几种微孔分子筛上丁烯异构化和甲烷氧化反应路径的理论计算
发布时间：2018-08-04 15:36
【摘要】：分子筛催化剂由于其具有较高的催化活性以及优异的择形性而广泛的应用于各类化学反应中。近年来,在分子筛催化剂上进行反应机理研究成为人们关注的重点,而采用实验方法往往受到实验条件及技术手段的限制不能获得较详细的实验结果。本论文通过采用密度泛函理论(DFT),系统研究在几种不同的微孔分子筛上孔道结构和酸性质对丁烯异构化反应机理的影响;同时,在金属负载的微孔分子筛中,又讨论了不同金属活性位点上甲烷氧化反应机理的差异,进一步揭示了分子筛自身属性对催化反应机理可能存在的影响,为制备和选择高效催化剂提供理论基础。本论文的第一部分采用ONIOM (B3LYP/6-31G(d,p),UFF)计算方法,选取57T,80T,96T和64T的模型来分别代表FER,ZSM-23,ZSM-48与ZSM-5四种具有十元环孔道但却有不同孔道尺寸和形状的分子筛,来比较孔道尺寸和形状对丁烯骨架异构为异丁烯反应机理的影响。通过比较丁烯和异丁烯分子在不同催化剂上的吸附能,可以发现,在具有相同椭圆形孔的FER、ZSM-48和ZSM-5分子筛中,随着孔道尺寸的增大,对丁烯和异丁烯的吸附能也在增大。但由于异丁烯的支链结构会受到分子筛孔道较大的空间位阻作用,因此,异丁烯的吸附能均小于正丁烯的吸附能。而对于具有水滴状孔口形状的ZSM-23来说,由于异丁烯分子位于水滴形的最宽处所受空间位阻作用较小,而且与分子筛骨架氧形成两个氢键作用,从而使其吸附能高于正丁烯。在FER、ZSM-23和ZSM-48上,丁烯骨架异构的单分子反应机理包括三个步骤:1)吸附态正丁烯质子化;2)2-丁基氧化物的环化;3)异丁基氧化物去质子化。其中,FER上的速控步骤为第三步,而ZSM-23和ZSM-48上则为第二步。但在ZSM-5上,虽然反应机理的前两步与在另外三种催化剂上一致,但异丁基氧化物又经过叔丁基碳正离子的过渡态生成叔丁基烷氧化物,这步也是整个反应的速控步骤,然后,再通过叔丁基烷氧化物的去质子化生成异丁烯。由此可以看出,由于ZSM-5分子筛的孔道尺寸比FER大1.0 A更易于碳正离子的形成,导致具有相似二维交叉孔道结构的FER和ZSM-5分子筛上具有不同的丁烯骨架异构反应机理。虽然具有一维直孔道十元环的ZSM-23和ZSM-48具有相同的反应路径和速控步骤,但由于ZSM-48较大的孔道尺寸可以降低反应物在孔道中所受的空间位阻,因此,ZSM-48上的速控步骤的活化能为31.8 kcal/mol,比ZSM-23上35.1 kcal/mol的活化能较低。总的来看,FER,ZSM-23,ZSM-48和ZSM-5四种分子筛上丁烯异构速控步骤活化能的大小顺序为ZSM-5 (36.1 kcal/mol) ZSM-23 (35.1 kcal/mol) FER (32.9 kcal/mol) ZSM-48 (31.8 kcal/mol),表明在 ZSM-5上最不易发生丁烯骨架异构反应,在ZSM-48上丁烯骨架异构反应最易发生,但由于ZSM-48分子筛过大的孔道尺寸,会降低对异丁烯的选择性。因此,FER依然被认为是这四种分子筛中最适合用于丁烯骨架异构的催化剂,这一结论不仅与前人所报道的实验结果相吻合,而且也很好的解释了产生这一结果的原因。在第二部分中,以孔道结构差异较小的FER和ZSM-5分子筛为研究对象,分别选择包含十元环孔道的90T和128T模型,采用ONIOM(B3LYP/6-31G(d,p),AM1)计算方法系统的研究不同数量Al原子取代Si原子后所形成的Bronsted酸位(B酸位)分布的稳定性及酸强度等性质,并比较不同性质的B酸位作为催化反应活性中心对丁烯双键异构反应过程的影响。在FER的1-Al取代中,Al4-06-Si2具有最强的稳定性,Al3-O7-Si4具有最高的B酸强度。在2-Al取代模型中,Al4-OH-(SiO)2-Al4-OH 和 Al1-OH-(SiO)2Si-HO-Al4 分别成为稳定性最高和酸强度最大的B酸位点。在ZSM-5上的1-Al取代中,Al9-018-Si6具有较强的稳定性,Al6-018-Si9具有较高的酸强度。HO-Al6-OSiOSi-OH-Al6 和 Al6-OH-SiOSi-OH-Al6 成为 2-Al 取代形成 B酸位中稳定性最高和酸性最强的B酸位点。通过分析发现,无论在FER还是ZSM-5上,H质子的落位对B酸稳定性及酸强度都有重要的影响,在1-Al取代模型中H质子的落位会导致与分子筛骨架结构形成特殊的相互作用(氢键),从而提高该处B酸位点的稳定性及酸强度。在2-Al取代模型中,H质子落位在两个Al原子同侧形成的B酸位比落位在两个Al原子之间形成的B酸位具有较低的稳定性和较高的酸强度。另外,2-Al取代所形成B酸的酸强度和稳定性随着两个取代Al原子之间距离的增大而增强。与此同时,随着分子筛中Al原子取代数量的增加,所形成的B酸位点的稳定性增强,但是酸强度却呈现减小的趋势。B酸位点酸性质的不同对丁烯双键异构反应的影响体现在:B酸性质不同的活性位点并不会改变丁烯双键异构过程的反应路径,但酸性质的不同却会导致反应能垒、中间产物结构以及过渡态构型的较大差异。在FER中,酸强度最强的B酸位点Al3-07-Si4和Al1-O-(SiO)3-Al4上反应活化能分别为21.8 kcal/mol和 18.1 kcal/mol,而稳定性最强的 B 酸位点 Al4-O6-Si2 和 Al4-O-(SiO)2-Al4上反应活化能分别为25.1 kcal/mol和27.6 kcal/mol。在ZSM-5中,酸强度最强和稳定性最高的B酸位点分别是Al6-O18-Si9、Al6-OH-SiOSi-OH-Al6 和 Al9-O18-Si6、HO-Al6-O-SiOSi-OH-Al6, 丁烯双键异构在这些位点上的活化能分别为20.7 kcal/mol、17.8 kcal/mol、24.0 kcal/mol以及26.7 kcal/mol,由此看出,相对于稳定性,B酸位点的酸强度可以直接决定丁烯双键异构化过程的反应活性。但B酸位点较高的催化活性是适宜的稳定性和酸强度协调作用的结果。在本论文的第三部分,针对金属改性分子筛Fe/ZSM-5,采用ONIOM分层的方法,讨论金属离子Fe2+和[FeO]2+在ZSM-5不同结构中的分布稳定性。同时,又选择含有8T的双五元环模型,以B3LYP/6-31G(d,p)方法考察了双铁位点[Fe(μ-O)Fe]2+、[Fe(μ-O)2Fe]2+、[Fe(μ-O)(μ-OH)Fe]+ 和[HOFe(μ-O)FeOH]2+上甲烷氧化过程的反应机理。对于金属在ZSM-5上的落位,无论是Fe2+还是[FeO]2+最稳定的落位在ZSM-5直孔道中的六圆环上(δ-6MR)两个T1 1位被Al取代时形成的对称构型中,位于直孔道δ-6MR上的其他位置和α-6MR上的稳定性次之,位于直孔道与正弦孔道交叉处的β-6MR中较不易落位,稳定性较低。另外,对Fe2+来说,在正弦孔道中的八元环上的分布也具有比较高的稳定性。总的来说,金属离子的分布稳定性主要依赖于金属阳离子与铝氧四面体中的氧负离子之间的配位作用,除此之外,分子筛自身骨架结构的可延展性也有能提高金属离子分布的稳定性。在四种不同的双铁位点[Fe(μ-O)Fe]2+、[Fe(μ-O)2Fe]2+、[Fe(μ-O)(μ-OH)Fe]+和[HOFe(μ-O)FeOH]2+上,甲烷氧化生成甲醇具有相同的反应机理:1)甲烷C-H键断裂;2)甲醇的生成。但在[Fe(μ-O)Fe]2+位点上,甲醇的形成是整个反应过程的速控步骤,反应活化能高达43.3 kcal/mol;而在另外其他三个位点上,速控步骤都是甲烷C-H键的断裂,在四种不同位点上反应速控步骤的活化能为:[Fe(μ-O)Fe]2+ ( 43.3 kcal/mol) [Fe(μ-O)2Fe]2+ (41.5 kcal/mol) [Fe(μ-O)(μ-OH)Fe]+ (26.2 kcal/mol) [HOFe(μ-O)FeOH]2+ (20.2kcal/mol)。通过观察,在含有羟基的双铁位点[Fe(μ-O)(μ-OH)Fe]+和[HOFe(μ-O)FeOH]2+上,羟基的存在可以降低甲烷C-H键断裂的活化能,提高其反应速控步骤的反应速率。而在不含羟基的[Fe(μ-O)Fe]2+和[Fe(μ-O)2Fe]2+位点上,通过加入水分子,可以明显的降低甲醇形成过程的活化能,与无水分子加入时相比,分别降低了13.2 kcal/mol和37.4 kcal/mol。这主要是加入的水分子与活性位之间存在竞争吸附导致的。我们的计算结果不仅很好的解释实验现象,而且为制备高效的甲烷氧化反应的催化剂提供理论依据。
