A Mechanistic Study of Methanol-to-Aromatics Reaction over Ga-Modified ZSM-5 Zeolites: Understanding the Dehydrogenation Process

Unraveling the Structure-Activity-Stability Relationship over Gallium-Promoted HZSM-5 Nanocrystalline Aggregates for Propane Aromatization

The aromatization of light alkane is an important process for increasing the aromatic production and utilization efficiency of light alkane resources simultaneously. Herein, Ga-modified HZSM-5 catalysts were prepared and investigated by a series of characterization techniques such as X-ray diffraction, nuclear magnetic resonance spectroscopy, transmission electron microscopy, N2 adsorption-desorption, and NH3 temperature-programmed desorption to study their physicochemical properties. The catalytic performance in propane aromatization was also tested. Importantly, the structure-activity relationship, reaction pathway, and coke formation mechanism in propane aromatization were systematically explored. It was found that different Ga introduction methods would affect the amounts of Brønsted and Lewis acid sites, and Ga-HZSM-5 prepared by the hydrothermal method exhibited higher amounts of Brønsted and Lewis acid sites but a lower B/L ratio. As a result, Ga-HZSM-5 showed higher propane conversion and benzene, toluene, and xylene yield compared with that of Ga2O3/HZSM-5. The propane aromatization reaction pathway indicated that propane dehydrogenation to propene was a crucial step for aromatic formation. The increase of the Lewis acid density in Ga-HZSM-5 can effectively improve the dehydrogenation rate and promote the aromatization reaction. Furthermore, the formation of coke species was studied by thermogravimetry-mass spectrometry and Raman approaches, the results of which indicated that the graphitization degree of coke formed over spent Ga-HZSM-5 is lower, resulting in enhanced anticoking stability.

A density functional theory study on the mechanism of toluene from dimethylcyclopentane catalyzed by the [GaH]2+ active site of Ga-ZSM-5

The ONIOM (ωb97xd/6-31G(d,p):pm6) method was used to study the reaction mechanism of dimethylcyclopentane to toluene by the [GaH]2+ active site of Ga-ZSM-5. The results showed that the rate-determining step in the dimethylcyclopentane aromatization process is the ring expansion process. Compared to those of methylcyclopentane to benzene (D. D. Zhang, H. Y. Liu, L. X. Ling, H. R. Zhang, R. G. Zhang, P. Liu and B. J. Wang, Phys. Chem. Chem. Phys., 2021, 23, 10988-11003.), the free energy barriers of dimethylcyclopentane to toluene are significantly decreased, indicating that toluene is easier to produce than benzene, which confirmed the experimental results that a higher proportion of toluene than benzene is produced in the MTA process.

Kinetic modelling of methanol transformation into p-xylene on a 3Zn-3Si/ZSM-5 catalyst

In this work, the study of the kinetics of the transformation of methanol to aromatics was conducted over a 3Zn-3Si/ZSM-5 catalyst for the high selectivity of p-xylene.

Study of Catalytic Properties of the HoxMg1-xAl2O4 Modified HZSM-5 Zeolite in Conversion of Methanol to C2-C4 Alkenes and p-Xylene

