Cavity Controls the Selectivity: Insights of Confinement Effects on MTO Reaction

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.

Enhanced ethene to propene ratio over Zn-modified SAPO-34 zeolites in methanol-to-olefin reaction

The enrichment of incorporated Zn-related species and the accommodation of facilitated aromatics impose increased diffusion restriction for bulky hydrocarbons, thus contributing to significant improvements in ethene selectivity and ethene to propene ratio over Zn modified SAPO-34 zeolites.

Direct production of olefins via syngas conversion over Co2C-based catalyst in slurry bed reactor

Direct production of olefins via syngas conversion over a Co₂C-based catalyst was investigated in a slurry bed reactor (SBR). It was found that the total selectivities to olefins and oxygenates reached 88.8C% at a CO conversion of 29.5% at 250 °C, 5 bar and H₂/CO = 0.5. The hydrocarbon distribution greatly deviated from the classical Anderson–Schulz–Flory (ASF) distribution, with only 2.6C% methane selectivity was obtained. XRD and TEM characterization verified that the Co₂C nanoprisms with special exposed facts of (101) and (020) constitutes the Fischer–Tropsch to olefins (FTO) active site. The catalytic activity increased gradually with rising the reaction temperature, while the product distribution almost kept unchanged under various reaction condition in SBR. Compared to the reaction in FBR, the Co₂C-based catalyst exhibited relative better catalytic performance during FTO process in SBR. Specifically, a higher CO conversion, a lower methane selectivity and a higher total selectivities to olefins and oxygenates were achieved in SBR. In addition, the catalyst can be in situ reduced in slurry bed reactor at mild temperature (300 °C) and no obvious deactivation was found within nearly 100 h time-on-stream, which suggested a promising route for the direct production of olefins via syngas in industrial application.

Co₂C-based catalyst exhibited a promising catalytic performance for direct production of olefins via syngas conversion in slurry bed reactor.

Brønsted/Lewis acid sites synergistically promote the initial C-C bond formation in the MTO reaction

The Lewis acid site combined with a Brønsted acid site in zeolite catalysts facilitates first C–C bond formation in the initiation step of the MTO reaction.

The methanol-to-olefin (MTO) reaction is an active field of research due to conflicting mechanistic proposals for the initial carbon–carbon (C–C) bond formation. Herein, a new methane–formaldehyde pathway, a Lewis acid site combined with a Brønsted acid site in zeolite catalysts can readily activate dimethyl ether (DME) to form ethene, is identified theoretically. The mechanism involves a hydride transfer from Al–OCH₃ on the Lewis acid site to the methyl group of the protonated methanol molecule on the adjacent Brønsted acid site leading to synchronous formation of methane and Al–COH₂⁺ (which can be considered as formaldehyde (HCHO) adsorbed on the Al³⁺ Lewis acid sites). The strong electrophilic character of the Al–COH₂⁺ intermediate can strongly accelerate the C–C bond formation with CH₄, as indicated by the significant decrease of activation barriers in the rate-determining-step of the catalytic processes. These results highlight a synergy of extra-framework aluminum (EFAl) Lewis and Brønsted sites in zeolite catalysts that facilitates initial C–C bond formation in the initiation step of the MTO reaction via the Al–COH₂⁺ intermediate.

Methanol-to-olefin conversion in ABC-6 zeolite cavities: unravelling the role of cavity shape and size from density functional theory calculations

We report a density functional theory (DFT) study to investigate methanol-to-olefin (MTO) conversion in four types of ABC-6 zeolite cavities (cha, avl, aft and h1) by varying their shape and size. Based on a side-chain alkylation mechanism, the reaction energies and barriers of methylations, deprotonations and eliminations for hydrocarbon pool (HP) intermediates are calculated. In the cha cavity, methylations and eliminations are found to possess low barriers as attributed to the strong confinement effect of the elliptical and small cha cavity. The avl and aft cavities also exhibit low barriers of the first and second methylations, and similar barriers of eliminations for producing olefins as in cha. Due to the narrow shape and large size of the h1 cavity, most of the reaction barriers in h1 are the highest. The stabilities of HP species and transition states in the four cavities are quantified by the Gibbs energy profiles. It is found that aft with a wide dimension is favorable for the stability, especially for the charged HP species. The DFT calculation results reveal that the activity and selectivity of MTO conversion in zeolite cavities are strongly governed by the confinement effect, which depends on cavity shape and size. We also predict that zeolites with aft cavities might have good performance for MTO conversion.

