HZSM-5 分子筛上乙烯芳构化过程中 C4 至 C6 中间体的反应机理

 应用量子力学和分子力学联合的 ONIOM2 (B3LYP/6-31G(d,p):UFF) 方法, 采用包含分子筛孔道结构的 78T 簇模型, 对 HZSM-5 分子筛上乙烯芳构化过程中 C4 至 C6 中间体的反应历程进行了研究, 探讨了分子筛的酸催化机理和择形催化作用. 结果表明, 作为乙烯二聚产物的表面正丁基烷氧络合物 (C4) 直接与乙烯作用得到正己基烷氧络合物 (C6), 在分子筛孔穴尺寸的限制下, 很难实现碳链的折叠环化. 按照间歇反应历程, 丁基烷氧络合物先发生 C–O 键断裂, 脱质子生成 1-丁烯, 然后在酸性位上再与乙烯加成, 在分子筛表面生成 3-甲基戊基烷氧络合物. 该烷氧络合物脱除质子给分子筛, 同时环化生成甲基环戊烷, 后者再与分子筛酸性质子共同脱除氢分子, 生成不稳定的碳正离子中间体, 然后重构成环己烷正离子. 丁基烷氧络合物脱质子的活化能为 158.42 kJ/mol; 1-丁烯与乙烯加成反应的活化能为 130.71 kJ/mol; 3-甲基戊基烷氧络合物脱氢环化生成甲基环戊烷的活化能为 122.06 kJ/mol. 由于孔穴的限域作用, 五员环的甲基环戊烷是重要的中间体.     

Ga/HZSM-5分子筛上乙烯二聚反应的理论研究

应用理论计算方法研究了Ga/HZSM-5及Al/HZSM-5 分子筛上乙烯二聚生成1-丁烯的反应历程, 比较了分子筛酸性对反应能量的影响. 计算采用分为两层的76T簇模型, 应用量子力学和分子力学联合的 ONIOM2 (B3LYP/6-31G(d, p):UFF) 方法. 乙烯二聚过程可按照分步机理和协同机理进行, 均得到表面丁基烷氧中间产物. 计算结果表明, 与在Al/HZSM-5分子筛上的反应过程相比, 乙烯在 Ga/HZSM-5分子筛上的吸附能低 20.62 kJ·mol^-1, 但质子化反应的活化能只高出1.26 kJ·mol^-1; 而乙基烷氧中间体与乙烯分子结合过程的活化能高出 62.55 kJ·mol^-1, 原因是Ga 原子半径大, 降低了六元环过渡态的稳定性. 若按协同机理, 质子转移和C―C键聚合同时进行, 在 Ga/HZSM-5分子筛上的活化能较Al/HZSM-5的高16.44 kJ·mol^-1. 因此乙烯二聚按照协同机理有利. 研究还表明, 表面丁基烷氧中间体脱质子, 生成1-丁烯并吸附在复原的分子筛酸性位上. 该反应在两种酸中心上的活化能几乎相同, 但明显高于其他各步的活化能, 因此成为整个反应的速度控制步骤.     

n‐Butane Monomolecular Cracking on Acidic Zeolites: A Density Functional Study

采用5T簇模型,利用密度泛函理论在B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d)水平下研究正丁烷在酸性分子筛上的单分子催化裂解反应。本文重点详细研究了正丁烷在分子筛表面不同C位的脱氢反应。在B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d)水平下计算所得第一和第二位C-C键裂解的活化能垒分别为 238、217 kJ/mol。而第一第二序位脱氢反应能垒分别为296、242 kJ/mol。正丁烷不同序位脱氢反应的活化能垒相差54 kJ/mol。从计算结果可以看出,正丁烷在分子筛上催化裂解脱氢反应优先发生在第二位C原子上。此外,本文还讨论了簇模型结构与酸性的关系,结果显示改变封端Si-H键的键长的方法可以用来模拟分子筛酸性变化。最后研究了分子筛酸性变化与正丁烷催化裂解反应能垒的关系。     

