Combined Strategies Enable Highly Selective Light Olefins and para-Xylene Production on Single Catalyst Bed.

Achieving multiple high-value-added chemical production through novel reaction processes and shape-selective catalytic strategy is the key to realizing efficient low-carbon catalytic processes. In this work, a methanol-toluene coreaction system was developed, and combined control strategies of reaction pathway guidance and shape-selective catalysis were applied for the successful production of light olefins and para-xylene on single HZSM-5 catalyst bed. Cofeeding toluene additionally provides reactive and flowing aromatic hydrocarbon pool species that change the dominant reaction pathway in the complex network of the methanol reaction on HZSM-5 and promote the formation of ethylene. For the first time, the key reaction intermediates methylmethylenecyclodiene are directly captured and identified by experimental and theoretical techniques. This helps to propose the catalytic cycle for the dominant generation of ethylene and, more importantly, enriches the methanol-to-hydrocarbons (MTH) chemistry and hydrocarbon pool mechanism. Furthermore, 0.4HZSM-5@S-1-CLD, an optimized HZSM-5 catalyst modified by the silicalite-1 epitaxial growth followed by silanization approach, realizes highly selective production of light olefins (especially ethylene) and para-xylene, while excellent reactant activity is maintained. This highly efficient coreaction route gives an important leading significance in synthesizing the raw materials for the polyolefin and polyester industries. The establishment of the combined control strategies provides a model for the joint production of multiple target chemicals in complex catalytic processes.

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.

Atomic Insight into the Local Structure and Microenvironment of Isolated Co-Motifs in MFI Zeolite Frameworks for Propane Dehydrogenation.

Embedding metal species into zeolite frameworks can create framework-bond metal sites in a confined microenvironment. The metals sitting in the specific T sites of zeolites and their crystalline surroundings are both committed to the interaction with the reactant, participation in the activation, and transient state achievement during the whole catalytic process. Herein, we construct isolated Co-motifs into purely siliceous MFI zeolite frameworks (Co-MFI) and reveal the location and microenvironment of the isolated Co active center in the MFI zeolite framework particularly beneficial for propane dehydrogenation (PDH). The isolated Co-motif with the distorted tetrahedral structure ({(≡SiO)2Co(HO-Si≡)2}, two Co-O-Si bonds, and two pseudobridging hydroxyls (Co···OH-Si) is located at T1(7) and T3(9) sites of the MFI zeolite. DFT calculations and deuterium-labeling reactions verify that the isolated Co-motif together with the MFI microenvironment collectively promotes the PDH reaction by providing an exclusive microenvironment to preactivate C3H8, polarizing the oxygen in Co-O-Si bonds to accept H* ({(≡SiO)CoHδ- (Hδ+O-Si≡)3}), and a scaffold structure to stabilize the C3H7* intermediate. The Co-motif active center in Co-MFI goes through the dynamic evolutions and restoration in electronic states and coordination states in a continuous and repetitive way, which meets the requirements from the series of elementary steps in the PDH catalytic cycle and fulfills the successful catalysis like enzyme catalysis.

Frustrated Lewis Pair in Zeolite Cages for Alkane Activations.

The frustrated Lewis pair (FLP) concept in homogeneous catalysis was extended to heterogeneous catalysis via the supramolecular system of FLP between deprotonated zeolite framework oxygens and confined carbocations in methanol-to-olefin (MTO) reactions. In this FLP, the polymethylbenzenium (PMB+ ) functioned as the Lewis acid to accept an electron pair, and the deprotonated framework oxygen site acted as the Lewis base to donate an electron pair. This FLP theoretically demonstrated the ability to undergo H2 heterolysis and alkanes dehydrogenation, and this was further confirmed by gas chromatography-mass spectrometer (GC-MS) catalytic experiments inside FLP-containing chabazite zeolites. All these findings not only bring new recognition to the carbocation chemistry in zeolite cages but also put forward a new reaction pathway as one part of MTO reactions.

Increasing the Number of Aluminum Atoms in T3 Sites of a Mordenite Zeolite by Low-Pressure SiCl4 Treatment to Catalyze Dimethyl Ether Carbonylation.