Small-Pore Zeolites: Synthesis and Catalysis

In the past decade or so, small-pore zeolites have received greater attention than large- and medium-pore molecular sieves that have historically dominated the literature. This is primarily due to the commercialization of two major catalytic processes, NOx exhaust removal and methanol conversion to light olefins, that take advantage of the properties of these materials with smaller apertures. Small-pore zeolites possess pores that are constructed of eight tetrahedral atoms (Si⁴⁺ and Al³⁺), each time linked by a shared oxygen These eight-member ring pores (8MR) provide small molecules access to the intracrystalline void space, e.g., to NOx during car exhaust cleaning (NOx removal) or to methanol en route to its conversion into light olefins, while restricting larger molecule entrance and departure that is critical to overall catalyst performance. In total, there are forty-four structurally different small-pore zeolites. Forty-one of these zeolites can be synthesized, and the first synthetic zeolite (KFI, 1948) was in fact a small-pore material. Although the field of 8MR zeolite chemistry has expanded in many directions, the progress in synthesis is framework-specific, leaving insights and generalizations difficult to realize. This review first focuses on the relevant synthesis details of all 8MR zeolites and provides some generalized findings and related insights. Next, catalytic applications where 8MR zeolites either have been commercialized or have dominated investigations are presented, with the aim of providing structure-activity relationships. The review ends with a summary that discusses (i) both synthetic and catalytic progress, (ii) a list of opportunities in the 8MR zeolite field, and (iii) a brief future outlook.

Increasing the selectivity to ethylene in the MTO reaction by enhancing diffusion limitation in the shell layer of SAPO-34 catalyst

The zinc cation accommodation in the cavities of the shell layer and the facilitated aromatic formation over the zinc cation modified SAPO-34 with a core-shell like structure introduce extra diffusion limitation for bulky hydrocarbons, which increases the selectivity to ethylene and the ethylene to propylene ratio at the initial stage of the MTO reaction.

Revealing Lattice Expansion of Small-Pore Zeolite Catalysts during the Methanol-to-Olefins Process Using Combined Operando X-ray Diffraction and UV-vis Spectroscopy

In small-pore zeolite catalysts, where the size of the pores is limited by eight-ring windows, aromatic hydrocarbon pool molecules that are formed inside the zeolite during the Methanol-to-Olefins (MTO) process cannot exit the pores and are retained inside the catalyst. Hydrocarbon species whose size is comparable to the size of the zeolite cage can cause the zeolite lattice to expand during the MTO process. In this work, the formation of retained hydrocarbon pool species during MTO at a reaction temperature of 400 °C was followed using operando UV-vis spectroscopy. During the same experiment, using operando X-ray Diffraction (XRD), the expansion of the zeolite framework was assessed, and the activity of the catalyst was measured using online gas chromatography (GC). Three different small-pore zeolite frameworks, i.e., CHA, DDR, and LEV, were compared. It was shown using operando XRD that the formation of retained aromatic species causes the zeolite lattice of all three frameworks to expand. Because of the differences in the zeolite framework dimensions, the nature of the retained hydrocarbons as measured by operando UV-vis spectroscopy is different for each of the three zeolite frameworks. Consequently, the magnitude and direction of the zeolite lattice expansion as measured by operando XRD also depends on the specific combination of the hydrocarbon species and the zeolite framework. The catalyst with the CHA framework, i.e., H-SSZ-13, showed the biggest expansion: 0.9% in the direction along the c-axis of the zeolite lattice. For all three zeolite frameworks, based on the combination of operando XRD and operando UV-vis spectroscopy, the hydrocarbon species that are likely to cause the expansion of the zeolite cages are presented; methylated naphthalene and pyrene in CHA, 1-methylnaphthalene and phenalene in DDR, and methylated benzene and naphthalene in LEV. Filling of the zeolite cages and, as a consequence, the zeolite lattice expansion causes the deactivation of these small-pore zeolite catalysts during the MTO process.

Synthesis of high-Si hierarchical beta zeolites without mesoporogen and their catalytic application in the methanol to propene reaction

High-Si single-crystalline beta zeolites with intracrystal mesopores were synthesized for the first time and investigated as catalysts for the methanol to propene (MTP) reaction.

