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By employing ssNMR techniques on PVCN-SPE, a cross-linked SPE, two types of Li ions associated with different polymer segments are identified. The VC to EO unit ratio in the network affects crystallization prevention, enhancing ion mobility.

Porous metal–organic frameworks (MOFs) have found many technological applications in fields such as carbon capture and storage, catalysis, and selective guest adsorption. Post-synthetic modification (PSM) approaches can influence MOF properties by introducing new functional groups or metals. MIL-121 is a prototypical aluminum MOF containing free uncoordinated carboxylic acid groups, which can act as adsorption sites for metal exchange as they are accessible to guests from within the pores. The introduction of metal species has been proven to enhance the gas adsorption capacity and catalytic properties of MIL-121. A deeper understanding of how MOF carboxylic acid groups interact with metals is imperative for the development of advanced industrially relevant materials. In this work, we demonstrate the remarkable capability of 17O solid-state NMR at 35.2 and 19.6 T to assign each 17O resonance in MIL-121 to its chemical/crystallographic oxygen site, provide site-specific structural information, and probe both the location and binding mode of metal guests within the framework. A series of 1D and 2D 17O NMR experiments on 17O-enriched MIL-121 supported by computational methods have been employed to study changes in the local environments of oxygen upon the activation of the material as well as upon metal loading. These results clearly show that the high spectral resolution achieved via high-field magic-angle spinning (MAS), REDOR (rotational-echo double-resonance), multiple-quantum MAS (MQMAS), and D-HMQC (dipolar heteronuclear multiple-quantum coherence) experiments yields unprecedented insight into this MOF. The 17O NMR parameters provide molecular-level information regarding the local oxygen environment, intermolecular interactions, and host–guest connectivity. This experimental approach can be applied to a wide variety of oxygen-containing MOFs.

Because of the growing interest in the applications of zeolitic materials and the various challenges associated with traditional synthesis methods, the development of novel synthesis approaches remains of fundamental importance. Herein, we report a general route for the synthesis of aluminophosphate (AlPO) zeotypes by simple calcination of amorphous precursors at moderate temperatures (250–450 °C) for short reaction times (3–60 min). Accordingly, highly crystalline AlPO zeotypes with various topologies of AST, SOD, LTA, AEL, AFI, and -CLO, ranging from ultra-small to extra-large pores, have been successfully synthesized. Multinuclear multidimensional solid-state NMR techniques combined with complementary operando mass spectrometry (MS), powder X-ray diffraction, high-resolution transmission electron microscopy, and Raman characterizations reveal that covalently bonded fluoride in the intermediates catalyze the bond breaking and remaking processes. The confined organic structure-directing agents with high thermal stability direct the ordered rearrangement. This novel synthesis strategy not only shows excellent synthesis efficiency in terms of a simple synthesis procedure, a fast crystallization rate, and a high product yield, but also sheds new light on the crystallization mechanism of zeolitic materials.

Current needs for extending zeolite catalysts beyond traditional gas-phase hydrocarbon chemistry demand detailed characterization of active site structures, distributions, and hydrothermal impacts. A broad suite of homonuclear and heteronuclear NMR correlation experiments on dehydrated H-ZSM-5 catalysts with isotopically enriched 17O frameworks reveals that at least two types of paired active sites exist, the amount of which depends on the population of fully framework-coordinated tetrahedral Al (Al(IV)-1) and partially framework-coordinated tetrahedral Al (Al(IV)-2) sites, both of which can be denoted as (SiO)4–n–Al(OH)n. The relative amounts of Al(IV)-1 and Al(IV)-2 sites, and subsequent pairing, cannot be inferred from the catalyst Si/Al ratio, but depend on synthetic and postsynthetic modifications. Correlation experiments demonstrate that, on average, acidic hydroxyl groups from Al(IV)-1/Al(IV)-2 pairs are closer to one another than those from Al(IV)-1/Al(IV)-1 pairs, as supported by computational DFT calculations. Through-bond and through-space polarization transfer experiments exploiting 17O nuclei reveal a number of different acidic hydroxyl groups in varying Si/Al catalysts, the relative amounts of which change following postsynthetic modifications. Using room-temperature isotopic exchange methods, it was determined that 17O was homogeneously incorporated into the zeolite framework, while 17O → 27Al polarization transfer experiments demonstrated that 17O incorporation does not occur for extra-framework AlnOmspecies. Data from samples exposed to controlled hydrolysis indicates that nearest neighbor Al pairs in the framework are more susceptible to hydrolytic attack. The data reported here suggest that Al(IV)-1/Al(IV)-2 paired sites are synergistic sites leading to increased reactivity in both low- and high-temperature reactions. No evidence was found for paired framework/nonframework sites.