酸性分子筛催化乙烯二聚反应

应用密度泛函理论(DFT), 采用5T簇模型来模拟分子筛催化剂的酸性位, 在B3LYP/6-311+G(3df, 2p)的条件下通过理论计算研究了乙烯在酸性分子筛上的二聚反应. 对反应各驻点进行了全局优化, 经过零点能校正后, 计算得出乙烯二聚反应的活化能. 研究表明, 乙烯在分子筛上的二聚反应分三步进行: 单个乙烯分子化学吸附→第二个乙烯分子的物理吸附→两乙烯分子二聚反应. 乙烯化学吸附生成的烷氧化合物与物理吸附的乙烯分子发生二聚反应生成新的C—C键同时生成新的烷氧化合物. 计算得到的乙烯化学吸附和二聚反应的反应能垒分别为108和149 kJ·mol-1. 反应的逆过程也就是1-丁烯在酸性分子筛表面的1-丁基烷氧化合物发生β分裂反应, 计算所得相应的1-丁烯β分裂反应的能垒为217 kJ·mol-1, 远高于相应的乙烯二聚反应能垒. 此外还进一步研究了所用基组对计算结果的影响.     

分子筛催化cis-2-丁烯的双键异构反应机理的DFT研究

基于含有两个Si和一个Al的分子筛3T簇模型, 利用密度泛函方法(DFT)研究了分子筛催化1-丁烯双键异构为cis-2-丁烯的反应机理. 在B3LYP/6-31G(d,p)计算水平上对反应各驻点进行了全优化, 并计算了反应的活化能. 研究发现, 分子筛上的酸性OH基团首先通过物理吸附靠近1-丁烯的双键, 形成了π配位复合物后, 丁烯双键的端基C原子逐渐抽取这个质子, 同时相邻酸性位的一个O原子也抽取丁烯碳链上的一个H原子, 形成吸附态的cis-2-丁烯, 最后通过脱附形成产物, 使分子筛复原, 反应按照协同反应机理发生. 计算得到的表观活化能是55.9 kJ/mol, 与实验结果接近.     

密度泛函理论研究分子筛相邻双酸性位对乙烯质子化反应的影响

采用密度泛函理论B3LYP/6-31G(d,p)方法,研究了HZSM-5分子筛上不同酸性位以及相邻双酸性位对乙烯质子化反应的影响,考察了乙烯吸附、质子化反应过渡态和表面乙氧中间体的几何结构、原子电荷和相关能量.结果表明,存在紧邻酸性位的分子筛酸性明显减弱,乙烯分子的吸附能降低,但质子转移过渡态的活化能仅略有降低;次邻酸...     

ONIOM 法研究分子筛骨架对噻吩裂解反应的影响

 采用 ONIOM 法研究了分子筛上噻吩的裂解反应, 考察了分子筛骨架环境对反应中各物种的几何构型、电荷分布以及整个反应的影响. 比较了 B3LYP 和 M05-2X 泛函的计算结果. 结果表明, 分子筛骨架可很好地稳定反应带电中间体, 促进了吸附分子的电荷分离, 从而使反应活化能降低. 与现在应用广泛的 B3LYP 泛函相比, 最近开发的 M05-2X 泛函能够更准确地描述反应物分子和分子筛骨架的相互作用, 计算得到的电荷分布和活化能也更为合理. 整个反应的决速步是亲电取代反应, 其活化能为 122.4 kJ/mol, 随后的 C–S 键解离活化能较低, 为 75.5 kJ/mol.     

环已胺在沸石分子筛上的吸脱附行为研究

本文以程序升温脱附(TPD)为主要实验手段,对环己胺在5种不同沸石分子筛上的吸脱附行为进行了研究。结果表明,沸石分子筛对环己胺有着较强的吸附作用,不同的沸石分子筛对环己胺的吸附能力受其结构和表面酸性特征的影响而异。有效吸附部位为与沸石分子筛表面酸性有关的弱化学吸附位;环己胺从不同沸石分子筛表面脱附的动力学与晶内扩散有关,其表观脱附活化能分别为:63.6kJ/mol(5A),68.6kJ/mol(13X),20.1kJ/mol(菱沸石),46.9kJ/mol(NaY)和47.3KJ/mol(ZSM-5)。     