Influence of the Reaction Temperature on the Nature of the Active and Deactivating Species During Methanol-to-Olefins Conversion over H-SAPO-34

The selectivity toward lower olefins during the methanol-to-olefins conversion over H-SAPO-34 at reaction temperatures between 573 and 773 K has been studied with a combination of operando UV-vis diffuse reflectance spectroscopy and online gas chromatography. It was found that the selectivity toward propylene increases in the temperature range of 573-623 K, while it decreases in the temperature range of 623-773 K. The high degree of incorporation of olefins, mainly propylene, into the hydrocarbon pool affects the product selectivity at lower reaction temperatures. The nature and dynamics of the active and deactivating hydrocarbon species with increasing reaction temperature were revealed by a non-negative matrix factorization of the time-resolved operando UV-vis diffuse reflectance spectra. The active hydrocarbon pool species consist of mainly highly methylated benzene carbocations at temperatures between 573 and 598 K, of both highly methylated benzene carbocations and methylated naphthalene carbocations at 623 K, and of only methylated naphthalene carbocations at temperatures between 673 and 773 K. The operando spectroscopy results suggest that the nature of the active species also influences the olefin selectivity. In fact, monoenylic and highly methylated benzene carbocations are more selective to the formation of propylene, whereas the formation of the group of low methylated benzene carbocations and methylated naphthalene carbocations at higher reaction temperatures (i.e., 673 and 773 K) favors the formation of ethylene. At reaction temperatures between 573 and 623 K, catalyst deactivation is caused by the gradual filling of the micropores with methylated naphthalene carbocations, while between 623 and 773 K the formation of neutral poly aromatics and phenanthrene/anthracene carbocations are mainly responsible for catalyst deactivation, their respective contribution increasing with increasing reaction temperature. Methanol pulse experiments at different temperatures demonstrate the dynamics between methylated benzene and methylated naphthalene carbocations. It was found that methylated naphthalene carbocations species are deactivating and block the micropores at low reaction temperatures, while acting as the active species at higher reaction temperatures, although they give rise to the formation of extended hydrocarbon deposits.

Insights into the Activity and Deactivation of the Methanol-to-Olefins Process over Different Small-Pore Zeolites As Studied with Operando UV-vis Spectroscopy

The nature and evolution of the hydrocarbon pool (HP) species during the Methanol-to-Olefins (MTO) process for three small-pore zeolite catalysts, with a different framework consisting of large cages interconnected by small eight-ring windows (CHA, DDR, and LEV) was studied at reaction temperatures between 350 and 450 °C using a combination of operando UV-vis spectroscopy and online gas chromatography. It was found that small differences in cage size, shape, and pore structure of the zeolite frameworks result in the generation of different hydrocarbon pool species. More specifically, it was found that the large cage of CHA results in the formation of a wide variety of hydrocarbon pool species, mostly alkylated benzenes and naphthalenes. In the DDR cage, 1-methylnaphthalene is preferentially formed, while the small LEV cage generally contains fewer hydrocarbon pool species. The nature and evolution of these hydrocarbon pool species was linked with the stage of the reaction using a multivariate analysis of the operando UV-vis spectra. In the 3-D pore network of CHA, the reaction temperature has only a minor effect on the performance of the MTO catalyst. However, for the 2-D pore networks of DDR and LEV, an increase in the applied reaction temperature resulted in a dramatic increase in catalytic activity. For all zeolites in this study, the role of the hydrocarbon species changes with reaction temperature. This effect is most clear in DDR, in which diamantane and 1-methylnaphthalene are deactivating species at a reaction temperature of 350 °C, whereas at higher temperatures diamantane formation is not observed and 1-methylnaphthalene is an active species. This results in a different amount and nature of coke species in the deactivated catalyst, depending on zeolite framework and reaction temperature.

Comparative investigation of the deactivation behaviors over HZSM-5 and HSAPO-34 catalysts during low-temperature methanol conversion

Deactivation over HZSM-5 originated from overloaded HCP species, while that over HSAPO-34 was caused by methyladamantanes generated from the external surface.

