Cai and coworkers conducted in situ 13 C NMR to investigate the adsorption mechanism of CO2 on diamine-cross-linked GO with controllable interlayer spacing. [114] The 13 C NMR CSA pattern served as an effective probe of molecular dynamics of confined CO2. In the VT static 13 C NMR spectra of 13 CO2 adsorbed on GOethylenediamine (EDA)-0.2 (Figure 10e), the spectra consisted of a broad component arising from adsorbed CO2 and the sharp component associated with gas phase CO2 at temperature above 213 K. The CSA pattern of adsorbed CO2 broadens below 193 K, suggesting the presence of restricted motion at that temperature point. For CO2 adsorbed on the GOs with different EDA loadings, the 13 C CSA patterns of adsorbed CO2 can be deconvoluted into several components (Figure 10f). As manifested in Figure 10g, both physically and chemically adsorbed CO2 were present in diamine-cross-linked GO matrices. Phys. I type CO2 was trapped by electrostatic interactions and undergone uniaxial rotation which was not significantly affected by the interlayer spacing, whereas Phys. II CO2 was only restricted by van der Waals interactions and exhibited isotropic motion. Chem. I type corresponded to CO2 chemically interacting with diamines. The interlayer spacing of GOs played a key role in the CO2 adsorption, resulting in the elevation of CO2 uptake.

The host-guest chemistry in graphene oxide derivatives is also of great interest for SSNMR investigations. [115] 1 H→ 13 C CP/MAS NMR and 2D 1 H-13 C HETCOR NMR provided direct evidence for the reaction between graphene oxide and amine functions occurring via ring opening of the epoxides and ruled out the possibility of amidation reactions with amine derivatives. [115b] The distribution and variation pattern of electrolyte ions including weakly and strongly adsorbed BF4 -in graphene-film electrodes were revealed by 11 B MAS NMR, which shed insights into the fabrication of high performance graphene-film based electric double layer capacitors.

[115d]

LDHs

LDHs is a type of inorganic supramolecular materials, which is widely applied in environmental, energy, catalysis, and biomaterials. [116] Detailed information on the local structure of the metal hydroxide layers is prerequisite for assessing their properties and performances. Different Mg local environments in the LDHs with various Al for Mg substitution levels have been uncovered by Sideris and co-workers [117] using high resolution 25 Mg MAS NMR spectroscopy. Three different magnesium environments were revealed in the 2D 25 Mg MQMAS NMR spectrum of MgAl-19-NO 3-(Figure 11a). The three signals resolved at 26.6, 40.6 and 47.5 ppm in the F1 dimension indicated the presence of Mg(OMg)6, Mg(OAl)3(OMg)3 and Mg(OAl)2(OMg)4 units, respectively. The relative ratio for the three distinct components was quantitatively determined from the 25 Mg MAS NMR spectra with the input of the NMR parameters (δiso, Qcc and ηQ) extracted from the 25 Mg MQMAS NMR spectra. [117] The inter-nuclear proximities and cation-ordering of LDHs can be determined using SSNMR spectroscopy. 1 H MAS and 1 H{ 27 Al}TRAPDOR NMR experiments were conducted on deuterated MgAl-25.1 (Figure 11b). [118] Deuterated samples were used to obtain highresolution 1 H NMR spectra by reducing the 1 H-1 H dipole-dipole interaction. Different species including Mg3OH (1.1 ppm), Mg2AlOH (2.9 ppm) and water molecule (4.6 ppm) were resolved on the 1 H NMR spectra. Two structural units (Mg3OH and Mg2AlOH) can be identified by the different 1 H- 27 Al dipolar dephasing in the 1 H{ 27 Al}TRAPDOR spectra (Figure 11b). The mole percentage of Mg2AlOH increased linearly at the expense of Mg3OH as the increase of aluminum loading, suggesting the absence of Al-O-Al linkage in the LDHs (Figure 11c). The high resolution 1 H and 1 H{ 27 Al} TRAPDOR NMR experiments revealed the local structure and cation orderings of LDHs. [118] Figure 11. (a) Natural abundance 25 Mg MQMAS spectrum of MgAl-19-NO3 -(19% of Mg 2+ is substituted by Al 3+ and NO3 -is the interlayer anion). The dashed lines show the experimental and simulated anisotropic spectra of three distinct Mg sites. Reproduced with permission. [117] Copyright 2012, American Chemical Society. Reproduced with permission. [118] Copyright 2014, American Chemical Society.