[Abstract]:Molecular sieve catalysts are widely used in various chemical reactions because of their high catalytic activity and excellent shape selectivity. In recent years, the research on the reaction mechanism on the molecular sieve catalyst has become the focus of attention, and the experimental methods are often limited by the restrictions of the experimental strip and the technical means. In this paper, the effect of the pore structure and acid properties of several different microporous molecular sieves on the mechanism of butylene isomerization was systematically studied by using the density functional theory (DFT). Meanwhile, the difference in the mechanism of methane oxidation on different metal active sites was discussed in the metal loaded microporous molecular sieve. The possible influence of the molecular sieve's self properties on the catalytic reaction mechanism is revealed. The first part of this paper uses the ONIOM (B3LYP/6-31G (D, P), UFF) calculation method, and selects the models of 57T, 80T, 96T and 64T to represent FER, ZSM-23, ZSM-48 and four kinds of ten ring channels. Molecular sieves with different pore sizes and shapes are used to compare the influence of the pore size and shape on the reaction mechanism of the isomerization of butene skeleton. By comparing the adsorption energy of butene and isobutene molecules on different catalysts, it is found that in the FER, ZSM-48 and ZSM-5 molecular sieves with the same oval holes, the size of the channel increases with the pore size. The adsorption energy of butylene and isobutene is also increased, but the branched chain structure of isobutene will be greatly hindered by the molecular sieve channel, so the adsorption energy of isobutene is less than that of the butene. For ZSM-23 with the shape of water droplet, the ISO butylene is located at the width of the water droplet. Space hindrance is smaller, and two hydrogen bonds are formed with molecular ethmoid shelf oxygen, which makes its adsorption energy higher than n-butene. On FER, ZSM-23 and ZSM-48, the single molecular reaction mechanism of the isomerization of butene consists of three steps: 1) the adsorbed state of n-butene; 2) 2- Ding Ji oxide cyclization; 3) isobutyl oxide deprotation. In the FER, the speed control step is third steps, while the ZSM-23 and ZSM-48 are second steps. But on ZSM-5, although the first two steps of the reaction mechanism are consistent with the other three catalysts, the isobutyl oxide produces tert butylalkanes through the transition state of tert butyl cations, which is also the speed control step of the whole reaction, then, then, then, From the deionization of TERT butene oxides to isobutenes, it is shown that because the pore size of the ZSM-5 molecular sieve is 1 A larger than that of FER, it is easier to form a carbon positive ion, which leads to a different Ding Xigu frame isomerization mechanism on the FER and ZSM-5 molecular sieves with similar two-dimensional cross channel structures. The ZSM-23 and ZSM-48 of the ten membered ring have the same reaction path and the speed control step, but because the larger pore size of ZSM-48 can reduce