Selective conversion of methanol to C2-C4 alkenes and p-xylene is one of the appealing chemical routes. Currently, there are no effective catalysts for the co-production of C2-C4 alkenes and p-xylene from methanol. To date, modified medium-pore ZSM-5 zeolites are considered one of the excellent candidates for the development of selective catalysts for the conversion of methanol to lower alkenes and aromatic hydrocarbons. In this paper, nanosized (30-33nm) powders of HoхMg1-хAl2O4 spinel structure were obtained by the method of combustion of nitrate solutions of aluminium, magnesium, holmium, diethylmalonate and hydrazine monohydrate with the further calcination of nanopowders at 1000 °C. Obtained nanopowders used in the preparation of a solid-phase catalytic composition of HoхMg1-хAl2O4-HZSM-5. Various physico-chemical properties of the catalytic composition were investigated using X-ray diffraction (XRD), pyridine adsorption (BİO-RAD FTS 3000 MX) and low-temperature nitrogen adsorption (BET) techniques. The textural properties and acidity of the catalysts were altered by adjusting the nanopowder concentration (1.0-5.0 wt.%) in the catalytic composition. The conversion of methanol in the presence of the catalytic compositions was carried out in flow-type fixed-bed catalytic reactor at 400 °C, in the presence of nitrogen carrier gas with 1.0 h-1 flow rate. A correlation between the selectivity to C2-C4 alkenes and p-xylene with a ratio of Lewis (L) and Brønsted (B) acid sites and the volume of the catalyst pore, the amount of the modifier in the catalytic system has been established. As the amount of HoхMg1-хAl2O4 nanopowder increases, the ratio of B/L acid sites and the volume of the catalyst pore decrease, which play a significant role in the increase of the selectivity to C2-C4 alkenes and p-xylene. Maximum yield of C2-C4 alkenes (31.6%) and selectivity to p-xylene (80.5%) is achieved on a catalytic composition containing 5.0 wt.% HoxMg1-xAl2O4. Copyright © 2022 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Role of formaldehyde in promoting aromatic selectivity during methanol conversion over gallium-modified zeolites

Gallium-modified HZSM-5 zeolites are known to increase aromatic selectivity in methanol conversion. However, there are still disputes about the exact active sites and the aromatic formation mechanisms over Ga-modified zeolites. In this work, in situ synchrotron radiation photoionization mass spectrometry (SR-PIMS) experiments were carried out to study the behaviors of intermediates and products during methanol conversion over Ga-modified HZSM-5. The increased formaldehyde (HCHO) yield over Ga-modified HZSM-5 was found to play a key role in the increase in aromatic yields. More HCHO was deemed to be generated from the direct dehydrogenation of methanol, and Ga₂O₃ in Ga-modified HZSM-5 was found to be the active phase. The larger increase in aromatic production over Ga-modified HZSM-5 after reduction‒oxidation treatment was found to be the result of redispersed Ga₂O₃ with smaller size generating a larger amount of HCHO. This study provides some new insights into the internal driving force for promoting the production of aromatics over Ga-modified HZSM-5.

Investigating the Interaction between Methanol and the Heulandite-type Zeolite using First Principle Molecular Dynamic

The interaction between methanol and the Heulandite-type zeolite has been unveiled to give an atomic scale detail regarding the catalytic activity of this zeolite for methanol conversion. The study was carried out by first principle molecular dynamics to get an insight into the structure and electronic behaviour of methanol inside the zeolite structure at different temperatures. The behaviour of methanol was studied when the location of the proton of Bronsted acid sites was varied to give both possible direct and less interaction with methanol. The results show that methanol interacts with the proton from zeolite to give a cationic species of [CH3OH2]+ both in 300K and 573K conditions. However, when the proton is located at different location far from possible interaction with methanol, the formation of a cationic species is hindered. This study provides an insight into the design of Heulandite type zeolite to give a catalytic activity toward methanol transformation.

Catalytic Enhancement of Cyclohexene Hydration by Ga-Doped ZSM-5 Zeolites

Ga-doped ZSM-5 zeolites were directly synthesized by a facile one-step hydrothermal method without organic templates and calcination and then investigated in the cyclohexene hydration reaction. The structure, component, textural properties, and acidity of the as-prepared samples were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray fluorescence (XRF), Brunauer-Emmett-Teller (BET), ammonia temperature-programmed desorption (NH₃-TPD), pyridine-chemisorbed IR (Py-IR), and ⁷¹Ga, ²⁷Al, ²⁹Si, and ¹H magic-angle spinning (MAS) NMR techniques. The characterization results showed that the introduction of Ga atoms into the ZSM-5 zeolite framework is much easier than Al atoms and beneficial to promote the formation of small-sized crystals. The number of Brønsted acid sites of Ga-doped ZSM-5 samples obviously increased compared with Ga0-ZSM-5. Additionally, the highest cyclohexanol yield (10.1%) was achieved over the Ga3-ZSM-5 sample, while the cyclohexanol yield of the Ga0-ZSM-5 sample was 8.6%. This result indicated that the improved catalytic performance is related to its larger external surface area, smaller particle size, and more Brønsted acid sites derived from Si-OH-Al and Si-OH-Ga of Ga3-ZSM-5. Notably, the green route reduces harmful gas emission and provides a basis for doping other heteroatoms to regulate the catalytic performance of zeolites, especially in industrial production.