Versatile SSNMR techniques have been applied to extract detailed structural information on various types of LDHs. [119] The dehydration and dehydroxylation processes of LDHs were examined using 1 H and 27 Al MAS NMR. Dehydration of Mg and Al containing LDHs proceeded below thermal treatment of 150 °C, above which dehydroxylation of Mg2AlOH and Mg3OH species accompanied with the structure changes of LDHs occured. [119d] The local structures of a LDHs containing paramagnetic Ni 2+ cations have been revealed from 1 H, 27 Al, and 13 C MAS NMR spectroscopy. The intralayer Ni/Al distributions of the LDHs could be determined from the pseudocontact chemical shifts. [119f, g] 1 H and 71 Ga SSNMR spectroscopy has been employed to reveal the ordering of Ga 3+ and various proton environments including Mg3OH, Mg2GaOH and water in magnesium gallium (MgGa) LDHs.

[119h]

Layered silicates

Layered silicates have attracted broad interests in many applications such as catalysis and adsorption due to their high capacities for ion exchange and easy functionalization by various modifications such as silylation and pillaring. Layered silicates can also serve as precursors for the formation of zeolites by conversion of the 2D frameworks into structurally related 3D porous materials. 29 Si, 1 H, 13 C multi-nuclear SSNMR was frequently utilized to explore the coordination, stacking geometries, interlayer structures of various layered silicates including RUB-51, HUS-1, PLS-3, PLS-4, HUS-2, HUS-3, HUS-4, and HUS-7. [120] The silicate framework structure of a new layered silicate material named CLS-1 was effectively solved using state-of-the-art NMR crystallography technique (2D 29 Si DQ-SQ MAS NMR jointed with XRD and DFT calculations). [121] Recently, the possible crystal structure of magadiite as one of representative layered silicates has been determined using X-ray pair distribution functions and synchrotron powder diffractometry. [122] A combination of diffraction technique, SSNMR and DFT theoretical calculation would provide promising opportunities to extract long-and short-range structural constraints and enable determination of the molecular structure of the layered silicates in the presence of various extents of stacking disorder.

Conclusion and outlook

From inorganic to inorganic-organic hybrid and organic framework and from 2D layers to 3D architecture, the microporous materials undergo enormous development. The increasing application of these materials is fuelled by the interest in taking advantage of their diverse structure and properties. The in-depth characterization of the microporous materials at atomiclevel plays a critical role for a better understanding of their functions that could lead to new applications.

The structural and chemical complexity involved in the porous materials is beyond the use of some characterization techniques. Multidimensional and multinuclear SSNMR spectroscopy Further enhancement in NMR sensitivity is expected from the development of new instrumentation and pulse sequences. For instance, the utility of SSNMR could be significantly improved by using advanced instruments, such as ultra-high field spectrometer equipped with ultra-fast MAS probe. High-resolution SSNMR at magnetic field of 35.2 T is now accessible. [123] The dramatic sensitivity enhancement was demonstrated on 17 O [124] and 43

Ca spectra [125] , which provides opportunities to explore the porous materials with halfinteger quadrupolar nuclei at very high field strength. The use of 1 H MAS NMR at moderate MAS speed in organic framework porous material is often problematic due to the low spectral resolution caused by strongly coupled 1 H-1 H spin interactions. The MAS frequency on commercially available probes has reached beyond 110 kHz, the important role of which has been recognized in biochemistry system. [126] The benefit of ultra-fast MAS probe would be demonstrated on porous materials containing paramagnetic ions by reducing the signal broadening caused by paramagnetic shift. DNP technique undoubtedly continues to push the frontiers of SSNMR in material science as it enables sensitivity enhancement larger than two orders of magnitude, which leads to experimental time-saving by four orders of magnitude.

