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

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.

Dynamic Catalytic Mechanism of the Methanol-to-Hydrocarbons Reaction over Zeolites

ConspectusThe methanol-to-hydrocarbons (MTH) process has provided a new route to obtaining basic chemicals without relying on an oil resource. Acidity and shape selectivity endow the zeolite with a decisive role in MTH catalysis. However, the inherent reaction characteristics of the MTH reaction over zeolites, such as the complexity of catalytic reaction kinetics, the diversity of catalytic reaction modes, and even the limitations of catalytic and diffusive decoupling, have all confused people with respect to obtaining a comprehensive mechanistic understanding. By examining the zeolite-catalyzed MTH reaction from the perspective of chemical bonding, one would realize that this reaction reflects the dynamic assembly process of C-C bonds from C1 components to multicarbon products. The key to understanding the MTH reaction lies in the mechanism by which C-C bonds are formed and rearranged in the confined microenvironment of the channel or cage structures of zeolite catalysts to achieve shape-selective production.The applications of advanced in situ spectroscopy as well as computational chemistry provide tremendous opportunities for capturing and identifying the details of the structure and properties of reactants, intermediates, and products in the confined reaction space of zeolite channels or cages, observing the real-time dynamic evolution of the catalytic surface, and modeling the elementary reaction steps at the molecular and atomic levels.In this Account, the dynamic catalytic mechanism of the zeolite-catalyzed MTH reaction will be outlined based on decades of continuous research and in-depth understanding. The combination of advanced in situ spectroscopy and theoretical methods allowed us to observe and simulate the formation, growth, and aging process on the working catalyst surface and thus map the dynamical evolution of active sites from a Brønsted acid site (BAS) to an organic-inorganic hybrid supramolecule (OIHS) in the MTH reaction. Moreover, the ever-evolving dynamic succession of the OIHS from surface methoxy species (SMS) to active ion-pair complexes (AIPC) to inert complexes (IC) guided the dynamic autocatalytic process from initiation to sustaining and then to termination, resulting in a complex interlaced hypercycle reaction network. The concept of dynamic catalysis will provide deep insight into the complex catalytic mechanisms as well as the structure-activity relationships in MTH chemistry. More importantly, we are now getting closer to the nature of zeolite catalysis beyond the traditional view of BAS catalysis.

Molecular Views on Mechanisms of Brønsted Acid-Catalyzed Reactions in Zeolites

The Brønsted acidity of proton-exchanged zeolites has historically led to the most impactful applications of these materials in heterogeneous catalysis, mainly in the fields of transformations of hydrocarbons and oxygenates. Unravelling the mechanisms at the atomic scale of these transformations has been the object of tremendous efforts in the last decades. Such investigations have extended our fundamental knowledge about the respective roles of acidity and confinement in the catalytic properties of proton exchanged zeolites. The emerging concepts are of general relevance at the crossroad of heterogeneous catalysis and molecular chemistry. In the present review, emphasis is given to molecular views on the mechanism of generic transformations catalyzed by Brønsted acid sites of zeolites, combining the information gained from advanced kinetic analysis, in situ, and operando spectroscopies, and quantum chemistry calculations. After reviewing the current knowledge on the nature of the Brønsted acid sites themselves, and the key parameters in catalysis by zeolites, a focus is made on reactions undergone by alkenes, alkanes, aromatic molecules, alcohols, and polyhydroxy molecules. Elementary events of C-C, C-H, and C-O bond breaking and formation are at the core of these reactions. Outlooks are given to take up the future challenges in the field, aiming at getting ever more accurate views on these mechanisms, and as the ultimate goal, to provide rational tools for the design of improved zeolite-based Brønsted acid catalysts.

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.