ZSM-5分子筛催化1-己烯生成cis-2-己烯的理论研究

基于54T团簇模型, 采用ONIOM分层计算方法, 研究了1-己烯在ZSM-5分子筛上进行顺式双键异构的反应机理. 计算结果表明, 1-己烯的顺式双键异构反应通过只有分子筛Brønsted酸部分起作用的机理进行. 首先, 1-己烯与分子筛的Brønsted酸性位形成π配位复合物. 接着, 酸质子发生迁移使1-己烯的双键端基碳原子被质子化, 同时双键的另一碳原子与失去质子的Brønsted酸羟基的氧原子成键, 形成稳定的烷氧基中间体. 然后, 烷氧基中间体中的C―O共价键被打断, 同时Brønsted酸羟基的氧原子从C₆H₁₃基团提取一个氢原子还原分子筛的酸性位, 并且生成cis-2-己烯. 这一反应路径与借助于分子筛活性位的酸-碱双功能性质的反应路径是相互竞争的. 计算得到的表观活化能是59.37 kJ·mol^-1, 该值与实验值非常接近. 这一结果合理解释了双键异构过程中的能量特征, 并且扩展了对分子筛活性位本质的理解.     

H-ZSM-5分子筛上的乙烯二聚反应机理的理论计算研究

应用分子力学和量子力学联合的ONIOM2(B3LYP/6-31G(d,p):UFF)计算方法研究了H-ZSM-5分子筛上乙烯二聚反应的机理. 用40T簇模型模拟ZSM-5分子筛位于孔道交叉点的酸性位,对乙烯二聚过程的分步反应和协同反应两种机理进行了考察. 对于分步反应机理,乙烯分子首先通过π-氢键作用在酸性位形成稳定的吸附络合物,再进一步发生质子化并生成乙醇盐中间体,随后乙醇盐与第二个乙烯分子发生碳-碳键结合形成丁醇盐产物. 第一步质子化和第二步碳链聚合的活化能分别为152.88和119.45 kJ/mol, 表明乙烯质子化反应为速控步骤. 对于协同反应机理,乙烯质子化、碳-碳键和碳-氧键生成同时进行,生成丁醇盐,反应的活化能为162.30 kJ/mol, 略高于分步反应机理中的速控步骤. 计算结果表明这两种反应机理之间存在相互竞争.     

Theoretical ONIOM2 study on pyridine adsorption in the channels and intersection of ZSM-5

The structures of the acid sites in the channels and intersections of H-, Li-, and Na-ZSM-5 (ZSM = zeolite socony mobil) and their interactions with pyridine molecule have been computed by using three corresponding models containing 22 tetrahedral sites. The calculated adsorption energies of pyridine in the intersection regions of H-, Li-, and Na-ZSM-5 are 197.0, 172.5, and 122.3 kJ/mol, respectively, in good agreement with the respective experimental values of 200 +/- 5, 155-195, and 120 kJ/mol, while those in the straight and sinusoidal channels are much smaller (157.9 and 127.6, 152.2 and 149.4, and 150.4 and 109.9 kJ/mol, respectively). These indicate that the most probable adsorption site for pyridine in ZSM-5 is the acidic site located in the intersection region. The structural parameters of the adsorption complexes show that the acidic proton in the three models of H-ZSM-5 has been transferred to the nitrogen of pyridine, while in alkali cation-exchanged ZSM-5, the coordination of the alkali cation to the nitrogen atom of pyridine dominates the overall interaction. In addition, the adsorption complexes were further stabilized by the long-range electrostatic interaction between the positively charged pyridine hydrogen atoms and the negatively charged lattice oxygen atoms of the zeolite framework. In the intersection regions of H-, Li-, and Na-ZSM-5, the coordination energy of the charge-compensating cation to the pyridine nitrogen amounts to 58, 60, and 68% of the total adsorption energy, respectively, while another 42, 40, and 32%, respectively, is due to long-range electrostatic interactions. This indicates that the zeolite lattice framework surrounding the adsorption site has important contributions to the adsorption energy of the pyridine molecule.     

