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A two-layer ONIOM study on the hydrodesulfurization mechanism of thiophene in H-FAU and M-FAU (M = Li(+), Na(+), and K(+)) has been carried out. The calculated results reveal that in H-FAU, for a unimolecular mechanism, the rate-determining step is hydrogenation of alkoxide intermediate. The assistance of H2O and H2S molecules does not reduce the difficulty of the C-S bond cracking step more effectively. A bimolecular hydrodesulfurization mechanism is more favorable due to the lower activation barriers. The rate-determining step is the formation of 2-methylthiophene, not the C-S bond cracking of thiophene. Moreover, the ring opening of thiophene is much easier to occur than the desulfurization step. A careful analysis of energetics indicates that H2S, propene, and methyl thiophene are the major products for the hydrodesulfurization process of thiophene over H-FAU zeolite, in good agreement with experimental findings. In M-FAU zeolites, both unimolecular and bimolecular cracking processes are difficult to occur because of the high energy barriers. Compared to the case on H-FAU, the metal cations on M-FAU increase the difficulty of occurrence of bimolecular polymerization and subsequent C-S bond cracking steps. Graphical abstract Hydrodesulfurization process of thiophene can take place in H-FAU zeolite. Two different mechanisms, unimolecular and bimolecular ones, have been proposed and evaluated in detail. The bimolecular mechanism is more favorable due to lower activation barrier as described in the picture above. Our calculated data indicate that H2S, propene, and methylthiophene are the major products, in good agreement with experimental observations. The effect of metal cations on the reaction mechanism is also investigated in this work.

Two-layer ONIOM calculations have been carried out to study methanol to propene (MTP) conversion reactions catalyzed by H-beta zeolite. On the basis of the so-called side-chain hydrocarbon pool (HCP) mechanism, this work proposes the complete catalytic cycle pathway for the MTP reaction. The cycle starts from the methylation of pentamethylbenzene (PMB), which leads to the formation of hexamethylbenzenium ion (hexaMB(+)). Subsequent steps involving deprotonation, methylation, an internal H-shift, and a unimolecular CH₃-shift are required to produce propene and ethene. The calculated activation barriers and reaction energy data indicate that propene is the more favored product, rather than ethene, from both kinetic and thermodynamic perspectives, which is consistent with experimental observations. In addition, the calculations suggest that the activation barriers of the reaction steps decrease in the order: internal H-shift > methylation > unimolecular CH₃-shift ≥ deprotonation. In the methylation step, methylation of the exocyclic double bond is easier than methylation of the ring carbons on the aromatic benzene derivative.

The reaction mechanisms of methanol to propene (MTP) on a phosphorus-modified acidic FAU (H-FAU) zeolite have been theoretically investigated by a two-layer ONIOM (our Own N-layered Integrated molecular Orbital and molecular Mechanics) method. The MTP mechanisms include the polyMB cycle and the alkene cycle. A careful analysis of the potential energy surface (PES) suggests that the rate-determining steps for different pathways are the internal H-shift step of INT-direct-5 (the direct internal H-shift pathway), the internal H-shift step of INT-paring-1 (the paring pathway), the deprotonation step of INT-direct-5 (the spiro pathway), the methylation step of INT-direct-2 (the methyl transfer pathway), and the methylation step of 3-methyl-1-butene (alkene cycle). The different reaction steps in the polyMB cycle occur in the following order of reactivity: protonation > internal CH3-shift > methylation > ring contraction/expansion > internal H-shift > deprotonation. In the alkene cycle, the order of reactivity is different: beta scission > internal H-shift > deprotonation > methylation. The addition of phosphorus atoms to the FAU zeolite decreases the apparent free energy barriers of the rate-determining steps. The differential charge density (DCD), local orbital locator (LOL) and reduced density gradient (RDG) plots reveal the direction of electron migration and the TS nature. The complicated van der Waals (VDW) interactions and weak covalent interactions between the atoms in the forming or breaking of chemical bonds are found in the different TS structures. [ABSTRACT FROM AUTHOR]

We have theoretically studied the hydrodesulphurisation (HDS) mechanism of thiophene and its hydrogenated derivatives over sulphided Co–Mo catalysts supported by four zeolites (ZSM-5, FAU, Beta, MCM-22) by a two-layer ONIOM (our Own N-layered Integrated molecular Orbital and molecular Mechanics) method. Due to low energy barriers, the thiophene can be hydrogenated to 2,3-dihydrothiophene (2,3-DHT), 2,5-dihydrothiophene (2,5-DHT) and tetrahydrothiophene (THT). Direct desulphurisation (DDS) and hydrogenolysis desulphurisation (HYD) pathways were investigated in detail. The calculated results show that in DDS mechanism of thiophene, the rate-determining step is the C–S bond cleavage. In HYD mechanisms of 2,5-DHT, 2,3-DHT and THT, the rate-determining step is hydrogen transfer on ZSM-5, FAU and Beta zeolites except MCM-22 zeolite. The HYD of 2,3-DHT and THT should occur by the stepwise pathway. But for the 2,5-DHT, the concerted pathway is probably more favourable than the stepwise pathway. The reduced density gradient (RDG), differential charge density (DCD), and local orbital locator (LOL) maps show that for all transition states (TSs), there are complicated van der Waals (VDW) and electrostatic interactions between the organic fragment and the catalytic centre. [ABSTRACT FROM AUTHOR]

