Solid-state NMR studies of internuclear correlations for characterizing catalytic materials

Fabricating a stable interface of tetracoordinated-phosphorus and framework Al within P-doping ZSM-5 zeolite for catalytic methanol-to-propylene reaction

Phosphorus (P)-doping H-ZSM-5 zeolites, which is crucial for industrial applications, aim to adjust both acidity and framework stability while optimizing product distribution in heterogeneous catalysis. Nonetheless, current phosphating methods often suffer from inadequate phosphorus dispersion and unclear interfacial interactions with framework aluminum (Al). In this work, P-doping ZSM-5 zeolites were successfully one-step prepared by using tributylphosphine served as an organophosphorus precursor, assisting by density functional theory calculations. On account of this, a stable interface of tetracoordinated-phosphorus and framework Al was fabricated uniformly. Such an interfacial structure not only made more framework Al sit in the straight/sinusoidal channels, but also remodeled Brønsted acid sites and reinforced its acidity. Under comparable conditions, methanol-to-propylene (MTP) evaluations indicated that the high-silica PZ-75 zeolite catalyst displayed an appreciable catalytic lifetime (30 h) and a higher C3H6 selectivity (52.4 %). Additionally, in-situ Fourier transform infrared spectroscopy further revealed that this exceptional MTP performance was attributable to the predominant olefinic pathway and lower hydrogen transfer ability. These results provide valuable information for cognizing phosphorus-zeolite chemistry and creating high-performance MTP catalytic materials.

Deuteron-proton isotope correlation spectroscopy at high magnetic fields

A cross-polarization 2H-1H isotope correlation spectroscopy (CP-iCOSY) approach is presented for characterizing a deuterated amino acid, pharmaceutical compound and a solid formulation. This can be achieved by isotopic enrichment in conjunction with high magnetic field (28.2 T) and fast magic-angle spinning (MAS), enabling the rapid detection of 2H NMR spectra in a few seconds to minutes. Specifically, two-dimensional (2D) 2H-1H CP-iCOSY experiment allows the local structures and through-space interactions in a partially deuterated compounds to be elucidated. In doing so, we compare conventional spin-lock and rotor-echo-short-pulse-irradiation RESPIRATIONCP sequences for acquiring 2D 1H-2H correlation spectra. The RESPIRATIONCP sequence allows the detection of 2D peaks at lower CP contact times (0.1-1 ms) than the conventional CP (0.2-4 ms) sequence. Analysis of partially deuterated L-histidine·HCl·H2O and dopamine.HCl is presented, in which the detection of 2D peaks corresponding to 2H-1H pairs separated by greater than 4 Å distance demonstrates the potential of the presented approach for the characterization of packing interactions. These results are corroborated by NMR crystallography analysis using the Gauge-Including Projector Augmented-Wave (GIPAW) approach.

Identification and Quantification of Al Pairs and Their Impact on Dealumination in Zeolites

Understanding the precise quantity and spatial distribution of paired aluminum (Al) sites in zeolite catalysts is crucial, as they significantly impact the catalytic performance via synergistic effects and long-term stability. In this study, a novel strategy by employing divalent cation titration with varying cation sizes, in combination with advanced quantitative 1H NMR and 1H-1H homonuclear correlation techniques, has been developed to accurately identify and classify three distinct types of Al pairs. These include two types of Al pairs aligned along six-membered rings (6-MRs) and 10-membered rings (10-MRs), the latter of which are essentially composed of Al atoms located in different 6-MR or 5-MR. The third type comprises two Al atoms located in different channels. The second and third types had been challenging to probe in the past, yet they may be critical for catalysis, particularly the second type demonstrating proximity close enough to accommodate Ba2+ (with a radius of 1.49 Å). Our strategy for quantifying each type of Al pair marks a significant advancement in the understanding of the zeolite framework. Furthermore, controlled hydrothermal treatments using stepwise steaming reveal that a higher concentration of Al pairs accelerates dealumination, primarily for dynamic reasons of water molecules but not intrinsic structural instability induced by Al pairs. To address this, we propose a "bi-Al" vs "mono-Al" hydrolysis model, offering fresh insights into the pivotal role of Al pairs on zeolite stability. This work opens new avenues for optimizing zeolite-based catalysts for enhanced performance and longevity.