Theoretical study of modes of adsorption of water dimer on H-ZSM-5 and H-Faujasite zeolites

Modes of adsorption of water dimer on H-ZSM-5 and H-Faujasite (H-FAU) zeolites have been investigated by a quantum embedded cluster approach, using the hybrid B3LYP density functional theory. The results indicate that there are two possible adsorption pathways, namely the stepwise process where only one water binds strongly to the (-O)3-Al-O(H) tetrahedral unit while the other weakly binds to the zeolite framework and the concerted process where both water molecules form a large ring of hydrogen-bonding network with the Br?nsted proton and an oxygen framework. With inclusion of the effects of the Madelung potential from the extended zeolite framework, for adsorption on H-ZSM-5 zeolite, both the neutral and ion-pair complexes exist with adsorption energies of -15.13 and -14.73 kcal/mol, respectively. For adsorption on the H-FAU, only the ion-pair complex exists with the adsorption energy of -14.63 kcal/mol. Our results indicate that adsorption properties depend not only on the acidity of the Br?nsted acidic site but also on the topology of the zeolite framework, such as on the spatial confinement effects which lead to very different adsorption structures for the ion-pair complexes in H-ZSM-5 and H-FAU, even though their adsorption energies are quite similar. Our calculated vibrational spectra for these ion-pair complexes support previous experimental IR interpretations.     

Density functional study of the mechanism of the Beckmann rearrangement catalyzed by H-ZSM-5: a cluster and embedded cluster study

The mechanism of the Beckmann rearrangement (BR) catalyzed by the ZSM-5 zeolite has been investigated by both the quantum cluster and embedded cluster approaches at the B3LYP level of theory using the 6-31G(d,p) basis set. Single-point calculations were carried out at the MP2/6-311G(d,p) level of theory to improve energetic properties. The embedded cluster model suggests that the initial step of the Beckmann rearrangement is not the O-protonated oxime but the N-protonated oxime. The energy barriers derived from the proton shuttle of the N-bound to the O-bound isomer are determined to be approximately 99 and approximately 40 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. The difference in the activation energy is due mainly to the effect of the Madelung potential from the zeolite framework. The next step is the rearrangement step, which is the transformation of the O-protonated oxime to be an enol-formed amide compound, formimidic acid. The activation energy, at the rearrangement step, is calculated to be approximately 125 and approximately 270 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. The final step is the tautomerization step which transforms the enol-form to the keto-form, formamide compound. The energy barrier for tautomerization is calculated to be 123 and 151 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. These calculated results suggest that the rate-determining step of the vapor phase of the Beckmann rearrangement on H-ZSM-5 is the rearrangement or tautomerization step.     

Reactivity of alkanes on zeolites: a computational study of propane conversion reactions

In this work, quantum chemical methods were used to study propane conversion reactions on zeolites; these reactions included protolytic cracking, primary hydrogen exchange, secondary hydrogen exchange, and dehydrogenation reactions. The reactants, products, and transition-state structures were optimized at the B3LYP/6-31G level and the energies were calculated with CBS-QB3, a complete basis set composite energy method. The computed activation barriers were 62.1 and 62.6 kcal/mol for protolytic cracking through two different transition states, 30.4 kcal/mol for primary hydrogen exchange, 29.8 kcal/mol for secondary hydrogen exchange, and 76.7 kcal/mol for dehydrogenation reactions. The effects of basis set for the geometry optimization and zeolite acidity on the reaction barriers were also investigated. Adding extra polarization and diffuse functions for the geometry optimization did not affect the activation barriers obtained with the composite energy method. The largest difference in calculated activation barriers is within 1 kcal/mol. Reaction activation barriers do change as zeolite acidity changes, however. Linear relationships were found between activation barriers and zeolite deprotonation energies. Analytical expressions for each reaction were proposed so that accurate activation barriers can be obtained when using different zeolites as catalysts, as long as the deprotonation energies are first acquired.     