Elucidation of radical- and oxygenate-driven paths in zeolite-catalysed conversion of methanol and methyl chloride to hydrocarbons

Understanding hydrocarbon generation in the zeolite-catalysed conversions of methanol and methyl chloride requires advanced spectroscopic approaches to distinguish the complex mechanisms governing C-C bond formation, chain growth and the deposition of carbonaceous species. Here operando photoelectron photoion coincidence (PEPICO) spectroscopy enables the isomer-selective identification of pathways to hydrocarbons of up to C₁₄ in size, providing direct experimental evidence of methyl radicals in both reactions and ketene in the methanol-to-hydrocarbons reaction. Both routes converge to C₅ molecules that transform into aromatics. Operando PEPICO highlights distinctions in the prevalence of coke precursors, which is supported by electron paramagnetic resonance measurements, providing evidence of differences in the representative molecular structure, density and distribution of accumulated carbonaceous species. Radical-driven pathways in the methyl chloride-to-hydrocarbons reaction(s) accelerate the formation of extended aromatic systems, leading to fast deactivation. By contrast, the generation of alkylated species through oxygenate-driven pathways in the methanol-to-hydrocarbons reaction extends the catalyst lifetime. The findings demonstrate the potential of the presented methods to provide valuable mechanistic insights into complex reaction networks.

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.

Mechanistic differences between methanol and dimethyl ether in zeolite-catalyzed hydrocarbon synthesis

Methanol conversion to hydrocarbons has emerged as a key reaction for synthetic energy carriers and light alkenes. The autocatalytic nature and complex reaction network make a mechanistic understanding very challenging and widely debated. Water is not only part of the overall conversion, it is also frequently used as diluent, influencing, in turn, activity, selectivity, and stability of the catalysts. Water directly and indirectly influences the processes that initiate the C–C formation via adjusting the chemical potential of methanol and dimethyl ether, with the latter being more efficient to generate highly reactive C₁ species via hydride transfer. The insight shows paths to optimize the stability of catalysts and to tailor the product distribution for H-ZSM-5–based catalysts.

Water influences critically the kinetics of the autocatalytic conversion of methanol to hydrocarbons in acid zeolites. At very low conversions but otherwise typical reaction conditions, the initiation of the reaction is delayed in presence of H₂O. In absence of hydrocarbons, the main reactions are the methanol and dimethyl ether (DME) interconversion and the formation of a C₁ reactive mixture—which in turn initiates the formation of first hydrocarbons in the zeolite pores. We conclude that the dominant reactions for the formation of a reactive C₁ pool at this stage involve hydrogen transfer from both MeOH and DME to surface methoxy groups, leading to methane and formaldehyde in a 1:1 stoichiometry. While formaldehyde reacts further to other C₁ intermediates and initiates the formation of first C–C bonds, CH₄ is not reacting. The hydride transfer to methoxy groups is the rate-determining step in the initiation of the conversion of methanol and DME to hydrocarbons. Thus, CH₄ formation rates at very low conversions, i.e., in the initiation stage before autocatalysis starts, are used to gauge the formation rates of first hydrocarbons. Kinetics, in good agreement with theoretical calculations, show surprisingly that hydrogen transfer from DME to methoxy species is 10 times faster than hydrogen transfer from methanol. This difference in reactivity causes the observed faster formation of hydrocarbons in dry feeds, when the concentration of methanol is lower than in presence of water. Importantly, the kinetic analysis of CH₄ formation rates provides a unique quantitative parameter to characterize the activity of catalysts in the methanol-to-hydrocarbon process.

Trimethyloxonium ion – a zeolite confined mobile and efficient methyl carrier at low temperatures: a DFT study coupled with microkinetic analysis

The reaction network of ethene methylation over H-ZSM-5, including methanol dehydration, ethene methylation, and C3H7+ conversion, is investigated by employing a multiscale approach combining DFT calculations and microkinetic modeling.