•Theoretical insights were provided on the influence of second-order quadrupolar-dipolar (2nd-QD) cross interaction on solid-state NMR spectroscopy.•An efficient simulation strategy was introduced to account for 2nd-QD interaction between a spin-1/2 and half-integer quadrupolar nuclear spin pair.•The influence of 1H-17O 2nd-QD interaction was demonstrated on 17O-enriched dehydrated zeolites, MOR (8-MR and 12-MR channel structure) and FER (8-MR and 10-MR channel structure).

Five-coordinated Als (Al(V)) on the surface of aluminas play important roles when they are used as catalysts or catalyst supports. However, the comprehensive characterization and understanding of the intrinsic structural properties of the Al(V) remain a challenge, due to the very small amount in commonly used aluminas. Herein, the surface structures of γ-Al2O3and Al(V)-rich Al2O3nanosheets (Al2O3–NS) have been investigated and compared in detail by multinuclear high-field solid-state NMR. Thanks to the high resolution and sensitivity of ultra-high-field (up to 35.2 T) NMR, the arrangements of surface Als were clearly demonstrated, which are substantially different from the bulk phase in γ-Al2O3due to the structure reconstruction. It reveals for the first time that most of the commonly observed Al(V)s tend to exist as aggregated states on the surface of γ-Al2O3, like those in amorphous Al2O3–NS liable to structure reconstruction. Our new insights into surface Al(V) species may help in understanding the structure–function relationship of alumina.

The structure of aluminum-containing moieties in and within zeolite H-ZSM-5 catalysts is a complex function of the elemental composition of the catalyst, synthesis conditions, exposure to moisture, and thermal history. 27Al NMR data collected at field strengths ranging from 7.05 to 35.2 T, i.e., 1H Larmor frequencies from 300 to 1500 MHz, reveal that Al primarily exists as framework or partially coordinated framework species in commercially available dehydrated H-ZSM-5 catalysts with Si/Al ranging from 11.5 to 40. Quantitative direct-excitation and sensitivity-enhanced 27Al NMR techniques applied over the wide range of magnetic field strengths used in this study show that prior to significant hydrothermal exposure, detectable amounts of nonframework Al species do not exist. Two-dimensional 27Al multiple-quantum magic-angle spinning (MQMAS) along with 1H–27Al and 29Si–27Al dipolar correlation (D-HMQC) NMR experiments confirm this conclusion and show that generation of nonframework species following varying severities of hydrothermal exposure are clearly resolved from partially coordinated framework sites. The impact of hydration on the appearance and interpretation of conventional direct-excitation 27Al spectra, commonly used to assess framework and nonframework Al, is discussed. Aluminum sites in dehydrated catalysts, which are representative of typical operating conditions, are characterized by large quadrupole interactions and are best assigned by obtaining data at multiple field strengths. On the basis of the results here, an accurate initial assessment of Al sites in high-Al content MFI catalysts prior to any hydrothermal treatment can be used to guide reaction conditions, anticipate potential water impacts, and identify contributions from hydroxyl groups other than those associated with the framework bridging acid site.

