Theoretical Study On The Distribution And Strength Of Acid Sites In HZSM-5 Zeolite And The Mechanisms Of Catalytic Hydrocracking Of Polynuclear Aromatic Hydrocarbons (PAHs)

With the decrease of conventional crude oil reserves,the utilization of heavy oil has become an important alternative in the global oil refining industry and lightening heavy oil through catalytic hydrocracking to produce gasoline,diesel and liquefied gas with high octane number is an important petroleum process.The main catalysts used in petroleum hydrocracking are bifunctional catalysts with zeolite as acidic component for hydrocracking and transition metal as active component for hydrogenation and dehydrogenation.The zeolite provides an active center for the cracking and isomerization of hydrocarbons.Alumina,amorphous aluminum silicates,zeolites and composite supports containing zeolite are the most used supports for the bifunctional catalysts.Zeolites express higher cracking and catalytic activity as well as higher product selectivity in petroleum hydrocracking due to their high hydrothermal stability,high Br?nsted acid strength,large specific surface area and unique porous structures.And the zeolite performs as an acidic component in a composite carrier to adjust the distribution of Lewis acid and Br?nsted acid as well as the proximity of the hydrogenation/dehydrogenation active component to the acid sites for cracking.Although many experimental studies have been carried out to investigate the catalytic hydrocracking process of heavy oil,it is difficult to determine the corresponding elementary reactions due to the complex composition of heavy oil and the multiple step mechanism.Even though the adsorption properties and catalytic mechanism of reactant molecules can be studied at molecular scale using theoretical calculation,the exact location of the acidic sites in the framework of zeolite cannot be characterized and identified by current experimental methods,which leads to the lack in the structural details of catalyst and active components.Thus,the difficulty in selecting a reasonable support model based on experimental data limits the computational study of heavy oil hydrocracking mechanism.In this Thesis,the stability and acid strength of HZSM-5 zeolite with different Si/Al ratios and framework-Al distribution were systematically studied by density functional theory(DFT)calculation.Based on the rationalized computational models,we further investigated the hydro-cracking mechanism of the model substrate at the acidic sites in each Al-substituted structure and the corresponding catalytic activity of each structure.HZSM-5 zeolite in MFI topology has 12 distinct Al substitution T sites.We used 1Al substituted HZSM-5 periodic models with different framework Al location to study the corresponding stability and acid strength.It is found that the energy span of the 12 structures with different Al location is only 15.2 k J/mol,indicating that thermodynamically all substitution sites and distributions of framework Al are possible under appropriate conditions.The comparison between the computational and experimental adsorption enthalpies of pyridine and ammonia at all acid sites shows that the structures with 1Al substituted at T1,T3,T5,T7 and T11 have the closest agreement in adsorption enthalpies,indicating that the most possible distribution sites for framework Al are T1,T3,T5,T7 and T11,which are different from the previously often used T12 site.Based on the above results,we systematically investigated the stability and adsorption performance of multiple Al substitution in HZSM-5 models to explore the intrinsic relationship between the number of acid centers and their acid strength distribution.for the dispersed model for Al atoms at the equivalent sites in different intersections(up to 4Al)and the aggregated model for Al atoms at different sites within one intersection(up to 6Al),there are no general and direct correlations among stability,interatomic Al-Al distances,Si-O(H)-Al bond angle,O-H distance,O-H stretching frequency and O-H heterolytic deprotonation energy as well as adsorption enthalpy of pyridine and ammonia.In the multiple Al substitution aggregated model,the stepwise adsorption enthalpies and the corresponding desorption temperatures of pyridine and ammonia increases with the number of Al atoms,in agreement with the experimental observations,suggesting that the location and distribution of acidic sites should not be averaged.According to the above studies on the substitution locations and distribution of Al in HZSM-5 with different Si/Al ratios,we can obtain relatively reasonable models which describe the distribution of Al atoms in the framework of zeolite to study the effect of framework Al distribution on the hydrocracking reactions of polycyclic aromatic hydrocarbons.We used 9,10-dihydroanthracene as a model substrate to investigate the hydrocracking mechanism in 1Al substituted HZSM-5 models at different Al substitution site(T1,T3,T5,T7,T11 and T12)to determine the most favorable acid site,the optimal reaction pathway,the formation mechanism of carbocation intermediates,the cleavage process of C-C bonds,the activation mechanism of H2 and the selectivity of products as well as the relationship between the acidity and reactivity of zeolites.It is found that the intersection of the straight and sinusoidal channel is thermodynamically more favored than the sinusoidal and straight channels.The rate-determining step for the formation of 1-benzyl-2-methylbenzene as the first intermediate is the heterolytic activation of H2,which is encapsulated in a conjugated frustrated Lewis pair(FLP),generated from the acidic O-H proton transfer to the hydrocarbon substrate.For the subsequent formation of toluene or benzene and oxylene from 1-benzyl-2-methylbenzene hydrocracking,the rate-determining step,which is the heterolytic activation of H2 encapsulated in a conjugated FLP,determines the selectivity.Based on the apparent Gibbs free energy barriers at500°C,the activity has the decreasing order of T5 > T11 > T1 ≈ T12 ≈ T3 ≈T7.There is no correlation between the Gibbs free energy barriers and the acidic strength based on O-H deprotonation energy and the adsorption enthalpy of pyridine as a probe molecule.The calculated results provide a rational basis for further investigation into the location,structure,and catalytic performance of active components in the catalysts consist of HZSM-5 zeolites as acidic component.