In Situ Synthesis of Metal-Containing ZSM-5 and Its Catalytic Performance in Aromatization of Methanol

Metal-containing ZSM-5 zeolites, denoted as M/ZSM-5 (M = Ga, Zn, and La), were prepared by an in situ synthetic method. The structural, textural, and surface acid properties of M/ZSM-5 were characterized by XRD, N₂ adsorption-desorption, NH₃-TPD, and FT-IR. The catalyst pore size and the distribution of strong acid and weak acid were affected by the introduction of metal. Considering the methanol-to-aromatics model reaction, the catalytic performance of these samples was investigated in a fixed bed reactor under the conditions of 450 °C and a liquid weight hourly space velocity (WHSV) of 5 h^-1. The results indicated that the Zn/ZSM-5 molecular sieve was most beneficial to aromatization; the relationship between the change of B/L caused by Zn content and aromatics selectivity was further studied. When the B/L value decreased, the selectivity of aromatics showed an initial increase and then a decrease. The B/L value was optimal, and the aromatics yield was the highest when the Zn loading was 0.35%.

Highly Enhanced Aromatics Selectivity by Coupling of Chloromethane and Carbon Monoxide over H-ZSM-5

The transformation of methane into high value-added chemicals such as aromatics provides a more desired approach towards sustainable chemistry but remains a critical challenge due to the low selectivity of aromatics and poor stability. Herein, we first report a coupling reaction of CH₃ Cl and CO (CCTA) based on methane conversion, which achieves extremely high aromatics selectivity (82.2 %) with the selectivity of BTX up to ca. 60 % over HZSM-5. The promoting effects have been demonstrated on other zeolites especially 10-membered rings (10 MR) zeolites. Multiple characterizations show that 2,3-dimethyl-2-cyclopentene-1-one (DMCPO) is generated from acetyl groups and olefins. Furthermore, isotopic labeling analysis confirms that CO is inserted into the DMCPO and aromatics rings. A new aromatization mechanism is proposed, including the formation of the above intermediates, which conspicuously weakens the hydrogen transfer reaction, leading to a considerable increase of aromatics selectivity and a dramatic drop in alkanes.

High Stability of Methanol to Aromatic Conversion over Bimetallic Ca,Ga-Modified ZSM-5

The production of valuable aromatics and the rapid catalyst deactivation due to coking are intimately related in the zeolite-catalyzed aromatization reactions. Here, we demonstrate that these two processes can be decoupled by promoting the Ga/HZSM-5 aromatization catalyst with Ca. The resulting bimetallic catalysts combine high selectivity to light aromatics with extended catalyst lifetime in the methanol-to-aromatics process. Evaluation of the catalytic performance combined with detailed catalyst characterization suggests that the added Ca interacts with the Ga-LAS, with a strong effect on the aromatization processes. A genetic algorithm approach complemented by ab initio thermodynamic analysis is used to elucidate the possible structures of bimetallic extraframework species formed under reaction conditions. The promotion effect of minute amounts of Ca is attributed to the stabilization of the intra-zeolite extraframework gallium oxide clusters with moderated dehydrogenation activity.

Production of aromatics from butanol over Ga-promoted HZSM5 catalysts: Tuning of benzene-toluene-xylene and ethylbenzene (BTEX) selectivity

The present investigation demonstrates the conversion of n-butanol to aromatics (BTA) including building block aromatics in one step over Ga modified HZSM5 catalysts in a fixed bed reactor under varying...