the space hindrance of the reactants in the channel, the activation energy of the speed control step on the ZSM-48 is 31.8 kcal/mol, which is lower than the activation energy of the 35.1 kcal/mol on the ZSM-23. In general, FER, ZSM-23, ZSM-48 and Z. The order of activation energy of the tachybutene Isomerization on SM-5 four molecular sieves is ZSM-5 (36.1 kcal/mol) ZSM-23 (35.1 kcal/mol) FER (32.9 kcal/mol) ZSM-48 (31.8 kcal/mol), indicating that the most difficult to occur on the ZSM-5 is the skeleton isomerization of butene, and the skeleton isomerization of the ZSM-48 alkene is the most easy to occur, but because the ZSM-48 molecular sieve is too large The pore size will reduce the selectivity to isobutylene. Therefore, FER is still considered the most suitable catalyst for the isomerization of the butene skeleton in the four molecular sieves. This conclusion is not only consistent with the experimental results reported by predecessors, but also a good explanation of the cause of the result. In the second part, the pore structure is poor. FER and ZSM-5 molecular sieves with different sizes are selected as the research objects, and the 90T and 128T models containing ten membered ring channels are selected respectively. The stability and acid strength of the Bronsted acid sites in different quantities of Al atoms are systematically studied by ONIOM (B3LYP/6-31G (D, P) and AM1) calculation methods. In the 1-Al substitution of FER, Al4-06-Si2 has the strongest stability and the Al3-O7-Si4 has the highest B acid strength. In the 2-Al substitution model, Al4-OH- (SiO) 2-Al4-OH and Al1-OH- (SiO) 2Si-HO-Al4, respectively, become the highest stable acid sites and the highest acid strength. Point. In the substitution of 1-Al on ZSM-5, Al9-018-Si6 has strong stability, Al6-018-Si9 has high acid strength.HO-Al6-OSiOSi-OH-Al6 and Al6-OH-SiOSi-OH-Al6 to become the B acid site with the highest stability and the strongest acid in the formation of B acid sites. It is found that the falling of the H protons on FER and ZSM-5. The stability and acid strength have important effects. In the 1-Al substitution model, the drop of H protons will lead to a special interaction with the molecular sieve framework (hydrogen bonds), thus improving the stability and acid strength of the B acid site at the site. In the 2-Al substitution model, the B acid position of the H proton drop at the same side of the two Al atoms falls in the two Al The B acid sites formed between the atoms have lower stability and higher acid strength. In addition, the acid strength and stability of the 2-Al substituted B acids increase with the increase of the distance between the two substitutions of Al atoms. At the same time, the stability of the B acid site is enhanced with the increase of the number of Al atoms in the molecular sieve, but the acid is strong, but the acid is strong. The difference in the acid properties of the.B acid site has the effect on the reaction of the double bond isomerization of butene: the active sites with different properties of B acid do not change the reaction path of the double bond isomerization of butene, but the difference in acid properties will lead to the reaction energy barrier, the large difference in the intermediate product and the transition state. In FER, The activation energy of the B acid site Al3-07-Si4 and Al1-O- (SiO) 3-Al4 with the strongest acid strength was 