Insights into the Interaction between Immobilized Biocatalysts and Metal-Organic Frameworks: A Case Study of PCN-333

The immobilization of enzymes in metal-organic frameworks (MOFs) with preserved biofunctionality paves a promising way to solve problems regarding the stability and reusability of enzymes. However, the rational design of MOF-based biocomposites remains a considerable challenge as very little is known about the state of the enzyme, the MOF support, and their host-guest interactions upon immobilization. In this study, we elucidate the detailed host-guest interaction for MOF immobilized enzymes in the biointerface. Two enzymes with different sizes, lipase and insulin, have been immobilized in a mesoporous PCN-333(Al) MOF. The dynamic changes of local structures of the MOF host and enzyme guests have been experimentally revealed for the existence of the confinement effect to enzymes and van der Waals interaction in the biointerface between the aluminum oxo-cluster of the PCN-333 and the -NH2 species of enzymes. This kind of host-guest interaction renders the immobilization of enzymes in PCN-333 with high affinity and highly preserved enzymatic bioactivity.

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.

Revealing Brønsted Acidic Bridging SiOHAl Groups on Amorphous Silica-Alumina by Ultrahigh Field Solid-State NMR

Amorphous silica-aluminas (ASAs) are important acidic catalysts and supports for many industrially essential and sustainable processes. The identification of surface acid sites with their local structures on ASAs is of critical importance for tuning their catalytic properties but still remains a great challenge and is under debate. Here, ultrahigh magnetic field (35.2 T) ²⁷Al-{¹H} D-HMQC (dipolar-mediated heteronuclear multiple-quantum correlation) two-dimensional NMR experiments demonstrate two types of Brønsted acid sites in ASA catalysts. In addition to the known pseudobridging silanol acid sites, the use of ultrahigh field NMR provides the first direct experimental evidence for the existence of bridging silanol (BS: SiOHAl) acid sites in ASAs, which has been hotly debated in the past few decades. This discovery provides new opportunities for scientists and engineers to develop and apply ASAs in various reaction processes due to the significance of BS in chemical and fuel productions based on its strong Brønsted acidity.

Selectivity descriptors for the direct hydrogenation of CO2 to hydrocarbons during zeolite-mediated bifunctional catalysis

Cascade processes are gaining momentum in heterogeneous catalysis. The combination of several catalytic solids within one reactor has shown great promise for the one-step valorization of C1-feedstocks. The combination of metal-based catalysts and zeolites in the gas phase hydrogenation of CO2 leads to a large degree of product selectivity control, defined mainly by zeolites. However, a great deal of mechanistic understanding remains unclear: metal-based catalysts usually lead to complex product compositions that may result in unexpected zeolite reactivity. Here we present an in-depth multivariate analysis of the chemistry involved in eight different zeolite topologies when combined with a highly active Fe-based catalyst in the hydrogenation of CO2 to olefins, aromatics, and paraffins. Solid-state NMR spectroscopy and computational analysis demonstrate that the hybrid nature of the active zeolite catalyst and its preferred CO2-derived reaction intermediates (CO/ester/ketone/hydrocarbons, i.e., inorganic-organic supramolecular reactive centers), along with 10 MR-zeolite topology, act as descriptors governing the ultimate product selectivity.

Accurate heteronuclear distance measurements at all magic-angle spinning frequencies in solid-state NMR spectroscopy

Heteronuclear dipolar coupling is indispensable in revealing vital information related to the molecular structure and dynamics, as well as intermolecular interactions in various solid materials. Although numerous approaches have been developed to selectively reintroduce heteronuclear dipolar coupling under MAS, most of them lack universality and can only be applied to limited spin systems. Herein, we introduce a new and robust technique dubbed phase modulated rotary resonance (PMRR) for reintroducing heteronuclear dipolar couplings while suppressing all other interactions under a broad range of MAS conditions. The standard PMRR requires the radiofrequency (RF) field strength of only twice the MAS frequency, can efficiently recouple the dipolar couplings with a large scaling factor of 0.50, and is robust to experimental imperfections. Moreover, the adjustable window modification of PMRR, dubbed wPMRR, can improve its performance remarkably, making it well suited for the accurate determination of dipolar couplings in various spin systems. The robust performance of such pulse sequences has been verified theoretically and experimentally via model compounds, at different MAS frequencies. The application of the PMRR technique was demonstrated on the H-ZSM-5 zeolite, where the interaction between the Brønsted acidic hydroxyl groups of H-ZSM-5 and the absorbed trimethylphosphine oxide (TMPO) were probed, revealing the detailed configuration of super acid sites.