Fatty acids are ubiquitous in biological systems and widely used in materials science, including for the formulation of drugs and the surface-functionalization of nanoparticles. However, important questions regarding the structure and reactivity of these molecules are still to be elucidated, including their mode of binding to certain metal cations or materials surfaces. In this context, we have developed novel, efficient, user-friendly, and cost-effective synthetic protocols based on ball-milling, for the 17O and 18O isotopic labeling of two key fatty acids which are widely used in (nano)materials science, namely stearic and oleic acid. Labeled molecules were analyzed by 1H and 13C solution NMR, IR spectroscopy, and mass spectrometry (ESI-TOF and LC-MS), as well as 17O solid state NMR (for the 17O labeled species). In both cases, the labeling procedures were scaled-up to produce up to gram quantities of 17O- or 18O-enriched molecules in just half-a-day, with very good synthetic yields (all ≥84%) and enrichment levels (up to an average of 46% per carboxylic oxygen). The 17O-labeled oleic acid was then used for the synthesis of a metal soap (Zn-oleate) and the surface-functionalization of ZnO nanoparticles (NPs), which were characterized for the first time by high-resolution 17O NMR (at 14.1 and 35.2 T). This allowed very detailed insight into (i) the coordination mode of the oleate ligand in Zn-oleate to be achieved (including information on Zn···O distances) and (ii) the mode of attachment of oleic-acid at the surface of ZnO (including novel information on its photoreactivity upon UV-irradiation). Overall, this work demonstrates the high interest of these fatty acid-enrichment protocols for understanding the structure and reactivity of a variety of functional (nano)materials systems using high resolution analyses like 17O NMR.

While 17O NMR is increasingly being used for elucidating the structure and reactivity of complex molecular and materials systems, much effort is still required for it to become a routine analytical technique. One of the main difficulties for its development comes from the very low natural abundance of 17O (0.04%), which implies that isotopic labeling is generally needed prior to NMR analyses. However, 17O-enrichment protocols are often unattractive in terms of cost, safety, and/or practicality, even for compounds as simple as metal oxides. Here, we demonstrate how mechanochemistry can be used in a highly efficient way for the direct 17O isotopic labeling of a variety of s-, p-, and d-block oxides, which are of major interest for the preparation of functional ceramics and glasses: Li2O, CaO, Al2O3, SiO2, TiO2, and ZrO2. For each oxide, the enrichment step was performed under ambient conditions in less than 1 h and at low cost, which makes these synthetic approaches highly appealing in comparison to the existing literature. Using high-resolution solid-state 17O NMR and dynamic nuclear polarization, atomic-level insight into the enrichment process is achieved, especially for titania and alumina. Indeed, it was possible to demonstrate that enriched oxygen sites are present not only at the surface but also within the oxide particles. Moreover, information on the actual reactions occurring during the milling step could be obtained by 17O NMR, in terms of both their kinetics and the nature of the reactive species. Finally, it was demonstrated how high-resolution 17O NMR can be used for studying the reactivity at the interfaces between different oxide particles during ball-milling, especially in cases when X-ray diffraction techniques are uninformative. More generally, such investigations will be useful not only for producing 17O-enriched precursors efficiently but also for understanding better mechanisms of mechanochemical processes themselves.

The spectroscopic study of oxygen, a vital element in materials, physical, and life sciences, is of tremendous fundamental and practical importance. 17O solid-state NMR (SSNMR) spectroscopy has evolved into an ideal site-specific characterization tool, furnishing valuable information on the local geometric and bonding environments about chemically distinct and, in some favorable cases, crystallographically inequivalent oxygen sites. However, 17O is a challenging nucleus to study via SSNMR, as it suffers from low sensitivity and resolution, owing to the quadrupolar interaction and low 17O natural abundance. Herein, we report a significant advance in 17O SSNMR spectroscopy. 17O isotopic enrichment and the use of an ultrahigh 35.2 T magnetic field have unlocked the identification of many inequivalent carboxylate oxygen sites in the as-made and activated phases of the metal–organic framework (MOF) α-Mg3(HCOO)6. The subtle 17O spectral differences between the as-made and activated phases yield detailed information about host–guest interactions, including insight into nonconventional O···H–C hydrogen bonding. Such weak interactions often play key roles in the applications of MOFs, such as gas adsorption and biomedicine, and are usually difficult to study via other characterization routes. The power of performing 17O SSNMR experiments at an ultrahigh magnetic field of 35.2 T for MOF characterization is further demonstrated by examining activation of the MIL-53(Al) MOF. The sensitivity and resolution enhanced at 35.2 T allows partially and fully activated MIL-53(Al) to be unambiguously distinguished and also permits several oxygen environments in the partially activated phase to be tentatively identified. This demonstration of the very high resolution of 17O SSNMR recorded at the highest magnetic field accessible to chemists to date illustrates how a broad variety of scientists can now study oxygen-containing materials and obtain previously inaccessible fine structural information.

