【摘要】：分子篩催化劑由于其具有較高的催化活性以及優(yōu)異的擇形性而廣泛的應(yīng)用于各類化學反應(yīng)中。近年來,在分子篩催化劑上進行反應(yīng)機理研究成為人們關(guān)注的重點,而采用實驗方法往往受到實驗條件及技術(shù)手段的限制不能獲得較詳細的實驗結(jié)果。本論文通過采用密度泛函理論(DFT),系統(tǒng)研究在幾種不同的微孔分子篩上孔道結(jié)構(gòu)和酸性質(zhì)對丁烯異構(gòu)化反應(yīng)機理的影響;同時,在金屬負載的微孔分子篩中,又討論了不同金屬活性位點上甲烷氧化反應(yīng)機理的差異,進一步揭示了分子篩自身屬性對催化反應(yīng)機理可能存在的影響,為制備和選擇高效催化劑提供理論基礎(chǔ)。本論文的第一部分采用ONIOM (B3LYP/6-31G(d,p),UFF)計算方法,選取57T,80T,96T和64T的模型來分別代表FER,ZSM-23,ZSM-48與ZSM-5四種具有十元環(huán)孔道但卻有不同孔道尺寸和形狀的分子篩,來比較孔道尺寸和形狀對丁烯骨架異構(gòu)為異丁烯反應(yīng)機理的影響。通過比較丁烯和異丁烯分子在不同催化劑上的吸附能,可以發(fā)現(xiàn),在具有相同橢圓形孔的FER、ZSM-48和ZSM-5分子篩中,隨著孔道尺寸的增大,對丁烯和異丁烯的吸附能也在增大。但由于異丁烯的支鏈結(jié)構(gòu)會受到分子篩孔道較大的空間位阻作用,因此,異丁烯的吸附能均小于正丁烯的吸附能。而對于具有水滴狀孔口形狀的ZSM-23來說,由于異丁烯分子位于水滴形的最寬處所受空間位阻作用較小,而且與分子篩骨架氧形成兩個氫鍵作用,從而使其吸附能高于正丁烯。在FER、ZSM-23和ZSM-48上,丁烯骨架異構(gòu)的單分子反應(yīng)機理包括三個步驟:1)吸附態(tài)正丁烯質(zhì)子化;2)2-丁基氧化物的環(huán)化;3)異丁基氧化物去質(zhì)子化。其中,FER上的速控步驟為第三步,而ZSM-23和ZSM-48上則為第二步。但在ZSM-5上,雖然反應(yīng)機理的前兩步與在另外三種催化劑上一致,但異丁基氧化物又經(jīng)過叔丁基碳正離子的過渡態(tài)生成叔丁基烷氧化物,這步也是整個反應(yīng)的速控步驟,然后,再通過叔丁基烷氧化物的去質(zhì)子化生成異丁烯。由此可以看出,由于ZSM-5分子篩的孔道尺寸比FER大1.0 A更易于碳正離子的形成,導致具有相似二維交叉孔道結(jié)構(gòu)的FER和ZSM-5分子篩上具有不同的丁烯骨架異構(gòu)反應(yīng)機理。雖然具有一維直孔道十元環(huán)的ZSM-23和ZSM-48具有相同的反應(yīng)路徑和速控步驟,但由于ZSM-48較大的孔道尺寸可以降低反應(yīng)物在孔道中所受的空間位阻,因此,ZSM-48上的速控步驟的活化能為31.8 kcal/mol,比ZSM-23上35.1 kcal/mol的活化能較低�？偟膩砜�,FER,ZSM-23,ZSM-48和ZSM-5四種分子篩上丁烯異構(gòu)速控步驟活化能的大小順序為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上最不易發(fā)生丁烯骨架異構(gòu)反應(yīng),在ZSM-48上丁烯骨架異構(gòu)反應(yīng)最易發(fā)生,但由于ZSM-48分子篩過大的孔道尺寸,會降低對異丁烯的選擇性。因此,FER依然被認為是這四種分子篩中最適合用于丁烯骨架異構(gòu)的催化劑,這一結(jié)論不僅與前人所報道的實驗結(jié)果相吻合,而且也很好的解釋了產(chǎn)生這一結(jié)果的原因。在第二部分中,以孔道結(jié)構(gòu)差異較小的FER和ZSM-5分子篩為研究對象,分別選擇包含十元環(huán)孔道的90T和128T模型,采用ONIOM(B3LYP/6-31G(d,p),AM1)計算方法系統(tǒng)的研究不同數(shù)量Al原子取代Si原子后所形成的Bronsted酸位(B酸位)分布的穩(wěn)定性及酸強度等性質(zhì),并比較不同性質(zhì)的B酸位作為催化反應(yīng)活性中心對丁烯雙鍵異構(gòu)反應(yīng)過程的影響。在FER的1-Al取代中,Al4-06-Si2具有最強的穩(wěn)定性,Al3-O7-Si4具有最高的B酸強度。在2-Al取代模型中,Al4-OH-(SiO)2-Al4-OH 和 Al1-OH-(SiO)2Si-HO-Al4 分別成為穩(wěn)定性最高和酸強度最大的B酸位點。在ZSM-5上的1-Al取代中,Al9-018-Si6具有較強的穩(wěn)定性,Al6-018-Si9具有較高的酸強度。HO-Al6-OSiOSi-OH-Al6 和 Al6-OH-SiOSi-OH-Al6 成為 2-Al 取代形成 B酸位中穩(wěn)定性最高和酸性最強的B酸位點。通過分析發(fā)現(xiàn),無論在FER還是ZSM-5上,H質(zhì)子的落位對B酸穩(wěn)定性及酸強度都有重要的影響,在1-Al取代模型中H質(zhì)子的落位會導致與分子篩骨架結(jié)構(gòu)形成特殊的相互作用(氫鍵),從而提高該處B酸位點的穩(wěn)定性及酸強度。