Enhancing Methane Conversion by Modification of Zn States in Co-Reaction of MTA

Limited by harsh reaction conditions, the activation and utilization of methane were regarded as holy grail reaction. Co-reaction with methanol, successfully realizing mild conversion below 450 °C, provides practical strategies for methane conversion on metal-loaded ZSM-5 zeolites, especially for highly efficient Zn loaded ones. However, Zn species, regarded as active acid sites on the zeolite, have not been sufficiently studied. In this paper, Zn-loaded ZSM-5 zeolite was prepared, and Zn was modified by capacity, loading strategy, and treating atmosphere. Apparent methane conversion achieves 15.3% for 1.0Zn/Z-H2 (16.8% as calculated net conversion) with a significantly reduced loading of 1.0 wt.% against deactivation, which is among the best within related zeolite materials. Besides, compared to the MTA reaction, the addition of methane promotes the high-valued aromatic production from 49.4% to 54.8%, and inhibits the C10+ production from 7.8% to 3.6%. Notably, Zn2+ is found to be another active site different from the reported ZnOH+. Medium strong acid sites are proved to be beneficial for methane activation. This work provides suggestions for the modification of the Zn active site, in order to prepare highly efficient catalysts for methane activation and BTX production in co-reaction with methanol.

Zn-P Co-Modified Hierarchical ZSM-5 Zeolites Directly Synthesized via Dry Gel Conversion for Enhanced Methanol to Aromatics Reaction

A unique method to prepare Zn-P co-modified hierarchical ZSM-5 zeolites was developed. The ZSM-5 zeolite was directly synthesized by a dry gel conversion without adding any templates or seeds. Afterwards, the hierarchical structure was endowed to the ZSM-5 zeolite by the sequential desilication-dealumination. Zn and P species were then introduced into the hierarchical ZSM-5 zeolites by the impregnation method and their activity in methanol to aromatics process was investigated. It was found that the Zn-P co-modified hierarchical ZSM-5 zeolites possessed more Zn-related Lewis acid sites, and the ratio of Zn(OH)+/ZnO was increased. The catalytic evaluation results revealed that the benzene, toluene and xylene (BTX) and aromatics selectivity were significantly improved from 20.59% and 29.41% of pristine ZSM-5 zeolite to 28.12% and 41.88% of Zn-P co-modified hierarchical counterpart (1.5Zn0.3P/HZSM-5), respectively. Owing to the introduced highly stable Zn-P co-modified hierarchical structures, the lifetime (conversion not less than 99%) of ZSM-5 zeolite during methanol to aromatics reaction was increased from 6 h to 18 h.

The effect of introducing Ga on the ZSM-5-catalyzed methanol to aromatics reaction: taking methylcyclopentane to benzene as an example

Naphthenes are key intermediates in the formation of aromatic compounds during the methanol to aromatics (MTA) reaction, and the dehydrogenation process is more important than the hydrogen transfer process. Theoretical studies were performed to investigate the methylcyclopentane, which represents a naphthene, to benzene MTA process catalyzed by ZSM-5 before and after introducing Ga, showing that Ga-ZSM-5 was more favorable for carrying out the reaction than two H-type ZSM-5 (H-Z₁ and H-Z₂) models. H-Z₁ and H-Z₂ are favorable for the transfer of H during ring expansion reactions and the reformation of Brønsted acids, but the dehydrogenation reactions involving H-Z₁ and H-Z₂ require high free-energy barriers to be overcome. Although introducing Ga to ZSM-5 is not conducive to the transfer of H after dehydrogenation, it can reduce the extremely high dehydrogenation free-energy barrier compared with H-Z₁ and H-Z₂; this is mainly because Ga at dehydrogenation active centers, [GaH]²⁺, can accept electrons and donate them to the H atoms of [GaH]²⁺, giving H negative charge and making it easy to combine with positive B-acid H atoms that come from methylcyclopentane, cyclohexene, and cyclohexadiene to produce H₂. Also, analysis of the transition state structures of all DH processes shows that Ga-ZSM-5 is more favorable for promoting the combination of H to produce H₂ than H-Z₁ and H-Z₂.

