Chuan-Ming Wang

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

Zeolite catalyzed methanol-to-olefins (MTO) conversion proceeds through a hydrocarbon pool mechanism involving a series of elementary steps. The nature of the active hydrocarbon pool species is yet to be made clear in different zeolites. In this work, both aromatic-based and olefin-based cycles in H-SAPO-34 and H-SSZ-13 were systematically investigated using periodic DFT calculations with a van der Waals (vdW) interaction corrected XC functional. Combining static adsorption energies and interconversion thermodynamics, we theoretically proved that 1,2,4,5-tetramethylbenzene (1,2,4,5-TMB) is the primary component of methylbenzenes in CHA-structured zeolites. The energetic span model was employed to compare the kinetics of both cycles in which 1,2,4,5-TMB and 2,3-dimethyl-2-butene (iso-C6) were taken as hydrocarbon pool species. Both cycles follow a similar sequence of elementary steps. We demonstrate that the iso-C6-based cycle is kinetically facile for the MTO conversion in H-SAPO-34 and H-SSZ-13. The rate-determining transition states are identified as the propagation of ethyl side chain in the 1,2,4,5-TMB-based cycle and the cracking of alkyl chain in the iso-C6-based cycle. Our results show that the reactivity of 1,2,4,5-TMB increases from H-SAPO-34 to H-SSZ-13. The stabilities of carbenium ions, important intermediates in the olefin-based cycle, increase with their size and zeolite acidity. These theoretical insights from the energetic span model enable us to highlight the importance of the olefin-based cycle in MTO conversion and understand the dependence of the reaction mechanism on zeolite frameworks.

Identification of tert‐Butyl Cations in Zeolite H‐ZSM‐5: Evidence from NMR Spectroscopy and DFT Calculations

Experimental evidence for the presence of tert-butyl cations, which are important intermediates in acid-catalyzed heterogeneous reactions, on solid acids has still not been provided to date. By combining density functional theory (DFT) calculations with (1)H/(13)C magic-angle-spinning NMR spectroscopy, the tert-butyl cation was successfully identified on zeolite H-ZSM-5 upon conversion of isobutene by capturing this intermediate with ammonia.

Verification of the dual cycle mechanism for methanol-to-olefin conversion in HSAPO-34: a methylbenzene-based cycle from DFT calculations

Understanding the reaction mechanism of methanol-to-olefin (MTO) conversion is a challenging issue in zeolite catalysis. Using the BEEF-vdW functional with van der Waals (vdW) correction, we systematically investigated the methylbenzene (MB)-based side chain hydrocarbon pool (HP) mechanism in a HSAPO-34 zeotype catalyst. The inclusion of vdW correction is very important, especially in stabilizing the intermediates and transition states with delocalized ion pair structures. The rate-determining step is identified as the propagation of side alkyl chains via the methylation of exocyclic double bonds. No obvious difference was observed in catalytic activity between different hydrocarbon pool species (hexamethylbenzene, tetramethylbenzene, and p-xylene). Ethene appears to be more favorable than propene as the product. These theoretical results strongly support the dual cycle mechanism in which MB-based and olefin-based routes run simultaneously during the MTO conversion, and ethene is produced through the MB-based route.

Insight into the topology effect on the diffusion of ethene and propene in zeolites: A molecular dynamics simulation study

Selectivity control is a difficult scientific and industrial challenge in methanol-to-olefins (MTO) conversion. It has been experimentally established that the topology of zeolite catalysts influenced the distribution of products. Besides the topology effect on reaction kinetics, the topology influences the diffusion of reactants and products in catalysts as well. In this work, by using COMPASS force-field molecular dynamics method, we investigated the intracrystalline diffusion of ethene and propene in four different zeolites, CHA, MFI, BEA and FAU, at different temperatures. The self-diffusion coefficients and diffusion activation barriers were calculated. A strong restriction on the diffusion of propene in CHA was observed because the self-diffusion coefficient ratio of ethene to propene is larger than 18 and the diffusion activation barrier of propene is more than 20 kJ/mol in CHA. This ratio decreases with the increase of temperature in the four investigated zeolites. The shape selectivity on products from diffusion perspective can provide some implications on the understanding of the selectivity difference between HSAPO-34 and HZSM-5 catalysts for the MTO conversion.