The reaction mechanism of ethanol to propene (ETP) on phosphorus-modified acidic FAU (H-FAU) zeolite has been theoretically investigated by a two-layer ONIOM (our Own N-layered Integrated molecular Orbital and molecular Mechanics) method. The ETP mechanism is divided into four reaction pathways from I to IV. The calculated data suggest that the rate-determining steps are the dehydration of ethanol (for pathways I and II) and the ethylation of propene (for pathways III and IV). Different reaction steps are in the following order of reactivity: dimerization > proton transfer ≥ deprotonation > beta-scission > ethylation > dehydration of ethanol. Pathways I and II have almost the same reactivity as pathways III and IV. The addition of phosphorus atom decreases the energy barriers of elementary steps and is more favorable for the ETP process than the acidic Si/Al zeolite without phosphorus modification. The differential charge density (DCD), local orbital locator (LOL) and reduced density gradient (RDG) plots reveal the electron migration and the nature of transition states. The RDG plots show that there are attractive and spatial repulsive van der Waals (VDW) interactions between different organic fragments of TSs. The DCD analysis reveals that the electrons migrate from an organic fragment to another fragment. The LOL maps suggest that there are weak covalent interactions between the atoms in the forming or breaking chemical bonds. [ABSTRACT FROM AUTHOR]

We have theoretically investigated the hydrodesulphurisation (HDS) mechanism of hydrogenated thiophene over Cu–Mo-modified FAU zeolite using a two-layer ONIOM (our Own N-layered Integrated molecular Orbital and molecular Mechanics) study. The thiophene is hydrogenated to 2,3-dihydrothiophene (2,3-DHT), 2,5-dihydrothiophene (2,5-DHT) and tetrahydrothiophene (THT) due to moderate free energy barriers. Hydrogenolysis desulphurisation (HYD) and concerted direct desulphurisation (DDS) are discussed. Ring-opening, hydrogen transfer and C−S bond cleavage steps of thiophene derivatives are involved in the HYD process. The rate-determining steps are the hydrogen transfer step for 2,5-DHT and C−S bond cracking step for 2,3-DHT and THT. The concerted DDS pathway is probably more favourable than the HYD pathway in the desulphurisation of 2,5-DHT. The difference charge density (DCD) analysis reveals that for the ring-opening process, the electrons are migrated from the organic chain to the Cu–Mo catalytic centre. The reduced density gradient (RDG) plots indicate that both steric hindrance and a weak van der Waals (VDW) interaction exist between organic fragment and catalytic centre for all transition states (TSs). The localised orbital locator (LOL) maps suggest that there are strong covalent interactions and weak VDW interactions between the atoms in the forming chemical bonds. [ABSTRACT FROM AUTHOR]

A two-layer ONIOM study has been carried out to investigate the unimolecular and bimolecular cracking mechanisms of thiophene catalyzed by acidic and metal cation-exchanged (Li + , Na + , K + , Cu + , and Ag + ) beta zeolites. The unimolecular cracking reactions of 3,4-dimethylthiophene (3,4-DMT) and benzothiophene (BT) catalyzed by acidic beta zeolite are also calculated. It was found that the rate-determining steps in these two mechanisms are both C S bond cracking. Furthermore, the activation energy of C S bond cracking in bimolecular mechanism is lower than that in unimolecular mechanism, indicating that the bimolecular cracking process is more favorable. In addition, although the activation energies of C S bond cleavage of 3,4-DMT and BT over H-beta zeolite are relatively lower than thiophene, the practical process would be difficult to occur due to still high energy barriers. A close analysis of activation energies over cation-exchanged zeolites suggests that the metal cations do not reduce and contrarily increase their difficulty of thiophene cracking reactions. [ABSTRACT FROM AUTHOR]

This paper presents diffusion data of N2, O2, H2S and SO2in MFI and 4A zeolites obtained by molecular dynamics simulations, especially its dependence on temperature and loading. At high loadings and temperatures, the order of self-diffusivity of guests in two zeolites is O2> N2> H2S>SO2. The diffusion behaviour is different in different zeolites at lower loadings, reflecting different influences from straight channels (MFI) and α-cages (4A). Furthermore, with increasing loading, the self-diffusivity of guest molecules decreases in MFI but generally increases in 4A. The centre of mass (COM) probability densities and diffusion trajectories of guests give insight into molecular-level diffusion process. The simulation results reveal that with increasing loading, the diffusion mechanism would change from the inter-pore to intra-pore diffusion in MFI. However, in 4A, the intra-pore diffusion is predominant at low and high loadings, but inter-pore diffusion is more important at moderate loadings. [ABSTRACT FROM AUTHOR]