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

Understanding the nature of heterogeneous catalysts is critical for the rational design of highly active catalysts, which necessitates in-depth characterization of the structure and properties of catalysts as well as reaction mechanisms. Solid-state NMR correlation spectroscopy is becoming increasingly recognized as a powerful tool in the study of catalysts and catalytic reactions because of its capability to provide atomic-level insights into the structure, interaction and dynamics of molecules by establishing connectivity and proximity between the same or distinct nuclei. This tutorial review focuses on the fundamentals and state-of-the-art applications of solid-state NMR correlation techniques to structural characterization of catalytic materials including zeolites, metal oxides, organometallic complexes and MOFs as well as relevant studies regarding synthesis, synergistic catalysis, host-guest interactions and reaction mechanisms. Various correlation NMR methods that have been employed to address the challenging issues in heterogeneous catalysis are highlighted. This review concludes with outlooks on the promising applications and potential developments of solid-state NMR correlation spectroscopy in catalytic materials.

Molecular Routes of Dynamic Autocatalysis for Methanol-to-Hydrocarbons Reaction

The industrially important methanol-to-hydrocarbons (MTH) reaction is driven and sustained by autocatalysis in a dynamic and complex manner. Hitherto, the entire molecular routes and chemical nature of the autocatalytic network have not been well understood. Herein, with a multitechnique approach and multiscale analysis, we have obtained a full theoretical picture of the domino cascade of autocatalytic reaction network taking place on HZSM-5 zeolite. The autocatalytic reaction is demonstrated to be plausibly initiated by reacting dimethyl ether (DME) with the surface methoxy species (SMS) to generate the initial olefins, as evidenced by combining the kinetic analysis, in situ DRIFT spectroscopy, 2D ¹³C-¹³C MAS NMR, electronic states, and projected density of state (PDOS) analysis. This process is operando tracked and visualized at the picosecond time scale by advanced ab initio molecular dynamics (AIMD) simulations. The initial olefins ignite autocatalysis by building the first autocatalytic cycle-olefins-based cycle-followed by the speciation of methylcyclopentenyl (MCP) and aromatic cyclic active species. In doing so, the active sites accomplish the dynamic evolution from proton acid sites to supramolecular active centers that are experimentally identified with an ever-evolving and fluid feature. The olefins-guided and cyclic-species-guided catalytic cycles are interdependently linked to forge a previously unidentified hypercycle, being composed of one "selfish" autocatalytic cycle (i.e., olefins-based cycle with lighter olefins as autocatalysts for catalyzing the formation of olefins) and three cross-catalysis cycles (with olefinic, MCP, and aromatic species as autocatalysts for catalyzing each other's formation). The unraveled dynamic autocatalytic cycles/network would facilitate the catalyst design and process control for MTH technology.

