| Zeolites have been applied extensively in the petrochemical industry due to their environmentally friendly properties. The acidity and pore confinement effect are key subjects in the zeolite science, which have great influence on the mechanisms and activities of acid-catalyzed reactions and have received extensive attentions from experimental and theoretical aspects. In this work, the quantum chemical calculations are applied to explore the acid strength of the solid acid catalysts, the influence of acid strength and pore confinement effect on several important reactions, and some meaningful results are obtained.(1) Adsoption of basic molecules is one of the widely used methods to character acid strength of solid acids. We theoretically built a series of Br(?)nsted and Lewis acid models with different acid strengths, and investigate the adsorptions of TMP on these Br(?)nsted and Lewis models, in order to elucidate the relationship between31P chemical shift and the strengths of these acid models. Based on the calculational result, it is found that31P chemical shifts can be used to discern the hydrogen-bonded TMP…H complex and the TMPH+adducts. For the TMP-Lewis acid complex, a linear correlation between the calculated31P chemical shifts and corresponding binding energies was observed for the B-, A1-, and Ti-containing Lewis acids, respectively, indicating the feasibility of using the31P chemical shift of adsorbed TMP as a scale for Lewis acidic strength.(2) Alkane activations include C-H and C-C bond activation and dissociation, which are important elementary reactions in the petrochemical industry.Based on the quantitative characterization of the solid acid strengths; we investigated the influence of Br(?)nsted acid strength on the reactivities of alkane activations by density functional theory (DFT) calculations. On the basis of the calculated energy barriers and rate coefficients, it’s demonstrated that stronger acidity could improve the reactivity of all the reactions studied. However, the sensitivity of the reaction activities to acid strengths is different. The propane cracking reaction is the most sensitive reaction, and the rate can be considerably improved by increasing acid strength; while methane hydrogen exchange is least sensitive to acid strength. It’s also demonstrated that such a sensitivity relationship could be closely related to the ionic character of the transition state. Compared to the other reactions, the transition state of propane cracking holds the most net charge.(3) It is well known that the dimensions of zeolite pores strongly control the reaction activity and selectivity in the zeolite catalysts. The methnol-to-olefines (MTO) process is one of the most successful non-petrochemical routes for production of light olefins from abundant resources of natural gas or coal. The influence of0.3A difference in the zeolite pore sizes on the generation of polymethylbenzenium cation, which is an important activated intermediates in the hydrocarbon-pool (HCP) species during the MTO reaction, has been systematically explored by DFT calculations. Base on the calculational results, the formation of pentamethylbenzenium cation was favered in the larger channel of ZSM-12zeolite from both kinetic and thermodynamic points of view and the pentamethylbenzenium cation would be the active HCP specie in the MTO reaction. However, for the HZSM-22zeolite with a0.3A smaller pore structure, the methylation on C-H sites of polymethylbenzene was selectively occurred by kinetic control. Therefore, it is demonstrated that the zeolite pore structure makes a dramatic influence on the transition state selectivity in the confined zeolite pore. Our results may provide a theoretical guide for the rational design of zeolite catalysts for MTO reaction.(4) Dimerization of lower alkenes to form higher hydrocarbons is one of available routes for the production of high octane number gasoline. The influence of both Bransted acid strength and pore confinement effect on ethylene dimerization reaction has been systematically studied by DFT calculations. It is demonstrated that the reactivity of ethylene dimerization reaction can be significantly enhanced by increasing acid strength no matter which mechanism is considered, while on the basis of activated barriers, the concerted mechanism is preferred on weak acids and two mechanisms are competitive when the acid strength increases to medium-strong acid.Due to the pore confinement effect that can effectively stabilize the ionic transition states of the dimerization reaction, the activity of the dimerization reaction is considerably improved inside the zeolite pore and the step-wise mechanism turns to be the preferable route. Additionally, on the basis of the systematic investigations on the alkene dimerization reactions over zeolites with varying pore sizes (such as ZSM-22, ZSM-5and SSZ-13), it is demonstrated that ZSM-22and ZSM-5zeolite are effective catalysts for the ethylene dimerization. The present results might provide a theoretical guide for the design, modification, and application of solid acid catalysts in the petrochemical industry.(5) The Beckmann rearrangement reaction of cyclohexanone oxime is an important industrial reaction for the production of ε-caprolactam which is a valuable compound for the manufacture of nylon fibers. The influence of acid strength and pore confinement effect on Beckmann rearrangement reaction has been systematically studied by DFT calculations. It is revealed that acidity and pore confinement effect are two main factors that affect the Beckmann rearrangement reaction. On the isolated models, the rearrangement is rate-determining step and increasing acid strength can effectively improve the reactivity. When the acid strength exceeds the medium strong acidity, the rate-determining step alters to1,2-H shift step, and the reaction activity decreases with increasing the acid strength.In zeolite pore, the effective pore confinement effect results in rate-determining step alters to1,2-H shift step, and decreasing acid strength improves the reactivity; for the smaller reactant, the inadequately confinement effect makes rearrangement step is rate-determining step and increasing acidity enhances the reaction activity. However, under any conditions, the reaction activity could be improved obviously inside zeolite pore.In this work, the quantitative characterization of the solid acid strengths has been established, and on this basis, the quantitive relationships between both acid strength and pore confinement effect of the solid acid catalysts and some important reactions in petrochemical industries were established by DFT calculations. Our results may provide a theoretical guide for the design, modification, and applications of solid acid catalysts in the petrochemical industry. |
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