Ultrahigh field 27Al{1H} 2D correlation NMR experiments demonstrate that at least two framework Al(IV) sites with hydroxyl groups can exist in acidic zeolite catalysts in their dehydrated and catalytically active states. In addition to the known Al(IV) at the framework bridging acid site (BAS), a new site created by a second tetrahedral Al atom and its hydroxyl group protons in zeolite HZSM-5 is clearly resolved at 35.2 T field strengths, enabled by recently developed series-connected hybrid (SCH) magnet technology. Coupled with computational modeling, extensive 27Al MQMAS experiments at multiple field strengths, and 1H MAS NMR experiments, these data indicate that this second tetrahedrally coordinated Al site (denoted Al(IV)-2) experiences an increased chemical shift and unique quadrupolar parameters relative to the BAS in both dehydrated and hydrated states. These new experimental data, supported by computational and catalytic reaction work, indicate that the second site arises from partially bonded framework (SiO)4−n-Al(OH)nspecies that significantly increase catalyst reactivity in benzene hydride-transfer and n-hexane cracking reactions. Al(IV)-2 sites result either from framework crystallization defects or from incomplete postsynthetic hydrolysis of a framework Al, prior to the formation of extraframework Al. Populations of this second acidic proton site created by the Al(IV)-2 species are shown to be controlled via postsynthetic catalyst treatments, should be general to different catalyst structures, and significantly enhance catalyst reactivity in the cited probe reactions when they are present. The results herein communicate the highest magnetic field strength data on active zeolite catalyst structures to date and enable for the first time the detection of Al and H association on a dry HZSM-5 catalyst, i.e., under conditions representative of typical end-use processes.

Direct observation of multiple reactive sites in the zeolite HZSM-5, a member of the MFI family of zeolite structures, contradicts the traditional view of only one type of active protonic species in industrially important zeolites. In addition to the well-known Brönsted acid site proton, two other protonic species undergo room-temperature hydrogen–deuterium exchange with an alkane hydrocarbon reagent, including one zeolite moiety characterized by a broad 1H chemical shift at ca. 12–15 ppm that is reported here for the first time. Although the ca. 13 ppm chemical shift value is consistent with computational predictions from the literature for a surface-stabilized hydroxonium ion in a zeolite, data suggest that the signal does not arise from hydroxonium species but rather from hydroxyls on extra-lattice aluminol species proximate to Brönsted lattice sites, i.e., a small population of highly deshielded acid sites. Double-resonance experiments show that this species is proximate to Al atoms, similar to the Brönsted acid site proton. These sites can be removed by appropriate postsynthesis chemical treatment, yielding a catalyst with reduced activity for isotopic H/D exchange reactions. Additionally, other extra-lattice aluminum hydroxyl groups previously discussed in the literature but whose protons were considered unreactive are also shown for the first time to react with hydrocarbon probe molecules. Two-dimensional exchange NMR reveals direct proton exchange between the Brönsted site and these two types of extra-lattice Al–OH species, and it also reveals unexpected proton exchange between extra-lattice Al–OH species and an alkane reagent.

Despite the extensive industrial and research interests in zeolites, their intrinsic catalytic nature is not fully understood due to the complexity of the hydroxyl-aluminum moieties. 17O NMR would provide irreplaceable opportunities for much-needed fine structural determination given the ubiquitous presence of oxygen atoms in nearly all species; however, the low sensitivity and quadrupolar nature of oxygen-17 make its NMR spectroscopic elucidation challenging. Here, we show that state-of-the-art double resonance solid-state NMR techniques have been combined with spectral editing methods based on scalar (through-bond) and dipolar (through-space) couplings, which allowed us to address the subtle protonic structures in zeolites. Notably, the often-neglected and undesired second-order quadrupolar-dipolar cross-term interaction (“2nd-QD interaction”) can actually be exploited and can help gain invaluable information. Eventually, a comprehensive set of 1H–17O/1H–27Al double resonance NMR with J-/D-coupling spectral editing techniques have been designed in this work and enabled us to reveal atomic-scale precise structural and dynamical details in zeolites including: 1) The jump rate of the bridging acid site (BAS) proton is relatively low, i.e., far less than 100 s–1at room temperature. 2) The Al–OH groups with 1H chemical shift at 2.6–2.8 ppm, at least for nonseverely dealuminated H-ZSM-5 catalysts, exhibit a rigid bridging environment similar to that of BAS. 3) The Si–OH groups at 2.0 ppm are not hydrogen bonded and undergo fast cone-rotational motion. The results in this study predict the 2nd-QD interaction to be universal for any rigid −17O–H environment, such as those in metal oxide surfaces or biomaterials.

