| The zeolites are widely used in various chemical reactions because of its high catalytic activity and excellent selectivity. In recent years, the research of the reaction mechanism on zeolites has been a primary point of concern.Experimental researches were often limited by experimental conditions and technical means so that could not obtain more detailed results. Therefore, we employed theoretical methods contained the density functional theory (DFT)and ONIOM combined method to systematically discuss the influence of the pore structure and acidity of zeolites on the 1-butene isomerization. At the same time, in the metal-modified zeolites, We also innovative discussed the differences from the mechanism of methane oxidation on different metal active sites. It further reveals the possible influence of the properties and characteristics of zeolites on the reaction mechanism and provides some theoretical basis for preparing and selecting the high efficiency catalysts.We selected 57T, 80T, 96T and 64T cluster models to represent FER,ZSM-23, ZSM-48 and ZSM-5, respectively. The influence of the pore size and shape on the research mechanism of 1-butene isomerization was studies on different zeolites by means of the ONIOM (B3LYP / 6-31G (d, p), UFF)method. By comparing the adsorption energies of 1-butene and isobutene on FER, ZSM-48 and ZSM-5 which has a similar elliptical pores, it can be found that the adsorption energy is increased with the pore size increasing. However,due to the steric hindrance of the molecular sieve pore, the adsorption energies of isobutene is less than that of 1-butene. For the ZSM-23 with the pore shape of the water drop-shaped, the isobutene was located at the widest point of the water droplet lead to a small steric hindrance, and two hydrogen bonds with the lattice oxygen of the ZSM-23 also resulted in a higher adsorption energies than 1-butene. On the FER, ZSM-23 and ZSM-48 zeolites, the monomolecular reaction mechanism of 1-butene isomerization consists of three steps: 1) the protonation of adsorbed 1-butene; 2) the cyclization of 2-butyl oxide; 3) the deprotonation of isobutyl oxide. The third step was the rate-limiting step on FER while the on ZSM-23 and ZSM-48 the second step was the speed control step. Interesting, although the first and second step of the reaction mechanism on ZSM-5 were the same with other three zeolites, the isobutyl oxide on ZSM-5 underwent the transition state of t-butylcarbonate ions to form t-butyl alkoxides, then, isobutene was formed by the deprotonation of the t-butyl alkoxide. This may caused by the larger pore size of ZSM-5 than that of FER which contributed to the formation of carbon orthonuclear ions. Although 1-butene isomerization have the same reaction mechanism and rate-limiting step on ZSM-23 and ZSM-48, the larger pore size of ZSM-48 can reduce the steric hindrance leading to the lower activation energy (31.8 kcal/mol) than that of ZSM-23 (35.1 kcal/mol). In general, the order of activation energy of rate-limiting step were ZSM-5 (36.1 kcal/mol)>ZSM-23 (35.1 kcal/mol)> FER (32.8 kcal/mol)> ZSM-48 (31.8 kcal/mol),which indicated that the isomerization of 1-butene was difficult to occur on ZSM-5 and easy to occur on ZSM-48. However, due to the large pore size, the selectivity of isobutene was reduced on ZSM-48. Therefore, FER was still considered to be the most suitable catalyst for the 1-butene isomerization, this results also confirmed the previous reported results.Then, the FER and ZSM-5 zeolites with small difference in pore structure were selected in this part. The 90T and 128T cluster models including ten-membered ring channels were employed by means of the ONIOM(B3LYP / 6-31G (d, p), AMl) method to study the stability and acid strength of the Bronsted acid sites (B acid sites) which were formed by the substitution of different numbers of Al atoms by Si atoms. And the effects of different B acid sites as the catalytic reaction center on the double bonds isomerization of 1 -butene were also compared. For FER zeolite, the A14-06-Si2 and A13-07-Si4 sites were the strongest stability and B acid strength in the 1-Al substitution, respectively. In the 2-Al substitution model,the A14-OH-(SiO)2-A14-OH and Al1-OH-(SiO)2Si-HO-Al4 were the strongest stability and B acid strength, respectively. For ZSM-5 zeolite, Al9-O18-Si6 was the high stability acid site and Al6-O18-Si9 was the strongest acid site in 1-Al substitution. HO-Al6-OSiOSi-OH-Al6 and Al6-OH-SiOSi-OH-Al6 were the strongest stability and B acid strength