Revealing the Bro̷nsted Acidic Nature of Penta-Coordinated Aluminum Species in Dealuminated Zeolite Y with Solid-State NMR Spectroscopy

The inevitable dealumination process of zeolite Y is closely related to ultrastabilization, enhanced Bro̷nsted acidity, and deactivation throughout its life cycle, producing complex aluminum and acidic hydroxyl species. Most investigations on dehydrated zeolites have focused on the Bro̷nsted acidity of tetra-coordinated Al (Al(IV)) and Lewis acidity associated with tricoordinated Al (Al(III)) sites, which has left the penta-coordinated Al (Al(V)) in dealuminated zeolites scarcely discussed. This is largely due to the oversimplified view of detectable Al(V) as an exclusively extra-framework species with Lewis acidity. Here we report the formation of Bro̷nsted acidic penta-coordinated Al species (Al(V)-BAS) in the dealumination process. Two-dimensional (2D) through-bond and multiquantum 1H-{27Al} correlation solid-state NMR experiments demonstrate the presence of a bridging Si-OH-Al(V) structure in dealuminated Y zeolites. Different from the conventional belief that water attack leads to the breaking of zeolite framework Al-O bonds in the initial stage of zeolite dealumination, the observed Al(V) as a dealumination intermediate is directly correlated with a BAS pair because of the direct dissociation of water on the framework tetrahedral aluminum (Al(IV)), thus bypassing the cleavage of Al-O bonds. 1H double-quantum solid-state NMR experiments and theoretical calculations provide further evidence for this mechanism, whereas pyridine adsorption experiments confirm stronger acidity of Al(V)-BASs than the traditional bridging hydroxyl groups associated with Al(IV). We were also able to detect the Al(V)-BAS site from dealuminated SSZ-13 zeolite with CHA topology, suggesting that its creation is not specific to the framework structure of zeolites.

Advanced solid-state NMR spectroscopy and its applications in zeolite chemistry

Solid-state NMR spectroscopy (ssNMR) can provide details about the structure, host-guest/guest-guest interactions and dynamic behavior of materials at atomic length scales. A crucial use of ssNMR is for the characterization of zeolite catalysts that are extensively employed in industrial catalytic processes. This review aims to spotlight the recent advancements in ssNMR spectroscopy and its application to zeolite chemistry. We first review the current ssNMR methods and techniques that are relevant to characterize zeolite catalysts, including advanced multinuclear and multidimensional experiments, in situ NMR techniques and hyperpolarization methods. Of these, the methodology development on half-integer quadrupolar nuclei is emphasized, which represent about two-thirds of stable NMR-active nuclei and are widely present in catalytic materials. Subsequently, we introduce the recent progress in understanding zeolite chemistry with the aid of these ssNMR methods and techniques, with a specific focus on the investigation of zeolite framework structures, zeolite crystallization mechanisms, surface active/acidic sites, host-guest/guest-guest interactions, and catalytic reaction mechanisms.

Oxidative treatment of micropollutants present in wastewater: A special emphasis on transformation products, their toxicity, detection, and field-scale investigations

Micropollutants have become ubiquitous in aqueous environments due to the increased use of pharmaceuticals, personal care products, pesticides, and other compounds. In this review, the removal of micropollutants from aqueous matrices using various advanced oxidation processes (AOPs), such as photocatalysis, electrocatalysis, sulfate radical-based AOPs, ozonation, and Fenton-based processes has been comprehensively discussed. Most of the compounds were successfully degraded with an efficiency of more than 90%, resulting in the formation of transformation products (TPs). In this respect, degradation pathways with multiple mechanisms, including decarboxylation, hydroxylation, and halogenation, have been illustrated. Various techniques for the analysis of micropollutants and their TPs have been discussed. Additionally, the ecotoxicity posed by these TPs was determined using the toxicity estimation software tool (T.E.S.T.). Finally, the performance and cost-effectiveness of the AOPs at the pilot scale have been reviewed. The current review will help in understanding the treatment efficacy of different AOPs, degradation pathways, and ecotoxicity of TPs so formed.