Controlling the location of aluminum atoms in a zeolite framework is critical for understanding structure-performance relationships of catalytic reaction systems and tailoring catalyst design. Herein, we report a strategy to preferentially relocate mordenite (MOR) framework Al atoms into the desired T3 sites by low-pressure SiCl4 treatment (LPST). High-field 27 Al NMR was used to identify the exact location of framework Al for the MOR samples. The results indicate that 73 % of the framework Al atoms were at the T3 sites after LPST under optimal conditions, which leads to controllably generating and intensifying active sites in MOR zeolite for the dimethyl ether (DME) carbonylation reaction with higher methyl acetate (MA) selectivity and much longer lifetime (25 times). Further research reveals that the Al relocation mechanism involves simultaneous extraction, migration, and reinsertion of Al atoms from and into the parent MOR framework. This unique method is potentially applicable to other zeolites to control Al location.

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.

Enhanced Propene/Propane Separation by Directional Decoration of the 12-Membered Rings of Mordenite with ZIF Fragments.

Propene/propane separation is challenging due to the very small difference in molecular sizes, boiling points and condensabilities between these molecules. Herein, we report a strategy of introducing ZIF fragments into traditional mordenite (MOR) zeolite to decorate the 12-membered ring of MOR. After decoration, the originally ineffective zeolite MOR exhibited high kinetic propene/propane selectivities (139 at 25 °C) and achieved efficient propene/propane separation. The propene/propane separation potentials of the resulting adsorbents were further confirmed by breakthrough experiments with equimolar propene/propane (50/50) mixtures.

Water-Induced Structural Dynamic Process in Molecular Sieves under Mild Hydrothermal Conditions: Ship-in-a-Bottle Strategy for Acidity Identification and Catalyst Modification.

Water is the most important substance in nature. Imitating the formation of natural materials, molecular sieves have been synthesized under hydrothermal conditions and applied in industry. Herein, we reveal an unforeseen observation on a very special water-induced structural dynamic process of these materials. Dynamic and reversible breaking and forming of T-O-T bonds in silicoaluminophosphate (SAPO) occurs through interactions between gaseous water and the molecular-sieve framework under mild hydrothermal conditions and is confirmed by detection of the incorporation of 17 O from H2 17 O into molecular-sieve framework. Encapsulation of the bulky molecules trimethylphosphine and pyridine (kinetic diameters much larger than the pore size of SAPO-34) into CHA cavities consolidated the water-induced dynamic process. Consequently, new insights into the dynamic features of molecular sieves in water are provided. The ship-in-a-bottle strategy based on these findings also open new fields for fine acidity identification and gives extra boost in shape-selective catalysis.

Coupling of Methanol and Carbon Monoxide over H-ZSM-5 to Form Aromatics.

The conversion of methanol into aromatics over unmodified H-ZSM-5 zeolite is generally not high because the hydrogen transfer reaction results in alkane formation. Now circa 80 % aromatics selectivity for the coupling reaction of methanol and carbon monoxide over H-ZSM-5 is reported. Carbonyl compounds and methyl-2-cyclopenten-1-ones (MCPOs), which were detected in the products and catalysts, respectively, are considered as intermediates. The latter species can be synthesized from the former species and olefins. 13 C isotope tracing and 13 C liquid-state NMR results confirmed that the carbon atoms of CO molecules were incorporated into MCPOs and aromatic rings. A new aromatization mechanism that involves the formation of the above intermediates and co-occurs with a dramatically decreased hydrogen transfer reaction is proposed. A portion of the carbons in CO molecules are incorporated into aromatic, which is of great significance for industrial applications.

Direct Mechanism of the First Carbon-Carbon Bond Formation in the Methanol-to-Hydrocarbons Process.

In the past two decades, the reaction mechanism of C-C bond formation from either methanol or dimethyl ether (DME) in the methanol-to-hydrocarbons (MTH) process has been a highly controversial issue. Described here is the first observation of a surface methyleneoxy analogue, originating from the surface-activated DME, by in situ solid-state NMR spectroscopy, a species crucial to the first C-C bond formation in the MTH process. New insights into the first C-C bond formation were provided, thus suggesting DME/methanol activation and direct C-C bond formation by an interesting synergetic mechanism, involving C-H bond breakage and C-C bond coupling during the initial methanol reaction within the chemical environment of the zeolite catalyst.

Direct observation of cyclic carbenium ions and their role in the catalytic cycle of the methanol-to-olefin reaction over chabazite zeolites.