The increasing availability of DNP equipment in research community and the introduction of novel experimental protocols (e.g, polarizing agent, solvent and sample preparation) opens numerous opportunities for its routine use for characterization of porous materials. In situ NMR technique for operando detection is crucial for the investigation of porous materials.

Despite the technical challenges, SSNMR probes capable of working under harsh condition such as extremely high/low temperature and high pressure have been built. [58c] Advanced SSNMR opens new avenue for structural and functional characterization in material science. The combination of NMR spectroscopy with complementary techniques, such as XRD, X-ray adsorption spectroscopy (XAS) and microscopy, would allow the composition and structure of materials to be determined on wide range of length scale and in different dimensions. The obtained atomic-level insights will benefit materials properties improvement and new material discovery.

  were correlated with the peaks at ca. -112 ppm from Si4b and Si3 sites, which were spatially connected with the signals at ca. -116 ppm from Si5b and Si6 sites. The 2D 29 Si{ 29 Si} J-Si2b-O-Si4b-O-Si5b and Si1b-O-Si2b-O-Si3-O-Si6 (blue lines), confirming the proposed model 1 (Figure 1d). Similarly, the Si-O-Si covalent interconnectivities between the Q 3 Si2b silanol moieties and Q 4 sites at the intralayer channel surfaces (Si2b-O-Si4b-O-Si5b and Si2b-O-Si3-O-Si6) were clearly observed in red lines,

Figure 1 .

 1 Figure 1. (a) 1D 29 Si MAS NMR spectrum, (b) DNP-enhanced 2D 29 Si{ 1 H} dipolar-mediated HETCOR spectrum and (c) DNP-enhanced 2D 29 Si{ 29 Si} J-mediated correlation spectrum of

  or P atoms from the amorphous phase to crystalline framework as revealed by multinuclear SSNMR provied detailed insights into the crytallizaiton mechanisms of these microporous materials of industrial interest. Sn(IV) isomorphously substituted into Sn-β zeolite represents a breakthrough in the exploitation of atom-efficient solid Lewis acid catalysts because of its unparalleled catalytic performance in biomass conversion. The characterization of framework Sn site is a subject of intensive studies. The so-called open (e.g. (SiO)3Sn-OH) and closed (e.g. (SiO)4Sn) Sn sites were proposed. Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy (DNP-SENS) technique [22] was recently applied on Sn-β zeolites. On the basis of 119 Sn chemical shifts, closed and open Sn sites were differentiated in Sn-β prepared with postsynthetic and hydrothermal method. Recently, two types of open Sn sites containing Sn-OH groups were selectively observed by proton-detected 1D and 2D 1 H{ 119 Sn} dipolar-mediated D-HMQC

  Figure 2. (a) 67 Zn HS-QCPMG NMR spectrum of Zn/ZSM-5, (b) 1 H-67 Zn S-RESPDOR NMR

Figure 3 .

 3 Figure 3. (a) 13 C{ 27 Al} S-RESPDOR spectrum of acetone adsorbed dealuminated HY, (b) 13 C{ 27 Al} build-up curve of 13 C signal at 234 ppm, (c) 27 Al{ 13 C} D-HMQC spectrum of [2-13 C]-acetone loaded on dealuminated HY and (d) schematic model of acetone adsorbed on the Brønsted acid site on dealuminated HY. Reproduced with permission. [17b] Copyright 2014,

  Figure 4. (a) 13 C MAS NMR spectra of 13 CH3OH reacted at SAPO-34 and H-SSZ-13, and (b) illustration of the formation of heptaMB + and pentaMCP + in H-SSZ-13. Reproduced with permission. [59c] Copyright 2013, Wiley-VCH. (c) 2D 13 C-13 C INADEQUATE MAS NMR spectrum of reaction of 13 CH3 13 CH2OH on H-ZSM-5, (d) real-time in situ 13 C MAS NMR of 13 CH3 13 CH2OH dehydration on zeolite H-ZSM-5 as a function of reaction temperatures and

  The NMR parameters including Qcc, η and δiso for the activated and organic molecule (DMF and benzene) loaded in α-Mg3-(HCOO)6 can be accurately determined from the 25 Mg MQMAS NMR experiment.