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

The nature of active hydrocarbon pool (HP) species for the methanol-to-olefins (MTO) conversion in zeolite catalysis still remains controversial. In this work, the catalytic cycles in which aromatics and olefins act as the HP species were investigated in H-ZSM-5 using periodic density functional theory calculations. Distribution of polymethylbenzenes (MBs) was qualitatively evaluated using static adsorption and dynamic interconversion analysis. It is revealed that 1,3,5-trimethylbenzene exhibits the strongest adsorption while 1,2,3,5-tetramethylbenzene (TMB) is the primary component of MBs in H-ZSM-5. The aromatic-based side chain cycle was found to be kinetically more demanding using p-xylene, 1,2,3,5-TMB, and 1,2,4,5-TMB as HP species to propagate ethyl side chain. The olefin-based cycle was illustrated using 2,3-dimethyl-2-butene (iso-C6) as HP species in a way similar to the aromatic-based side chain cycle, enabling the direct comparison of the overall kinetics. The involved methylation and cracking steps in the iso-C6-based cycle are more facile than those of the aromatic-based side chain cycle in H-ZSM-5. The aromatic-based paring cycle can also be excluded as the involved intermediate ions with five-membered rings are extremely unstable. As a result, it is most likely that olefins themselves rather than the aromatics serve as the HP species for the MTO conversion in H-ZSM-5.

Aromatic-based hydrocarbon pool mechanism for methanol-to-olefins conversion in H-SAPO-18: A van der Waals density functional study

Abstract The reaction mechanism of zeolite-catalyzed methanol-to-olefins (MTO) conversion is still debated. Aromatics and/or olefins themselves may act as hydrocarbon pool species in the reaction. In this work we used periodic density functional theory calculations with the van der Waals density functional to study the aromatic-based hydrocarbon pool mechanism in H-SAPO-18 zeotype with eight-membered ring openings. The distribution of different polymethylbenzenes (MBs) in H-SAPO-18 was evaluated from adsorption and interconversion analysis. Hexamethylbenzene was calculated to be the primary component of MBs in H-SAPO-18. Gibbs free energy analysis on the process of ethyl side chain propagation indicated that hexamethylbenzene was not more reactive than pentamethylbenzene and tetramethylbenzene. The overall Gibbs free energy barriers were calculated to be more than 200 kJ/mol at MTO reaction temperature (673 K). These calculated results would provide some implications for understanding the reaction mechanism and the role of aromatics in MTO conversion.

A First-Principle Study of Oxonium Ylide Mechanism over HSAPO-34 Zeolite

Abstract Based on density functional theory calculation with periodic boundary conditions, the possibility of the direct coupling of methanol into ethene by oxonium ylide mechanism was investigated. The calculated results indicate that the energy barriers for the formation of dimethyl ether and trimethyl oxonium ion inside HSAPO-34 zeolite are 1.68 and 0.93 eV, respectively. The suggested intermediate oxonium ylide is very unstable and the energy barriers for the formation of C-C bond are over 3.0 eV by concerted pathway. It is thus concluded that the methanol to olefin reaction cannot follow the oxonium ylide mechanism.

