Host–Guest Interactions and Their Catalytic Consequences in Methanol to Olefins Conversion on Zeolites Studied by 13C–27Al Double-Resonance Solid-State NMR Spectroscopy

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

Zeolite chemistry and catalysis are expected to play a decisive role in the next decade(s) to build a more decentralized renewable feedstock-dependent sustainable society owing to the increased scrutiny over carbon emissions. Therefore, the lack of fundamental and mechanistic understanding of these processes is a critical "technical bottleneck" that must be eliminated to maximize economic value and minimize waste. We have identified, considering this objective, that the chemistry related to the first-generation reaction intermediates (i.e., carbocations, radicals, carbenes, ketenes, and carbanions) in zeolite chemistry and catalysis is highly underdeveloped or undervalued compared to other catalysis streams (e.g., homogeneous catalysis). This limitation can often be attributed to the technological restrictions to detect such "short-lived and highly reactive" intermediates at the interface (gas-solid/solid-liquid); however, the recent rise of sophisticated spectroscopic/analytical techniques (including under in situ/operando conditions) and modern data analysis methods collectively compete to unravel the impact of these organic intermediates. This comprehensive review summarizes the state-of-the-art first-generation organic reaction intermediates in zeolite chemistry and catalysis and evaluates their existing challenges and future prospects, to contribute significantly to the "circular carbon economy" initiatives.

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.

Progressive steps and catalytic cycles in methanol-to-hydrocarbons reaction over acidic zeolites

Understanding the complete reaction network and mechanism of methanol-to-hydrocarbons remains a key challenge in the field of zeolite catalysis and C1 chemistry. Inspired by the identification of the reactive surface methoxy species on solid acids, several direct mechanisms associated with the formation of the first C-C bond in methanol conversion have been recently disclosed. Identifying the stepwise involvement of the initial intermediates containing the first C-C bond in the whole reaction process of methanol-to-hydrocarbons conversion becomes possible and attractive for the further development of this important reaction. Herein, several initial unsaturated aldehydes/ketones containing the C-C bond are identified via complementary spectroscopic techniques. With the combination of kinetic and spectroscopic analyses, a complete roadmap of the zeolite-catalyzed methanol-to-hydrocarbons conversion from the initial C-C bonds to the hydrocarbon pool species via the oxygen-containing unsaturated intermediates is clearly illustrated. With the participation of both Brønsted and Lewis acid sites in H-ZSM-5 zeolite, an initial aldol-cycle is proposed, which can be closely connected to the well-known dual-cycle mechanism in the methanol-to-hydrocarbons conversion.

Influence of Substrate Concentration on Kinetic Parameters of Ethanol Dehydration in MFI and CHA Zeolites and Relation of These Kinetic Parameters to Acid–Base Properties

The catalytic activity of zeolites is often related to their acid–base properties. In this work, the relationship between the value of apparent activation energy of ethanol dehydration, measured in a fixed bed reactor and by means of a temperature-programmed surface reaction (TPSR) depending on the amount of ethanol in the zeolite lattice and the value of activation energy of H/D exchange as a measure of acid–base properties of MFI and CHA zeolites, was studied. Tests in a fixed bed reactor were unable to provide reliable reaction kinetics data due to internal diffusion limitations and rapid catalyst deactivation. Only the TPSR method was able to provide activation energy values comparable to the activation energy values obtained from the H/D exchange rate measurements. In addition, for CHA zeolite, it has been shown that the values of ethanol dehydration activation energies depend on the amount of ethanol in the CHA framework, and this effect can be attributed to the substrate clustering effects supporting the deprotonation of zeolite Brønsted centers.

Conversion of methanol to hydrocarbons over H-MCM-22 zeolite: deactivation behaviours related to acid density and distribution

The deactivation of H-MCM-22 zeolites with different Si/Al ratios can be roughly divided into three stages: first the rapid deactivation of the supercages, the second reaction with slow coking and the deactivation stage with rapid coking mainly on the external pockets.

Stabilizing the framework of SAPO-34 zeolite toward long-term methanol-to-olefins conversion

As a commercial MTO catalyst, SAPO-34 zeolite exhibits excellent recyclability probably due to its intrinsic good hydrothermal stability. However, the structural dynamic changes of SAPO-34 catalyst induced by hydrocarbon pool (HP) species and the water formed during the MTO conversion as well as its long-term stability after continuous regenerations are rarely investigated and poorly understood. Herein, the dynamic changes of SAPO-34 framework during the MTO conversion were identified by 1D ²⁷Al, ³¹P MAS NMR, and 2D ³¹P-²⁷Al HETCOR NMR spectroscopy. The breakage of T-O-T bonds in SAPO-34 catalyst during long-term continuous regenerations in the MTO conversion could be efficiently suppressed by pre-coking. The combination of catalyst pre-coking and water co-feeding is established to be an efficient strategy to promote the catalytic efficiency and long-term stability of SAPO-34 catalysts in the commercial MTO processes, also sheds light on the development of other high stable zeolite catalyst in the commercial catalysis.