Carbenium ions in zeolites: Two important carbenium ions have been observed for the first time under working conditions of the methanol-to-olefins (MTO) reaction over chabazite zeolites using (13) C NMR spectroscopy. Their crucial roles in the MTO reaction cycles have been demonstrated by combining experiments and theoretical calculations.

Cavity-controlled methanol conversion over zeolite catalysts.

The successful development and application in industry of methanol-to-olefins (MTO) process brought about an innovative and efficient route for olefin production via non-petrochemical resources and also attracted attention of C1 chemistry and zeolite catalysis. Molecular sieve catalysts with diversified microenvironments embedding unique channel/cavity structure and acid properties, exhibit demonstrable features and advantages in the shape-selective catalysis of MTO. Especially, shape-selective catalysis over 8-MR and cavity-type zeolites with acidic supercage environment and narrow pore opening manifested special host-guest interaction between the zeolite catalyst and guest reactants, intermediates and products. This caused great differences in product distribution, catalyst deactivation and molecular diffusion, revealing the cavity-controlled methanol conversion over 8-MR and cavity-type zeolite catalyst. Furthermore, the dynamic and complicated cross-talk behaviors of catalyst material (coke)-reaction-diffusion over these types of zeolites determines the catalytic performance of the methanol conversion. In this review, we shed light on the cavity-controlled principle in the MTO reaction including cavity-controlled active intermediates formation, cavity-controlled reaction routes with the involvement of these intermediates in the complex reaction network, cavity-controlled catalyst deactivation and cavity-controlled diffusion. All these were exhibited by the MTO reaction performances and product selectivity over 8-MR and cavity-type zeolite catalysts. Advanced strategies inspired by the cavity-controlled principle were developed, providing great promise for the optimization and precise control of MTO process.

Observation of heptamethylbenzenium cation over SAPO-type molecular sieve DNL-6 under real MTO conversion conditions.

The heptamethylbenzenium cation (heptaMB(+)) has been speculated to be one of the most important active intermediates involved in the "hydrocarbon pool" mechanism of methanol-to-olefin (MTO) conversion. By the use of DNL-6, a newly synthesized SAPO-type molecular sieve with large cavities, heptaMB(+) has for the first time been directly observed during methanol conversion under real working conditions. (13)C-labeling experiments suggested that olefin formation mediated by heptaMB(+) mainly follows the side-chain mechanism.

Quantitatively Mapping the Distribution of Intrinsic Acid Sites in Mordenite Zeolite by High-Field 23Na Solid-State Nuclear Magnetic Resonance.

It is of great significance to accurately quantify the Brønsted acid sites (BASs) at different positions of mordenite (MOR) zeolite. However, H-MOR obtained from Na-MOR can hardly avoid dealumination under hydrothermal conditions, which causes difficulty in the acid characterization. Herein, 23Na-27Al D-HMQC was performed combined with high-field 23Na MQ MAS NMR and DFT calculation, which provided an unambiguous attribution of the 23Na chemical shifts and further helped to improve the resolution of 27Al MAS NMR. By fitting the 23Na and 1H MAS NMR spectra of Na/H-MOR, the intrinsic BAS contents in different T-sites were measured by characterizing the location and content of sodium ions. These Na/H-MOR zeolites with various acid distributions were used for DME carbonylation and showed that the amount of BASs in the T3 site was proportional to the activity of carbonylation. This study provides a new method for investigating the intrinsic acid properties of zeolites.

Multiscale dynamical cross-talk in zeolite-catalyzed methanol and dimethyl ether conversions.

Establishing a comprehensive understanding of the dynamical multiscale diffusion and reaction process is crucial for zeolite shape-selective catalysis and is urgently demanded in academia and industry. So far, diffusion and reaction for methanol and dimethyl ether (DME) conversions have usually been studied separately and focused on a single scale. Herein, we decipher the dynamical molecular diffusion and reaction process for methanol and DME conversions within the zeolite material evolving with time, at multiple scales, from the scale of molecules to single catalyst crystal and catalyst ensemble. Microscopic intracrystalline diffusivity is successfully decoupled from the macroscopic experiments and verified by molecular dynamics simulation. Spatiotemporal analyses of the confined carbonaceous species allow us to track the migratory reaction fronts in a single catalyst crystal and the catalyst ensemble. The constrained diffusion of DME relative to methanol alleviates the high local chemical potential of the reactant by attenuating its local enrichment, enhancing the utilization efficiency of the inner active sites of the catalyst crystal. In this context, the dynamical cross-talk behaviors of material, diffusion and reaction occurring at multiple scales is uncovered. Zeolite catalysis not only reflects the reaction characteristics of heterogeneous catalysis, but also provides enhanced, moderate or suppressed local reaction kinetics through the special catalytic micro-environment, which leads to the heterogeneity of diffusion and reaction at multiple scales, thereby realizing efficient and shape-selective catalysis.