21.8 kcal/mol and 18.1 kcal/mol respectively, while the most stable B acid site Al4-O6-Si2 and Al4-O- (SiO) 2-Al4 reaction activation energy was 25.1 and 27.6 respectively. The acid site with the strongest acid strength and the highest stability was found. Al6-O18-Si9, Al6-OH-SiOSi-OH-Al6 and Al9-O18-Si6, HO-Al6-O-SiOSi-OH-Al6, butene double bond isomerization at these sites are 20.7 kcal/mol, 17.8 kcal/mol, 24 kcal/mol and 26.7 kcal/mol respectively. Thus, the acid strength of the B acid site can directly determine the process of the double bond isomerization of butene relative to stability. The high catalytic activity of the B acid site is the result of the suitable stability and the coordination of acid strength. In the third part of this paper, the distribution stability of metal ions Fe2+ and [FeO]2+ in different ZSM-5 structures is discussed by ONIOM stratification method for the metal modified molecular sieve Fe/ZSM-5. At the same time, the 8T is also selected. The five element ring model is used to investigate the reaction mechanism of the methane oxidation process on the double iron site [Fe (P) [Fe (mu -O) Fe]2+, [Fe (mu -O) 2Fe]2+, [Fe (mu -O) and micron (mu -OH). In the symmetric configuration formed by the substitution of the 11 Al, the other positions on the straight channel delta -6MR and the stability on the alpha -6MR are second, and in the beta -6MR at the intersection of the straight channel and the sinusoidal channel, the stability is lower and the stability is lower. In addition, the distribution of the distribution on the eight element ring in the sinusoidal channel is also relatively high stability for Fe2+. The distribution stability of metal ions is mainly dependent on the coordination between the metal cations and the oxygen negative ions in the alumino tetrahedron. In addition, the ductility of the molecular sieves' self skeleton structure can also improve the stability of the distribution of metal ions. At four different diiron sites, [Fe (mu -O) Fe]2+, [Fe (mu -O) 2Fe]2+, [Fe (mu -O) (MU) (MU). -OH) on Fe]+ and [HOFe (-O) FeOH]2+, methane oxidation produces methanol with the same reaction mechanism: 1) the methane C-H bond breaks; 2) the formation of methanol. But at [Fe (mu -O) Fe]2+ site, the formation of methanol is the speed control step of the whole reaction process and the activation energy is up to 43.3 kcal/mol; and at the other three loci, the speed control step is methane The activation energy of the reaction of the C-H bond at four different sites is [Fe (mu -O) Fe]2+ (43.3 kcal/mol) [Fe (41.5 kcal/mol) [Fe (41.5 kcal/mol) [Fe (mu -O) (mu -OH). The presence of hydroxyl group can reduce the activation energy of the fracture of the methane C-H bond and increase the reaction rate of the reaction speed control step. On the [Fe (mu -O) Fe]2+ and [Fe (mu -O) 2Fe]2+ loci without hydroxyl group, the activation energy of the methanol formation process can be obviously reduced by adding water molecules, which is reduced by 13.2 kcal/mol respectively compared with the addition of anhydrous molecules. And 37.4 kcal/mol., which is mainly caused by competitive adsorption between the added water molecules and the active sites. Our calculation results not only explain the experimental phenomena well, but also provide a theoretical basis for the preparation of highly efficient catalysts for methane oxidation.