in 2-Al substitution model,respectively. According to the results, the location of the H atom has an important effect on the stability and acid strength of B acid site. The hydrogen bond formed by the location of H atom could increase the stability and acid strength in 1-Al substitution models on FER and ZSM-5. The acid sites which the proton located on the same side of two Al atoms have the lower stability and higher acid strength than that H located between two Al atoms. In addition,the acid strength and stability of the B acid sites were enhanced with the distance between the two substituted Al atoms increased in 2-Al substitution models. At the same time, with the increase of the number of Al atoms in FER and ZSM-5 zeolites, the stability of the B acid sites were increased, while the acid strength showed a decreasing trend. The effect of different acidity of B acid sites on the 1-butene double bond isomerization reaction is mainly showed that B acid active sites does not change the reaction path, but could influence on the reaction energy barrier, intermediate structure and transition state configuration. In the FER, the activation energies of A13-O7-Si4 and All-O-(SiO)3-Al4, which were the most stable acid sites, were 21.8 kcal / mol and 18.1 kcal / mol, respectively. The activation energies of the A14-06-Si2 and Al4-O-(SiO)2-Al, which were the strongest acid strength sites, were 25.1 kcal/mol and 27.6 kcal/mol, respectively. In ZSM-5, the strongest and the most stable acid sites were A16-018-Si9, Al6-O-SiOSi-O-Al6 and Al9-O18-Si6, Al6-O-SiOSi-O-Al6, The activation energy of these acid sites were 20.7 kcal/mol, 17.8 kcal/mol, 24.0 kcal/mol and 26.7 kcal/mol,respectively. It can be seen that the acid strength of the B acid site can directly determine the reactivity of the butadiene double bond isomerization process relative to the stability. But the higher the catalytic activity of the B acid site is the result of the coordination effect of the appropriate stability and acid strength.Finally, the distribution of Fe2+ and [FeO]2+ in different structures of ZSM-5 was discussed with 96T cluster model by using ONIOM method. Then, 8T cluster model and B3LYP/6-31G(d,p) method were used in four types of biiron species [Fe(?-O)Fe]2+, [Fe(?-O)2Fe]2+, [Fe(?-O)(?-OH)Fe]+ and[HOFe(?-O)FeOH]2+ to investigate the effect of different active sites on the mechanism of the methane oxidation. For the distribution of the metal ions on the ZSM-5, the six-membered ring of the ZSM-5 straight channel (?-6MR)which the two T11 sites were substituted by Al was the most stable site for Fe2+ and [FeO]2+. Followed by other positions on the straight channel ?-6MR and a-6MR, and finally in the ?-6MR at the intersection of the straight and sinusoidal channels. In addition, the distribution in the eight-membered ring in the sinusoidal channel also has a relatively high stability for Fe2+. According to our results, we found that the distribution of metal ions mainly depends on the coordination effect between the metal cation and the oxygen atoms in the aluminum oxide tetrahedron of ZSM-5. And, the ductility of the ZSM-5 structure was also beneficial to the distribution of iron ions. The methane oxidation to methanol has the same reaction mechanism on four types of biiron species: 1) the C-H bond of methane activation; 2) the methanol formation. However, in, the methanol formation was the rate-limiting step for the whole reaction process on the [Fe(?-O)Fe]2+ site, the activation energy was as high as 43.3 kcal/mol. While the cleavage of C-H bond was the rate-limiting step on the other three sites. The order of the rate-limiting step barrier energies in different sites was: [Fe(?-O)Fe]2+ (43.3 kcal/mol)>[Fe(?-O)2Fe]2+ (41.5 kcal/mol)>[Fe(?-O)(?-OH)Fe]+ (26.2 kcal/mol)>[HOFe(?-O)FeOH]2+ (20.2 kcal/mol). The hydroxyl groups which contained in [Fe(?-O)(?-OH)Fe]+ and [HOFe(?-O)FeOH]2+ sites could improve reaction rate through a low barrier energies. In addition, water introduced to the [Fe(?-O)Fe]2+ and [Fe(?-O)2Fe]2+ sites can also significantly reduce the activation energy of the methanol formation process. The barrierenergies were decreased by 13.2 kcal/mol and 37.4 kcal/mol on [Fe(?-O)Fe]2+and [Fe(?-O)2Fe]2+ sites with water added, This may caused by the competitive adsorption between water molecules and ZSM-5. Our results are not only consistent with the relevant experimental phenomena, but also a good explanation of the experimental phenomenon. |
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