Consequences of secondary zeolite growth on catalytic performance in DMTO studied over DDR and CHA

Zeolites with DDR (Sigma-1 and ZSM-58) and CHA (SSZ-13) topology were synthesized by seed assisted and direct hydrothermal synthesis in order to investigate the effects of fast crystal growth on catalytic performance.

Investigation of methanol conversion over high-Si beta zeolites and the reaction mechanism of their high propene selectivity

Large pore high-Si beta zeolites (Si/Al = 136 to 340) were synthesized by a HF-assisted method, and their catalytic performance for the conversion of methanol to propene was explored.

Synthesis of SAPO-34 nanoaggregates with the assistance of an inexpensive three-in-one non-surfactant organosilane

SAPO-34 nanoaggregates with improved MTO reaction catalytic activity were synthesized by the assistance of tri-functional organosilane PZPMS.

Bi2O3 modification of HZSM-5 for methanol-to-propylene conversion: evidence of olefin-based cycle

Bi₂O₃-modified HZSM-5 catalysts were prepared via a traditional wetness impregnation approach, and used for the methanol-to-propylene conversion reaction.

Direct observation of methylcyclopentenyl cations (MCP+) and olefin generation in methanol conversion over TON zeolite

An induction period and MCP⁺ formed in the methanol conversion have been observed successfully for the first time, indicating that the hydrocarbon pool (HCP) mechanism cannot be ruled out over H-ZSM-22.

Kinetics and thermodynamics of polymethylbenzene formation over zeolites with different pore sizes for understanding the mechanisms of methanol to olefin conversion – a computational study

The most kinetically and thermodynamically favored intermediates from methylation and geminal methylation.

Evolution of the reaction mechanism during the MTH induction period over the 2-dimensional FER zeolite

During the MTH reaction over HZSM-35, the dual-cycle mechanism was found to evolve evidently as the induction reaction progressed.

Oxygen-containing coke species in zeolite-catalyzed conversion of methanol to hydrocarbons

Oxygen-containing coke species are identified during methanol-to-hydrocarbons reactions, and their influence on the deactivation of catalyst is investigated.

Methanol conversion on ZSM-22, ZSM-35 and ZSM-5 zeolites: effects of 10-membered ring zeolite structures on methylcyclopentenyl cations and dual cycle mechanism

The different behaviors of retained organic species and their reaction routes in three 10-membered ring zeolites are presented.

Influence of acid site density on the three-staged MTH induction reaction over HZSM-5 zeolite

High acid site density can accelerate formation of active species and shorten the initial two stages of MTH induction reaction.

Recent advances of pore system construction in zeolite-catalyzed chemical industry processes

The kaleidoscopic applications of zeolite catalysts (zeo-catalysts) in petrochemical processes has been considered as one of the major accomplishments in recent decades. About twenty types of zeolite have been industrially applied so far, and their versatile porous architectures have contributed their most essential features to affect the catalytic efficiency. This review depicts the evolution of pore models in zeolite catalysts accompanied by the increase in industrial and environmental demands. The indispensable roles of modulating pore models are outlined for zeo-catalysts for the enhancement of their catalytic performances in various industrial processes. The zeolites and related industrial processes discussed range from the uni-modal micropore system of zeolite Y (12-ring micropore, 12-R) in fluid catalytic cracking (FCC), zeolite ZSM-5 (10-R) in xylene isomerization and SAPO-34 (8-R) in olefin production to the multi-modal micropore system of MCM-22 (10-R and 12-R pocket) in aromatic alkylation and the hierarchical pores in FCC and catalytic cracking of C4 olefins. The rational construction of pore models, especially hierarchical features, is highlighted with a careful classification from an industrial perspective accompanied by a detailed analysis of the theoretical mechanisms.

Similarities and differences between aromatic-based and olefin-based cycles in H-SAPO-34 and H-SSZ-13 for methanol-to-olefins conversion: insights from energetic span model

Both aromatic-based and olefin-based cycles involve similar sequences of elementary steps. Energetic span model analysis indicates olefins are active hydrocarbon pool species in H-SAPO-34 and H-SSZ-13 for MTO conversion.

Slight channel difference influences the reaction pathway of methanol-to-olefins conversion over acidic H-ZSM-22 and H-ZSM-12 zeolites

The mechanisms by which slight channel differences influence the reaction pathway of MTO conversion over acidic zeolites have been revealed.