【学位授予单位】：北京化工大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O621.25;O643.36
本文编号：2164331
[Abstract]:Molecular sieve catalysts are widely used in various chemical reactions because of their high catalytic activity and excellent shape selectivity. In recent years, the research on the reaction mechanism on the molecular sieve catalyst has become the focus of attention, and the experimental methods are often limited by the restrictions of the experimental strip and the technical means. In this paper, the effect of the pore structure and acid properties of several different microporous molecular sieves on the mechanism of butylene isomerization was systematically studied by using the density functional theory (DFT). Meanwhile, the difference in the mechanism of methane oxidation on different metal active sites was discussed in the metal loaded microporous molecular sieve. The possible influence of the molecular sieve's self properties on the catalytic reaction mechanism is revealed. The first part of this paper uses the ONIOM (B3LYP/6-31G (D, P), UFF) calculation method, and selects the models of 57T, 80T, 96T and 64T to represent FER, ZSM-23, ZSM-48 and four kinds of ten ring channels. Molecular sieves with different pore sizes and shapes are used to compare the influence of the pore size and shape on the reaction mechanism of the isomerization of butene skeleton. By comparing the adsorption energy of butene and isobutene molecules on different catalysts, it is found that in the FER, ZSM-48 and ZSM-5 molecular sieves with the same oval holes, the size of the channel increases with the pore size. The adsorption energy of butylene and isobutene is also increased, but the branched chain structure of isobutene will be greatly hindered by the molecular sieve channel, so the adsorption energy of isobutene is less than that of the butene. For ZSM-23 with the shape of water droplet, the ISO butylene is located at the width of the water droplet. Space hindrance is smaller, and two hydrogen bonds are formed with molecular ethmoid shelf oxygen, which makes its adsorption energy higher than n-butene. On FER, ZSM-23 and ZSM-48, the single molecular reaction mechanism of the isomerization of butene consists of three steps: 1) the adsorbed state of n-butene; 2) 2- Ding Ji oxide cyclization; 3) isobutyl oxide deprotation. In the FER, the speed control step is third steps, while the ZSM-23 and ZSM-48 are second steps. But on ZSM-5, although the first two steps of the reaction mechanism are consistent with the other three catalysts, the isobutyl oxide produces tert butylalkanes through the transition state of tert butyl cations, which is also the speed control step of the whole reaction, then, then, then, From the deionization of TERT butene oxides to isobutenes, it is shown that because the pore size of the ZSM-5 molecular sieve is 1 A larger than that of FER, it is easier to form a carbon positive ion, which leads to a different Ding Xigu frame isomerization mechanism on the FER and ZSM-5 molecular sieves with similar two-dimensional cross channel structures. The ZSM-23 and ZSM-48 of the ten membered ring have the same reaction path and the speed control step, but because the larger pore size of ZSM-48 can reduce the space hindrance of the reactants in the channel, the activation energy of the speed control step on the ZSM-48 is 31.8 kcal/mol, which is lower than the activation energy of the 35.1 