Acidic zeolites are solid aluminosilicate catalysts whose utility arises from Brønsted sites that predominately reside in their crystalline framework structure. Data described herein indicate that extra-framework aluminum (EFAl) moieties, often proposed as important species in overall catalyst activity via Brønsted–Lewis synergies, can themselves contribute protons that are also reactive Brønsted acid centers. While the MFI family of zeolites is a relatively simple channel-structure type, the quantitative spectroscopic detection of all protons shows that the distribution of reactive Brønsted acid site protons arising from framework and extra-framework moieties can be complex. Experiments show that postsynthetic treatments can be used to modify this distribution, in theory enabling routes to HZSM-5 catalysts with only one type of reactive Brønsted site. Quantitative spin-counting NMR experiments combined with chemical washing using ammonium hexafluorosilicate (AHFS) show that the number of framework bridging acid sites (BAS) in typical commercial MFI catalysts (Si/Al equal 15 and 40) is between 50 and 60% of that expected based on the total Al content. Acidic protons from EFAl constitute the major fraction of remaining Brønsted sites. Probe-molecule reactions demonstrate that catalysts with only framework BAS are significantly less reactive than those with both extra-framework and framework Brønsted acid sites. Various postsynthetic methods are compared to optimize the desired Brønsted acid site distribution in MFI catalysts, including both removal and re-introduction of acidic protons from EFAl sites.

Surface metal hydrides (M–H) are ubiquitous in heterogeneous catalytic reactions, while the detailed characterizations are frequently hindered by their high reactivity/low concentration, and the complicated surface structures of the host solids, especially in terms of practical solid catalysts. Herein, combining instant quenching capture and advanced solid-state NMR methodology, we report the first direct and unambiguous NMR evidence on the highly reactive surface gallium hydrides (Ga–H) over a practical Ga2O3catalyst during direct H2activation. The spectroscopic effects of 69Ga and 71Ga isotopes on the 1H NMR signal are clearly differentiated and clarified, allowing a concrete discrimination of the Ga–H signal from the hydroxyl crowd. Accompanied with quantitative and two-dimensional NMR spectroscopical methods, as well as density functional theory calculations, information on the site specification, structural configuration, and formation mechanism of the Ga–H species has been revealed, along with the H2dissociation mechanism. More importantly, the successful spectroscopic identification and isolation of the surface Ga–H allow us to clearly reveal the critical but ubiquitous intermediate role of this species in catalytic reactions, such as propane dehydrogenation and CO2hydrogenation reactions. The analytic approach presented in this work can be extended to other M–H analysis, and the insights will benefit the design of more efficient Ga-based catalysts.

The dehydration of alcohols to value-added chemicals using alumina catalysts has been industrialized decades ago; however, despite years of fundamental research, the molecular-level understanding of the reaction process is still lacking. Herein, the dehydration reactions of ethanol on two representative aluminas, i.e., γ-Al2O3and five-coordinated Al-rich Al2O3-nanosheet (Al2O3-NS), are comparatively investigated by a combination of solid-state NMR spectroscopy and reaction evaluations. NMR results reveal that the presence of hydroxyl groups nearby the Lewis acid sites (LASs) leads to different ethanol adsorption modes. The surface hydroxyls nearby LASs interfere with ethanol adsorption, which causes the methyl group to move away from the alumina surface and hinders the elimination of β-H, resulting in low reaction activity. In addition, the desorption property of surface hydroxyls will affect the dehydration reaction as they participate in the catalytic reaction cycles. Such effects may also explain the lower reaction activities for diethyl ether to ethylene compared with ethanol. Although both four- and five-coordinated Als are the possible active sites for ethanol dehydration, the reactivity of alcohol dehydration may also be substantially affected by the local environment of undercoordinated Als besides coordination numbers. These new insights demonstrate that the host–guest interaction regulated by the local environment of active sites plays an important role in the catalytic reaction.

