Methanol to Olefins over H-MCM-22 Zeolite: Theoretical Study on the Catalytic Roles of Various Pores

Carbocation chemistry confined in zeolites: spectroscopic and theoretical characterizations

Acid-catalyzed reactions inside zeolites are one type of broadly applied industrial reactions, where carbocations are the most common intermediates of these reaction processes, including methanol to olefins, alkene/aromatic alkylation, and hydrocarbon cracking/isomerization. The fundamental research on these acid-catalyzed reactions is focused on the stability, evolution, and lifetime of carbocations under the zeolite confinement effect, which greatly affects the efficiency, selectivity and deactivation of zeolite catalysts. Therefore, a profound understanding of the carbocations confined in zeolites is not only beneficial to explain the reaction mechanism but also drive the design of new zeolite catalysts with ideal acidity and cages/channels. In this review, we provide both an in-depth understanding of the stabilization of carbocations by the pore confinement effect and summary of the advanced characterization methods to capture carbocations in zeolites, including UV-vis spectroscopy, solid-state NMR, fluorescence microscopy, IR spectroscopy and Raman spectroscopy. Also, we clarify the relationship between the activity and stability of carbocations in zeolite-catalyzed reactions, and further highlight the role of carbocations in various hydrocarbon conversion reactions inside zeolites with diverse frameworks and varying acidic properties.

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.

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

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

Superior ZSM-5@γ-Al2O3 Composite Catalyst for Methanol and Ethanol Coconversion to Light Olefins

This paper proposes a ZSM-5@γ-Al₂O₃ composite with a core-shell structure for the high-efficiency cocatalytic conversion of a methanol-ethanol system to light olefins. Using ZSM-5 and γ-Al₂O₃ as sole catalysts for comparison, the effects of physical blending, impregnation, and liquid-phase precipitation coating strategies on the catalytic performance and physicochemical properties of the composite catalysts were systematically investigated. The results indicated that the ZSM-5@γ-Al₂O₃ composite catalyst prepared by a liquid-phase precipitation coating exhibited excellent catalytic performance. When the ethanol content was 25 wt % and the reaction occurred at 350 °C, the conversion rates of methanol and ethanol were 96.1 and 99.9%, respectively; the selectivity and yield of light olefins reached 92.3 and 89.9%, respectively. The introduction of ethanol into methanol enhanced the selectivity of light olefins as target products. The interfacial composite phase formed by in situ nucleation growth of pseudoboehmite produced distinct Brønsted-Lewis acid synergistic active centers. It also increased the mesopore/micropore ratio in the composite catalyst.

Solvent-free selective oxidation of toluene over metal-doped MCM-22

The synthesis of MCM-22, subsequent metal doping, and physicochemical characterization of the products are reported. The solvent-free catalytic oxidation of toluene using hydrogen peroxide is explored, and the reaction parameters are optimized. A possible reaction mechanism is also described.

Molecular design of chiral zirconium metal–organic frameworks for asymmetric transfer hydrogenation of imines

Two chiral zirconium metal–organic frameworks are designed with high enantioselectivity for asymmetric transfer hydrogenation of imines.

Computational QM/MM investigation of the adsorption of MTH active species in H-Y and H-ZSM-5

The transformation of methanol-to-hydrocarbons (MTH) has significant potential as a route to synthesise low-cost fuels; however, the initial stages of the zeolite catalysed MTH process are not well understood.

Direct synthesis of acetic acid from carbon dioxide and methane over Cu-modulated BEA, MFI, MOR and TON zeolites: a density functional theory study

Cu-Modulated zeolites can be promising candidate catalysts in the direct conversion of carbon dioxide and methane to acetic acid.

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.

Reaction Mechanism for Direct Cyclization of Linear C5 , C6 , and C7 Alkenes over H-ITQ-13 Zeolite Investigated Using Density Functional Theory

Although dienes or trienes have been shown to be possible precursors for cyclization, direct cyclization of alkenes or alkoxides has not been systematically studied yet. Thus, the reaction mechanism of cyclization of linear alkenes over H-ITQ-13 was investigated here by density functional theory considering dispersive interactions (DFT-D). The similar free energy of different linear alkoxides of the same carbon number suggests that they can co-exist in the H-ITQ-13 intersection at 673.15 K during the methanol to olefins (MTO) process. The formation of linear alkenes by olefins methylation with methoxyl groups (ZOCH₃ ), trimethyloxonium ions (TMO⁺ ), and methanol are kinetically more favorable than by dimerization of olefins. Linear alkoxides or alkenes prefer direct cyclization to cycloalkanes rather than hydride transfer to diene. This study provides new insight into the alkene cyclization and aromatization mechanisms in MTO process.

Mechanistic insights into the catalytic role of various acid sites on ZSM-5 zeolite in the carbonylation of methanol and dimethyl ether

The catalytic performance of ZSM-5 in carbonylation is strongly related to the location of acid sites in the zeolite framework.

Reaction mechanism for the conversion of methanol to olefins over H-ITQ-13 zeolite: a density functional theory study

For MTO over H-ITQ-13, the alkene cycle takes priority over the aromatic one, giving a high selectivity to propene.

Elucidating the reaction pathway for ethene and propene formation in the methanol-to-hydrocarbons reaction over high silica H-Beta

High silica H-Beta exhibits high propene selectivity in methanol conversion due to the participation of an olefins-based cycle and domination of higher methylbenzenes in an aromatics-based cycle.

Insight into the effect of incorporation of boron into ZSM-11 on its catalytic performance for conversion of methanol to olefins

The MTO catalytic performance of ZSM-11 can be finely tuned by incorporating boron through regulation of aluminum location in the framework.

Insights into the catalytic cycle and activity of methanol-to-olefin conversion over low-silica AlPO-34 zeolites with controllable Brønsted acid density

The catalytic cycle and activity of methanol-to-olefin conversion over low-silica AlPO-34 zeolites can be effectively altered by changing the Brønsted acid density.

Computational insights into the reaction mechanism of methanol-to-olefins conversion in H-ZSM-5: nature of hydrocarbon pool

Periodic DFT calculations in H-ZSM-5 revealed that 1,2,3,5-tetramethylbenzene is the primary component of methylbenzene, and olefins themselves are the active hydrocarbon pool species for the methanol-to-olefins conversion.

Theoretical insights into how the first C–C bond forms in the methanol-to-olefin process catalysed by HSAPO-34

A mechanism is proposed for the role of the carbenium ion in the first C–C coupling in the methanol-to-olefin process catalysed by HSAPO-34.

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.

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.