Figure 5 .

 5 Figure 5. (a) Framework and Mg coordination environments of α-Mg3-(HCOO)6 and (b) natural abundance 25 Mg 3QMAS NMR spectrum of the activated α-Mg3-(HCOO)6.

Figure 6 .

 6 Figure 6. (a) The specified 13 C-15 N spin pair is indicated in MTV-MOF-5-BF, (b) fitting of experimental 13 C{ 15 N} REDOR decay curves using different models, large clusters (LC),

Figure 7 .

 7 Figure 7. (a) Experimental (blue) and simulated (red) VT 2 H NMR spectra of D2 adsorbed in

  Figure 8a-8b shows the 13 C and 15 N CP/MAS NMR spectra of dmpn-Mg2(dobpdc) upon adsorption of CO2, confirming the formation of ammonium carbamate and carbamic acid in Mg2(dobpdc) channel. The detailed assignments in the 1 H-13 C HETCOR spectrum (Figure 8c) of CO2 adsorbed on dmpn-Mg2(dobpdc) was manifested in Figure 8d. The 1 H-13 C HETCOR spectrum shows identification of the adsorption sites by correlating the C atoms from CO2

Figure 8 .

 8 Figure 8. (a) 1 H→ 13 C and (b) 1 H→ 15 N CP/MAS, and (c) 1 H-13 C HETCOR NMR spectra of

  Figure 9. (a) Schematic structure of PR(x)-COFs, (b) DNP-enhanced 1 H→ 13 C and 1 H→ 15 N

  Figure 10. (a) 1 H→ 13 C CP/MAS NMR spectra of graphene oxide (GO) and unreduced

  (b) 1 H MAS, 1 H{27 Al} TRAPDOR NMR spectra along with 1 H signals assignments of deuterated MgAl-25.1 (25.1% of Mg 2+ is substituted by Al 3+ ). (c) The mole percentages of Mg3OH and Mg2AlOH environments as a function of Al loadings.

  has been successfully used to investigate several representative porous materials including zeolites, MOFs, COFs, PAFs, and layered materials. Due to the advancement in experimental method and hardware, SSNMR has provided detailed information on the local structures, dynamic behaviors of molecules, and host-guest interactions in these porous materials, which is essential for the establishment of their structure-property relationships and hence, the rational design of new functional porous materials.Although the SSNMR characterization has reached a new level, the rapid development of porous materials also brings novel challenges for the application of SSNMR in this exciting field. The heterogeneous distribution of the framework or extra-framework elements in porous materials, paramagnetic components and the nuclei with low natural abundance and/or large very large quadrupolar interactions significantly lower the NMR detection sensitivity and spectral resolution, which limits the application of the SSNMR with available methodologies. Additionally, obtaining the information at working condition relevant for synthesis, phase change, gas adsorption/separation and catalytic reactions remains one of the most sought after goals in the SSNMR study of porous materials.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grants U1932218, 22061130202, 21773295, 21733013, 91745111), key program for frontier science of the Chinese Academy of Sciences (QYZDB-SSW-SLH027), Key projects of international partnership plan for foreign cooperation (112942KYSB20180009), Hubei Provincial Natural Science Foundation (2017CFA032) and 111 Project (Grant No. B17020). J.X. and O.L. acknowledge the support from the Sino-French Cai Yuanpei Programme (201904910879).

ToC figure

Recent progresses in the application of solid-state NMR in the study of microporous materials are reviewed. The state-of-the-art of solid-state NMR spectroscopies for characterization of structures and dynamics of microporous materials are summarized. The obtained atomic-level information that allows understanding of structure-performance relationships is highlighted.