Mesoporous hexagonal boron nitride (p-BN) has received significant attention over the last decade as a promising candidate for water cleaning/pollutant removal and hydrogen storage applications. Here, high-resolution solid-state NMR spectroscopy and plane-wave density-functional theory (DFT) calculations are used to obtain an atomic-level description of p-BN. 1H–15N or 1H–14N heteronuclear (HETCOR) correlation experiments recorded with either conventional NMR at room temperature or dynamic nuclear polarization surface-enhanced spectroscopy (DNP-SENS) at ca.100 K reveal NB2H, NBH2, NBH3+species residing on the edges of BN sheets. Ultra-high field 35.2 T 11B NMR spectroscopy was used to resolve 11B NMR signals from BN3, BN2Ox(OH)1–x(x= 0–1), BNOx(OH)2–x(x= 0–2), BOx(OH)3–x(x= 0–3), and BOx(OH)4–x–(x= 0–4). Importantly, 2D 11B dipolar double-quantum–single-quantum homonuclear correlation spectra reveal that many pore/defect sites are composed of boron oxide/hydroxide clusters connected to the BN framework through BN2O units. 1D and 2D 11B{15N} HETCOR NMR experiments, in addition to plane-wave DFT calculations of nine different structural models, further confirm the assignment of all NMR signals. The detailed structure determination of the pore and edge/defect sites within p-BN should further enable the rational design and development of next-generation p-BN-based materials. In addition, the techniques outlined here should be applicable to determine structure within other porous and/or boron-based materials.

Zeolite ZSM-5 catalysts were modified with varying amounts of H3PO4to yield a suite of catalysts with phosphorus loadings ranging from 0.3 to 5 wt %, corresponding to the P/Al molar ratio ranging from ca. 0.25 to 4.2 based on the Si/Al ratio = 40. A combination of 31P, 27Al, 29Si, and 1H NMR experiments at multiple field strengths suggests that discrete phosphate structures form at Al lattice sites at low H3PO4loadings, with increased phosphorus structural heterogeneity at high loadings. Al–O–P bond formation is detected for H3PO4loadings corresponding to a P/Al ratio as low as 1:4. Control experiments with similar phosphorus treatment on silicalite, the silica analogue of ZSM-5, show only free phosphate and small oligomeric phosphate formation, with none of the spectroscopic signatures indicative of proximate Al and P atoms that are observed in similar ZSM-5 catalysts. Two-dimensional heteronuclear multiple quantum coherence experiments involving multiple nuclei (27Al–31P, 27Al–1H, 27Al–29Si, and 1H–31P) indicate that framework-bound phosphates and a distribution of acidic hydroxyl groups form at the expense of original Brønsted acid sites (BAS) in the catalyst. However, even at the highest loadings investigated, some residual BASs remain. Prior to any hydrothermal treatments, multiple-quantum magic-angle spinning experiments indicate that P-modified catalysts contain several different Al sites, versus only two that are observed in the untreated ZSM-5. Density functional theory calculations provide further support for the formation of framework Al–O–P species, in addition to strongly hydrogen-bonded H3PO4/BAS complexes.

Hydroxyl groups are among the major active surface sites over metal oxides. However, their spectroscopic characterizations have been challenging due to limited resolutions, especially on hydroxyl-rich surfaces where strong hydroxyl networks are present. Here, using nanostructured In2O3as an example, we show significantly enhanced discrimination of the surface hydroxyl groups, owing to the high-resolution 1H NMR spectra performed at a high magnetic field (18.8 T) and a fast magic angle spinning (MAS) of up to 60 kHz. A total of nine kinds of hydroxyl groups were distinguished and their assignments (μ1, μ2, and μ3) were further identified with the assistance of 17O NMR. The spatial distribution of these hydroxyl groups was further explored via two-dimensional (2D) 1H-1H homonuclear correlation experiments with which the complex surface hydroxyl network was unraveled at the atomic level. Moreover, the quantitative analysis of these hydroxyl groups with such high resolution enables further investigations into the physicochemical property and catalytic performance characterizations (in CO2reduction) of these hydroxyl groups. This work provides insightful understanding on the surface structure/property of the In2O3nanoparticles and, importantly, may prompt general applications of high-field ultrafast MAS NMR techniques in the study of hydroxyl-rich surfaces on other metal oxide materials.