Dynamic Activation of C1 Molecules Evoked by Zeolite Catalysis

Direct observation of the activation and transformation of reactant molecules is extremely attractive but very challenging in the study of most chemical processes. Here is reported the first case of dynamic activation of C1 molecules in zeolite-catalyzed chemistry. During the methanol conversion over the HZSM-5 zeolite, a sequence of progressive activation states of dimethyl ether (DME) evoked by the special catalysis from CH₃-Zeo, a hybrid supramolecular catalytic system formed by the organic methylic species growing on the inorganic silico-aluminate zeolite framework, has been directly observed by in situ ssNMR spectroscopy at programmed temperatures. Operando simulations visually display the variability of this hybrid supramolecular system of which the C-O bond property goes through a dynamic transition from covalent to ionic with the temperature increase, and thus the gradually enhanced electrophilicity of CH₃ ^δ+ and nucleophilicity of Zeo ^δ- lead to the dynamic activation of DME. This dynamic transition is generally reflected in the alkyl-Zeo system with other alkoxy groups, which linked the alkoxy species and carbocations in zeolite catalysis. Consequently, this work not only sheds light on the key issue of the first carbon-carbon (C-C) bond formation in the methanol to hydrocarbons (MTH) process but also brings a new awareness on the essence of acid catalysis in zeolite mediated chemical processes.

Engineering the Distinct Structure Interface of Subnano-alumina Domains on Silica for Acidic Amorphous Silica-Alumina toward Biorefining

Amorphous silica-aluminas (ASAs) are important solid catalysts and supports for many industrially essential and sustainable processes, such as hydrocarbon transformation and biorefining. However, the wide distribution of acid strength on ASAs often results in undesired side reactions, lowering the product selectivity. Here we developed a strategy for the synthesis of a unique class of ASAs with unvarying strength of Brønsted acid sites (BAS) and Lewis acid sites (LAS) using double-flame-spray pyrolysis. Structural characterization using high-resolution transmission electron microscopy (TEM) and solid-state nuclear magnetic resonance (NMR) spectroscopy showed that the uniform acidity is due to a distinct nanostructure, characterized by a uniform interface of silica-alumina and homogeneously dispersed alumina domains. The BAS population density of as-prepared ASAs is up to 6 times higher than that obtained by classical methods. The BAS/LAS ratio, as well as the population densities of BAS and LAS of these ASAs, could be tuned in a broad range. In cyclohexanol dehydration, the uniform Brønsted acid strength provides a high selectivity to cyclohexene and a nearly linear correlation between acid site densities and cyclohexanol conversion. Moreover, the concerted action of these BAS and LAS leads to an excellent bifunctional Brønsted-Lewis acid catalyst for glucose dehydration, affording a superior 5-hydroxymethylfurfural yield.

Nonuniform Electric Field-Enhanced In-Source Declustering in High-Pressure Photoionization/Photoionization-Induced Chemical Ionization Mass Spectrometry for Operando Catalytic Reaction Monitoring

Photoionization mass spectrometry (PI-MS) is a powerful and highly sensitive analytical technique for online monitoring of volatile organic compounds (VOCs). However, due to the large difference of PI cross sections for different compounds and the limitation of photon energy, the ability of lamp-based PI-MS for detection of compounds with low PI cross sections and high ionization energies (IEs) is insufficient. Although the ion production rate can be improved by elevating the ion source pressure, the problem of generating plenty of cluster ions, such as [MH]⁺·(H₂O)_n (n = 1 and 2) and [M₂]⁺, needs be solved. In this work, we developed a new nonuniform electric field high-pressure photoionization/photoionization-induced chemical ionization (NEF-HPPI/PICI) source with the abilities of both HPPI and PICI, which was accomplished through ion-molecule reactions with high-intensity H₃O⁺ reactant ions generated by photoelectron ionization (PEI) of water molecules. By establishing a nonuniform electric field in a three-zone ionization region to enhance in-source declustering and using 99.999% helium as the carrier gas, not only the formation of cluster ions was significantly diminished, but the ion transmission efficiency was also improved. Consequently, the main characteristic ion for each analyte both in HPPI and PICI occupied more than 80%, especially [HCOOH·H]⁺ with a yield ratio of 99.2% for formic acid. The analytical capacity of this system was demonstrated by operando monitoring the hydrocarbons and oxygenated VOC products during the methanol-to-olefins and methane conversion catalytic reaction processes, exhibiting wide potential applications in process monitoring, reaction mechanism research, and online quality control.