在2-Al取代模型中,H質(zhì)子落位在兩個Al原子同側(cè)形成的B酸位比落位在兩個Al原子之間形成的B酸位具有較低的穩(wěn)定性和較高的酸強度。另外,2-Al取代所形成B酸的酸強度和穩(wěn)定性隨著兩個取代Al原子之間距離的增大而增強。與此同時,隨著分子篩中Al原子取代數(shù)量的增加,所形成的B酸位點的穩(wěn)定性增強,但是酸強度卻呈現(xiàn)減小的趨勢。B酸位點酸性質(zhì)的不同對丁烯雙鍵異構(gòu)反應(yīng)的影響體現(xiàn)在:B酸性質(zhì)不同的活性位點并不會改變丁烯雙鍵異構(gòu)過程的反應(yīng)路徑,但酸性質(zhì)的不同卻會導致反應(yīng)能壘、中間產(chǎn)物結(jié)構(gòu)以及過渡態(tài)構(gòu)型的較大差異。在FER中,酸強度最強的B酸位點Al3-07-Si4和Al1-O-(SiO)3-Al4上反應(yīng)活化能分別為21.8 kcal/mol和 18.1 kcal/mol,而穩(wěn)定性最強的 B 酸位點 Al4-O6-Si2 和 Al4-O-(SiO)2-Al4上反應(yīng)活化能分別為25.1 kcal/mol和27.6 kcal/mol。在ZSM-5中,酸強度最強和穩(wěn)定性最高的B酸位點分別是Al6-O18-Si9、Al6-OH-SiOSi-OH-Al6 和 Al9-O18-Si6、HO-Al6-O-SiOSi-OH-Al6, 丁烯雙鍵異構(gòu)在這些位點上的活化能分別為20.7 kcal/mol、17.8 kcal/mol、24.0 kcal/mol以及26.7 kcal/mol,由此看出,相對于穩(wěn)定性,B酸位點的酸強度可以直接決定丁烯雙鍵異構(gòu)化過程的反應(yīng)活性。但B酸位點較高的催化活性是適宜的穩(wěn)定性和酸強度協(xié)調(diào)作用的結(jié)果。在本論文的第三部分,針對金屬改性分子篩Fe/ZSM-5,采用ONIOM分層的方法,討論金屬離子Fe2+和[FeO]2+在ZSM-5不同結(jié)構(gòu)中的分布穩(wěn)定性。同時,又選擇含有8T的雙五元環(huán)模型,以B3LYP/6-31G(d,p)方法考察了雙鐵位點[Fe(μ-O)Fe]2+、[Fe(μ-O)2Fe]2+、[Fe(μ-O)(μ-OH)Fe]+ 和[HOFe(μ-O)FeOH]2+上甲烷氧化過程的反應(yīng)機理。對于金屬在ZSM-5上的落位,無論是Fe2+還是[FeO]2+最穩(wěn)定的落位在ZSM-5直孔道中的六圓環(huán)上(δ-6MR)兩個T1 1位被Al取代時形成的對稱構(gòu)型中,位于直孔道δ-6MR上的其他位置和α-6MR上的穩(wěn)定性次之,位于直孔道與正弦孔道交叉處的β-6MR中較不易落位,穩(wěn)定性較低。另外,對Fe2+來說,在正弦孔道中的八元環(huán)上的分布也具有比較高的穩(wěn)定性�？偟膩碚f,金屬離子的分布穩(wěn)定性主要依賴于金屬陽離子與鋁氧四面體中的氧負離子之間的配位作用,除此之外,分子篩自身骨架結(jié)構(gòu)的可延展性也有能提高金屬離子分布的穩(wěn)定性。在四種不同的雙鐵位點[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應(yīng)機理:1)甲烷C-H鍵斷裂;2)甲醇的生成。但在[Fe(μ-O)Fe]2+位點上,甲醇的形成是整個反應(yīng)過程的速控步驟,反應(yīng)活化能高達43.3 kcal/mol;而在另外其他三個位點上,速控步驟都是甲烷C-H鍵的斷裂,在四種不同位點上反應(yīng)速控步驟的活化能為:[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)。通過觀察,在含有羥基的雙鐵位點[Fe(μ-O)(μ-OH)Fe]+和[HOFe(μ-O)FeOH]2+上,羥基的存在可以降低甲烷C-H鍵斷裂的活化能,提高其反應(yīng)速控步驟的反應(yīng)速率。而在不含羥基的[Fe(μ-O)Fe]2+和[Fe(μ-O)2Fe]2+位點上,通過加入水分子,可以明顯的降低甲醇形成過程的活化能,與無水分子加入時相比,分別降低了13.2 kcal/mol和37.4 kcal/mol。這主要是加入的水分子與活性位之間存在競爭吸附導致的。我們的計算結(jié)果不僅很好的解釋實驗現(xiàn)象,而且為制備高效的甲烷氧化反應(yīng)的催化劑提供理論依據(jù)。
[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.
【學位授予單位】：北京化工大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：O621.25;O643.36

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本文編號：2164331