Sen Wang

Theoretical insights into the mechanism of olefin elimination in the methanol-to-olefin process over HZSM-5, HMOR, HBEA, and HMCM-22 zeolites.

The mechanism of olefin elimination in the process of methanol-to-olefins (MTO) over a series of zeolites like HZSM-5, HMOR, HBEA, and HMCM-22 was investigated by DFT-D calculations, which is a crucial step that controls the MTO product distribution. The results demonstrate that the manners of olefin elimination are related to the pore structure of zeolite catalyst and the interaction between proton transfer reagent (water or methanol) and zeolite acidic framework. The indirect spiro mechanism is preferable to the direct mechanism over HMOR, HBEA, and HMCM-22 zeolites with large pores, as suggested by the energy barrier of rate-determining step and the potential energy surface (PES), but is unfavorable over HZSM-5 with medium-sized pores due to the steric hindrance of spiro intermediates. Over various zeolites, water and methanol perform differently in proton transfer to form the spiro intermediates; over HMOR and HBEA with strong acidity, water is superior to methanol in promoting propene elimination, whereas over HMCM-22 with relatively weaker acidity, methanol is more favorable as a proton transfer reagent.

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

On the basis of density functional theory including dispersion correction (ωB97XD), the thermodynamics and kinetics of the formation of polymethylbenzene intermediates in methanol to olefin conversion over zeolites with different pore sizes have been systematically computed. The agreement between the experimental and theoretical adsorption enthalpies of the several polymethylbenzenes over H-FAU reasonably validates the applied models and methods, and reveals the importance of dispersion correction in the space confinement and electrostatic stabilization of the zeolite framework. The free energies of the stepwise formation of the polymethylbenzenes show that the most favorable active hydrocarbon pool intermediates are pentamethylbenzene and hexamethylbenzene over H-BEA and H-SAPO-34, as well as tetramethylbenzene over H-ZSM-5 and H-ZSM-22. These stable polymethylbenzenes are also precursors for the formation of geminal methylated cationic intermediates on the basis of kinetic and thermodynamic analyses. The agreement of the thermodynamic and kinetic results on the favorable intermediates validates the use of Gibbs free reaction energies to estimate the primary component of the intermediates in the various zeolites. All these pore-size-dependent differences among the zeolites show their enhanced confinement effect, which is mainly influenced by the short-range electrostatic potential including stabilization and repulsion.

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

It has been found in recent years that H-ITQ-13 zeolite as a catalyst performs excellently in the conversion of methanol to olefins (MTO); however, the relationship between the catalytic performance and its unique pore structure remains rather ambiguous. In this work, the reaction mechanism of MTO over H-ITQ-13 was investigated by density functional theory considering dispersive interactions (DFT-D). The results illustrate that both the aromatic and alkene cycles are viable for MTO over H-ITQ-13. Via the aromatic cycle, the selectivity to ethene is higher than that to propene (with pentamethylbenzene (5MB) and 1,4-dimethylnaphthalene (2MN) as the active hydrocarbon pool (HCP) species), whereas via the alkene cycle (with lower alkenes as the HCP species), propene is the primary product. The alkene cycle takes priority over the aromatic cycle, since the transition states formed in the alkene cycle can be well stabilized in the three-dimensional framework of small pores in H-ITQ-13; as a result, high selectivity to propene can be achieved for MTO over H-ITQ-13. The insights shown in this work help to clarify the reaction mechanism of MTO over H-ITQ-13 and reveal the relationship between the catalytic performance and the unique pore structure of H-ITQ-13, which is of great benefit to the development of better MTO catalysts and reaction processes with high selectivity to propene.

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

A series of ZSM-11 zeolites was synthesized by adding different amounts of boron to a synthesis gel for the purpose of improving its catalytic performance for conversion of methanol to olefins (MTO). It was found that boron has marginal effects on the crystal structure, texture properties and acid site strength of ZSM-11, but it significantly increases its catalytic stability and hydride transfer activity that results in an increase in the amounts of aromatics and alkanes in products. DFT calculations revealed that aluminum and boron atoms competitively enter the ZSM-11 framework. In combination with 27Al MAS NMR, GC-MS, m-xylene isomerization and Co2+-exchanged ZSM-11 diffuse reflectance UV-vis spectroscopy results, it can be deduced that incorporation of boron compels some aluminum atoms to shift towards the intersection T sites, which can greatly alleviate the coking process although it allows the formation of more bulky molecules. This shows that the MTO catalytic performance of ZSM-11 can be largely adjusted by changing the amount of incorporated boron through regulation of aluminum or acid site locations in the framework.

Synthesis of Chainlike ZSM-5 Zeolites: Determination of Synthesis Parameters, Mechanism of Chainlike Morphology Formation, and Their Performance in Selective Adsorption of Xylene Isomers

Chainlike zeolites are advantageous to various applications as a catalyst or an adsorbent with specific selectivity; however, it is often very difficult to get desired morphology due to the complexity of zeolite synthesis process. In this work, appropriate parameters for the synthesis of perfect chainlike ZSM-5 zeolites were well determined, which illustrates that the chain length can be controlled by the composition of synthesis mixture, the amount of residual alcohol in the synthesis system, and the crystallization time. Moreover, the mechanism of chainlike crystal growth was investigated by analyzing the surface species during the synthesis process, with the help of density functional theory (DFT) calculation. The results indicate that the formation of disk crystals with proper dimension and flat surface having abundant hydroxyl groups is crucial to the growth of chainlike ZSM-5 crystals; the condensation of Si-OH groups on the (010) facet is energetically more favorable than that on other facets, leading to the growth of MFI crystals along the b-orientation. Through finely tuning the multifarious synthesis parameters, chainlike ZSM-5 zeolites with controllable length in b-orientation are obtained without using any other extra organic additives except the necessary template agent such as tetrapropylammonium hydroxide (TPAOH). Owing to the increased tortuosity of pore channels in the chainlike ZSM-5, the difference between p-xylene and o/m-xylenes in their adsorption behavior and diffusivity is greatly enhanced. These results help to clarify the formation mechanism of zeolites with chainlike morphology and then bring forward an effective approach to get zeolite materials with specific properties in adsorption and catalysis.

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 role of various acid sites located at the straight channel, sinusoidal channel and intersection cavity on ZSM-5 zeolite in the carbonylation of methanol and dimethyl ether (DME) was investigated through combined DFT-D calculation and micro-kinetic analysis. The results indicate that the formation of acetate takes priority in the straight channel, whereas the acid sites in the intersection cavity may induce the conversion of methanol and DME into unexpected by-products that lead to the rapid deactivation of the ZSM-5 catalyst. The catalytic behavior of various acid sites in carbonylation is strongly influenced by their local environment and the surrounding framework structure which determine the space confinement and electrostatic stabilization effects. Meanwhile, DME as feedstock shows a higher carbonylation activity than methanol in the straight channel, producing preferably methyl acetate. As a result, the catalytic activity and product selectivity of ZSM-5 in carbonylation can be finely tuned through purposely regulating the location of acid sites. Moreover, the carbonylation reaction rate can be further elevated through incorporating Cu into the ZSM-5 zeolites, especially mononuclear copper (Cu) species, due to the strong electrostatic interaction between the nucleophilic CH3+–CO intermediate and electrophilic Cu sites. This work helps to clarify the catalytic role of various acid sites on ZSM-5, as well as the nature of active Cu species in the carbonylation reaction, which is of great benefit to the design of efficient zeolite catalysts for the carbonylation process.