kcal/mol on the ZSM-23. In general, FER, ZSM-23, ZSM-48 and Z. The order of activation energy of the tachybutene Isomerization on SM-5 four molecular sieves is ZSM-5 (36.1 kcal/mol) ZSM-23 (35.1 kcal/mol) FER (32.9 kcal/mol) ZSM-48 (31.8 kcal/mol), indicating that the most difficult to occur on the ZSM-5 is the skeleton isomerization of butene, and the skeleton isomerization of the ZSM-48 alkene is the most easy to occur, but because the ZSM-48 molecular sieve is too large The pore size will reduce the selectivity to isobutylene. Therefore, FER is still considered the most suitable catalyst for the isomerization of the butene skeleton in the four molecular sieves. This conclusion is not only consistent with the experimental results reported by predecessors, but also a good explanation of the cause of the result. In the second part, the pore structure is poor. FER and ZSM-5 molecular sieves with different sizes are selected as the research objects, and the 90T and 128T models containing ten membered ring channels are selected respectively. The stability and acid strength of the Bronsted acid sites in different quantities of Al atoms are systematically studied by ONIOM (B3LYP/6-31G (D, P) and AM1) calculation methods. In the 1-Al substitution of FER, Al4-06-Si2 has the strongest stability and the Al3-O7-Si4 has the highest B acid strength. In the 2-Al substitution model, Al4-OH- (SiO) 2-Al4-OH and Al1-OH- (SiO) 2Si-HO-Al4, respectively, become the highest stable acid sites and the highest acid strength. Point. In the substitution of 1-Al on ZSM-5, Al9-018-Si6 has strong stability, Al6-018-Si9 has high acid strength.HO-Al6-OSiOSi-OH-Al6 and Al6-OH-SiOSi-OH-Al6 to become the B acid site with the highest stability and the strongest acid in the formation of B acid sites. It is found that the falling of the H protons on FER and ZSM-5. The stability and acid strength have important effects. In the 1-Al substitution model, the drop of H protons will lead to a special interaction with the molecular sieve framework (hydrogen bonds), thus improving the stability and acid strength of the B acid site at the site. In the 2-Al substitution model, the B acid position of the H proton drop at the same side of the two Al atoms falls in the two Al The B acid sites formed between the atoms have lower stability and higher acid strength. In addition, the acid strength and stability of the 2-Al substituted B acids increase with the increase of the distance between the two substitutions of Al atoms. At the same time, the stability of the B acid site is enhanced with the increase of the number of Al atoms in the molecular sieve, but the acid is strong, but the acid is strong. The difference in the acid properties of the.B acid site has the effect on the reaction of the double bond isomerization of butene: the active sites with different properties of B acid do not change the reaction path of the double bond isomerization of butene, but the difference in acid properties will lead to the reaction energy barrier, the large difference in the intermediate product and the transition state. In FER, The activation energy of the B acid site Al3-07-Si4 and Al1-O- (SiO) 3-Al4 with the strongest acid strength was 21.8 kcal/mol and 18.1 kcal/mol respectively, while the most stable B acid site Al4-O6-Si2 and Al4-O- (SiO) 2-Al4 reaction activation energy was 25.1 and 27.6 respectively. The acid site with