Nature of the metal-support interaction in bifunctional catalytic Pt/H-ZSM-5 zeolite

The metal-support interaction of a dispersed Pt atom on H-ZSM-5 zeolite has been investigated by using an embedded cluster and cluster models with the density functional theory/B3LYP functional method. We found that the Pt atom interacts with a Br?nsted proton and a nearby framework oxygen. Interaction with the framework oxygen causes electron transfer from the zeolite to the Pt atom. Concurrently, a Br?nsted proton stabilizes the Pt atom on the zeolite surface by withdrawing excess electron density from the Pt atom. These charge transfers result in a zero net charge on the Pt atom while changing its orbital occupation. The binding energy of Pt on the Br?nsted acid was 15 kcal/mol. Inclusion of the Madelung potential by Surface Charge Representation of the Electrostatic Embedded Potential method (SCREEP) had small effects on structure and charge density of Pt/H-ZSM-5 but it shifted the stretching mode of CO toward a higher frequency by almost 40 cm(-1). The frequency shift of absorbed CO calculated with embedded cluster models was from 8 to 11 cm(-1) red shift, compared to 20 cm(-1) red shift from experiment. This implies that not only the electronic state of the Pt atom but also the Madelung potential of the support is responsible for the observed small red shift of CO on the Pt-H-ZSM-5.     

Reactivity of isobutane on zeolites: a first principles study

In this work, ab initio and density functional theory methods are used to study isobutane protolytic cracking, primary hydrogen exchange, tertiary hydrogen exchange, and dehydrogenation reactions catalyzed by zeolites. The reactants, products, and transition-state structures are optimized at the B3LYP/6-31G* level, and the final energies are calculated using the CBS-QB3 composite energy method. The computed activation barriers are 52.3 kcal/mol for cracking, 29.4 kcal/mol for primary hydrogen exchange, 29.9 kcal/mol for tertiary hydrogen exchange, and 59.4 kcal/mol for dehydrogenation. The zeolite acidity effects on the reaction barriers are also investigated by changing the cluster terminal Si-H bond lengths. The analytical expressions between activation barriers and zeolite deprotonation energies for each reaction are proposed so that accurate activation barriers can be obtained when using different zeolites as catalysts.     

DNA polymerase beta catalysis: are different mechanisms possible?

DNA polymerases are crucial constituents of the complex cellular machinery for replicating and repairing DNA. Discerning mechanistic pathways of DNA polymerase on the atomic level is important for revealing the origin of fidelity discrimination. Mammalian DNA polymerase beta (pol beta), a small (39 kDa) member of the X-family, represents an excellent model system to investigate polymerase mechanisms. Here, we explore several feasible low-energy pathways of the nucleotide transfer reaction of pol beta for correct (according to Watson-Crick hydrogen bonding) G:C basepairing versus the incorrect G:G case within a consistent theoretical framework. We use mixed quantum mechanics/molecular mechanics (QM/MM) techniques in a constrained energy minimization protocol to effectively model not only the reactive core but also the influence of the rest of the enzymatic environment and explicit solvent on the reaction. The postulated pathways involve initial proton abstraction from the terminal DNA primer O3'H group, nucleophilic attack that extends the DNA primer chain, and elimination of pyrophosphate. In particular, we analyze several possible routes for the initial deprotonation step: (i) direct transfer to a phosphate oxygen O(Palpha) of the incoming nucleotide, (ii) direct transfer to an active site Asp group, and (iii) transfer to explicit water molecules. We find that the most probable initial step corresponds to step (iii), involving initial deprotonation to water, which is followed by proton migration to active site Asp residues, and finally to the leaving pyrophosphate group, with an activation energy of about 15 kcal/mol. We argue that initial deprotonation steps (i) and (ii) are less likely as they are at least 7 and 11 kcal/mol, respectively, higher in energy. Overall, the rate-determining step for both the correct and the incorrect nucleotide cases is the initial deprotonation in concert with nucleophilic attack at the phosphate center; however, the activation energy we obtain for the mismatched G:G case is 5 kcal/mol higher than that of the matched G:C complex, due to active site structural distortions. Taken together, our results support other reported mechanisms and help define a framework for interpreting nucleotide specificity differences across polymerase families, in terms of the concept of active site preorganization or the so-called "pre-chemistry avenue".