Boosting the synthesis of value-added aromatics directly from syngas via a Cr2O3 and Ga doped zeolite capsule catalyst

Even though the transformation of syngas into aromatics has been realized via a methanol-mediated tandem process, the low product yield is still the bottleneck, limiting the industrial application of this technology. Herein, a tailor-made zeolite capsule catalyst with Ga doping and SiO2 coating was combined with the methanol synthesis catalyst Cr2O3 to boost the synthesis of value-added aromatics, especially para-xylene, from syngas. Multiple characterization studies, control experiments, and density functional theory (DFT) calculation results clarified that Ga doped zeolites with strong CO adsorption capability facilitated the transformation of the reaction intermediate methanol by optimizing the first C-C coupling step under a high-pressure CO atmosphere, thereby driving the reaction forward for aromatics synthesis. This work not only reveals the synergistic catalytic network in the tandem process but also sheds new light on principles for the rational design of a catalyst in terms of oriented conversion of syngas.

Recent Advances on Gallium-Modified ZSM-5 for Conversion of Light Hydrocarbons

Light olefins are key components of modern chemical industry and are feedstocks for the production of many commodity chemicals widely used in our daily life. It would be of great economic significance to convert light alkanes, produced during the refining of crude oil or extracted during the processing of natural gas selectively to value-added products, such as light alkenes, aromatic hydrocarbons, etc., through catalytic dehydrogenation. Among various catalysts developed, Ga-modified ZSM-5-based catalysts exhibit superior catalytic performance and stability in dehydrogenation of light alkanes. In this mini review, we summarize the progress on synthesis and application of Ga-modified ZSM-5 as catalysts in dehydrogenation of light alkanes to olefins, and the dehydroaromatization to aromatics in the past two decades, as well as the discussions on in-situ formation and evolution of reactive Ga species as catalytic centers and the reaction mechanisms.

Direct Ink Writing of Metal Oxide/H-ZSM-5 Catalysts for n-Hexane Cracking: A New Method of Additive Manufacturing with High Metal Oxide Loading

Previously, 3D printing of porous materials and metal oxides was limited to low loading metal loadings, as increasing the nitrate salt concentrations, which are used to generate the oxide component, gave rise to poor rheological properties beyond 10 wt %. In this study, we addressed this problem by directly printing insoluble oxides alongside H-ZSM-5 zeolite, which allowed for increased oxide loadings. Various metal oxides such as V₂O₅, ZrO₂, Cr₂O₃, and Ga₂O₃ were doped onto H-ZSM-5 through the additive manufacturing method. Characterization and correlation between the X-ray diffraction, NH₃-temperature-programmed desorption, O₂-temperature programmed oxidation, temperature-programmed reduction, scanning electron microscopy-energy dispersive spectroscopy, and in situ CO₂ DRIFTS experiments revealed that directly 3D printing metal oxides/H-ZSM-5 inks leads to significant modification in the surface properties and oxide bulk dispersion, thereby enhancing the composites' reducibility and giving rise to widely differing product distributions in n-hexane cracking reaction. The obtained metal oxide/zeolite structured materials were used as bifunctional structured catalysts for the selective formation of light olefins from hexane at 550-600 °C and GHSV = 9000 mL/g_catalst·h in a packed-bed reactor. Among the various compositions of metal oxides/H-ZSM-5 examined (i.e., 15 wt % Ga₂O₃, 15 wt % ZrO₂, 15 wt % V₂O₅, 15 wt % Cr₂O₃, or 5 wt % Cr/10 wt % ZrO₂/10 wt % V₂O₅/10 wt % Ga₂O₃ balanced with H-ZSM-5), the 15 wt % Cr/ZSM-5 monolith displayed the best n-hexane cracking performance, as it achieved 80-85% conversion of hexane with a 40% selectivity toward propylene, 30% selectivity toward ethylene, and 10% selectivity toward butene at 550 °C. The sample also showed zero benzene/toluene/xylene selectivity and no deactivation after 6 h. This study represents a proof-of-concept for tailoring customizable heterogeneous structured catalysts by directly 3D printing high loading of metal oxides/porous zeolite and is a breakthrough in material science.

First-principles microkinetic analysis of Lewis acid sites in Zn-ZSM-5 for alkane dehydrogenation and its implication to methanol-to-aromatics conversion

Stabilities and dehydrogenation activities of butane and cyclohexane on four different Zn sites in ZSM-5 zeolite were theoretically revealed. ZnOH⁺ was identified as the most active site at low temperature and the activity increases with the sequence of dehydrogenation.