Calculating 13 C NMR chemical shifts of large molecules using the eXtended ONIOM method at high accuracy with a low cost

Fragmentation-based methods for nuclear magnetic resonance (NMR) chemical shift calculations have become more and more popular in first-principles calculations of large molecules. However, there are many options for a fragmentation-based method to select, such as theoretical methods, fragmentation schemes, the number of levels of theory, etc. It is important to study the optimal combination of the options to achieve a good balance between accuracy and efficiency. Here we investigate different combinations of options used by a fragmentation-based method, the eXtended ONIOM (XO) method, for 13 C chemical shift calculations on a set of organic and biological molecules. We found that: (1) introducing Hartree-Fock exchange into density functional theory (DFT) could reduce the calculation error due to fragmentation in contrast to pure DFT functionals, while a hybrid functional, xOPBE, is generally recommended; (2) fragmentation schemes generated from the molecular tailoring approach (MTA) with small level parameter n, for example, n = 2 and the degree-based fragmentation method (DBFM) with n = 1, are sufficient to achieve satisfactory accuracy; (3) the two-level XO (XO2) NMR calculation is superior to the calculation with only one level of theory, as the second level (i.e., low level) of theory provides a way to well describe the long-range effect. These findings are beneficial to practical applications of fragmentation-based methods for NMR chemical shift calculations of large molecules.

Site Diversity and Mechanism of Metal-Exchanged Zeolite Catalyzed Non-Oxidative Propane Dehydrogenation

Metal-exchanged zeolites are well-known propane dehydrogenation (PDH) catalysts; however, the structure of the active species remains unresolved. In this review, existing PDH catalysts are first surveyed, and then the current understanding of metal-exchanged zeolite catalysts is described in detail. The case of Ga/H-ZSM-5 is employed to showcase that advances in the understanding of structure-activity relations are often accompanied by technological or conceptional breakthroughs. The understanding of Ga speciation at PDH conditions has evolved owing to the advent of in situ/operando characterizations and to the realization that the local coordination environment of Ga species afforded by the zeolite support has a decisive impact on the active site structure. In situ/operando quantitative characterization of catalysts, rigorous determination of intrinsic reaction rates, and predictive computational modeling are all significant in identifying the most active structure in these complex systems. The reaction mechanism could be both intricately related to and nearly independent of the details of the assumed active structure, as in the two main proposed PDH mechanisms on Ga/H-ZSM-5, that is, the carbenium mechanism and the alkyl mechanism. Perspectives on potential approaches to further elucidate the active structure of metal-exchanged zeolite catalysts and reaction mechanisms are discussed in the final section.

High-Field NMR, Reactivity, and DFT Modeling Reveal the γ-Al2 O3 Surface Hydroxyl Network

Aluminas are strategic materials used in many major industrial processes, either as catalyst supports or as catalysts in their own right. The transition alumina γ-Al2 O3 is a privileged support, whose reactivity can be tuned by thermal activation. This study provides a qualitative and quantitative assessment of the hydroxyl groups present on the surface of γ-Al2 O3 at three different dehydroxylation temperatures. The principal [AlOH] configurations are identified and described in unprecedented detail at the molecular level. The structures were established by combining information from high-field 1 H and 27 Al solid-state NMR, IR spectroscopy and DFT calculations, as well as selective reactivity studies. Finally, the relationship between the hydroxyl structures and the molecular-level structures of the active sites in catalytic alkane metathesis is discussed.

Recent advances in solid-state NMR of zeolite catalysts

Zeolites are important inorganic crystalline microporous materials with a broad range of applications in the areas of catalysis, ion exchange, and adsorption/separations. Solid-state nuclear magnetic resonance (NMR) spectroscopy has proven to be a powerful tool in the study of zeolites and relevant catalytic reactions because of its advantage in providing atomic-level insights into molecular structure and dynamic behavior. In this review, we provide a brief discussion on the recent progress in exploring framework structures, catalytically active sites and intermolecular interactions in zeolites and metal-containing ones by using various solid-state NMR methods. Advances in the mechanistic understanding of zeolite-catalysed reactions including methanol and ethanol conversions are presented as selected examples. Finally, we discuss the prospect of the solid-state NMR technique for its application in zeolites.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis

Zeolites with ordered microporous systems, distinct framework topologies, good spatial nanoconfinement effects, and superior (hydro)thermal stability are an ideal scaffold for planting diverse active metal species, including single sites, clusters, and nanoparticles in the framework and framework-associated sites and extra-framework positions, thus affording the metal-in-zeolite catalysts outstanding activity, unique shape selectivity, and enhanced stability and recyclability in the processes of Brønsted acid-, Lewis acid-, and extra-framework metal-catalyzed reactions. Especially, thanks to the advances in zeolite synthesis and characterization techniques in recent years, zeolite-confined extra-framework metal catalysts (denoted as metal@zeolite composites) have experienced rapid development in heterogeneous catalysis, owing to the combination of the merits of both active metal sites and zeolite intrinsic properties. In this review, we will present the recent developments of synthesis strategies for incorporating and tailoring of active metal sites in zeolites and advanced characterization techniques for identification of the location, distribution, and coordination environment of metal species in zeolites. Furthermore, the catalytic applications of metal-in-zeolite catalysts are demonstrated, with an emphasis on the metal@zeolite composites in hydrogenation, dehydrogenation, and oxidation reactions. Finally, we point out the current challenges and future perspectives on precise synthesis, atomic level identification, and practical application of the metal-in-zeolite catalyst system.

Emerging Analytical Methods to Characterize Zeolite-Based Materials

Zeolites and zeolitic materials are, through their use in numerous conventional and sustainable applications, very important to our daily lives, including to foster the necessary transition to a more circular society. The characterization of zeolite-based materials has a tremendous history and a great number of applications and properties of these materials have been discovered in the past decades. This review focuses on recently developed novel as well as more conventional techniques applied with the aim of better understanding zeolite-based materials. Recently explored analytical methods, e.g. atom probe tomography, scanning transmission X-ray microscopy, confocal fluorescence microscopy and photo-induced force microscopy, are discussed on their important contributions to the better understanding of zeolites as they mainly focus on the micro- to nanoscale chemical imaging and the revelation of structure–composition–performance relationships. Some other techniques have a long and established history, e.g. nuclear magnetic resonance, infrared, neutron scattering, electron microscopy and X-ray diffraction techniques, and have gone through increasing developments allowing the techniques to discover new and important features in zeolite-based materials. Additional to the increasing application of these methods, multiple techniques are nowadays used to study zeolites under working conditions (i.e. the in situ/operando mode of analysis) providing new insights in reaction and deactivation mechanisms.

Application of solid-state NMR techniques for structural characterization of metal-organic frameworks

Solid-state NMR can afford the structural information about the chemical composition, local environment, and spatial coordination at the atomic level, which has been extensively applied to characterize the detailed structure and host-guest interactions in metal-organic frameworks (MOFs). In this review, recent advances for the structural characterizations of MOFs using versatile solid-state NMR techniques were briefly introduced. High-field sensitivity-enhanced solid-state NMR method enabled the direct observation of metal centers in MOFs containing low-γ nuclei. Two-dimensional (2D) homo- and hetero-nuclear correlation MAS NMR experiments provided the spatial proximity among linkers, metal clusters and the introduced guest molecules. Moreover, quantitative measurement of inter-nuclear distances using solid-state NMR provided valuable structural information about the connectivity geometry as well as the host-guest interactions within MOFs. Furthermore, solid-state NMR has exhibited great potential for unraveling the structure property of MOFs containing paramagnetic metal centers.

Preferential population of Al atoms at the T4 site of ZSM-35 for the carbonylation of dimethyl ether

ZSM-35, synthesized using dioxane as the structure-directing agent, featured preferential population of Al atoms at the T4 site in the 8-MR pore, and the Brønsted acid site, thus generated, catalyzed DME carbonylation actively and stably.

Insight into Carbocation-Induced Noncovalent Interactions in the Methanol-to-Olefins Reaction over ZSM-5 Zeolite by Solid-State NMR Spectroscopy

Carbocations such as cyclic carbenium ions are important intermediates in the zeolite-catalyzed methanol-to-olefins (MTO) reaction. The MTO reaction propagates through a complex hydrocarbon pool process. Understanding the carbocation-involved hydrocarbon pool reaction on a molecular level still remains challenging. Here we show that electron-deficient cyclopentenyl cations stabilized in ZSM-5 zeolite are able to capture the alkanes, methanol, and olefins produced during MTO reaction via noncovalent interactions. Intermolecular spatial proximities/interactions are identified by using two-dimensional ¹³ C-¹³ C correlation solid-state NMR spectroscopy. Combined NMR experiments and theoretical analysis suggests that in addition to the dispersion and CH/π interactions, the multiple functional groups in the cyclopentenyl cations produce strong attractive force via cation-induced dipole, cation-dipole and cation-π interactions. These carbocation-induced noncovalent interactions modulate the product selectivity of hydrocarbon pool reaction.