Pentacoordinated Aluminum Species: New Frontier for Tailoring Acidity-Enhanced Silica-Alumina Catalysts

Silica-alumina catalysts, including zeolites and amorphous silica-aluminas (ASAs), are among the most widely used solid acid catalysts and supports to produce petrochemicals, fine chemicals, and renewable energy. The coordination, distribution, and interactions of aluminum in ASAs have an enormous impact on their acidic properties and catalytic performance. Unsaturated tetracoordinated aluminum (Al^IV) species are commonly accepted as the key sites in generating catalytically active Brønsted acid sites (BASs) in silica-alumina catalysts. Extensive efforts focus on increasing the concentration of Al^IV as the main route to enhance their Brønsted acidity for efficient catalysis. However, increasing the Al^IV concentration either weakens the acid strength in zeolites or lowers Brønsted acidity in ASAs at high Al/Si ratios, impeding acidity enhancement of these popular catalysts."Pentacoordinated aluminum (Al^V) species" are potential unsaturated Al species like Al^IV but rarely observed in silica-aluminas, and thus, are widely considered unavailable for BAS formation or surface reactions. In this Account, we will describe novel strategies for the controlled synthesis of Al^V-enriched ASAs using flame-spray pyrolysis (FSP) techniques and highlight the contribution of Al^V species in acidity enhancement, together with their structure-activity relationship in the conversion of biomass-derived compounds into valuable chemicals. Using various in situ and advanced 2D solid-state NMR (SSNMR) experiments, the studies of the acidic properties and local structure of Al^V-enriched ASAs reveal that Al^V species can highly populate on ASA surfaces, promote BASs formation, and facilitate adaptable tuning of BASs from moderate to zeolitic strength by synergy with neighboring Al sites. Moreover, the BASs with enhanced acidity can work jointly with surface Lewis acid sites or metal active species for bifunctional catalysis on Al^V-enriched ASAs. Compared to zeolites, these Al^V-enriched ASAs are highly active in acid-catalyzed biomass conversion, including alcohol dehydration and sugar conversion reactions, as well as in promoting the performance of supported metal catalysts in chemoselective hydrogenation of aromatic ketones. These new insights provide a state-of-the-art strategy for strongly enhancing the acidity of these popular silica-alumina catalysts, which offers an interesting potential for a wide range of acid and multifunctional catalysis.

Applications of Zeolites to C1 Chemistry: Recent Advances, Challenges, and Opportunities

C1 chemistry, which is the catalytic transformation of C1 molecules including CO, CO₂ , CH₄ , CH₃ OH, and HCOOH, plays an important role in providing energy and chemical supplies while meeting environmental requirements. Zeolites are highly efficient solid catalysts used in the chemical industry. The design and development of zeolite-based mono-, bi-, and multifunctional catalysts has led to a booming application of zeolite-based catalysts to C1 chemistry. Combining the advantages of zeolites and metallic catalytic species has promoted the catalytic production of various hydrocarbons (e.g., methane, light olefins, aromatics, and liquid fuels) and oxygenates (e.g., methanol, dimethyl ether, formic acid, and higher alcohols) from C1 molecules. The key zeolite descriptors that influence catalytic performance, such as framework topologies, nanoconfinement effects, Brønsted acidities, secondary-pore systems, particle sizes, extraframework cations and atoms, hydrophobicity and hydrophilicity, and proximity between acid and metallic sites are discussed to provide a deep understanding of the significance of zeolites to C1 chemistry. An outlook regarding challenges and opportunities for the conversion of C1 resources using zeolite-based catalysts to meet emerging energy and environmental demands is also presented.

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.

Two-way desorption coupling to enhance the conversion of syngas into aromatics by MnO/H-ZSM-5

We herein report a composite catalyst containing partially reducible and highly active manganese oxide and nano-size H-ZSM-5 with short b-axis, prepared for the direct conversion of syngas into aromatics.