Stability of zeolite catalysts is a highly desirable property for commercial methanol to olefins conversion but extremely challenging to achieve. Here, the authors combine the catalyst pre-coking and water co-feeding to develop an efficient strategy to enhance the long-term stability of SAPO-34 catalyst.

Recent advances in probing host–guest interactions with solid state nuclear magnetic resonance

A recent update on how solid state NMR has aided the interpretation and understanding of host–guest interactions in the field of supramolecular assemblies is provided.

Composite Nanostructure of Manganese Cluster and CHA-Type Silicoaluminaphosphates: Enhanced Catalytic Performance in Dimethylether to Light Olefins Conversion

Light olefins, especially ethylene and propylene, are important chemicals in petrochemical industries with an increasing demand and play an essential role in the global consumption. In this regard, there have been extensive studies to design efficient catalysts for the light olefins productions. In this study, we report a new protocol to induce Mn nanoclusters (MnNC) into the mesopore of a CHA-type silicoaluminaphosphates via a one-pot synthesis of MnNC@SAPO-34 catalysts. The catalysts are characterized by a series of technology, such as TEM, XRD, NH₃-TPD, ²⁷Al MAS NMR, ICP-MS, XPS, and as well as N₂-physical adsorption methods. The Mn nanoclusters of Mn₂O₃ and MnO₂ species are well dispersed in the framework of the SAPO-34 silicoaluminaphosphates, modifying the porosity and acidic property of the SAPO-34: Giving rise to more mesoporous and improving the acid density. The MnNC@SAPO-34 catalysts exhibit decent 100% conversion and 92.2% olefins selectivity in the dimethyl ether to olefins (DTO) reactions, which is considerably higher than that for SAPO-34 silicoaluminaphosphates (79.6% olefins selectivity). The higher olefins selectivity over the MnNC@SAPO-34 is deemed to associate with the strong acid density and intensity of the silicoaluminaphosphates. Further, the Mn particles largely improve silicoaluminaphosphates’s durability.

Elucidating Zeolite Channel Geometry-Reaction Intermediate Relationships for the Methanol-to-Hydrocarbon Process

The chemical industry has exploited zeolite shape selectivity for more than 50 years, yet our fundamental understanding remains incomplete. Herein, the zeolite channel geometry-reactive intermediate relationships are studied in detail using anisotropic zeolite ZSM-5 crystals for the methanol-to-hydrocarbon (MTH) process, and advanced magic-angle spinning solid-state NMR (ssNMR) spectroscopy. The utilization of anisotropic ZSM-5 crystals enabled the preferential formation of reaction intermediates in single-orientation zeolite channels, as revealed by molecular dynamics simulations and in situ UV/Vis diffuse-reflectance spectroscopy. The ssNMR results show that the slightly more constrained sinusoidal zeolite channels favor the olefin cycle by promoting the homologation of alkanes, whereas the more extended straight zeolite channels facilitate the aromatic cycle with a higher degree of alkylation of aromatics. Dynamic nuclear polarization experiments further indicate the preferential formation of heavy aromatics at the zeolite surface dominated by the sinusoidal channels, providing further insight into catalyst deactivation.

Recent Advances of Solid-State NMR Spectroscopy for Microporous Materials

Microporous materials have attracted a rapid growth of research interest in materials science and the multidisciplinary area because of their wide applications in catalysis, separation, ion exchange, gas storage, drug release, and sensing. A fundamental understanding of their diverse structures and properties is crucial for rational design of high-performance materials and technological applications in industry. Solid-state NMR (SSNMR), capable of providing atomic-level information on both structure and dynamics, is a powerful tool in the scientific exploration of solid materials. Here, advanced SSNMR instruments and methods for characterization of microporous materials are briefly described. The recent progress of the application of SSNMR for the investigation of microporous materials including zeolites, metal-organic frameworks, covalent organic frameworks, porous aromatic frameworks, and layered materials is discussed with representative work. The versatile SSNMR techniques provide detailed information on the local structure, dynamics, and chemical processes in the confined space of porous materials. The challenges and prospects in SSNMR study of microporous and related materials are discussed.