Methylation of olefins with ketene in zeotypes and its implications for the direct conversion of syngas to light olefins: a periodic DFT study

The direct conversion of syngas to light olefins with high selectivity is of great significance as it offers an option to produce ethene, propene, or butenes from nonpetroleum resources. Recent studies (Science, 2016, 351, 1065–1068) reported a process named OX-ZEO, activating CO and H2 to light olefins with selectivity as high as 80% using bifunctional catalysts. It was verified that ketene, produced from partially reduced oxide (ZnCrOx), is an important intermediate to be transformed into the desired olefins in acidic zeolite (H-SAPO-34). In this work, we theoretically illustrated the evolution pathway of ketene with olefins, a key step in the hydrocarbon pool mechanism for chain propagation, to understand the conversion from ketene to olefins in H-SAPO-34. We revealed that the framework-bounded CH3CO species (CH3COZ), an intermediate produced via the protonation of ketene, is an important methylating agent towards hydrocarbon pool in zeotypes. It is the direct associative pathway other than the sequential dissociative pathway that contributes to the methylation between CH3COZ and tetramethylethene (TME) as a representative olefin-based hydrocarbon pool. The effect of acid strength is also studied in a series of metal isomorphically substituted CHA-structured zeolites or zeotypes. The scaling relations of the transition state enthalpies with the acid strength using the adsorption enthalpy of ammonia as a descriptor can be established in both key elementary steps, i.e. the decarbonylation of CH3COZ and the methylation of CH3COZ with TME; the enthalpy barrier of the latter step is more sensitive to acid strength than the former one while both decreases with the increase of acid strength. These theoretical results may provide some implications to understand the key role of ketene and tailor catalyst structures in the OX-ZEO process.

Direct Conversion of Syngas into Light Olefins over Zirconium‐Doped Indium(III) Oxide and SAPO‐34 Bifunctional Catalysts: Design of Oxide Component and Construction of Reaction Network

The direct synthesis of light olefins from syngas over a bifunctional catalyst containing an oxide and zeolite has been proven to be a promising strategy. Nevertheless, an unclear reaction network hinders any further enhancement in catalytic performance, such as increasing the conversion of CO. We herein report a novel bifunctional catalyst composed of a InZr binary oxide and SAPO‐34 zeolite displaying superior CO conversion (27.7 %) with selectivity to light olefins (73.6 %) at 400 °C, 2.0 MPa. We demonstrate that the Zr‐doped body‐centered cubic In2O3 phase, exhibiting higher stability than pure In2O3 under a reducing atmosphere, is the active oxide component for the initial activation of CO. A complete reaction network is proposed by DFT calculations and model reactions, revealing that CO activation over Zr‐In2O3 follows a quasi‐CO2 hydrogenation pathway and methanol is the key intermediate to be transformed into light olefins in zeolites. Moreover, inhibiting excessive hydrogenation is an effective strategy to achieve higher performance.

Elucidating the dominant reaction mechanism of methanol-to-olefins conversion in H-SAPO-18: A first-principles study

Abstract The reaction mechanism of zeolite- or zeotype-catalyzed methanol-to-olefins (MTO) conversion is still a subject of debate. Employing periodic density functional theory calculations, the olefin-based cycle was studied using tetramethylethene (TME) as a representative olefinic hydrocarbon pool in H-SAPO-18 zeotype. The overall free energy barrier at 673 K was calculated and found to be less than 150 kJ/mol in the TME-based cycle, much lower than those in the aromatic-based cycle (> 200 kJ/mol), indicating that olefins themselves are the dominant active hydrocarbon pool species in H-SAPO-18. The similarity of the intermediates involved between the aromatic-based cycle and the olefin-based cycle was also highlighted, revealing that both cycles were pattern-consistent. The selectivity related to the distribution of cracking precursors, such as higher olefins or carbenium ions, as a result of the olefin-based cycle for the MTO conversion. The enthalpy barrier of the cracking step scaled linearly with the number of carbon atoms of cracking precursors to produce ethene or propene with ethene being much less favored than propene for cracking of C7 and higher precursors. This work highlighted the importance of the olefin-based cycle in H-SAPO-18 for the MTO conversion and established the similarity between the olefin-based and aromatic-based cycles.