the strongest acid strength and the highest stability was found. Al6-O18-Si9, Al6-OH-SiOSi-OH-Al6 and Al9-O18-Si6, HO-Al6-O-SiOSi-OH-Al6, butene double bond isomerization at these sites are 20.7 kcal/mol, 17.8 kcal/mol, 24 kcal/mol and 26.7 kcal/mol respectively. Thus, the acid strength of the B acid site can directly determine the process of the double bond isomerization of butene relative to stability. The high catalytic activity of the B acid site is the result of the suitable stability and the coordination of acid strength. In the third part of this paper, the distribution stability of metal ions Fe2+ and [FeO]2+ in different ZSM-5 structures is discussed by ONIOM stratification method for the metal modified molecular sieve Fe/ZSM-5. At the same time, the 8T is also selected. The five element ring model is used to investigate the reaction mechanism of the methane oxidation process on the double iron site [Fe (P) [Fe (mu -O) Fe]2+, [Fe (mu -O) 2Fe]2+, [Fe (mu -O) and micron (mu -OH). In the symmetric configuration formed by the substitution of the 11 Al, the other positions on the straight channel delta -6MR and the stability on the alpha -6MR are second, and in the beta -6MR at the intersection of the straight channel and the sinusoidal channel, the stability is lower and the stability is lower. In addition, the distribution of the distribution on the eight element ring in the sinusoidal channel is also relatively high stability for Fe2+. The distribution stability of metal ions is mainly dependent on the coordination between the metal cations and the oxygen negative ions in the alumino tetrahedron. In addition, the ductility of the molecular sieves' self skeleton structure can also improve the stability of the distribution of metal ions. At four different diiron sites, [Fe (mu -O) Fe]2+, [Fe (mu -O) 2Fe]2+, [Fe (mu -O) (MU) (MU). -OH) on Fe]+ and [HOFe (-O) FeOH]2+, methane oxidation produces methanol with the same reaction mechanism: 1) the methane C-H bond breaks; 2) the formation of methanol. But at [Fe (mu -O) Fe]2+ site, the formation of methanol is the speed control step of the whole reaction process and the activation energy is up to 43.3 kcal/mol; and at the other three loci, the speed control step is methane The activation energy of the reaction of the C-H bond at four different sites is [Fe (mu -O) Fe]2+ (43.3 kcal/mol) [Fe (41.5 kcal/mol) [Fe (41.5 kcal/mol) [Fe (mu -O) (mu -OH). The presence of hydroxyl group can reduce the activation energy of the fracture of the methane C-H bond and increase the reaction rate of the reaction speed control step. On the [Fe (mu -O) Fe]2+ and [Fe (mu -O) 2Fe]2+ loci without hydroxyl group, the activation energy of the methanol formation process can be obviously reduced by adding water molecules, which is reduced by 13.2 kcal/mol respectively compared with the addition of anhydrous molecules. And 37.4 kcal/mol., which is mainly caused by competitive adsorption between the added water molecules and the active sites. Our calculation results not only explain the experimental phenomena well, but also provide a theoretical basis for the preparation of highly efficient catalysts for methane oxidation.
【学位授予单位】：北京化工大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O621.25;O643.36
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2 刘伯元;沸石及其开发应用[J];地质与勘探;1994年05期
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