How to Constrain Metal-Oxyl Bonds on a Solid Surface? Lesson from Isovalent Zn(II)-Oxyl and Ga(III)-Oxyl Bonds Isolated in Zeolite Matrix

Isolation of the atomic O radical anion bound to a metal ion (metal-oxyl) on solid surfaces is highly desirable for an understanding of how we should design the surface structure for using oxyl as the reactive site. Owing to the analytical difficulty of oxyl, however, even identification of oxyl remains scarce. Herein, we report isovalent Zn^II-oxyl and Ga^III-oxyl bonds isolated in the zeolite matrix. Close similarities in reactivity, spectroscopic property, and bonding nature were observed between them, but their site requirements were entirely different; the former is generated at the monovalent ion-exchangeable site, whereas the latter at the divalent ion-exchangeable site. This study strongly suggests that tuning the polarization of the metal-oxygen bond using the charge-controlled lattice oxygens is a useful way to constrain surface metal-oxyl bonds.

Ethylene Dehydroaromatization over Ga-ZSM-5 Catalysts: Nature and Role of Gallium Speciation

Bifunctional catalysis in zeolites possessing both Brønsted and Lewis acid sites offers unique opportunities to tailor shape selectivity and enhance catalyst performance. Here, we examine the impact of framework and extra-framework gallium species on enriched aromatics production in zeolite ZSM-5. We compare three distinct methods of preparing Ga-ZSM-5 and reveal direct (single step) synthesis leads to optimal catalysts compared to post-synthesis methods. Using a combination of state-of-the-art characterization, catalyst testing, and density functional theory calculations, we show that Ga Lewis acid sites strongly favor aromatization. Our findings also suggest Ga(framework)-Ga(extra-framework) pairings, which can only be achieved in materials prepared by direct synthesis, are the most energetically favorable sites for reaction pathways leading to aromatics. Calculated acid site exchange energies between extra-framework Ga at framework sites comprised of either Al or Ga reveal a site-specific preference for stabilizing Lewis acids, which is qualitatively consistent with experimental measurements. These findings indicate the possibility of tailoring Lewis acid siting by the placement of Ga heteroatoms at distinct tetrahedral sites in the zeolite framework, which can have a marked impact on catalyst performance relative to conventional H-ZSM-5.

Adsorption of acetylene on Sn-doped Ni(111) surfaces: a density functional study

First-principle density functional theory calculations have been performed to investigate the adsorption of C₂H₂ on Ni(111) and Sn@Ni(111) at different coverages. At low coverage, the C₂H₂ molecule is strongly adsorbed on Ni(111) and the dissociation of the H atom is not favorable. Furthermore, the more the H atom dissociated, the more unstable the system is. However, the dissociation structure of C₂H+H has the largest adsorption energy on Sn@Ni(111), indicating that the dissociation structure is more stable than molecular adsorbed C₂H₂. At moderate coverage, there is some repulsive interaction between two C₂H₂ molecules, inducing the decrease in adsorption energy. On Ni(111), the two C₂H₂ tend to adsorb separately, however, the dimer C₄H₄ has the largest adsorption energy on Sn@Ni(111). At high coverage, the trimer derivative benzene has the largest adsorption energy on both Ni(111) and Sn@Ni(111) surfaces. The adsorption energies of the formed benzene are very high on the two systems, even larger than those of three individual adsorbed C₂H₂.

Enhancing low-temperature methane conversion on Zn/ZSM-5 in the presence of methanol by regulating the methanol-to-aromatics reaction pathway

The co-reaction method provides an efficient strategy for methane conversion under mild conditions, which is of urgent importance for direct valorization of natural gas to liquid hydrocarbons.

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.

Verifying the olefin formation mechanism of the methanol-to-hydrocarbons reaction over H-ZSM-48

The product distributions and catalytic cycle of the methanol-to-hydrocarbons reaction over H-ZSM-48 zeolite can be altered by changing the reaction temperature.