Well-organized zeolite nanocrystal aggregates with interconnected hierarchically micro-meso-macropore systems showing enhanced catalytic performance.

Preparation and characterization of well-organized zeolitic nanocrystal aggregates with an interconnected hierarchically micro-meso-macro porous system are described. Amorphous nanoparticles in bimodal aluminosilicates were directly transformed into highly crystalline nanosized zeolites, as well as acting as scaffold template. All pores on three length scales incorporated in one solid body are interconnected with each other. These zeolitic nanocrystal aggregates with hierarchically micro-meso-macroporous structure were thoroughly characterized. TEM images and (29)Si NMR spectra showed that the amorphous phase of the initial material had been completely replaced by nanocrystals to give a micro-meso-macroporous crystalline zeolitic structure. Catalytic testing demonstrated their superiority due to the highly active sites and the presence of interconnected micro-meso-macroporosity in the cracking of bulky 1,3,5-triisopropylbenzene (TIPB) compared to traditional zeolite catalysts. This synthesis strategy was extended to prepare various zeolitic nanocrystal aggregates (ZSM-5, Beta, TS-1, etc.) with well-organized hierarchical micro-meso-macroporous structures.

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.

Understanding the Fundamentals of Microporosity Upgrading in Zeolites: Increasing Diffusion and Catalytic Performances.

Hierarchical zeolites are regarded as promising catalysts due to their well-developed porosity, increased accessible surface area, and minimal diffusion constraints. Thus far, the focus has been on the creation of mesopores in zeolites, however, little is known about a microporosity upgrading and its effect on the diffusion and catalytic performance. Here the authors show that the "birth" of mesopore formation in faujasite (FAU) type zeolite starts by removing framework T atoms from the sodalite (SOD) cages followed by propagation throughout the crystals. This is evidenced by following the diffusion of xenon (Xe) in the mesoporous FAU zeolite prepared by unbiased leaching with NH4 F in comparison to the pristine FAU zeolite. A new diffusion pathway for the Xe in the mesoporous zeolite is proposed. Xenon first penetrates through the opened SOD cages and then diffuses to supercages of the mesoporous zeolite. Density functional theory (DFT) calculations indicate that Xe diffusion between SOD cage and supercage occurs only in hierarchical FAU structure with defect-contained six-member-ring separating these two types of cages. The catalytic performance of the mesoporous FAU zeolite further indicates that the upgraded microporosity facilitates the intracrystalline molecular traffic and increases the catalytic performance.

Capture and identification of coke precursors to elucidate the deactivation route of the methanol-to-olefin process over H-SAPO-34.

The evolution of retained species during the whole methanol-to-olefins process was revealed with the aid of GC-MS, thermogravimetric analysis (TG) and density functional theory (DFT) calculations. Precise routes for the transformation of retained methylbenzenes to methylnaphthalenes were proposed, based on the direct capture of three possible organic intermediates, to explain the catalyst deactivation procedure.

Molecular elucidating of an unusual growth mechanism for polycyclic aromatic hydrocarbons in confined space.

Extension and clustering of polycyclic aromatic hydrocarbons (PAHs) are key mechanistic steps for coking and deactivation in catalysis reactions. However, no unambiguous mechanistic picture exists on molecule-resolved PAHs speciation and evolution, due to the immense experimental challenges in deciphering the complex PAHs structures. Herein, we report an effective strategy through integrating a high resolution MALDI FT-ICR mass spectrometry with isotope labeling technique. With this strategy, a complete route for aromatic hydrocarbon evolution is unveiled for SAPO-34-catalyzed, industrially relevant methanol-to-olefins (MTO) as a model reaction. Notable is the elucidation of an unusual, previously unrecognized mechanistic step: cage-passing growth forming cross-linked multi-core PAHs with graphene-like structure. This mechanistic concept proves general on other cage-based molecule sieves. This preliminary work would provide a versatile means to decipher the key mechanistic step of molecular mass growth for PAHs involved in catalysis and combustion chemistry.