Surface enhanced dynamic nuclear polarization solid-state NMR spectroscopy sheds light on Brønsted-Lewis acid synergy during the zeolite catalyzed methanol-to-hydrocarbon process

After a prolonged effort over two decades, the reaction mechanism of the zeolite-catalyzed methanol-to-hydrocarbon (MTH) process is now well-understood: the so-called 'direct mechanism' (via direct coupling of two methanol molecules) is responsible for the formation of the initial carbon-carbon bonds, while the hydrocarbon pool (HCP)-based dual cycle mechanism is responsible for the formation of reaction products. While most of the reaction events occur at zeolite Brønsted acid sites, the addition of Lewis acid sites (i.e., via the introduction of alkaline earth cations like calcium) has been shown to inhibit the formation of deactivating coke species and hence increase the catalyst lifetime. With the aim to have an in-depth mechanistic understanding, herein, we employ magic angle spinning surface-enhanced dynamic nuclear polarization solid-state NMR spectroscopy to illustrate that the inclusion of Lewis acidity prevents the formation of carbene/ylide species on the zeolite, directly affecting the equilibrium between arene and olefin cycles of the HCP mechanism and hence regulating the ultimate product selectivity and catalyst lifetime.

Metal Active Sites and Their Catalytic Functions in Zeolites: Insights from Solid-State NMR Spectroscopy

Zeolites are important heterogeneous catalysts widely used in the modern chemical and petrochemical industries. Metal-containing zeolites show distinct performance in the catalytic processes such as fluid catalytic cracking, activation and conversion of light alkanes, methanol-to-aromatics conversion, biomass transformation, and so on. The metal speciation, distribution, and interactions on zeolites have enormous impact on their property and catalytic performance. Significant efforts have been devoted to the synthesis of more active and selective zeolites by engineering the metal active sites. However, the nature of metal species and their role in the reactions are still poorly understood, which makes it difficult to establish the structure-activity relationship toward the rational design and application of zeolites. For example, synergic active sites are often present on the metal-containing zeolites, but their structure, property and quantification still remain to be resolved. Solid-state NMR is a powerful tool for the characterization of heterogeneous catalysts and catalytic reactions by providing information about both molecular structure and dynamics. The heterogeneity and low concentration of the metal sites on zeolites usually leads to a great challenge for their characterization. In this Account, we will describe our effort to study the metal active sites, host-guest interactions, and reaction intermediates by using solid-state NMR spectroscopy, with the aim to highlight recent advances in solid-state NMR techniques for probing the structure and property of metal-containing zeolites as well as the relevant reaction mechanisms. Using sensitivity-enhanced NMR methods such as ⁶⁷Zn, ⁷¹Ga, and ¹¹⁹Sn, NMR enables the identification of metal speciation on zeolites. The synergic active sites constituted by metal species (as Lewis acid sites) and acidic protons (as Brønsted acid sites) on zeolites that amount to only a small fraction of the whole system can be directly probed and quantified with advanced ¹H-⁶⁷Zn or ¹H-⁷¹Ga double-resonance solid-state NMR. We developed NMR methods to study the host-guest interactions in zeolites by observing the spatial interaction/proximity between aluminum sites (associated with Brønsted or Lewis acid sites) in zeolite host and carbon atoms in organic molecule guest formed during catalytic reaction, which leads to the formation of supramolecular reaction centers in the methanol-to-olefins reaction. The mechanisms underlying the catalytic reactions on metal-modified zeolite are revealed by the identification of key reaction intermediates with in situ ¹³C MAS NMR spectroscopy. Our discussion based on the representative examples shows how the metal species serving as active sites significantly affect the property and activity of zeolites and related reaction pathways. The structural information obtained by the state-of-the-art solid-state NMR techniques provides new insights into the structure-activity relationship of zeolites in heterogeneous catalysis, which should be beneficial for rational design of highly efficient zeolite catalysts.

Extra-Framework Aluminum-Assisted Initial C-C Bond Formation in Methanol-to-Olefins Conversion on Zeolite H-ZSM-5

Surface methoxy species bound to an extra-framework Al (SMS-EFAL) was unambiguously identified by advanced ¹³ C-{²⁷ Al} double-resonance solid-state NMR technique in the methanol-to-olefins reaction on H-ZSM-5 zeolite. The high reactivity of the SMS-EFAL leads to the formation of surface ethoxy species and ethanol as the key intermediates for ethene generation in the early reaction stage. A direct route for the initial C-C bond formation in ethene was proposed and corroborated by density functional theory calculations.